    Network Working Group                                           T. Smith
    Request for Comments: 2226                               IBM Corporation
    Category: Standards Track                                    G. Armitage
                                                         Lucent Technologies
                                                                October 1997

                         IP Broadcast over ATM Networks

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1997).  All Rights Reserved.

    Abstract

       This memo describes how the IP multicast service being developed by
       the IP over ATM working group may be used to support IP broadcast
       transmission. The solution revolves around treating the broadcast
       problem as a special case of multicast, where every host in the
       subnet or cluster is a member of the group.

       An understanding of the services provided by RFC 2022 is assumed.

    1.  Introduction.

       The IETF's first step in solving the problems of running IP over
       Asynchronous Transfer Mode (ATM) technology is described in RFC 1577
       [1].  It provides for unicast communication between hosts and routers
       within Logical IP Subnets (LISs), and proposes a centralized ATM ARP
       Server which provides IP to ATM address resolution services to LIS
       members.

       Two classes of IP service were omitted - multicast and broadcast
       transmissions. Multicasting allows a single transmit operation to
       cause a packet to be received by multiple remote destinations.

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       Broadcasting typically allows a single transmit operation to cause a
       packet to be received by all IP hosts that are members of a
       particular 'subnet'.

       To address the need for multicast support (represented by
       transmission to IP addresses in the Class D space), RFC 2022
       ("Support for Multicast over UNI 3.0/3.1 based ATM Networks") [2] was
       created.  This memo creates an analog of the RFC 1577 ARP Server - a
       new entity known as the MARS (Multicast Address Resolution Server).
       The MARS operates as a centralized registry and distribution
       mechanism for mappings between IP multicast addresses and groups of
       ATM unicast addresses. Host behavior is also defined for establishing
       and managing point to multipoint VCs, based on the information
       returned by the MARS, when hosts wish to transmit packets to a
       multicast group.

       This memo aims to show how RFC 2022 may be used to emulate IP
       broadcast within Logical IP Subnets. While the broadcast technique
       does not align itself well with the underlying point-to-point nature
       of ATM, clearly, some applications will still wish to use IP
       broadcasts.  Client-server applications where the client searches for
       a server by sending out a broadcast is one scenario.  Routing
       protocols, most notably RIP, are other examples.

    2.  Review of Unicast and Multicast.

       Both the unicast and multicast cases take advantage of the point-to-
       point and point-to-multipoint capabilities defined in the ATM Forum
       UNI 3.1 document [4].  A unicast IP address has a single ATM level
       destination.  Unicast transmissions occur over point to point Virtual
       Channels (VCs) between the source and destination. The ARP Server
       holds mappings between IP destination addresses and their associated
       ATM destination address. Hosts issue an ARP_REQUEST to the ARP Server
       when they wish to ascertain a particular mapping.  The ARP Server
       replies with either an ARP_REPLY containing the ATM address of the
       destination, or an ARP_NAK when the ARP Server is unable to resolve
       the address. If the request is successful the host establishes a VC
       to the destination interface. This VC is then used to forward the
       first (and subsequent) packets to that particular IP destination. RFC
       1577 describes in further detail how hosts are administratively
       grouped in to Logical IP Subnets (LISs), and how the ARP Server
       establishes the initial mappings for members of the LIS it serves.

       The basic host behavior for multicasting is similar - the sender must
       establish and manage a point to multipoint VC whose leaf nodes are
       the group's actual members. Under UNI 3.1 these VCs can only be
       established and altered by the source (root) interface.

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

       The MARS is an evolution of the ARP Server model, and performs two
       key functions.  The first function is the maintenance of a list of
       ATM addresses corresponding to the members for each group.  This list
       is created by a host registration process which involves two messages
       - a MARS_JOIN which declares that a host wishes to join the specified
       group(s), and a MARS_LEAVE which indicates that a host wishes to
       leave the specified group(s).

       MARS_JOIN and MARS_LEAVE messages are also redistributed to all
       members of the group so that active senders may dynamically adjust
       their point to multipoint VCs accordingly.

       The other major function is the retrieval of group membership from
       MARS (analogous to the ARP Server providing unicast address
       mappings). When faced with the need to transmit an IP packet with a
       Class D destination address, a host issues a MARS_REQUEST to the
       MARS. If the group has members the MARS returns a MARS_MULTI
       (possibly in multiple segments) carrying a set of ATM addresses. The
       host then establishes an initial point to multipoint VC using these
       ATM addresses as the leaf nodes. If the MARS had no mapping it would
       return a MARS_NAK.

