RFC 1045

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Network Working Group                                     David Cheriton
Request for Comments:  1045                          Stanford University
                                                           February 1988

                         Protocol Specification


This RFC describes a protocol proposed as a standard for the Internet
community.  Comments are encouraged.  Distribution of this document is


This memo specifies the Versatile Message Transaction Protocol (VMTP)
[Version 0.7 of 19-Feb-88], a transport protocol specifically designed
to support the transaction model of communication, as exemplified by
remote procedure call (RPC).  The full function of VMTP, including
support for security, real-time, asynchronous message exchanges,
streaming, multicast and idempotency, provides a rich selection to the
VMTP user level.  Subsettability allows the VMTP module for particular
clients and servers to be specialized and simplified to the services
actually required.  Examples of such simple clients and servers include

Table of Contents

1. Introduction                                                        1

   1.1. Motivation                                                     2
       1.1.1. Poor RPC Performance                                     2
       1.1.2. Weak Naming                                              3
       1.1.3. Function Poor                                            3
   1.2. Relation to Other Protocols                                    4
   1.3. Document Overview                                              5

2. Protocol Overview                                                   6

   2.1. Entities, Processes and Principals                             7
   2.2. Entity Domains                                                 9
   2.3. Message Transactions                                          10
   2.4. Request and Response Messages                                 11
   2.5. Reliability                                                   12
       2.5.1. Transaction Identifiers                                 13
       2.5.2. Checksum                                                14
       2.5.3. Request and Response Acknowledgment                     14
       2.5.4. Retransmissions                                         15
       2.5.5. Timeouts                                                15
       2.5.6. Rate Control                                            18
   2.6. Security                                                      19
   2.7. Multicast                                                     21
   2.8. Real-time Communication                                       22
   2.9. Forwarded Message Transactions                                24
   2.10. VMTP Management                                              25
   2.11. Streamed Message Transactions                                25
   2.12. Fault-Tolerant Applications                                  28
   2.13. Packet Groups                                                29
   2.14. Runs of Packet Groups                                        31
   2.15. Byte Order                                                   32
   2.16. Minimal VMTP Implementation                                  33
   2.17. Message vs. Procedural Request Handling                      33
   2.18. Bibliography                                                 34

3. VMTP Packet Formats                                                37

   3.1. Entity Identifier Format                                      37
   3.2. Packet Fields                                                 38
   3.3. Request Packet                                                45
   3.4. Response Packet                                               47

4. Client Protocol Operation                                          49

   4.1. Client State Record Fields                                    49
   4.2. Client Protocol States                                        51
   4.3. State Transition Diagrams                                     51
   4.4. User Interface                                                52
   4.5. Event Processing                                              53
   4.6. Client User-invoked Events                                    54
       4.6.1. Send                                                    54
       4.6.2. GetResponse                                             56
   4.7. Packet Arrival                                                56
       4.7.1. Response                                                58
   4.8. Management Operations                                         61
       4.8.1. HandleNoCSR                                             62
   4.9. Timeouts                                                      64

5. Server Protocol Operation                                          66

   5.1. Remote Client State Record Fields                             66
   5.2. Remote Client Protocol States                                 66
   5.3. State Transition Diagrams                                     67
   5.4. User Interface                                                69
   5.5. Event Processing                                              70
   5.6. Server User-invoked Events                                    71
       5.6.1. Receive                                                 71
       5.6.2. Respond                                                 72
       5.6.3. Forward                                                 73
       5.6.4. Other Functions                                         74
   5.7. Request Packet Arrival                                        74
   5.8. Management Operations                                         78
       5.8.1. HandleRequestNoCSR                                      79
   5.9. Timeouts                                                      82

6. Concluding Remarks                                                 84

I. Standard VMTP Response Codes                                       85

II. VMTP RPC Presentation Protocol                                    87
   II.1. Request Code Management                                      87

III. VMTP Management Procedures                                       89

   III.1. Entity Group Management                                    100
   III.2. VMTP Management Digital Signatures                         101

IV. VMTP Entity Identifier Domains                                   102

   IV.1. Domain 1                                                    102
   IV.2. Domain 3                                                    104
   IV.3. Other Domains                                               105
   IV.4. Decentralized Entity Identifier Allocation                  105

V. Authentication Domains                                            107

   V.1. Authentication Domain 1                                      107
   V.2. Other Authentication Domains                                 107

VI. IP Implementation                                                108

VII. Implementation Notes                                            109

   VII.1. Mapping Data Structures                                    109
   VII.2. Client Data Structures                                     111
   VII.3. Server Data Structures                                     111
   VII.4. Packet Group transmission                                  112
   VII.5. VMTP Management Module                                     113
   VII.6. Timeout Handling                                           114
   VII.7. Timeout Values                                             114
   VII.8. Packet Reception                                           115
   VII.9. Streaming                                                  116
   VII.10. Implementation Experience                                 117

VIII. UNIX 4.3 BSD Kernel Interface for VMTP                         118

Index                                                                120
                            List of Figures

   Figure 1-1:   Relation to Other Protocols                           4
   Figure 3-1:   Request Packet Format                                45
   Figure 3-2:   Response Packet Format                               47
   Figure 4-1:   Client State Transitions                             52
   Figure 5-1:   Remote Client State Transitions                      68
   Figure III-1:   Authenticator Format                               92
   Figure VII-1:   Mapping Client Identifier to CSR                  109
   Figure VII-2:   Mapping Server Identifiers                        110
   Figure VII-3:   Mapping Group Identifiers                         111

1. Introduction Go to the top of this page...top

The Versatile Message Transaction Protocol (VMTP) is a transport
protocol designed to support remote procedure call (RPC) and general
transaction-oriented communication.  By transaction-oriented
communication, we mean that:

   - Communication is request-response:  A client sends a request
     for a service to a server, the request is processed, and the
     server responds.  For example, a client may ask for the next
     page of a file as the service.  The transaction is terminated
     by the server responding with the next page.

   - A transaction is initiated as part of sending a request to a
     server and terminated by the server responding.  There are no
     separate operations for setting up or terminating associations
     between clients and servers at the transport level.

   - The server is free to discard communication state about a
     client between transactions without causing incorrect behavior
     or failures.

The term message transaction (or transaction) is used in the reminder of
this document for a request-response exchange in the sense described

VMTP handles the error detection, retransmission, duplicate suppression
and, optionally, security required for transport-level end-to-end

The protocol is designed to provide a range of behaviors within the
transaction model, including:

   - Minimal two packet exchanges for short, simple transactions.

   - Streaming of multi-packet requests and responses for efficient
     data transfer.

   - Datagram and multicast communication as an extension of the
     transaction model.

Example Uses:

   - Page-level file access - VMTP is intended as the transport
     level for file access, allowing simple, efficient operation on
     a local network.  In particular, VMTP is appropriate for use
     by diskless workstations accessing shared network file

   - Distributed programming - VMTP is intended to provide an
     efficient transport level protocol for remote procedure call
     implementations, distributed object-oriented systems plus
     message-based systems that conform to the request-response

   - Multicast communication with groups of servers to:  locate a
     specific object within the group, update a replicated object,
     synchronize the commitment of a distributed transaction, etc.

   - Distributed real-time control with prioritized message
     handling, including datagrams, multicast and asynchronous

The protocol is designed to operate on top of a simple unreliable
datagram service, such as is provided by IP.

1.1. Motivation Go to the top of this page...top

VMTP was designed to address three categories of deficiencies with
existing transport protocols in the Internet architecture.  We use TCP
as the key current transport protocol for comparison.

1.1.1. Poor RPC Performance Go to the top of this page...top

First, current protocols provide poor performance for remote procedure
call (RPC) and network file access.  This is attributable to three key

   - TCP requires excessive packets for RPC, especially for
     isolated calls.  In particular, connection setup and clear
     generates extra packets over that needed for VMTP to support

   - TCP is difficult to implement, speaking purely from the
     empirical experience over the last 10 years.  VMTP was
     designed concurrently with its implementation, with focus on
     making it easy to implement and providing sensible subsets of
     its functionality.

   - TCP handles packet loss due to overruns poorly.  We claim that
     overruns are the key source of packet loss in a
     high-performance RPC environment and, with the increasing
     performance of networks, will continue to be the key source.
     (Older machines and network interfaces cannot keep up with new
     machines and network interfaces.  Also, low-end network
     interfaces for high-speed networks have limited receive

VMTP is designed for ease of implementation and efficient RPC.  In
addition, it provides selective retransmission with rate-based flow
control, thus addressing all of the above issues.

1.1.2. Weak Naming Go to the top of this page...top

Second, current protocols provide inadequate naming of transport-level
endpoints because the names are based on IP addresses.  For example, a
TCP endpoint is named by an Internet address and port identifier.
Unfortunately, this makes the endpoint tied to a particular host
interface, not specifically the process-level state associated with the
transport-level endpoint.  In particular, this form of naming causes
problems for process migration, mobile hosts and multi-homed hosts.
VMTP provides host-address independent names, thereby solving the above
mentioned problems.

In addition, TCP provides no security and reliability guarantees on the
dynamically allocated names.  In particular, other than well-known
ports, (host-addr, port-id)-tuples can change meaning on reboot
following a crash.  VMTP provides large identifiers with guarantee of
stability, meaning that either the identifier never changes in meaning
or else remains invalid for a significant time before becoming valid

1.1.3. Function Poor Go to the top of this page...top

TCP does not support multicast, real-time datagrams or security.  In
fact, it only supports pair-wise, long-term, streamed reliable
interchanges.  Yet, multicast is of growing importance and is being
developed for the Internet (see RFC 966 and 988).  Also, a datagram
facility with the same naming, transmission and reception facilities as
the normal transport level is a powerful asset for real-time and
parallel applications.  Finally, security is a basic requirement in an
increasing number of environments.  We note that security is natural to
implement at the transport level to provide end-to-end security (as
opposed to (inter)network level security).  Without security at the
transport level, a transport level protocol cannot guarantee the
standard transport level service definition in the presence of an
intruder.  In particular, the intruder can interject packets or modify
packets while updating the checksum, making mockery out of the
transport-level claim of "reliable delivery".

In contrast, VMTP provides multicast, real-time datagrams and security,
addressing precisely these weaknesses.

In general, VMTP is designed with the next generation of communication
systems in mind.  These communication systems are characterized as
follows.  RPC, page-level file access and other request-response
behavior dominates.  In addition, the communication substrate, both
local and wide-area, provides high data rates, low error rates and
relatively low delay.  Finally, intelligent, high-performance network
interfaces are common and in fact required to achieve performance that
approximates the network capability.  However, VMTP is also designed to
function acceptably with existing networks and network interfaces.

1.2. Relation To Other Protocols Go to the top of this page...top

VMTP is a transport protocol that fits into the layered Internet
protocol environment.  Figure 1-1 illustrates the place of VMTP in the
protocol hierarchy.

 +-----------+ +----+ +-----------------+ +------+
 |File Access| |Time| |Program Execution| |Naming|... Application
 +-----------+ +----+ +-----------------+ +------+      Layer
       |           |           |             |      |
                        | RPC Presentation |          Presentation
                        +------------------+          Layer
            +------+          +--------+
            |  TCP |          | VMTP   |              Transport
            +------+          +--------+              Layer
                |                  |
           |       Internet Protocol & ICMP    |      Internetwork
           +-----------------------------------+      Layer


Figure 1-1:   Relation To Other Protocols Go to the top of this page...top

The RPC presentation level is not currently defined in the Internet
suite of protocols.  Appendix II defines a proposed RPC presentation
level for use with VMTP and assumed for the definition of the VMTP
management procedures.  There is also a need for the definition of the
Application layer protocols listed above.

If internetwork services are not required, VMTP can be used without the
IP layer, layered directly on top of the network or data link layers.

1.3. Document Overview Go to the top of this page...top

The next chapter gives an overview of the protocol, covering naming,
message structure, reliability, flow control, streaming, real-time,
security, byte-ordering and management.  Chapter 3 describes the VMTP
packet formats.  Chapter 4 describes the client VMTP protocol operation
in terms of pseudo-code for event handling.  Chapter 5 describes the
server VMTP protocol operation in terms of pseudo-code for event
handling.  Chapter 6 summarizes the state of the protocol, some
remaining issues and expected directions for the future.  Appendix I
lists some standard Response codes.  Appendix II describes the RPC
presentation protocol proposed for VMTP and used with the VMTP
management procedures.  Appendix III lists the VMTP management
procedures.  Appendix IV proposes initial approaches for handling entity
identification for VMTP.  Appendix V proposes initial authentication
domains for VMTP.  Appendix VI provides some details for implementing
VMTP on top of IP.  Appendix VII provides some suggestions on host
implementation of VMTP, focusing on data structures and support
functions.  Appendix VIII describes a proposed program interface for
UNIX 4.3 BSD and its descendants and related systems.

2. Protocol Overview Go to the top of this page...top

VMTP provides an efficient, reliable, optionally secure transport
service in the message transaction or request-response model with the
following features:

   - Host address-independent naming with provision for multiple
     forms of names for endpoints as well as associated (security)
     principals.  (See Sections 2.1, 2.2, 3.1 and Appendix IV.)

   - Multi-packet request and response messages, with a maximum
     size of 4 megaoctets per message.  (Sections 2.3 and 2.14.)

   - Selective retransmission. (Section 2.13.)  and rate-based flow
     control to reduce overrun and the cost of overruns.  (Section

   - Secure message transactions with provision for a variety of
     encryption schemes.  (Section 2.6.)

   - Multicast message transactions with multiple response messages
     per request message.  (Section 2.7.)

   - Support for real-time communication with idempotent message
     transactions with minimal server overhead and state (Section
     2.5.3), datagram request message transactions with no
     response, optional header-only checksum, priority processing
     of transactions, conditional delivery and preemptive handling
     of requests (Section 2.8)

   - Forwarded message transactions as an optimization for certain
     forms of nested remote procedure calls or message
     transactions.  (Section 2.9.)

   - Multiple outstanding (asynchronous) message transactions per
     client.  (Section 2.11.)

   - An integrated management module, defined with a remote
     procedure call interface on top of VMTP providing a variety of
     communication services (Section 2.10.)

   - Simple subset implementation for simple clients and simple
     servers.  (Section 2.16.)

This chapter provides an overview of the protocol as introduction to the
basic ideas and as preparation for the subsequent chapters that describe
the packet formats and event processing procedures in detail.
In overview, VMTP provides transport communication between network-
visible entities via message transactions.  A message transaction
consists of a request message sent by the client, or requestor, to a
group of server entities followed by zero or more response messages to
the client, at most one from each server entity.  A message is
structured as a message control portion and a segment data portion.  A
message is transmitted as one or more packet groups.  A packet group  is
one or more packets (up to a maximum of 32 packets) grouped by the
protocol for acknowledgment, sequencing, selective retransmission and
rate control.

Entities and VMTP operations are managed using a VMTP management
mechanism that is accessed through a procedural interface (RPC)
implemented on top of VMTP.  In particular, information about a remote
entity is obtained and maintained using the Probe VMTP management
operation.  Also, acknowledgment information and requests for
retransmission are sent as notify requests to the management module.
(In the following description, reference to an "acknowledgment" of a
request or a response refers to a management-level notify operation that
is acknowledging the request or response.)

2.1. Entities, Processes And Principals Go to the top of this page...top

VMTP defines and uses three main types of identifiers:  entity
identifiers, process identifiers and principal identifiers, each 64-bits
in length.  Communication takes place between network-visible entities,
typically mapping to, or representing, a message port or procedure
invocation.  Thus, entities are the VMTP communication endpoints.  The
process associated with each entity designates the agent behind the
communication activity for purposes of resource allocation and
management.  For example, when a lock is requested on a file, the lock
is associated with the process, not the requesting entity, allowing a
process to use multiple entity identifiers to perform operations without
lock conflict between these entities.  The principal associated with an
entity specifies the permissions, security and accounting designation
associated with the entity.  The process and principal identifiers are
included in VMTP solely to make these values available to VMTP users
with the security and efficiency provided by VMTP.  Only the entity
identifiers are actively used by the protocol.

Entity identifiers are required to have three properties;

Uniqueness      Each entity identifier is uniquely defined at any given
                time.  (An entity identifier may be reused over time.)

Stability       An entity identifier does not change between valid
                meanings without suitable provision for removing
                references to the entity identifier.  Certain entity
                identifiers are strictly stable, (i.e. never changing
                meaning), typically being administratively assigned
                (although they need not be bound to a valid entity at
                all times), often called well-known identifiers.  All
                other entity identifiers are required to be T-stable,
                not change meaning without having remained invalid for
                at least a time interval T.

Host address independent
                An entity identifier is unique independent of the host
                address of its current host.  Moreover, an entity
                identifier is not tied to a single Internet host
                address.  An entity can migrate between hosts, reside on
                a mobile host that changes Internet addresses or reside
                on a multi-homed host.  It is up to the VMTP
                implementation to determine and maintain up to date the
                host addresses of entities with which it is

The stability of entity identifiers guarantees that an entity identifier
represents the same logical communication entity and principal (in the
security sense) over the time that it is valid.  For example, if an
entity identifier is authenticated as having the privileges of a given
user account, it continues to have those privileges as long as it is
continuously valid (unless some explicit notice is provided otherwise).
Thus, a file server need not fully authenticate the entity on every file
access request.  With T-stable identifiers, periodically checking the
validity of an entity identifier with period less than T seconds detects
a change in entity identifier validity.

A group of entities can form an entity group, which is a set of zero or
more entities identified by a single entity identifier.  For example,
one can have a single entity identifier that identifies the group of
name servers.  An entity identifier representing an entity group is
drawn from the same name space as entity identifiers.  However, single
entity identifiers are flagged as such by a bit in the entity
identifier, indicating that the identifier is known to identify at most
one entity.  In addition to the group bit, each entity identifier
includes other standard type flags.  One flag indicates whether the
identifier is an alias for an entity in another domain (See Section 2.2
below.).  Another flag indicates, for an entity group identifier,
whether the identifier is a restricted group or not.  A restricted group
is one in which an entity can be added only by another entity with group
management authorization.  With an unrestricted group, an entity is
allowed to add itself.  If an entity identifier does not represent a
group, a type bit indicates whether the entity uses big-endian or
little-endian data representation (corresponding to Motorola 680X0 and
VAX byte orders, respectively).  Further specification of the format of
entity identifiers is contained in Section 3.1 and Appendix IV.

An entity identifier identifies a Client, a Server or a group of
Servers <1>.  A Client is always identified by a T-stable identifier.  A
server or group of servers may be identified by a a T-stable identifier
(group or single entity) or by strictly stable (statically assigned)
entity group identifier.  The same T-stable identifier can be used to
identify a Client and Server simultaneously as long as both are
logically associated with the same entity.  The state required for
reliable, secure communication between entities is maintained in client
state records (CSRs), which include the entity identifier of the Client,
its principal, its current or next transaction identifier and so on.

2.2. Entity Domains Go to the top of this page...top

An entity domain is an administration or an administration mechanism
that guarantees the three required entity identifier properties of
uniqueness, stability and host address independence for the entities it
administers.  That is, entity identifiers are only guaranteed to be
unique and stable within one entity domain.  For example, the set of all
Internet hosts may function as one domain.  Independently, the set of
hosts local to one autonomous network may function as a separate domain.
Each entity domain is identified by an entity domain identifier, Domain.
Only entities within the same domain may communicate directly via VMTP.
However, hosts and entities may participate in multiple entity domains
simultaneously, possibly with different entity identifiers.  For
example, a file server may participate in multiple entity domains in
order to provide file service to each domain.  Each entity domain
specifies the algorithms for allocation, interpretation and mapping of
entity identifiers.

Domains are necessary because it does not appear feasible to specify one
universal VMTP entity identification administration that covers all
entities for all time.  Domains limit the number of entities that need
to be managed to maintain the uniqueness and stability of the entity


<1>   Terms such as Client, Server, Request, Response, etc.  are
capitalized in this document when they refer to their specific meaning
in VMTP.
name space.  Domains can also serve to separate entities of different
security levels.  For instance, allocation of a unclassified entity
identifier cannot conflict with secret level entity identifiers because
the former is interpreted only in the unclassified domain, which is
disjoint from the secret domain.

It is intended that there be a small number of domains.  In particular,
there should be one (or a few) domains per installation "type", rather
than per installation.  For example, the Internet is expected to use one
domain per security level, resulting in at most 8 different domains.
Cluster-based internetwork architectures, those with a local cluster
protocol distinct from the wide-area protocol, may use one domain for
local use and one for wide-area use.

Additional details on the specification of specific domains is provided
in Appendix IV.

2.3. Message Transactions Go to the top of this page...top

The message transaction is the unit of interaction between a Client that
initiates the transaction and one or more Servers.  A message
transaction starts with a request message  generated by a client.  At
the service interface, a server becomes involved with a transaction by
receiving and accepting the request.  A server terminates its
involvement with a transaction by sending a response message.  In a
group message transaction, the server entity designated by the client
corresponds to a group of entities.  In this case, each server in the
group receives a copy of the request.  In the client's view, the
transaction is terminated when it receives the response message or, in
the case of a group message transaction, when it receives the last
response message.  Because it is normally impractical to determine when
the last response message has been received.  the current transaction is
terminated by VMTP when the next transaction is initiated.

Within an entity domain, a transaction is uniquely identified by the
tuple (Client, Transaction, ForwardCount).  where Transaction is a
32-bit number and ForwardCount is a 4-bit value.  A Client uses
monotonically increasing Transaction identifiers for new message
transactions.  Normally, the next higher transaction number, modulo
2**32, is used for the next message transaction, although there are
cases in which it skips a small range of Transaction identifiers.  (See
the description of the STI control flag.)  The ForwardCount is used when
a message transaction is forwarded and is zero otherwise.

A Client generates a stream of message transactions with increasing
transaction identifiers, directed at a diversity of Servers.  We say a
Client has a transaction outstanding if it has invoked a message
transaction, but has not received the last Response (or possibly any
Response).  Normally, a Client has only one transaction outstanding at a
time.  However, VMTP allows a Client to have multiple message
transactions outstanding simultaneously, supporting streamed,
asynchronous remote procedure call invocations.  In addition, VMTP
supports nested calls where, for example, procedure A calls procedure B
which calls procedure C, each on a separate host with different client
entity identifiers for each call but identified with the same process
and principal.

