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Unit I

Frame relay Networks

Frame Relay often is described as a streamlined version of X.25, offering fewer of therobust capabilities, such as windowing and retransmission of last data that are offered in

X.25.

Frame Relay Devices

Devices attached to a Frame Relay W ! fall into the following two general categories"

# Data terminal e$uipment %D&'( # Data circuit)terminating e$uipment %D*'(

D&'s generally are considered to be terminating e$uipment for a specific networ+ andtypically are located on the premises of a customer. n fact, they may be owned by thecustomer. '-amples of D&' devices are terminals, personal computers, routers, and

bridges.

D*'s are carrier)owned internetwor+ing devices. &he purpose of D*' e$uipment is to provide cloc+ing and switching services in a networ+, which are the devices that actuallytransmit data through the W !. n most cases, these are pac+et switches. Figure /)shows the relationship between the two categories of devices.

Standard Frame Relay Frame

0tandard Frame Relay frames consist of the fields illustrated in Figure /)1.

Figure Five Fields *omprise the Frame Relay Frame

'ach frame relay D3 consists of the following fields"

. Flag Field. &he flag is used to perform high level data lin+ synchroni4ation whichindicates the beginning and end of the frame with the uni$ue pattern / /. &oensure that the / / pattern does not appear somewhere inside the frame, bitstuffing and destuffing procedures are used.

2. ddress Field. 'ach address field may occupy either octet 2 to , octet 2 to 1, or octet 2 to 5, depending on the range of the address in use. two)octet address

http://en.wikipedia.org/wiki/Protocol_data_unithttp://en.wikipedia.org/wiki/Protocol_data_unit

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field comprising the ' 6 DDR'00 F '7D 'X&'!0 8! 9 &0 and the*:R6*8;; !D:R'0 8!0' 9 &.

. D7* )Data 7in+ *onnection dentifier 9its. &he D7* serves to identify thevirtual connection so that the receiving end +nows which information connectiona frame belongs to. !ote that this D7* has only local significance. single

physical channel can multiple- several different virtual connections.1. F'*!, 9'*!, D' bits. &hese bits report congestion"

o F'*!6Forward '-plicit *ongestion !otification bito 9'*!69ac+ward '-plicit *ongestion !otification bito D'6Discard 'ligibility bit

5. nformation Field. system parameter defines the ma-imum number of data bytes that a host can pac+ into a frame.

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X.25 specifies processing at layers , 2 and of the 80 model, while frame relayoperates at layers and 2 only. &his means that frame relay has significantly less

processing to do at each node, which improves throughput by an order of magnitude.

X.25 prepares and sends pac+ets, while frame relay prepares and sends frames. X.25 pac+ets contain several fields used for error and flow control, none of which frame relayneeds. &he frames in frame relay contain an e-panded address field that enables framerelay nodes to direct frames to their destinations with minimal processing .

X.25 has a fi-ed bandwidth available. t uses or wastes portions of its bandwidth as theload dictates. Frame relay can dynamically allocate bandwidth during call setupnegotiation at both the physical and logical channel level.

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is an nternational &elecommunication 3nion)&elecommunications 0tandards 0ection % &3)&( standard for cell relay whereininformation for multiple service types, such as voice, video, or data, is conveyed in small,fi-ed)si4e cells. &; networ+s are connection)oriented.

&; is a cell)switching and multiple-ing technology that combines the benefits of circuit switching %guaranteed capacity and constant transmission delay( with those of

pac+et switching %fle-ibility and efficiency for intermittent traffic(. t provides scalable bandwidth from a few megabits per second %;bps( to many gigabits per second %>bps(.9ecause of its asynchronous nature, &; is more efficient than synchronoustechnologies, such as time-division multiplexing (TDM) .

With &D;, each user is assigned to a time slot, and no other station can send in that timeslot. f a station has much data to send, it can send only when its time slot comes up, evenif all other time slots are empty.

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ATM Protocol architecture:

&; is almost similar to cell relay and pac+ets witching using X.25and framerelay.li+e pac+et switching and frame relay, &; involves the transfer of data in discrete pieces.also,li+e pac+et switching and frame relay , &; allows multiple logical

connections to multiple-ed over a single physical interface. in the case of &;,theinformation flow on each logical connection is organised into fi-ed)si4e pac+ets, calledcells. &; is a streamlined protocol with minimal error and flow control capabilities "thisreduces the overhead of processing &; cells and reduces the number of overhead bitsre$uired with each cell, thus enabling &; to operate at high data rates.the use of fi-ed)si4e cells simplifies the processing re$uired at each &; node,again supporting the useof &; at high data rates. &he &; architecture uses a logical model to describe thefunctionality that it supports. &; functionality corresponds to the physical layer and

part of the data lin+ layer of the 80 reference model. . the protocol referencce modelshown ma+es reference to three separate planes"

user plane provides for user information transfer ,along with associated controls%e.g.,flow control ,error control(.

control plane performs call control and connection control functions.

management plane includes plane management ,which performs management functionrelated to a system as a whole and provides coordination between all the planes ,and layer management which performs management functions relating to resource and parametersresiding in its protocol entities .

&he &; reference model is composed of the following &; layers"

# Physical layer @ nalogous to the physical layer of the 80 reference model, the&; physical layer manages the medium)dependent transmission.

# ATM layer @*ombined with the &; adaptation layer, the &; layer is roughlyanalogous to the data lin+ layer of the 80 reference model. &he &; layer is responsiblefor the simultaneous sharing of virtual circuits over a physical lin+ %cell multiple-ing(and passing cells through the &; networ+ %cell relay(. &o do this, it uses the A andA* information in the header of each &; cell.

# ATM adaptation layer (AAL) @*ombined with the &; layer, the 7 is roughly

analogous to the data lin+ layer of the 80 model. &he 7 is responsible for isolatinghigher)layer protocols from the details of the &; processes. &he adaptation layer prepares user data for conversion into cells and segments the data into 1?)byte cell payloads.

Finally, the higher layers residing above the 7 accept user data, arrange it into pac+ets,and hand it to the 7. Figure "illustrates the &; reference model.

1

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Structure of an ATM cell

n &; cell consists of a 5 byte header and a 1? byte payload. &he payload si4e of 1? bytes was a compromise between the needs of voice telephony and pac+et networ+s,obtained by a simple averaging of the 30 proposal of =1 bytes and 'uropean proposal of

2, said by some to be motivated by a 'uropean desire not to need echo)cancellers onnational trun+s.

&; defines two different cell formats" !! %!etwor+)networ+ interface( and 3!%3ser)networ+ interface(. ;ost &; lin+s use 3! cell format.

iagram o! the UNI ATM "ell

B 1 /

>F* A

A A*

A*

A* & *7

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>F* 6 >eneric Flow *ontrol %1 bits( %default" 1)4ero bits(A 6 Airtual ath dentifier %? bits 3! ( or % 2 bits !! (A* 6 Airtual channel identifier % = bits(

& 6 ayload &ype % bits(*7 6 *ell 7oss riority % )bit(

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encoded in the cell header. virtual path, in other words, is a grouping of virtualchannels which connect the same end)points. &his two layer approach results in improvednetwor+ performance. 8nce a virtual path is set up, the addition:removal of virtualchannels is straightforward

ATM Classes of Services

ATM is connection oriented and allows the user to specify the resources required on a per-connectionbasis (per SVC) dynamically. There are the fi e classes of ser ice defined for ATM (as per ATM !orum"#$ %.& specification). The 'oS parameters for these ser ice classes are summari ed in Table 1 .

$er%ice "lass &uality o! $er%ice Parameter

constant bit rate%*9R(

&his class is used for emulating circuit switching. &he cell rate isconstant with time. *9R applications are $uite sensitive to cell)delayvariation. '-amples of applications that can use *9R are telephone

traffic %i.e., n-=1 +bps(, videoconferencing, and television.

variable bit rateE non)real time%A9RE!R&(

&his class allows users to send traffic at a rate that varies with timedepending on the availability of user information. 0tatisticalmultiple-ing is provided to ma+e optimum use of networ+ resources.;ultimedia e)mail is an e-ample of A9RE!R&.

variable bit rateE real time %A9RE R&(

&his class is similar to A9RE!R& but is designed for applications thatare sensitive to cell)delay variation. '-amples for real)time A9R arevoice with speech activity detection %0 D( and interactive compressedvideo.

available bit rate% 9R(

&his class of &; services provides rate)based flow control and isaimed at data traffic such as file transfer and e)mail. lthough thestandard does not re$uire the cell transfer delay and cell)loss ratio to beguaranteed or minimi4ed, it is desirable for switches to minimi4e delayand loss as much as possible. Depending upon the state of congestionin the networ+, the source is re$uired to control its rate. &he users areallowed to declare a minimum cell rate, which is guaranteed to theconnection by the networ+.

unspecified bitrate %39R(

&his class is the catch)all, other class and is widely used today for &* : .

TechnicalParameter e!inition

cell loss ratio *7R is the percentage of cells not delivered at their destination

B

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%*7R( because they were lost in the networ+ due to congestion and buffer overflow.

cell transfer delay %*&D(

&he delay e-perienced by a cell between networ+ entry and e-it points is called the *&D. t includes propagation delays,$ueuing delays at various intermediate switches, and servicetimes at $ueuing points.

cell delayvariation%*DA(

*DA is a measure of the variance of the cell transfer delay.igabit 'thernet , , Frame Relay ,08!'& :0D< , 3;&0 :Wireless, etc.

