Computer Networksnemo/cen5501/slides/Chapter1.pdfCEN 5501C - Computer Networks - Spring 2007 -...

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1

Computer Networks

Chapter 1 - Fundamentals


Computer Networks

• Need to share– Information– Resources

• Communication vs. StorageTransmission across:– Space (communication)– Time (storage)


Virtuality• Architecture

– Layered– Hierarchical

• Algorithms– Information hiding, ADTs, objects

• Protocols – Distributed coordination algorithms

• Programs– Modularity


Layered vs. Hierarchical

• Both– Peer-to-peer communication– Encapsulation– Protocol = common language/behaviors

• Layered– Layer i serves layer i+1 ONLY– Layer i gets service from layer i-1 ONLY

• Hierarchical– Lower layers serve higher layers


Layered vs. Hierarchical

• Issues– Flexibility– Efficiency– Modularity– Maintainability– Scalability/manageability– Future adaptability


Comparative Architectures7 - Application

6 - Presentation

5 - Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

UserApplication

Data Link ControlDDCMP

Physical 1 - Physical

Path Control

Transmission Ctl

Data Flow Control

Transport

Functional Management Data Services

Network Services Protocol

NetworkApplication

ISO - OSI IBM - SNA DEC - DECnet


OSI Reference Architecture

• Physical (L1 = PHY)• Data Link (L2 = MAC/Link)• Network (L3)• Transport (L4)• Session (L5)• Presentation (L6)• Application (L7)


PHY Layer

• Hardware• Physical manipulation of medium

(modulation)• Physical sensing of medium (detection)• Low level synchronization

(bits/symbols/frames)• Forward error correction/error detection• Mechanical/electrical interconnect and

medium


Link Layer• Firmware• Framing• Addressing• Medium access control (MAC)• Backward error detection/correction• Reliable delivery of frames from one STA

to a directly connected STA• Pacing• Upward multiplexing


Network Layer

• Software/firmware• Packets/cells• Routing• Packet fragmentation/reassembly• Backward error correction• Delivery of frames from source to an

indirectly connected destination• Congestion control


Transport Layer• Software on end host: End-to-end layer• Reliable communication stream

– Messages– Byte stream– Ordering– BEC

• Upward Multiplexing• Delivery of messages/byte stream from

source process to destination process• Congestion control


Session Layer

• Software on end host: end-to-end layer• Stream management• Dialog control• Packet chaining (atomic delivery)• Downward Multiplexing• Authentication• Connection-oriented


Presentation Layer

• Software on end host• Common utilities

– Encryption– Compression– Uniform formatting (XML, ASN.1,…)

• Standardized representations• Interfacing to local resources


Application Layer

• Software on end host• Specific application programs

– FTP– Remote terminal (rlogin, telnet, ssh,…)– Email– HTTP

• May also be layered in distributed software system


Encapsulation

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destinationMMMM

Ht

HtHnHtHnHl

MMMM

Ht

HtHnHtHnHl

messagesegmentdatagramframe

(thanks – Kurose & Ross)

• Receive SDU from higher layer• Hide uninterpreted SDU as payload of PDU


End-to-end Data Transport

(thanks – Kurose & Ross)


Service Models

• Interface– Connectionless– Connection-oriented

• Reliability– Best effort– Reliable

• Combinations


Service Interface

• Interface– Connectionless: memoryless

• Send packet• Receive packet

– Connection-oriented: stateful• Initialize (set up connection)• Use (send/receive)• Close (release state)


Service Reliability

• Reliability– Best effort

• Lost packets• Duplicate packets• Delayed/reordered delivery• Damaged packets

– Reliable• Undamaged packets• All packets sent delivered in timely fashion• Delivered in order sent


Message Conventions

1-REQ

2,3

2 - Tx 3 - Rx

4 - IND5 - RSP8 - CNF

6 - Tx7 - Rx6,7

Request/Confirm Indication/Response


Combining Service Models

Best Effort

Reliable

Connectionless Connection-Oriented

UDP, IP, IPX, CLNP, DECnet, Appletalk, CLNS,…

ATM

TCP, X.25, CONS???


