RFC 2186                          ICP                     September 1997


   Message Length

     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Opcode    |    Version    |         Message Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Request Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Options                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Option Data                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Sender Host Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                            Payload                            |
   /                                                               /
   /                                                               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     FIGURE 1: ICP message format.

      The total length (octets) of the ICP message.  ICP messages MUST
      not exceed 16,384 octets in length.

   Request Number
      An opaque identifier.  When responding to a query, this value must
      be copied into the reply message.

   Options
      A 32-bit field of option flags that allows extension of this
      version of the protocol in certain, limited ways.  See "ICP Option
      Flags" below.

   Option Data
      A four-octet field to support optional features.  The following
      ICP features make use of this field:

      The ICP_FLAG_SRC_RTT option uses the low 16-bits of Option Data to
      return RTT measurements.  The ICP_FLAG_SRC_RTT option is further
      described below.









 
RFC 2186                          ICP                     September 1997


   Sender Host Address
      The IPv4 address of the host sending the ICP message.  This field
      should probably not be trusted over what is  provided by getpeer-
      name(), accept(), and recvfrom().  There is some ambiguity over
      the original purpose of this field.  In practice it is not used.

   Payload
      The contents of the Payload field vary depending on the Opcode,
      but most often it contains a null-terminated URL string.

2.  ICP Opcodes

   The following table shows currently defined ICP opcodes:

   Value    Name
   -----    -----------------
       0    ICP_OP_INVALID
       1    ICP_OP_QUERY
       2    ICP_OP_HIT
       3    ICP_OP_MISS
       4    ICP_OP_ERR
     5-9    UNUSED
      10    ICP_OP_SECHO
      11    ICP_OP_DECHO
   12-20    UNUSED
      21    ICP_OP_MISS_NOFETCH