Chapter 7: Multimedia Networking. Chapter 7: Multimedia Networking. Contents: Multimedia, QoS, CDN, P2P. Multimedia. Multimedia Networking Map

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7: Multimedia Networking 7-1

Chapter 7: Multimedia Networking

Jim Kurose, Keith Ross: “Computer Networking: A Top-Down Approach”

3rd_edition: Addison-Wesley, July 2004 4th_edition: Addison-Wesley, July 2007

Slides modified (edited, reordered, reduced, extended) by Lidia Yamamoto, Univ. Basel, May 26, 2008

Chapter 7: Multimedia Networking

Jim Kurose, Keith Ross: “Computer Networking: A Top-Down Approach” Slides modified (edited, reordered, reduced, extended) by

Lidia Yamamoto, Univ. Basel, June 11, 2007

(most material comes from the 3rdedition, some from other sources, plus some gif figures from the 4thedition) A note on the use of these ppt slides:

We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lotof work on our part. In return for use, we only ask the following:

If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Contents: Multimedia, QoS, CDN, P2P

Introduction Definitions Performance requirements Multimedia networking Streaming • Client Buffering • RTSP Real-time Interactive • RTP/RTCP • SIP, SDP, H.323 • Mitigating delay and loss

QoS

Intserv, Diffserv

Content Distribution

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Multimedia

Media: stimulate one of our 5 senses

Sight: Text, Graphics, Images, Video Hearing: Audio

Smell? Taste? Touch?

Multimedia strict definition: more than one medium

combined and synchronized together

Web browsing or e-mail with text + images Audio-visual communication, e.g. videoconferencing Multimedia documents

Multimedia networking

, Internet context:

continuous

audio and/or video

(also when single medium)

Streaming: Stored or live audio/video streaming Real-timeand/or interactiveaudio/video

Multimedia Networking Map

media data transmission Streaming presentation control by user presentation description

Streaming multimedia:

synchronization across media loss/delay compensation Media player Media server main direction of information flow 7: Multimedia Networking 7-6

Multimedia Networking Map

call setup agree on encoding schemes user location interactive session description

statistics on media transmission synchronization across media media data transmission loss/delay compensation Application clock information information for accounting, billing invite users

Interactive multimedia:

fixed or mobile phone networks gateway

(2)

Multimedia Networking Requirements

How to transmit audio or video over the Internet?

Network performance requirements

Parameters: delay, packet loss, bandwidth, jitter(delay variation)

File transfer, web browsing, e-mail are elastic

applications: flexible in terms of delay/bandwidth, but no loss tolerated

Multimedia applications are typically delay-sensitiveand require some minimum bandwidthbut are loss tolerant: infrequent losses cause minor glitches

Technical challenges

Currently all packets receive equal, best effort service Except for TCP reliability (loss=0) guarantee, no other

performance guarantees are available

Scalability: one to many, many to many transmissions

Multimedia Networking Requirements

File transfer E-mail Web browsing Streaming

multimedia multimediaReal-time

No loss (max loss = 0) Max loss

Max delay, jitter performance requirements TCP UDP Available transport protocols Min bandwidth

IP Network Layer: Best Effort Delivery Service elastic applications delay-sensitive, loss tolerant

?

Multimedia Networking: Solutions

Add performance guarantees to the Internet

stack

Quality of Service (QoS), e.g. Intserv, Diffserv IP Multicast

Several attempts, no real success so far

Adaptive applications: make the best of best

effort

Streaming and (semi-)real-time support over UDP and even TCP

Application-layer solutions: CDNs, P2P, Application-layer multicast,…

Increasingly successful

Contents: Multimedia, QoS, CDN, P2P

QoS

Intserv, Diffserv

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Streaming

Stored

Multimedia

file transmission

client playout begins as soon as possible

(i.e. before whole file is transmitted)

control:

VCR or DVD-like:

pause, move

forwards, backwards,…

time

constraint: media data should

arrive in time for playout

Media server

client

media data

control

Streaming

Live

Multimedia

examples

Internet radio talk show Live sports event

playout buffer instead of file transmission

store a few tens of seconds before playback: cope with jitter

no fast-forwarding possible!

time constraint same as stored streaming

Internet “broadcast station” client media data control

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7: Multimedia Networking 7-13 constant bit rate video transmission Cu m ul at iv e da ta time variable network delay client video

reception _{rate video}constant bit

playout at client client playout delay bu ffe re d vide o

Streaming Multimedia: Client Buffering

Client-side buffering with playout delay to

compensate for network-added delay and jitter

Streaming Multimedia: Client Buffering

Buffer size: b > s*d

If large buffer (~file size) possible then avg(r(t)) > s allowed Small buffer requires avg(r(t)) = s

If avg(r(t)) < s: starvation buffered video variable fill rate, r(t) constant drain rate, s playout delay: d 7: Multimedia Networking 7-15

Streaming Multimedia: UDP or TCP?

