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PPPoE connection to UK BT ADSL, MTU with RFC 4638

Wed Sep 30, 2015 12:49 pm

Hello all!

I've recently started using an RB2011 as my main router with all its PCQing and configurability and visibility :)

I've been trying to get a 1500 MTU working with RFC 4638. I've set the ether1 interface MTU to 1508 (as required) and set the MTU and MRU to 1500 on the PPPoE client connection. I did have it going through a bridge interface before, but this didn't seem to broach the 1492 MTU limit, (even after changing MTU in the bridge and the interface) so I removed that and put it directly on ether1. Since sorting that out, I seem to be connecting with an MTU of 1492 and an MRU of 1500, which would suggest that everything is setup pretty much correctly, but I don't know why its working one way and not the other.

I'm using the RB with a Technicolor TG582n, connected to ADSL which I had to reflash with a suitable image to make work (PITA!). I set up the bridge on the TG using no special options as recommended by someone else I spoke to that's made this work with a TG and a different router.

Any ideas of what is stopping the MTU go to 1492? Could it be anything to do with the MSS clamping that seems to automatically be added in to my mangle rules when I connect? Likewise, if I do manage to correct the MTU, will the clamping rules automatically disappear?

I've set the L2 MTU to 1600 or atleast 1530 (?) as recommended by some other posts that I managed to find on the subject. I'm not at home at the moment so can't check. What am I missing?

Thanks in advance!
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Re: PPPoE connection to UK BT ADSL, MTU with RFC 4638

Fri Oct 02, 2015 12:01 am

You'll need to be on the bleeding edge if you want RFC4638: ... 50#p501049
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Re: PPPoE connection to UK BT ADSL, MTU with RFC 4638

Fri Oct 02, 2015 12:19 am

Forcing a higher setting can cause problems.
Since you're using ADSL your router isnt directly connected to the ATM lines so you cannot use 1500MTU.
In PPP some bits are required as overhead as a PPP header so your MTU will decrease.

Think of it this way
You have the link connected at layer 1 with some bandwidth.
You get logical link at layer 2 with an MTU (typically this is slightly above 1500)
VLANs which is layer 2 and if you use it require a few bits so it takes a few from that 1500 which decreases it
If you use PPP which is layer 2 it will also be decreased.
If you use layer 3 in order to fit the packet into the remaining MTU the MTU for layer 3 will be decreased further.
If you use layer 4 such as VPN it will have to fit the packet into the remaining space of the MTU which will contain the headers from layer 2 and layer 3 so it is further decreased.

So no you cannot use 1500MTU because your router isnt doing layer 2 over the line directly. In a router/modem combo you will see 1500MTU. With the router behind a modem it needs to negotiate a link using PPP over layer 2 which has to go through the modem first so the modem has to take a way a bit of the MTU hence you get lower MTU(same amount if you used layer 2 logic +PPP). Since DSL is analogue and prone to losses it has a lower MTU than ethernet.

This also depends on your ISP. Some ISPs can use higher or lower MTUs but many use the default one for compatibility reasons and because even though networks have improved there may be those that will still struggle from the same old lines.

Lower MTUs increase network speeds in a network with packet losses compared to higher MTUs, Higher MTUs only reduce overheads and network processing if the network is error free and doesnt always give better speeds.
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Re: PPPoE connection to UK BT ADSL, MTU with RFC 4638

Fri Oct 02, 2015 1:55 am

Thanks for the heads up troffasky! That neatly answers my question as to why I'm not getting the MTU I should. I'm quite amazed I didn't come across that post sooner in my myriad of googling that I did.

SystemErrorMessage, thank you for your informative post, unfortunately in this situation it was already something that I knew already and understood.

In the UK, on any BT Openreach supplied lines using one of their products (99.9% of all DSL and FTTC connections in the UK) they have implemented a standard called RFC4638. This uses baby jumbo frames, with an MTU of 1508, which allows for the extra overhead in the packets carrying the PPPoE, so you can have full a 1500MTU on the other side. It has to be implemented in everything in the chain.

If you get FTTC in the UK, you generally get given a standalone Ethernet only modem, like the HG612 which has always supported it (afaik). For ADSL its a little more difficult because only a handful of modems support RFC4638. This information isn't really listed anywhere which makes things difficult. I asked some knowledgeable folks on an IRC channel and someone suggested the TG582n, it just happened I had one knocking about and managed to re-flash it appropriately. I kinda assumed that MikroTik had implemented it by now. If you had two MTiks and made a link between them, you could use 1500 MTU, but its slightly different with RFC4638, it requires extra connection parameters.

I was a little confused that I was getting an MRU of 1500 and an MTU of 1492 and I was hoping someone who'd made it work might be able to tell me if I've got something specific wrong with my setup, or something I needed to enable somewhere which I'd missed. I'm glad I now know why its not working and I'm hoping that once I install the update, it will just 'work' :)

I will report back with my results.
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Re: PPPoE connection to UK BT ADSL, MTU with RFC 4638

Fri Oct 02, 2015 2:58 pm

RFC4638 has only just been implemented in the latest RouterOS Release Candidate. 6.33rc16

Changelog says:
*) pppoe - added support for MTU > 1492 on PPPoE;

Which version are you using ?

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