Network Working Group F. Baker, Editor
Request for Comments: 1812 Cisco Systems
Obsoletes: 1716, 1009 June 1995
Category: Standards Track
Requirements for IP Version 4 Routers
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
PREFACE
This document is an updated version of RFC1716, the historical
Router Requirements document. That RFCpreserved the significant
work that went into the working group, but failed to adequately
describe current technology for the IESG to consider it a current
standard.
The current editor had been asked to bring the document up to date,
so that it is useful as a procurement specification and a guide to
implementors. In this, he stands squarely on the shoulders of those
who have gone before him, and depends largely on eXPert contributors
for text. Any credit is theirs; the errors are his.
The content and form of this document are due, in large part, to the
working group"s chair, and document"s original editor and author:
Philip Almquist. It is also largely due to the efforts of its
previous editor, Frank Kastenholz. Without their efforts, this
document would not exist.
Table of Contents
1. INTRODUCTION ........................................ 6
1.1 Reading this Document .............................. 8
1.1.1 Organization ..................................... 8
1.1.2 Requirements ..................................... 9
1.1.3 Compliance ....................................... 10
1.2 Relationships to Other Standards ................... 11
1.3 General Considerations ............................. 12
1.3.1 Continuing Internet Evolution .................... 12
1.3.2 Robustness Principle ............................. 13
1.3.3 Error Logging .................................... 14
1.3.4 Configuration .................................... 14
1.4 Algorithms ......................................... 16
2. INTERNET ARCHITECTURE ............................... 16
2.1 Introduction ....................................... 16
2.2 Elements of the Architecture ....................... 17
2.2.1 Protocol Layering ................................ 17
2.2.2 Networks ......................................... 19
2.2.3 Routers .......................................... 20
2.2.4 Autonomous Systems ............................... 21
2.2.5 Addressing Architecture .......................... 21
2.2.5.1 Classical IP Addressing Architecture ........... 21
2.2.5.2 Classless Inter Domain Routing (CIDR) .......... 23
2.2.6 IP Multicasting .................................. 24
2.2.7 Unnumbered Lines and Networks Prefixes ........... 25
2.2.8 Notable Oddities ................................. 26
2.2.8.1 Embedded Routers ............................... 26
2.2.8.2 Transparent Routers ............................ 27
2.3 Router Characteristics ............................. 28
2.4 Architectural Assumptions .......................... 31
3. LINK LAYER .......................................... 32
3.1 INTRODUCTION ....................................... 32
3.2 LINK/INTERNET LAYER INTERFACE ...................... 33
3.3 SPECIFIC ISSUES .................................... 34
3.3.1 Trailer Encapsulation ............................ 34
3.3.2 Address Resolution Protocol - ARP ................ 34
3.3.3 Ethernet and 802.3 Coexistence ................... 35
3.3.4 Maximum Transmission Unit - MTU .................. 35
3.3.5 Point-to-Point Protocol - PPP .................... 35
3.3.5.1 Introduction ................................... 36
3.3.5.2 Link Control Protocol (LCP) Options ............ 36
3.3.5.3 IP Control Protocol (IPCP) Options ............. 38
3.3.6 Interface Testing ................................ 38
4. INTERNET LAYER - PROTOCOLS .......................... 39
4.1 INTRODUCTION ....................................... 39
4.2 INTERNET PROTOCOL - IP ............................. 39
4.2.1 INTRODUCTION ..................................... 39
4.2.2 PROTOCOL WALK-THROUGH ............................ 40
4.2.2.1 Options: RFC791 Section 3.2 ................... 40
4.2.2.2 Addresses in Options: RFC791 Section 3.1 ...... 42
4.2.2.3 Unused IP Header Bits: RFC791 Section 3.1 ..... 43
4.2.2.4 Type of Service: RFC791 Section 3.1 ........... 44
4.2.2.5 Header Checksum: RFC791 Section 3.1 ........... 44
4.2.2.6 Unrecognized Header Options: RFC791,
Section 3.1 .................................... 44
4.2.2.7 Fragmentation: RFC791 Section 3.2 ............. 45
4.2.2.8 Reassembly: RFC791 Section 3.2 ................ 46
4.2.2.9 Time to Live: RFC791 Section 3.2 .............. 46
4.2.2.10 Multi-subnet Broadcasts: RFC922 .............. 47
4.2.2.11 Addressing: RFC791 Section 3.2 ............... 47
4.2.3 SPECIFIC ISSUES .................................. 50
4.2.3.1 IP Broadcast Addresses ......................... 50
4.2.3.2 IP Multicasting ................................ 50
4.2.3.3 Path MTU Discovery ............................. 51
4.2.3.4 Subnetting ..................................... 51
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ........... 52
4.3.1 INTRODUCTION ..................................... 52
4.3.2 GENERAL ISSUES ................................... 53
4.3.2.1 Unknown Message Types .......................... 53
4.3.2.2 ICMP Message TTL ............................... 53
4.3.2.3 Original Message Header ........................ 53
4.3.2.4 ICMP Message Source Address .................... 53
4.3.2.5 TOS and Precedence ............................. 54
4.3.2.6 Source Route ................................... 54
4.3.2.7 When Not to Send ICMP Errors ................... 55
4.3.2.8 Rate Limiting .................................. 56
4.3.3 SPECIFIC ISSUES .................................. 56
4.3.3.1 Destination Unreachable ........................ 56
4.3.3.2 Redirect ....................................... 57
