Cisco Cisco Packet Data Gateway (PDG)
Evolved Packet Data Gateway Overview
▀ How the ePDG Works
▄ ePDG Administration Guide, StarOS Release 17
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The ePDG detects the duplicate session and clears the previous established session at its ends. Then the ePDG
establishes a new session on the P-GW using the following steps:
establishes a new session on the P-GW using the following steps:
Table 16. WiFi-to-WiFi Re-Attach - GTPv2
Step Description
13.
ePDG -> P-GW: Create Session Request - The ePDG selects the P-GW based on DNS response from the APN-FQDN.The
ePDG sends Create Session Request (IMSI, [MSIDSN], Serving Network, RAT Type (WLAN), Indication Flags, Sender
F-TEID for C-plane, APN, Selection Mode, PAA, APN-AMBR, Bearer Contexts), [Recovery], [Charging Characteristics],
Private IE (P-CSCF). The TEID shall be set to zero so that P-GW shall handle the same as create-on-create case.
ePDG sends Create Session Request (IMSI, [MSIDSN], Serving Network, RAT Type (WLAN), Indication Flags, Sender
F-TEID for C-plane, APN, Selection Mode, PAA, APN-AMBR, Bearer Contexts), [Recovery], [Charging Characteristics],
Private IE (P-CSCF). The TEID shall be set to zero so that P-GW shall handle the same as create-on-create case.
14.
P-GW -> ePDG: Create Session Response - The P-GW terminates the previous session by handling it as create on create
case and establishes a new session. The P-GW allocates the requested IP address session and responds back to the ePDG
with a Create Session Response (Cause, P-GW S2b Address C-plane, PAA, [Recovery], APN-AMBR, Additional Protocol
Configuration Option (APCO) Bearer Contexts Created, Private IE (P-CSCF)) message.
case and establishes a new session. The P-GW allocates the requested IP address session and responds back to the ePDG
with a Create Session Response (Cause, P-GW S2b Address C-plane, PAA, [Recovery], APN-AMBR, Additional Protocol
Configuration Option (APCO) Bearer Contexts Created, Private IE (P-CSCF)) message.
15.
ePDG -> UE: IKE_AUTH - The ePDG sends IKE_AUTH (AUTH, CP, SA, CFG_REPLY
([INTERNAL_IP4_ADDRESS], [INTERNAL_IP4_NETMASK], [INTERNAL_IP4_DNS], INTERNAL_IP6_ADDRESS,
INTERNAL_IP6_SUBNET, INTERNAL_IP6_DNS, P-CSCF) TSi, TSr).
The ePDG calculates the AUTH parameter, which authenticates the second IKE_SA_INIT message. The ePDG sends the
assigned IP address in the configuration payload (CFG_REPLY). The AUTH parameter is sent to the UE together with the
configuration payload, security associations and the rest of the IKEv2 parameters and the IKEv2 negotiation terminates.
([INTERNAL_IP4_ADDRESS], [INTERNAL_IP4_NETMASK], [INTERNAL_IP4_DNS], INTERNAL_IP6_ADDRESS,
INTERNAL_IP6_SUBNET, INTERNAL_IP6_DNS, P-CSCF) TSi, TSr).
The ePDG calculates the AUTH parameter, which authenticates the second IKE_SA_INIT message. The ePDG sends the
assigned IP address in the configuration payload (CFG_REPLY). The AUTH parameter is sent to the UE together with the
configuration payload, security associations and the rest of the IKEv2 parameters and the IKEv2 negotiation terminates.
16.
ePDG -> UE: Router Advertisement - ePDG sends Router Advertisement to ensure IP Stack is fully initialized.
WiFi to LTE Handoff with Dedicated Bearer (UE initiated)
When a VoLTE call is ongoing, the P-GW will install the bearers on the LTE network using piggyback procedure.