       (RFC 2022 also discusses how the MARS can arrange for Class D groups
       to be supported by either multicast servers, or meshes of point to
       multipoint VCs from host to host.  However, from the host's
       perspective this is transparent, and is not central to this
       discussion of IP broadcast support.)

       This memo describes how a host may utilize the registration and group
       management functions in an existing MARS based IP/ATM network to
       emulate IP broadcasts.

    3.  Broadcast as a special case of Multicast.

       Many of the problems that occur when implementing a broadcast
       solution also occur in when implementing a multicast solution.  In
       fact, broadcast may be considered a special case of multicast.  That
       is, broadcast is a multicast group whose members include all members
       in the LIS.

       There are two broadcast groups which this memo addresses:

          1) 255.255.255.255 - "All ones" broadcast

          2) x.z - CIDR-prefix (subnet) directed broadcast

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       Broadcast (1) is sometimes referred to as a limited broadcast to this
       physical network.  Broadcast (2) can be thought of as the the
       broadcast for subnets or networks in the old paradigm. As described
       in [6] and [7], the notion of subnets and networks is being replaced
       with a more efficient utilization of the routing address space known
       as Classless Inter-Domain Routing.  The CIDR-prefix (x) is the
       combination of IP address and subnet mask that denotes the subnet
       number.  The host portion of the address (z) is all ones.  One should
       note that while these broadcasts have different scopes at the IP or
       network layer, they have precisely the same scope at the link layer
       -- namely that all members of the LIS will receive a copy.

       These addresses may be used in two environments:

          o  Broadcasting to all members of a given LIS where
             a priori knowledge of a host's IP address and
             subnet mask are known (e.g. the CIDR-prefix directed
             broadcast).

          o  Broadcasting to all members of a physical network
             without knowledge of a host's IP address and
             subnet mask (e.g. the all ones broadcast).

       On a broadcast medium like Ethernet, these two environments result in
       the same physical destination.  That is, all stations on that network
       will receive the broadcast even if they are on different logical
       subnets, or are non-IP stations.  With ATM, this may not be the case.
       Because ATM is non-broadcast, a registration process must take place.
       And if there are stations that register to some broadcast groups, but
       not others, then the different broadcast groups will have different
       memberships.  The notion of broadcast becomes inconsistent.

       One case that requires the use of the all ones broadcast is that of
       the diskless boot, or bootp client, where the host boots up, and does
       not know its own IP address or subnet mask.  Clearly, the host does
       not know which subnet it belongs to.   So, to send a broadcast to its
       bootp server, the diskless workstation must use the group which
       contains no subnet information, i.e. the 255.255.255.255 broadcast
       group.  Carrying the example a little further, the bootp server,
       after receiving the broadcast, can not send either a directed frame
       nor a subnet directed broadcast to respond to the diskless
       workstation.  Instead, the bootp server must also use the
       255.255.255.255 group to communicate with the client.

       While the all ones broadcast is required at the IP layer, it also has
       relevance at the link layer when deciding which broadcast group to
       register with in MARS.  In other words, a bootp client wishing to
       register for a link layer broadcast, can only register for

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       255.255.255.255 in the MARS address space because the client's subnet
       is unknown at the time.  Given that some applications must use the
       all ones address in MARS for their broadcast group, and that we wish
       to minimize the number of broadcast groups used by LIS members, the
       all ones group in MARS MUST be used by all members of the LIS when
       registering to receive broadcast transmissions.  The VCC used for
       transmitting any broadcast packet will be based on the members
       registered in the MARS under the 255.255.255.255 address position.
       This VCC will be referred to as the "broadcast channel" through the
       remainder of this memo.

    4.  The MARS role in broadcast.

       Many solutions have been proposed, some of which are listed in
       Appendix A.  This memo addresses a MARS solution which appears to do
       the best job of solving the broadcast problem.

       There are a number of characteristics of the MARS architecture that
       should be kept intact.  They include:

       o  MARS contains no knowledge of subnet prefixes and subnet masks.
          Each group address registered with MARS is managed independently.

       o  A MARS may only serve one LIS. This insures that the
          broadcast group 255.255.255.255 is joined by hosts from one
          LIS, keeping its scope bound to conventional interpretation.

       o  The Multicast Server (MCS) described in [2] may be used to service
          the broadcast groups defined in this memo without modification.
          The MCS will reduce the number of channels used by the network.

