1. Presented by Rajan Includes slides presented by Andrew Sprouse, Northeastern University CISC 856 TCP/IP and Upper Layer Protocols Date:May 03, 2011

2. References Low Extra Delay Background Transport (LEDBAT) draft-ietf-ledbat-congestion-04.txt LEDBAT: the new BitTorrent congestion control protocol - Dario Rossi, Luca Muscariello, Computer Communications and Networks (ICCCN), 2010 Proceedings of 19th International Conference

3. Peer to peer computing Tasks or workloads partitioned between peers. Peers are both suppliers and consumers of resources.

4. BitTorrent A peer-to-peer file transfer protocol Very popular today. [2009]

5. BitTorrent mechanism: Files are broken into pieces. – Users each download different pieces from the original uploader (seed). – Users exchange the pieces with their peers to obtain the ones they are missing. This process is organized by a centralized server called the Tracker.

6. 1. Seeder generates a torrent file Uploads torrent to a web server. Seeder – A client sharing 100% of the shared file.

7. 2. The seeder notifies the tracker that it is sharing the file described in the torrent file.

8. 3. A bitTorrent user downloads the torrent file from the web server bitTorrent user – client downloading the shared file from the seeder.

9. 4. The bitTorrent user connects to the tracker specified in the torrent file. The tracker returns a list of other peers who are sharing the file.

10. 5. The bitTorrent user connects to its peers to retrieve pieces of the files.

11. Seeder: A Downloader B {1,2,3,4,5,6,7,8,9,10} {} {1,2,3} Downloader C {} {1,2,3} {1,2,3,4} {1,2,3,5} {1,2,3,4,5}

12. Motivation User A downloading files User B checking email Modem BitTorrent traffic Other traffic Send buffer of modem

13. Motivation Fair? – BitTorrent traffic - background transfers. – Need higher priority than checking mail. Why such unfairness? – BitTorrent uses multiple TCP connections. – More connections, the larger share of the bandwidth BitTorrent gets. Problems: – Apart from priority, TCP fills buffer, increasing delay.

14. Motiivation Traditional solution – Cap the upload rate of the BitTorrent client to 80% of the up-link capacity. Disadvantage: – Only 20% for interactive traffic. – 20% wasted if no interactive traffic.

15. Goals

16. LEDBAT Overview Timestamp Ack. delay base_delay queuing_delay off_target cwnd B A

17. One way delay: LEDBAT use one-way delay to estimate queuing delay. Why one way delay? – Avoid reverse path congestion.

18. Base delay & Queuing delay: AB Remote_timestamp =Time.now() Ack delay =local timestamp – remote timestamp Current_delay = Ack delay base_delay = min(current delay, base_delay) Queuing delay= current delay - base delay

19. Base delay estimate End-to-end delay: – Transmission delay – Propagation delay – Processing delay – Queuing delay Barring noise, these 3 delays are constant

20. Initial estimate of base delay: Initial base_delay = infinity No queuing delay. – Acknowledgement Delay = Transmission delay + Propagation delay + Processing delay – Base_delay = min(infinity, Acknowledgement delay) ARouterB

21. TARGET, off_target and cwnd TARGET is the maximum queuing delay that LEDBAT tolerates. Off_target = TARGET - queuing delay. – For example if TARGET = 20 ms – If queuing delay > 20, then sender rate is decreased – If queuing delay < 20, then sender rate is increased off_target < 0, decrease sender rate. off_target > 0, increase sender rate. cwnd += GAIN * off_target / cwnd – Where GAIN = the rate at which the congestion window responds to changes in queuing delay.

22. Max Rampup rate fastest increase if and only if queuing = 0 Sender rate is 1 MSS per RTT as in TCP. in case of loss, LEDBAT should behave like TCP halve its congestion window Halves on congestion

23. Controller A proportional-integral-derivative (PID) is used to govern dynamic of the congestion window. Controller is characterized by 2 parameters: – TARGET set to 25 ms – GAIN = 1/TARGET queuing > target → slower sender rate queuing < target → faster sender rate queuing ≈ target → steady (stability)

24. A B ssthreshold =4 Additive Increase starts A B TARGET =4 TCP LEDBAT

25. Example Settings: – Packet size = 1500 bytes – All flows have same RTT = 50ms. (one way base delay = 25ms) – Link capacity = 10 Mbps – Buffer size = 40 packets. – TARGET = 20 packets LEDBAT source TCP source Modem

26. LEDBAT performance LEDBAT against TCP: at t =0 LEDBAT vs LEDBAT In both case, the flow starts in the same time. time 0 2 4 6 8 10 12 14

27. Micro Transport Protocol UDP-based variant of the bitTorrent peer to peer file sharing protocol. congestion control algorithm used by µTP, known as Ledbat

29. vertype extension Connection_id Timestamp_microseconds Timestamp_difference_microseconds Wnd_size Seq_nrAck_nr