1. Angelo Sapello University of Delaware sapello@udel.edu
BitTorrent
Angelo Sapello
University of Delaware

2. Server vs. P2P
Standard client/server model – all clients connect to a single server
Peer to peer (P2P) – clients get the data they need from one or more other clients
P2P may still require a server, but with a greatly reduced load

3. BitTorrent Terminology
Torrent file – the meta-data file indicating what tracker to connect to and what the file should look like
Tracker – an HTTP server that keeps track of the swarm
Swarm – the group of peers downloading/uploading the file
Seeder – A peer that has all the pieces of the file
Leecher – A peer that only downloads

4. BitTorrent model User downloads a meta-data “torrent” file
Peer connects to tracker server to get peer information
Peer connects to other peers in the swarm
Peer downloads pieces in a rarest first fashion
Peer simultaneously uploads pieces to other peers
Model defined by Bram Cohen

5. BitTorrent Transport Layer
BitTorrent is a three part application layer protocol
TCP provides all transport layer services
Reliable: No need for ACKs by client
Connection oriented: Minimal client handshakes
Down-side:
Multiple connections cause “fairness” issues
Multiple port usage causes NAT/firewall clogging

6. Initial Seeder/Web Server: 142.178.26.3:62189
Sally:
:53182
Bobbi (Leecher):
:52516
Allen:
:53182
Angelo:
:55103
Linda (Seeder):
:57812
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Initial Seeder/Web Server:
:62189

7. Initial Seeder/Web Server: 142.178.26.3:62189 Uninterested Choked
Sally:
:53182
Uninterested
Choked
Bobbi (Leecher):
:52516
Uninterested
Choked
Allen:
:53182
Uninterested
Choked
Angelo:
:55103
Uninterested
Choked
Linda (Seeder):
:57812
Uninterested
Choked
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189
Uninterested
Choked

8. Initial Seeder/Web Server: 142.178.26.3:62189 Uninterested Choked
Sally:
:53182
Uninterested
Unchoked
Bobbi (Leecher):
:52516
Uninterested
Choked
Allen:
:53182
Uninterested
Unchoked
Angelo:
:55103
Interested
Choked
Linda (Seeder):
:57812
Uninterested
Unchoked
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189
Uninterested
Choked

9. Initial Seeder/Web Server: 142.178.26.3:62189 Uninterested Choked
Sally:
:53182
Interested
Unchoked
Bobbi (Leecher):
:52516
Uninterested
Choked
Allen:
:53182
Uninterested
Unchoked
Angelo:
:55103
Interested
Unchoked
Linda (Seeder):
:57812
Uninterested
Unchoked
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189
Uninterested
Choked

10. Initial Seeder/Web Server: 142.178.26.3:62189 Uninterested Choked
Sally:
:53182
Interested
Unchoked
Bobbi (Leecher):
:52516
Uninterested
Choked
Allen:
:53182
Uninterested
Unchoked
Angelo:
:55103
Interested
Unchoked
Linda (Seeder):
:57812
Uninterested
Unchoked
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189
Uninterested
Choked

11. Initial Seeder/Web Server: 142.178.26.3:62189 Uninterested Choked
Sally:
:53182
Interested
Unchoked
Bobbi (Leecher):
:52516
Uninterested
Choked
Allen:
:53182
Uninterested
Unchoked
Angelo:
:55103
Uninterested
Unchoked
Linda (Seeder):
:57812
Uninterested
Unchoked
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189
Uninterested
Choked

12. Initial Seeder/Web Server: 142.178.26.3:62189
Sally:
:53182
Bobbi (Leecher):
:52516
Allen:
:53182
Angelo:
:55103
Linda (Seeder):
:57812
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Angelo: :55103
Initial Seeder/Web Server:
:62189

13. Initial Seeder/Web Server: 142.178.26.3:62189
Sally:
:53182
Bobbi (Leecher):
:52516
Allen:
:53182
Angelo:
:55103
Linda (Seeder):
:57812
Tracker:
/mytorrent:
Linda: :57812
Bobbi: :52516
Mandriva: :62189
Sally: :53182
Allen: :49552
Initial Seeder/Web Server:
:62189

