The "direct peering" scenario consists in having one of the two roaming partners provide support for both variants of roaming flavour (e.g. a PMIP operator would support GTP-based roaming interface towards a GTP-only roaming partner, or vice versa) in order to make roaming possible. The support for such roaming flavour can be provided either on the same GW node or on different GW nodes. Upon establishment of connectivity for a specific roaming UE, the Visited network chooses a GTP-based or a PMIP-based S8 interface (on the same GW node or on different GW nodes, note that for a single user only a single Serving-GW is allocated when connecting to EPC), depending on the preferences of the roaming partner that owns the subscriber.

Depicted in Figure A.1-1 (a) is an example of "direct peering" interworking between a GTP-based HPLMN and a PMIP-based VPLMN. When roamers whose subscription is owned by the GTP-based operator attach to the EPS network of the PMIP-based operator, they are assigned a GTP-capable GW acting in the role of S-GW. The S-GW selection is carried out by MME or SGSN based on the subscriber's HPLMN. In case of the Serving-GW supporting both GTP and PMIP, the MME/SGSN should indicate the Serving-GW which protocol should be used over S5/S8 interface. Depicted in Figure A.1-1 (b) is an example of "direct peering" interworking between a PMIP-based HPLMN and a GTP-based VPLMN. When roamers whose subscription is owned by the PMIP-based operator attach to the EPS network of the GTP-based operator, they are assigned a GTP-capable S-GW. The information provided by the PMIP-based HPLMN for the P-GW selection function must take into account that the Visited network is GTP-only, in order to return either the IP address (or an APN that can be resolved to an IP address according to the PDN-GW resolution mechanism) that points to a GTP-capable PDN-GW. Figure A.1-2 depicts the scenario in which a UE from a GTP-based network roams in a PMIP-based network, local breakout is used, and home-routed bearers are also possible. As with the home-routed case, the MME or SGSN in the PMIP-based VPLMN selects a GTP-capable Serving-GW, but it selects a PMIP capable PDN-GW. As a result, the S-GW in this example supports both GTP and PMIP based S5/S8. This allows the local breakout bearer and any associated home-routed bearer for the user (e.g. the default bearer) to be served by the same Serving-GW. Support of S9 may not be required in all local breakout scenarios.

Figure A.1-3 depicts the scenario in which a UE from a PMIP-based network roams into a GTP-based network and local breakout is used. As with the home-routed case, the MME/SGSN in the GTP-based VPLMN selects a GTP-capable Serving-GW and the PDN-GW selection function selects a GTP-capable PDN-GW. This allows the local breakout bearer and any associated home-routed bearer for the user (e.g., the default bearer) to be served by the same Serving-GW. Support of S9 may not be required in all local breakout scenarios.

In this scenario an Interworking Proxy (IWP) sits between the GTP-based PLMN and the PMIP-based PLMN to perform protocol conversion between the GTP protocol on one side and the PMIP and Diameter protocols on the other side.

The IWP is inserted transparently in the signalling and bearer path i.e. no changes to the GTP, PMIP and Diameter protocols are required.