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rfc:rfc922

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.

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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

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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
 knowing exactly what other host can supply it, or when a host wants
 to provide information to a large set of hosts in a timely manner.
 When a host needs information that one or more of its neighbors might
 have, it could have a list of neighbors to ask, or it could poll all
 of its possible neighbors until one responds.  Use of a wired-in list
 creates obvious network management problems (early binding is
 inflexible).  On the other hand, asking all of one's neighbors is
 slow if one must generate plausible host addresses, and try them
 until one works.  On the ARPANET, for example, there are roughly 65
 thousand plausible host numbers.  Most IP implementations have used
 wired-in lists (for example, addresses of "Prime" gateways.)
 Fortunately, broadcasting provides a fast and simple way for a host
 to reach all of its neighbors.
 A host might also use a broadcast to provide all of its neighbors
 with some information; for example, a gateway might announce its
 presence to other gateways.
 One way to view broadcasting is as an imperfect substitute for
 multicasting, the sending of messages to a subset of the hosts on a
 network.  In practice, broadcasts are usually used where multicasts
 are what is wanted; datagrams are broadcast at the hardware level,
 but filtering software in the receiving hosts gives the effect of
 multicasting.
 For more examples of broadcast applications, see [1, 3].

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

4. Broadcast Classes

 There are several classes of IP broadcasting:
  1. Single-destination datagrams broadcast on the local hardware

net: A datagram is destined for a specific IP host, but the

      sending host broadcasts it at the data link layer, perhaps to
      avoid having to do routing.  Since this is not an IP broadcast,
      the IP layer is not involved, except that a host should discard
      datagram not meant for it without becoming flustered (i.e.,
      printing an error message).
  1. Broadcast to all hosts on the local hardware net: A

distinguished value for the host-number part of the IP address

      denotes broadcast instead of a specific host.  The receiving IP
      layer must be able to recognize this address as well as its own.
      However, it might still be useful to distinguish at higher
      levels between broadcasts and non-broadcasts, especially in
      gateways.  This is the most useful case of broadcast; it allows
      a host to discover gateways without wired-in tables, it is the
      basis for address resolution protocols, and it is also useful
      for accessing such utilities as name servers, time servers,
      etc., without requiring wired-in addresses.
  1. Broadcast to all hosts on a remote hardware network: It is

occasionally useful to send a broadcast to all hosts on a

      non-local network; for example, to find the latest version of a
      hostname database, to bootload a host on a subnet without a
      bootserver, or to monitor the timeservers on the subnet.  This
      case is the same as local-network broadcasts; the datagram is
      routed by normal mechanisms until it reaches a gateway attached
      to the destination hardware network, at which point it is
      broadcast.  This class of broadcasting is also known as
      "directed broadcasting", or quaintly as sending a "letter bomb"
      [1].
  1. Broadcast to all hosts on a subnetted IP network (Multi-subnet

broadcasts): A distinguished value for the subnet-number part of

      the IP address is used to denote "all subnets".  Broadcasts to
      all hosts of a remote subnetted IP network are done just as
      directed broadcasts to a single subnet.
  1. Broadcast to the entire Internet: This is probably not useful,

and almost certainly not desirable.

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

 For reasons of performance or security, a gateway may choose not to
 forward broadcasts; especially, it may be a good idea to ban
 broadcasts into or out of an autonomous group of networks.

5. Broadcast Methods

 A host's IP receiving layer must be modified to support broadcasting.
 In the absence of broadcasting, a host determines if it is the
 recipient of a datagram by matching the destination address against
 all of its IP addresses.  With broadcasting, a host must compare the
 destination address not only against the host's addresses, but also
 against the possible broadcast addresses for that host.
 The problem of how best to send a broadcast has been extensively
 discussed [1, 3, 4, 13, 14].  Since we assume that the problem has
 already been solved at the data link layer, an IP host wishing to
 send either a local broadcast or a directed broadcast need only
 specify the appropriate destination address and send the datagram as
 usual.  Any sophisticated algorithms need only reside in gateways.
 The problem of broadcasting to all hosts on a subnetted IP network is
 apparently somewhat harder.  However, even in this case it turns out
 that the best known algorithms require no additional complexity in
 non-gateway hosts.  A good broadcast method will meet these
 additional criteria:
  1. No modification of the IP datagram format.
  1. Reasonable efficiency in terms of the number of excess copies

generated and the cost of paths chosen.

  1. Minimization of gateway modification, in both code and data

space.

