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

[Łukasz Zemczak]

Hey Jerry! This looks good and I can certainly sponsor it for you.

One question before I do that though. Looking at the original source commit:
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/499?commit_id=212758ea05a4c13d65f36b55c90aee7919642631
It is mentioned there by some user that MM can also expose this via MTU from the IPv6 bearer, not only IPv4. Not sure if this is an use case we are concerned about, and if there was a follow up patch for this use case.
Can you take a look and give me a sign if we should still continue as-is? Thanks!

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

Status in network-manager package in Ubuntu:
  Fix Released
Status in network-manager source package in Focal:
  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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