Network Working Group F. Baker, Editor Request for Comments: 1812 Cisco Systems Obsoletes: 1716, 1009 June 1995 Category: Standards Track Requirements for IP Version 4 Routers Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. PREFACE This document is an updated version of RFC 1716, the historical Router Requirements document. That RFC preserved the significant work that went into the working group, but failed to adequately describe current technology for the IESG to consider it a current standard. The current editor had been asked to bring the document up to date, so that it is useful as a procurement specification and a guide to implementors. In this, he stands squarely on the shoulders of those who have gone before him, and depends largely on expert contributors for text. Any credit is theirs; the errors are his. The content and form of this document are due, in large part, to the working group's chair, and document's original editor and author: Philip Almquist. It is also largely due to the efforts of its previous editor, Frank Kastenholz. Without their efforts, this document would not exist. Table of Contents 1. INTRODUCTION ........................................ 6 1.1 Reading this Document .............................. 8 1.1.1 Organization ..................................... 8 1.1.2 Requirements ..................................... 9 1.1.3 Compliance ....................................... 10 1.2 Relationships to Other Standards ................... 11 1.3 General Considerations ............................. 12 1.3.1 Continuing Internet Evolution .................... 12 1.3.2 Robustness Principle ............................. 13 1.3.3 Error Logging .................................... 14 Baker Standards Track [Page 1] RFC 1812 Requirements for IP Version 4 Routers June 1995 1.3.4 Configuration .................................... 14 1.4 Algorithms ......................................... 16 2. INTERNET ARCHITECTURE ............................... 16 2.1 Introduction ....................................... 16 2.2 Elements of the Architecture ....................... 17 2.2.1 Protocol Layering ................................ 17 2.2.2 Networks ......................................... 19 2.2.3 Routers .......................................... 20 2.2.4 Autonomous Systems ............................... 21 2.2.5 Addressing Architecture .......................... 21 2.2.5.1 Classical IP Addressing Architecture ........... 21 2.2.5.2 Classless Inter Domain Routing (CIDR) .......... 23 2.2.6 IP Multicasting .................................. 24 2.2.7 Unnumbered Lines and Networks Prefixes ........... 25 2.2.8 Notable Oddities ................................. 26 2.2.8.1 Embedded Routers ............................... 26 2.2.8.2 Transparent Routers ............................ 27 2.3 Router Characteristics ............................. 28 2.4 Architectural Assumptions .......................... 31 3. LINK LAYER .......................................... 32 3.1 INTRODUCTION ....................................... 32 3.2 LINK/INTERNET LAYER INTERFACE ...................... 33 3.3 SPECIFIC ISSUES .................................... 34 3.3.1 Trailer Encapsulation ............................ 34 3.3.2 Address Resolution Protocol - ARP ................ 34 3.3.3 Ethernet and 802.3 Coexistence ................... 35 3.3.4 Maximum Transmission Unit - MTU .................. 35 3.3.5 Point-to-Point Protocol - PPP .................... 35 3.3.5.1 Introduction ................................... 36 3.3.5.2 Link Control Protocol (LCP) Options ............ 36 3.3.5.3 IP Control Protocol (IPCP) Options ............. 38 3.3.6 Interface Testing ................................ 38 4. INTERNET LAYER - PROTOCOLS .......................... 39 4.1 INTRODUCTION ....................................... 39 4.2 INTERNET PROTOCOL - IP ............................. 39 4.2.1 INTRODUCTION ..................................... 39 4.2.2 PROTOCOL WALK-THROUGH ............................ 40 4.2.2.1 Options: RFC 791 Section 3.2 ................... 40 4.2.2.2 Addresses in Options: RFC 791 Section 3.1 ...... 42 4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 ..... 43 4.2.2.4 Type of Service: RFC 791 Section 3.1 ........... 44 4.2.2.5 Header Checksum: RFC 791 Section 3.1 ........... 44 4.2.2.6 Unrecognized Header Options: RFC 791, Section 3.1 .................................... 44 4.2.2.7 Fragmentation: RFC 791 Section 3.2 ............. 45 4.2.2.8 Reassembly: RFC 791 Section 3.2 ................ 46 4.2.2.9 Time to Live: RFC 791 Section 3.2 .............. 46 4.2.2.10 Multi-subnet Broadcasts: RFC 922 .............. 