&he main services provided by 7 % &; daptation 7ayer( are"

0egmentation and reassembly

?

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S#0e$uence number. !umbers the stream of 0 R D3s of a * *0 D3 %modulo =(. &hese$uence number is comprised of the *0 and the 0!.

CS$*onvergence sublayer indicator. 3sed for residual time stamp for cloc+ing.

SC0e$uence count. &he se$uence number for the entire *0 D3, which is generated by the

*onvergence 0ublayer.

S#0e$uence number protection. *omprised of the *R* and the ' *.

C*C*yclic redundancy chec+ calculated over the 0 R header.

+ C'ven parity chec+ calculated over the *R*.

SA* ," payload1B)byte user information field.

AAL

72 provides bandwidth)efficient transmission of low)rate, short and variable pac+etsin delay sensitive applications. t supports A9R and *9R. 72 also provides for variable payload within cells and across cells. 7 type 2 is subdivided into the*ommon art 0ublayer %* 0 ( and the 0ervice 0pecific *onvergence 0ublayer %00*0 (.

AAL CPS Pac!et

&he * 0 pac+et consists of a octet header followed by a payload. &he structure of the72 * 0 pac+et is shown in the following illustration.

* D 7 33

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*hannel associated signalling bits Facsimile demodulated control data larms 3ser state control operations.

&he following illustration gives the general sturcture of 72 00*0 &ype D3s. &heformat varies and each message has its own format according to the actual message type.

Redundancy &imestamp

;essagedependantinformation

;essagetype

*R*) /

2 1 = = / bits

AAL! ##"# Type % PD

Redundancyac+ets are sent times to ensure error correction. &he value in this field signifies the

transmission number.

&ime stamp*ounters pac+et delay variation and allows a receiver to accurately reproduce the relativetiming of successive events separated by a short interval.

;essage dependant informationac+et content that varies, depending on the message type.

;essage type&he message type code.

*R*) /&he /)bit *R*.

AAL*+,

7 :1 consists of message and streaming modes. t provides for point)to)point and point)to)multipoint % &; layer( connections. &he *onvergence 0ublayer %*0( of the

&; daptation 7ayer % 7( is divided into two parts" service specific %00*0 ( andcommon part %* *0 (. &his is illustrated in the following diagram"

7 :1 pac+ets are used to carry computer data, mainly 0;D0 traffic.

AAL*+, "P"$ P U

&he functions of the 7 :1 * *0 include connectionless networ+ layer %*lass D(,meaning no need for an 00*0G and frame relaying telecommunication service in *lass *.&he * *0 D3 is composed of the following fields"

2

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AAL%&' #A PD

0&0egment type. Aalues may be as follows"

0!0e$uence number. !umbers the stream of 0 R D3s of a * *0 D3 %modulo =(.

; D;ultiple-ing identification. &his is used for multiple-ing several 7 :1 connectionsover one &; lin+.

nformation&his field has a fi-ed length of 11 bytes and contains parts of * *0 D3.

77ength indication. *ontains the length of the 0 R 0D3 in bytes, as follows"

*R**yclic redundancy chec+.

Functions of 7 :1 0 R include identification of 0 R 0D3sG error indication andhandlingG 0 R 0D3 se$uence continuityG multiple-ing and demultiple-ing.

AAL- &he type 5 adaptation layer is a simplified version of 7 :1. t also consists of message and streaming modes, with the *0 divided into the service specific and common

part. 75 provides point)to)point and point)to)multipoint % &; layer( connections.

75 is used to carry computer data such as &* : . t is the most popular 7 and issometimes referred to as 0' 7 %simple and easy adaptation layer(.

AAL- "P"$ P U

&he 75 * *0 D3 is composed of the following fields"

nfo &railer

* *0 payload ad 33 * 7ength *R*

/)=55 5 /)1B 2 1 bytes

AAL "P"# PD

* *0&he actual information that is sent by the user. !ote that the information comes beforeany length indication %as opposed to 7 :1 where the amount of memory re$uired is+nown in advance(.

1

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adadding bytes to ma+e the entire pac+et %including control and *R*( fit into a 1?)byte

boundary.

33* *0 user)to)user indication to transfer one byte of user information.

**ommon part indicator is a filling byte %of value /(. &his field is to be used in the futurefor layer management message indication.

7ength7ength of the user information without the ad.

*R**R*) 2. 3sed to allow identification of corrupted transmission.

AAL- $A/ P U &he structure of the 75 *0 D3 is as follows"

nformation D 33 * 7ength *R*) 2

)1? /)1B 2 1 bytes

?)byte trailer

AAL #A PD

0igh1$peed LANs2mergence o! 0igh1$peed LANs

2 0ignificant trends E *omputing power of *s continues to grow rapidly E !etwor+ computing

'-amples of re$uirements E *entrali4ed server farms E ower wor+groups E

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*0; :*D*arrier 0ense ;ultiple ccess: *arrier Detection

f the medium is idle, transmit.f the medium is busy, continue to listen until the channel is idle, then transmit

immediately.f a collision is detected during transmission, immediately cease transmitting.

fter a collision, wait a random amount of time, then attempt to transmit again%repeat from step (.

;edium 8ptions at /;bpsHdata rateI Hsignaling methodI Hma- lengthI

/9ase5 E / ;bps E 5/)ohm coa-ial cable bus E ;a-imum segment length 5// meters

/9ase)& E &wisted pair, ma-imum length // meters

=

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E 0tar topology %hub or multipoint repeater at central point(

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9ridge Frame handling done in softwarenaly4e and forward one frame at a time

0tore)and)forward

7ayer 2 0witchFrame handling done in hardware;ultiple data paths and can handle multiple frames at a time*an do cut)through

7ayer 2 0witchesFlat address space9roadcast storm8nly one path between any 2 devices

0olution " subnetwor+s connected by routers0olution 2" layer switching, pac+et)forwarding logic in hardware

?

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9enefits of / >bps 'thernet over &;

!o e-pensive, bandwidth consuming conversion between 'thernet pac+ets and&; cells

!etwor+ is 'thernet, end to end plus 'thernet offers Jo0 and traffic policing capabilities approach that of

&;Wide variety of standard optical interfaces for / >bps 'thernet

Fibre *hannel2 methods of communication with processor"

E :8 channel E !etwor+ communications

Fibre channel combines both E 0implicity and speed of channel communications E Fle-ibility and interconnectivity of networ+ communications

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:8 channel

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Fibre *hannel !etwor+)8riented FacilitiesFull multiple-ing between multiple destinations

eer)to)peer connectivity between any pair of portsnternetwor+ing with other connection technologies

Fibre *hannel Re$uirementsFull duple- lin+s with 2 fibres:lin+

// ;bps E ?// ;bpsDistances up to / +m

0mall connectorshigh)capacity>reater connectivity than e-isting multidrop channels9road availability0upport for multiple cost:performance levels0upport for multiple e-isting interface command sets

Fibre *hannel rotocol rchitectureF*)/ hysical ;ediaF*) &ransmission rotocolF*)2 Framing rotocolF*) *ommon 0ervicesF*)1 ;apping

3ireless LAN /e4uirements&hroughput

!umber of nodes*onnection to bac+bone0ervice area9attery power consumption&ransmission robustness and security*ollocated networ+ operation7icense)free operation

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Unit II

Jueing analysis

n $ueueing theory , a 4ueueing model is used to appro-imate a real $ueueingsituation or system, so the $ueueing behaviour can be analysed mathematically.Jueueing models allow a number of useful steady state performance measuresto be determined, including"

the average number in the $ueue, or the system, the average time spent in the $ueue, or the system, the statistical distribution of those numbers or times, the probability the $ueue is full, or empty, and the probability of finding the system in a particular state.

&hese performance measures are important as issues or problems caused by$ueueing situations are often related to customer dissatisfaction with service or may be the root cause of economic losses in a business. nalysis of therelevant $ueueing models allows the cause of $ueueing issues to be identified

and the impact of any changes that might be wanted to be assessed.

Notation

&ueueing models can be represented using Kendall s notation "

:9:0:K:!:Disc

where" is the interarrival time distribution 9 is the service time distribution 0 is the number of servers K is the system capacity ! is the calling population Disc is the service discipline assumed

0ome standard notation for distributions % or 9( are"

; for a ;ar+ovian %e-ponential ( distribution 'L for an 'rlang distribution with L phases D for Deterministic %constant( > for >eneral distribution < for a hase)type distribution

Models

Constr"ction and analysis

Jueueing models are generally constructed to represent the steady state of a$ueueing system, that is, the typical, long run or average state of the system.

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s a conse$uence, these are stochastic models that represent the probabilitythat a $ueueing system will be found in a particular configuration or state .

general procedure for constructing and analysing such $ueueing models is"

. dentify the parameters of the system, such as the arrival rate, service time,Jueue capacity, and perhaps draw a diagram of the system.

