RFC 1812 (rfc1812)
Requirements for IP Version 4 Routers
Network Working Group F. Baker, Editor
Request for Comments: 1812 Cisco Systems
Obsoletes: 1716, 1009 June 1995
Category: Standards Track
Requirements for IP Version 4 Routers
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
PREFACE
This document is an updated version of RFC 1716, the historical
Router Requirements document. That RFC preserved the significant
work that went into the working group, but failed to adequately
describe current technology for the IESG to consider it a current
standard.
The current editor had been asked to bring the document up to date,
so that it is useful as a procurement specification and a guide to
implementors. In this, he stands squarely on the shoulders of those
who have gone before him, and depends largely on expert contributors
for text. Any credit is theirs; the errors are his.
The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author:
Philip Almquist. It is also largely due to the efforts of its
previous editor, Frank Kastenholz. Without their efforts, this
document would not exist.
Table of Contents
1. INTRODUCTION ........................................ 6
1.1 Reading this Document .............................. 8
1.1.1 Organization ..................................... 8
1.1.2 Requirements ..................................... 9
1.1.3 Compliance ....................................... 10
1.2 Relationships to Other Standards ................... 11
1.3 General Considerations ............................. 12
1.3.1 Continuing Internet Evolution .................... 12
1.3.2 Robustness Principle ............................. 13
1.3.3 Error Logging .................................... 14
1.3.4 Configuration .................................... 14
1.4 Algorithms ......................................... 16
2. INTERNET ARCHITECTURE ............................... 16
2.1 Introduction ....................................... 16
2.2 Elements of the Architecture ....................... 17
2.2.1 Protocol Layering ................................ 17
2.2.2 Networks ......................................... 19
2.2.3 Routers .......................................... 20
2.2.4 Autonomous Systems ............................... 21
2.2.5 Addressing Architecture .......................... 21
2.2.5.1 Classical IP Addressing Architecture ........... 21
2.2.5.2 Classless Inter Domain Routing (CIDR) .......... 23
2.2.6 IP Multicasting .................................. 24
2.2.7 Unnumbered Lines and Networks Prefixes ........... 25
2.2.8 Notable Oddities ................................. 26
2.2.8.1 Embedded Routers ............................... 26
2.2.8.2 Transparent Routers ............................ 27
2.3 Router Characteristics ............................. 28
2.4 Architectural Assumptions .......................... 31
3. LINK LAYER .......................................... 32
3.1 INTRODUCTION ....................................... 32
3.2 LINK/INTERNET LAYER INTERFACE ...................... 33
3.3 SPECIFIC ISSUES .................................... 34
3.3.1 Trailer Encapsulation ............................ 34
3.3.2 Address Resolution Protocol - ARP ................ 34
3.3.3 Ethernet and 802.3 Coexistence ................... 35
3.3.4 Maximum Transmission Unit - MTU .................. 35
3.3.5 Point-to-Point Protocol - PPP .................... 35
3.3.5.1 Introduction ................................... 36
3.3.5.2 Link Control Protocol (LCP) Options ............ 36
3.3.5.3 IP Control Protocol (IPCP) Options ............. 38
3.3.6 Interface Testing ................................ 38
4. INTERNET LAYER - PROTOCOLS .......................... 39
4.1 INTRODUCTION ....................................... 39
4.2 INTERNET PROTOCOL - IP ............................. 39
4.2.1 INTRODUCTION ..................................... 39
4.2.2 PROTOCOL WALK-THROUGH ............................ 40
4.2.2.1 Options: RFC 791 Section 3.231'>Section 3.2 ................... 40
4.2.2.2 Addresses in Options: RFC 791 Section 3.130'>Section 3.1 ...... 42
4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.130'>Section 3.1 ..... 43
4.2.2.4 Type of Service: RFC 791 Section 3.130'>Section 3.1 ........... 44
4.2.2.5 Header Checksum: RFC 791 Section 3.130'>Section 3.1 ........... 44
4.2.2.6 Unrecognized Header Options: RFC 791,
Section 3.1 .................................... 44
4.2.2.7 Fragmentation: RFC 791 Section 3.231'>Section 3.2 ............. 45
4.2.2.8 Reassembly: RFC 791 Section 3.231'>Section 3.2 ................ 46
4.2.2.9 Time to Live: RFC 791 Section 3.231'>Section 3.2 .............. 46
4.2.2.10 Multi-subnet Broadcasts: RFC 922 .............. 47
4.2.2.11 Addressing: RFC 791 Section 3.231'>Section 3.2 ............... 47
4.2.3 SPECIFIC ISSUES .................................. 50
4.2.3.1 IP Broadcast Addresses ......................... 50
4.2.3.2 IP Multicasting ................................ 50
4.2.3.3 Path MTU Discovery ............................. 51
4.2.3.4 Subnetting ..................................... 51
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ........... 52
4.3.1 INTRODUCTION ..................................... 52
4.3.2 GENERAL ISSUES ................................... 53
4.3.2.1 Unknown Message Types .......................... 53
4.3.2.2 ICMP Message TTL ............................... 53
4.3.2.3 Original Message Header ........................ 53
4.3.2.4 ICMP Message Source Address .................... 53
4.3.2.5 TOS and Precedence ............................. 54
4.3.2.6 Source Route ................................... 54
4.3.2.7 When Not to Send ICMP Errors ................... 55
4.3.2.8 Rate Limiting .................................. 56
4.3.3 SPECIFIC ISSUES .................................. 56
4.3.3.1 Destination Unreachable ........................ 56
4.3.3.2 Redirect ....................................... 57
4.3.3.3 Source Quench .................................. 57
4.3.3.4 Time Exceeded .................................. 58
4.3.3.5 Parameter Problem .............................. 58
4.3.3.6 Echo Request/Reply ............................. 58
4.3.3.7 Information Request/Reply ...................... 59
4.3.3.8 Timestamp and Timestamp Reply .................. 59
4.3.3.9 Address Mask Request/Reply ..................... 61
4.3.3.10 Router Advertisement and Solicitations ........ 62
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .......... 62
5. INTERNET LAYER - FORWARDING ......................... 63
5.1 INTRODUCTION ....................................... 63
5.2 FORWARDING WALK-THROUGH ............................ 63
5.2.1 Forwarding Algorithm ............................. 63
5.2.1.1 General ........................................ 64
5.2.1.2 Unicast ........................................ 64
5.2.1.3 Multicast ...................................... 65
5.2.2 IP Header Validation ............................. 67
5.2.3 Local Delivery Decision .......................... 69
5.2.4 Determining the Next Hop Address ................. 71
5.2.4.1 IP Destination Address ......................... 72
5.2.4.2 Local/Remote Decision .......................... 72
5.2.4.3 Next Hop Address ............................... 74
5.2.4.4 Administrative Preference ...................... 77
5.2.4.5 Load Splitting ................................. 79
5.2.5 Unused IP Header Bits: RFC-791 Section 3.130'>Section 3.1 ....... 79
5.2.6 Fragmentation and Reassembly: RFC-791,
Section 3.2 ...................................... 80
5.2.7 Internet Control Message Protocol - ICMP ......... 80
5.2.7.1 Destination Unreachable ........................ 80
5.2.7.2 Redirect ....................................... 82
5.2.7.3 Time Exceeded .................................. 84
5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........ 84
5.3 SPECIFIC ISSUES .................................... 85
5.3.1 Time to Live (TTL) ............................... 85
5.3.2 Type of Service (TOS) ............................ 86
5.3.3 IP Precedence .................................... 87
5.3.3.1 Precedence-Ordered Queue Service ............... 88
5.3.3.2 Lower Layer Precedence Mappings ................ 89
5.3.3.3 Precedence Handling For All Routers ............ 90
5.3.4 Forwarding of Link Layer Broadcasts .............. 92
5.3.5 Forwarding of Internet Layer Broadcasts .......... 92
5.3.5.1 Limited Broadcasts ............................. 93
5.3.5.2 Directed Broadcasts ............................ 93
5.3.5.3 All-subnets-directed Broadcasts ................ 94
5.3.5.4 Subnet-directed Broadcasts .................... 94
5.3.6 Congestion Control ............................... 94
5.3.7 Martian Address Filtering ........................ 96
5.3.8 Source Address Validation ........................ 97
5.3.9 Packet Filtering and Access Lists ................ 97
5.3.10 Multicast Routing ............................... 98
5.3.11 Controls on Forwarding .......................... 98
5.3.12 State Changes ................................... 99
5.3.12.1 When a Router Ceases Forwarding ............... 99
5.3.12.2 When a Router Starts Forwarding ............... 100
5.3.12.3 When an Interface Fails or is Disabled ........ 100
5.3.12.4 When an Interface is Enabled .................. 100
5.3.13 IP Options ...................................... 101
5.3.13.1 Unrecognized Options .......................... 101
5.3.13.2 Security Option ............................... 101
5.3.13.3 Stream Identifier Option ...................... 101
5.3.13.4 Source Route Options .......................... 101
5.3.13.5 Record Route Option ........................... 102
5.3.13.6 Timestamp Option .............................. 102
6. TRANSPORT LAYER ..................................... 103
6.1 USER DATAGRAM PROTOCOL - UDP ....................... 103
6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................ 104
7. APPLICATION LAYER - ROUTING PROTOCOLS ............... 106
7.1 INTRODUCTION ....................................... 106
7.1.1 Routing Security Considerations .................. 106
7.1.2 Precedence ....................................... 107
7.1.3 Message Validation ............................... 107
7.2 INTERIOR GATEWAY PROTOCOLS ......................... 107
7.2.1 INTRODUCTION ..................................... 107
7.2.2 OPEN SHORTEST PATH FIRST - OSPF .................. 108
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -
DUAL IS-IS ....................................... 108
7.3 EXTERIOR GATEWAY PROTOCOLS ........................ 109
7.3.1 INTRODUCTION .................................... 109
7.3.2 BORDER GATEWAY PROTOCOL - BGP .................... 109
7.3.2.1 Introduction ................................... 109
7.3.2.2 Protocol Walk-through .......................... 110
7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
.................................................. 110
7.4 STATIC ROUTING ..................................... 111
7.5 FILTERING OF ROUTING INFORMATION ................... 112
7.5.1 Route Validation ................................. 113
7.5.2 Basic Route Filtering ............................ 113
7.5.3 Advanced Route Filtering ......................... 114
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........ 114
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
..................................................... 115
8.1 The Simple Network Management Protocol - SNMP ...... 115
8.1.1 SNMP Protocol Elements ........................... 115
8.2 Community Table .................................... 116
8.3 Standard MIBS ...................................... 118
8.4 Vendor Specific MIBS ............................... 119
8.5 Saving Changes ..................................... 120
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ......... 120
9.1 BOOTP .............................................. 120
9.1.1 Introduction ..................................... 120
9.1.2 BOOTP Relay Agents ............................... 121
10. OPERATIONS AND MAINTENANCE ......................... 122
10.1 Introduction ...................................... 122
