Re: [tcpm] Exceeding value in MSS option?

[Frode Kileng <[e-mail]>]

Hi,

please see below

On 20 Oct 2020, at 13:56, Mirja Kuehlewind 
<[e-mail]> wrote:
> Hi Martin, hi Gorry,
>
> I think Gorry’s paper does indicate that at least for MSS, there are 
> middleboxes that add this option. However, I don’t think there is 
> evidence that there is a general problem about middlebox adding option 
> (rather than removing them). I also reviewed the draft in tsv-art 
> review and given that they say it’s actually not recommended and 
> should only be done with care, I thought that text is okay. However, I 
> guess there is room to improve the wording and explain a bit better, 
> when and if this might be consider or not.
>
Although maybe not  a "general problem",  a study of MSS middlebox 
manipluation in  23 European and Asian mobile networks did reveal some 
issues:

- 20 of 23 networks always inserts MSS. Value for IPv4 varies from 1360 
to 1420 and the value typically seems to reflect  vendor defaults

- The 3 network that didn't clamp MSS do reduce an end-point specified 
values  if it's higher than some network specific threshold

- 2 Networks overrode a end-point specified lower MSS value and changed 
it to the higher network specific default! I.e. a potential source of 
problems.

- 1 Network did not clamp if some other TCP options was used (MPTCP, 
TFO, etc)! Also a source of problems.

I'm afraid that the results from this study has not been published.


Best regards

Frode Kileng