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Network Working Group                                         Jon PostelRequest for Comments: 895                                            ISI                                                              April 1984A Standard for the Transmission of IP Datagramsover Experimental Ethernet NetworksStatus 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.Postel                                                          [Page 1]

RFC 895                                                       April 1984Address 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 inAppendix B of the Internet Protocol   specification [1], the IP datagram is transmitted over the Ethernet   as a series of 8-bit bytes.Postel                                                          [Page 2]

RFC 895                                                       April 1984References   [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.Postel                                                          [Page 3]