6.3 TCP

TCP is a connection-oriented protocol that provides reliable, stream-oriented connections in an IP environment. TCP corresponds to the transport layer (Layer 4) of the OSI reference model.

TCP guarantees delivery of packets to the application layer. This reliable delivery feature is based on sequence numbers that coordinate which data has been transmitted and received. TCP can retransmit any lost data. In addition, TCP senses network delay patterns and dynamically throttles data to prevent bottlenecks. Faster-sending hosts can be slowed down to let slower hosts catch up. TCP uses a number of control flags to manage the connection.

6.3.1 TCP Features

Features of TCP include the following:

Stream data transfer

TCP delivers data as a continuous stream of bytes identified by sequence numbers. This saves time, since applications do not have to break data into smaller bits before sending. Instead, TCP groups bytes into segments and passes them to IP for delivery. The segments are later assembled at the destination according to the packet sequence numbers.

Reliability

TCP ensures reliability by sequencing bytes with a forwarding acknowledgment number. Bytes that are not acknowledged within a specified time period are retransmitted.

Efficient flow control

TCP provides efficient flow control: when sending acknowledgments back to the source, the receiving TCP process indicates the highest sequence number it can receive without overflowing its internal buffers.

Full-duplex operation

Full-duplex operation allows TCP to both send and receive data at the same time.

6.3.2 TCP Packet Field Descriptions

The following descriptions summarize the TCP packet fields illustrated in Figure 6-2:

Source port and destination port

Indicates the ports on the sending and receiving end of the connection

Sequence number

Indicates the unique number assigned to the first byte of data in the segment

Acknowledgment number

Provides the sequence number of the next byte of data expected on the receiving end

Data offset

Indicates the number of 32-bit words in the TCP header

Reserved

Reserved for future use

Flags

Provide control markers such as the SYN, ACK, and FIN bits used for connection establishment and termination

Window

Indicates the size of the receiving window (buffer) for incoming data

Checksum

Verifies the integrity of received data

Urgent pointer

Marks the start of urgent data

Options

Includes numerous TCP options

Data

Includes the information payload

Figure 6-2. A representation of TCP packet fields