rfc2080.txt

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Network Working Group                                          G. Malkin
Request for Comments: 2080                                      Xylogics
Category: Standards Track                                     R. Minnear
                                                        Ipsilon Networks
                                                            January 1997

                             RIPng for IPv6

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.

Abstract

   This document specifies a routing protocol for an IPv6 internet.  It
   is based on protocols and algorithms currently in wide use in the
   IPv4 Internet.

   This specification represents the minimum change to the Routing
   Information Protocol (RIP), as specified in RFC 1058 [1] and RFC 1723
   [2], necessary for operation over IPv6 [3].

Acknowledgements

   This document is a modified version of RFC 1058, written by Chuck
   Hedrick [1].  The modifications reflect RIP-2 and IPv6 enhancements,
   but the original wording is his.

   We'd like to thank Dennis Ferguson and Thomas Narten for their input.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   1.1   Theoretical Underpinnings  . . . . . . . . . . . . . . . . .  3
   1.2   Limitations of the Protocol  . . . . . . . . . . . . . . . .  3
   2.  Protocol Specification . . . . . . . . . . . . . . . . . . . .  4
   2.1   Message Format . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.1   Next Hop . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.2   Addressing Considerations  . . . . . . . . . . . . . . . . .  8
   2.3   Timers . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   2.4   Input Processing . . . . . . . . . . . . . . . . . . . . . . 10
   2.4.1   Request Messages . . . . . . . . . . . . . . . . . . . . . 10
   2.4.2   Response Messages  . . . . . . . . . . . . . . . . . . . . 11



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   2.5   Output Processing  . . . . . . . . . . . . . . . . . . . . . 14
   2.5.1   Triggered Updates  . . . . . . . . . . . . . . . . . . . . 14
   2.5.2   Generating Response Messages . . . . . . . . . . . . . . . 15
   2.6   Split Horizon  . . . . . . . . . . . . . . . . . . . . . . . 16
   3.  Control Functions  . . . . . . . . . . . . . . . . . . . . . . 17
   4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 18
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

1. Introduction

   This memo describes one protocol in a series of routing protocols
   based on the Bellman-Ford (or distance vector) algorithm.  This
   algorithm has been used for routing computations in computer networks
   since the early days of the ARPANET.  The particular packet formats
   and protocol described here are based on the program "routed," which
   is included with the Berkeley distribution of Unix.

   In an international network, such as the Internet, it is very
   unlikely that a single routing protocol will used for the entire
   network.  Rather, the network will be organized as a collection of
   Autonomous Systems (AS), each of which will, in general, be
   administered by a single entity.  Each AS will have its own routing
   technology, which may differ among AS's.  The routing protocol used
   within an AS is referred to as an Interior Gateway Protocol (IGP).  A
   separate protocol, called an Exterior Gateway Protocol (EGP), is used
   to transfer routing information among the AS's.  RIPng was designed
   to work as an IGP in moderate-size AS's.  It is not intended for use
   in more complex environments.  For information on the context into
   which RIP version 1 (RIP-1) is expected to fit, see Braden and Postel
   [6].

   RIPng is one of a class of algorithms known as Distance Vector
   algorithms.  The earliest description of this class of algorithms
   known to the author is in Ford and Fulkerson [8].  Because of this,
   they are sometimes known as Ford-Fulkerson algorithms.  The term
   Bellman-Ford is also used, and derives from the fact that the
   formulation is based on Bellman's equation [4].  The presentation in
   this document is closely based on [5].  This document contains a
   protocol specification.  For an introduction to the mathematics of
   routing algorithms, see [1].  The basic algorithms used by this
   protocol were used in computer routing as early as 1969 in the
   ARPANET.  However, the specific ancestry of this protocol is within
   the Xerox network protocols.  The PUP protocols [7] used the Gateway
   Information Protocol to exchange routing information.  A somewhat
   updated version of this protocol was adopted for the Xerox Network
   Systems (XNS) architecture, with the name Routing Information
   Protocol [9].  Berkeley's routed is largely the same as the Routing



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   Information Protocol, with XNS addresses replaced by a more general
   address format capable of handling IPv4 and other types of address,
   and with routing updates limited to one every 30 seconds.  Because of
   this similarity, the term Routing Information Protocol (or just RIP)
   is used to refer to both the XNS protocol and the protocol used by
   routed.

1.1  Theoretical Underpinnings

   An introduction to the theory and math behind Distance Vector
   protocols is provided in [1].  It has not been incorporated in this
   document for the sake of brevity.

