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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


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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
      Internet default maximum packet size of 576 octets.  Hosts
      connected to an 802.3 network should keep this in mind when
      sending datagrams to hosts not on the same 802.3 network.  It may




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


      be appropriate to send smaller datagrams to avoid unnecessary
      fragmentation at intermediate gateways.  Please see [9] for
      further information on this point.

   Address Mappings

      The mapping of 32-bit Internet addresses to 16-bit or 48-bit 802.3
      addresses can be done in several ways.  A static table could be
      used, or a dynamic discovery procedure could be used.

      Static Table

         Each host could be provided with a table of all other hosts on
         the local network with both their 802.3 and Internet addresses.

      Dynamic Discovery

         Mappings between 32-bit Internet addresses and 802.3 addresses
         could be accomplished through a protocol similar to the
         Ethernet Address Resolution Protocol (ARP) [10].  Internet
         addresses are assigned arbitrarily on some Internet networks.
         Each host's implementation must know its own Internet address
         and respond to 802.3 Address Resolution packets appropriately.
         It should also use ARP to translate Internet addresses to 802.3
         addresses when needed.

      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 802.3 address (of all binary ones).

         The use of the ARP dynamic discovery procedure is strongly
         recommended.

   Trailer Formats

      Some versions of Unix 4.2bsd use a different encapsulation method
      in order to get better network performance with the VAX virtual
      memory architecture.  Consenting systems on the same 802.3 network
      may use this format between themselves.  Details of the trailer
      encapsulation method may be found in [11].







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


   Byte Order

      As described in Appendix B of the Internet Protocol specification
      [1], the IP datagram is transmitted over 802.2/802.3 networks as a
      series of 8-bit bytes.

Conclusion

   The two encapsulation methods presented can be mixed on the same
   local area network; however, this would partition the network into
   two incompatible subnetworks.  One host on a network could support
   both methods and act as a gateway between the two subnetworks;
   however, this would introduce a significant performance penalty and
   should be avoided.

   The IEEE 802.2/802.3 compatible encapsulation method is preferable to
   the Ethernet compatible method because the IEEE 802.2 and IEEE 802.3
   standards have been accepted both nationally and internationally and
   because the same encapsulation method could be used on other IEEE 802
   Physical Layer implementations.  However, there are many existing
   installations that are using IP on Ethernet and a controlled
   transition from Ethernet to IEEE 802.2/802.3 is necessary.

   To this end, all new implementations should allow for a static choice
   of encapsulation methods and all existing implementations should be
   modified to provide this static choice as well.  During the
   transition, all hosts on the same network would use the Ethernet
   compatible method.  After 802.2/802.3 support has been added to all
   existing implementations, the IEEE 802.2/802.3 method would be used
   and the transition would be complete.

References

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

   [2]  The Institute of Electronics and Electronics Engineers, Inc.
        "IEEE Standards for Local Area Networks: Carrier Sense Multiple
        Access with Collision Detection (CSMA/CD) Access Method and
        Physical Layer Specifications".  The Institute of Electronics
        and Electronics Engineers, Inc., New York, New York, 1985.

   [3]  The Institute of Electronics and Electronics Engineers, Inc.
        "IEEE Standards for Local Area Networks: Token-Passing Bus
        Access Method and Physical Layer Specifications".  The Institute
        of Electronics and Electronics Engineers, Inc., New York, New
        York, 1985.


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


   [4]  The Institute of Electronics and Electronics Engineers, Inc.
        "IEEE Standards for Local Area Networks: Token Ring Access
        Method and Physical Layer Specifications".  The Institute of
        Electronics and Electronics Engineers, Inc., New York, New York,
        1985.

   [5]  The Institute of Electronics and Electronics Engineers, Inc.
        "IEEE Standards for Local Area Networks: Logical Link Control".
        The Institute of Electronics and Electronics Engineers, Inc.,
        New York, New York, 1985.

   [6]  "The Ethernet, Physical and Data Link Layer Specifications,
        Version 2.0".  Digital Equipment Corporation, Intel Corporation,
        and Xerox Corporation, 1982.

   [7]  Hornig, C.  "A Standard for the Transmission of IP Datagrams
        over Ethernet Networks".  RFC-894, Symbolics Cambridge Research
        Center, April 1984.

   [8]  Reynolds, J., and Postel, J.  "Assigned Numbers".  RFC-943,
        USC/Information Sciences Institute, April 1985.

   [9]  Postel, J.  "The TCP Maximum Segment Size Option and Related
        Topics".  RFC-879, USC/Information Sciences Institute,
        November 1983.

   [10] Plummer, D.  "An Ethernet Address Resolution Protocol".
        RFC-826, Symbolics Cambridge Research Center, November 1982.

   [11] Leffler, S., and Karels, M.  "Trailer Encapsulations".  RFC-893,
        University of California at Berkeley, April 1984.


















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