Page 505 - 5G Basics - Core Network Aspects

Core network aspects 1

7.3.4.1 Explicit control of handover execution at the UE

In this case, when the HDC-FE has finished preparation of the new path as described in clause 7.3.3, it issues
requests to the tunnel upper and lower ends to commit to the new path. Details of the protocol required to
do this without losing packets are out of scope. Once the HDC-FE is informed that commitment is complete,
the HDC-FE issues requests to the tunnel upper and lower ends to delete the old tunnel. It sends a request
to RACF to deallocate the QoS resources assigned to the old path, from the old access point to the anchor
point.

7.3.4.2 Link down indication or unbind indication
If the HDC-FE receives a 'link down' indication from the L2HE-FE instance associated with the old link, or an
unbind indication from the MLM-FE, the procedure is similar to the procedure just described, with the
exception that if the tunnel lower end was the UE, it does not have to be directed to commit to the new path
or to delete the old tunnel. The details of the protocol required to avoid packet loss in this case are out of
the scope of this Recommendation. They may differ from what is required in the case described by
clause 7.3.4.1.
7.3.5 Routing optimization considerations for network-based handover

When a correspondent UE communicates with the mobile UE, normally packets from the correspondent UE
will be intercepted by the transport function associated with the MLM-FE(C) (i.e., the L3HEF at the anchor
point), then encapsulated and tunneled to the current location of UE. For data path optimization, the MLM-
FE(P) to which a correspondent UE is associated may, based on administrative policies, cooperate with the
MLM-FE(P) to which the mobile UE is associated to establish a direct tunnel between their respective
networks. At that point the L3HEF at each end will encapsulate packets based on the location binding
information exchanged between the two MLM-FE(P)s and forward them directly to its peer L3HEF through
the tunnel between them. The peer L3HEF delivers the decapsulated packets to the correspondent or mobile
UE respectively.

The L3HEF is required to be aware of active communication between the mobile UE and correspondent UE.
The process is triggered when it detects a flow that matches policy indicating that routing optimization may
be attempted. It then indicates route optimization to its associated MLM-FE(P) via the HDC-FE. The MLM-
FE(P) sends a location management message to the MLM-FE(C) which contains a routing optimization (RO)
indication.

Upon receiving an RO request message from the MLM-FE(P), the MLM-FE(C) will perform routing
optimization operation not only with the MLM-FE(P) associated with the mobile UE, but also with the MLM-
FE(P) associated with the correspondent UE. The MLM-FE(C) may look up its location binding list and
determine whether the two MLM-FE(P) instances have both registered to it. In the case where both MLM-
FE(P) instances are registered to the same MLM-FE(C) instance, the latter will pass the location of each MLM-
FE(P) to the other.

If the two MLM-FE(P) instances register to different MLM-FE(C) instances, the MLM-FE(C) associated with
the mobile UE and MLM-FE(C) associated with the correspondent UE will coordinate with each other to
provide the location of each MLM-FE(P) instance to the other. If the MLM-FE(C) associated with the mobile
UE fails to retrieve the location of the MLM-FE(C) associated with the correspondent UE, it will notify the
MLM-FE(P) associated with the mobile UE that route optimization is not available.

After routing optimization operations are completed between the two MLM-FE(P)s, the location binding
cache in each MLM-FE(P) is updated and the optimized tunnel is installed. The details for the common MLM-
FE(C) case are shown in Figure 10.
NOTE 1 – The messages required for routing optimization are shown in Figure 10, but their detailed definition is for
further study.
NOTE 2 – The HDC-FE is shown as a centralized function for convenience, and MLM-FE(P) A is arbitrarily chosen as the
initiating entity to start tunnel set-up. It is more realistic to assume that both MLM-FE(P) instances will notify HDC-FE
and tunnel set-up and take-down will proceed in a more distributed fashion.

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