Network Working Group                                           C. Graff
Request for Comments: 1770                                 US Army CECOM
Category: Informational                                       March 1995


      IPv4 Option for Sender Directed Multi-Destination Delivery

Status of this Memo

  This memo provides information for the Internet community.  This memo
  does not specify an Internet standard of any kind.  Distribution of
  this memo is unlimited.

Abstract

  This memo defines an IPv4 option to provide a sender directed multi-
  destination delivery mechanism called Selective Directed Broadcast
  Mode (SDBM).  The SDBM provides unreliable UDP delivery to a set of
  IP addresses included in the option field of an IPv4 datagram.  Data
  reliability if required will be provided by the application layer.
  This approach was developed to support sender directed multi-
  destination delivery to sparsely populated groups with no additional
  control traffic.  This approach will find application in the
  extremely bandwidth constrained tactical military environment, as
  well as in some commercial applications requiring sender control of
  data delivery.

Background

  The Selective Directed Broadcast Mode (SDBM) is an integral part of
  the U.S. Army standard for tactical data communication networks as
  defined in MIL-STD-188-220() (Reference 1). The MIL-STD-188-220()
  defines a protocol architecture for the lower four layers of the
  ISO-OSI Reference model. The MIL-STD-188-220() is currently
  undergoing a reformatting to be consistent with other DoD standards
  that deal with IP networking. These efforts will provide tactical IP
  internetting of tactical Army broadcast radio networks, and will
  support fully IP compliant internetworking to other types of IP
  networks via commercial IP routers.  It is the goal of the U.S. Army
  to move toward a fully IP compliant internetwork architecture for all
  tactical battlefield data communications. The Army does, however,
  have a critical need for a reliable, sender directed multi-
  destination data transfer capability that is not currently supported
  by the existing or emerging internet standards. The SDBM IP option
  was developed to meet this need. The required data reliability will
  be provided by incorporating an acknowledgement strategy at the
  application layer. It is hoped that this IP option, providing multi-
  destination capability not currently provided by the current and



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RFC 1770         Selective Directed Broadcast Protocol        March 1995


  emerging internet standards, will be embraced by the internet
  community and become an integal part of the IP family of protocols
  and be incorporated in commercial IP software products.

SDBM Format

  The SDBM provides the ability for an application to explicitly list a
  set of intended IP destinations. This capability will be implemented
  as an option in the IP layer, as shown in Figure 1. This option field
  is variable in length, up to a maximum of 40 octets due to the
  limitation of the HLEN field as specified in STD 5, RFC 791
  (Reference 2). Under this option 38 of the 40 octets would be used to
  contain the 2 octet control field and a maximum of 9 IP addresses.


      1            8           16                      31

      ***************************************************
      |            |            |                       |
      |            |            |                       |
      |  TYPE      |   LENGTH   |     IP ADDRESS 1      |
      |            |            |                       |
      |            |            |                       |
      |*************************************************|
      |                         |                       |
      |  IP ADDRESS 1(Cont)     |     IP ADDRESS 2      |
      |                         |                       |
      |                         |                       |
      |*************************************************|
      |                         |                       |
      |  IP ADDRESS 2(Cont)     |     ..........        |
      |                         |                       |
      |                         |                       |
      |*************************************************|
      |                         |                       |
      |                         |                       |
      |      ..........         |     IP ADDRESS N      |
      |                         |                       |
      |                         |                       |
      |*************************************************|
      |                         |                       |
      |    IP ADDRESS N(Cont)   |        UNUSED         |
      |                         |                       |
      |                         |                       |
      ***************************************************


                 Figure 1 IP Option Field Layout



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RFC 1770         Selective Directed Broadcast Protocol        March 1995


  The TYPE field specifies the copy flag, class, and option number.
  The copy field indicates whether or not this option field is to be
  copied into each fragment if the IP datagram is fragmented. The class
  field and option number field are set to 0 and 21 respectively. The
  format of the TYPE field is shown at Figure 2.

         1                                                8
         **************************************************
         |      |           |                             |
         | COPY |   CLASS   |    OPTION NUMBER            |  =  149
         |      |           |                             |
         **************************************************

                  Figure 2 Type Field Layout

  Since the IP multi-address list shall always be copied to all IP
  headers during fragmentation, the COPY bit should be set to 1.

