Link Layer

1
Link Layer

• 4/19/2012

2
Admin

• Written AssignmentNetwork new due date Monday,
  April 23
• If you are considering replacement work, please
  stop by to talk to me
• Any feedback/suggestions on the course will be
  appreciated.

3
Recap Internet Routing

• Intradomain routing and interdomain routing
• CIDR to allow flexibility in aggregation of
  destination addresses to improve routing
  scalability
• Longest prefix matching to determine the next hop
  to a destination
• Basic switching fabric design

4
Putting it Together Example 1 (same network)
A-gtB
src
dst
misc fields
data
223.1.1.1
223.1.1.3

• Look up dest address
• find dest is on same net
• Hand datagram to link layer to send inside a
  link-layer frame

To Internet
223.1.1.1
223.1.4.1
223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
223.1.3.2
223.1.3.1
5
Putting it Together Example 2 (Different
Networks) A-gt E
misc fields
data
223.1.1.1
223.1.2.3

• look up dest address in forwarding table
• routing table next hop router to dest is
  223.1.1.4
• Hand datagram to link layer to send to router
  223.1.1.4 inside a link-layer frame
• the dest. of the link layer frame is 223.1.1.4

0.0.0.0/0 223.1.1.4 -
To Internet
223.1.4.1
6
Summary of Network Layer

• We have covered the basics of the network layer
• routing and forwarding
• There are multiple other topics that we did not
  cover
• Multicast/anycast
• QoS
• slides will be linked on the schedule page just
  in case you need reading in the summer

7
Recap The Hourglass Architecture of the Internet
Telnet
Email
FTP
WWW
TCP
UDP
IP
Ethernet
FDDI
Wireless
ADSL
CableDOCSIS
8
Link Layer Introduction
link

• Some terminology
• hosts and routers are nodes
• (bridges and switches too)
• communication channels that connect adjacent
  nodes along a communication path are links
• wired, wireless
• dedicated, shared
• 2-PDU is called a frame, encapsulates 3-PDU
  datagram

9
Link layer Context

• Data-link layer has responsibility of
  transferring datagram from one node to another
  node
• Datagram may be transferred by different link
  protocols over different links, e.g.,
• Ethernet on first link,
• frame relay on intermediate links
• 802.11 on last link

• transportation analogy
• trip from New Haven to San Francisco
• taxi home to union station
• train union station to JFK
• plane JFK to San Francisco airport
• shuttle airport to hotel

10
Link Layer Services

• Framing
• encapsulate datagram into frame, adding header,
  trailer and error detection/correction
• Multiplexing/demultiplexing
• frame headers to identify src, dest
• Media access control
• Forwarding/switching with a link-layer (Layer 2)
  domain
• Reliable delivery between adjacent nodes
• we learned how to do this already !
• seldom used on low bit error link (fiber, some
  twisted pair)
• common for wireless links high error rates

11
Adaptors Communicating
datagram
receiving node
link layer protocol
sending node
adapter
adapter

• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.
• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to receiving node

• link layer typically implemented in adaptor
  (aka NIC)
• Ethernet card, modem, 802.11 card
• adapter is semi-autonomous, implementing link
  physical layers

12
LAN/MAC/Physical Address

• In most link-layer, each adapter has a unique
  link layer address (also called MAC address)
• used as address in datalink frames to identify
  the interface
• 48 bit MAC address (for most types of LANs)
  burned in the adapter ROM
• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)

13
Recall Earlier Routing Discussion

• Starting at A, given IP datagram addressed to E
• look up net. address of E, find C
• link layer sends datagram to C inside link-layer
  frame the dest. address should be Cs MAC
  address

14
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

yry3_at_cicada yry3 /sbin/arp Address
HWtype HWaddress Flags Mask
Iface zoo-gatew.cs.yale.edu ether
AA00040020D4 C
eth0 artemis.zoo.cs.yale.edu ether
00065B3F6E21 C
eth0 lab.zoo.cs.yale.edu ether
00B0D0F3C7A5 C eth0
15
ARP Protocol

• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator
• A broadcast protocol
• source broadcasts query frame, containing queried
  IP address
• all machines on LAN receive ARP query
• destination D receives ARP frame, replies
• frame sent to As MAC address (unicast)

16
Comparison of IP address and MAC Address

• IP address is locator
• address depends on network to which an interface
  is attached
• NOT portable
• introduces features (e.g., CIDR) for routing
  scalability
• IP address needs to be globally unique (if no
  NAT)

• MAC address is an identifier
• dedicated to a device
• portable
• flat
• MAC address does not need to be globally unique,
  but the current assignment ensures uniqueness

17
Outline

• Admin
• Link layer overview
• Error detection

18
Error Detection

• D Data protected by error checking, may
  include header fields
• ED Error Detection bits (redundancy)
• Error detection not 100 reliable!
• a good error detector may miss some errors, but
  rarely
• larger ED field generally yields better
  detection
• Error detection design considers computation
  primitives.

19
Cyclic Redundancy Check Background

• Widely used in practice, e.g.,
• Ethernet, DOCSIS (Cable Modem), FDDI, PKZIP,
  WinZip, PNG
• For a given data D, consider it as a polynomial
  D(x)
• consider the string of 0 and 1 as the
  coefficients of a polynomial
• e.g. consider string 10011 as x4x1
• addition and subtraction are modular 2, thus the
  same as xor
• Choose generator polynomial G(x) with r1 bits,
  where r is called the degree of G(x)

20
Cyclic Redundancy Check Encode

• Given data G(x) and D(x), choose R(x) with r
  bits, such that
• D(x)xrR(x) is exactly divisible by G(x)
• The bits correspond to D(x)xrR(x) are sent to
  the receiver

21
Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble 8 bytes
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011 (why the preamble?)
• Source and dest. addresses 6 bytes
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk
• CRC CRC-32 checked at receiver, if error is
  detected, the frame is simply dropped

22
Cyclic Redundancy Check Decode

• Since G(x) is global, when the receiver receives
  the transmission T(x), it divides T(x) by G(x)
• if non-zero remainder error detected!
• if zero remainder, assumes no error

T
T D(x)xrR(x)
EncodeCRC(G)
check
D
23
CRC Steps and an Example

• Suppose the degree of G(x) is r
• Append r zero to D(x), i.e. consider D(x)xr
• Divide D(x)xr by G(x). Let R(x) denote the
  reminder
• Send ltD, Rgt to the receiver

24
The Power of CRC

• Let T(x) denote D(x)xrR(x), and E(x) the
  polynomial of the error bits
• the received signal is T(x) T(x)E(x)
• Since T(x) is divisible by G(x), we only need to
  consider if E(x) is divisible by G(x)

T
T D(x)xrR(x)
EncodeCRC(G)
check
D
25
Designing CRC

• Detect a single-bit error E(x) xi
• if G(x) contains two or more terms, E(x) is not
  divisible by G(x)
• Detect an odd number of errors E(x) has an odd
  number of terms
• lemma if E(x) has an odd number of terms, E(x)
  cannot be divisible by (x1)
• suppose E(x) (x1)F(x), let x1, the left hand
  will be 1, while the right hand will be 0
• thus if G(x) contains x1 as a factor, E(x) will
  not be divided by G(x)
• Many more errors can be detected by designing the
  right G(x)

26
Example G(x)

• 32 bits CRC
• CRC32 x32 x26 x23 x22 x16 x12 x11
  x10 x8 x7 x5 x4 x2 x 1
• used by Ethernet, FDDI, PKZIP, WinZip, and PNG
• GSM phones
• For more details see the link below and further
  links it contains
• http//en.wikipedia.org/wiki/Cyclic_redundancy_che
  ck

                      .
27
Outline

• Admin
• Link layer overview
• Error detection/correction
• Link access

28
Multiple Access Links and Protocols

• Two types of links
• point-to-point
• e.g., a leased dedicated line, PPP for dial-up
  access
• broadcast (shared wire or medium)
• traditional Ethernet Cable networks
• 802.11 wireless LAN cellular networks
• satellite

29
Multiple Access Protocols

• Single shared broadcast channel
• thus, if two or more simultaneous transmissions
  by nodes, due to interference, only one node can
  send successfully at a time (see CDMA later for
  an exception)
• multiple access protocol
• Protocol that determines how nodes share channel,
  i.e., determines when nodes can transmit
• Communication about channel sharing must use
  channel itself !
• Discussion properties of an ideal multiple
  access protocol.

30
Ideal Mulitple Access Protocol

• Broadcast channel of rate R bps
• Efficiency when only one node wants to transmit,
  it can send at full rate R
• Rate allocation
• simple fairness when N nodes want to transmit,
  each can send at average rate R/N
• we may need more complex rate control
• Decentralized
• no special node to coordinate transmissions
• no synchronization of clocks
• Simple

31
MAC Protocols a Taxonomy

• Goals
• efficient, rate control, decentralized, simple
• Three broad classes
• channel partitioning
• divide channel into smaller pieces (time slot,
  frequency, code)
• non-partitioning
• random access
• allow collisions
• taking-turns
• a token coordinates shared access to avoid
  collisions

32
Outline

• Admin. and recap
• Link layer overview
• Error detection and correction
• Media access control (MAC) protocols
• channel partitioning

33
Channel Partitioning TDMA

• TDMA time division multiple access
• Access to channel in "rounds"
• Each station gets fixed length slot (length pkt
  trans time) in each round
• Unused slots go idle
• Example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

34
Channel Partitioning FDMA

• FDMA frequency division multiple access
• Channel spectrum divided into frequency bands
• Each station assigned fixed frequency band
• Unused transmission time in frequency bands go
  idle
• Example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle

time
1
2
3
frequency bands
4
5
6
35
GSM - TDMA/FDMA
935-960 MHz 124 channels (200 kHz) downlink
frequency
890-915 MHz 124 channels (200 kHz) uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
guard space
guard space
tail
user data
Training
S
S
user data
tail
57 bits
1
1
3
3 bits
57 bits
26 bits
S indicates data or control
36
Channel Partitioning CDMA

• CDMA (Code Division Multiple Access)
• Used mostly in wireless broadcast channels
  (cellular, satellite, etc)
• A spread-spectrum technique

History http//people.seas.harvard.edu/jones/csc
ie129/nu_lectures/lecture7/hedy/lemarr.htm
37
CDMA Encoding

• All users share same frequency, but each user m
  has its own unique chipping sequence (i.e.,
  code) cm to encode data, i.e., code set
  partitioning
• e.g. cm 1 1 1 -1 1 -1 -1 -1
• Assume original data are represented by 1 and -1
• Encoded signal (original data) modulated by
  (chipping sequence)
• assume cm 1 1 1 -1 1 -1 -1 -1
• if data is d, send d cm,
• if data d is 1, send cm
• if data d is -1 send -cm

38
CDMA Encoding
tb
user data d(t)
1
-1
X
tc
chipping sequence c(t)
-1
1
1
-1
1
-1
1
-1
1
-1
-1
1
1
1

resulting signal
-1
1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
tb bit period tc chip period
39
CDMA Decoding

• Inner-product (summation of bit-by-bit product)
  of encoded signal and chipping sequence
• if inner-product gt 0, the data is 1 else -1

40
CDMA Encode/Decode
Encode
Code of user m cm 1 1 1 -1 1 -1 -1 -1
Decode

• The number of bitsof each chipping sequence is
  M

41
CDMA Deal with Multiple-User Interference

• Two codes Ci and Cj are orthogonal, if
• , where we use . to denote inner
  product, e.g.
• If codes are orthogonal, multiple users can
  coexist and transmit simultaneously with
  minimal interference

C1 1 1 1 -1 1 -1
-1 -1 C2 1 -1 1 1
1 -1 1 1 ------------------------
----------------- C1 . C2 1 (-1) 1
(-1) 1 1 (-1)(-1)0
Analogy Speak in different languages!
42
CDMA Two-Sender Interference
Code 1 1 1 1 -1 1 -1 -1 -1 Code 2 1 -1 1 1
1 -1 1 1
43
Discussions

• Advantages of channel partitioning
• Problems of channel partitioning

44
Outline

• Recap
• Link layer overview
• Error detection and correction
• MAC protocols
• Partitioning protocols
• Non-partitioning MAC protocols
• Random access

44
45
Random Access Protocols

• When a node has packets to send
• transmit at full channel data rate R
• no a priori coordination among nodes
• Two or more transmitting nodes -gt collision
• Random access MAC protocol specifies
• when to access channel?
• how to detect collisions?
• how to recover from collisions?
• Examples of random access MAC protocols
• slotted ALOHA and pure ALOHA
• CSMA and CSMA/CD, CSMA/CA

46
Slotted Aloha Norm Abramson

• Time is divided into equal size slots ( pkt
  trans. time)
• Node with new arriving pkt transmit at beginning
  of next slot
• If collision retransmit pkt in future slots with
  probability p, until successful.

Success (S), Collision (C), Empty (E) slots
47
Slotted Aloha Efficiency

• Q What is the fraction of successful slots?
• suppose n stations have packets to send
• suppose each transmits in a slot with probability
  p
• - prob. of succ. by a specific node p
  (1-p)(n-1)
• - prob. of succ. by any one of the N nodes
• S(p) n Prob (only one transmits)
• n p (1-p)(n-1)

48
Goodput vs. Offered Load

S throughput goodput (success rate)
1.5
0.5
1.0
2.0
G offered load np

• when p n lt 1, as p (or n) increases
• probability of empty slots reduces
• probability of collision is still low, thus
  goodput increases
• when p n gt 1, as p (or n) increases,
• probability of empty slots does not reduce much,
  but
• probability of collision increases, thus goodput
  decreases
• goodput is optimal when p n 1

49
Maximum Efficiency vs. n
1/e 0.37
50
Pure (unslotted) Aloha

• Unslotted Aloha simpler, no clock
  synchronization
• Whenever pkt needs transmission
• send without awaiting for the beginning of slot
• Collision probability increases
• pkt sent at t0 collide with other pkts sent in
  t0-1, t01

51
Pure Aloha (cont.)

• Assume a node transmit with probability p in one
  unit of time
• P(success by a given node) P(node transmits)
• 
  P(no other node transmits in t0-1,t0
• 
  P(no other node transmits in t0, t01
• p .
  (1-p)n-1 . (1-p)n-1
• p .
  (1-p)2(n-1)
• P(success by any of N nodes) n p . (1-p)2(n-1)
• 
  
• - Bound 1/(2e) .18

52
Goodput vs. Offered Load

0.4
0.3
S throughput goodput (success rate)
0.2
0.1
1.5
0.5
1.0
2.0
G offered load Np
53
Dynamics of (Slotted) Aloha

• In reality, the number of stations backlogged is
  changing
• we need to study the dynamics when using a fixed
  transmission probability p
• Assume we have a total of m stations (the
  machines on a LAN)
• n of them are currently backlogged, each tries
  with a (fixed) probability p
• the remaining m-n stations are not backlogged.
  They may start to generate packets with a
  probability pa, where pa is much smaller than p

54
Model
n backlogged each transmits with prob. p
m-n unbacklogged
each transmits with prob. pa
55
Dynamics of Aloha Effects of Fixed Probability

• - assume a total of
• m stations
• pa ltlt p
• success rate is thedeparture rate, the rate
  the backlog is reducing

dep. and arrival rate of backlogged stations
n number of backlogged stations
m
0
offered load 1
Lesson if we fix p, but n varies, we may have an
undesirable stable point
56
Summary of Problems of Aloha Protocols

• Problems
• slotted Aloha has better efficiency than pure
  Aloha but clock synchronization is hard to
  achieve
• Aloha protocols have low efficiency due to
  collision or empty slots
• when offered load is optimal (p 1/N), the
  goodput is only about 37
• when the offered load is not optimal, the goodput
  is even lower
• undesirable steady state at a fixed transmission
  rate, when the number of backlogged stations
  varies
• Ethernet design address the problems
• approximate slotted Aloha without clock
  synchronization
• reduce the penalty of collision or empty slots
• infer optimal transmission rate

57
The Basic MAC Mechanisms of Ethernet
get a packet from upper layer K 0 n
0 // K control wait time n no. of
collisions repeat wait for K 512 bit-time
while (network busy) wait wait for 96
bit-time after detecting no signal transmit
and detect collision if detect collision
stop and transmit a 48-bit jam signal
n m min(n, 10), where n is the
number of collisions choose K randomly
from 0, 1, 2, , 2m-1. if n lt 16 goto
repeat else give up
58
Ethernet

• Dominant LAN technology
• First widely used LAN technology
• Kept up with speed race 10 Mbps, 100 Mbps, 1
  Gbps, 10 Gbps

Metcalfes Ethernet sketch
59
Course Topics Summary

• The Internet is a general-purpose, large-scale,
  distributed computer network
• Major design features/principles
• packet switching/statistical multiplexing
• hour-glass architecture
• end-to-end principle
• decentralized architecture
• E.g., DNS, interdomain routing
• resource allocation framework
• optimization decomposition through duality
• adaptive control
• e.g., AIMD sliding window self clocking, Ethernet
• queueing modeling/performance analysis and design
• tradeoff between theoretical impossibility and
  practice

60
Evolution

• Driven by Technology, Infrastructure, Policy,
  Applications, and Understanding
• technology
• e.g., wireless/optical communication technologies
  and device miniaturization (sensors)
• infrastructure
• e.g., cloud computing
• applications
• e.g., content distribution, game, tele presence,
  sensing, grid computing, VoIP,
• understanding
• e.g., resource sharing principle, routing
  principles, mechanism design, optimal stochastic
  control (randomized access)
• Complexity comes from evolution.
• Dont be afraid to challenge the foundation and
  redesign!

61
(No Transcript)
62
Backup Slides
63
Ethernets Exponential Backoff

• Goal adapt retransmission attempts to estimated
  current load
• compared with CSMA, 1/2m can be considered as p
• not a static p---adjusted using exponential
  backoff
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

64
Many Issues

• How to make it faster
• How to make it more efficient
• How to make it more reliable/robust/secure

65
CSMA Carrier Sense Multiple Access

• CSMA listen before transmit
• Objective approximate slotted Aloha (compared
  with pure Aloha)
• If backlogged, wait until channel sensed idle,
  then transmit pkt with prob. p
• human analogy dont interrupt others !

66
CSMA Collisions
spatial layout of nodes along Ethernet
D
A
B
C
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
t0
time
Collision entire packet transmission time
wasted still not very efficient!
67
CSMA/CD (Collision Detection)

• Human analogy the polite conversationalist
• CSMA/CD
• observations
• collisions can be detected within short time
• if colliding transmissions are aborted, we can
  reduce channel wastage
• carrier sensing, deferral as in CSMA
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals
• difficult in wireless LANs receiver shuts off
  while transmitting

68
CSMA/CD Collision Detection
spatial layout of nodes along Ethernet
spatial layout of nodes along Ethernet
D
D
A
A
B
C
B
C
t0
t0
time
time
B detects collision, aborts
D detects collision, aborts
instead of wasting the whole packettransmission
time, abort after detection.
69
Efficiency of CSMA/CD

• Given collision detection, instead of wasting the
  whole packet transmission time (a slot), we waste
  only the time needed to detect collision.
• Use a contention slot of 2 T, where T is one-way
  propagation delay (why 2 T ?)
• When the transmission probability p is
  approximately optimal (p 1/N), we try
  approximately e times before each successful
  transmission

P packet size, e.g. 1000 bitsC link capacity,
e.g. 10Mbps
P/C
70
Efficiency of CSMA/CD

• The efficiency (the percentage of useful time) is
  approximately
• The value of a plays a fundamental role in the
  efficiency of CSMA/CD protocols.
• Question you want to increase the capacity of a
  link layer technology (e.g., , 10 Mbps Ethernet
  to 100 Mbps), but still want to maintain the same
  efficiency, what can you do?

71
Summary of Problems to be Addressed

• Approximate slotted Aloha
• Reduce the penalty of collision or empty slots
• Infer optimal transmission rate

72
Physical Layer
73
Internet Bandwidth Growth
Source TeleGeograph Research
74
What Determines Transmission Rate?

• Service transmit a bit stream from a sender to a
  receiver

sender
receiver
channel
Encoding
Decoding
output bit stream
input bit stream
Question to be addressed how much can we send
through the channel ?
75
Basic Theory Channel Capacity

• The maximum number of bits that can be
  transmitted per second (bps) by a physical media
  is
• where W is the frequency range, S/N is the signal
  noise ratio. We assume Gaussian noise.

76
Fourier Transform

• Suppose the period of a data unit is f (1/T),
  then the data unit can be represented as the sum
  of many harmonics (sin(), cos()) with frequencies
  f, 2f, 3f, 4f,
• A reasonably behaved periodic function g(t), with
  minimal period T, can be constructed as the sum
  of a series of sines and cosines

77
char b
78
Signal Attenuation

• The quality of signal will degrade when it
  travels
• loss, frequency passing

79
Frequency Dependent Attenuation

• The received signal will be distorted even when
  there is no interference and the transmitted
  signal is perfect square waveform

Example Voltage-attenuation magnitude ratios of
Category 5 cable. For example, 500 feet of cable
attenuates a 10-MHz, 1-V signal to 0.32 V, which
corresponds to about 9.90 dB ( 20 log 1/0.32)
80
Example
V.34 (33.6kbps Dialup Modem)
channel
telephone network
sender modem
ISP modem
Analog to Digital quantization for
transmitting throughthe digital telephone
backbone
Modem Modulation(digit-gtanalog)
ISPdemodulation
input bit stream
3Khz bandwidth(add white noise)
output bit stream

• Example W3000Hz, S/N ? 4000

81
Example ADSL

• Spectrum allocation divided into a total of
  256 downstream and 32 upstream tones, where
  each tone is a standard 4kHz voice channel
• During initial negotiation, a tone is used only
  if the S/N is above 6 db (?4)

82
Faster
83
The Wire Fiber

• A look at a fiber
• How it works?

A graded index fiber
84
The Wire Fiber

• Wide spectrum at low loss 0.3db/km (c.f.
  copper 190db/km _at_100Mhz), 30-100km without
  repeater
• Bandwidth of a single fiber
• theoretical 100-200Tbps http//www.trnmag.com/St
  ories/080101/Study_shows_fiber_has_room_to_grow_08
  0101.html
• Lightweight 33 tons of copper to transmit the
  same amount of information carried by ¼ pound of
  optical fiber

85
Advantages of Fibers
86
How to Do Switching?

• Optical-Electrical-Optical
• Optical switch optical micro-electro-mechanical
  systems (MEMS)

Optical path
One optical switch
http//www.qwest.com/largebusiness/enterprisesolut
ions/networkMaps/preloader.swf
87
Example MEMS Optical Switch

• Using mirrors, e.g. Lambda Router

88
Implications

• Fine-grained switching may not be feasible
• What is the architecture of optical networks
  packet switching, circuit switching, or others?