  1. High likelihood of delivery.
 The algorithm that appears best is the Reverse Path Forwarding (RPF)
 method [4].  While RPF is suboptimal in cost and reliability, it is
 quite good, and is extremely simple to implement, requiring no
 additional data space in a gateway.

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

6. Gateways and Broadcasts

 Most of the complexity in supporting broadcasts lies in gateways.  If
 a gateway receives a directed broadcast for a network to which it is
 not connected, it simply forwards it using the usual mechanism.
 Otherwise, it must do some additional work.
 6.1. Local Broadcasts
    When a gateway receives a local broadcast datagram, there are
    several things it might have to do with it.  The situation is
    unambiguous, but without due care it is possible to create
    infinite loops.
    The appropriate action to take on receipt of a broadcast datagram
    depends on several things: the subnet it was received on, the
    destination network, and the addresses of the gateway.
  1. The primary rule for avoiding loops is "never broadcast a

datagram on the hardware network it was received on". It is

         not sufficient simply to avoid repeating datagram that a
         gateway has heard from itself; this still allows loops if
         there are several gateways on a hardware network.
  1. If the datagram is received on the hardware network to which

it is addressed, then it should not be forwarded. However,

         the gateway should consider itself to be a destination of the
         datagram (for example, it might be a routing table update.)
  1. Otherwise, if the datagram is addressed to a hardware network

to which the gateway is connected, it should be sent as a

         (data link layer) broadcast on that network.  Again, the
         gateway should consider itself a destination of the datagram.
  1. Otherwise, the gateway should use its normal routing

procedure to choose a subsequent gateway, and send the

         datagram along to it.

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

 6.2. Multi-subnet broadcasts
    When a gateway receives a broadcast meant for all subnets of an IP
    network, it must use the Reverse Path Forwarding algorithm to
    decide what to do.  The method is simple: the gateway should
    forward copies of the datagram along all connected links, if and
    only if the datagram arrived on the link which is part of the best
    route between the gateway and the source of the datagram.
    Otherwise, the datagram should be discarded.
    This algorithm may be improved if some or all of the gateways
    exchange among themselves additional information; this can be done
    transparently from the point of view of other hosts and even other
    gateways.  See [4, 3] for details.
 6.3. Pseudo-Algol Routing Algorithm
    This is a pseudo-Algol description of the routing algorithm a
    gateway should use.  The algorithm is shown in figure 1.  Some
    definitions are:
    RouteLink(host)
       A function taking a host address as a parameter and returning
       the first-hop link from the gateway to the host.
    RouteHost(host)
       As above but returns the first-hop host address.
    ResolveAddress(host)
       Returns the hardware address for an IP host.
    IncomingLink
       The link on which the packet arrived.
    OutgoingLinkSet
       The set of links on which the packet should be sent.
    OutgoingHardwareHost
       The hardware host address to send the packet to.

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

    Destination.host
       The host-part of the destination address.
    Destination.subnet
       The subnet-part of the destination address.
    Destination.ipnet
       The IP-network-part of the destination address.

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

BEGIN

 IF Destination.ipnet IN AllLinks THEN
    BEGIN
       IF IsSubnetted(Destination.ipnet) THEN
          BEGIN
             IF Destination.subnet = BroadcastSubnet THEN
                BEGIN      /* use Reverse Path Forwarding algorithm */
                   IF IncomingLink = RouteLink(Source) THEN
                      BEGIN IF Destination.host = BroadcastHost THEN
                            OutgoingLinkSet <- AllLinks -
                         IncomingLink;
                         OutgoingHost <- BroadcastHost;
                         Examine packet for possible internal use;
                      END
                   ELSE  /* duplicate from another gateway, discard */
                      Discard;
                END
             ELSE
                IF Destination.subnet = IncomingLink.subnet THEN
                   BEGIN           /* forwarding would cause a loop */
                      IF Destination.host = BroadcastHost THEN
                         Examine packet for possible internal use;
                      Discard;
                   END
                ELSE BEGIN    /* forward to (possibly local) subnet */
                      OutgoingLinkSet <- RouteLink(Destination);
                      OutgoingHost <- RouteHost(Destination);
                   END
          END
       ELSE BEGIN         /* destined for one of our local networks */
             IF Destination.ipnet = IncomingLink.ipnet THEN
                BEGIN              /* forwarding would cause a loop */
                   IF Destination.host = BroadcastHost THEN
                      Examine packet for possible internal use;
                   Discard;
                END
             ELSE BEGIN                     /* might be a broadcast */
                   OutgoingLinkSet <- RouteLink(Destination);
                   OutgoingHost <- RouteHost(Destination);
                END
          END
    END
 ELSE BEGIN                    /* forward to a non-local IP network */
       OutgoingLinkSet <- RouteLink(Destination);
       OutgoingHost <- RouteHost(Destination);
    END
 OutgoingHardwareHost <- ResolveAddress(OutgoingHost);

