Novell Directory Agents can be installed and configured so that the Local mode operation can do the following:
Directory Agents function as a centralized data store for service URLs that are registered by Service Agents and solicited by User Agents. Because Directory Agents hold all the services for each configured scope, User Agents can obtain all desired service information with a single request and reply. By contrast, in networks without Directory Agents, User Agents issue a multicast request and might receive many replies.
Directory Agents are configured to support one or more SLP scopes. (An unscoped operation is similar to supporting a single scope.) Directory Agents collect and store service URLs and their associated attributes according to the scope in which the services are registered. Service Agents and User Agents obtain the scopes supported by a Directory Agent from a Directory Agent's DA Advert message. In this way, User Agents and Service Agents can dynamically detect and utilize the scopes configured for each Directory Agent. In networks without Directory Agents, Service Agents and User Agents must be configured with the SLP scopes they will use.
Novell Directory Agents allow the network administrator to create customized scopes by pulling service information from one scope and storing it in a different scope. This is a variation of the scope proxy feature because the custom scope name is different than the scope being proxied.
For example, if a network administrator wants to create a custom scope for a single group of users containing only specific service URLs and attributes, the custom scope is configured on the local Directory Agent and the address of the scope authority servicing a target scope and the target scope's name is configured as a proxy address for the custom scope. The content of the custom scope can be further controlled by adding filters that apply only to the custom scope.
When the services are retrieved from the scope authority and registered in the custom scope, the attributes of the service are modified to indicate that the service is now part of the custom scope. The group of users can then be configured to use only the custom scope with the network administrator controlling the service information available to them. Using this same technique, a hierarchy of scopes can be created to reflect the administrative groupings of services that best fit your network user's needs.
Novell Directory Agents can be configured to proxy scopes supported natively by other Directory Agents, also referred to as scope authorities. Instead of having every Service Agent register with every Directory Agent in the network, Service Agents can be configured to register with a single or small subset of Directory Agents. The other Directory Agents in the network are then configured to proxy the scopes of the central Directory Agents, which act as the authorities for the proxied scopes.
When a Directory Agent is configured to proxy a scope supported by another Directory Agent, the proxy agent downloads the scope information at configured intervals and then acts as a local service cache for that scope. This can be advantageous for remote sites reachable over WAN segments. Rather than having User Agents in remote sites interacting with Directory Agents over the WAN, a proxy Directory Agent can be deployed in the remote site, keeping all SLP service queries within the local site's network.
Because service information can be registered and obtained with a single unicast request and reply, the operation of SLP becomes more efficient and hence more scalable. Because each interaction with a Directory Agent always results in a reply, the time required to resolve a service request is kept to a minimum. When a User Agent issues a multicast request, it must wait a period of time before determining if all answers have been received. This is because Service Agents and Directory Agents do not respond unless they can answer the query. As a result, the User Agent must pause while waiting for replies, estimating when all possible answers have been received. But as soon as a User Agent receives a reply from a Directory Agent it can process the response immediately.
All protocol interactions with a Directory Agent are performed using unicast messages. If multicast is not supported on your network, deploying a Directory Agent and configuring the Service Agents and User Agents with the IP address of the Directory Agent (through local configuration or DHCP) allows SLP to be used in networks that do not support multicast addressing.
In addition to the features listed above that are defined by the SLP protocol, Novell Directory Agents support other value-added features that assist the network administrator in deploying SLP within their network. Novell Directory Agents can be configured to operate in Private mode. When configured for Private mode, the Directory Agent does not multicast Directory Agent Advert messages or answer multicast requests, thus making the Directory Agent undiscoverable by dynamic means. To use a Directory Agent configured in Private mode, User Agents and Service Agents must be configured with the address of the private Directory Agent.
This allows the network administrator to create closed groups of users of one or more private Directory Agents. Private Directory Agents are also a valuable tool in piloting new versions of the Directory Agent or testing new configurations without disturbing the operating network.
When a Directory Agent is operating in Local mode, network administrators can configure filters that control which service URLs are accepted for registration and which service URLs are returned in service replies. The filters are configured on a per scope basis, allowing network administrators to customize the content of each scope separately. The filtering criteria include service type, specific URLs, service lifetime, and the address of the Service Agent or User Agent making a request. One or more filter criteria can be specified for each filter.