[Ecn-sane] [Bloat] sce materials from ietf

[Sebastian Moeller]

Hi Jonathan,


> On Nov 30, 2019, at 23:23, Jonathan Morton <chromatix99 at gmail.com> wrote:
> 
>> On 1 Dec, 2019, at 12:17 am, Carsten Bormann <cabo at tzi.org> wrote:
>> 
>>> There are unfortunate problems with introducing new TCP options, in that some overzealous firewalls block traffic which uses them.  This would be a deployment hazard for SCE, which merely using a spare header flag avoids.  So instead we are still planning to use the spare bit - which happens to be one that AccECN also uses, but AccECN negotiates in such a way that SCE can safely use it even with an AccECN capable partner.
>> 
>> This got me curious:  Do you have any evidence that firewalls are friendlier to new flags than to new options?
> 
> Mirja Kuhlewind said as much during the TCPM session we attended, and she ought to know.  There appear to have been several studies performed on this subject; reserved TCP flags tend to get ignored pretty well, but unknown TCP options tend to get either stripped or blocked.
> 
> This influenced the design of AccECN as well; in an early version it would have used only a TCP option and left the TCP flags alone.  When it was found that firewalls would often interfere with this, the three-bit field in the TCP flags area was cooked up.


	Belt and suspenders, eh? But realistically, the idea of using an accumulating SCE counter to allow for a lossy reverse ACK path seems sort of okay (after all TCP relies on the same, so there would be a nice symmetry ).
I really wonder whether SCE could not, in addition to its current bit, borrow the URG pointer field in cases when it is not used, or not fully used (if the MSS is smaller than 64K there might be a few bits leftover, with an MTU < 2000 I would expect that ~5 bits might still be usable in that rate case). I might be completely of to lunch here, but boy a nice rarely used contiguous 16bit field in the TCP header, what kind of mischief one could arrange with that ;) Looking at the AccECN draft, I see that my idea is not terribly original... But, hey for SCE having an additional higher fidelity SCE counter might be a nice addition, assuming URG(0), urgent pointer > 0 will not bleached/rejected by uninitiated TCP stacks/middleboxes...

Best Regards
	Sebastian




> 
> - Jonathan Morton
>