Re. Overhead on ADSL with PPPoA; optimising MTU
Andy Furniss
> Hey all,
>
> Ever since I came across Lawrence Baldwin's site, I've been trying to
> work out what the optimal max MTU for a ADSL connection with PPPoA.
> His site discusses PPPoE but he mentions and I think he's right he's
> made a few mistakes and he's given up on it.
>
> Anyway, I've been looking into it and best as I can make out, for
> VC-mux the optimal highest MTU commonly supported would be 1480. AAL5
> with VC-mux has a 8 byte SAR trailer at the end of the last cell and
> is padded if necessary. With a 1480 byte MTU, 30 of the cells with
> have a full 48 bytes of data, the last cell will have 40 bytes of data
> and the 8 byte trailer.
>
> With SNAP/LLC, the optimal MTU should be 1472 since it adds a 8 byte
> header.
>
> I think all this is correct assuming I'm right that a 1480 byte IP
> transmission unit means 1480 bytes through the ATM link. I'm still not
> clear if there is any other overhead I'm missing. Lawrence's site
> suggests there is ethernet overhead but I think he's wrong??? If there
> is a 38 byte ethernet overhead that is sent through the ADSL ATM link,
> then the optimal max MTU would be 1490 I think (for VC-mux)?
>
> For PPPoE, there is another 8 bytes of overhead to consider of
> course...
>
> P.S. This is only relevant if you want to get the maximum out of your
> modem's ADSL link. If, for example, your ISP does not give the maximum
> from your link but does traffic shaping later on, it doesn't really
> matter, in fact a max MTU of 1500 is optimal since this minimises
> TCP/UDP overhead.
Sorry it's posted top level - it got rejected as a proper reply.
For pppoa/vcmux in the UK the optimal MTU is 1478 - the pppoa overhead is
10. (1478+10) / 48 = 31 whole cells, each which get another 5 bytes ATM
header added.
IIRC the RFC says 8 byte trailer and 1 or 2 header. I know what my overhead
is from the cell count on my modem and from the fact that I can see less
throughput with MTU 1479 compared to 1478.
(linux) ping -s 10 sends one cell -s 11 two. For pppoe users I think
sometimes the headers do get sent aswell - which makes any tweak less
noticable for big packets as there is less padding needed for 1492/1500 + a
big overhead, compared to the amount for pppoa at 1500. Finding what your
overhead is still be usefull if you want to run a smaller MTU. If you can
get a cell count you should be able to find a data figure that changes from
2 to 3 cells and work it out from there. If you can't get a cell count then
you may be able to notice a difference in IP throughput between MTU = X and
MTU = X+1. 53 bytes per packet is noticable in pppstats.
If you can't see a difference then it maybe you are being throttled at IP
level to below link speed already - I heard of some phone exchanges in the
UK being like this.
Andy.
THe NuTTeR
"Andy Furniss" <spam....@surfanytime.co.uk> wrote in message
news:qodr32...@amd.localdomain...
<snip>
> For pppoa/vcmux in the UK the optimal MTU is 1478 - the pppoa overhead
> is
> 10. (1478+10) / 48 = 31 whole cells, each which get another 5 bytes
> ATM
> header added.
Unless you use AOhelL when the MTU has to be 1400, a "standard" UK MTU
destroys the ability of the connection. Not really sure why, but I
couldn't get internet traffic to work properly until the MTU was 1400.
Someone elses connection of course.
Andy Furniss
>
> "Andy Furniss" <spam....@surfanytime.co.uk> wrote in message
> news:qodr32...@amd.localdomain...
> <snip>
> > For pppoa/vcmux in the UK the optimal MTU is 1478 - the pppoa overhead
> > is
> > 10. (1478+10) / 48 = 31 whole cells, each which get another 5 bytes
> > ATM
> > header added.
>
> Unless you use AOhelL when the MTU has to be 1400,
strange - if I were forced to use <=1400 I would choose 1382.
>a "standard" UK MTU
> destroys the ability of the connection. Not really sure why, but I
> couldn't get internet traffic to work properly until the MTU was 1400.
Probably down to their kit - I have heard of people having problems that
reducing MTU (ususlly to 1430) fixes.
I and many others have no problems with using 1478 or 1500.
Andy.
THe NuTTeR
For reference this was a belkin 54g wireless router (sold from PC world
with freeserve branding on the box) with AOL on ADSL in the UK
Set MTU to 1400 and it worked properly after not working at the default
(1478 i think)
G