DM200 Support for 1508-byte MTU for RFC4638?

[dermoth]

Hi,

I have a DM200 VDSL2 modem working like a charm in bridge mode, and when I checked it seems my ISP supports 1508-byte MTU PPPoE negociation (RFC4638) - when I enable it on my router I see the PPP-Max-Payload tags with 1500 value on all PPPoE discovery packets and the sessions gets established. I can also send 1508-sized frames to the modem's IP address (ICMP) and they get responded (no fragment), and I can see 1508-sized PPPoE frames (1500 payload) going into the the DSL modem through the Ethernet port. The default 1500 mtu on eth0 modem port doesn't seem to cause any issue.

However if I trace the ATM side on the modem I can see 1500-sized frames going out and back (1492 payload) but the 1508 ones gets dropped (nothing going out at all). The wan port specifically refuses any MTU settings over 1500, as can be seen here:

root@DM200:/# ip link set ptm0.35 mtu 1500

Here setting MTU to 1500 returns no error, but with 1508:

root@DM200:/# ip link set ptm0.35 mtu 1508
RTNETLINK answers: Numerical result out of range

It appears similar modems using the same chipset do support 1508 frames, so why is it limited on the DM200? See for example this patch on OpenWRT for the same lantiq driver:

https://git.openwrt.org/?p=openwrt/openwrt.git;a=patch;h=8a2a20e71e2909f84dab47e51dfda9e292a6c1ae

Regards

[michaelkenward]

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@dermoth wrote:

Hi,

I have a DM200 VDSL2 modem working like a charm in bridge mode

.... so why is it limited on the DM200? 

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I wonder if you are over thinking this. The DM200 works.

Are you having problems connecting a router to  the DM200?

If so, what is the router?

[dermoth]

I'm not overthinking this. The stardard MTU for Ethernet equipment is 1500 bytes; DSL suffers a small loss of efficiency over Cable and other technologies because the PPP encapsulation shinks this window, meaning packets needs to be smaller (more overhead) and in worst cases, if PMTU doen't work properly, they can be fragmented which causes even greater overhead and delays.

To resolve this RFC4638 implemented a method in which PPPoE sessions can be established using a slightly higher MTU. The Netgear DM200 modem support MTUs of that size on the ethernet port, and the Lantiq hardware seems to support it as well provided proper driver support (which custom opensource firmwares do). So I'm wondering why Netgear doesn's support it, at least in bridge mode which doesn't require any special changes besides allowing 1508-MTU size in wan settings page.

Regards,

Thomas