10.2 Router Initialization ............................. 123
10.2.1 Minimum Router Configuration .................... 123
10.2.2 Address and Prefix Initialization ............... 124
10.2.3 Network Booting using BOOTP and TFTP ............ 125
10.3 Operation and Maintenance ......................... 126
10.3.1 Introduction .................................... 126
10.3.2 Out Of Band Access .............................. 127
10.3.2 Router O&M Functions ............................ 127
10.3.2.1 Maintenance - Hardware Diagnosis .............. 127
10.3.2.2 Control - Dumping and Rebooting ............... 127
10.3.2.3 Control - Configuring the Router .............. 128
10.3.2.4 Net Booting of System Software ................ 128
10.3.2.5 Detecting and responding to misconfiguration
............................................... 129
10.3.2.6 Minimizing Disruption ......................... 130
10.3.2.7 Control - Troubleshooting Problems ............ 130
10.4 Security Considerations ........................... 131
10.4.1 Auditing and Audit Trails ....................... 131
10.4.2 Configuration Control ........................... 132
11. REFERENCES ......................................... 133
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ...... 145
APPENDIX B. GLOSSARY ................................... 146
APPENDIX C. FUTURE DIRECTIONS .......................... 152
APPENDIX D. Multicast Routing Protocols ................ 154
D.1 Introduction ....................................... 154
D.2 Distance Vector Multicast Routing Protocol -
DVMRP .............................................. 154
D.3 Multicast Extensions to OSPF - MOSPF ............... 154
D.4 Protocol Independent Multicast - PIM ............... 155
APPENDIX E Additional Next-Hop Selection Algorithms
................................................... 155
E.1. Some Historical Perspective ....................... 155
E.2. Additional Pruning Rules .......................... 157
E.3 Some Route Lookup Algorithms ....................... 159
E.3.1 The Revised Classic Algorithm .................... 159
E.3.2 The Variant Router Requirements Algorithm ........ 160
E.3.3 The OSPF Algorithm ............................... 160
E.3.4 The Integrated IS-IS Algorithm ................... 162
Security Considerations ................................ 163
APPENDIX F: HISTORICAL ROUTING PROTOCOLS ............... 164
F.1 EXTERIOR GATEWAY PROTOCOL - EGP .................... 164
F.1.1 Introduction ..................................... 164
F.1.2 Protocol Walk-through ............................ 165
F.2 ROUTING INFORMATION PROTOCOL - RIP ................. 167
F.2.1 Introduction ..................................... 167
F.2.2 Protocol Walk-Through ............................ 167
F.2.3 Specific Issues .................................. 172
F.3 GATEWAY TO GATEWAY PROTOCOL - GGP .................. 173
Acknowledgments ........................................ 173
Editor's Address ....................................... 175
1. INTRODUCTION 
top
This memo replaces for RFC 1716, "Requirements for Internet Gateways"
([INTRO:1]).
This memo defines and discusses requirements for devices that perform
the network layer forwarding function of the Internet protocol suite.
The Internet community usually refers to such devices as IP routers or
simply routers; The OSI community refers to such devices as
intermediate systems. Many older Internet documents refer to these
devices as gateways, a name which more recently has largely passed out
of favor to avoid confusion with application gateways.
An IP router can be distinguished from other sorts of packet switching
devices in that a router examines the IP protocol header as part of
the switching process. It generally removes the Link Layer header a
message was received with, modifies the IP header, and replaces the
Link Layer header for retransmission.
The authors of this memo recognize, as should its readers, that many
routers support more than one protocol. Support for multiple protocol
suites will be required in increasingly large parts of the Internet in
the future. This memo, however, does not attempt to specify Internet
requirements for protocol suites other than TCP/IP.
This document enumerates standard protocols that a router connected to
the Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols. It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.
For each protocol, this memo also contains an explicit set of
requirements, recommendations, and options. The reader must
understand that the list of requirements in this memo is incomplete by
itself. The complete set of requirements for an Internet protocol
router is primarily defined in the standard protocol specification
documents, with the corrections, amendments, and supplements contained
in this memo.
This memo should be read in conjunction with the Requirements for
Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
routers must both be capable of originating IP datagrams and receiving
IP datagrams destined for them. The major distinction between
Internet hosts and routers is that routers implement forwarding
algorithms, while Internet hosts do not require forwarding
capabilities. Any Internet host acting as a router must adhere to the
requirements contained in this memo.
The goal of open system interconnection dictates that routers must
function correctly as Internet hosts when necessary. To achieve this,
this memo provides guidelines for such instances. For simplification
and ease of document updates, this memo tries to avoid overlapping
discussions of host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by
reference. In some cases the requirements stated in [INTRO:2] and
[INTRO:3] are superseded by this document.
A good-faith implementation of the protocols produced after careful
reading of the RFCs should differ from the requirements of this memo
in only minor ways. Producing such an implementation often requires
some interaction with the Internet technical community, and must
follow good communications software engineering practices. In many
cases, the requirements in this document are already stated or implied
in the standard protocol documents, so that their inclusion here is,
in a sense, redundant. They were included because some past
implementation has made the wrong choice, causing problems of
interoperability, performance, and/or robustness.
This memo includes discussion and explanation of many of the
requirements and recommendations. A simple list of requirements would
be dangerous, because:
o Some required features are more important than others, and some
features are optional.
o Some features are critical in some applications of routers but
irrelevant in others.
o There may be valid reasons why particular vendor products that are
designed for restricted contexts might choose to use different
specifications.
However, the specifications of this memo must be followed to meet the
general goal of arbitrary router interoperation across the diversity
and complexity of the Internet. Although most current implementations
fail to meet these requirements in various ways, some minor and some
major, this specification is the ideal towards which we need to move.
These requirements are based on the current level of Internet
architecture. This memo will be updated as required to provide
additional clarifications or to include additional information in
those areas in which specifications are still evolving.
1.1 Reading This Document top
1.1.1 Organization top
This memo emulates the layered organization used by [INTRO:2] and
[INTRO:3]. Thus, Chapter 2 describes the layers found in the Internet
architecture. Chapter 3 covers the Link Layer. Chapters 4 and 5 are
concerned with the Internet Layer protocols and forwarding algorithms.
Chapter 6 covers the Transport Layer. Upper layer protocols are
divided among Chapters 7, 8, and 9. Chapter 7 discusses the protocols
which routers use to exchange routing information with each other.
Chapter 8 discusses network management. Chapter 9 discusses other
upper layer protocols. The final chapter covers operations and
maintenance features. This organization was chosen for simplicity,
clarity, and consistency with the Host Requirements RFCs. Appendices
to this memo include a bibliography, a glossary, and some conjectures
about future directions of router standards.
In describing the requirements, we assume that an implementation
strictly mirrors the layering of the protocols. However, strict
layering is an imperfect model, both for the protocol suite and for
recommended implementation approaches. Protocols in different layers
interact in complex and sometimes subtle ways, and particular
functions often involve multiple layers. There are many design
choices in an implementation, many of which involve creative breaking
of strict layering. Every implementor is urged to read [INTRO:4] and
[INTRO:5].
Each major section of this memo is organized into the following
subsections:
(1) Introduction
(2) Protocol Walk-Through - considers the protocol specification
documents section-by-section, correcting errors, stating
requirements that may be ambiguous or ill-defined, and providing
further clarification or explanation.
(3) Specific Issues - discusses protocol design and implementation
issues that were not included in the walk-through.
Under many of the individual topics in this memo, there is
parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
material is intended to give a justification, clarification or
explanation to the preceding requirements text. The implementation
material contains suggested approaches that an implementor may want to
consider. The DISCUSSION and IMPLEMENTATION sections are not part of
the standard.
1.1.2 Requirements top
In this memo, the words that are used to define the significance of
each particular requirement are capitalized. These words are:
o MUST
This word means that the item is an absolute requirement of the
specification. Violation of such a requirement is a fundamental
error; there is no case where it is justified.
o MUST IMPLEMENT
This phrase means that this specification requires that the item be
implemented, but does not require that it be enabled by default.
o MUST NOT
This phrase means that the item is an absolute prohibition of the
specification.
o SHOULD
This word means that there may exist valid reasons in particular
circumstances to ignore this item, but the full implications should
be understood and the case carefully weighed before choosing a
different course.
o SHOULD IMPLEMENT
This phrase is similar in meaning to SHOULD, but is used when we
recommend that a particular feature be provided but does not
necessarily recommend that it be enabled by default.
o SHOULD NOT
This phrase means that there may exist valid reasons in particular
circumstances when the described behavior is acceptable or even
useful. Even so, the full implications should be understood and
the case carefully weighed before implementing any behavior
described with this label.
o MAY
This word means that this item is truly optional. One vendor may
choose to include the item because a particular marketplace
requires it or because it enhances the product, for example;
another vendor may omit the same item.
1.1.3 Compliance top
Some requirements are applicable to all routers. Other requirements
are applicable only to those which implement particular features or
protocols. In the following paragraphs, relevant refers to the union
of the requirements applicable to all routers and the set of
requirements applicable to a particular router because of the set of
features and protocols it has implemented.
Note that not all Relevant requirements are stated directly in this
memo. Various parts of this memo incorporate by reference sections of
the Host Requirements specification, [INTRO:2] and [INTRO:3]. For
purposes of determining compliance with this memo, it does not matter
whether a Relevant requirement is stated directly in this memo or
merely incorporated by reference from one of those documents.