UDP

server sends at rate appropriate for client (oblivious to

network congestion !)

often send rate = encoding rate= constant rate then, fill rate = constant rate - packet loss

short playout delay (a few seconds) to compensate for jitter

error recovery if time permits

TCP

send at maximum possible rate under TCP

buffer fill rate fluctuates due to TCP congestion control larger playout delay in order to smooth TCP delivery rate popular: HTTP/TCP passes more easily through firewalls

Streaming Multimedia: client rate(s)

Q:

how to handle different client receive rate

capabilities?

28.8 Kbps dialup

100 Mbps Ethernet

A:

server stores, transmits multiple copies

of video, encoded at different rates

1.5 Mbps encoding 28.8 Kbps encoding

Multimedia Networking: Partial Map

media data transmission Streaming presentation control by user presentation description

Streaming multimedia:

Media player

Media server

main direction of information flow

RTSP

Real Time Streaming Protocol (RTSP)

IETF RFC 2326

www.rtsp.org

User control of streaming media: VCR-like

rewind, fast forward, pause, resume, repositioning, etc…

Client-server application-layer protocol

request-response: requests from player to server; each request

confirmed with a response message from server Syntax and operation intentionally similar to HTTP

(4)

RTSP Operation

Client (browser) requests page

containing multimedia presentation Client obtains XML metafile: a

description of the multimedia presentation, which can consist of several media streams.

Link to each media stream: rtsp:// The browser invokes corresponding

media player(helper application) based on the content type of the presentation description.

Each player sets up an RTSP control

connection, and a data connection to the streaming server

RTSP: (out-of band) control protocol request-response

RTSP SETUP, PLAY, PAUSE, TEARDOWN HTTP GET SETUP PLAY media stream PAUSE TEARDOWN media player Web server media server Web browser client server presentation desc. 7: Multimedia Networking 7-20

XML Metafile Example

<title>Twister</title> <session>

<group language=en lipsync> <switch> <track type=audio e="PCMU/8000/1" src = "rtsp://audio.example.com/twister/audio.en/lofi"> <track type=audio e="DVI4/16000/2" pt="90 DVI4/8000/1" src="rtsp://audio.example.com/twister/audio.en/hifi"> </switch> <track type="video/jpeg" src="rtsp://video.example.com/twister/video"> </group> </session> 7: Multimedia Networking 7-21

RTSP Exchange Example

C: SETUP rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Transport: rtp/udp; compression;port=3056; mode=PLAY

S: RTSP/1.0200 1 OK Session 4231 C: PLAY rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Range: npt=0-C: PAUSE rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 Range: npt=37 C: TEARDOWN rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 S: 200 3OK 7: Multimedia Networking 7-22

Multimedia Networking: Protocol Map

media data transmission Streaming presentation control by user presentation description

Streaming multimedia:

synchronization across media loss/delay compensation Media player Media server XML metafile RTP/UDP, TCP RTSP main direction of information flow

Contents: Multimedia, QoS, CDN, P2P

QoS

Intserv, Diffserv

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Interactive, Real-Time Multimedia

end-end delay requirements:

audio: < 150 msec good, < 400 msec OK

loss tolerance:

1% to 20% depending on encoding and application

session initialization

locate participants, agree on encoding formats, etc.

applications:

VoIP, IP

telephony, video conference,

distributed interactive worlds

(5)

statistics on media transmission synchronization across media RTP/UDP media data transmission RTCP loss/delay compensation Application clock information

Interactive multimedia:

data transmission and control

RTP and RTCP

RTP (Real-Time Transport Protocol): RFC 3550

Transport-level packet format to carry monomedia

data (video, audio, animations,…). Payload contains:

Payload type, sequence number, timestamp, source

identifier(s), media samples

Multimedia via multiple synchronized RTP sessions

Support for multipoint, bidirectional flows,

translators and mixers

Generally runs over UDP: unicast or multicast

Companion Control Protocol: RTCP:

