W3C standard for (stateful) SOAP conversations?

Hi,

I'm looking for a SOAP extension that could fulfill the following use case:

Airlines want to allow customers to book flights online. This is a
multi-step process and some information needs to be stored on the server
during the process. Basically I want a conversation id (or something
similar) associated with several messages so that the server can associate
this id with some information kept locally.

Is this already addressed by some current protocol specification?

Note: I'm rather limited in my selection of protocols (because of
political reasons), more specifically I can't go with a solution that is
completely stateless and thus imbeds state information, ala cookies, in
the messages themselves. Of course if you know of such a solution you
could still point it out as it would be interesting to me even if I can’t
use it.

Best regards,

Johan Tibell