TCP Header

Posted on October 6, 2009. Filed under: CCNA, ICND1 break down |


TCP segment structure

Bit offset 0–3 4–7 8–15 16–31
0 Source port Destination port
32 Sequence number
64 Acknowledgment number
96 Data offset Reserved CWR ECE URG ACK PSH RST SYN FIN Window Size
128 Checksum Urgent pointer
160 Options (optional)
160/192+  Data 

The TCP header consists of 11 fields, of which only 10 are required. The eleventh field is optional (pink background in table) and is aptly named “options”.

  • Source port (16 bits) – identifies the sending port
  • Destination port (16 bits) – identifies the receiving port
  • Sequence number (32 bits) – has a dual role
  • If the SYN flag is set, then this is the initial sequence number. The sequence number of the actual first data byte will then be this sequence number plus 1.
  • If the SYN flag is clear, then this is the sequence number of the first data byte
  • 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 (4 bits) – for future use and should be set to zero
  • Flags (8 bits) (aka Control bits) – contains 8 1-bit flags
  • CWR (1 bit) – Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism (added to header by RFC 3168).
  • ECE (1 bit) – ECN-Echo indicates
  • If the SYN flag is set, that the TCP peer is ECN capable.
  • If the SYN flag is clear, that a packet with Congestion Experienced flag in IP header set is received during normal transmission(added to header by RFC 3168).
  • URG (1 bit) – indicates that the Urgent pointer field is significant
  • ACK (1 bit) – indicates that the Acknowledgment field is significant. All packets after the initial SYN packet sent by the client should have this flag set.
  • PSH (1 bit) – Push function
  • RST (1 bit) – Reset the connection
  • SYN (1 bit) – Synchronize sequence numbers. Only the first packet sent from each end should have this flag set. Some other flags change meaning based on this flag, and some are only valid for when it is set, and others when it is clear.
  • FIN (1 bit) – No more data from sender
  • Window (16 bits) – the size of the receive window, which specifies the number of bytes (beyond the sequence number in the acknowledgment field) that the receiver is currently willing to receive (see Flow control)
  • Checksum (16 bits) – The 16-bit checksum field is used for error-checking of the header and data
  • Urgent pointer (16 bits) – if the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte
  • Options (Variable 0-320 bits, divisible by 32) – The length of this field is determined by the data offset field. Options 0 and 1 are a single byte (8 bits) in length. The remaining options indicate the total length of the option (expressed in bytes) in the second byte.
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