Network Service vs. Implementation

Service

Implementation

Connectionless Connection-Oriented

UDP, CLNS

TCP, DNA

???Connectionless

Connection-Oriented X.25, ATM, CONS, SNA


Network Properties

• Scope• Scalability• Robustness• Autoconfigurability• Tweakability• Determinism• Migration


Network Properties - Robustness

• Types of Errors– Link/node failure– Data errors (esp. undetected!)– S/W errors– H/W errors– Human Errors

• Features


Network Properties - Robustness

• Types of Errors• Features

– Safety Barriers– Self-stabilization– Fault detection– Byzantine robustness


Reliable Data Transfer

• Models– Errors– Receiver capacity

• Requirements– Duplexity– Timers– State


Data Transfer Model• Events

– What can happen at node, channel• Frames

– What do they hold• Duplexity

– Simplex, half duplex, full duplex• Time costs

– What does it take to complete transfer• Metrics

– How do we measure the costs


Data Transfer - Events

• Node– New frame to send from HLE– Frame/ACK arrival – good frame– Frame/ACK arrival – damaged frame– Timeout– Attempt to receive next frame by HLE

• Channel– Error – damage frame– Error – lose frame


Data Transfer - Frames• Forward Control Info

– Type– Sequence number– Timestamp– Length– Addressing– Error Detection (FCS)

• Reverse Control Info– ACKs– Flow control/pacing– Piggybacking

• Information– payload

Dst Src Type SN TS Len … Payload FCS


Channel Model - Duplexity

• Simplex – only one way• Half duplex – one way at a time• Full duplex – simultaneously both ways

A BA B A B

SimplexHalf Duplex Full Duplex


Data Transfer - Delays

• Processing– Source– Destination

• Transmission– Time to put bits on wire

• Propagation– Time for bit to traverse

channel

Src Dest

Src Proc

Transmission

Propagation

Propagation

Src Proc

Dest Proc

Data frame

ACK frameACK Tx

Tx REQ

Rx REQ

Rx CNFTx CNF


Data Transfer - Metrics• Utilization

– Time sending info/total time• Storage requirements

– At source– At destination

• Channel type– duplexity

• Timers– Retransmission– ACK transmission


Reliable Data Xfer - Utopia• Infinitely fast receiver• Simplex channel• No errors• 100% utilization by

protocol

Src Dest


Stop&Wait Data Xfer - Pacing• Finitely fast receiver• Half duplex channel• No errors• <100% utilization by

protocol– UProtocol = TTx/Tcycle– TTx= L (bits)/R (bps)– Tcycle = TTx+Tprop+Tproc+

Tprop+Tproc

Src Dest

OK

OK

Data time

Cycle time


Reliable Data Xfer – PAR(Positive ACK and Retransmit)

• Finitely fast receiver• Full duplex channel• Channel errors• Utilization factor due to

errors– Uerrors = Tgood/ Tgood+ Tbad

• U = Uprotocol x Uerrors

Src Dest

ACK-

*

ACK-

X


Reliable Data Xfer – PARneed for sequence numbers

Src Dest

ACK-

*

ACK-

X

Src Dest

ACK-

*

ACK-

X

First Frame

Second Frame

Second Frame - duplicate

Missing ACK Missing ACK

ACK

Frame 1

Frame 2

Frame 2 - duplicate

Missing ACK


Reliable Data Xfer – 1-bit ARQSrc Dest

ACK 1-

*

ACK 0-

X

Src Dest

ACK 1-

*

ACK 0-

X

Frame 0

Frame 1

Frame 1

Missing ACK Missing ACK

Frame 0

Frame 1

Frame 1

Missing ACK

ACK 0

accept accept

accept

accept

discardduplicate


Protocol Utilization – 1-bit ARQ• Protocol util.

