Understanding Default Bearers and PDP Contexts
Jaden Young
jyoung at appareo.com
Mon May 8 15:31:38 UTC 2023
Hello,
Thanks for ModemManager!
Should a bearer of type MM_BEARER_TYPE_DEFAULT correspond 1-1 with a
modem PDP context?
Said another way, if I have a default bearer with APN B, should `mmcli
-m 0 --3gpp-profile-list` and `AT+CGDCONT` show a profile/PDP context
with APN B?
If there isn't a direct correspondence then the rest of this message can
be ignored. Pessimistically dumping more information here in case this
is a bug, and for searchability should another find themselves as
confused as I am.
Configuration Info:
- Modem: Quectel EG95-NAXD, FW version EG95NAXDGAR07A01M1G
- Location: US
- Carrier: AT&T
- Platform: Yocto Linux (kirkstone)
- ModemManager version: 1.18.6
- NetworkManager version: 1.36.2
Problem Description:
My goal is verifying that traffic is being sent directly to a private
APN without flowing through the PDN addressed by the initial-attach APN.
My intuition says that this should be possible, and that there would be
a correspondence between default bearers and PDP contexts. A source [1]
for which I have no sense of reputation quotes:
> According to 3GPP TS 23.401 [82], there is a 1 to 1 mapping between
> active EPS bearer context and active PDP context
Reading through 3GPP TS 23.401 version 18.1.0 [2] I haven't been able to
verify that claim.
In NetworkManager, I have a connection config with
`[gsm]\napn="m2m.com.attz"` (well, goal is a different APN, but this
suffices for example). I can see that this creates bearer 1, and
generating traffic will augment stats on `mmcli -b 1`. However,
`AT+CGDCONT` and `AT+CGREG` indicate that there isn't a PDP context
defined with `m2m.com.attz` as APN, and that the active PDP context is
using the APN of the default-attach bearer 0, `broadband`.
#+begin_quote
# mmcli -m 0
-----------------------------------
General | path: /org/freedesktop/ModemManager1/Modem/0
| device id: <snip, valid>
-----------------------------------
Hardware | manufacturer: QUALCOMM INCORPORATED
| model: QUECTEL Mobile Broadband Module
| firmware revision: EG95NAXDGAR07A01M1G
| carrier config: VoLTE-ATT
| carrier config revision: 0501033C
| h/w revision: 10000
| supported: gsm-umts, lte
| current: gsm-umts, lte
| equipment id: <snip, valid>
-----------------------------------
System | device: /sys/devices/platform/bus at 5b000000/5b0e0000.usb/ci_hdrc.1/usb1/1-1
| drivers: option, qmi_wwan
| plugin: quectel
| primary port: cdc-wdm0
| ports: cdc-wdm0 (qmi), ttyUSB0 (qcdm), ttyUSB2 (at), wwan0 (net)
-----------------------------------
Status | lock: sim-pin2
| unlock retries: sim-pin (3), sim-puk (10), sim-pin2 (3), sim-puk2 (10)
| state: connected
| power state: on
| access tech: lte
| signal quality: 67% (cached)
-----------------------------------
Modes | supported: allowed: 3g; preferred: none
| allowed: 4g; preferred: none
| allowed: 3g, 4g; preferred: 4g
| allowed: 3g, 4g; preferred: 3g
| current: allowed: 2g, 3g, 4g; preferred: 4g
-----------------------------------
Bands | supported: utran-4, utran-5, utran-2, eutran-2, eutran-4, eutran-5,
| eutran-12, eutran-13, eutran-25, eutran-26
| current: utran-4, utran-5, utran-2, eutran-2, eutran-4, eutran-5,
| eutran-12, eutran-13, eutran-25, eutran-26
-----------------------------------
IP | supported: ipv4, ipv6, ipv4v6
-----------------------------------
3GPP | imei: <snip, valid>
| enabled locks: fixed-dialing
| operator id: 310410
| operator name: AT&T
| registration: home
-----------------------------------
3GPP EPS | ue mode of operation: csps-2
| initial bearer path: /org/freedesktop/ModemManager1/Bearer/0
| initial bearer apn: broadband
| initial bearer ip type: ipv4
-----------------------------------
SIM | primary sim path: /org/freedesktop/ModemManager1/SIM/0
| sim slot paths: slot 1: /org/freedesktop/ModemManager1/SIM/0 (active)
| slot 2: none
-----------------------------------
Bearer | paths: /org/freedesktop/ModemManager1/Bearer/1
# 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: broadband
| ip type: ipv4
# mmcli -b 1
------------------------------------
General | path: /org/freedesktop/ModemManager1/Bearer/1
| type: default
------------------------------------
Status | connected: yes
| suspended: no
| multiplexed: no
| interface: wwan0
| ip timeout: 20
------------------------------------
Properties | apn: m2m.com.attz
| roaming: allowed
| ip type: ipv4v6
------------------------------------
IPv4 configuration | method: static
| address: 10.45.22.52
| prefix: 29
| gateway: 10.45.22.53
| dns: 8.8.8.8, 8.8.4.4
| mtu: 1430
------------------------------------
Statistics | duration: 119
| bytes rx: 27608
| bytes tx: 26331
| attempts: 1
| total-duration: 119
| total-bytes rx: 27608
| total-bytes tx: 26331
# at 'AT+CGDCONT?'
+CGDCONT: 1,"IP","broadband","0.0.0.0",0,0,0,0
+CGDCONT: 16,"IPV4V6","attm2mglobal","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT: 2,"IPV4V6","ims","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT: 3,"IPV4V6","sos","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,1
OK
# at 'AT+CGREG?'
+CGREG: 0,1
OK
#+end_quote
[1] https://itecspec.com/spec/3gpp-27-007-10-1-0-general-remark-about-eps-bearer-contexts-and-pdp-contexts/
[2] https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=849
Thank you,
Jaden Young
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