Transmission Control Protocol TCP

The Transmission Control Protocol (TCP) address transports functions of the OSI model. The Transport layer is primarily concerned with end-to-end data transport to the upper layers of the OSI model. TCP delivery is guaranteed. TCP also provides error checking. TCP runs over the IP protocol and is a connection-orientated protocol. It also uses windowing and acknowledgments to ensure segments are sent as efficiently as possible. Unlike IP, TCP will reorder segments that arrive at the destination. Figure 2.10 shows the TCP frame format.

Bits Specified

Source Port (16 bits)

Destination Port (16 bits)

Sequence Number (32 bits)

Acknowledgment Number (32 bits)

Data Offset



(4 bits)

(6 bits)

(16 bits)

Reserved ^ (6 bits)

Checksum (16 bits)

Urgent Pointer (16 bits)

Options + Padding (0 or 32 bits)

End User Data (variable)

Figure 2.10 TCP frame format.

The fields in the TCP segment have the following functions:

► Source Port—Identifies the source port number, which is assigned locally.

► Destination Port—Identifies the destination port number.

► Sequence Number—Ensures that the order of packets are assembled correctly.

► Acknowledgment Number—Specifies the next expected packet.

► Data Offset—Identifies the number of 32-bit words in the TCP header.

► Reserved—Specifies a setting of zero. This option is unused and set aside for future use.

► Flags—Provides some form of urgency for certain packets. The following are the six flags that can be used in a TCP segment:

► URG Flag—Specifies that the urgent pointer field is significant.

► ACK Flag—Specifies that the acknowledgment is significant.

► PSHFlag—Represents the push function.

► RST Flag—Identifies the reset connection.

► SYN Flag—Synchronizes the sequence numbers.

► FIN Flag—Indicates that there is no more data from the device.

► Window—Indicates the number of bytes that the sender is willing to receive.

► Checksum—Checks the TCP header and data.

► Urgent Pointer—Allows the identification of urgent data.

► Padding + Options—If the packet is less than the minimum required to access the physical medium, padding is used to ensure the packet ends on a 32-bit boundary. Options are exactly as the name implies. The most common option is the maximum fragment size, which identifies to the remote device the largest segment size the receiving device is willing to accept.

Common, or well-known, TCP numbers are:






—Active Users


—File Transfer Protocol (FTP) Data


-FTP Control




-Simple Mail Transfer Protocol (SMTP)



CCIE candidates should be aware of the most common port numbers.

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