Thank you very much for your contribution to Netplan, this PR is looking great! You covered all the backends and added nice testing around it. Kudos!

Could you please move the integration test (in routinging.py) up into the _CommonTests class? So we can check it actually works on NetworkManager, in addition to test_route_advmss (__main__.TestNetworkd) ... eth42 .............ok, e.g. next to test_per_route_advertised_receive_window.

Also, please add the new setting to the fuzzer config in tests/config_fuzzer/schemas/common.js:

                "advertised-mss": {
                    type: "integer",
                    minimum: 0
                }

Besides that, I left a few small inline comments. Also, let's wait for some comments from @vorlonofportland and @daniloegea.

Thank you very much for adding proper integration tests!

The PR overall LGTM!

But: CI is still failing the NetworkManager tests... this seems to be because the advmss route_option (https://networkmanager.dev/docs/api/latest/nm-settings-nmcli.html) is only supported since NetworkManager 1.39.8 (1.40.0 stable) (NetworkManager/NetworkManager@90e7afc), which is not available in Ubuntu Jammy, that is still used in our CI system currently.

I see this output on Jammy, while it works fine on Noble:

# nmcli connection modify netplan-eth0 +ipv4.routes "192.168.122.0/24 10.10.10.1 advmss=1400"
Error: failed to modify ipv4.routes: invalid option 'advmss=1400': unknown attribute 'advmss'.

So we might need to block this PR on #465, or alternatively, mark the corresponding NetworkManager tests to be skipped on NetworkManager < 1.39.8, e.g. something along those lines:

import gi

gi.require_version("NM", "1.0")
from gi.repository import NM

print("version:", NM.Client.new(None).get_version())

And then @unittest.skipif() backend is NetworkManager and version is too small.