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[Weaver]

I have read in various places claims that AOL UK needs an MTU of 1400? Is this still true? Or is this an ancient rumour?

If it is still true...

Does that mean that a router using PPPoA needs to impose an MTU of 1390 at the LAN interface immediately below the IP layer, to be quite clear, using the term MTU to mean the total size of an IP packet including the IP header, but not including PPP overhead, AAL5 overhead etc? Or 1400?

Where does the fragmentation happen?

If it is true that an MTU <=1390 or 1400 is still a necessity, then I suppose an MTU of 29*48-10=1382 would be the optimum for PPPoA giving an exact 29 full ATM cells.

I went to visit an AOL victim yesterday but couldn't do any testing as the CHAP server was down. Of course my best advice was to get them away from AOL as quickly as possible, but getting them up and running again temporarily would make it easier so that they could visit some websites of real ISPs. Their router was set to have an MTU of 1458 which is completely useless, but seeing as AOL was inaccessible I couldn't investigate further at the time.

[jeffbb]

HI

I am not sure what MTU AOL needs but this might help
http://www.kitz.co.uk/adsl/MTU2.htm
until some more knowledgeable Kitizen arrives   


Regards Jeff

[Oranged]

From Kitz's "Tweaking your MTU settings" :

AOL Suggested Settings

MTU RWin Comments

1400 13600 512kb connection 
1400 27200 1Mb connection 
1400 54400 2Mb connection

[kitz]

Its something that does still occur on occasions (as it does with Orange).  I think it may depend which platform you are on IPStream/LLU but Im not 100% certain on that.  There was an incident last year or so when CPW started amalgamating the AOL platform with their TT customers, and all of a sudden lots and lots of TalkTalk users suddenly found themselves with the typical MTU type problems such as being unable to view certain sites.

MTU is end to end point.. and packets will fragment at the point where the MTU is set at the lowest - hence why really IMHO youre best setting the MTU on each PC if possible.  If the MTU is lower on the router than on the PC and/or if the router is blocking pings (in effect turning off ICMP which stops PMTUD from working), then theres a good chance that the router itself could become the blackhole.

1400 seems to do the trick regardless if its PPPoA or PPPoE. 
afaik the PPPoA/PPPoE on the router is only setting up the connection for the backhaul?.  Despite what AOL say, the backhaul is generally PPPoA and theres only Be* that uses anything else.  Much of the confusion may have originally arisen due to non-UK AOL being PPPoE.
Although PPPoE will work, its only emulated at BTws exchanges and is in fact PPPoEoA therefore you are only adding even more overheads.

The AOL/TT MTU problems (same with orange) seem to come in fits and starts.. it will all be quiet for a while..  then suddenly you will get a whole batch of users within a few days moaning "I can't access M$/ebay/mybanking" which can soon be rectified by setting the MTU down to 1400.  This has been happening on and off for several years.. the lastest Orange one was only about a month ago.  Fragmentation will be occurring in these instances somewhere on the ISPs own network.
If IPStream perhaps one of their core routers... or if LLU after it comes of the BTw network at one of their PoPs, and on either the Opal Network or AOL/TT Network... before its routed out onto the general internet.

TT/AOL dont disclose their architecture but it will very likely be something like.

LLU

Code: [Select]

local exchange <-------- BTw ATM -------> PoP(at a large town or city) <--------Opal backhaul ---------> CoLo (possibly Docklands/London) <---- ISP Routing ----> Internet
although the MSAN may be LLU'd and owned by the ISP, many LLU ISPs still use 'BTw fibre' to do routing for the link from the smaller location to a larger location... they just purchase/rent a chunk of bandwidth fibre.

IPStream.

Code: [Select]

Local exchange <------- BTw ATM ------> RAS (at city) <------ BTw Colossus IP Network -----> CoLo (possibly Docklands/London <----- ISP Routing ----->  Internet.

L2TP tunnelling will hide all the BTw part of the network so that the first hop will only show at the CoLo/ISP Edgerouter.

At the point of the CoLo there will be many different edge and core routers.. A large ISP will have 100's of routers and switches..   it could be that just one of them may be set wrong, and if you land on that one then thats when things go to pot.  Then a few weeks later your routing may change.

[orainsear]

I've got a feeling that it's been changed to 1450

Yep it's 1450 Link to AOL help

[Weaver]

This particular AOL victim was on the same non-LLU exchange as I am, so the usual BT Wholesale IPStream was being resold.

Their router's MTU of 1458 (Netgear having this as a default), I reduced this to be below 1390 so in the light of kitizens' info 1430 would be perfect.

I hear kitz warning about routers and turning off ICMP _altogether_ if "don't respond to ping" is set. Nasty. I'll try to think up some method of testing that directly. Of course routers deciding not to respond to echo request is nothing to do with not implementing correct ICMP responses to DF packets. Ugh.

[hake]

I still find that a MTU of 1400 reduces packet fragmentation to negligible amounts on AOL/Talktalk.  The slight increase in overheads through a modest reduction in the MTU value is repaid with better overall Internet performance.

[Weaver]

> I still find that a MTU of 1400 reduces packet fragmentation to negligible amounts on AOL/Talktalk.  The slight increase in overheads through a modest reduction in the MTU value is repaid with better overall Internet performance.

If you are able to do proper comparison testing with an MTU at or below 1400 vs one above 1400 vs say 1478, and you do definitely get problems with reliability or a decrease in performance with higher MTU values, of say 1478 or 1440, and if you do eventually establish that the MTU absolutely needs to be <=1400 say, then I would strongly suggest that you should use an MTU of 1382 then, definitely not 1400, because that way you will save an entire ATM cell (since 1392=29*48-10), so saving 53 bytes at a cost of only 18 bytes less payload capacity. But if the MTU 1400 restriction is now just a legend, then either 1430 or best of all 1478 is the way to go. But there's no conceiveable reason to use 1400 even if the actual limit is 1400.

[kitz]

My head just started spinning at those calculations and I'd need more time to think about it.. but MTU is TCP/IP end to end point.

Granted I hear what youre saying about the ATM cells and thats where I'd need to sit down and think, but over the backhaul theres varying different protocols which each have their own overheads - eg atm for the BTw backhaul, l2tp tunnelling undertaken by BTw to hide their backhaul. PPPoA or PPPoE or PPPoAoE.

With AOL/TT (and even the Orange one which crops up on occasions) fragmentation doesnt normally occur on the ATM backhaul.. its often on the ISPs own network which will be carrying its own protocol overheads.  BTw recommends 1500 due to that being how their network is set up (and it being the maximum TCP/IP packet size) and in their calculations for the ATM backhaul they allow an additional amount to cover the additional protocol overheads, so that 1500 is the amount that can pass through their part of the network unhindered.   Ive no idea why AOL started to be different and chose 1400..  but there still on occasions seem to be some transit routes that will still only accept 1400.
In years gone by BTw recommended 1458 due to overheads.

In the days when speeds were slower... and more consistant it was able to repeatedly check MTU figures to find the best and most optimum setting.   Thats now gone out of the window as faster speeds mean increases/decreases are much more difficult to accurately observe.
It was in those days that it was observed that the 1430 seemed to work best.

MTU problems certainly still do materialise not only on AOL/TT/Orange, but not quite as often. Lowering your MTU ensures that packets are not blackholed..  which is the most frequent MTU problem.

As speeds have progressed and are much faster its the RWIN that makes the most beneficial use of speed increases by reducing the amount of ACKs.