Understanding Default Bearers and PDP Contexts

Jaden Young jyoung at appareo.com
Mon May 8 15:31:38 UTC 2023


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`.

# 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:
                     |         prefix: 29
                     |        gateway:
                     |            dns:,
                     |            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
+CGDCONT: 16,"IPV4V6","attm2mglobal","",0,0,0,0
+CGDCONT: 2,"IPV4V6","ims","",0,0,0,0
+CGDCONT: 3,"IPV4V6","sos","",0,0,0,1


# at 'AT+CGREG?'

+CGREG: 0,1


[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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