The Transmission Control Protocol (TCP), a connection-oriented protocol on the Transport
layer that provides reliable delivery of data, is an integral part of the IP suite. Look at the structure
of the TCP packet. The following EtherPeek frame was taken during a POP3 transaction:
TCP - Transmission Control Protocol
Source Port: 110 POP3
Destination Port: 1097
Sequence Number: 997270908
Ack Number: 7149472
Offset: 5
Reserved: 0000
Code: %010000
Ack is valid
Window: 8760
Checksum: 0x8064
Urgent Pointer: 0
No TCP Options
No More POP Command or Reply Data
Extra bytes (Padding):
UUUUUU 55 55 55 55 55 55
Frame Check Sequence: 0x04020000
This structure is similar to the IP packet structure. The TCP header is 32 bits long and has a
minimum length of five fields, but can be six fields deep when options are specified. The first layer
starts with the Source Port and Destination Port fields. Each of these fields is 16 bits long.
A Sequence Number field occupies the entire second layer, meaning that it is 32 bits long.
TCP is a connection-oriented protocol, and this field is used to keep track of the various requests
that have been sent.
The third layer is a 32-bit length field containing the acknowledgment sequence number that
is used to track responses.
The fourth layer begins with the Offset field, which is four bits and specifies the number of
32-bit words present in the header. Six bits are reserved for future use (this is called the Reserved
field). This field follows the Offset field.
The next field, called the Flag or Code field, is also a six-bit field, and it contains control
information. Look at Table 36.6 for an explanation of the six bits within the Flag field.
The Window field specifies the buffer size for incoming data. Once the buffer is filled, the
sending system must wait for a response from the receiving system. This field is 16 bits long.
Layer 5 of the TCP header begins with the Checksum parameter, which also occupies 16 bits.
It is used to verify the integrity of the transmitted data.
The Urgent Pointer field references the last byte of data, so the receiver knows how much
urgent data it will receive. This is also a 16-bit field.
Finally, there is the Option field, which must also be 32 bits long. If the options do not
occupy 32 bits, padding is added to reach the correct length.