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Sequenced packet exchange

Sequenced Packet Exchange (SPX) is the OSI-model Transport layer protocol in the IPX/SPX protocol stack. It is a reliable, connection-oriented protocol, similar to the TCP protocol of the TCP/IP, but it is datagram rather than stream protocol.

SPX packet structure

Each SPX packet begins with a header with the following structure:

Octets Field
1 Connection Control
1 Datastream Type
2 Source Connection Id
2 Destination Connection Id (0xFFFF = unknown)
2 Sequence Number
2 Acknowledgement Number
2 Allocation Number (The number of outstanding receive buffers available)
0-534 data

The Connection Control fields contains 4 single-bit flags:

Weight Meaning
0x10 End-of-message
0x20 Attention
0x40 Acknowledgement Required
0x80 System Packet

The Datastream Type serves to close the SPX connection. For this purpose two values are used:

Value Meaning
0x00-0xFD Available for client use
0xFE End-of-Connection
0xFF End-of-Connection Acknowledgement

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