Incentives Build Robustness in BitTorrent

1. Incentives Build Robustnessin BitTorrent Bram Cohen

2. Motivation Single server • Scalability issues for client/server systems • Server’s workload grows linearly with number of clients • Flash crowd problem

3. An “Ideal” Solution: IP Multicast Single server • Same stream is shared by all clients receiving same data • Requires infrastructure-level changes • Security issues • No widely accepted transport protocol on IP multicast layer

4. Current Approaches • Server farms replace central server • Scalability problem continues • Expensive to maintain • Content delivery networks (CDNs) • Akamai, … • Still expensive • Peer-to-peer systems (P2P) • Newer approach

5. P2P Systems (I) • Let clients, now called peers, share the server workload • Peers forward all the data they receive to other peers

6. P2P Systems (II)

7. Advantages • P2P solutions are • Scalable: • Downloading bandwidth grows with number of peers • Easy to deploy: • No additional hardware • No change to network infrastructure • Cheap

8. Issues • Organizing data transfers: • Figuring which peers have which chunks of data • Deciding where to send these chunks • Dealing with churning: • Peers come and go • Enforcing fairness: • Some peers do not upload as many data as they download

9. BitTorrent • Unstructured P2P System • Peers have no parent peers or child peers • Centralized tracker • Collects information on peers • Responds to requests for that information • Built-in fairness incentive • Rechoking favors cooperative peers • Simple user interface

10. Importance

11. Deployment (I) • Decision to use BitTorrent is made by publisher of file • Users join BitTorrent to get a file they want • Most users stops uploading once they have downloaded the file • Standard implementation keeps uploading until the BT window closes

12. Deployment (II) • In a typical deployment • Number of downloaders having parts of the file (leeches) increases very fast then peaks at a maximum before decreasing exponentially • Number of downloaders having the whole file (seeds) increases more slowly then peaks at a maximum before decreasing exponentially

13. Evolution

14. Starting a BT (I) • To start a BT, publisher puts on a web server a static file with information about • The file • Its length • Its name, • Hashing information • The URL of a tracker

15. The tracker • Helps downloaders find each other • Uses a simple protocol layered on top of HTTP • New downloader sends information about • What file it’s downloading • What port it’s listening on • … • Tracker replies with a list of peers that are downloading the same file

16. Starting a BT (II) • Next step is starting a downloader having the whole contents of the file • The seed • Web server and tracker havevery low bandwidth requirements • Seed must upload at least once whole file contents

17. Connecting with peers • Standard tracker algorithm returns random lists of peers • Random graphs are very robust • BitTorrent cuts files into pieces of fixed size, typically a quarter megabyte • Each downloader • Reports to its peers what pieces it has • Starts exchanging these pieces with them

18. Ensuring data integrity • Torrent file on web server has SHA1 hashes of all the pieces • Peers don’t report that they have a piece until they’ve checked its hash • Could have used erasure codes

19. Pipelining • Must avoid delays between pieces being sent • BT breaks pieces further into sub-pieces (typically 16KB in size) • Always keeps several requests pipelined at once • Sends a new request each time a sub-piece arrives

20. Piece Selection (I) • Downloaders requesting pieces follow four policies • Strict Priority • Finish first downloading pieces of which downloader has one or more sub-pieces • Gets complete pieces as quickly as possible

21. Piece Selection (II) • Rarest First • Download first the pieces that the fewest of their own peers have • Ensures that peers have the pieces that most of their peers want

22. Piece Selection (III) • Random First Piece • New peer should get its first complete piece as quickly as possible • Rare pieces can be downloaded from fewer peers than other pieces • New peer will select first pieces to download at random until it has obtained a complete piece

23. Piece Selection (IV) • Endgame Mode • At end of download • Peer will send to all other peers requests for sub-pieces it doesn’t have yet from all other • Will send cancels for all sub-pieces which arrive • Objective is to speed up end of download

24. Overview Torrent file Tracker Two nodes and theirpeers Downloaders A seed

25. BT Choking Algorithm • Penalizes peers that do not reciprocate • Tit-for-tat policy • Every ten seconds, each peer selects four less cooperating peers it will choke • Will refuse to upload data to these peers for ten seconds • Long enough for TCP to reach full capacity with the new transfers

26. Optimistic Unchoking • At all times, each BT peer has single‘optimistic unchoke’, • Unchoked regardless of its current upload • Rotated every third rechoke period (30 seconds) • “Correspond very strongly to always cooperating on the first move in prisoner’s dilemma.”

27. Upload only mode • A peer that has the whole content of the file it wanted to download starts privileging peers that use best its upload bandwidth

28. Anti-snubbing • A peer might be sometimes choked by all peers from which it was downloading • “To mitigate this problem, when over a minute goes by without getting a single piece from a particular peer, BitTorrent assumes it is ’snubbed’ by that peer and doesn’t upload to it except as an optimistic unchoke”

29. Actual deployments • BT routinely serves • Files hundreds of megabytes in size • To hundreds of simultaneous downloaders • Can have over a thousand concurrent downloaders. • Sole scaling bottleneck appears to be the bandwidth overhead of the tracker • One thousandth of total traffic