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Network Working Group                                      Jeffrey MogulRequest for Comments: 919                    Computer Science Department                                                     Stanford University                                                            October 1984BROADCASTING INTERNET DATAGRAMSStatus 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 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 [13], 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 [11].) 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 [10], token ring   networks [2], etc.   We do not assume, however, that broadcasts are reliably delivered.   (One might consider providing a reliable 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 919                                                     October 1984Broadcasting Internet Datagrams   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.   Note: some organizations have divided their IP networks into subnets,   for which a standard [8] has been proposed.  This RFC does not cover   the numerous complications arising from the interactions between   subnets and broadcasting; see [9] for a complete discussion.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.Mogul                                                           [Page 2]

RFC 919                                                     October 1984Broadcasting Internet Datagrams3. 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; packets 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].4. Broadcast Classes   There are several classes of IP broadcasting:      - Single-destination datagram broadcast on the local IP net: A        datagrams 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        datagrams not meant for it without becoming flustered (i.e.,        printing an error message).      - Broadcast to all hosts on the local IP 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.Mogul                                                           [Page 3]

RFC 919                                                     October 1984Broadcasting Internet Datagrams        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.      - Broadcast to all hosts on a remote IP 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 an IP network without a        bootserver, or to monitor the timeservers on the IP network.        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 IP 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].      - Broadcast to the entire Internet: This is probably not useful,        and almost certainly not desirable.   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, 14, 15].  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.Mogul                                                           [Page 4]

RFC 919                                                     October 1984Broadcasting Internet Datagrams6. 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.   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.      - 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 datagrams that a gateway        has heard from itself; this still allows loops if there are        several gateways on a hardware network.      - 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.)      - 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.      - Otherwise, the gateway should use its normal routing procedure        to choose a subsequent gateway, and send the datagram along to        it.7. Broadcast IP Addressing - Proposed Standards   If different IP implementations are to be compatible, there must be a   distinguished number to denote "all hosts".   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.Mogul                                                           [Page 5]

RFC 919                                                     October 1984Broadcasting Internet Datagrams   The address 255.255.255.255 denotes a broadcast on a local hardware   network, which 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:      - broadcast to all of its immediate neighbors by using        255.255.255.255      - broadcast to all of net 36 by using 36.255.255.255   (Note that unless the network has been broken up into subnets, these   two methods have identical effects.)   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".   7.1. ARP Servers and Broadcasts      The Address Resolution Protocol (ARP) described in [12] 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.Mogul                                                           [Page 6]

RFC 919                                                     October 1984Broadcasting Internet Datagrams8. 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, 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, Bolt Beranek and Newman, 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.   8.   Jeffrey Mogul.  Internet Subnets.RFC-917, Stanford University,        October 1984.   9.   Jeffrey Mogul.  Broadcasting Internet Packets in the Presence of        Subnets.RFC-922, Stanford University, October 1984.   10.  David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts        Institute of Technology Artificial Intelligence Laboratory, June        1981.   11.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, Bolt        Beranek and Newman, March 1977.   12.  David Plummer.  An Ethernet Address Resolution Protocol.RFC-826, Symbolics, September 1982.   13.  Jon Postel.  Internet Protocol.RFC 791, ISI, September 1981.Mogul                                                           [Page 7]

RFC 919                                                     October 1984Broadcasting Internet Datagrams   14.  David W. Wall.  Mechanisms for Broadcast and Selective        Broadcast.  Ph.D. Th., Stanford University, June 1980.   15.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.        Computer Systems Lab Technical Report TR189, Stanford        University, June 1980.Mogul                                                           [Page 8]