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Web Architecture:HTTP, URIs & URLs

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Some NETWORK terms

Term Definition

Host Machine connected to a networkPacket Basic unit of Internet communicationIP Internet Protocol to coordinate delivery of individual packets

between hostsIP address Numerical address for an Internet hostHostname Case-insensitive string identifier for an Internet hostDNS DistributedDomain Name System to translate between IP

addresses and hostnamesConnection Logical communication channel between hostsTCP

Transmission Control Protocol providing abstraction of a reliable,bidirectional connection

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Some WEB terms

Term Definition

Web Network of interlinked informationHypertext Linking related information for navigationInternet Worldwide network of networks using IPWeb page Document accessible on the Web via a URIWeb site Collection of related Web pagesBrowser/Web Client Application (user agent) to request and display Web pagesWeb server Program that receives & responds to HTTP requestOrigin server Server where requested resource residesIntermediary Web component in the request path between client and origin

server (proxies, gateways)

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Web architecture:Key Components

HTTP/1.1: HyperText Transfer ProtocolFormat and semantics of request/response messages

URI: Uniform Resource IdentifierFormatted string that identifies a resource

HTML/XHTML: HyperText Markup Language

Plus

DNS: Domain Name System

TCP/IP: Internet Protocol Suite

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URI: Anatomy

A Uniform Resource Identifier identifies a resource on theinternet and is independent of current location

Generic

Scheme Authority Path Query Fragment

http: //www.cs.ncl.ac.uk /teaching/ ?m=3504 #cwork

https://internal.cs.ncl.ac.uk/modules/2011-12/csc3504/https://internal.cs.ncl.ac.uk/modules/2011-12/modules.php?m=3504https://internal.cs.ncl.ac.uk/modules/2011-12/csc3504/index.html#cwork

Scheme specific

Scheme Scheme specific syntax Query

mailto: some.body@ncl.ac.uk ?subject=Hello

Mail me

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When is a URI a URL?

Uniform Resource Locator

http: is a URI schemeand

an http: URI is a URL

"...a URL is a type of URI that identifies a resource via a representation of itsprimary access mechanism (e.g., its network "location"), rather than by some

other attributes it may have."http://www.w3.org/TR/uri-clarification/#contemporary

http://www.w3.org/TR/uri-clarification/http://www.w3.org/TR/uri-clarification/http://www.w3.org/TR/uri-clarification/http://www.w3.org/TR/uri-clarification/

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HTTP:A request-response protocol

HyperText Transfer Protocol makes the Web work! Clientsask for resources from servers by assembling and sending an

HTTP requestmessage Serversrespond with the appropriate HTTP responsemessage, including

any content to be displayed

HTTP requests & responses travel as TCP/IP packets: Metadata in headers& content in an (optional) entity body

HTTP is stateless Each request/response pair is an independent exchange. No protocol level maintenance of state (for scalability)

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HTTP methods

The operations carried out over HTTP HEAD, GET, POST, PUT, DELETE etc

HEAD and GET are mandatory (all resources support them) Others are optional (depends on what you're up to)

GET returns current state and content of resource

This is the "default" method for HTTP

HEAD just returns response metadata i.e. a GET without the body (content)

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HTTP headers

Headers are the metadata for the request/response exchange

Some are generic and apply to both request and response e.g.Date -> the date/time stamp for the messageCache-Control -> instructions for en-route caching (or not)

Understanding the reading, setting and manipulation HTTP

headers is very useful for managing a web site

Be aware that headers can be spoofed not good!

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More HTTP Headers

Request headers (19 in total in 4 classes)Response preferences -> Accept, Accept-Charsetetc

Additional request info -> Authorization, From etcConditional headers -> If-Modified-SinceetcConstrain server behaviour -> Max-Forwardsetc

Response headers (9 in total in 4 classes)Redirection -> Location

Additonal information -> Server, Retry-AfteretcAuthentication -> WWW-Authenticate, Proxy-AuthenticateCaching -> Age etc

Entity & Hop-by-hop headers Info about the resource (content) Content-Typeetc Hop-by-hops can be read, stripped or added to en-route

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Common HTTP methods

Method Use Safe* Idempotent** Mandatory

HEAD Exchange of request/response

headersYes Yes Yes

GETRequest and return the currentstate and content of a resource e.g.access a web page

Yes Yes Yes

POSTRequest uses entity body to updateresource or as input for processinge.g. form input

No No No

PUTServer stores entity body contentsat request URI location e.g. fileuploads

No Yes No

DELETE Deletes identified resource i.e.opposite of PUT

No Yes No

* Safe does not change state of resource

** Idempotent... the side effects of repeated, identical requests are the same as for a single request

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HTTP response codes

Generated by the server tell a client the status of a request 41 response codes in total (some you'll never see!)

Class Range Examples

Informational 1XX100 Continue101 Switching Protocols

Success 2XX200 OK201 Created204 NoContent

Redirection 3XX 300 MultipleChoices301 MovedPermanently

Client Error 4XX

400 BadRequest401 Unauthorized403 Forbidden404 NotFound

Server Error 5XX 500 InternalServerError

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Basic HTTP request structure

Request line GET /index.html HTTP/1.1

General headers Date: Mon, 11 Oct 2010 11:00:00

Request headers Host: www.internal.cs.ncl.ac.ukUser-Agent: Mozilla/5.0

Entity & Hop-by-hopheaders

CRLF

Entity body

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Basic HTTP response structure

Status line HTTP/1.1 200 OK

General headers Date: Mon, 11 Oct 2010 11:00:01

Response headers Server: Apache/2.2.16 (Unix)

Entity headers Content-Length: 4488Content-Type: text/html

Hop-by-hop headers

CRLF

Entity body

XHTML WEB PAGE CONTENT

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Proxies and caches

A proxy acts as a server to clients and as a client to other proxiesor to an origin server

Use to share access, to anonymise clients, as a gateway to othersystems, as a filter/firewall, as a cache

A cache stores HTTP messages to reduce user-perceived latency,network traffic and server load

HTTP provides extensive cache control support (what can/cannot becached, when to invalidate entries etc.)

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