An implementation is said to be conditionally compliant if it
satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
requirements. An implementation is said to be unconditionally
compliant if it is conditionally compliant and also satisfies all the
Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An
implementation is not compliant if it is not conditionally compliant
(i.e., it fails to satisfy one or more of the Relevant MUST, MUST
IMPLEMENT, or MUST NOT requirements).
This specification occasionally indicates that an implementation
SHOULD implement a management variable, and that it SHOULD have a
certain default value. An unconditionally compliant implementation
implements the default behavior, and if there are other implemented
behaviors implements the variable. A conditionally compliant
implementation clearly documents what the default setting of the
variable is or, in the absence of the implementation of a variable,
may be construed to be. An implementation that both fails to
implement the variable and chooses a different behavior is not
compliant.
For any of the SHOULD and SHOULD NOT requirements, a router may
provide a configuration option that will cause the router to act other
than as specified by the requirement. Having such a configuration
option does not void a router's claim to unconditional compliance if
the option has a default setting, and that setting causes the router
to operate in the required manner.
Likewise, routers may provide, except where explicitly prohibited by
this memo, options which cause them to violate MUST or MUST NOT
requirements. A router that provides such options is compliant
(either fully or conditionally) if and only if each such option has a
default setting that causes the router to conform to the requirements
of this memo. Please note that the authors of this memo, although
aware of market realities, strongly recommend against provision of
such options. Requirements are labeled MUST or MUST NOT because
experts in the field have judged them to be particularly important to
interoperability or proper functioning in the Internet. Vendors
should weigh carefully the customer support costs of providing options
that violate those rules.
Of course, this memo is not a complete specification of an IP router,
but rather is closer to what in the OSI world is called a profile.
For example, this memo requires that a number of protocols be
implemented. Although most of the contents of their protocol
specifications are not repeated in this memo, implementors are
nonetheless required to implement the protocols according to those
specifications.
1.2 Relationships To Other Standards top
There are several reference documents of interest in checking the
status of protocol specifications and standardization:
o INTERNET OFFICIAL PROTOCOL STANDARDS
This document describes the Internet standards process and lists
the standards status of the protocols. As of this writing, the
current version of this document is STD 1, RFC 1780, [ARCH:7].
This document is periodically re-issued. You should always
consult an RFC repository and use the latest version of this
document.
o Assigned Numbers
This document lists the assigned values of the parameters used in
the various protocols. For example, it lists IP protocol codes,
TCP port numbers, Telnet Option Codes, ARP hardware types, and
Terminal Type names. As of this writing, the current version of
this document is STD 2, RFC 1700, [INTRO:7]. This document is
periodically re-issued. You should always consult an RFC
repository and use the latest version of this document.
o Host Requirements
This pair of documents reviews the specifications that apply to
hosts and supplies guidance and clarification for any
ambiguities. Note that these requirements also apply to routers,
except where otherwise specified in this memo. As of this
writing, the current versions of these documents are RFC 1122 and
RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].
o Router Requirements (formerly Gateway Requirements)
This memo.
Note that these documents are revised and updated at different times;
in case of differences between these documents, the most recent must
prevail.
These and other Internet protocol documents may be obtained from the:
The InterNIC
DS.INTERNIC.NET
InterNIC Directory and Database Service
info@internic.net
+1-908-668-6587
URL: http://ds.internic.net/
1.3 General Considerations top
There are several important lessons that vendors of Internet software
have learned and which a new vendor should consider seriously.
1.3.1 Continuing Internet Evolution top
The enormous growth of the Internet has revealed problems of
management and scaling in a large datagram based packet communication
system. These problems are being addressed, and as a result there
will be continuing evolution of the specifications described in this
memo. New routing protocols, algorithms, and architectures are
constantly being developed. New internet layer protocols, and
modifications to existing protocols, are also constantly being
devised. Routers play a crucial role in the Internet, and the number
of routers deployed in the Internet is much smaller than the number
of hosts. Vendors should therefore expect that router standards will
continue to evolve much more quickly than host standards. These
changes will be carefully planned and controlled since there is
extensive participation in this planning by the vendors and by the
organizations responsible for operation of the networks.
Development, evolution, and revision are characteristic of computer
network protocols today, and this situation will persist for some
years. A vendor who develops computer communications software for
the Internet protocol suite (or any other protocol suite!) and then
fails to maintain and update that software for changing
specifications is going to leave a trail of unhappy customers. The
Internet is a large communication network, and the users are in
constant contact through it. Experience has shown that knowledge of
deficiencies in vendor software propagates quickly through the
Internet technical community.
1.3.2 Robustness Principle top
At every layer of the protocols, there is a general rule (from
[TRANS:2] by Jon Postel) whose application can lead to enormous
benefits in robustness and interoperability:
Be conservative in what you do,
be liberal in what you accept from others.
Software should be written to deal with every conceivable error, no
matter how unlikely. Eventually a packet will come in with that
particular combination of errors and attributes, and unless the
software is prepared, chaos can ensue. It is best to assume that the
network is filled with malevolent entities that will send packets
designed to have the worst possible effect. This assumption will
lead to suitably protective design. The most serious problems in the
Internet have been caused by unforeseen mechanisms triggered by low
probability events; mere human malice would never have taken so
devious a course!
Adaptability to change must be designed into all levels of router
software. As a simple example, consider a protocol specification
that contains an enumeration of values for a particular header field
- e.g., a type field, a port number, or an error code; this
enumeration must be assumed to be incomplete. If the protocol
specification defines four possible error codes, the software must
not break when a fifth code is defined. An undefined code might be
logged, but it must not cause a failure.
The second part of the principal is almost as important: software on
hosts or other routers may contain deficiencies that make it unwise
to exploit legal but obscure protocol features. It is unwise to
stray far from the obvious and simple, lest untoward effects result
elsewhere. A corollary of this is watch out for misbehaving hosts;
router software should be prepared to survive in the presence of
misbehaving hosts. An important function of routers in the Internet
is to limit the amount of disruption such hosts can inflict on the
shared communication facility.
1.3.3 Error Logging top
The Internet includes a great variety of systems, each implementing
many protocols and protocol layers, and some of these contain bugs
and misguided features in their Internet protocol software. As a
result of complexity, diversity, and distribution of function, the
diagnosis of problems is often very difficult.
Problem diagnosis will be aided if routers include a carefully
designed facility for logging erroneous or strange events. It is
important to include as much diagnostic information as possible when
an error is logged. In particular, it is often useful to record the
header(s) of a packet that caused an error. However, care must be
taken to ensure that error logging does not consume prohibitive
amounts of resources or otherwise interfere with the operation of the
router.
There is a tendency for abnormal but harmless protocol events to
overflow error logging files; this can be avoided by using a circular
log, or by enabling logging only while diagnosing a known failure.
It may be useful to filter and count duplicate successive messages.
One strategy that seems to work well is to both:
o Always count abnormalities and make such counts accessible through
the management protocol (see Chapter 8); and
o Allow the logging of a great variety of events to be selectively
enabled. For example, it might useful to be able to log
everything or to log everything for host X.
This topic is further discussed in [MGT:5].
1.3.4 Configuration top
In an ideal world, routers would be easy to configure, and perhaps
even entirely self-configuring. However, practical experience in the
real world suggests that this is an impossible goal, and that many
attempts by vendors to make configuration easy actually cause
customers more grief than they prevent. As an extreme example, a
router designed to come up and start routing packets without
requiring any configuration information at all would almost certainly
choose some incorrect parameter, possibly causing serious problems on
any networks unfortunate enough to be connected to it.
Often this memo requires that a parameter be a configurable option.
There are several reasons for this. In a few cases there currently
is some uncertainty or disagreement about the best value and it may
be necessary to update the recommended value in the future. In other
cases, the value really depends on external factors - e.g., the
distribution of its communication load, or the speeds and topology of
nearby networks - and self-tuning algorithms are unavailable and may
be insufficient. In some cases, configurability is needed because of
administrative requirements.
Finally, some configuration options are required to communicate with
obsolete or incorrect implementations of the protocols, distributed
without sources, that persist in many parts of the Internet. To make
correct systems coexist with these faulty systems, administrators
must occasionally misconfigure the correct systems. This problem
will correct itself gradually as the faulty systems are retired, but
cannot be ignored by vendors.
When we say that a parameter must be configurable, we do not intend
to require that its value be explicitly read from a configuration
file at every boot time. For many parameters, there is one value
that is appropriate for all but the most unusual situations. In such
cases, it is quite reasonable that the parameter default to that
value if not explicitly set.
This memo requires a particular value for such defaults in some
cases. The choice of default is a sensitive issue when the
configuration item controls accommodation of existing, faulty,
systems. If the Internet is to converge successfully to complete
interoperability, the default values built into implementations must
implement the official protocol, not misconfigurations to accommodate
faulty implementations. Although marketing considerations have led
some vendors to choose misconfiguration defaults, we urge vendors to
choose defaults that will conform to the standard.
Finally, we note that a vendor needs to provide adequate
documentation on all configuration parameters, their limits and
effects.
1.4 Algorithms top
In several places in this memo, specific algorithms that a router
ought to follow are specified. These algorithms are not, per se,
required of the router. A router need not implement each algorithm
as it is written in this document. Rather, an implementation must
present a behavior to the external world that is the same as a
strict, literal, implementation of the specified algorithm.
Algorithms are described in a manner that differs from the way a good
implementor would implement them. For expository purposes, a style
that emphasizes conciseness, clarity, and independence from
implementation details has been chosen. A good implementor will
choose algorithms and implementation methods that produce the same
results as these algorithms, but may be more efficient or less
general.
We note that the art of efficient router implementation is outside
the scope of this memo.
2. INTERNET ARCHITECTURE top
This chapter does not contain any requirements. However, it does
contain useful background information on the general architecture of
the Internet and of routers.
General background and discussion on the Internet architecture and
supporting protocol suite can be found in the DDN Protocol Handbook
[ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
[ARCH:4]. The Internet architecture and protocols are also covered
in an ever-growing number of textbooks, such as [ARCH:5] and
[ARCH:6].
2.1 Introduction top
The Internet system consists of a number of interconnected packet
networks supporting communication among host computers using the
Internet protocols. These protocols include the Internet Protocol
(IP), the Internet Control Message Protocol (ICMP), the Internet
Group Management Protocol (IGMP), and a variety transport and
application protocols that depend upon them. As was described in
Section [1.2], the Internet Engineering Steering Group periodically
releases an Official Protocols memo listing all the Internet
protocols.