4.3.3.3 Source Quench .................................. 57
4.3.3.4 Time Exceeded .................................. 58
4.3.3.5 Parameter Problem .............................. 58
4.3.3.6 Echo Request/Reply ............................. 58
4.3.3.7 Information Request/Reply ...................... 59
4.3.3.8 Timestamp and Timestamp Reply .................. 59
4.3.3.9 Address Mask Request/Reply ..................... 61
4.3.3.10 Router Advertisement and Solicitations ........ 62
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .......... 62
5. INTERNET LAYER - FORWARDING ......................... 63
5.1 INTRODUCTION ....................................... 63
5.2 FORWARDING WALK-THROUGH ............................ 63
5.2.1 Forwarding Algorithm ............................. 63
5.2.1.1 General ........................................ 64
5.2.1.2 Unicast ........................................ 64
5.2.1.3 Multicast ...................................... 65
5.2.2 IP Header Validation ............................. 67
5.2.3 Local Delivery Decision .......................... 69
5.2.4 Determining the Next Hop Address ................. 71
5.2.4.1 IP Destination Address ......................... 72
5.2.4.2 Local/Remote Decision .......................... 72
5.2.4.3 Next Hop Address ............................... 74
5.2.4.4 Administrative Preference ...................... 77
5.2.4.5 Load Splitting ................................. 79
5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ....... 79
5.2.6 Fragmentation and Reassembly: RFC-791,
Section 3.2 ...................................... 80
5.2.7 Internet Control Message Protocol - ICMP ......... 80
5.2.7.1 Destination Unreachable ........................ 80
5.2.7.2 Redirect ....................................... 82
5.2.7.3 Time Exceeded .................................. 84
5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........ 84
5.3 SPECIFIC ISSUES .................................... 85
5.3.1 Time to Live (TTL) ............................... 85
5.3.2 Type of Service (TOS) ............................ 86
5.3.3 IP Precedence .................................... 87
5.3.3.1 Precedence-Ordered Queue Service ............... 88
5.3.3.2 Lower Layer Precedence Mappings ................ 89
5.3.3.3 Precedence Handling For All Routers ............ 90
5.3.4 Forwarding of Link Layer Broadcasts .............. 92
5.3.5 Forwarding of Internet Layer Broadcasts .......... 92
5.3.5.1 Limited Broadcasts ............................. 93
5.3.5.2 Directed Broadcasts ............................ 93
5.3.5.3 All-subnets-directed Broadcasts ................ 94
5.3.5.4 Subnet-directed Broadcasts .................... 94
5.3.6 Congestion Control ............................... 94
5.3.7 Martian Address Filtering ........................ 96
5.3.8 Source Address Validation ........................ 97
5.3.9 Packet Filtering and Access Lists ................ 97
5.3.10 Multicast Routing ............................... 98
5.3.11 Controls on Forwarding .......................... 98
5.3.12 State Changes ................................... 99
5.3.12.1 When a Router Ceases Forwarding ............... 99
5.3.12.2 When a Router Starts Forwarding ............... 100
5.3.12.3 When an Interface Fails or is Disabled ........ 100
5.3.12.4 When an Interface is Enabled .................. 100
5.3.13 IP Options ...................................... 101
5.3.13.1 Unrecognized Options .......................... 101
5.3.13.2 Security Option ............................... 101
5.3.13.3 Stream Identifier Option ...................... 101
5.3.13.4 Source Route Options .......................... 101
5.3.13.5 Record Route Option ........................... 102
5.3.13.6 Timestamp Option .............................. 102
6. TRANSPORT LAYER ..................................... 103
6.1 USER DATAGRAM PROTOCOL - UDP ....................... 103
6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................ 104
7. APPLICATION LAYER - ROUTING PROTOCOLS ............... 106
7.1 INTRODUCTION ....................................... 106
7.1.1 Routing Security Considerations .................. 106
7.1.2 Precedence ....................................... 107
7.1.3 Message Validation ............................... 107
7.2 INTERIOR GATEWAY PROTOCOLS ......................... 107
7.2.1 INTRODUCTION ..................................... 107
7.2.2 OPEN SHORTEST PATH FIRST - OSPF .................. 108
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -
DUAL IS-IS ....................................... 108
7.3 EXTERIOR GATEWAY PROTOCOLS ........................ 109
7.3.1 INTRODUCTION .................................... 109
7.3.2 BORDER GATEWAY PROTOCOL - BGP .................... 109
7.3.2.1 Introduction ................................... 109
7.3.2.2 Protocol Walk-through .......................... 110
7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
.................................................. 110
7.4 STATIC ROUTING ..................................... 111
7.5 FILTERING OF ROUTING INFORMATION ................... 112
7.5.1 Route Validation ................................. 113
7.5.2 Basic Route Filtering ............................ 113
7.5.3 Advanced Route Filtering ......................... 114
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........ 114
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
..................................................... 115
8.1 The Simple Network Management Protocol - SNMP ...... 115
8.1.1 SNMP Protocol Elements ........................... 115
8.2 Community Table .................................... 116
8.3 Standard MIBS ...................................... 118
