RFCs in HTML Format


RFC 1812

                 Requirements for IP Version 4 Routers


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 ::= { <Network-prefix>, <Host-number> } To finally deliver the datagram, the last router in its path must map the Host-number (or rest) part of an IP address to the host's Link Layer address. 2.2.5.1 Classical IP Addressing Architecture Although well documented elsewhere [INTERNET:2], it is useful to describe the historical use of the network prefix. The language developed to describe it is used in this and other documents and permeates the thinking behind many protocols. The simplest classical network prefix is the Class A, B, C, D, or E network prefix. These address ranges are discriminated by observing the values of the most significant bits of the address, and break the address into simple prefix and host number fields. This is described in [INTERNET:18]. In short, the classification is: 0xxx - Class A - general purpose unicast addresses with standard 8 bit prefix 10xx - Class B - general purpose unicast addresses with standard 16 bit prefix 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 <Host-number> field into two parts: a subnet number, and a true host number on that subnet: IP-address ::= { <Network-number>, <Subnet-number>, <Host-number> } The interconnected physical networks within an organization use the same network prefix but different subnet numbers. The distinction between the subnets of such a subnetted network is not normally visible outside of that network. Thus, routing in the rest of the Internet uses only the <Network-prefix> part of the IP destination address. Routers outside the network treat <Network-prefix> and <Host-number> together as an uninterpreted rest part of the 32-bit IP address. Within the subnetted network, the routers use the extended network prefix: { <Network-number>, <Subnet-number> } The bit positions containing this extended network number have historically been indicated by a 32-bit mask called the subnet mask. The <Subnet-number> bits SHOULD be contiguous and fall between the <Network-number> and the <Host-number> fields. More up to date protocols do not refer to a subnet mask, but to a prefix length; the "prefix" portion of an address is that which would be selected by a subnet mask whose most significant bits are all ones and the rest are zeroes. The length of the prefix equals the number of ones in the subnet mask. This document assumes that all subnet masks are expressible as prefix lengths. The inventors of the subnet mechanism presumed that each piece of an organization's network would have only a single subnet number. In practice, it has often proven necessary or useful to have several subnets share a single physical cable. For this reason, routers should be capable of configuring multiple subnets on the same 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 ::= { <Network-prefix>, <Host-number> } An effect of the use of CIDR is that the set of destinations associated with address prefixes in the routing table may exhibit subset relationship. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorter prefix); similarly, a route describing a larger set of destinations (a shorter prefix) is said to be less specific than a route describing a smaller set of destinations (a longer prefix). Routers must use the most specific matching route (the longest matching network prefix) when forwarding traffic. 2.2.6 IP Multicasting IP multicasting is an extension of Link Layer multicast to IP internets. Using IP multicasts, a single datagram can be addressed to multiple hosts without sending it to all. In the extended case, these hosts may reside in different address domains. This collection of hosts is called a multicast group. Each multicast group is represented as a Class D IP address. An IP datagram sent to the group is to be delivered to each group member with the same best- effort delivery as that provided for unicast IP traffic. The sender of the datagram does not itself need to be a member of the destination group. The semantics of IP multicast group membership are defined in [INTERNET:4]. That document describes how hosts and routers join and leave multicast groups. It also defines a protocol, the Internet Group Management Protocol (IGMP), that monitors IP multicast group membership. Forwarding of IP multicast datagrams is accomplished either through static routing information or via a multicast routing protocol. Devices that forward IP multicast datagrams are called multicast routers. They may or may not also forward IP unicasts. Multicast datagrams are forwarded on the basis of both their source and destination addresses. Forwarding of IP multicast packets is described in more detail in Section [5.2.1]. Appendix D discusses multicast routing protocols. 