Broadcasting Internet datagrams in the presence of subnets
RFC 922

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Network Working Group                                      Jeffrey Mogul
Request for Comments: 922                    Computer Science Department
                                                     Stanford University
                                                            October 1984

       BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS

Status of this Memo

   We propose simple rules for broadcasting Internet datagrams on local
   networks that support broadcast, for addressing broadcasts, and for
   how gateways should handle them.

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

Acknowledgement

   This proposal here is the result of discussion with several other
   people, especially J. Noel Chiappa and Christopher A. Kent, both of
   whom both pointed me at important references.

1. Introduction

   The use of broadcasts, especially on high-speed local area networks,
   is a good base for many applications.  Since broadcasting is not
   covered in the basic IP specification [12], there is no agreed-upon
   way to do it, and so protocol designers have not made use of it. (The
   issue has been touched upon before, e.g. [6], but has not been the
   subject of a standard.)

   We consider here only the case of unreliable, unsequenced, possibly
   duplicated datagram broadcasts (for a discussion of TCP broadcasting,
   see [10].) Even though unreliable and limited in length, datagram
   broadcasts are quite useful [1].

   We assume that the data link layer of the local network supports
   efficient broadcasting.  Most common local area networks do support
   broadcast; for example, Ethernet [7, 5], ChaosNet [9], token ring
   networks [2], etc.

   We do not assume, however, that broadcasts are reliably delivered.
   (One might consider providing a reliable datagram broadcast protocol
   as a layer above IP.) It is quite expensive to guarantee delivery of
   broadcasts; instead, what we assume is that a host will receive most
   of the broadcasts that are sent.  This is important to avoid
   excessive use of broadcasts; since every host on the network devotes
   at least some effort to every broadcast, they are costly.

Mogul                                                           [Page 1]



RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets

   When a datagram is broadcast, it imposes a cost on every host that
   hears it.  Therefore, broadcasting should not be used
   indiscriminately, but rather only when it is the best solution to a
   problem.

2. Terminology

   Because broadcasting depends on the specific data link layer in use
   on a local network, we must discuss it with reference to both
   physical networks and logical networks.

   The terms we will use in referring to physical networks are, from the
   point of view of the host sending or forwarding a broadcast:

   Local Hardware Network

      The physical link to which the host is attached.

   Remote Hardware Network

      A physical network which is separated from the host by at least
      one gateway.

   Collection of Hardware Networks

      A set of hardware networks (transitively) connected by gateways.

   The IP world includes several kinds of logical network.  To avoid
   ambiguity, we will use the following terms:

   Internet

      The DARPA Internet collection of IP networks.

   IP Network

      One or a collection of several hardware networks that have one
      specific IP network number.

   Subnet

      A single member of the collection of hardware networks that
      compose an IP network.  Host addresses on a given subnet share an
      IP network number with hosts on all other subnets of that IP
      network, but the local-address part is divided into subnet-number

Mogul                                                           [Page 2]



RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets

      and host-number fields to indicate which subnet a host is on.  We
      do not assume a particular division of the local-address part;
      this could vary from network to network.

   The introduction of a subnet level in the addressing hierarchy is at
   variance with the IP specification [12], but as the use of
   addressable subnets proliferates it is obvious that a broadcasting
   scheme should support subnetting.  For more on subnets, see [8].

   In this paper, the term "host address" refers to the host-on-subnet
   address field of a subnetted IP network, or the host-part field
   otherwise.

   An IP network may consist of a single hardware network or a
   collection of subnets; from the point of view of a host on another IP
   network, it should not matter.

3. Why Broadcast?

   Broadcasts are useful when a host needs to find information without