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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.

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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.)

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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.

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