All Internet protocols use IP as the basic data transport mechanism.
IP is a datagram, or connectionless, internetwork service and
includes provision for addressing, type-of-service specification,
fragmentation and reassembly, and security. ICMP and IGMP are
considered integral parts of IP, although they are architecturally
layered upon IP. ICMP provides error reporting, flow control,
first-hop router redirection, and other maintenance and control
functions. IGMP provides the mechanisms by which hosts and routers
can join and leave IP multicast groups.
Reliable data delivery is provided in the Internet protocol suite by
Transport Layer protocols such as the Transmission Control Protocol
(TCP), which provides end-end retransmission, resequencing and
connection control. Transport Layer connectionless service is
provided by the User Datagram Protocol (UDP).
2.2 Elements Of The Architecture top
2.2.1 Protocol Layering top
To communicate using the Internet system, a host must implement the
layered set of protocols comprising the Internet protocol suite. A
host typically must implement at least one protocol from each layer.
The protocol layers used in the Internet architecture are as follows
[ARCH:7]:
o Application Layer
The Application Layer is the top layer of the Internet protocol
suite. The Internet suite does not further subdivide the
Application Layer, although some application layer protocols do
contain some internal sub-layering. The application layer of the
Internet suite essentially combines the functions of the top two
layers - Presentation and Application - of the OSI Reference Model
[ARCH:8]. The Application Layer in the Internet protocol suite
also includes some of the function relegated to the Session Layer
in the OSI Reference Model.
We distinguish two categories of application layer protocols: user
protocols that provide service directly to users, and support
protocols that provide common system functions. The most common
Internet user protocols are:
- Telnet (remote login)
- FTP (file transfer)
- SMTP (electronic mail delivery)
There are a number of other standardized user protocols and many
private user protocols.
Support protocols, used for host name mapping, booting, and
management include SNMP, BOOTP, TFTP, the Domain Name System (DNS)
protocol, and a variety of routing protocols.
Application Layer protocols relevant to routers are discussed in
chapters 7, 8, and 9 of this memo.
o Transport Layer
The Transport Layer provides end-to-end communication services.
This layer is roughly equivalent to the Transport Layer in the OSI
Reference Model, except that it also incorporates some of OSI's
Session Layer establishment and destruction functions.
There are two primary Transport Layer protocols at present:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
TCP is a reliable connection-oriented transport service that
provides end-to-end reliability, resequencing, and flow control.
UDP is a connectionless (datagram) transport service. Other
transport protocols have been developed by the research community,
and the set of official Internet transport protocols may be
expanded in the future.
Transport Layer protocols relevant to routers are discussed in
Chapter 6.
o Internet Layer
All Internet transport protocols use the Internet Protocol (IP) to
carry data from source host to destination host. IP is a
connectionless or datagram internetwork service, providing no
end-to-end delivery guarantees. IP datagrams may arrive at the
destination host damaged, duplicated, out of order, or not at all.
The layers above IP are responsible for reliable delivery service
when it is required. The IP protocol includes provision for
addressing, type-of-service specification, fragmentation and
reassembly, and security.
The datagram or connectionless nature of IP is a fundamental and
characteristic feature of the Internet architecture.
The Internet Control Message Protocol (ICMP) is a control protocol
that is considered to be an integral part of IP, although it is
architecturally layered upon IP - it uses IP to carry its data
end-to-end. ICMP provides error reporting, congestion reporting,
and first-hop router redirection.
The Internet Group Management Protocol (IGMP) is an Internet layer
protocol used for establishing dynamic host groups for IP
multicasting.
The Internet layer protocols IP, ICMP, and IGMP are discussed in
chapter 4.
o Link Layer
To communicate on a directly connected network, a host must
implement the communication protocol used to interface to that
network. We call this a Link Layer protocol.
Some older Internet documents refer to this layer as the Network
Layer, but it is not the same as the Network Layer in the OSI
Reference Model.
This layer contains everything below the Internet Layer and above
the Physical Layer (which is the media connectivity, normally
electrical or optical, which encodes and transports messages).
Its responsibility is the correct delivery of messages, among
which it does not differentiate.
Protocols in this Layer are generally outside the scope of
Internet standardization; the Internet (intentionally) uses
existing standards whenever possible. Thus, Internet Link Layer
standards usually address only address resolution and rules for
transmitting IP packets over specific Link Layer protocols.
Internet Link Layer standards are discussed in chapter 3.
2.2.2 Networks top
The constituent networks of the Internet system are required to
provide only packet (connectionless) transport. According to the IP
service specification, datagrams can be delivered out of order, be
lost or duplicated, and/or contain errors.
For reasonable performance of the protocols that use IP (e.g., TCP),
the loss rate of the network should be very low. In networks
providing connection-oriented service, the extra reliability provided
by virtual circuits enhances the end-end robustness of the system,
but is not necessary for Internet operation.
Constituent networks may generally be divided into two classes:
o Local-Area Networks (LANs)
LANs may have a variety of designs. LANs normally cover a small
geographical area (e.g., a single building or plant site) and
provide high bandwidth with low delays. LANs may be passive
(similar to Ethernet) or they may be active (such as ATM).
o Wide-Area Networks (WANs)
Geographically dispersed hosts and LANs are interconnected by
wide-area networks, also called long-haul networks. These
networks may have a complex internal structure of lines and
packet-switches, or they may be as simple as point-to-point
lines.
2.2.3 Routers top
In the Internet model, constituent networks are connected together by
IP datagram forwarders which are called routers or IP routers. In
this document, every use of the term router is equivalent to IP
router. Many older Internet documents refer to routers as gateways.
Historically, routers have been realized with packet-switching
software executing on a general-purpose CPU. However, as custom
hardware development becomes cheaper and as higher throughput is
required, special purpose hardware is becoming increasingly common.
This specification applies to routers regardless of how they are
implemented.
A router connects to two or more logical interfaces, represented by
IP subnets or unnumbered point to point lines (discussed in section
[2.2.7]). Thus, it has at least one physical interface. Forwarding
an IP datagram generally requires the router to choose the address
and relevant interface of the next-hop router or (for the final hop)
the destination host. This choice, called relaying or forwarding
depends upon a route database within the router. The route database
is also called a routing table or forwarding table. The term
"router" derives from the process of building this route database;
routing protocols and configuration interact in a process called
routing.
The routing database should be maintained dynamically to reflect the
current topology of the Internet system. A router normally
accomplishes this by participating in distributed routing and
reachability algorithms with other routers.
Routers provide datagram transport only, and they seek to minimize
the state information necessary to sustain this service in the
interest of routing flexibility and robustness.
Packet switching devices may also operate at the Link Layer; such
devices are usually called bridges. Network segments that are
connected by bridges share the same IP network prefix forming a
single IP subnet. These other devices are outside the scope of this
document.
2.2.4 Autonomous Systems top
An Autonomous System (AS) is a connected segment of a network
topology that consists of a collection of subnetworks (with hosts
attached) interconnected by a set of routes. The subnetworks and the
routers are expected to be under the control of a single operations
and maintenance (O&M) organization. Within an AS routers may use one
or more interior routing protocols, and sometimes several sets of
metrics. An AS is expected to present to other ASs an appearence of
a coherent interior routing plan, and a consistent picture of the
destinations reachable through the AS. An AS is identified by an
Autonomous System number.
The concept of an AS plays an important role in the Internet routing
(see Section 7.1).
2.2.5 Addressing Architecture top
An IP datagram carries 32-bit source and destination addresses, each
of which is partitioned into two parts - a constituent network prefix
and a host number on that network. Symbolically:
IP-address ::= { <Network-prefix>, <Host-number> }
To finally deliver the datagram, the last router in its path must map
the Host-number (or rest) part of an IP address to the host's Link
Layer address.
2.2.5.1 Classical IP Addressing Architecture top
Although well documented elsewhere [INTERNET:2], it is useful to
describe the historical use of the network prefix. The language
developed to describe it is used in this and other documents and
permeates the thinking behind many protocols.
The simplest classical network prefix is the Class A, B, C, D, or E
network prefix. These address ranges are discriminated by observing
the values of the most significant bits of the address, and break the
address into simple prefix and host number fields. This is described
in [INTERNET:18]. In short, the classification is:
0xxx - Class A - general purpose unicast addresses with standard
8 bit prefix
10xx - Class B - general purpose unicast addresses with standard
16 bit prefix
110x - Class C - general purpose unicast addresses with standard
24 bit prefix
1110 - Class D - IP Multicast Addresses - 28 bit prefix, non-
aggregatable
1111 - Class E - reserved for experimental use
This simple notion has been extended by the concept of subnets.
These were introduced to allow arbitrary complexity of interconnected
LAN structures within an organization, while insulating the Internet
system against explosive growth in assigned network prefixes and
routing complexity. Subnets provide a multi-level hierarchical
routing structure for the Internet system. The subnet extension,
described in [INTERNET:2], is a required part of the Internet
architecture. The basic idea is to partition the <Host-number> field
into two parts: a subnet number, and a true host number on that
subnet:
IP-address ::=
{ <Network-number>, <Subnet-number>, <Host-number> }
The interconnected physical networks within an organization use the
same network prefix but different subnet numbers. The distinction
between the subnets of such a subnetted network is not normally
visible outside of that network. Thus, routing in the rest of the
Internet uses only the <Network-prefix> part of the IP destination
address. Routers outside the network treat <Network-prefix> and
<Host-number> together as an uninterpreted rest part of the 32-bit IP
address. Within the subnetted network, the routers use the extended
network prefix:
{ <Network-number>, <Subnet-number> }
The bit positions containing this extended network number have
historically been indicated by a 32-bit mask called the subnet mask.
The <Subnet-number> bits SHOULD be contiguous and fall between the
<Network-number> and the <Host-number> fields. More up to date
protocols do not refer to a subnet mask, but to a prefix length; the
"prefix" portion of an address is that which would be selected by a
subnet mask whose most significant bits are all ones and the rest are
zeroes. The length of the prefix equals the number of ones in the
subnet mask. This document assumes that all subnet masks are
expressible as prefix lengths.
