Using Multiple Protocols

An application uses the WSAEnumProtocols function to determine which transport protocols and protocol chains are present, and to obtain information about each as contained in the associated WSAPROTOCOL_INFO structure.

In most instances, there is a single WSAPROTOCOL_INFO structure for each protocol or protocol chain. However, some protocols exhibit multiple behaviors. For example, the SPX protocol is message oriented (that is, the sender's message boundaries are preserved by the network), but the receiving socket can ignore these message boundaries and treat it as a byte stream. Thus, two different WSAPROTOCOL_INFO structure entries could exist for SPXpics/SOCK200090000.gifone for each behavior.

In Windows Sockets 2, several new address family, socket type, and protocol values appear. Windows Sockets 1.1 supported a single address family (AF_INET) comprising a small number of well-known socket types and protocol identifiers. The existing address family, socket type, and protocol identifiers are retained for compatibility reasons, but new transport protocols with new media types are supported.

A Windows Sockets 2 clearinghouse has been established for protocol stack vendors to obtain unique identifiers for new address families, socket types, and protocols. FTP and World Wide Web servers are used to supply current identifier/value mappings, and email is used to request allocation of new ones. This is the World Wide Web URL for the Windows Sockets 2 Identifier Clearinghouse:

New, unique identifiers are not necessarily well known, but this should not pose a problem. Applications that need to be protocol-independent are encouraged to select a protocol on the basis of its suitability rather than the values assigned to their socket_type or protocol fields. Protocol suitability is indicated by the communications attributes, such as message versus byte stream, and reliable versus unreliable, that are contained in the protocol WSAPROTOCOL_INFO structure. Selecting protocols on the basis of suitability as opposed to well-known protocol names and socket types lets protocol-independent applications take advantage of new transport protocols and their associated media types, as they become available.

The server half of a client/server application benefits by establishing listening sockets on all suitable transport protocols. Then, the client can establish its connection using any suitable protocol. For example, this would let a client application be unmodified whether it was running on a desktop system connected through LAN or on a laptop using a wireless network.

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