7/29/2019 Week 3 Lec 1-Bit

1/34

Chapter 1:IntroductionComputer Networking:A Top Down Approach ,

4th edition.Jim Kurose, Keith Ross

Computer Networks

7/29/2019 Week 3 Lec 1-Bit

2/34

Last Lecture

Queuing Delay

TCP/IP Protocol Stack

7/29/2019 Week 3 Lec 1-Bit

3/34

Todays Lecture

TCP/IP Protocol Stack

Encapsulation

Chapter 2

Application Layer

7/29/2019 Week 3 Lec 1-Bit

4/34

InternetProtocolStack To provide structure to design of

network protocols, networkdesigners organize protocols inlayers

Service says what a layer doesProtocol says how the service is

implementedAdvantagesDrawbacksWhen taken together the

protocols of various layers are

called the Protocol Stack.Internet Protocol Stack consistsof Five layers

Physical, Link, Network,Transport and Application layers .

Organization of Book

7/29/2019 Week 3 Lec 1-Bit

5/34

Internet Protocol Stack

Application Layer:

Network applications and their application layer protocolsreside.

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 endsystems

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

7/29/2019 Week 3 Lec 1-Bit

6/34

Internet Protocol Stack Transport Layer:

Transports application-layer messages between applicationend 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 service

Guaranteed delivery of application layer messages

Flow control

Congestion Control

User Datagram Protocol (UDP)Connectionless service

No reliability, flow control and congestion control

7/29/2019 Week 3 Lec 1-Bit

7/34

Internet Protocol Stack Network Layer:

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

Transport layer passes a transport layer

segment and a destination address to thenetwork layer.

Network layer includes IP Protocol

Defines the fields in the datagram as well as

how end systems and routers act on thesefields

Different routing protocols.

Determine the route that datagrams take

between sources and destinations

7/29/2019 Week 3 Lec 1-Bit

8/34

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 calledframes.

Transforms a raw transmission facility to areliable link.

Mechanism to detect and retransmit damagedor lost frames

Example of link layer protocols include WiFi,Ethernet etc.

7/29/2019 Week 3 Lec 1-Bit

9/34

Internet Protocol Stack Physical Layer:

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

Representation of bits

Physical Layer data consists of a stream of bits (0 or 1)

To be transmitted bits must be encoded into signals. Thephysical layer defines the type of encoding.

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

7/29/2019 Week 3 Lec 1-Bit

10/34

Internet Protocol Stack Application:Provides user interfaces and

support for services such as e-mail, filetransfer etc. FTP, HTTP

Transport:Transports application-layermessages between application end points.

Segmentation and reassembly TCP, UDP

Network:Routing of Datagrams fromsource to destination IP, routing protocols

Link:Move a packet from one node (host orrouter) 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

7/29/2019 Week 3 Lec 1-Bit

11/34

OSI Reference Model

In 1970 International Organization forStandardization proposed a seven layeredmodel called Open SystemsInterconnection (OSI) model.

Presentation Layer: Provide services such

as data encryption, compression. Session Layer: Synchronization points

(checkpointing) and recovery of dataexchange.

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

implemented in the application by theapplication developer.

7/29/2019 Week 3 Lec 1-Bit

12/34

source

applicationtransport

networklinkphysical

segment

datagram

destination

applicationtransportnetwork

linkphysical

router

switch

Encapsulationmessage

HtHn M

Ht

HtHnHl M

HtHn M

Ht MM

network

linkphysical

linkphysical

Ht

Hn

Hl

M

HtHn M HtHn M

HtHnHl M

M

Ht M

Hnframe

7/29/2019 Week 3 Lec 1-Bit

13/34

Chapter 2Application LayerComputer Networking: A

Top Down Approach,

4th edition.

Jim Kurose, Keith Ross

Addison-Wesley, July

2007.

7/29/2019 Week 3 Lec 1-Bit

14/34

Chapter 2: Application layer

2.1 Principles of network applications

2.2 Web and HTTP

2.3 FTP2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS2.6 P2P applications

7/29/2019 Week 3 Lec 1-Bit

15/34

Chapter 2: Application Layer

Our goals: conceptual,

implementationaspects of networkapplicationprotocols

client-server

paradigmpeer-to-peer

paradigm

learn about protocolsby examining popularapplication-levelprotocols

HTTP

FTP

SMTP / POP3 /

IMAPDNS

7/29/2019 Week 3 Lec 1-Bit

16/34

Some Network Applications

E-mail

Web

Instant messaging

Remote login P2P file sharing

Multi-user network games

Streaming stored video clips

Voice over IP Real-time video conferencing

Many more interesting applications

7/29/2019 Week 3 Lec 1-Bit

17/34

Creating a Network Application

write programs that run on (different) end systems

communicate over network

e.g., web server softwarecommunicates with browser

softwareNo need to write software fornetwork-core devices

Network-core devices do not runuser applications

Function at lower layers

Basic design is to confineapplication software to the endsystems

Facilitate rapid networkapplication development and

deployment

application

transport

network

data link

physical

application

transport

network

data linkphysical

application

transport

network

data link

physical

7/29/2019 Week 3 Lec 1-Bit

18/34

Application Architectures

Application Architecture isDesigned by the application developer

Dictates how the application isstructured over various end systems

Two architectures used in modern dayNetwork Applications:Client-Server Architecture

Peer-to-Peer (P2P) Architecture

7/29/2019 Week 3 Lec 1-Bit

19/34

Client-Server ArchitectureServer:

Always-on host

permanent well defined IPaddress

A single server is incapable ofkeeping up with all the requestsof the clients.

A cluster of hosts referred asserver farm is often used.

Clients:

Communicate with server

May have dynamic IP addresses

Do not communicate directly witheach other

Client/Server architecture isinfrastructure intensive

Require service providers toinstall and maintain servers.

Client/Server

7/29/2019 Week 3 Lec 1-Bit

20/34

P2P Architecture

Direct communicationbetween pairs ofintermittently connectedhosts called peers

Peers are not owned by anyservice provider

P2P Peers communicate without

passing through anydedicated server

e.g. Bit Torrent, eMule, Skype

Security issues Highly Distributive

Cost Effective

Detailed study later on in the

chapter

peer-peer

h d

7/29/2019 Week 3 Lec 1-Bit

21/34

What Transport Service does anApplication need?

Data Loss

Loss Tolerant Applications Some apps (e.g., audio, VoIP)

can tolerate some loss 2% tolerable for VoIP

Other apps (e.g., file transfer,

email) require 100% reliabledata transfer

Timing Application may require

timing guarantee

Tight timing constraints multiplayer games, VoIP,teleconferencing.

In Non-real time lower delaysare preferred but no tightconstraint on end-to-enddelays.

Throughput

Bandwidth sensitiveapplications (e.g., multimedia)require minimum amount ofthroughput

Other apps (elastic apps)

make use of whateverthroughput they get e.g .Email, file transferSecurity Encryption and decryption

7/29/2019 Week 3 Lec 1-Bit

22/34

Transport Service Requirements of CommonApplications

Application

file transfer

e-mail

Web documentsreal-time audio/video

stored audio/video

interactive games

Data loss

no loss

no loss

no lossloss-tolerant

loss-tolerant

loss-tolerant

Throughput

elastic

elastic

elasticaudio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps -10kbps

Time Sensitive

no

no

noyes, 100s msec

yes, few secs

yes, 100s msec

7/29/2019 Week 3 Lec 1-Bit

23/34

Internet transport protocols services

TCP service: connection-oriented:setup

required between client andserver processes

reliable transportbetweensending and receiving process

flow control:sender wontoverwhelm receiver

congestion control:throttle

sender when networkoverloaded

does not provide:timing,minimum throughputguarantees, security

UDP service: unreliable data transfer

between sending andreceiving process

does not provide:connection setup,reliability, flow control,security

Throughput and timing

guarantee not provided

7/29/2019 Week 3 Lec 1-Bit

24/34

Internet Applications: application, transportprotocols

Application

e-mail

remote terminal access

Webfile transfer

streaming multimedia

Internet telephony

Applicationlayer protocol

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]FTP [RFC 959]

RTP [RFC 1889]

SIP, Skype

Underlyingtransport protocol

TCP

TCP

TCPTCP

UDP

UDP

7/29/2019 Week 3 Lec 1-Bit

25/34

Application -layer Protocol Defines

Types of messagesexchanged, e.g., request, response

Message syntax: what fields in messages

Message semantics meaning of information in

fields

Rules for when and howprocesses send &respond to messages

Public-domain protocols:

defined in RFCs

allows for

interoperability e.g., HTTP, SMTP

Proprietary protocols:

e.g., Skype

7/29/2019 Week 3 Lec 1-Bit

26/34

Addressing processes

To receive messages,

process must haveidentifier

host device has unique 32-bit IP address

Q:does IP address of hoston which process runssuffice for identifying theprocess?

A:No, manyprocessescan be running on samehost

To identify the receiving process

two pieces of information need tobe specified:

IP address of the host

Port number

Identifier that specifies the

receiving process in thedestination host

Popular applications have beenassigned a specific port number.

Example port numbers:

HTTP server: 80 Mail server: 25

www.iana.org (well known portnumbers for all protocols)

More in chapter 3

7/29/2019 Week 3 Lec 1-Bit

27/34

Hyper Text Transfer Protocol (HTTP) HTTP defined in RFC 1945 and RFC 2616

HTTP implemented on both client and server sides

HTTP definesstructure of messages exchanged between client and

server.

How the client and server exchange the messages

Web browsers implement the client side of HTTP

Web Servers implement the server side of HTTP Host web objects each addressable by a URL

Web page consists of objects Object can be HTML file, JPEG image, audio file

Web pages consist of base HTML file and severalreferenced objects

Each object is addressable by a URL

7/29/2019 Week 3 Lec 1-Bit

28/34

HTTP overview

HTTP: hypertexttransfer protocol

Webs application layerprotocol

client/server model client:browser that

requests, receives,displays Web objects

server:Web server

sends objects inresponse to requests

PC running

Explorer

Server

running

Apache Web

server

Linux running

Firefox

7/29/2019 Week 3 Lec 1-Bit

29/34

HTTP overview (continued)Uses TCP:

Client initiates TCP connection to server, port 80 Server accepts TCP connection from client

HTTP messages (application-layer protocolmessages) exchanged between browser (HTTP

client) and Web server (HTTP server) Advantage of Layered architecture

HTTP does not worry about lost data or how TCPrecovers from loss

HTTP is stateless Server maintains no information about past client

requests

7/29/2019 Week 3 Lec 1-Bit

30/34

HTTP connections

Nonpersistent HTTP

At most one object issent over a TCPconnection.

Persistent HTTP

Multiple objects canbe sent over singleTCP connectionbetween client andserver.

7/29/2019 Week 3 Lec 1-Bit

31/34

Nonpersistent HTTPSuppose user enters URL

www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCPconnection to HTTP server(process) atwww.someSchool.edu on port

80

2. HTTP client sends HTTPrequest message (containing

URL) into TCP connection

socket. Message indicates thatclient wants object

someDepartment/home.index

1b. HTTP server at hostwww.someSchool.edu waiting

for TCP connection at port 80.accepts connection, notifying

client

3. HTTP server receives request

message, forms responsemessage containing requested

object, and sends message to

the client

time

(contains text,

references to 10

jpeg images)

7/29/2019 Week 3 Lec 1-Bit

32/34

Nonpersistent HTTP (cont.)

5. HTTP client receives responsemessage containing html file,displays html. Parsing htmlfile, finds 10 referenced jpegobjects

6.Steps 1-4 repeated for eachof 10 jpeg objects

4. HTTP server closes TCPconnection.

time

When the user requests the webpage 11 TCP connectionsare generated

7/29/2019 Week 3 Lec 1-Bit

33/34

Non-Persistent HTTP: Response time

Definition of RTT: time fora packet to travel fromclient to server and back.

Response time:

one RTT to initiate TCPconnection

one RTT for HTTPrequest response

total = 2RTT+transmittime

time to

transmit

file

initiate TCP

connection

RTT

request

file

RTT

file

received

time time

7/29/2019 Week 3 Lec 1-Bit

34/34

Persistent HTTP

Nonpersistent HTTPissues:

Requires 2 RTTs perobject

Maintain TCP buffers inboth client and server

Burden on the WebServer

High overhead

Persistent HTTP Server leaves TCP

connection open aftersending response

Subsequent HTTPmessages betweensame client/serversent over open

connection Default mode is

Persistent withpipelining