Section: 4.3.15 Selected IP Traffic Offload (SIPTO) function
Content: The SIPTO function enables an operator to offload certain types of traffic at a network node close to that UE's point of attachment to the access network. The SIPTO function specified in this TS applies to IP PDN Types and Ethernet and Non-IP PDN Types. SIPTO above RAN can be achieved by selecting a set of GWs (S-GW and P-GW) that is geographically/topologically close to a UE's point of attachment. SIPTO above RAN corresponds to a traffic offload through a P-GW located in the mobile operator's core network. SIPTO applies to both the non-roaming case and, provided appropriate roaming agreements are in place between the operators, to the roaming case. Offload of traffic for a UE is available for UTRAN and E-UTRAN accesses only. When the UE enters to UTRAN/E-UTRAN from another type of access network (e.g., from GERAN), it is the responsibility of the new SGSN/MME to decide whether to perform deactivation with reactivation request for a given PDN connection, depending on SIPTO permissions for the relevant APN. Realization for SIPTO above RAN relies on the same architecture models and principles as for local breakout described in clause 4.2. In order to select a set of appropriate GW (S-GW and P-GW) based on geographical/topological proximity to UE, the GW selection function specified in TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61] uses the UE's current location information. In order for the operator to allow/prohibit SIPTO on per user and per APN basis, subscription data in the HSS is configured to indicate to the MME if offload is allowed or prohibited. If the SIPTO permissions information from the HSS conflicts with MME's configuration for that UE, then SIPTO is not used. If HSS indicates VPLMN address not allowed, then VPLMN (i.e. MME) shall not provide SIPTO. In the absence of any SIPTO permissions indication from the HSS the VPLMN (i.e MME) shall not provide SIPTO. The MME may be configured on a per APN basis as to whether or not to use SIPTO (e.g. to handle the case where the HSS is not configured with SIPTO information for the UE). For SIPTO above RAN, as a result of UE mobility (e.g. detected by the MME at TAU or SGSN at RAU or movement from GERAN), the target MME may wish to redirect a PDN connection towards a different GW that is more appropriate for the UE's current location, e.g. MME may know whether the UE's new location is served by the same GW as the old one. When the MME decides upon the need for GW relocation, the MME deactivates the impacted PDN connections indicating "reactivation requested" as specified in clause 5.10.3. If all of the PDN connections for the UE need to be relocated, the MME may initiate the "explicit detach with reattach required" procedure as specified in clause 5.3.8.3. NOTE: If either of the above procedures for GW relocation are initiated while the UE has active applications, it may cause disruption of services that are affected if the IP address changes.