Protocol-Independent Networking

This section provides information on protocol-independent networking features available in Windows. These features allow applications to communicate using various network protocols without needing to bind directly to a specific protocol like TCP or UDP. This abstraction simplifies network programming and enhances application portability.

Key Concepts

Address Families

Winsock supports multiple address families, enabling communication across different network environments. The most common address families are:

• AF_INET: For IPv4 communication.
• AF_INET6: For IPv6 communication.
• AF_UNSPEC: Allows the system to choose the most appropriate address family.

Socket Types and Protocols

Winsock allows you to specify socket types and protocols when creating a socket. This flexibility is central to protocol independence:

• SOCK_STREAM: Provides a reliable, connection-oriented byte stream (e.g., TCP).
• SOCK_DGRAM: Provides a connectionless datagram service (e.g., UDP).
• IPPROTO_TCP: Specifies the TCP protocol.
• IPPROTO_UDP: Specifies the UDP protocol.

By using AF_UNSPEC and allowing the underlying Winsock provider to determine the protocol, applications can often operate seamlessly with different network configurations.

Core APIs and Structures

WSAStartup and WSACleanup

Every Winsock application must initialize the Winsock DLL by calling WSAStartup before making any other Winsock API calls and then terminate its use of the Winsock DLL by calling WSACleanup when finished.

int WSAStartup(
  _In_  WORD      wVersionRequested,
  _Out_ LPWSADATA lpWSAData
);

int WSACleanup();
        

getaddrinfo and freeaddrinfo

These functions are crucial for protocol-independent name resolution and address manipulation. getaddrinfo translates a node name and service name into a set of socket addresses. It is the recommended modern approach over older functions like gethostbyname.

int getaddrinfo(
  _In_opt_  PCSTR           pNodeName,
  _In_opt_  PCSTR           pServiceName,
  _In_opt_  const ADDRINFOA *pHints,
  _Out_     LPADDRINFOA     *ppResult
);

void freeaddrinfo(
  _In_ __drv_freesMem(Mem) PADDRINFOA pAddrInfo
);
        

The ADDRINFOA structure contains information about socket addresses, including the address family, socket type, and protocol.

socket

This function creates a socket that can be used for communication. When used with AF_UNSPEC, it allows Winsock to select the appropriate address family based on other parameters or system configuration.

SOCKET socket(
  _In_ int af,
  _In_ int type,
  _In_ int protocol
);
        

Examples

Client Example (Conceptual)

A protocol-independent client would typically:

1. Call WSAStartup.
2. Use getaddrinfo to resolve the server's hostname and port, specifying AF_UNSPEC to allow for IPv4 or IPv6.
3. Create a socket using socket, again potentially with AF_UNSPEC and letting getaddrinfo provide the correct family and type.
4. Connect to the server using connect.
5. Send and receive data using send and recv.
6. Close the socket.
7. Call WSACleanup.

Server Example (Conceptual)

A protocol-independent server would typically:

1. Call WSAStartup.
2. Use getaddrinfo to prepare an address structure to bind to, often binding to all available interfaces (e.g., NULL for node name, specific port, with AI_PASSIVE flag).
3. Create a socket using socket.
4. Bind the socket to the address using bind.
5. Listen for incoming connections using listen.
6. Accept connections using accept, which returns a new socket for communication with the client.
7. Send and receive data.
8. Close sockets.
9. Call WSACleanup.

Further Reading