create in handle_f_teid is wrong

[RoadRunnr]

somewhat related to #34

For Update PDR, handle_f_teid is called with create = false. Despite that setting, a new TEID would still be allocated, only the registration of the new TEID would be skipped. This is clearly wrong, when a new TEID is allocated is must also registered.

The other question is if a Update PDR should be permitted to allocate a new TEID. There is no clear guidance spelled out in the specification. All references to F-TEID allocation always put this into relation of PDR creation. The is no mentioning of PDR modification in this context. It seems therefore safe to assume that a Update PDR might only reference an already allocated F-TEID, but might not by itself allocated a new F-TEID.

The choose mechanism in handle_f_teid therefore needs to be adjusted to only allocate a new F-TEID when create is true.

Note: it would be legal in a modify request to have a Create PDR that creates a new F-TEID and a Update PDR that references that new F-TEID. There is no order requirement for PFCP IEs, the Update PDR could therefore be encoded in the PFCP request before the Update PDR. This is not a problem for our current implementation that uses a fixed ordering when processing IE and will always process the Create PDR before the Update PDR

[sergeymatov]

ack, starting work on this

[sergeymatov]

I suppose it can be closed