8.4 Vendor Specific MIBS ............................... 119
8.5 Saving Changes ..................................... 120
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ......... 120
9.1 BOOTP .............................................. 120
9.1.1 Introduction ..................................... 120
9.1.2 BOOTP Relay Agents ............................... 121
10. OPERATIONS AND MAINTENANCE ......................... 122
10.1 Introduction ...................................... 122
10.2 Router Initialization ............................. 123
10.2.1 Minimum Router Configuration .................... 123
10.2.2 Address and Prefix Initialization ............... 124
10.2.3 Network Booting using BOOTP and TFTP ............ 125
10.3 Operation and Maintenance ......................... 126
10.3.1 Introduction .................................... 126
10.3.2 Out Of Band Access .............................. 127
10.3.2 Router O&M Functions ............................ 127
10.3.2.1 Maintenance - Hardware Diagnosis .............. 127
10.3.2.2 Control - Dumping and Rebooting ............... 127
10.3.2.3 Control - Configuring the Router .............. 128
10.3.2.4 Net Booting of System Software ................ 128
10.3.2.5 Detecting and responding to misconfiguration
............................................... 129
10.3.2.6 Minimizing Disruption ......................... 130
10.3.2.7 Control - Troubleshooting Problems ............ 130
10.4 Security Considerations ........................... 131
10.4.1 Auditing and Audit Trails ....................... 131
10.4.2 Configuration Control ........................... 132
11. REFERENCES ......................................... 133
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ...... 145
APPENDIX B. GLOSSARY ................................... 146
APPENDIX C. FUTURE DIRECTIONS .......................... 152
APPENDIX D. Multicast Routing Protocols ................ 154
D.1 Introduction ....................................... 154
D.2 Distance Vector Multicast Routing Protocol -
DVMRP .............................................. 154
D.3 Multicast Extensions to OSPF - MOSPF ............... 154
D.4 Protocol Independent Multicast - PIM ............... 155
APPENDIX E Additional Next-Hop Selection Algorithms
................................................... 155
E.1. Some Historical Perspective ....................... 155
E.2. Additional Pruning Rules .......................... 157
E.3 Some Route Lookup Algorithms ....................... 159
E.3.1 The Revised Classic Algorithm .................... 159
E.3.2 The Variant Router Requirements Algorithm ........ 160
E.3.3 The OSPF Algorithm ............................... 160
E.3.4 The Integrated IS-IS Algorithm ................... 162
Security Considerations ................................ 163
APPENDIX F: HISTORICAL ROUTING PROTOCOLS ............... 164
F.1 EXTERIOR GATEWAY PROTOCOL - EGP .................... 164
F.1.1 Introduction ..................................... 164
F.1.2 Protocol Walk-through ............................ 165
F.2 ROUTING INFORMATION PROTOCOL - RIP ................. 167
F.2.1 Introduction ..................................... 167
F.2.2 Protocol Walk-Through ............................ 167
F.2.3 Specific Issues .................................. 172
F.3 GATEWAY TO GATEWAY PROTOCOL - GGP .................. 173
Acknowledgments ........................................ 173
Editor"s Address ....................................... 175
1. INTRODUCTION
This memo replaces for RFC1716, "Requirements for Internet Gateways"
([INTRO:1]).
This memo defines and discusses requirements for devices that perform
the network layer forwarding function of the Internet protocol suite.
The Internet community usually refers to such devices as IP routers or
simply routers; The OSI community refers to such devices as
intermediate systems. Many older Internet documents refer to these
devices as gateways, a name which more recently has largely passed out
of favor to avoid confusion with application gateways.
An IP router can be distinguished from other sorts of packet switching
devices in that a router examines the IP protocol header as part of
the switching process. It generally removes the Link Layer header a
message was received with, modifies the IP header, and replaces the
Link Layer header for retransmission.
The authors of this memo recognize, as should its readers, that many
routers support more than one protocol. Support for multiple protocol
suites will be required in increasingly large parts of the Internet in
the future. This memo, however, does not attempt to specify Internet
requirements for protocol suites other than TCP/IP.
This document enumerates standard protocols that a router connected to
the Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols. It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.
For each protocol, this memo also contains an explicit set of
requirements, recommendations, and options. The reader must
understand that the list of requirements in this memo is incomplete by
itself. The complete set of requirements for an Internet protocol
router is primarily defined in the standard protocol specification
documents, with the corrections, amendments, and supplements contained
in this memo.
This memo should be read in conjunction with the Requirements for
Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
routers must both be capable of originating IP datagrams and receiving
IP datagrams destined for them. The major distinction between
Internet hosts and routers is that routers implement forwarding
algorithms, while Internet hosts do not require forwarding
capabilities. Any Internet host acting as a router must adhere to the
requirements contained in this memo.
