BitTorrent
	Made by Bram Cohen
		Wife, 3 kids
		Aspergers

Purpose- Share Files
	Support flash crowds
	Self-scaling
	Dishonest clients (somewhat, riaa)
	Leechers
	Not anonymous 
		MPAA (http://blog.newworldodor.net/2007/01/11/mpaa-uploads-fake-torrents-to-trap-you/)
		Peergardian
	Not secret content
	Not unblockable ???? (http://www.msnbc.msn.com/id/21376597/)
	Not streaming (only can view when download complete) (Cohen in Jan 2007 says will be possible)
	Not searching

How Much Traffic
	http://www.businesswire.com/portal/site/google/index.jsp?ndmViewId=news_view&newsId=20070618005912&newsLang=en	
	http://www.ipoque.com/resources/internet-studies/internet-study-2007
	http://torrentfreak.com/shocking-61-of-all-upstream-internet-traffic-is-p2p-081021/


Torrent files
	Must get some other way (Meganova?)
	Contain
		URL of the tracker
		Block size
		Checksum of every block
		Suggested filename

Tracker
	Keeps track of who is part of the swarm.
	Connect using HTTP Get command with parameters that describe statistics
	Does not know who has which pieces.
	There is a trackerless option
	Trackers can be private
	Normally one tracker per swarm, can have multitracker

Swarm
	Anyone who is part of the same tracker for the same file

What each client does
	Connect to peers
		Chocked -- send me no requests
		Interested -- He has something I want
		How many peers?
			When you get a block you announce, so more than 25 too high
	Downloading
		Which order
		First->Last
			Works for streaming
			Horrible for network
		Random
		Rarest First
			Best for network
			Best for you
		Queuing .. 
			Keep a bunch in the queue to avoid a latency lag problem
			math :  queue_size = block_size / bandwidth * latency + a bit more
	Uploading and leechers
		tit-for-tat
			Proven wonderful in game context
			Horrible for newbies
		standard choking algorithm
			Goals:
				Try new connections once in a while
				Reward good citizens
				Keep number of active peers to a reasonable minimum
				Don't fibrulate (switch states often)
			Choke Algorithm:
				Keep 4 peers unchocked by best download rate
				Add any peer with better download rate that is uninterested
					When he becomes interested, lowest of four is bumped
				Unchoke one other peer at random, prefering new connections 3x as much
		Snubbing
			If no download for 1 minute, then no uploads to that peer
			
	Recontact Tracker
		Helps keeps stats
	Peer Exchange
		Helps unload trackers
		Can cause partitioning of the swarm
		Once a peer exchange graph is partitioned, it is partitioned forever
	Updating peers
		HAVE messages
			Can suppress to peers known to already have
				50% reduction
				What about rarest first?
			choke and unchoke
			interested and unintersted
			request (block#)
	Super Seed Mode
		Must have the whole file
		Goal:  Distribute the pieces of the file
		Goal:  Get the peers excanging with each other
		For each new peer, 
			Only annouce one piece as available.
			When that piece is announced back to you via someone else, annouce to the original peer a second piece.

		


Avoiding Blocking and Traffic Shaping
	Attack the cliet-tracker connection
		Based on IP number (Sandvine application does this)
		Defenses
			Encrypt
			Use UDP
			Filter RST packets
	Attack the peer-peer connection
		Cannot be based on IP number
		Look inside packet for torrent header
			Send rst packets
			throttle
				always
				when congested
			Encryption can hide torrent header
				Must not introduce a new recognizable header
				Even weak encryption is OK
				Sometimes this is broken


Distributed Hash table
http://bittorrent.org/beps/bep_0005.html

Bittorrent Protocol
http://wiki.theory.org/BitTorrentSpecification