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CS E 679: Computer Network Review

CS E 679: Computer Network Review. Review of the uncounted quiz Computer network review. Network Layers. Transport Layers. TCP/UDP. TCP. Transport Control Protocol Flow control and Responds to congestion Reliable In-order delivery “Nice” Protocol. 32 bits. source port #. dest port #.

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CS E 679: Computer Network Review

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  1. CSE679: Computer Network Review • Review of the uncounted quiz • Computer network review

  2. Network Layers

  3. Transport Layers • TCP/UDP

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

  5. 32 bits source port # dest port # sequence number acknowledgement number head len not used rcvr window size U A P R S F checksum ptr urgent data Options (variable length) application data (variable length) TCP segment structure URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP)

  6. 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

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

  8. 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.

  9. Flow Control

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

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

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

  13. “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired UDP: User Datagram Protocol [RFC 768]

  14. often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses (why?): DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recover! UDP segment structure 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Application data (message) UDP segment format

  15. 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

  16. Unfairness • When UDP and TCP compete, UDP wins by pushing TCP into congestion

  17. TCP-Friendly • Throughput of a TCP connection • P: the packet size • p: the lost probability of a packet • Limit flows to TCP-style BW • Don’t know RTT exactly • Why should everyone follow this exactly? • Monitoring individual flows difficult

  18. Conclusion • TCP • 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. • UDP • UDP can be tuned to better support multimedia applications. • TCP-friendly • Rate-based adaptation • Packet-pair • Hop-by-hop control

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