p2p protocol (2)

8/2/2019 p2p Protocol (2)

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Peer-to-Peer Protocols

Peer-to-Peer Protocols and Service ModelsFlow ControlARQ Protocols

Data Link Control

UNIT-II


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Peer-to-Peer protocols: many protocolsinvolve the interaction between two peers Service Models

Detailed discussion of ARQ provides example ofdevelopment of peer-to-peer protocols

Data Link Control Framing

PPP

HDLC


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n 1 peer process n 1 peer process

n peer process n peer process

n + 1 peer process n + 1 peer process

Peer-to-Peer Protocols

Peer-to-Peer processesexecute layer-n protocolto provide service tolayer-(n+1)

Layer-(n+1) peer callslayer-n and passesService Data Units(SDUs) for transfer

Layer-n peers exchangeProtocol Data Units(PDUs) to effect transfer

Layer-n delivers SDUs todestination layer-(n+1)

peer

SDU SDU

PDU


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Service Models

Theservice model specifies the information transferservice layer-n provides to layer-(n+1)

The most important distinction is whether the serviceis:

Connection-oriented Connectionless

Service model possible features: Arbitrary message size or structure

Sequencing and Reliability

Timing and Flow control

Multiplexing

Privacy, integrity, and authentication


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Message Size and Structure

What message size and structure will a service

model accept?

Different services impose restrictions on size &

structure of data it will transfer

Single bit? Block of bytes? Byte stream?

Ex: Transfer of voice mail = 1 long message

Ex: Transfer of voice call = byte stream1 voice mail= 1 message = entire sequence of speech samples

(a)

1 call = sequence of 1-byte messages

(b)


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1 long message

2 or more blocks

2 or more short messages

1 block

Segmentation & Blocking

To accommodate arbitrary message size, a layer mayhave to deal with messages that are too long or too shortfor its protocol

Segmentation & Reassembly: a layer breaks long

messages into smaller blocks and reassembles these atthe destination

Blocking & Unblocking: a layer combines smallmessages into bigger blocks prior to transfer


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Reliability & Sequencing

Reliability: Are messages or information

stream delivered error-free and without loss or

duplication?

Sequencing: Are messages or information

stream delivered in order?

ARQ protocols combine error detection,

retransmission, and sequence numbering to

provide reliability & sequencing

Examples: TCP and HDLC


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Flow Control

Messages can be lost if receiving system does nothave sufficient buffering to store arriving messages

If destination layer-(n+1) does not retrieve its

information fast enough, destination layer-n buffersmay overflow

Flow Control provide backpressure mechanisms thatcontrol transfer according to availability of buffers at

the destination Examples: TCP and HDLC


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Timing

Applications involving voice and video generate units

of information that are related temporally

Destination application must reconstruct temporal

relation in voice/video units Network transfer introduces delay & jitter

Timing Recovery protocols use timestamps &

sequence numbering to control the delay & jitter indelivered information

Examples: RTP & associated protocols in Voice over

IP


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Multiplexing

Multiplexing enables multiple layer-(n+1)

users to share a layer-n service

A multiplexing tag is required to identify

specific users at the destination

Examples: UDP, IP


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Privacy, Integrity & Authentication

Privacy: ensuring that information transferred cannot

be read by others

Integrity: ensuring that information is not altered

during transfer Authentication: verifying that sender and/or receiver

are who they claim to be

Security protocols provide these services Examples:IPSec, SSL


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End-to-End vs. Hop-by-Hop

A service feature can be provided by implementing a protocol

end-to-end across the network

across every hop in the network

Example: Perform error control at every hop in the network or only

between the source and destination?

Perform flow control between every hop in the network or

only between source & destination? We next consider the tradeoffs between the two

approaches


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End-to-End Approach Preferred

1 2 5

Data

ACK/NAK

End-to-end

More scalable if

complexity at

the edge

Simple

inside the

network

Hop-by-hop

cannot ensure E2E

correctness

1 2 5

Data

ACK/

NAK

Hop-by-hop

3

Data

ACK/

NAK

4

Data

ACK/

NAK

Data

ACK/

NAK

3

Data

4

Data Data

Faster recovery


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Flow Control: Initial Assumptions

Simplex Channel

Infinite buffer capacity with the receiver

Error free transmission Network layer at the senders end is always

ready with data

10/06/11 14Computer Networks and Protocols


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Stop and Wait



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Sliding Window


Sliding Window Protocols are a feature of packet-based datatransmission protocols. They are used where reliable in-order delivery

of packets is required, such as in the data link layer (OSI model) as well

as in TCP (transport layer of the OSI model).

Conceptually, each portion of the transmission (packets in most data

link layers, but bytes in TCP) is assigned a unique consecutivesequence number, and the receiver uses the numbers to place received

packets in the correct order, discarding duplicate packets and

identifying missing ones. The problem with this is that there is no limit

of the size of the sequence numbers that can be required.


