[ALUG] Mostly-OT: Strange ADSL issues

[Adam Bower]

On Sat, Oct 07, 2006 at 06:05:11PM +0100, Mark Rogers wrote:
> Adam Bower wrote:
> > MTU should be set to 1458, it might seem to work with a higher setting,
> > but some sites will break and you may see other intermittent problems. 
> >   
> 
> I've not seen 1458 mentioned anywhere before.
> 
> Most people seem to say 1500/1500/1460 (MTU/MRU/MSS) for BT, but I'm
> happy to try 1458 - what about MRU/MSS? I guess 1458/1418 respectively?

That's interesting, as the recommended amount has been 1458 for over 3
years now, and this is a BT Wholesale recommendation, not just a random
suggestion found on the net. I've never seen any advice that suggested
otherwise. (although, googling for the "wrong" settings does reveal some
people suggesting them, but with no good reason) The one time I had 
problems with lost/broken packets and dsl was when I was running a USB 
speedtouch and i'd forgotten to adjust the mtu down to 1458 on a box
running ipcop and a usb speedtouch modem.

> NB: Most of the routers I've tried have had default settings for a BT
> connection and usually they're 1500/1500/1460 I think, I just noticed
> that the MSS looks out on this one, probably left over from one of the
> many tests I've tried recently.

Just out of interest, which routers? Again, every router i've used has
come pre-configured to 1458 apart from a couple of old routers that were
set to 1500 prior to the BT wholesale advice. I've never had to change 
(or bothered to change) MSS or MRU, or even look to what they are set 
at, which suggests I've not had a problem with them before.

> <edit>I've now seen reference to 1458 as BT's preferred MTU so I'm
> trying it now.</edit>

Good good, let's see how you get on for now.

> When tweaking my router's MTU settings, should I need to change my PC
> settings at all? Presumably the router will fragment packets as
> required? I guess dropping the network down to 1458 would improve
> throughput, but it should work if I don't, yes?

Nope, you shouldn't need to adjust the PC settings at all, the router 
should do the fragmentation (although, what routers are you using, I
find it strange that you've found something with an "odd" default? I
wonder if the hardware is doing something else or if anyone else who has
the same router has had any problems).

> Just to be clear: if I ping with a packet size of 1472 and fragmentation
> disabled, I get consistent success at MTU=1500 (although having said
> that maybe I should recheck to be sure). If your suspicion is correct,
> presumably I'd be losing packets above 1458 (ie 1430 ICMP packet size)?

Not necessarily, it might be a red herring and nothing to do with your
problems at all. I just noticed that you were setting (what looked to me
to be) "odd" MTU sizes and figured it would be best to start with and 
stick to the BT "recommended" size for now and see what happens as the 
only problems I ever had were when I had MTU set wrong which broke 
sites like ebay.co.uk and a couple of others, oh, and it totally 
screwed up a vpn too.

Thanks
Adam
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