The inventors of the subnet mechanism presumed that each piece of an
organization's network would have only a single subnet number. In
practice, it has often proven necessary or useful to have several
subnets share a single physical cable. For this reason, routers
should be capable of configuring multiple subnets on the same
physical interfaces, and treat them (from a routing or forwarding
perspective) as though they were distinct physical interfaces.
2.2.5.2 Classless Inter Domain Routing (CIDR) top
The explosive growth of the Internet has forced a review of address
assignment policies. The traditional uses of general purpose (Class
A, B, and C) networks have been modified to achieve better use of
IP's 32-bit address space. Classless Inter Domain Routing (CIDR)
[INTERNET:15] is a method currently being deployed in the Internet
backbones to achieve this added efficiency. CIDR depends on
deploying and routing to arbitrarily sized networks. In this model,
hosts and routers make no assumptions about the use of addressing in
the internet. The Class D (IP Multicast) and Class E (Experimental)
address spaces are preserved, although this is primarily an
assignment policy.
By definition, CIDR comprises three elements:
o topologically significant address assignment,
o routing protocols that are capable of aggregating network layer
reachability information, and
o consistent forwarding algorithm ("longest match").
The use of networks and subnets is now historical, although the
language used to describe them remains in current use. They have
been replaced by the more tractable concept of a network prefix. A
network prefix is, by definition, a contiguous set of bits at the
more significant end of the address that defines a set of systems;
host numbers select among those systems. There is no requirement
that all the internet use network prefixes uniformly. To collapse
routing information, it is useful to divide the internet into
addressing domains. Within such a domain, detailed information is
available about constituent networks; outside it, only the common
network prefix is advertised.
The classical IP addressing architecture used addresses and subnet
masks to discriminate the host number from the network prefix. With
network prefixes, it is sufficient to indicate the number of bits in
the prefix. Both representations are in common use. Architecturally
correct subnet masks are capable of being represented using the
prefix length description. They comprise that subset of all possible
bits patterns that have
o a contiguous string of ones at the more significant end,
o a contiguous string of zeros at the less significant end, and
o no intervening bits.
Routers SHOULD always treat a route as a network prefix, and SHOULD
reject configuration and routing information inconsistent with that
model.
IP-address ::= { <Network-prefix>, <Host-number> }
An effect of the use of CIDR is that the set of destinations
associated with address prefixes in the routing table may exhibit
subset relationship. A route describing a smaller set of
destinations (a longer prefix) is said to be more specific than a
route describing a larger set of destinations (a shorter prefix);
similarly, a route describing a larger set of destinations (a shorter
prefix) is said to be less specific than a route describing a smaller
set of destinations (a longer prefix). Routers must use the most
specific matching route (the longest matching network prefix) when
forwarding traffic.
2.2.6 IP Multicasting top
IP multicasting is an extension of Link Layer multicast to IP
internets. Using IP multicasts, a single datagram can be addressed
to multiple hosts without sending it to all. In the extended case,
these hosts may reside in different address domains. This collection
of hosts is called a multicast group. Each multicast group is
represented as a Class D IP address. An IP datagram sent to the
group is to be delivered to each group member with the same best-
effort delivery as that provided for unicast IP traffic. The sender
of the datagram does not itself need to be a member of the
destination group.
The semantics of IP multicast group membership are defined in
[INTERNET:4]. That document describes how hosts and routers join and
leave multicast groups. It also defines a protocol, the Internet
Group Management Protocol (IGMP), that monitors IP multicast group
membership.
Forwarding of IP multicast datagrams is accomplished either through
static routing information or via a multicast routing protocol.
Devices that forward IP multicast datagrams are called multicast
routers. They may or may not also forward IP unicasts. Multicast
datagrams are forwarded on the basis of both their source and
destination addresses. Forwarding of IP multicast packets is
described in more detail in Section [5.2.1]. Appendix D discusses
multicast routing protocols.
2.2.7 Unnumbered Lines And Networks Prefixes top
Traditionally, each network interface on an IP host or router has its
own IP address. This can cause inefficient use of the scarce IP
address space, since it forces allocation of an IP network prefix to
every point-to-point link.
To solve this problem, a number of people have proposed and
implemented the concept of unnumbered point to point lines. An
unnumbered point to point line does not have any network prefix
associated with it. As a consequence, the network interfaces
connected to an unnumbered point to point line do not have IP
addresses.
Because the IP architecture has traditionally assumed that all
interfaces had IP addresses, these unnumbered interfaces cause some
interesting dilemmas. For example, some IP options (e.g., Record
Route) specify that a router must insert the interface address into
the option, but an unnumbered interface has no IP address. Even more
fundamental (as we shall see in chapter 5) is that routes contain the
IP address of the next hop router. A router expects that this IP
address will be on an IP (sub)net to which the router is connected.
That assumption is of course violated if the only connection is an
unnumbered point to point line.
To get around these difficulties, two schemes have been conceived.
The first scheme says that two routers connected by an unnumbered
point to point line are not really two routers at all, but rather two
half-routers that together make up a single virtual router. The
unnumbered point to point line is essentially considered to be an
internal bus in the virtual router. The two halves of the virtual
router must coordinate their activities in such a way that they act
exactly like a single router.
This scheme fits in well with the IP architecture, but suffers from
two important drawbacks. The first is that, although it handles the
common case of a single unnumbered point to point line, it is not
readily extensible to handle the case of a mesh of routers and
unnumbered point to point lines. The second drawback is that the
interactions between the half routers are necessarily complex and are
not standardized, effectively precluding the connection of equipment
from different vendors using unnumbered point to point lines.
Because of these drawbacks, this memo has adopted an alternate
scheme, which has been invented multiple times but which is probably
originally attributable to Phil Karn. In this scheme, a router that
has unnumbered point to point lines also has a special IP address,
called a router-id in this memo. The router-id is one of the
router's IP addresses (a router is required to have at least one IP
address). This router-id is used as if it is the IP address of all
unnumbered interfaces.
2.2.8 Notable Oddities top
2.2.8.1 Embedded Routers top
A router may be a stand-alone computer system, dedicated to its IP
router functions. Alternatively, it is possible to embed router
functions within a host operating system that supports connections to
two or more networks. The best-known example of an operating system
with embedded router code is the Berkeley BSD system. The embedded
router feature seems to make building a network easy, but it has a
number of hidden pitfalls:
(1) If a host has only a single constituent-network interface, it
should not act as a router.
For example, hosts with embedded router code that gratuitously
forward broadcast packets or datagrams on the same net often
cause packet avalanches.
(2) If a (multihomed) host acts as a router, it is subject to the
requirements for routers contained in this document.
For example, the routing protocol issues and the router control
and monitoring problems are as hard and important for embedded
routers as for stand-alone routers.
Internet router requirements and specifications may change
independently of operating system changes. An administration
that operates an embedded router in the Internet is strongly
advised to maintain and update the router code. This might
require router source code.
(3) When a host executes embedded router code, it becomes part of the
Internet infrastructure. Thus, errors in software or
configuration can hinder communication between other hosts. As
a consequence, the host administrator must lose some autonomy.
In many circumstances, a host administrator will need to disable
router code embedded in the operating system. For this reason,
it should be straightforward to disable embedded router
functionality.
(4) When a host running embedded router code is concurrently used for
other services, the Operation and Maintenance requirements for
the two modes of use may conflict.
For example, router O&M will in many cases be performed remotely
by an operations center; this may require privileged system
access that the host administrator would not normally want to
distribute.
2.2.8.2 Transparent Routers top
There are two basic models for interconnecting local-area networks
and wide-area (or long-haul) networks in the Internet. In the first,
the local-area network is assigned a network prefix and all routers
in the Internet must know how to route to that network. In the
second, the local-area network shares (a small part of) the address
space of the wide-area network. Routers that support this second
model are called address sharing routers or transparent routers. The
focus of this memo is on routers that support the first model, but
this is not intended to exclude the use of transparent routers.
The basic idea of a transparent router is that the hosts on the
local-area network behind such a router share the address space of
the wide-area network in front of the router. In certain situations
this is a very useful approach and the limitations do not present
significant drawbacks.
The words in front and behind indicate one of the limitations of this
approach: this model of interconnection is suitable only for a
geographically (and topologically) limited stub environment. It
requires that there be some form of logical addressing in the network
level addressing of the wide-area network. IP addresses in the local
environment map to a few (usually one) physical address in the wide-
area network. This mapping occurs in a way consistent with the { IP
address <-> network address } mapping used throughout the wide-area
network.
Multihoming is possible on one wide-area network, but may present
routing problems if the interfaces are geographically or
topologically separated. Multihoming on two (or more) wide-area
networks is a problem due to the confusion of addresses.
The behavior that hosts see from other hosts in what is apparently
the same network may differ if the transparent router cannot fully
emulate the normal wide-area network service. For example, the
ARPANET used a Link Layer protocol that provided a Destination Dead
indication in response to an attempt to send to a host that was off-
line. However, if there were a transparent router between the
ARPANET and an Ethernet, a host on the ARPANET would not receive a
Destination Dead indication for Ethernet hosts.
2.3 Router Characteristics top
An Internet router performs the following functions:
(1) Conforms to specific Internet protocols specified in this
document, including the Internet Protocol (IP), Internet Control
Message Protocol (ICMP), and others as necessary.
(2) Interfaces to two or more packet networks. For each connected
network the router must implement the functions required by that
network. These functions typically include:
o Encapsulating and decapsulating the IP datagrams with the
connected network framing (e.g., an Ethernet header and
checksum),
o Sending and receiving IP datagrams up to the maximum size
supported by that network, this size is the network's Maximum
Transmission Unit or MTU,
o Translating the IP destination address into an appropriate
network-level address for the connected network (e.g., an
Ethernet hardware address), if needed, and
o Responding to network flow control and error indications, if
any.
See chapter 3 (Link Layer).
(3) Receives and forwards Internet datagrams. Important issues in
this process are buffer management, congestion control, and
fairness.
o Recognizes error conditions and generates ICMP error and
information messages as required.
o Drops datagrams whose time-to-live fields have reached zero.
o Fragments datagrams when necessary to fit into the MTU of the
next network.
See chapter 4 (Internet Layer - Protocols) and chapter 5
(Internet Layer - Forwarding) for more information.
