1. University of Pittsburgh
CS 1652
Jack Lange
University of Pittsburgh

2. Multimedia and Quality of Service: What is it?
multimedia applications: network audio and video
(“continuous media”)
network provides application with level of performance needed for application to function.
QoS
7: Multimedia Networking

3. Multimedia Networking Application

4. Multimedia networking: 3 application types
streaming, stored audio, video
streaming: can begin playout before downloading entire file
stored (at server): can transmit faster than audio/video will be rendered (implies storing/buffering at client)
e.g., YouTube, Netflix, Hulu
conversational voice/video over IP
interactive nature of human-to-human conversation limits delay tolerance
e.g., Skype
streaming live audio, video
e.g., live sporting event
Multimedia Networking

5. MM Networking Applications
Fundamental characteristics:
typically delay sensitive
end-to-end delay
delay jitter
loss tolerant: infrequent losses cause minor glitches
antithesis of bulk data, which is loss intolerant but delay tolerant.
Jitter is the variability
of packet delays within
the same packet stream
7: Multimedia Networking

6. 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. Streaming Stored Multimedia: What is it?
streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
3. video received,
played out at client
Cumulative data
2. video
sent
1. video
recorded
network
delay
time
7: Multimedia Networking

8. Streaming Multimedia: Client Buffering
constant bit
rate video
transmission
variable
network
Delay
(jitter)
client video
reception
constant bit
rate video
playout at client
client playout
delay
Cumulative data
buffered
video
time
client-side buffering, playout delay compensate for network-added delay, delay jitter
7: Multimedia Networking

9. Client-side buffering, playout
buffer fill level, Q(t)
variable fill
rate, x(t)
playout rate,
e.g., CBR r
client application
buffer, size B
video server
client
Multimedia Networking

10. Streaming multimedia: HTTP
multimedia file retrieved via HTTP GET
send at maximum possible rate under TCP
fill rate fluctuates due to TCP congestion control, retransmissions (in-order delivery)
larger playout delay: smooth TCP delivery rate
HTTP/TCP passes more easily through firewalls
variable rate, x(t)
video
file
TCP send buffer
TCP receive buffer
application playout buffer
server
client
Multimedia Networking

11. Voice-over-IP (VoIP)
VoIP end-end-delay requirement: needed to maintain “conversational” aspect
higher delays noticeable, impair interactivity
< 150 msec: good
> 400 msec bad
includes application-level (packetization, playout), network delays
session initialization: how does callee advertise IP address, port number, encoding algorithms?
value-added services: call forwarding, screening, recording
emergency services: 911
Multimedia Networking

12. VoIP: packet loss, 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, loss concealment, packet loss rates between 1% and 10% can be tolerated
Multimedia Networking

13. VoIP: 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
Multimedia Networking

14. VoIP: 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’
Multimedia Networking

15. Video Streaming and CDNs: context
video traffic: major consumer of Internet bandwidth
Netflix, YouTube: 37%, 16% of downstream residential ISP traffic
~1B YouTube users, ~75M Netflix users
challenge: scale - how to reach ~1B users?
single mega-video server won’t work
challenge: heterogeneity
different users have different capabilities (e.g., wired versus mobile; bandwidth rich versus bandwidth poor)
solution: distributed, application-level infrastructure
Application Layer

16. Content distribution networks
challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users?
option 1: single, large “mega-server”
single point of failure
point of network congestion
long path to distant clients
multiple copies of video sent over outgoing link
….quite simply: this solution doesn’t scale
Application Layer

17. Content distribution networks
challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users?
option 2: store/serve multiple copies of videos at multiple geographically distributed sites (CDN)
enter deep: push CDN servers deep into many access networks
close to users
used by Akamai, 1700 locations
bring home: smaller number (10’s) of larger clusters in POPs near (but not within) access networks
used by Limelight
Application Layer

18. Content Distribution Networks (CDNs)
CDN: stores copies of content at CDN nodes
e.g. Netflix stores copies of MadMen
subscriber requests content from CDN
directed to nearby copy, retrieves content
may choose different copy if network path congested …
Akamai: 100,000+ servers in clusters in networks in 70+ countries serving trillions of requests a day.
How many people use Netflix?
manifest file
where’s Madmen?
Application Layer 18

19. Case study: Netflix
upload copies of multiple versions of video to CDN servers
Amazon cloud
CDN
server
Netflix registration,
accounting servers
3. Manifest file
returned for
requested video
CDN
server
2. Bob browses
Netflix video 2 3 1
CDN
server
1. Bob manages Netflix account
4. DASH streaming
Application Layer

20. CDN content access: a closer look
Bob (client) requests video
video stored in CDN at
1. Bob gets URL for video
from netcinema.com web page 1
2. resolve
via Bob’s local DNS 2
6. request video from
KINGCDN server,
streamed via HTTP 5
Bob’s
local DNS
server
netcinema.com
3. netcinema’s DNS returns URL
4&5. Resolve
via KingCDN’s authoritative DNS,
which returns IP address of KingCDN
server with video 4 3
netcinema’s
authoratative DNS
KingCDN.com
KingCDN
authoritative DNS
Application Layer

21. VLANs

22. VLANs: motivation consider:
CS user moves office to EE, but wants connect to CS switch?
single broadcast domain:
all layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN
security/privacy, efficiency issues
Computer
Science
Computer
Engineering
Electrical
Engineering
Link Layer and LANs

23. VLANs
port-based VLAN: switch ports grouped (by switch management software) so that single physical switch ……
Virtual Local
Area Network 1 7 9 15 2 8 10 16
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Electrical Engineering
(VLAN ports 1-8) … 1 8 2 7 9 16 10 15
Computer Science
(VLAN ports 9-16)
… operates as multiple virtual switches
Link Layer and LANs

24. Electrical Engineering
Port-based VLAN
router
traffic isolation: frames to/from ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints, rather than switch port
forwarding between VLANS: done via routing (just as with separate switches)
in practice vendors sell combined switches plus routers 1 7 9 15 2 8 10 16
dynamic membership: ports can be dynamically assigned among VLANs … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Link Layer and LANs

25. VLANS spanning multiple switches1 7 9 16 1 15 3 5 7 2 8 10 2 4 6 8 … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Ports 2,3,5 belong to EE VLAN
Ports 4,6,7,8 belong to CS VLAN
trunk port: carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches can’t be vanilla frames (must carry VLAN ID info)
802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports
Link Layer and LANs

26. 802.1Q VLAN frame format 802.1 frame 802.1Q frame
type
dest.
address
source
address
preamble
data (payload)
CRC
802.1 frame
type
dest.
address
source
preamble
802.1Q frame
data (payload)
CRC
2-byte Tag Protocol Identifier
(value: 81-00)
Recomputed
CRC
Tag Control Information (12 bit VLAN ID field,
3 bit priority field like IP TOS)
Link Layer and LANs

27. ZeroConf
Multimedia Networking

28. ZeroConf Networking What: Only need to plug computer into network
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

29. 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

30. 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
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

31. Multicast DNS Dedicated TLD: .local
Dedicated Multicast Address:
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

32. 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