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 line are not really two routers at all, but rather two half-routers that together make up a single virtual router. The unnumbered point to point line is essentially considered to be an internal bus in the virtual router. The two halves of the virtual router must coordinate their activities in such a way that they act exactly like a single router. This scheme fits in well with the IP architecture, but suffers from two important drawbacks. The first is that, although it handles the common case of a single unnumbered point to point line, it is not readily extensible to handle the case of a mesh of routers and unnumbered point to point lines. The second drawback is that the interactions between the half routers are necessarily complex and are not standardized, effectively precluding the connection of equipment from different vendors using unnumbered point to point lines. Because of these drawbacks, this memo has adopted an alternate scheme, which has been invented multiple times but which is probably originally attributable to Phil Karn. In this scheme, a router that has unnumbered point to point lines also has a special IP address, called a router-id in this memo. The router-id is one of the Baker Standards Track [Page 25]
RFC 1812 Requirements for IP Version 4 Routers June 1995 router's IP addresses (a router is required to have at least one IP address). This router-id is used as if it is the IP address of all unnumbered interfaces. 2.2.8 Notable Oddities 2.2.8.1 Embedded Routers A router may be a stand-alone computer system, dedicated to its IP router functions. Alternatively, it is possible to embed router functions within a host operating system that supports connections to two or more networks. The best-known example of an operating system with embedded router code is the Berkeley BSD system. The embedded router feature seems to make building a network 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 is subject to the requirements for routers 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. 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 maintain and update the router code. This might require router source code. (3) When a host executes embedded router code, it becomes part of the Internet infrastructure. Thus, errors in software or configuration can hinder communication between other hosts. As a consequence, the host administrator must lose some autonomy. In many circumstances, a host administrator will need to disable router code embedded in the operating system. For this reason, it should be straightforward to disable embedded router functionality. Baker Standards Track [Page 26]
RFC 1812 Requirements for IP Version 4 Routers June 1995
RFC 1812 Requirements for IP Version 4 Routers June 1995 E.3.4 The Integrated IS-IS Algorithm Integrated IS-IS uses an algorithm that is similar to but not quite identical to the OSPF Algorithm. Integrated IS-IS uses a different set of route classes, and differs slightly in its handling of type of service. The algorithm is: 1. Basic Match 2. IS-IS Route Classes 3. Longest Match 4. Weak TOS 5. Best Metric 6. Policy Although Integrated IS-IS uses Weak TOS, the protocol is only capable of carrying routes for a small specific subset of the possible values for the TOS field in the IP header. Packets containing other values in the TOS field are routed using the default TOS. Type of service support is optional; if disabled, the fourth step would be omitted. As in OSPF, the specification does not include the Policy step. This algorithm has some advantages over the Revised Classic Algorithm: (1) It supports type of service routing. (2) Its rules are written down, rather than merely being a part of the Internet folklore. (3) It (obviously) works with Integrated IS-IS. However, this algorithm also retains some of the disadvantages of the Revised Classic Algorithm: (1) Path properties other than type of service (e.g., MTU) are ignored. (2) As in the Revised Classic Algorithm, the details (or even the existence) of the Policy step are left to the discretion of the implementor. (3) It doesn't work with OSPF because of the differences between IS- IS route classes and OSPF route classes. Also, because IS-IS supports only a subset of the possible TOS values, some obvious implementations of the Integrated IS-IS algorithm would not support OSPF's interpretation of TOS. The Integrated IS-IS Algorithm also has a further disadvantage (which is not shared by the Revised Classic Algorithm): IS-IS internal (intra-area or inter-area) routes are always considered to be Baker Standards Track [Page 162]
RFC 1812 Requirements for IP Version 4 Routers June 1995 superior to routes learned from other routing protocols, even in cases where the IS-IS route matches fewer bits of the destination address and doesn't provide the requested type of service. This is a policy decision that may not be appropriate in all cases. Finally, it is worth noting that the Integrated IS-IS Algorithm's TOS support suffers from the same deficiency noted for the OSPF Algorithm. Security Considerations Although the focus of this document is interoperability rather than security, there are obviously many sections of this document that have some ramifications on network security. Security means different things to different people. Security from a router's point of view is anything that helps to keep its own networks