Empty APN, 2 PDP contexts and 1 PDP context
Aleksander Morgado
aleksandermj at chromium.org
Tue Apr 9 09:02:19 UTC 2024
Hey!
>
> First of all I wanted to say thanks for developing and supporting a great piece of software.
>
Thanks to you for using it!
> 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.
>
Ok, so if I'm reading this right, you do NOT explicitly configure the
attach APN settings, you only provide the data APN settings. The
attach APN settings are provided by the modem itself.
> 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
I don't know what AutoSel does in these modems, can you elaborate?
> 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:
>
Yes.
> # 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
>
In the above output, your initial EPS default bearer that is brought
up during attach is using the "super" APN. I don't think this is what
you want, based on the previous comments? Not sure.
> 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.
>
Yes. As long as you don't modify state of the modem with the AT
commands, i.e. only use commands to read the state, you can safely run
them and they will give you the latest up to date info.
> 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.
>
I assume the modem is reporting the PDP context in CID 1 with empty
APN because the attach APN settings have an empty APN. You can confirm
this running the following:
$ sudo qmicli -d /dev/cdc-wdm0 -p --wds-get-lte-attach-pdn-list
// will likely return that profile 1 is the one used for attach
$ sudo qmicli -d /dev/cdc-wdm0 -p --wds-get-profile-list=3gpp
// and check contents of the profile 1
If you see a blank APN there, it means you're telling the network you
don't want any specific APN, and the network should assign one to you.
If this is the case, looking at the output above, it could be the
network assigning to you the "super" APN that you don't want.
You can manually configure the attach APN settings to be the same as
the data APN settings with an explicit mmcli operation:
$ sudo mmcli -m a
--3gpp-set-initial-eps-bearer-settings="apn=de1.super,ip-type=ipv4"
If this is the problem, you should try to ensure those settings are in
place before creating the data APN. E.g. you can first check that the
settings are the expected ones in the mmcli output, and if they
aren't, you set them.
$ mmcli -m a -K | grep "modem.3gpp.eps.initial-bearer.settings"
modem.3gpp.eps.initial-bearer.settings.apn : --
modem.3gpp.eps.initial-bearer.settings.ip-type : --
modem.3gpp.eps.initial-bearer.settings.user : --
modem.3gpp.eps.initial-bearer.settings.password : --
--
Aleksander
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