Network protocol
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.
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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