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Chapter 2Application LayerComputer Networking:A Top Down Approach,4th edition.Jim Kurose, Keith RossAddison-Wesley, July2007.

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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

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HTTP connections

Nonpersistent HTTP

At most one object issent over a TCPconnection.

Persistent HTTP

Multiple objects canbe sent over singleTCP connectionbetween client andserver.

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Nonpersistent HTTPSuppose user enters URL

www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCPconnection to HTTP server(process) at

www.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)

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Nonpersistent HTTP (cont.)

5. HTTP client receives responsemessage containing html file,displays html. Parsing html

file, finds 10 referenced jpegobjects

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

4. HTTP server closes TCP

connection.

time

When the user requests the webpage 11 TCP connectionsare generated

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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

RTTrequest

file

RTT

file

received

time time

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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

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HTTP Request MessageTwo types of HTTP messages:

Request, ResponseHTTP request message:

Human-readable format

A typical HTTP request message

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu

User-agent: Mozilla/4.0Connection: closeAccept-language:fr

(Blank Line)

Body

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HTTP Request Message

Request Line

First Line of HTTP request message

The request line has three fields

Method field can take on several values

GET,POST, PUT, DELETE etcMajority of HTTP request messages use GET

URL field

HTTP version field

Header Lines Host: Host on which the object resides

User-agent: Identifies the client program e.g. Mozilla/4.0

Connection: Shows whether the connection should be closedor not

Accept-language: Shows the language the client prefers

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HTTP Request Message

Body is empty withGET method

Used with the POSTmethod

Web page oftenincludes form input

Input is sent toserver in body

PUT method:

To upload objects toweb server

DELETE method Allows a user or an

application to deletean object on web

server

Find out about OPTIONS, TRACE, HEAD?

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HTTP Response Message

HTTP/1.1 200 OKConnection: closeDate: Thu, 06 Aug 1998 12:00:15

GMTServer: Apache/1.3.0 (Unix)Last-Modified: Mon, 22 Jun 1998Content-Length: 6821Content-Type: text/html

Body contains Requested Object

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HTTP Response Message

Status Line Protocol Version

Status Code and Status MessageThe status code and associated phrase indicate the result of

the request Header Line Connection: Shows whether the connection should be closed

or not

Date and time: When the HTTP response was created

Server: Analogous to user-agent in request message

Last-modified: When the Object was last created ormodified

Content-length: Number of bytes in object being sent

Content-Type: Indicates the object type (HTML text etc.)

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HTTP Response Status Codes

200 OK Request succeeded and information is returned in

response

301 Moved Permanently Requested object moved, new location specified later in

this message (Location:)

400 Bad Request Request message not understood by server

404 Not Found Requested document not found on this server

505 Version not supported HTTP version is not supported by the server

The status code and associated phrase indicate the result ofthe request

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User-Server Interaction: Cookies

It is often desirable for webservers to identify users Server wishes to restrict

user access Wants to serve content as a

function of user identity Many major Web sites use

cookies

Four components:

1) Cookie header line ofHTTP responsemessage

2) Cookie header line inHTTP requestmessage3) Cookie file kept on users

host, managed by usersbrowser

4) Back-end database atWeb site

Example: Susan accesses

Internet always from PC visits specific e-

commerce site for firsttime When initial HTTP

requests arrives atserver, server creates: unique ID entry in backend

database for ID

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Cookies: keeping state (cont.)

ClientServer

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msgcookie: 1678 cookie-

specific

action

access

usual http request msgAmazon server

creates ID

1678 for user create

entry

usual http responseSet-cookie: 1678

amazon 1678

usual http request msgcookie: 1678 cookie-

spectific

action

accessamazon 1678

backend

database

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Cookies (continued)

Cookies Pages visited in which

order and at whattimes

Shopping carts One click shopping Recommendations of

other products

Cookies and privacy: Cookies permit sites to

learn a lot about you

Can inform thirdparty You may supply name

and e-mail to sites

For more info on Cookies see RFC-2965 HTTP State Management Mechanism www.cookiecentral.com

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Web Caches (Proxy Server)

Web Cache has its owndisk storage

Keeps copies of recentlyrequested objects in

storage User sets browser: Web

accesses via cache

Browser sends all HTTPrequests to cache

object in cache: cachereturns object

else cache requestsobject from origin

server, then returnsobject to client

Goal: Satisfy client request without involving origin server

Client

Proxy

Server

Client

OriginServer

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More about Web Caching

Cache acts as bothclient and server

Typically cache isinstalled by ISP(university, company)

Why Web Caching?

Reduce response timefor client request

Reduce traffic on aninstitutions access link. Does not have to upgrade

bandwidth as quickly,thereby reducing costs.