ZXDNET CONTROL MESSAGE PROTOCOL
PROTOCOL SPECIFICATION
ZXDSTD: 2
author: tntxxd@ztx6d
last-update: 2025/01/31


1.  INTRODUCTION

  The Zxdnet Protocol (ZP) is used for host-to-host datagram service in a system of interconnected networks called the Catenet. The network connecting devices are called Gateways. Occasionally a gateway or destination host will communicate with a source host, for example, to report an error in datagram processing or to ensure if the device is alive. For such purposes this protocol, the Zxdnet Control Message Protocol (ZCMP), is used. ZCMP, uses the basic support of ZP as if it were a higher level protocol, however, ZCMP is actually an integral part of ZP, and must be implemented by every ZP module.

  ZCMP messages are sent in several situations: for example, when a datagram cannot reach its destination and when the gateway does not have the buffering capacity to forward a datagram.

  The Zxdnet Protocol is not designed to be absolutely reliable. The purpose of these control messages is to provide feedback about problems in the communication environment, not to make ZP reliable. There are still no guarantees that a datagram will be delivered or a control message will be returned. Some datagrams may still be undelivered without any report of their loss. The higher level protocols that use ZP must implement their own reliability procedures if reliable communication is required.

  The ZCMP messages typically report errors in the processing of datagrams. To avoid the infinite regress of messages about messages etc., no ZCMP messages are sent about ZCMP messages.

2.  FORMAT

  ZCMP messages are sent using the basic ZP header. The Zxdnet Protocol Number is 1. The first octet of the data portion of the datagram is a ZCMP type field; the value of this field determines the format of the remaining data.

  2.1.  Error Report Message

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Zxdnet Header + Maximum 12 bytes of Original Data Datagram  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


        ZP Fields:

          Destination Address

            The source network and address from the original datagram's data.

        ZCMP Fields:

          Type = 1.

          Code

            0 = net unreachable.

            1 = host unreachable.

            2 = protocol unreachable.

            3 = port unreachable.

            4 = out of MTU.

            5 = time to live exceeded in transit.

            6 = data missing.

            7 = data corruption.

          Checksum

            The checksum is the 16-bit ones's complement of the one's complement sum of the ZCMP message starting with the ZCMP Type. For computing the checksum, the checksum field should be zero. If the computed checksum is zero, it is transmitted as all ones. If the checksum field in the received datagram is zero, the receiver must skip the verification process.

  2.2.  Echo or Echo Reply Message

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Data (0-24) bytes...
     +-+-+-+-+-


        ZP Fields:

          Addresses

            The address of the source in an echo message will be the destination of the echo reply message. To form an echo reply message, the source and destination addresses are simply reversed, the code changed to 1, and the checksum recomputed.

        ZCMP Fields:

          Type = 2.

          Code

            0 = echo.

            1 = echo reply.

          Checksum

            The checksum is the 16-bit ones's complement of the one's complement sum of the ZCMP message starting with the ZCMP Type. For computing the checksum, the checksum field should be zero. If the computed checksum is zero, it is transmitted as all ones. If the checksum field in the received datagram is zero, the receiver must skip the verification process.

      Description

          The data received in the echo message must be returned in the echo reply message.