1.2  Limitations of the Protocol

   This protocol does not solve every possible routing problem.  As
   mentioned above, it is primarily intended for use as an IGP in
   networks of moderate size.  In addition, the following specific
   limitations are be mentioned:

   - The protocol is limited to networks whose longest path (the
     network's diameter) is 15 hops.  The designers believe that the
     basic protocol design is inappropriate for larger networks.  Note
     that this statement of the limit assumes that a cost of 1 is used
     for each network.  This is the way RIPng is normally configured.
     If the system administrator chooses to use larger costs, the upper
     bound of 15 can easily become a problem.

   - The protocol depends upon "counting to infinity" to resolve certain
     unusual situations (see section 2.2 in [1]).  If the system of
     networks has several hundred networks, and a routing loop was formed
     involving all of them, the resolution of the loop would require
     either much time (if the frequency of routing updates were limited)
     or bandwidth (if updates were sent whenever changes were detected).
     Such a loop would consume a large amount of network bandwidth
     before the loop was corrected.  We believe that in realistic cases,
     this will not be a problem except on slow lines.  Even then, the
     problem will be fairly unusual, since various precautions are taken
     that should prevent these problems in most cases.

   - This protocol uses fixed "metrics" to compare alternative routes.
     It is not appropriate for situations where routes need to be chosen
     based on real-time parameters such a measured delay, reliability,
     or load.  The obvious extensions to allow metrics of this type are
     likely to introduce instabilities of a sort that the protocol is
     not designed to handle.





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2. Protocol Specification

   RIPng is intended to allow routers to exchange information for
   computing routes through an IPv6-based network.  RIPng is a distance
   vector protocol, as described in [1].  RIPng should be implemented
   only in routers; IPv6 provides other mechanisms for router discovery
   [10].  Any router that uses RIPng is assumed to have interfaces to
   one or more networks, otherwise it isn't really a router.  These are
   referred to as its directly-connected networks.  The protocol relies
   on access to certain information about each of these networks, the
   most important of which is its metric.  The RIPng metric of a network
   is an integer between 1 and 15, inclusive.  It is set in some manner
   not specified in this protocol; however, given the maximum path limit
   of 15, a value of 1 is usually used.  Implementations should allow
   the system administrator to set the metric of each network.  In
   addition to the metric, each network will have an IPv6 destination
   address prefix and prefix length associated with it.  These are to be
   set by the system administrator in a manner not specified in this
   protocol.

   Each router that implements RIPng is assumed to have a routing table.
   This table has one entry for every destination that is reachable
   throughout the system operating RIPng.  Each entry contains at least
   the following information:

   - The IPv6 prefix of the destination.

   - A metric, which represents the total cost of getting a datagram
     from the router to that destination.  This metric is the sum of the
     costs associated with the networks that would be traversed to get
     to the destination.

   - The IPv6 address of the next router along the path to the
     destination (i.e., the next hop).  If the destination is on one of
     the directly-connected networks, this item is not needed.

   - A flag to indicate that information about the route has changed
     recently.  This will be referred to as the "route change flag."

   - Various timers associated with the route.  See section 2.3 for more
     details on timers.

   The entries for the directly-connected networks are set up by the
   router using information gathered by means not specified in this
   protocol.  The metric for a directly-connected network is set to the
   cost of that network.  As mentioned, 1 is the usual cost.  In that
   case, the RIPng metric reduces to a simple hop-count.  More complex
   metrics may be used when it is desirable to show preference for some



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   networks over others (e.g., to indicate of differences in bandwidth
   or reliability).

   Implementors may also choose to allow the system administrator to
   enter additional routes.  These would most likely be routes to hosts
   or networks outside the scope of the routing system.  They are
   referred to as "static routes."  Entries for destinations other than
   these initial ones are added and updated by the algorithms described
   in the following sections.

   In order for the protocol to provide complete information on routing,
   every router in the AS must participate in the protocol.  In cases
   where multiple IGPs are in use, there must be at least one router
   which can leak routing information between the protocols.

2.1  Message Format

   RIPng is a UDP-based protocol.  Each router that uses RIPng has a
   routing process that sends and receives datagrams on UDP port number
   521, the RIPng port.  All communications intended for another
   router's RIPng process are sent to the RIPng port.  All routing
   update messages are sent from the RIPng port.  Unsolicited routing
   update messages have both the source and destination port equal to
   the RIPng port.  Those sent in response to a request are sent to the
   port from which the request came.  Specific queries may be sent from
   ports other than the RIPng port, but they must be directed to the
   RIPng port on the target machine.

   The RIPng packet format is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  command (1)  |  version (1)  |       must be zero (2)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                Route Table Entry 1 (20)                       ~
      |                                                               |