TCP structure

Structure of a TCP segment

When a TCP application sends a data, it divides into number of TCP segments. These segments include part of the data along with the header that defines various parameters used in the TCP communication between the source and the destination. These TCP segments are then encapsulated within the data portion of an IP datagram and sent on their way. If TCP are sent inside IP datagrams, and I just said that IP is unreliable.

How can TCP possibly be reliable?

The trick is that, unlike straight IP, TCP expects a response from its TCP computer on receiving end. Think of this way: Imagine mailing a letter again!!, to someone and including a Post-it Note on the letter that specifies your phone number and tells the recipient to call you when she receives the letter. If you don't hear from her, you know she didn't get the letter. To ensure reliable communication, TCP includes a "Post-it Note" in its header that does two things:

When the application requests that the data be sent a remote location, TCP constructs an initial segment that attempts to set up the socket interface between the two systems. No data is sent until TCP hears back from the receiving system that the sockets are in place and that's ready to receive the data.

When the sockets are ready to go, TCP starts sending within its segments and always asks receiving TCP to acknowledge that these data segments have arrived.If no acknowledgement is received, the sending TCP retransmits the segments.

Here is the exact format of TCP header.

Field	Bits	Description
Source Port	0 to 15	The source port number
Destination Port	16 to 31	The destination number
Sequence Number	32 to 63	In the overall sequence of bytes being sent, this field specifies the position in this sequence of the segment's first data byte.
Acknowledgement	64 to 95	In the ACK control Bit is set, this field contains the value of the next sequence number the sender of the segments is expecting the receiver to acknowledge.
Data Offset	96 to 99	The length of the TCP segment header, in 32 bit words.This tells the receiving socket where the data starts.
Reserved	100 to 105	This field is reserved for future use!
Control Bits	106 to 111	Those codes specify various aspects of the communication.When set to 1, each bit controls a particular code as listed here: 106 URG: Urgent pointer field significant. 107 ACK:Acknowledgment Number field to be used. 108 PSH: Push Function. 109 RST: Reset the connection. 110 SYN: Synchronize sequence numbers.This bit is set when the connection is opened. 111 FIN: No more data form sender, so close the connection.
Windows	112 to 127	The number of data bytes that the sender can currently accept. This sliding window lets the sender and the receiver vary the number of bytes sent and thus increase efficiency.
Checksum	128 to 143	This value lets the receiver determine the integrity of the data.
Urgent Pointer	144 to 159	If the URG control bit is set, this field indicates the location in the data where the urgent resides.
Options	160 and over	This variable-length field specifies extra TCP options such as the maximum segment size.