[Bug 1955797] [NEW] [SRU] network-manager can’t modify MTU automatically based on what ModemManager exposes for WWAN modems.

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[Impact]

Some 4G/5G mobile networks(for ex., AT&T) requires a specific MTU
setting, this setting will be exposed by the ModemManager for network-
manger to configure the MTU of the modem network interface .

The current modem-manager v1.22.10-1ubuntu2.2  in focal can’t pass
AT&T’s modem system certification.

[Fix]

This upstream patch can resolve this bug:
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/499/diffs?commit_id=212758ea05a4c13d65f36b55c90aee7919642631

[Test Plan]

1. Use a Lenovo platform SE30 including 4G Quectel EM160 modem and an
AT&T SIM card in US

2. Enable 4G modem and connect to AT&T apn

$ sudo mmcli -i 0 --pin=0000
$ sudo mmcli --modem 0 --enable
$ sudo mmcli -m 0 --simple-connect="pin=0000,apn=emome"
$ sudo nmcli radio wwan on
$ sudo nmcli connection add type gsm ifname wwan0p2MBIM con-name mymodem apn emome
$ sudo nmcli connection up id mymodem

3. Check if the MTU got from the following results are the same:

$ sudo mmcli --bearer 5 | mtu
$ ifconfig mhi_mbim0 | grep mtu

* Expected result:
    Both MTUs are the same value 1430
* Actual tested result:
    Different values (one is 1430 , another is 1500)

[Where problems could occur]

Very low.
This is a simple fix: network-manager gets the MTU from the ModemManager then sets the MTU of the modem interface.

[Other info]

1. Network-manager v1.30.0-1ubuntu3 in hirsute has included the fix.

** Affects: network-manager (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: oem-priority patch
-- 
[SRU] network-manager can’t modify MTU automatically based on what ModemManager exposes for WWAN modems.
https://bugs.launchpad.net/bugs/1955797
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