[Cake] Status of sch_cake and sctp with ECN

[Jonathan Morton]

> On 7 Oct, 2018, at 11:45 am, Ruben <ruben at vfn-nrw.de> wrote:
> 
> I googled this but I found no answers. RFC4960 (the sctp RFC) is mentioning ECN, but just in the appendix. I was wondering if the Linux library has ECN support, and how one might active it (as socket option or as system setting?) and if this tags are currently supported to be handled by sch_cake?

ECN itself is an IP-level protocol, using two bits in the former TOS byte.  Cake will treat any packets advertising themselves as ECN-capable in this way as supporting ECN.  Additionally, Cake uses Linux' packet-dissection mechanism (which I believe supports SCTP) to identify and isolate traffic flows, so it doesn't need to understand every possible transport protocol or encapsulation itself.

Of course, it is the responsibility of the sender to set those bits, and of the receiver to interpret them and echo their meaning back to the sender, and of the transport-layer protocol as a whole to respond appropriately to any congestion signal encoded in them.  I don't personally know how well SCTP does that.

 - Jonathan Morton