The goal of open system interconnection dictates that routers must
function correctly as Internet hosts when necessary. To achieve this,
this memo provides guidelines for such instances. For simplification
and ease of document updates, this memo tries to avoid overlapping
discussions of host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by
reference. In some cases the requirements stated in [INTRO:2] and
[INTRO:3] are superseded by this document.
A good-faith implementation of the protocols produced after careful
reading of the RFCs should differ from the requirements of this memo
in only minor ways. Producing such an implementation often requires
some interaction with the Internet technical community, and must
follow good communications software engineering practices. In many
cases, the requirements in this document are already stated or implied
in the standard protocol documents, so that their inclusion here is,
in a sense, redundant. They were included because some past
implementation has made the wrong choice, causing problems of
interoperability, performance, and/or robustness.
This memo includes discussion and explanation of many of the
requirements and recommendations. A simple list of requirements would
be dangerous, because:
o Some required features are more important than others, and some
features are optional.
o Some features are critical in some applications of routers but
irrelevant in others.
o There may be valid reasons why particular vendor products that are
designed for restricted contexts might choose to use different
specifications.
However, the specifications of this memo must be followed to meet the
general goal of arbitrary router interoperation across the diversity
and complexity of the Internet. Although most current implementations
fail to meet these requirements in various ways, some minor and some
major, this specification is the ideal towards which we need to move.
These requirements are based on the current level of Internet
architecture. This memo will be updated as required to provide
additional clarifications or to include additional information in
those areas in which specifications are still evolving.
1.1 Reading this Document
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 among Chapters 7, 8, and 9. Chapter 7 discusses the protocols
which routers use to exchange routing information with each other.
Chapter 8 discusses network management. Chapter 9 discusses other
upper layer protocols. The final chapter covers operations and
maintenance features. This organization was chosen for simplicity,
clarity, and consistency with the Host Requirements RFCs. Appendices
to this memo include a bibliography, a glossary, and some conjectures
about future directions of router standards.
In describing the requirements, we assume that an implementation
strictly mirrors the layering of the protocols. However, strict
layering is an imperfect model, both for the protocol suite and for
recommended implementation approaches. Protocols in different layers
interact in complex and sometimes suBTle ways, and particular
functions often involve multiple layers. There are many design
choices in an implementation, many of which involve creative breaking
of strict layering. Every implementor is urged to read [INTRO:4] and
[INTRO:5].
Each major section of this memo is organized into the following
subsections:
(1) Introduction
(2) Protocol Walk-Through - considers the protocol specification
documents section-by-section, correcting errors, stating
requirements that may be ambiguous or ill-defined, and providing
further clarification or explanation.
(3) Specific Issues - discusses protocol design and implementation
issues that were not included in the walk-through.
Under many of the individual topics in this memo, there is
parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
material is intended to give a justification, clarification or
explanation to the preceding requirements text. The implementation
material contains suggested approaches that an implementor may want to
consider. The DISCUSSION and IMPLEMENTATION sections are not part of
the standard.
1.1.2 Requirements
In this memo, the Words that are used to define the significance of
each particular requirement are capitalized. These words are:
o MUST
This word means that the item is an absolute requirement of the
specification. Violation of such a requirement is a fundamental
error; there is no case where it is justified.
o MUST IMPLEMENT
This phrase means that this specification requires that the item be
implemented, but does not require that it be enabled by default.
o MUST NOT
This phrase means that the item is an absolute prohibition of the
specification.
o SHOULD
This word means that there may exist valid reasons in particular
circumstances to ignore this item, but the full implications should
be understood and the case carefully weighed before choosing a
different course.
o SHOULD IMPLEMENT
This phrase is similar in meaning to SHOULD, but is used when we
recommend that a particular feature be provided but does not
necessarily recommend that it be enabled by default.
o SHOULD NOT
This phrase means that there may exist valid reasons in particular
circumstances when the described behavior is acceptable or even
useful. Even so, the full implications should be understood and
the case carefully weighed before implementing any behavior
described with this label.
o MAY
This word means that this item is truly optional. One vendor may
choose to include the item because a particular marketplace
requires it or because it enhances the product, for example;
another vendor may omit the same item.
1.1.3 Compliance
Some requirements are applicable to all routers. Other requirements
are applicable only to those which implement particular features or
protocols. In the following paragraphs, relevant refers to the union
of the requirements applicable to all routers and the set of
requirements applicable to a particular router because of the set of
features and protocols it has implemented.
Note that not all Relevant requirements are stated directly in this
memo. Various parts of this memo incorporate by reference sections of
the Host Requirements specification, [INTRO:2] and [INTRO:3]. For
purposes of determining compliance with this memo, it does not matter
whether a Relevant requirement is stated directly in this memo or
merely incorporated by reference from one of those documents.