2. dentify the system states. % state will generally represent the integer number of customers, people, Mobs, calls, messages, etc. in the system and may or maynot be limited.(

. Draw a state transition diagram that represents the possible system states andidentify the rates to enter and leave each state. &his diagram is a representationof a ;ar+ov chain .

1. 9ecause the state transition diagram represents the steady state situation between state there is a balanced flow between states so the probabilities of being in adMacent states can be related mathematically in terms of the arrivaland service rates and state probabilities.

5. '-press all the state probabilities in terms of the empty state probability, usingthe inter)state transition relationships.

=. Determine the empty state probability by using the fact that all state probabilities always sum to .

Whereas specific problems that have small finite state models are often able to be analysed numerically, analysis of more general models, using calculus ,yields useful formulae that can be applied to whole classes of problems.

Single-server #"e"e

0ingle)server $ueues are, perhaps, the most commonly encountered $ueueingsituation in real life. 8ne encounters a $ueue with a single server in manysituations, including business %e.g. sales cler+(, industry %e.g. a productionline(, transport %e.g. a bus, a ta-i ran+, an intersection(, telecommunications%e.g. &elephone line(, computing %e.g. processor sharing(. 'ven where there aremultiple servers handling the situation it is possible to consider each server individually as part of the larger system, in many cases. %e.g supermar+etchec+out has several single server $ueues that the customer can select from.(*onse$uently, being able to model and analyse a single server $ueue s

behaviour is a particularly useful thing to do.

Poisson arri%als and ser%ice

M+M+ +5+5 represents a single server that has unlimited $ueue capacity andinfinite calling population, both arrivals and service are oisson %or random( processes, meaning the statistical distribution of both the inter)arrival timesand the service times follow the e-ponential distribution. 9ecause of themathematical nature of the e-ponential distribution, a number of $uite simplerelationships are able to be derived for several performance measures based on+nowing the arrival rate and service rate.

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&his is fortunate because, an ;:;: $ueuing model can be used toappro-imate many $ueuing situations.

Poisson arri%als and general ser%ice

M+6+ +5+5 represents a single server that has unlimited $ueue capacity andinfinite calling population, while the arrival is still oisson process, meaningthe statistical distribution of the inter)arrival times still follow the e-ponential

distribution, the distribution of the service time does not. &he distribution of the service time may follow any general statistical distribution, not Muste-ponential. Relationships are still able to be derived for a %limited( number of

performance measures if one +nows the arrival rate and the mean and varianceof the service rate. : provide specific solutions that give broadinsights into the best model to choose for specific $ueueing situations becausethey permit the comparison of those solutions to the performance of an ;:;:model.

M"lti$le-servers #"e"e;ultiple %identical()servers $ueue situations are fre$uently encountered intelecommunications or a customer service environment. When modelling thesesituations care is needed to ensure that it is a multiple servers $ueue, not anetwor+ of single server $ueues, because results may differ depending on howthe $ueuing model behaves.

8ne observational insight provided by comparing $ueuing models is that asingle $ueue with multiple servers performs better than each server havingtheir own $ueue and that a single large pool of servers performs better than two

or more smaller pools, even though there are the same total number of serversin the system.

8ne simple e-ample to prove the above fact is as follows" *onsider a systemhaving ? input lines, single $ueue and ? servers.&he output line has a capacityof =1 +bit:s. *onsidering the arrival rate at each input as 2 pac+ets:s. 0o, thetotal arrival rate is = pac+ets:s. With an average of 2/// bits per pac+et, theservice rate is =1 +bit:s:2///b 6 2 pac+ets:s.

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models there is no $ueue, as such, instead each arriving customer receivesservice. When viewed from the outside, the model appears to delay or storeeach customer for some time.

ueuein! System "lassification

With 7ittle s &heorem, we have developed some basic understanding of a$ueueing system. &o further our understanding we will have to dig deeper intocharacteristics of a $ueueing system that impact its performance. For e-ample,$ueueing re$uirements of a restaurant will depend upon factors li+e"

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; %;ar+ov( '-ponential probability densityD %Deterministic( ll customers have the same value> %>eneral( ny arbitrary probability distribution

'-amples of $ueueing systems that can be defined with this convention are"

M+M+ . &his is the simplest $ueueing system to analy4e.

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7ittle s law

n$ueueing theory , Little8s result , theorem , lemma , or law says"

&he average number of customers in a stable system %over some time interval(, !, is e$ual to their average arrival rate, O, multiplied by their average time inthe system, &, or"

lthough it loo+s intuitively reasonable, it s a $uite remar+able result, as itimplies that this behavior is entirely independent of any of the detailed

probability distributions involved, and hence re$uires no assumptions about theschedule according to which customers arrive or are serviced, or whether theyare served in the order in which they arrive.

t is also a comparatively recent result ) it was first proved by ohn 7ittle , annstitute rofessor and the *hair of ;anagement 0cience at the ; & 0loan

0chool of ;anagement , in = .

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e-ists and is given by 7ittle s theorem,

Ideal Per!ormance

2

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(ffects of "on!estion

)

"on!estion*"ontrol Mechanisms

Backpressure

Request from destination to source to reduce rate Useful only on a logical connection basis Requires hop-by-hop flow control mechanism

Policing Measuring and restricting packets as they enter the network

hoke packet !pecific message back to source "#g#$ % MP !ource &uench

%mplicit congestion signaling !ource detects congestion from transmission delays and lost packets

and reduces flow

/

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(+#licit con!estion si!nalin!

Frame Relay reduces networ+ overhead by implementing simple congestion)notification mechanisms rather than e-plicit, per)virtual)circuit flow control. FrameRelay typically is implemented on reliable networ+ media, so data integrity is notsacrificed because flow control can be left to higher)layer protocols. Frame Relayimplements two congestion)notification mechanisms"

# Forward)e-plicit congestion notification %F'*!(

# 9ac+ward)e-plicit congestion notification %9'*!(

F'*! and 9'*! each is controlled by a single bit contained in the Frame Relay

frame header. &he Frame Relay frame header also contains a Discard 'ligibility %D'( bit, which is used to identify less important traffic that can be dropped during periodsof congestion.

&he ./" *it is part of the ddress field in the Frame Relay frame header. &he F'*!mechanism is initiated when a D&' device sends Frame Relay frames into thenetwor+. f the networ+ is congested, D*' devices %switches( set the value of theframes F'*! bit to . When the frames reach the destination D&' device, the

ddress field %with the F'*! bit set( indicates that the frame e-perienced congestionin the path from source to destination. &he D&' device can relay this information to ahigher)layer protocol for processing. Depending on the implementation, flow controlmay be initiated, or the indication may be ignored.

&he 0/" *it is part of the ddress field in the Frame Relay frame header. D*'devices set the value of the 9'*! bit to in frames traveling in the opposite directionof frames with their F'*! bit set. &his informs the receiving D&' device that a

particular path through the networ+ is congested. &he D&' device then can relay thisinformation to a higher)layer protocol for processing. Depending on theimplementation, flow)control may be initiated, or the indication may be ignored.

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Frame Relay Discard &ligi'ility

&he Discard /ligi*ility (D/) *it is used to indicate that a frame has lower importancethan other frames. &he D' bit is part of the ddress field in the Frame Relay frameheader.

D&' devices can set the value of the D' bit of a frame to to indicate that the framehas lower importance than other frames. When the networ+ becomes congested, D*'devices will discard frames with the D' bit set before discarding those that do not.

&his reduces the li+elihood of critical data being dropped by Frame Relay D*'devices during periods of congestion.

Frame Relay &rror Chec!ing

Frame Relay uses a common error)chec+ing mechanism +nown as the cyclicredundancy chec$ (" ") . &he *R* compares two calculated values to determinewhether errors occurred during the transmission from source to destination. FrameRelay reduces networ+ overhead by implementing error chec+ing rather than error correction. Frame Relay typically is implemented on reliable networ+ media, so dataintegrity is not sacrificed because error correction can be left to higher)layer protocolsrunning on top of Frame Relay.

Tra!!ic Management in "ongested Network 9 $ome"onsiderations

Fairness Aarious flows should sufferY e$ually 7ast)in)first)discarded may not be fair

Juality of 0ervice %Jo0(

Flows treated differently, based on need Aoice, video" delay sensitive, loss insensitive File transfer, mail" delay insensitive, loss sensitive nteractive computing" delay and loss sensitive

Reservations olicing" e-cess traffic discarded or handled on best)effort basis

Frame /elay "ongestion "ontrol

;inimi4e frame discard;aintain Jo0 %per)connection bandwidth(;inimi4e monopoli4ation of networ+ 0imple to implement, little overhead;inimal additional networ+ trafficResources distributed fairly7imit spread of congestion

2

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8perate effectively regardless of flow

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Fi-ed rate lin+ between user and networ+ %for networ+ access(*ommitted 9urst 0i4e %9c(

;a-imum data over an interval agreed to by networ+ '-cess 9urst 0i4e %9e(

;a-imum data, above 9c, over an interval that networ+ will attempt totransfer

/elationship o! "ongestion Parameters

1

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5

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Unit III

T"P Flow "ontrol3ses a form of sliding windowDiffers from mechanism used in 77*,

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0uppose last message 9 issued was ! 6 i, W 6 M

&o increase credit to + %+ I M( when no new data, 9 issues ! 6 i, W 6 + &o ac+nowledge segment containing m octets %m H M(, 9 issues ! 6 i C m, W 6 M

E m

Flow "ontrol Perspecti%es

"redit PolicyReceiver needs a policy for how much credit to give sender *onservative approach" grant credit up to limit of available buffer space;ay limit throughput in long)delay situations

B

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8ptimistic approach" grant credit based on e-pectation of freeing space beforedata arrives

2!!ect o! 3indow $i;eW 6 &* window si4e %octets(R 6 Data rate %bps( at &* sourceD 6 ropagation delay %seconds(

fter &* source begins transmitting, it ta+es D seconds for first octet to arrive,and D seconds for ac+nowledgement to return

&* source could transmit at most 2RD bits, or RD:1 octets

Normali;ed Throughput $

W I RD : 10 6

1W:RD W H RD : 1 3indow $cale Parameter

"omplicating Factors;ultiple &* connections are multiple-ed over same networ+ interface, reducingR and efficiencyFor multi)hop connections, D is the sum of delays across each networ+ plusdelays at each router

f source data rate R e-ceeds data rate on one of the hops, that hop will be a bottlenec+ 7ost segments are retransmitted, reducing throughput. mpact depends onretransmission policy

/etransmission $trategy&* relies e-clusively on positive ac+nowledgements and retransmission on

ac+nowledgement timeout

?

