TCP

TCP is a standard transport layer protocol described in RFC 793. It provides reliable stream-based connections over unreliable networks, and forms the foundation for HTTP, SMTP, and many other protocol standards. 

Connections made over TCP guarantee data transfer at the expense of throughput. Connections are made through a three-way handshake process, ensuring a one-to-one connection. Remote nodes advertise how much data they are ready to receive, and all data transmitted must be acknowledged. If a remote node fails to acknowledge the receipt of data, it is automatically retransmitted. This ensures that network errors such as lost, corrupted, or out-of-order packets are automatically corrected. 

To accomplish this, TCP must operate in a buffer. Once the transmit buffer is full, no more data can be sent until the remote node has acknowledged receipt. For the Microchip TCP/IP Stack, the application must return to the main stack loop in order for this to happen. Likewise, the remote node cannot transmit more data until the local device has acknowledged receipt and that space is available in the buffer. When a local application needs to read more data, it must return to the main stack loop and wait for a new packet to arrive. 

The TCP flow diagram below provides an overview for the use of the TCP module: 

Sockets are opened using TCPOpen. This function can either open a listening socket to wait for client connections, or can make a client connection to the remote node. The remote node can be specified by a host name string to be resolved in DNS, an IP address, or a NODE_INFO struct containing previously resolved IP and MAC address information. 

Once connected, applications can read and write data. On each entry, the application must verify that the socket is still connected. For most applications a call to TCPIsConnected will be sufficient, but TCPWasReset may also be used for listening sockets that may turn over quickly. 

To write data, call TCPIsPutReady to check how much space is available. Then, call any of the TCPPut family of functions to write data as space is available. Once complete, call TCPFlush to transmit data immediately. Alternately, return to the main stack loop. Data will be transmitted when either a) half of the transmit buffer becomes full or b) a delay time has passed (usually 40ms). 

To read data, call TCPIsGetReady to determine how many bytes are ready to be retrieved. Then use the TCPGet family of functions to read data from the socket, and/or the TCPFind family of functions to locate data in the buffer. When no more data remains, return to the main stack loop to wait for more data to arrive. 

If the application needs to close the connection, call TCPDisconnect, then return to the main stack loop and wait for the remote node to acknowledge the disconnection. Client sockets will return to the idle state, while listening sockets will wait for a new connection. 

For more information, refer to the GenericTCPClient or GenericTCPServer examples, or read the associated RFC.