47 Baker Standards Track [Page 2] RFC 1812 Requirements for IP Version 4 Routers June 1995 4.2.2.11 Addressing: RFC 791 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 Baker Standards Track [Page 3] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 4] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 5] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 RFC 1716, "Requirements for Internet Gateways" ([INTRO:1]). This memo defines and discusses requirements for devices that perform the network layer forwarding function of the Internet protocol suite. The Internet community usually refers to such devices as IP routers or simply routers; The OSI community refers to such devices as intermediate systems. Many older Internet documents refer to these devices as gateways, a name which more recently has largely passed out of favor to avoid confusion with application gateways. An IP router can be distinguished from other sorts of packet switching devices in that a router examines the IP protocol header as part of the switching process. It generally removes the Link Layer header a message was received with, modifies the IP header, and replaces the Link Layer header for retransmission. Baker Standards Track [Page 6] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 7] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 8] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 9] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 10] RFC 1812 Requirements for IP Version 4 Routers June 1995 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, RFC 1780, [ARCH:7]. This document is periodically re-issued. You should always consult an RFC repository and use the latest version of this document. Baker Standards Track [Page 11] RFC 1812 Requirements for IP Version 4 Routers June 1995 o Assigned Numbers This document lists the assigned values of the parameters used in the various protocols. For example, it lists IP protocol codes, TCP port numbers, Telnet Option Codes, ARP hardware types, and Terminal Type names. As of this writing, the current version of this document is STD 2, RFC 1700, [INTRO:7]. This document is periodically re-issued. You should always consult an RFC repository and use the latest version of this document. o Host Requirements This pair of documents reviews the specifications that apply to hosts and supplies guidance and clarification for any ambiguities. Note that these requirements also apply to routers, except where otherwise specified in this memo. As of this writing, the current versions of these documents are RFC 1122 and RFC 1123 (STD 3), [INTRO:2] and [INTRO:3]. o Router Requirements (formerly Gateway Requirements) This memo. Note that these documents are revised and updated at different times; in case of differences between these documents, the most recent must prevail. These and other Internet protocol documents may be obtained from the: The InterNIC DS.INTERNIC.NET InterNIC Directory and Database Service info@internic.net +1-908-668-6587 URL: http://ds.internic.net/ 1.3 General Considerations 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 Baker Standards Track [Page 12] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 13] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 14] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 15] RFC 1812 Requirements for IP Version 4 Routers June 1995 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, Baker Standards Track [Page 16] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 17] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 18] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 Baker Standards Track [Page 19] RFC 1812 Requirements for IP Version 4 Routers June 1995 (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 Baker Standards Track [Page 20] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 ::= { , } 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 Baker Standards Track [Page 21] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 field into two parts: a subnet number, and a true host number on that subnet: IP-address ::= { , , } 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 part of the IP destination address. Routers outside the network treat and together as an uninterpreted rest part of the 32-bit IP address. Within the subnetted network, the routers use the extended network prefix: { , } The bit positions containing this extended network number have historically been indicated by a 32-bit mask called the subnet mask. The bits SHOULD be contiguous and fall between the and the 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 Baker Standards Track [Page 22] RFC 1812 Requirements for IP Version 4 Routers June 1995 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. Baker Standards Track [Page 23] RFC 1812 Requirements for IP Version 4 Routers June 1995 Routers SHOULD always treat a route as a network prefix, and SHOULD reject configuration and routing information inconsistent with that model. IP-address ::= { , } 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. Baker Standards Track [Page 24] RFC 1812 Requirements for IP Version 4 Routers June 1995 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 lin