IP over ARCNET: Standard for the transmission of IP datagrams and ARP packets over ARCNET networks
How to carry IP and ARP packets inside ARCNET frames.
Prindeville, P.A., "Standard for the transmission of IP datagrams and ARP packets over ARCNET networks", RFC 1051, DOI 10.17487/RFC1051, March 1988, <https://www.rfc-editor.org/info/rfc1051>.
@misc{rfc1051,
series = {Request for Comments},
number = 1051,
howpublished = {RFC 1051},
publisher = {RFC Editor},
doi = {10.17487/RFC1051},
url = {https://www.rfc-editor.org/info/rfc1051},
author = {P.A. Prindeville},
title = {{Standard for the transmission of IP datagrams and ARP packets over ARCNET networks}},
pagetotal = 4,
year = 1988,
month = mar,
} RFC text
Contents
Network Working Group P. Prindeville
Request for Comments: 1051 McGill University
March 1988
A Standard for the Transmission of IP Datagrams
and ARP Packets over ARCNET Networks
Status of this Memo
This RFC specifies a standard protocol for the Internet community.
Distribution of this memo is unlimited.
Introduction
This RFC specifies a standard method of encapsulating Internet
Protocol (IP) [1] and Address Resolution Protocol (ARP) [2] datagrams
on an ARCNET [3].
Acknowledgements
The author wishes to express thanks to Robert Craig of the McGill
University Computing Centre and Bruce Hughes of Datapoint Corporation
for their generous support of facilities and information. I also
extend my gratitude to the readers of the PCIP mailing list for their
helpful ideas and comments.
Frame Format
IP and ARP datagrams are transmitted in standard ARCNET packets. As
required by Datapoint Corporation, the first octet of the data field
is reserved for the network layer protocol identification (the
"system code" in Datapoint nomenclature), and must contain the value
240 (F0 hex) for IP or 241 (F1 hex) for ARP. The ARP hardware
address type for ARCNET is 7 [9].
ARCNET supports packet formats containing 1-253 octets of data
(normal format) and 257-508 octets of data (extended format),
inclusive of system code. Note that there exists a range of data
lengths (254-256) which are 'forbidden'. IP packets within this
range should be padded (with octets of zero) to meet the minimum
extended packet size of 257 data octets. This padding is not part of
the IP packet and is not included in the total length field of the IP
header.
On networks where some hosts do not support extended packet format,
the IP Maximum Transmission Unit (MTU) should be set to 253, though
implementors are encouraged to support the extended packet format
mode of operation.
Because the ARCNET maximum packet length is less than the Internet
default MTU, implementations are strongly encouraged to support IP
level fragmentation and reassembly. Hosts not supporting this should
take steps to discourage others from sending fragmented packets, such
as using the TCP Maximum Segment Size option [4].
The frame format is:
Normal Packet Extended Packet
+----------------+ +----------------+
| ALERT* | | ALERT* |
+----------------+ +----------------+
| SOH (1) | | SOH (1) |
+----------------+ +----------------+
| SID | | SID |
+----------------+ +----------------+
| | | |
+ DID + + DID +
| | | |
+----------------+ +----------------+
| COUNT | | NUL (0) |
+----------------+ + +
| SYSTEM CODE | | COUNT |
+----------------+ +----------------+
| | | SYSTEM CODE |
: DATA : +----------------+
| | | |
+----------------+ : DATA :
| | | |
+ CRC + +----------------+
| | | |
+----------------+ + CRC +
| |
+----------------+
ALERT*: Six mark bits signifying the beginning of a frame.
SID: Sender's node ID.
DID: Receipient's node ID (repeated for reliability).
COUNT: Length of data and system code (one's complement).
SYSTEM CODE: 240 for IP, 241 for ARP (decimal).
DATA: Is either an IP or an ARP packet, padded with NULs so
as to not be between 254 and 256 octets long.
CRC: Cyclic redundancy check (CRC-16).
Address Mappings
The mappings between 32-bit Internet addresses to 8-bit ARCNET
addresses can be done several ways, recommended are:
Host Number Extraction
The easiest thing to do is to use the last eight bits of host
number part of the Internet address as the host's node id. This
has been implemented on Experimental Ethernet [5] and ProNET-10
[6].
Dynamic Discovery
Mappings between 32-bit Internet addresses and 8-bit ARCNET node
ids could be accomplished through ARP. Internet addresses are
assigned arbitrarily on some Internet networks. All
implementations supporting ARP must have a means of disabling ARP
and using the above Host Number Extraction method of address
mapping so that systems may interoperate.
The use of ARP is optional. However, ARP is desirable when using
IP implementations that don't support subnetting [7], as in the
Proxy ARP scenario [8].
Broadcast Address
The broadcast Internet address (the address on the network with a
host part of all binary ones) should be mapped to the broadcast node
id 0.
References
[1] Postel, J., "Internet Protocol", RFC-791, Network Information
Center, SRI, September 1981.
[2] Plummer, D., "An Ethernet Address Resolution Protocol", RFC- 826,
Network Information Center, SRI, November 1982.
[3] "ARCNET Designer's Handbook", Order Number 61610, Datapoint
Corporation, 1983.
[4] Postel, J., "The TCP Maximum Segment Size Option and Related
Topics", RFC-879, Network Information Center, SRI, November 1983.
[5] Postel, J., "A Standard for the Transmission of IP Datagrams over
Experimental Ethernet Networks", RFC-895, Network Information
Center, SRI, April 1984.
[6] "ProNET-10 Model p1300 IBM PC Interface System Installation and
Programming Guide", Version 4.0, Proteon Inc., July 1986.
[7] Mogul, J. and J. Postel, "Internet Standard Subnetting
Procedure", RFC-950, Network Information Center, SRI, October
1984.
[8] Carl-Mitchell, S. and J.S. Quarterman, "Using ARP to Implement
Transparent Subnet Gateways", RFC-1027, Network Information
Center, SRI, October 1987.
[9] Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1010,
Network Information Center, SRI, May 1987.
ISO Plain Language
On this page
§1Carrying IP and ARP
RFC 1051 defines a method for encapsulating Internet Protocol (IP) and Address Resolution Protocol (ARP) datagrams on ARCNET. Each is transmitted in a standard ARCNET packet. The first data octet identifies the network-layer protocol: decimal 240 (hexadecimal F0) means IP, and 241 (F1) means ARP.
ARCNET carries the IP or ARP packet as data inside its own packet. The first data byte tells the receiver which kind it is.
§2ARCNET Packet Sizes
ARCNET supports normal packet data lengths of 1 to 253 octets and extended lengths of 257 to 508 octets, counting the system code. Lengths from 254 to 256 are forbidden. An IP packet in that range is padded with zero octets to reach 257; that padding is not counted in the IP header's total length. Where some hosts lack extended packet support, the IP MTU should be 253.
ARCNET has a gap in its allowed packet sizes. An IP packet that falls in the gap gets zero padding so the ARCNET packet reaches the minimum extended size.
§3Mapping Network Addresses
The RFC describes two recommended ways to map 32-bit Internet addresses to 8-bit ARCNET node IDs. One method uses the last eight bits of the Internet host number. Another uses ARP to discover mappings dynamically; ARP is optional. Implementations that support ARP must also be able to disable it and use host-number extraction. The broadcast Internet address maps to ARCNET node ID 0.
Internet addresses and ARCNET node IDs have different lengths. A system can use the last eight host-number bits or look up a mapping with ARP.