(4) Chooses a next-hop destination for each IP datagram, based on the
information in its routing database. See chapter 5 (Internet
Layer - Forwarding) for more information.
(5) (Usually) supports an interior gateway protocol (IGP) to carry
out distributed routing and reachability algorithms with the
other routers in the same autonomous system. In addition, some
routers will need to support an exterior gateway protocol (EGP)
to exchange topological information with other autonomous
systems. See chapter 7 (Application Layer - Routing Protocols)
for more information.
(6) Provides network management and system support facilities,
including loading, debugging, status reporting, exception
reporting and control. See chapter 8 (Application Layer -
Network Management Protocols) and chapter 10 (Operation and
Maintenance) for more information.
A router vendor will have many choices on power, complexity, and
features for a particular router product. It may be helpful to
observe that the Internet system is neither homogeneous nor fully
connected. For reasons of technology and geography it is growing
into a global interconnect system plus a fringe of LANs around the
edge. More and more these fringe LANs are becoming richly
interconnected, thus making them less out on the fringe and more
demanding on router requirements.
o The global interconnect system is composed of a number of wide-area
networks to which are attached routers of several Autonomous
Systems (AS); there are relatively few hosts connected directly to
the system.
o Most hosts are connected to LANs. Many organizations have clusters
of LANs interconnected by local routers. Each such cluster is
connected by routers at one or more points into the global
interconnect system. If it is connected at only one point, a LAN
is known as a stub network.
Routers in the global interconnect system generally require:
o Advanced Routing and Forwarding Algorithms
These routers need routing algorithms that are highly dynamic,
impose minimal processing and communication burdens, and offer
type-of-service routing. Congestion is still not a completely
resolved issue (see Section [5.3.6]). Improvements in these areas
are expected, as the research community is actively working on
these issues.
o High Availability
These routers need to be highly reliable, providing 24 hours a
day, 7 days a week service. Equipment and software faults can
have a wide-spread (sometimes global) effect. In case of failure,
they must recover quickly. In any environment, a router must be
highly robust and able to operate, possibly in a degraded state,
under conditions of extreme congestion or failure of network
resources.
o Advanced O&M Features
Internet routers normally operate in an unattended mode. They
will typically be operated remotely from a centralized monitoring
center. They need to provide sophisticated means for monitoring
and measuring traffic and other events and for diagnosing faults.
o High Performance
Long-haul lines in the Internet today are most frequently full
duplex 56 KBPS, DS1 (1.544 Mbps), or DS3 (45 Mbps) speeds. LANs,
which are half duplex multiaccess media, are typically Ethernet
(10Mbps) and, to a lesser degree, FDDI (100Mbps). However,
network media technology is constantly advancing and higher speeds
are likely in the future.
The requirements for routers used in the LAN fringe (e.g., campus
networks) depend greatly on the demands of the local networks. These
may be high or medium-performance devices, probably competitively
procured from several different vendors and operated by an internal
organization (e.g., a campus computing center). The design of these
routers should emphasize low average latency and good burst
performance, together with delay and type-of-service sensitive
resource management. In this environment there may be less formal
O&M but it will not be less important. The need for the routing
mechanism to be highly dynamic will become more important as networks
become more complex and interconnected. Users will demand more out
of their local connections because of the speed of the global
interconnects.
As networks have grown, and as more networks have become old enough
that they are phasing out older equipment, it has become increasingly
imperative that routers interoperate with routers from other vendors.
Even though the Internet system is not fully interconnected, many
parts of the system need to have redundant connectivity. Rich
connectivity allows reliable service despite failures of
communication lines and routers, and it can also improve service by
shortening Internet paths and by providing additional capacity.
Unfortunately, this richer topology can make it much more difficult
to choose the best path to a particular destination.
2.4 Architectural Assumptions top
The current Internet architecture is based on a set of assumptions
about the communication system. The assumptions most relevant to
routers are as follows:
o The Internet is a network of networks.
Each host is directly connected to some particular network(s); its
connection to the Internet is only conceptual. Two hosts on the
same network communicate with each other using the same set of
protocols that they would use to communicate with hosts on distant
networks.
o Routers do not keep connection state information.
To improve the robustness of the communication system, routers are
designed to be stateless, forwarding each IP packet independently
of other packets. As a result, redundant paths can be exploited
to provide robust service in spite of failures of intervening
routers and networks.
All state information required for end-to-end flow control and
reliability is implemented in the hosts, in the transport layer or
in application programs. All connection control information is
thus co-located with the end points of the communication, so it
will be lost only if an end point fails. Routers control message
flow only indirectly, by dropping packets or increasing network
delay.
Note that future protocol developments may well end up putting
some more state into routers. This is especially likely for
multicast routing, resource reservation, and flow based
forwarding.
o Routing complexity should be in the routers.
Routing is a complex and difficult problem, and ought to be
performed by the routers, not the hosts. An important objective
is to insulate host software from changes caused by the inevitable
evolution of the Internet routing architecture.
o The system must tolerate wide network variation.
A basic objective of the Internet design is to tolerate a wide
range of network characteristics - e.g., bandwidth, delay, packet
loss, packet reordering, and maximum packet size. Another
objective is robustness against failure of individual networks,
routers, and hosts, using whatever bandwidth is still available.
Finally, the goal is full open system interconnection: an Internet
router must be able to interoperate robustly and effectively with
any other router or Internet host, across diverse Internet paths.
Sometimes implementors have designed for less ambitious goals.
For example, the LAN environment is typically much more benign
than the Internet as a whole; LANs have low packet loss and delay
and do not reorder packets. Some vendors have fielded
implementations that are adequate for a simple LAN environment,
but work badly for general interoperation. The vendor justifies
such a product as being economical within the restricted LAN
market. However, isolated LANs seldom stay isolated for long.
They are soon connected to each other, to organization-wide
internets, and eventually to the global Internet system. In the
end, neither the customer nor the vendor is served by incomplete
or substandard routers.
The requirements in this document are designed for a full-function
router. It is intended that fully compliant routers will be
usable in almost any part of the Internet.
3. LINK LAYER top
Although [INTRO:1] covers Link Layer standards (IP over various link
layers, ARP, etc.), this document anticipates that Link-Layer
material will be covered in a separate Link Layer Requirements
document. A Link-Layer Requirements document would be applicable to
both hosts and routers. Thus, this document will not obsolete the
parts of [INTRO:1] that deal with link-layer issues.
3.1 INTRODUCTION top
Routers have essentially the same Link Layer protocol requirements as
other sorts of Internet systems. These requirements are given in
chapter 3 of Requirements for Internet Gateways [INTRO:1]. A router
MUST comply with its requirements and SHOULD comply with its
recommendations. Since some of the material in that document has
become somewhat dated, some additional requirements and explanations
are included below.
DISCUSSION
It is expected that the Internet community will produce a
Requirements for Internet Link Layer standard which will supersede
both this chapter and the chapter entitled "INTERNET LAYER
PROTOCOLS" in [INTRO:1].
3.2 LINK/INTERNET LAYER INTERFACE top
This document does not attempt to specify the interface between the
Link Layer and the upper layers. However, note well that other parts
of this document, particularly chapter 5, require various sorts of
information to be passed across this layer boundary.
This section uses the following definitions:
o Source physical address
The source physical address is the Link Layer address of the host
or router from which the packet was received.
o Destination physical address
The destination physical address is the Link Layer address to
which the packet was sent.
The information that must pass from the Link Layer to the
Internetwork Layer for each received packet is:
(1) The IP packet [5.2.2],
(2) The length of the data portion (i.e., not including the Link-
Layer framing) of the Link Layer frame [5.2.2],
(3) The identity of the physical interface from which the IP packet
was received [5.2.3], and
(4) The classification of the packet's destination physical address
as a Link Layer unicast, broadcast, or multicast [4.3.2],
[5.3.4].
In addition, the Link Layer also should provide:
(5) The source physical address.
The information that must pass from the Internetwork Layer to the
Link Layer for each transmitted packet is:
(1) The IP packet [5.2.1]
(2) The length of the IP packet [5.2.1]
(3) The destination physical interface [5.2.1]
(4) The next hop IP address [5.2.1]
In addition, the Internetwork Layer also should provide:
(5) The Link Layer priority value [5.3.3.2]
The Link Layer must also notify the Internetwork Layer if the packet
to be transmitted causes a Link Layer precedence-related error
[5.3.3.3].
3.3 SPECIFIC ISSUES top
3.3.1 Trailer Encapsulation top
Routers that can connect to ten megabit Ethernets MAY be able to
receive and forward Ethernet packets encapsulated using the trailer
encapsulation described in [LINK:1]. However, a router SHOULD NOT
originate trailer encapsulated packets. A router MUST NOT originate
trailer encapsulated packets without first verifying, using the
mechanism described in [INTRO:2], that the immediate destination of
the packet is willing and able to accept trailer-encapsulated
packets. A router SHOULD NOT agree (using these mechanisms) to
accept trailer-encapsulated packets.
3.3.2 Address Resolution Protocol - ARP top
Routers that implement ARP MUST be compliant and SHOULD be
unconditionally compliant with the requirements in [INTRO:2].
The link layer MUST NOT report a Destination Unreachable error to IP
solely because there is no ARP cache entry for a destination; it
SHOULD queue up to a small number of datagrams breifly while
performing the ARP request/reply sequence, and reply that the
destination is unreachable to one of the queued datagrams only when
this proves fruitless.
A router MUST not believe any ARP reply that claims that the Link
Layer address of another host or router is a broadcast or multicast
address.
3.3.3 Ethernet And 802.3 Coexistence top
Routers that can connect to ten megabit Ethernets MUST be compliant
and SHOULD be unconditionally compliant with the Ethernet
requirements of [INTRO:2].
3.3.4 Maximum Transmission Unit - MTU top
The MTU of each logical interface MUST be configurable within the
range of legal MTUs for the interface.
Many Link Layer protocols define a maximum frame size that may be
sent. In such cases, a router MUST NOT allow an MTU to be set which
would allow sending of frames larger than those allowed by the Link
Layer protocol. However, a router SHOULD be willing to receive a
packet as large as the maximum frame size even if that is larger than
the MTU.
