[ALUG] Mostly-OT: Strange ADSL issues

[Mark Rogers]

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?

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.

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

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?

> BT (iirc) said some time ago they were going to fix the MTU "problem",
> so it could be that on the ATM backend it is working sometimes but when
> a different router comes into play for whatever reason on the ATM
> network that it breaks because you've got a higher mtu set and this
> could explain the behaviour you are seeing.
>   

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)?

-- 
Mark Rogers
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