prof. younghee lee 1 1 computer networks u lecture 3: data link, lan, atm and mpls prof. younghee...

1Prof. Younghee Lee1

Computer Networks Lecture 3: Data Link, LAN, ATM and MPL

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Prof. Younghee Lee

* Some part of this teaching materials are prepared referencing the lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts)


Link Layer Services

Framing, link access: – encapsulate datagram into frame, adding header, trailer– implement channel access if shared medium, – ‘physical addresses’ used in frame headers to identify source, dest

» different from IP address!

Reliable delivery between two physically connected devices: – we learned how to do this already – seldom used on low bit error link (fiber, some twisted pair)– wireless links: high error rates

» Q: why both link-level and end-end reliability?


Link Layer Services (more)

Flow Control: – pacing between sender and receivers

Error Detection: – errors caused by signal attenuation, noise. – receiver detects presence of errors:

» signals sender for retransmission or drops frame

Error Correction: – receiver identifies and corrects bit error(s) without

resorting to retransmission


The need for flow and error control Flow control

– protocol mechanism that enables a destination entity to regulate the flow of PDUs sent by a source entity» examples; different flow control characteristics

X.25 virtual circuit: network layer LAPB: data link layer TCP connections: transport layer


The need for flow and error control Flow control

– employed in various contexts» Hop Scope, Network interface, Entry to exit, End to end

Error control– used to recover from the loss or damage of PDUs in transit


Link control mechanism-Stop and Wait Simplest form of flow control CRC ACK0:

– acknowledges receipt of a frame numbered 1

– indicates that the receiver is ready for a frame numbered 0

– positive acknowledgement ARQ: Automatic Repeat reQuest Time out interval rarely used because of inefficiency

– Example: 1.5Mbps link x 45ms RTT = 67.5Kb (8KB). Assuming frame size of 1KB, stop-and-wait uses about one-eighth of the link's capacity. Want the sender to be able to transmit up to 8 frames before having to wait for an ACK.

– Case: link with very small RTT?


Link control mechanism-Stop and Wait

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Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0


Cyclic Redundancy Check Add k bits of redundant data to an n-bit message. Represent n-bit message as an n-1 degree polynomial; e.g.,

MSG=10011010 corresponds to M(x) = x7+ x4 + x3 + x1. Let k be the degree of some divisor polynomial C(x); e.g.,

C(x) = x3+ x2 + 1. Transmit polynomial P(x) that is evenly divisible by C(x), and

receive polynomial P(x) + E(x); E(x)=0 implies no errors. Recipient divides (P(x) + E(x)) by C(x); the remainder will be

zero in only two cases: E(x) was zero (i.e. there was no error), or E(x) is exactly divisible by C(x). Choose C(x) to make second case extremely rare.


Cyclic Redundancy Check Sender:

– multiply M(x) by xk; for our example, we get x10 + x7 + x6 + x4 (10011010000);

– divide result by C(x) (1101);

– Send 10011010000 - 101 = 10011010101, since this must be exactly divisible by C(x);


Cyclic Redundancy Check Common polynomials for C(x):

CRC

CRC-8

CRC-10

CRC-12

CRC-16

CRC-CCITT

CRC-32

C(x)

x8+x2+x1+1

x10+x9+x5+x4+x1+1

x12+x11+x3+x2+x1+1

x16+x15+x2+1

x16+x12+x5+1

x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1


Link control mechanism-Sliding window Sliding-window Techniques

– Receiver accept n frames, and A is allowed to send n frames without waiting for any acknowledgements

– 12~ 0 :number sequence k


Link control mechanism-Sliding window Go-back-N: http://wps.aw.com/aw_kurose_network_2/0,7240,227091-,00.html

– most commonly used– The form of error control based on sliding-window flow control– RR– REJ:negative acknowledgement– Damaged frame:

» Case A: errored frame» Case B: lost in transit» Case C:lost in transit. No additional frame to send soon. Sender time out

Sender sends RR with a P bit of 1 as a command Receiver acknowledges by sending a RR indicating the next frame that it expect Sender retransmit

– Damaged RR» damaged RR. Subsequent RR» damaged RR. A’s timer expires.