Clock timestamp for media synchronization Periodic sender/receiver statistics

RTP Header

Payload Type

(7 bits)

: type of encoding, e.g.:

0 = PCM mu-law, 64 kbps 3 = GSM, 13 kbps 33 = MPEG2 video

Sequence Number

(16 bits)

of RTP packet

Timestamp

(32 bits)

: sample number

incremented by one for each sample (not real-time clock) even if source inactive

SSRC

(32 bits)

: synchronization source identifier

CSRC

(32 bits): contributing source id (e.g. mixing)

Seq. no. Timestamp SSRC cod.

type CSRC1 CSRC2 Samples…

call setup agree on encoding schemes user location interactive session description synchronization across media SDP loss/delay compensation

Application invite _users SIP

Interactive multimedia:

Session Initiation Protocol (SIP)

IETF RFC 3261

Creation and management of multimedia sessions

Establish, modify, terminate sessions Invite new participants to existing sessions Add or remove media

User location for personal mobility (name mapping and redirection)

SIP user identity

URI = Universal Resource Identifier: location-independent, e.g. sip:[email protected], sip:[email protected]

Address, e.g. sip:[email protected]

Multimedia session descriptions in SIP (mainly):

SDP= Session Description Protocol(RFC 2327)

SIP Call Set Up: Known IP Address

Alice’s SIP invite message

contains SDP session description

C= protocol (IPv4) & IP address. M= media (audio), port

number (38060), preferred encoding (“RTP/AVP 0” = PCM ulaw) to receive audio Bob’s SIP “200 OK” message

contains his own SDP descr.: IP address, port &

preferred encoding to recv. (“RTP/AVP 3” = GSM) SIP messages can be sent

over TCP or UDP; here sent over RTP/UDP. Default SIP port number is

5060. SDP

(6)

SIP Call Set Up: Location- independent URI

sip:[email protected] sip:[email protected]

SIP Proxy atlanta.com

Call routing via SIP proxy servers SIP Registrar Biloxi.com Location database SIP Proxy biloxi.com

•SIP Registrar: registers SIP user locations into database

•SIP Proxy: informs clients about locations, routes calls

(both functions may be combined into the same machine) 2 INVITE sip:[email protected] 3 4 5 1 6 7: Multimedia Networking 7-32

SIP Call Set Up: Location- independent URI

sip:[email protected] sip:[email protected] SIP Proxy atlanta.com _SIP Proxy unibas.ch SIP Registrar Biloxi.com Location database SIP Proxy biloxi.com 2 INVITE sip:[email protected] 4 5 REGISTER 3 REDIRECT unibas.ch 6 7 ₈ 1

Call routing via SIP proxy servers

Another Example

Caller [email protected] places a call to [email protected]

(1) Jim sends INVITE message to umass SIP proxy. (2) Proxy forwards request to upenn registrar server. (3) upenn server returns redirect response, indicating that it should try [email protected]

(4) umass proxy sends INVITE to eurecom registrar. (5) eurecom registrar forwards INVITE to 197.87.54.21, which is running keith’s SIP client. (6-8) SIP response sent back (9) media sent directly

between clients.

Note:also a SIP ack message, which is not shown. SIP client 217.123.56.89 SIP client 197.87.54.21 SIP proxy umass.edu SIP registrar upenn.edu SIP registrar eurecom.fr 1 2 3 4 5 6 7 8 9 _Keith (now) Jim Keith (home) 7: Multimedia Networking 7-34

H.323 in a Nutshell

analogous to SIP registrar, plus accounting and billing functions

H.323 in a Nutshell

ITU recommendation for real-time audio and video

across IP networks

Comprehensive suite of protocols including:

Call set-up, transmission (via RTP), synchronization, gatekeepers for accounting and billing, gateways to traditional phone networks,...

Intensive support from industry and telecom

industry (late 90’s), e.g. Microsoft NetMeeting

SIP then gained a large momentum: simple, web

flavor, support from the IETF and Internet

community

Today battle still not decided; other players

emerged (e.g. skype with proprietary protocols)

Multimedia Networking: Protocol Map

call setup agree on encoding schemes user location interactive session description

statistics on media transmission synchronization across media RTP/UDP media data transmission SDP RTCP loss/delay compensation Application clock information information for accounting, billing invite users SIP

Interactive multimedia:

H.323 gateway fixed or mobile phone networks

(7)

Interactive, Real-Time Multimedia

Mitigating the impact of loss and delay

Example: Internet Phone

Mitigating delay

Adaptive playout delay adjustment

Mitigating loss

Forward error correction (FEC) Interleaving

Loss concealment

Mitigating delay: Adaptive Playout

packets time packets generated packets received loss r p p' playout schedule p' - r playout schedule p - r • First packet received at time r • First playout schedule: begins at p • Second playout schedule: begins at p’

Adjust buffer size (hence playout delay) by compressing/elongating silent periods Valid only for voice!