An implementation is said to be conditionally compliant if it
satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
requirements. An implementation is said to be unconditionally
compliant if it is conditionally compliant and also satisfies all the
Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An
implementation is not compliant if it is not conditionally compliant
(i.e., it fails to satisfy one or more of the Relevant MUST, MUST
IMPLEMENT, or MUST NOT requirements).
This specification occasionally indicates that an implementation
SHOULD implement a management variable, and that it SHOULD have a
certain default value. An unconditionally compliant implementation
implements the default behavior, and if there are other implemented
behaviors implements the variable. A conditionally compliant
implementation clearly documents what the default setting of the
variable is or, in the absence of the implementation of a variable,
may be construed to be. An implementation that both fails to
implement the variable and chooses a different behavior is not
compliant.
For any of the SHOULD and SHOULD NOT requirements, a router may
provide a configuration option that will cause the router to act other
than as specified by the requirement. Having such a configuration
option does not void a router"s claim to unconditional compliance if
the option has a default setting, and that setting causes the router
to operate in the required manner.
Likewise, routers may provide, except where explicitly prohibited by
this memo, options which cause them to violate MUST or MUST NOT
requirements. A router that provides such options is compliant
(either fully or conditionally) if and only if each such option has a
default setting that causes the router to conform to the requirements
of this memo. Please note that the authors of this memo, although
aware of market realities, strongly recommend against provision of
such options. Requirements are labeled MUST or MUST NOT because
experts in the field have judged them to be particularly important to
interoperability or proper functioning in the Internet. Vendors
should weigh carefully the customer support costs of providing options
that violate those rules.
Of course, this memo is not a complete specification of an IP router,
but rather is closer to what in the OSI world is called a profile.
For example, this memo requires that a number of protocols be
implemented. Although most of the contents of their protocol
specifications are not repeated in this memo, implementors are
nonetheless required to implement the protocols according to those
specifications.
1.2 Relationships to Other Standards
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, RFC1780, [ARCH:7].
This document is periodically re-issued. You should always
consult an RFCrepository and use the latest version of this
document.
o Assigned Numbers
This document lists the assigned values of the parameters used in
the various protocols. For example, it lists IP protocol codes,
TCP port numbers, Telnet Option Codes, ARP hardware types, and
Terminal Type names. As of this writing, the current version of
this document is STD 2, RFC1700, [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 RFC1122 and
RFC1123 (STD 3), [INTRO:2] and [INTRO:3].
o Router Requirements (formerly Gateway Requirements)
This memo.
Note that these documents are revised and updated at different times;
in case of differences between these documents, the most recent must
prevail.
These and other Internet protocol documents may be obtained from the:
The InterNIC
DS.INTERNIC.NET
InterNIC Directory and Database Service
info@internic.net
+1-908-668-6587
URL: http://ds.internic.net/
1.3 General Considerations
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
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 many
attempts by vendors to make configuration easy actually cause
customers more grief than they prevent. As an extreme example, a
router designed to come up and start routing packets without
requiring any configuration information at all would almost certainly
choose some incorrect parameter, possibly causing serious problems on
any networks unfortunate enough to be connected to it.
Often this memo requires that a parameter be a configurable option.
There are several reasons for this. In a few cases there currently
is some uncertainty or disagreement about the best value and it may
be necessary to update the recommended value in the future. In other
cases, the value really depends on external factors - e.g., the
distribution of its communication load, or the speeds and topology of
nearby networks - and self-tuning algorithms are unavailable and may
be insufficient. In some cases, configurability is needed because of
administrative requirements.
Finally, some configuration options are required to communicate with
obsolete or incorrect implementations of the protocols, distributed
without sources, that persist in many parts of the Internet. To make
correct systems coexist with these faulty systems, administrators
must occasionally misconfigure the correct systems. This problem
will correct itself gradually as the faulty systems are retired, but
cannot be ignored by vendors.
When we say that a parameter must be configurable, we do not intend
to require that its value be explicitly read from a configuration
file at every boot time. For many parameters, there is one value
that is appropriate for all but the most unusual situations. In such
cases, it is quite reasonable that the parameter default to that
value if not explicitly set.
This memo requires a particular value for such defaults in some
cases. The choice of default is a sensitive issue when the
configuration item controls accommodation of existing, faulty,
systems. If the Internet is to converge successfully to complete
interoperability, the default values built into implementations must
implement the official protocol, not misconfigurations to accommodate
faulty implementations. Although marketing considerations have led
some vendors to choose misconfiguration defaults, we urge vendors to
choose defaults that will conform to the standard.
Finally, we note that a vendor needs to provide adequate
documentation on all configuration parameters, their limits and
effects.
1.4 Algorithms
In several places in this memo, specific algorithms that a router
ought to follow are specified. These algorithms are not, per se,
required of the router. A router need not implement each algorithm
as it is written in this document. Rather, an implementation must
present a behavior to the external world that is the same as a
strict, literal, implementation of the specified algorithm.
Algorithms are described in a manner that differs from the way a good
implementor would implement them. For expository purposes, a style
that emphasizes conciseness, clarity, and independence from
implementation details has been chosen. A good implementor will
choose algorithms and implementation methods that produce the same
results as these algorithms, but may be more efficient or less
general.