DISCUSSION
Note that this is a stricter requirement than imposed on hosts by
[INTRO:2], which requires that the MTU of each physical interface
be configurable.
If a network is using an MTU smaller than the maximum frame size
for the Link Layer, a router may receive packets larger than the
MTU from misconfigured and incompletely initialized hosts. The
Robustness Principle indicates that the router should successfully
receive these packets if possible.
3.3.5 Point-to-Point Protocol - PPP top
Contrary to [INTRO:1], the Internet does have a standard point to
point line protocol: the Point-to-Point Protocol (PPP), defined in
[LINK:2], [LINK:3], [LINK:4], and [LINK:5].
A point to point interface is any interface that is designed to send
data over a point to point line. Such interfaces include telephone,
leased, dedicated or direct lines (either 2 or 4 wire), and may use
point to point channels or virtual circuits of multiplexed interfaces
such as ISDN. They normally use a standardized modem or bit serial
interface (such as RS-232, RS-449 or V.35), using either synchronous
or asynchronous clocking. Multiplexed interfaces often have special
physical interfaces.
A general purpose serial interface uses the same physical media as a
point to point line, but supports the use of link layer networks as
well as point to point connectivity. Link layer networks (such as
X.25 or Frame Relay) use an alternative IP link layer specification.
Routers that implement point to point or general purpose serial
interfaces MUST IMPLEMENT PPP.
PPP MUST be supported on all general purpose serial interfaces on a
router. The router MAY allow the line to be configured to use point
to point line protocols other than PPP. Point to point interfaces
SHOULD either default to using PPP when enabled or require
configuration of the link layer protocol before being enabled.
General purpose serial interfaces SHOULD require configuration of the
link layer protocol before being enabled.
3.3.5.1 Introduction top
This section provides guidelines to router implementors so that they
can ensure interoperability with other routers using PPP over either
synchronous or asynchronous links.
It is critical that an implementor understand the semantics of the
option negotiation mechanism. Options are a means for a local device
to indicate to a remote peer what the local device will accept from
the remote peer, not what it wishes to send. It is up to the remote
peer to decide what is most convenient to send within the confines of
the set of options that the local device has stated that it can
accept. Therefore it is perfectly acceptable and normal for a remote
peer to ACK all the options indicated in an LCP Configuration Request
(CR) even if the remote peer does not support any of those options.
Again, the options are simply a mechanism for either device to
indicate to its peer what it will accept, not necessarily what it
will send.
3.3.5.2 Link Control Protocol (LCP) Options top
The PPP Link Control Protocol (LCP) offers a number of options that
may be negotiated. These options include (among others) address and
control field compression, protocol field compression, asynchronous
character map, Maximum Receive Unit (MRU), Link Quality Monitoring
(LQM), magic number (for loopback detection), Password Authentication
Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP),
and the 32-bit Frame Check Sequence (FCS).
A router MAY use address/control field compression on either
synchronous or asynchronous links. A router MAY use protocol field
compression on either synchronous or asynchronous links. A router
that indicates that it can accept these compressions MUST be able to
accept uncompressed PPP header information also.
DISCUSSION
These options control the appearance of the PPP header. Normally
the PPP header consists of the address, the control field, and the
protocol field. The address, on a point to point line, is 0xFF,
indicating "broadcast". The control field is 0x03, indicating
"Unnumbered Information." The Protocol Identifier is a two byte
value indicating the contents of the data area of the frame. If a
system negotiates address and control field compression it
indicates to its peer that it will accept PPP frames that have or
do not have these fields at the front of the header. It does not
indicate that it will be sending frames with these fields removed.
Protocol field compression, when negotiated, indicates that the
system is willing to receive protocol fields compressed to one
byte when this is legal. There is no requirement that the sender
do so.
Use of address/control field compression is inconsistent with the
use of numbered mode (reliable) PPP.
IMPLEMENTATION
Some hardware does not deal well with variable length header
information. In those cases it makes most sense for the remote
peer to send the full PPP header. Implementations may ensure this
by not sending the address/control field and protocol field
compression options to the remote peer. Even if the remote peer
has indicated an ability to receive compressed headers there is no
requirement for the local router to send compressed headers.
A router MUST negotiate the Asynchronous Control Character Map (ACCM)
for asynchronous PPP links, but SHOULD NOT negotiate the ACCM for
synchronous links. If a router receives an attempt to negotiate the
ACCM over a synchronous link, it MUST ACKnowledge the option and then
ignore it.
DISCUSSION
There are implementations that offer both synchronous and
asynchronous modes of operation and may use the same code to
implement the option negotiation. In this situation it is
possible that one end or the other may send the ACCM option on a
synchronous link.
A router SHOULD properly negotiate the maximum receive unit (MRU).
Even if a system negotiates an MRU smaller than 1,500 bytes, it MUST
be able to receive a 1,500 byte frame.
A router SHOULD negotiate and enable the link quality monitoring
(LQM) option.
DISCUSSION
This memo does not specify a policy for deciding whether the
link's quality is adequate. However, it is important (see Section
[3.3.6]) that a router disable failed links.
A router SHOULD implement and negotiate the magic number option for
loopback detection.
A router MAY support the authentication options (PAP - Password
Authentication Protocol, and/or CHAP - Challenge Handshake
Authentication Protocol).
A router MUST support 16-bit CRC frame check sequence (FCS) and MAY
support the 32-bit CRC.
3.3.5.3 IP Control Protocol (IPCP) Options top
A router MAY offer to perform IP address negotiation. A router MUST
accept a refusal (REJect) to perform IP address negotiation from the
peer.
Routers operating at link speeds of 19,200 BPS or less SHOULD
implement and offer to perform Van Jacobson header compression.
Routers that implement VJ compression SHOULD implement an
administrative control enabling or disabling it.
3.3.6 Interface Testing top
A router MUST have a mechanism to allow routing software to determine
whether a physical interface is available to send packets or not; on
multiplexed interfaces where permanent virtual circuits are opened
for limited sets of neighbors, the router must also be able to
determine whether the virtual circuits are viable. A router SHOULD
have a mechanism to allow routing software to judge the quality of a
physical interface. A router MUST have a mechanism for informing the
routing software when a physical interface becomes available or
unavailable to send packets because of administrative action. A
router MUST have a mechanism for informing the routing software when
it detects a Link level interface has become available or
unavailable, for any reason.
DISCUSSION
It is crucial that routers have workable mechanisms for
determining that their network connections are functioning
properly. Failure to detect link loss, or failure to take the
proper actions when a problem is detected, can lead to black
holes.
The mechanisms available for detecting problems with network
connections vary considerably, depending on the Link Layer
protocols in use and the interface hardware. The intent is to
maximize the capability to detect failures within the Link-Layer
constraints.
4. INTERNET LAYER - PROTOCOLS top
4.1 INTRODUCTION top
This chapter and chapter 5 discuss the protocols used at the Internet
Layer: IP, ICMP, and IGMP. Since forwarding is obviously a crucial
topic in a document discussing routers, chapter 5 limits itself to
the aspects of the protocols that directly relate to forwarding. The
current chapter contains the remainder of the discussion of the
Internet Layer protocols.
4.2 INTERNET PROTOCOL - IP top
4.2.1 INTRODUCTION top
Routers MUST implement the IP protocol, as defined by [INTERNET:1].
They MUST also implement its mandatory extensions: subnets (defined
in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and
Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).
Router implementors need not consider compliance with the section of
[INTRO:2] entitled "Internet Protocol -- IP," as that section is
entirely duplicated or superseded in this document. A router MUST be
compliant, and SHOULD be unconditionally compliant, with the
requirements of the section entitled "SPECIFIC ISSUES" relating to IP
in [INTRO:2].
In the following, the action specified in certain cases is to
silently discard a received datagram. This means that the datagram
will be discarded without further processing and that the router will
not send any ICMP error message (see Section [4.3]) as a result.
However, for diagnosis of problems a router SHOULD provide the
capability of logging the error (see Section [1.3.3]), including the
contents of the silently discarded datagram, and SHOULD count
datagrams discarded.
4.2.2 PROTOCOL WALK-THROUGH top
RFC 791 [INTERNET:1] is the specification for the Internet Protocol.
4.2.2.1 Options: RFC 791 Section 3.2 top31'>Section 3.2
In datagrams received by the router itself, the IP layer MUST
interpret IP options that it understands and preserve the rest
unchanged for use by higher layer protocols.
Higher layer protocols may require the ability to set IP options in
datagrams they send or examine IP options in datagrams they receive.
Later sections of this document discuss specific IP option support
required by higher layer protocols.
DISCUSSION
Neither this memo nor [INTRO:2] define the order in which a
receiver must process multiple options in the same IP header.
Hosts and routers originating datagrams containing multiple
options must be aware that this introduces an ambiguity in the
meaning of certain options when combined with a source-route
option.
Here are the requirements for specific IP options:
(a) Security Option
Some environments require the Security option in every packet
originated or received. Routers SHOULD IMPLEMENT the revised
security option described in [INTERNET:5].
DISCUSSION
Note that the security options described in [INTERNET:1] and RFC
1038 ([INTERNET:16]) are obsolete.
(b) Stream Identifier Option
This option is obsolete; routers SHOULD NOT place this option
in a datagram that the router originates. This option MUST be
ignored in datagrams received by the router.
(c) Source Route Options
A router MUST be able to act as the final destination of a
source route. If a router receives a packet containing a
completed source route, the packet has reached its final
destination. In such an option, the pointer points beyond the
last field and the destination address in the IP header
addresses the router. The option as received (the recorded
route) MUST be passed up to the transport layer (or to ICMP
message processing).
In the general case, a correct response to a source-routed
datagram traverses the same route. A router MUST provide a
means whereby transport protocols and applications can reverse
the source route in a received datagram. This reversed source
route MUST be inserted into datagrams they originate (see
[INTRO:2] for details) when the router is unaware of policy
constraints. However, if the router is policy aware, it MAY
select another path.
Some applications in the router MAY require that the user be
able to enter a source route.
A router MUST NOT originate a datagram containing multiple
source route options. What a router should do if asked to
forward a packet containing multiple source route options is
described in Section [5.2.4.1].
When a source route option is created (which would happen when
the router is originating a source routed datagram or is
inserting a source route option as a result of a special
filter), it MUST be correctly formed even if it is being
created by reversing a recorded route that erroneously includes
the source host (see case (B) in the discussion below).
