Peer-to-Peer Intro Jani & Sami Peltotalo

Overview of P2P Overlay networks Current P2P applications –P2P file sharing –Instant messaging / voice over IP –P2P distributed computing

P2P Architectures

1G P2P: Centralized Network Fast search/query response times Simple Protocol Provides a high degree of performance and resilience Susceptible to being shutdown, single server or server farm e.g. Napster

2G P2P: Decentralized Network Slow search/query response times which generates large volumes of network traffic Network resilience and performance governed by users' PCs and their network connectivity No central points of failure or control e.g. Gnutella 0.4

3G P2P: Hybrid Architecture super-peers Improved search/query response times, with less traffic generated per query than decentralized networks The deployment of super-peers provides a high degree of performance and resilience No central points of failure or control e.g. FastTrack, Gnutella 0.6

4G P2P: Different type of architectures BitTorrent: centralized eDonkey2000: semi-centralized Overnet: decentralized

FastTrack Overview

FastTrack Clients: KaZaA, iMesh, Grokster... MP3s & entire albums, videos, games Decentralized network, supernodes act as temporary indexing servers (hierarchical architecture) Control data encrypted Everything in HTTP request and response messages Optional parallel downloading of files

supernodes originalnodes Each peer is either a supernode or is assigned to a supernode Selection criterias: CPU, memory, network connection Each SN has about children nodes and has TCP connections with other supernodes SN tracks the content and IP of its children nodes, not content under its neighboring SNs FastTrack: Architecture

FastTrack: Metadata When ON connects to SN, it uploads its metadata For each file: –File name –File size –Content Hash (MD5+CRC) –File descriptors: used for keyword matches during query Content Hash: –When peer A selects file at peer B, peer A sends ContentHash in HTTP request –If download for a specific file fails (partially completes), ContentHash is used to search for new copy of file

FastTrack: Overlay Maintenance List of potential supernodes included within software download New peer goes through the list until it finds operational supernode –Node “pings” (5-6) supernodes on the list and connects with the first replied SN –Connects and obtains more up-to-date list, with 200 entries –SNs in the updated list are “close” to ON If supernode goes down, node goes through the updated list and finds new supernode

FastTrack: Queries Node first sends query to supernode –Supernode responds with matches –If x matches found, done Otherwise, supernode forwards query to subset of supernodes –If total of x matches found, done Otherwise, query further forwarded –Probably by original supernode rather than recursively

FastTrack: Parallel Downloading and Recovery If file is found in multiple nodes, user can select parallel downloading Identical copies identified by ContentHash HTTP byte-range header used to request different portions of the file from different nodes Automatic recovery when server peer stops sending file –ContentHash is used to search for new copy of file

eDonkey2000 Overview

eDonkey2000 (ED2K) Semi-centralized network, includes index servers Many clients: eDonkey2000, MLDonkey, eMule, Shareaza... Index server: Lugdunum Used also for legal content delivery Files identified by hash (MD4) Possible to search files using web, founded ed2k links can be used to start file download –ed2k://|file|gentoo.linux.install-x86-minimal [found via 40F09DB7400B6|/)

ED2K Communication: –client-connected index server: TCP –client-other index servers: UDP –index server-index server: UDP –client-client: TCP File transfer using Multisource File Transmission Protocol (MFTP) –also HTTP and BitTorrent supported

Index Server 1 Register to server, tell server own shared files Peer 1 Index Server 3 Index Server 2 Peer 4 (registered to index server 3) Index Server 1 XXXX.exePeer 2 YYYY.txtPeer 2 YYYY.exePeer 2 XXXX.txt XXXX.exe ZZZZ.txt ZZZZ.exe YYYY.txt ZZZZ.exe YYYY.txt YYYY.exe XXXX.exe Peer 2 (registered to index server 1) Peer 3 (registered to index server 2) Index Server 2 YYYY.txtPeer 3 ZZZZ.exePeer 3 Index Server 3 ZZZZ.txtPeer 4 ZZZZ.exePeer 4 ED2K: Registration

