Transmission Control Protocol (TCP)
supports the network at the transport layer.
A D V E R T I S E M E N T
Transmission Control
Protocol (TCP) provides a reliable connection oriented service.
Connection oriented means both the client and server must open the
connection before data is sent. TCP is defined by RFC 793 and 1122. TCP
provides:
End to end reliability.
Data packet re sequencing.
Flow control.
TCP relies on the IP service at the network layer to deliver data to
the host. Since IP is not reliable with regard to message quality or
delivery, TCP must make provisions to be sure messages are delivered on
time and correctly (Federal Express?).
TCP Message Format
The format of the TCP header is as follows:
Source port number (16 bits)
Destination port number (16 bits)
Sequence number (32 bits) - The byte in the data stream that the
first byte of this packet represents.
Acknowledgement number (32 bits) - Contains the next sequence
number that the sender of the acknowledgement expects to receive
which is the sequence number plus 1 (plus the number of bytes
received in the last message?). This number is used only if the ACK
flag is on.
Header length (4 bits) - The length of the header in 32 bit
words, required since the options field is variable in length.
Reserved (6 bits)
URG (1 bit) - The urgent pointer is valid.
ACK (1 bit) - Makes the acknowledgement number valid.
PSH (1 bit) - High priority data for the application.
RST (1 bit) - Reset the connection.
SYN (1 bit) - Turned on when a connection is being established
and the sequence number field will contain the initial sequence
number chosen by this host for this connection.
FIN (1 bit) - The sender is done sending data.
Window size (16 bits) - The maximum number of bytes that the
receiver will to accept.
TCP checksum (16 bits) - Calculated over the TCP header, data,
and TCP pseudo header.
Urgent pointer (16 bits) - It is only valid if the URG bit is
set. The urgent mode is a way to transmit emergency data to the
other side of the connection. It must be added to the sequence
number field of the segment to generate the sequence number of the
last byte of urgent data.
Options (variable length)
The header is followed by data. TCP data is full duplex.
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