Computer networks on TCP

EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 22.05.2006 Session IV Topic: TCP Faculty: Anita Kanavalli MSRIT • Solution • Uses threshold • initially some value in addition to the receiver and congestion window • When timeout threshold is set to half of the current congestion window • Congestion window is set to one max segment • Slow start is used to find what the network can handle • Exponential growth stops when threshold hit • From that point congestion window grow linearly • Example • Segment size=1K • Congwin=64KB • when timeout threshold=34KB • Congwin=1KB • the congstion window grows exponentially until it hits threshold and then linearly TCP timer management Round trip time RTT how to set TCP timeout value? • longer than RTT – note: RTT will vary • too short: premature timeout /* slowstart is over */ /* Congwin > threshold */ Until (loss event) { every w segments ACKed: Congwin++ } threshold = Congwin/2 Congwin = 1 perform slowstart Congestion avoidance 1 EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 22.05.2006 Session IV Topic: TCP Faculty: Anita Kanavalli MSRIT – unnecessary retransmissions • too long: slow reaction to segment loss • The algorithm is based on continuous measurements of network performance • Jacobson algorithm is one used for calculating RTT • For each connection a value RTT is maintained • RTT updated using the formula • RTT=αRTT+(1- α)M • Where M is time taken for ACK • And α is the smoothing factor • And α=7/8 TCP RTT • Jacobson algorithm • Another smoothed value D deviation it is the difference between the expected and observed value |RTT-M| • D= α D+(1- α )|RTT-M| • Timeout interval =RTT+4*D • The problem with retransmission answered by Karn’s algorithm • RTT not updated for retransmitted segment timeout is doubled on each failure till the segment gets through first time • There is another timer called the persistence timer- it is used when the sender is made to wait due to lack of buffer space at the receiver. Once this timer goes off the sender sends the probe to find about the receiver buffer space otherwise a deadlock occurs so this timer is used to resolve the same • The third timer is the keepalive timer- it is used for the connections which are idle for a long time suppose this timer goes off then the connection is closed Wireless TCP EDUSAT SESSION FOR COMPUTER NETWORKS-I (CS64) Date: 22.05.2006 Session IV Topic: TCP Faculty: Anita Kanavalli MSRIT • Indirect TCP to split the TCP connection into two separate connections • first one from sender to base station the second from base station to receiver • the advantage is both connections are homogeneous • The disadvantage is that it breaks the semantics of TCP • There is another solution for keeping the semantics of TCP is the Transactional TCP Transactional TCP The above figure (a) shows the normal RPC call where nine messages are exchanged between the client and the server Figure (b) shows the one with Transactional TCP T/TCP where req

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