Chapter 6: Multimedia Networking

Chapter 6: Multimedia Networking
Our goals:
❒ principles: network,
application-level support
for multimedia
❍ different forms of
network multimedia,
requirements
❍ making the best of
best effort service
❍ mechanisms for
providing QoS
❒ specific streaming
protocols
❒ architectures for QoS
Overview:
❒ multimedia applications and
requirements
❒ making the best of today’s
best effort service
❒ scheduling and policing
mechanisms
❒ next generation Internet
❍ Intserv
❍ RSVP
❍ Diffserv
Multimedia, Quality of Service: What is it?
Multimedia applications:
network audio and video
QoS
network provides
application with level of
performance needed for
application to function.
1
Multimedia Performance Requirements
Requirement: deliver data in “timely” manner
❒ interactive multimedia: short end-end delay
e.g., IP telephony, teleconf., virtual worlds, DIS
❍ excessive delay impairs human interaction
❒ streaming (non-interactive) multimedia:
❍ data must arrive in time for “smooth” playout
❍ late arriving data introduces gaps in rendered
audio/video
❍
❒ reliability: 100% reliability not always required
MM Networking Applications
Classes of MM applications:
1) Streaming stored audio
and video
2) Streaming live audio and
video
3) Real-time interactive
audio and video
Jitter is the variability
of packet delays within
the same packet stream
Fundamental
characteristics:
❒ Typically delay sensitive
❍
❍
end-to-end delay
delay jitter
❒ But loss tolerant:
infrequent losses cause
minor glitches
❒ Antithesis of data,
which are loss intolerant
but delay tolerant
2
Streaming Stored Multimedia
Streaming:
❒ media stored at source
❒ transmitted to client
❒ streaming: client playout begins
before all data has arrived
❒ timing constraint for still-to-be
transmitted data: in time for playout
Cumulative data
Streaming Stored Multimedia:
What is it?
1. video
recorded
2. video
sent
network
delay
3. video received,
played out at client
time
streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
3
Streaming Multimedia - Interactivity
Types of interactivity:
❒ none: like broadcast radio, TV
❍ initial startup delays of < 10 secs OK
❒ VCR-functionality: client can pause,
rewind, FF
❍ 1-2 sec until command effect OK
❒ timing constraint for still-to-be
transmitted data: in time for playout
Streaming Live Multimedia
Examples:
❒ Internet radio talk show
❒ Live sporting event (e.g., soccer game)
Streaming
❒ playback buffer
❒ playback can lag tens of seconds after
transmission
❒ still have timing constraint
Interactivity
❒ fast forward impossible
❒ rewind, pause possible!
4
Interactive, Real-Time Multimedia
❒ applications: IP telephony,
video conference, distributed
interactive worlds
❒ end-end delay requirements:
❍ audio: < 150 msec good, < 400 msec OK
• includes application-level (packetization) and network
delays
• higher delays noticeable, impair interactivity
❒ session initialization
❍
how does callee advertise its IP address, port
number, encoding algorithms?
Multimedia Over Today’s Internet
TCP/UDP/IP: “best-effort service”
❒
no guarantees on delay, loss
?
?
?
?
?
?
But you said multimedia apps requires ?
QoS and level of performance to be
?
? effective!
?
?
Today’s Internet multimedia applications
use application-level techniques to mitigate
(as best possible) effects of delay, loss
5
How should the Internet evolve to
better support multimedia?
Integrated services philosophy:
❒ Fundamental changes in
Internet so that apps can
reserve end-to-end
bandwidth
❒ Requires new, complex
software in hosts & routers
Laissez-faire
❒ no major changes
❒ more bandwidth when
needed
❒ content distribution,
application-layer mechanisms
❍
application layer
Differentiated services
philosophy:
❒ Fewer changes to Internet
infrastructure, yet provide
1st and 2nd class service.
What’s your opinion?
Streaming Stored Multimedia
Application-level streaming
techniques for making the
best out of best effort
service:
❍ client side buffering
❍ use of UDP versus TCP
❍ multiple encodings of
multimedia
Media Player
❒ jitter removal
❒ decompression
❒ error concealment
❒ graphical user interface
w/ controls for
interactivity
6
Internet multimedia: simplest approach
❒ audio or video stored in file
❒ files transferred as HTTP object
received in entirety at client
❍ then passed to player
❍
audio, video not streamed:
❒ no, “pipelining,” long delays until playout!
