Internetwork Packet Exchange/Sequenced Packet Exchange developed by Novell and is used primarily on networks that use the Novell NetWare network operating system. The IPX and SPX protocols provide services similar to those offered by IP and TCP. Like IP, IPX is a connectionless network layer protocol. SPX runs on top of IPX at the transport layer and, like TCP, provides connection oriented, guaranteed delivery.
IPX nodes do not have to be configured with a unique node identifier; instead, they copy the MAC address of the network interface card into the IPX node address field. The IPX header contains information about which transport layer protocol receives a particular packet. With IPX, this information is contained in the destination socket field. Servers have pre specified destination socket numbers, so workstations always know what value to use to send information to the server. In contrast, these workstations assign source socket numbers dynamically for their own protocols outside the server socket number's range.
IPX routing protocols require each logical network to have a different network number in order to forward IPX packets correctly. But, unlike IP, with IPX only servers and routers must be configured with a network number. New network stations first use dynamic Routing Information Protocol (RIP) routing packets to learn network topography and configuration from servers and routers and then configure themselves accordingly.
Because IPX is a connectionless protocol, NetWare servers are unable to tell if a station's connection to the server is currently active. To avoid reserving resources for inactive users, the NetWare server sends a watchdog packet to a client after a predetermined length of inactivity. The packet asks if the client is still connected and, if the client does not respond, the server terminates the connection.
SPX is connection oriented and, thus, does not require the use of watchdog packets. However, network devices will keep an SPX session open by sending keep alive packets to verify the connection.
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