2.4. Request And Response Messages Go to the top of this page...top

A message transaction consists of a request message and one or more
Response messages.  A message is structured as message control block
(MCB) and segment data, passed as parameters, as suggested below.

  | Message Control Block |
  |       segment data                |

In the request message, the MCB specifies control information about the
request plus an optional data segment.  The MCB has the following
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +                         ServerEntityId  (8 octets)            +
 |   Flags       |         RequestCode                           |
 +                         CoresidentEntity (8 octets)           +
 >                         User Data (12 octets)                 <
 |                         MsgDelivery                           |
 |                         SegmentSize                           |

The ServerEntityId is the entity to which the Request MCB is to be sent
(or was sent, in the case of reception).  The Flags indicate various
options in the request and response handling as well as whether the
CoresidentEntity, MsgDelivery and SegmentSize fields are in use.  The
RequestCode field specifies the type of Request.  It is analogous to a
packet type field of the Ethernet, acting as a switch for higher-level
protocols.  The CoresidentEntity field, if used, designates a subgroup
of the ServerEntityId group to which the Request should be routed,
namely those members that are co-resident with the specified entity (or
entity group).  The primary intended use is to specify the manager for a
particular service that is co-resident with a particular entity, using
the well-known entity group identifier for the service manager in the
ServerEntityId field and the identifier for the entity in the
CoresidentEntity field.  The next 12 octets are user- or

The MsgDelivery field is optionally used by the RPC or user level to
specify the portions of the segment data to transmit and on reception,
the portions received.  It provides the client and server with
(optional) access to, and responsibility for, a simple selective
transmission and reception facility.  For example, a client may request
retransmission of just those portions of the segment that it failed to
receive as part of the original Response.  The primary intended use is
to support highly efficient multi-packet reading from a file server.
Exploiting user-level selective retransmission using the MsgDelivery
field, the file server VMTP module need not save multi-packet Responses
for retransmission.  Retransmissions, when needed, are instead handled
directly from the file server buffers.

The SegmentSize field indicates the size of the data segment, if
present.  The CoresidentEntity, MsgDelivery and SegmentSize fields are
usable as additional user data if they are not otherwise used.

The Flags field provides a simple mechanism for the user level to
communicate its use of VMTP options with the VMTP module as well as for
VMTP modules to communicate this use among themselves.  The use of these
options is generally fixed for each remote procedure so that an RPC
mechanism using VMTP can treat the Flags as an integral part of the
RequestCode field for the purpose of demultiplexing to the correct stub.

A Response message control block follows the same format except the
Response is sent from the Server to the Client and there is no
Coresident Entity field (and thus 20 octets of user data).

2.5. Reliability Go to the top of this page...top

VMTP provides reliable, sequenced transfer of request and response
messages as well as several variants, such as unreliable datagram
requests.  The reliability mechanisms include: transaction identifiers,
checksums, positive acknowledgment of messages and timeout and
retransmission of lost packets.

2.5.1. Transaction Identifiers Go to the top of this page...top

Each message transaction is uniquely identified by the pair (Client,
Transaction).  (We defer discussion of the ForwardCount field to Section
2.9.)  The 32-bit transaction identifier is initialized to a random
value when the Client entity is created or allocated its entity
identifier.  The transaction identifier is incremented at the end of
each message transaction.  All Responses with the same specified
(Client, Transaction) pair are associated with this Request.

The transaction identifier is used for duplicate suppression at the
Server.  A Server maintains a state record for each Client for which it
is processing a Request, identified by (Client, Transaction).  A Request
with the same (Client, Transaction) pair is discarded as a duplicate.
(The ForwardCount field must also be equal.)  Normally, this record is
retained for some period after the Response is sent, allowing the Server
to filter out subsequent duplicates of this Request.  When a Request
arrives and the Server does not have a state record for the sending
Client, the Server takes one of three actions:

   1. The Server may send a Probe request, a simple query
      operation, to the VMTP management module associated with the
      requesting Client to determine the Client's current
      Transaction identifier (and other information), initialize a
      new state record from this information, and then process the
      Request as above.

   2. The Server may reason that the Request must be a new request
      because it does not have a state record for this Client if it
      keeps these state records for the maximum packet lifetime of
      packets in the network (plus the maximum VMTP retransmission
      time) and it has not been rebooted within this time period.
      That is, if the Request is not new either the Request would
      have exceeded the maximum packet lifetime or else the Server
      would have a state record for the Client.

   3. The Server may know that the Request is idempotent or can be
      safely redone so it need not care whether the Request is a
      duplicate or not.  For example, a request for the current
      time can be responded to with the current time without being
      concerned whether the Request is a duplicate.  The Response
      is discarded at the Client if it is no longer of interest.

2.5.2. Checksum Go to the top of this page...top

Each VMTP packet contains a checksum to allow the receiver to detect
corrupted packets independent of lower level checks.  The checksum field
is 32 bits, providing greater protection than the standard 16-bit IP
checksum (in combination with an improved checksum algorithm).  The
large packets, high packet rates and general network characteristics
expected in the future warrant a stronger checksum mechanism.

The checksum normally covers both the VMTP header and the segment data.
Optionally (for real-time applications), the checksum may apply only to
the packet header, as indicated by the HCO control bit being set in the
header.  The checksum field is placed at the end of the packet to allow
it to be calculated as part of a software copy or as part of a hardware
transmission or reception packet processing pipeline, as expected in the
next generation of network interfaces.  Note that the number of header
and data octets is an integral multiple of 8 because VMTP requires that
the segment data be padded to be a multiple of 64 bits.  The checksum
field is appended after the padding, if any.  The actual algorithm is
described in Section 3.2.

A zero checksum field indicates that no checksum was transmitted with
the packet.  VMTP may be used without a checksum only when there is a
host-to-host error detection mechanism and the VMTP security facility is
not being used.  For example, one could rely on the Ethernet CRC if
communication is restricted to hosts on the same Ethernet and the
network interfaces are considered sufficiently reliable.

2.5.3. Request And Response Acknowledgment Go to the top of this page...top

VMTP assumes an unreliable datagram network and internetwork interface.
To guarantee delivery of Requests and Response, VMTP uses positive
acknowledgments, retransmissions and timeouts.

A Request is normally acknowledged by receipt of a Response associated
with the Request, i.e. with the same (Client, Transaction).  With
streamed message transactions, it may also be acknowledged by a
subsequent Response that acknowledges previous Requests in addition to
the transaction it explicitly identifies.  A Response may be explicitly
acknowledged by a NotifyVmtpServer operation requested of the manager
for the Server.  In the case of streaming, this is a cumulative
acknowledgment, acknowledging all Responses with a lower transaction
identifier as well.)  In addition, with non-streamed communication, a
subsequent Request from the same Client acknowledges Responses to all
previous message transactions (at least in the sense that either the
client received a Response or is no longer interested in Responses to
those earlier message transactions).  Finally, a client response timeout
(at the server) acknowledges a Response at least in the sense that the
server need not be prepared to retransmit the Response subsequently.
Note that there is no end-to-end guarantee of the Response being
received by the client at the application level.

2.5.4. Retransmissions Go to the top of this page...top

In general, a Request or Response is retransmitted periodically until
acknowledged as above, up to some maximum number of retransmissions.
VMTP uses parameters RequestRetries(Server) and ResponseRetries(Client)
that indicate the number of retransmissions for the server and client
respectively before giving up.  We suggest the value 5 be used for both
parameters based on our experience with VMTP and Internet packet loss.
Smaller values (such as 3) could be used in low loss environments in
which fast detection of failed hosts or communication channels is
required.  Larger values should be used in high loss environments where
transport-level persistence is important.

In a low loss environment, a retransmission only includes the MCB and
not the segment data of the Request or Response, resulting in a single
(short) packet on retransmission.  The intended recipient of the
retransmission can request selective retransmission of all or part of
the segment data as necessary.  The selective retransmission mechanism
is described in Section 2.13.

If a Response is specified as idempotent, the Response is neither
retransmitted nor stored for retransmission.  Instead, the Client must
retransmit the Request to effectively get the Response retransmitted.
The server VMTP module responds to retransmissions of the Request by
passing the Request on to the server again to have it regenerate the
Response (by redoing the operation), rather than saving a copy of the
Response.  Only Request packets for the last transaction from this
client are passed on in this fashion; older Request packets from this
client are discarded as delayed duplicates.  If a Response is not
idempotent, the VMTP module must ensure it has a copy of the Response
for retransmission either by making a copy of the Response (either
physically or copy-on-write) or by preventing the Server from continuing
until the Response is acknowledged.

2.5.5. Timeouts Go to the top of this page...top

There is one client timer for each Client with an outstanding
transaction.  Similarly, there is one server timer for each Client
transaction that is "active" at the server, i.e. there is a transaction
record for a Request from the Client.

When the client transmits a new Request (without streaming), the client
timer  is set to roughly the time expected for the Response to be
returned.  On timeout, the Request is retransmitted with the APG
(Acknowledge Packet Group) bit set.  The timeout is reset to the
expected roundtrip time to the Server because an acknowledgment should
be returned immediately unless a Response has been sent.  The Request
may also be retransmitted in response to receipt of a VMTP management
operation indicating that selected portions of the Request message
segment need to be retransmitted.  With streaming, the timeout applies
to the oldest outstanding message transaction in the run of outstanding
message transactions.  Without streaming, there is one message
transaction in the run, reducing to the previous situation.  After the
first packet of a Response is received, the Client resets the timeout to
be the time expected before the next packet in the Response packet group
is received, assuming it is a multi-packet Response.  If not, the timer
is stopped.  Finally, the client timer is used to timeout waiting for
second and subsequent Responses to a multicast Request.

The client timer is set at different times to four different values:

TC1(Server)     The expected time required to receive a Response from
                the Server.  Set on initial Request transmission plus
                after its management module receives a NotifyVmtpClient
                operation, acknowledging the Request.

TC2(Server)     The estimated round trip delay between the client and
                the server.  Set when retransmitting after receiving no
                Response for TC1(Server) time and retransmitting the
                Request with the APG bit set.

TC3(Server)     The estimated maximum expected interpacket time for
                multi-packet Responses from the Server.  Set when
                waiting for subsequent Response packets within a packet
                group before timing out.

TC4             The time to wait for additional Responses to a group
                Request after the first Response is received.  This is
                specified by the user level.

These values are selected as follows.  TC1 can be set to TC2 plus a
constant, reflecting the time within which most servers respond to most
requests.  For example, various measurements of VMTP usage at Stanford
indicate that 90 percent of the servers respond in less than 200
milliseconds.  Setting TC1 to TC2 + 200 means that most Requests receive
a Response before timing out and also that overhead for retransmission
for long running transactions is insignificant.  A sophisticated
implementation may make the estimation of TC1 further specific to the

TC2 may be estimated by measuring the time from when a Probe request is
sent to the Server to when a response is received.  TC2 can also be
measured as the time between the transmission of a Request with the APG
bit set to receipt of a management operation acknowledging receipt of
the Request.

When the Server is an entity group, TC1 and TC2 should be the largest of
the values for the members of the group that are expected to respond.
This information may be determined by probing the group on first use
(and using the values for the last responses to arrive).  Alternatively,
one can resort to default values.

TC3 is set initially to 10 times the transmission time for the maximum
transmission unit (MTU) to be used for the Response.  A sophisticated
implementation may record TC3 per Server and refine the estimate based
on measurements of actual interpacket gaps.  However, a tighter estimate
of TC3 only improves the reaction time when a packet is lost in a packet
group, at some cost in unnecessary retransmissions when the estimate
becomes overly tight.

The server timer, one per active Client, takes on the following values:

TS1(Client)     The estimated maximum expected interpacket time.  Set
                when waiting for subsequent Request packets within a
                packet group before timing out.

TS2(Client)     The time to wait to hear from a client before
                terminating the server processing of a Request.  This
                limits the time spent processing orphan calls, as well
                as limiting how out of date the server's record of the
                Client state can be.  In particular, TS2 should be
                significantly less than the minimum time within which it
                is reasonable to reuse a transaction identifier.

TS3(Client)     Estimated roundtrip time to the Client,

TS4(Client)     The time to wait after sending a Response (or last
                hearing from a client) before discarding the state
                associated with the Request which allows it to filter
                duplicate Request packets and regenerate the Response.

TS5(Client)     The time to wait for an acknowledgment after sending a
                Response before retransmitting the Response, or giving
                up (after some number of retransmissions).

TS1 is set the same as TC3.

The suggested value for TS2 is TC1 + 3*TC2 for this server, giving the
Client time to timeout waiting for a Response and retransmit 3 Request
packets, asking for acknowledgments.

TS3 is estimated the same as TC1 except that refinements to the estimate
use measurements of the Response-to-acknowledgment times.

In the general case, TS4 is set large enough so that a Client issuing a
series of closely-spaced Requests to the same Server reuses the same
state record at the Server end and thus does not incur the overhead of
recreating this state.  (The Server can recreate the state for a Client
by performing a Probe on the Client to get the needed information.)  It
should also be set low enough so that the transaction identifier cannot
wrap around and so that the Server does not run out of CSR's.  We
suggest a value in the range of 500 milliseconds.  However, if the
Server accepts non-idempotent Requests from this Client without doing a
Probe on the Client, the TS4 value for this CSR is set to at least 4
times the maximum packet lifetime.

TS5 is TS3 plus the expected time for transmission and reception of the
Response.  We suggest that the latter be calculated as 3 times the
transmission time for the Response data, allowing time for reception,
processing and transmission of an acknowledgment at the Client end.  A
sophisticated implementation may refine this estimate further over time
by timing acknowledgments to Responses.

2.5.6. Rate Control Go to the top of this page...top

VMTP is designed to deal with the present and future problem of packet
overruns.  We expect overruns to be the major cause of dropped packets
in the future.  A client is expected to estimate and adjust the
interpacket gap times so as to not overrun a server or intermediate
nodes.  The selective retransmission mechanism allows the server to
indicate that it is being overrun (or some intermediate point is being
overrun).  For example, if the server requests retransmission of every
Kth block, the client should assume overrun is taking place and increase
the interpacket gap times.  The client passes the server an indication
of the interpacket gap desired for a response.  The client may have to
increase the interval because packets are being dropped by an
intermediate gateway or bridge, even though it can handle a higher rate.
A conservative policy is to increase the interpacket gap whenever a
packet is lost as part of a multi-packet packet group.
The provision of selective retransmission allows the rate of the client
and the server to "push up" against the maximum rate (and thus lose
packets) without significant penalty.  That is, every time that packet
transmission exceeds the rate of the channel or receiver, the recovery
cost to retransmit the dropped packets is generally far less than
retransmitting from the first dropped packet.

The interpacket gap is expressed in 1/32nd's of the MTU packet
transmission time.  The minimum interpacket gap is 0 and the maximum gap
that can be described in the protocol is 8 packet times.  This places a
limit on the slowest receivers that can be efficiently used on a
network, at least those handling multi-packet Requests and Responses.
This scheme also limits the granularity of adjustment.  However, the
granularity is relative to the speed of the network, as opposed to an
absolute time.  For entities on different networks of significantly
different speed, we assume the interconnecting gateways can buffer
packets to compensate<2>. With different network speeds and intermediary
nodes subject to packet loss, a node must adjust the interpacket gap
based on packet loss.  The interpacket gap parameter may be of limited

2.6. Security Go to the top of this page...top

VMTP provides an (optional) secure mode that protects against the usual
security threats of peeking, impostoring, message tampering and replays.
Secure VMTP must be used to guarantee any of the transport-level
reliability properties unless it is guaranteed that there are no
intruders or agents that can modify packets and update the packet
checksums.  That is, non-secure VMTP provides no guarantees in the
presence of an intelligent intruder.

The design closely follows that described by Birrell [1].  Authenticated
information about a remote entity, including an encryption/decryption
key, is obtained and maintained using a VMTP management operation, the
authenticated Probe operation, which is executed as a non-secure VMTP
message transaction.  If a server receives a secure Request for which
the server has no entity state, it sends a Probe request to the VMTP


<2>   Gateways must also employ techniques to preserve or intelligently
modify (if appropriate) the interpacket gaps.  In particular, they must
be sure not to arbitrarily remove interpacket gaps as a result of their
forwarding of packets.
management module of the client, "challenging" it to provide an
authenticator that both authenticates the client as being associated
with a particular principal as well as providing a key for
encryption/decryption.  The principal can include a real and effective
principal, as used in UNIX <3>.  Namely, the real principal is the
principal on whose behalf the Request is being performed whereas the
effective principal is the principal of the module invoking the request
or remote procedure call.

Peeking is prevented by encrypting every Request and Response packet
with a working Key that is shared between Client and Server.
Impostoring and replays are detected by comparing the Transaction
identifier with that stored in the corresponding entity state record
(which is created and updated by VMTP as needed).  Message tampering is
detected by encryption of the packet including the Checksum field.  An
intruder cannot update the checksum after modifying the packet without
knowing the Key.  The cost of fully encrypting a packet is close to the
cost of generating a cryptographic checksum (and of course, encryption
is needed in the general case), so there is no explicit provision for
cryptographic checksum without packet encryption.

A Client determines the Principal of the Server and acquires an
authenticator for this Server and Principal using a higher level
protocol.  The Server cannot decrypt the authenticator or the Request
packets unless it is in fact the Principal expected by the Client.

An encrypted VMTP packet is flagged by the EPG bit  in the VMTP packet
header.  Thus, encrypted packets are easily detected and demultiplexed
from unencrypted packets.  An encrypted VMTP packet is entirely
encrypted except for the Client, Version, Domain, Length and Packet
Flags fields at the beginning of the packet.  Client identifiers can be
assigned, changed and used to have no real meaning to an intruder or to
only communicate public information (such as the host Internet address).
They are otherwise just a random means of identification and
demultiplexing and do not therefore divulge any sensitive information.
Further secure measures must be taken at the network or data link levels
if this information or traffic behavior is considered sensitive.

VMTP provides multiple authentication domains  as well as an encryption
qualifier to accommodate different encryption algorithms and their


<3>   Principal group membership must be obtained, if needed, by a
higher level protocol.
corresponding security/performance trade-offs.  (See Appendix V.)  A
separate key distribution and authentication protocol is required to
handle generation and distribution of authenticators and keys.  This
protocol can be implemented on top of VMTP and can closely follow the
Birrell design as well.

Security is optional in the sense that messages may be secure or
non-secure, even between consecutive message transactions from the same
client.  It is also optional in that VMTP clients and servers are not
required to implement secure VMTP (although they are required to respond
intelligently to attempts to use secure VMTP).  At worst, a Client may
fail to communicate with a Server if the Server insists on secure
communication and the Client does not implement security or vice versa.
However, a failure to communicate in this case is necessary from a
security standpoint.

2.7. Multicast Go to the top of this page...top

The Server entity identifier in a message transaction can identify an
entity group, in which case the Request is multicast to every Entity in
this group (on a best-efforts basis).  The Request is retransmitted
until at least one Response is received (or an error timeout occurs)
unless it is a datagram Request.  The Client can receive multiple
Responses to the Request.

The VMTP service interface does not directly provide reliable multicast
because it is expensive to provide, rarely needed by applications, and
can be implemented by applications using the multiple Response feature.
However, the protocol itself is adequate for reliable multicast using
positive acknowledgments.  In particular, a sophisticated Client
implementation could maintain a list of members for each entity group of
interest and retransmit the Request until acknowledged by all members.
No modifications are required to the Server implementations.

VMTP supports a simple form of subgroup addressing.  If the CRE  bit is
set in a Request, the Request is delivered to the subgroup of entities
in the Server group that are co-resident with one or more entities in
the group (or individual entity) identified by the CoresidentEntity
field of the Request.  This is commonly used to send to the manager
entity for a particular entity, where Server specifies the group of such
managers.  Co-resident means "using the same VMTP module", and logically
on the same network host.  In particular, a Probe request can be sent to
the particular VMTP management module for an entity by specifying the
VMTP management group as the Server and the entity in question as the
As an experimental aspect of the protocol, VMTP supports the Server
sending a group Response which is sent to the Client as well as members
of the destination group of Servers to which the original Request was
sent.  The MDG bit indicates whether the Client is a member of this
group, allowing the Server module to determine whether separately
addressed packet groups are required to send the Response to both the
Client and the Server group.  Normally, a Server accepts a group
Response only if it has received the Request and not yet responded to
the Client.  Also, the Server must explicitly indicate it wants to
accept group Responses.  Logically, this facility is analogous to
responding to a mail message sent to a distribution list by sending a
copy of the Response to the distribution list.

2.8. Real-time Communication Go to the top of this page...top

VMTP provides three forms of support for real-time communication, in
addition to its standard facilities, which make it applicable to a wide
range of real-time applications.  First, a priority is transmitted in
each Request and Response which governs the priority of its handling.
The priority levels are intended to correspond roughly to:

   - urgent/emergency.

   - important

   - normal

   - background.

with additional gradations for each level.  The interpretation and
implementation of these priority levels is otherwise host-specific, e.g.
the assignment to host processing priorities.

Second, datagram Requests allow the Client to send a datagram to another
entity or entity group using the VMTP naming, transmission and delivery
mechanism, but without blocking, retransmissions or acknowledgment.
(The client can still request acknowledgment using the APG bit although
the Server does not expect missing portions of a multi-packet datagram
Request to be retransmitted even if some are not received.)  A datagram
Request in non-streamed mode supersedes all previous Requests from the
same Client.  A datagram Request in stream mode is queued (if necessary)
after previous datagram Requests on the same stream.  (See Section

Finally, VMTP provides several control bit flags to modify the handling
of Requests and Responses for real-time requirements.  First, the
conditional message delivery (CMD) flag causes a Request to be discarded
if the recipient is not waiting for it when it arrives, similarly for
the Response.  This option allows a client to send a Request that is
contingent on the server being able to process it immediately.  The
header checksum only (HCO) flag indicates that the checksum has been
calculated only on the VMTP header and not on the data segment.
Applications such as voice and video can avoid the overhead of
calculating the checksum on data whose utility is insensitive to typical
bit errors without losing protection on the header information.
Finally, the No Retransmission (NRT) flag indicates that the recipient
of a message should not ask for retransmission if part of the message is
missing but rather either use what was received or discard it.