Internet multimedia: streaming approach
❒ browser GETs metafile
❒ browser launches player, passing metafile
❒ player contacts server
❒ server streams audio/video to player
7
Streaming from a streaming server
❒ This architecture allows for non-HTTP protocol between
server and media player
❒ Can also use UDP instead of TCP.
constant bit
rate video
transmission
variable
network
delay
client video
reception
constant bit
rate video
playout at client
buffered
video
Cumulative data
Streaming Multimedia: Client Buffering
client playout
delay
time
❒ Client-side buffering, playout delay compensate
for network-added delay, delay jitter
8
Streaming Multimedia: Client Buffering
constant
drain
rate, d
variable fill
rate, x(t)
buffered
video
❒ Client-side buffering, playout delay compensate
for network-added delay, delay jitter
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 (2-5 seconds) to compensate for network
delay jitter
❒ error recover: time permitting
TCP
❒ send at maximum possible rate under TCP
❒ congestion loss: fill rate fluctuates
❒ larger playout delay: smooth TCP delivery rate
❒ HTTP/TCP passes more easily through firewalls
9
Streaming Multimedia: client rate(s)
1.5 Mbps encoding
28.8 Kbps encoding
Q: how to handle different client receive rate
capabilities?
❍ 28.8 Kbps dialup
❍ 100Mbps Ethernet
A: server stores, transmits multiple copies
of video, encoded at different rates
Real-time interactive applications
❒ PC-2-PC phone
❍ instant messaging
services are providing
this
Lets look at a PC-2PC Internet phone
example in detail.
❒ PC-2-phone
Dialpad
❍ Net2phone
❒ videoconference with
Webcams
❍
10
Interactive Multimedia: Internet Phone
Introduce Internet Phone by way of an example
❒ speaker’s audio: alternating talk spurts, silent
periods.
❍
64 kbps during talk spurt
❒ pkts generated only during talk spurts
❍
20 msec chunks at 8 Kbytes/sec: 160 bytes data
❒ application-layer header added to each chunk
❒ chunk+header encapsulated into UDP segment
❒ application sends UDP segment into socket every
20 msec during talk spurt
Internet Phone: Packet Loss and Delay
❒ network loss: IP datagram lost due to network
congestion (router buffer overflow)
❒ delay loss: IP datagram arrives too late for
playout at receiver
❍
❍
delays: processing, queueing in network; end-system
(sender, receiver) delays
typical maximum tolerable delay: 400 ms
❒ loss tolerance: depending on voice encoding, losses
concealed, packet loss rates between 1% and 10%
can be tolerated.
11
constant bit
rate
transmission
variable
network
delay
(jitter)
client
reception
constant bit
rate playout
at client
buffered
data
Cumulative data
Delay Jitter
client playout
delay
time
❒ Consider the end-to-end delays of two consecutive
packets: difference can be more or less than 20
msec
Internet Phone: Fixed Playout Delay
❒ Receiver attempts to playout each chunk exactly q
msecs after chunk was generated.
❍ chunk has time stamp t: play out chunk at t+q .
❍ chunk arrives after t+q: data arrives too late
for playout, data “lost”
❒ Tradeoff for q:
❍ large q: less packet loss
❍ small q: better interactive experience
12
Fixed Playout Delay
• Sender generates packets every 20 msec during talk spurt.
• First packet received at time r
• First playout schedule: begins at p
• Second playout schedule: begins at p’
packets
loss
packets
generated
packets
received
playout schedule
p' - r
playout schedule
p-r
time
r
p
p'
Recovery From Packet Loss
❒ loss: packet never arrives or arrives too late
❒ real-time constraints: little (no) time for
retransmissions!
❍
What to do?
