LAN/WAN Protocols - SMUlyle.smu.edu/~nair/courses/7344/3_protocols_v4e.pdf · 2004-08-31 · LAN/WAN Protocols Class 3 Closer examination of L2 protocols. SMU CSE 5344 / 7344 2 Review

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LAN/WAN Protocols

Class 3Closer examination of L2

protocols

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Review of Concepts

Communication subnet

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data Data

User application, process And management functionsData interpretation, formatAnd control transformationAdministration and controlOf session between two nodesNetwork transparent data transfer and transmission controlRouting, switching and flowControl over a networkMaintain an d release data:Link, error and flow controlElectrical and mechanicalcharacteristics

Data Flow

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Protocol Concepts

• What if the transmitted message is too long?

• How do we react to damaged transmission • What if the transmitter and receiver run

different speeds? • What if both the parties want to transmit

and receive at the same time?

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L2 Functions

1. DLL Functionsa. Network Layer Servicesb. Framing / Error Detection / Error Correctionc. Error Controld. Flow Control

2. Data Link Protocols to Deliver the Functionsa. Stop-and-Wait Protocolb. Sliding Window Protocolsc. Sliding-window Flow Controld. A One bit Sliding-Windowe. A Protocol Using Go-Back-Nf. Selective Reject

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L2 Functional Requirements

• Frame synchronization. Data are sent in blocks called frames. The beginning and end of each frame must be recognized.

• Error control. Any bit errors introduced by the transmission system must be checked & corrected.

• Flow control. The sending station must not send frames at a rate faster then the receiving station can absorb them.

• Addressing. The identity of the two stations involved in a transmission must be specified.

• Link management. The initiation, maintenance, and termination of a data exchange requires a fair amount of coordination and cooperation among stations.

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DLL Functions – Network LayerDLL processes data transfer between the PHY and NL (Network Layer)Specifically deliver frames by:

– Handling Transmission Errors– Regulating Data Flow

Three methods to deliver frames:1. Unacknowledged connectionless service2. Acknowledged connectionless service3. Acknowledged connection-oriented service

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Unack’ed Connectionless Service1. No logical connection is established beforehand2. Source sends frames to the destination without having the

destination acknowledge them

3. If a frame is lost– no attempt is made to detect the loss or recover from it in the DLL.

This class of service is appropriate– when the error rate is very low so that recovery task is left for

solution to higher layers

– for real-time traffic in which late data are worse than bad data.

Most LANs use unacknowledged connectionless service in the DLL

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Ack’ed Connectionless Service1. No logical connection is established beforehand2. Source “learns” whether or not a frame has arrived correctly3. If no ACK is received within a specified time interval

– it can be sent again

4. If an ACK is lost– a packet may be sent several times and thus received several times

5. What advantage is there to breaking up a large packet? – individual “lost” frames are acknowledged or retransmitted– entire packets get through much faster than a lost unbroken frame

This class of service is appropriate– when over unreliable channels, such as wireless system

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Ack’ed Connection-oriented Service

1. A connection is established between SRC&DST before transfer

2. Any frame sent over the connection is numbered

3. The DLL guarantees that • each frame sent is received

• Frames are received in the correct order

4. With a connection-oriented service, transfers go through 3distinct phases:1. connection is established and counters are initialized

– Counters keep track of received/lost frames2. Frames are transmitted and acknowledged3. Connection is released, freeing up counters and buffers

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DLL Functions – Framing/ED/ECFraming

• encapsulates datagram into frame, adding header, trailer– two types of sources of framing errors:

• Lost frame• Damaged frame

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 errors without

retransmission

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DLL Functions – Error Control1. Error Detection2. Positive Acknowledgment3. Retransmission after time-out4. Optional: Negative acknowledgment and retransmission

These mechanisms are referred to as Automatic Repeat reQuest (ARQ)

Three “classic” versions Of ARQ:1. Stop-and-wait2. Go-back-N3. Selective-reject

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DLL Functions – Flow Control1.Feedback-based

• receiver sends back information to the sender • giving it permission to send more data (send “n” frames)

• telling the sender to stop sending (overflow)

2.Rate based• protocol has a built-in mechanism that limits the rate at

which senders may transmit data.