END

Figure 1: Pseudo-Algol algorithm for routing broadcasts by gateways

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

7. Broadcast IP Addressing - Conventions

 If different IP implementations are to be compatible, there must be
 convention distinguished number to denote "all hosts" and "all
 subnets".
 Since the local network layer can always map an IP address into data
 link layer address, the choice of an IP "broadcast host number" is
 somewhat arbitrary.  For simplicity, it should be one not likely to
 be assigned to a real host.  The number whose bits are all ones has
 this property; this assignment was first proposed in [6].  In the few
 cases where a host has been assigned an address with a host-number
 part of all ones, it does not seem onerous to require renumbering.
 The "all subnets" number is also all ones; this means that a host
 wishing to broadcast to all hosts on a remote IP network need not
 know how the destination address is divided up into subnet and host
 fields, or if it is even divided at all.  For example, 36.255.255.255
 may denote all the hosts on a single hardware network, or all the
 hosts on a subnetted IP network with 1 byte of subnet field and 2
 bytes of host field, or any other possible division.
 The address 255.255.255.255 denotes a broadcast on a local hardware
 network that must not be forwarded.  This address may be used, for
 example, by hosts that do not know their network number and are
 asking some server for it.
 Thus, a host on net 36, for example, may:
  1. broadcast to all of its immediate neighbors by using

255.255.255.255

  1. broadcast to all of net 36 by using 36.255.255.255
 without knowing if the net is subnetted; if it is not, then both
 addresses have the same effect. A robust application might try the
 former address, and if no response is received, then try the latter.
 See [1] for a discussion of such "expanding ring search" techniques.
 If the use of "all ones" in a field of an IP address means
 "broadcast", using "all zeros" could be viewed as meaning
 "unspecified".  There is probably no reason for such addresses to
 appear anywhere but as the source address of an ICMP Information
 Request datagram.  However, as a notational convention, we refer to
 networks (as opposed to hosts) by using addresses with zero fields.
 For example, 36.0.0.0 means "network number 36" while 36.255.255.255
 means "all hosts on network number 36".

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

 7.1. ARP Servers and Broadcasts
    The Address Resolution Protocol (ARP) described in [11] can, if
    incorrectly implemented, cause problems when broadcasts are used
    on a network where not all hosts share an understanding of what a
    broadcast address is.  The temptation exists to modify the ARP
    server so that it provides the mapping between an IP broadcast
    address and the hardware broadcast address.
    This temptation must be resisted.  An ARP server should never
    respond to a request whose target is a broadcast address.  Such a
    request can only come from a host that does not recognize the
    broadcast address as such, and so honoring it would almost
    certainly lead to a forwarding loop.  If there are N such hosts on
    the physical network that do not recognize this address as a
    broadcast, then a datagram sent with a Time-To-Live of T could
    potentially give rise to T**N spurious re-broadcasts.

8. References

 1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
      University, January 1982.
 2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
      Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516,
      November 1978.
 3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
      Computer Networks.  Ph.D. Th., Stanford University, April 1977.
 4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
      of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048,
      December 1978.
 5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
      Layer Specifications.  Version 1.0, Digital Equipment
      Corporation, Intel, Xerox, September 1980.
 6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
      Addressing Issues.  IEN-212, BBN, September 1982.
 7.   R.M. Metcalfe and D.R. Boggs.  "Ethernet: Distributed Packet
      Switching for Local Computer Networks".  Comm. ACM 19, 7,
      pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
      Center, reprinted in CSL-80-2.

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

 8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
      October 1984.
 9.   David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
      Institute of Technology Artificial Intelligence Laboratory,
      June 1981.
 10.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, BBN,
      March 1977.
 11.  David Plummer.  An Ethernet Address Resolution Protocol.
      RFC-826, Symbolics, September 1982.
 12.  Jon Postel.  Internet Protocol.  RFC-791, ISI, September 1981.
 13.  David W. Wall.  Mechanisms for Broadcast and Selective
      Broadcast.  Ph.D. Th., Stanford University, June 1980.
 14.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
      Computer Systems Lab Technical Report TR189, Stanford
      University, June 1980.

Mogul [Page 12]

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