4.16 issue with mbim modem and ping with size > 14552 bytes

Hello,

I have an issue with an USB mbim modem when trying to send with ping
more than 14552 bytes: it looks like to me a kernel issue, but not at
the cdc_mbim or cdc_ncm level, anyway not sure, so I'm reporting the
issue.

My kernel is 4.16. The device is the following:

root@L2122:~# ifconfig
wwp0s20u7i2 Link encap:Ethernet  HWaddr be:3d:f2:f4:0d:e9
          inet addr:2.193.7.73  Bcast:0.0.0.0  Mask:255.255.255.252
          inet6 addr: fe80::bc3d:f2ff:fef4:de9/64 Scope:Link
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:5 errors:0 dropped:0 overruns:0 frame:0
          TX packets:55 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:759 (759.0 B)  TX bytes:6275 (6.2 KB)

Sending ping sized 14552 no issue:

root@L2122:~# ping -s 14552 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 14552(14580) bytes of data.
1448 bytes from 8.8.8.8: icmp_seq=1 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=2 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=3 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=4 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=5 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=6 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=7 ttl=52 (truncated)
1448 bytes from 8.8.8.8: icmp_seq=8 ttl=52 (truncated)
^C

--- 8.8.8.8 ping statistics ---

8 packets transmitted, 8 received, 0% packet loss, time 7008ms
rtt min/avg/max/mdev = 54.887/83.154/102.563/18.502 ms

root@L2122:~# ifconfig
wwp0s20u7i2 Link encap:Ethernet  HWaddr be:3d:f2:f4:0d:e9
          inet addr:2.193.7.73  Bcast:0.0.0.0  Mask:255.255.255.252
          inet6 addr: fe80::bc3d:f2ff:fef4:de9/64 Scope:Link
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:15 errors:0 dropped:0 overruns:0 frame:0
          TX packets:161 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:12583 (12.5 KB)  TX bytes:125999 (125.9 KB)

If I try ping sized 14554, it does not work

root@L2122:~# ping -s 14554 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 14554(14582) bytes of data.
^C

--- 8.8.8.8 ping statistics ---

7 packets transmitted, 0 received, 100% packet loss, time 6122ms

and I see tx errors in the network interface

root@L2122:~# ifconfig
wwp0s20u7i2 Link encap:Ethernet  HWaddr be:3d:f2:f4:0d:e9
          inet addr:2.193.7.73  Bcast:0.0.0.0  Mask:255.255.255.252
          inet6 addr: fe80::bc3d:f2ff:fef4:de9/64 Scope:Link
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:20 errors:0 dropped:0 overruns:0 frame:0
          TX packets:190 errors:5 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:12943 (12.9 KB)  TX bytes:142476 (142.4 KB)

but the real problem is that the network interface seems not to be
working anymore:

root@L2122:~# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
^C

--- 8.8.8.8 ping statistics ---

10 packets transmitted, 0 received, 100% packet loss, time 9193ms

root@L2122:~#
root@L2122:~# ifconfig
wwp0s20u7i2 Link encap:Ethernet  HWaddr be:3d:f2:f4:0d:e9
          inet addr:2.193.7.73  Bcast:0.0.0.0  Mask:255.255.255.252
          inet6 addr: fe80::bc3d:f2ff:fef4:de9/64 Scope:Link
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:20 errors:0 dropped:0 overruns:0 frame:0
          TX packets:190 errors:20 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:12943 (12.9 KB)  TX bytes:142476 (142.4 KB)

Nothing relevant in the kernel log.

Anyone can suggest me how to debug this further?

Thanks in advance,
Daniele