Caching Overview :
ISA Server supports caching as a way to improve the speed of retrieving information from the Internet. From the internal user’s point of view, caching improves Internet access performance. From the network administrator’s point of view, caching provides the added benefit of reducing the use of network bandwidth. ISA Server extends the benefits of caching by enabling scheduled content download jobs in which ISA Server downloads Internet content to the cache before any user requests the object. You can also combine Web caching with Web proxy chaining to optimize performance further if your organization has multiple locations. This lesson provides an overview of how caching works and how ISA Server 2004 implements caching.
What Is Caching?
One of the primary deployment scenarios for ISA Server 2004 is as a Web proxy server in which ISA Server retrieves information from the Internet for internal clients. ISA Server supports Web caching as a way to improve the speed with which this information is returned to Web clients.
Caching stores Web content on the ISA Server computer in memory or on the server’s hard disk. When a user requests Web information that is in the cache, ISA Server provides the information from the cache, enabling a quicker response to the client. ISA Server 2004 can be configured to enable the caching of Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP) objects.
ISA Server 2004 caching provides the following benefits:
- Improved performance Web caching speeds client response for Internet access by bringing the Web content closer to the user. When a user behind the ISA Server 2004 firewall requests Web content, ISA Server checks to see whether the content is contained in its cache. If it is, the cached content is returned to the user. Accessing Web content from a cache on the corporate network is faster than
requiring a connection to a remote Web server located on the Internet. Recently accessed information is stored in the cache and remains in the cache as long as that content continues to be accessed by users.
- Reduced bandwidth usage Web caching can help reduce the overall bandwidth usage on the organization’s Internet connection. When users request Web content already contained in cache, that content is returned to the user immediately from the cache without requesting the content again from the Internet.
How Caching Works
When a user requests an HTTP or FTP object, the Web proxy client sends the request to the Web proxy filter on ISA Server.
In a forward caching scenario, the following actions occur to complete the client request:
1. The Web proxy client sends a request for content located on an Internet Web server. The Web request is intercepted by ISA Server 2004 and forwarded to the Web proxy filter.
2. ISA Server checks whether the requested content is contained in its cache. If the content is not in the cache, or if the content has expired (that is, the header information in the content indicates that it should no longer be served from a cache), ISA Server 2004 forwards the request to the Web server on the Internet.
3. The Web server on the Internet returns the information requested.
4. The ISA Server Web proxy filter places the Web content in its in-memory cache. ISA Server 2004 uses an in-memory cache to store the most frequently requested content.
5. After placing the Web content in the in-memory cache, ISA Server 2004 Web caching server returns the content to the requesting user.
6. After a time, the ISA Server 2004 Web proxy filter will copy the contents of the inmemory cache to the disk-based cache. If the content is not frequently accessed, the in-memory cache will flush the content and the only copy of the content on ISA server will reside in the disk-based cache.
Caching Scenarios
ISA Server supports both forward and reverse caching.
1- Forward caching occurs when a user on the corporate network makes a request for Web content located on an Internet Web server. The user initiates an HTTP, Hypertext Transfer Protocol Secure (HTTPS), or FTP request to an Internet Web server and the request is intercepted by ISA Server. ISA Server retrieves the content from the Internet Web server, stores that content in its cache, and returns the content to the user.
2- Reverse caching occurs when users on the Internet request Web content located on a server on the corporate network that is accessible through a Web publishing rule. When an Internet user requests content from the internal server, ISA Server forwards the request to the Web server. The Web server sends the requested content to ISA Server, which then returns the content to the Internet user who made the request. In this scenario, ISA Server will cache a copy of the requested information so that the next request for the same information can be provided from the ISA Server cache rather than again accessing the internal Web server.
What Is Caching?
One of the primary deployment scenarios for ISA Server 2004 is as a Web proxy server in which ISA Server retrieves information from the Internet for internal clients. ISA Server supports Web caching as a way to improve the speed with which this information is returned to Web clients.
Caching stores Web content on the ISA Server computer in memory or on the server’s hard disk. When a user requests Web information that is in the cache, ISA Server provides the information from the cache, enabling a quicker response to the client. ISA Server 2004 can be configured to enable the caching of Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP) objects.
ISA Server 2004 caching provides the following benefits:
- Improved performance Web caching speeds client response for Internet access by bringing the Web content closer to the user. When a user behind the ISA Server 2004 firewall requests Web content, ISA Server checks to see whether the content is contained in its cache. If it is, the cached content is returned to the user. Accessing Web content from a cache on the corporate network is faster than
requiring a connection to a remote Web server located on the Internet. Recently accessed information is stored in the cache and remains in the cache as long as that content continues to be accessed by users.
- Reduced bandwidth usage Web caching can help reduce the overall bandwidth usage on the organization’s Internet connection. When users request Web content already contained in cache, that content is returned to the user immediately from the cache without requesting the content again from the Internet.
How Caching Works
When a user requests an HTTP or FTP object, the Web proxy client sends the request to the Web proxy filter on ISA Server.
In a forward caching scenario, the following actions occur to complete the client request:
1. The Web proxy client sends a request for content located on an Internet Web server. The Web request is intercepted by ISA Server 2004 and forwarded to the Web proxy filter.
2. ISA Server checks whether the requested content is contained in its cache. If the content is not in the cache, or if the content has expired (that is, the header information in the content indicates that it should no longer be served from a cache), ISA Server 2004 forwards the request to the Web server on the Internet.
3. The Web server on the Internet returns the information requested.
4. The ISA Server Web proxy filter places the Web content in its in-memory cache. ISA Server 2004 uses an in-memory cache to store the most frequently requested content.
5. After placing the Web content in the in-memory cache, ISA Server 2004 Web caching server returns the content to the requesting user.
6. After a time, the ISA Server 2004 Web proxy filter will copy the contents of the inmemory cache to the disk-based cache. If the content is not frequently accessed, the in-memory cache will flush the content and the only copy of the content on ISA server will reside in the disk-based cache.
Caching Scenarios
ISA Server supports both forward and reverse caching.
1- Forward caching occurs when a user on the corporate network makes a request for Web content located on an Internet Web server. The user initiates an HTTP, Hypertext Transfer Protocol Secure (HTTPS), or FTP request to an Internet Web server and the request is intercepted by ISA Server. ISA Server retrieves the content from the Internet Web server, stores that content in its cache, and returns the content to the user.
2- Reverse caching occurs when users on the Internet request Web content located on a server on the corporate network that is accessible through a Web publishing rule. When an Internet user requests content from the internal server, ISA Server forwards the request to the Web server. The Web server sends the requested content to ISA Server, which then returns the content to the Internet user who made the request. In this scenario, ISA Server will cache a copy of the requested information so that the next request for the same information can be provided from the ISA Server cache rather than again accessing the internal Web server.
