With the Cache Array Routing Protocol (CARP), an array containing multiple Forefront TMG computers can act as a single logical cache. CARP allows an array to efficiently balance Web-based client load and distribute cached content among array members. CARP provides client computers with the information and algorithms required to identify the best server in the array to serve their request, eliminating the need for array members to forward requests between them. CARP also supports array member selection by the servers and by chained proxy servers.
Note Forefront TMG Medium Business Edition does not support arrays that contain more than one Forefront TMG computer.
CARP uses hash-based routing to determine the best path for resolving a request within an array. The request resolution path is based upon hashes of the array member identities and the host name in the Uniform Resource Locator (URL) specified in the request. For any given host name from a URL, a Web browser can determine exactly where in the array the information is stored if the information has already been cached from a previous request, and array members can determine where the information is to be cached for future requests.
CARP provides powerful benefits:
The CARP process provides efficient routing for requests on the client side and on the server side.
Clients can be Web browsers or downstream proxy servers. Client-side CARP works by having the client select an array member for each host name contained in URLs. On the client side, Forefront TMG uses CARP as follows:
With server-side CARP, clients select the array member for sending a request in a predetermined, round-robin, or random fashion. If the array member receiving the request cannot serve the requested object from its cache, it uses the CARP algorithm to create a prioritized list of array members for the host name in the URL and forwards the request to the IP address for intra-array communication (IntraArrayAddress) of the first server in the list. If that array member cannot serve the requested object, the next server in the list is contacted until all the servers in the list have been contacted.
Server-side CARP is enabled for Web requests coming from a particular network when the ResolveInArray property of the network's Web listener is set to True. However, no Web requests from the network are handled unless the EnableWebProxyClients property for the network is also set to True. Note also that no objects are cached on any server in the array unless the CacheLimitInMegs property of at least one cache drive of at least one server belonging to the array is set to a nonzero value.
The script generated by Forefront TMG implements the CARP algorithm. The script includes information about the configuration and current status of the array. The script ensures that the URL space is divided evenly and in accordance with configurable load factors between the array members.
The result is a specific location for each cached object, meaning that the Web browser or downstream server can know exactly where a requested URL is either already stored locally or will be located after caching. The hashing functions used ensure that the load is statistically distributed and balanced across the array.
Because Forefront TMG computers in an array may have different hardware and some may be more powerful than others, you may want to divide the cache load accordingly. For this reason, you can configure the CARP functions by adjusting the load factor for each server in the array using the LoadFactor property.
Some Web sites require that client IP addresses remain unchanged throughout a session. Requests should be sent to these Web sites without using the CARP algorithm. CARP can be disabled for these Web sites by adding them to a domain name set referenced by the CARPExceptions property of the FPCWebProxy object.
Build date: 11/30/2009
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