CSC 535 Communication Networkls I Chapter 5b Peer-to-Peer Protocols Dr. Cheer-Sun Yang.

CSC 535Communication Networkls I

Chapter 5bPeer-to-Peer Protocols

Dr. Cheer-Sun Yang

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

2

n+1entity

SAP1

n+1entity

SAP2

n entity n entity

n-SDU

n-SDU

n-SDU

H

H n-SDU

n-PDU

Figure 5.1

3

Peer-to-Peer Protocols(1)Used for linking a host to another host or

point-to-point (2)used for linking an end system with another end system.

Physical layer delas with bits; data link layer deals with frames; network layer deals with datagrams or packets; transport layer deals with segments.

In this chapter, we focus on data link layer protocols: stop-and-wait (with) ARQ, Go-Back-N (with) ARQ, and Selective Repeat (with) ARQ


4

3 2 11 2

21

3 2 11 2

21

21

Medium

1

2

Physical layer entity

Data link layer entity3 Network layer entity

PhysicalLayer

Data linkLayer

PhysicalLayer

Data linkLayer

A B

A B

Packets Packets

Frames

(a)

(b)

Figure 5.2


5

PhysicalLayer

Data linkLayer

PhysicalLayer

Data linkLayer

End system

NetworkLayer

NetworkLayer

PhysicalLayer

Data linkLayer

NetworkLayer

PhysicalLayer

Data linkLayer

NetworkLayer

TransportLayer

TransportLayer

MessagesMessages

Segments

End system

Network

Figure 5.3


6

3 2 11 2

21

3 2 11 2

21

21

Medium

A B

3 2 11 2

21

C

21

21

2 134 1 2 3 4

End System End System

Network1

2

Physical layer entity

Data link layer entity3 Network layer entity

3 Network layer entity

Transport layer entity4

Figure 5.4

7

ARQ ProtocolsA frame level error recovery technique. Their

purpose is to ensure that frame level error can be recovered automatically without the request of upper layer (i.e., network layer) software.

ARQ forms the basis for point-to-point protocols that provide for reliable transfer of information.

There are three protocols: Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ

8

Point-to-Point RequirementsData link layer provides a point-to-point reliable

service.This service provides an adaptation function for

a host to attach to a network.Major issues include but are not limited to:

arbitrary message size (major) reliability and sequencing (=>transport layer) pacing and flow control (=>transport layer) timing (major) addressing (major) privacy, integrity, and authentication

(=>presentation)


9

Application Network Application

AdaptationFunction

AdaptationFunction

End-to-end application requirements

Network service

Figure 5.5


10

1 call = 1 message = entire sequence of speech samples

1 call = sequence of 1-byte messages

(a)

(b)

(c)

1 long message

2 or more blocks

2 or more short messages

1 block

Figure 5.6

11

End-to-End vs. Hop-by-HopData link layer provides hop-by-hop (or

point-to-point) services, whereas transport layer provides end-to-end (or host-to-host) services.

In this chapter, we will only talk about data link layer functions. (Skip 5.1.3-it causes more confusion to a beginner than clarification.)

We first take a look at a point-to-point communication model.


12

Packet sequence

Error-free packet

sequence

Informationframes

Control frames

Transmitter Receiver

CRC

Informationpacket

Header

Station A Station B

Information Frame

Control frame

CRC Header

Figure 5.8

13

Other Basic Elements:Information frames (I-frames): user

packets (to/ from network layer)Control frames: ACKs,NAKs, or enquiry

frame ENQTime-out mechanism

We begin with Stop-and-Wait ARQ

14

Stop and WaitSource transmits frameDestination receives frame and replies

with acknowledgementSource waits for ACK before sending next

frameDestination can stop flow by not send ACKWorks well for a few large frames(Add time out mechanism)

=>Stop-and-Wait ARQ


15

(a) Frame 1 lost

A

B

frame 0

frame1

ACK

frame1

ACK

timeTime-out

frame2

(b) ACK lost

A

B

frame 0

frame1

ACK

frame1

ACK

timeTime-out

frame2

ACK

In parts (a) and (b) transmitting station A acts the same way, but part (b) receiving station B accepts frame 1 twice.

