Two methods for the transmission of IP datagrams over IEEE 802.3 networks
RFC 948

Document Type RFC - Unknown (June 1985; No errata)
Obsoleted by RFC 1042
Last updated 2013-03-02
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< INC-PROJECT, WINSTON-RFC.NLS.6, >, 11-Jun-85 21:31-PDT JBP ;;;;

Winston                                                         [Page 0]



Network Working Group                                        Ira Winston
Request for Comments: 948                     University of Pennsylvania
                                                               June 1985

         TWO METHODS FOR THE TRANSMISSION OF IP DATAGRAMS OVER
                          IEEE 802.3 NETWORKS

Status of this Memo

   This memo describes two methods of encapsulating Internet
   Protocol (IP) [1] datagrams on an IEEE 802.3 network [2].  This RFC
   suggests a proposed protocol for the ARPA-Internet community, and
   requests discussion and suggestions for improvements.  Distribution
   of this memo is unlimited.

Introduction

   The IEEE 802 project has defined a family of standards for Local Area
   Networks (LANs) that deals 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) [2, 3, 4] and one Data Link Layer Standard (802.2) [5] 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.

   The 802.3 standard is based on the Ethernet Version 2.0 standard [6].
   The Ethernet Physical Layer and the 802.3 Physical Layer are
   compatible for all practical purposes however, the Ethernet Data Link
   Layer and the 802.3/802.2 Data Link Layer are incompatible.

   There are many existing Ethernet network installations that transmit
   IP datagrams using the Ethernet compatible standard described in [7].
   IEEE 802.3 Physical Layer compatible connections can be added to
   these networks using an an Ethernet Data Link Layer compatible method
   for transmitting IP datagrams without violating the 802.3 standard.
   Alternatively, an 802.2/802.3 Data Link Layer compatible method for
   transmitting IP datagrams can be used.

Ethernet Compatible Method

   IEEE 802.3 networks must use 48-bit physical addresses and 10
   megabit/second bandwidth in order to be Ethernet compatible.

   The IEEE 802.3 packet header is identical to Ethernet packet header
   except for the meaning assigned to one of the fields in the header.
   In an Ethernet packet header this field is used as a protocol type
   field and in an 802.3 packet header the field is used as a length
   field.  The maximum allowed length field value on a 10 megabit/second

Winston                                                         [Page 1]



RFC 948                                                        June 1985
Transmission of IP Datagrams Over IEEE 802.3 Networks

   802.3 network is 1500.  The 802.3 standard states that packets with a
   length field greater than the maximum allowed length field may be
   ignored, discarded, or used in a private manner.  Therefore, the
   length field can be used in a private manner as a protocol type field
   as long as the protocol types being used are greater than 1500.  The
   protocol type for IP, ARP and trailer encapsulation are all greater
   than 1500.  Using this technique, the method for transmitting IP
   datagrams on Ethernet networks described in [7] can be used to
   transmit IP datagrams on IEEE 802.3 networks in an Ethernet
   compatible manner.

IEEE 802.2/802.3 Compatible Method

   Frame Format

      IP datagrams are transmitted in standard 802.2/802.3 LLC Type 1
      Unnumbered Information format with the DSAP and SSAP fields of the
      802.2 header set to 96, the IEEE assigned global SAP value for
      IP [8].  The data field contains the IP header followed
      immediately by the IP data.

      IEEE 802.3 packets have minimum size restrictions based on network
      bandwidth.  When necessary, the data field should be padded (with
      octets of zero) to meet the 802.3 minimum frame size requirements.
      This padding is not part of the IP packet and is not included in
      the total length field of the IP header.

      IEEE 802.3 packets have maximum size restrictions based on the
      network bandwidth.  Implementations are encouraged to support
      full-length packets.

         Gateway implementations MUST be prepared to accept full-length
         packets and fragment them when necessary.

         Host implementations should be prepared to accept full-length
         packets, however hosts MUST NOT send datagrams longer than 576
         octets unless they have explicit knowledge that the destination
         is prepared to accept them.  A host may communicate its size
         preference in TCP based applications via the TCP Maximum
         Segment Size option [9].

      Note:  Datagrams on 802.3 networks may be longer than the general
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