LAN MTU for a PPPoE connection to avoid packet fragmentation

[fantom]

Hi, does it make sense to set MTU to 1492 for LAN clients to completely avoid packet fragmentation completly?

 

[Nullity]

Hi, does it make sense to set MTU to 1492 for LAN clients to completely avoid packet fragmentation completly?

Maybe. Local clients should be able to figure out the MTU for a given path. https://en.wikipedia.org/wiki/Path_MTU_Discovery

 

[JDB]

Maybe. Local clients should be able to figure out the MTU for a given path. https://en.wikipedia.org/wiki/Path_MTU_Discovery

Only yesterday I was having an issue with PMTU due to ICMP being blocked (regularly across the web). It is by no means an infallible protocol.
I ended up reducing my MTU as the solution.

The problem is more prevalent when using VPNs for data transfer as the overhead is significant and the packet payload is large (this was the scenario I was grappling with).


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[Vexira]

try reading this http://whirlpool.net.au/wiki/adsl_theory_ppp

 

[Vexira]

other wise just keep pinging a server for exaple google, ping -l 1472 -f 8.8.8.8 start with a low value, keep increasing it once it says fragmented. after that you will have your value.
also what are you running, dsl, fiber, cable, etc.......?

 

[Shonk]

Hi, does it make sense to set MTU to 1492 for LAN clients to completely avoid packet fragmentation completly?

depends what country
i know in the uk bt's vdsl2 offerings support baby jumbo frames so you can use 1500 and not eat into the vdsl side

 

[Nullity]

Only yesterday I was having an issue with PMTU due to ICMP being blocked (regularly across the web). It is by no means an infallible protocol.
I ended up reducing my MTU as the solution.

The problem is more prevalent when using VPNs for data transfer as the overhead is significant and the packet payload is large (this was the scenario I was grappling with).


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You reduced it at the LAN clients or at the gateway router?

 

[Vexira]

Hi, does it make sense to set MTU to 1492 for LAN clients to completely avoid packet fragmentation completly?

you would only set the mtu in the router rather than each indvidual client, and mtu is depdnat on weather or not your running pppoe pppoa, ipoe, ipoa etc

 

[JDB]

You reduced it at the LAN clients or at the gateway router?

On the router.
PMTU then worked locally in my LAN to dynamically inform my clients of the limited MTU size.


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[Nullity]

On the router.
PMTU then worked locally in my LAN to dynamically inform my clients of the limited MTU size.


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I think OP is questioning whether to force all LAN clients (via DHCP or manually?) to 1492 or allow PMTU negotiation at the router to do it's job.

I dunno. More clarification is needed from OP.

 

[JDB]

I think OP is questioning whether to force all LAN clients (via DHCP or manually?) to 1492 or allow PMTU negotiation at the router to do it's job.

I dunno. More clarification is needed from OP.

If he forced the LAN ports of the router to be a smaller MTU it will guarantee PMTU to work as it will be local and not relying on ICMP on the WAN side.

Whether or not MTU size is really an issue is the real question as it depends on a lot of the ISP infrastructure.


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[fantom]

Thanks for the replies. I was indeed referring to setting MTU=1492 on all LAN clients. The WAN is already set with 1492 and if the LAN is 1500, I thought the router would have to split every packet from LAN (1500) into two, because of PPPoE (1492).

I configured dnsmasq with "dhcp-option=lan,26,1492" and all Linux clients obey it. Windows ignores this option, so that one is set manually. I also set MTU=1492 on all router's LAN side interfaces including wireless.

The goal was to make sure the router is not in the business of fragmenting the outgoing packets. Am I over thinking it?

Update. Before this change I did not have any issues with my Internet connection. The thinking was to optimize the lights out of it so the the router does not do unnecessary work

 

[Nullity]

Thanks for the replies. I was indeed referring to setting MTU=1492 on all LAN clients. The WAN is already set with 1492 and if the LAN is 1500, I thought the router would have to split every packet from LAN (1500) into two, because of PPPoE (1492).

I configured dnsmasq with "dhcp-option=lan,26,1492" and all Linux clients obey it. Windows ignores this option, so that one is set manually. I also set MTU=1492 on all router's LAN side interfaces including wireless.

The goal was to make sure the router is not in the business of fragmenting the outgoing packets. Am I over thinking it?

Update. Before this change I did not have any issues with my Internet connection. The thinking was to optimize the lights out of it so the the router does not do unnecessary work

Within your LAN, PMTU should work fine so fragmentation shouldn't happen... I think.

You will lose <1% efficiency by forcing 1492 but that probably wouldn't be noticeable.


With my PPPoE ADSL internet I've also wondered about the impact of forcing 1492 locally myself. Neither seemed to impact anything in any negative way so I ultimately gave up properly researching it but I'm still interested in it. Currently, I just let PMTU (or some edge-case fragmentation I'm unaware of...) do whatever it does.

 

[JDB]

All seems reasonable to me. Even if it was not technically necessary so long as you see no detrimental effect then there's no harm.


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[fantom]

Thx to all; I did see no difference and now I understand that there is a chance PMTU was actually working fine in my network.