Presented By
LACHIRAM.G

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TCP Congestion Control
 Essential strategy :: The TCP host sends packets into the network without a reservation and then the host reacts to observable events.
 Originally TCP assumed FIFO queuing.
 Basic idea :: each source determines how much capacity is available to a given flow in the network.
 ACKs are used to ‘pace’ the transmission of packets such that TCP is “self-clocking”.
AIMD (Additive Increase / Multiplicative Decrease)
 CongestionWindow (cwnd) is a variable held by the TCP source for each connection.
cwnd is set based on the perceived level of congestion. The Host receives implicit (packet drop) or explicit (packet mark) indications of internal congestion.
Additive Increase
 Additive Increase is a reaction to perceived available capacity.
 Linear Increase basic idea:: For each “cwnd’s worth” of packets sent, increase cwnd by 1 packet.
 In practice, cwnd is incremented fractionally for each arriving ACK.
Multiplicative Decrease
* The key assumption is that a dropped packet and the resultant timeout are due to congestion at a router or a switch.
Multiplicate Decrease:: TCP reacts to a timeout by halving cwnd.
 Although cwnd is defined in bytes, the literature often discusses congestion control in terms of packets (or more formally in MSS == Maximum Segment Size).
 cwnd is not allowed below the size of a single packet.
Slow Start
 Linear additive increase takes too long to ramp up a new TCP connection from cold start.
 Beginning with TCP Tahoe, the slow start mechanism was added to provide an initial exponential increase in the size of cwnd.
Remember mechanism by: slow start prevents a slow start. Moreover, slow start is slower than sending a full advertised window’s worth of packets all at once.
 The source starts with cwnd = 1.
 Every time an ACK arrives, cwnd is incremented.
 cwnd is effectively doubled per RTT “epoch”.
 Two slow start situations:
 At the very beginning of a connection {cold start}.
 When the connection goes dead waiting for a timeout to occur (i.e, the advertized window goes to zero!)