RFC 1716
Hyperlinked version
Network Working Group P. Almquist, Author
Request for Comments: 1716 Consultant
Category: Informational F. Kastenholz, Editor
FTP Software, Inc.
November 1994
Towards Requirements for IP Routers
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
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RFC 1716 Towards Requirements for IP Routers November 1994
Table of Contents
0. PREFACE ....................................................... 1
1. INTRODUCTION .................................................. 2
1.1 Reading this Document ........................................ 4
1.1.1 Organization ............................................... 4
1.1.2 Requirements ............................................... 5
1.1.3 Compliance ................................................. 6
1.2 Relationships to Other Standards ............................. 7
1.3 General Considerations ....................................... 8
1.3.1 Continuing Internet Evolution .............................. 8
1.3.2 Robustness Principle ....................................... 9
1.3.3 Error Logging .............................................. 9
1.3.4 Configuration .............................................. 10
1.4 Algorithms ................................................... 11
2. INTERNET ARCHITECTURE ......................................... 13
2.1 Introduction ................................................. 13
2.2 Elements of the Architecture ................................. 14
2.2.1 Protocol Layering .......................................... 14
2.2.2 Networks ................................................... 16
2.2.3 Routers .................................................... 17
2.2.4 Autonomous Systems ......................................... 18
2.2.5 Addresses and Subnets ...................................... 18
2.2.6 IP Multicasting ............................................ 20
2.2.7 Unnumbered Lines and Networks and Subnets .................. 20
2.2.8 Notable Oddities ........................................... 22
2.2.8.1 Embedded Routers ......................................... 22
2.2.8.2 Transparent Routers ...................................... 23
2.3 Router Characteristics ....................................... 24
2.4 Architectural Assumptions .................................... 27
3. LINK LAYER .................................................... 29
3.1 INTRODUCTION ................................................. 29
3.2 LINK/INTERNET LAYER INTERFACE ................................ 29
3.3 SPECIFIC ISSUES .............................................. 30
3.3.1 Trailer Encapsulation ...................................... 30
3.3.2 Address Resolution Protocol - ARP .......................... 31
3.3.3 Ethernet and 802.3 Coexistence ............................. 31
3.3.4 Maximum Transmission Unit - MTU ............................ 31
3.3.5 Point-to-Point Protocol - PPP .............................. 32
3.3.5.1 Introduction ............................................. 32
3.3.5.2 Link Control Protocol (LCP) Options ...................... 33
3.3.5.3 IP Control Protocol (ICP) Options ........................ 34
3.3.6 Interface Testing .......................................... 35
4. INTERNET LAYER - PROTOCOLS .................................... 36
4.1 INTRODUCTION ................................................. 36
4.2 INTERNET PROTOCOL - IP ....................................... 36
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4.2.1 INTRODUCTION ............................................... 36
4.2.2 PROTOCOL WALK-THROUGH ...................................... 37
4.2.2.1 Options: RFC-791 Section 3.2 ............................. 37
4.2.2.2 Addresses in Options: RFC-791 Section 3.1 ................ 40
4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1 ............... 40
4.2.2.4 Type of Service: RFC-791 Section 3.1 ..................... 41
4.2.2.5 Header Checksum: RFC-791 Section 3.1 ..................... 41
4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1 ......... 41
4.2.2.7 Fragmentation: RFC-791 Section 3.2 ....................... 42
4.2.2.8 Reassembly: RFC-791 Section 3.2 .......................... 43
4.2.2.9 Time to Live: RFC-791 Section 3.2 ........................ 43
4.2.2.10 Multi-subnet Broadcasts: RFC-922 ........................ 43
4.2.2.11 Addressing: RFC-791 Section 3.2 ......................... 43
4.2.3 SPECIFIC ISSUES ............................................ 47
4.2.3.1 IP Broadcast Addresses ................................... 47
4.2.3.2 IP Multicasting .......................................... 48
4.2.3.3 Path MTU Discovery ....................................... 48
4.2.3.4 Subnetting ............................................... 49
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ..................... 50
4.3.1 INTRODUCTION ............................................... 50
4.3.2 GENERAL ISSUES ............................................. 50
4.3.2.1 Unknown Message Types .................................... 50
4.3.2.2 ICMP Message TTL ......................................... 51
4.3.2.3 Original Message Header .................................. 51
4.3.2.4 ICMP Message Source Address .............................. 51
4.3.2.5 TOS and Precedence ....................................... 51
4.3.2.6 Source Route ............................................. 52
4.3.2.7 When Not to Send ICMP Errors ............................. 53
4.3.2.8 Rate Limiting ............................................ 54
4.3.3 SPECIFIC ISSUES ............................................ 55
4.3.3.1 Destination Unreachable .................................. 55
4.3.3.2 Redirect ................................................. 55
4.3.3.3 Source Quench ............................................ 56
4.3.3.4 Time Exceeded ............................................ 56
4.3.3.5 Parameter Problem ........................................ 57
4.3.3.6 Echo Request/Reply ....................................... 57
4.3.3.7 Information Request/Reply ................................ 58
4.3.3.8 Timestamp and Timestamp Reply ............................ 58
4.3.3.9 Address Mask Request/Reply ............................... 59
4.3.3.10 Router Advertisement and Solicitations .................. 61
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .................... 61
