Byte Ordering - Sockets 2 Reference

Byte Ordering

Care must always be taken to account for any differences between the byte ordering used by Intel Architecture and the byte ordering used on the wire by individual transport protocols. Any reference to an address or port number passed to or from a Windows Sockets routine must be in the network order for the protocol being utilized. In the case of IP, this includes the IP address and port fields of a sockaddr_in structure (but not the sin_family field).

Consider an application which normally contacts a server on the TCP port corresponding to the "time" service, but which provides a mechanism for the user to specify that an alternative port is to be used. The port number returned by getservbyname() is already in network order, which is the format required for constructing an address, so no translation is required. However if the user elects to use a different port, entered as an integer, the application must convert this from host to TCP/IP network order (using the WSAHtons() function) before using it to construct an address. Conversely, if the application wishes to display the number of the port within an address (returned via, e.g., getpeername()), the port number must be converted from network to host order (using WSANtohs()) before it can be displayed.

Since the Intel Architecture and Internet byte orders are different, the conversions described above are unavoidable. Application writers are cautioned that they should use the standard conversion functions provided as part of WinSock rather than writing their own conversion code, since future implementations of WinSock are likely to run on systems for which the host order is identical to the network byte order. Only applications which use the standard conversion functions are likely to be portable.

Consider an application that normally contacts a server on the TCP port corresponding to the "time" service, but provides a mechanism for the user to specify an alternative port. The port number returned by getservbyname is already in network order, which is the format required for constructing an address so no translation is required. However, if the user elects to use a different port, entered as an integer, the application must convert this from host to TCP/IP network order (using the WSAHtons function) before using it to construct an address. Conversely, if the application were to display the number of the port within an address (returned by getpeername for example), the port number must be converted from network to host order (using WSANtohs) before it can be displayed.

Since the Intel and Internet byte orders are different, the conversions described above are unavoidable. Application writers are cautioned that they should use the standard conversion functions provided as part of Windows Sockets rather than writing their own conversion code since future implementations of Windows Sockets are likely to run on systems for which the host order is identical to the network byte order. Only applications that use the standard conversion functions are likely to be portable.

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