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Link Layer

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Title: 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
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