Sequenced Packet Exchange (SPX) is a protocol in the IPX/SPX protocol stack that corresponds to a connection-oriented transport layer protocol in the OSI model. Being reliable and connection-oriented, it is analogous to the Transmission Control Protocol (TCP) of TCP/IP, but it is a datagram protocol, rather than a stream protocol.
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 |