1 EE 627 Lecture 11 Review of Last Lecture UDP & Multimedia TCP & UDP Interaction

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

3 UDP & 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

4 TCP/UDP 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.

5 UDP Consequences Most applications today use TCP Stability of network relies on congestion response of applications Large scale use of UDP could lead to problems - no congestion response Large number of multimedia applications expected - move larger amounts of data

6 Unfairness When UDP and TCP compete, UDP wins by pushing TCP into congestion

7 Unfairness - FIFO

8 Unfairness - WRR

9 Loss of goodput -FIFO Packets dropped later in network

10 Loss of goodput -WRR

11 Multimedia Delivery Even when using UDP, applications should respond to congestion end-to-end. Need to promote “nice” behavior or “TCP- friendly” behavior. Emerging applications shouldn’t kill the performance of “nice” applications.

12 TCP-Friendly Throughput of a TCP connection Limit flows to TCP-style BW Don’t know RTT exactly Why should everyone follow this exactly? Monitoring individual flows difficult

13 Equation-based control Don’t have to respond to congestion exactly like TCP –As long as steady-state BW is about the same Design a protocol that claims on an average the same BW as TCP RTT is available to end-host, try to estimate drop probability –Adjust rate based on the BW using the equation

14 Drop Probability Don’t want to use instantaneous drop probability - varies too much, noisy. Use some kind of averaging –Tends to dampen response to congestion –Important to respond quickly in times of heavy congestion –Uses a limited history -remember last 8 events –Weigh the more recent ones higher

15 Equation-based control Shown to work well when competing with TCP –Lower variance in flow’s BW than TCP –Fairer distribution than TCP A little complicated Spurred a lot of interest in new protocols.

16 Binomial congestion control Generalize congestion response Showed that these protocols are TCP- friendly if l+k = 1, through analysis Varying l and k, keeping l+k = 1 -- can get multiple protocols.

17 Binomial Congestion control Showed that steady-state analysis did not tell the complete picture –Depending on congestion response, the drop rates could be different for different protocols –Could respond to congestion in a TCP-friendly way, but may force TCP do have more drops –Conjecture: RED is better than droptail to make drop probabilities equal across flows

18 Open Issues Much interest in this area of research Not clear at what time scales, a flow needs to be TCP-friendly –clearly steady state analysis not sufficient. –Instantaneous TCP like response not needed. Other possible mechanisms simpler?

19 Other Mechanisms Multiple connections - seem to respond to congestion, but claim larger BW. –Used in web browsers Pricing - make user pay more when sending more bits. Adjust pricing based on congestion.

20 Rate-based adaptation Have a notion of allowed rate -adjust rate to avoid congestion - reduce rate before packet loss. Packet-pair: Send a pair of packets, watch the time separation of acks The delay between acks gives an indication of bottleneck BW

21 Packet-pair

22 Packet-pair Technique Ack compression leads to incorrect BW estimation. Timestamp packets on receipt - t1, t2 Inform sender d = t2 - t1, bottleneck BW = (d)/P, P = size of first packet. Need to send multiple times and use min d. Hard to get an estimate of available BW

23 Packet-pair With parallel transfers, both packets may arrive simultaneously at the receiver - inflating available BW Can be improved by sending more packets Possible to decouple rate adaptation and reliable delivery

24 Hop-by-Hop Possible to do flow control hop-by-hop Send backward pressure to reduce rate when queues are building up Tough to control individual flows Every network element need to implement - not just endpoints.