Sender send RR in Case C. set P-bit timer. If receiver fails to respond, then sender’s P-bit timer will expire Sender tries again multiple times. No ACK -> reset procedure

– Damaged REJ: equivalent to Case C


Link control mechanism-Sliding window Selective-reject ARQ

– retransmit only the frame that receive a negative ACK: SREJ– much less widely used than go-back-N– more efficient than go-back-N– receiver must maintain a buffer large enough to save post-

SREJ frames until the frame in error is retransmitted– transmitter, too, requires complex logic.


ARQ performance: Stop-and-wait ARQ Error-free Stop-and-wait

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Comparison L2, L4 Flow control + Error control

– RTT» Delay variance

– Timeout value

Throughput– Error control at each L2’s + L4; Loss at L3 buffer– Error control only at L4;– TCP throughput in case of many loss due to intensive errors at L2?

» Satellite link, wireless link…


HDLC

High Level Data Link Control : ISO Service

– Connectionless service» Acknowledged service

» Unacknowledged service: most common

– Connection-oriented service

Balanced mode, unbalanced mode


LLC

Logical Link Control Data link layer for LAN consists of MAC & LLC Service

– Type 1: unacknowledged connectionless service» most common in LAN

– Type2: reliable connection-oriented service:» End system without TCP

– Type3: acknowledged connectionless service» The receiving node will send acknowledgments for individual frames.

No virtual circuit


LANs

Data link layer • LLC sublayer • MAC sublayer


Multiple Access Links and Protocols

Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g, Ethernet, Wa

velan, etc.)

switched (e.g., switched Ethernet, ATM etc)


Channel Partitioning MAC protocols: 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

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

bands time


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) unique “code” assigned to each user; ie, code set partition

ing used mostly in wireless broadcast channels (cellular, satel

lite,etc) all users share same frequency, but each user has own “c

hipping” sequence (ie, code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping s

equence allows multiple users to “coexist” and transmit simultaneo

usly with minimal interference (if codes are “orthogonal”)


CDMA Encode/Decode


CDMA: two-sender interference


Random Access protocols

When node has packet to send– transmit at full channel data rate R.– no a priori coordination among nodes

two or more transmitting nodes -> “collision”, random access MAC protocol specifies:

– how to detect collisions– how to recover from collisions (e.g., via delayed

retransmissions)

Examples of random access MAC protocols:– slotted ALOHA– ALOHA– CSMA and CSMA/CD


Slotted Aloha

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


Slotted Aloha efficiency

Q: what is max fraction slots successful?A: Suppose N stations have packets to send

– each transmits in slot with probability p– prob. successful transmission S is:

by single node: S= p (1-p)(N-1)

by any of N nodes

S = Prob (only one transmits) = N p (1-p)(N-1)

… choosing optimum p as n -> infty ...

= 1/e = .37 as N -> infty

At best: channeluse for useful transmissions 37%of time!


Pure (unslotted) ALOHA

unslotted Aloha: simpler, no synchronization pkt needs transmission:

– send without awaiting for beginning of slot

collision probability increases:– pkt sent at t0 collide with other pkts sent in [t0-1, t0+1]


Pure Aloha (cont.)P(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1, p0] .

P(no other node transmits in [p0, p0+1]

= p . (1-p)(N-1) . (1-p)(N-1)

P(success by any of N nodes) = N p . (1-p)2(N-1)

… choosing optimum p as n -> infty ...

= 1/(2e) = .18

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Slotted Alohaprotocol constrainseffective channelthroughput!


CSMA: Carrier Sense Multiple Access)

CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission

– Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)

– Non-persistent CSMA: retry after random interval

human analogy: don’t interrupt others!


CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability


CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time– colliding transmissions aborted, reducing channel

wastage – persistent or non-persistent retransmission

collision detection: – easy in wired LANs: measure signal strengths,

compare transmitted, received signals– difficult in wireless LANs: receiver shut off while

transmitting human analogy: the polite conversationalist http://wps.aw.com/

aw_kurose_network_2/0,7240,227091-,00.html


CSMA/CD collision detection


“Taking Turns” MAC protocols

channel partitioning MAC protocols:– share channel efficiently at high load– inefficient at low load: delay in channel access, 1/N

bandwidth allocated even if only 1 active node!

Random access MAC protocols– efficient at low load: single node can fully utilize

channel– high load: collision overhead

“taking turns” protocols

look for best of both worlds!