Uproto= T/(T+2τ)T = Tx time = D/RD = size (bits/frame)R = data rate (bps)τ = propagation delay

• Uproto= 1/(1+2α)α = τ /T = τ /(D/R)

τ

Src Dest

ACK 1

Frame 0

T

τ

ACK 0

Frame 1

Cycle Tim

e


Protocol Utilization – 1-bit ARQ1-bit ARQ Efficiency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.25 0.5 1 2 4 8 16 32 64

Normalize d de lay

Effic

ienc

y


Reliable Data Xfer – GBN ARQSrc Dest

*

-

D 0D 1ACK 1

Missing D3

Accept D0D 2

D 3 (resent)

D 4D 5

ACK 2ACK 3

ACK 3ACK 3 Discard D4

Discard D5

D 4D 5

ACK 6ACK 5

Accept D1Accept D2

Accept D3

Accept D4Accept D5

ACK 4

• Multiple sequence #’s• Channel errors• Discard out of order

frames on Rx• Resend all frames

from missing on forward

• Rx buffer size of 1• Tx buffer size of N


Reliable Data Xfer – SR ARQSrc Dest

*

-

Missing D3

D 0D 1ACK 1 Accept D0D 2

D 3 (resent)

D 4D 5

ACK 2ACK 3

ACK 3ACK 3 Buffer D4

Buffer D5

D 6D 7

ACK 8ACK 7

Accept D1Accept D2

Accept D3

Accept D6Accept D7

ACK 6

• Multiple sequence #’s• Channel errors• Buffer out of order

frames on Rx• Resend only missing

frame• Rx buffer size of N• Tx buffer size of k


Protocol Utilization – ARQSrc Dest

kT data

T cycle

• Protocol util.depends on kUproto= kT/(T+2τ)T = Tx time = D/Rτ = propagation delay

• Uproto= min(1,k/(1+2α))α = τ /T = τ /(D/R)

• Utilization factor due to errors different for GBN and SR

τ

τ

T


Protocol Utilization –ARQARQ Protocol Efficiency

0

0.2

0.4

0.6

0.8

1

1.2

0.25 0.5 1 2 4 8 16 32 64

a = normalized delay

effic

ienc

y

k=1k=2k=3k=5k=9k=17


Error Utilization Factor• SR-ARQ

Only resend missing framesUerr,SR= 1-pp = frame error prob

• GBN-ARQSend all frames after missing frameUerr,GBN=(1-p)/(1+2αp) when k>1+2αUerr,GBN=(1-p)/(1-p+kp) when k<1+2αDepends on k


Error Utilization FactorEfficiency Loss due to Errors

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Frame Error Rate p

Effic

ienc

y

GBN k=1GBN k=2GBN k=4GBN k=8GBN k=16GBN k=32

(large a)


Overall ARQ Utilization• SR-ARQ

USR= min{(1-p),k(1-p)/(1+2α)}

• GBN-ARQUGBN= (1-p)/(1+2αp) when k>1+2αUGBN= k(1-p)/(1+2α)(1-p+kp) when k<1+2α


Overall UtilizationARQ Efficiency, p=0.001

0

0.2

0.4

0.6

0.8

1

1.2

0.25 0.5 1 2 4 8 16 32 641282565121024

Normalized delay (a)

Effic

ienc

y

both k=1GBN k=9GBN k=65GBN k=129SR k=9SR k=65SR k=129


Sequence Numbers• Sequence numbers are finite (usually)

Assume n bits• Hence, they wrap around

i.e., 0 follows 2n-1• Need to use circular “<“ relation

i.e., 0 > 2n-1• Need to limit range usable by sender and

acceptable to receiver (SW and RW)|SW| + |RW| <= 2n

0 12

2n-1

2n-1-12n-1

2n-1+1

Computer Networksnemo/cen5501/slides/Chapter1.pdfCEN 5501C - Computer Networks - Spring 2007 -...

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