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&here is no e-plicit negative ac+nowledgementRetransmission re$uired when"0egment arrives damaged, as indicated by chec+sum error, causing receiver to

discard segment0egment fails to arrive

Timers timer is associated with each segment as it is sent

f timer e-pires before segment ac+nowledged, sender must retransmitKey Design ssue"value of retransmission timer

&oo small" many unnecessary retransmissions, wasting networ+ bandwidth&oo large" delay in handling lost segment

Two $trategies&imer should be longer than round)trip delay %send segment, receive ac+(Delay is variable

$trategies.

Fi-ed timer daptive

Pro7lems with Adapti%e $chemeeer &* entity may accumulate ac+nowledgements and not ac+nowledge

immediatelyFor retransmitted segments, canZt tell whether ac+nowledgement is response to

original transmission or retransmission !etwor+ conditions may change suddenly

Adapti%e /etransmission Timerverage Round)&rip &ime % R&&(

K C R&&%K C ( 6 [ R&&%i(

K C i 6

6 K R&%K( C R&&%K C ( K C K C

/F"

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/F"

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T"P $egment Pacing

T"P Flow and "ongestion "ontrol

/etransmission Timer Management&hree &echni$ues to calculate retransmission timer %R&8("

R&& Aariance 'stimation'-ponential R&8 9ac+off KarnZs lgorithm

1

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/TT #ariance 2stimation(>aco7son?s Algorithm)

sources of high variance in R&&f data rate relative low, then transmission delay will be relatively large, with

larger variance due to variance in pac+et si4e7oad may change abruptly due to other sources

eer may not ac+nowledge segments immediately

>aco7son?s Algorithm0R&&%K C ( 6 % E g( \ 0R&&%K( C g \ R&&%K C (

0'RR%K C ( 6 R&&%K C ( E 0R&&%K(

0D'A%K C ( 6 % E h( \ 0D'A%K( C h \]0'RR%K C (]

R&8%K C ( 6 0R&&%K C ( C f \ 0D'A%K C (

g 6 /. 25h 6 /.25f 6 2 or f 6 1 %most current implementations use f 6 1(

Two @ther FactorsacobsonZs algorithm can significantly improve &* performance, but"

What R&8 to use for retransmitted segmentsQ !0W'R" e-ponential R&8 bac+off algorithm

Which round)trip samples to use as input to acobsonZs algorithmQ!0W'R" KarnZs algorithm

2:ponential /T@ 'acko!! ncrease R&8 each time the same segment retransmitted E bac+off process

;ultiply R&8 by constant"R&8 6 $ \ R&8

$ 6 2 is called binary e-ponential bac+off 3hich /ound1trip $amples

f an ac+ is received for retransmitted segment, there are 2 possibilities"c+ is for first transmission

c+ is for second transmission&* source cannot distinguish 2 cases

!o valid way to calculate R&&" E From first transmission to ac+, or E From second transmission to ac+Q

12

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E Barn?s AlgorithmDo not use measured R&& to update 0R&& and 0D'A*alculate bac+off R&8 when a retransmission occurs3se bac+off R&8 for segments until an ac+ arrives for a segment that has not been

retransmitted&hen use acobsonZs algorithm to calculate R&8

3indow Management0low startDynamic window si4ing on congestionFast retransmitFast recovery7imited transmit

$low $tartawnd 6 ; !T credit, cwndUwhereawnd 6 allowed window in segmentscwnd 6 congestion window in segmentscredit 6 amount of unused credit granted in most recent ac+ cwnd 6 for a new connection and increased by for each ac+ received, up to ama-imum

2!!ect o! $low $tart

1

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ynamic 3indow $i;ing on "ongestion lost segment indicates congestionrudent to reset cwsd 6 and begin slow start process

;ay not be conservative enough" easy to drive a networ+ into saturation buthard for the net to recoverY % acobson(

nstead, use slow start with linear growth in cwnd

Illustration o! $low $tart and "ongestion A%oidance

Fast /etransmitR&8 is generally noticeably longer than actual R&&

f a segment is lost, &* may be slow to retransmit&* rule" if a segment is received out of order, an ac+ must be issued immediately

for the last in)order segmentFast Retransmit rule" if 1 ac+s received for same segment, highly li+ely it was

lost, so retransmit immediately, rather than waiting for timeoutFast /eco%ery

When &* retransmits a segment using Fast Retransmit, a segment was assumedlost

*ongestion avoidance measures are appropriate at this point'.g., slow)start:congestion avoidance procedure&his may be unnecessarily conservative since multiple ac+s indicate segments are

getting throughFast Recovery" retransmit lost segment, cut cwnd in half, proceed with linear

increase of cwnd&his avoids initial e-ponential slow)start

11

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Limited Transmitf congestion window at sender is small, fast retransmit may not get triggered,

e.g., cwnd 6 3nder what circumstances does sender have small congestion windowQ

s the problem commonQf the problem is common, why not reduce number of duplicate ac+s needed to

trigger retransmitQLimited Transmit Algorithm0ender can transmit new segment when conditions are met"

&wo consecutive duplicate ac+s are receivedDestination advertised window allows transmission of segment

mount of outstanding data after sending is less than or e$ual to cwnd C 2Per!ormance o! T"P o%er ATM

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pac+et si4es of 5 2 octets to ?/&* window si4es from ? Kbytes to =1 Kbytes

&; switch buffer si4e per port from 25= cells to ?///8ne)to)one mapping of &* connections to &; virtual circuits&* sources have infinite supply of data ready

@7ser%ationsf a single cell is dropped, other cells in the same datagram are unusable, yet

&; networ+ forwards these useless cells to destination

0maller buffer increase probability of dropped cells7arger segment si4e increases number of useless cells transmitted if a single celldroppedPartial Packet and 2arly Packet iscard

Reduce the transmission of useless cellsWor+ on a per)virtual circuit basis

artial ac+et Discard E f a cell is dropped, then drop all subse$uent cells in that segment %i.e., loo+ forcell with 0D3 type bit set to one(

'arly ac+et Discard E When a switch buffer reaches a threshold level, preemptively discard all cells in asegment$electi%e rop

deally, !:A cells buffered for each of the A virtual circuitsW%i( 6 !%i( 6 !%i( \ A

!:A !f ! I R and W%i( I _

then drop ne-t new pac+et on A* i_ is a parameter to be chosen

ATM $witch 'u!!er Layout

Fair 'u!!er Allocation;ore aggressive dropping of pac+ets as congestion increasesDrop new pac+et when"

1=

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! I R and W%i( I _ \ 9 E R ! ) R T"P o%er A'/

>ood performance of &* over 39R can be achieved with minor adMustments toswitch mechanisms

&his reduces the incentive to use the more comple- and more e-pensive 9Rservice

erformance and fairness of 9R $uite sensitive to some 9R parameter settings8verall, 9R does not provide significant performance over simpler and less

e-pensive 39R)' D or 39R)' D)F9

Tra!!ic and "ongestion "ontrol in ATM NetworksIntroduction

*ontrol needed to prevent switch buffer overflowuaranteed Frame Rate %>FR(

/e4uirements !or ATM Tra!!ic and "ongestion "ontrol;ost pac+et switched and frame relay networ+s carry non)real)time bursty data

E !o need to replicate timing at e-it node E 0imple statistical multiple-ing E 3ser !etwor+ nterface capacity slightly greater than average of channels

*ongestion control tools from these technologies do not wor+ in &;

Pro7lems with ATM "ongestion "ontrol;ost traffic not amenable to flow control E Aoice ` video can not stop generating

Feedbac+ slow E 0mall cell transmission time v propagation delay

Wide range of applications E From few +bps to hundreds of ;bps E Different traffic patterns

1B

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E Different networ+ services

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!o flow control or error control processing E &; switches have e-tremely high throughput E *ongestion can cause cell delay variation

9uild up of $ueuing effects at switches&otal load accepted by networ+ must be controlled