Mitigating loss: Forward Error Correction

(FEC) with Redundant Packets

A B C R R = A xor B xor C A B C R B = A xor C xor R Lost packet Recovery: Redundant packet (inserted one every n packets)

+ : simple, valid for any media -: consumes 1/n more bandwidth -: increases playout delay

Mitigating loss: Forward Error Correction

(FEC) with Redundant Streams

Redundant stream (lower quality, e.g. GSM) Nominal stream (higher quality, e.g. PCM) 7: Multimedia Networking 7-41

Mitigating loss: Interleaving

Small gaps 7: Multimedia Networking 7-42

Interleaving

Used in GSM

Pros:

+ : significantly improves perceived quality

+ : low overhead

+ : does not increase bandwidth usage

+ : can handle lost bursts

Cons:

(8)

Loss concealment

A B C A B C A C (silence) sent received played No concealment: A B C A B C A C packet repetition sent received played A Repetition: sent received played A B C A B C A C B’ = f(A, C) B’ Interpolation: A _B’ C time decoded signal 7: Multimedia Networking 7-44

QoS

Intserv, Diffserv

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Multimedia Networking Requirements

File transfer E-mail Web browsing Streaming

multimedia multimediaReal-time

No loss (max loss = 0) Max loss

Max delay, jitter performance requirements TCP UDP Available transport protocols Min bandwidth

IP Network Layer: Best Effort Delivery Service elastic applications delay-sensitive, loss tolerant

?

Multimedia Quality of Service (QoS)

Multimedia applications:

network audio and video

(“continuous media”)

network provides

application with

level of

performance needed for

application to function.

QoS

Multimedia, Quality of Service

Internet currently offers only best effort service

QoS proposals:

IntServ: QoS guarantees possible but complex Diffserv: Loose guarantees (per class). Simpler, but still

complex to deploy in practice.

Future directions

IETF Integrated Services (Intserv)

Per flow QoS guarantees through

resource

reservation

at all nodes along the path

call set-up signaling (RSVPprotocol, RFC 2205) traffic, QoS declaration

per-element admission control and reservation

QoS-sensitive scheduling

request/ reply

(9)

IETF Differentiated Services

Main concern with Intserv:

Scalability:

signaling, maintaining per-flow router

state difficult with large number of flows in core

routers

Diffserv approach:

simple functions in network core, relatively

complex functions at edge routers (or hosts)

achieved through differentiated service classes,

as opposed to per-flow resource reservation

Edge router:

per-flowtraffic management marks packets

diffserv packet field: IPv4: Type of Service (ToS) IPv6: Traffic Class

Classes:

Expedited Forwarding (EF): 5 Assured Forwarding (AF)

AF 4 AF 3 AF 2 AF 1 Best Effort: 0

Core router:

per classtraffic management buffering and scheduling based

on diffserv field

Diffserv Architecture

scheduling

...

marking preced en ce 7: Multimedia Networking 7-51

How should the Internet evolve to better

support multimedia?

Integrated services (IntServ) philosophy:

QoS guarantees through

fine-grain end-to-end resource reservations (per flow)

Requires new, more complex

software in hosts & routers

Differentiated services (DiffServ) philosophy: Fewer changes to Internet

infrastructure

Coarse grain service levels or “classes” of service

"Laissez-faire" no major changes more bandwidth when

needed, over-provisioning

content distribution,

application-layer solutions

New architecture attempts Earlier Asynchronous Transfer Mode (ATM), MPLS Active/Programmable Networks More recent: NewArch (USA) Autonomic Communication 7: Multimedia Networking 7-52

Summary

Many limitations of current networked multimedia

applications are due to network constraints

Main problems with both Intserv and Diffserv

Complexity, deployment & management cost/effort Crossing legacy best-effort portions of the Internet Socio-economic barriers: who uses ≠ who pays

