Sequenced Packet Exchange


Sequenced Packet Exchange 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 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:
OctetsField
1Connection Control
1Datastream Type
2Source Connection Id
2Destination Connection Id
2Sequence Number
2Acknowledgement Number
2Allocation Number
0–534data

The Connection Control fields contains 4 single-bit flags:
WeightMeaning
0x10End-of-message
0x20Attention
0x40Acknowledgement Required
0x80System Packet

The Datastream Type serves to close the SPX connection. For this purpose two values are used:
ValueMeaning
0x00–0xFDAvailable for client use
0xFEEnd-of-Connection
0xFFEnd-of-Connection Acknowledgement