“Taking Turns” MAC protocols

Polling: master node “invite

s” slave nodes to transmit in turn

Request to Send, Clear to Send msgs

concerns:– polling overhead – latency– single point of failur

e (master)

Token passing: control token passed from one n

ode to next sequentially. token message concerns:

– token overhead – latency– single point of failure (token)

Good thing:– Fairness control– QoS control– Throughput– Use of Fiber optic links


Reservation-based protocols

Distributed Polling: time divided into slots begins with N short reservation slots

– reservation slot time equal to channel end-end propagation delay

– station with message to send posts reservation– reservation seen by all stations

after reservation slots, message transmissions ordered by known

priority


Ethernet LAN History

– Developed by Xerox PARC in mid-1970s– Roots in ALOHA packet-radio network– Standardized by Xerox, DEC, and Intel in 1978– Similar to IEEE 802.3 standard

CSMA/CD– carrier sense– multiple access– collision detection

Bandwidth: 10Mbps and 100Mbps Problem: Distributed algorithm that provides fair access to a shared medium CDPD: Cellular Digital Packet Data

– Mac protocol for CDPD: Digital sense Multiple access; a variation of CSMA/CD


Ethernet LAN

Addresses: Unique, 48-bit unicast address assigned to each adaptor Example: 8:0:2b:e4:b1:2

Broadcast: all 1s Multicast: first bit is 1


LAN Addresses and ARP

32-bit IP address: network-layer address used to get datagram to destination network (recall IP

network definition)

LAN (or MAC or physical) address: used to get datagram from one interface to another

physically-connected interface (same network) 48 bit MAC address (for most LANs)

burned in the adapter ROM


LAN Addresses and ARP

Each adapter on LAN has unique LAN address


LAN Address (more)

MAC address allocation administered by IEEE manufacturer buys portion of MAC address

space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability

– can move LAN card from one LAN to another IP hierarchical address NOT portable

– depends on network to which one attaches


ARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

– TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88


ARP protocol: Same LAN (network)

A wants to send datagram to B, and B’s MAC address not in A’s ARP table.

A broadcasts ARP query packet, containing B's IP address

– Dest MAC address = FF-FF-FF-FF-FF-FF

– all machines on LAN receive ARP query

B receives ARP packet, replies to A with its (B's) MAC address– frame sent to A’s MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that

times out (goes away) unless refreshed

ARP is “plug-and-play”:– nodes create their ARP

tables without intervention from net administrator


Transmitter AlgorithmIf line is idle: Send immediately Upper bound message size of 1500 bytes Must wait 51s between back-to-back framesIf line is busy: Wait until idle and transmit immediately Called 1-persistent (special case of p-persistent)If collision: jam for 512 bits, then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes of data) delay and try again

– 1st time: uniformly distributed between 0 and 51.2s– 2nd time: uniformly distributed between 0 and 102.4s– 3rd time: uniformly distributed between 0 and 204.8s– give up after several tries (usually 16)– exponential backoff


ExperiencesObserve in Practice 10-200 hosts (not 1024) Length shorter than 1500m (RTT closer to 5 than 51) Packet length is bimodal High-level flow control and host performance limit load

Recommendations Do not overload (30% utilization is about max) Implement controllers correctly Use large packets Get the rest of the system right (broadcast, retransmission)


Hub and Switch Hub:

– Shared medium bus – Shared medium hub– Switching hub

» to achieve greater performance» throughput on the LAN in figure is

20Mbps» advantages

No change is required aggregated capacity scales easily.

» Types Store-and-forward switch: buffers

input frame, and then routes to output line

Cut-through switch: repeating the incoming frame onto the output line after recognizing the destination address at the beginning of MAC frame. Highest possible throughput but at some risk of propagating bad frames(CRC)

N x 10M


IEEE 802.11 Wireless LAN wireless LANs: untethered (often mobile) networ

king IEEE 802.11 standard:

– MAC protocol– unlicensed frequency spectrum: 900Mhz, 2.4Ghz

Basic Service Set (BSS) (a.k.a. “cell”) contains:– wireless hosts– access point (AP): base station

BSS’s combined to form distribution system (DS)


