HTTP - Hypertext Transfer Protocol

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HTTP - Hypertext Transfer Protocol

Arthur : Yigal Eliaspur

Date : 28.1.2001

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HTTP Overview

Web’s application-layer protocol in use by the WWW since 1990 client/server paradigm

– in the web:• clients : browsers (IExplorer,Netscape..) • server : web servers (Apache,IIS..)

Request/Response Protocol: Web servers usually using TCP port 80

response

requestSC

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HTTP Overview (cont.)

Stateless protocol - HTTP server maintains no information about the client.

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HTTP Versions HTTP 0.9

• Simple GET protocol for the Web

• limits on data transfer (1024 characters)

HTTP 1.0 • Headers give information about the data transferred.

• Greater data type/quantity transfer in both directions

HTTP 1.1 • Supports hierarchical proxy servers

• caching

• persistent connections

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HTTP 0.9 GET example

telnet www.cs.huji.ac.il 80 GET /~dbsi/index.html <CRLF> output :

• <HTML><HEAD>.......</HEAD><BODY>...............</BODY></HTML>

• Connection closed by foreign host

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HTTP 1.0 developed between 1992 and 1996. Exchange more than simple text Headers allowed in both requests and

responses Extends GET request to allow headers Adds HEAD request to get information Adds POST request, sends information with

the request

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Request message format

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Response message format

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HTTP Request/Response exampleRequest:

GET /~sdbi/index.html HTTP/1.0User-agent: Mosilla/4.0Accept-language: fr<CRLF>

Response:

HTTP/1.0 200 OKDate: Mon, 22 Jan 2001 12:41:51 GMTServer: Apache/1.3.9 (Unix)Last-Modified: Thu, 04 Jan 2001 17:45:19 GMTContent-Length: 6381Content-Type: text/html

(data data data data ...)

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Response-codes

Response Meaning

1XX Information2XX Request successful200 Request succeeded202 Request accepted, processing incomplete3XX Client error301 Requested URL assigned a new permanent URL302 Requested URL temporarily assigned a new URL304 Document not modified4XX Server error400 Bad request401 Request not accepted, need user authentication403 Forbidden for unspecified reason404 Not found5XX System failure500 Internal server error501 Not implemented502 Invalid response from gateway or upstream server503 Service temporarily unavailable

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Headers types General

– Date, Pragma ..

Request – Authorization, From, If-Modifed-Since, Referer, User-

Agent ..

Response – Location, Server, WWW-Authenticate ...

Entity – Allow, Content-Encoding, Content-Length, Content-Type,

Expires, Last-Modified, extension-header...

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POST & HEAD messages POST

sends information with the request in the Entity Body.

Useful when the user fills out a form.

HEAD return only the request result without the data itself

(I.e. only the Status line and the Header lines)

use for debugging HTTP servers and for page update checking.

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Upgrading Header

allows the client to specify what additional communication protocols it supports

The server may choose to switch protocols, but this is not mandatory.

Example:• Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9,

RTA/x11

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Caching

Why?– Reduces response time

Request is satisfied from cache closest to browser

Takes less time to get the page and display it

– Reduces traffic Each page only accessed from the server once

Reduces bandwidth used by browser

– Saves money if client is paying by traffic

– Keeps bandwidth requirements down

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Caching (cont.)

Risks?– Might not be ``semantically transparent'’

• the response is different from what would have been returned by the origin server.

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Caching in HTTP/1.0 simple caching mechanism: Origin server may mark a response, using the Expires header

cache validity checking using a ”conditional request“ which include : If-Modified-Since & Last-Modified headers.

server responds:– 304 (Not Modified)

– 200 (OK) + the New entry.

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Caching in HTTP/1.0 (cont.)

The Pragma: no-cache request Header indicate that a request should not be satisfied from a cache.

PROBLEM - origin servers/clients can’t give full and explicit instructions to caches (will be explained later)

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Caching in HTTP/1.1

retains the basic HTTP/1.0 design– new features

– more careful specifications of the existing features.

Entry start as fresh. Become stale - when reaches its

expiration time.• must revalidate it with the origin server.

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cache validator string : entity tag. – two responses resource with the same

entity tag must be identical.

– Can include : fine-grained timestamp, internal database pointer . . .

– If-None-Match header with one or more entity tags.

