Page 183 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes acknowledgement, timers are used. When a response to a FIN is not forthcoming within two
maximum packet lifetimes, the sender of the FIN releases the connection. Eventually, the other
host notices that nobody seems to be listening to it any more and that host also time out. The
procedure followed for releasing and establishing a connection is represented as follows:

S. No State Description
1 CLOSED Indicates no connection is active or pending.
2 LISTEN The server is waiting for an incoming call.
3 SYN RECEIVED A connection request has arrived; wait for acknowledgement.
4 SYN SENT The application or process has started to open a connection.
5 ESTABLISHED Indicates normal data transfer state.
6 FIN WAIT 1 The process or application has finished.
7 FIN WAIT 2 The hosts agreed to release the connection.
8 TIMED WAIT Indicates wait for all packets die off.
9 CLOSING Indicates that both hosts have attempted to close simultaneously.
10 CLOSE WAIT One of the host has initiated a release.
11 LAST ACK Indicates wait for all packets die off.

TCP Transmission Policy

Window management in TCP is not directly related to the acknowledgement. When the window
is set to 0, the host machine at the sending end may not normally send segments. We may
consider an example in which the receiving host has a 4096-byte buffer and sending host transmits
a 2048-byte segment. The 2048-byte segment is correctly received and the receiving host
acknowledges the segment. However, the buffer left with only 2048 of buffer space and the host
at receiving side advertises a window of 2048 bytes starting at the next byte expected. Now the
host at sending end transmits another 2048 bytes, which is acknowledged and the advertised
window is 0. The host at sending end has to stop until the application process on the host at
receiving end has removed some data from the buffer so that the TCP can advertise a larger
window. It is observed if the window is zero, the host at sending side may not send segments.
There are two exceptions. One of them is related to the urgent data that may be sent. Example of
urgent data is to enable the host machine to finish the process running on the remote host
machine. Figure 11.9 illustrates this concept.
Another exception is that the host machine at the sending may send a 1-byte segment to make
the receiver re announce the next byte expected and the size of window. This is quite important
in preventing deadlock if a window announcement ever gets lost. Senders are not required to
transmit data immediately when they appear from the application as well as the remote host
machine is not required to send acknowledgements as soon as possible. For example, consider
a telnet connection to an interactive editor that reacts to every keystroke. In the worst case, if a
character arrives at the TCP software, a 21-byte TCP segment is provided to IP to send as a 41-
byte datagram. At the remote host machine at the receiving side, TCP returns a 40-byte
acknowledgement (20-byte TCP header + 20-byte IP header). If the editor reacts to the character,
it echoes it as a 41-byte packet, which is acknowledged with a 40-byte packet. Thus, 162 bytes and
four segments are sent for each character typed. There are a number of issues that can degrade
TCP performance. One of them is the silly window syndrome in which when data are passed in
large blocks, but an application on the host machine at receiving side reads data one byte at a
time. This is illustrated in Figure 11.9.

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