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

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https://bugs.launchpad.net/bugs/1955797

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

Status in network-manager package in Ubuntu:
  New

Bug description:
  [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.

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