  Returning to Figure 1, the LENGTH octet indicates how many octets are
  in the option field. It is calculated as:

          LENGTH = 2 + 4*(number of IP addresses)

  The remaining octets contain the IP addresses of the specified
  destination hosts. Each IP address occupies 4 octets.

Transmission of SDBM datagrams

  The procedures for a source host, transit router, and destination
  router are provided below. When a source host has a message to send
  to multiple destination hosts, it shall,

  a. Group the destination host internet addresses by their network
     identifiers (Net IDs). If there are N distinct Net IDs, there will
     be at least N distinct directed broadcast packets. If there are
     more that 9 destination hosts on a single net, multiple directed
     broadcast datagrams must be sent to that net.

  b. For each Net ID, form the directed broadcast address as defined in
     STD 3, RFC 1122 (Reference 3) for that network. The directed
     broadcast address is used as the destination address in the IP
     datagram and the source address is the address of the host sending
     the message.

  c. Place the entire IP address for up to 9 destination hosts in the in
     the same net in the option field defined above. The total length of
     all IP options in a given datagram is limited to 40 octets as
     determined by the HLEN (Header Length) field which defines the



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RFC 1770         Selective Directed Broadcast Protocol        March 1995


     number of 32 bit words in the header. If other options are to be
     included in addition to the SDBM option, the number of addresses in
     the option field must be reduced accordingly.

  d. The thusly formed datagram shall be transmitted and processed
     according to normal datagram handling procedures.

  When a IP SDBM datagram encounters a transit router (router not
  connected to the destination network), the datagram shall be
  processed in accordance with normal IP datagram handling procedures.
  When encountering the destination router (the destination network is
  directly attached to the router), the destination router shall
  perform a, b or c below:

  a. If the local subnet has a broadcast capability, broadcast to all
     hosts in the network and let the hosts perform address filtering.

  b. If the local subnet does not support broadcast, form a local subnet
     packet for each destination host in the SDBM datagram and transmit
     into the network.

  c. If the local subnet supports reliable layer 2 multi-address
     capability as provided by MIL-STD-188-220() networks, use a layer 2
     multi-address frame to deliver the datagram to addresses found in
     the IP option field.

Reception of SDBM datagrams

  In processing received SDBM datagrams, receiving hosts shall look
  inside the IP option field for their address. Processing shall
  continue only if the host's IP address is found inside this option
  field. Thus the source host has explicit control over which hosts
  will process its datagrams. Since SDBM uses a broadcast address in
  its destination field, the SDBM can only be used with UDP (Reference
  4) and not TCP (Reference 5) as the TCP supports only point-to-point
  connections and not point-to-multi-point.















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RFC 1770         Selective Directed Broadcast Protocol        March 1995


Source for MIL-STD-188-220()

  The above mentioned MIL-STD-188-220() may be obtained by contacting

  US Army Communications Electronics Command
  AMSEL-RD-SE-AIN-E (ATTN: Mr. Ted Dzik)
  Fort Monmouth, NJ 07703

  Comm: (908) 532-1780
  Fax:  (908) 532-3398
  EMail: [email protected]

Acknowledgements

  The author wishes to acknowledge the major contributions to this work
  made by Mr. Dave Macauley of ATT and Ms. Barbara Denny of SRI
  International.  Other contributions were made by members of the 188-
  220() committee.

References

  (1) "MIL-STD-188-220() For Task Force XXI, Interoperability Standard
      for Digital Message Transfer Device Subsystems, 23 December 1994.

  (2) Postel, J., "Internet Protocol - DARPA Internet Program Protocol
      Specification", STD 5, RFC 791, DARPA, September 1981.

  (3) Braden, R., Editor, "Requirements for Internet Hosts --
      Communication Layers" STD 3, RFC 1122, IETF, October 1989.

  (4) Postel, J., "User Datagram Protocol", STD 6, RFC 768,
      USC/Information Sciences Institute, August 1980.

  (5) Postel, J., "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", STD 7, RFC 793, September 1981.

Security Considerations

      Security issues are not discussed in this memo.












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RFC 1770         Selective Directed Broadcast Protocol        March 1995


Author's Address

      US Army Communications Electronics Command
      AMSEL-RD-ST-LA-L ( ATTN: Charles Graff )
      Ft. Monmouth, NJ 07703

      Phone: (908) 544 3264
      Fax:   (908) 544 2150
      EMail: [email protected]










































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