None of these facilities introduce new protocol states.  In fact, the
total processing overhead in the normal case is a bit flag test for CMD,
HCO or NRT plus assignment of priority on packet transmission and
reception.  (In fact, CMD and NRT are not tested in the normal case.)
The additional code complexity is minimal.  We feel that the overhead
for providing these real-time facilities is minimal and that these
facilities are both important and adequate for a wide class of real-time

Several of the normal facilities of VMTP appear useful for real-time
applications.  First, multicast is useful for distributed, replicated
(fault-tolerant) real-time applications, allowing efficient state query
and update for (for example) sensors and control state.  Second, the DGM
or idempotent flag for Responses has some real-time benefits, namely:  a
Request is redone to get the latest values when the Response is lost,
rather than just returning the old values.  The desirability of this
behavior is illustrated by considering a request for the current time of
day.  An idempotent handling of this request gives better accuracy in
returning the current time in the case that a retransmission is
necessary.  Finally, the request-response semantics (in the absence of
streaming) of each new Request from a Client terminating the previous
message transactions from that Client, if any, provides the "most recent
is most important" handling of processing that most real-time
applications require.

In general, a key design goal of VMTP was provide an efficient
general-purpose transport protocol with the features required for
real-time communication.  Further experience is required to determine
whether this goal has been achieved.

2.9. Forwarded Message Transactions Go to the top of this page...top

A Server may invoke another Server to handle a Request.  It is fairly
common for the invocation of the second Server to be the last action
performed by the first Server as part of handling the Request.  For
example, the original Server may function primarily to select a process
to handle the Request.  Also, the Server may simply check the
authorization on the Request.  Describing this situation in the context
of RPC, a nested remote procedure call may be the last action in the
remote procedure and the return parameters are exactly those of the
nested call.  (This situation is analogous to tail recursion.)

As an optimization to support this case, VMTP provides a Forward
operation that allows the server to send the nested Request to the other
server and have this other server respond directly to the Client.

If the message transaction being forwarded was not multicast, not secure
or the two Servers are the same principal and the ForwardCount of the
Request is less than the maximum forward count of 15, the Forward
operation is implemented by the Server sending a Request onto the next
Server with the forwarded Request identified by the same Client and
Transaction as the original Request and a ForwardCount one greater than
the Request received from the Client.  In this case, the new Server
responds directly to the Client.  A forwarded Request is illustrated in
the following figure.

 +---------+   Request       +----------+
 | Client  +---------------->| Server 1 |
 +---------+                 +----------+
      ^                        |
      |                        | forwarded Request
      |                        V
      |   Response           +----------+
      +----------------------| Server 2 |

If the message transaction does not meet the above requirements, the
Server's VMTP module issues a nested call and simply maps the returned
Response to a Response to original Request without further Server-level
processing.  In this case, the only optimization over a user-level
nested call is one fewer VMTP service operation; the VMTP module handles
the return to the invoking call directly.  The Server may also use this
form of forwarding when the Request is part of a stream of message
transactions.  Otherwise, it must wait until the forwarded message
transaction completes before proceeding with the subsequent message
transactions in the stream.
Implementation of the user-level Forward operation is optional,
depending on whether the server modules require this facility.  Handling
an incoming forwarded Request is a minor modification of handling a
normal incoming Request.  In particular, it is only necessary to examine
the ForwardCount field when the Transaction of the Request matches that
of the last message transaction received from the Client.  Thus, the
additional complexity in the VMTP module for the required forwarding
support is minimal; the complexity is concentrated in providing a highly
optimized user-level Forward primitive, and that is optional.

2.10. VMTP Management Go to the top of this page...top

VMTP management includes operations for creating, deleting, modifying
and querying VMTP entities and entity groups.  VMTP management is
logically implemented by a VMTP management server module that is invoked
using a message transaction addressed to the Server, VMTP_MANAGER_GROUP,
a well-known group entity identifier, in conjunction with Coresident
Entity mechanism introduced in Section 2.7.  A particular Request may
address the local module, the module managing a particular entity, the
set of modules managing those entities contained in a specific group or
all management modules, as appropriate.

The VMTP management procedures are specified in Appendix III.

2.11. Streamed Message Transactions Go to the top of this page...top

Streamed message transactions refer to two or more message transactions
initiated by a Client before it receives the response to the first
message transaction, with each transaction being processed and responded
to in order but asynchronous relative to the initiation of the
transactions.  A Client streams messages transactions, and thereby has
multiple message transactions outstanding, by sending them as part of a
single run of message transactions.  A run  of message transactions is a
sequence of message transactions with the same Client and Server and
consecutive Transaction identifiers, with all but the first and last
Requests and Responses flagged with the NSR (Not Start Run)  and NER
(Not End Run)  control bits.  (Conversely, the first Request and
Response does not have the NSR set and the last Request and Response
does not have the NER bit set.)  The message transactions in a run use
consecutive transaction identifiers (except if the STI bit <4> is used
in one, in which case the transaction identifier for the next message
transaction is 256 greater, rather than 1).

The Client retains a record for each outstanding transaction until it
gets a Response or is timed out in error.  The record provides the
information required to retransmit the Request.  On retransmission
timeout, the client retransmits the last Request for which it has not
received a Response the same as is done with non-streamed communication.
(I.e. there need be only one timeout for all the outstanding message
transactions associated with a single client.)

The consecutive transaction identifiers within a run of message
transactions are used as sequence numbers for error control.  The Server
handles each message transaction in the sequence specified by its
transaction identifier.  When it receives a message transaction that is
not marked as the beginning of a run, it checks that it previously
received a message transaction with the predecessor transaction
identifier, either 1 less than the current one or 256 less if the
previous one had the STI bit set.  If not, the Server sends a
NotifyVmtpClient operation to the Client's manager indicating either:
(1) the first message transaction was not fully received, or else (2) it
has no record of the last one received.  If the NRT control flag is set,
it does not await nor expect retransmission but proceeds with handling
this Request.  This flag is used primarily when datagram Requests are
used as part of a stream of message transactions.  If NRT was not
specified, the Client must retransmit from the first message transaction
not fully received (either at all or in part) before the Server can
proceed with handling this run of Requests or else restart the run of
message transactions.

The Client expects to receive the Responses in a consecutive sequence,
using the Transaction identifier to detect missing Responses.  Thus, the
Server must return Responses in sequence except possibly for some gaps,
as follows.  The Server can specify in the PGcount field in a Response,
the number of consecutively previous Responses that this Response


<4>   The STI bit is used by the Client to effectively allocate 255
transaction identifiers for use by the Server in returning a large
Response or stream of Responses.
corresponds to, up to a maximum of 255 previous Responses <5>.  Thus,
for example, a Response with Transaction identifier 46 and PGcount 3
represents Responses 43, 44, 45 and 46.  This facility allows the Server
to eliminate sending Responses to Requests that require no Response,
effectively batching the Responses into one.  It also allows the Server
to effectively maintain strictly consecutive sequencing when the Client
has skipped 256 Transaction identifiers using the STI bit and the Server
does not have that many Responses to return.

If the Client receives a Response that is not consecutive, it
retransmits the Request(s) for which the Response(s) is/are missing
(unless, of course, the corresponding Requests were sent as datagrams).
The Client should wait at the end of a run of message transactions for
the last one to complete.

When a Server receives a Request with the NSR bit clear and a higher
transaction identifier than it currently has for the Client, it
terminates all processing and discards Responses associated with the
previous Requests.  Thus, a stream of message transactions is
effectively aborted by starting a new run, even if the Server was in the
middle of handling the previous run.

Using a mixture of datagram and normal Requests as part of a stream of
message transactions, particularly with the use of the NRT bit, can lead
to complex behavior under packet loss.  It is recommended that a run of
message transactions be all of one type to avoid problems, i.e. all
normal or all datagrams.  Finally, when a Server forwards a Request that
is part of a run, it must suspend further processing of the subsequent
Requests until the forwarded Request has been handled, to preserve order
of processing.  The simplest handling of this situation is to use a real
nested call when forwarding with streamed message transactions.

Flow control of streamed message transactions relies on rate control at
the Client plus receipt (or non-receipt) of management notify operations
indicating the presence of overrunning.  A Client must reduce the number
of outstanding message transactions at the Server when it receives a
NotifyVmtpServer operation with the MSGTRANS_OVERFLOW ResponseCode.  The
transact parameter indicates the last packet group that was accepted.


<5>  PGcount actually corresponds to packet groups which are described
in Section 2.13.  This (simplified) description is accurate when there
is one Request or Response per packet group.
The implementation of multiple outstanding message transactions requires
the ability to record, timeout and buffer multiple outstanding message
transactions at the Client end as well as the Server end.  However, this
facility is optional for both the Client and the Server.  Client systems
with heavy-weight processes and high network access cost are most likely
to benefit from this facility.  Servers that serve a wide variety of
client machines should implement streaming to accommodate these types of

2.12. Fault-Tolerant Applications Go to the top of this page...top

One approach to fault-tolerant systems is to maintain a log of all
messages sent at each node and replay the messages at a node when the
node fails, after restarting it from the last checkpoint <6>.  As an
experimental facility, VMTP provides a Receive Sequence Number field in
the NotifyVmtpClient and NotifyVmtpServer operations as well as the Next
Receive Sequence (NRS) flag in the Response packet to allow a sender to
log a receive sequence number with each message sent, allowing the
packets to be replayed at a recovering node in the same sequence as they
were originally received, thereby recovering to the same state as

Basically, each sending node maintains a receive sequence number for
each receiving node.  On sending a Request to a node, it presume that
the receive sequence number is one greater than the one it has recorded
for that node.  If not, the receiving node sends a notify operation
indicating the receive sequence number assigned the Request.  The NRS in
the Response confirms that the Request message was the next receive
sequence number, so the sender can detect if it failed to receive the
notify operation in the previous case.  With Responses, the packets are
ordered by the Transaction identifier except for multicast message
transactions, in which there may be multiple Responses with the same
identification.  In this case, NotifyVmtpServer operations are used to
provide receive sequence numbers.

This experimental extension of the protocol is focused on support for
fault-tolerant real-time distributed systems required in various
critical applications.  It may be removed or extended, depending on
further investigations.


<6>  The sender-based logging is being investigated by Willy Zwaenepoel
of Rice University.

2.13. Packet Groups Go to the top of this page...top

A message (whether Request or Response) is sent as one or more packet
groups.  A packet group is one or more packets, each containing the same
transaction identification and message control block.  Each packet is
formatted as below with the message control block logically embedded in
the VMTP header.

 |            VMTP Header             ||                     |
 +------------+-----------------------||   segment data      |
 |VMTP Control| Message Control Block ||                     |

The some fields of the VMTP control portion of the packet and data
segment portion can differ between packets within the same packet group.

The segment data portion of a packet group represents up to 16
kilooctets of the segment specified in the message control block.  The
portion contained in each packet is indicated by the PacketDelivery
field contained in the VMTP header.  The PacketDelivery field as a bit
mask has a similar interpretation to the MsgDelivery field in that each
bit corresponds to a segment data block of 512 octets.  The
PacketDelivery field limits a packet group to 16 kilooctets and a
maximum of 32 VMTP packets (with a minimum of 1 packet).  Data can be
sent in fewer packets by sending multiple data blocks per packet.  We
require that the underlying datagram service support delivery of (at
minimum) the basic 580 octet VMTP packet <7>.  To illustrate the use of
the PacketDelivery field, consider for example the Ethernet which has a
MTU of 1536 octets.  so one would send 2 512-octet segment data blocks
per packet.  (In fact, if a third block is last in the segment and less
than 512 octets and fits in the packet without making it too big, an
Ethernet packet could contain three data blocks.  Thus, an Ethernet
packet group for a segment of size 0x1D00 octets (14.5 blocks) and
MsgDelivery 0x000074FF consists of 6 packets indicated as follows <8>.


<7>  Note that with a 20 octet IP header, a VMTP packet is 600
octets.  We propose the convention that any host implementing VMTP
implicitly agrees to accept IP/VMTP packets of at least 600 octets.

<8>  We use the C notation 0xHHHH to represent a hexadecimal number.
 Delivery  1 1  1 1  1 1  1 1  0 0  1 0  1 0  1 0  0 0 0 0 0 . . .
           0000 0400 0800 0C00 1000 1400 1800 1C00
 Segment  |....|....|....|....|....|....|....|.|
          :    :    :    :    :    :  : /  /   :
          v    v    v    v    v    v  v   /|   v
          +----+----+----+----+    +----+  +---+
 Packets  |  1 |  2 |  3 |  4 |    |  5 |  | 6 |
          +----+----+----+----+    +----+  +---+

Each '.' is 256 octets of data.  The PacketDelivery masks for the 6
packets are: 0x00000003, 0x0000000C, 0x00000030, 0x000000C0, 0x00001400
and 0x00006000, indicating the segment blocks contained in each of the
packets.  (Note that the delivery bits are in little endian order.)

A packet group is sent as a single "blast" of packets with no explicit
flow control.  However, the sender should estimate and transmit at a
rate of packet transmission to avoid congesting the network or
overwhelming the receiver, as described in Section 2.5.6.  Packets in a
packet group can be sent in any order with no change in semantics.

When the first packet of a packet group is received (assuming the Server
does not decide to discard the packet group), the Server saves a copy of
the VMTP packet header, indicates it is currently receiving a packet
group, initializes a "current delivery mask" (indicating the data in the
segment received so far) to 0, accepts this packet (updating the current
delivery mask) and sets the timer for the packet group.  Subsequent
packets in the packet group update the current delivery mask.

Reception of a packet group is terminated when either the current
delivery mask indicates that all the packets in the packet group have
been received or the packet group reception timer expires (set to TC3 or
TS1).  If the packet group reception timer expires, if the NRT bit is
set in the Control flags then the packet group is discarded if not
complete unless MDM is set.  In this case, the MsgDelivery field in the
message control block is set to indicate the segment data blocks
actually received and the message control block and segment data
received is delivered to application level.

If NRT is not set and not all data blocks have been received, a
NotifyVmtpClient (if a Request) or NotifyVmtpServer (if a Response) is
sent back with a PacketDelivery field indicating the blocks received.
The source of the packet group is then expected to retransmit the
missing blocks.  If not all blocks of a Request are received after
RequestAckRetries(Client) retransmissions, the Request is discarded and
a NotifyVmtpClient operation with an error response code is sent to the
client's manager unless MDM is set.  With a Response, there are
ResponseAckRetries(Server) retransmissions and then, if MDM is not set,
the requesting entity is returned the message control block with an
indication of the amount of segment data received extending contiguously
from the start of the segment.  E.g. if the sender sent 6 512-octet
blocks and only the first two and the last two arrived, the receiver
would be told that 1024 octets were received.  The ResponseCode field is
set to BAD_REPLY_SEGMENT.  (Note that VMTP is only able to indicate the
specific segment blocks received if MDM is set.)

The parameters RequestAckRetries(Client) and ResponseAckRetries(Server)
could be set on a per-client and per-server basis in a sophisticated
implementation based on knowledge of packet loss.

If the APG flag is set, a NotifyVmtpClient or NotifyVmtpServer
operation is sent back at the end of the packet group reception,
depending on whether it is a Request or a Response.

At minimum, a Server should check that each packet in the packet group
contains the same Client, Server, Transaction identifier and SegmentSize
fields.  It is a protocol error for any field other than the Checksum,
packet group control flags, Length and PacketDelivery in the VMTP header
to differ between any two packets in one packet group.  A packet group
containing a protocol error of this nature should be discarded.

Notify operations should be sent (or invoked) in the manager whenever
there is a problem with a unicast packet.  i.e. negative acknowledgments
are always sent in this case.  In the case of problems with multicast
packets, the default is to send nothing in response to an error
condition unless there is some clear reason why no other node can
respond positively.  For example, the packet might be a Probe for an
entity that is known to have been recently existing on the receiving
host but now invalid and could not have migrated.  In this case, the
receiving host responds to the Probe indicating the entity is
nonexistent, knowing that no other host can respond to the Probe.  For
packets and packet groups that are received and processed without
problems, a Notify operation is invoked only if the APG bit is set.

2.14. Runs Of Packet Groups Go to the top of this page...top

A run of packet groups is a sequence of packet groups, all Request
packets or all Response packets, with the same Client and consecutive
transaction identifiers, all but the first and last packets flagged with
the NSR (Not Start Run) and NER (Not End Run) control bits.  When each
packet group in the run corresponds to a single Request or Response, it
is identical to a run of message transactions. (See Section 2.11)
However, a Request message or a Response message may consists of up to
256 packet groups within a run, for a maximum of 4 megaoctets of segment
data.  A message that is continued in the next packet group in the run
is flagged in the current packet group by the CMG flag.  Otherwise, the
next packet group in the run (if any) is treated as a separate Request
or Response.

Normally, each Request and Response message is sent as a single packet
group and each run consists of a single packet group.  In this case
neither NSR or NER are set.  For multi-packet group messages, the
PacketDelivery mask in the i-th packet group of a message corresponds to
the portion of the segment offset by i-1 times 16 kilooctets,
designating the the first packet group to have i = 1.

2.15. Byte Order Go to the top of this page...top

For purposes of transmission and reception, the MCB is treated as
consisting of 8 32-bit fields and the segment is a sequence of bytes.
VMTP transmits the MCB in big-endian order, performing byte-swapping, if
necessary, before transmission.  A little-endian host must byte-swap the
MCB on reception.  (The data segment is transmitted as a sequence of
bytes with no reordering.)  The byte order of the sender of a message is
indicated by the LEE  bit in the entity identifier for the sender, the
Client field if a Request and the Server field if a Response.  The
sender and receiver of a message are required to agree in some higher
level protocol (such as an RPC presentation protocol) on who does
further swapping of the MCB and data segment if required by the types of
the data actually being transmitted.  For example, the segment data may
contain a record with 8-bit, 16-bit and 32-bit fields, so additional
transformation is required to move the segment from a host of one byte
order to another.

VMTP to date has used a higher-level presentation protocol in which
segment data is sent in the native order of the sending host and
byte-swapped as necessary by the receiving host.  This approach
minimizes the byte-swapping overhead between machines of common byte
order (including when the communication is transparently local to one
host), avoids a strong bias in the protocol to one byte-order, and
allows for the sending entity to be sending to a group of hosts with
different byte orders.  (Note that the byte-swap overhead for the MCB is
minimal.)  The presentation-level overhead is minimal because most
common operations, such as file access operations, have parameters that
fit the MCB and data segment data types exactly.

2.16. Minimal VMTP Implementation Go to the top of this page...top

A minimal VMTP client needs to be able to send a Request packet group
and receive a Response packet group as well as accept and respond to
Requests sent to its management module, including Probe and NotifyClient
operations.  It may also require the ability to invoke Probe and Notify
operations to locate a Server and acknowledge responses.  (the latter
only if it is involved in transactions that are not idempotent or
datagram message transactions.  However, a simple sensor, for example,
can transmit VMTP datagram Requests indicating its current state with
even less mechanism.)  The minimal client thus requires very little code
and is suitable as a basis for (e.g.) a network boot loader.

A minimal VMTP server implements idempotent, non-encrypted message
transactions, possibly with no segment data support.  It should use an
entity state record for each Request but need only retain it while
processing the Request.  Without segment data larger than a packet,
there is no need for any timers, buffering (outside of immediate request
processing) or queuing.  In particular, it needs only as many records as
message transactions it handles simultaneously (e.g. 1).  The entity
state record is required to recognize and respond to Request
retransmissions during request processing.

The minimal server need only receive Requests and and be able to send
Response packets.  It need have only a minimal management module
supporting Probe operations.  (Support for the NotifyVmtpClient
operation is only required if it does not respond immediately to a
Request.)  Thus the VMTP support for say a time server, sensor, or
actuator can be extremely simple.  Note that the server need never issue
a Probe operation if it uses the host address of the Request for the
Response and does not require the Client information returned by the
Probe operation.  The minimal server should also support reception of
forwarded Requests.

2.17. Message Vs. Procedural Request Handling Go to the top of this page...top

A request-response protocol can be used to implement two forms of
semantics on reception.  With procedural handling of a Request, a
Request is handled by a process associated with the Server that
effectively takes on the identity of the calling process, treating the
Request message as invoking a procedure, and relinquishing its
association to the calling process on return.  VMTP supports multiple
nested calls spanning multiple machines.  In this case, the distributed
call stack that results is associated with a single process from the
standpoint of authentication and resource management, using the
ProcessId field supported by VMTP.  The entity identifiers effectively
link these call frames together.  That is, the Client field in a Request
is effectively the return link to the previous call frame.

With message handling of a Request, a Request message is queued for a
server process.  The server process dequeues, reads, processes and
responds to the Request message, executing as a separate process.
Subsequent Requests to the same server are queued until the server asks
to receive the next Request.

Procedural semantics have the advantage of allowing each Request (up to
the resource limits of the Server) to execute concurrently at the
Server, with Request-specific synchronization.  Message semantics have
the advantage that Requests are serialized at the Server and that the
request processing logically executes with the priority, protection and
independent execution of a separate process.  Note that procedural and
message handling of a request appear no differently to the client
invoking the message transaction, except possibly for differences in

We view the two Request handling approaches as appropriate under
different circumstances.  VMTP supports both models.

2.18. Bibliography Go to the top of this page...top

The basic protocol is similar to that used in the original form of the V
kernel [3, 4] as well as the transport protocol of Birrell and
Nelson's [2] remote procedure call mechanism.  An earlier version of the
protocol was described in SIGCOMM'86 [6].  The rate-based flow control
is similar to the techniques of Netblt [9].  The support for idempotency
draws, in part, on the favorable experience with idempotency in the V
distributed system.  Its use was originally inspired by the Woodstock
File Server [11].  The multicast support draws on the multicast
facilities in V [5] and is designed to work with, and is now implemented
using, the multicast extensions to the Internet [8] described in RFC 966
and 988.  The secure version of the protocol is similar to that
described by Birrell [1] for secure RPC.  The use of runs of packet
groups is similar to Fletcher and Watson's delta-T protocol [10].  The
use of "management" operations implemented using VMTP in place of
specialized packet types is viewed as part of a general strategy of
using recursion to simplify protocol architectures [7].

