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Network Working Group Jon Postel Request for Comments: 895 ISI

                                                            April 1984

          A Standard for the Transmission of IP Datagrams
                over Experimental Ethernet Networks

Status of this Memo

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

Introduction

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

Frame Format

 IP datagrams are transmitted in standard Experimental Ethernet
 frames.  The type field of the Ethernet frame must contain the value
 513 (1001 octal).  The data field contains the IP header followed
 immediately by the IP data.

 If necessary, the data field should be padded to meet the
 Experimental 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 maximum length of an IP datagram sent over an Experimental
 Ethernet is 1536 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.

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RFC 895 April 1984

Address Mappings

 The mapping between 32-bit Internet addresses to 8-bit Experimental
 Ethernet addresses can be done several ways.

 The easiest thing to do is to use the last eight bits of host number
 part of the Internet address as the host's address on the
 Experimental Ethernet.  This is the recommended approach.

 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
    Experimental Ethernet address (address zero).

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 will also be changed in the future.)

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.

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RFC 895 April 1984

References

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

 [2]  Metcalfe, R. and D. Boggs, "Ethernet: Distributed Packet
 Switching for Local Computer Networks", Communications of the ACM,
 V.19, N.7, pp 395-402, July 1976.

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

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