Introduction
This document describes the collision observed between Modify Bearer Request (MBReq) and Create Bearer Request (CBReq) in the SGW during N26 handover.
Prerequisites
Requirements
Cisco recommends that you have knowledge of these topics:
Components Used
The information in this document is based on these software and hardware versions:
- Cisco StarOS® 21.28.m5
- Ultra Packet Core CUPS
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Problem
During an N26 inter-RAT 5G SA -> 4G LTE handover for an EPS-Fallback VoLTE voice call, the QCI1 Dedicated Bearer creation and the Access Modify Bearer Request procedures occur independently of each other and can collide. When this collision occurs, the SGW fails to correctly set up the QCI5 ims bearer leading to voice calls and IMS traffic failing on LTE until the UE returns to 5G SA.
Analysis Performed
1. Handover from 5G to 4G completes successfully (TAU Request/Authentication/Encryption/Create Session Requests to move both bearers):
Frame 76 S1AP/NAS-EPS InitialUEMessage, Tracking area update request
Frame 98 GTPv2 EBI= 5,5 EUTRAN QCI= 5 Create Session Request
Frame 99 GTPv2 EBI= 6,6 EUTRAN QCI= 8 Create Session Request
Frame 109 S1AP/NAS-EPS InitialContextSetupRequest, Tracking area update accept
2. Then, there is the Initial-Context-Setup Response Success and TAU Complete from the eNB with the eNB S1-U IPs. At this point, the MME has not provided these to the SGW and the SGW does not know where to send down-link UP packets:
Frame 111 S1AP InitialContextSetupResponse
Frame 113 S1AP/NAS-EPS UplinkNASTransport, Tracking area update complete
3. MME sends a Modify Bearer Request on all EBIs (5 & 6) to provide the eNB S1-U IP to the SGW (frames 114 and 115). However, at the same time, the SGW gets a Create Bearer Request on EBI= 5 (QCI-5) to create the QCI-1 bearer (frame 117). When this collision happens, the Modify Bearer Request on EBI= 5 does not get processed; therefore, the SGW never learns the eNB S1-U IP on the ims default bearer so down link packets cannot be delivered:
Frame 114 GTPv2 EBI= 6 EUTRAN Modify Bearer Request
Frame 115 GTPv2 EBI= 5 EUTRAN Modify Bearer Request
Frame 116 GTPv2 EBI= 6 Modify Bearer Response
Frame 117 GTPv2 EBI= 5,0 QCI= 1 Create Bearer Request
Frame 118 S1AP UECapabilityInfoIndication, UECapabilityInformation
Frame 121 GTPv2 EBI= 5 Downlink Data Notification
Frame 122 GTPv2 Downlink Data Notification Acknowledgement
Frame 129 GTPv2 EBI= 5 EUTRAN Modify Bearer Request
Frame 130 GTPv2 EBI= 5 Modify Bearer Response
Frame 131 S1AP/NAS-EPS E-RABSetupRequest, Activate dedicated EPS bearer context request
Frame 132 S1AP E-RABSetupResponse
Based on 3GPP specification 29.274, SGW must be handling Bearer Context mismatch (for example, the collision between CBReq and MBReq):
14.3 Handling of Bearer Context Mismatch
14.3.2 Exceptional scenarios
During a dedicated bearer creation procedure, temporary Bearer Context mismatch may occur at the SGW, (for example) due to the collision between Create Bearer Request and Modify (Access) Bearer Request messages.
[...]
The SGW should handle such Bearer Context mismatch in an implementation specific way, but in such a way to accept the Modify (Access) Bearer Request message and to not locally delete the missing Bearer Contex.
Solution
This issue is caused by Cisco bug ID CSCwf63318. To handle this collision at SGW as per standards, the Modify Bearer Request takes a higher precedence over the Create Bearer Request. SGW aborts the Create Bearer Request temporarily, then proceeds with the Modify Bearer Request and forwards it to PGW.
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