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L2 Protocols

1. DLL Functionsa. Network Layer Servicesb. Framing / Error Detection / Error Correctionc. Error Controld. Flow Control

2. Data Link Protocols to Deliver the Functionsa. Stop-and-Wait Protocolb. Sliding Window Protocolsc. Sliding-window Flow Controld. A One bit Sliding-Windowe. A Protocol Using Go-Back-Nf. Selective Reject

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DLL Protocols – Stop-and-Wait• Sender sends one frame and waits for an ACK

– seq numbers on Frames and/or ACKs

• Packet size manipulation– the source breaks up a large block of data into smaller blocks

and transmits the data in many frames

• Reasons:– The buffer size of the receiver may be limited– The larger the packet, the more bit errors– With smaller frames

• errors are detected sooner • smaller amount of data needs retransmission

– On a shared medium it is not desirable to permit one station to occupy the medium for an extended period, as this causes long delay at the other sending stations.

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DLL Protocols – Stop-and-Wait ARQ

B discards duplicateframe

Frame 0

ACK1

Frame 1

ACK0

Frame 0Timeout

Frame 0

ACK1Timeout

Frame 0

Frame lost --A retransmits

ACK1 lost A retransmits

• 1-bit for sequence size• Allows 1 outstanding frame

A B

• 1 ACKs frame0• Indicates ready for frame1

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Why is this Inefficient?

• Pipeline capacity is BW x RTT• For 1.5 Mbps BW & 45ms RTT

– What’s the pipe’s capacity?• Capacity is 1500 Kbps x 0.045 s = 67.5 Kb• Approx 8 KB

• A 1 KB data stream is approx 1/8th the capacity of the pipe (i.e. of the BW)

So need to improve efficiency . . . Send more data and/or change protocol

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Sliding Window Protocol• Piggybacked ACKs

– Less overhead (bw, interrupts, buffer etc.)– Sends more data – by piggybacking with ACK– Deal with timeouts

• Sequence numbers– expands on the 1-bit sequence size more numbers

• Sending Window (Size)– number of unack’ed frames the sender can send

• Receiving Window (Size)– number of out-of-order frames acceptable

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Sliding Window Protocol• Sending Window Size (SWS)

– LAR seqnum of Last ACK Received– LFS seqnum of Last Frame Sent– LFS – LAR SWS

0 1 2 3 5 6 7 0 14 0765432 4321

Frames already receivedWindow of frames thatmay be transmitted

FrameSequencenumber

Last frametransmitted

(a) Transmitter’s perspective

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Sliding Window Protocol• Receiving Window Size (RWS)

– LAF seqnum of Last Acceptable Frame– LFR seqnum of Last Frame Received– LAF – LFR RWS

(b) Receiver’s perspective

0 554321 6 2107 43

Frames already receivedWindow of frames thatmay be accepted

321076 4

Last frame ACK’ed

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Sliding Window ProtocolAssume there is traffic in both directions

– data frames from A to B are intermixed with the ACK frames from B to A (Duplex Channel)

• Station B can buffer n frames– B can accept a “window” of n frames– A is allowed to send n frames without waiting for

any ACKs.

• 3-bit field– sequence number range from 0 to 7

• Both ACKs (RR) and NACKs (RNR) are used

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Sliding-WindowFrames already received

Window of frames thatmay be transmitted

0 1 2 3 5 6 7 0 14 0765432

0

4321

554321 6 2107 43

Frames already receivedWindow of frames thatmay be accepted

321076 4

FrameSequencenumber

Last frametransmitted

Window shrinksfrom trailing edgeas frames are sent

Window expands fromleading edge as ACKsare received

Last frame ACK’ed

Window shrinksfrom trailing edgeas frames are received

Window expands fromleading edge as ACKsare sent

(a) Transmitter’s perspective

(b) Receiver’s perspective

Pipeline

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Example: Sliding-window protocol

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 30 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

F3F4F5F6

0 1 2 3 4 5 6 7 0 1 2 3RR7

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3F0F1F2

Source system A Destination system B

Maximum window size=7

0 1 2 3 4 5 6 7 0 1 2 3RR3

Sent without ACK

Implication of Window Size = 1?

One-Bit Sliding Window (piggybacking)

A sends (0,1,A0)

A gets (0,0,B0)*A sends (1,0,A1)



B gets (0,1,A0)*B sends (0,0,B0)




A sends (0,1,A0)


A gets (0,0,B0)A sends (1,0,A1)


B sends (0,1,B0)B gets (0,1,A0)*B sends (0,0,B0)

B gets (0,0,A0)B sends (1,0,B1)


B gets (1,1,A1)B sends (0,1,B2)

a b

Scenarios: (a) Normal case. (b) Abnormal case – A & B transmit at the same time. Notation: (seq, ack, packet #). Asterisk indicates where a network layer accepts a packet.

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Pipelining

• Idea– Do not block transmitter during the

roundtrip time– Increase the Receiver window size

• RWS = 1 | RWS = SWS

• Methods to recover from errors1. Go back n2. Selective repeat

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Pipelining

Given– channel capacity is b bits/sec– frame size f bits and– round-trip propagation time R sec

the time required to transmit a single frame is f/b sec.