We note that the art of efficient router implementation is outside
the scope of this memo.
2. INTERNET ARCHITECTURE
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 the Internet
protocols.
All Internet protocols use IP as the basic data transport mechanism.
IP is a datagram, or connectionless, internetwork service and
includes provision for addressing, type-of-service specification,
fragmentation and reassembly, and security. ICMP and IGMP are
considered integral parts of IP, although they are architecturally
layered upon IP. ICMP provides error reporting, flow control,
first-hop router redirection, and other maintenance and control
functions. IGMP provides the mechanisms by which hosts and routers
can join and leave IP multicast groups.
Reliable data delivery is provided in the Internet protocol suite by
Transport Layer protocols such as the Transmission Control Protocol
(TCP), which provides end-end retransmission, resequencing and
connection control. Transport Layer connectionless service is
provided by the User Datagram Protocol (UDP).
2.2 Elements of the Architecture
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.
There are two primary Transport Layer protocols at present:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
TCP is a reliable connection-oriented transport service that
provides end-to-end reliability, resequencing, and flow control.
UDP is a connectionless (datagram) transport service. Other
transport protocols have been developed by the research community,
and the set of official Internet transport protocols may be
expanded in the future.
Transport Layer protocols relevant to routers are discussed in
Chapter 6.
o Internet Layer
All Internet transport protocols use the Internet Protocol (IP) to
carry data from source host to destination host. IP is a
connectionless or datagram internetwork service, providing no
end-to-end delivery guarantees. IP datagrams may arrive at the
destination host damaged, duplicated, out of order, or not at all.
The layers above IP are responsible for reliable delivery service
when it is required. The IP protocol includes provision for
addressing, type-of-service specification, fragmentation and
reassembly, and security.
The datagram or connectionless nature of IP is a fundamental and
characteristic feature of the Internet architecture.
The Internet Control Message Protocol (ICMP) is a control protocol
that is considered to be an integral part of IP, although it is
architecturally layered upon IP - it uses IP to carry its data
end-to-end. ICMP provides error reporting, congestion reporting,
and first-hop router redirection.
The Internet Group Management Protocol (IGMP) is an Internet layer
protocol used for establishing dynamic host groups for IP
multicasting.
The Internet layer protocols IP, ICMP, and IGMP are discussed in
chapter 4.
o Link Layer
To communicate on a directly connected network, a host must
implement the communication protocol used to interface to that
network. We call this a Link Layer protocol.
Some older Internet documents refer to this layer as the Network
Layer, but it is not the same as the Network Layer in the OSI
Reference Model.
This layer contains everything below the Internet Layer and above
the Physical Layer (which is the media connectivity, normally
electrical or optical, which encodes and transports messages).
Its responsibility is the correct delivery of messages, among
which it does not differentiate.
Protocols in this Layer are generally outside the scope of
Internet standardization; the Internet (intentionally) uses
existing standards whenever possible. Thus, Internet Link Layer
standards usually address only address resolution and rules for
transmitting IP packets over specific Link Layer protocols.
Internet Link Layer standards are discussed in chapter 3.
2.2.2 Networks
The constituent networks of the Internet system are required to
provide only packet (connectionless) transport. According to the IP
service specification, datagrams can be delivered out of order, be
lost or duplicated, and/or contain errors.
For reasonable performance of the protocols that use IP (e.g., TCP),
the loss rate of the network should be very low. In networks
providing connection-oriented service, the extra reliability provided
by virtual circuits enhances the end-end robustness of the system,
but is not necessary for Internet operation.
Constituent networks may generally be divided into two classes:
o Local-Area Networks (LANs)
LANs may have a variety of designs. LANs normally cover a small
geographical area (e.g., a single building or plant site) and
provide high bandwidth with low delays. LANs may be passive
(similar to Ethernet) or they may be active (such as ATM).
o Wide-Area Networks (WANs)
Geographically dispersed hosts and LANs are interconnected by
wide-area networks, also called long-haul networks. These
networks may have a complex internal structure of lines and
packet-switches, or they may be as simple as point-to-point
lines.
2.2.3 Routers
In the Internet model, constituent networks are connected together by
IP datagram forwarders which are called routers or IP routers. In
this document, every use of the term router is equivalent to IP
router. Many older Internet documents refer to routers as gateways.
Historically, routers have been realized with packet-switching
software executing on a general-purpose CPU. However, as custom
hardware development becomes cheaper and as higher throughput is
required, special purpose hardware is becoming increasingly common.
This specification applies to routers regardless of how they are
implemented.
A router connects to two or more logical interfaces, represented by
IP subnets or unnumbered point to point lines (discussed in section
[2.2.7]). Thus, it has at least one physical interface. Forwarding
an IP datagram generally requires the router to choose the address
and relevant interface of the next-hop router or (for the final hop)
the destination host. This choice, called relaying or forwarding
depends upon a route database within the router. The route database
is also called a routing table or forwarding table. The term
"router" derives from the process of building this route database;
routing protocols and configuration interact in a process called
routing.