Index Server 1 Index Server 3 Index Server 2 Peer 4 (registered to index server 3) XXXX.txt XXXX.exe ZZZZ.txt ZZZZ.exe YYYY.txt ZZZZ.exe YYYY.txt YYYY.exe XXXX.exe Peer 2 (registered to index server 1) Peer 3 (registered to index server 2) ED2K: Registration Reply List of other index servers known by index server 1 Peer 1 (registered to index server 1) Index Server 1 XXXX.txtPeer 1 XXXX.exePeer 1 & Peer 2 YYYY.txtPeer 2 YYYY.exePeer 2

Index Server 1 Index Server 3 Index Server 2 Peer 4 Peer 2Peer 3 ED2K: File Search Peer 1 Search Files (TCP) Search Files (UDP) Search Files message includes: keyword optionally - min file size - max file size - availability - etc.

Index Server 1 Index Server 3 Index Server 2 Peer 4 Peer 2Peer 3 ED2K: File Search Reply Peer 1 Search File Results (TCP) Search File Results (UDP) Search File Results message includes one or more file info: file hash client IP and port (optional?) file name

Index Server 1 Index Server 3 Index Server 2 Peer 4 Peer 2Peer 3 ED2K: File Downloading 1/3 Peer 1 Get Sources (TCP) Get Sources (UDP) Done if Search File Results message(s) don’t include client IP and port pair(s) or ED2K link is used to start downloading includes: - file hash

Index Server 1 Index Server 3 Index Server 2 Peer 4 Peer 2Peer 3 ED2K: File Downloading 2/3 Peer 1 Found Sources (TCP) Found Sources (UDP) Found Sources message includes: file hash address list - client IP and port

Index Server 1 Index Server 3 Index Server 2 Peer 4 Peer 2Peer 3 ED2K: File Downloading 3/3 Peer 1 File requests and downloading

BitTorrent Overview

BitTorrent Centralized network, includes tracker.torrent files Google search for.torrents Legal material available

Tracker HTTP Server GET.torrent file Downloader Seed 2 Leecher 1 Seed 1 In.torrent file: file size file name hash of file (SHA1) url of tracker.torrent file BitTorrent: Get.torrent

Tracker HTTP Server Seed 2 Leecher 1 Seed 1 BitTorrent: Get Peer List Response-peer list GET-announce Downloader

Tracker HTTP Server Downloader Seed 2 Leecher 1 Seed 1 BitTorrent: Query File Pieces GET pieces of file

Tracker HTTP Server Leecher 2 Seed 2 Leecher 1 Seed 1 BitTorrent: File Pieces Info about download status pieces of file

Tracker HTTP Server Seed 4 Seed 2 Seed 3 Seed 1 BitTorrent: Status Information Info about complete download

Skype Overview

Skype Skype is a P2P VoIP client developed by the people who did KaZaA Allows its users to place voice calls and send text messages to other users of Skype clients Two types of nodes in the overlay network, ordinary hosts (OH) and super nodes (SN) OH is a Skype application that can be used to place voice calls and send text messages SN is an ordinary host’s end-point on the Skype network Any node with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a SN

Skype OH must connect to a SN and must register itself with the Skype login server for a successful login 7 bootstrap super nodes The host cache (HC) is a list of super node IP address and port pairs that OH builds and refreshes regularly HC contains a maximum of 200 entries

Skype Network Super Nodes Skype Login Server Message exchange during login

Skype Uses its Global Index technology to search for a user Firewall traversal: First UDP, second TCP, third TCP port 80 (HTTP), fourth TCP port 443 (HTTPS) Call signaling is carried always over TCP

NAT and Firewall Traversal -If caller is behind port- restricted NAT, call signaling (TCP) is forwarded through a node, which has a public IP address -If either caller or callee or both are behind port- restricted NAT voice traffic (UDP) is forwarded through the same node - If both caller and callee have a public IP address, call signaling (TCP) and voice traffic (UDP) flow directly between them - If both caller and callee are behind port-restricted NAT and UDP- restricted firewall, then signaling traffic and voice traffic is forwarded through another node over TCP