Incentives Build Robustness in BitTorrent - Bram Cohen

Presented by Venkatesh Samprati

What is the Problem Being Solved Here?

• Sharing a fairly large file

• Involves making a replica

• Problem is somewhat similar to, but not the same as, replication in a distributed file system, a Content Delivery Network or a Distributed Hash Table overlay network

Simple Solution: One Big Server

• Make the file available on a central server

• Each client downloads file from this server

• Problems– Solution does not scale very well– With a large number of clients, the server’s

resources get overwhelmed

The Brilliance of Napster: P2P

• In the original Napster, nodes connected to a central server and gave it a listing of all the files they had.

• Nodes relay searches to the central server, which performs them locally

• The actual file transfer occurs peer-to-peer• The big weakness of this approach was that the

directory server was a single point of failure

Bit Torrent

• Chop file into many pieces

• Replicate DIFFERENT pieces on different peers as soon as possible

• As soon as a peer has a complete piece, it can trade it with other peers

• Hopefully, we will be able to assemble the entire file at the end

Basic Components• Seed

– Peer that has the entire file

• Leacher– Peer that has an incomplete copy of the file

• A Torrent file– Passive component– Length and Name of file.– Files are typically fragmented into 256KB pieces– The torrent file lists SHA1 hashes of all the pieces to allow peers

to verify integrity– Typically hosted on a web server– Contains IP address of tracker.

Tracker

• - Centralized process, keeps track of peers

• - Does not distribute actual contents of file. Only holds metadata.

• - two types of interaction with peers

• - Returns random list of peers (max 50)

Pieces and Sub-Pieces

• A piece is broken into sub-pieces ... typically 16KB in size

• Policy: Until a piece is assembled, only download sub-pieces for that piece

• This policy lets complete pieces assemble quickly

Piece Selection

• The order in which pieces are selected by different peers is critical for good performance

• If a bad algorithm is used, we could end up in a situation where every peer has all the pieces that are currently available and none of the missing ones

• If the original seed is taken down, the file cannot be completely downloaded!

Random First Piece

• Initially, a peer has nothing to trade

• Important to get a complete piece ASAP

• Rare pieces are typically available at fewer peers, so downloading a rare piece initially is not a good idea

• Policy: Select a random piece of the file and download it

Rarest Piece First

• Policy: Determine the pieces that are most rare among your peers and download those first

• This ensures that the most common pieces are left till the end to download

• Rarest first also ensures that a large variety of pieces are downloaded from the seed

Endgame Mode

• Policy: When all the sub-pieces that a peer doesn’t have are actively being requested, these are requested from EVERY peer

• When the sub-piece arrives, the replicated requests are cancelled

• This ensures that a download doesn’t get prevented from completion due to a single peer with a slow transfer rate

• Some bandwidth is wasted, but in practice, this is not too much

Pareto Efficiency

• Vilfredo Pareto – Italian Economist• “Given a set of alternative allocations and a set of individuals, a

movement from one alternative allocation to another that can make at least one individual better off, without making any other individual worse off, is called a Pareto optimization. An allocation of resource is Pareto Optimal when no further Pareto optimization can be made”

• Algorithm, in which pairs of counterparties see if they can improve their lot together, and leads to global optima.

Prisoner’s DilemmaPrisoner B stays Silent Prisoner B Betrays

Prisoner A Stays Silent Both server Six Months

Prisoner A serves 10 Yrs and Prisoner B goes free

Prisoner A Betrays Prisoner A goes free and Prisoner B serves 10 Yrs

Both serve 2 yrs.

Choking• One of BitTorrent’s most powerful idea is the choking

mechanism• It ensures that nodes cooperate and eliminates the free-

rider problem• Cooperation involves uploaded sub-pieces that you have

to your peer• Choking is a temporary refusal to upload; downloading

occurs as normal• Connection is kept open so that setup costs are not borne

again and again• Based on game-theoretic concepts

– Tit-for-tat strategy in Repeated Games

Repeated Games

• Over time, more complex strategies can evolve

• For instance, Tit-for-tat– Do onto others as they do onto you– If someone cheats, you must retaliate back– Have a recovery mechanism to ensure eventual

cooperation

Choking Algorithm

• Goal is to have several bidirectional connections running continuously

• Upload to peers who have uploaded to you recently

• Unutilized connections are uploaded to on a trial basis to see if better transfer rates could be found using them

Choking Specifics• A peer always unchokes a fixed number of its peers

(default of 4)• Decision to choke/unchoke done based on current

download rates, which is evaluated on a rolling 20-second average

• Evaluation on who to choke/unchoke is performed every 10 seconds– This prevents wastage of resources by rapidly choking/unchoking

peers– Supposedly enough for TCP to ramp up transfers to their full capacity

• Which peer is the optimistic unchoke is rotated every 30 seconds

Anti-Snubbing

• Policy: When over a minute has gone by without receiving a single sub-piece from a particular peer, do not upload to it except as an optimistic unchoke

• A peer might find itself being simultaneously choked by all its peers that it was just downloading from

• Download will lag until optimistic unchoke finds better peers

• Policy: If choked by everyone, increase the number of simultaneous optimistic unchokes to more than one

Upload-Only mode

• Once download is complete, a peer has no download rates to use for comparison nor has any need to use them

• The question is, which nodes to upload to?• Policy: Upload to those with the best upload rate.• This ensures that pieces get replicated faster• Also, peers that have good upload rates are

probably not being served by others

Operation

References

• "BitTorrent Economics Paper" , Bram Cohen

• "BitTorrent protocol specification" , Bram Cohen

• "BitTorrent Resource Availability Analysis" , Brian Greinke and James Hsia. (Rice)

• "Dissecting BitTorrent: Five Months in a Torrent's Lifetime" , M. Izal, G. Urvoy-Keller, E.W. Biersack, P.A. Felber, A. Al Hamra, and L.Garc es-Erice. (Institut Eurecom, France)