RFC1716 - Towards Requirements for IP Routers

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.
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
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
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
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
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
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 RFCso 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.
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.
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.
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
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.
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
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, RFC1610, [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, 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 (December, 1993) the current versions of these
documents are RFC1122 and RFC1123, (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.
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.
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.
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.
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.
We note that the art of efficient router implementation is outside of
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 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).
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, 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.
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.
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.
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 ::=
{ <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
(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
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.
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 hos