TCP

TCP: (Transport Control Protocol)

TCP (Transport Control Protocol) is a protocol that provides a reliable, connection-oriented, byte stream, transport layer service.

TCP provides the following features:

packetizes the user data into segments
sets a time out 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
calculates and verifies mandatory end to end checksum

The TCP header and data are encapsulated within an IP datagram.

<--- IP datagram --->
IP header	TCP header	TCP data

Format for a TCP header

16-bit source port number			16-bit destination port number
32-bit sequence number
32-bit acknowledgment number
4-bit header length	6-bits reserved	Flags	16-bit window size
16-bit TCP checksum			16-bit urgent pointer
options (if any)
data (if any)

Source port number - This port number along with the IP address in the IP header uniquely identifies the source.

Destination port number - This port number along with the IP address in the header uniquely identifies the destination.

Sequence number - TCP provides full duplex to the application layer. With data flowing independantly in both directions each end must maintain its own sequence number.

Acknowledgment number - This number indicates the next sequence number the sending host expects to receive next from the destination host. TCP is a sliding window protocol, ie: the window slide back and forth depending on the acknowledment number.

Header length - The length of the header in 32 bit words. TCP is limited to 60 byte header.

Flags - There are 6 flags:

URG - Urgent pointer is valid.
ACK - Acknowledgment number is valid.
PSH - Receiver should pass this data to the application ASAP.
RST - Reset the connection.
SYN - Synchronize sequence numbers to initiate a connection.
FIN - The sender is finished sending data.

Window size - This is the number of bytes, starting with the one specified by the acknowledgement number field, that the receiver is willing to accept. This is a type of flow control where each end advertises a window size.

Checksum - Is the checksum of the bytes in the TCP header + TCP data

Urgent pointer - This pointer is valid only if the Urgent flag is set. It 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.