Network Technologies

Network Technologies

Glenn Strong

Department of Computer Science School of Computer Science and Statistics

Trinity College, Dublin

What Happens When Browser Contacts Server I

The browser determines the hostname (the address that was clicked upon or typed) www.adobe.com (host name is Mnemonic - assists human memory).

The browser asks the DNS (Domain Name Service) for the IP address of www.adobe.com

DNS - a distributed database that translates hostnames to IP addresses

DNS replies with the corresponding IP address. For example 192.150.14.120

The browser then makes a TCP connection to port 80 on 192.150.14.120

Handshakes with web server process running on 192.150.14.120 Sets up a reliable connection between the two machines

What Happens When Browser Contacts Server II

The web server running on 192.150.14.120 retrieves index.html from its file system. It attaches some HTTP header information and passes the file stream to its TCP process. Encapsulation again. The browser receives the file and closes the TCP connection Browser reads HTML and renders page

TCP and Ports

Weve already mentioned that a server machine might have several server processes running on it, each providing different type of service - HTTP, FTP, Telnet, SMTP

Each server process can be identified by the port at which it listens on that machine

What exactly is a port?

We use a port to set up a TCP connection

Programming term specifying a logical connection place In TCP/IP, allows a client program specify a particular server process on a computer in a network

Port numbers are from 0 to 65536

Ports 0 to 1024 are reserved for use by certain privileged services. See RFC 1700

Ports

When taken together, the source port, the destination port, source and destination IP numbers uniquely identify an application process This allows several processes to communicate without the signals becoming mixed up

This is all carried out by the transport layer

It identifies the data destined for each process by examining the

source/destination IP address source/destination port number

of each incoming segment

HTTP

Ok - weve introduced what happens when a client contacts a web server

After it establishes a TCP connection, we saw that it issues a GET command followed by the file name it requires and version of HTTP it is running

HTTP is the application layer protocol for retrieving web pages The browser uses TCP as the transport mechanism to send HTTP information between client and server.

HTTP is the Hypertext Transfer Protocol WWWs application layer protocol Client/server model

client: browser that requests, receives, displays WWW objects server: WWW server sends objects in response to requests

HTTP Example

Suppose a user enters the URL: www.tcd.ie/Library/index.htm 1 _{HTTP client initiates TCP connection to HTTP server (process) at}

www.tcd.ie. Port 80 is default for HTTP server.

2 _{HTTP server at host www.tcd.ie waiting for TCP connection at port} 80 accepts connection, notifying client.

3 HTTP client sends http request message (containing URL) into TCP connection socket.

4 _{HTTP server receives request message, forms response message} containing requested object (Library/index.htm), sends message into socket.

5 _{HTTP server closes TCP connection.}

6 HTTP client receives response message containing HTML file, displays HTML. Parsing the HTML file, the browser finds 10 referenced jpeg objects

How many TCP connections do we need to establish?

Non-persistent connection: one object in each TCP connection Some browsers create multiple TCP connections simultaneously -one per object

A Note on Virtual Hosting

Ive mentioned servers in the context of a single machine hosting a single web domain.

However, a single machine might have several host names associated with its IP address.

This is called virtual hosting; It is made possible by HTTP/1.1 Every HTTP/1.1 request includes a HOST field which specifies the domain name associated with the request

This allows the server know which part of its file system has been allocated to the domain name.

Offering a virtual host facility is often mis-named as offering virtual server facility

A virtual server is a facility whereby you are given complete control of a server process

Essentially, you are running your own remote server, giving

Complete configuration control You have your own IP address

Back to the HTTP Protocol

The TCP transport service for HTTP:

Client initiates TCP connection (creates socket) to server, port 80 Server accepts TCP connection from client

HTTP messages (application-layer protocol messages) exchanged between browser (http client) and WWW server (http server) TCP connection closeda

Note that each request-response transaction is independent:

HTTP is “stateless”. Server maintains no information about past client requests

Protocols that maintain “state” are complex!

past history (state) must be maintained

HTTP Message Format: Request

There are two types of HTTP message: request

response

The HTTP request message is ASCII text, in a human-readable format: GET /somedir/page.html HTTP/1.1

Host: www.virtualhost.com Connection: close

User-agent: Mozilla/4.0

Accept: text/html, image/gif,image/jpeg Accept-language:fr

HTTP Message Format: Request

There are several different kinds of request; the previous example was a GET; here is a POST

POST /somedir/script.php HTTP/1.0 User-Agent: Mozilla/4.0

Content-Type: application/x-www-form-urlencoded Content-Length: 51

username=fred&password=mumble&sweeties=cola+bottles In which a block of data is sent along with the request. This is a common way to submit form data (in a GET request the form contents are sent as part of the URI).

HTTP Message Format: Reply

The response sent by the server consists of a status line (the protocol status code and status phrase), followed by header lines and the data requested (e.g. an HTML file).

HTTP/1.1 200 OK Connection: close

Date: Thu, 06 Apr 2004 12:00:15 GMT Server: Apache/1.3.0 (Unix)

Last-Modified: Mon, 22 Mar 2004 ... Content-Length: 6821

HTTP Reply Status Codes

In first line in server to client response message. A few sample codes: 200 OK

request succeeded, requested object later in this message 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 HTTP Version Not Supported

User-Server Interaction: Authentication

HTTP is stateless. So how do we: control access to server documents?

Serve content specifically tailored to a particular user?

We need some form of authentication. The goal of authentication is to control access to server documents.

stateless: client must present authorization in each request authorization: typically name and password

authorization: header line in request

if no authorization presented, server refuses access, sends WWW authenticate:

User-Server Interaction: Cookies [RFC 2109]

Server sends cookie to client in response, as part of the headers Set-cookie: UniqueID=1243af54e

Client presents cookie in later requests as part of the request headers cookie: UniqueID=1243af54e

Server matches presented-cookie with server-stored cookies

authentication

remembering user preferences, previous choices

If the cookie has options they are set as part of the value

How Amazon.com Redirects your Browser and Sets a

Cookie

When you type www.amazon.com into your browser for the first time several things occur before you receive a web page

Firstly Amazon.com (like many large sites) do not keep their front page at index.html. Your browser is redirected to the front page by a HTTP 302 response

When your browser then requests this new page, the Amazons server checks to see if a cookie has already been set

If not, it sends your browser another 302 response this time with a cookie

Your browser sets the cookie

Your browser requests the new Amazon front page, this time returning the cookie ID

The Amazon server at last sends you the front page of its site After the cookie has been set your browser will include a cookie field in every request it makes to www.Amazon.com

Client Caching: Conditional GET

Goal: dont send object if client has up-to-date stored (cached) version

What if the cached version is stale? Client does a quick check:

Client specifies last-modified date of cached copy in a HTTP request If-modified-since: <date>

Web Caches (Proxy Server)

Goal: satisfy client request without involving origin server

User sets browser so that it accesses the web a via the proxy server The proxy has a cache of pages already downloaded

Client sends all HTTP requests to web cache

if object at web cache, web cache immediately returns object in HTTP response

else proxy server requests object from origin server, then returns http response to client

Note: client first opens TCP connection to proxy server and sends HTTP request over connection.

If proxy doesnt have the item, it opens a TCP connection to remote web server and sends request for the item