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Sender Sliding Window



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Receiver Sliding Window



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Sliding Window Example



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Sender


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Receiver



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Automatic Repeat Request (ARQ)

Purpose: to ensure a sequence of information packetsis delivered in order and without errors or duplicationsdespite transmission errors & losses

We will look at:

Stop-and-Wait ARQ Go-Back N ARQ

Selective Repeat ARQ

Basic elements of ARQ:

Error-detecting code with high error coverage ACKs (positive acknowledgments)

NAKs (negative acknowlegments)

Timeout mechanism



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ERROR CONTROL


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Stop-and-Wait ARQ

Transmitter

Ready state Await request from higher layer for

packet transfer

When request arrives, transmit framewith updated S and CRC

Go to Wait StateWait state Wait for ACK or timer to expire; block

requests from higher layer

If timeout expires retransmit frame and reset timer

If ACK received: If sequence number is incorrect or iferrors detected: ignore ACK

If sequence number is EQUAL acceptframe, go to Ready state

Receiver

Always in Ready State Wait for arrival of new frame When frame arrives, check for errors If no errors detected and sequence

number is correct (S=R), then

accept frame, update Rnext,

send ACK frame with Rnext,

deliver packet to higher layer

If no errors detected and wrongsequence number

discard frame

send ACK frame with Rnext

If errors detected discard frame



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Applications of Stop-and-Wait ARQ

IBM Binary Synchronous Communications

protocol (Bisync): character-oriented data link

control

Xmodem: modem file transfer protocol

Trivial File Transfer Protocol (RFC 1350):

simple protocol for file transfer over UDP



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CRCInformation

packet

Header

Information frame Control frame: ACKs

CRCHeader

PacketError-free

packet

Information frame

Control frame

Transmitter

(Process A)

Receiver

(Process B)

Stop-and-Wait ARQ

Timer set after each

frame transmission

Transmit a frame, wait for ACK



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Damaged Frame



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Stop-and-Wait ARQ, lost ACK frame



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Go-Back-N

Improve Stop-and-Wait by not waiting!

Keep channel busy by continuing to send frames

Allow a window of up to Ws outstanding frames

Use m-bit sequence numbering

If ACK for oldest frame arrives before window is

exhausted, we can continue transmitting

If window is exhausted, pull back and retransmit alloutstanding frames

Alternative: Use timeout



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Go-Back-N ARQ

Frame transmission arepipelinedto keep the channel busy Frame with errors and subsequent out-of-sequence frames are ignored Transmitter is forced to go back when window of 4 is exhausted

A

B

fr

0Timefr

1

fr

2fr

3

fr

4

fr

5fr

6

fr

3

A

C

K

1

out of sequence

frames

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

fr

5fr

6

fr

4fr

7

fr

8

fr

9

A

C

K

2

A

C

K

3

A

C

K

4

A

C

K

5

A

C

K

6

A

C

K

7

A

C

K

8

A

C

K

9

Rnext 0 1 2 3 3 4 5 6 7 8 9



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Damaged Frame



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Lost Frame



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Lost ACK



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Applications of Go-Back-N ARQ

HDLC(High-Level Data Link Control): bit-

oriented data link control

V.42 modem: error control over telephone

modem links



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Selective Repeat ARQ

Go-Back-N ARQ inefficient because multiple frames areresent when errors or losses occur

Selective Repeat retransmits only an individual frame

Timeout causes individual corresponding frame to be resent

NAK causes retransmission of oldest un-acked frame

Receiver maintains a receive window of sequence numbers

that can be accepted

Error-free, but out-of-sequence frames with sequence numbers

within the receive window are buffered

Arrival of frame with R causes window to slide forward by 1 or

more



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Selective Reject



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Applications of Selective Repeat ARQ

TCP (Transmission Control Protocol):

transport layer protocol uses variation of

selective repeat to provide reliable stream

service

Service Specific Connection Oriented

Protocol: error control for signaling messages

in ATM networks



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Point-to-Point

ProtocolPPP



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PPP provides router-to-router and host-to-

network connections over synchronous andasynchronous circuits, which can be either

dialup or leased lines.

PPP is the protocol of choice when

configuring serial links in a multivendor

environment.



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PPP actually uses HDLC as a basis for

encapsulating datagrams.

PPP's frame format is based on the HDLC

frame format.

PPP defines the Link Control Protocol(LCP). The job of LCP is to establish,

configure, and testthe data-link connection.



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Point-to-Point Link



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Transition States



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PPP Layers



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PPP Frames



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LCP Packet Encapsulated in a Frame


PAP


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PAP


Password Authentication Protocol-

provides a simple method for the peer to establish its

identity using a 2-way handshake. This is done only upon

initial link establishment.After the Link Establishment phase is complete, an

Id/Password pair is repeatedly sent by the peer to the

authenticator until authentication is acknowledged or the

connection is terminated


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PAP Packets



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IPCP Packet Encapsulated in PPP Frame


Internet Protocol Control Protocol

- Is responsible for configuring, enabling, and disablingthe IP protocol modules on both ends of the point-to-point

link.


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HDLC

High Level Data Link Control

Early Link Layer Protocol.

Used in many LAN and WAN Protocols.

Support Broad range of Application.



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HDLC Basis

Why do we use it ?

Framing

Frame Protection

Error Recovery

Three types of StationsPrimary Station.

Secondary Station.

Combined Station.

Three Modes

Normal Response Mode[NRM]Asynchronous Response Mode[ARM]

Asynchronous Balance Mode[ABM]



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HDLC Configuration


C C fi i


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HDLC Configuration



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HDLC Configuration



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HDLC Modes



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HDLC Frame Types



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HDLC Frame Types



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HDLC Frame Types



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HDLC Flag Field


Bit St ffi g


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Bit Stuffing


HDLC Address Field


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HDLC Address Field


HDLC C l Fi ld


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HDLC Control Field


P ll/Fi l


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Poll/Final


HDLC I f i Fi ld


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HDLC Information Field


HDLC FCS Fi ld


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HDLC FCS Field


Peer-to-Peer Example


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p


Peer-to-Peer Example


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p

p2p protocol (2)

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