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
External links SPX, Sequenced Packet Exchange IPX / SPX Packet Structures