Layered Protocols and Protocol Chains

Windows Sockets 2 incorporates the concept of a layered protocol. A layered protocol is one that implements only higher level communications functions while relying on an underlying transport stack for the actual exchange of data with a remote endpoint. An example of this type of layered protocol is a security layer that adds a protocol to the socket connection process in order to perform authentication and establish an encryption scheme. Such a security protocol generally requires the services of an underlying, reliable transport protocol such as TCP or SPX.

The term base protocol refers to a protocol, such as TCP or SPX, that is fully capable of performing data communications with a remote endpoint. A layered protocol is a protocol that cannot stand alone, while a protocol chain is one or more layered protocols strung together and anchored by a base protocol.

A protocol chain is created by having the layered protocols support the Windows Sockets 2 SPI at both their upper and lower edges. A special WSAPROTOCOL_INFO structure is created that refers to the protocol chain as a whole, and that describes the explicit order in which the layered protocols are joined. This is illustrated in the figure Layered Protocol Architecture. Since only base protocols and protocol chains are directly usable by applications, they are the only ones listed when the installed protocols are enumerated with the WSAEnumProtocols function.

Layered Protocol Architecture

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