LTE7490 MTU setting missing

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June6
June6 Posts: 12
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edited January 4 in Mobile Broadband

According to the LTE7490 manual, the following page (Network Setting -> Broadband -> Modify/Edit WAN Interface) should have the MTU setting, but it's missing on my LTE7490 GUI (latest firmware), showing only the following options:

Screenshot 2023-09-29 010436.png

The problem: images won't load properly (very slow) on hs.fi & is.fi news sites (image servers: hs-mediadelivery-fi.sanoma-sndp.fi & is-mediadelivery-fi.sanoma-sndp.fi respectively).
The problem only manifests when connecting through LTE7490.

The cause: IP fragmentation as reported by Wireshark.

The fix: Setting the default MTU of 1500 (frame length 1434 as reported by Wireshark) of my Windows network interface to 1277 (frame length 1291, TCP payload 1237 bytes as reported by Wireshark) or lower → no more IP fragmentation → images always load properly.

  1. What is the default MTU of LTE7490 and can it be changed somehow?
  2. Is there a bug in the firmware of LTE7490 as images on hs.fi & is.fi won't load if the client MTU (e.g. on a Windows PC) is set to the default of 1500 ?

PMTUD/ICMP seems to work with LTE7490 as tested with:

http://icmpcheck.popcount.org/ and http://icmpcheckv6.popcount.org

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  • tonygibbs16
    tonygibbs16 Posts: 845  Guru Member
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    Hello June6

    Can you please post the version number of the firmware you have?

  • June6
    June6 Posts: 12
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    Sure, it's V1.00(ABQY.6)C0

  • June6
    June6 Posts: 12
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    Any update on this issue ?

  • tonygibbs16
    tonygibbs16 Posts: 845  Guru Member
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    edited October 2023
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    Hello @June6 ,

    I think that maybe the way forward for you is to reduce the MTU on your clients for now.

    If the application says to TCP do not fragment with the DF flag, then I think that the LTE7490:cannot do any fragmentation even if Path MTU Discovery (PMTUD) is working, or if the LTE7490 was set to use a smaller MTU.

    I hope that this is helpful.

    Kind regards,

    Tony

  • June6
    June6 Posts: 12
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    https://community.zyxel.com/en/discussion/comment/58122#Comment_58122

    My ISP requires MTU 1400 and MSS 1360.

    When clicking Network Setting -> Broadband -> Broadband → Modify/Edit WAN Interface, the following screen is shown for a split second (suggesting LTE7490 MTU is 1500), but the final page is like in the first post.

    Could you fix this in the upcoming firmware version?

    lte7490_mtu.png

    Also, don't know if you've noticed or if it's a problem, but setting "IPv4/IPv6 Mode" to "IPv4 only" in the same screen doesn't persist, the value is always reverted to "IPv4 IPv6 DualStack". Same goes for the "IPv6 Default Gateway" setting.

  • Bob_C
    Bob_C Posts: 170  Zyxel Employee
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    Hi June6,

    In order to set IPv4 and IPv6 mode, please configure corresponding APN profile from Cellular APN tab (Broadband > Cellular APN).

    Regards,

    Bob

  • June6
    June6 Posts: 12
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    https://community.zyxel.com/en/discussion/comment/58122#Comment_58122

    By editing the Backup/Restore file I tried lowering the MTU of my LTE7490 (thanks Bob_C for the tip), but it didn't help with my issue.

    Why is it needed to reduce client (Windows network adapter) MTU to 1277 or lower to avoid IP fragmentation on is.fi & hs.fi sites ? The issue source is hs-mediadelivery-fi.sanoma-sndp.fi & is-mediadelivery-fi.sanoma-sndp.fi servers (maybe they don't support PMTUD) ?

    If I put MTU on Windows back to the default 1500, the max successful "ping 1.1.1.1 -f -l" is 1436 (= MTU 1464).

  • tonygibbs16
    tonygibbs16 Posts: 845  Guru Member
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    edited October 2023
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    Hello @June6

    The following article about changing MTU might be useful to you: https://homenetworkgeek.com/mtu-size/#:~:text=Larger%20packets%20are%20more%20likely%20to%20suffer%20from,smaller%20number%20can%20help%20improve%20the%20overall%20latency.

    Why change the MTU on client hosts and server hosts?

    * because you want the communication to succeed.

    * because you want to use the largest datagrams without fragmentation if possible.

    * because changing the MTU allows smaller datagrams to be used for lower latency.

    * because the MTU might be smaller on some parts of the path from source IP address to destination IP address and be outside of the control of the sender.

    - the ping -f -l <size> test you have done allowed you to find the biggest MTU (i.e. <size> + 28 octets) usable on the path when fragmentation is not permitted (DF flag set.)

    - The MTU size can be different depending on the physical and datalink layers used on the path, which might be of different MTU sizes per link and could be smaller than 1500 octets.

    * because the MTU settings on the path can be outside of your control except what can be set in client hosts and server hosts that are initiating the communication and responding to the communication.

    - routers might not allow MTU to be set.

    - ISPs might have different MTUs outside of your control.

    Professionally, I have had to change MTU sizes in order for communications to work, including reducing the MTU size on clients and servers in order for communications to work across links that use IPSec.

    I hope that this is helpful.

    Kind regards,

    Tony

    PS: I do not work for Zyxel.

    Hei Hei :-)

  • June6
    June6 Posts: 12
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    edited November 2023
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    Thank you for the information.

    I tried a USB wifi-adapter on my PC and connected to my smartphone hotspot, and the same result: MTU has to be set less than or equal to 1277 to avoid IP fragmentation on is.fi & hs.fi

    So it's not an issue with Zyxel LTE7490.

    If I had an ethernet internet connection, I could test whether the sites above work with higher client MTU.

    This is how the IP fragmentation looks like on Wireshark, when client MTU is above 1277:

    Screenshot (586) S22 hotspot_2.png
    Screenshot (585) S22 hotspot.png

    I see the IPv4 fragments are 1400 + 60 bytes.
    I guess my ISP is using MTU 1400 for their mobile network ?

    Maybe someone can extract some useful information out of these screenshots ?

  • tonygibbs16
    tonygibbs16 Posts: 845  Guru Member
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    Hello @June6

    Thanks very much for your reply. You are welcome.

    Wireshark is my friend. :-)

    It appears from the Wireshark screenshots that fragmentation is happening, and it seems to be successful.

    I think that you are right that your ISP has an MTU of 1400 octets, hence in the first screenshot item 4305 is only 60 octets with the first fragment being in item 4303 of 1400 octets.

    I hope that this is helpful.

    Kind regards,

    Tony

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