The routing database should be maintained dynamically to reflect the
current topology of the Internet system. A router normally
accomplishes this by participating in distributed routing and
reachability algorithms with other routers.
Routers provide datagram transport only, and they seek to minimize
the state information necessary to sustain this service in the
interest of routing flexibility and robustness.
Packet switching devices may also operate at the Link Layer; such
devices are usually called bridges. Network segments that are
connected by bridges share the same IP network prefix forming a
single IP subnet. These other devices are outside the scope of this
document.
2.2.4 Autonomous Systems
An Autonomous System (AS) is a connected segment of a network
topology that consists of a collection of subnetworks (with hosts
attached) interconnected by a set of routes. The subnetworks and the
routers are expected to be under the control of a single operations
and maintenance (O&M) organization. Within an AS routers may use one
or more interior routing protocols, and sometimes several sets of
metrics. An AS is expected to present to other ASs an appearence of
a coherent interior routing plan, and a consistent picture of the
destinations reachable through the AS. An AS is identified by an
Autonomous System number.
The concept of an AS plays an important role in the Internet routing
(see Section 7.1).
2.2.5 Addressing Architecture
An IP datagram carries 32-bit source and destination addresses, each
of which is partitioned into two parts - a constituent network prefix
and a host number on that network. Symbolically:
IP-address ::= { <Network-prefix>, <Host-number> }
To finally deliver the datagram, the last router in its path must map
the Host-number (or rest) part of an IP address to the host"s Link
Layer address.
2.2.5.1 Classical IP Addressing Architecture
Although well documented elsewhere [INTERNET:2], it is useful to
describe the historical use of the network prefix. The language
developed to describe it is used in this and other documents and
permeates the thinking behind many protocols.
The simplest classical network prefix is the Class A, B, C, D, or E
network prefix. These address ranges are discriminated by observing
the values of the most significant bits of the address, and break the
address into simple prefix and host number fields. This is described
in [INTERNET:18]. In short, the classification is:
0xxx - Class A - general purpose unicast addresses with standard
8 bit prefix
10xx - Class B - general purpose unicast addresses with standard
16 bit prefix
110x - Class C - general purpose unicast addresses with standard
24 bit prefix
1110 - Class D - IP Multicast Addresses - 28 bit prefix, non-
aggregatable
1111 - Class E - reserved for experimental use
This simple notion has been extended by the concept of subnets.
These were introduced to allow arbitrary complexity of interconnected
LAN structures within an organization, while insulating the Internet
system against explosive growth in assigned network prefixes and
routing complexity. Subnets provide a multi-level hierarchical
routing structure for the Internet system. The subnet extension,
described in [INTERNET:2], is a required part of the Internet
architecture. The basic idea is to partition the <Host-number> field
into two parts: a subnet number, and a true host number on that
subnet:
IP-address ::=
{ <Network-number>, <Subnet-number>, <Host-number> }
The interconnected physical networks within an organization use the
same network prefix but different subnet numbers. The distinction
between the subnets of such a subnetted network is not normally
visible outside of that network. Thus, routing in the rest of the
Internet uses only the <Network-prefix> part of the IP destination
address. Routers outside the network treat <Network-prefix> and
<Host-number> together as an uninterpreted rest part of the 32-bit IP
address. Within the subnetted network, the routers use the extended
network prefix:
{ <Network-number>, <Subnet-number> }
The bit positions containing this extended network number have
historically been indicated by a 32-bit mask called the subnet mask.
The <Subnet-number> bits SHOULD be contiguous and fall between the
<Network-number> and the <Host-number> fields. More up to date
protocols do not refer to a subnet mask, but to a prefix length; the
"prefix" portion of an address is that which would be selected by a
subnet mask whose most significant bits are all ones and the rest are
zeroes. The length of the prefix equals the number of ones in the
subnet mask. This document assumes that all subnet masks are
expressible as prefix lengths.
The inventors of the subnet mechanism presumed that each piece of an
organization"s network would have only a single subnet number. In
practice, it has often proven necessary or useful to have several
subnets share a single physical cable. For this reason, routers
should be capable of configuring multiple subnets on the same
physical interfaces, and treat them (from a routing or forwarding
perspective) as though they were distinct physical interfaces.
2.2.5.2 Classless Inter Domain Routing (CIDR)
The explosive growth of the Internet has forced a review of address
assignment policies. The traditional uses of general purpose (Class
A, B, and C) networks have been modified to achieve better use of
IP"s 32-bit address space. Classless Inter Domain Routing (CIDR)
[INTERNET:15] is a method currently being deployed in the Internet
backbones to achieve this added efficiency. CIDR depends on
deploying and routing to arbitrarily sized networks. In this model,
hosts and routers make no assumptions about the use of addressing in
the internet. The Class D (IP Multicast) and Class E (Experimental)
address spaces are preserved, although this is primarily an
assignment policy.
By definition, CIDR comprises three elements:
o topologically significant address assignment,
o routing protocols that are capable of aggregating network layer
reachability information, and
o consistent forwarding algorithm ("longest match").
