TCP datacenter features

[GrumpyOldTroll]

From @michael-scharf 's review (https://mailarchive.ietf.org/arch/msg/mops/eGGPYW2vytPsLwRBlJ38MezPFh0/)
(in Section 6.2. TCP and Its Behavior)

• Does ECN, DCTCP, etc. really not matter at all, for instance, if a data
  center hosts video playout servers?

[GrumpyOldTroll]

This might be a scoping matter. I think this section was meant to be talking about delivery to end users, but maybe some discussion of datacenters is right to at least briefly mention somewhere?

If so, maybe that gets a small section somewhere that mostly says the topic is out of scope as its own too-broad topic.
Not sure if DCTCP/ECN needs a mention as it usually it wouldn't be required until you get below ultra low latency, but it can (presumably? I don't have ready measurements...) help with datacenter efficiency and reliability on your delay targets and maybe has some worthwhile references.

Note: we dropped a bit of explanation about ECN (#42), and although we didn't really capture why in our notes IIRC the decision came from the low occurrence of marking in end user delivery.

[GrumpyOldTroll]

closed by #173