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.