- The format of the
TCP message is as
follows:
Notes:
- Source port #
and destination port#:
- Used to implement the multiplexing function (just like UDP)
- FLAGS: used to
indicate
a TCP control message
- SYN flag:
(synchronize)
- SYN is set when the TCP source wants to establish a connection
- FIN flag:
(finish)
- FIN is set when the either TCP party wants to close an existing TCP connection
- RESET flag:
(reset)
- RESET is
used
to
abort a
TCP connection
(Abnormal exit)
- RESET is
used
to
abort a
TCP connection
- PUSH flag:
(flush the connection)
- The PUSH bit is set when the user application invoked the push operation that flushes/clears the transmission buffer.
- URG flag::
- indicates that
urgent data
is sent inside the
TCP payload.
- The Urgent pointer marks the end of the urgent data
- indicates that
urgent data
is sent inside the
TCP payload.
- ACK flag:
(acknowledgement)
- If the ACK bit is
set:
- The Acknowledgement (receive) sequence number in the TCP header is valid
Otherwise:
- The Acknowledgement (receive) sequence number in the TCP header is invalid and must be ignored !!!
Note:
- The send sequence number in the TCP header is always valid
- If the ACK bit is
set:
- SYN flag:
(synchronize)
- SequenceNumber:
- Eevery item
that require
reliable transmission
must be:
- identified by a unique (send) sequence number
Example:
- A SYN request (used to establish a TCP connection)
- A FIN request (used to tear down a TCP connection)
- Each byte that the user application transmits
- Eevery item
that require
reliable transmission
must be:
- Acknowlegdement Number:
- In TCP,
ACK(n) will
acknowledge:
- All send sequence numbers that are < n
Note:
- the Acknowlegdement Number is only valid if the ACK flag bit is set !!!!!!!
- In TCP,
ACK(n) will
acknowledge:
- Advertised WindowSize:
- Use in flow control
- Advertised window size = the many bytes (free space) that is available in the receiver buffer
The sender cannot transmit more data than it is given in the Adv. window size
( More on the Advertised WindowSize field soon...)
- Use in flow control
- CheckSum: checksums the TCP header & message.
- Source port #
and destination port#:
- Notable fact:
- TCP
is a byte oriented protocol.
I.e.:
- The unit of data transmitted in TCP is 1 byte.
- TCP
is a byte oriented protocol.
- In other words:
- Each byte is
assigned a
(unique)
send sequence number
- Each byte will be acknowledged individually
(It's not too bad since TCP uses cumulative ACK !!!)
- Each byte is
assigned a
(unique)
send sequence number
- Example:
- Suppose the
send sequence number = 10:
- The sender
transmits
1 (one) TCP message contains
2 bytes:
- The receiver must
acknowledge the reception
of 2 bytes:
- Suppose the
send sequence number = 10: