1. Advance Computer Networks Lecture#15
Instructor:
Engr. Muhammad Mateen Yaqoob

2. Multiprotocol label switching (MPLS)
IETF (Internet Engineering Task Force) approved a standard = MPLS
Conventional routers in the Internet can be replaced by MPLS routers
MPLS Router: It can work like a switch and a router
When behaving like a router; it can forward the packets based on destination address
When behaving like a switch; it can forward a packet based on label
Mateen Yaqoob Department of Computer Science

3. Multiprotocol label switching (MPLS)
initial goal: high-speed IP forwarding using fixed length label (instead of IP address)
fast lookup using fixed length identifier (rather than shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
IP header
remainder of link-layer frame
PPP-Point-to-point protocol
Label: 20-bit; label used to index forwarding table in router
Exp: 3-bit; reserved for experimental purposes
S: 1-bit; situation of sub-header in stack. When bit is 1, it means the header is last one in stack
TTL: 8-bit; similar to TTL in IP datagram
label 20
Exp 3 S 1
TTL 8
Mateen Yaqoob Department of Computer Science

4. MPLS capable routers a.k.a. label-switched router
forward packets to outgoing interface based only on label value (don’t inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
flexibility: MPLS forwarding decisions can differ from those of IP
use destination and source addresses to route flows to same destination differently (traffic engineering)
re-route flows quickly if link fails: pre-computed backup paths (useful for VoIP)
Mateen Yaqoob Department of Computer Science

5. Data center networks
10’s to 100’s of thousands of hosts, often closely coupled, in close proximity:
e-business (e.g. Amazon)
content-servers (e.g., YouTube, Akamai, Apple, Microsoft)
search engines, data mining (e.g., Google)
challenges:
multiple applications, each serving massive numbers of clients
managing/balancing load, avoiding processing, networking, data bottlenecks
Inside a 40-ft Microsoft container,
Chicago data center
Mateen Yaqoob Department of Computer Science

6. Data center networks load balancer: application-layer routing
receives external client requests
directs workload within data center
returns results to external client (hiding data center internals from client)
Internet
Border router
Load
balancer
Load
balancer
Access router
Tier-1 switches B A C
Tier-2 switches
TOR switches
Server racks 1 2 3 4 5 6 7 8
Mateen Yaqoob Department of Computer Science

7. Data center networks rich interconnection among switches, racks:
increased throughput between racks (multiple routing paths possible)
increased reliability via redundancy
Server racks
TOR switches
Tier-1 switches
Tier-2 switches 1 2 3 4 5 6 7 8
Mateen Yaqoob Department of Computer Science

8. Synthesis: a day in the life of a web request
journey down protocol stack complete!
application, transport, network, link
putting-it-all-together: synthesis!
goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page
scenario: student attaches laptop to campus network, requests/receives
Mateen Yaqoob Department of Computer Science

9. A day in the life: scenario
browser
DNS server
/13
school network
/24
web page
web server
Google’s network
/19
Mateen Yaqoob Department of Computer Science

10. A day in the life… connecting to the Internet
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
router
(runs DHCP)
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
DHCP
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Mateen Yaqoob Department of Computer Science

11. A day in the life… connecting to the Internet
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client
A day in the life… connecting to the Internet
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCP
DHCP
UDP IP
Eth
Phy
router
(runs DHCP)
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
DHCP client receives DHCP ACK reply
Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router
DHCP
Mateen Yaqoob Department of Computer Science

12. A day in the life… ARP (before DNS, before HTTP)
before sending HTTP request, need IP address of DNS
DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP
DNS
UDP IP
Eth
Phy
DNS
router
(runs DHCP)
ARP
ARP query
ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface
Eth
Phy
ARP
ARP reply
client now knows MAC address of first hop router, so can now send frame containing DNS query
Mateen Yaqoob Department of Computer Science

13. A day in the life… using DNS
UDP IP
Eth
Phy
DNS
DNS server
DNS
UDP IP
Eth
Phy
DNS
router
(runs DHCP)
DNS
DNS
DNS
DNS
Comcast network
/13
IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
demux’ed to DNS server
DNS server replies to client with IP address of
Mateen Yaqoob Department of Computer Science