14. Meta-data format Information bencoded
String “#:” ; any array of characters
Integer “i” - “e” ; a signed integer
Dictionary “d” - “e” ; key-value pairs
List “l” - “e” ; an array of the above types
Announce section – tracker information
Info section – file and pieces information
Pieces – a list of SHA-1 hash values of data chunks in the files

15. Meta-data file (cont'd)
Multiple files are ordered and concatenated to download like a single file
Example:
d8:announce96: 4133c0cb8a59ca4bb2d203be13:creation datei e4:infod5:filesld6:lengthi341985e4:pathl35:mandriva-linux-free i586.idxeed6:lengthi e4:pathl35:mandriva-linux-free i586.isoeed6:lengthi70e4:pathl39:mandriva-linux-free i586.iso.md5eed6:lengthi78e4:pathl40:mandriva-linux-free i586.iso.sha1eee4:name31:mandriva-linux-free i58612:piece lengthi262144e6:pieces349160: To
{“announce”:” 133c0cb8a59ca4bb2d203be”,”creation date”: ,
“info”:{“files”:[{“length”:341985,”path”:[“mandriva-linux-free i586.idx”]}, {“length”: ,”path”:[“mandriva-linux-free i586.iso”]}, {“length”:70,”path”:[“mandriva-linux-free i586.iso.md5”]}, {“length”:78,”path”:[“mandriva-linux-free i586.iso.sha1”]}], “name”:”mandriva-linux-free i586”,”piece length”:262144,”pieces”:
Binary array of 20 byte SHA-1 hash codes of each 256KB data chunk }}

16. Peace of mind How do you ensure that a peer has not tainted the piece?
SHA-1 hash algorithm proposed by NSA requires 2^80 hash operations to find a match pair by brute force. Mathematically broken in 2^63 operations.
Poisoned pieces are rejected and peer is no longer recognized as part of the swarm

17. Tracker protocol Standard HTTP GET request/response Example:
GET /announce/Mandrivalinux/282cb77c390627c44133c0cb8a59ca4bb2d203be?info_hash=%06g%e0g%98%a f%beN%f2%f6%40E%ad%0a%23U%dd%5d%d4%ba&peer_id=M %afF%3bE%b8%c9%2b%94%f1%ac%bf%93&port=57917&uploaded=0&downloaded=0&left= &corr upt=0&key=66D4D19D&event=started&numwant=200&compact=1&no_peer_id=1&ipv6=2002%3a80af%3ae e1e%3a%3a80af%3aee1e HTTP/1.1
Host: torrent.mandriva.com:6969
User-Agent: BitTorrent/6400(18095)
Accept-Encoding: gzip
HTTP/ OK
Content-Length: 1230
Content-Type: text/plain
Pragma: no-cache
d8:intervali1800e5:peers1200:
Binary encoded IP address, port number pairs for the entire (or subset of the) swarm or beconded dictionary or Peer ID, IP address, port triplets

18. BitTorrent protocol BitTorrent Handshake All other messages
Protocol Name Length (1-byte = 19)
Protocol Name (“BitTorrent protocol”)
64-bit reserved word
Info dictionary hash (SHA1 20-bytes)
Peer ID (20 bytes)
All other messages
Message length (32-bits/4-bytes)
Message ID (8-bits/1-byte)
Payload (Message length – 1 bytes)

19. BitTorrent Message IDs
None (len = 0): Keep Alive
0: Choke = Not willing to send data
1: Unchoke = Willing to send data
2: Interested = Would like data
3: Not interested = Would not like data
4: Have = I have just downloaded this piece
5: Bitfield = I have these pieces
6: Request = I would like this block – index (piece), begin(offset) and length
7: Piece = Here is a block – index (piece), begin(offset) and block (actual data)
8: Cancel = I no longer want this block – index, begin, length

20. Wireshark Example

21. Other issues Leeching Firewall/NAT crashing
Connection throttling (Comcast)
Copyright protection

22. Questions? Additional resources BitTorrent Specification:
BitTorrent Unofficial Explanation: