Chapter 1:Introduction Computer

Networking: A Top Down Approach ,

4th edition. Jim Kurose, Keith Ross

Computer Networks

Today’s Lecture

Layered Architecture Brief description of Five Layers

Internet Protocol Stack

To provide structure to design of network protocols, network designers organize protocols in layers

Service – says what a layer doesProtocol – says how the service is

implementedAdvantages DrawbacksWhen taken together the protocols

of various layers are called the Protocol Stack.

Internet Protocol Stack consists of Five layers

Physical, Link, Network, Transport and Application layers .

Organization of Book

Internet Protocol Stack

To provide structure to design of network protocols, network designers organize protocols in layers

Service – says what a layer doesProtocol – says how the service is

implementedAdvantages DrawbacksWhen taken together the protocols

of various layers are called the Protocol Stack.

Internet Protocol Stack consists of Five layers

Physical, Link, Network, Transport and Application layers .

Organization of Book

Internet Protocol Stack

• Application Layer:

Network applications and their application layer protocols reside.

Provides user interfaces and support for services such as e-mail, file transfer etc.

Hyper Text Transfer Protocol (HTTP)File Transfer Protocol (FTP)Session Initiation Protocol (SIP)

An application layer protocol is distributed over multiple end systems

The packets of information at the application layer is called as a message.

Internet Protocol Stack• Transport Layer:

Transports application-layer messages between application end points.

Transport layer packet is called as a segment Breaks long messages into shorter segments There are two Transport Layer Protocols Transmission Control Protocol (TCP)

Connection Oriented serviceGuaranteed delivery of application layer messagesFlow controlCongestion Control

User Datagram Protocol (UDP)Connectionless serviceNo reliability, flow control and congestion control

Internet Protocol Stack• Network Layer:

Responsible for moving network layer packets known as datagrams from one host to another.

Transport layer passes a transport layer segment and a destination address to the network layer.

Network layer includes IP ProtocolDefines the fields in the datagram as well

as how end systems and routers act on these fields

Commonly referred as IP layer.Different routing protocols.

Determine the route that datagrams take between source and destination

Internet Protocol Stack• Link Layer:

Moves a packet from one node (host or router) to the next node in the route.

Divide the stream of bits received from the network layer into manageable data units called frames.

Transforms a raw transmission facility to a reliable link.Mechanism to detect and retransmit

damaged or lost framesExample of link layer protocols include

WiFi, Ethernet etc.

Internet Protocol Stack• Physical Layer:

The job of this layer is to move the individual bits with in frames from one node to next.

Representation of bitsPhysical Layer data consists of a stream of

bits (0 or 1)To be transmitted bits must be encoded

into signals. The physical layer defines the type of encoding.

The protocol in this layer depend on the actual transmission medium of the link.

Internet Protocol Stack Application: Provides user interfaces

and support for services such as e-mail, file transfer etc. FTP, HTTP

Transport: Transports application-layer messages between application end points. Segmentation and reassembly TCP, UDP

Network: Routing of Datagrams from source to destination IP, routing protocols

Link: Move a packet from one node (host or router) to the next node in the route. Ethernet, WiFi

Physical: Move the individual bits with in frames from one node to next

Application

Transport

Network

Link

Physical

OSI Reference Model In 1970 International Organization for

Standardization proposed a seven layered model called Open Systems Interconnection (OSI) model.

Presentation Layer: Provide services such as data encryption, compression.

Session Layer: Synchronization points (checkpointing) and recovery of data exchange.

Internet stack “missing” these layers! these services, if needed, must be

implemented in the application by the application developer.

source

application

transportnetwork

linkphysical

segment

datagram

destination

application

transportnetwork

linkphysical

router

switch

Encapsulationmessage

HtHn M

Ht

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M HtHn M

HtHnHl M

M

Ht M

Hn

frame

Throughput The rate (bits/sec) at which bits are transferred

between sender/receiver Difference between Bandwidth and Throughput?

ISPs sell bandwidth In computer networks, the throughput is less

than the bandwidth for several reasons • The channel may be shared by other users• Packet loss due to congestion• Packet loss due to bit errors• Noise in the channel• Transmission rates of the link over which

the data flows.

Throughput

Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Throughput is min {Rs,Rc} Transmission Rate of the bottleneck link

Throughput

10 connections share bottleneck link R

Rs

Rs

Rs

Rc

Rc

Rc

R

10 clients/servers pairs, Common link R traversed

by all 10. Rate of the link R is very

large then the throughput is min {Rs , Rc}

Rs=2Mbps, Rc=1Mbps,

R=5Mbps Common link divides

transmission rate equally among the 10 downloads

500kbps to each download

Shared Link R is now the bottleneck.