14. A day in the life…TCP connection carrying HTTPIP
Eth
Phy
router
(runs DHCP)
SYN
SYNACK
SYN
to send HTTP request, client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP IP
Eth
Phy
SYN
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
web server
TCP connection established!
Mateen Yaqoob Department of Computer Science

15. A day in the life… HTTP request/reply
web page finally (!!!) displayed
HTTP
HTTP
TCP IP
Eth
Phy
router
(runs DHCP)
HTTP
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to
HTTP
TCP IP
Eth
Phy
HTTP
HTTP
web server responds with HTTP reply (containing web page)
web server
IP datagram containing HTTP reply routed back to client
Mateen Yaqoob Department of Computer Science

16. Wireless and Mobile Networks
Background:
# wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)!
# wireless Internet-connected devices equals # wireline Internet-connected devices
laptops, Internet-enabled phones promise anytime untethered Internet access
two important (but different) challenges
wireless: communication over wireless link
mobility: handling the mobile user who changes point of attachment to network
Mateen Yaqoob Department of Computer Science

17. Elements of a wireless network
wireless hosts
laptop, smartphone
run applications
may be stationary (non-mobile) or mobile
wireless does not always mean mobility
network
infrastructure
Mateen Yaqoob Department of Computer Science

18. Elements of a wireless network
base station
typically connected to wired network
relay - responsible for sending packets between wired network and wireless host(s) in its “area”
e.g., cell towers, access points
network
infrastructure
Mateen Yaqoob Department of Computer Science

19. Elements of a wireless network
wireless link
typically used to connect mobile(s) to base station
also used as backbone link
multiple access protocol coordinates link access
various data rates, transmission distance
network
infrastructure
Mateen Yaqoob Department of Computer Science

20. Characteristics of selected wireless links
200
802.11n 54
802.11a,g
802.11a,g point-to-point
5-11
802.11b
4G: LTWE WIMAX 4
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
Data rate (Mbps) 1
802.15
.384
2.5G: UMTS/WCDMA, CDMA2000
.056
2G: IS-95, CDMA, GSM
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
5Km – 20 Km
Mateen Yaqoob Department of Computer Science

21. Elements of a wireless network
infrastructure mode
base station connects mobiles into wired network
handoff: mobile changes base station providing connection into wired network
network
infrastructure
Mateen Yaqoob Department of Computer Science

22. Elements of a wireless network
ad hoc mode
no base stations
nodes can only transmit to other nodes within link coverage
nodes organize themselves into a network: route among themselves
Mateen Yaqoob Department of Computer Science

23. Wireless network taxonomy
single hop
multiple hops
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
infrastructure
(e.g., APs)
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET no
infrastructure
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
Mateen Yaqoob Department of Computer Science

24. Wireless Link Characteristics
important differences from wired link ….
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
Mateen Yaqoob Department of Computer Science

25. Wireless Link Characteristics
10-1
SNR: signal-to-noise ratio
larger SNR – easier to extract signal from noise (a “good thing”)
SNR versus BER tradeoffs
given physical layer: increase power -> increase SNR->decrease BER
given SNR: choose physical layer that meets BER requirement, giving highest thruput
SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)
10-2
10-3
BER
10-4
10-5
10-6
10-7 10 20 30 40
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
Mateen Yaqoob Department of Computer Science

26. IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed spectrum
up to 11 Mbps
direct sequence spread spectrum (DSSS) in physical layer
all hosts use same chipping code
802.11a
5-6 GHz range
up to 54 Mbps
802.11g
2.4-5 GHz range
802.11n: multiple antennae
up to 200 Mbps
all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions
Mateen Yaqoob Department of Computer Science

27. 802.11 LAN architecture wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:
wireless hosts
access point (AP): base station
ad hoc mode: hosts only
Internet
hub, switch
or router
BSS 1
BSS 2
Mateen Yaqoob Department of Computer Science

28. 802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for AP
interference possible: channel can be same as that chosen by neighboring AP!
host: must associate with an AP
scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address
selects AP to associate with
may perform authentication [Chapter 8]
will typically run DHCP to get IP address in AP’s subnet
Mateen Yaqoob Department of Computer Science

