Network Working Group                                  L. McLaughlin III
Request for Comments: 1132                          The Wollongong Group
                                                           November 1989


   A Standard for the Transmission of 802.2 Packets over IPX Networks


Status of this Memo

   This document specifies a standard method of encapsulating 802.2 [1]
   packets on networks supporting Novell's Internet Packet Exchange
   Protocol [2] (IPX).  It obsoletes earlier documents detailing the
   transmission of Internet packets over IPX networks.  It differs from
   these earlier documents in that it allows for the transmission of
   multiple network protocols over IPX and for the transmission of
   packets through IPX bridges.  Distribution of this memo is unlimited.

Introduction

   The goal of this specification is to allow compatible and
   interoperable implementations for transmitting Internet packets such
   as the Internet Protocol [3] (IP) and Address Resolution Protocol [4]
   (ARP) as well as the Connectionless-mode Network Protocol [5] (CLNP)
   over IPX networks.

   IPX is a proprietary standard developed by Novell derived from
   Xerox's Internet Datagram Protocol [6] (IDP). Defining the
   encapsulation of the IEEE 802.2 Data Link Layer Standard over IPX in
   terms of yet another 802.X Physical Layer standard allows for the
   transmission of IP Datagrams as described in RFC 1042 [7].  This
   document will focus on the implementation of that RFC over IPX
   networks.

Description

   In general, this specification allows IPX networks to be used to
   support any network protocol which can use the IEEE 802.2 Data Link
   Layer specification.

   More specifically, IPX networks may be used to support IP networks
   and subnetworks of any class.  By encapsulating IP datagrams within
   IPX datagrams and assigning IP numbers to the hosts on a IPX network,
   IP-based applications are supported on these hosts.  The addition of
   an IP Gateway capable of encapsulating IP packets within 802.IPX
   datagrams would allow those hosts on an IPX network to communicate
   with the Internet.

Maximum Transmission Unit

   The maximum data size of a IPX datagram is 546 bytes.  As the
   combined size of the 802.2 LLC and SNAP headers is 8 bytes, this
   results in a Maximum Transmission Unit (MTU) of 538 bytes.

Address Mappings

   The mapping of Internet Protocol addresses to 802.IPX addresses is
   done using the Address Resolution Protocol in the same fashion as
   with other IEEE 802.X physical addresses.  However, the length of an
   802.IPX physical address is 10 bytes rather than 2 or 6.  This 10
   byte physical address consists of the 4 bytes of the IPX network
   address followed by the 6 bytes of the IPX node address.

Byte Order

   The byte transmission order is "big-endian" [8].

Broadcast Addresses

   IPX packets may be broadcast by setting the IPX header Packet Type
   field to 0x14, the Destination Network field to the local network
   number, the the Destination Node field to 0xffffff, and the Immediate
   Address field of the IPX Event Control Block to 0xffffff.

Unicast Addresses

   IPX packets may be unicast by setting the IPX header Packet Type
   field to 0x04, the Destination Network field and Destination Node
   field to those values found by address resolution, and the Immediate
   Address field of the IPX Event Control Block to the physical address
   of the destination node or the appropriate IPX bridge.

Checksum

   Like most IPX applications, this specification does not use IPX
   checksum.

Reserved values

   The IPX socket 0x8060 has been reserved by Novell for the
   implementation of this protocol.

Implementation

   The encapsulation of Internet packets within IPX networks has proved
   to be quite useful.  Because the IPX interface insulates knowledge of

   the physical layer from an application, 802.2 over IPX networks work
   over any physical medium.  A typical IP over IPX packet is shown
   below:

                              --------------------
                    N bytes   |  physical header |
                              |------------------|
                   30 bytes   |    IPX header    |
                              |------------------|
                    8 bytes   |   802.2 header   |
                              |------------------|
           usually 20 bytes   |     IP header    |
                              |------------------|
           usually 20 bytes   |    TCP header    |
                              |------------------|
            up to 498 bytes   |    TCP data      |
                              --------------------

   On workstations supporting an IPX programming interface,
   implementation of this specification has proved fairly
   straightforward.  The only change which was done was to modify the
   existing address resolution protocol code to allow for cache entries
   larger than the hardware address length.  This was done to allow room
   for the immediate address of a possible intervening IPX bridge in
   addition to the destination node and network addresses to be
   associated with a given IP address.

   Thus far, no implementations have been attempted on systems which do
   not already support an IPX programming interface (e.g., a dedicated
   router) though a few implementation details can be noted.  First,
   obviously any such implementation will have to distinguish IPX
   packets from other packets; this process will be media dependent.
   Second, note that no unicast packet is ever sent from host1 to host2
   without a prior broadcast packet from host2 to host1.  Thus, the
   immediate address of a possible intervening IPX bridge between host1
   and host2 can be learned from the physical header of that prior
   broadcast packet.  Third, any such implementation will need to
   discover the local IPX network number from a Novell bridge or file
   server.  The mechanisms for doing this exist but documentation for
   their use is not commonly available.

References

  [1]  IEEE, "IEEE Standards for Local Area Networks: Logical Link
       Control", IEEE, New York, 1985.

  [2]  Novell, Inc., "Advanced NetWare V2.1 Internetwork Packet Exchange
       Protocol (IPX) with Asynchronous Event Scheduler (AES)", October

       1986.

  [3]  Postel, J., "Internet Protocol", RFC-791, USC/Information
       Sciences Institute, September 1981.

  [4]  Plummer, D., "An Ethernet Address Resolution Protocol", RFC-826,
       November 1982.

  [5]  ISO DIS 8473: "Information Processing Systems - Data
       Communications - Protocol for Providing the Connectionless-mode
       Network Service".

  [6]  Xerox Corporation, "Xerox Network Systems Architecture", XNSG
       068504, April 1985.

  [7]  Postel, J., and J. Reynolds, "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", RFC-1042, USC/Information
       Sciences Institute, February 1988.

  [8]  Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
       October 1981.

Security Considerations

   Security issues are not addressed in this memo.


Author's Address:

   Leo J. McLaughlin III
   The Wollongong Group
   1129 San Antonio Road
   Palo Alto, CA 94303

   Phone: (415) 962-7100

   EMail: ljm@TWG.COM