20.8 Comparison of UDP and TCP

Both UDP and TCP are standard protocols used by applications to send information over a network. The choice of which to use for a given application depends on the design goals of the application. This section summarizes the main differences between UDP and TCP from the viewpoint of the application.

1. TCP is connection-oriented and UDP is not . To send over a TCP communication endpoint, a client first makes a connection request and the server accepts it. Once the client and server have established the connection, they can enjoy symmetric bidirectional communication with standard read and write functions. The endpoints are associated with the client and server pair. Either side can close the connection, in which case the other side finds out about it when it tries to read or write. Thus, applications communicating with TCP can tell when the other side is done. In contrast, an application can use a UDP communication endpoint to send to or receive from anyone . Each message must include the destination address (usually an IP address and port number). UDP does not provide an application with knowledge about the status of the remote end.

2. UDP is based on messages, and TCP is based on byte streams . If an application sends a UDP message with a single sendto , then (if the buffer is large enough) a call to recvfrom on the destination endpoint either retrieves the entire message or nothing at all. (Remember that we only consider unconnected UDP sockets.) In contrast, an application that sends a block of data with a single TCP write has no guarantee that the receiver retrieves the entire block in a single read. A single read retrieves a contiguous sequence of bytes in the stream. This sequence may contain all or part of the block or may extend over several blocks.

3. TCP delivers streams of bytes in the same order in which they were sent . UDP can deliver messages out of order, even if no errors occur anywhere in the network. UDP delivers messages to the application in the order they are received. Since individual UDP packets may travel different routes on the Internet, they may not arrive in the order they were sent. In contrast, the network subsystem of the receiving host buffers TCP packets and uses sequence numbers to deliver bytes to the application in the order they were sent.

4. TCP is reliable and UDP is unreliable . If TCP cannot deliver data to the remote host, it eventually reports the failure by returning an error. UDP is unreliable. The network might drop UDP packets and never deliver them to the remote host. UDP does not notify either the sender or the receiver that an error has occurred.

5. The UDP sendto and the TCP write functions return after successfully copying their message into a buffer of the network subsystem . The point of return for UDP does not depend in any way on the status of the receiver. For TCP, the point of return depends indirectly on the status of the receiver and the network. The TCP network subsystem may hold outgoing data in its buffers because the receiving host has no available buffers, the receiver has not acknowledged packets, or the network is congested . The held data may cause subsequent TCP write calls to block. Although TCP has flow control, you should not interpret a return from a TCP write call as an indication that data has arrived at the destination host .