Page 185 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes are rare nowadays. Each sending host happens to maintain two windows. One of them is the
receiving host window indicating the receiving host capacity and another is the congestion
window indicating the network capacity. The number of bytes to be transmitted should be the
minimum of the 2 windows.
Initially the congestion window is the MTU that is increased to a double capacity on each burst
successfully sent because an acknowledgement is received before the timeout is forwarded. This
exponential increase is referred as the slow start that continues until the threshold, which is
initially 64K, is reached. Thereafter, the increase is linearly with 1 MTU. If timeout happens, the
threshold is set to half the current congestion window and the slow start is repeated. When an
ICMP source quench packet comes in, it is treated the same way as a timeout.

TCP Timer Management

The retransmission timer intends to handle the large variation in round trip time occurring in
TCP. The round trip time M is determined for each segment and the estimates of the mean and
mean deviation are updated as:
Round trip time (RTT) = βRTT + (1-β)M
D = βD + (1-β) |RTT-M|

with β a smoothing parameter, typically 7/8. The timeout is then set to: RTT + 4 D
The Karn’s algorithm does not update RTT and D for retransmitted segments. As an alternative,
the timeout is doubled on each failure till the segments complete the first time. On receipt of a
window size equals to 0, the persistence timer is used to guard against the lost of the next
window update. Keepalive timer is also used in which if a connection is idle for a long time, the
timeout causes a packet to be transmitted to check whether the other side is still alive. If the
packet fails to respond, the connection is terminated. This feature, however is not recommended
because it adds overhead and may terminate an otherwise healthy connection due to a transient
network problem. The last timer is the one used in the TIMED WAIT state while closing, running
for twice the maximum packet lifetime to make sure that when a connection is closed; all packets
created by it have died off.

Self Assessment

Fill in the blanks:

8. TCP is an example of ……………………, reliable transport protocol.
9. The example of connectionless protocol is ……………... Its advantage is low overhead.
10. A ……………… protocol set up a connection before it transmits information to a host.
11. The advantage of ……………… protocols are more reliable and keeps track of the delivery
of the message.
12. The three errors that may be experienced and corrected by TCP service are lost frames;
frames arrive out of order and …………….. frames.

11.4 Summary

Layer four of the OSI reference model is the transport layer that provides transparent
transfer of data between the source and destination machines using the services of the
network layer such as IP below to move PDUs of data between the two communicating
machines.

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