Finally, this protocol was designed, in part, to respond to the
requirements identified by Braden in RFC 955.  We believe that VMTP
satisfies the requirements stated in RFC 955.
[1]   A.D. Birrell, "Secure Communication using Remote Procedure
      Calls", ACM. Trans. on Computer Systems 3(1), February, 1985.

[2]   A. Birrell and B. Nelson, "Implementing Remote Procedure Calls",
      ACM Trans. on Computer Systems 2(1), February, 1984.

[3]   D.R. Cheriton and W. Zwaenepoel, "The Distributed V Kernel and its
      Performance for Diskless Workstations", In Proceedings of the 9th
      Symposium on Operating System Principles,  ACM, 1983.

[4]   D.R. Cheriton, "The V Kernel: A Software Base for Distributed
      Systems", IEEE Software 1(2), April, 1984.

[5]   D.R. Cheriton and W. Zwaenepoel, "Distributed Process Groups in
      the V Kernel", ACM Trans. on Computer Systems 3(2), May, 1985.

[6]   D.R. Cheriton, "VMTP: A Transport Protocol for the Next
      Generation of Communication Systems", In Proceedings of
      SIGCOMM'86, ACM, Aug 5-7, 1986.

[7]   D.R. Cheriton, "Exploiting Recursion to Simplify an RPC 
      Communication Architecture", in preparation, 1988.

[8]   D.R. Cheriton and S.E. Deering, "Host Groups: A Multicast 
      Extension for Datagram Internetworks", In 9th Data Communication 
      Symposium, IEEE Computer Society and ACM SIGCOMM, September, 1985.

[9]   D.D. Clark and M. Lambert and L. Zhang, "NETBLT: A Bulk Data 
      Transfer Protocol", Technical Report RFC 969, Defense Advanced 
      Research Projects Agency, 1985.

[10]  J.G. Fletcher and R.W. Watson, "Mechanism for a Reliable Timer-
      based Protocol", Computer Networks 2:271-290, 1978.
[11]  D. Swinehart and G. McDaniel and D. Boggs, "WFS: A Simple File 
      System for a Distributed Environment", In Proc. 7th Symp. 
      Operating Systems Principles, 1979.

3. VMTP Packet Formats Go to the top of this page...top

VMTP uses 2 basic packet formats corresponding to Request packets and
Response packets.  These packet formats are identical in most of the
fields to simplify the implementation.

We first describe the entity identifier format and the packet fields
that are used in general, followed by a detailed description of each of
the packet formats.  These fields are described below in detail.  The
individual packet formats are described in the following subsections.
The reader and VMTP implementor may wish to refer to Chapters 4 and 5
for a description of VMTP event handling and only refer to this detailed
description as needed.

3.1. Entity Identifier Format Go to the top of this page...top

The 64-bit non-group entity identifiers have the following substructure.

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |R| |L|R|
 |A|0|E|E|      Domain-specific structure
 |E| |E|S|
                Domain-specific structure                        |

The field meanings are as follows:

RAE             Remote Alias Entity - the entity identifier identifies
                an entity that is acting as an alias for some entity
                outside this entity domain.  This bit is used by
                higher-level protocols.  For instance, servers may take
                extra security and protection measures with aliases.

GRP             Group - 0, for non-group entity identifiers.

LEE             Little-Endian Entity - the entity transmits data in
                little-endian (VAX) order.

RES              Reserved - must be 0.

The 64-bit entity group identifiers have the following substructure.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |R| |U|R|
 |A|1|G|E|      Domain-specific structure
 |E| |P|S|
                Domain-specific structure                        |

The field meanings are as follows:

RAE             Remote Alias Entity - same as for non-group entity

GRP             Group - 1, for entity group identifiers.

UGP             Unrestricted Group - no restrictions are placed on
                joining this group.  I.e. any entity can join limited
                only by implementation resources.

RES              Reserved - must be 0.

The all-zero entity identifier is reserved and guaranteed to be
unallocated in all domains.  In addition, a domain may reserve part of
the entity identifier space for statically allocated identifiers.
However, this is domain-specific.

Description of currently defined entity identifier domains is provided
in Appendix IV.

3.2. Packet Fields Go to the top of this page...top

Client          64-bit identifier for the client entity associated with
                this packet.  The structure, allocation and binding of
                this identifier is specific to the specified Domain.  An
                entity identifier always includes 4 types bits as
                specified in Section 3.1.

Version         The 3-bit identifier specifying the version of the
                protocol.  Current version is version 0.

Domain          The 13-bit identifier specifying the naming and
                administration domain for the client and server named in
                the packet.
Packet Flags: 3 bits. (The normal case has none of the flags set.)

  HCO           Header checksum only - checksum has only been calculated
                on the header.  This is used in some real-time
                applications where the strict correctness of the data is
                not needed.

  EPG           Encrypted packet group - part of a secure message

  MPG           Multicast packet group - packet was multicast on

Length          A 13-bit field that specifies the number of 32-bit words
                in the segment data portion of the packet (if any),
                excluding the checksum field.  (Every VMTP packet is
                required to be a multiple of 64 bits, possibly by
                padding out the segment data.)  The minimum legal Length
                is 0, the maximum length is 4096 and it must be an even

Control Flags: 9 bits. (The normal case has none of the flags set.)

  NRS           Next Receive Sequence - the associated Request message
                (in a Response) or previous Response (if a Request) was
                received consecutive with the last Request from this
                entity.  That is, there was no interfering messages

  APG           Acknowledge Packet Group - Acknowledge packet group on
                receipt.  If a Request, send back a Request to the
                client's manager providing an update on the state of the
                transaction as soon as the request packet group is
                received, independent of the response being available.
                If a Response, send an update to the server's manager as
                soon as possible after response packet group is received
                providing an update on the state of the transaction at
                the client

  NSR           Not Start Run - 1 if this packet is not part of the
                first packet group of a run of packet groups.

  NER           Not End Run - 1 if this packet is not part of the last
                packet group of a run of packet groups.

  NRT           No Retransmission - do not ask for retransmissions of
                this packet group if not all received within timeout
                period, just deliver or discard.

  MDG           Member of Destination Group - this packet is sent to a
                group and the client is a member of this group.

  CMG           Continued Message - the message (Request or Response) is
                continued in the next packet group.  The next packet
                group has to be part of the same run of packet groups.

  STI           Skip Transaction Identifiers - the next transaction
                identifier that the Client plans to use is the current
                transaction plus 256, if part of the same run and at
                least this big if not.  In a Request, this authorizes
                the Server to send back up to 256 packet groups
                containing the Response.

  DRT           Delay Response Transmission - set by request sender if
                multiple responses are expected (as indicated by the MRD
                flag in the RequestCode) and it may be overrun by
                multiple responses.  The responder(s) should then
                introduce a short random delay in sending the Response
                to minimize the danger of overrunning the Client.  This
                is normally only used for responding to multicast
                Requests where the Client may be receiving a large
                number of Responses, as indicated by the MRD flag in the
                Request flags.  Otherwise, the Response is sent

                3 bits - the ordinal number of transmissions of this
                packet group prior to this one, modulo 8.  This field is
                used in estimation of roundtrip times.  This count may
                wrap around during a message transaction.  However, it
                should be sufficient to match acknowledgments and
                responses with a particular transmission.

ForwardCount:   4 bits indicating the number of times this Request has
                been forwarded.  The original Request is always sent
                with a ForwardCount of 0.

Interpacket Gap: 8 bits.  
                Indicates the recommended time to use between subsequent
                packet transmissions within a multi-packet packet group
                transmission.  The Interpacket Gap time is in 1/32nd of
                a network packet transmission time for a packet of size
                MTU for the node.  (Thus, the maximum gap time is 8
                packet times.)
PGcount: 8 bits 
                The number of packet groups that this packet group
                represents in addition to that specified by the
                Transaction field.  This is used in acknowledging
                multiple packet groups in streamed communication.

Priority        4-bit identifier for priority for the processing of this
                request both on transmission and reception.  The
                interpretation is:

                1100            urgent/emergency

                1000            important

                0000            normal

                0100            background

                Viewing the higher-order bit as a sign bit (with 1
                meaning negative), low values are high priority and high
                values are low priority.  The low-order 2 bits indicate
                additional (lower) gradations for each level.

Function Code: 1 bit - types of VMTP packets.  If the low-order bit of
                the function code is 0, the packet is sent to the
                Server, else it is sent to the Client.

                0               Request

                1               Response

Transaction: 32 bits:  
                Identifier for this message transaction.

PacketDelivery: 32 bits:  
                Delivery indicates the segment blocks contained in this
                packet.  Each bit corresponds to one 512-octet block of
                segment data.  A 1 bit in the i-th bit position
                (counting the LSB as 0) indicates the presence of the
                i-th segment block.

Server: 64 bits 
                Entity identifier for the server or server group
                associated with this transaction.  This is the receiver
                when a Request packet and the sender when a Response
Code: 32 bits   The Request Code and Response Code, set either at the
                user level or VMTP level depending on use and packet
                type.  Both the Request and Response codes include 8
                high-order bits from the following set of control bits:

  CMD           Conditional Message Delivery -  only deliver the request
                or response if the receiving entity is waiting for it at
                the time of delivery, otherwise drop the message.

  DGM           DataGram Message - indicates that the message is being
                sent as a datagram.  If a Request message, do not wait
                for reply, or retransmit.  If a Response message, treat
                this message transaction as idempotent.

  MDM           Message Delivery Mask - indicates that the MsgDelivery
                field is being used.  Otherwise, the MsgDelivery field
                is available for general use.

  SDA           Segment Data Appended - segment data is appended to the
                message control block, with the total size of the
                segment specified by the SegmentSize field.  Otherwise,
                the segment data is null and the SegmentSize field is
                not used by VMTP and available for user- or RPC-level

  CRE           CoResident Entity - indicates that the CoResidentEntity
                field in the message should be interpreted by VMTP.
                Otherwise, this field is available for additional user

  MRD           Multiple Responses Desired - multiple Responses are
                desired to to this Request if it is multicast.
                Otherwise, the VMTP module can discard subsequent
                Responses after the first Response.

  PIC           Public Interface Code - Values for Code with this bit
                set are reserved for definition by the VMTP
                specification and other standard protocols defined on
                top of VMTP.

  RES           Reserved for future use. Must be 0.

                64-bit Identifier for an entity or group of entities
                with which the Server entity or entities must be
                co-resident, i.e. route only to entities (identified by
                Server) on the same host(s) as that specified by
                CoResidentEntity, Only meaningful if CRE is set in the
                Code field.

User Data       12 octets Space in the header for the VMTP user to
                specify user-specific control and data.

MsgDelivery: 32 bits 
                The segment blocks being transmitted (in total) in this
                packet group following the conventions for the
                PacketDelivery field.  This field is ignored by the
                protocol and treated as an additional user data field if
                MDM is 0.  On transmission, the user level sets the
                MsgDelivery to indicate those portions of the segment to
                be transmitted.  On receipt, the MsgDelivery field is
                modified by the VMTP module to indicate the segment data
                blocks that were actually received before the message
                control block is passed to the user or RPC level.  In
                particular, the kernel does not discard the packet group
                if segment data blocks are missing.  A Server or Client
                entity receiving a message with a MsgDelivery in use
                must check the field to ensure adequate delivery and
                retry the operation if necessary.

SegmentSize: 32 bits 
                Size of segment in octets, up to a maximum of 16
                kilooctets without streaming and 4 megaoctets with
                streaming, if SDA is set.  Otherwise, this field is
                ignored by the protocol and treated as an additional
                user data field.

Segment Data: 0-16 kilooctets 
                0 octets if SDA is 0, else the portion of the segment
                corresponding to the Delivery Mask, limited by the
                SegmentSize and the MTU, padded out to a multiple of 64

Checksum: 32 bits.  
                The 32-bit checksum for the header and segment data.

The VMTP checksum algorithm <9> develops a 32-bit checksum by computing


<9>  This algorithm and description are largely due to Steve Deering of
Stanford University.
two 16-bit, ones-complement sums (like IP), each covering different
parts of the packet.  The packet is divided into clusters of 16 16-bit
words.  The first, third, fifth,... clusters are added to the first sum,
and the second, fourth, sixth,... clusters are added to the second sum.
Addition stops at the end of the packet; there is no need to pad out to
a cluster boundary (although it is necessary that the packet be an
integral multiple of 64 bits; padding octets may have any value and are
included in the checksum and in the transmitted packet).  If either of
the resulting sums is zero, it is changed to 0xFFFF.  The two sums are
appended to the transmitted packet, with the first sum being transmitted
first.  Four bytes of zero in place of the checksum may be used to
indicate that no checksum was computed.

The 16-bit, ones-complement addition in this algorithm is the same as
used in IP and, therefore, subject to the same optimizations.  In
particular, the words may be added up 32-bits at a time as long as the
carry-out of each addition is added to the sum on the following
addition, using an "add-with-carry" type of instruction.  (64-bit or
128-bit additions would also work on machines that have registers that

A particular weakness of this algorithm (shared by IP) is that it does
not detect the erroneous swapping of 16-bit words, which may easily
occur due to software errors.  A future version of VMTP is expected to
include a more secure algorithm, but such an algorithm appears to
require hardware support for efficient execution.

Not all of these fields are used in every packet.  The specific packet
formats are described below.  If a field is not mentioned in the
description of a packet type, its use is assumed to be clear from the
above description.

3.3. Request Packet Go to the top of this page...top

The Request packet (or packet group) is sent from the client to the
server or group of servers to solicit processing plus the return of zero
or more responses.  A Request packet is identified by a 0 in the LSB of
the fourth 32-bit word in the packet.

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +                       Client (8 octets)                       +
 |Ver  |                         |H|E|M|                         |
 |sion |          Domain         |C|P|P|      Length             |
 |     |                         |O|G|G|                         |
 |N|A|N|N|N|M|C|S|D|Retra|Forward|    Inter-     |       |R|R|R| |
 |R|P|S|E|R|D|M|T|R|nsmit| Count |    Packet     | Prior |E|E|E|0|
 |S|G|R|R|T|G|G|I|T|Count|       |     Gap       | -ity  |S|S|S| |
 |                      Transaction                              |
 |                     PacketDelivery                            |
 +                    Server (8 octets)                          +
 |C|D|M|S|R|C|M|P|                                               |
 |M|G|D|D|E|R|R|I|        RequestCode                            |
 |D|M|M|A|S|E|D|C|                                               |
 +                 CoResidentEntity (8 octets)                   +
 >                   User Data (12 octets)                       <
 |                      MsgDelivery                              |
 |                       SegmentSize                             |
 >                  segment data, if any                         <
 |                        Checksum                               |


Figure 3-1:   Request Packet Format Go to the top of this page...top

The fields of the Request packet are set according to the semantics
described in Section 3.2 with the following qualifications.
InterPacketGap  The estimated interpacket gap time the client would like
                for the Response packet group to be sent by the Server
                in responding to this Request.

Transaction     Identifier for transaction, at least one greater than
                the previously issued Request from this Client.

Server          Server to which this Request is destined.

RequestCode     Request code for this request, indicating the operation
                to perform.

3.4. Response Packet Go to the top of this page...top

The Response packet is sent from the Server to the Client in response to
a Request, identified by a 1 in the LSB of the fourth 32-bit word in the

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +                       Client (8 octets)                       +
 |Ver  |                         |H|E|M|                         |
 |sion |          Domain         |C|P|P|      Length             |
 |     |                         |O|G|G|                         |
 |N|A|N|N|N|R|C|S|R|Retra|Forward|               |       |R|R|R| |
 |R|P|S|E|R|E|M|T|E|nsmit| Count |    PGcount    | Prior |E|E|E|1|
 |S|G|R|R|T|S|G|I|S|Count|       |               | -ity  |S|S|S| |
 |                      Transaction                              |
 |                      PacketDelivery                           |
 +                        Server (8 octets)                      +
 |C|D|M|S|R|R|R|R|                                               |
 |M|G|D|D|E|E|E|E|        ResponseCode                           |
 |D|M|M|A|S|S|S|S|                                               |
 >                   UserData (20 octets)                        <
 |                     MsgDelivery                               |
 |                    Segment Size                               |
 >                  segment data, if any                         <
 |                       Checksum                                |


Figure 3-2:   Response Packet Format Go to the top of this page...top

The fields of the Response packet are set according to the semantics
described in Section 3.2 with the following qualifications.

Client, Version, Domain, Transaction
                Match those in the Request packet group to which this is
                a response.

STI             1 if this Response is using one or more of the
                transaction identifiers skipped by the Client after the
                Request to which this is a Response.  STI in the Request
                essentially allocates up to 256 transaction identifiers
                for the Server to use in a run of Response packet

RetransmitCount The retransmit count from the last Request packet
                received to which this is a response.

ForwardCount    The number of times the corresponding Request was
                forwarded before this Response was generated.

PGcount         The number of consecutively previous packet groups that
                this response is acknowledging in addition to the one
                identified by the Transaction identifier.

Server          Server sending this response.  This may differ from that
                originally specified in the Request packet if the
                original Server was a server group, or the request was

The next two chapters describes the protocol operation using these
packet formats, with the the Client and the Server portions described

4. Client Protocol Operation Go to the top of this page...top

This chapter describes the operation of the client portion of VMTP in
terms of the procedures for handling VMTP user events, packet reception
events, management operations and timeout events.  Note that the client
portion of VMTP is separable from the server portion.  It is feasible to
have a node that only implements the client end of VMTP.

To simplify the description, we define a client state record (CSR) plus
some standard utility routines.

4.1. Client State Record Fields Go to the top of this page...top

In the following protocol description, there is one client state record
(CSR) per (client,transaction) outstanding message transaction.  Here is
a suggested set of fields.

Link            Link to next CSR when queued in one of the transmission,
                timeout or message queues.

QueuePtr        Pointer to queue head in which this CSR is contained or
                NULL if none.  Queue could be one of transmission queue,
                timeout queue, server queue or response queue.

                The process identification and address space.

Priority        Priority for processing, network service, etc.

State           One of the client states described below.

FinishupFunc    Procedure to be executed on the CSR when it is completes
                its processing in transmission or timeout queues.

TimeoutCount    Time to remain in timeout queue.

TimeoutLimit    User-specified time after which the message transaction
                is aborted. The timeout is infinite if set to zero.

RetransCount    Number of retransmissions since last hearing from the

                The time at which the last packet was sent.  This field
                is used to calculate roundtrip times, using the
                RetransmitCount to match the responding packet to a
                particular transmission.  I.e. Response or management
                NotifyVmtpClient operation to Request and a management
                NotifyVmtpServer operation to a Response.

TimetoLive      Time to live to be used on transmission of IP packets.

                Bit mask indicating the portions of the segment to
                transmit.  Set before entering the transmission queue
                and cleared incrementally as the 512-byte segment blocks
                of the segment are transmitted.

LocalClientLink Link to next CSR hashing to same hash index in the

LocalClient     Entity identifier for client when this CSR is used to
                send a Request packet.

                Transaction identifier for current message transaction
                the local client has outstanding.

LocalPrincipal  Account identification, possibly including key and key

LocalDelivery   Bit mask of segment blocks that have not been
                acknowledged in the Request or have been received in the
                Response, depending on the state.

ResponseQueue   Queue of CSR's representing the queued Responses for
                this entity.

VMTP Header     Prototype VMTP header, used to generate and store the
                header portion of a Request for transmission and
                retransmission on timeout.

SegmentDesc     Description of the segment data associated with the CSR,
                either the area storing the original Request data, the
                area for receiving Request data, or the area storing the
                Response data that is returned.

HostAddr        The network or internetwork host address to which the
                Client last transmitted.  This field also indicates the
                type of the address, e.g. IP, Ethernet, etc.

Note: the CSR can be combined with a light-weight process descriptor
with considerable benefit if the process is designed to block when it
issues a message transaction.  In particular, by combining the two
descriptors, the implementation saves time because it only needs to
locate and queue one descriptor with various operations (rather than
having to locate two descriptors).  It also saves space, given that the
VMTP header prototype provides space such as the user data field which
may serve to store processor state for when the process is preempted.
Non-preemptive blocking can use the process stack to store the processor
state so only a program counter and stack pointer may be required in the
process descriptor beyond what we have described.  (This is the approach
used in the V kernel.)

4.2. Client Protocol States Go to the top of this page...top

A Client State Record records the state of message transaction generated
by this host, identified by the (Client, Transaction) values in the CSR.
As a client originating a transaction, it is in one of the following

                Waiting for a Response packet group to arrive with the
                same (Client,Transaction) identification.

                Waiting for additional packets in the Response packet
                group it is currently receiving.

"Other"         Not waiting for a response, which can be Processing or
                some other operating system state, or one of the Server
                states if it also acts as a server.

This covers all the states for a client.

4.3. State Transition Diagrams Go to the top of this page...top

The client state transitions are illustrated in Figure 4-1.  The client
goes into the state AwaitingResponse on sending a request unless it is a
datagram request.  In the AwaitingResponse state, it can timeout and
retry and eventually give up and return to the processing state unless
it receives a Response.  (A NotifyVmtpClient operation resets the
timeout but does not change the state.)  On receipt of a single packet
response, it returns to the processing state.  Otherwise, it goes to
ReceivingResponse state.  After timeout or final response packet is
received, the client returns to the processing state.  The processing
state also includes any other state besides those associated with
issuing a message transaction.
   | Processing |<--------------------|
   |            |<-------------|      |
   |            |<---|         |      |
   +|------^--^-+  Single    Last     |
 Transmit  |  |    Packet    Response |
    |      |  |    Response  Packet   |
    |      |  |      |         |      |
    +-DGM->+ Timeout |         |   Final timeout
    |         |      |         |      |
   +V-----------+    |       +-----------+
   |  Awaiting  |----+       | Receiving |->Response-+
   |  Response  |->Response->| Response  |           |
   |            |  (multi-   |           |<----------+
   +-|--------^-+   packet)  +----------^+
     V        |                |        |
     +-Timeout+                +>Timeout+


Figure 4-1:   Client State Transitions Go to the top of this page...top

4.4. User Interface Go to the top of this page...top

The RPC or user interface to VMTP is implementation-dependent and may
use systems calls, functions or some other mechanism.  The list of
requests that follow is intended to suggest the basic functionality that
should be available.