5. INTERNET LAYER - FORWARDING ................................... 62
5.1 INTRODUCTION ................................................. 62
5.2 FORWARDING WALK-THROUGH ...................................... 62
5.2.1 Forwarding Algorithm ....................................... 62
5.2.1.1 General .................................................. 63
5.2.1.2 Unicast .................................................. 64
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5.2.1.3 Multicast ................................................ 65
5.2.2 IP Header Validation ....................................... 66
5.2.3 Local Delivery Decision .................................... 68
5.2.4 Determining the Next Hop Address ........................... 70
5.2.4.1 Immediate Destination Address ............................ 71
5.2.4.2 Local/Remote Decision .................................... 71
5.2.4.3 Next Hop Address ......................................... 72
5.2.4.4 Administrative Preference ................................ 77
5.2.4.6 Load Splitting ........................................... 78
5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ................. 79
5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2 .......... 79
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 .............................................. 84
5.3.1 Time to Live (TTL) ......................................... 84
5.3.2 Type of Service (TOS) ...................................... 85
5.3.3 IP Precedence .............................................. 87
5.3.3.1 Precedence-Ordered Queue Service ......................... 88
5.3.3.2 Lower Layer Precedence Mappings .......................... 88
5.3.3.3 Precedence Handling For All Routers ...................... 89
5.3.4 Forwarding of Link Layer Broadcasts ........................ 92
5.3.5 Forwarding of Internet Layer Broadcasts .................... 92
5.3.5.1 Limited Broadcasts ....................................... 94
5.3.5.2 Net-directed Broadcasts .................................. 94
5.3.5.3 All-subnets-directed Broadcasts .......................... 95
5.3.5.4 Subnet-directed Broadcasts ............................... 97
5.3.6 Congestion Control ......................................... 97
5.3.7 Martian Address Filtering .................................. 99
5.3.8 Source Address Validation .................................. 99
5.3.9 Packet Filtering and Access Lists .......................... 100
5.3.10 Multicast Routing ......................................... 101
5.3.11 Controls on Forwarding .................................... 101
5.3.12 State Changes ............................................. 101
5.3.12.1 When a Router Ceases Forwarding ......................... 102
5.3.12.2 When a Router Starts Forwarding ......................... 102
5.3.12.3 When an Interface Fails or is Disabled .................. 103
5.3.12.4 When an Interface is Enabled ............................ 103
5.3.13 IP Options ................................................ 103
5.3.13.1 Unrecognized Options .................................... 103
5.3.13.2 Security Option ......................................... 104
5.3.13.3 Stream Identifier Option ................................ 104
5.3.13.4 Source Route Options .................................... 104
5.3.13.5 Record Route Option ..................................... 104
5.3.13.6 Timestamp Option ........................................ 105
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6. TRANSPORT LAYER ............................................... 106
6.1 USER DATAGRAM PROTOCOL - UDP ................................. 106
6.2 TRANSMISSION CONTROL PROTOCOL - TCP .......................... 106
7. APPLICATION LAYER - ROUTING PROTOCOLS ......................... 109
7.1 INTRODUCTION ................................................. 109
7.1.1 Routing Security Considerations ............................ 109
7.1.2 Precedence ................................................. 110
7.2 INTERIOR GATEWAY PROTOCOLS ................................... 110
7.2.1 INTRODUCTION ............................................... 110
7.2.2 OPEN SHORTEST PATH FIRST - OSPF ............................ 111
7.2.2.1 Introduction ............................................. 111
7.2.2.2 Specific Issues .......................................... 111
7.2.2.3 New Version of OSPF ...................................... 112
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS
.............................................................. 112
7.2.4 ROUTING INFORMATION PROTOCOL - RIP ......................... 113
7.2.4.1 Introduction ............................................. 113
7.2.4.2 Protocol Walk-Through .................................... 113
7.2.4.3 Specific Issues .......................................... 118
7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP .......................... 119
7.3 EXTERIOR GATEWAY PROTOCOLS ................................... 119
7.3.1 INTRODUCTION ............................................... 119
7.3.2 BORDER GATEWAY PROTOCOL - BGP .............................. 120
7.3.2.1 Introduction ............................................. 120
7.3.2.2 Protocol Walk-through .................................... 120
7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP ............................ 121
7.3.3.1 Introduction ............................................. 121
7.3.3.2 Protocol Walk-through .................................... 122
7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL .............. 124
7.4 STATIC ROUTING ............................................... 125
7.5 FILTERING OF ROUTING INFORMATION ............................. 127
7.5.1 Route Validation ........................................... 127
7.5.2 Basic Route Filtering ...................................... 127
7.5.3 Advanced Route Filtering ................................... 128
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE .................. 129
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS .............. 131
8.1 The Simple Network Management Protocol - SNMP ................ 131
8.1.1 SNMP Protocol Elements ..................................... 131
8.2 Community Table .............................................. 132
8.3 Standard MIBS ................................................ 133
