Dual Address Bearer Flag - DAF

Functionality:

DAF or Dual Address Bearer Flag is used by the MME to indicate to the PGW if all the other MME’s and SGSN’s it can talk to are ipv4v6 capable

Values:

DAF bit is present in the Indication Flag IE. It is sent on a Create Session Request message.

DAF is set to 1 when the following conditions are satisfied

1. UE requests for an IPV4V6 bearer ( i.e. PDN TYPE is set to ipv4v6 )

2. Operator configures in MME that all SGSN’s and MME’s connected to the MME support ipv4v6

“The MME shall set the Dual Address Bearer Flag when the PDN type is set to IPv4v6 and all SGSNs which the UE may be handed over to are Release 8 or above supporting dual addressing, which is determined based on node pre-configuration by the operator”

DAF is set to 0 otherwise.

Usage:

Problem:

Consider a scenario where UE requests for an IPV4V6 bearer.

The PGW can take any one of the two actions:

1. Assign 1 ipv4v6 bearer

2. Assign 1 ipv4 bearer and 1 ipv6 bearer

Now suppose PGW assigns ipv4v6 bearer to the UE.

UE now moves to a 3G network, which triggers a Context Transfer from MME to SGSN

If the SGSN isn’t capable to handle a dual address bearer (i.e. ipv4v6 bearer) the context transfer will not be successful and hence the UE’s existing bearer will be lost.

The SGSN will now have to create 2 new bearers (1 ipv4 and another ipv6)

Solution:

DAF can be used to avoid such scenarios.

If all the SGSN’s and MME’s do not support ipv4v6, PGW should not assign an ipv4v6 bearer to the UE.

The MME is aware if all SGSN’s and MME’s connected to it support dual addressing. If MME finds any SGSN incapable of handling dual addressing, MME will set the DAF bit to 0.

PGW will notice that though the UE has requested for an ipv4v6 bearer, not all SGSN’s or MME’s support dual addresses. As such PGW can now assign 2 bearers (1 ipv4 and another ipv6) instead of assigning 1 ipv4v6 bearer as in the earlier case.

This time when the UE does a handover from 4G to 3G, the context transfer will map 2 bearers from 4G to 3G (instead of 1). Thus the context transfer will be successful.

Mapping between P-TMSI and GUTI during inter RAT handover

Section describes usage of GUTI/P-TMSI as part of Inter RAT Handover.

GUTI:

GUTI is an identifier for the UE allocated by MME so that IMSI is not required to be transmitted over radio interface (where it is susceptible to interception). Structure of GUTI is designed such that it is possible to identify the node (MME) that allocated it.

GUTI = MCC + MNC + MME ID + M-TMSI

MME ID identifies an MME in the operator’s network and M-TMSI identifies a UE served by that MME.MME ID contains MME group id which is used to identify MME pool and MME code which is used to indicate a node within the pool

PTMSI:

P-TMSI (Packet domain TMSI) is an identifier for the UE allocated by SGSN. P-TMSI contains NRI (Network resource identifier) which is used to identify the node that allocated it. In 3G, LAC (present as part of RAI) is used to identify the SGSN pool and NRI is used to identify a node within the pool.

Note:  Mapping between GUTI and P-TMSI/RAI is done such that the information regarding the allocating node is still retained.

Usage of GUTI/P-TMSI during Inter-RAT handover (4G – 3G- 4G):

•  As part of Attach procedure, MME allocates a GUTI to the UE (Attach Accept).  
• UE moves from 4G to 3G network.
• UE includes a P-TMSI and RAI values derived from GUTI as part of RAU request to SGSN as old P-TMSI/old RAI.
• SGSN uses the P-TMSI to identify the associated MME and requests the MME to provide UE context information.
•  MME maps the P-TMSI to GUTI to identify the subscriber and passes the necessary information to SGSN.
• Once the context transfer is done, SGSN assigns a P-TMSI.
• UE stores this information as part of its 3G subscriber context and uses for all subsequent 3G procedures.
• When UE moves from 3G to 4G, UE sends GUTI information mapped from P-TMSI as part of TAU request (old GUTI). Further since UE has a valid GUTI, UE sends this information as part of TAU request as additional GUTI.
• MME uses P-TMSI and RAI mapped from GUTI to identify the responsible SGSN and sends a request to perform context transfer including the P-TMSI.
• SGSN uses P-TMSI to identify the subscriber and passes necessary information to MME.
• Further MME checks if the GUTI passed as additional GUTI is valid. If it is not valid, a new GUTI is allocated for the subscriber and sent as part of TAU accept.
• It must be noted that MME is aware that it communicates with a 3G SGSN. However SGSN communicates to MME as if it is communicating to another SGSN. Hence only MME will do the mapping (P-TMSI <-> GUTI) in the network whenever necessary.
• The values of LAC and MME group id shall be disjoint, so that they can be differentiated. The most significant bit of the LAC is set to zero and the most significant bit of MME group id is set to one. This is required because MME should be able to identify from the old GUTI if it needs to talk to SGSN or another MME.