Figure 5.9


16

A

B

frame 0 frame

0ACK

frame1

ACK

timetime-out

frame2

Transmitting station A misinterprets duplicate ACKs

Figure 5.10

Possible ambiguities when ACKs are unnumbered.


17


SlastRnext

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

(0,0) (0,1)

(1,0) (1,1)

Timer

Global State:(Slast, Rnext)

Error-free frame 0arrives at receiver

ACK forframe 0arrives attransmitter

ACK forframe 1arrives attransmitter Error-free frame 1

arrives at receiver

Station A Station BRnext

Slast

Figure 5.11


18

(a) Frame 1 lost

A

B

frame 0

frame1ACK

1

ENQ

ACK1

time

Time-out

frame1

(b) ACK for frame 1 lost

A

B

frame 0

frame1ACK

1

ENQACK

0

timeTime-out

frame0ACK

0

Figure 5.12

19

FragmentationLarge block of data may be split into small

frames Limited buffer size Errors detected sooner (when whole frame

received) On error, retransmission of smaller frames is

needed Prevents one station occupying medium for

long periods

Stop and wait becomes inadequate

20

Go-Back-N ARQStop-and-Wait is too conservative and the

effective information transmission rate and protocol transmission efficiency (discussed later) are too low.


21

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr6

fr3

ACK1 error

Out-of-sequence frames

Go-Back-4: 4 frames are outstanding; so go back 4

fr5

fr6

fr4

fr7

fr8

fr9

ACK2

ACK3

ACK4

ACK5

ACK6

ACK7

ACK8

ACK9

Figure 5.13


22

A

B

fr0

timefr1

fr2

fr3

fr0

error

Out-of-sequence frames

4 frames are outstanding; so go back 4

fr2

fr3

fr1

fr4

fr5

fr6

A

B

timefr0

fr0

error

Time-out expires

fr1

ACK1

Stop-and-Wait

Go-Back-N

ACK1

ACK2

ACK3

ACK4

ACK5

ACK6

Figure 5.14


23

Timer Slast

Slast+1

Srecent

Slast+Ws-1

Timer

Timer


...

Receive Window

Buffers

Slast Slast+Ws-1

...Send Window

Srecent

Framestransmittedand ACKed

Rnext

framesreceived

The receiver will only accepta frame that is error-free and that has sequence number Rnext

...

Figure 5.15


24

A

B

fr0

timefr1

fr2

fr3

fr0

fr1

fr2

fr3

ACK1

M =22 = 4, Go-Back - 4:

ACK4

ACK2

ACK3

Transmitter goes back 4

Receiver has Rnext=0, but it does not know whether its ACK for frame 0 was received, so it does not know whether this is the old frame 0 or a new frame 0

A

B

fr0

timefr1

fr2

fr0

fr1

fr2

ACK1

M=22=4, Go-Back-3:

ACK2

ACK3

Transmitter goes back 3

Receiver has Rnext=3 , so it rejects the old frame 0

Figure 5.16


25

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr1

fr2

ACK1

error

Out-of-sequenceframes

Go-Back-7:

fr4

fr5

fr3

fr6

fr7

fr0

NAK1

ACK3

ACK4

ACK5

ACK6

ACK7

ACK2

Transmitter goes back to frame 1

Figure 5.17


26


TransmitterReceiver

ACKs are piggybacked in headers

Station A Station B

RA next

“A” Receive Window

RB next

“B” Receive Window

SA last

SA last

SA last+1

SArecent

SA last+WA s-1Timer

SA last+WA s-1

...

...

Buffers

“A” Send Window

...

SB last

SB last

SBlast+1

SBrecent

SB last+WB s-1

SB last+WB s-1

...

...

Buffers

“B” Send Window

...

SArecent RA next

SBrecent RB next

Timer

Timer

Timer

Timer

Timer

Timer

Timer

Figure 5.18


27

TfTf

Tproc

TpropTprop

Tout

Figure 5.19

28

Selective Repeat ARQData link layer provides hop-by-hop (or

point-to-point) services, whereas transport layer provides end-to-end (or host-to-host) services.