"ell elay #ariation at UNI*aused by processing in three layers of &; model

E 0ee ne-t slide for details

!one of these delays can be predicted !one follow repetitive pattern0o, random element e-ists in time interval between reception by &; stac+ andtransmission

ATM Tra!!ic1/elated Attri7utes0i- service categories %see chapter 5(

E *onstant bit rate %*9R( E Real time variable bit rate %rt)A9R( E !on)real)time variable bit rate %nrt)A9R( E 3nspecified bit rate %39R( E vailable bit rate % 9R(

E >uaranteed frame rate %>FR(*haracteri4ed by &; attributes in four categories

E &raffic descriptors E Jo0 parameters E *ongestion E 8ther

Tra!!ic Parameters

&raffic pattern of flow of cells E ntrinsic nature of traffic

0ource traffic descriptor E ;odified inside networ+

*onnection traffic descriptor $ource Tra!!ic escriptor

ea+ cell rate E 3pper bound on traffic that can be submitted E Defined in terms of minimum spacing between cells T E *R 6 : T E ;andatory for *9R and A9R services

0ustainable cell rate E 3pper bound on average rate E *alculated over large time scale relative to T E Re$uired for A9R E 'nables efficient allocation of networ+ resources between A9R sources E 8nly useful if 0*R H *R

;a-imum burst si4e E ;a- number of cells that can be sent at *R E f bursts are at ;90, idle gaps must be enough to +eep overall rate below

0*R

1

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E Re$uired for A9R ;inimum cell rate

E ;in commitment re$uested of networ+ E *an be 4ero E 3sed with 9R and >FR E 9R ` >FR provide rapid access to spare networ+ capacity up to *R E *R E ;*R represents elastic component of data flow E 0hared among 9R and >FR flows

;a-imum frame si4e E ;a- number of cells in frame that can be carried over >FR connection E 8nly relevant in >FR

"onnection Tra!!ic escriptorncludes source traffic descriptor plus")

*ell delay variation tolerancemount of variation in cell delay introduced by networ+ interface and 3!

9ound on delay variability due to slotted nature of &;, physical layeroverhead and layer functions %e.g. cell multiple-ing(

Represented by time variable *onformance definition

0pecify conforming cells of connection at 3!'nforced by dropping or mar+ing cells over definition

&uality o! $er%ice Parameters1ma:"T*ell transfer delay %*&D(

&ime between transmission of first bit of cell at source and reception of last bit at destination

&ypically has probability density function %see ne-t slide(Fi-ed delay due to propagation etc.*ell delay variation due to buffering and scheduling;a-imum cell transfer delay %ma-*&D(is ma- re$uested delay for connectionFraction S of cells e-ceed threshold

Discarded or delivered latePeak1to1peak " # C "L/

ea+)to)pea+ *ell Delay AariationRemaining % )S( cells within Jo0Delay e-perienced by these cells is between fi-ed delay and ma-*&D&his is pea+)to)pea+ *DA*DA& is an upper bound on *DA

*ell loss ratioRatio of cells lost to cells transmitted

"ell Trans!er elay P F

5/

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"ongestion "ontrol Attri7utes8nly feedbac+ is defined

9R and >FR

ctions ta+en by networ+ and end systems to regulate traffic submitted9R flow control

daptively share available bandwidth@ther Attri7utes

9ehaviour class selector %9*0( E 0upport for differentiated services %chapter =( E rovides different service levels among 39R connections E ssociate each connection with a behaviour class E ;ay include $ueuing and scheduling

;inimum desired cell rate

Tra!!ic Management Framework 8bMectives of &; layer traffic and congestion control

E 0upport Jo0 for all foreseeable services E !ot rely on networ+ specific 7 protocols nor higher layer application

specific protocols E ;inimi4e networ+ and end system comple-ity E ;a-imi4e networ+ utili4ation

Timing Le%els*ell insertion timeRound trip propagation time*onnection duration7ong term

Tra!!ic "ontrol and "ongestion Functions

5

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Tra!!ic "ontrol $trategyDetermine whether new &; connection can be accommodated

gree performance parameters with subscriber &raffic contract between subscriber and networ+ &his is congestion avoidance

f it fails congestion may occur E nvo+e congestion control

Tra!!ic "ontrolResource management using virtual paths*onnection admission control3sage parameter control

0elective cell discard&raffic shaping'-plicit forward congestion indication

/esource Management Using #irtual Pathsllocate resources so that traffic is separated according to service characteristics

Airtual path connection %A *( are groupings of virtual channel connections%A**(

Applications3ser)to)user applications

E A * between 3! pair E !o +nowledge of Jo0 for individual A** E 3ser chec+s that A * can ta+e A**sZ demands

3ser)to)networ+ applications E A * between 3! and networ+ node E !etwor+ aware of and accommodates Jo0 of A**s

!etwor+)to)networ+ applications E A * between two networ+ nodes E !etwor+ aware of and accommodates Jo0 of A**s

52

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/esource Management "oncerns*ell loss ratio;a- cell transfer delay

ea+ to pea+ cell delay variationll affected by resources devoted to A *f A** goes through multiple A *s, performance depends on consecutive A *s

and on node performance E A * performance depends on capacity of A * and traffic characteristicsof A**s

E A** related function depends on switching:processing speed and priority

#""s and #P"s "on!iguration

Allocation o! "apacity to #P"ggregate pea+ demand

E ;ay set A * capacity %data rate( to total of A** pea+ rates'ach A** can give Jo0 to accommodate pea+ demandA * capacity may not be fully used

0tatistical multiple-ing E A * capacity I6 average data rate of A**s but H aggregate pea+ demand E >reater *DA and *&D E ;ay have greater *7R

E ;ore efficient use of capacity E For A**s re$uiring lower Jo0 E >roup A**s of similar traffic together

"onnection Admission "ontrol3ser must specify service re$uired in both directions

E *ategory

5

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E *onnection traffic descriptor 0ource traffic descriptor *DA&Re$uested conformance definition

E Jo0 parameter re$uested and acceptable value !etwor+ accepts connection only if it can commit resources to support re$uestsProcedures to $et Tra!!ic "ontrol Parameters

"ell Loss Priority&wo levels re$uested by user

E riority for individual cell indicated by *7 bit in header E f two levels are used, traffic parameters for both flows specified

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Peak "ell /ate Algorithmeneric cell rate algorithm E 7ea+y buc+et algorithm E

6eneric "ell /ate Algorithm

#irtual $cheduling Algorithm

55

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Leaky 'ucket Algorithm

"ontinuous Leaky 'ucket Algorithm

$ustaina7le "ell /ate Algorithm8perational definition of relationship between sustainable cell rate and bursttolerance3sed by 3 * to monitor compliance0ame algorithm as pea+ cell rate

UP" Actions*ompliant cell pass, non)compliant cells discarded

5=

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f no additional resources allocated to *7 6 traffic, *7 6/ cells *f two level cell loss priority cell with"

E *7 6/ and conforms passes E *7 6/ non)compliant for *7 6/ traffic but compliant for *7 6/C is

tagged and passes E *7 6/ non)compliant for *7 6/ and *7 6/C traffic discarded E *7 6 compliant for *7 6/C passes E *7 6 non)compliant for *7 6/C discarded

Possi7le Actions o! UP"

2:plicit Forward "ongestion Indication'ssentially same as frame relay

f node e-periencing congestion, set forward congestion indication is cell headers E &ells users that congestion avoidance should be initiated in this direction E 3ser may ta+e action at higher level

A'/ Tra!!ic ManagementJo0 for *9R, A9R based on traffic contract and 3 * described previously

!o congestion feedbac+ to source8pen)loop control

!ot suited to non)real)time applications E File transfer, web access, R *, distributed file systems E !o well defined traffic characteristics e-cept *R E *R not enough to allocate resources

3se best efforts or closed)loop control

'est 2!!orts0hare unused capacity between applications

s congestion goes up" E *ells are lost E 0ources bac+ off and reduce rate E Fits well with &* techni$ues %chapter 2(

5B

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E nefficient*ells dropped causing re)transmission

"losed1Loop "ontrol0ources share capacity not used by *9R and A9R

rovide feedbac+ to sources to adMust loadvoid cell loss

0hare capacity fairly3sed for 9R

"haracteristics o! A'/ 9R connections share available capacity

E ccess instantaneous capacity unused by *9R:A9R E ncreases utili4ation without affecting *9R:A9R Jo0

0hare used by single 9R connection is dynamic E Aaries between agreed ;*R and *R

!etwor+ gives feedbac+ to 9R sources E 9R flow limited to available capacity E 9uffers absorb e-cess traffic prior to arrival of feedbac+

7ow cell loss E ;aMor distinction from 39R

Feed7ack Mechanisms*ell transmission rate characteri4ed by"

E llowable cell rate*urrent rate

E ;inimum cell rate;in for *R ;ay be 4ero

E ea+ cell rate;a- for *R

E nitial cell rate0tart with *R6 *R

dMust *R based on feedbac+ Feedbac+ in resource management %R;( cells

E *ell contains three fields for feedbac+ *ongestion indicator bit %* (

!o increase bit %! ('-plicit cell rate field %'R(

$ource /eaction to Feed7ack f * 6

E Reduce *R by amount proportional to current *R but not less than *R

'lse if ! 6/ E ncrease *R by amount proportional to *R but not more than *R

f *RI'R set *RH)ma-T'R,;*RU

"ell Flow on A'/ &wo types of cell

E Data ` resource management %R;(0ource receives regular R; cells

5?

