Internet-Draft                                         Timothy J. Smith,
                                                        IBM Corporation.
                                                     Grenville Armitage,
                                                         August 12, 1996

                     IP Broadcast over ATM Networks

Status of this Memo

   This document was submitted to the IETF Internetworking Over NBMA
   Working Group (ion).  Publication of this document does not imply
   acceptance by the Internetworking Over NBMA Working Group of any
   ideas expressed within.  Comments should be submitted to the mailing list.

   Distribution of this memo is unlimited.

   This memo is an internet draft. Internet Drafts are working documents
   of the Internet Engineering Task Force (IETF), its Areas, and its
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   To learn the current status of any Internet-Draft, please check the
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   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 draft-ietf-ipatm-ipmc-
   12.txt is assumed.

1.  Introduction.

   The IETF's first step in solving the problems of running IP over

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   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

   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.
   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), the Internet-
   Draft draft-ietf-ipatm-ipmc-12.txt ("Support for Multicast over UNI
   3.1 based ATM Networks") [2] was created.  This draft 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 draft-ietf-ipatm-ipmc-12.txt 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

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   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.

   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.

   (draft-ietf-ipatm-ipmc-12.txt 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 almost completely 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

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   is, broadcast is a multicast group whose members include all members
   in the LIS.

   There are three broadcast groups which this memo addresses:

      1) - "All ones" broadcast

      2) x.y.z - subnet directed broadcast

      3) x.z - network directed broadcast

   Broadcast (1) is sometimes referred to as a limited broadcast to this
   physical network.  Broadcast (2) is the subnet broadcast where x is
   the network number, y is the subnet number, and z is the all ones
   remainder of the address.  Broadcast (3) is the network broadcast
   where x is the network number, and z is the all ones remainder of the
   address.  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 subnet directed

      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 broadcast
   group.  Carrying the example a little further, the bootp server,

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   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 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 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 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 serve more than one LIS.  An implication of this is
      that broadcast group is joined
      by hosts from each LIS, increasing its scope beyond its
      conventional interpretation.  It is RECOMMENDED that
      a network administrator designate a separate MARS entity for
      each LIS that uses the broadcast function, unless they
      are sure the extended scope of is of no

   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

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   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.

   In cases where a MARS must serve more than one LIS, the use of the
   all ones broadcast will potentially cause broadcast packets to be
   sent to to members of a different LIS.  While this characteristic is
   similar to what happens when an Ethernet network has multiple logical
   subnets, it could be optimized by using a more specific broadcast
   group.  Further optimization is not discussed in this memo, and may
   be discussed in future memos.

   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.

   draft-ietf-ipatm-ipmc-12.txt includes mechanisms for addressing the
   refreshing of group entries and also the issue of a host verifying
   that it has been added to the list of members in a specific group.

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.

   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 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.

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   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

   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.

6.  Implications of IP broadcast on ATM level resources.

   draft-ietf-ipatm-ipmc-12.txt 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. draft-ietf-ipatm-ipmc-12.txt contains further discussion on the
   possible application of multiple MCSs to provide fault-tolerant

   (Current discussion in the ip-atm working group on encapsulation
   mechanisms to solve the problem of reflected packets returning from
   multicast servers are outside the scope of this memo. Here we simply
   assume the underlying multicast service works.)

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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.

Security Considerations

   This memo does not address security issues.


   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.


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

   [2]  G. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM
   Networks", Internet-Draft, IP over ATM Working Group, draft-ietf-
   ipatm-ipmc-12.txt, November 1995.

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

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

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

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Author's Address

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

Phone: (919) 254-4723

Grenville Armitage
Internetworking Research Group,
445 South Street,
Morristown, NJ, 07960

Phone: (201) 829 2635

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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

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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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
   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 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.

   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

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       scalability issues.  That is, Multicast Servers (MCSs)
       may be used to set up the point-to-multipoint channels
       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.  Subnet Broadcast solution.

   One of the earliest solutions was to simply state that broadcast
   support would be implemented by using a single multicast group --
   namely, the 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 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 subnet information) to
   register with and to use until subnet knowledge is known.  In essence,
   all stations first register to the group, then as
   they know their subnet information, they could optionally de-register
   from the all one's group and register to the 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 (which may
       span multiple LISs) because the preferred transactions will be
       on the subnet broadcast channel

   Unfortunately the first bullet contains a flaw.  Namely that 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.

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   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 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.

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