       The MARS needs no additional code or special algorithms to handle the
       resolution of IP broadcast addresses. It is simply a general database
       that holds {Protocol address, ATM.1, ATM.2, ... ATM.n} mappings, and
       imposes no constraints on the type and length of the 'Protocol
       address'. Whether the hosts view it as Class D or 'broadcast' (or
       even IP) is purely a host side issue.

       It is likely that end points will want to use the IP broadcast
       emulation described here in order to support boot time location of
       the end point's IP address. This leads to the observation that the
       MARS should NOT expect to see both the IP source and ATM source
       address fields of the MARS_JOIN filled in.  This is reasonable, since
       only the ATM source address is used when registering the end point as
       a group member.

       The MARS architecture is sufficient to insure the integrity of the
       broadcast group list without any modification.

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    5.  Host Requirements for Broadcast.

       The following list of bullets describes additional characteristics of
       a MARS-compliant host.  These characteristics are required to take
       advantage of the broadcast function.

       o  A host must register as a MARS client.

       o  A host, soon after registration MUST issue a MARS_JOIN to the
          all ones broadcast address (i.e. 255.255.255.255) with the
          mar$flags.layer3grp reset.

       o  When transmitting packets, the host should map all IP layer
          broadcasts to the VCC (broadcast channel) created and maintained
          based on the all ones entry in MARS.

       o  A host MUST monitor the MARS_JOIN/MARS_LEAVE messages
          for 255.255.255.255 to keep the broadcast channel current.

       o  A broadcast channel should be torn down after a period of
          inactivity.  The corresponding timeout period MAY be specified
          with a minimum value of one minute, and a RECOMMENDED
          default value of 20 minutes.

       One should note that while every member participating in the
       broadcast MUST be a member of the all ones group, not all members
       will choose to transmit broadcast information.  Some members will
       only elect to receive broadcast information passively.  Therefore, in
       a LIS with n stations, there may be less than n channels terminated
       at each station for broadcast information.  Further reductions may be
       gained by adding a Multicast Server (MCS) to the broadcast
       environment which could reduce the number of VCs to two (one
       incoming, one outgoing), or one for a station that only wishes to
       listen.

       It is well understood that broadcasting in this environment may tax
       the resources of the network and of the hosts that use it.
       Therefore, an implementer MAY choose to provide a mechanism for
       retracting the host's entry in the broadcast group after it has been
       established or prior to joining the group.  The MARS_LEAVE is used to
       request withdrawal from the group if the host wishes to disable
       broadcast reception after it has joined the group.  The default
       behavior SHALL be to join the all ones broadcast group in MARS.

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    6.  Implications of IP broadcast on ATM level resources.

       RFC 2022 discusses some of the implications of large multicast groups
       on the allocation of ATM level resources, both within the network and
       within end station ATM interfaces.

       The default mechanism is for IP multicasting to be achieved using
       meshes of point to multipoint VCs, direct from source host to group
       members. Under certain circumstances system administrators may, in a
       manner completely transparent to end hosts, redirect multicast
       traffic through ATM level Multicast Servers (MCSs). This may be
       performed on an individual group basis.

       It is sufficient to note here that the IP broadcast 'multicast group'
       will constitute the largest consumer of VCs within your ATM network
       when it is active. For this reason it will probably be the first
       multicast group to have one or more ATM MCSs assigned to support it.
       However, there is nothing unique about an MCS assigned to support IP
       broadcast traffic, so this will not be dealt with further in this
       memo. RFC 2022 contains further discussion on the possible
       application of multiple MCSs to provide fault-tolerant architectures.

    7.  Further discussion.

       A point of discussion on the ip-atm forum revolved around "auto
       configuration" and "diskless boot".  This memo describes a broadcast
       solution that requires the use of the MARS.  Therefore, at a minimum,
       the ATM address of the MARS must be manually configured into a
       diskless workstation.  Suggestions such as universal channel numbers,
       and universal ATM addresses have been proposed, however, no agreement
       has been reached.

       Another topic for discussion is multiprotocol support.  MARS is
       designed for protocol independence.  This memo specifically addresses
       the IP broadcast case, identifying which addresses are most effective
       in the IP address space.  However, the principles apply to any layer
       3 protocol.  Further work should be performed to identify suitable
       addresses for other layer 3 protocols.

       Finally, there has been support voiced for a link layer broadcast
       that would be independent of the layer 3 protocol.  Such a solution
       may provide a simpler set of rules through which broadcast
       applications may be used.  In addition, some solutions also provide
       for more efficient use of VCCs.

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

    Security Considerations

       This memo addresses a specific use of the MARS architecture and
       components to provide the broadcast function.  As such, the security
       implications are no greater or less than the implications of using
       any of the other multicast groups available in the multicast address
       range.  Should enhancements to security be required, they would need
       to be added as an extension to the base architecture in RFC 2022.

    Acknowledgments

       The apparent simplicity of this memo owes a lot to the services
       provided in [2], which itself is the product of much discussion on
       the IETF's IP-ATM working group mailing list.  Grenville Armitage
       worked on this document while at Bellcore.

    References

       [1]  Laubach, M., "Classical IP and ARP over ATM", RFC 1577,
            December 1993.

       [2]  Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
            Networks", RFC 2022, November 1995.

       [3]  Deering, S., "Host Extensions for IP Multicasting", STD 5,
            RFC 1112, August 1989.

       [4]  ATM Forum, "ATM User-Network Interface Specification Version
            3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.

       [5]  Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E. and
            A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
            February 1995.

       [6]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
            Domain Routing (CIDR): an Address Assignment and Aggregation
            Strategy", RFC 1519, September 1993.

       [7]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
            June 1995.

       [8]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
            Levels, BCP 14, RFC 2119, March 1997.

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

    Authors' Addresses

       Timothy J. Smith
       Network Routing Systems,
       International Business Machines Corporation.
       N21/664
       P.O.Box 12195
       Research Triangle Park, NC 27709

       Phone: (919) 254-4723
       EMail: tjsmith@vnet.ibm.com

       Grenville Armitage
       Bell Labs, Lucent Technologies.
       101 Crawfords Corner Rd,
       Holmdel, NJ, 07733

       EMail: gja@lucent.com

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

    Appendix A.  Broadcast alternatives

       Throughout the development of this memo, there have been a number of
       alternatives explored and discarded for one reason or another.  This
       appendix documents these alternatives and the reason that they were
       not chosen.

    A.1  ARP Server Broadcast Solutions.

       The ARP Server is a good candidate to support broadcasting.  There is
       an ARP Server for every LIS.  The ARP Server contains the entire LIS
       membership.  These are fundamental ingredients for the broadcast
       function.

    A.1.1  Base Solution without modifications to ARP Server.

       One may choose as an existing starting point to use only what is
       available in RFC 1577.  That is, a host can easily calculate the
       range of members in its LIS based on its own IP address and subnet
       mask.  The host can then issue an ARP Request for every member of the
       LIS.  With this information, the host can then set up point-to-point
       connections with all members, or can set up a point-to-multipoint
       connection to all members.  There you have it, the poor man's
       broadcast.

       While this solution is very straight forward, it suffers from a
       number of problems.

       o  The load on the ARP Server is very large.  If all stations on
          a LIS choose to implement broadcasting, the initial surge of ARP
          Requests will be huge.  Some sort of slow start sequence would be
          needed.

       o  The amount of resource required makes this a non-scalable
          solution.  The authors believe that broadcasting will require an
          MCS to reduce the number of channel resources required to support
          each broadcast 'group'.  Using the ARP Server in this manner does
          not allow an MCS to be transparently introduced. (Basic RFC1577
          interfaces also do not implement the extended LLC/SNAP
          encapsulation required to safely use more than one MCS).

       o  The diskless boot solution can not function in this environment
          because it may be unable to determine which subnet to which it
          belongs.

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

    A.1.2  Enhanced ARP Server solution.

       This solution is similar to the base solution except that it takes
       some of the (MARS) multicast solution and embeds it in the ARP
       Server.  The first enhancement is to add the MARS_MULTI command to
       the set of opcodes that the ARP Server supports.  This would allow a
       host to issue a single request, and to get back the list of members
       in one or more MARS_REPLY packets.  Rather than have a registration
       mechanism, the ARP Server could simply use the list of members that
       have already been registered.  When a request comes in for the subnet
       broadcast address, the ARP Server would aggregate the list, and send
       the results to the requester.

       This suffers from two drawbacks.

       1)  Scalability with regard to number of VCs is still an issue.
           One would eventually need to add in some sort of multicast
           server solution to the ARP Server.

       2)  The diskless boot scenario is still broken.  There is no
           way for a station to perform a MARS_MULTI without first
           knowing its IP address and subnet mask.

