Network Working Group F. Baker, Editor
Request for Comments: 1812 Cisco Systems
Obsoletes: 1716, 1009 June 1995
Category: Standards Track
Requirements for IP Version 4 Routers
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document is an updated version of RFC 1716, the historical
Router Requirements document. That RFC preserved the significant
work that went into the working group, but failed to adequately
describe current technology for the IESG to consider it a current
The current editor had been asked to bring the document up to date,
so that it is useful as a procurement specification and a guide to
implementors. In this, he stands squarely on the shoulders of those
who have gone before him, and depends largely on expert contributors
for text. Any credit is theirs; the errors are his.
The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author:
Philip Almquist. It is also largely due to the efforts of its
previous editor, Frank Kastenholz. Without their efforts, this
document would not exist.
Table of Contents
1. INTRODUCTION ........................................ 61.1 Reading this Document .............................. 81.1.1 Organization ..................................... 81.1.2 Requirements ..................................... 91.1.3 Compliance ....................................... 101.2 Relationships to Other Standards ................... 111.3 General Considerations ............................. 121.3.1 Continuing Internet Evolution .................... 121.3.2 Robustness Principle ............................. 131.3.3 Error Logging .................................... 14
APPENDIX B. GLOSSARY ................................... 146APPENDIX C. FUTURE DIRECTIONS .......................... 152APPENDIX D. Multicast Routing Protocols ................ 154D.1 Introduction ....................................... 154D.2 Distance Vector Multicast Routing Protocol -
DVMRP .............................................. 154D.3 Multicast Extensions to OSPF - MOSPF ............... 154D.4 Protocol Independent Multicast - PIM ............... 155APPENDIX E Additional Next-Hop Selection Algorithms
................................................... 155E.1. Some Historical Perspective ....................... 155E.2. Additional Pruning Rules .......................... 157E.3 Some Route Lookup Algorithms ....................... 159E.3.1 The Revised Classic Algorithm .................... 159E.3.2 The Variant Router Requirements Algorithm ........ 160E.3.3 The OSPF Algorithm ............................... 160E.3.4 The Integrated IS-IS Algorithm ................... 162
Security Considerations ................................ 163APPENDIX F: HISTORICAL ROUTING PROTOCOLS ............... 164F.1 EXTERIOR GATEWAY PROTOCOL - EGP .................... 164F.1.1 Introduction ..................................... 164F.1.2 Protocol Walk-through ............................ 165F.2 ROUTING INFORMATION PROTOCOL - RIP ................. 167F.2.1 Introduction ..................................... 167F.2.2 Protocol Walk-Through ............................ 167F.2.3 Specific Issues .................................. 172F.3 GATEWAY TO GATEWAY PROTOCOL - GGP .................. 173
Acknowledgments ........................................ 173
Editor's Address ....................................... 1751. INTRODUCTION
This memo replaces for RFC 1716, "Requirements for Internet Gateways"
This memo defines and discusses requirements for devices that perform
the network layer forwarding function of the Internet protocol suite.
The Internet community usually refers to such devices as IP routers or
simply routers; The OSI community refers to such devices as
intermediate systems. Many older Internet documents refer to these
devices as gateways, a name which more recently has largely passed out
of favor to avoid confusion with application gateways.
An IP router can be distinguished from other sorts of packet switching
devices in that a router examines the IP protocol header as part of
the switching process. It generally removes the Link Layer header a
message was received with, modifies the IP header, and replaces the
Link Layer header for retransmission.
The authors of this memo recognize, as should its readers, that many
routers support more than one protocol. Support for multiple protocol
suites will be required in increasingly large parts of the Internet in
the future. This memo, however, does not attempt to specify Internet
requirements for protocol suites other than TCP/IP.
This document enumerates standard protocols that a router connected to
the Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols. It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.
For each protocol, this memo also contains an explicit set of
requirements, recommendations, and options. The reader must
understand that the list of requirements in this memo is incomplete by
itself. The complete set of requirements for an Internet protocol
router is primarily defined in the standard protocol specification
documents, with the corrections, amendments, and supplements contained
in this memo.
