
Network Working Group                                     Charles Hornig
Request for Comments: 894            Symbolics Cambridge Research Center
                                                              April 1984

 A Standard for the Transmission of IP Datagrams over Ethernet Networks


Status of this Memo

   This RFC specifies a standard method of encapsulating Internet
   Protocol (IP) [1] datagrams on an Ethernet [2].  This RFC specifies a
   standard protocol for the ARPA-Internet community.

Introduction

   This memo applies to the Ethernet (10-megabit/second, 48-bit
   addresses).  The procedure for transmission of IP datagrams on the
   Experimental Ethernet (3-megabit/second, 8-bit addresses) is
   described in [3].

Frame Format

   IP datagrams are transmitted in standard Ethernet frames.  The type
   field of the Ethernet frame must contain the value hexadecimal 0800.
   The data field contains the IP header followed immediately by the IP
   data.

   The minimum length of the data field of a packet sent over an
   Ethernet is 46 octets.  If necessary, the data field should be padded
   (with octets of zero) to meet the Ethernet minimum frame size.  This
   padding is not part of the IP packet and is not included in the total
   length field of the IP header.

   The minimum length of the data field of a packet sent over an
   Ethernet is 1500 octets, thus the maximum length of an IP datagram
   sent over an Ethernet is 1500 octets.  Implementations are encouraged
   to support full-length packets.  Gateway implementations MUST be
   prepared to accept full-length packets and fragment them if
   necessary.  If a system cannot receive full-length packets, it should
   take steps to discourage others from sending them, such as using the
   TCP Maximum Segment Size option [4].

   Note:  Datagrams on the Ethernet may be longer than the general
   Internet default maximum packet size of 576 octets.  Hosts connected
   to an Ethernet should keep this in mind when sending datagrams to
   hosts not on the same Ethernet.  It may be appropriate to send
   smaller datagrams to avoid unnecessary fragmentation at intermediate
   gateways.  Please see [4] for further information on this point.





Hornig                                                          [Page 1]



RFC 894                                                       April 1984


Address Mappings

   The mapping of 32-bit Internet addresses to 48-bit Ethernet addresses
   can be done 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 Ethernet and Internet addresses.

   Dynamic Discovery

      Mappings between 32-bit Internet addresses and 48-bit Ethernet
      addresses could be accomplished through the Address Resolution
      Protocol (ARP) [5].  Internet addresses are assigned arbitrarily
      on some Internet network.  Each host's implementation must know
      its own Internet address and respond to Ethernet Address
      Resolution packets appropriately.  It should also use ARP to
      translate Internet addresses to Ethernet 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
      Ethernet address (of all binary ones, FF-FF-FF-FF-FF-FF hex).

   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 Ethernet may use this
   format between themselves.

   No host is required to implement it, and no datagrams in this format
   should be sent to any host unless the sender has positive knowledge
   that the recipient will be able to interpret them.  Details of the
   trailer encapsulation may be found in [6].

   (Note:  At the present time Unix 4.2bsd will either always use
   trailers or never use them (per interface), depending on a boot-time
   option.  This is expected to be changed in the future.  Unix 4.2bsd
   also uses a non-standard Internet broadcast address with a host part
   of all zeroes, this may also be changed in the future.)



Hornig                                                          [Page 2]



RFC 894                                                       April 1984


Byte Order

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

References

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

   [2]  "The Ethernet - A Local Area Network", Version 1.0, Digital
   Equipment Corporation, Intel Corporation, Xerox Corporation,
   September 1980.

   [3]  Postel, J., "A Standard for the Transmission of IP Datagrams
   over Experimental Ethernet Networks", RFC-895, USC/Information
   Sciences Institute, April 1984.

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

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

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























Hornig                                                          [Page 3]