89
More Efficient
90
Problem Inefficient Interactions

• Large deployment of highly adaptive, multipoint
  applications
• An iterative process between two sets of
  adaptation
• ISP traffic engineering to change routing to
  shift traffic away from higher utilized links
• current traffic pattern ? new routing matrix
• App direct traffic to better performing end
  points
• current routing matrix ? new traffic pattern

91
ISP Traffic Engineering App Latency Optimizer

• red App adjust alone fixed ISP routing
• blue ISP traffic engineering adapt alone fixed
  App communications

ISP optimizer interacts poorly with App.
92
The Fundamental Problem

• Traditional Internet architectural feedback to
  application efficiency is limited
• routing (hidden)
• rate control through coarse-grained TCP
  congestion feedback
• To achieve better efficiency, needs explicit
  communications between network resource providers
  and applications

93
P4P Framework Design Goals

• Performance improvement
• Scalability and extensibility support diverse
  ISP objectives and applications scenarios in
  large networks
• Privacy preservation
• Ease of implementation
• Open standard any ISP, provider, applications
  can easily implement it

94
Current Status

• ATT
• Bezeq Intl
• BitTorrent
• CacheLogic
• Cisco Systems
• Grid Networks
• Joost
• LimeWire
• Manatt
• Oversi
• Pando Networks
• PeerApp
• Telefonica Group
• VeriSign
• Verizon
• Vuze
• Univ of Washington
• Yale University

• Abacast
• AHT Intl
• Akamai
• Alcatel Lucent
• CableLabs
• Cablevision
• Comcast
• Cox Comm
• Juniper Networks
• Microsoft
• MPAA
• NBC Universal
• Nokia
• RawFlow
• Solid State Networks
• Thomson
• Time Warner Cable
• Turner Broadcasting

• P4P-WG
• Next step
• wider integration
• IETF standard

95
Reliability
96
Is the Internet Reliable?

• A key design objective of the Internet (i.e.,
  packet-switched networks) is robustness
• Does the Internet infrastructure achieve the
  target reliability objective of a highly
  reliable system (99.999)?

97
Perspective

• 911 Phone service (1993 NRIC report )
• 29 minutes per year per line
• 99.994 availability
• Std. Phone service (various sources)
• 53 minutes per line per year
• 99.99 availability
• what about the Internet?
• Various studies about 99.5
• Need to reduce down time by 500 times to achieve
  five nines 50 times to match phone service

98
Unreachable Networks 10 days
99
Internet Disaster Recovery Response

• Why slow response?
• the cable repairing is slow not until 21 days
  after quake
• BGP is not designed to create business
  relationship
• Objective
• a meta-BGP to facilitate discovery and creation
  of BGP business relationship

100
(No Transcript)
101
Backup IP Multicast
102
IP Fragmentation Reassembly

• Network links have MTU (max.transfer size) -
  largest possible link-level frame.
• different link types, different MTUs, e.g.
  Ethernet MTU is 1500 bytes
• Large IP datagram divided (fragmented)
• one datagram becomes several datagrams
• reassembled only at final destination
• IP header bits used to identify, order related
  fragments

fragmentation in one large datagram out 3
smaller datagrams
reassembly
103
IP Fragmentation and Reassembly

• Example
• 4000 byte datagram
• MTU 1500 bytes

104
IP Multicast Service Model

• Multicast group concept use of indirection
• A group is identified by a location-independent
  logical address (class D IP address prefix 1110)
• Open group model
• Anyone can send packets to the logical group
  address
• Anyone can join a group and receive packets
• Normal, best-effort delivery semantics of IP

Needed infrastructure to deliver mcast-addressed
datagrams to all hosts that have joined that
multicast group
105
Multicast Across LANs