operational and in addition helps to keep the Internet as a whole healthy. For the purposes of this document, the security services we are concerned with are denial of service, integrity, and authentication as it applies to the first two. Privacy as a security service is important, but only peripherally a concern of a router - at least as of the date of this document. In several places in this document there are sections entitled ... Security Considerations. These sections discuss specific considerations that apply to the general topic under discussion. Rarely does this document say do this and your router/network will be secure. More likely, it says this is a good idea and if you do it, it *may* improve the security of the Internet and your local system in general. Unfortunately, this is the state-of-the-art AT THIS TIME. Few if any of the network protocols a router is concerned with have reasonable, built-in security features. Industry and the protocol designers have been and are continuing to struggle with these issues. There is progress, but only small baby steps such as the peer-to-peer authentication available in the BGP and OSPF routing protocols. In particular, this document notes the current research into developing and enhancing network security. Specific areas of research, development, and engineering that are underway as of this writing (December 1993) are in IP Security, SNMP Security, and common authentication technologies. Notwithstanding all the above, there are things both vendors and users can do to improve the security of their router. Vendors should Baker Standards Track [Page 163]
RFC 1812 Requirements for IP Version 4 Routers June 1995 get a copy of Trusted Computer System Interpretation [INTRO:8]. Even if a vendor decides not to submit their device for formal verification under these guidelines, the publication provides excellent guidance on general security design and practices for computing devices. APPENDIX F: HISTORICAL ROUTING PROTOCOLS Certain routing protocols are common in the Internet, but the authors of this document cannot in good conscience recommend their use. This is not because they do not work correctly, but because the characteristics of the Internet assumed in their design (simple routing, no policy, a single "core router" network under common administration, limited complexity, or limited network diameter) are not attributes of today's Internet. Those parts of the Internet that still use them are generally limited "fringe" domains with limited complexity. As a matter of good faith, collected wisdom concerning their implementation is recorded in this section. F.1 EXTERIOR GATEWAY PROTOCOL - EGP F.1.1 Introduction The Exterior Gateway Protocol (EGP) specifies an EGP that is used to exchange reachability information between routers of the same or differing autonomous systems. EGP is not considered a routing protocol since there is no standard interpretation (i.e. metric) for the distance fields in the EGP update message, so distances are comparable only among routers of the same AS. It is however designed to provide high-quality reachability information, both about neighbor routers and about routes to non-neighbor routers. EGP is defined by [ROUTE:6]. An implementor almost certainly wants to read [ROUTE:7] and [ROUTE:8] as well, for they contain useful explanations and background material. DISCUSSION The present EGP specification has serious limitations, most importantly a restriction that limits routers to advertising only those networks that are reachable from within the router's autonomous system. This restriction against propagating third party EGP information is to prevent long-lived routing loops. This effectively limits EGP to a two-level hierarchy. RFC 975 is not a part of the EGP specification, and should be ignored. Baker Standards Track [Page 164]
RFC 1812 Requirements for IP Version 4 Routers June 1995 F.1.2 Protocol Walk-through Indirect Neighbors: RFC 888, page 26 An implementation of EGP MUST include indirect neighbor support. Polling Intervals: RFC 904, page 10 The interval between Hello command retransmissions and the interval between Poll retransmissions SHOULD be configurable but there MUST be a minimum value defined. The interval at which an implementation will respond to Hello commands and Poll commands SHOULD be configurable but there MUST be a minimum value defined. Network Reachability: RFC 904, page 15 An implementation MUST default to not providing the external list of routers in other autonomous systems; only the internal list of routers together with the nets that are reachable through those routers should be included in an Update Response/Indication packet. However, an implementation MAY elect to provide a configuration option enabling the external list to be provided. An implementation MUST NOT include in the external list routers that were learned through the external list provided by a router in another autonomous system. An implementation MUST NOT send a network back to the autonomous system from which it is learned, i.e. it MUST do split- horizon on an autonomous system level. If more than 