In this chapter, we will only talk about data link layer functions. (Skip 5.1.3-it causes more confusion to a beginner than clarification.)

We first take a look at a point-to-point communication model.


29


Buffers

Slast Slast+Ws-1

...Send Window

Srecent

Framestransmittedand ACKed

Timer Slast

Slast+1

Srecent

Slast+Ws-1

Timer

Timer

...

...

Receive Window

Rnext

Framesreceived Rnext +Wr-1

Rnext+1

Rnext+2

Rnext+Wr-1

...

Buffers

Figure 5.20


30

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr6

fr2

ACK1 error

fr8

fr9

fr7

fr10

fr11

fr12

ACK2

NAK2

ACK7

ACK8

ACK9

ACK10

ACK11

ACK12

ACK2

ACK2

ACK2

Figure 5.21


31

A

B

fr0

timefr1

fr2

fr0

ACK1

M=22=4, Selective Repeat: Send Window = Receive Window = 3

ACK2

ACK3

Receive Window {3,0,1}

Frame 0 resent

A

B

fr0

timefr1

fr0

ACK1

Send Window = Receive Window = 2

ACK2

Receive Window {2,3}

Frame 0 resent

frame 0 rejected

Figure 5.22

32

Transmission EfficiencyIntuitively, stop-and-wait is the worst; go-

back-n could be better if error rate is not too high; selective repeat could be better is error is high.

Now, we will discuss them formally.


33

frametf time

A

B

tproptack tproc

tprop

tproc

t0

Figure 5.23


34

0

0.2

0.4

0.6

0.8

11.

E-0

1

5.E

-02

1.E

-02

5.E

-03

1.E

-03

5.E

-04

1.E

-04

5.E

-05

1.E

-05

5.E

-06

1.E

-06

5.E

-07

1.E

-07

Selective RepeatGo-Back-N

Stop & Wait

p

Figure 5.24


35

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

32 64 128 256 512 1024 1536 2048nf

Selective Repeat

Go-Back-N

Stop &Wait

62 32 1710

63

33

Figure 5.25

36

Flow ControlFlow control refers to procedures that

prevent a transmitter from overrunning a receiver’s buffer.

Congestion control refers to a network layer function that prevents a source to overwhelm a router’s buffer.

Techniques: (physical layer) X-ON/X-OFF(“server” to “client”) (physical layer) RTS/CTS (host to/from modem,

respectively) (data link layer) sliding window protocols


37

on off offon

A

B

transmit transmit

2Tprop

time

time

Figure 5.26

38

Sliding Windows Flow ControlAllow multiple frames to be in transitReceiver has buffer W longTransmitter can send up to W frames

without ACKEach frame is numberedACK includes number of next frame

expectedSequence number bounded by size of field

(k) Frames are numbered modulo 2k

39

Sliding Window Diagram

40

Sliding Window EnhancementsReceiver can acknowledge frames without

permitting further transmission (Receive Not Ready)

Must send a normal acknowledge to resume

If duplex, use piggybacking If no data to send, use acknowledgement

frame If data but no acknowledgement to send, send

last acknowledgement number again, or have ACK valid flag (TCP)


41

A

B

tcycle

return of permits

time

time

Figure 5.27


42

Network

Synchronous source sends periodic information blocks

Network output not periodic

Figure 5.28


43

sendtimes

playouttimes

arrivaltimes

Tplayout time

time

Receiver too slowbuffer overflow

Tplayout time

timeReceiver too fastbuffer starvation

Many latepackets

Tplayout time

timeReceiver speedjust right

Figure 5.29


44

t4 t3 t2 t1

Timestamps

Add Smoothingfilter

Adjustfrequency Recovered

clock

Counter

+

-

Buffer for information blocks

Errorsignal

Playoutcommand

Figure 5.30


45

Network clock

fn

Transmitter

Network

Receiver

fs fr

f f

Figure 5.31


46

byte stream

Send buffer

Segments

Receive buffer

byte stream

Application Application

ACKs


Figure 5.32

* SKIP SECTION 5.3.3 !!!

