1. Internet Applications
INTERNET SERVICES
Anwar Alhenshiri

2. Internet Services Domain Name Service Proxy Service Mail Service
Internet Applications
Internet Services
Domain Name Service
Proxy Service
Mail Service
Web Service

3. DNS
DNS

4. Internet Naming Hierarchy
DNS
Internet Naming Hierarchy
The silent dot at the
end of all addresses
.com
.net
.org
.in
.tcd
.ac
.co
.iitk
www
www

5. DNS Operation DNS Setup
A DNS server maintains the name to IP address mapping of the domain for which it is the name server.
The DNS server for a domain is registered with the domain registrar and the entry is maintained by the Internet Root-Servers (13) or Country Level Root-Servers.
Whenever a server is queried, if doesn’t have the answer, the root servers are contacted.
The root servers refer to the DNS server for that domain (in case the domain is a top level domain) or the Country Root Server (in case the domain is country level domain).

6. Proxy Server
PROXY SERVER

7. Internet Connections Customers connect to an ISP
Proxy Server
Internet Connections
ISP network
ISP network
Backbone
networks
Bandwidth-limited links
ISP network
Customers connect to an ISP
ISPs connect to backbone
Customer Networks

8. Internet Connections Cost of connections is based on bandwidth
Proxy Server
Internet Connections
Cost of connections is based on bandwidth
Cost of connection is a major part of network cost
Organisations only obtain as much bandwidth as they can afford
Many organisations in Asia-Pacific only have 64kb/s – 2Mb/s connections (as compared to their counterpart in US and Europe who have bandwidths of 2.4 Gbps – Gbps)

9. Proxy Server
What is a Web Proxy?
A proxy is a host which relays web access requests from clients
Used when clients do not access the web directly
Used for security, logging, accounting and performance
browser
proxy
web

10. What is Web Caching? Storing copies of recently accessed web pages
Proxy Server
What is Web Caching?
Storing copies of recently accessed web pages
Pages are delivered from the cache when requested again
Browser caches
Proxy caches

11. Why Cache? Shorter response time Reduced bandwidth requirement
Proxy Server
Why Cache?
Shorter response time
Reduced bandwidth requirement
Reduced load on servers
Access control and logging

12. Popular Proxy Caches Apache proxy MS proxy server WinProxy Squid
Squid is popular because it is powerful, configurable and free
Many others

13. Web Server
WEB SERVER

14. Web Server
Web Server
HTTP (Hyper Text Transfer Protocol) is used to transfer web pages from a Web Server to Web Client (Browser)
Web Pages are arranged in a directory structure in the Web Server
HTTP supports CGI (Common Gateway interface)
HTTP supports Virtual Hosting (Hosting multiple sites on the same server)
Popular Web Servers
Apache
Windows IIS
IBM Websphere

16. Mail Architecture Internet Mail Client Mail Client Mail Server
Mail Architecture
Internet
Mail Server
Mail Server
Mail Client
Mail Client

17.
Mail Architecture

18.
Mail Architecture
Simple Mail Transfer Protocol (SMTP) is used to transfer mail between Mail Servers over Internet
Post Office Protocol (PoP) and Interactive Mail Access Protocol (IMAP) is used between Client and Mail Server to retrieve mails
The mail server of a domain is identified by the MX record of that domain
Popular Mail Servers
Sendmail/Postfix
Microsoft Exchange Server
IBM Lotus

19. Web Caching: Replication on the World Wide Web
Anwar Alhenshiri
Credit Goes to: Dr. Paul Schragger

20. Types of Web Caches User Agent Cache (Browser Cache)
Single user
Shared Cache (Proxy Cache)
Multi-user
Gateway Cache

21. Deployment

22. Browser Cache Implemented on a single user’s machine.
Local hard drive space stores representation of viewed content.
Simple rules for control.
Usefulness
Recently viewed pages (Back/Forward button)
Commonly used images

23. Proxy Cache Similar concept, but for multiple users.
Usually implemented on a firewall or separate device known as intermediaries.
Request routing can be manually or through an interception proxy.
Usefulness
Latency and network traffic are reduced

24. Gateway Cache Also known as reverse proxy caches.
Implemented on separate devices by a website administrator.
Distributed gateway caches are used in Content Delivery Networks (CDN).
Usefulness
Increased scalability, reliability, and performance

25. Content Delivery Network (CDN)
4a. Get embedded documents from local cache or server (if not already cached)
Cache
CDN
server
4b. Embedded documents
5. Embedded documents
3. Get embedded documents
1. Get base documents
Client
Original
server
2. Document with refs to embedded documents
Cache producer within Akamai CDN

26. Controlling Cache HTML Meta Tags HTTP Headers
Written in <head> section of an HTML page.
Can mark expiration date or as uncacheable.
Only used by some browser caches and not seen by proxy caches.
HTTP Headers
Automatically created by Web server.
Sent before HTML.
Seen by browser and proxy caches.

27. HTTP Defining Mechanisms
Freshness – Content is able to be loaded from cache without having to check with the original server.
Validation – The process of confirming with the original server whether or not cached content is still valid to load.

28. HTTP Response Headers Example
HTTP/ OK
Date: Fri, 30 Oct :19:41 GMT
Server: Apache/1.3.3 (Unix)
Cache-Control: max-age=3600, must-revalidate
Expires: Fri, 30 Oct :19:41 GMT
Last-Modified: Mon, 29 Jun :28:12 GMT
ETag: "3e fbbc"
Content-Length: 1040
Content-Type: text/html
Validation
Freshness

29. Expires Header and Freshness
Expires: Fri, 30 Oct :19:41 GMT
Indicates how long the representation is fresh. After this time passes, the cache will communicate with the original server to see if there have been any changes.
Beneficial for static page images as well as continually changing content.
Web server and cache must be synchronized.

30. Cache-Control Header and Freshness
Cache-Control: max-age=3600, must-revalidate
Gives control beyond Expires header.
Cache control response-headers:
max-age=[seconds]
s-maxage=[seconds]
public
no-cache
no-store
must-revalidate
proxy-revalidate

31. Last-Modified and Validation
Last-Modified: Mon, 29 Jun :28:12 GMT
Validate cache by looking at the last time the document was altered (Last-Modified).
If-Modified-Since request is sent to the original server. If changes have been made since the date given, the entire document is returned. Otherwise, the cached document can be loaded.

32. ETag and Validation Unique identifiers created by server.
ETag: "3e fbbc"
Unique identifiers created by server.
Changed each time the representation is altered on the original server.
If-None-Match request is sent to server and a simple comparison is used to validate the content.

33. References
Nottingham, Mark. “Caching Tutorial: for Web Authors and Webmasters.” July <
Tanenbaum, Andrew S. Distributed Systems: Principles and Paradigms. New Jersey: Prentice-Hall, 2002.
“Web cache.” Wikipedia. <

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