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Appendix C     Packet Header Formats

TCP Packet Header

TCP packet header is discussed in detail in RFC 793 which is available at ftp://

ftp.isi.edu/in notes/rfc793.txt for download. Figure C 4 shows structure of TCP header.

Source Port

Destination Port

Sequence Number

Acknowledgement Number

Offset

Reserved

Flags

Window

Checksum

Urgent Pointer

Options and Padding

Figure C 4 TCP header

Different parts of TCP header are explained in Table C 3. Again for a detailed

explanation of TCP, refer to the RFC 793.

Table C 3 TCP Packet Header Fields

Field

Explanation

Source Port

This part is 16 bits long and shows source port number.

Destination Port

This is a 16 bit long field and shows the destination port number.

Sequence Number

This is the sequence number for the TCP packet. It is 32 bits long. It 

shows the sequence number of the first data octet in the packet. How 

ever if SYN bit is set, this number shows the initial sequence number.

Acknowledgement Number

This number is used for acknowledging packets. It is 32 bits long. This 

number shows the sequence number of the octet that the sender is 

expecting.

Offset

This is a 4  bit field and shows the length of  the TCP header. Length is 

measured in 32 bit numbers.

Reserved

Six bits are reserved.

Flags or Control bits

The flags are six bits in length and are used for control purposes. 

These bits are URG, ACK, PSH, RST, SYN and FIN. A value of 1 in 

any bit place indicates the flag is set.

Window

This is 16 bits long and is used to tell the other side about the length of 

TCP window size.