TCP: Transmission Control Protocol

In this chapter we provide a description of the services provided by TCP for the application layer. We also look at the fields in the TCP header. In the chapters that follow we examine all of these header fields in more detail, as we see how TCP operates.

Our description of TCP starts in this chapter and continues in the next seven chapters. Chapter 18 describes how a TCP connection is established and terminated, and Chapters 19 and 20 look at the normal transfer of data, both for interactive use (remote login) and bulk data (file transfer). Chapter 21 provides the details of TCP's timeout and retransmission, followed by two other TCP timers in Chapters 22 and 23. Finally Chapter 24 takes a look at newer TCP features and TCP performance.

The original specification for TCP is RFC 793 [Postel 1981c], although some errors in that RFC are corrected in the Host Requirements RFC.

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.

17.3 TCP Header

Recall that TCP data is encapsulated in an IP datagram, as shown in Figure 17.1.

Figure 17.1 Encapsulation of TCP data in an IP datagram.

Figure 17.2 shows the format of the TCP header. Its normal size is 20 bytes, unless options are present.

Figure 17.2 TCP header.

Each TCP segment contains the source and destination port number to identify the sending and receiving application. These two values, along with the source and destination IP addresses in the IP header, uniquely identify each connection.

The combination of an IP address and a port number is sometimes called a socket. This term appeared in the original TCP specification (RFC 793), and later it also became used as the name of the Berkeley-derived programming interface (Section 1.15). It is the socket pair (the 4-tuple consisting of the client IP address, client port number, server IP address, and server port number) that specifies the two end points that uniquely identifies each TCP connection in an internet.

The sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP that the first byte of data in this segment represents. If we consider the stream of bytes flowing in one direction between two applications, TCP numbers each byte with a sequence number. This sequence number is a 32-bit unsigned number that wraps back around to 0 after reaching 2³² - 1.

When a new connection is being established, the SYN flag is turned on. The sequence number field contains the initial sequence number (ISN) chosen by this host for this connection. The sequence number of the first byte of data sent by this host will be the ISN plus one because the SYN flag consumes a sequence number. (We describe additional details on exactly how a connection is established and terminated in the next chapter where we'll see that the FIN flag consumes a sequence number also.)

Since every byte that is exchanged is numbered, the acknowledgment number contains the next sequence number that the sender of the acknowledgment expects to receive. This is therefore the sequence number plus 1 of the last successfully received byte of data. This field is valid only if the ACK flag (described below) is on.

Sending an ACK costs nothing because the 32-bit acknowledgment number field is always part of the header, as is the ACK flag. Therefore we'll see that once a connection is established, this field is always set and the ACK flag is always on.

TCP provides a full-duplex service to the application layer. This means that data can be flowing in each direction, independent of the other direction. Therefore, each end of a connection must maintain a sequence number of the data flowing in each direction.

TCP can be described as a sliding-window protocol without selective or negative acknowledgments. (The sliding window protocol used for data transmission is described in Section 20.3.) We say that TCP lacks selective acknowledgments because the acknowledgment number in the TCP header means that the sender has successfully received up through but not including that byte. There is currently no way to acknowledge selected pieces of the data stream. For example, if bytes 1-1024 are received OK, and the next segment contains bytes 2049-3072, the receiver cannot acknowledge this new segment. All it can send is an ACK with 1025 as the acknowledgment number. There is no means for negatively acknowledging a segment. For example, if the segment with bytes 1025-2048 did arrive, but had a checksum error, all the receiving TCP can send is an ACK with 1025 as the acknowledgment number. In Section 21.7 we'll see how duplicate acknowledgments can help determine that packets have been lost.

The header length gives the length of the header in 32-bit words. This is required because the length of the options field is variable. With a 4-bit field, TCP is limited to a 60-byte header. Without options, however, the normal size is 20 bytes.

There are six flag bits in the TCP header. One or more of them can be turned on at the same time. We briefly mention their use here and discuss each flag in more detail in later chapters.

URG	The urgent pointer is valid (Section 20.8).
ACK	The acknowledgment number is valid.
PSH	The receiver should pass this data to the application as soon as possible (Section 20.5).
RST	Reset the connection (Section 18.7).
SYN	Synchronize sequence numbers to initiate a connection. This flag and the next are described in Chapter 18.
FIN	The sender is finished sending data.

TCP's flow control is provided by each end advertising a window size. This is the number of bytes, starting with the one specified by the acknowledgment number field, that the receiver is willing to accept. This is a 16-bit field, limiting the window to 65535 bytes. In Section 24.4 we'll look at the new window scale option that allows this value to be scaled, providing larger windows.

The checksum covers the TCP segment: the TCP header and the TCP data. This is a mandatory field that must be calculated and stored by the sender, and then verified by the receiver. The TCP checksum is calculated similar to the UDP checksum, using a pseudo-header as described in Section 11.3.

The urgent pointer is valid only if the URG flag is set. This pointer is a positive offset that must be added to the sequence number field of the segment to yield the sequence number of the last byte of urgent data. TCP's urgent mode is a way for the sender to transmit emergency data to the other end. We'll look at this feature in Section 20.8.

The most common option field is the maximum segment size option, called the MSS. Each end of a connection normally specifies this option on the first segment exchanged (the one with the SYN flag set to establish the connection). It specifies the maximum sized segment that the sender wants to receive. We describe the MSS option in more detail in Section 18.4, and some of the other TCP options in Chapter 24.

In Figure 17.2 we note that the data portion of the TCP segment is optional. We'll see in Chapter 18 that when a connection is established, and when a connection is terminated, segments are exchanged that contain only the TCP header with possible options. A header without any data is also used to acknowledge received data, if there is no data to be transmitted in that direction. There are also some cases dealing with timeouts when a segment can be sent without any data.

17.4 Summary

TCP provides a reliable, connection-oriented, byte stream, transport layer service. We looked briefly at all the fields in the TCP header and will examine them in detail in the following chapters.

TCP packetizes the user data into segments, sets a timeout any time it sends data, acknowledges data received by the other end, reorders out-of-order data, discards duplicate data, provides end-to-end flow control, and calculates and verifies a mandatory end-to-end checksum.

TCP is used by many of the popular applications, such as Telnet, Rlogin, FTP, and electronic mail (SMTP).

Exercises

17.1 We've covered the following packet formats, each of which has a checksum in its corresponding header: IP, ICMP, IGMP, UDP, and TCP. For each one, describe what portion of an IP datagram the checksum covers and whether the checksum is mandatory or optional.

17.2 Why do all the Internet protocols that we've discussed (IP, ICMP, IGMP, UDP, TCP) quietly discard a packet that arrives with a checksum error?

17.3 TCP provides a byte-stream service where record boundaries are not maintained between the sender and receiver. How can applications provide their own record markers?

17.4 Why are the source and destination port numbers at the beginning of the TCP header?

17.5 Why does the TCP header have a header length field while the UDP header (Figure 11.2) does not?