Standard for the transmission of IP datagrams and ARP packets over ARCNET networks
RFC 1051

Document Type RFC - Historic (March 1988; No errata)
Obsoleted by RFC 1201
Last updated 2018-08-22
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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.

Prindeville                                                     [Page 1]
RFC 1051                  IP and ARP on ARCNET                March 1988

   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).

Prindeville                                                     [Page 2]
RFC 1051                  IP and ARP on ARCNET                March 1988

Address Mappings

   The mappings between 32-bit Internet addresses to 8-bit ARCNET
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