This memo should be read in conjunction with the Requirements for
Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
routers must both be capable of originating IP datagrams and receiving
IP datagrams destined for them. The major distinction between
Internet hosts and routers is that routers implement forwarding
algorithms, while Internet hosts do not require forwarding
capabilities. Any Internet host acting as a router must adhere to the
requirements contained in this memo.
The goal of open system interconnection dictates that routers must
function correctly as Internet hosts when necessary. To achieve this,
this memo provides guidelines for such instances. For simplification
and ease of document updates, this memo tries to avoid overlapping
discussions of host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by
reference. In some cases the requirements stated in [INTRO:2] and
[INTRO:3] are superseded by this document.
A good-faith implementation of the protocols produced after careful
reading of the RFCs should differ from the requirements of this memo
in only minor ways. Producing such an implementation often requires
some interaction with the Internet technical community, and must
follow good communications software engineering practices. In many
cases, the requirements in this document are already stated or implied
in the standard protocol documents, so that their inclusion here is,
in a sense, redundant. They were included because some past
implementation has made the wrong choice, causing problems of
interoperability, performance, and/or robustness.
This memo includes discussion and explanation of many of the
requirements and recommendations. A simple list of requirements would
be dangerous, because:
o Some required features are more important than others, and some
features are optional.
o Some features are critical in some applications of routers but
irrelevant in others.
o There may be valid reasons why particular vendor products that are
designed for restricted contexts might choose to use different
However, the specifications of this memo must be followed to meet the
general goal of arbitrary router interoperation across the diversity
and complexity of the Internet. Although most current implementations
fail to meet these requirements in various ways, some minor and some
major, this specification is the ideal towards which we need to move.
These requirements are based on the current level of Internet
architecture. This memo will be updated as required to provide
additional clarifications or to include additional information in
those areas in which specifications are still evolving.
1.1 Reading this Document
This memo emulates the layered organization used by [INTRO:2] and
[INTRO:3]. Thus, Chapter 2 describes the layers found in the Internet
architecture. Chapter 3 covers the Link Layer. Chapters 4 and 5 are
concerned with the Internet Layer protocols and forwarding algorithms.
Chapter 6 covers the Transport Layer. Upper layer protocols are
divided among Chapters 7, 8, and 9. Chapter 7 discusses the protocols
which routers use to exchange routing information with each other.
Chapter 8 discusses network management. Chapter 9 discusses other
upper layer protocols. The final chapter covers operations and
maintenance features. This organization was chosen for simplicity,
clarity, and consistency with the Host Requirements RFCs. Appendices
to this memo include a bibliography, a glossary, and some conjectures
about future directions of router standards.
In describing the requirements, we assume that an implementation
strictly mirrors the layering of the protocols. However, strict
layering is an imperfect model, both for the protocol suite and for
recommended implementation approaches. Protocols in different layers
interact in complex and sometimes subtle ways, and particular
functions often involve multiple layers. There are many design
choices in an implementation, many of which involve creative breaking
of strict layering. Every implementor is urged to read [INTRO:4] and
Each major section of this memo is organized into the following
(2) Protocol Walk-Through - considers the protocol specification
documents section-by-section, correcting errors, stating
requirements that may be ambiguous or ill-defined, and providing
further clarification or explanation.
(3) Specific Issues - discusses protocol design and implementation
issues that were not included in the walk-through.
Under many of the individual topics in this memo, there is
parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
material is intended to give a justification, clarification or
explanation to the preceding requirements text. The implementation
material contains suggested approaches that an implementor may want to
consider. The DISCUSSION and IMPLEMENTATION sections are not part of
In this memo, the words that are used to define the significance of
each particular requirement are capitalized. These words are:
This word means that the item is an absolute requirement of the
specification. Violation of such a requirement is a fundamental
error; there is no case where it is justified.
o MUST IMPLEMENT
This phrase means that this specification requires that the item be
implemented, but does not require that it be enabled by default.
o MUST NOT
This phrase means that the item is an absolute prohibition of the
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
o SHOULD IMPLEMENT
This phrase is similar in meaning to SHOULD, but is used when we
recommend that a particular feature be provided but does not
necessarily recommend that it be enabled by default.
o SHOULD NOT
This phrase means that there may exist valid reasons in particular
circumstances when the described behavior is acceptable or even
useful. Even so, the full implications should be understood and
the case carefully weighed before implementing any behavior
described with this label.