After the last bit of data frame has been sent, – there is a delay of R/2 before that bit arrives at the rcvr and– another delay of at least R/2 for ACK to come back,

for a total delay of R

In stop-and-wait the line is busy for f/b and idle for R, giving: Line utilization = f / (f+bR) = x%

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PipeliningExample – send 1000-bit frame assuming:

– 50 kbps satellite channel– 500ms round-trip delay = 2*(125ms to satellite + 125 ms to rcvr)

What is the Bandwidth Utilization?– At t=0 ms the frame has been started and t=20 ms sent– At t=270 ms the frame is received and ACK sent – At t=520 ms ACK received by the sender;

sender was blocked during 500/520 or 96% of the time4 % of the bandwidth was used

To increase bandwidth utilization the sender transmits up to w frames before blocking instead of just 1 frame

For this example, w should be 26 – why?– sender sends Frame 0 as before – at t=520 ms finishes sending 26 frames – the ACK for frame 0 will have just arrived– ACKs arrive every 20 msec – PIPLINING – so the sender always

gets permission to continue when it needs it

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Recovery – 1. Go back n

• If an error occurs– Receiver discards subsequent frames– No ACKs for the discarded frames

• ACK is not needed as time-out is used– Receiver window size = 1– Transmitter times-out and resends the UNACKed frames

• Inefficient if the error rate is high

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Go-Back-NSender:• k-bit seq # in frame header• “window” of up to n consecutive UNACK’ed frames

allowed

• ACK(n): ACKs all frames up to, including seq # n -“cumulative ACK”– may receive duplicate ACKs

• timer is set for each in-flight frame• timeout(n): retransmit frame n and all higher seq #

frames in window

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Protocol Using Go Back N

0 31 2 4 5 6 7 8 765432

Time interval, Time out

D DD D DD

Frames discarded by DLL

5432

Timea

Error

0 1 E

0 1 2 3 4 5Go Back N With window size 1

Data flow ACK flowError recovery, when:(a) receiver’s window size is 1

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Recovery – 2. Selective Repeat• If an error occurs

– Receiver still stores the subsequent good frames

– Transmitter retransmits the bad ones– Receiver window size > 1

• Efficient at higher error rate (BER)

• Trade off between bandwidth and buffer space

0 31 2 4 5 6 7 8

0 E

765432

D

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Error recovery, when:(b) receiver’s window size is large Selective Repeat

1 5432DD D DD

Time interval, Time out

5320 1 64 9872 13121110

9872 13121110

Error Frames discarded by DLL Timea

0 1 2 3 4 5

Protocol Using Selective Repeat

Go Back N With window size 1

E0 1

Error

53 64

Frames buffered by DLLb

0 1 NAK2 1 1 1 6 7 8 9 10 11Selective repeat (NAK)

Data flow ACK flow

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Optimal Window Size

• Go back n– MaxSeq (not MaxSeq +1)– Why?

• Selective repeat– (MaxSeq+1)/2– Why?

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Optimal Window Size

• Go back n (MaxSeq, not MaxSeq +1)– # of bits allotted to SeqNum = 1– MaxSeqNum = 1– If RWS = 1, MaxSeqNum SWS + 1

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Optimal Window Size

• Selective repeat – (MaxSeq+1)/2– # of bits allotted to SeqNum = 3– MaxSeq = 7

SWS = (7+1)/2 = 4send 3 frames, wait, send 4 frames

How many buffers are needed and why?

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Summary of DLL FunctionsConvert a raw bit stream (from physical layer) into a stream of frames (for network layer)

– Various framing methods used• character count• byte stuffing• bit stuffing

Provide Reliable delivery– Error control to retransmit damaged or lost frames– Flow control to prevent a fast sender from overrunning a

slow receiver– Preserve frame sequence order

Sliding window mechanism integrates error control and flow control in a convenient way

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Protocol Specification

Sender Receiver Channel0/1 0/1 0/1/A/--

Content lost

Frame 0 delivered

Transitions

TimeoutTimeout

Formalizing protocol specification

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ATM Data Link Layer• Header Error Control(HEC)

– Protects the 4 bytes in the header• Cell reception

– Hard to recognize cell boundaries– Use HEC for boundary detection

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End of Class 3

Documents

LAN/WAN Protocols - SMUlyle.smu.edu/~nair/courses/7344/3_protocols_v4e.pdf · 2004-08-31 · LAN/WAN Protocols Class 3 Closer examination of L2 protocols. SMU CSE 5344 / 7344 2 Review