• Much stronger then If-Modified-Since.

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Caching in HTTP/1.1 (cont.) Cache-Control header

– server/client implicit directives to caches

– directives examples:•max-age - relative expiration time.

– HTTP/1.0 Expires header can lead to clock skew failure.

•no-transform - prevent proxies response transformations.

– like reduce image complexity over a slow link (WAP)

•private & no-store - prevent the storage of some or all of a response.

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Vary header - include list of headers that identical the request beside the URL field.– For example : Accept-Language, Accept-Charset ...

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Cooperative Cashing

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Cooperative Cashing (cont.) Higher level cache ( e.g. national cash)

– larger user population – higher hit rates.

Multiple Web cashes which cooperate => Improve overall performance.

Cooperative cashes usualy built from clusters – divide the traffic overhead– improve storage capacity

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Cooperative Cashing (cont.)– which of the cashes we sould ask for a particular

doc?• Hash routing (of URLs) - an object want be present in

more then one cash.

HTTP/1.1 introduces the concept of hop-by-hop headers: – message headers that apply only to a given

connection, and not to the entire path.

– This enable much more power with proxies (cashes) usage.

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Cooperative Cashing (cont.) HTTP 1.1 hop-by-hop headers:

• Connection

– options that are desired for that particular connection (e.g connection:close.)

• Public

– lists the set of methods supported by the server

• Proxy-Authenticate

– enable authentication methods between two hops.

• Transfer-Encoding -

– compression method between two hops.

• Upgrade

– additional communication protocols supported.

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Persistent & Non Persistent Connections. Persistent Connections:

• Opens new TCP connection for each request.• For example : for a web page with 10 image - 11 new

TCP connections is needed.• Used in HTTP/1.0

nonpersistent connections :• one TCP connection can serve more then one

request/response pair.• Less connection establishing overhead, smaller slow-

start delay.• Used as default in HTTP/1.1

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Persistent & Non Persistent Connections.(cont.) nonpersistent connections, two types:

– without pipelining• the client issues a new request only when the

previous response has been arrived.

– with pipelining• client send the request as soon as it encounters a

reference.• Multiple request/response on the same TCP

packet.• Or on back-to-back packets.

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Compression

most image formats (GIF, JPEG, MPEG) are precompressed.

many other data types used in the Web are not.

compression could save almost 40% of the bytes sent via HTTP

need for negotiating the use of codings.

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Compression (cont.) Client send : Accept-Encoding header

– indicate what content-codings it can handle, and which ones it prefers.

Server Send :– Content-Encoding header - for end-to-

end coding indication.

– Transfer-Encoding header - for hop-to-hop coding indication. (supported only in HTTP/1.1)

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W3C Performance Measurements

"Microscape" Benchmark, 43 inline images Scenarios HTTP/1.0: using 4 simultaneous connections HTTP/1.1: using 1 persistent connection HTTP/1.1 pipeline: using 1 persistent

connection HTTP/1.1 pipeline + compression: using 1

connection

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W3C Performance Measurements (cont.)

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Authentication Many sites require users to provide a username and

password in order to access the documents housed on the server.

Provide mechanism for keeping track of users (more then security mechanism).

How does it’s work?• Client send

– ordinary request message

• server responds with– 401 Authorization Required status code – WWW-Authenticate header which specified how to perform

authentication

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Authentication (cont.)

• Client resend– the requested message but this time including Authorization header (e.g. user-name & password.)

• The client continue to add this header for each following request to that server.

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Cookies Another site mechanism for keeping tracks of users. Example:

– Client contact a web site for the first time.– Server response with :

•Set-cookie : 1678453 header

– client store the cookie value and the server name in a special “cookie file”.

– For each further request for that server the client will add the •Cookie : 1678453 header

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Cookies (cont.) Usage:

– server requires authentication but doesn’t want to hassle a user with a user-name and password.

– Remembering user’s preferences for advertising.– Enable creating a virtual shopping cart.

Problems– users who accesses the same site from different

machines.

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References

http://www.ietf.org/rfc/rfc2068.txt http://www.ietf.org/rfc/rfc1945.txt http://www.w3.org/Protocols/ http://www8.org/w8-papers/5c-protocols

/key/key.html Computer Networks by Joames

Fokurose & Keith W.Ross.

Documents

HTTP - Hypertext Transfer Protocol