8.4 Vendor Specific MIBS ......................................... 134
8.5 Saving Changes ............................................... 135
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ................... 137
9.1 BOOTP ........................................................ 137
9.1.1 Introduction ............................................... 137
9.1.2 BOOTP Relay Agents ......................................... 137
10. OPERATIONS AND MAINTENANCE ................................... 139
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10.1 Introduction ................................................ 139
10.2 Router Initialization ....................................... 140
10.2.1 Minimum Router Configuration .............................. 140
10.2.2 Address and Address Mask Initialization ................... 141
10.2.3 Network Booting using BOOTP and TFTP ...................... 142
10.3 Operation and Maintenance ................................... 143
10.3.1 Introduction .............................................. 143
10.3.2 Out Of Band Access ........................................ 144
10.3.2 Router O&M Functions ...................................... 144
10.3.2.1 Maintenance - Hardware Diagnosis ........................ 144
10.3.2.2 Control - Dumping and Rebooting ......................... 145
10.3.2.3 Control - Configuring the Router ........................ 145
10.3.2.4 Netbooting of System Software ........................... 146
10.3.2.5 Detecting and responding to misconfiguration ............ 146
10.3.2.6 Minimizing Disruption ................................... 147
10.3.2.7 Control - Troubleshooting Problems ...................... 148
10.4 Security Considerations ..................................... 149
10.4.1 Auditing and Audit Trails ................................. 149
10.4.2 Configuration Control ..................................... 150
11. REFERENCES ................................................... 152
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ................ 162
APPENDIX B. GLOSSARY ............................................. 164
APPENDIX C. FUTURE DIRECTIONS .................................... 169
APPENDIX D. Multicast Routing Protocols .......................... 172
D.1 Introduction ................................................. 172
D.2 Distance Vector Multicast Routing Protocol - DVMRP ........... 172
D.3 Multicast Extensions to OSPF - MOSPF ......................... 173
APPENDIX E Additional Next-Hop Selection Algorithms .............. 174
E.1. Some Historical Perspective .................................. 174
E.2. Additional Pruning Rules ..................................... 176
E.3 Some Route Lookup Algorithms ................................. 177
E.3.1 The Revised Classic Algorithm ............................... 178
E.3.2 The Variant Router Requirements Algorithm ................... 179
E.3.3 The OSPF Algorithm .......................................... 179
E.3.4 The Integrated IS-IS Algorithm .............................. 180
Security Considerations ........................................... 182
Acknowledgments ................................................... 183
Editor's Address .................................................. 186
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0. PREFACE
This document is a snapshot of the work of the Router Requirements
working group as of November 1991. At that time, the working group had
essentially finished its task. There were some final technical matters
to be nailed down, and a great deal of editing needed to be done in
order to get the document ready for publication. Unfortunately, these
tasks were never completed.
At the request of the Internet Area Director, the current editor took
the last draft of the document and, after consulting the mailing list
archives, meeting minutes, notes, and other members of the working
group, edited the document to its current form. This effort included
the following tasks: 1) Deleting all the parenthetical material (such as
editor's comments). Useful information was turned into DISCUSSION
sections, the rest was deleted. 2) Completing the tasks listed in the
last draft's To be Done sections. As a part of this task, a new "to be
done" list was developed and included as an appendix to the current
document. 3) Rolling Philip Almquist's "Ruminations on the Next Hop"
and "Ruminations on Route Leaking" into this document. These represent
significant work and should be kept. 4) Fulfilling the last intents of
the working group as determined from the archival material. The intent
of this effort was to get the document into a form suitable for
publication as an Historical RFC so that the significant work which went
into the creation of this document would be preserved.
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. Without his efforts, this document would not exist.
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1. INTRODUCTION
The goal of this work is to replace RFC-1009, Requirements for Internet
Gateways ([INTRO:1]) with a new document.
This memo is an intermediate step toward that goal. It defines and
discusses requirements for devices which 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 has to modify the IP header and to
strip off and replace the Link Layer framing.
The authors of this memo recognize, as should its readers, that many
routers support multiple protocol suites, and that 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 final version of 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 are required to implement forwarding
algorithms and Internet hosts do not require forwarding capabilities.
Any Internet host acting as a router must adhere to the requirements
contained in the final version of this memo.
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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 the final version of this document.
A good-faith implementation of the protocols produced after careful
reading of the RFCs, with some interaction with the Internet technical
community, and that follows good communications software engineering
practices, should differ from the requirements of this memo in only
minor ways. Thus, 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. However, 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.
