Application Server Concept: Proxy Caching, Web/Mail/DNS Server Optimization

Proxies

In proxies content (particularly stored, web content) is available from one source, but may also be available from other sources.

Terminology:

• Origin Server:- It represents the original source of an object
• Proxy Server:- It supplies the object instead of origin server

Some servers (caches) are demand-driven which acts as both:

• Server:- responding to client’s requests
• Client:- forwarding requests that it cannot respond to towards the origin server.

Some proxies are that we find are driven by supply and demand. For Example: proactive caching. Some are supply-driven like Content Distribution Networks

Caching Principle

In caching principle, fast resources are scarce e.g. storage close to clients (low propagation delay & fast links) It is aimed to locate commonly used objects in fast resource, other objects can remain in slower resources. After determining which objects are commonly accessed then common Accesses are often correlated, and are said to exhibit “locality”.

• Temporal Locality:- Those Information which are accessed at one time is likely to believed to be accessed again in near upcoming future.
• Spatial Locality:- Information accessed at one point is likely to be accessed also by nearby points.

Proxy Server (Web Caching)

A proxy server offers a computer network service which allows clients to make indirect network connections between other network services of a system.If a client connects to the proxy server, then it requests a connection, file, or other resource available on a different server. The caching information between the client and the server is shared in a shared location where all of the client can use the same data.

The proxy server always provides the resource either by connecting to the specified server or by serving it from a cache. In some cases, the proxy possess some right to alter the client request and may alter the client's request or the server's response for various purposes.

Web Proxies

Web proxiesprovides a nearby cache of Web pages and files available on remote Web servers.Web proxies allows the local network clients to access those web pages and filesmore quickly or reliably.

When it receives a request for a Web resource (by the means of specified URL),the available caching proxy search forthe resulting URL in its local cache. If it is found, it returns the document immediately to the client. Otherwise it fetches the information from the remote server then it returns it to the requester and saves a copy in the cache for future. The cache is based on the expiry algorithm to remove documents from the cache which helps to securely store the more usable information in the server, according to their age, size, and access history of those informations. Two simple cache algorithms that are mostly used are Least Recently Used (LRU) and Least Frequently Used (LFU). LRU is based to remove the least-recently used documents and other informations, and LFU is based to remove the least-frequently used documents.

Web proxies also has the function to filter down the content of Web pages that it has served. There is the use of some censorware applications - which major function is to have an attempt to block offensive Web content. And the other web proxies reformat the web pages for a specific purpose. For example, Skweezer is used to reformat the web pages for cell phones and PDAs.Using such web proxies can also helps the server to maintain the security against the intruders who dares to access or intercept the informations through the computer viruses or other medium.

Four Different types of proxy servers that are available are:

Transparent Proxy

Transparent Proxy makes the original IP address available through the http headers. Since, transparent proxy have their own right and power to cache the webistes and doesnot provide any anonymity to the user it is widely used. However, the use of a transparent proxy will lead you to around simple IP bans.It is called transparent because the your IP address is exposed, not because you do not know that you are using it (your system is not specifically configured to use the transparent proxy).

Anonymous Proxy

It does not make the original IP address available to the users and alsoitis detectable andidentifies itself as a proxy server but provides reasonable anonymity for most users.

Distorting Proxy

It makes an incorrect original IP address available through the http headers butidentifies itself as a proxy server.

High Anonymity Proxy

It does not make available the original IP address anddoes not identify itself as a proxy server.

Main Uses of proxy server:

1. Caching:

When a user accesses a web page, that page is temporarily stored in the proxy cache. Then, when a subsequent user requests for the same web page which is stored in the proxy cache they access the copy in the proxy cache, rather than having the web page sent again from the originating server. It improves performance and frees up Internet bandwidth for other tasks.

2. Filtering: Allows to block specific sites
3. Maintain Privacy: Can hide actual IP address of Client from the outside world

Benefits of Caching

1. Send request to origin server (reducing delay, and link use).
2. Send full response from origin server (reducing link use)
3. Directly benefits end-user.
4. Caching benefits the service providers by making their service more popular.
5. Reduced traffic
6. Reduces load on network links
7. Reduces load on server: e.g. “flash crowds” reduction
8. Mask unavailability of origin server: e.g. when working offline, or during faults
9. Filter Requests
10. Security schemes
11. Logging

Reference

1. Tanenbaum, WetherRall, Andrew S. , David J. ,Computer Networks. 5th Ed. Boston: Pearson Education, Inc
2. https://en.wikipedia.org/wiki/Proxy_server
3. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
4. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
5. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers