Empty APN, 2 PDP contexts and 1 PDP context

[Filip Kubicz]

Hi ModemManager Community!

First of all I wanted to say thanks for developing and supporting a
great piece of software.

We have a few thousand devices running ModemManager 1.18.6 and Quectel EC21
modem. Our service provider Twilio/Kore keeps reporting that on some
devices we have 2 simultaneous data sessions with different APNs: "super"
and "de1.super".

We use ModemManager's API to find the modem, enable it, then wait for
auto-register done by the modem FW, and finally create a bearer with
"de1.super" APN and connect. Operations are run over QMI protocol.

We have autoselect disabled, so I'd expect to always use European APN
"de1.super", instead of US "super" which is the default.
mmcli -m 0 --command='AT+QMBNCFG="AutoSel"' response: '+QMBNCFG:
"AutoSel",0
We have made an experiment to show PDP contexts on the devices with
AT+CGDCONT?, and noticed an interesting thing:
On most of them, it only shows 1 PDP context:
*PdpContext { ctx_id: 1, pdp_type: IpV4V6, pdp_addr:
\"0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0\", apn: \"\", data_comp: Off, head_comp:
Off, ipv4_addr_alloc: NasSignaling, request_type:
ContextEstablishmentOrNon3GppAccessNetworkHandover }*

However, on 1 of the 200 devices, we see 2 PDP contexts:
*PdpContext { ctx_id: 2, pdp_type: Ip, pdp_addr: \"0.0.0.0\", apn:
\"de1.super\", data_comp: Off, head_comp: Off, ipv4_addr_alloc:
NasSignaling, request_type:
ContextEstablishmentOrNon3GppAccessNetworkHandover }*

*PdpContext { ctx_id: 1, pdp_type: IpV4V6, pdp_addr:
\"0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0\", apn: \"\", data_comp: Off, head_comp:
Off, ipv4_addr_alloc: NasSignaling, request_type:
ContextEstablishmentOrNon3GppAccessNetworkHandover }*

However, even on device with just 1 PDP context received from AT command
# mmcli -m 0 --command='AT+CGDCONT?'
response: '+CGDCONT:
1,"IPV4V6","","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0'

AT commands are run ~10 minutes after modem setup and connection is done
with ModemManager.
ModemManager always reports 2 bearers, I understand the 1st bearer is LTE
default EPS bearer, while the 2nd one is the actual data session bearer:

# mmcli -b 0
  -------------------------
  General    |        path: /org/freedesktop/ModemManager1/Bearer/0
             |        type: default-attach
  -------------------------
  Status     |   connected: yes
             |   suspended: no
             | multiplexed: no
             |  ip timeout: 20
  -------------------------
  Properties |         apn: super
             |     ip type: ipv4

# mmcli -b 3
  ------------------------------------
  General            |           path:
/org/freedesktop/ModemManager1/Bearer/3
                     |           type: default
  ------------------------------------
  Status             |      connected: yes
                     |      suspended: no
                     |    multiplexed: no
                     |      interface: wwan0
                     |     ip timeout: 20
  ------------------------------------
  Properties         |            apn: de1.super
                     |        roaming: allowed
                     |        ip type: ipv4
  ------------------------------------
  IPv4 configuration |         method: static
                     |        address: 100.79.145.117
                     |         prefix: 30
                     |        gateway: 100.79.145.118
                     |            dns: 8.8.4.4, 8.8.8.8
                     |            mtu: 1360
  ------------------------------------
  Statistics         |       duration: 3810
                     |       bytes rx: 513071
                     |       bytes tx: 296673
                     |       attempts: 1
                     | total-duration: 3810
                     | total-bytes rx: 513071
                     | total-bytes tx: 296673

I have some questions:
1. When using QMI, is it reliable to check modem state with AT commands? I
don't care about immediate / race conditions, but I'd like to know that if
I run some AT command e.g. 1 hour after it was set up by ModemManager with
QMI it will tell me the accurate modem state.

2. Why do some devices have only 1 PDP context with empty APN, while others
have 2 PDP contexts with the right APN in context 2?
We'd like to make sure all devices use European APN as it positively
impacts the latency and download speed.

Thank you!
Filip Kubicz
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