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1.1 Reading this Document
1.1.1 Organization
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 between Chapter 7, which
discusses the protocols which routers use to exchange routing
information with each other, Chapter 8, which discusses network
management, and Chapter 9, which 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].
In general, 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
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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
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.
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, but 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.
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1.1.3 Compliance
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 of 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
of 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).
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 as long as the option has a default
setting, and that leaving the option at its default setting causes
the router to operate in a manner which conforms to the
requirement.
Likewise, routers may provide, except where explicitly prohibited
by this memo, options which cause them to violate MUST or MUST NOT
requirements. A router which provides such options is compliant
(either fully or conditionally) if and only if each such option
has a default setting which 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
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support costs of providing options which 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
There are several reference documents of interest in checking the
current 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 1610, [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, 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 (December, 1993) the current versions of these
documents are RFC 1122 and RFC 1123, (STD 3) [INTRO:2], and
[INTRO:3] respectively.
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.
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These and other Internet protocol documents may be obtained from
the:
The InterNIC
DS.INTERNIC.NET
InterNIC Directory and Database Service
+1 (800) 444-4345 or +1 (619) 445-4600
info@internic.net
1.3 General Considerations
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
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 and additional
internet-layer protocols are also constantly being devised.
Because routers play such a crucial role in the Internet, and
because the number of routers deployed in the Internet is much
smaller than the number of hosts, vendors should 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.
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1.3.2 Robustness Principle
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; sooner or later a packet will come in with
that particular combination of errors and attributes, and unless
the software is prepared, chaos can ensue. In general, 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 shows up. An undefined code might be
logged, but it must not cause a failure.
The second part of the principle 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
The Internet includes a great variety of systems, each
implementing many protocols and protocol layers, and some of these
contain bugs and misfeatures in their Internet protocol software.
As a result of complexity, diversity, and distribution of
function, the diagnosis of problems is often very difficult.
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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
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
in fact 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.
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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
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 which produce the same
results as these algorithms, but may be more efficient or less
general.
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We note that the art of efficient router implementation is outside of
the scope of this memo.
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2. INTERNET ARCHITECTURE
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
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 of 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).
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2.2 Elements of the Architecture
2.2.1 Protocol Layering
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.
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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, i.e., 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 its directly-connected network, a host must
implement the communication protocol used to interface to that
network. We call this a Link Layer layer protocol.
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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.
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
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. In general, a LAN will
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.
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2.2.3 Routers
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, but special-purpose hardware is becoming increasingly
common. This specification applies to routers regardless of how
they are implemented.
A router is connected to two or more networks, appearing to each
of these networks as a connected host. Thus, it has (at least)
one physical interface and (at least) one IP address on each of
the connected networks (this ignores the concept of un-numbered
links, which is discussed in section [2.2.7]). Forwarding an IP
datagram generally requires the router to choose the address of
the next-hop router or (for the final hop) the destination host.
This choice, called routing, depends upon a routing database
within the router. The routing database is also sometimes known
as a routing table or forwarding table.
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 which are
connected by bridges share the same IP network number, i.e., they
logically form a single IP network. These other devices are
outside of the scope of this document.
Another variation on the simple model of networks connected with
routers sometimes occurs: a set of routers may be interconnected
with only serial lines, to form a network in which the packet
switching is performed at the Internetwork (IP) Layer rather than
the Link Layer.
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2.2.4 Autonomous Systems
For technical, managerial, and sometimes political reasons, the
routers of the Internet system are grouped into collections called
autonomous systems. The routers included in a single autonomous
system (AS) are expected to:
o Be under the control of a single operations and maintenance
(O&M) organization;
o Employ common routing protocols among themselves, to
dynamically maintain their routing databases.
A number of different dynamic routing protocols have been
developed (see Section [7.2]); the routing protocol within a
single AS is generically called an interior gateway protocol or
IGP.
An IP datagram may have to traverse the routers of two or more ASs
to reach its destination, and the ASs must provide each other with
topology information to allow such forwarding. An exterior
gateway protocol (generally BGP or EGP) is used for this purpose.
2.2.5 Addresses and Subnets
An IP datagram carries 32-bit source and destination addresses,
each of which is partitioned into two parts - a constituent
network number and a host number on that network. Symbolically:
IP-address ::= { <Network-number>, <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 into the
physical address of a host connection to the constituent network.
This simple notion has been extended by the concept of subnets,
which were introduced in order to allow arbitrary complexity of
interconnected LAN structures within an organization, while
insulating the Internet system against explosive growth in network
numbers and routing complexity. Subnets essentially provide a
multi-level hierarchical routing structure for the Internet
system. The subnet extension, described in [INTERNET:2], is now 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 ::=
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{ <Network-number>, <Subnet-number>, <Host-number> }
The interconnected physical networks within an organization will
be given the same network number but different subnet numbers.