Characteristics of selected wireless link standards

384 Kbps384 Kbps

56 Kbps56 Kbps

54 Mbps54 Mbps

5-11 Mbps5-11 Mbps

1 Mbps1 Mbps

802.15

802.11b

802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid rangeoutdoor

200m – 4Km

Long rangeoutdoor

5Km – 20Km


MAC MAC Problems in wireless network: to use CSMA/CD

– Collision Detection(CD) does not work– CS might not work in some case( if a terminal is “hidden”)

Hidden terminal problem– Nodes A and C cannot hear each other

» Node A : currently transmitting to B» Node C : wants to transmit to B» Transmissions by nodes A and C can collide at node B

• Nodes A and C are hidden from each other

A B C


MAC Exposed terminal problem

– Node C cannot send to D due to carrier of B sense » Node B : currently transmitting to A» Node C : wants to transmit to D» Carrier of C doesn’t interfere A’s reception, Carrier of B doesn’t interfere D’s

reception Waiting is not necessary

» But C is waiting since it sense carrier of B

– C is exposed to B

A B C D


IEEE 802.11 Wireless LAN

802.11b– 2.4-5 GHz unlicensed

radio spectrum– up to 11 Mbps– direct sequence spread

spectrum (DSSS) in physical layer

» all hosts use same chipping code

– widely deployed, using base stations

802.11a – 5-6 GHz range– up to 54 Mbps

802.11g – 2.4-5 GHz range– up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions


IEEE 802.11: multiple access Like Ethernet, uses CSMA:

– random access– carrier sense: don’t collide with ongoing transmission

Unlike Ethernet:– no collision detection – transmit all frames to completion– acknowledgment – because without collision detection, you don’

t know if your transmission collided or not

Why no collision detection?– difficult to receive (sense collisions) when transmitting due to we

ak received signals (fading)– can’t sense all collisions in any case: hidden terminal, fading

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


MAC IFS: interframe space: depend on the type of frame to transmit

– SIFS: Short IFS» High priority frame before contending for channel» ACK frame, CTS frame, data transfer of a segmented MSDU, frames from station that are responding to a poll

from an AP, any frame from an AP during CFP

– PIFS: PCF-IFS» Used by PCF to gain priority access to the medium at the start of a CFP(Contention Free Period)

– DIFS: DCF-IFS» Used by the DCF to transmit data and management MPDU

DCF : CSMA/CA– Initial MAC PDU: medium idle for a period DIFS or greater -> transmit

» Medium busy -> wait random backoff time to schedule a reattempt

– Reattempt: decrement a counter each time an idle contention slot transpire – After successful frame transmission -> backoff procedure to transmit next frame

Busy Medium SIFS

PIFS

DIFSContentionWIndow

Next

Frame


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then

- transmit entire frame (no CD)

2 if sense channel busy then

- start random backoff time

- timer counts down while channel idle

- transmit when timer expires

- if no ACK, increase random backoff interval, repeat 2

802.11 receiverif frame received OK

- return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

optional; not typically used sender first transmits small request-to-send (RTS) packets to A

P using CSMA– RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

– sender transmits data frame– other stations defer transmissions

Avoid data frame collisions completely using small reservation packets!


Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer


hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router

H1 remains in same IP subnet: IP address can remain same

switch: which AP is associated with H1?– self-learning (Ch. 5):

switch will see frame from H1 and “remember” which switch port can be used to reach H1


Mobile Switching

Center

Public telephonenetwork, andInternet

Mobile Switching

Center

Components of cellular network architecture

connects cells to wide area net manages call setup (more later!) handles mobility (more later!)

MSC

covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS

cell

wired network


Cellular standards: brief survey

2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions general packet radio service (GPRS)

– evolved from GSM – data sent on multiple channels (if available)

enhanced data rates for global evolution (EDGE)– also evolved from GSM, using enhanced modulation – Date rates up to 384K

CDMA-2000 (phase 1)– data rates up to 144K– evolved from IS-95


Cellular standards: brief survey

3G systems: voice/data Universal Mobile Telecommunications Service (UMTS)

– GSM next step, but using CDMA CDMA-2000

….. more (and more interesting) cellular topics due to mobility (stay tuned for details)


Ad Hoc Networks

Ad hoc network: IEEE 802.11 stations can dynamically form network without AP

Applications:– “laptop” meeting in conference room, car– interconnection of “personal” devices– battlefield

IETF MANET (Mobile Ad hoc Networks) working group


Bridges

Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

Bridge isolates collision domains since it buffers frames

When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit


Bridges (more)

Bridge advantages:– Isolates collision domains resulting in higher total

max throughput, and does not limit the number of nodes nor geographical coverage

– Can connect different type Ethernet since it is a store and forward device

– Transparent: no need for any change to hosts LAN adapters


Bridges: frame filtering, forwarding

bridges filter packets – same-LAN -segment frames not forwarded onto other

LAN segments

forwarding: – how to know which LAN segment on which to forward

frame?– looks like a routing problem (more shortly!)