255 internal or 255 external routers need to be specified in a Network Reachability update, the networks reachable from routers that can not be listed MUST be merged into the list for one of the listed routers. Which of the listed routers is chosen for this purpose SHOULD be user configurable, but SHOULD default to the source address of the EGP update being generated. An EGP update contains a series of blocks of network numbers, where each block contains a list of network numbers reachable at a particular distance through a particular router. If more than 255 networks are reachable at a particular distance through a particular router, they are split into multiple blocks (all of which have the same distance). Similarly, if more than 255 blocks are required to list the networks reachable through a particular router, the router's address is listed as many times as necessary to include all the blocks in the update. Baker Standards Track [Page 165]
RFC 1812 Requirements for IP Version 4 Routers June 1995 Unsolicited Updates: RFC 904, page 16 If a network is shared with the peer, an implementation MUST send an unsolicited update upon entry to the Up state if the source network is the shared network. Neighbor Reachability: RFC 904, page 6, 13-15 The table on page 6 that describes the values of j and k (the neighbor up and down thresholds) is incorrect. It is reproduced correctly here: Name Active Passive Description ----------------------------------------------- j 3 1 neighbor-up threshold k 1 0 neighbor-down threshold The value for k in passive mode also specified incorrectly in RFC- 904, page 14 The values in parenthesis should read: (j = 1, k = 0, and T3/T1 = 4) As an optimization, an implementation can refrain from sending a Hello command when a Poll is due. If an implementation does so, it SHOULD provide a user configurable option to disable this optimization. Abort timer: RFC 904, pages 6, 12, 13 An EGP implementation MUST include support for the abort timer (as documented in section 4.1.4 of RFC 904). An implementation SHOULD use the abort timer in the Idle state to automatically issue a Start event to restart the protocol machine. Recommended values are P4 for a critical error (Administratively prohibited, Protocol Violation and Parameter Problem) and P5 for all others. The abort timer SHOULD NOT be started when a Stop event was manually initiated (such as through a network management protocol). Cease command received in Idle state: RFC 904, page 13 When the EGP state machine is in the Idle state, it MUST reply to Cease commands with a Cease-ack response. Hello Polling Mode: RFC 904, page 11 An EGP implementation MUST include support for both active and passive polling modes. Baker Standards Track [Page 166]
RFC 1812 Requirements for IP Version 4 Routers June 1995 Neighbor Acquisition Messages: RFC 904, page 18 As noted the Hello and Poll Intervals should only be present in Request and Confirm messages. Therefore the length of an EGP Neighbor Acquisition Message is 14 bytes for a Request or Confirm message and 10 bytes for a Refuse, Cease or Cease-ack message. Implementations MUST NOT send 14 bytes for Refuse, Cease or Cease-ack messages but MUST allow for implementations that send 14 bytes for these messages. Sequence Numbers: RFC 904, page 10 Response or indication packets received with a sequence number not equal to S MUST be discarded. The send sequence number S MUST be incremented just before the time a Poll command is sent and at no other times. F.2 ROUTING INFORMATION PROTOCOL - RIP F.2.1 Introduction RIP is specified in [ROUTE:3]. Although RIP is still quite important in the Internet, it is being replaced in sophisticated applications by more modern IGPs such as the ones described above. A router implementing RIP SHOULD implement RIP Version 2 [ROUTE:?], as it supports CIDR routes. If occasional access networking is in use, a router implementing RIP SHOULD implement Demand RIP [ROUTE:?]. Another common use for RIP is as a router discovery protocol. Section [4.3.3.10] briefly touches upon this subject. F.2.2 Protocol Walk-Through Dealing with changes in topology: [ROUTE:3], page 11 An implementation of RIP MUST provide a means for timing out routes. Since messages are occasionally lost, implementations MUST NOT invalidate a route based on a single missed update. Implementations MUST by default wait six times the update interval before invalidating a route. A router MAY have configuration options to alter this value. DISCUSSION It is important to routing stability that all routers in a RIP autonomous system use similar timeout value for invalidating routes, and therefore it is important that an implementation default to the timeout value specified in the RIP specification. Baker Standards Track [Page 167]
