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Author Topic: Changing AdvLinkMTU when using NPt  (Read 221 times)

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Offline paddy76

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Changing AdvLinkMTU when using NPt
« on: October 28, 2017, 05:36:18 pm »
For various reasons I've opted to use NPt between my LAN and WAN. Because of this the MTU set in radvd.conf is wrong as it seems to be following the LAN side MTU when not using interface tracking.
This becomes an issue when using, for example, a GIF tunnel to HE as MTU has to be lowered to 1280.

For now I've fixed it by changing the services.inc file so it always sets AdvLinkMTU to 1280 instead of interface MTU if interface tracking is not used.

A more permanent solution would be if it were possible to manually set the MTU in the radvd gui configuration, follow option 26 from DHCPv6 or possibility to select interface that should be followed to get MTU for radvd.

Offline JKnott

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Re: Changing AdvLinkMTU when using NPt
« Reply #1 on: October 28, 2017, 08:47:27 pm »
One thing that's mandatory with IPv6 is path MTU discovery.  That means that a too small MTU along the path is automagically discovered and adjusted for.  If that didn't work, you couldn't use paths where a link had a smaller MTU than your LAN.  Does the different MTU actually cause a problem?  Having a different MTU on LAN vs WAN has always been part of networking.  Years ago, 576 was a common MTU for dial up links, yet connected just fine with Ethernet & 1500 byte MTU or token ring with 4K byte MTU.  The main difference back then was fragmentation of too large packets.  With IPv6 and even IPv4 now. MTU discovery is used to prevent too large packets from being sent, so fragmentation is no longer needed.

Also, for the first 6 years I had IPv6, I used a tunnel broker with 1280 MTU and never had a problem, even though my LAN was the usual 1500.


« Last Edit: October 28, 2017, 08:56:30 pm by JKnott »

Offline Dave M.

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Re: Changing AdvLinkMTU when using NPt
« Reply #2 on: October 29, 2017, 06:51:20 am »
I also modify services.inc to set the MTU to 1280. PMTU discovery is nice in theory but in practice too much traffic ends up going nowhere.

Offline JKnott

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Re: Changing AdvLinkMTU when using NPt
« Reply #3 on: October 29, 2017, 07:52:21 am »
I also modify services.inc to set the MTU to 1280. PMTU discovery is nice in theory but in practice too much traffic ends up going nowhere.

If it fails, it's because someone blocked ICMP somewhere.  IP has long been designed to work with smaller MTUs along the path and I've certainly not had a problem with MTU discovery.  For example, if someone has an ADSL connection, they'd normally have 1500 on the LAN, 1492 on the WAN and may hit 1280 somewhere, even without a tunnel.  IP has to deal with it and does.

As I mentioned above, I used a tunnel for 6 years and never had an issue with MTU.

Offline Napsterbater

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Re: Changing AdvLinkMTU when using NPt
« Reply #4 on: November 14, 2017, 04:40:39 pm »
For various reasons I've opted to use NPt between my LAN and WAN. Because of this the MTU set in radvd.conf is wrong as it seems to be following the LAN side MTU when not using interface tracking.
This becomes an issue when using, for example, a GIF tunnel to HE as MTU has to be lowered to 1280.

Ummm NPt has nothing to do with MTU, it is not the reason the "MTU is wrong" (its not). An Ethernet LAN has a MTU of 1500, thus will be advertised as such. It is NOT suposed to advertise that WAN MTU to the LAN.

Nothing is wrong with it advertising 1500 MTU, that is as designed/intended.

Offline JKnott

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Re: Changing AdvLinkMTU when using NPt
« Reply #5 on: November 14, 2017, 09:08:17 pm »
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An Ethernet LAN has a MTU of 1500, thus will be advertised as such.

Actually, I was experimenting with 9K byte jumbo frames the other day.  Even had the pfSense DHCP server configured with option 26 to do that.  The only thing that has a limit of 1500 bytes is 802.3 Ethernet.  IP uses Ethernet II, which does not have a size limit, though older hardware might be limited to 1500.  Gigabit gear generally supports jumbo frames.

Offline Napsterbater

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Re: Changing AdvLinkMTU when using NPt
« Reply #6 on: November 15, 2017, 08:25:24 am »
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An Ethernet LAN has a MTU of 1500, thus will be advertised as such.

Actually, I was experimenting with 9K byte jumbo frames the other day.  Even had the pfSense DHCP server configured with option 26 to do that.  The only thing that has a limit of 1500 bytes is 802.3 Ethernet.  IP uses Ethernet II, which does not have a size limit, though older hardware might be limited to 1500.  Gigabit gear generally supports jumbo frames.
godd point.. I should have mentioned that in general an MTU of 1500 is used/standard. But you are right, its not the max.

But my point of it not being related to NPt, no more having to be the same as or even related to the WAN MTU stands.