       The diskless boot problem could be solved by adding to the ARP Server
       a registration process where anyone could register to the
       255.255.255.255 address.  These changes would make the ARP Server
       look more and more like MARS.

    A.2  MARS Solutions.

       If we wish to keep the ARP Server constant as described in RFC 1577,
       the alternative is to use the Multicast Address Resolution Server
       (MARS) described in [2].

       MARS has three nice features for broadcasting.

       1)  It has a generalized registration approach which allows
           for any address to have a group of entities registered.
           So, if the subnet address is not known, a host can
           register for an address that is known (e.g. 255.255.255.255).

       2)  The command set allows for lists of members to be passed
           in a single MARS_MULTI packet.   This reduces traffic.

       3)  MARS contains an architecture for dealing with the
           scalability issues.  That is, Multicast Servers (MCSs)
           may be used to set up the point-to-multipoint channels

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           and reduce the number of channels that a host needs to
           set up to one.  Hosts wishing to broadcast will instead
           send the packet to the MCS who will then forward it to
           all members of the LIS.

    A.2.1.  CIDR-prefix (Subnet) Broadcast solution.

       One of the earliest solutions was to simply state that broadcast
       support would be implemented by using a single multicast group in the
       class D address space -- namely, the CIDR-prefix (subnet) broadcast
       address group.  All members of a LIS would be required to register to
       this address, and use it as required.  A host wishing to use either
       the 255.255.255.255 broadcast, or the network broadcast addresses
       would internally map the VC to the subnet broadcast VC.  The all ones
       and network broadcast addresses would exist on MARS, but would be
       unused.

       The problem with this approach goes back to the diskless workstation
       problem.  Because the workstation may not know which subnet it
       belongs to, it doesn't know which group to register with.

    A.2.2.  All one's first, subnet broadcast second

       This solution acknowledges that the diskless boot problem requires a
       generic address (one that does not contain CIDR-prefix (subnet)
       information) to register with and to use until subnet knowledge is
       known.  In essence, all stations first register to the
       255.255.255.255 group, then as they know their subnet information,
       they could optionally de-register from the all one's group and
       register to the CIDR-prefix (subnet) broadcast group.

       This solution would appear to solve a couple of problems:

       1)  The bootp client can function if the server remains
           registered to the all one's group continuously.

       2)  There will be less traffic using the all ones group
           because the preferred transactions will be on the
           subnet broadcast channel.

       Unfortunately the first bullet contains a flaw.  The server must
       continually be registered to two groups -- the all ones group and the
       subnet broadcast group.  If this server has multiple processes that
       are running different IP applications, it may be difficult for the
       link layer to know which broadcast VC to use.  If it always uses the
       all ones, then it will be missing members that have removed
       themselves from the all ones and have registered to the subnet
       broadcast.  If it always uses the subnet broadcast group, the

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       diskless boot scenario gets broken.  While making the decision at the
       link layer may require additional control flows be built into the
       path, it may also require the rewriting of application software.

       In some implementations, a simple constant is used to indicate to the
       link layer that this packet is to be transmitted to the broadcast
       "MAC" address.  The assumption is that the physical network broadcast
       and the logical protocol broadcast are one and the same.  As pointed
       out earlier, this is not the case with ATM.  Therefore applications
       would need to specifically identify the subnet broadcast group
       address to take advantage of the smaller group.

       These problems could be solved in a number of ways, but it was
       thought that they added unnecessarily to the complexity of the
       broadcast solution.

    Appendix B.  Should MARS Be Limited to a Single LIS?

       RFC 2022 explicitly states that a network administrator MUST ensure
       that each LIS is served by a separate MARS, creating a one-to-one
       mapping between cluster and a unicast LIS.  But, it also mentions
       that relaxation of this restriction MAY occur after future research
       warrants it.  This appendix discusses some to the potential
       implications to broadcast should this restriction be removed.

       The most obvious change would be that the notion of a cluster would
       span more than one LIS.  Therefore, the broadcast group of
       255.255.255.255 would contain members from more than one LIS.

       It also should be emphasized that the one LIS limitation is not a
       restriction of the MARS architecture.  Rather, it is only enforced if
       an administrator chooses to do so.

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    RFC 2226             IP Broadcast over ATM Networks         October 1997

    Full Copyright Statement

       Copyright (C) The Internet Society (1997).  All Rights Reserved.

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