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Summary of Major Data Link Layer Functions:Framing: where to begin and where to endError control: error detection and frame

level error recoveryFlow control: error control is not enough!

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Example Data Link Layer Protocols (HDLC, PPP)High Level Data Link Control (HDLC)Point-to-Point Protocol (PPP)


49

PhysicalLayer

Data LinkLayer

Data LinkLayer

NetworkLayer

DLSDU DLSDU

NetworkLayer

PhysicalLayer

DLPDU

NLPDU

“packet”

“frame”

DLSAP DLSAP

Figure 5.32

50

HDLC Station TypesPrimary station

Controls operation of link Frames issued are called commands Maintains separate logical link to each

secondary station

Secondary station Under control of primary station Frames issued called responses

Combined station May issue commands and responses

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HDLC Link ConfigurationsUnbalanced

One primary and one or more secondary stations

Supports full duplex and half duplex

Balanced Two combined stations Supports full duplex and half duplex


52

Primary SecondaryCommands

Responses

Unbalanced Point-to-point link

PrimaryCommands

Responses

Secondary Secondary Secondary

Unbalanced Multipoint link

Primary

Secondary

Commands Responses

Balanced Point-to-point link between Combined Stations

Primary

Secondary

CommandsResponses

Figure 5.33


53

Flag FlagAddress Control Information FCS

Figure 5.35


54

0 N(S) N(R)P/F

1 2-4 5 6-8

Information Frame

N(R)P/F

Supervisory Frame

Unnumbered Frame

1 0 S S

P/F1 1 M M M M M

Figure 5.36


55

SABM UAUA DISCDatatransfer

Figure 5.37


56

Primary A Secondaries B, C

B, RR, 0, P

B, I, 0, 0B, I, 1, 0B, I, 2, 0,F

X

B, SREJ, 1

C, RR, 0, P

C, RR, 0, F

B, SREJ, 1,P

B, I, 1, 0B, I, 3, 0B, I, 4, 0, F

B, I, 0, 5

Figure 5.38


57

Combined Station A Combined Station B

B, I, 0, 0 A, I, 0, 0

B, I, 1, 0

B, I, 2, 1

A, I, 1, 1

A, I, 2, 1

X

B, REJ, 1B, I, 3, 2

B, I, 4, 3

B, I, 1, 3

B, I, 2, 4

B, I, 3, 4

A, I, 3, 1

B, RR, 2

B, RR, 3

Figure 5.39

58

Other DLC Protocols (LAPB,LAPD)Link Access Procedure, Balanced (LAPB)

Part of X.25 (ITU-T) Subset of HDLC - ABM Point to point link between system and packet

switching network node

Link Access Procedure, D-Channel ISDN (ITU-D) ABM Always 7-bit sequence numbers (no 3-bit) 16 bit address field contains two sub-addresses

One for device and one for user (next layer up)

59

Other DLC Protocols (PPP)Provides a method for encapsulating

datagrams over point-to-point linkscan be used to connect two routerscan be used to connect a PC to the

Internet via a modem over a phone line


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Flag flagAddress Control Information CRCProtocol01111110 011111101111111 00000011

Unnumbered frame

Specifies what kind of packet is contained in the payload, e.g., LCP, NCP, IP, OSI CLNP, IPX

All stations are toaccept the frame

Figure 5.40


61

A Typical Scenario

Home PC to Internet Service Provider

1. PC calls router via modem.2. PC and router exchange LCP

packets to negotiate PPP parameters.

3. Check on identities.4. NCP packets exchanged to

configure the network layer, e.g., TCP/IP ( requires IP address assignment).

5. Data transport, e.g. send/receive IP packets.

6. NCP used to tear down the network layer connection (free up IP address); LCP used to shut down data link layer connection.

7. Modem hangs up.

Dead

Establish

Authenticate

Network

Terminate

Open

failed

failed

1. CarrierDetected

2. OptionsNegotiated

3. AuthenticationCompleted4. NCP

Configuration

6. Done

7. CarrierDropped

5.

Figure 5.41

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Required ReadingChapter 5

CSC 535 Communication Networkls I Chapter 5b Peer-to-Peer Protocols Dr. Cheer-Sun Yang.

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