RFC 1812 Requirements for IP Version 4 Routers June 1995 However, that timeout value is too conservative in environments where packet loss is reasonably rare. In such an environment, a network manager may wish to be able to decrease the timeout period to promote faster recovery from failures. IMPLEMENTATION There is a very simple mechanism that a router may use to meet the requirement to invalidate routes promptly after they time out. Whenever the router scans the routing table to see if any routes have timed out, it also notes the age of the least recently updated route that has not yet timed out. Subtracting this age from the timeout period gives the amount of time until the router again needs to scan the table for timed out routes. Split Horizon: [ROUTE:3], page 14-15 An implementation of RIP MUST implement split horizon, a scheme used for avoiding problems caused by including routes in updates sent to the router from which they were learned. An implementation of RIP SHOULD implement Split horizon with poisoned reverse, a variant of split horizon that includes routes learned from a router sent to that router, but sets their metric to infinity. Because of the routing overhead that may be incurred by implementing split horizon with poisoned reverse, implementations MAY include an option to select whether poisoned reverse is in effect. An implementation SHOULD limit the time in which it sends reverse routes at an infinite metric. IMPLEMENTATION Each of the following algorithms can be used to limit the time for which poisoned reverse is applied to a route. The first algorithm is more complex but does a more thorough job of limiting poisoned reverse to only those cases where it is necessary. The goal of both algorithms is to ensure that poison reverse is done for any destination whose route has changed in the last Route Lifetime (typically 180 seconds), unless it can be sure that the previous route used the same output interface. The Route Lifetime is used because that is the amount of time RIP will keep around an old route before declaring it stale. The time intervals (and derived variables) used in the following algorithms are as follows: Tu The Update Timer; the number of seconds between RIP updates. This typically defaults to 30 seconds. Baker Standards Track [Page 168]
RFC 1812 Requirements for IP Version 4 Routers June 1995 Rl The Route Lifetime, in seconds. This is the amount of time that a route is presumed to be good, without requiring an update. This typically defaults to 180 seconds. Ul The Update Loss; the number of consecutive updates that have to be lost or fail to mention a route before RIP deletes the route. Ul is calculated to be (Rl/Tu)+1. The +1 is to account for the fact that the first time the ifcounter is decremented will be less than Tu seconds after it is initialized. Typically, Ul will be 7: (180/30)+1. In The value to set ifcounter to when a destination is newly learned. This value is Ul-4, where the 4 is RIP's garbage collection timer/30 The first algorithm is: - Associated with each destination is a counter, called the ifcounter below. Poison reverse is done for any route whose destination's ifcounter is greater than zero. - After a regular (not triggered or in response to a request) update is sent, all the non-zero ifcounters are decremented by one. - When a route to a destination is created, its ifcounter is set as follows: - If the new route is superseding a valid route, and the old route used a different (logical) output interface, then the ifcounter is set to Ul. - If the new route is superseding a stale route, and the old route used a different (logical) output interface, then the ifcounter is set to MAX(0, Ul - INT(seconds that the route has been stale/Ut). - If there was no previous route to the destination, the ifcounter is set to In. - Otherwise, the ifcounter is set to zero - RIP also maintains a timer, called the resettimer below. Poison reverse is done on all routes whenever resettimer has not expired (regardless of the ifcounter values). Baker Standards Track [Page 169]
RFC 1812 Requirements for IP Version 4 Routers June 1995 - When RIP is started, restarted, reset, or otherwise has its routing table cleared, it sets the resettimer to go off in Rl seconds. The second algorithm is identical to the first except that: - The rules which set the ifcounter to non-zero values are changed to always set it to Rl/Tu, and - The resettimer is eliminated. Triggered updates: [ROUTE:3], page 15-16; page 29 Triggered updates (also called flash updates) are a mechanism for immediately notifying a router's neighbors when the router adds or deletes routes or changes their metrics. A router MUST send a triggered update when routes are deleted or their metrics are increased. A router MAY send a triggered update when routes are added or their metrics decreased. Since triggered updates can cause excessive routing overhead, implementations MUST use the following mechanism to limit the frequency of triggered updates: (1) When a router sends a triggered update, it sets a timer to a random time between one and five seconds in the future. The router must not generate additional triggered updates before this timer expires. (2) If the router would generate a triggered update during this interval it sets a flag indicating that a triggered update is desired. The router also logs the desired triggered update. (3) When the triggered update timer expires, the router checks the triggered update flag. If the flag is set then the router sends a single triggered update which includes all the changes that were logged. The router then clears the flag and, since a triggered update was sent, restarts this algorithm. (4) The flag is also cleared whenever a regular update is sent. Triggered updates SHOULD include all routes that have changed since the most recent regular (non-triggered) update. Triggered updates MUST NOT include routes that have not changed since the most recent regular update. Baker Standards Track [Page 170]