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E Feedbac+ 9ul+ of R; cells initiated by source

E 8ne forward R; cell %FR;( per %!rm) ( data cells !rm preset E usually 2

E 'ach FR; is returned by destination as bac+wards R; %9R;( cell E FR; typically * 6/, ! 6/ or 'R desired transmission rate in range

*RH6'RH6 *R E ny field may be changed by switch or destination before return

ATM $witch /ate "ontrol Feed7ack 'F* mar+ing

'-plicit forward congestion indication*auses destination to set * bit in 'R;

Relative rate mar+ing0witch directly sets * or ! bit of R;

f set in FR;, remains set in 9R;Faster response by setting bit in passing 9R;Fastest by generating new 9R; with bit set

'-plicit rate mar+ing0witch reduces value of 'R in FR; or 9R;

Flow o! ata and /M "ells

A/' Feed7ack % T"P A"B 9R feedbac+ controls rate of transmission

E Rate control&* feedbac+ controls window si4e

E *redit controlR9 feedbac+ from switches or destination

&* feedbac+ from destination only

/M "ell Format

5

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/M "ell Format Notes&; header has &6 / to indicate R; cell

8n virtual channel A and A* same as data cells on connection8n virtual path A same, A* 6=

rotocol id identifies service using R; % R96 (;essage type

E Direction FR;6/, 9R;6 E 9'*! cell. 0ource %9!6/( or switch:destination %9!6 ( E * %6 for congestion( E ! %6 for no increase( E Re$uest: c+nowledge %not used in &; forum spec(

A/' Parameters

=/

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A/' "apacity Allocation

&; switch must perform"*ongestion control

;onitor $ueue lengthFair capacity allocation

&hrottle bac+ connections using more than fair share&; rate control signals are e-plicit

&* are implicitncreasing delay and cell loss

"ongestion "ontrol Algorithms1'inary Feed7ack 3se only 'F* , * and ! bits0witch monitors buffer utili4ation

When congestion approaches, binary notification E 0et 'F* on forward data cells or * or ! on FR; or 9R;

&hree approaches to which to notify E 0ingle F F8 $ueue E ;ultiple $ueues E Fair share notification

$ingle FIF@ &ueue

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When buffer use e-ceeds threshold %e.g. ?/ ( E 0witch starts issuing binary notifications E *ontinues until buffer use falls below threshold E *an have two thresholds

8ne for start and one for stop0tops continuous on:off switching

E 9iased against connections passing through more switchesMultiple &ueues

0eparate $ueue for each A* or group of A*s0eparate threshold on each $ueue8nly connections with long $ueues get binary notifications

E Fair E 9adly behaved source does not affect other A*s E Delay and loss behaviour of individual A*s separated

*an have different Jo0 on different A*sFair $hare

0elective feedbac+ or intelligent mar+ing&ry to allocate capacity dynamically

'.g.fairshare 6%target rate(:%number of connections(;ar+ any cells where **RIfairshare

2:plicit /ate Feed7ack $chemes*ompute fair share of capacity for each A*Determine current load or congestion*ompute e-plicit rate %'R( for each connection and send to source&hree algorithms

E 'nhanced proportional rate control algorithm

' R* E '-plicit rate indication for congestion avoidance'R *

E *ongestion avoidance using proportional control* *

2nhanced Proportional /ate "ontrol Algorithm(2P/"A0witch trac+s average value of current load on each connection

E ;ean allowed cell rate %; R*( E ; *R% (6% )S( %; *R% ) ( C S **R% ( E **R% ( is **R field in th FR; E &ypically S6 : =

E 9ias to past values of **R over current E >ives estimated average load passing through switch E f congestion, switch reduces each A* to no more than D F ; *R

D F6down pressure factor, typically B:?'RH)minT'R, D F ; *RU

Load FactordMustments based on load factor

7F6 nput rate:target rate

=2

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E nput rate measured over fi-ed averaging interval E &arget rate slightly below lin+ bandwidth %?5 to / ( E 7FI congestion threatened

A*s will have to reduce rate

2:plicit /ate Indication !or "ongestion A%oidance (2/I"A)ttempt to +eep 7F close to

Define"

fairshare 6 %target rate(:%number of connections(A*share 6 **R:7F 6 %**R:% nput Rate(( %&arget Rate('R * selectively adMusts A* rates

E &otal 'R allocated to connections matches target rate E llocation is fair E 'R 6 ma-Tfairshare, A*shareU E A*s whose A*share is less than their fairshare get greater increase

"ongestion A%oidance Using Proportional "ontrol ("AP")f 7FH fairshareH)fairshare minT'R3, C% )7F( RupUf 7FI fairshareH)fairshare minT'R3, )% )7F( RdnU

'R3I , determines ma- increaseRup between /./25 and /. , slope parameter Rdn, between /.2 and /.?, slope parameter 'RF typically /.5, ma- decrease in allottment of fair share

f fairshare H 'R value in R; cells, 'RH)fairshare0impler than 'R **an show large rate oscillations if R F %Rate increase factor( too high*an lead to unfairness6/F @%er%iew0imple as 39R from end system view

E 'nd system does no policing or traffic shaping E ;ay transmit at line rate of &; adaptor

;odest re$uirements on &; networ+ !o guarantee of frame delivery

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Mechanisms !or supporting /ate 6uarantees&agging and policing9uffer management0cheduling

Tagging and Policing&agging identifies frames that conform to contract and those that donZt

E *7 6 for those that donZt

0et by networ+ element doing conformance chec+ ;ay be networ+ element or source showing less important frames E >et lower Jo0 in buffer management and scheduling E &agged cells can be discarded at ingress to &; networ+ or subse$uent

switch E Discarding is a policing function

'u!!er Management&reatment of cells in buffers or when arriving and re$uiring buffering

f congested %high buffer occupancy( tagged cells discarded in preference tountaggedDiscard tagged cell to ma+e room for untagged cell

;ay buffer per)A*Discards may be based on per $ueue thresholds$cheduling>ive preferential treatment to untagged cells0eparate $ueues for each A*

E er A* scheduling decisions E '.g. F F8 modified to give *7 6/ cells higher priority

0cheduling between $ueues controls outgoing rate of A*s E ndividual cells get fair allocation while meeting traffic contract

"omponents o! 6F/ Mechanism

=1

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6F/ "on!ormance e!inition3 * function

E 3 * monitors A* for traffic conformance E &ag or discard non)conforming cells

Frame conforms if all cells in frame conform

E Rate of cells within contract>eneric cell rate algorithm *R and *DA& specified forconnection

E ll cells have same *7 E Within ma-imum frame si4e %;F0(

&o$ 2ligi7ility Test&est for contract conformance

E Discard or tag non)conforming cells7oo+ing at upper bound on traffic

E Determine frames eligible for Jo0 guarantee3nder >FR contract for A*

7oo+ing at lower bound for trafficFrames are one of"

E !onconforming" cells tagged or discarded E *onforming ineligible" best efforts E *onforming eligible" guaranteed delivery

$impli!ied Frame 'ased 6"/A

=5

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Unit I#Integrated and i!!erentiated $er%ices

Introduction !ew additions to nternet increasing traffic

E raphics E Real time voice and video

!eed to manage traffic and control congestion'F& standards

E ntegrated services*ollective service to set of traffic demands in domain

E 7imit demand ` reserve resources E Differentiated services

*lassify traffic in groupsDifferent group traffic handled differently

Integrated $er%ices Architecture (I$A)v1 header fields for precedence and type of service usually ignored

&; only networ+ designed to support &* , 3D and real)time traffic E ;ay need new installation !eed to support Juality of 0ervice %Jo0( within &* :

E dd functionality to routers E ;eans of re$uesting Jo0Internet Tra!!ic 9 2lastic

*an adMust to changes in delay and throughput'.g. common &* and 3D application

E ');ail E insensitive to delay changes E F& E 3ser e-pect delay proportional to file si4e

0ensitive to changes in throughput E 0!; E delay not a problem, e-cept when caused by congestion E Web %

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Internet Tra!!ic 9 InelasticDoes not easily adapt to changes in delay and throughput

E Real time traffic&hroughput

E ;inimum may be re$uiredDelay

E '.g. stoc+ tradingitter ) Delay variation

E ;ore Mitter re$uires a bigger buffer E '.g. teleconferencing re$uires reasonable upper boundac+et loss

Inelastic Tra!!ic Pro7lemsDifficult to meet re$uirements on networ+ with variable $ueuing delays and

congestion !eed preferential treatment

pplications need to state re$uirements E head of time %preferably( or on the fly E 3sing fields in header E Resource reservation protocol

;ust still support elastic traffic E Deny service re$uests that leave too few resources to handle elastic trafficdemands

I$A Approachrovision of Jo0 over

0haring available capacity when congestedRouter mechanisms

E Routing lgorithms0elect to minimi4e delay

E ac+et discard*auses &* sender to bac+ off and reduce load'nahnced by 0

Flow pac+et can be associated with a flow

E Distinguishable stream of related pac+ets E From single user activity E Re$uiring same Jo0 E '.g. one transport connection or one video stream E 3nidirectional E *an be more than one recipient

;ulticast E ;embership of flow identified by source and destination address, portnumbers, protocol type

E v= header flow identifier can be used but isnot necessarily e$uivalent to 0flowI$A Functions

dmission control E For Jo0, reservation re$uired for new flow

=B

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E R0A usedRouting algorithm

E 9ase decision on Jo0 parametersJueuing discipline

E &a+e account of different flow re$uirementsDiscard policy

E ;anage congestion E ;eet Jo0

I$A Implementation in /outer9ac+ground Functions

Forwarding functions

I$A "omponents 9 'ackground FunctionsReservation rotocol

E R0Admission control

;anagement agent E *an use agent to modify traffic control database and direct admission control

Routing protocol

I$A "omponents 9 Forwarding*lassifier and route selection

E ncoming pac+ets mapped to classes0ingle flow or set of flows with same Jo0

E '.g. all video flows9ased on header fields

E Determines ne-t hopac+et scheduler

=?