Send( mcb, timeout, segptr, segsize )
                Initiate a message transaction to the server and request
                message specified by mcb and return a response in mcb,
                if it is received within the specified timeout period
                (or else return USER_TIMEOUT in the Code field).  The
                segptr parameter specifies the location from which the
                segment data is sent and the location into which the
                response data is to be delivered.  The segsize field
                indicates the maximum length of this area.

GetResponse( responsemcb, timeout, segptr, segsize )
                Get the next response sent to this client as part of the
                current message transaction, returning the segment data,
                if any, into the memory specified by segptr and segsize.

This interface assumes that there is a client entity associated with the
invoking process that is to be used with these operations.  Otherwise,
the client entity must be specified as an additional parameter.

4.5. Event Processing Go to the top of this page...top

The following events may occur in the VMTP client:

   - User Requests

        * Send

        * GetResponse

   - Packet Arrival

        * Response Packet

        * Request

     The minimal Client implementation handles Request packets for
     its VMTP management (server) module and sends NotifyVmtpClient
     requests in response to others, indicating the specified
     server does not exist.

   - Management Operation - NotifyVmtpClient

   - Timeouts

        * Client Retransmission Timeout

The handling of these events is described in detail in the following

We first describe some conventions and procedures used in the
description.  A field of the received packet is indicated as (for
example) p.Transaction, for the Transaction field.  Optional portions of
the code, such as the streaming handling code are prefixed with a "|" in
the first column.

MapClient( client )
                Return pointer to CSR for client with the specified
                clientId, else NULL.

SendPacketGroup( csr )
                Send the packet group (Request, Response) according to
                that specified by the CSR.

NotifyClient( csr, p, code )
                Invoke the NotifyVmtpClient operation with the
                parameters csr.RemoteClient, p.control,
                csr.ReceiveSeqNumber, csr.RemoteTransaction and
                csr.RemoteDelivery, and code.  If csr is NULL, use
                p.Client, p.Transaction and p.PacketDelivery instead and
                the global ReceiveSequenceNumber, if supported.  This
                function simplifies the description over calling
                NotifyVmtpClient directly in the procedural
                specification below.  (See Appendix III.)

NotifyServer( csr, p, code )
                Invoke the NotifyVmtpServer operation with the
                parameters p.Server, csr.LocalClient,
                csr.LocalTransaction, csr.LocalDelivery and code.  Use
                p.Client, P.Transaction and 0 for the clientId, transact
                and delivery parameters if csr is NULL.  This function
                simplifies the description over calling NotifyVmtpServer
                directly in the procedural specification below.  (See
                Appendix III.)

DGMset(p)       True if DGM bit set in packet (or csr) else False.
                (Similar functions are used for other bits.)

Timeout( csr, timeperiod, func )
                Set or reset timer on csr record for timeperiod later
                and invoke func if the timeout expires.

4.6. Client User-invoked Events Go to the top of this page...top

A user event occurs when a VMTP user application invokes one of the VMTP
interface procedures.

4.6.1. Send Go to the top of this page...top

Send( mcb, timeout, segptr, segsize )
    map to main CSR for this client.
    increment csr.LocalTransaction
    Init csr and check parameters and segment if any.
    Set SDA if sending appended data.
    Flush queued replies from previous transaction, if any.
    if local non-group server then
        deliver locally
        await response
    if GroupId(server) then
        Check for and deliver to local members.
        if CRE request and non-group local CR entity then
           await response
        set MDG if member of this group.
    clear csr.RetransCount
    set csr.TransmissionMask
    set csr.TimeLimit to timeout
    set csr.HostAddr for csr.Server
    SendPacketGroup( csr )
    if DGMset(csr) then
    set csr.State to AwaitingResponse
    Timeout( rootcsr, TC1(csr.Server), LocalClientTimeout )
end Send


   1. Normally, the HostAddr is extracted from the ServerHost
      cache, which maps server entity identifiers to host
      addresses.  However, on cache miss, the client first queries
      the network using the ProbeEntity operation, as specified in
      Appendix III, determining the host address from the Response.
      The ProbeEntity operation is handled as a separate message
      transaction by the Client.

The stream interface incorporates a parameter to pass a responseHandler
procedure that is invoked when the message transaction completes.

StreamSend( mcb, timeout, segptr, segsize, responseHandler )
    map to main CSR for this client.
|   Allocate a new csr if root in use.
|   lastcsr := First csr for last request.
|   if STIset(lastcsr)
|       csr.LocalTransaction := lastcsr.LocalTransaction + 256
|   else
|       csr.LocalTransaction := lastcsr.LocalTransaction + 1
    Init csr and check parameters and segment if any.
    . . . ( rest is the same as for the normal Send)


   1. Each outstanding message transaction is represented by a CSR
      queued on the root CSR for this client entity.  The root CSR
      is used to handle timeouts, etc.  On timeout, the last packet
      from the last packet group is retransmitted (with or without
      the segment data).

4.6.2. GetResponse Go to the top of this page...top

GetResponse( req, timeout, segptr, segsize )
    csr := CurrentCSR;
    if responses queued then return next response
      (in req, segptr to max of segsize )
    if timeout is zero then return KERNEL_TIMEOUT error
    set state to AWAITING_RESPONSE
    Timeout( csr, timeout, ReturnKernelTimeout );
end GetResponse


   1. GetResponse is only used with multicast Requests, which is
      the only case in which multiple (different) Responses should
      be received.

   2. A response must remain queued until the next message
      transaction is invoked to filter out duplicates of this

   3. If the response is incomplete (only relevant if a
      multi-packet response), then the client may wait for the
      response to be fully received, including issuing requests for
      retransmission (using NotifyVmtpServer operations) before
      returning the response.

   4. As an optimization, a response may be stored in the CSR of
      the client.  In this case, the response must be transferred
      to a separate buffer (for duplicate suppression) before
      waiting for another response.  Using this optimization, a
      response buffer is not allocated in the common case of the
      client receiving only one response.

4.7. Packet Arrival Go to the top of this page...top

In general, on packet reception, a packet is mapped to the client state
record, decrypted if necessary using the key in the CSR.  It then has
its checksum verified and then is transformed to the right byte order.
The packet is then processed fully relative to its packet function code.
It is discarded immediately if it is addressed to a different domain
than the domain(s) in which the receiving host participates.
For each of the 2 packet types, we assume a procedure called with a
pointer p to the VMTP packet and psize, the size of the packet in
octets.  Thus, generic packet reception is:

if not LocalDomain(p.Domain) then return;

csr := MapClient( p.Client )

if csr is NULL then
    HandleNoCsr( p, psize )

if Secure(p) then
    if SecureVMTP not supported then
        { Assume a Request. }
        if not Multicast(p) then
            NotifyClient(NULL, p, SECURITY_NOT_SUPPORTED )
|   Decrypt( csr.Key, p, psize )

if p.Checksum not null then
    if not VerifyChecksum(p, psize) then return;
if OppositeByteOrder(p) then ByteSwap( p, psize )
if psize not equal sizeof(VmtpHeader) + 4*p.Length then
    NotifyClient(NULL, p, VMTP_ERROR )
Invoke Procedure[p.FuncCode]( csr, p, psize )
Discard packet and return


   1. The Procedure[p.FuncCode] refers to one of the 2 procedures
      corresponding to the two different packet types of VMTP,
      Requests and Responses.

   2. In all the following descriptions, a packet is discarded on
      "return" unless otherwise stated.

   3. The procedure HandleNoCSR is a management routine that
      allocates and initializes a CSR and processes the packet or
      else sends an error indication to the sender of the packet.
      This procedure is described in greater detail in Section

4.7.1. Response Go to the top of this page...top

This procedure handles incoming Response packets.

HandleResponse( csr, p, psize )
    if not LocalClient( csr ) then
        if Multicast then return
|       if Migrated( p.Client ) then
|           NotifyServer(csr, p ENTITY_MIGRATED )
|       else
            NotifyServer(csr, p, ENTITY_NOT_HERE )

    if NSRset(p) then
        if Streaming not supported then
            NotifyServer(csr, p, STREAMING_NOT_SUPPORTED )
            return STREAMED_RESPONSE
|       Find csr corresponding to p.Transaction
|       if none found then
|           NotifyServer(csr, p, BAD_TRANSACTION_ID )
|           return
      if csr.LocalTransaction not equal p.Transaction then
        NotifyServer(csr, p, BAD_TRANSACTION_ID )
    Locate reply buffer rb for this p.Server
    if found then
        if rb.State is not ReceivingResponse then
          { Duplicate }
            if APGset(p) or NERset(p) then
                { Send Response to stop response packets. }
                NotifyServer(csr, p, RESPONSE_DISCARDED )
         { rb.State is ReceivingRequest}
         if new segment data then retain in CSR segment area.
         if packetgroup not complete then
             Timeout( rb, TC3(p.Server), LocalClientTimeout )
          goto EndPacketGroup
    { Otherwise, a new response message. }
    if (NSRset(p) or NERset(p)) and NoStreaming then
        NotifyServer(csr, p, VMTP_ERROR )
|    if NSRset(p) then
|      { Check consecutive with previous packet group }
|       Find last packet group CSR from p.Server.
|       if p.Transaction not
|             lastcsr.RemoteTransaction+1 mod 2**32 then
|         { Out of order packet group }
|            NotifyServer(csr, p, BAD_TRANSACTION_ID)
|           return
|       endif
|       if lastcsr not completed then
|           NotifyServer(lastcsr, p, RETRY )
|       endif
|       if CMG(lastcsr) then
|           Add segment data to lastcsr Response
|           Notify lastcsr with new packet group.
|           Clear lastcsr.VerifyInterval
|       else
|           if lastcsr available then
|                 use it for this packet group
|           else allocate and initialize new CSR
|           Save message and segment data in new CSR area.
|       endif
|   else { First packet group }
        Allocate and init reply buffer rb for this response.
        if allocation fails then
            NotifyServer(csr, p, BUSY )
        Set rb.State to ReceivingResponse
        Copy message and segment data to rb's segment area
         and set rb.PacketDelivery to that delivered.
        Save p.Server host address in ServerHost cache.
    if packetgroup not complete then
        Timeout( rb, TS1(p.Client), LocalClientTimeout )
    { We have received last packet in packet group. }
    if APGset(p) then NotifyServer(csr, p, OK )
|   if NERset(p) and CMGset(p) then
|       Queue waiting for continuation packet group.
|       Timeout( rb, TC2(rb.Server), LocalClientTimeout )
|       return
|   endif
    { Deliver response message. }
    Deliver response to Client, or queue as appropriate.
end HandleResponse


   1. The mechanism for handling streaming is optional and can be
      replaced with the tests for use of streaming.  Note that the
      server should never stream at the Client unless the Client
      has streamed at the Server or has used the STI control bit.
      Otherwise, streamed Responses are a protocol error.

   2. As an optimization, a Response can be stored into the CSR for
      the Client rather than allocating a separate CSR for a
      response buffer.  However, if multiple responses are handled,
      the code must be careful to perform duplicate detection on
      the Response stored there as well as those queued.  In
      addition, GetResponse must create a queued version of this
      Response before allowing it to be overwritten.

   3. The handling of Group Responses has been omitted for brevity.
      Basically, a Response is accepted if there has been a Request
      received locally from the same Client and same Transaction
      that has not been responded to.  In this case, the Response
      is delivered to the Server or queued.

4.8. Management Operations Go to the top of this page...top

VMTP uses management operations (invoked as remote procedure calls) to
effectively acknowledge packet groups and request retransmissions.  The
following routine is invoked by the Client's management module on
request from the Server.

NotifyVmtpClient( clientId,ctrl,receiveSeqNumber,transact,delivery,code)
    Get csr for clientId
    if none then return
    if RemoteClient( csr ) and not NotifyVmtpRemoteClient then
|   else (for streaming)
|      Find csr with same LocalTransaction as transact
|      if csr is NULL then return
    if csr.State not AwaitingResponse then return
    if ctrl.PGcount then ack previous packet groups.
    select on code
      case OK:
        Notify ack'ed segment blocks from delivery
        Clear csr.RetransCount;
        Timeout( csr, TC1(csr.Server), LocalClientTimeout )
      case RETRY:
        Set csr.TransmissionMask to missing segment blocks,
            as specified by delivery
        SendPacketGroup( csr )
        Timeout( csr, TC1(csr.Server), LocalClientTimeout )
      case RETRY_ALL
        Set csr.TransmissionMask to retransmit all blocks.
        SendPacketGroup( csr )
        Timeout( csr, TC1(csr.Server), LocalClientTimeout )
|       if streaming then
|          Restart transmission of packet groups,
|                starting from transact+1
      case BUSY:
         if csr.TimeLimit exceeded then
             Set csr.Code to USER_TIMEOUT
             return Response to application
        Set csr.TransmissionMask for full retransmission
        Clear csr.RetransCount
        Timeout( csr, TC1(csr.Server), LocalClientTimeout )
        Get new host address for entity
        Set csr.TransmissionMask for full retransmission
        Clear csr.RetransCount
        SendPacketGroup( csr )
        Timeout( csr, TC1(csr.Server), LocalClientTimeout )

        Record that server does not support streaming
        if CMG(csr) then forget this packet group
        else resend Request as separate packet group.
         Set csr.Code to code
         return Response to application
end NotifyVmtpClient


   1. The delivery parameter indicates the segment blocks received
      by the Server.  That is, a 1 bit in the i-th position
      indicates that the i-th segment block in the segment data of
      the Request was received.  All subsequent NotifyVmtpClient
      operations for this transaction should be set to acknowledge
      a superset of the segment blocks in this packet.  In
      particular, the Client need not be prepared to retransmit the
      segment data once it has been acknowledged by a Notify

4.8.1. HandleNoCSR Go to the top of this page...top

HandleNoCSR is called when a packet arrives for which there is no CSR
matching the client field of the packet.

HandleNoCSR( p, psize )
    if Secure(p) then
        if SecureVMTP not supported then
            { Assume a Request }
            if not Multicast(p) then
        HandleRequestNoCSR( p, psize )
    if p.Checksum not null then
        if not VerifyChecksum(p, psize) then return;
    if OppositeByteOrder(p) then ByteSwap( p, psize )
    if psize not equal sizeof(VmtpHeader) + 4*p.Length then
        NotifyClient(NULL, p, VMTP_ERROR )

    if p.FuncCode is Response then
|        if Migrated( p.Client ) then
|           NotifyServer(csr, p ENTITY_MIGRATED )
|       else
            NotifyServer(csr, p, NONEXISTENT_ENTITY )

    if p.FuncCode is Request then
       HandleRequestNoCSR( p, psize )
end HandleNoCSR


   1. The node need only check to see if the client entity has
      migrated if in fact it supports migration of entities.

   2. The procedure HandleRequestNoCSR is specified in Section
      5.8.1.  In the minimal client version, it need only handle
      Probe requests and can do so directly without allocating a
      new CSR.

4.9. Timeouts Go to the top of this page...top

A client with a message transaction in progress has a single timer
corresponding to the first unacknowledged request message.  (In the
absence of streaming, this request is also the last request sent.)  This
timeout is handled as follows:

LocalClientTimeout( csr )
  select on csr.State
    case AwaitingResponse:
      if csr.RetransCount > MaxRetrans(csr.Server) then
             terminate Client's message transactions up to
             and including the current message transaction.
             set return code to KERNEL_TIMEOUT
      increment csr.RetransCount
      Resend current packet group with APG set.
      Timeout( csr, TC2(csr.Server), LocalClientTimeout )
    case ReceivingResponse:
      if DGMset(csr) or csr.RetransCount > Max then
         if MDMset(csr) then
            Set MCB.MsgDeliveryMask to blocks received.
            Set csr.Code to BAD_REPLY_SEGMENT
         return to user Client
      increment csr.RetransCount
      NotifyServer with RETRY
      Timeout( csr, TC3(csr.Server), LocalClientTimeout )
  end select
end LocalClientTimeout


   1. A Client can only request retransmission of a Response if the
      Response is not idempotent.  If idempotent, it must
      retransmit the Request.  The Server should generally support
      the MsgDeliveryMask for Requests that it treats as idempotent
      and that require multi-packet Responses.  Otherwise, there is
      no selective retransmission for idempotent message

   2. The current packet group is the last one transmitted.  Thus,
      with streaming, there may be several packet groups
      outstanding that precede the current packet group.
   3. The Request packet group should be retransmitted without the
      segment data, resulting in a single short packet in the
      retransmission.  The Server must then send a
      NotifyVmtpClient with a RETRY or RETRY_ALL code to get the
      segment data transmitted as needed.  This strategy minimizes
      the overhead on the network and the server(s) for

5. Server Protocol Operation Go to the top of this page...top

This section describes the operation of the server portion of the
protocol in terms of the procedures for handling VMTP user events,
packet reception events and timeout events.  Each server is assumed to
implement the client procedures described in the previous chapter.
(This is not strictly necessary but it simplifies the exposition.)

5.1. Remote Client State Record Fields Go to the top of this page...top

The CSR for a server is extended with the following fields, in addition
to the ones listed for the client version.

RemoteClient    Identifier for remote client that sent the Request that
                this CSR is handling.

                Link to next CSR hashing to same hash index in the

                Transaction identifier for Request from remote client.

RemoteDelivery  The segment blocks received so far as part of a Request
                or yet to be acknowledged as part of a Response.

VerifyInterval  Time interval since there was confirmation that the
                remote Client was still valid.

RemotePrincipal Account identification, possibly including key and key
                timeout for secure communication.

5.2. Remote Client Protocol States Go to the top of this page...top

A CSR in the server end is in one of the following states.

AwaitingRequest Waiting for a Request packet group.  It may be marked as
                waiting on a specific Client, or on any Client.

                Waiting to receive additional Request packets in a
                multi-packet group Request.

Responded       The Response has been sent and the CSR is timing out,
                providing duplicate suppression and retransmission (if
                the Response was not idempotent).

                Response has been acknowledged or has timed out so
                cannot be retransmitted.  However, duplicates are still
                filtered and CSR can be reused for new message

Processing      Executing on behalf of the Client.

Forwarded       The message transaction has been forwarded to another
                Server that is to respond directly to the Client.

5.3. State Transition Diagrams Go to the top of this page...top

The CSR state transitions in the server are illustrated in Figure 5-1.
The CSR generally starts in the AwaitingRequest state.  On receipt of a
Request, the Server either has an up-to-date CSR for the Client or else
it sends a Probe request (as a separate VMTP message transaction) to the
VMTP management module associated with the Client.  In the latter case,
the processing of the Request is delayed until a Response to the Probe
request is received.  At that time, the CSR information is brought up to
date and the Request is processed.  If the Request is a single-packet
request, the CSR is then set in the Processing state to handle the
request.  Otherwise (a multi-packet Request), the CSR is put into the
ReceivingResponse state, waiting to receive subsequent Request packets
that constitute the Request message.  It exits the ReceivingRequest
state on timeout or on receiving the last Request packet.  In the former
case, the request is delivered with an indication of the portion
received, using the MsgDelivery field if MDM is set.  After request
processing is complete, either the Response is sent and the CSR enters
the Responded state or the message transaction is forwarded and the CSR
enters the Forwarded state.

In the Responded state, if the Response is not marked as idempotent, the
Response is retransmitted on receipt of a retransmission of the
corresponding Request, on receipt of a NotifyVmtpServer operation
requesting retransmission or on timeout at which time APG is set,
requesting an acknowledgment from the Client.  The Response is
retransmitted some maximum number of times at which time the Response is
discarded and the CSR is marked accordingly.  If a Request or a
NotifyVmtpServer operation is received expecting retransmission of the
Response after the CSR has entered the ResponseDiscarded state, a
NotifyVmtpClient operation is sent back (or invoked in the Client
management module) indicating that the response was discarded unless the
Request was multicast, in which case no action is taken.  After a
     (Retransmit Forwarded Request and NotifyVmtpClient)
                   V       |
                 |           |
          +-Time-| Forwarded |<-------------+
          |  out +-----------+              |
          |                                 |
          |          (Retransmit Response)  |
          |                      Request    |
          V                      Ack        |
          |                    +-Timeout-+  |
          |                    V         |  |
        +---------+ Ack/ +|---------^+ |
 +-Time-|Response |<-Timeout--| Responded | |
 |  out |Discarded|           +----^------+ |
 |      +---------+                |        |
 |  +------------+                 |        |
 |  |            |->-Send Response-+        |
 |  |            |->-forward Request--------+
 +->| Processing |<----------------------+
 |  |            |<----------------+     |
 |  |            |<---|            |     |
 |  +-|--------^-+    |          Last    |
 | Receive     |      |          Request |
 |    |   Timeout   Single       Packet  |
 |    |        |    Packet         |   Timeout
 |    |        |    Request        ^     ^
 |    |        |      ^           +|-----|--+
 |  +-V--------|-+    |           |Receiving|<-+Time
 +->|  Awaiting  |->--+->Request->| Request |--+ out
    |  Request   |    |  (multi-  +---------+
    +------|-----+    ^  packet)
        Request       |
           |        Response
      Send Probe     to
           |        Probe
       +---V----+     |
       |Awaiting|     ^
       |to Probe|


Figure 5-1:   Remote Client State Transitions Go to the top of this page...top
CSR returns either to the AwaitingRequest state or to the Processing
state.  Note that "Ack" refers to acknowledgment by a Notify operation.

A Request that is forwarded leaves the CSR in the Forwarded state.  In
the Forwarded state, the forwarded Request is retransmitted
periodically, expecting NotifyRemoteClient operations back from the
Server to which the Request was forwarded, analogous to the Client
behavior in the AwaitingResponse state.  In this state, a
NotifyRemoteClient from this Server acknowledges the Request or asks
that it be retransmitted or reports an error.  A retransmission of the
Request from the Client causes a NotifyVmtpClient to be returned to the
Client if APG is set.  The CSR leaves the Forwarded state after timing
out in the absence of NotifyRemoteClient operations from the forward
Server or on receipt of a NotifyRemoteClient operation indicating the
forward Server has sent a Response and received an acknowledgement.  It
then enters the ResponseDiscarded state.

Receipt of a new Request from the same Client aborts the current
transaction, independent of its state, and initiates a new transaction
unless the new Request is part of a run of message transactions.  If it
is part of a run of message transactions, the handling follows the state
diagram except the new Request is not Processed until there has been a
response sent to the previous transaction.