DISCUSSION
Suppose a source routed datagram is to be routed from source _S to
destination D via routers G1, G2, Gn. Source S constructs a
datagram with G1's IP address as its destination address, and a
source route option to get the datagram the rest of the way to its
destination. However, there is an ambiguity in the specification
over whether the source route option in a datagram sent out by S
should be (A) or (B):
(A): {>>G2, G3, ... Gn, D} <--- CORRECT
(B): {S, >>G2, G3, ... Gn, D} <---- WRONG
(where >> represents the pointer). If (A) is sent, the datagram
received at D will contain the option: {G1, G2, ... Gn >>}, with S
and D as the IP source and destination addresses. If (B) were
sent, the datagram received at D would again contain S and D as
the same IP source and destination addresses, but the option would
be: {S, G1, ...Gn >>}; i.e., the originating host would be the
first hop in the route.
(d) Record Route Option
Routers MAY support the Record Route option in datagrams
originated by the router.
(e) Timestamp Option
Routers MAY support the timestamp option in datagrams
originated by the router. The following rules apply:
o When originating a datagram containing a Timestamp Option, a
router MUST record a timestamp in the option if
- Its Internet address fields are not pre-specified or
- Its first pre-specified address is the IP address of the
logical interface over which the datagram is being sent
(or the router's router-id if the datagram is being sent
over an unnumbered interface).
o If the router itself receives a datagram containing a
Timestamp Option, the router MUST insert the current time
into the Timestamp Option (if there is space in the option
to do so) before passing the option to the transport layer
or to ICMP for processing. If space is not present, the
router MUST increment the Overflow Count in the option.
o A timestamp value MUST follow the rules defined in [INTRO:2].
IMPLEMENTATION
To maximize the utility of the timestamps contained in the
timestamp option, the timestamp inserted should be, as nearly as
practical, the time at which the packet arrived at the router.
For datagrams originated by the router, the timestamp inserted
should be, as nearly as practical, the time at which the datagram
was passed to the Link Layer for transmission.
The timestamp option permits the use of a non-standard time clock,
but the use of a non-synchronized clock limits the utility of the
time stamp. Therefore, routers are well advised to implement the
Network Time Protocol for the purpose of synchronizing their
clocks.
4.2.2.2 Addresses In Options: RFC 791 Section 3.1 top30'>Section 3.1
Routers are called upon to insert their address into Record Route,
Strict Source and Record Route, Loose Source and Record Route, or
Timestamp Options. When a router inserts its address into such an
option, it MUST use the IP address of the logical interface on which
the packet is being sent. Where this rule cannot be obeyed because
the output interface has no IP address (i.e., is an unnumbered
interface), the router MUST instead insert its router-id. The
router's router-id is one of the router's IP addresses. The Router
ID may be specified on a system basis or on a per-link basis. Which
of the router's addresses is used as the router-id MUST NOT change
(even across reboots) unless changed by the network manager.
Relevant management changes include reconfiguration of the router
such that the IP address used as the router-id ceases to be one of
the router's IP addresses. Routers with multiple unnumbered
interfaces MAY have multiple router-id's. Each unnumbered interface
MUST be associated with a particular router-id. This association
MUST NOT change (even across reboots) without reconfiguration of the
router.
DISCUSSION
This specification does not allow for routers that do not have at
least one IP address. We do not view this as a serious
limitation, since a router needs an IP address to meet the
manageability requirements of Chapter [8] even if the router is
connected only to point-to-point links.
IMPLEMENTATION
One possible method of choosing the router-id that fulfills this
requirement is to use the numerically smallest (or greatest) IP
address (treating the address as a 32-bit integer) that is
assigned to the router.
4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 top30'>Section 3.1
The IP header contains two reserved bits: one in the Type of Service
byte and the other in the Flags field. A router MUST NOT set either
of these bits to one in datagrams originated by the router. A router
MUST NOT drop (refuse to receive or forward) a packet merely because
one or more of these reserved bits has a non-zero value; i.e., the
router MUST NOT check the values of thes bits.
DISCUSSION
Future revisions to the IP protocol may make use of these unused
bits. These rules are intended to ensure that these revisions can
be deployed without having to simultaneously upgrade all routers
in the Internet.
4.2.2.4 Type Of Service: RFC 791 Section 3.1 top30'>Section 3.1
The Type-of-Service byte in the IP header is divided into three
sections: the Precedence field (high-order 3 bits), a field that is
customarily called Type of Service or TOS (next 4 bits), and a
reserved bit (the low order bit).
Rules governing the reserved bit were described in Section [4.2.2.3].
A more extensive discussion of the TOS field and its use can be found
in [ROUTE:11].
The description of the IP Precedence field is superseded by Section
[5.3.3]. RFC 795, Service Mappings, is obsolete and SHOULD NOT be
implemented.
4.2.2.5 Header Checksum: RFC 791 Section 3.1 top30'>Section 3.1
As stated in Section [5.2.2], a router MUST verify the IP checksum of
any packet that is received, and MUST discard messages containing
invalid checksums. The router MUST NOT provide a means to disable
this checksum verification.
A router MAY use incremental IP header checksum updating when the
only change to the IP header is the time to live. This will reduce
the possibility of undetected corruption of the IP header by the
router. See [INTERNET:6] for a discussion of incrementally updating
the checksum.
IMPLEMENTATION
A more extensive description of the IP checksum, including
extensive implementation hints, can be found in [INTERNET:6] and
[INTERNET:7].
4.2.2.6 Unrecognized Header Options: RFC 791 Section 3.1
A router MUST ignore IP options which it does not recognize. A
corollary of this requirement is that a router MUST implement the End
of Option List option and the No Operation option, since neither
contains an explicit length.
DISCUSSION
All future IP options will include an explicit length.
4.2.2.7 Fragmentation: RFC 791 Section 3.2 top31'>Section 3.2
Fragmentation, as described in [INTERNET:1], MUST be supported by a
router.
When a router fragments an IP datagram, it SHOULD minimize the number
of fragments. When a router fragments an IP datagram, it SHOULD send
the fragments in order. A fragmentation method that may generate one
IP fragment that is significantly smaller than the other MAY cause
the first IP fragment to be the smaller one.
DISCUSSION
There are several fragmentation techniques in common use in the
Internet. One involves splitting the IP datagram into IP
fragments with the first being MTU sized, and the others being
approximately the same size, smaller than the MTU. The reason for
this is twofold. The first IP fragment in the sequence will be
the effective MTU of the current path between the hosts, and the
following IP fragments are sized to minimize the further
fragmentation of the IP datagram. Another technique is to split
the IP datagram into MTU sized IP fragments, with the last
fragment being the only one smaller, as described in [INTERNET:1].
A common trick used by some implementations of TCP/IP is to
fragment an IP datagram into IP fragments that are no larger than
576 bytes when the IP datagram is to travel through a router.
This is intended to allow the resulting IP fragments to pass the
rest of the path without further fragmentation. This would,
though, create more of a load on the destination host, since it
would have a larger number of IP fragments to reassemble into one
IP datagram. It would also not be efficient on networks where the
MTU only changes once and stays much larger than 576 bytes.
Examples include LAN networks such as an IEEE 802.5 network with a
MTU of 2048 or an Ethernet network with an MTU of 1500).
One other fragmentation technique discussed was splitting the IP
datagram into approximately equal sized IP fragments, with the
size less than or equal to the next hop network's MTU. This is
intended to minimize the number of fragments that would result
from additional fragmentation further down the path, and assure
equal delay for each fragment.
Routers SHOULD generate the least possible number of IP fragments.
Work with slow machines leads us to believe that if it is
necessary to fragment messages, sending the small IP fragment
first maximizes the chance of a host with a slow interface of
receiving all the fragments.
4.2.2.8 Reassembly: RFC 791 Section 3.2 top31'>Section 3.2
As specified in the corresponding section of [INTRO:2], a router MUST
support reassembly of datagrams that it delivers to itself.
4.2.2.9 Time To Live: RFC 791 Section 3.2 top31'>Section 3.2
Time to Live (TTL) handling for packets originated or received by the
router is governed by [INTRO:2]; this section changes none of its
stipulations. However, since the remainder of the IP Protocol
section of [INTRO:2] is rewritten, this section is as well.
Note in particular that a router MUST NOT check the TTL of a packet
except when forwarding it.
A router MUST NOT originate or forward a datagram with a Time-to-Live
(TTL) value of zero.
A router MUST NOT discard a datagram just because it was received
with TTL equal to zero or one; if it is to the router and otherwise
valid, the router MUST attempt to receive it.
On messages the router originates, the IP layer MUST provide a means
for the transport layer to set the TTL field of every datagram that
is sent. When a fixed TTL value is used, it MUST be configurable.
The number SHOULD exceed the typical internet diameter, and current
wisdom suggests that it should exceed twice the internet diameter to
allow for growth. Current suggested values are normally posted in
the Assigned Numbers RFC. The TTL field has two functions: limit the
lifetime of TCP segments (see RFC 793 [TCP:1], p. 28), and terminate
Internet routing loops. Although TTL is a time in seconds, it also
has some attributes of a hop-count, since each router is required to
reduce the TTL field by at least one.
TTL expiration is intended to cause datagrams to be discarded by
routers, but not by the destination host. Hosts that act as routers
by forwarding datagrams must therefore follow the router's rules for
TTL.
A higher-layer protocol may want to set the TTL in order to implement
an "expanding scope" search for some Internet resource. This is used
by some diagnostic tools, and is expected to be useful for locating
the "nearest" server of a given class using IP multicasting, for
example. A particular transport protocol may also want to specify
its own TTL bound on maximum datagram lifetime.
A fixed default value must be at least big enough for the Internet
"diameter," i.e., the longest possible path. A reasonable value is
about twice the diameter, to allow for continued Internet growth. As
of this writing, messages crossing the United States frequently
traverse 15 to 20 routers; this argues for a default TTL value in
excess of 40, and 64 is a common value.
4.2.2.10 Multi-subnet Broadcasts: RFC 922 top
All-subnets broadcasts (called multi-subnet broadcasts in
[INTERNET:3]) have been deprecated. See Section [ 