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E ;anages one or more $ueues for each output E 8rder $ueued pac+ets sent

9ased on class, traffic control database, current and past activity on outgoing port E olicing

I$A $er%ices&raffic specification %&0pec( defined as service for flow8n two levels

E >eneral categories of service>uaranteed*ontrolled load9est effort %default(

E articular flow within category&0pec is part of contract

Token 'ucket;any traffic sources can be defined by to+en buc+et scheme

rovides concise description of load imposed by flow

E 'asy to determine resource re$uirementsrovides input parameters to policing function

Token 'ucket iagram

I$A $er%ices 9 6uaranteed $er%ice

ssured capacity level or data rate0pecific upper bound on $ueuing delay through networ+

E ;ust be added to propagation delay or latency to get total delay E 0et high to accommodate rare long $ueue delays

!o $ueuing losses E .e. no buffer overflow

'.g. Real time play bac+ of incoming signal can use delay buffer for incomingsignal but will not tolerate pac+et loss

=

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I$A $er%ices 9"ontrolled Load

&ightly appro-imates to best efforts under unloaded conditions !o upper bound on $ueuing delay

E reedy &* connection can crowd out altruistic connections E f one connection does not bac+ off, others may bac+ off moreFair &ueuing (F&)

;ultiple $ueues for each port E 8ne for each source or flow E Jueues services round robin E 'ach busy $ueue %flow( gets e-actly one pac+et per cycle E 7oad balancing among flows E !o advantage to being greedy

our $ueue gets longer, increasing your delay E 0hort pac+ets penali4ed as each $ueue sends one pac+et per cycle

FIF@ and F&

Processor $haring;ultiple $ueues as in FJ0end one bit from each $ueue per round

E 7onger pac+ets no longer get an advantage*an wor+ out virtual %number of cycles( start and finish time for a given pac+et

B/

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ood appro-imation to performance of 0

E &hroughput and delay converge as time increases

"omparison o! FIF@D F& and '/F&

6enerali;ed Processor $haring (6P$)9RFJ can not provide different capacities to different flows'nhancement called Weighted fair $ueue %WFJ(From 0, allocate weighting to each flow that determines how many bots are sent

during each round E f weighted 5, then 5 bits are sent per round

>ives means of responding to different service re$uests>uarantees that delays do not e-ceed bounds

3eighted Fair &ueue'mulates bit by bit > 00ame strategy as 9RFJ

B

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FIF@ % 3F&

Proacti%e Packet iscard*ongestion management by proactive pac+et discard

E 9efore buffer full E 3sed on single F F8 $ueue or multiple $ueues for elastic traffic E '.g. Random 'arly Detection %R'D(

/andom 2arly etection (/2 )Moti%ation

0urges fill buffers and cause discards8n &* this is a signal to enter slow start phase, reducing load

E 7ost pac+ets need to be resent

B2

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dds to load and delay E >lobal synchroni4ation

&raffic burst fills $ueues so pac+ets lost;any &* connections enter slow start&raffic drops so networ+ under utili4ed*onnections leave slow start at same time causing burst9igger buffers do not help&ry to anticipate onset of congestion and tell one connection to slow down

/2 esign 6oals*ongestion avoidance>lobal synchroni4ation avoidance

E *urrent systems inform connections to bac+ off implicitly by dropping pac+etsvoidance of bias to bursty traffic

E Discard arriving pac+ets will do this9ound on average $ueue length

E

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B1

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i!!erentiated $er%ices ( $)

0 and R0A comple- to deploy;ay not scale well for large volumes of traffic

E mount of control signals E ;aintenance of state information at routers

D0 architecture designed to provide simple, easy to implement, low overheadtool

E 0upport range of networ+ servicesDifferentiated on basis of performance

"haracteristics o! $3se v1 header &ype of 0ervice or v= &raffic *lass field

E !o change to 0ervice level agreement %07 ( established between provider %internet

domain( and customer prior to use of D0 E D0 mechanisms not needed in applications

9uild in aggregation E ll traffic with same D0 field treated same

'.g. multiple voice connections E D0 implemented in individual routers by $ueuing and forwarding based onD0 field

0tate information on flows not saved by routers

$er%ices rovided within D0 domain E *ontiguous portion of nternet over which consistent set of D0 policiesadministered

E &ypically under control of one administrative entityDefined in 07

E *ustomer may be user organi4ation or other D0 domain E ac+et class mar+ed in D0 field

0ervice provider configures forwarding policies routers E 8ngoing measure of performance provided for each class

D0 domain e-pected to provide agreed service internally

f destination in another domain, D0 domain attempts to forward pac+etsthrough other domains E ppropriate service level re$uested from each domain$LA Parameters

Detailed service performance parameters E &hroughput, drop probability, latency

*onstraints on ingress and egress points E ndicate scope of service

B5

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&raffic profiles to be adhered to E &o+en buc+et

Disposition of traffic in e-cess of profile2:ample $er%ices

Jualitative E " 7ow latency E 9" 7ow loss

Juantitative

E *" / in)profile traffic delivered with no more than 5/ms latency E D" 5 in)profile traffic delivered

;i-ed E '" &wice bandwidth of F E F" &raffic with drop precedence X has higher delivery probability than thatwith drop precedence D0 Field Detail

7eftmost = bits are D0 codepoint E =1 different classes available E pools

-----/ " reserved for standards

E ////// " default pac+et class E ---/// " reserved for bac+wards compatibility with v1 &80

---- " reserved for e-perimental or local use----/ " reserved for e-perimental or local use but may be allocated for

future standards if neededRightmost 2 bits unused

"on!iguration iagram

"on!iguration 9 Interior /outersDomain consists of set of contiguous routers

nterpretation of D0 codepoints within domain is consistent

B=

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nterior nodes %routers( have simple mechanisms to handle pac+ets based oncodepoints

E Jueuing gives preferential treatment depending on codepointer

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Drop out pac+ets before in pac+ets if necessaryDifferent levels of service because different number of in pac+ets for each user

P0' 1 Assured ForwardingFour classes defined

E 0elect one or more to meet re$uirementsWithin class, pac+ets mar+ed by customer or provider with one of three drop

precedence values

E 3sed to determine importance when dropping pac+ets as result of congestion

"odepoints !or AF P0'

B?

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Unit #Protocols !or &o$ $upport

Increased emands !eed to incorporate bursty and stream traffic in &* : architecturencrease capacity

E Faster lin+s, switches, routers E ntelligent routing policies E 'nd)to)end flow control

;ulticastingJuality of 0ervice %Jo0( capability&ransport protocol for streaming

/esource /eser%ation 1 Unicastrevention as well as reaction to congestion re$uired

*an do this by resource reservation3nicast

E 'nd users agree on Jo0 for tas+ and re$uest from networ+ E ;ay reserve resources E Routers pre)allocate resources E f Jo0 not available, may wait or try at reduced Jo0

/esource /eser%ation 9 Multicast>enerate vast traffic

E

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Receivers can select one of multiple sources %channel selection(Deal gracefully with changes in routes

E Re)establish reservations*ontrol protocol overhead ndependent of routing protocol

/$#P "haracteristics3nicast and ;ulticast0imple-

E 3nidirectional data flow

E 0eparate reservations in two directionsReceiver initiated E Receiver +nows which subset of source transmissions it wants

;aintain soft state in internet E Responsibility of end users

roviding different reservation styles E 3sers specify how reservations for groups are aggregated

&ransparent operation through non)R0A routers0upport v1 %&o0 field( and v= %Flow label field(

ata Flows 1 $essionData flow identified by destinationResources allocated by router for duration of sessionDefined by

E Destination address3nicast or multicast

E protocol identifier &* , 3D etc.

E Destination port;ay not be used in multicast

Flow escriptorReservation Re$uest

E Flow specDesired Jo03sed to set parameters in nodeZs pac+et scheduler 0ervice class, Rspec %reserve(, &spec %traffic(

E Filter spec0et of pac+ets for this reservation0ource address, source prot

Treatment o! Packets o! @ne $ession at @ne /outer

?/

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/$#P @peration iagram

/$#P @peration> , >2, > members of multicast group0 , 02 sources transmitting to that group has reservation filter spec including 0 and 02> , >2 from 0 only

?