5.4. User Interface Go to the top of this page...top

The RPC or user interface to VMTP is implementation-dependent and may
use systems calls, functions or some other mechanism.  The list of
requests that follow is intended to suggest the basic functionality that
should be available.

AcceptMessage( reqmcb, segptr, segsize, client, transid, timeout ) 
                Accept a new Request message in the specified reqmcb
                area, placing the segment data, if any, in the area
                described by segptr and segsize.  This returns the
                Server in the entityId field of the reqmcb and actual
                segment size in the segsize parameters.  It also returns
                the Client and Transaction for this message transaction
                in the corresponding parameters.  This procedure
                supports message semantics for request processing.  When
                a server process executes this call, it blocks until a
                Request message has been queued for the server.
                AcceptMessage returns after the specified timeout period
                if a message has not been received by that time.

RespondMessage( responsemcb, client, transid, segptr ) 
                Respond to the client with the specified response
                message and segment, again with message semantics.

RespondCall( responsemcb, segptr ) 
                Respond to the client with the specified response
                message and segment, with remote procedure call
                semantics.  This procedure does not return.  The
                lightweight process that executes this procedure is
                matched to a stack, program counter, segment area and
                priority from the information provided in a
                ModifyService call, as specified in Appendix III.

ForwardMessage( requestmcb, transid, segptr, segsize, forwardserver ) 
                Forward the client to the specified forwardserver with
                the request specified in mcb.

ForwardCall( requestmcb, segptr, segsize, forwardserver ) 
                Forward the client transaction to the specified
                forwardserver with the request specified by requestmcb.
                This procedure does not return.

                Return the entityId for the remote client on whose
                behave the process is executing.  This is only
                applicable in the procedure call model of request

GetForwarder( client )
                Return the entity that forwarded this Request, if any.

GetProcess( client )
                Return an identifier for the process associated with
                this client entity-id.

GetPrincipal( client )
                Return the principal associated with this client

5.5. Event Processing Go to the top of this page...top

The following events may occur in VMTP servers.

   - User Requests

        * Receive
        * Respond

        * Forward

        * GetForwarder

        * GetProcess

        * GetPrincipal

   - Packet Arrival

        * Request Packet

   - Management Operations

        * NotifyVmtpServer

   - Timeouts

        * Client State Record Timeout

The handling of these events is described in detail in the following
subsections.  The conventions of the previous chapter are followed,
including the use of the various subroutines in the description.

5.6. Server User-invoked Events Go to the top of this page...top

A user event occurs when a VMTP server invokes one of the VMTP interface

5.6.1. Receive Go to the top of this page...top

AcceptMessage(reqmcb, segptr, segsize, client, transid, timeout)
    Locate server's request queue.
    if request is queued then
        Remember CSR associated with this Request.
        return Request in reqmcb, segptr and segsize
               and client and transaction id.
    Wait on server's request queue for next request
    up time timeout seconds.
end ReceiveCall

   1. If a multi-packet Request is partially received at the time
      of the AcceptMessage, the process waits until it completes.

   2. The behavior of a process accepting a Request as a
      lightweight thread is similar except that the process
      executes using the Request data logically as part of the
      requesting Client process.

5.6.2. Respond Go to the top of this page...top

RespondCall is described as one case of the Respond transmission
procedure; RespondMessage is similar.

RespondCall( responsemcb, responsesegptr )
    Locate csr for this client.
    Check segment data accessible, if any
    if local client then
        Handle locally
    if responsemcb.Code is RESPONSE_DISCARDED then
    SendPacketGroup( csr )
    set csr.State to Responded.
    if DGM reply then { Idempotent }
        release segment data
        Timeout( csr, TS4(csr.Client), FreeCsr );
    else { Await acknowledgement or new Request else ask for ack. }
        Timeout( csr, TS5(csr.Client), RemoteClientTimeout )
end RespondCall


   1. RespondMessage is similar except the Server process must be
      synchronized with the release of the segment data (if any).

   2. The non-idempotent Response with segment data is sent first
      without a request for an acknowledgement.  The Response is
      retransmitted after time TS5(client) if no acknowledgment or
      new Request is received from the client in the meantime.  At
      this point, the APG bit is sent.

   3. The MCB of the Response is buffered in the client CSR, which
      remains for TS4 seconds, sufficient to filter old duplicates.
      The segment data (if any) must be retained intact until:  (1)
      after transmission if idempotent or (2) after acknowledged or
      timeout has occurred if not idempotent.  Techniques such as
      copy-on-write might be used to keep a copy of the Response
      segment data without incurring the cost of a copy.

5.6.3. Forward Go to the top of this page...top

Forwarding is logically initiating a new message transaction between the
Server (now acting as a Client) and the server to which the Request is
forwarded.  When the second server returns a Response, the same Response
is immediately returned to the Client.  The forwarding support in VMTP
preserves these semantics while providing some performance optimizations
in some cases.

ForwardCall( req, segptr, segsize, forwardserver )
    Locate csr for this client.
    Check segment data accessible, if any

    if local client or Request was multicast or secure
       or csr.ForwardCount == 15 then
        Handle as a new Send operation
    if forwardserver is local then
        Handle locally
    Set csr.funccode to Request
    Increment csr.ForwardCount
    Set csr.State to Responded
    SendPacketGroup( csr ) { To ForwardServer }
    Timeout( csr, TS4(csr.Client), FreeAlien )
end ForwardCall


   1. A Forward is logically a new call or message transaction.  It
      must be really implemented as a new message transaction if
      the original Request was multicast or secure (with the
      optional further refinement that it can be used with a secure
      message transaction when the Server and ForwardServer are the
      same principal and the Request was not multicast).

   2. A Forward operation is never handled as an idempotent
      operation because it requires knowledge that the
      ForwardServer will treat the forwarded operation as
      idempotent as well.  Thus, a Forward operation that includes
      a segment should set APG on the first transmission of the
      forwarded Request to get an acknowledgement for this data.
      Once the acknowledgement is received, the forwarding Server
      can discard the segment data, leaving only the basic CSR to
      handle retransmissions from the Client.

5.6.4. Other Functions Go to the top of this page...top

GetRemoteClient is a simple local query of the CSR.  GetProcess and
GetPrincipal also extract this information from the CSR.  A server
module may defer the Probe callback to the Client to get that
information until it is requested by the Server (assuming it is not
using secure communication and duplicate suppression is adequate without
callback.)  GetForwarder is implemented as a callback to the Client,
using a GetRequestForwarder VMTP management operation.  Additional
management procedures for VMTP are described in Appendix III.

5.7. Request Packet Arrival Go to the top of this page...top

The basic packet reception follows that described for the Client
routines.  A Request packet is handled by the procedure HandleRequest.

HandleRequest( csr, p, psize )

    if LocalClient(csr) then
        { Forwarded Request on local Client }
        if csr.LocalTransaction != p.Transaction then return
        if csr.State != AwaitingResponse then return
        if p.ForwardCount < csr.ForwardCount then
           Discard Request and return.
        Find a CSR for Client as a remote Client.
        if not found then
            if packet group complete then
                handle as a local message transaction
            Allocate and init CSR
            goto newTransaction
        { Otherwise part of current transaction }
        { Handle directly below. }n
    if csr.RemoteTransaction = p.Transaction then
      { Matches current transaction }
        if OldForward(p.ForwardCount,csr.ForwardCount) then
        if p.ForwardCount > csr.ForwardCount then
          { New forwarded transaction }
            goto newTransaction
        { Otherwise part of current transaction }
        if csr.State = ReceivingRequest then
            if new segment data then retain in CSR segment area.
            if Request not complete then
               Timeout( csr, TS1(p.Client), RemoteClientTimeout )
            goto endPacketGroup
        if csr.State is Responded then
          { Duplicate }
            if csr.Code is RESPONSE_DISCARDED
               and Multicast(p) then
            if not DGM(csr) then { Not idempotent }
                if SegmentData(csr) then set APG
                { Resend Response or Request, if Forwarded }
                SendPacketGroup( csr )
                timeout=if SegmentData(csr) then TS5(csr.Client)
                          else TS4(csr.Client)
                Timeout( csr, timeout, RemoteClientTimeout )
            { Else idempotent - fall thru to newTransaction }
        else { Presume it is a retransmission }
            NotifyClient( csr, p, OK )
   else if OldTransaction(csr.RemoteTransact,p.Transaction) then
    { Otherwise, a new message transaction. }
    Abort handling of previous transactions for this Client.

    if (NSRset(p) or NERset(p)) and NoStreaming then
        NotifyClient( csr, p, STREAMING_NOT_SUPPORTED )
|   if NSRset(p) then { Streaming }
|     { Check that consecutive with previous packet group }
|       Find last packet group CSR from this client.
|      if p.Transaction not lastcsr.RemoteTransaction+1 mod 2**32
|         and not STIset(lastcsr) or
|        p.Transaction not lastcsr.RemoteTransaction+256 mod **32
|        then
|         { Out of order packet group }
|         NotifyClient(csr, p, BAD_TRANSACTION_ID )
|         return
|       endif
|       if lastcsr not completed then
|           NotifyClient( lastcsr, p, RETRY )
|       endif
|       if lastcsr available then use it for this packet group
|       else allocate and initialize new CSR
|       if CMG(lastcsr) then
|          Add segment data to lastcsr Request
|          Keep csr as record of this packet group.
|          Clear lastcsr.VerifyInterval
|      endif
|   else { First packet group }
        if MultipleRemoteClients(csr) then ScavengeCsrs(p.Client)
        Set csr.RemoteTransaction, csr.Priority
        Copy message and segment data to csr's segment area
         and set csr.PacketDelivery to that delivered.
        Clear csr.PacketDelivery
        Clear csr.VerifyInterval
        SaveNetworkAddress( csr, p )
    if packetgroup not complete then
        Timeout( csr, TS3(p.Client), RemoteClientTimeout )
    { We have received complete packet group. }
    if APG(p) then NotifyClient( csr, p, OK )
|   if NERset(p) and CMG(p) then
|       Queue waiting for continuation packet group.
|       Timeout( csr, TS3(csr.Client), RemoteClientTimeout )
|       return
|   endif
    { Deliver request message. }
    if GroupId(csr.Server) then
        For each server identified by csr.Server
            Replicate csr and associated data segment.
            if CMDset(csr) and Server busy then
               Discard csr and data
               Deliver or invoke csr for each Server.
            if not DGMset(csr) then queue for Response
            else Timeout( csr, TS4(csr.Client), FreeCsr )
       if CMDset(csr) and Server busy then
           Discard csr and data
           Deliver or invoke csr for this server.
        if not DGMset(csr) then queue for Response
        else Timeout( csr, TS4(csr.Client), FreeCsr )
end HandleRequest


   1. A Request received that specifies a Client that is a local
      entity should be a Request forwarded by a remote server to a
      local Server.

   2. An alternative structure for handling a Request sent to a
      group when there are multiple local group members is to
      create a remote CSR for each group member on reception of the
      first packet and deliver a copy of each packet to each such
      remote CSR as each packet arrives.

5.8. Management Operations Go to the top of this page...top

VMTP uses management operations (invoked as remote procedure calls) to
effectively acknowledge packet groups and request retransmissions.  The
following routine is invoked by the Server's management module on
request from the Client.

    Find csr with same RemoteTransaction and RemoteClient
    as clientId and transact.
    if not found or csr.State not Responded then return
    if DGMset(csr) then
        if transmission of Response in progress then
            Abort transmission
            if code is migrated then
               restart transmission with new host addr.
        if Retry then Report protocol error
    select on code
      case RETRY:
        if csr.RetransCount > MaxRetrans(clientId) then
             if response data segment then
                 Discard data and mark as RESPONSE_DISCARDED
|                if NERset(csr) and subsequent csr then
|                    Deallocate csr and use later csr for
|                    future duplicate suppression
|                endif
        increment csr.RetransCount
        Set csr.TransmissionMask to missing segment blocks,
            as specified by delivery
        SendPacketGroup( csr )
        Timeout( csr, TS3(csr.Client), RemoteClientTimeout )
      case BUSY:
        if csr.TimeLimit exceeded then
            if response data segment then
                Discard data and mark as RESPONSE_DISCARDED
|               if NERset(csr) and subsequent csr then
|                   Deallocate csr and use later csr for
|                   future duplicate suppression
|               endif
        Set csr.TransmissionMask for full retransmission
        Clear csr.RetransCount
        Timeout( csr, TS3(csr.Server), RemoteClientTimeout )

        Get new host address for entity
        Set csr.TransmissionMask for full retransmission
        Clear csr.RetransCount
        SendPacketGroup( csr )
        Timeout( csr, TS3(csr.Server), RemoteClientTimeout )

      case default:
        Abort transmission of Response if in progress.
        if response data segment then
           Discard data and mark as RESPONSE_DISCARDED
           if NERset(csr) and subsequent csr then
               Deallocate csr and use later csr for
               future duplicate suppression
end NotifyVmtpServer


   1. A NotifyVmtpServer operation requesting retransmission of
      the Response is acceptable only if the Response was not
      idempotent.  When the Response is idempotent, the Client must
      be prepared to retransmit the Request to effectively request
      retransmission of the Response.

   2. A NotifyVmtpServer operation may be received while the
      Response is being transmitted.  If an error return, as an
      efficiency, the transmission should be aborted, as suggested
      when the Response is a datagram.

   3. A NotifyVmtpServer operation indicating OK or an error
      allows the Server to discard segment data and not provide for
      subsequent retransmission of the Response.

5.8.1. HandleRequestNoCSR Go to the top of this page...top

When a Request is received from a Client for which the node has no CSR,
the node allocates and initializes a CSR for this Client and does a
callback to the Client's VMTP management module to get the Principal,
Process and other information associated with this Client.  It also
checks that the TransactionId is correct in order to filter out

HandleRequestNoCSR( p, psize )
|   if Secure(p) then
|       Allocate and init CSR
|       SaveSourceHostAddr( csr, p )
|       ProbeRemoteClient( csr, p, AUTH_PROBE )
|       if no response or error then
|          delete CSR
|          return
|       Decrypt( csr.Key, p, psize )
|        if p.Checksum not null then
|       if not VerifyChecksum(p, psize) then return;
|       if OppositeByteOrder(p) then ByteSwap( p, psize )
|       if psize not equal sizeof(VmtpHeader) + 4*p.Length then
|          NotifyClient(NULL, p, VMTP_ERROR )
|          return
|       HandleRequest( csr, p, psize )
|       return
    if Server does not exist then
        NotifyClient( csr, p, NONEXISTENT_ENTITY )
    if security required by server then
        NotifyClient(csr, p, SECURITY_REQUIRED )
    Allocate and init CSR
    SaveSourceHostAddr( csr, p );
    if server requires Authentication then
        ProbeRemoteClient( csr, p, AUTH_PROBE )
        if no response or error then
           delete CSR
    { Setup immediately as a new message transaction }
    set csr.Server to p.Server
    set csr.RemoteTransaction to p.Transaction-1

    HandleRequest( csr, p, psize )


   1. A Probe request is always handled as a Request not requiring
      authentication so it never generates a callback Probe to the

   2. If the Server host retains remote client CSR's for longer
      than the maximum packet lifetime and the Request
      retransmission time, and the host has been running for at
      least that long, then it is not necessary to do a Probe
      callback unless the Request is secure.  A Probe callback can
      take place when the Server asks for the Process or
      PrincipalId associated with the Client.

5.9. Timeouts Go to the top of this page...top

The server must implement a timeout for remote client CSRs.  There is a
timeout for each CSR in the server.

RemoteClientTimeout( csr )
  select on csr.State
    case Responded:
        if RESPONSE_DISCARDED then
            mark as timed out
            Make a candidate for reuse.
        if csr.RetransCount > MaxRetrans(Client) then
            discard Response
            mark CSR as RESPONSE_DISCARDED
            Timeout(csr, TS4(Client), RemoteClientTimeout)
        increment csr.RetransCount
        { Retransmit Response or forwarded Request }
        Set APG to get acknowledgement.
        SendPacketGroup( csr )
        Timeout( csr, TS3(Client), RemoteClientTimeout )
    case ReceivingRequest:
      if csr.RetransCount > MaxRetrans(csr.Client)
         or DGMset(csr) or NRTset(csr) then
          Modify csr.segmentSize and csr.MsgDelivery
          to indicate packets received.
          if MDMset(csr) then
              Invoke processing on Request
              discard Request and reuse CSR
              (Note: Need not remember Request discarded.)
      increment csr.RetransCount
      NotifyClient( csr, p, RETRY )
      Timeout( csr, TS3(Client), RemoteClientTimeout )
        Report error - invalid state for RemoteClientTimeout
end RemoteClientTimeout


   1. When a CSR in the Responded state times out after discarding
      the Response, it can be made available for reuse, either by
      the same Client or a different one.  The CSR should be kept
      available for reuse by the Client for as long as possible to
      avoid unnecessary callback Probes.

6. Concluding Remarks Go to the top of this page...top

This document represents a description of the current state of the VMTP
design.  We are currently engaged in several experimental
implementations to explore and refine all aspects of the protocol.
Preliminary implementations are running in the UNIX 4.3BSD kernel and in
the V kernel.

Several issues are still being discussed and explored with this
protocol.  First, the size of the checksum field and the algorithm to
use for its calculation are undergoing some discussion.  The author
believes that the conventional 16-bit checksum used with TCP and IP is
too weak for future high-speed networks, arguing for at least a 32-bit
checksum.  Unfortunately, there appears to be limited theory covering
checksum algorithms that are suitable for calculation in software.

Implementation of the streaming facilities of VMTP is still in progress.
This facility is expected to be important for wide-area, long delay

I. Standard VMTP Response Codes Go to the top of this page...top

The following are the numeric values of the response codes used in VMTP.

0               OK

1               RETRY

2               RETRY_ALL

3               BUSY

4               NONEXISTENT_ENTITY

5               ENTITY_MIGRATED

6               NO_PERMISSION

7               NOT_AWAITING_MSG

8               VMTP_ERROR

9               MSGTRANS_OVERFLOW

10              BAD_TRANSACTION_ID


12              NO_RUN_RECORD

13              RETRANS_TIMEOUT

14              USER_TIMEOUT

15              RESPONSE_DISCARDED


17              BAD_REPLY_SEGMENT

18              SECURITY_REQUIRED

19              STREAMED_RESPONSE

20              TOO_MANY_RETRIES

21              NO_PRINCIPAL
22              NO_KEY


24              NO_AUTHENTICATOR

25-63           Reserved for future VMTP assignment.

Other values of the codes are available for use by higher level
protocols.  Separate protocol documents will specify further standard

Applications are free to use values starting at 0x00800000 (hex) for
application-specific return values.

II. VMTP RPC Presentation Protocol Go to the top of this page...top

For complete generality, the mapping of the procedures and the
parameters onto VMTP messages should be defined by a RPC presentation
protocol.  In the absence of an accepted standard protocol, we define an
RPC presentation protocol for VMTP as follows.

Each procedure is assigned an identifying Request Code.  The Request
code serves effectively the same as a tag field of variant record,
identifying the format of the Request and associated Response as a
variant of the possible message formats.

The format of the Request for a procedure is its Request Code followed
by its parameters sequentially in the message control block until it is

The remaining parameters are sent as part of the message segment data
formatted according to the XDR protocol (RFC ??).  In this case, the
size of the segment is specified in the SegmentSize field.

The Response for a procedure consists of a ResponseCode field followed
by the return parameters sequentially in the message control block,
except if there is a parameter returned that must be transmitted as
segment data, its size is specified in the SegmentSize field and the
parameter is stored in the SegmentData field.

Attributes associated with procedure definitions should indicate the
Flags to be used in the RequestCode.  Request Codes are assigned as
described below.

II.1. Request Code Management Go to the top of this page...top

Request codes are divided into Public Interface Codes and
application-specific, according to whether the PIC value is set.  An
interface is a set of request codes representing one service or module
function.  A public interface is one that is to be used in multiple
independently developed modules.  In VMTP, public interface codes are
allocated in units of 256 structured as

 | ControlFlags|  Interface     | Version/Procedure |
    8 bits          16 bits              8 bits
An interface is free to allocate the 8 bits for version and procedure as
desired.  For example, all 8 bits can be used for procedures.  A module
multiple Interface values.  They need not be consecutive Interface

III. VMTP Management Procedures Go to the top of this page...top

Standard procedures are defined for VMTP management, including creation,
deletion and query of entities and entity groups, probing to get
information about entities, and updating message transaction information
at the client or the server.

The procedures are implemented by the VMTP manager that constitutes a
portion of every complete VMTP module.  Each procedure is invoked by
sending a Request to the VMTP manager that handles the entity specified
in the operation or the local manager.  The Request sent using the
normal Send operation with the Server specified as the well-known entity
group VMTP_MANGER_GROUP, using the CoResident Entity mechanism to direct
the request to the specific manager that should handle the Request.
(The ProbeEntity operation is multicast to the VMTP_MANAGER_GROUP if the
host address for the entity is not known locally and the host address is
determined as the host address of the responder.  For all other
operations, a ProbeEntity operation is used to determine the host
address if it is not known.)  Specifying co-resident entity 0 is
interpreted as the co-resident with the invoking process.  The
co-resident entity identifier may also specify a group in which case,
the Request is sent to all managers with members in this group.

The standard procedures with their RequestCode and parameters are listed
below with their semantics.  (The RequestCode range 0xVV000100 to
0xVV0001FF is reserved for use by the VMTP management routines, where VV
is any choice of control flags with the PIC bit set.  The flags are set
below as required for each procedure.)

0x05000101 - ProbeEntity(CREntity, entityId, authDomain) -> (code,
                Request and return information on the specified entity
                in the specified authDomain, sending the Request to the
                VMTP management module coresident with CREntity.  An
                error return is given if the requested information
                cannot be provided in the specified authDomain.  The
                <staterec> returned is structured as the following

                Transaction identifier
                                The current or next transaction
                                identifier being used by the probed

                ProcessId: 64 bits 
                                Identifier for client process.  The
                                meaning of this is specified as part of
                                the Domain definition.

                PrincipalId     The identifier for the principal or
                                account associated with the process
                                specified by ProcessId.  The meaning of
                                this field is specified as part of the
                                Domain definition.

