Structure of the USP header

The header contains the identifier, two version fields, a flag field, a type field and a length field. openUTM expects the protocol described on the input side. This has the following structure:

Identifier ASCII, 4 bytes	Major version 1 byte	Minor version 1 byte	Flags 1 byte	MsgType 1 byte	MsgSize 4 bytes	Data Max. 31988 bytes
| UTMS(55544D53)	| 01	| 01		| Message type	| 0000xxxx	| Data

Explanation

Identifier

The identification field must always the contain the ASCII string "UTMS" (=0x55544D53) for openUTM.

Major version

The major version field must always contain the value 0x01.

Minor version

The minor version field must always contain the value 0x01.

Flags

The flag field is an information field. Only bit 0x02 is evaluated: if it is set, the message is followed by another fragment. If it is not set, it is not followed by a fragment. All the other bits are reserved for future versions.

MsgType

The type field can contain the values 0x00, 0x01 or 0x07:

• • • Receive messages from the client:
      In the case of unfragmented messages, the client sets the value 0x00 for MsgType. In the case of fragmented messages, the client sets the value 0x00 for MsgTyp when the first fragment is received (bit 0x02 is set in the flag field). For the fragments that follow, the value 0x07 is set for MsgType (bit 0x02 remains set). Bit 0x02 is not set for the last of these fragments.

    • Send messages to the client:
      In the case a USP header is created (e.g. with USP-HDR=ALL), openUTM sets the value 0x01 for MsgType when the first fragment is sent and the value 0x07 for fragments that follow.

MsgSize

The length field MsgSize contains the length of the message or message segment, including the header. This length must not exceed 32000 bytes on the input side and 32767 bytes on the output side. The message length is transferred in network byte order (big endian). The C functions htonl (host to network long) and ntohl (network to host long) can be used to carry out conversions between the local view and the network view.

Examples

1. The client sends messages to openUTM, see the sample source file SOCBSP.C shipped with the product:

   • • The client sends one message (without fragmentation)

     • The client sends two message segments

     • The client sends three message segments

   Number of message segments	Identifier	Major version	Minor version	Flag	MsgType	Size
   1 whole message	55544D53	01	01	00	00	0000nnnn
   2 segments
   segment 1	55544D53	01	01	02	00	0000nnnn
   segment 2	55544D53	01	01	00	07	0000mmmm
   3 segments
   segment 1	55544D53	01	01	02	00	0000kkkk
   segment 2	55544D53	01	01	02	07	0000nnnn
   segment 3	55544D53	01	01	00	07	0000mmmm

2. The server sends messages to the client (USP header is created):

   • one whole message (without fragmentation)

   • three message segments

   Number of message segments	Identifier	Major version	Minor version	Flag	MsgType	Size
   1 whole message	55544D53	01	01	00	01	0000nnnn
   3 segments
   segment 1	55544D53	01	01	02	01	0000kkkk
   segment 2	55544D53	01	01	02	07	0000nnnn
   segment 3	55544D53	01	01	00	07	0000mmmm

kkkk, nnnn, mmmm are the lengths of the different message segments.