Backbone Bridge


Interconnection Without Backbone

Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment


Bridge Filtering

bridges learn which hosts can be reached through which interfaces: maintain filtering tables– when frame received, bridge “learns” location of

sender: incoming LAN segment– records sender location in filtering table

filtering table entry: – (Node LAN Address, Bridge Interface, Time Stamp)– stale entries in Filtering Table dropped (TTL can be 60

minutes)


Bridge Learning: example

Suppose C sends frame to D and D replies back with frame to C

C sends frame, bridge has no info about D, so floods to both LANs – bridge notes that C is on port 1 – frame ignored on upper LAN – frame received by D


Bridge Learning: example

D generates reply to C, sends – bridge sees frame from D – bridge notes that D is on interface 2 – bridge knows C on interface 1, so selectively

forwards frame out via interface 1


Bridges vs. Routers both store-and-forward devices

– routers: network layer devices (examine network layer headers)– bridges are Link Layer devices

routers maintain routing tables, implement routing algorithms bridges maintain filtering tables, implement filtering, learning

and spanning tree algorithms


High Speed LANs Most important high speed LANs

– Fast Ethernet and Gigabit Ethernet– ATM LAN


Fast Ethernet 100BASE-T 100BASE-X: two physical links between nodes, one for reception and one for transmission 100BASE-T4: the use of four twisted pair lines making use of 3 pairs in one direction at a time

– category 3: voice grade UTP, standard telephone wire, – category 5: more tightly twisted for higher quality


Fast Ethernet Mixed Configuration

– readily supports a mixture of existing 10-Mbps LANs and newer 100-Mbps LANs

3

1


Gigabit Ethernet Gigabit Ethernet

– compatible with 100Base-T, 10Base-T– use of optical fiber over relatively short distance, although UTP,

STP, and coaxial cable configurations are also allowed.


Gigabit Ethernet Application of Gigabit Ether

net

Discussion: ATM LAN vs Gigabit Ethernet


Metropolitan Ethernet Ethernet services for metro transport network Carrier-class Ethernet-based transport technologies

Metro Ethernet Forum(MEF)– Forum to accelerate the adoption of optical Ethernet as the technology of choice in metro network world wide– Move beyond LAN origin to become a full duplex, switched technology capable of meeting the long-range transport, bandwidth, geographical and capacity requirements of metro networking

» A compelling combination of speed, scalability, operational simplicity, and economics

– Deliverables» Implementation agreements, Test procedure, positioning statements, technical specifications, Marketing

– Services» Ethernet virtual private line service(EVPLS), Ethernet virtual private LAN services(EVPLns) and Circuit emulation services(CES)


Metropolitan Ethernet IEEE 802.3 Ethernet in the First Mile(EFM)

– The optimal subscriber access network» For faster, simpler, better and more profitable

– Point to point on fiber(P2P), Point to multipoint fiber(EPON), Point to point copper(Copper)

– Provide a family of physical layer specifications– OAM– To expand the application of Ethernet to include subscriber access

networks in order to provide a significant increase in performance while maintaining equipment, operation, and maintenance cost


ATM LANs Typical requirements for a third generation LAN

– support multiple, guaranteed classes of services– provide scalable throughput– facilitate the internetworking between LAN and WAN technology

ATM ideally suited to these requirements The Internet: connectionless, ATM: connection oriented possible types of ATM LANs:

– Gateway to ATM WAN– Backbone ATM switch: interconnect other LANs– Workgroup ATM: high performance MM WSs, connect directly to switch


ATM LANs Need perform some sort of protocol conversion from the MAC

protocol to the ATM cell stream


ATM LANs ATM hub includes a number of ports that operate at different

data rates and use different protocols pure ATM LAN?


