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A TCP/UDP Primer

By François Piette

Most people don't know which protocol to choose between TCP and UDP when they begin to work with TCP/IP. And when they have chosen, they begin to have some problems because it doesn't work as they expected.

This document tries to explain in easy words the pros and cons for each one in the context of the freeware TWSocket component for Delphi and C++Builder.


ADDRESSING

TCP and UDP use the same addressing scheme, either an IPv4 address (32 bits number, always written as four 8-bit numbers expressed as unsigned 3-digit decimal numbers separated by dots such as 193.174.25.26) and a port number (a 16-bit number expressed as a unsigned decimal number) or an IPv6 address (128 bits number, written in full as eight groups of four hexadecimal digits separated by colons) and a port number.

The IP address is used by the low-level protocol (IP) to route the datagram to the correct host on the specified network. Then the port number is used to route the datagram to the correct host process (a program on the host).

For a given protocol (TCP or UDP), a single host process can exist at a time to receive data sent to the given port. Usually one port is dedicated to one process.

UDP

UDP stands for User Datagram Protocol. It is described in STD-6/RFC-768 and provides a connectionless host-to-host communication path. UDP has minimal overhead:; each packet on the network is composed of a small header and user data. It is called a UDP datagram.

UDP preserves datagram boundaries between the sender and the receiver. It means that the receiver socket will receive an OnDataAvailable event for each datagram sent and the Receive method will return a complete datagram for each call. If the buffer is too small, the datagram will be truncated. If the buffer is too large, only one datagram is returned, the remaining buffer space is not touched.

UDP is connectionless. It means that a datagram can be sent at any moment without prior advertising, negotiation or preparation. Just send the datagram and hope the receiver is able to handle it.

UDP is an unreliable protocol. There is absolutely no guarantee that the datagram will be delivered to the destination host. But to be honest, the failure rate is very low on the Internet and nearly null on a LAN unless the bandwidth is full.

Not only the datagram can be undelivered, but it can be delivered in an incorrect order. It means you can receive a packet before another one, even if the second has been sent before the first you just received. You can also receive the same packet twice.

Your application must be prepared to handle all those situations: missing datagram, duplicate datagram or datagram in the incorrect order. You must program error detection and correction. For example, if you need to transfer some file, you'd better set up a kind of Zmodem protocol.

The main advantages for UDP are that datagram boundaries are respected, you can broadcast, and it is fast.


The main disadvantage is unreliability and therefore complicated to program at the application level.


TCP

TCP stands for Transmission Control Protocol. It is described in STD-7/RFC-793. TCP is a connection-oriented protocol that is responsible for reliable communication between two end processes. The unit of data transferred is called a stream, which is simply a sequence of bytes.

Being connection-oriented means that before actually transmitting data, you must open the connection between the two end points. The data can be transferred in full duplex (send and receive on a single connection). When the transfer is done, you have to close the connection to free system resources. Both ends know when the session is opened (begin) and is closed (end). The data transfer cannot take place before both ends have agreed upon the connection. The connection can be closed by either side; the other is notified. Provision is made to close gracefully or just abort the connection.

Being stream oriented means that the data is an anonymous sequence of bytes. There is nothing to make data boundaries apparent. The receiver has no means of knowing how the data was actually transmitted. The sender can send many small data chunks and the receiver receive only one big chunk, or the sender can send a big chunk, the receiver receiving it in a number of smaller chunks. The only thing that is guaranteed is that all data sent will be received without any error and in the correct order. Should any error occur, it will automatically be corrected (retransmitted as needed) or the error will be notified if it can't be corrected.

At the program level, the TCP stream look like a flat file. When you write data to a flat file, and read it back later, you are absolutely unable to know if the data has been written in only one chunk or in several chunks. Unless you write something special to identify record boundaries, there is nothing you can do to learn it afterward. You can, for example, use CR or CR LF to delimit your records just like a flat text file.

At the programming level, TWSocket is fairly simple to use. To send data, you just need to call the Send method (or any variation such as SendStr) to give the data to be transmitted. TWSocket will put it in a buffer until it can be actually transmitted. Eventually the data will be sent in the background (the Send method returns immediately without waiting for the data to be transmitted) and the OnDataSent event will be generated once the buffer is emptied.

To receive data, a program must wait until it receives the OnDataAvailable event. This event is triggered each time a data packet comes from the lower level. The application must call the Receive method to actually get the data from the low-level buffers. You have to receive all the data available or your program will go in an endless loop because TWSocket will trigger the OnDataAvailable again if you didn't receive all the data.


As the data is a stream of bytes, your application must be prepared to receive data as sent from the sender, fragmented in several chunks or merged in bigger chunks. For example, if the sender sent "Hello " and then "World!", it is possible to get only one OnDataAvailable event and receive "Hello World!" in one chunk, or to get two events, one for "Hello " and the other for "World!". You can even receive more smaller chunks like "Hel", "lo wo" and "rld!". What happens depends on traffic load, router algorithms, random errors and many other parameters you can't control.

On the subject of client/server applications, most applications need to know command boundaries before being able to process data. As data boundaries are not always preserved, you cannot suppose your server will receive a single complete command in one OnDataAvailable event. You can receive only part of a request or maybe two or more request merged in one chunk. To overcome this difficulty, you must use delimiters.

Most TCP/IP protocols, like SMTP, POP3, FTP and others, use CR/LF pair as command delimiter. Each client request is sent as is with a CR/LF pair appended. The server receives the data as it arrives, assembles it in a receive buffer, scans for CR/LF pairs to extract commands from the received stream, and removes them from the receive buffer.

Using CR/LF as delimiter is very handy because you can test your server using the well know application "telnet".

Thanks to Tony Wells and Don Peoples who reviewed this text.