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.
Some requirements are applicable to all routers. Other requirements
are applicable only to those which implement particular features or
protocols. In the following paragraphs, relevant refers to the union
of the requirements applicable to all routers and the set of
requirements applicable to a particular router because of the set of
features and protocols it has implemented.
Note that not all Relevant requirements are stated directly in this
memo. Various parts of this memo incorporate by reference sections of
the Host Requirements specification, [INTRO:2] and [INTRO:3]. For
purposes of determining compliance with this memo, it does not matter
whether a Relevant requirement is stated directly in this memo or
merely incorporated by reference from one of those documents.
An implementation is said to be conditionally compliant if it
satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
requirements. An implementation is said to be unconditionally
compliant if it is conditionally compliant and also satisfies all the
Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An
implementation is not compliant if it is not conditionally compliant
(i.e., it fails to satisfy one or more of the Relevant MUST, MUST
IMPLEMENT, or MUST NOT requirements).
This specification occasionally indicates that an implementation
SHOULD implement a management variable, and that it SHOULD have a
certain default value. An unconditionally compliant implementation
implements the default behavior, and if there are other implemented
behaviors implements the variable. A conditionally compliant
implementation clearly documents what the default setting of the
variable is or, in the absence of the implementation of a variable,
may be construed to be. An implementation that both fails to
implement the variable and chooses a different behavior is not
For any of the SHOULD and SHOULD NOT requirements, a router may
provide a configuration option that will cause the router to act other
than as specified by the requirement. Having such a configuration
option does not void a router's claim to unconditional compliance if
the option has a default setting, and that setting causes the router
to operate in the required manner.
Likewise, routers may provide, except where explicitly prohibited by
this memo, options which cause them to violate MUST or MUST NOT
requirements. A router that provides such options is compliant
(either fully or conditionally) if and only if each such option has a
default setting that causes the router to conform to the requirements
of this memo. Please note that the authors of this memo, although
aware of market realities, strongly recommend against provision of
such options. Requirements are labeled MUST or MUST NOT because
experts in the field have judged them to be particularly important to
interoperability or proper functioning in the Internet. Vendors
should weigh carefully the customer support costs of providing options
that violate those rules.
Of course, this memo is not a complete specification of an IP router,
but rather is closer to what in the OSI world is called a profile.
For example, this memo requires that a number of protocols be
implemented. Although most of the contents of their protocol
specifications are not repeated in this memo, implementors are
nonetheless required to implement the protocols according to those
1.2 Relationships to Other Standards
There are several reference documents of interest in checking the
status of protocol specifications and standardization:
o INTERNET OFFICIAL PROTOCOL STANDARDS
This document describes the Internet standards process and lists
the standards status of the protocols. As of this writing, the
current version of this document is STD 1, RFC 1780, [ARCH:7].
This document is periodically re-issued. You should always
consult an RFC repository and use the latest version of this
o Assigned Numbers
This document lists the assigned values of the parameters used in
the various protocols. For example, it lists IP protocol codes,
TCP port numbers, Telnet Option Codes, ARP hardware types, and
Terminal Type names. As of this writing, the current version of
this document is STD 2, RFC 1700, [INTRO:7]. This document is
periodically re-issued. You should always consult an RFC
repository and use the latest version of this document.
o Host Requirements
This pair of documents reviews the specifications that apply to
hosts and supplies guidance and clarification for any
ambiguities. Note that these requirements also apply to routers,
except where otherwise specified in this memo. As of this
writing, the current versions of these documents are RFC 1122 and
RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].
o Router Requirements (formerly Gateway Requirements)
Note that these documents are revised and updated at different times;
in case of differences between these documents, the most recent must
These and other Internet protocol documents may be obtained from the:
InterNIC Directory and Database Service
URL: http://ds.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 internet layer protocols, and
modifications to existing protocols, are also constantly being
devised. Routers play a crucial role in the Internet, and the number
of routers deployed in the Internet is much smaller than the number
of hosts. Vendors should therefore expect that router standards will
continue to evolve much more quickly than host standards. These
changes will be carefully planned and controlled since there is
extensive participation in this planning by the vendors and by the
organizations responsible for operation of the networks.
