17.2 TCP Services

Even though TCP and UDP use the same network layer (IP), TCP provides a totally different service to the application layer than UDP does. TCP provides a connection-oriented, reliable, byte stream service.

The term connection-oriented means the two applications using TCP (normally considered a client and a server) must establish a TCP connection with each other before they can exchange data. The typical analogy is dialing a telephone number, waiting for the other party to answer the phone and say "hello," and then saying who's calling. In Chapter 18 we look at how a connection is established, and disconnected some time later when either end is done.

There are exactly two end points communicating with each other on a TCP connection. Concepts that we talked about in Chapter 12, broadcasting and multicasting, aren't applicable to TCP.

TCP provides reliability by doing the following:

The application data is broken into what TCP considers the best sized chunks to send. This is totally different from UDP, where each write by the application generates a UDP datagram of that size . The unit of information passed by TCP to IP is called a segment. (See Figure 1.7.) In Section 18.4 we'll see how TCP decides what this segment size is.
When TCP sends a segment it maintains a timer, waiting for the other end to acknowledge reception of the segment. If an acknowledgment isn't received in time, the segment is retransmitted. In Chapter 21 we'll look at TCP's adaptive timeout and retransmission strategy.
When TCP receives data from the other end of the connection, it sends an acknowledgment. This acknowledgment is not sent immediately, but normally delayed a fraction of a second, as we discuss in Section 19.3.
TCP maintains a checksum on its header and data. This is an end-to-end checksum whose purpose is to detect any modification of the data in transit. If a segment arrives with an invalid checksum, TCP discards it and doesn't acknowledge receiving it. (It expects the sender to time out and retransmit.)
Since TCP segments are transmitted as IP datagrams, and since IP datagrams can arrive out of order, TCP segments can arrive out of order. A receiving TCP resequences the data if necessary, passing the received data in the correct order to the application.
Since IP datagrams can get duplicated , a receiving TCP must discard duplicate data.
TCP also provides flow control. Each end of a TCP connection has a finite amount of buffer space. A receiving TCP only allows the other end to send as much data as the receiver has buffers for. This prevents a fast host from taking all the buffers on a slower host.

A stream of 8-bit bytes is exchanged across the TCP connection between the two applications. There are no record markers automatically inserted by TCP. This is what we called a byte stream service. If the application on one end writes 10 bytes, followed by a write of 20 bytes, followed by a write of 50 bytes, the application at the other end of the connection cannot tell what size the individual writes were. The other end may read the 80 bytes in four reads of 20 bytes at a time. One end puts a stream of bytes into TCP and the same, identical stream of bytes appears at the other end.

Also, TCP does not interpret the contents of the bytes at all. TCP has no idea if the data bytes being exchanged are binary data, ASCII characters, EBCDIC characters , or whatever. The interpretation of this byte stream is up to the applications on each end of the connection.

This treatment of the byte stream by TCP is similar to the treatment of a file by the Unix operating system. The Unix kernel does no interpretation whatsoever of the bytes that an application reads or write ” that is up to the applications. There is no distinction to the Unix kernel between a binary file or a file containing lines of text.