ATM LANs: LANE Need Connectionless service for TCP/IP software implementation over a connection-oriented ATM

network– connectionless server approach

» Every host initially sets up a connection to this server

– ATM LAN emulation» Every hosts has a ATM virtual circuit to every other host» These VC can be established and released dynamically as needed or can be permanent virtual circuit» LES(LAN Emulation Server): ATM ARP server» BUS(Broadcast/Unknown server): broadcasting, multicasting


High Speed LANs Token Ring MAN:

– DQDB(802.6): Distributed Queue Dual Bus» high speed broadcast bus» slotted access

FDDI II: packet/circuit switched traffic FFOL: FDDI Follow - on LAN

– scalable beyond one Gbps– support cell-based ATM traffic– connect to SONET/SDH link

S-NET, Expressnet, Datakit HIPPI: High Performance Parallel Interface FC: Fiber Channel Broadband LAN, Baseband LAN, data PABX 100VG-AnyLAN: IEEE 802.12

– demand priority: » hub polls each computer in turn» computer transmit only when the hub grants» support a simple priority scheme» support frame format other than Ethernet


ATM-MPLS


ATM protocol architecture ATM protocol architecture

– ITU-T SG 13, SG 11– ATM Forum– Connection-oriented packet-switched network– Used in both WAN and LAN settings– Signaling (connection setup) Protocol: Q.2931– Packets are called cells: 5-byte header + 48-byte payload– Commonly transmitted over SONET (but not necessarily)


Cell Network

Why Cell?– Taking advantages of the characteristics of both packet and bit --> good for multimedia communications– Interference

o interference at gigabit speed?


ATM architecture

adaptation layer: only at edge of ATM network– data segmentation/reassembly– roughly analagous to Internet transport layer

ATM layer: “network” layer– cell switching, routing

physical layer


ATM: network or link layer?

Vision: end-to-end transport: “ATM from desktop to desktop”– ATM is a network

technology

Reality: used to connect IP backbone routers – “IP over ATM”– ATM as switched link

layer, connecting IP routers


ATM Adaptation Layer Adaptation layer:

– to support information transfer protocols not based on ATM– PCM voice: need to assemble PCM bits into cells– LAPF: need to map LAPF frame into a number of cells

» Frame Relay Data Link Layer (LAPF) AAL Services

– Handling of transmission errors– Segmentation and reassembly– Handling of lost and misinserted cell conditions– Flow control and timing control


ATM Layer

Service: transport cells across ATM network analagous to IP network layer very different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?


ATM Service Categories Real Time Service

– Constant bit rate(CBR): videoconferencing, interactive audio(telephony), Audio/video distribution(TV, distance learning, PPV), Audio/video retrieval(VOD, audio library.. Really needed?)

– Real-time Variable Bit Rate(rt-VBR): compressed video Non-real-time service

– Non-real-time variable bit rate(nrt-VBR): data transfers that have critical response-time requirements; airline reservations, banking transactions,...

– Unspecified bit rate: best effort service; text/data/image transfer, messaging, distribution, retrieval, remote terminal(telecommuting)

– Available bit rate(ABR): PCR(Peak Cell Rate), MCR(Minimum Cell Rate), explicit feedback to to sources for fair allocation of capacity, unused capacity by ABR remains available for UBR traffic; LAN interconnection


ATM Logical Connections VCC: Virtual Channel Connection VPC: Virtual Path Connection Advantages of the use of VP

– Simplified network architecture– Increased network performance and reliability– Reduced processing and short connection setup time– Enhanced network service


ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest

– call setup, teardown for each call before data can flow– each packet carries VC identifier (not destination ID)– every switch on source-dest path maintain “state” for each

passing connection– link,switch resources (bandwidth, buffers) may be allocate

d to VC: to get circuit-like perf. Permanent VCs (PVCs)

– long lasting connections– typically: “permanent” route between to IP routers

Switched VCs (SVC):– dynamically set up on per-call basis


ATM VCs

Advantages of ATM VC approach:– QoS performance guarantee for connection

mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach:

– Inefficient support of datagram traffic– one PVC between each source/dest pair) doe

s not scale (N*2 connections needed) – SVC introduces call setup latency, processin

g overhead for short lived connections


ABR Rate Control Cell Flow

– forward RM (FRM) cell– backward RM (BRM) cell

ways to provide rate control feedback– EFCI marking: switch set EFCI, destination end system set the CI in BR

