I've been looking for the same answers, here is what I found: In a book about UML Style, there was indicated, that executing a use-case is to be shown, by puting the use-case itself as an object with a life-line. Rational
Rose actually allows you to do this.
As for the preconditions, I choose to modell that logic into the diagram itself. For example, no beeing logged in, takes the user to executing the "log in" use-case. There is a posibility to represent if-else blocks in a seq. diagr.
And SSL - I would say that is something that I would not modell into the Seq. Diag. It is maybe the deployment diagram I would put it (Web-server to Browser connection), and mention it in the text
doc I submit with the asignment.
I only put important objects/components in the seq. diag., like Cade did - Sequence Locator, Business Delegate, Session Facade, some value objects, but nothing like remote or local interfaces. And there is only a customer interacting with these, I didn't show any trav. ag., asuming that the travel agent will mascarade as different customers in order to do the job, and that is a detail included in the app. client, that builds on the same structure and uses the same BDs, as the web client - so i assumed there is no need to modell that.
I would really apreciate your comments,
Rudi