Overview of Network

Protocols, Services & LayeringOSI Reference Model

TCP/IP ArchitectureError Detection and Correction

What is Computer Networks?

A collection of autonomous computers interconnected by a single technology Interconnected via:

Copper wireFiber opticsMicrowavesInfraredCommunication satellites, etc.

Protocols, Services & Layering

Layers, Services & Protocols

The overall communications process between two or more machines connected across one or more networks is very complex

Layering partitions related communications functions into groups that are manageable

Each layer provides a service to the layer above

Each layer operates according to a protocol

Protocols

A protocol is a set of rules that governs how two or more communicating entities in a layer are to interact

Messages that can be sent and received Actions that are to be taken when a certain

event occurs, e.g. sending or receiving messages, expiry of timers

The purpose of a protocol is to provide a service to the layer above

Layers

A set of related communication functions that can be managed and grouped together

Application Layer: communications functions that are used by application programs HTTP, DNS, SMTP (email)

Transport Layer: end-to-end communications between two processes in two machines TCP, User Datagram Protocol (UDP)

Network Layer: node-to-node communications between two machines Internet Protocol (IP)

Example: TCP

TCP is a transport layer protocol Provides reliable byte stream service between two

processes in two computers across the Internet Sequence numbers keep track of the bytes that have

been transmitted and received Error detection and retransmission used to recover

from transmission errors and losses TCP is connection-oriented: the sender and receiver

must first establish an association and set initial sequence numbers before data is transferred

Connection ID is specified uniquely by (send port #, send IP address, receive port #, receiver IP address)

Example: UDP

UDP is a transport layer protocol Provides best-effort datagram service

between two processes in two computers across the Internet

Port numbers distinguish various processes in the same machine

UDP is connectionless Datagram is sent immediately Quick, simple, but not reliable

Summary Layers: related communications functions

Application Layer: HTTP, DNS Transport Layer: TCP, UDP Network Layer: IP

Services: a protocol provides a communications service to the layer above TCP provides connection-oriented reliable byte transfer

service UDP provides best-effort datagram service

Each layer builds on services of lower layers HTTP builds on top of TCP DNS builds on top of UDP TCP and UDP build on top of IP

Chapter 2 Applications and

Layered Architectures

OSI Reference Model

Why Layering?

Layering simplifies design, implementation, and testing by partitioning overall communications process into parts

Protocol in each layer can be designed separately from those in other layers

Protocol makes “calls” for services from layer below Layering provides flexibility for modifying and

evolving protocols and services without having to change layers below

Monolithic non-layered architectures are costly, inflexible, and soon obsolete

Open Systems Interconnection

Network architecture: Definition of all the layers Design of protocols for every layer

By the 1970s every computer vendor had developed its own proprietary layered network architecture

Problem: computers from different vendors could not be networked together

Open Systems Interconnection (OSI) was an international effort by the International Organization for Standardization (ISO) to enable multivendor computer interconnection

OSI Reference Model

Describes a seven-layer abstract reference model for a network architecture

Purpose of the reference model was to provide a framework for the development of protocols

OSI also provided a unified view of layers, protocols, and services which is still in use in the development of new protocols

Detailed standards were developed for each layer, but most of these are not in use

TCP/IP protocols preempted deployment of OSI protocols

7-Layer OSI Reference Model

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Application Application

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Communicating End SystemsOne or More Network Nodes

End-to-End Protocols

Physical Layer

Transfers bits across link Definition & specification of the physical

aspects of a communications link Mechanical: cable, plugs, pins... Electrical/optical: modulation, signal strength,

voltage levels, bit times, … functional/procedural: how to activate, maintain, and

deactivate physical links… Ethernet, DSL, cable modem, telephone

modems… Twisted-pair cable, coaxial cable, optical fiber,

radio, infrared, …

Data Link Layer

Transfers frames across direct connections Groups bits into frames Detection of bit errors; Retransmission of frames Activation, maintenance, & deactivation of data link

connections Medium access control for local area networks Flow control

Data LinkLayer

PhysicalLayer

Data LinkLayer

PhysicalLayer

frames

bits

Network Layer

Transfers packets across multiple links and/or multiple networks

Addressing must scale to large networks Nodes jointly execute routing algorithm to

determine paths across the network Forwarding transfers packet across a node Congestion control to deal with traffic surges Connection setup, maintenance, and

teardown when connection-based

Internetworking Internetworking is part of network layer and provides transfer

of packets across multiple possibly dissimilar networks Gateways (routers) direct packets across networks