M– Relative rate marking: switch set CI or NI in FRM. Set CI or NI in BRM fo

r raapid result – Explicit rate marking: reduce the value of the ER field of an FRM, BRM


Datagram Journey in IP-over-ATM Network

at Source Host:– IP layer finds mapping between IP, ATM dest address (usi

ng ARP)– passes datagram to AAL5– AAL5 encapsulates data, segments to cells, passes to AT

M layer ATM network: moves cell along VC to destination

at Destination Host:– AAL5 reassembles cells into original datagram– if CRC OK, datgram is passed to IP


X.25 and Frame Relay

Like ATM: wide area network technologies virtual circuit oriented origins in telephony world can be used to carry IP datagrams

– can thus be viewed as Link Layers by IP protocol


X.25

X.21(EIA-232) LAPB(subset of

HDLC) virtual circuit

– logical connection between two stations through the network

(specific preplanned route through the network between two station): internal virtual circuit


IP versus X.25

X.25: reliable in-sequence end-end delivery from end-to-end– “intelligence in the network”

IP: unreliable, out-of-sequence end-end delivery– “intelligence in the endpoints”

gigabit routers: limited processing possible 2000: IP wins


Frame Relay

Designed in late ‘80s, widely deployed in the ‘90s

Frame relay service:– no error control– end-to-end congestion control


Frame Relay (more)

Designed to interconnect corporate customer LANs– typically permanent VC’s: “pipe” carrying aggregate traff

ic between two routers

– switched VC’s: as in ATM corporate customer leases FR service from public Frame

Relay network (eg, Sprint, ATT)


Frame Relay Networks

Frame Relay Architecture– LAPF(Link Access Protocol for Frame Mode Bearer Services)

» Frame delimiting, alignment, and transparency

» Frame multiplexing/demultiplexing using the address field

» Inspection of the frame

» Detection of transmission errors

» Congestion control function

User Data Transfer– DLCI swapping

Frame Relay Call Control


IP Forwarding Architecture

High Performance Forwarder– Pure destination based forwarding

Lookup based on IP address Internal high speed switch instead of router’s internal backplane bus Multiple lookup engine at each interface

– Switched forwarding» Overlay model

Overlay an IP network onto an ATM network CLIP(Classical IP over ATM), LANE, NHRP, MPOA

» Peer model Uses the existing IP address(or algorithmically derived ATM addresses)

to identify end systems and uses IP routing protocols to set up ATM connections

MPLS


IP Forwarding IP over ATM

– LANE: ATMF, to make an ATM LAN appear as a set of logical shared medium LANs interconnected via router.

– IPOA: IETF, group of ATM stations are divided into Logical IP Subnet(LIS), interconnected via router. Each LIS has an ATM ARP server for address resolution

– NHRP(Next Hop Resolution Protocol): IETF, to locate an exit point in the ATM cloud closest to the destination and to obtain ATM address.

– Multiprotocol over ATM(MPOA): ATMF, to provide internetworking service such as IP, IPX, and AppleTalk over an ATM network: an extension of LANE. Uses NHRP.

IP switching: Ipsilon Tag switching: Cisco Cell Switched Router: Toshiba Aggregate Route Based IP Switching(ARIS): IBM MPLS: IETF(~Tag switching)


IP over ATM CLIP(Classical IP over ATM):

– group of ATM stations are divided into Logical IP Subnet(LIS), interconnected via router. Each Logical IP subnetwork(LIS) has an ATM ARP server for address resolution

– All members(IP end system) in the same LIS must use the same IP address prefix

LIS1

LIS3 LIS4 LIS5LIS6

LIS2

RouterRouter

Router RouterRouter

ATM network


IP over ATM

NHRP(Next Hop Resolution Protocol):– IETF, to locate an exit point in the ATM cloud closest to the destination

and to obtain ATM address.: shortcut path through ATM network -> intermediate routers can be bypassed

LIS1

LIS3 LIS4 LIS5LIS6

LIS2

RouterRouter

Router RouterRouter

ATM network

Source

Destination

Shortest path(NHRP)

Default path(CLIP)


ATM Discussion

At one point, ATM was viewed as a replacement for IP.– Could carry both traditional telephone traffic (CBR circuits) and oth

er traffic (data, VBR)– Better than IP, since it supports QoS

Complex technology.– Switching core is fairly simple, but– Support for different traffic classes– Signaling software is very complex– Technology did not match people’s experience with IP

» deploying ATM in LAN is complex (e.g. broadcast)» supporting connection-less service model on connection-based technology

– With IP over ATM, a lot of functionality is replicated


IP Switching How to use ATM hardware without the software.

