1 689 Lecture 2 Review of Last Lecture Networking basics TCP/UDP review

2 Multimedia Applications Video-on-demand Near-video-on-demand Interactive games Teleconferencing IP Telephony Training/Travel videos

3 Multimedia Requirements Guarantees over long periods of time Throughput guarantees Audio requires loss/delay guarantees Interactive apps. Require low delay Visual Perception allows adaptivity Audio not as forgiving as video

4 Multimedia requirements CBR VBR Variable bit rate places extra burden Systems/devices/network support harder Adaptive applications compensate for bandwidth/performance degradation and packet losses.

5 Typical System

6 Delivering multimedia Network transport layers responsible

7 Transport layers TCP/UDP

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

9 TCP Header

10 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.

11 Flow control

12 Flow control 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.

13 Slow Start

14 Additive Increase

15 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

16 Reliable Delivery

17 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.

18 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

19 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.

20 Congestion Response

21 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.

22 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.

23 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.

24 UDP Unreliable Datagram Protocol

25 UDP Provides multiplexing and demultiplexing of sources. No reliability, flow control, congestion control. Sends data in a burst. Most multimedia applications using UDP

26 UDP and multimedia Put flow control, congestion control into application. Retransmit if packet deadline not past Move on if packet deadline is past Don’t respond to Congestion Not a “nice” citizen. Possible to cause congestion collapse.

27 Summary TCP not well suited to multimedia. TCP is a well understood, ‘nice’ protocol. Multiplicative decrease/additive increase allows fair sharing of BW and avoids congestion collapse. UDP is being used by multimedia developers.