G = gateway H = host

Net 1

Net 5

Net 3

Net 2

HNet 3

G

H

H

H

GG

GG

G

Net 1

Net 2 Net 4

Net 5

Ethernet LAN

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMNetwork

Transport Layer

Transfers data end-to-end from process in a machine to process in another machine

Reliable stream transfer or quick-and-simple single-block transfer

Multiplexing Message segmentation and reassembly Connection setup, maintenance, and release

TransportLayer

NetworkLayer

TransportLayer

NetworkLayer

NetworkLayer

NetworkLayer

Communication Network

Application & Upper Layers

Application Layer: Provides services that are frequently required by applications: DNS, web access, file transfer, email…

Presentation Layer: machine-independent representation of data…

Session Layer: dialog management, recovery from errors, …

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

Application

ApplicationLayer

TransportLayer

Application

Incorporated into Application Layer

Headers & Trailers Each protocol uses a header that carries addresses,

sequence numbers, flag bits, length indicators, etc… CRC check bits may be appended for error detection

ApplicationLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Application ApplicationAPP DATA

AH APP DATA

TH AH APP DATA

NH TH AH APP DATA

DH NH TH AH APP DATA CRC

bits

Summary:7-Layer OSI Reference Model

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Application Application

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

One or More Network Nodes

End-to-End Protocols

Chapter 2 Applications and

Layered Architectures

TCP/IP Architecture

How the Layers Work Together

TCP/IP Protocol Suite

(ICMP, ARP)

Diverse network technologies

Reliable stream service

Userdatagram service

Distributed applications

HTTP SMTP RTP

TCP UDP

IP

Network

interface 1

Network

interface 3

Network

interface 2

DNS

Best-effort connectionless packet transfer

Encapsulation

TCP Header contains source & destination

port numbers

IP Header contains source and destination

IP addresses; transport protocol type

Ethernet Header contains source & destination MAC addresses; network protocol type

HTTP Request

TCP header HTTP Request

IP header

TCP header HTTP Request

Ethernet header

IP header

TCP header HTTP Request FCS

Recap: TCP/IP Protocol Suite

(ICMP, ARP)

Diverse network technologies

Transport Layer

HTTP SMTP RTP

TCP UDP

IP

Network

interface 1

Network

interface 3

Network

interface 2

DNS

Internet Layer

Application Layer

Network Interface Layer

Summary

Encapsulation is key to layering IP provides for transfer of packets across

diverse networks TCP and UDP provide universal

communications services across the Internet Distributed applications that use TCP and

UDP can operate over the entire Internet Internet names, IP addresses, port numbers,

sockets, connections, physical addresses

Error Detection and Correction

Error Control

Digital transmission systems introduce errors Applications require certain reliability level

Data applications require error-free transfer Voice & video applications tolerate some errors

Error control used when transmission system does not meet application requirement

Error control ensures a data stream is transmitted to a certain level of accuracy despite errors

Two basic approaches: Error detection & retransmission (ARQ: Automatic

Retransmission Request) Forward error correction (FEC)

Key Idea All transmitted data blocks (“codewords”) satisfy a

pattern If received block doesn’t satisfy pattern, it is in error Redundancy: Only a subset of all possible blocks

can be codewords Blindspot: when channel transforms a codeword

into another codeword

ChannelEncoderUserinformation

Patternchecking

All inputs to channel satisfy pattern or condition

Channeloutput

Deliver user information orset error alarm

Single Parity Check

Append an overall parity check to k information bits

Info Bits: b1, b2, b3, …, bk

Check Bit: bk+1= b1+ b2+ b3+ …+ bk modulo 2

Codeword: (b1, b2, b3, …, bk,, bk+1)

All codewords have even # of 1s Receiver checks to see if # of 1s is even

All error patterns that change an odd # of bits are detectable All even-numbered patterns are undetectable

Parity bit used in ASCII code

Example of Single Parity Code

Information (7 bits): (0, 1, 0, 1, 1, 0, 0) Parity Bit: b8 = 0 + 1 +0 + 1 +1 + 0 = 1 Codeword (8 bits): (0, 1, 0, 1, 1, 0, 0, 1)

If single error in bit 3 : (0, 1, 1, 1, 1, 0, 0, 1) # of 1’s =5, odd Error detected

If errors in bits 3 and 5: (0, 1, 1, 1, 0, 0, 0, 1) # of 1’s =4, even Error not detected

Two-Dimensional Parity Check

1 0 0 1 0 0

0 1 0 0 0 1

1 0 0 1 0 0

1 1 0 1 1 0

1 0 0 1 1 1

Bottom row consists of check bit for each column

Last column consists of check bits for each row

More parity bits to improve coverage Arrange information as columns Add single parity bit to each column Add a final “parity” column Used in early error control systems

