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Computer Networks

HTTP

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Web and HTTP

First some jargon

Web page consists of objects Object can be HTML le, JPEG image, Java applet, audio le, Web page consists of base HTML-le which includes several

referenced objects

Each object is addressable by a URL Example URL:

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www.someschool.edu/someDept/pic.gif

host name path name

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HTTP overview HTTP: hypertext transfer protocol Webs applicaOon layer protocol client/server model

client: browser that requests, receives, displays Web objects

server: Web server sends objects in response to requests

Uses TCP: client iniOates TCP connecOon (creates socket) to server, port 80 server accepts TCP connecOon from client HTTP messages (applicaOon-layer protocol messages) exchanged between browser (HTTP

client) and Web server (HTTP server) TCP connecOon closed HTTP is stateless server maintains no informaOon about past client requests Protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent, must be reconciled

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PC running Explorer

Server running Apache Web server Mac running

Navigator

HTTP request

HTTP re

quest

HTTP response

HTTP re

sponse

HTTP connecOons

Nonpersistent HTTP

At most one object is sent over a TCP connecOon.

Persistent HTTP

MulOple objects can be sent over single TCP connecOon between client and server.

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Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index (contains text, references to 10 jpeg images)

1a. HTTP client iniOates TCP connecOon to HTTP server (process) at www.someSchool.edu on port 80

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2. HTTP client sends HTTP request message (containing URL) into TCP connecOon socket. Message indicates that client wants object someDepartment/home.index

1b. HTTP server at host www.someSchool.edu waiOng for TCP connecOon at port 80. accepts connecOon, noOfying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket

Ome

5. HTTP client receives response message containing html le, displays html. Parsing html le, nds 10 referenced jpeg objects

4. HTTP server closes TCP connecOon.

6. Steps 1-5 repeated for each of 10 jpeg objects

Non-Persistent HTTP: Response Ome

DeniOon of RTT: Ome for a small packet to travel from client to server and back.

Response Ome:

one RTT to iniOate TCP connecOon

one RTT for HTTP request and rst few bytes of HTTP response to return

le transmission Ome total = 2RTT+transmit Ome

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time to transmit file

initiate TCP connection

RTT request file

RTT

file received

time time

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Persistent HTTP

Nonpersistent HTTP issues:

requires 2 RTTs per object OS overhead for each TCP

connecOon

browsers oeen open parallel TCP connecOons to fetch referenced objects

Persistent HTTP

server leaves connecOon open aeer sending response

subsequent HTTP messages between same client/server sent over open connecOon

client sends requests as soon as it encounters a referenced object

as lifle as one RTT for all the referenced objects

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HTTP request message

two types of HTTP messages: request, response HTTP request message:

ASCII (human-readable format)

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GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr

(extra carriage return, line feed)

request line (GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

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HTTP request message: general format

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Uploading form input

Post method:

Web page oeen includes form input

Input is uploaded to server in enOty body

URL method:

Uses GET method Input is uploaded in URL

eld of request line:

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www.somesite.com/animalsearch?monkeys&banana

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Method types

HTTP/1.0

GET POST HEAD

asks server to leave requested object out of response

HTTP/1.1

GET, POST, HEAD PUT

uploads le in enOty body to path specied in URL eld

DELETE deletes le specied in the

URL eld

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HTTP response message

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HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 ... Content-Length: 6821 Content-Type: text/html data data data data data ...

status line (protocol

status code status phrase)

header lines

data, e.g., requested HTML file

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HTTP response status codes

200 OK request succeeded, requested object later in this message

301 Moved Permanently requested object moved, new locaOon specied later in this

message (LocaOon:)

304 Not modied See condiOonal GET

400 Bad Request request message not understood by server

404 Not Found requested document not found on this server

505 HTTP Version Not Supported 13

In rst line in server -> client response message.

A few sample codes:

User-server state: cookies

Many major Web sites use cookies

Four components: 1) cookie header line of HTTP

response message 2) cookie header line in HTTP

request message 3) cookie le kept on users host,

managed by users browser 4) back-end database at Web

site

Example:

Susan always access Internet always from PC

visits specic e-commerce site for rst Ome

when iniOal HTTP requests arrives at site, site creates:

unique ID entry in backend database for ID

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Cookies: keeping state (cont.)

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client server

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msg cookie: 1678 cookie- specific action

access

ebay 8734 usual http request msg Amazon server

creates ID 1678 for user create entry

usual http response Set-cookie: 1678 ebay 8734 amazon 1678

usual http request msg cookie: 1678 cookie- spectific

action

access ebay 8734 amazon 1678

backend database

Cookies (conOnued) What cookies can bring:

authorizaOon shopping carts recommendaOons user session state (Web e-

mail)

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Cookies and privacy:

cookies permit sites to learn a lot about you

you may supply name and e-mail to sites

aside

How to keep state:

protocol endpoints: maintain state at sender/receiver over mulOple transacOons

cookies: hfp messages carry state

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Web caches (proxy server)

user sets browser: Web accesses via cache

browser sends all HTTP requests to cache object in cache: cache

returns object

else cache requests object from origin server, then returns object to client

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Goal: satisfy client request without involving origin server

client

Proxy server

client

HTTP re

quest

HTTP re

sponse

HTTP request HTTP req

uest

origin server

origin server

HTTP response HTTP res

ponse

More about Web caching

cache acts as both client and server

typically cache is installed by ISP (university, company, residenOal ISP)

Why Web caching?

reduce response Ome for client request

reduce trac on an insOtuOons access link.

Internet dense with caches: enables poor content providers to eecOvely deliver content (but so does P2P le sharing)

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Caching example AssumpOons average object size = 1,000,000

bits avg. request rate from insOtuOons

browsers to origin servers = 15/sec delay from insOtuOonal router to

any origin server and back to router = 2 sec

Consequences uOlizaOon on LAN = 15% uOlizaOon on access link = 100% total delay = Internet delay + access

delay + LAN delay = 2 sec + minutes + milliseconds

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origin servers

public Internet

institutional network 100 Mbps LAN

15 Mbps access link

institutional cache

Caching example (cont) possible soluOon increase bandwidth of access link

to, say, 100 Mbps

consequence uOlizaOon on LAN = 15% uOlizaOon on access link = 15% Total delay = Internet delay + access

delay + LAN delay

= 2 sec + msecs + msecs oeen a costly upgrade

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origin servers

public Internet

institutional network 100 Mbps LAN

100 Mbps access link

institutional cache

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Caching example (cont)

possible soluOon: install cache suppose hit rate is 0.4 consequence 40% requests will be saOsed

almost immediately 60% requests saOsed by origin

server uOlizaOon of access link reduced

to 60%, resulOng in negligible delays (say 10 msec)

total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

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origin servers

public Internet

institutional network 100 Mbps LAN

15 Mbps access link

institutional cache

CondiOonal GET

Goal: dont send object if cache has up-to-date cached version

cache: specify date of cached copy in HTTP request If-modified-since:

server: response contains no

object if cached copy is up-to-date: HTTP/1.0 304 Not

Modified

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cache server HTTP request msg

If-modified-since:

HTTP response HTTP/1.0

304 Not Modified

object not

modified

HTTP request msg If-modified-since:

HTTP response HTTP/1.0 200 OK

object modified