Optimizing ISA 2004 caching (Part 1)

If you would like to read the next part in this article series please go to Optimizing ISA 2004 caching (Part 2).

ISA server 2004 aids in the provisioning of efficient communication between local and remote locations. ISA has particularly been designed to provide caching facilities to branch offices and using this feature properly will maximize internet performance in distributed organizations and will maximize bandwidth usage.

Optimization of internet traffic is essential and by doing so increases performance and capacity. This is achieved through ISA’s specialized caching properties. Enabling caching can improve network performance and response time by saving copies of images, text, and other data that clients download from web and FTP sites on the Internet and making them available to the next client that requests information from that particular site quickly on the LAN rather than slowly over the WAN link. In most scenarios savings of 30-60% on internet bandwidth will be evident, if the solution is properly configured; this is a significant saving that can boost internet utilization. Bandwidth = money and saving bandwidth effectively means saving money especially if speed plays a role as it does in caching.

ISA services that use caching

The bundled web proxy and firewall service in ISA 2004 uses caching to improve the performance of the browsing. The caching works by means of checking if the requested object already exists in the local ISA cache. If the object is stored in the local cache and if the object is current, the proxy server sends the object to the client from the cache. If the page is not in the cache, the proxy server sends the request to the appropriate server on the Internet. The retrieved content is then passed back to the requesting client.

This portion of the caching can be streamlined by investigating what is being downloaded from the internet. This is done to optimize ISA caching process. For example, everyday users visit their webmail.com website. ISA caches this site but new daily content is uploaded. A caching job rule can be created that regularly caches the content prior to the user requesting the website. If this is for a large organization, significant bandwidth gains may be achieved.

Rule structure improves cache performance

The order of the ISA rules may affect the speed of the object’s retrieval. Let’s examine the process. ISA server first checks if the content is not blocked, ISA Server then saves a copy of the content in its cache and then the object is returned to the client application that made the original request. The firewall rules should be ordered correctly so that the content is quickly filtered, some mis-configurations that I have seen have resulted in slow responses.

Branch offices

If an organization has more than one ISA server, consider the disturbed caching approach as this will allow you to leverage off the sum of the accumulated caching across all branch office and head office ISA servers. ISA server 2004 has this feature incorporated, and it is a great advantage of the caching mechanism. All the servers running ISA Server in the branch offices can be configured to forward their requests to the head-office ISA Server. The head office ISA Server will accumulate a large cache that contains the requested items from the subordinate offices. This ensures that the Internet content can be delivered to the client with the least use of WAN bandwidth. Further distributed caching at branch offices can be configured for greater efficiency.

Storage of the cached content

RAM caching storage

ISA server 2004 Caching stores the web content on the computer running ISA Server, in the server RAM or on the hard disk. The more RAM that is available, the faster the performance of stored objects in RAM. However note that the default amount of RAM that will be used for caching purposes is 10%. I typically recommend that, for best results, at least 1 gigabyte of RAM be used on caching ISA servers that have significant load.

Hard disk caching storage

Caching location may also be stored on the hard disk depending on the sequence of the request. Recent content is typically stored in RAM while older requests are stored on the hard disk. On large installations of ISA it is recommended that the hard disk also be one of high performance. New disks with low seek time and 15000rpm are now available and improve cache performance substantially.

The Cache file

The cache file is a single file. For the purpose of efficiency this file can be located on each disk partition. However it’s recommended that the file be located on a separate physical disc other than where the ISA, operating system and the page files are installed. This will reduce contention on the system and boot disk. On start up of the ISA services ISA will re-index the cached objects. The disk partition has to be formatted with NTFS. The cache file will have a .cdat extension and will grow to the limit that the ISA admin has preset.

Network systems with low internet bandwidth

Millions of networks are connected to the internet and continuously millions are being connected as connectivity becomes more wide spread and inexpensive. The fact still remains that there is a cost associated with bandwidth and the more users connected, the less bandwidth is available, both on the internet, but more importantly, behind the firewall. In developed countries, 100mbps connections are common; in developing countries the equivalent in terms of cost is a 1-4 Mbps connection. Both need to be managed and optimized. It is recommended that monitoring and logging of the usage be done in order to understand the dynamics of the load and protocol share. This practice helps in designing the cache system and optimizing its performance. Protocols other than HTTP and FTP do not benefit from caching, for this reason it is important to know what percentage of traffic is HTTP, HTTPS and FTP.

ISAs two types of caching

Figure 1: Forward Caching

Forward caching occurs when a user on the internal network initiates a request for Web content located on an external Internet Web server. The user initiates an HTTP, HTTPS or FTP request to an Internet Web server, the request passes though ISA Server, ISA Server retrieves the content from the external Internet Web server, on the client’s behalf, stores the content in its cache and returns the content to the requesting user.

Figure 2: Reverse Caching

Reverse caching occurs when users on the Internet request Web content located on the internal network and are allowed through a Web publishing rule. The Internet user requests content from the internal web server, ISA Server forwards the request to the Web server on the user’s behalf. The Web server sends the requested content to ISA Server, which then returns the content to the originating Internet user. A cached copy of the requested information is kept in the ISA cache for the next request for the same information.

Optimizing the cache configuration

ISA Server caches can be configured to limit certain types of content that are being cached; the size of certain cached objects can be manipulated to reserve cache space and capacity for additional smaller objects. Each environment has different needs so some analysis maybe required.

Another key consideration would be the location of the caching server within the organization. The centralized approach is not always the best approach as some remote sites may have slow links to the central Internet stream that originates at Head office. The preferred solution would be to layer the caching environment so that the ISA server at the branch office chains upstream to the main ISA server but the subordinate ISA server also keeps a local cache. The reason for this is that the ISA server will not need to traverse across a slow link to retrieve content if it caches the content locally. In the next article in this series, this will be explained in more detail.


In the first part of this series we covered performance improvements that can be implemented when using ISA 2004. We also looked at the different types of caching and how ISA handles regularly accessed content both inbound and outbound. In the following article different optimization methods will be described that further improve caching on ISA 2004.

If you would like to read the next part in this article series please go to Optimizing ISA 2004 caching (Part 2).

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