1 0 0 1 0 0

0 0 0 1 0 1

1 0 0 1 0 0

1 0 0 0 1 0

1 0 0 1 1 1

1 0 0 1 0 0

0 0 0 0 0 1

1 0 0 1 0 0

1 0 0 1 1 0

1 0 0 1 1 1

1 0 0 1 0 0

0 0 0 1 0 1

1 0 0 1 0 0

1 0 0 1 1 0

1 0 0 1 1 1

1 0 0 1 0 0

0 0 0 0 0 1

1 0 0 1 0 0

1 1 0 1 1 0

1 0 0 1 1 1

Arrows indicate failed check bits

Two errorsOne error

Three errors Four errors

(undetectable)

Error-detecting capability

1, 2, or 3 errors can always be

detected; Not all patterns >4 errors can be detected

Other Error Detection Codes

Many applications require very low error rate Need codes that detect the vast majority of errors Single parity check codes do not detect enough

errors Two-dimensional codes require too many check bits The following error detecting codes used in practice:

CRC Polynomial Codes

Polynomial Codes

Polynomials instead of vectors for codewords Polynomial arithmetic instead of check sums Implemented using shift-register circuits Also called cyclic redundancy check (CRC) codes Most data communications standards use

polynomial codes for error detection Polynomial codes also basis for powerful error-

correction methods

Addition:

Multiplication:

Binary Polynomial Arithmetic Binary vectors map to polynomials

(ik-1 , ik-2 ,…, i2 , i1 , i0) ik-1xk-1 + ik-2xk-2 + … + i2x2 + i1x + i0

(x7 + x6 + 1) + (x6 + x5) = x7 + x6 + x6 + x5 + 1

= x7 +(1+1)x6 + x5 + 1

= x7 +x5 + 1 since 1+1=0 mod2

(x + 1) (x2 + x + 1) = x(x2 + x + 1) + 1(x2 + x + 1)

= x3 + x2 + x + (x2 + x + 1)

= x3 + 1

Binary Polynomial Division Division with Decimal Numbers

32

35 ) 12223

10517 2

4

140divisor

quotient

remainder

dividend1222 = 34 x 35 + 32

dividend = quotient x divisor +remainder

Polynomial Divisionx3 + x + 1 ) x6 + x5

x6 + x4 + x3

x5 + x4 + x3

x5 + x3 + x2

x4 + x2

x4 + x2 + x

x

= q(x) quotient

= r(x) remainder

divisordividend

+ x+ x2x3

Note: Degree of r(x) is less than degree of divisor

Polynomial Coding Code has binary generator polynomial of degree n–k

k information bits define polynomial of degree k – 1

Find remainder polynomial of at most degree n – k – 1

g(x) ) xn-k i(x)

q(x)

r(x)xn-ki(x) = q(x)g(x) + r(x)

Define the codeword polynomial of degree n – 1

b(x) = xn-ki(x) + r(x)

n bits k bits n-k bits

g(x) = xn-k + gn-k-1xn-k-1 + … + g2x2 + g1x + 1

i(x) = ik-1xk-1 + ik-2xk-2 + … + i2x2 + i1x + i0

Transmitted codeword:b(x) = x6 + x5 + xb = (1,1,0,0,0,1,0)

1011 ) 1100000

1110

1011

1110

1011

10101011

010

x3 + x + 1 ) x6 + x5

x3 + x2 + x

x6 + x4 + x3

x5 + x4 + x3

x5 + x3 + x2

x4 + x2

x4 + x2 + x

x

Polynomial example: k = 4, n–k = 3Generator polynomial: g(x)= x3 + x + 1

Information: (1,1,0,0) i(x) = x3 + x2

Encoding: x3i(x) = x6 + x5

The Pattern in Polynomial Coding

All codewords satisfy the following pattern:

All codewords are a multiple of g(x)! Receiver should divide received n-tuple by g(x) and check if remainder is zero If remainder is nonzero, then received n-tuple is not a codeword

b(x) = xn-ki(x) + r(x) = q(x)g(x) + r(x) + r(x) = q(x)g(x)

Standard Generator Polynomials

CRC-8:

CRC-16:

CCITT-16:

CCITT-32:

CRC = cyclic redundancy check

HDLC, XMODEM, V.41

IEEE 802, DoD, V.42

Bisync

ATM

= x8 + x2 + x + 1

= x16 + x15 + x2 + 1= (x + 1)(x15 + x + 1)

= x16 + x12 + x5 + 1

= x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1