Introduction to P2P - Distributed Software Develop | CS 682, Study notes of Software Engineering

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Distributed Software Development

Introduction to P2P

Chris Brooks Department of Computer Science University of San Francisco Department of Computer Science — University of San Francisco – p. 1/

14-2: Peer-to-Peer

• Peer-to-peer is often touted as a hot new technology ◦ Really, it’s as old as the Internet (or earlier) - Fundamental idea: all nodes in a network should be capable ofthe same functionality. - Contrast this with client-server. - In practice, many systems are a hybrid of p2p and client-server. Department of Computer Science — University of San Francisco – p. 2/

14-4: Examples

• File sharing apps (Morpheus, Kazaa, Limewire, etc) -

IRC/ICQ

• SETI@Home/distributed.net - Web caching - VoIP -

SMTP

• etc Department of Computer Science — University of San Francisco – p. 4/

14-5: Historical examples

USENET

◦ Peers within a domain would collect usenet posts andforward them via UUCP, then NNTP. ◦ No central server or repository.

DNS

◦ A hierarchical p2p system Department of Computer Science — University of San Francisco – p. 5/

14-7: Potential advantages of peer-to-peer

• Reduce server bottlenecks - Move content closer to users - Allow users to publish as well as consume information ◦ Note that publishing and authoring can be different things. - Scalability - More able to deal with adaptive, dynamic networks. Department of Computer Science — University of San Francisco – p. 7/

14-8: Challenges of peer-to-peer

• Finding peers (addressing) ◦ How does a peer join the network? ◦ How do peers identify each other? ◦ Is the network constructed statically, or dynamically? ◦ Does a peer always connect to the same set of peers? Department of Computer Science — University of San Francisco – p. 8/

14-10: Challenges of Peer-to-Peer

• Other challenges: ◦ Interoperability and standards^ • Currently, every P2P network has its own protocol - youcan’t use a BitTorrent client to search on Gnutella. ◦ Dealing with dynamic networks ◦ Security^ • This includes the distribution of malicious files, protectionfrom snooping, and user authentication. Department of Computer Science — University of San Francisco – p. 10/

14-11: First-generation applications

• So-called first-generation applications use a centralized nameserver. - All user addresses and file indexes are kept there. ◦ This can be used to build an application-leveladdress-resolution protocol. - Napster is the canonical example of this. - Most IM programs also work this way. - Central server is used to search. - Files are transferred between peers. Department of Computer Science — University of San Francisco – p. 11/

14-13: Second-generation models

• Second-generation models remove the central server. - All file information is distributed. - Advantage: ◦ Potentially more scalable ◦ Legal issues avoided. - Disadvantages: ◦ Search is more complex ◦ Discovery of peers more complicated. Department of Computer Science — University of San Francisco – p. 13/

14-14: Example: Gnutella

• Gnutella is an example of an open P2P system. - Many clients use the same protocol ◦ LimeWire, BearShare, Gnucleus, etc - Gnutella uses no centralized server - all information is stored atthe peers. Department of Computer Science — University of San Francisco – p. 14/

14-16: Example: Gnutella

• When a peer wants to perform a search, it sends a query to allknown nodes. ◦ This request is then forwarded on to nodes one level away,who forard it to all nodes they know about, and so on. ◦ This is known as query flooding. ◦ If a hit is found, the node containing the file contacts thesearcher and download begins. Department of Computer Science — University of San Francisco – p. 16/

14-17: Example: Gnutella

• Problems with Gnutella: ◦ Searching is unreliable - the network is often partitioned. ◦ Search based only on keywords - this produces namingissues.^ • If I want “Lost S1, Ep 3”, what do I search for? ◦ Wastes bandwidth forwarding searches to all nodes. ◦ Does not take advantage of network structure^ • Some peers may have better bandwidth, moreinformation, or a more central network structure. Department of Computer Science — University of San Francisco – p. 17/

14-19: Hierarchical P2P

• This allows peers with greater resources to do more. - Searches don’t need to be propagated more than is necessary. - Kazaa is a closed network, so most of its inner workings havebeen figured out through reverse engineering. Department of Computer Science — University of San Francisco – p. 19/

14-20: Issues with file distribution

• File integrity - how do you guarantee you’re getting the file youthink you’re getting? ◦ md5 or SHA hash of file contents - How can files be shared anonymously? ◦ Virtual addresses and randomized routing can hide identity. - How to distribute large files? ◦ Download chunks simultaneously from many peers at once. Department of Computer Science — University of San Francisco – p. 20/