IP over Exp. Ethernet: Standard for the transmission of IP datagrams over experimental Ethernet networks
How IP datagrams ride inside frames on the 3-megabit Experimental Ethernet.
Postel, J., "Standard for the transmission of IP datagrams over experimental Ethernet networks", STD 42, RFC 895, DOI 10.17487/RFC0895, April 1984, <https://www.rfc-editor.org/info/rfc895>.
@misc{rfc895,
series = {Request for Comments},
number = 895,
howpublished = {RFC 895},
publisher = {RFC Editor},
doi = {10.17487/RFC0895},
url = {https://www.rfc-editor.org/info/rfc895},
author = {J. Postel},
title = {{Standard for the transmission of IP datagrams over experimental Ethernet networks}},
pagetotal = 3,
year = 1984,
month = apr,
} RFC text
Contents
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.
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.
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.
AI ELI5
On this page
§1Scope: The Experimental Ethernet
RFC 895 specifies a standard method of encapsulating Internet Protocol (IP) datagrams on an Experimental Ethernet, which runs at 3 megabits per second and uses 8-bit addresses. Sending IP datagrams on the 10-megabit, 48-bit address Ethernet is described separately in RFC 894.
This is the rulebook for sending Internet letters over the very first, slower Ethernet cable.
§2Frame Format and Size
IP datagrams travel in standard Experimental Ethernet frames. The frame's type field must contain 513 (1001 octal), and the data field holds the IP header followed immediately by the IP data. If needed, the data field should be padded to meet the minimum frame size; the padding is not part of the IP packet and is not counted in the IP total length field.
The maximum length of an IP datagram on this network is 1536 octets. Implementations are encouraged to support full-length packets, and gateways MUST accept them and fragment them if necessary. A system that cannot receive full-length packets should discourage others from sending them, for example with the TCP Maximum Segment Size option (RFC 879).
Datagrams on the Ethernet may be longer than the general Internet default maximum of 576 octets, so hosts sending to machines on other networks may want smaller datagrams to avoid fragmentation at gateways.
Each frame is an envelope stamped with a special number, 513, so receivers know an Internet letter is inside. If the letter is too small, padding fills the envelope.
§3Address Mapping and Broadcast
Mapping a 32-bit Internet address to an 8-bit Experimental Ethernet address can be done several ways. The easiest, and the recommended approach, is to use the last eight bits of the host number part of the Internet address as the host's Ethernet address.
The broadcast Internet address (the host part is all binary ones) should be mapped to the broadcast Experimental Ethernet address, which is address zero.
Each computer's short street number is just the last part of its Internet address. Shouting to everyone uses the special number zero.
§4Trailers and Byte Order
Some versions of Unix 4.2bsd use a different encapsulation, called trailers, for better performance on the VAX. Consenting systems on the same Ethernet may use it between themselves, but no host is required to implement it, and trailer-format datagrams should be sent only when the sender knows the recipient can interpret them. Details are in RFC 893.
The IP datagram is sent over the Ethernet as a series of 8-bit bytes, as described in Appendix B of the Internet Protocol specification (RFC 791).
Some machines use a different way of packing the envelope. It is optional, and you should only use it with machines you know understand it.