The distinction between the subnets of such a subnetted network is
normally not visible outside of that network. Thus, routing in
the rest of the Internet will be based only upon the <Network-
number> part of the IP destination address; routers outside the
network will combine <Subnet-number> and <Host-number> together to
form an uninterpreted rest part of the 32-bit IP address. Within
the subnetted network, the routers must route on the basis of an
extended network number:
{ <Network-number>, <Subnet-number> }
Under certain circumstances, it may be desirable to support
subnets of a particular network being interconnected only via a
path which is not part of the subnetted network. Even though many
IGP's and no EGP's currently support this configuration
effectively, routers need to be able to support this configuration
of subnetting (see Section [4.2.3.4]). In general, routers should
not make assumptions about what are subnets and what are not, but
simply ignore the concept of Class in networks, and treat each
route as a { network, mask }-tuple.
DISCUSSION:
It is becoming clear that as the Internet grows larger and
larger, the traditional uses of Class A, B, and C networks will
be modified in order to achieve better use of IP's 32-bit
address space. Classless Interdomain Routing (CIDR)
[INTERNET:15] is a method currently being deployed in the
Internet backbones to achieve this added efficiency. CIDR
depends on the ability of assigning and routing to networks
that are not based on Class A, B, or C networks. Thus, routers
should always treat a route as a network with a mask.
Furthermore, for similar reasons, a subnetted network need not
have a consistent subnet mask through all parts of the network.
For example, one subnet may use an 8 bit subnet mask, another 10
bit, and another 6 bit. Routers need to be able to support this
type of configuration (see Section [4.2.3.4]).
The bit positions containing this extended network number are
indicated by a 32-bit mask called the subnet mask; it is
recommended but not required that the <Subnet-number> bits be
contiguous and fall between the <Network-number> and the <Host-
number> fields. No subnet should be assigned the value zero or -1
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(all one bits).
Although the inventors of the subnet mechanism probably expected
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.
There are special considerations for the router when a connected
network provides a broadcast or multicast capability; these will
be discussed later.
2.2.6 IP Multicasting
IP multicasting is an extension of Link Layer multicast to IP
internets. Using IP multicasts, a single datagram can be
addressed to multiple hosts. 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.
In general, 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 and Subnets
Traditionally, each network interface on an IP host or router has
its own IP address. Over the years, people have observed that
this can cause inefficient use of the scarce IP address space,
since it forces allocation of an IP network number, or at least a
subnet number, to every point-to-point link.
To solve this problem, a number of people have proposed and
implemented the concept of unnumbered serial lines. An unnumbered
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serial line does not have any IP network or subnet number
associated with it. As a consequence, the network interfaces
connected to an unnumbered serial 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 that IP address will be on an IP (sub)net that
the router is connected to. That assumption is of course violated
if the only connection is an unnumbered serial line.
To get around these difficulties, two schemes have been invented.
The first scheme says that two routers connected by an unnumbered
serial line aren't really two routers at all, but rather two
half-routers which together make up a single (virtual) router.
The unnumbered serial 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 serial line, it is
not readily extensible to handle the case of a mesh of routers and
unnumbered serial 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 serial lines.
Because of these drawbacks, this memo has adopted an alternative
scheme, which has been invented multiple times but which is
probably originally attributable to Phil Karn. In this scheme, a
router which has unnumbered serial 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.
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2.2.8 Notable Oddities
2.2.8.1 Embedded Routers
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 which 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
internetting 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 must implement
ALL the relevant router requirements 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.
Since 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 have the ability to
maintain and update the router code (e.g., this might
require router code source).
(3) Once a host runs embedded router code, it becomes part of
the Internet system. 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, and
any embedded router code must be organized so that it can
be easily disabled.
(4) If a host running embedded router code is concurrently
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used for other services, the O&M (Operation and
Maintenance) requirements for the two modes of use may be
in serious conflict.
For example, router O&M will in many cases be performed
remotely by an operations center; this may require
privileged system access which the host administrator
would not normally want to distribute.
2.2.8.2 Transparent Routers
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
number 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. All of the 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
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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 which was powered off. 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 if it sent a datagram to a host that was
powered off and was connected to the ARPANET via the
transparent router instead of directly.
2.3 Router Characteristics
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 the network flow control and error indication,
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 various error conditions and generates ICMP error
and information messages as required.
o Drops datagrams whose time-to-live fields have reached zero.
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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 comprised 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
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These routers need routing algorithms which are highly dynamic and
also 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 56 Kbps,
DS1 (1.4Mbps), and DS3 (45Mbps) speeds. LANs are typically
Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
However, network media technology is constantly advancing and even
higher speeds are likely in the future. Full-duplex operation is
provided at all of these speeds.
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
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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
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 don't 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 effect flow
control 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
resource reservation and flows.
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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 spelled out 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.
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3. LINK LAYER
Although [INTRO:1] covers Link Layer standards (IP over foo, 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
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 chapter 3 of [INTRO:1].
