A Standard for the Transmission of IP Datagrams over IEEE 802 Networks

Wed, 14 Oct 87 19:04:08 PDT

Your comments please


Network Working Group J. Postel
Request for Comments: DRAFT J. Reynolds
Obsoletes: RFC-948 mmmm 1987

 A Standard for the Transmission of IP Datagrams over IEEE 802 Networks

Status of this Memo

   This RFC specifies a standard method of encapsulating the Internet
   Protocol (IP) [1] datagrams and Address Resolution Protocol (ARP) [2]
   requests and replies on IEEE 802 Networks. This RFC specifies a
   protocol standard for the Internet community. Distribution of this
   memo is unlimited.


   This memo would not exist with out the very significant contributions
   of Drew Perkins of Carnegie Mellon University and Jacob Rekhter of
   the T.J. Watson Research Center, IBM Corporation.


   The goal of this specification is to have implementations for
   transmitting IP datagrams and ARP request and replies be compatible
   and interwork. To achieve this it may be necessary in a few cases to
   limit the use that IP datagrams make of the capabilities of a
   particular IEEE 802 network.

   This memo describes the use of IP and ARP on three types of networks.
   It is not necessary that the use of IP and ARP be consistent across
   all three types of networks, only that it be consistent within each

   The IEEE 802 specifications define a family of standards for Local
   Area Networks (LANs) that deal with the Physical and Data Link Layers
   as defined by the ISO Open System Interconnection Reference Model
   (ISO/OSI). Several Physical Layer standards (802.3, 802.4, and
   802.5) [3,4,5] and one Data Link Layer Standard (802.2) [6] have been
   defined. The IEEE Physical Layer standards specify the ISO/OSI
   Physical Layer and the Media Access Control Sublayer of the ISO/OSI
   Data Link Layer. The 802.2 Data Link Layer standard specifies the
   Logical Link Control Sublayer of the ISO/OSI Data Link Layer.

   All communication is performed using 802.2 type 1 communication. The

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RFC DRAFT IP and ARP on IEEE 802 Networks mmmm 1987

   802.2 type 2 communication is not used.

   The 802.x networks may have 16-bit or 48-bit physical addresses.

   It is the goal of this memo to specify enough about the use of IP and
   ARP on each type of network such that:

      (1) all equipment using IP or ARP on 802.3 networks will

      (2) all equipment using IP or ARP on 802.4 networks will

      (3) all equipment using IP or ARP on 802.5 networks will

   Of course, the goal of IP is interoperability between computers
   attached to different networks, when those networks are
   interconnected via an IP gateway [8].


   IEEE 802 networks may be used as IP networks of any class (A, B, or
   C). These systems use two Link Service Access Point (LSAP) fields of
   the LLC header in much the same way the ARPANET uses the "link"
   field. Further, there is an extension of the LLC header called the
   Sub-Network Access Protocol (SNAP).

   IP datagrams are sent on 802 networks encapsulated within the 802.2
   LLC and SNAP data link layers, and the 802.3, 802.4, or 802.5
   physical networks layers. The SNAP is used with an Organization Code
   indicating that the following 16 bits specify the EtherType code (as
   listed in Assigned Numbers [7]).

   Note that the 802.3 standard specifies a transmission rate of from 1
   to 20 megabit/second, the 802.4 standard specifies 1, 5, and 10
   megabit/second, and the 802.5 standard specifies 1 and 4
   megabit/second. The typical transmission rates used are 10
   megabit/second (802.3) or 4 megabit/second (802.5). However, this
   specification for the transmission of IP Datagrams does not depend on
   the transmission rate.

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RFC DRAFT IP and ARP on IEEE 802 Networks mmmm 1987


              MAC Header | 802.{3/4/5} MAC

   | DSAP=K1| SSAP=K1| Control| 802.2 LLC

   |Protocol Id or Org Code =K2| EtherType | 802.2 SNAP

   The total length of the LLC Header and the SNAP header is 8-octets,
   making the 802.2 protocol overhead come out on a nice boundary.

   The K1 value is 170 (decimal).

   The K2 value is 0 (zero).

   The control value is 3 (for Unnumbered Information).

Address Mappings

   The mapping of 32-bit Internet addresses to 16-bit or 48-bit 802
   addresses must be done via the dynamic discovery procedure of the
   Address Resolution Protocol (ARP) [2].

   Internet addresses are assigned arbitrarily on Internet networks.
   Each host's implementation must know its own Internet address and
   respond to Address Resolution requests appropriately. It must also
   use ARP to translate Internet addresses to 802 addresses when needed.

   The ARP Details

      The ARP protocol has several fields that parameterize its use in
      any specific context [2]. These fields are:

         hrd 16 - bits The Hardware Type Code
         pro 16 - bits The Protocol Type Code
         hln 8 - bits Bytes in each hardware address
         pln 8 - bits Bytes in each protocol address
         op 16 - bits Operation Code

      The hardware type code assigned for the 802 networks (of all
      kinds) is 6 (see [7] page 16).

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RFC DRAFT IP and ARP on IEEE 802 Networks mmmm 1987

      The protocol type code for IP is 2048 (see [7] page 14).

      The hardware address length is 2 (for 16-bit 802 addresses), or 6
      (for 48-bit 802 addresses).

      The protocol address length (for IP) is 4.

      The operation code is 1 for request and 2 for reply.

Broadcast Address

   The broadcast Internet address (the address on that network with a
   host part of all binary ones) should be mapped to the broadcast 802
   address (of all binary ones) (see [8] page 14).

Trailer Formats

   Some versions of Unix 4.x bsd use a different encapsulation method in
   order to get better network performance with the VAX virtual memory
   architecture. Consenting systems on the same 802 network may use
   this format between themselves. Details of the trailer encapsulation
   method may be found in [9]. However, all hosts must be able to
   communicate using the standard (non-trailer) method.

Byte Order

   As described in Appendix B of the Internet Protocol specification
   [1], the IP datagram is transmitted over 802 networks as a series of

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