Transcript Tuesday, November 27th - University of Pittsburgh

November 27th, 2012
CS1652
Jack Lange
University of Pittsburgh
1
OMET Question
 Compared to other Pitt CS classes I learned:
 (1) Much Less
 (2) Slightly Less
 (3) The same amount
 (4) Slightly More
 (5) A lot more
7: Multimedia Networking
7-2
Multimedia and Quality of Service: What is it?
multimedia applications:
network audio and video
(“continuous media”)
QoS
network provides
application with level of
performance needed for
application to function.
7: Multimedia Networking
7-3
Multimedia Networking
Application
4
MM Networking Applications
Classes of MM applications:
1) stored streaming
2) live streaming
3) interactive, real-time
Fundamental
characteristics:
 typically delay sensitive


end-to-end delay
delay jitter
 loss tolerant: infrequent
Jitter is the variability
of packet delays within
the same packet stream
losses cause minor
glitches
 antithesis of bulk data,
which is loss intolerant
but delay tolerant.
7: Multimedia Networking
7-5
Streaming Stored Multimedia
Stored 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
7: Multimedia Networking
7-6
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
7: Multimedia Networking
7-7
Streaming Multimedia: Client Buffering
variable
network
Delay
(jitter)
client video
reception
constant bit
rate video
playout at client
buffered
video
constant bit
rate video
transmission
time
client playout
delay
 client-side buffering, playout delay compensate
for network-added delay, delay jitter
7: Multimedia Networking
7-8
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
7: Multimedia Networking
7-9
Streaming Live Multimedia
Examples:
 Internet radio talk show
 live sporting event
Streaming (as with streaming stored multimedia)
 playback buffer
 playback can lag tens of seconds after
transmission
 still have timing constraint
Interactivity
 fast forward impossible
 rewind, pause possible
7: Multimedia Networking 7-10
Real-Time Interactive 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?
7: Multimedia Networking
7-11
Multimedia Over Today’s Internet
TCP/UDP/IP: “best-effort service”
 no guarantees on delay, loss
?
?
?
?
?
?
?
?
But multimedia apps require
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
7: Multimedia Networking 7-12
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; endsystem (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.
7: Multimedia Networking 7-13
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 in choosing q:
 large q: less packet loss
 small q: better interactive experience
7: Multimedia Networking 7-14
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'
7: Multimedia Networking 7-15
Content distribution networks (CDNs)
Content replication
 challenging to stream large
files (e.g., video) from single
origin server in real time
 solution: replicate content at
hundreds of servers
throughout Internet
 content downloaded to CDN
servers ahead of time
 placing content “close” to
user 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 7-16
Content distribution networks (CDNs)
Content replication
 CDN (e.g., Akamai)
customer is the content
provider (e.g., CNN)
 CDN replicates
customers’ content in
CDN servers.
 when provider updates
content, CDN updates
servers
origin server
in North America
CDN distribution node
CDN server
in S. America CDN server
in Europe
CDN server
in Asia
7: Multimedia Networking 7-17
CDN example
HTTP request for
www.foo.com/sports/sports.html
origin server
1
2
client
3
DNS query for www.cdn.com
CDN’s authoritative
DNS server
HTTP request for
www.cdn.com/www.foo.com/sports/ruth.gif
CDN server near client
origin server (www.foo.com)
 distributes HTML
 replaces:
http://www.foo.com/sports.ruth.gif
with
http://www.cdn.com/www.foo.com/sports/ruth.gif
CDN company (cdn.com)
 distributes gif files
 uses its authoritative
DNS server to route
redirect requests
7: Multimedia Networking 7-18
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, error concealment
 retransmissions, time permitting
 CDN: bring content closer to clients
7: Multimedia Networking 7-19
ZeroConf Networking
 What:
 Non-administered networks
 Full Plug-and-Play architecture
 No centralized services to configure (DNS, DHCP, etc)
 Only need to plug computer into network
 Automatically assigns address
 Automatically discovers services
 Automatically negotiates protocols and connections
 For local networks mostly
 Not easy to allow external (internet) connectivity
7: Multimedia Networking 7-20
Finding an Address
 Do not require anyone to provide an address
 No network administrator
 No DHCP server
 Step 1: Select a random address
 Special range dedicated to internal zeroconf networks
 Step 2: Probe to check if address is already taken
 How?
 Step 3: Assign address to interface
7: Multimedia Networking 7-21
What is multicast
 So far we’ve talked about unicast and broadcast
 Multicast is “in between”
 Each interface has an IP address
 But also selectively listens for multicast addresses
 Each network device has MAC address
 But also selectively listens for multicast addresses
 One host can send a packet to anyone who wants
to receive it


Mechanism for self defined group communication
TCP doesn’t really work, but UDP is fine
7: Multimedia Networking 7-22
Multicast DNS
 Dedicated TLD: .local
 Dedicated Multicast Address: 224.0.0.251
 Send out a DNS request to a multicast group
 If a group member has that name they respond
 Example
 When you buy a new computer you give it a name
• E.g. “My Mac Laptop”

The DNS address is now “My Mac Laptop.local”
• If someone queries for that hostname your laptop responds
• Sort of like an ARP query
7: Multimedia Networking 7-23
Service Discovery
 Service Discovery extension to DNS (DNS-SD)
 DNS used to identify services (not computers)
 Based on Multicast
 Special domain name format
 Instance.ServiceType.Domain
 E.g. My iTunes._daap._tcp.local
 Query for ServiceTypes
 Enumerate all instances of a service
 DNS is based on Multicast!
 So we can send a query for _daap._tcp.local to EVERYONE!
 If a computer is running a given service they respond with
their instance name
7: Multimedia Networking 7-24