Standard for the transmission of 802.2 packets over IPX networks
RFC 1132 also known as STD 49

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.
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RFC 1132 802.2 Packets over IPX Networks November 1989
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
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RFC 1132 802.2 Packets over IPX Networks November 1989
 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
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RFC 1132 802.2 Packets over IPX Networks November 1989
 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
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