                                The identifier for the principal or
                                account associated with the Client port,
                                which may be different from the
                                PrincipalId especially if this is an
                                nested call.  The meaning of this field
                                is specified as part of the Domain

                The code field indicates whether this is an error
                response or not.  The codes and their interpretation

                No error. Probe was completed OK.

                Specified entity does not exist.

                The entity has migrated and is no longer at the host to
                which the request was sent.

                Entity has refused to provide ProbeResponse.

                The Request packet group was in error relative to the
                VMTP protocol specification.

                Some type of error - discard ProbeResponse.

0x0D000102 - AuthProbeEntity(CREntity,entityId,authDomain,randomId) ->
                Request authentication of the entity specified by
                entityId from the VMTP manager coresident with CREntity
                in authDomain authentication domain, returning the
                information contained in the return parameters.  The
                fields are set the same as that specified for the basic
                ProbeResponse except as noted below.

                                20 bytes consisting of the EntityId, the
                                randomId and the probed Entity's current
                                Transaction value plus a 32-bit checksum
                                for these two fields (checksummed using
                                the standard packet Checksum algorithm),
                                all encrypted with the Key supplied in
                                the Authenticator.

                EncryptType     An identifier that identifies the
                                variant of encryption method being used
                                by the probed Entity for packets it
                                transmits and packets it is able to
                                receive.  (See Appendix V.)  The
                                high-order 8 bits of the EncryptType
                                contain the XOR of the 8 octets of the
                                PrincipalId associated with private key
                                used to encrypt the EntityAuthenticator.
                                This value is used by the requestor or
                                Client as an aid in locating the key to
                                decrypt the authenticator.

                                (returned as segment data) The
                                ProcessId, PrincipalId,
                                EffectivePrincipal associated with the
                                ProbedEntity plus the private
                                encryption/decryption key and its
                                lifetime limit to be used for
                                communication with the Entity.  The
                                authenticator is encrypted with a
                                private key associated with the Client
                                entity such that it can be neither read
                                nor forged by a party not trusted by the
                                Client Entity.  The format of the
                                Authenticator in the message segment is
                                shown in detail in Figure III-1.

                Key: 64 bits    Encryption key to be used for encrypting
                                and decrypting packets sent to and
                                received from the probed Entity.  This
                                is the "working" key for packet
                                transmissions.  VMTP only uses private
                |            ProcessId   (8 octets)             |
                |           PrincipalId  (8 octets)             |
                |           EffectivePrincipalId  (8 octets)    |
                |            Key  (8 octets)                    |
                |              KeyTimeLimit                     |
                |              AuthDomain                       |
                |               AuthChecksum                    |


Figure III-1:   Authenticator Format Go to the top of this page...top

                                key encryption for data transmission.

                KeyTimeLimit: 32 bits 
                                The time in seconds since Dec. 31st,
                                1969 GMT at which one should cease to
                                use the Key.

                AuthDomain: 32 bits 
                                The authentication domain in which to
                                interpret the principal identifiers.
                                This may be different from the
                                authDomain specified in the call if the
                                Server cannot provide the authentication
                                information in the request domain.

                AuthChecksum: 32 bits 
                                Contains the checksum (using the same
                                Checksum algorithm as for packet) of
                                KeyTimeLimit, Key, PrincipalId and


                   1. A authentication Probe Request and Response
                      are sent unencrypted in general because it is
                      used prior to there being a secure channel.
                      Therefore, specific fields or groups of
                      fields checksummed and encrypted to prevent
                      unauthorized modification or forgery.  In
                      particular, the ProbeAuthenticator is
                      checksummed and encrypted with the Key.

                   2. The ProbeAuthenticator authenticates the
                      Response as responding to the Request when
                      its EntityId, randomId and Transaction values
                      match those in the Probe request.  The
                      ProbeAutenticator is bound to the
                      EntityAutenticator by being encrypted by the
                      private Key contained in that authenticator.

                   3. The authenticator is encrypted such that it
                      can be decrypted by a private key, known to
                      the Client.  This authenticator is presumably
                      obtained from a key distribution center that
                      the Client trusts.  The AuthChecksum prevents
                      undetected modifications to the

0x05000103 - ProbeEntityBlock( entityId ) -> ( code, entityId ) 
                Check whether the block of 256 entity identifiers
                associated with this entityId are in use.  The entityId
                returned should match that being queried or else the
                return value should be ignored and the operation redone.

0x05000104 - QueryVMTPNode( entityId ) -> (code, MTU, flags, authdomain,
                domains, authdomains, domainlist) 
                Query the VMTP management module for entityId to get
                various module- or node-wide parameters, including:  (1)
                MTU - Maximum transmission unit or packet size handled
                by this node.  (2) flags- zero or more of the following
                bit fields:

                1               Handles streamed Requests.

                2               Can issue streamed message transactions
                                for clients.

                4               Handles secure Requests.

                8               Can issue secure message transactions.

                The authdomain indicates the primary authentication
                domain supported.  The domains and authdomains
                parameters indicate the number of entity domains and
                authentication domains supported by this node, which are
                listed in the data segment parameter domainlist if
                either parameter is non-zero. (All the entity domains
                precede the authentication domains in the data segment.)

0x05000105 - GetRequestForwarder( CREntity, entityId1 ) -> (code,
                entityId2, principal, authDomain) 
                Return the forwarding server's entity identifer and
                principal for the forwarder of entityId1.  CREntity
                should be zero to get the local VMTP management module.

0x05000106 - CreateEntity( entityId1 ) -> ( code, entityId2 ) 
                Create a new entity and return its entity identifier in
                entityId2.  The entity is created local to the entity
                specified in entityId1 and local to the requestor if
                entityId1 is 0.

0x05000107 - DeleteEntity( entityId ) -> ( code ) 
                Delete the entity specified by entityId, which may be a
                group.  If a group, the deletion is only on a best
                efforts basis.  The client must take additional measures
                to ensure complete deletion if required.

0x0D000108 -QueryEntity( entityId ) -> ( code, descriptor ) 
                Return a descriptor of entityId in arg of a maximum of
                segmentSize bytes.

0x05000109 - SignalEntity( entityId, arg )->( code ) 
                Send the signal specified by arg to the entity specified
                by entityId.  (arg is 32 bits.)

0x0500010A - CreateGroup(CREntity,entityGroupId,entityId,perms)->(code)
                Request that the VMTP manager local to CREntity create
                an new entity group, using the specified entityGroupId
                with entityId as the first member and permissions
                "perms", a 32-bit field described later.  The invoker is
                registered as a manager of the new group, giving it the
                permissions to add or remove members.  (Normally
                CREntity is 0, indicating the VMTP manager local to the

0x0500010B - AddToGroup(CREntity, entityGroupId, entityId,
                Request that the VMTP manager local to CREntity add the
                specified entityId to the entityGroupId with the
                specified permissions.  If entityGroupId specifies a
                restricted group, the invoker must have permission to
                add members to the group, either because the invoker is
                a manager of the group or because it was added to the
                group with the required permissions.  If CREntity is 0,
                then the local VMTP manager checks permissions and
                forwards the request with CREntity set to entityId and
                the entityId field set to a digital signature (see
                below) of the Request by the VMTP manager, certifying
                that the Client has the permissions required by the
                Request.  (If entityGroupId specifies an unrestricted
                group, the Request can be sent directly to the handling
                VMTP manager by setting CREntity to entityId.)

0x0500010C - RemoveFromGroup(CREntity, entityGroupId, entityId)->(code) 
                Request that the VMTP manager local to CREntity remove
                the specified entityId from the group specified by
                entityGroupId.  Normally CREntity is 0, indicating the
                VMTP manager local to the requestor.  If CREntity is 0,
                then the local VMTP manager checks permissions and
                forwards the request with CREntity set to entityId and
                the entityId field a digital signature of the Request by
                the VMTP manager, certifying that the Client has the
                permissions required by the Request.

0x0500010D - QueryGroup( entityId )->( code, record )...  
                Return information on the specified entity.  The
                Response from each responding VMTP manager is (code,
                record).  The format of the record is (memberCount,
                member1, member2, ...).  The Responses are returned on a
                best efforts basis; there is no guarantee that responses
                from all managers with members in the specified group
                will be received.

0x0500010E - ModifyService(entityId,flags,count,pc,threadlist)->(code,
                Modify the service associated with the entity specified
                by entityId.  The flags may indicate a message service
                model, in which case the call "count" parameter
                indicates the maximum number of queued messages desired;
                the return "count" parameter indicates the number of
                queued message allowed.  Alternatively, the "flags"
                parameters indicates the RPC thread service model, in
                which case "count" threads are requested, each with an
                inital program counter as specified and stack, priority
                and message receive area indicated by the threadlist.
                In particular, "threadlist" consists of "count" records
                of the form
                (priority,stack,stacksize,segment,segmentsize), each one
                assigned to one of the threads.  Flags defined for the
                "flags" parameter are:

                1               THREAD_SERVICE - otherwise the message

                2               AUTHENTICATION_REQUIRED - Sent a Probe
                                request to determine principal
                                associated with the Client, if not

                4               SECURITY_REQUIRED - Request must be
                                encrypted or else reject.

                8               INCREMENTAL - treat the count value as
                                an increment (or decrement) relative to
                                the current value rather than an
                                absolute value for the maximum number of
                                queued messages or threads.

                In the thread model, the count must be a positive
                increment or else 0, which disables the service.  Only a
                count of 0 terminates currently queued requests or
                in-progress request handling.

0x4500010F -
                Update the state associated with the transaction
                specified by client and transact, an entity identifier
                and transaction identifier, respectively.  This
                operation is normally used only by another VMTP
                management module.  (Note that it is a datagram
                operation.)  The other parameters are as follows:

                ctrl            A 32-bit value corresponding to 4th
                                32-bit word of the VMTP header of a
                                Response packet that would be sent in
                                response to the Request that this is
                                responding to.  That is, the control
                                flags, ForwardCount, RetransmitCount and
                                Priority fields match those of the
                                Request.  (The NRS flag is set if the
                                receiveSeqNumber field is used.)  The
                                PGCount subfield indicates the number of
                                previous Request packet groups being
                                acknowledged by this Notify operation.
                                (The bit fields that are reserved in
                                this word in the header are also
                                reserved here and must be zero.)

                recSeq          Sequence number of reception at the
                                Server if the NRS flag is set in the
                                ctrl parameter, otherwise reserved and
                                zero.  (This is used for sender-based
                                logging of message activity for replay
                                in case of failure - an optional

                delivery        Indicates the segment blocks of the
                                packet group have been received at the

                code            indicates the action the client should
                                take, as described below.

                The VMTP management module should take action on this
                operation according to the code, as specified below.

                OK              Do nothing at this time, continue
                                waiting for the response with a reset

                RETRY           Retransmit the request packet group
                                immediately with at least the segment
                                blocks that the Server failed to
                                receive, the complement of those
                                indicated by the delivery parameter.

                RETRY_ALL       Retransmit the request packet group
                                immediately with at least the segment
                                blocks that the Server failed to
                                receive, as indicated by the delivery
                                field plus all subsequently transmitted
                                packets that are part of this packet
                                run.  (The latter is applicable only for
                                streamed message transactions.)

                BUSY            The server was unable to accept the
                                Request at this time.  Retry later if
                                desired to continue with the message

                                Specified Server entity does not exist.
                ENTITY_MIGRATED The server entity has migrated and is no
                                longer at the host to which the request
                                was sent.  The Server should attempt to
                                determine the new host address of the
                                Client using the VMTP management
                                ProbeEntity operation (described

                NO_PERMISSION   Server has not authorized reception of
                                messages from this client.

                                The conditional message delivery bit was
                                set for the Request packet group and the
                                Server was not waiting for it so the
                                Request packet group was discarded.

                VMTP_ERROR      The Request packet group was in error
                                relative to the VMTP protocol

                                Transaction identifier is old relative
                                to the transaction identifier held for
                                the Client by the Server.

                                Server does not support multiple
                                outstanding message transactions from
                                the same Client, i.e. streamed message

                                The Request was secure and this Server
                                does not support security.

                                The Server is refusing the Request
                                because it was not encrypted.

                NO_RUN_RECORD   Server has no record of previous packets
                                in this run of packet groups.  This can
                                occur if the first packet group is lost
                                or if the current packet group is sent
                                significantly later than the last one
                                and the Server has discarded its client
                                state record.
0x45000110 - NotifyVmtpServer(server,client,transact,delivery,code)->() 
                Update the server state associated with the transaction
                specified by client and transact, an entity identifier
                and transaction identifier, respectively.  This
                operation is normally used only by another VMTP
                management module.  (Note that it is a datagram
                operation.)  The other parameters are as follows:

                delivery        Indicates the segment blocks of the
                                Response packet group that have been
                                received at the Client.

                code            indicates the action the Server should
                                take, as listed below.

                The VMTP management module should take action on this
                operation according to the code, as specified below.

                OK              Client is satisfied with Response data.
                                The Server can discard the response
                                data, if any.

                RETRY           Retransmit the Response packet group
                                immediately with at least the segment
                                blocks that the Client failed to
                                receive, as indicated by the delivery
                                parameter.  (The delivery parameter
                                indicates those segment blocks received
                                by the Client).

                RETRY_ALL       Retransmit the Response packet group
                                immediately with at least the segment
                                blocks that the Client failed to
                                receive, as indicated by the (complement
                                of) the delivery parameter.  Also,
                                retransmit all Response packet groups
                                send subsequent to the specified packet

                                Specified Client entity does not exist.

                ENTITY_MIGRATED The Client entity has migrated and is no
                                longer at the host to which the response
                                was sent.

                                The Response was discarded and no longer
                                of interest to the Client.  This may
                                occur if the conditional message
                                delivery bit was set for the Response
                                packet group and the Client was not
                                waiting for it so the Response packet
                                group was discarded.

                VMTP_ERROR      The Response packet group was in error
                                relative to the VMTP protocol

0x41000111 -
                The same as NotifyVmtpClient except the co-resident
                addressing is not used.  This operation is used to
                update client state that is remote when a Request is

Note the use of the CRE bit in the RequestCodes to route the request to
the correct VMTP management module(s) to handle the request.

III.1. Entity Group Management Go to the top of this page...top

An entity in a group has a set of permissions associated with its
membership, controling whether it can add or remove others, whether it
can remove itself, and whether others can remove it from the group.  The
permissions for entity groups are as follows:
VMTP_GRP_MANAGER    0x00000001 { Manager of group. }
VMTP_REM_BY_SELF    0x00000002 { Can be removed self. }
VMTP_REM_BY_PRIN    0x00000004 { Can be rem'ed by same principal}
VMTP_REM_BY_OTHE    0x00000008 { Can be removed any others. }
VMTP_ADD_PRIN       0x00000010 { Can add by same principal. }
VMTP_ADD_OTHE       0x00000020 { Can add any others. }
VMTP_REM_PRIN       0x00000040 { Can remove same principal. }
VMTP_REM_OTHE       0x00000080 { Can remove any others. }

To remove an entity from a restricted group, the invoker must have
permission to remove that entity and the entity must have permissions
that allow it to be removed by that entity.  With an unrestricted group,
only the latter condition applies.

With a restricted group, a member can only be added by another entity
with the permissions to add other entities.  The creator of a group is
given full permissions on a group.  A entity adding another entity to a
group can only give the entity it adds a subset of its permissions.
With unrestricted groups, any entity can add itself to the group.  It
can also add other entities to the group providing the entity is not
marked as immune to such requests.  (This is an implementation
restriction that individual entities can impose.)

III.2. VMTP Management Digital Signatures Go to the top of this page...top

As mentioned above, the entityId field of the AddToGroup and
RemoveFromGroup is used to transmit a digital signature indicating the
permission for the operation has been checked by the sending kernel.
The digital signature procedures have not yet been defined.  This field
should be set to 0 for now to indicate no signature after the CREntity
parameter is set to the entity on which the operation is to be

IV. VMTP Entity Identifier Domains Go to the top of this page...top

VMTP allows for several disjoint naming domains for its endpoints.  The
64-bit entity identifier is only unique and meaningful within its
domain.  Each domain can define its own algorithm or mechanism for
assignment of entity identifiers, although each domain mechanism must
ensure uniqueness, stability of identifiers and host independence.

IV.1. Domain 1 Go to the top of this page...top

For initial use of VMTP, we define the domain with Domain identifier 1
as follows:

 | TypeFlags | Discriminator  |    Internet Address    |
    4 bits          28 bits                32 bits
The Internet address is the Internet address of the host on which this
entity-id is originally allocated.  The Discriminator is an arbitrary
value that is unique relative to this Internet host address.  In
addition, the host must guarantee that this identifier does not get
reused for a long period of time after it becomes invalid.  ("Invalid"
means that no VMTP module considers in bound to an entity.)  One
technique is to use the lower order bits of a 1 second clock.  The clock
need not represent real-time but must never be set back after a crash.
In a simple implementation, using the low order bits of a clock as the
time stamp, the generation of unique identifiers is overall limited to
no more than 1 per second on average.  The type flags were described in
Section 3.1.

An entity may migrate between hosts.  Thus, an implementation can
heuristically use the embedded Internet address to locate an entity but
should be prepared to maintain a cache of redirects for migrated
entities, plus accept Notify operations indicating that migration has

Entity group identifiers in Domain 1 are structured in one of two forms,
depending on whether they are well-known or dynamically allocated
identifiers.  A well-known entity identifier is structured as:

 | TypeFlags |  Discriminator |Internet Host Group Addr|
    4 bits          28 bits                32 bits
with the second high-order bit (GRP) set to 1.  This form of entity
identifier is mapped to the Internet host group address specified in the
low-order 32 bits.  The Discriminator distinguishes group identifiers
using the same Internet host group.  Well-known entity group identifiers
should be allocated to correspond to the basic services provided by
hosts that are members of the group, not specifically because that
service is provided by VMTP.  For example, the well-known entity group
identifier for the domain name service should contain as its embedded
Internet host group address the host group for Domain Name servers.

A dynamically allocated entity identifier is structured as:

 | TypeFlags |  Discriminator |   Internet Host Addr   |
    4 bits          28 bits             32 bits

with the second high-order bit (GRP) set to 1.  The Internet address in
the low-order 32 bits is a Internet address assigned to the host that
dynamically allocates this entity group identifier.  A dynamically
allocated entity group identifier is mapped to Internet host group
address 232.X.X.X where X.X.X are the low-order 24 bits of the
Discriminator subfield of the entity group identifier.

We use the following notation for Domain 1 entity identifiers <10> and
propose it use as a standard convention.

        <flags>-<discriminator>-<Internet address>

where <flags> are [X]{BE,LE,RG,UG}[A]

    X = reserved
    BE = big-endian entity
    LE = little-endian entity
    RG = restricted group
    UG = unrestricted group
    A  = alias

and <discriminator> is a decimal integer and <Internet address> is in
standard dotted decimal IP address notation.



<10>  This notation was developed by Steve Deering.
BE-25593- is big-endian entity #25593 created on host

RG-1- is the well-known restricted VMTP managers group.

UG-565338- is unrestricted entity group #565338 created on host

LEA-7823- is a little-endian alias entity #7823 created on host

This notation makes it easy to communicate and understand entity
identifiers for Domain 1.

The well-known entity identifiers specified to date are:

                Managers for VMTP operations.

                Client entity identifier to use when a (big-endian) host
                has not determined or been allocated any client entity

                Client entity identifier to use when a (little-endian)
                host has not determined or been allocated any client
                entity identifiers.

Note that is the host group address assigned to VMTP and to
which all VMTP hosts belong.

Other well-known entity group identifiers will be specified in
subsequent extensions to VMTP and in higher-level protocols that use

IV.2. Domain 3 Go to the top of this page...top

Domain 3 is reserved for embedded systems that are restricted to a
single network and are independent of IP.  Entity identifiers are
allocated using the decentralized approach described below.  The mapping
of entity group identifiers is specific to the type of network being
used and not defined here.  In general, there should be a simple
algorithmic mapping from entity group identifier to multicast address,
similar to that described for Domain 1.  Similarly, the values for
default client identifier are specific to the type of network and not
defined here.

IV.3. Other Domains Go to the top of this page...top

Definition of additional VMTP domains is planned for the future.
Requests for allocation of VMTP Domains should be addressed to the
Internet protocol administrator.

IV.4. Decentralized Entity Identifier Allocation Go to the top of this page...top

The ProbeEntityBlock operation may be used to determine whether a block
of entity identifiers is in use.  ("In use" means valid or reserved by a
host for allocation.)  This mechanism is used to detect collisions in
allocation of blocks of entity identifiers as part of the implementation
of decentralized allocation of entity identifiers.  (Decentralized
allocation is used in local domain use of VMTP such as in embedded
systems- see Domain 3.)

Basically, a group of hosts can form a Domain or sub-Domain, a group of
hosts managing their own entity identifier space or subspace,
respectively.  As an example of a sub-Domain, a group of hosts in Domain
1 all identified with a particular host group address can manage the
sub-Domain corresponding to all entity identifiers that contain that
host group address.  The ProbeEntityBlock operation is used to allocate
the random bits of these identifiers as follows.

When a host requires a new block of entity identifiers, it selects a new
block (randomly or by some choice algorithm) and then multicasts a
ProbeEntityBlock request to the members of the (sub-)Domain some R
times.  If no response is received after R (re)transmissions, the host
concludes that it is free to use this block of identifiers.  Otherwise,
it picks another block and tries again.


   1. A block of 256 identifiers is specified by an entity
      identifier with the low-order 8 bits all zero.

   2. When a host allocates an initial block of entity identifiers
      (and therefore does not yet have a specified entity
      identifier to use) it uses VMTP_DEFAULT_BECLIENT (if
      big-endian, else VMTP_DEFAULT_LECLIENT if little-endian) as
      its client identifier in the ProbeEntityBlock Request and a
      transaction identifier of 0.  As soon as it has allocated a
      block of entity identifiers, it should use these identifiers
      for all subsequent communication.  The default client
      identifier values are defined for each Domain.

   3. The set of hosts using this decentralized allocation must not
      be subject to network partitioning.  That is, the R
      transmissions must be sufficient to ensure that every host
      sees the ProbeEntityBlock request and (reliably) sends a
      response.  (A host that detects a collision can retransmit
      the response multiple times until it sees a new
      ProbeEntityBlock operation from the same host/Client up to a
      maximum number of times.)  For instance, a set of machines
      connected by a single local network may able to use this type
      of allocation.

