1. CIS679: TCP and Multimedia r Review of last lecture r TCP and Multimedia

2. Review of Last Lecture r MPEG-2 m Targets at high resolution m Profiles and levels m Compatible to MPEG-1 r Multimedia application classes m Streaming m Unidirectional real-time applications m Interactive real-time applications r Multimedia requirements r Challenges

3. Delivering Multimedia r Network and transport layers

4. Transport Layers r TCP/UDP

5. TCP r Transport Control Protocol r Flow control and Responds to congestion r Reliable In-order delivery r “Nice” Protocol

6. TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number rcvr window size ptr urgent data checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

7. TCP seq. #’s and ACKs Seq. #’s: m byte stream “number” of first byte in segment’s data ACKs: m seq # of next byte expected from other side m cumulative ACK Q: how receiver handles out-of-order segments m A: TCP spec doesn’t say, - up to implementer Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ time simple telnet scenario

8. Flow Control

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

10. TCP Slowstart r exponential increase (per RTT) in window size (not so slow!) r loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP) initialize: Congwin = 1 for (each segment ACKed) Congwin++ until (loss event OR CongWin > threshold) Slowstart algorithm Host A one segment RTT Host B time two segments four segments

11. TCP Congestion Avoidance /* slowstart is over */ /* Congwin > threshold */ Until (loss event) { every w segments ACKed: Congwin++ } threshold = Congwin/2 Congwin = 1 perform slowstart Congestion avoidance 1 1: TCP Reno skips slowstart (fast recovery) after three duplicate ACKs

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

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

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