Network Technologies essentials Week 9: Distributed file sharing & multimedia Compilation made by Tim Moors, UNSW Australia Original slides by David Wetherall,

Network Technologies essentials

Week 9: Distributed file sharing & multimedia

Compilation made by Tim Moors, UNSW AustraliaOriginal slides by David Wetherall, University of WashingtonISBN-10: 0133766624 © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5th ed, Prentice-Hall, 2011

Videos 8-6, 8-7, 8-9 and 9-1 to 9-3

Computer Networks 2

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?

NetworkCache

Local copies

Server

8-6.3

Computer Networks 3

Web Caching (4)• Putting the pieces together:

8-6.6

Computer Networks 4

After CDNs (2)• Benefits assuming popular content:– Reduces server, network load– Improves user experience (PLT)

Source

User

User

. . .Replica

8-7.6

Content Delivery Network

Computer Networks 58-7.9

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 68-7.10

Computer Networks 7

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

Peer

Peer

PeerPeer

8-9.5

Computer Networks 8

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 Networks 9

BitTorrent Protocol• Steps to download a torrent:

1. Start with torrent description2. Contact tracker to join and get list of

peers (with at least seed peer)2. Or, use DHT index for peers3. Trade pieces with different peers4. Favor peers that upload to you rapidly;

“choke” peers that don’t by slowing your upload to them

8-9.13

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

Computer Networks 108-9.14

Computer Networks 11

Network Delay (2)• Network delay is variable– Message latency plus queuing delay– Variability in delay is called jitter

Delay (msec)

Frac

tion

(PD

F)

Latency Queuing

9-2.4

Computer Networks 12

Playout Buffer (2)• Media arrival curve determines

time in playout buffer and deadline

Time

Med

ia ti

mes

tam

p

PlayoutBuffering

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 139-2.13

Computer Networks 14

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)

INVITE180 RINGING

200 OK

Acceptcall

BYE

RTP media

200 OKACK

Hangup

9-2.16

Handling Bandwidth• Send file in one of multiple encodings– Higher quality encodings require more bandwidth– Select best encoding given available bandwidth

Computer Networks 15

By Toytoy, CC-BY-SA-3.0, from Wikimedia Commons

23:1 46:1 144:115:1

Higher qualityMore bandwidth

Lower qualityLess bandwidth

(JPEG example)

9-3.6

Computer Networks 16

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

GET MEDIA

GET MEDIA

GET MEDIA

9-3.10

Computer Networks 17

END

© 2013 D. Wetherall

Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey