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Project 3 Flow and congestion control

Learn about the challenges in transport protocol design and implementation using UDP. Generate WHOHAS queries and IHAVE replies, download chunks using stop and wait, and add flow and congestion control.

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Project 3 Flow and congestion control

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  1. Project 3Flow and congestion control 15-441 Spring 2010 Recitation #10

  2. Context – Project 3 • Bit torrent-like file transfer app • Use UDP to understand the challenges in transport protocol design and implementation • Checkpoints • #1: Generate WHOHAS queries and IHAVE replies (due today!) • #2: Download a chunk using stop and wait • #3: Add flow control • #4: Add congestion control

  3. Pkt 0 ACK 0 ACK 0 ACK 1 Why do we need flow control • Stop and wait • Don’t transmit a new packet until you get an ACK • Limited performance • 1 packet per RTT Pkt 0 Pkt 1

  4. Sliding window - idea • Enough in-flight packets to “hide” the RTT • Have a buffer of packets on each side • Advance window when sender and receiver agree packets at the beginning have been received • Common case • Receiver: send cumulative ACK, slide window • Sender: slide window, send next packet

  5. Sliding window - visualization Sender Receiver Next expected Max acceptable Max ACK received Next seqnum … … … … Sender window Receiver window Sent & Acked Sent Not Acked Received & Acked Acceptable Packet OK to Send Not Usable Not Usable

  6. Sliding window – reliability • Lost packet (with cumulative ACKs) • When detecting a missing packet, the receiver sends an ACK with the sequence number of the last correctly ordered packet • If the sender timeouts while waiting for an ACK, it will retransmit • Accommodating out of order packets • The sender waits for 3 duplicate ACKs before retransmitting

  7. Sliding window – implementation • Sender keeps 3 pointers • LastPacketAcked, LastPacketSent, LastPacketAvailable • Ensure invariants • LastPacketAcked <= LastPacketSent • LastPacketSent <= LastPacketAvailable • LastPacketAvailable-LastPacketAcked <= WindowSize • Simplify by ignoring receiver issues: he can always process whatever he gets

  8. Sliding window – testing • Use assert statements to ensure invariants • Inject some losses to test loss behavior • Write time-stamped packet sequence numbers and ACKs to a file and plot them

  9. Packets Acks Sliding window – testing fast retrans Retransmission X Duplicate Acks Sequence No Time

  10. Congestion control • Adapt to the network conditions by changing your window size • Losses assumed to be due to congestion, so throttle back the sender when you see one • Implement: • Slow start • Congestion Avoidance • Fast retransmit

  11. Slow start • Start with window of size 1 (SW=1) and define the slow start threshold to be 64 (ssthresh=64) • Now, upon: • ACK received: SW++ • Loss detected: ssthresh = max(SW/2,2), SW=1 • SW = ssthresh: move into congestion avoidance

  12. Congestion avoidance • Upon ACK received: • SW+=(1/SW) (increases by ~1 every RTT) • Upon loss detected: • ssthresh = max(SW/2,2), SW=1 • Revert back to slow start • Detecting losses • Retransmission timeout • 3 duplicate ACKs received (fast retransmit)

  13. Estimating RTT – Karn’s algorithm • Estimated RTT (ERRT), based on the observed RTT (ORTT) • ERTT (i+1) = α * ERTT(i) + ( 1-α ) * ORTT (i) • α typically = 0.875 • Retransmit timer set to 2 * ERTT, increase backoff on retransmission • Ignore any ORTTs from packets with retransmissions – prevents situations where you are stuck with a erroneously low RTT (http://www.opalsoft.net/qos/TCP-10.htm)

  14. Congestion avoidance - testing Plot the window size as a function of time for a given flow Congestion Window Cut Congestion Window and Rate Time Grabbing back Bandwidth Packet loss + retransmit

  15. Packets Acks Congestion avoidance - testing Write time-stamped sequence nos. and ACKS Window size increases by 1 every RTT. Sequence No Time

  16. Fast recovery (optional) • When a loss is detected, don’t go back to slow start, leverage the outstanding packets: • Wait to receive SW/2 ACKs • Set SW = max(SW/2,2) and resume congestion avoidance

  17. Packets Acks Fast recovery (optional) Sent for each dupack after W/2 dupacks arrive Sequence No X Time

  18. Selective ACKs (optional) • Instead of having the sequence no. of the packet you last received in order • send a bitmask of all received packets • Enables faster retransmissions

  19. Packets Acks Without selective ACKs X X X Now what? - timeout X Sequence No Time

  20. Packets Acks With selective ACKs (optional) Now what? – send retransmissions as soon as detected X X X X Sequence No Time

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