rfc1812.txt

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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



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   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



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   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



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   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



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   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