GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc948

< 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
    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

Winston [Page 2]

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].

Winston [Page 3]

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.

Winston [Page 4]

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.

Winston [Page 5]

/data/webs/external/dokuwiki/data/pages/rfc/rfc948.txt · Last modified: 1992/09/23 19:45 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki