Sequenced Packet Exchange (SPX)
The Sequenced Packet Exchange (SPX) protocol works
with the Internetwork Packet Exchange protocol (IPX)
to provide reliable delivery. SPX guarantees that
packets are received intact, in the order they were sent, and
eliminates duplicate packets.
IPX receives packets from the network, and passes on those that SPX should handle.
SPX requests verification from the destination module for each packet sent. By checking a verification value agaianst a value calculated before transmission, SPX ensures that the packet arrives intact. In the case of a missing packet, the transmitting SPX module retransmits, and continues to do so up to a program-specified number of retries.
SPX does not provide group broadcast support; packets can only be sent to a single session partner. SPX has provisions for detecting if its partner has disappeared.
SPX uses IPX and a medium-access protocol for its transport.
The SPX packet structure is shown in Figure 4-4. This structure is enveloped within the data area of IPX.
Figure 4-4 SPX packet structure
An SPX packet contains the following fields: