Algorithm for entity tag generation in WebSphere Presence Server

Answer

The entity-tag token (SIP-E-Tag header returned in a 200 OK response to a SIP PUBLISH message) must be generated by the Presence Server using configured data.

To provide uniqueness and some level of authorization, the configured data should include

· A specific header field; chosen header field is the To: header field.
· PIDF document element attribute(s); chosen values are entity-id of the <presence> element and id of the <tuple> element.

both part of an incoming SIP PUBLISH message. Those values will be used for the generation of the entity-tag token returned in the associated response. It should also be possible to append constant string value(s) for the entity-tag token generation.

As such, the algorithm will use the header field, the tuple id, and the constant string to generate the entity-tag token for event-type presence. It will use the header field and the constant string to generate the entity-tag token for the event-type message summary.

For example,

• assume the header field specified for use in the entity-tag generation is the "To:" SIP header;
• assume the tuple id in the incoming message body is "company_vm"; and
• assume the configured constant string is "PresenceAdapter";
• then, per the simple algorithm, the entity token for event-type presence will be tel:+18563021008@ims.company.net~PresenceAdapter~company_vm; based on the "To:" header field value of tel:+18563021008@ims.company.net in the incoming message; and
• the entity token for event-type message-summary will be
  tel:+18563021008@ims.company.net~PresenceAdapter

It must be possible to apply the algorithm to specific services that interact with the Presence Server as well as replace the out-of-the-box algorithm with a custom one at point of deployment. Therefore the algorithm or the API that is used for the generation of the entity-tag token must be published.

For implementation purposes, the following properties have been added to the SystemConfiguration.xml

<!-- Configure entity tag generator -->
<customEtagGenerator enable="true" class="">
<!-- list of event package with parameters for entity tag generation used by the default publication service -->
<event name="presence" constant="pa" separator="~"/>
<event name="message-summary" constant="company_vm_pa" separator="~"/>
</customEtagGenerator>

The property customEtagGenerator enable="true"enables the new eTag algorithm. If this attribute is false, the Presence Server generates etags as before.

In the default implementation, the E-Tag is partially configurable based on the values in the configuration file. "To:" header and the "tuple id" are mandatory for the tag generation.

In addition, a SPI is exposed as part of WebSpeherePresenceServerAPI.jar to allow users to develop their own algorithm for etag generation if needed. In this case the new module / jar along with the WebSpeherePresenceServerAPI.jar should be added to the server classpath and the system configuration should include the class name of the third party generator.

For example:
<customEtagGenerator enable="true" class="com.abc.ThirdPartyGenerator">
...
</customEtagGenerator>

If a class is present in the SystemConfiguration.xml, the Presence Server will invoke this class for etag generation.