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Network Working Group                                     Charles HornigRequest for Comments: 894            Symbolics Cambridge Research Center                                                              April 1984A Standard for the Transmission of IP Datagrams over Ethernet NetworksStatus 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 1984Address 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 1984Byte 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.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]