1 TCP Transport Control Protocol Reliable In-order delivery Flow control Responds to congestion “Nice” Protocol

2 TCP Header

3 TCP flow control Window based Sender cannot send more data than a window without acknowledgements. Window is a minimum of receiver’s buffer and ‘congestion window’. After a window of data is transmitted, in steady state, acks control sending rate.

4 Flow control

5 Congestion window is increased gradually At the beginning, set cwnd = 1 For each ack, double the cwnd until a threshold Increase by 1 for a window of acks after that.

6 Slow Start

7 Additive Increase

8 Reliable delivery Sender, Receiver keep track of bytes sent and bytes received. Acks have an indication of next byte expected. Three duplicate acks considered a packet loss - sender retransmits

9 Reliable Delivery

10 Congestion Control Traffic on the network is constantly changing. Packets may be lost due to transmission errors, switch buffer overflows, receiver buffer overflows. Packet loss is taken as an indication of congestion.

11 Congestion Response TCP reduces sending rate on packet loss cwnd is halved on a packet loss cwnd is set to 1 on a timeout TCP follows -- Multiplicative Decrease and Additive Increase policy for window adjustments

12 Congestion Response Queue length reduction takes qudratic time Multiplicative decrease allows sufficient time to reduce queue lengths -- Jacobson Jain -- Multiplicative decrease and additive increase allows ‘fair’ sharing of bandwidth. TCP -- ‘good’ citizen - allows fair sharing, avoids congestion collapse.

13 Congestion Response

14 TCP Congestion Response TCP responds to congestion. Increases window size until a packet loss This allows maximization of utilization. No Congestion avoidance mechanism Number of Proposals - TCP Vegas -- includes a rate adjustment mechanism based on observed delay.

15 DECbit Indicate congestion by setting a bit in the packet. Receiver echoes the bit to the sender. Sender adjusts sending rate based on percentage of marked packets. Avoids congestion before it happens Reduces the stair-case affect of TCP.

16 TCP and multimedia Reliable delivery not needed for multimedia Timely delivery more important than in- order delivery. Late packet can be thrown away TCP’s reliability gets in the way.