hosted by the PGW and ePDG entities, respectively. The GRE protocol constructs the S2b tunnel from a key provided by the ePDG entity for downstream traffic and a key provided by the PGW for upstream traffic. server DIAMETER CCR DIAMETER CCA DIAMETER AAR DIAMETER SAR DIAMETER SAA DIAMETER AAA Figure 8.5. S2b tunnel establishment using PMIPv6 mechanism The procedure for establishing the S2b tunnel starts during the SWu tunnel establishment procedure described in Chapter 7 (Figure 8.5). S2a/S2b Tunnel Establishment 1) The ePDG entity transmits to the PGW entity the PBU extension containing the following fields: MN-NAI, Lifetime, APN, Access Technology Type, GRE key for downlink traffic, UE Address Info, Charging Characteristics and Additional Parameters. 2) The PGW entity sends the PCRF entity the CCR DIAMETER message to obtain the mobile traffic profile. 3) The PCRF entity responds to the PGW entity with the CCA DIAMETER message containing the rules to be applied to the default bearer (APN-AMBR rate parameters and QoS). 4) The PGW entity sends the AAA server the AAR DIAMETER message to communicate its identity and the access point name for the connection. 5) The AAA server transmits to the HSS entity the SAR DIAMETER message to transfer the information received from the PGW entity. 6) The HSS entity responds to the AAA server with the SAA DIAMETER message to transfer the information received from the PGW entity. 7) The AAA server responds to the PGW entity with the AAA DIAMETER message containing the information received from the HSS entity. The mobile profile is taken into account if the PCRF did not provide the information in step 3. 8) The PGW entity responds to the ePDG entity with the PBA extension containing the following fields: MN-NAI, UE Address Info, GRE Key for uplink traffic and Charging ID. The ePDG entity completes the SWu tunnel establishment procedure described in Chapter 6. 8.2. GTPv2 mechanism The GTPv2 (GPRS Tunneling Protocol version 2) mechanism comprises the GTPv2-C (Control) signaling that ma