Session Replacement - Cisco ASR 5000 Administration Manual

Enhanced Wireless Access Gateway Overview
Step Description
20 The GGSN processes the Create PDP Context Request and allocates the MPC IP address in the Create PDP Context
Response message. It also negotiates the QoS to be used for the subscriber session and sends the same in the Create PDP
Context Response message.
21 The eWAG processes the Create PDP Context Response message and creates the binding between the Wi-Fi IP address and
the MPC IP address in the eWAG session.
22 The eWAG sends the Accounting-Response message to the Wi-Fi AAA server to acknowledge the Accounting-Interim
message.
23 The UE initiates data transfer to the destination in APN network with Source IP set to its Wi-Fi IP address. This packet gets
routed to the eWAG from the WLAN network.
24 The eWAG performs NAT on this data packet (Layer 3 to Layer 7), from Wi-Fi IP address to MPC-IP address.
25 The eWAG sends the NATd IP packet encapsulated over the GTP-U tunnel created with the GGSN.
26 The GGSN decapsulates the IP packet received over the GTP-U tunnel, and sends it to the destination APN network. Note
that this IP packet contains the source IP address set to the MPC IP address.
27 The data packet received in the downlink direction from the APN network is processed by the GGSN. This downlink
packet contains the destination IP address set to the MPC IP address.
28 The GGSN encapsulates the IP packet over the GTP-U tunnel and sends it downlink to the eWAG.
29 The eWAG performs reverse-NAT on the downlink IP packet received over the GTP-U tunnel from the GGSN, and
converts all MPC IP addresses to Wi-Fi IP addresses from Layer 3 to Layer 7.
30 The eWAG sends the plain IP packet downlink to the UE.

Session Replacement

Session identification at eWAG is done using the following parameters:
 Username+MSISDN combination
 Wi-Fi IP address
If the eWAG cannot identify the session for the received Accounting-Start message using the above parameters, then
session replacement will happen if any one of the above parameters matches existing session as explained below:
1. Matching session found at eWAG with same Username+MSISDN combo but containing different Wi-Fi IP
address. This is the scenario where the subscriber lost connectivity with Wi-Fi and is trying to reconnect again
with a different IP address.
2. Matching session found at eWAG with same Wi-Fi IP address but containing different Username+MSISDN
combo. This is the scenario where the subscriber has disconnected from Wi-Fi network and released the IP
address but the Accounting-Stop sent from WLC is lost/not received by eWAG. So the session at eWAG will
be stale during this time and when new Accounting-Start message comes with the same Wi-Fi IP address as the
existing session it will get replaced as this Accounting-Start message is for new subscriber with different
Username+MSISDN combo.
OL-28188-02
Important:
In case of session replacement, old call will be disconnected with the session
disconnect reason "IPSG-session-replacement".
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