RFC 1812 Requirements for IP Version 4 Routers June 1995 DISCUSSION Sending all routes, whether they have changed recently or not, is unacceptable in triggered updates because the tremendous size of many Internet routing tables could otherwise result in considerable bandwidth being wasted on triggered updates. Use of UDP: [ROUTE:3], page 18-19. RIP packets sent to an IP broadcast address SHOULD have their initial TTL set to one. Note that to comply with Section [6.1] of this memo, a router SHOULD use UDP checksums in RIP packets that it originates, MUST discard RIP packets received with invalid UDP checksums, but MUST NOT discard received RIP packets simply because they do not contain UDP checksums. Addressing Considerations: [ROUTE:3], page 22 A RIP implementation SHOULD support host routes. If it does not, it MUST (as described on page 27 of [ROUTE:3]) ignore host routes in received updates. A router MAY log ignored hosts routes. The special address 0.0.0.0 is used to describe a default route. A default route is used as the route of last resort (i.e., when a route to the specific net does not exist in the routing table). The router MUST be able to create a RIP entry for the address 0.0.0.0. Input Processing - Response: [ROUTE:3], page 26 When processing an update, the following validity checks MUST be performed: o The response MUST be from UDP port 520. o The source address MUST be on a directly connected subnet (or on a directly connected, non-subnetted network) to be considered valid. o The source address MUST NOT be one of the router's addresses. DISCUSSION Some networks, media, and interfaces allow a sending node to receive packets that it broadcasts. A router must not accept its own packets as valid routing updates and process them. The last requirement prevents a router from accepting its own routing updates and processing them (on the assumption that they were sent by some other router on the network). Baker Standards Track [Page 171]
RFC 1812 Requirements for IP Version 4 Routers June 1995 An implementation MUST NOT replace an existing route if the metric received is equal to the existing metric except in accordance with the following heuristic. An implementation MAY choose to implement the following heuristic to deal with the above situation. Normally, it is useless to change the route to a network from one router to another if both are advertised at the same metric. However, the route being advertised by one of the routers may be in the process of timing out. Instead of waiting for the route to timeout, the new route can be used after a specified amount of time has elapsed. If this heuristic is implemented, it MUST wait at least halfway to the expiration point before the new route is installed. F.2.3 Specific Issues RIP Shutdown An implementation of RIP SHOULD provide for a graceful shutdown using the following steps: (1) Input processing is terminated, (2) Four updates are generated at random intervals of between two and four seconds, These updates contain all routes that were previously announced, but with some metric changes. Routes that were being announced at a metric of infinity should continue to use this metric. Routes that had been announced with a non-infinite metric should be announced with a metric of 15 (infinity - 1). DISCUSSION The metric used for the above really ought to be 16 (infinity); setting it to 15 is a kludge to avoid breaking certain old hosts that wiretap the RIP protocol. Such a host will (erroneously) abort a TCP connection if it tries to send a datagram on the connection while the host has no route to the destination (even if the period when the host has no route lasts only a few seconds while RIP chooses an alternate path to the destination). RIP Split Horizon and Static Routes Split horizon SHOULD be applied to static routes by default. An implementation SHOULD provide a way to specify, per static route, that split horizon should not be applied to this route. Baker Standards Track [Page 172]
