Network Technologies essentials Week 9: Distributed file sharing & multimedia Compilation made by Tim Moors, UNSW Australia Original slides by David Wetherall, University of Washington ISBN-10: © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011 Videos 8-6, 8-7, 8-9 and 9-1 to 9-3

Computer Networks2 Web Caching Users often revisit web pages – Big win from reusing local copy! – This is caching Key question: – When is it OK to reuse local copy? Network Cache Local copies Server 8-6.3

Web Caching (4) Putting the pieces together: Computer Networks

Computer Networks4 After CDNs (2) Benefits assuming popular content: – Reduces server, network load – Improves user experience (PLT) Source User... Replica 8-7.6

Content Delivery Network Computer Networks

Content Delivery Network (2) DNS resolution of site gives different answers to clients – Tell each client the site is the nearest replica (map client IP) Computer Networks

Computer Networks7 P2P Challenges No servers on which to rely – Communication must be peer-to-peer and self-organizing, not client-server – Leads to several issues at scale … Peer 8-9.5

Computer Networks8 P2P Challenges (2) 1.Limited capabilities – How can one peer deliver content to all other peers? 2.Participation incentives – Why will peers help each other? 3.Decentralization – How will peers find content? 8-9.6

Computer Networks9 BitTorrent Protocol Steps to download a torrent: 1.Start with torrent description 2.Contact tracker to join and get list of peers (with at least seed peer) 2.Or, use DHT index for peers 3.Trade pieces with different peers 4.Favor peers that upload to you rapidly; “choke” peers that don’t by slowing your upload to them

BitTorrent Protocol (2) All peers (except seed) retrieve torrent at the same time Computer Networks

Computer Networks11 Network Delay (2) Network delay is variable – Message latency plus queuing delay – Variability in delay is called jitter Delay (msec) Fraction (PDF) Latency Queuing 9-2.4

Computer Networks12 Playout Buffer (2) Media arrival curve determines time in playout buffer and deadline Time Media timestamp Playout Buffering Too late! 9-2.9

RTP (Real-time Transport Protocol) Used to carry media on top of best effort UDP (§6.4.3) – Header has media format, timestamp, sequence number, etc. – Media follows in standard formats, e.g., G.711, MP4 Computer Networks

Computer Networks14 SIP Signaling (2) Signaling for call control – Like HTTP, uses simple method/response codes – Runs on UDP or TCP – SIP proxy servers and registrars provide mobility (not shown) INVITE 180 RINGING 200 OK Accept call BYE RTP media 200 OK ACK Hang up

Handling Bandwidth Send file in one of multiple encodings – Higher quality encodings require more bandwidth – Select best encoding given available bandwidth Computer Networks15 By Toytoy, CC-BY-SA-3.0, from Wikimedia Commons 23:1 46:1 144:1 15:1 Higher quality More bandwidth Lower quality Less bandwidth (JPEG example) 9-3.6

Computer Networks16 Streaming with HTTP Fetch media description data – Gives index of clips, rates Fetch small segments – Put in playout buffer Adapt selection of encoding – Based on buffer occupancy Evolving standards, e.g., DASH – Leverages HTTP and HTML5 – Server is otherwise stateless GET INDEX GET MEDIA

END © 2013 D. Wetherall Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Computer Networks17