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R delivers from 02 to > but does not forward to R1> , >2 send R0A re$uest with filter e-cluding 02> , >2 only members of group reached through R1

E R1 doesnZt need to forward pac+ets from this session E R1 merges filter spec re$uests and sends to R

R no longer forwards this sessionZs pac+ets to R1 E

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'-plicit list of sendersFF%0 J , 02 J2 ,^(Aideo distribution

$hared 2:plicit $tyle0ingle reservation shared among specific list of senders0'%0 , 02, 0 , ^ J (

;ulticast applications with multiple data sources but unli+ely to transmitsimultaneously

/$#P Protocol Mechanisms&wo message types

E Resv8riginate at multicast group receivers

ropagate upstream;erged and pac+et when appropriate*reate soft states

Reach sender E llow host to set up traffic control for first hop E ath

rovide upstream routing informationssued by sending hosts

&ransmitted through distribution tree to all destinations

/$#P 0ost Model

Summary

R0A is a transport layer protocol that enables a networ+ to provide differentiated levelsof service to specific flows of data. 8stensibly, different application types have different

performance re$uirements. R0A ac+nowledges these differences and provides themechanisms necessary to detect the levels of performance re$uired by different appli)cations and to modify networ+ behaviors to accommodate those re$uired levels. 8vertime, as time and latency)sensitive applications mature and proliferate, R0A scapabilities will become increasingly important.

?

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Revie, uestions

&E s it necessary to migrate a2ay from your existing routing protocol to support #3P4

AE R0A is not a routing protocol. nstead, it was designed to wor+ in conMunction withe-isting routing protocols. &hus, it is not necessary to migrate to a new routing protocolto support R0A .

&E dentify the three #3P levels of service5 and explain the difference among them6

AE R0A s three levels of service include best)effort, rate)sensitive, and delay)sensitiveservice. 9est)effort service is used for applications that re$uire reliable delivery ratherthan a timely delivery. Rate)sensitive service is used for any traffic that is sensitive tovariation in the amount of bandwidth available. 0uch applications include

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!othing directly improves throughput or delay; 70 tries to match &; Jo0 support

'ackground'fforts to marry and &;

switching % psilon(&ag switching %*isco(

ggregate route based switching % 9;(

*ascade % navigator(ll use standard routing protocols to define paths between end pointsssign pac+ets to path as they enter networ+

3se &; switches to move pac+ets along paths E &; switching %was( much faster than routers E 3se faster technology

e%elopments'&F wor+ing group in B, proposed standard 2//

Routers developed to be as fast as &; switches

E Remove the need to provide both technologies in same networ+ ; 70 does provide new capabilities E Jo0 support E &raffic engineering E Airtual private networ+s E ;ultiprotocol support

"onnection @riented &o$ $upport>uarantee fi-ed capacity for specific applications*ontrol latency:Mitter 'nsure capacity for voice

rovide specific, guaranteed $uantifiable 07 s*onfigure varying degrees of Jo0 for multiple customers; 70 imposes connection oriented framewor+ on based internets

Tra!!ic 2ngineeringbility to dynamically define routes, plan resource commitments based on +nown

demands and optimi4e networ+ utili4ation9asic allows primitive traffic engineering

E '.g. dynamic routing; 70 ma+es networ+ resource commitment easy

E ble to balance load in face of demand E ble to commit to different levels of support to meet user traffic

re$uirements E ware of traffic flows with Jo0 re$uirements and predicted demand E ntelligent re)routing when congested

#PN $upport&raffic from a given enterprise or group passes transparently through an internet

?5

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0egregated from other traffic on interneterformance guarantees

0ecurity

Multiprotocol $upport; 70 can be used on different networ+ technologies

E Re$uires router upgrades*oe-ist with ordinary routers&;

E 'nables and ordinary switches co)e-istFrame relay

E 'nables and ordinary switches co)e-ist;i-ed networ+

MPL$ Terminology

MPL$ @peration7abel switched routers capable of switching and routing pac+ets based on labelappended to pac+et7abels define a flow of pac+ets between end points or multicast destinations'ach distinct flow %forward e$uivalence class E F'*( has specific path through70Rs defined

E *onnection oriented'ach F'* has Jo0 re$uirements

header not e-amined E Forward based on label value

MPL$ @peration iagram

?=

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2:planation 9 $etup7abelled switched path established prior to routing and delivery of pac+etsJo0 parameters established along path

E Resource commitment E Jueuing and discard policy at 70R E nterior routing protocol e.g. 80 F used E 7abels assigned

7ocal significance only;anually or using 7abel distribution protocol %7D ( or enhancedversion of R0A

2:planation 9 Packet 0andlingac+et enters domain through edge 70R

E rocessed to determine Jo070R assigns pac+et to F'* and hence 70

E ;ay need co)operation to set up new 70ppend label

Forward pac+etWithin domain 70R receives pac+etRemove incoming label, attach outgoing label and forward

'gress edge strips label, reads header and forwards

Notes; 70 domain is contiguous set of ; 70 enabled routers&raffic may enter or e-it via direct connection to ; 70 router or from non); 70router F'* determined by parameters, e.g.

E 0ource:destination address or networ+ address

?B

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E ort numbers E protocol id E Differentiated services codepoint E v= flow label

Forwarding is simple loo+up in predefined table E ;ap label to ne-t hop

*an define

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Position o! MPL$ La7el $tack

F2"sD L$PsD and La7els&raffic grouped into F'*s&raffic in a F'* transits an ;7 0 domain along an 70

ac+ets identified by locally significant labelt each 70R, labelled pac+ets forwarded on basis of label.

E 70R replaces incoming label with outgoing label'ach flow must be assigned to a F'*Routing protocol must determine topology and current conditions so 70 can beassigned to F'*

E ;ust be able to gather and use information to support Jo070Rs must be aware of 70 for given F'*, assign incoming label to 70 ,communicate label to other 70Rs

Topology o! L$Ps3ni$ue ingress and egress 70R E 0ingle path through domain

3ni$ue egress, multiple ingress 70Rs E ;ultiple paths, possibly sharing final few hops

;ultiple egress 70Rs for unicast traffic;ulticast

/oute $election0election of 70 for particular F'*

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"onstraint 'ased /outing Algorithm&a+e in to account traffic re$uirements of flows and resources available alonghops

E *urrent utili4ation, e-isting capacity, committed services E dditional metrics over and above traditional routing protocols %80 F(

;a- lin+ data rate*urrent capacity reservation

ac+et loss ratio7in+ propagation delay

La7el istri7ution0etting up 70

ssign label to 70nform all potential upstream nodes of label assigned by 70R to F'*

E llows proper pac+et labelling

E 7earn ne-t hop for 70 and label that downstream node has assigned toF'*

llow 70R to map incoming to outgoing label

/eal Time Transport Protocol&* not suited to real time distributed application

E oint to point so not suitable for multicast E Retransmitted segments arrive out of order E !o way to associate timing with segments

3D does not include timing information nor any support for real timeapplications0olution is real)time transport protocol R&

/TP Architecture*lose coupling between protocol and application layer functionality

E Framewor+ for application to implement single protocolpplication level framingntegrated layer processing

Application Le%el FramingRecovery of lost data done by application rather than transport layer

E pplication may accept less than perfect deliveryReal time audio and video

nform source about $uality of delivery rather than retransmit0ource can switch to lower $uality

E pplication may provide data for retransmission0ending application may recompute lost values rather than storingthem

/

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0ending application can provide revised values*an send new data to fi-Y conse$uences of loss

7ower layers deal with data in units provided by application E pplication data units % D3(

Integrated Layer Processing

dMacent layers in protocol stac+ tightly coupled

llows out of order or parallel functions from different layers

/TP Architecture iagram

/TP ata Trans!er Protocol&ransport of real time data among number of participants in a session, defined by"

E R& ort number 3D destination port number if using 3D

E R& *ontrol rotocol %R&* ( port numberDestination port address used by all participants for R&* transfer

E addresses;ulticast or set of unicast

Multicast $upport'ach R& data unit includes"0ource identifier &imestamp

ayload format/elays

ntermediate system acting as receiver and transmitter for given protocol layer ;i-ers

E Receives streams of R& pac+ets from one or more sources E *ombines streams

E Forwards new stream&ranslators E roduce one or more outgoing R& pac+ets for each incoming pac+et E '.g. convert video to lower $uality

/TP 0eader

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/TP "ontrol Protocol (/T"P)R& is for user dataR&* is multicast provision of feedbac+ to sources and session participants3ses same underlying transport protocol %usually 3D ( and different port number R&* pac+et issued periodically by each participant to other session members

/T"P FunctionsJo0 and congestion control

dentification0ession si4e estimation and scaling0ession control

/T"P Transmission !umber of separate R&* pac+ets bundled in single 3D datagram

E 0ender report

E Receiver report E 0ource description E >oodbye E pplication specific

/T"P Packet Formats

2

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Packet Fields (All Packets)Aersion %2 bit( currently version 2

adding % bit( indicates padding bits at end of control information, with numberof octets as last octet of padding

*ount %5 bit( of reception report bloc+s in 0R or RR, or source items in 0D'0 or

9 ' ac+et type %? bit(7ength % = bit( in 2 bit w