29. 802.11: passive/active scanning
AP 2
AP 1 H1
BBS 2
BBS 1 1 2 3 4
active scanning:
Probe Request frame broadcast from H1
Probe Response frames sent from APs
Association Request frame sent: H1 to selected AP
Association Response frame sent from selected AP to H1
BBS 1
BBS 2 1 1
AP 1
AP 2 2 3 H1
passive scanning:
beacon frames sent from APs
association Request frame sent: H1 to selected AP
association Response frame sent from selected AP to H1
Mateen Yaqoob Department of Computer Science

30. Multimedia: audio analog audio signal sampled at constant rate
telephone: 8,000 samples/sec
CD music: 44,100 samples/sec
each sample quantized, i.e., rounded
e.g., 28=256 possible quantized values
each quantized value represented by bits, e.g., 8 bits for 256 values
quantization error
quantized value of
analog value
analog
signal
audio signal amplitude
sampling rate
(N sample/sec)
time
Mateen Yaqoob Department of Computer Science

31. Multimedia: audio example rates
example: 8,000 samples/sec, 256 quantized values: 64,000 bps
receiver converts bits back to analog signal:
some quality reduction
example rates
CD: Mbps
MP3: 96, 128, 160 kbps
Internet telephony: 5.3 kbps and up
time
audio signal amplitude
analog
signal
quantized value of
analog value
quantization error
sampling rate
(N sample/sec)
Mateen Yaqoob Department of Computer Science

32. Multimedia: video video: sequence of images displayed at constant rate
……………………...…
spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and number of repeated values (N)
video: sequence of images displayed at constant rate
e.g. 24 images/sec
digital image: array of pixels
each pixel represented by bits
coding: use redundancy within and between images to decrease # bits used to encode image
spatial (within image)
temporal (from one image to next)
frame i
temporal coding example: instead of sending complete frame at i+1, send only differences from frame i
frame i+1
Mateen Yaqoob Department of Computer Science

33. Multimedia: video CBR: (constant bit rate): video encoding rate fixed
……………………...…
spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and number of repeated values (N)
CBR: (constant bit rate): video encoding rate fixed
VBR: (variable bit rate): video encoding rate changes as amount of spatial, temporal coding changes
examples:
MPEG 1 (CD-ROM) 1.5 Mbps
MPEG2 (DVD) 3-6 Mbps
MPEG4 (often used in Internet, < 1 Mbps)
frame i
temporal coding example: instead of sending complete frame at i+1, send only differences from frame i
frame i+1
Mateen Yaqoob Department of Computer Science

34. 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 (futbol)
Mateen Yaqoob Department of Computer Science

35. Streaming stored video:
3. video received,
played out at client
(30 frames/sec)
streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
Cumulative data
2. video
sent
video
recorded (e.g., 30 frames/sec)
network delay
(fixed in this example)
time
Mateen Yaqoob Department of Computer Science

36. Streaming stored video: challenges
continuous playout constraint: once client playout begins, playback must match original timing
… but network delays are variable (jitter), so will need client-side buffer to match playout requirements
other challenges:
client interactivity: pause, fast-forward, rewind, jump through video
video packets may be lost, retransmitted
Mateen Yaqoob Department of Computer Science

37. 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
Mateen Yaqoob Department of Computer Science

38. VoIP characteristics
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 of data
application-layer header added to each chunk
chunk+header encapsulated into UDP or TCP segment
application sends segment into socket every 20 msec during talkspurt
Mateen Yaqoob Department of Computer Science

39. 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
Mateen Yaqoob Department of Computer Science

40. Delay jitter
constant bit
rate
transmission
variable
network
delay
(jitter)
client
reception
constant bit
rate playout
at client
client playout
delay
Cumulative data
buffered
data
time
end-to-end delays of two consecutive packets: difference can be more or less than 20 msec (transmission time difference)
Mateen Yaqoob Department of Computer Science

41. Real-Time Protocol (RTP)
RTP specifies packet structure for packets carrying audio, video data
RFC 3550
RTP packet provides
payload type identification
packet sequence numbering
time stamping
RTP runs in end systems
RTP packets encapsulated in UDP segments
interoperability: if two VoIP applications run RTP, they may be able to work together
Mateen Yaqoob Department of Computer Science