The use of networks and subnets is now historical, although the
language used to describe them remains in current use. They have
been replaced by the more tractable concept of a network prefix. A
network prefix is, by definition, a contiguous set of bits at the
more significant end of the address that defines a set of systems;
host numbers select among those systems. There is no requirement
that all the internet use network prefixes uniformly. To collapse
routing information, it is useful to divide the internet into
addressing domains. Within such a domain, detailed information is
available about constituent networks; outside it, only the common
network prefix is advertised.
The classical IP addressing architecture used addresses and subnet
masks to discriminate the host number from the network prefix. With
network prefixes, it is sufficient to indicate the number of bits in
the prefix. Both representations are in common use. Architecturally
correct subnet masks are capable of being represented using the
prefix length description. They comprise that subset of all possible
bits patterns that have
o a contiguous string of ones at the more significant end,
o a contiguous string of zeros at the less significant end, and
o no intervening bits.
Routers SHOULD always treat a route as a network prefix, and SHOULD
reject configuration and routing information inconsistent with that
model.
IP-address ::= { <Network-prefix>, <Host-number> }
An effect of the use of CIDR is that the set of destinations
associated with address prefixes in the routing table may exhibit
subset relationship. A route describing a smaller set of
destinations (a longer prefix) is said to be more specific than a
route describing a larger set of destinations (a shorter prefix);
similarly, a route describing a larger set of destinations (a shorter
prefix) is said to be less specific than a route describing a smaller
set of destinations (a longer prefix). Routers must use the most
specific matching route (the longest matching network prefix) when
forwarding traffic.
2.2.6 IP Multicasting
IP multicasting is an extension of Link Layer multicast to IP
internets. Using IP multicasts, a single datagram can be addressed
to multiple hosts without sending it to all. In the extended case,
these hosts may reside in different address domains. This collection
of hosts is called a multicast group. Each multicast group is
represented as a Class D IP address. An IP datagram sent to the
group is to be delivered to each group member with the same best-
effort delivery as that provided for unicast IP traffic. The sender
of the datagram does not itself need to be a member of the
destination group.
The semantics of IP multicast group membership are defined in
[INTERNET:4]. That document describes how hosts and routers join and
leave multicast groups. It also defines a protocol, the Internet
Group Management Protocol (IGMP), that monitors IP multicast group
membership.
Forwarding of IP multicast datagrams is accomplished either through
static routing information or via a multicast routing protocol.
Devices that forward IP multicast datagrams are called multicast
routers. They may or may not also forward IP unicasts. Multicast
datagrams are forwarded on the basis of both their source and
destination addresses. Forwarding of IP multicast packets is
described in more detail in Section [5.2.1]. Appendix D discusses
multicast routing protocols.
2.2.7 Unnumbered Lines and Networks Prefixes
Traditionally, each network interface on an IP host or router has its
own IP address. This can cause inefficient use of the scarce IP
address space, since it forces allocation of an IP network prefix to
every point-to-point link.
To solve this problem, a number of people have proposed and
implemented the concept of unnumbered point to point lines. An
unnumbered point to point line does not have any network prefix
associated with it. As a consequence, the network interfaces
connected to an unnumbered point to point line do not have IP
addresses.
Because the IP architecture has traditionally assumed that all
interfaces had IP addresses, these unnumbered interfaces cause some
interesting dilemmas. For example, some IP options (e.g., Record
Route) specify that a router must insert the interface address into
the option, but an unnumbered interface has no IP address. Even more
fundamental (as we shall see in chapter 5) is that routes contain the
IP address of the next hop router. A router expects that this IP
address will be on an IP (sub)net to which the router is connected.
That assumption is of course violated if the only connection is an
unnumbered point to point line.
To get around these difficulties, two schemes have been conceived.
The first scheme says that two routers connected by an unnumbered
point to point line are not really two routers at all, but rather two
half-routers that together make up a single virtual router. The
unnumbered point to point line is essentially considered to be an
internal bus in the virtual router. The two halves of the virtual
router must coordinate their activities in such a way that they act
exactly like a single router.
This scheme fits in well with the IP architecture, but suffers from
two important drawbacks. The first is that, although it handles the
common case of a single unnumbered point to point line, it is not
readily extensible to handle the case of a mesh of routers and
unnumbered point to point lines. The second drawback is that the
interactions between the half routers are necessarily complex and are
not standardized, effectively precluding the connection of equipment
from different vendors using unnumbered point to point lines.
Because of these drawbacks, this memo has adopted an alternate
scheme, which has been invented multiple times but which is probably
originally attributable to Phil Karn. In this scheme, a router that
has unnumbered point to point lines also has a special IP address,
called a router-id in this memo. The router-id is one of the
router"s IP addresses (a router is required to have at least one IP
address). This router-id is used as if it is the IP address of all
unnumbered interfaces.
2.2.8 Notable Oddities
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 that supports connections to
two or more networks. The best-known example of an operating system
with embedded router code is the Berkeley BSD system. The embedded
router feature seems to make building a