3.2 LINK/INTERNET LAYER INTERFACE
Although this document does not attempt to specify the interface
between the Link Layer and the upper layers, it is worth noting here
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:
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(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
3.3.1 Trailer Encapsulation
Routers which can connect to 10Mb 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 section 2.3.1 of [INTRO:2], that
the immediate destination of the packet is willing and able to
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accept trailer-encapsulated packets. A router SHOULD NOT agree
(using these same mechanisms) to accept trailer-encapsulated
packets.
3.3.2 Address Resolution Protocol - ARP
Routers which implement ARP MUST be compliant and SHOULD be
unconditionally compliant with the requirements in section 2.3.2
of [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.
A router MUST not believe any ARP reply which 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
Routers which can connect to 10Mb Ethernets MUST be compliant and
SHOULD be unconditionally compliant with the requirements of
Section [2.3.3] of [INTRO:2].
3.3.4 Maximum Transmission Unit - MTU
The MTU of each logical interface MUST be configurable.
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 hosts which are in the process of
initializing themselves, or which have been misconfigured.
In general, the Robustness Principle indicates that these
packets should be successfully received, if at all possible.
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3.3.5 Point-to-Point Protocol - PPP
Contrary to [INTRO:1], the Internet does have a standard serial
line protocol: the Point-to-Point Protocol (PPP), defined in
[LINK:2], [LINK:3], [LINK:4], and [LINK:5].
A serial line interface is any interface which is designed to send
data over a telephone, leased, dedicated or direct line (either 2
or 4 wire) using a standardized modem or bit serial interface
(such as RS-232, RS-449 or V.35), using either synchronous or
asynchronous clocking.
A general purpose serial interface is a serial line interface
which is not solely for use as an access line to a network for
which an alternative IP link layer specification exists (such as
X.25 or Frame Relay).
Routers which contain such 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
serial line protocols other than PPP, all general purpose serial
interfaces MUST default to using PPP.
3.3.5.1 Introduction
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.
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3.3.5.2 Link Control Protocol (LCP) Options
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 do address/control field compression on either
synchronous or asynchronous links. A router MAY do protocol
field compression on either synchronous or asynchronous links.
A router MAY indicate that it can accept these compressions,
but MUST be able to accept uncompressed PPP header information
even if it has indicated a willingness to receive compressed
PPP headers.
DISCUSSION:
These options control the appearance of the PPP header.
Normally the PPP header consists of the address field (one
byte containing the value 0xff), the control field (one byte
containing the value 0x03), and the two-byte protocol field
that identifies 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. The protocol field may also be
compressed from two to one byte in most cases.
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 Async 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
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the option and then ignore it.
DISCUSSION:
There are implementations that offer both sync and async
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 (ICP) Options
A router MAY offer to perform IP address negotiation. A router
MUST accept a refusal (REJect) to perform IP address
negotiation from the peer.
A router SHOULD NOT perform Van Jacobson header compression of
TCP/IP packets if the link speed is in excess of 64 Kbps.
Below that speed the router MAY perform Van Jacobson (VJ)
header compression. At link speeds of 19,200 bps or less the
router SHOULD perform VJ header compression.
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3.3.6 Interface Testing
A router MUST have a mechanism to allow routing software to
determine whether a physical interface is available to send
packets or not. 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, since failure to do so (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 also in some cases on the interface
hardware chosen by the router manufacturer. The intent is to
maximize the capability to detect failures within the Link-
Layer constraints.
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4. INTERNET LAYER - PROTOCOLS
4.1 INTRODUCTION
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 which directly relate to forwarding.
The current chapter contains the remainder of the discussion of the
Internet Layer protocols.
4.2 INTERNET PROTOCOL - IP
4.2.1 INTRODUCTION
Routers MUST implement the IP protocol, as defined by
[INTERNET:1]. They MUST also implement its mandatory extensions:
subnets (defined in [INTERNET:2]), and IP broadcast (defined in
[INTERNET:3]).
A router MUST be compliant, and SHOULD be unconditionally
compliant, with the requirements of sections 3.2.1 and 3.3 of
[INTRO:2], except that:
o Section 3.2.1.1 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.2 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.3 should be ignored, since it is superseded by
Section [4.2.2.11] of this memo.
o Section 3.2.1.4 may be ignored, since it duplicates
requirements found in this memo.
o Section 3.2.1.6 should be ignored, since it is superseded by
Section [4.2.2.4] of this memo.
o Section 3.2.1.8 should be ignored, since it is superseded by
Section [4.2.2.1] of this memo.
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
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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 record the event in a statistics counter.
4.2.2 PROTOCOL WALK-THROUGH
RFC 791 is [INTERNET:1], the specification for the Internet
Protocol.
4.2.2.1 Options: RFC-791 Section 3.2
In datagrams received by the router itself, the IP layer MUST
interpret those 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
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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 (i.e., the pointer points beyond
the last field and the destination address in the IP
header addresses the router), the packet has reached its
final destination; the option as received (the recorded
route) MUST be passed up to the transport layer (or to
ICMP message processing).
