Request redirection with GRM

GRM components

The keys to achieving fast delivery of high-quality content are

Moving content stored in central locations close to end users
Directing a client to the delivery point closest to that client

NetCache appliances configured as GRM servers provide redirection services, redirecting client requests to NetCache appliances configured as GRM agent caches. GRM agent caches, which are located close to clients, cache content and resolve content requests on behalf of clients.

Typically, GRM agent caches are configured to handle only CDN traffic so that they can provide premium service to CDN subscribers. A GRM agent can be used to access content on the Internet, as well as to handle CDN content. However, Network Appliance recommends that you use your GRM agent caches to handle only CDN content delivery, which provides your customers with the most preferential service possible.

Center caches and edge caches

GRM agent caches in the data center (in a center group) are referred to as center caches. GRM agent caches in defined groups outside of the data center (edge groups) are referred to as edge caches. When a client in an edge group makes a request, the GRM server returns only the IP addresses of edge caches in that edge group. A center cache can provide failover for edge caches. A center cache can also provide content for clients that subscribe to CDN services but are not associated with an edge group.

GRM server redirection methods

GRM supports two different redirection methods for GRM servers:

DNS-based redirection services (GRM DNS server)
L7 redirection services (GRM L7 server)

When you configure a NetCache appliance as a GRM server, you configure general GRM server options plus specific options for the type of redirection that you need. Depending on your redirection requirements, you might deploy all GRM DNS servers, all GRM L7 servers, or a combination of the two types of servers (a mixed hierarchy). Typically, multiple GRM servers are deployed, for example, for redundancy.

No limit exists for the number of GRM agent caches that a GRM server can support.

About a GRM DNS server

A GRM DNS server provides standard DNS redirection plus a layer of intelligence that is not available with standard DNS-- including support for proximity measurements, detection of unavailable GRM agents, and filtering. As with standard DNS, the client's local DNS nameserver communicates with the GRM DNS server through a DNS request, and the GRM DNS server responds with a DNS response.

The GRM DNS server returns to the client a list of IP addresses of the available GRM agents that are close to the local DNS nameserver that the client uses. The client browser then selects a GRM agent cache from the list returned by the GRM DNS server. The cache that the browser selects might or might not be the closest cache to the client.

A GRM server can assume the identity of the existing authoritative DNS nameserver for the CDN name space, that is, the GRM server can assume the IP address that was previously assigned to the authoritative DNS nameserver. The authoritative DNS nameserver is then reconfigured with a new IP address. The authoritative DNS nameserver becomes the back-end to the GRM server in this case. Alternatively, URL rewriting can be used to direct content to the CDN.

About a GRM L7 server

Like the GRM DNS server, the GRM L7 server includes intelligence to support proximity measurements, detection of unavailable GRM agents, and filtering. However, the GRM L7 server can provide more precise redirection than the GRM DNS server. The reason is that the GRM server redirection is based on the client's IP address (rather than on the local DNS nameserver that the client uses). The GRM L7 server operates at Layer 7, the application layer of the OSI model, to rewrite a client request with the IP address of the agent cache that it determines is closest to the requesting client.

When you deploy a GRM L7 server, you must include in your deployment a means for client requests to locate the GRM L7 server. See Providing a means for locating a GRM L7 server.

Note
If a proxy.pac file is used to direct client requests to a GRM L7 server, the GRM L7 server might return more than one GRM agent cache close to the client. If other methods described in Providing a means for locating a GRM L7 server are used, the GRM L7 server rewrites the client request with the IP address of one GRM agent cache.

The same NetCache appliance can be configured as a GRM server and a GRM agent. If you are deploying GRM DNS servers to provide the means of locating GRM L7 servers, configuring the GRM L7 server as a GRM agent also enables the GRM DNS servers to select the closest GRM L7 server to a client.

General GRM deployment guidelines

The following table provides general guidelines for determining when you should include a GRM L7 server in your deployment.

If...	Then...
The local DNS nameserver and the clients are physically close.	You can use all GRM DNS servers in your deployment.
The local DNS nameserver and the clients it services are not physically close.	You will want to include one or more GRM L7 servers in your deployment. You must provide a means for redirecting client requests to the GRM L7 servers. See Providing a means for locating a GRM L7 server.

If...

Then...

The local DNS nameserver and the clients are physically close.

You can use all GRM DNS servers in your deployment.

The local DNS nameserver and the clients it services are not physically close.

You will want to include one or more GRM L7 servers in your deployment.

You must provide a means for redirecting client requests to the GRM L7 servers. See Providing a means for locating a GRM L7 server.

For example, some enterprise CDNs are spread over continents, with clients located on continent that is different from the continent on which their local DNS nameserver is located. If only GRM DNS servers are used in such cases, the GRM agent IP addresses that GRM DNS servers return would be on the same continent as the local DNS nameserver because the GRM DNS server sees only the IP address of the local DNS nameserver, not the IP address of the client.

If a GRM L7 server is deployed for redirection service to clients, the GRM L7 server will redirect requests based on client IP address. Therefore, the GRM agent IP address returned to the client will be truly close to the requesting client.

Providing a means for locating a GRM L7 server

If you are using a GRM L7 server, you must provide some means for the client requests to reach the GRM L7 server that is closest to the client. You can choose the methods shown in the following table.

Methods for directing requests to a GRM L7 server	Advantages and disadvantages
Using a standard DNS nameserver	Advantage This is a simple solution. Disadvantages There is no way to guarantee that the GRM L7 server to which the DNS nameserver redirects requests is the closest one to the client. For example, if one GRM L7 server is in Australia and another is in the United States, the DNS nameserver round-robin process might direct requests from clients in the United States to the GRM L7 server in Australia. The GRM L7 server can become overloaded if all client requests are sent to the same GRM L7 server.
Deploying a GRM DNS server	Advantages The GRM DNS server acts as an intelligent means of locating the closest GRM L7 server to the clients. The GRM DNS server can also be used to monitor the health of the GRM L7 servers. Disadvantages This solution is more complicated than using only a DNS nameserver. This solution requires additional NetCache appliances.
Using a locator solution in your network infrastructure	Advantage An existing locator device in the network infrastructure can be used. Some locator devices provide sophisticated features, for example, features for load balancing over GRM L7 servers. Disadvantages You must rely on the existing network infrastructure. You must have devices in your network infrastructure to support this function.
Using a proxy.pac file	Advantage This solution is inexpensive, scalable, and easy to implement. Disadvantages Client browsers must be manually configured to point to the proxy.pac file on the GRM L7 server that is closest to the clients. Load balancing must be programmed into the proxy.pac file (for example, hash on URL).

Licensing

Each GRM server and GRM agent cache requires a license. Contact your Network Appliance sales engineer or the Installed Base Group at ibg@netapp.com to purchase a license.