❒ Forward Error Correction (FEC): add error
correction bits (recall 2-dimensional parity)
❍
❍
❍
add redundant chunk made up of exclusive OR of n chunks
redundancy (overhead) is 1/n
can reconstruct if at most one lost chunk
❒ Interleaving: spread loss evenly over received data to
minimize impact of loss
13
FEC - Piggybacking Lower Quality Stream
FEC Scheme:
• “piggyback” lower quality stream
• send lower resolution audio stream as the redundant information
•Whenever there is non-consecutive loss, the receiver can conceal
the loss
• Can also append (n-1)st and (n-2)nd low-bit rate chunk
Interleaving
Interleaving Scheme
❒ no redundancy needed
❒ chunks are broken
up into smaller units
❍
for example, four 5 msec
units per chunk
❒ if packet is lost, still have
most of every chunk
❒ has no redundancy overhead
❒ but adds to playout delay
❒ packet contains small units
from different chunks
14
Summary: Internet Multimedia: bag of tricks
❒ use UDP to avoid TCP congestion control (delays)
for time-sensitive traffic
❒ client-side adaptive playout delay: to compensate
for delay
❒ server side matches stream bandwidth to available
client-to-server path bandwidth
❍
❍
chose among pre-encoded stream rates
dynamic server encoding rate
❒ error recovery (on top of UDP)
❍ FEC, interleaving
❍ retransmissions, time permitting
❍ conceal errors: repeat nearby data
Improving QOS in IP Networks
Thus far: “making the best of best effort”
Future: next generation Internet with QoS guarantees
❍ RSVP: signaling for resource reservations
❍ Differentiated Services: differential guarantees
❍ Integrated Services: firm guarantees
❒ simple model
for sharing and
congestion
studies:
15
Principles for QOS Guarantees
❒ Example: 1Mbps IP phone, FTP share 1.5 Mbps link.
❍ bursts of FTP can congest router, cause audio loss
❍ want to give priority to audio over FTP
Principle 1
packet marking needed for router to distinguish
between different classes; and new router policy
to treat packets accordingly
Principles for QOS Guarantees (more)
❒ what if applications misbehave (audio sends higher
than declared rate)
❍
policing: force source adherence to bandwidth allocations
❒ marking and policing at network edge:
❍ similar to ATM UNI (User Network Interface)
Principle 2
provide protection (isolation) for one class from others
16
Principles for QOS Guarantees (more)
fixed (non-sharable) bandwidth to flow:
inefficient use of bandwidth if flows doesn’t use
❒ Allocating
its allocation
Principle 3
While providing isolation, it is desirable to use
resources as efficiently as possible
Principles for QOS Guarantees (more)
❒
Basic fact of life: can not support traffic demands
beyond link capacity
Principle 4
Call Admission: flow declares its needs, network may
block call (e.g., busy signal) if it cannot meet needs
17
Summary of QoS Principles
Let’s next look at mechanisms for achieving this ….
Scheduling And Policing Mechanisms
❒ scheduling: choose next packet to send on link
❒ FIFO (first in first out) scheduling: send in order of
arrival to queue
❍
❍
real-world example?
discard policy: if packet arrives to full queue: who to discard?
• Tail drop: drop arriving packet
• priority: drop/remove on priority basis
• random: drop/remove randomly
18
Scheduling Policies: more
Priority scheduling: transmit highest priority queued
packet
❒ multiple classes, with different priorities
❍
❍
class may depend on marking or other header info, e.g. IP
source/dest, port numbers, etc..
Real world example?
Scheduling Policies: still more
round robin scheduling:
❒ multiple classes
❒ cyclically scan class queues, serving one from each
class (if available)
❒ real world example?
19
Scheduling Policies: still more
Weighted Fair Queuing:
❒ generalized Round Robin
❒ each class gets weighted amount of service in each
cycle
❒ real-world example?
Policing Mechanisms
Goal: limit traffic to not exceed declared parameters
Three common-used criteria:
❒
(Long term) Average Rate: how many pkts can be sent
per unit time (in the long run)
❍
crucial question: what is the interval length: 100 packets per
sec or 6000 packets per min have same average!
❒
Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500
❒
(Max.) Burst Size: max. number of pkts sent
ppm peak rate
consecutively (with no intervening idle)
20
Policing Mechanisms
Token Bucket: limit input to specified Burst Size
and Average Rate.