Development, evolution, and revision are characteristic of computer
network protocols today, and this situation will persist for some
years. A vendor who develops computer communications software for
the Internet protocol suite (or any other protocol suite!) and then
fails to maintain and update that software for changing
specifications is going to leave a trail of unhappy customers. The
Internet is a large communication network, and the users are in
constant contact through it. Experience has shown that knowledge of
deficiencies in vendor software propagates quickly through the
Internet technical community.
1.3.2 Robustness Principle
At every layer of the protocols, there is a general rule (from
[TRANS:2] by Jon Postel) whose application can lead to enormous
benefits in robustness and interoperability:
Be conservative in what you do,
be liberal in what you accept from others.
Software should be written to deal with every conceivable error, no
matter how unlikely. Eventually a packet will come in with that
particular combination of errors and attributes, and unless the
software is prepared, chaos can ensue. It is best to assume that the
network is filled with malevolent entities that will send packets
designed to have the worst possible effect. This assumption will
lead to suitably protective design. The most serious problems in the
Internet have been caused by unforeseen mechanisms triggered by low
probability events; mere human malice would never have taken so
devious a course!
Adaptability to change must be designed into all levels of router
software. As a simple example, consider a protocol specification
that contains an enumeration of values for a particular header field
- e.g., a type field, a port number, or an error code; this
enumeration must be assumed to be incomplete. If the protocol
specification defines four possible error codes, the software must
not break when a fifth code is defined. An undefined code might be
logged, but it must not cause a failure.
The second part of the principal is almost as important: software on
hosts or other routers may contain deficiencies that make it unwise
to exploit legal but obscure protocol features. It is unwise to
stray far from the obvious and simple, lest untoward effects result
elsewhere. A corollary of this is watch out for misbehaving hosts;
router software should be prepared to survive in the presence of
misbehaving hosts. An important function of routers in the Internet
is to limit the amount of disruption such hosts can inflict on the
shared communication facility.
1.3.3 Error Logging
The Internet includes a great variety of systems, each implementing
many protocols and protocol layers, and some of these contain bugs
and misguided features in their Internet protocol software. As a
result of complexity, diversity, and distribution of function, the
diagnosis of problems is often very difficult.
Problem diagnosis will be aided if routers include a carefully
designed facility for logging erroneous or strange events. It is
important to include as much diagnostic information as possible when
an error is logged. In particular, it is often useful to record the
header(s) of a packet that caused an error. However, care must be
taken to ensure that error logging does not consume prohibitive
amounts of resources or otherwise interfere with the operation of the
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].
In an ideal world, routers would be easy to configure, and perhaps
even entirely self-configuring. However, practical experience in the
real world suggests that this is an impossible goal, and that many
attempts by vendors to make configuration easy actually cause
customers more grief than they prevent. As an extreme example, a
router designed to come up and start routing packets without
requiring any configuration information at all would almost certainly
choose some incorrect parameter, possibly causing serious problems on
any networks unfortunate enough to be connected to it.
Often this memo requires that a parameter be a configurable option.
There are several reasons for this. In a few cases there currently
is some uncertainty or disagreement about the best value and it may
be necessary to update the recommended value in the future. In other
cases, the value really depends on external factors - e.g., the
distribution of its communication load, or the speeds and topology of
nearby networks - and self-tuning algorithms are unavailable and may
be insufficient. In some cases, configurability is needed because of
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
In several places in this memo, specific algorithms that a router
ought to follow are specified. These algorithms are not, per se,
required of the router. A router need not implement each algorithm
as it is written in this document. Rather, an implementation must
present a behavior to the external world that is the same as a
strict, literal, implementation of the specified algorithm.
Algorithms are described in a manner that differs from the way a good
implementor would implement them. For expository purposes, a style
that emphasizes conciseness, clarity, and independence from
implementation details has been chosen. A good implementor will
choose algorithms and implementation methods that produce the same
results as these algorithms, but may be more efficient or less
We note that the art of efficient router implementation is outside
the scope of this memo.