   4. To guarantee T-stability, a host must prevent reuse of a
      block of identifiers if any of the identifiers in the block
      are currently valid or have been valid less than T seconds
      previously.  To this end, a host must remember recently used
      identifiers and object to their reuse in response to a
      ProbeEntityBlock operation.

   5. Care is required in a VMTP implementation to ensure that
      Probe operations cannot be discarded due to lack of buffer
      space or queued or delayed so that a response is not
      generated quickly.  This is required not only to detect
      collisions but also to provide accurate roundtrip estimates
      as part of ProbeEntity operations.

V. Authentication Domains Go to the top of this page...top

A VMTP authentication domain defines the format and interpretation for
principal identifiers and encryption keys.  In particular, an
authentication domain must specify a means by which principal
identifiers are allocated and guaranteed unique and stable.  The
currently defined authentication domains are as follows (0 is reserved).

Ideally, all entities within one entity domain are also associated with
one authentication domain.  However, authentication domains are
orthogonal to entity domains.  Entities within one domain may have
different authentication domains.  (In this case, it is generally
necessary to have some correspondence between principals in the
different domains.)  Also, one entity identifier may be associated with
multiple authentication domains.  Finally, one authentication domain may
be used across multiple entity domains.

V.1. Authentication Domain 1 Go to the top of this page...top

A principal identifier is structured as follows.

 |     Internet Address      | Local User Identifier  |
             32 bits                    32 bits

The Internet Address may specify an individual host (such as a UNIX
machine) or may specify a host group address corresponding to a cluster
of machines operating under a single adminstration.  In both cases,
there is assumed to be an adminstration associated with the embedded
Internet address that guarantees the uniqueness and stability of the
User Identifier relative to the Internet address.  In particular, that
administration is the only one authorized to allocate principal
identifiers with that Internet address prefix, and it may allocate any
of these identifiers.

In authentication domain 1, the standard EncryptionQualifiers are:

0               Clear text - no encryption.

1               use 64-bit CBC DES for encryption and decryption.

V.2. Other Authentication Domains Go to the top of this page...top

Other authentication domains will be defined in the future as needed.

VI. IP Implementation Go to the top of this page...top

VMTP is designed to be implemented on the DoD IP Internet Datagram
Protocol (although it may also be implemented as a local network
protocol directly in "raw" network packets.)

VMTP is assigned the protocol number 81.

With a 20 octet IP header and one segment block, a VMTP packet is 600
octets.  By convention, any host implementing VMTP implicitly agrees to
accept VMTP/IP packets of at least 600 octets.

VMTP multicast facilities are designed to work with, and have been
implemented using, the multicast extensions to the Internet [8]
described in RFC 966 and 988.  The wide-scale use of full VMTP/IP
depends on the availability of IP multicast in this form.

VII. Implementation Notes Go to the top of this page...top

The performance and reliability of a protocol in operation is highly
dependent on the quality of its implementation, in addition to the
"intrinsic" quality of the protocol design.  One of the design goals of
the VMTP effort was to produce an efficiently implementable protocol.
The following notes and suggestions are based on experience with
implementing VMTP in the V distributed system and the UNIX 4.3 BSD
kernel.  The following is described for a client and server handling
only one domain.  A multi-domain client or server would replicate these
structures for each domain, although buffer space may be shared.

VII.1. Mapping Data Structures Go to the top of this page...top

The ClientMap procedure is implemented using a hash table that maps to
the Client State Record whether this entity is local or remote, as shown
in Figure VII-1.

 ClientMap   |   | x |                          |
                   |   +--------------+    +--------------+
                   +-->| LocalClient  |--->| LocalClient  |
                       +--------------+    +--------------+
                       | RemoteClient |    | RemoteClient |-> ...
                       +--------------+    +--------------+
                       |              |    |              |
                       |              |    |              |
                       +--------------+    +--------------+


Figure VII-1:   Mapping Client Identifier To CSR Go to the top of this page...top

Local clients are linked through the LocalClientLink, similarly for the
RemoteClientLink.  Once a CSR with the specified Entity Id is found,
some field or flag indicates whether it is identifying a local or remote
Entity.  Hash collisions are handled with the overflow pointers
LocalClientLink and RemoteClientLink (not shown) in the CSR for the
LocalClient and RemoteClient fields, respectively.  Note that a CSR
representing an RPC request has both a local and remote entity
identifier mapping to the same CSR.

The Server specified in a Request is mapped to a server descriptor using
the ServerMap (with collisions handled by the overflow pointer.).  The
server descriptor is the root of a queue of CSR's for handling requests
plus flags that modify the handling of the Request.  Flags include:
  ServerMap      |       | x |                         |
                           |   +--------------+
                           |   | OverflowLink |
                           |   +--------------+
                           +-->|   Server     |
                               | Flags | Lock |
                               | Head Pointer |
                               | Tail Pointer |


Figure VII-2:   Mapping Server Identifiers Go to the top of this page...top

THREAD_QUEUE    Request is to be invoked directly as a remote procedure
                invocation, rather than by a server process in the
                message model.

                Sent a Probe request to determine principal associated
                with the Client, if not known.

                Request must be encrypted or else reject.

REQUESTS_QUEUED Queue contains waiting requests, rather than free CSR's.
                Queue this request as well.

SERVER_WAITING  The server is waiting and available to handle incoming
                Request immediately, as required by CMD.

Alternatively, the Server identifiers can be mapped to a CSR using the
MapToClient mechanism with a pointer in the CSR refering to the server
descriptor, if any.  This scheme is attractive if there are client CSR's
associated with a service to allow it to communicate as a client using
VMTP with other services.

Finally, a similar structure is used to expand entity group identifiers
to the local membership, as shown in Figure VII-3.  A group identifier
is hashed to an index in the GroupMap.  The list of group descriptors
rooted at that index in the GroupMap contains a group descriptor for
each local member of the group.  The flags are the group permissions
defined in Appendix III.
  GroupMap       |       | x |                                  |
                           |   +--------------+
                           |   | OverflowLink |
                           |   +--------------+
                           +-->|EntityGroupId |
                               | Flags        |
                               | Member Entity|


Figure VII-3:   Mapping Group Identifiers Go to the top of this page...top

Note that the same pool of descriptors could be used for the server and
group descriptors given that they are similar in size.

VII.2. Client Data Structures Go to the top of this page...top

Each client entity is represented as a client state record.  The CSR
contains a VMTP header as well as other bookkeeping fields, including
timeout count, retransmission count, as described in Section 4.1.  In
addition, there is a timeout queue, transmission queue and reception
queue.  Finally, there is a ServerHost cache that maps from server
entity-id records to host address, estimated round trip time,
interpacket gap, MTU size and (optimally) estimated processing time for
this server entity.

VII.3. Server Data Structures Go to the top of this page...top

The server maintains a heap of client state records (CSR), one for each
(Client, Transaction).  (If streams are not supported, there is, at
worst, a CSR per Client with which the server has communicated with
recently.)  The CSR contains a VMTP header as well as various
bookkeeping fields including timeout count, retransmission count.  The
server maintains a hash table mapping of Client to CSR as well as the
transmission, timeout and reception queues.  In a VMTP module
implementing both the client and server functions, the same timeout
queue and transmission queue are used for both.

VII.4. Packet Group Transmission Go to the top of this page...top

The procedure SendPacketGroup( csr ) transmits the packet group
specified by the record CSR.  It performs:

   1. Fragmentation of the segment data, if any, into packets.
      (Note, segment data flagged by SDA bit.)

   2. Modifies the VMTP header for each packet as required e.g.
      changing the delivery mask as appropriate.

   3. Computes the VMTP checksum.

   4. Encrypts the appropriate portion of the packet, if required.

   5. Prepends and appends network-level header and trailer using
      network address from ServerHost cache, or from the responding

   6. Transmits the packet with the interpacket gap specified in
      the cache.  This may involve round-robin scheduling between
      hosts as well as delaying transmissions slightly.

   7. Invokes the finish-up procedure specified by the CSR record,
      completing the processing.  Generally, this finish-up
      procedure adds the record to the timeout queue with the
      appropriate timeout queue.

The CSR includes a 32-bit transmission mask that indicates the portions
of the segment to transmit.  The SendPacketGroup procedure is assumed to
handle queuing at the network transmission queue, queuing in priority
order according to the priority field specified in the CSR record.
(This priority may be reflected in network transmission behavior for
networks that support priority.)

The SendPacketGroup procedure only looks at the following fields of a

   - Transmission mask

   - FuncCode

   - SDA

   - Client

   - Server
   - CoResidentEntity

   - Key

It modifies the following fields

   - Length

   - Delivery

   - Checksum

In the case of encrypted transmission, it encrypts the entire packet,
not including the Client field and the following 32-bits.

If the packet group is a Response, (i.e. lower-order bit of function
code is 1) the destination network address is determined from the
Client, otherwise the Server.  The HostAddr field is set either from the
ServerHost cache (if a Request) or from the original Request if a
Response, before SendPacketGroup is called.

The CSR includes a timeout and TTL fields indicating the maximum time to
complete the processing and the time-to-live for the packets to be

SendPacketGroup is viewed as the right functionality to implement for
transmission in an "intelligent" network interface.

Finally, it appears preferable to be able to assume that all portions of
the segment remain memory-resident (no page faults) during transmission.
In a demand-paged systems, some form of locking is required to keep the
segment data in memory.

VII.5. VMTP Management Module Go to the top of this page...top

The implementation should implement the management operations as a
separate module that is invoked from within the VMTP module.  When a
Request is received, either from the local user level or the network,
for the VMTP management module, the management module is invoked as a
remote or local procedure call to handle this request and return a
response (if not a datagram request).  By registering as a local server,
the management module should minimize the special-case code required for
its invocation.  The management module is basically a case statement
that selects the operation based on the RequestCode and then invokes the
specified management operation.  The procedure implementing the
management operation, especially operations like NotifyVmtpClient and
NotifyVmtpServer, are logically part of the VMTP module because they
require full access to the basic data structures of the VMTP

The management module should be implemented so that it can respond
quickly to all requests, particularly since the timing of management
interactions is used to estimate round trip time.  To date, all
implementations of the management module have been done at the kernel
level, along with VMTP proper.

VII.6. Timeout Handling Go to the top of this page...top

The timeout queue is a queue of CSR records, ordered by timeout count,
as specified in the CSR record.  On entry into the timeout queue, the
CSR record has the timeout field set to the time (preferable in
milliseconds or similar unit) to remain in the queue plus the finishup
field set to the procedure to execute on removal on timeout from the
queue.  The timeout field for a CSR in the queue is the time relative to
the record preceding it in the queue (if any) at which it is to be
removed.  Some system-specific mechanism decrements the time for the
record at the front of the queue, invoking the finishup procedure when
the count goes to zero.

Using this scheme, a special CSR is used to timeout and scan CSR's for
non-recently pinged CSR's.  That is, this CSR times out and invokes a
finishup procedure that scans for non-recently pinged CSR that are
"AwaitingResponse" and signals the request processing entity and deletes
the CSR.  It then returns to the timeout queue.

The timeout mechanism tends to be specific to an operating system.  The
scheme described may have to be adapted to the operating system in which
VMTP is to be implemented.

This mechanism handles client request timeout and client response
timeout.  It is not intended to handle interpacket gaps given that these
times are expected to be under 1 millisecond in general and possibly
only a few microseconds.

VII.7. Timeout Values Go to the top of this page...top

Roundtrip timeout values are estimated by matching Responses or
NotifyVmtpClient Requests to Request transmission, relying on the
retransmitCount to identify the particular transmission of the Request
that generated the response.  A similar technique can be used with
Responses and NotifyVmtpServer Requests.  The retransmitCount is
incremented each time the Response is sent, whether the retransmission
was caused by timeout or retransmission of the Request.

The ProbeEntity request is recommended as a basic way of getting
up-to-date information about a Client as well as predictable host
machine turnaround in processing a request.  (VMTP assumes and requires
an efficient, bounded response time implementation of the ProbeEntity

Using this mechanism for measuring RTT, it is recommended that the
various estimation and smoothing techniques developed for TCP RTT
estimation be adapted and used.

VII.8. Packet Reception Go to the top of this page...top

Logically a network packet containing a VMTP packet is 5 portions:

   - network header, possibly including lower-level headers

   - VMTP header

   - data segment

   - VMTP checksum

   - network trailer, etc.

It may be advantageous to receive a packet fragmented into these
portions, if supported by the network module.  In this case, ideally the
VMTP header may be received directly into a CSR, the data segment into a
page that can be mapped, rather than copied, to its final destination,
with VMTP checksum and network header in a separate area (used to
extract the network address corresponding to the sender).

Packet reception is described in detail by the pseudo-code in Section

With a response, normally the CSR has an associated segment area
immediately available so delivery of segment data is immediate.
Similarly, server entities should be "armed" with CSR's with segment
areas that provide for immediate delivery of requests.  It is reasonable
to discard segment data that cannot be immediately delivered in this
way, providing that clients and servers are able to preallocate CSR's
with segment areas for requests and responses.  In particular, a client
should be able to provide some number of additional CSR's for receiving
multiple responses to a multicast request.
The CSR data structure is intended to be the interface data structure
for an intelligent network interface.  For reception, the interface is
"armed" with CSR's that may point to segment areas in main memory, into
which it can deliver a packet group.  Ideally, the interface handles all
the processing of all packets, interacting with the host after receiving
a complete Request or Response packet group.  An implementation should
use an interface based on SendPacketGroup(CSR) and
ReceivePacketGroup(CSR) to facilitate the introduction of an intelligent
network interface.

ReceivePacketGroup(csr) provides the interface with a CSR descriptor and
zero or more bytes of main memory to receive segment data.  The CSR
describes whether it is to receive responses (and if so, for which
client) or requests (and if so for which server).

The procedure ReclaimCSR(CSR) reclaims the specified record from the
interface before it has been returned after receiving the specified
packet group.

A finishup procedure is set in the CSR to be invoked when the CSR is
returned to the host by the normal processing sequence in the interface.
Similarly, the timeout parameter is set to indicate the maximum time the
host is providing for the routine to perform the specified function.
The CSR and associated segment memory is returned to the host after the
timeout period with an indication of progress after the timeout period.
It is not returned earlier.

VII.9. Streaming Go to the top of this page...top

The implementation of streaming is optional in both VMTP clients and
servers.  Ideally, all performance-critical servers should implement
streaming.  In addition, clients that have high context switch overhead,
network access overhead or expect to be communicating over long delay
links should also implement streaming.

A client stream is implemented by allocating a CSR for each outstanding
message transaction.  A stream of transactions is handled similarly to
multiple outstanding transactions from separate clients except for the
interaction between consecutive numbered transactions in a stream.

For the server VMTP module, streamed message transactions to a server
are queued (if accepted) subordinate to the first unprocessed CSR
corresponding to this Client.  Thus, streamed transactions from a given
Client are always performed in the order specified by the transaction
If a server does not implement streaming, it must refuse streamed
message transactions using the NotifyVmtpClient operation.  Also, all
client VMTP's that support streaming must support the streamed interface
to a server that does not support streaming.  That is, it must perform
the message transactions one at a time.  Consequently, a program that
uses the streaming interface to a non-streaming server experiences
degraded performance, but not failure.

VII.10. Implementation Experience Go to the top of this page...top

The implementation experience to date includes a partial implementation
(minus the streaming and full security) in the V kernel plus a similar
preliminary implementation in the 4.3 BSD Unix kernel.  In the V kernel
implementation, the CSR's are part of the (lightweight) process

The V kernel implementation is able to perform a VMTP message
transaction with no data segment between two Sun-3/75's connected by 10
Mb Ethernet in 2.25 milliseconds.  It is also able to transfer data at
4.7 megabits per second using 16 kilobyte Requests (but null checksums.)
The UNIX kernel implementation running on Microvax II's achieves a basic
message transaction time of 9 milliseconds and data rate of 1.9 megabits
per second using 16 kilobyte Responses.  This implementation is using
the standard VMTP checksum.

We hope to report more extensive implementation experience in future
revisions of this document.

VIII. UNIX 4.3 BSD Kernel Interface For VMTP Go to the top of this page...top

UNIX 4.3 BSD includes a socket-based design for program interfaces to a
variety of protocol families and types of protocols (streams,
datagrams).  In this appendix, we sketch an extension to this design to
support a transaction-style protocol.  (Some familiarity with UNIX 4.2/3
IPC is assumed.)  Several extensions are required to the system
interface, rather than just adding a protocol, because no provision was
made for supporting transaction protocols in the original design.  These
extensions include a new "transaction" type of socket plus new system
calls invoke, getreply, probeentity, recreq, sendreply and forward.

A socket of type transaction bound to the VMTP protocol type
IPPROTO_VMTP is created by the call 

    s = socket(AF_INET, SOCK_TRANSACT, VMTP);

This socket is bound to an entity identifier by 

    bind(s, &entityid, sizeof(entityid));

The first address/port bound to a socket is considered its primary name
and is the one used on packet transmission.  A message transaction is
invoked between the socket named by s and the Server specified by mcb by

    invoke(s, mcb, segptr, seglen, timeout );

The mcb is a message control block whose format was described in Section
2.4.  The message control block specifies the request to send plus the
destination Server.  The response message control block returned by the
server is stored in mcb when invoke returns.  The invoking process is
blocked until a response is received or the message transaction times
out unless the request is a datagram request.  (Non-blocking versions
with signals on completion could also be provided, especially with a
streaming implementation.)

For multicast message transactions (sent to an entity group), the next
response to the current message transaction (if it arrives in less than
timeout milliseconds) is returned by 

    getreply( s, mcb, segptr, maxseglen, timeout );

The invoke operation sent to an entity group completes as soon as the
first response is received.  A request is retransmitted until the first
reply is received (assuming the request is not a datagram).  Thus, the
system does not retransmit while getreply is timing out even if no
replies are available.
The state of an entity associated with entityId is probed using 

    probeentity( entityId, state );

A UNIX process acting as a VMTP server accepts a Request by the

    recvreq(s, mcb, segptr, maxseglen );

The request message for the next queued transaction request is returned
in mcb, plus the segment data of maximum length maxseglen, starting at
segptr in the address space.  On return, the message control block
contains the values as set in invoke except: (1) the Client field
indicates the Client that sent the received Request message.  (2) the
Code field indicates the type of request.  (3) the MsgDelivery field
indicates the portions of the segment actually received within the
specified segment size, if MDM is 1 in the Code field.  A segment block
is marked as missing (i.e. the corresponding bit in the MsgDelivery
field is 0) unless it is received in its entirety or it is all of the
data in last segment contained in the segment.

To complete a transaction, the reply specified by mcb is sent to the
client specified by the MCB using 

    sendreply(s, mcb, segptr );

The Client field of the MCB indicates the client to respond to.

Finally, a message transaction specified by mcb is forwarded to
newserver as though it were sent there by its original invoker using 

    forward(s, mcb, segptr, timeout );

Index Go to the top of this page...top

          Acknowledgment   14
          APG   16, 31, 39
          Authentication domain   20

          Big-endian   9

          Checksum   14, 43
          Checksum, not set   44
          Client   7, 10, 38
          Client timer   16
          CMD   42, 110
          CMG   32, 40
          Co-resident entity   25
          Code   42
          CoResidentEntity   42, 43
          CRE   21, 42

          DGM   42
          Digital signature, VMTP management   95, 101
          Diskless workstations   2
          Domain   9, 38
          Domain 1   102
          Domain 3   104

          Entity   7
          Entity domain   9
          Entity group   8
          Entity identifier   37
          Entity identifier allocation   105
          Entity identifier, all-zero   38
          EPG   20, 39

          Features   6
          ForwardCount   24
          Forwarding   24
          FunctionCode   41

          Group   8
          Group message transaction   10
          Group timeouts   16
          GRP   37

          HandleNoCSR   62
          HandleRequestNoCSR   79
          HCO   14, 23, 39
          Host independence   8

          Idempotent   15
          Interpacket gap   18, 40
          IP   108

          Key   91

          LEE   32, 37
          Little-endian   9

          MCB   118
          MDG   22, 40
          MDM   30, 42
          Message control block   118
          Message size   6
          Message transaction   7, 10
          MPG   39
          MsgDelivery   43
          MSGTRANS_OVERFLOW   27
          Multicast   4, 21, 120
          Multicast, reliable   21

          Naming   6
          Negative acknowledgment   31
          NER   25, 31, 39
          NRT   26, 30, 39
          NSR   25, 27, 31, 39

          Object-oriented   2
          Overrun   18

          Packet group   7, 29, 39
          Packet group run   31
          PacketDelivery   29, 31, 41
          PGcount   26, 41
          PIC   42
          Principal   11
          Priority   41
          Process   11
          ProcessId   89
          Protocol number,IP   108

          RAE   37
          Rate control   18
          Real-time   2, 4
          Realtime   22
          Reliability   12
          Request message   10
          RequestAckRetries   30
          RequestRetries   15
          Response message   10
          ResponseAckRetries   31
          ResponseRetries   15
          Restricted group   8
          Retransmission   15
          RetransmitCount   17
          Roundtrip time   17
          RPC   2
          Run   31, 39
          Run, message transactions   25

          SDA   42
          Security   4, 19
          Segment block   41
          Segment data   43
          SegmentSize   42, 43
          Selective retransmission   18
          Server   7, 10, 41
          Server group   8
          Sockets, VMTP   118
          STI   26, 40
          Streaming   25, 55
          Strictly stable   8
          Subgroups   21

          T-stable   8
          TC1(Server)   16
          TC2(Server)   16
          TC3(Server)   16
          TC4   16
          TCP   2
          Timeouts   15
          Transaction   10, 41
          Transaction identification   10
          TS1(Client)   17
          TS2(Client)   17
          TS3(Client)   17
          TS4(Client)   17
          TS5(Client)   17
          Type flags   8

          UNIX interface   118
          Unrestricted group   8, 38
          NotifyVmtpClient   7, 26, 27, 30
          NotifyVmtpServer   7, 14, 30
          User Data   43

          Version   38
          VMTP Management digital signature   95, 101