In order to respond correctly to source-routed datagrams
it receives, a router MUST provide a means whereby
transport protocols and applications can reverse the
source route in a received datagram and insert the
reversed source route into datagrams they originate (see
Section 4 of [INTRO:2] for details).
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, 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
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(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
timestamp 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.
o A timestamp value MUST follow the rules given in
Section [3.2.2.8] of [INTRO:2].
IMPLEMENTATION:
To maximize the utility of the timestamps contained in
the timestamp option, it is suggested that the
timestamp inserted 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
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transmission.
4.2.2.2 Addresses in Options: RFC-791 Section 3.1
When a router inserts its address into a Record Route, Strict
Source and Record Route, Loose Source and Record Route, or
Timestamp, 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. Which of the router's addresses is used as the
router-id MUST NOT change (even across reboots) unless changed
by the network manager or unless the configuration of the
router is changed 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 which 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
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.
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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
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
As stated in Section [5.2.2], a router MUST verify the IP
checksum of any packet which is received. The router MUST NOT
provide a means to disable this checksum verification.
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.
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DISCUSSION:
All future IP options will include an explicit length.
4.2.2.7 Fragmentation: RFC-791 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 MUST send the fragments in order. A fragmentation method
which may generate one IP fragment which 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 hopefully 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 per page 26 of [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. In general, this allows 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 (such as an 802.5 network
with a MTU of 2048 or an Ethernet network with an MTU of
1536).
One other fragmentation technique discussed was splitting
the IP datagram into approximately equal sized IP fragments,
with the size being smaller than the next hop network's MTU.
This is intended to minimize the number of fragments that
would result from additional fragmentation further down the
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path.
In most cases, routers should try and create situations that
will generate the lowest number of IP fragments possible.
Work with slow machines leads us to believe that if it is
necessary to send small packets in a fragmentation scheme,
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
As specified in Section 3.3.2 of [INTRO:2], a router MUST
support reassembly of datagrams which it delivers to itself.
4.2.2.9 Time to Live: RFC-791 Section 3.2
Time to Live (TTL) handling for packets originated or received
by the router is governed by [INTRO:2]. Note in particular
that a router MUST NOT check the TTL of a packet except when
forwarding it.
4.2.2.10 Multi-subnet Broadcasts: RFC-922
All-subnets broadcasts (called multi-subnet broadcasts in
[INTERNET:3]) have been deprecated. See Section [5.3.5.3].
4.2.2.11 Addressing: RFC-791 Section 3.2
There are now five classes of IP addresses: Class A through
Class E. Class D addresses are used for IP multicasting
[INTERNET:4], while Class E addresses are reserved for
experimental use.
A multicast (Class D) address is a 28-bit logical address that
stands for a group of hosts, and may be either permanent or
transient. Permanent multicast addresses are allocated by the
Internet Assigned Number Authority [INTRO:7], while transient
addresses may be allocated dynamically to transient groups.
Group membership is determined dynamically using IGMP
[INTERNET:4].
We now summarize the important special cases for Unicast (that
is class A, B, and C) IP addresses, using the following
notation for an IP address:
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{ <Network-number>, <Host-number> }
or
{ <Network-number>, <Subnet-number>, <Host-number> }
and the notation -1 for a field that contains all 1 bits and
the notation 0 for a field that contains all 0 bits. This
notation is not intended to imply that the 1-bits in a subnet
mask need be contiguous.
(a) { 0, 0 }
This host on this network. It MUST NOT be used as a
source address by routers, except the router MAY use this
as a source address as part of an initialization procedure
(e.g., if the router is using BOOTP to load its
configuration information).
Incoming datagrams with a source address of { 0, 0 } which
are received for local delivery (see Section [5.2.3]),
MUST be accepted if the router implements the associated
protocol and that protocol clearly defines appropriate
action to be taken. Otherwise, a router MUST silently
discard any locally-delivered datagram whose source
address is { 0, 0 }.
DISCUSSION:
Some protocols define specific actions to take in
response to a received datagram whose source address is
{ 0, 0 }. Two examples are BOOTP and ICMP Mask
Request. The proper operation of these protocols often
depends on the ability to receive datagrams whose
source address is { 0, 0 }. For most protocols,
however, it is best to ignore datagrams having a source
address of { 0, 0 } since they were probably generated
by a misconfigured host or router. Thus, if a router
knows how to deal with a given datagram having a { 0, 0
} source address, the router MUST accept it.
Otherwise, the router MUST discard it.
See also Section [4.2.3.1] for a non-standard use of { 0,
0 }.
(b) { 0, <Host-number> }
Specified host on this network. It MUST NOT be sent by
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routers except that the router MAY uses this as a source
address as part of an initialization procedure by which
the it learns its own IP address.
(c) { -1, -1 }
Limited broadcast. It MUST NOT be used as a source
address.