Current “winner” solution is “laissez-faire” with

over-provisioning

Research directions

Autonomic, self-organizing networks: self-deploying services, including QoS

Adaptive functionality, not only at application layer, also at transport and network layer

QoS

Intserv, Diffserv

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Content Distribution Networks (CDNs)

Problem: Streaming stored multimedia: large and

very popular files (e.g. video from TV broadcast)

from single origin server in real time

Bottleneck at server Delay, loss

Not a solution:

Multicast: not everyone asks content at the same time Web caching: content pulled on demand, fresh

information (e.g. news) not cached

Solution: CDN

Content pushed in advance to servers located as close as possible to potential users

New business: CDN company

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CDN: Proactive Content Replication

CDN company (e.g., Akamai): customers are large content providers (e.g. CNN)

CDN replicates customers’

content in CDN servers.

Content downloaded to CDN servers ahead of time

Placing content “close” to user

alleviates load at origin server avoids impairments (loss, delay)

of sending content over long paths CDN server typically in edge/access network origin server in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia 7: Multimedia Networking 7-56

An

Akamaized

Website (Wikipedia)

origin server Akamai CDN server 7: Multimedia Networking 7-57

CDN Redirection Mechanism

HTTP request for www.ocpn.com/sports/sports.html

DNS query for www.cdn.com Answer from map to determine nearest server HTTP request for www.cdn.com/www.ocpn.com/video.mpg2 1 2 3 Origin server CDNs authoritative DNS server Nearby CDN server client HTML page containing

link to www.cdn.com/www.ocpn.com/video.mpg2

1 4 2 4 3 Content Provider’s network CDN network www.ocpn.com www.cdn.com 7: Multimedia Networking 7-58

CDN: (Research) Issues

Content distribution: from origin server to CDN servers IP Multicast

Application Layer Multicast (Overcast, Scattercast, Yoid) Dedicated Content Managers

Replica placement: optimal placement of CDN servers

Minimum latency and overall bandwidth

Discovery: Assign clients to CDN servers: locate server that

gives best performance Min latency and load

Table (map) indicates distance (cost) from leaf ISP (range of

IP addresses) to CDN server

• Can be built based on BGP routing tables, RTT estimates, etc. Map lookup to chose CDN server with minimum distance (cost)

for an ISP

Contents: Multimedia, QoS, CDN, P2P

QoS

Intserv, Diffserv

Networks (CDNs)

Peer-to-Peer (P2P)

Case study: Skype

Client-server architecture

Server:

content provider

always-on host

permanent IP address

server farms for scaling

Clients:

content consumer communicate with server may be intermittently

connected

may have dynamic IP

addresses

do not communicate directly

with each other

client

server client

(11)

P2P architecture

Peer: both content providerand

consumer

Motivation: share information,

e.g. files

End systems communicate

directly (no 3rd_{party servers)}

Decentralized, scalable Issues:

content dissemination, search

and retrieval intermittent connectivity dynamic IP addresses NAT/firewall traversal non-cooperative nodes security 7: Multimedia Networking 7-62

Hybrid

Centralized index

e.g. original “Napster”

1) when peer connects, it informs central server:

IP address content, e.g. song titles 2) Alice queries index for song, index

returns Bob’s address

3) Alice requests and obtains song file from Bob directly

Centralized access control

e.g. Skype: register/authenticate with central server, rest P2P (search for users, media exchange)

centralized index server peers Alice Bob 1 1 1 1 2 3 7: Multimedia Networking 7-63

P2P Case Study: Skype

login server

Supernode structure:

P2P Case Study: Skype

P2P VoIP software, encrypted calls

NAT and firewall traversal

Gateways to mobile/fixed phone networks

Proprietary protocols over TCP and UDP

Supernode structure (precursor: KaZaA)

Supernode peer must be fully reachable (not behind NAT) among other criteria: Firewall/NAT traversal via supernodes

Media data: direct communication between peers

when possible; when not (e.g. firewall) then relay via

supernode

Codecs in the range ~16kbps-128kbps with loss

concealment

CDN+P2P Summary

CDN increasingly popular

Can P2P win over CDN?

Skype’s new Joost service: TV on demand

Bittorrent for sharing large files (including videos, software)

Legal issues

Copyright infringement

Dissemination of offensive content, malicious programs (e.g.

Chapter 7: Multimedia Networking. Chapter 7: Multimedia Networking. Contents: Multimedia, QoS, CDN, P2P. Multimedia. Multimedia Networking Map