ICMP Router Discovery Messages
RFC 1256

Document Type RFC - Proposed Standard (September 1991; No errata)
Last updated 2013-03-02
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Network Working Group                                 S. Deering, Editor
Request for Comments: 1256                                    Xerox PARC
                                                          September 1991

                     ICMP Router Discovery Messages

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   This document is a product of the IETF Router Discovery Working
   Group.  Distribution of this memo is unlimited.

Abstract

   This document specifies an extension of the Internet Control Message
   Protocol (ICMP) to enable hosts attached to multicast or broadcast
   networks to discover the IP addresses of their neighboring routers.

Table of Contents

   1. Terminology                                                      1
   2. Protocol Overview                                                3
   3. Message Formats                                                  5
   4. Router Specification                                             7
        4.1. Router Configuration Variables                            7
        4.2. Message Validation by Routers                             9
        4.3. Router Behavior                                           9
   5. Host Specification                                              12
        5.1. Host Configuration Variables                             12
        5.2. Message Validation by Hosts                              13
        5.3. Host Behavior                                            14
   6. Protocol Constants                                              17
   7. Security Considerations                                         17
   References                                                         18
   Author's Address                                                   19

1. Terminology

   The following terms have a precise meaning when used in this
   document:

   system        a device that implements the Internet Protocol, IP [9].

   router        a system that forwards IP datagrams, as specified

Router Discovery Working Group                                  [Page 1]
RFC 1256             ICMP Router Discovery Messages       September 1991

                 in [2].  This does not include systems that, though
                 capable of IP forwarding, have that capability turned
                 off.  Nor does it include systems that do IP forwarding
                 only insofar as required to obey IP Source Route
                 options.

   host          any system that is not a router.

   multicast     unless otherwise qualified, means the use of either IP
                 multicast [4] or IP broadcast [6] service.

   link          a communication facility or medium over which systems
                 can communicate at the link layer, i.e., the protocol
                 layer immediately below IP.  The term "physical
                 network" has sometimes been used (imprecisely) for
                 this. Examples of links are LANs (possibly bridged to
                 other LANs), wide-area store-and-forward networks,
                 satellite channels, and point-to-point links.

   multicast link
                 a link over which IP multicast or IP broadcast service
                 is supported.  This includes broadcast media such as
                 LANs and satellite channels, single point-to-point
                 links, and some store-and-forward networks such as SMDS
                 networks [8].

   interface     a system's attachment point to a link.  It is possible
                 (though unusual) for a system to have more than one
                 interface to the same link.  Interfaces are uniquely
                 identified by IP unicast addresses; a single interface
                 may have more than one such address.

   multicast interface
                 an interface to a multicast link, that is, an interface
                 to a link over which IP multicast or IP broadcast
                 service is supported.

   subnet        either a single subnet of a subnetted IP network [7] or
                 a single non-subnetted IP network, i.e., the entity
                 identified by an IP address logically ANDed with its
                 assigned subnet mask.  More than one subnet may exist
                 on the same link.

   neighboring   having an IP address belonging to the same subnet.

Router Discovery Working Group                                  [Page 2]
RFC 1256             ICMP Router Discovery Messages       September 1991

2. Protocol Overview

   Before a host can send IP datagrams beyond its directly-attached
   subnet, it must discover the address of at least one operational
   router on that subnet. Typically, this is accomplished by reading a
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