Understanding the network.pdf - Back to Home

The TCP Header
The TCP header provides the means for communication about the TCP
process between the two TCP endpoints. The header (as shown in
Figure 2.16) provides the data sequencing and acknowledgment
information, and serves as the facility for connection setup and
disconnection.
Figure 2.16. The TCP packet header.
The TCP packet can be up to 65KB in size. Because TCP is network
independent, packet size limitation is not a priority. IP fragments the
TCP packets into sizes it can transport efficiently. The TCP header is
40 bytes in size, which is rather large for a network packet header.
The header is so large because it needs to carry all the data control
information.
The header dedicates memory segments or fields to provide the
information needed to deliver and reassemble the ULP data into an
unaltered state. These fields are as follows:
• Source Port—This is the TCP port the process is coming from.
• Destination Port—This is the TCP port the process is sending
data to.
• Sequence Number—This is the sequence number for data
contained in the packet.
• Acknowledgment Number—This field contains the sequence
number the sender expects to receive from the destination.
• Data Offset—This field indicates how large the TCP header is.
• Control flags indicate the status of the TCP connection:
o SYN sets up TCP connections.