– ATM switches are very fast data switches– software adds overhead, cost

The idea is to identify flows at the IP level and to create specific VCs to support these flows.– flows are identified on the fly by monitoring traffic– flow classification can use addresses, protocol types, ...– can distinguish based on destination, protocol, QoS

Once established, data belonging to the flow bypasses level 3 routing.– never leaves the ATM switch

Interoperates fine with “regular” IP routers.– detects and collaborates with neighboring IP switches


IP Switching How would a node in the network determine which packets

should be assigned to flows for the purpose of switching? – (Without explicit guidance such as the IPv6 flow label..)– by flow statistics


An Alternative: Tag Switching Instead of monitoring traffic to identify flows to op

timize, use routing information to guide the creation of “switched” paths.– Switched paths are set up as a side effect of filling in fo

rwarding tables

Generalize to other types of hardware. Also introduced stackable tags.

– Made it possible to temporarily merge flows and to demultiplex them without doing an IP route lookup

– Requires variable size field for tag

A

B

A

B

A

B

C

C


Tag Switching Label(tag) switching technology for layer 3 packet forwarding

– Forwarding Component

» Tags: Packets carry

» Tag Information Base: A tag switch carries tag forwarding information in a database called TIB

» Forwarding procedure: tag as index to lookup information in TIB. Switch replaces the tag and the link level information, and sends the packet to the outgoing interface

independent of routing functionality and of the network layer

– Control Component

» a binding between a tag and network layer routing. A tag could be bound to an individual application flow, a single route, group of routes or a multicast tree

» to create tag bindings and distribute the tag binding information among the interconnected tag switch

QoS – assigning a tag to a class of packets once the classification has been done


Label Switching

Goal: speed up route look up.– IP addresses are long and a longest prefix match is expensive– Will get worse with IPv6

Replace IP address with a shorter fixed sized address that can be looked up quickly.– Easy look up in hardware, e.g. ATM VCID– Set up a connection for that address– Label packets at entry to the network

Use of short address is typically optional.– Regular IP look up is a fallback– May be slower

Multi-protocol label switching (MPLS).


IP Switching versus Tag Switching

Flows versus routes.– tags explicitly cover groups of routes– tag bindings set up as part of route establishment– flows in IP switching are driven by traffic and detected

by “filters”» Supports both fine grain application flows and coarser grain flow

groups

Stackable tags.– provides more flexibility

Generality– IP switching focuses on ATM– not clear that this is a fundamental difference


Multi-Protocol Label Switching: MPLS

Goal of MPLS group in IETF: – To develop a standard for integration of layer 2 switching with layer 3 routin

g in order to improve price/performance of network layer routing, scalability of network layer and provide great flexibility in providing new routing service

Map packet onto Forward Equivalence Class (FEC) based on its header.– Simple case: longest prefix match of destination address– More complex if QoS of policy routing is used

In MPLS, a label is associated with the packet when it enters the network and forwarding is based on the label in the network core.– Label is swapped (as ATM VCIs)

Potential advantages.– Packet forwarding can be faster– Routing can be based on ingress router and port– Can use more complex routing decisions– Can force packets to followed a pinned route


MPLS Mechanisms

Implementation of the label is technology specific.– Could be ATM VCI or an extra header

Label Distribution Protocols distributes information on label/FEC bindings.– Extensions of existing protocols (routing, RSVP) or

stand-alone protocols– Can be upstream or downstream

Supports stacked labels.


MPLS capable routers

a.k.a. label-switched router forwards packets to outgoing interface based

only on label value (don’t inspect IP address)– MPLS forwarding table distinct from IP forwarding

tables signaling protocol needed to set up forwarding

– RSVP-TE– forwarding possible along paths that IP alone would

not allow (e.g., source-specific routing) !!– use MPLS for traffic engineering

must co-exist with IP-only routers


Reading

[Degermark97] Small Forwarding Tables for Fast Routing Lookups

prof. younghee lee 1 1 computer networks u lecture 3: data link, lan, atm and mpls prof. younghee...

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