• Goal find a tree (or trees) connecting routers
  having local mcast group members
• source-based different tree from sender to each
  receiver
• Distance-vector multicast routing protocol
  (DVMRP)
• Protocol-independent multicast-dense mode
  (PIM-DM)
• shared-tree same tree used by all group members
• Core-Based Tree (CBT)
• Protocol-independent multicast-sparse mode
  (PIM-SM)

shared tree
106
Source Tree Reverse Path Flooding (RPF)

• A router x forwards a packet from source (S) iff
  it arrives via neighbor y, and y is on the
  shortest path from x back to S
• A packet is replicated to all but the incoming
  interface

S
1
1
y
x
1
z
1
1
t
a
107
Reverse Path Forwarding Improvement

• Basic idea forward a packet from S only on child
  links for S
• A child link of router x for source S
• a link that has x as parent on the shortest path
  from thelink to S
• a child x notifies its parent y(through the
  routing protocol)that it has selected y as
  itsparent

S
y
x
z
t
a
108
Reverse Path Forwarding Pruning

• No need to forward datagrams down subtree with no
  mcast group members
• prune msgs sent upstream by router with no
  downstream group members

LEGEND
S source
R1
router with attached group member
R4
router with no attached group member
R2
P
P
R5
prune message
links with multicast forwarding
P
R3
R7
R6
109
Pruning

• Prune (Source, Group) at a leaf router if no
  members
• send No-Membership Report (NMR) up tree
• If all children of router R prune (S,G)
• propagate prune for (S,G) to its parent
• What do you do when a member of a group
  (re)joins?
• send a Graft message to upstream parent
• How to deal with failures?
• prune dropped
• flow is reinstated
• down stream routers re-prune
• Note again a soft-state approach

110
Implementation of Source Trees in the Internet

• Multicast OSFP (MOSFP)
• Membership is part of the link state
  distribution calculate source specific,
  pre-pruned trees
• Reverse Path Forwarding
• Distance Vector Multicast Routing Protocol
  (DVMRP)
• Protocol Independent Multicast Dense Mode
  (PIM-DM)
• very similar to DVMRP
• Difference PIM uses any unicast routing
  algorithm to determine the path from a router to
  the source DVMRP uses distance vector
• Question the state requirement of Reverse Path
  Forwarding

111
Building a Shared Tree

• Steiner Tree minimum cost tree connecting all
  routerswith attached group members
• A Steiner tree is not a spanning tree because
  you do not need to connect all nodes in the
  network
• Problem is NP-hard
• Excellent heuristics exists
• Not used in practice
• computational complexity
• information about entire network needed
• monolithic rerun whenever a router needs to
  join/leave

112
Center (Core) based Shared Tree

• Single delivery tree shared by all
• One router identified as center of tree
• Tree construction is receiver-based
• edge router sends unicast join-msg addressed to
  center router
• join-msg processed by intermediate routers and
  forwarded towards center
• join-msg either hits existing tree branch for
  this center, or arrives at center
• path taken by join-msg becomes new branch of tree
  for this router
• A sender unicasts a packet to center
• The packet is distributed on the tree when it
  hits the tree

113
Example M3 Joins

• Group members M1, M2

core
M1
M2
M3
S1
Discussion what is property of the constructed
tree?
114
Example M1 Sends Data

• Group members M1, M2, M3
• M1 sends data

core
M1
M2
M3
control (join) messages
data
S1
115
Shared Tree Protocols in the Internet

• Core Based Tree
• Protocol Independent Multicast (PIM) Sparse mode
• The catch how do you know the center?
• session announcement

116
Mbone Tunneling
Q How to connect islands of multicast routers
in a sea of unicast routers?
logical topology
physical topology

• mcast datagram encapsulated inside normal
  (non-multicast-addressed) datagram
• normal IP datagram sent thru tunnel via regular
  IP unicast to receiving mcast router
• receiving mcast router unencapsulates to get
  mcast datagram