ICMP Router Discovery Messages (RFC1256)
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2019-Feb-11
Internet Society Requests For Comment (RFCs)
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. [STANDARDS-TRACK]
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.
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
The following terms have a precise meaning when used in this document:
system a device that implements the Internet Protocol, IP .
router a system that forwards IP datagrams, as specified
Router Discovery Working Group [Page 1]
RFC 1256 ICMP Router Discovery Messages September 1991
in . 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  or IP broadcast  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 .
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  or a single non-subnetted IP network, i.e., the entity identified by an IP address logically ANDed with...