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CMPE 150 – Winter 2009

CMPE 150 – Winter 2009. Lecture 17 March 5, 2009 P.E. Mantey. CMPE 150 -- Introduction to Computer Networks. Instructor: Patrick Mantey mantey@soe.ucsc.edu http://www.soe.ucsc.edu/~mantey/ Office: Engr. 2 Room 595J Office hours: Tues 3-5 PM, Mon 5-6 PM*

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CMPE 150 – Winter 2009

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  1. CMPE 150 – Winter 2009 • Lecture 17 • March 5, 2009 • P.E. Mantey

  2. CMPE 150 -- Introduction to Computer Networks • Instructor: Patrick Mantey mantey@soe.ucsc.edu http://www.soe.ucsc.edu/~mantey/ • Office: Engr. 2 Room 595J • Office hours: Tues 3-5 PM, Mon 5-6 PM* • TA: Anselm Kia akia@soe.ucsc.edu • Web site: http://www.soe.ucsc.edu/classes/cmpe150/Winter09/ • Text: Tannenbaum: Computer Networks (4th edition – available in bookstore, etc. )

  3. Syllabus

  4. Text Readings • Today: • Chapter 6, Section 6.6 (TCP Performance) • Tuesday March 10 • Chapter 7.1 (DNS), 7.2-3 • Chapter 7.4 (Multimedia)

  5. Internet Layering • Level 5 -- Application Layer • (rlogin, ftp, SMTP, POP3, IMAP, HTTP..) • Level 4 -- Transport Layer(a.k.a Host-to-Host) • (TCP, UDP) • Level 3 -- Network Layer (a.k.a. Internet) • (IP, ICMP, ARP) • Level 2 -- (Data)Link Layer / MAC sub-layer • (a.k.a. Network Interface or Network Access Layer) • Level 1 -- Physical Layer

  6. Transport Layer – Connection Oriented Service • Addressing / Access Points • Connection Request / Establishment • Delayed Duplicate Packet Problem • Timer management • Connection points • Connection Release

  7. Today’s Agenda • Transport Layer • TCP: Timers / RTT estimates • Exponential backoff • Wireless vs. wired • Transactional TCP • Performance

  8. Port Detective http://www.softpedia.com/get/Network-Tools/Network-Testing/Port-Detective.shtml

  9. 3389 Remote desktop (Windows XP) Reference: http://www.microsoft.com/windowsxp/using/networking/expert/northrup_03may16.mspx ==========================================================See also:http://www.chebucto.ns.ca/~rakerman/port-table.html

  10. TCP Congestion Control • (a) A fast network feeding a low capacity receiver. • (b) A slow network feeding a high-capacity receiver.

  11. Jacobson’s Algorithm 1 • Determining the round-trip time: • TCP keeps RTT variable. • When segment sent, TCP measures how long it takes to get ACK back (M). • RTT = alpha*RTT + (1-alpha)M. • alpha: smoothing factor; determines weight given to previous estimate. • Typically, alpha=7/8.

  12. Jacobson’s Algorithm 2 • Determining timeout value: • Measure RTT variation, or |RTT-M|. • Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|. • Alpha may or may not be the same as value used to smooth RTT. • Timeout = RTT+4*D.

  13. Karn’s Algorithm • How to account for ACKs of retransmitted segments? • Count it for first or second transmission? • Karn proposed not to update RTT on any retransmitted segment. • Instead RTT is doubled on each failure until segments get through.

  14. TCP Congestion Control: Example cwin timeout threshold threshold time

  15. Persistence Timer • Prevents deadlock if an window update packet is lost and advertised window = 0. • When persistence timer goes off, sender probes receiver; receiver replies with its current advertised window. • If = 0, persistence timer is set again.

  16. Keepalive Timer • Goes off when a connection is idle for a long time. • Causes one side to check whether the other side is still alive. • If no answer, connection terminated.

  17. TCP Timer Management • (a) Probability density of ACK arrival times in the data link layer. • (b) Probability density of ACK arrival times for TCP.

  18. TCP Retransmission Timer • When segment sent, retransmission timer starts. • If segment ACKed, timer stops. • If time out, segment retransmitted and timer starts again.

  19. How to set timer? • Based on round-trip time: time between a segment is sent and ACK comes back. • If timer is too short, unnecessary retransmissions. • If timer is too long, long retransmission delay.

  20. Jacobson’s Algorithm 1 • Determining the round-trip time: • TCP keeps RTT variable. • When segment sent, TCP measures how long it takes to get ACK back (M). • RTT = alpha*RTT + (1-alpha)M. • alpha: smoothing factor; determines weight given to previous estimate. • Typically, alpha=7/8.

  21. Jacobson’s Algorithm 2 • Determining timeout value: • Measure RTT variation, or |RTT-M|. • Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|. • Alpha may or may not be the same as value used to smooth RTT. • Timeout = RTT+4*D.

  22. Karn’s Algorithm • How to account for ACKs of retransmitted segments? • Count it for first or second transmission? • Karn proposed not to update RTT on any retransmitted segment. • Instead RTT is doubled on each failure until segments get through.

  23. Persistence Timer • Prevents deadlock if an window update packet is lost and advertised window = 0. • When persistence timer goes off, sender probes receiver; receiver replies with its current advertised window. • If = 0, persistence timer is set again.

  24. Keepalive Timer • Goes off when a connection is idle for a long time. • Causes one side to check whether the other side is still alive. • If no answer, connection terminated.

  25. Performance Issues • Performance Problems in Computer Networks • Network Performance Measurement • System Design for Better Performance • Fast TPDU Processing • Protocols for Gigabit Networks

  26. System Design for Better Performance • Rules: • CPU speed is more important than network speed. • Reduce packet count to reduce software overhead. • Minimize context switches. • Minimize copying. • You can buy more bandwidth but not lower delay. • Avoiding congestion is better than recovering from it. • Avoid timeouts.

  27. Wireless TCP and UDP • Splitting a TCP connection into two connections.

  28. Wireless TCP 1 • According to layered system design principles, transport protocol should be independent of underlying technology. • However, wireless networks invalidate this principle. • Ignoring properties of wireless medium can lead to poor TCP performance. • Problem: TCP’s congestion control.

  29. Wireless TCP 2 • Problem: packet loss as congestion indicator. • When retransmission timer times out, sender slows down. • Wireless links are lossy! • Dealing with losses in this case should be re-sending lost segments asap.

  30. Indirect TCP (I-TCP) • [Bakne and Badrinath, 1995]. • Split TCP connection in 2: one from sender to base station and the other from base station to receiver. • Base station serves as “repeater”: copies segments between connections in both directions. • Connections are homogeneous; timeouts on 1st. connection, slow down sender. • Problem: violates TCP’s e2e’ness. • Example: ACKs to sender mean base station received segments, not necessarily receiver.

  31. Snoop TCP • [Balakrishnan et al., 1995]. • Does not break connection. • Modifications to base station’s network layer code. • Snooping agent on base station observes and caches TCP segments sent to mobile host and ACKs coming back. • If it doesn’t see an ACK for a segment or sees duplicate ACKs, it times out and retransmits. • But source may time out anyway.

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