RFC 1812 Requirements for IP Version 4 Routers June 1995 F.3 GATEWAY TO GATEWAY PROTOCOL - GGP The Gateway to Gateway protocol is considered obsolete and SHOULD NOT be implemented. Acknowledgments O that we now had here But one ten thousand of those men in England That do no work to-day! What's he that wishes so? My cousin Westmoreland? No, my fair cousin: If we are mark'd to die, we are enow To do our country loss; and if to live, The fewer men, the greater share of honour. God's will! I pray thee, wish not one man more. By Jove, I am not covetous for gold, Nor care I who doth feed upon my cost; It yearns me not if men my garments wear; Such outward things dwell not in my desires: But if it be a sin to covet honour, I am the most offending soul alive. No, faith, my coz, wish not a man from England: God's peace! I would not lose so great an honour As one man more, methinks, would share from me For the best hope I have. O, do not wish one more! Rather proclaim it, Westmoreland, through my host, That he which hath no stomach to this fight, Let him depart; his passport shall be made And crowns for convoy put into his purse: We would not die in that man's company That fears his fellowship to die with us. This day is called the feast of Crispian: He that outlives this day, and comes safe home, Will stand a tip-toe when the day is named, And rouse him at the name of Crispian. He that shall live this day, and see old age, Will yearly on the vigil feast his neighbours, And say 'To-morrow is Saint Crispian:' Then will he strip his sleeve and show his scars. And say 'These wounds I had on Crispin's day.' Old men forget: yet all shall be forgot, But he'll remember with advantages What feats he did that day: then shall our names. Familiar in his mouth as household words Harry the king, Bedford and Exeter, Warwick and Talbot, Salisbury and Gloucester, Baker Standards Track [Page 173]
RFC 1812 Requirements for IP Version 4 Routers June 1995 Be in their flowing cups freshly remember'd. This story shall the good man teach his son; And Crispin Crispian shall ne'er go by, From this day to the ending of the world, But we in it shall be remember'd; We few, we happy few, we band of brothers; For he to-day that sheds his blood with me Shall be my brother; be he ne'er so vile, This day shall gentle his condition: And gentlemen in England now a-bed Shall think themselves accursed they were not here, And hold their manhoods cheap whiles any speaks That fought with us upon Saint Crispin's day. -- William Shakespeare This memo is a product of the IETF's Router Requirements Working Group. A memo such as this one is of necessity the work of many more people than could be listed here. A wide variety of vendors, network managers, and other experts from the Internet community graciously contributed their time and wisdom to improve the quality of this memo. The editor wishes to extend sincere thanks to all of them. The current editor also wishes to single out and extend his heartfelt gratitude and appreciation to the original editor of this document; Philip Almquist. Without Philip's work, both as the original editor and as the Chair of the working group, this document would not have been produced. He also wishes to express deep and heartfelt gratitude to the previous editor, Frank Kastenholz. Frank changed the original document from a collection of information to a useful description of IP technology - in his words, a "snapshot" of the technology in 1991. One can only hope that this snapshot, of the technology in 1994, is as clear. Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy Wittbrodt each wrote major chapters of this memo. Others who made major contributions to the document included Bill Barns, Steve Deering, Kent England, Jim Forster, Martin Gross, Jeff Honig, Steve Knowles, Yoni Malachi, Michael Reilly, and Walt Wimer. Additional text came from Andy Malis, Paul Traina, Art Berggreen, John Cavanaugh, Ross Callon, John Lekashman, Brian Lloyd, Gary Malkin, Milo Medin, John Moy, Craig Partridge, Stephanie Price, Yakov Rekhter, Steve Senum, Richard Smith, Frank Solensky, Rich Woundy, and others who have been inadvertently overlooked. Some of the text in this memo has been (shamelessly) plagiarized from earlier documents, most notably RFC 1122 by Bob Braden and the Host Baker Standards Track [Page 174]
RFC 1812 Requirements for IP Version 4 Routers June 1995 Requirements Working Group, and RFC 1009 by Bob Braden and Jon Postel. The work of these earlier authors is gratefully acknowledged. Jim Forster was a co-chair of the Router Requirements Working Group during its early meetings, and was instrumental in getting the group off to a good start. Jon Postel, Bob Braden, and Walt Prue also contributed to the success by providing a wealth of good advice before the group's first meeting. Later on, Phill Gross, Vint Cerf, and Noel Chiappa all provided valuable advice and support. Mike St. Johns coordinated the Working Group's interactions with the security community, and Frank Kastenholz coordinated the Working Group's interactions with the network management area. Allison Mankin and K.K. Ramakrishnan provided expertise on the issues of congestion control and resource allocation. Many more people than could possibly be listed or credited here participated in the deliberations of the Router Requirements Working Group, either through electronic mail or by attending meetings. However, the efforts of Ross Callon and Vince Fuller in sorting out the difficult issues of route choice and route leaking are especially acknowledged. The editor thanks his employer, Cisco Systems, for allowing him to spend the time necessary to produce the 1994 snapshot.



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