The transmission control protocol is a layer 4 protocol that allows for multiplexing and a duplex communication channel. It is defined in RFC793. This is a connection orientated protocol which establishes a connection with TCP 3 way handshake and closes connections with the TCP connection teardown. This gaurentees the following features.

• reliability: every message will be received and acknowledge or it will be redelivered,
• ordered delivery: messages have a sequence number and will only be parsed to the Application layer in order,
• error checking: Checksum in layer 4,
• Transmission control in TCP:
  • Flow control: allow the receiver to dictate how many message it can buffer.
  • Congestion control and fair usage: will use connection probing to determine a safe and fair transmission rate.

The TCP header has the following fields:

• Source port,
• Destination port,
• sequence number: the sequence number of the first data octect, this has a special role in the TCP 3 way handshake,
• Acknowledgement number: the next sequence number the receiver is expecting to get,
• Data offset: the number of 32 bit words in the TCP header,
• Reserved: set to all 0’s,
• control bits: these are 0 or 1 for the following 6 fields Urgent, Acknowledgement, Push, Reset, Synchronise, and Finish.
• Window: The number of data octets number of data octets the sender of this segment is willing to receive.
• Checksum: Checksum in layer 4,
• Urgent pointer,
• Options, and
• Padding to make the header a multiple of 32 bits.