❒ bucket can hold b tokens
❒ tokens generated at rate
full
❒
r token/sec unless bucket
over interval of length t: number of packets
admitted less than or equal to (r t + b).
Policing Mechanisms (more)
❒ token bucket, WFQ combine to provide guaranteed
upper bound on delay, i.e., QoS guarantee!
arriving
traffic
token rate, r
bucket size, b
WFQ
per-flow
rate, R
D = b/R
max
21
IETF Integrated Services
❒ architecture for providing QOS guarantees in IP
networks for individual application sessions
❒ resource reservation: routers maintain state info
(a la VC) of allocated resources, QoS req’s
❒ admit/deny new call setup requests:
Question: can newly arriving flow be admitted
with performance guarantees while not violated
QoS guarantees made to already admitted flows?
Intserv: QoS guarantee scenario
❒ Resource reservation
❍ call setup, signaling (RSVP)
❍ traffic, QoS declaration
❍ per-element admission control
request/
reply
❍
QoS-sensitive
scheduling (e.g.,
WFQ)
22
Call Admission
Arriving session must :
❒ declare its QoS requirement
R-spec: defines the QoS being requested
❒ characterize traffic it will send into network
❍ T-spec: defines traffic characteristics
❒ signaling protocol: needed to carry R-spec and Tspec to routers (where reservation is required)
❍ RSVP
❍
Intserv QoS: Service models [rfc2211, rfc 2212]
Controlled load service:
Guaranteed service:
❒ "a quality of service closely
worst case traffic arrival: leakyapproximating the QoS that
bucket-policed source
same flow would receive from an
❒ simple (mathematically provable)
unloaded network element."
bound on delay [Parekh 1992, Cruz
1988]
❒
arriving
traffic
token rate, r
bucket size, b
WFQ
per-flow
rate, R
D = b/R
max
23
IETF Differentiated Services
Concerns with Intserv:
❒ Scalability: signaling, maintaining per-flow router
state difficult with large number of flows
❒ Flexible Service Models: Intserv has only two
classes. Also want “qualitative” service classes
❍
❍
“behaves like a wire”
relative service distinction: Platinum, Gold, Silver
Diffserv approach:
❒ simple functions in network core, relatively
complex functions at edge routers (or hosts)
❒ Do’t define define service classes, provide
functional components to build service classes
Diffserv Architecture
Edge router:
r marking
scheduling
- per-flow traffic management
- marks packets as in-profile
and out-profile
b
..
.
Core router:
- per class traffic management
- buffering and scheduling
based on marking at edge
- preference given to in-profile
packets
- Assured Forwarding
24
Edge-router Packet Marking
profile: pre-negotiated rate A, bucket size B
packet marking at edge based on per-flow profile
❒
❒
Rate A
B
User packets
Possible usage of marking:
class-based marking: packets of different classes marked differently
❒ intra-class marking: conforming portion of flow marked differently than
non-conforming one
❒
Classification and Conditioning
❒ Packet is marked in the Type of Service (TOS) in
IPv4, and Traffic Class in IPv6
❒ 6 bits used for Differentiated Service Code Point
(DSCP) and determine PHB that the packet will
receive
❒ 2 bits are currently unused
25
Classification and Conditioning
may be desirable to limit traffic injection rate of
some class:
❒ user declares traffic profile (eg, rate, burst size)
❒ traffic metered, shaped if non-conforming
Forwarding (PHB)
❒ PHB result in a different observable (measurable)
forwarding performance behavior
❒ PHB does not specify what mechanisms to use to
ensure required PHB performance behavior
❒ Examples:
❍
❍
Class A gets x% of outgoing link bandwidth over time
intervals of a specified length
Class A packets leave first before packets from class B
26
Forwarding (PHB)
PHBs being developed:
❒ Expedited Forwarding: pkt departure rate of a
class equals or exceeds specified rate
❍
logical link with a minimum guaranteed rate
❒ Assured Forwarding: 4 classes of traffic
❍ each guaranteed minimum amount of bandwidth
❍ each with three drop preference partitions
Multimedia Networking: Summary
❒ multimedia applications and requirements
❒ making the best of today’s best effort
service
❒ scheduling and policing mechanisms
❒ next generation Internet
❍
Intserv, RSVP, Diffserv
27
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