Transport Layer Protocols
There were two primary transport layer protocols, both very different from their PUP predecessor:
- Sequenced Packet Protocol (SPP) was an acknowledged transport protocol, analogous to TCP; one chief technical difference is that the sequence numbers count the packets, and not the bytes as in TCP and PUP's BSP; it was the direct antecedent to Novell's IPX/SPX.
- Packet Exchange Protocol (PEP) was a connectionless non-reliable protocol similar in nature to UDP and the antecedent to Novell's PXP.
XNS, like PUP, also used EP, the Error Protocol, as a reporting system for problems such as dropped packets. This provided a unique set of packets which could be filtered to look for problems.
Read more about this topic: Xerox Network Systems
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