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