A datagram with this destination address will be received
by every host and router on the connected physical
network, but will not be forwarded outside that network.
(d) { <Network-number>, -1 }
Network Directed Broadcast - a broadcast directed to the
specified network. It MUST NOT be used as a source
address. A router MAY originate Network Directed
Broadcast packets. A router MUST receive Network Directed
Broadcast packets; however a router MAY have a
configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.
(e) { <Network-number>, <Subnet-number>, -1 }
Subnetwork Directed Broadcast - a broadcast sent to the
specified subnet. It MUST NOT be used as a source
address. A router MAY originate Network Directed
Broadcast packets. A router MUST receive Network Directed
Broadcast packets; however a router MAY have a
configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.
(f) { <Network-number>, -1, -1 }
All Subnets Directed Broadcast - a broadcast sent to all
subnets of the specified subnetted network. It MUST NOT
be used as a source address. A router MAY originate
Network Directed Broadcast packets. A router MUST receive
Network Directed Broadcast packets; however a router MAY
have a configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.
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(g) { 127, <any> }
Internal host loopback address. Addresses of this form
MUST NOT appear outside a host.
The <Network-number> is administratively assigned so that its
value will be unique in the entire world.
IP addresses are not permitted to have the value 0 or -1 for
any of the <Host-number>, <Network-number>, or <Subnet-number>
fields (except in the special cases listed above). This
implies that each of these fields will be at least two bits
long.
For further discussion of broadcast addresses, see Section
[4.2.3.1].
Since (as described in Section [4.2.1]) a router must support
the subnet extensions to IP, there will be a subnet mask of the
form: { -1, -1, 0 } associated with each of the host's local IP
addresses; see Sections [4.3.3.9], [5.2.4.2], and [10.2.2].
When a router originates any datagram, the IP source address
MUST be one of its own IP addresses (but not a broadcast or
multicast address). The only exception is during
initialization.
For most purposes, a datagram addressed to a broadcast or
multicast destination is processed as if it had been addressed
to one of the router's IP addresses; that is to say:
o A router MUST receive and process normally any packets with
a broadcast destination address.
o A router MUST receive and process normally any packets sent
to a multicast destination address which the router is
interested in.
The term specific-destination address means the equivalent
local IP address of the host. The specific-destination address
is defined to be the destination address in the IP header
unless the header contains a broadcast or multicast address, in
which case the specific-destination is an IP address assigned
to the physical interface on which the datagram arrived.
A router MUST silently discard any received datagram containing
an IP source address that is invalid by the rules of this
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section. This validation could be done either by the IP layer
or by each protocol in the transport layer.
DISCUSSION:
A misaddressed datagram might be caused by a Link Layer
broadcast of a unicast datagram or by another router or host
that is confused or misconfigured.
4.2.3 SPECIFIC ISSUES
4.2.3.1 IP Broadcast Addresses
For historical reasons, there are a number of IP addresses
(some standard and some not) which are used to indicate that an
IP packet is an IP broadcast. A router
(1) MUST treat as IP broadcasts packets addressed to
255.255.255.255, { <Network-number>, -1 }, { <Network-
number>, <Subnet-number>, -1 }, and { <Network-number>,
-1, -1 }.
(2) SHOULD silently discard on receipt (i.e., don't even
deliver to applications in the router) any packet
addressed to 0.0.0.0, { <Network-number>, 0 }, {
<Network-number>, <Subnet-number>, 0 }, or { <Network-
number>, 0, 0 }; if these packets are not silently
discarded, they MUST be treated as IP broadcasts (see
Section [5.3.5]). There MAY be a configuration option to
allow receipt of these packets. This option SHOULD
default to discarding them.
(3) SHOULD (by default) use the limited broadcast address
(255.255.255.255) when originating an IP broadcast
destined for a connected network or subnet (except when
sending an ICMP Address Mask Reply, as discussed in
Section [4.3.3.9]). A router MUST receive limited
broadcasts.
(4) SHOULD NOT originate datagrams addressed to 0.0.0.0, {
<Network-number>, 0 }, { <Network-number>, <Subnet-
number>, 0 }, or { <Network-number>, 0, 0 }. There MAY be
a configuration option to allow generation of these
packets (instead of using the relevant 1s format
broadcast). This option SHOULD default to not generating
them.
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DISCUSSION:
In the second bullet, the router obviously cannot recognize
addresses of the form { <Network-number>, <Subnet-number>, 0
} if the router does not know how the particular network is
subnetted. In that case, the rules of the second bullet do
not apply because, from the point of view of the router, the
packet is not an IP broadcast packet.
4.2.3.2 IP Multicasting
An IP router SHOULD satisfy the Host Requirements with respect
to IP multicasting, as specified in Section 3.3.7 of [INTRO:2].
An IP router SHOULD support local IP multicasting on all
connected networks for which a mapping from Class D IP
addresses to link-layer addresses has been specified (see the
