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Project 3 Flow and congestion control

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

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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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