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Exam Review

Exam Review. Networking CS 3470, Section 1 Sarah Diesburg. Coverage. Total: 100 points; 1 min / point Based on lectures, assignments, and projects Based on your ability to apply various principles learned in the class Multiple choice, short answer, and problem solving questions. Coverage.

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Exam Review

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  1. Exam Review Networking CS 3470, Section 1 Sarah Diesburg

  2. Coverage • Total: 100 points; 1 min / point • Based on lectures, assignments, and projects • Based on your ability to apply various principles learned in the class • Multiple choice, short answer, and problem solving questions

  3. Coverage • Two parts • Chapters 5-6 (last third of class) • Roughly 50 pts • Cumulative (over first 2/3 of class, exams 1-2) • Roughly 50 pts

  4. Chapters 5-6 Review

  5. Transport Layer • Definitions • Transport layer • What is the primary function? • Why is it necessary (say on top of IP)? • Packet Encapsulation • Layer headers are added or taken away • Multiplexing/demultiplexing

  6. Transport Layer • Definitions • TCP socket • Identified by 4-tuple (IP src, IP dest, port src, port dest) • UDP socket • Identified by 2-tuple (IP dst, IP port) • Pseudoheader • Flow control • Congestion control

  7. Transport Layer • Segments • Packets exchanged between TCP peers • TCP flags • SYN, ACK, FIN, RST

  8. Transport Layer • Short answer / problem solving • UDP advantages over TCP • TCP advantages over UDP • Checksum with 1’s compliment • Two-Army problem

  9. DNS • Definitions • What is DNS? • Host aliasing • Multiple names for one server • MX records • Load balancing • Many IP addresses associated with a hostname • Canonical name

  10. DNS • Short answers / problem solving • Role of a root DNS server • Role of a top-level domain (TLD) server • Be able to create and identify iterative and recursive DNS queries • Role of DHCP and DNS

  11. TCP Control Flow • Definitions • Sequence number field (often called SequenceNum) • Passive open • Active open • Maximum Segment Lifetime (TCP)

  12. TCP Control Flow • Short answers / problem solving • TCP 3-way handshake • Goals of TCP sliding window • AdvertisedWindow/EffectiveWindow (see next slide)

  13. Sliding Window Revisited Sender Receiver • Be able to look at a sliding window diagram and show me the AdvertisedWindow (on receiver) and EffectiveWindow (on sender)

  14. TCP Transmission Control • Definitions • Silly window syndrome • Fast retransmit (3 duplicate ACKs) • Nagle’s Algorithm • Generally, what does it do? (No need to memorize algorithm.) • Adaptive Retransmission • Generally, what does it do? (No need to memorize algorithm.)

  15. TCP Transmission Control • Short answer / problem solving • TCP’s three mechanisms to trigger the transmission of a segment • General TCP behavior -- ACK generation (see next slide)

  16. TCP ACK generation[RFC 1122, RFC 2581] TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediately send ACK, provided that segment starts at lower end of gap Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. # . Gap detected Arrival of segment that partially or completely fills gap

  17. Queuing Disciplines • Definitions • Flow • FIFO queuing • FIFO priority queuing • Tail drop • Fair queuing

  18. Queuing Disciplines • Short answers / problem solving • How do queuing disciplines relate to congestion control? • What is Fi in fair queuing? • Be prepared to answer queuing problems as seen in homework 9, question 3

  19. TCP Congestion Control • Definitions • Congestion window (or CongWin) • TCP loss event • Why do we care about losses? • Timeout or 3 duplicate ACKs • Additive Increase/Multiplicative Decrease (AIMD) • Slow Start • Fast Recovery

  20. TCP Congestion Control • Short answers / problem solving • Explain how AIMD and slow start work • Explain how fast recovery works • Explain how TCP is fair • Is all traffic fair? • Homework 9, questions 1-2

  21. Summary: TCP Congestion Control • When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. • When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. • When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. • When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

  22. Event State TCP Sender Action Commentary ACK receipt for previously unacked data Slow Start (SS) CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT ACK receipt for previously unacked data Congestion Avoidance (CA) CongWin = CongWin+MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT Loss event detected by triple duplicate ACK SS or CA Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. Timeout SS or CA Threshold = CongWin/2, CongWin = 1 MSS, Set state to “Slow Start” Enter slow start Duplicate ACK SS or CA Increment duplicate ACK count for segment being acked CongWin and Threshold not changed TCP sender congestion control

  23. Congestion Avoidance • Definitions • DECbit • RED gateways

  24. Network Programming • Look over your projects and be able to tell me the differences between setting up a TCP connection and a UDP connection • What does a pipe (“|”) and redirection (“>” or “<“) do?

  25. Putting it all together Review

  26. Putting it all together • Wireshark • I may paste a screenshot of a Wireshark trace and ask you a few questions about it • Review project 4 to prepare

  27. Putting it all together • Definitions • Review definitions and general concepts from previous reviews and old tests

  28. Putting it all together • The following problem-solving questions may show up on the final: • Calculate the total time required to transfer a file (recall the definition of latency) • SMTP protocol • Drawing windows in the sliding window protocol

  29. Putting it all together • ISO/OSI vs Internet Model • Determining next hop for a CIDR network • Dijkstra's algorithm • TCP simple talk control flow • Host vs network order

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