Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP)
TCP provides a communication service at an intermediate level between an application program and the Internet Protocol (IP). That is, when an application program desires to send a large chunk of data across the Internet using IP, instead of breaking the data into IP-sized pieces and issuing a series of IP requests, the software can issue a single request to TCP and let TCP handle the IP details.
- source port. 16 bits. The sender's port number.
- destination port. 16 bits. The receiver port number.
- sequence number. 32 bits.
Sequence number has 2 meanings depending on SYN flag in the datagram is on or off.
- If the SYN flag is 0, then this is the accumulated sequence number of the first data byte of this segment for the current session.
- If the SYN flag is 1, then this is the initial sequence number. The sequence number of the actual first data byte and the acknowledged number in the corresponding ACK are then this sequence number plus 1.
- Acknowledgment number (32 bits) – if the ACK flag is set then the value of this field is the next sequence number that the receiver is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.
- Data offset (4 bits) – specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.
- Reserved (3 bits) – for future use and should be set to zero