TCP/IP Protocols

Windows API Reference - Networking & Internet

The Transmission Control Protocol/Internet Protocol (TCP/IP) suite is the foundational set of protocols that enables communication across the internet and most private networks. Windows provides extensive support for TCP/IP through various APIs, allowing applications to leverage its robust capabilities for reliable data transfer and efficient network communication.

Core Concepts

TCP/IP is a layered model, with TCP and IP being the most prominent protocols at the transport and network layers, respectively. Understanding these concepts is crucial for effective network programming:

  • IP (Internet Protocol): Responsible for addressing and routing packets of data across networks. It provides an unreliable, connectionless datagram service.
  • TCP (Transmission Control Protocol): Provides a reliable, connection-oriented, byte-stream service. It handles segmentation, reassembly, flow control, and error correction to ensure data arrives in order and without loss.
  • UDP (User Datagram Protocol): An alternative transport layer protocol to TCP. It offers a faster, connectionless, but unreliable datagram service, suitable for applications where speed is prioritized over guaranteed delivery (e.g., streaming media, online gaming).
  • Sockets: The programming interface used to access network services. In Windows, this is primarily the Winsock API, which provides a standardized way to interact with network protocols like TCP/IP.

Key Windows Sockets (Winsock) Functions for TCP/IP

The Winsock API provides a rich set of functions for developing TCP/IP applications. Here are some of the most fundamental:

socket()

Creates a socket that is an endpoint for communication. It specifies the address family (e.g., AF_INET for IPv4), socket type (e.g., SOCK_STREAM for TCP, SOCK_DGRAM for UDP), and protocol.

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

Parameters:

  • af: The address family.
  • type: The socket type.
  • protocol: The protocol to be used with the socket.
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bind()

Associates a local address and port number with a socket. This is essential for servers to listen for incoming connections on a specific port.

int bind(
      SOCKET s,
      const struct sockaddr *name,
      int namelen
    );

Parameters:

  • s: The socket descriptor.
  • name: A pointer to a sockaddr structure containing the local address and port.
  • namelen: The size of the sockaddr structure.
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listen()

Puts a socket into a state where it can accept incoming connection requests. Used primarily for TCP sockets by servers.

int listen(
      SOCKET s,
      int backlog
    );

Parameters:

  • s: The socket descriptor.
  • backlog: The maximum length of the queue of pending connections.
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accept()

Accepts a connection request on a listening socket. For TCP, it creates a new socket for the connection and returns its descriptor.

SOCKET accept(
      SOCKET s,
      struct sockaddr *addr,
      int *addrlen
    );

Parameters:

  • s: The listening socket descriptor.
  • addr: Optional buffer to receive the address of the connecting client.
  • addrlen: The size of the address buffer.
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connect()

Establishes a connection to a remote host. Used by clients to initiate a connection to a server.

int connect(
      SOCKET s,
      const struct sockaddr *name,
      int namelen
    );

Parameters:

  • s: The socket descriptor.
  • name: A pointer to a sockaddr structure containing the remote address and port.
  • namelen: The size of the sockaddr structure.
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send() / sendto()

Sends data over a socket. send() is typically used for connection-oriented sockets (TCP), while sendto() is used for connectionless sockets (UDP).

int send(
      SOCKET s,
      const char *buf,
      int len,
      int flags
    );

    int sendto(
      SOCKET s,
      const char *buf,
      int len,
      int flags,
      const struct sockaddr *to,
      int tolen
    );
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recv() / recvfrom()

Receives data from a socket. recv() is used for connection-oriented sockets (TCP), while recvfrom() is used for connectionless sockets (UDP) and also retrieves the sender's address.

int recv(
      SOCKET s,
      char *buf,
      int len,
      int flags
    );

    int recvfrom(
      SOCKET s,
      char *buf,
      int len,
      int flags,
      struct sockaddr *from,
      int *fromlen
    );
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closesocket()

Closes a socket and releases its resources. Essential for proper network communication termination.

int closesocket(
      SOCKET s
    );
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Advanced Topics

Beyond the fundamental functions, Windows offers advanced features for TCP/IP networking:

  • Asynchronous I/O (WSAAsyncSelect, WSAEventSelect): Allows applications to handle network events without blocking the main thread.
  • Overlapped I/O (WSASend, WSARecv): Provides high-performance I/O operations that can be initiated and completed without blocking.
  • Quality of Service (QoS): Mechanisms for prioritizing network traffic.
  • IPv6 Support: Native support for the next generation of the Internet Protocol.
  • Network Address Translation (NAT) Traversal: Techniques to enable applications behind NAT devices to communicate.

Note on Protocol Selection

When creating a socket, choosing between SOCK_STREAM (TCP) and SOCK_DGRAM (UDP) is critical. TCP guarantees reliable, ordered delivery, making it suitable for most data transfer applications (e.g., HTTP, FTP, email). UDP is faster and more efficient but offers no guarantees, making it ideal for real-time applications like voice and video streaming or online gaming where occasional packet loss is acceptable.

Tip for Development

Always include robust error handling when working with network sockets. Winsock functions return specific error codes (via WSAGetLastError()) that provide valuable information for debugging connection issues, timeouts, and other network problems.