Transmission Control Protocol (TCP)

As you’ll see shortly, a TCP header carries significantly more information than its UDP counterpart. This is a result of the extra fields that are required as part of making TCP reliable. It is also the reason why you’ll hear it said that TCP as a protocol has higher overhead. The diagram below outlines the fields found in a TCP header.

Figure: TCP header.

The TCP header fields are described below:

  • Source Port. This field specifies the TCP port number of the system sending the segment.
  • Destination Port. This field specifies the TCP port number of the destination system.
  • Sequence Number. The ordered number of the segment that is used to ensure that data can be properly reassembled on the receiving system, since segments may arrive out of order.
  • Acknowledgement Number. The acknowledgement number is used to let the other system know that a segment was received.
  • HLEN. The header length field specifies the total TCP header length, again grouped into 32 bit numbers.
  • Reserved. This field is always set to zero, and is reserved for future use.
  • Code Bits. This field specifies the purpose of the segment. For example, it can be used to denote acknowledgements (ACK), closing a connection (FIN), a synchronization request (SYN), urgent data (URG) and so forth
  • Window. Acts as the window size advertisement, specifying how much data this system is currently willing to receive.
  • Checksum. A computed value that verifies the integrity of the TCP header, similar to a CRC.
  • Urgent Pointer. When the URG code bit is set, this field specifies where within the segment the urgent data can be found.
  • Option. If used, this field specifies the maximum segment size that can be received.
  • Data. The encapsulated data passed down from upper-layer protocols and applications.

Tip: For more information on Transmission Control Protocol, see RFC 793.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.