[ndnSIM] Access outgoing data pipeline from strategy

[Junxiao Shi]

Hi Rakesh

> However, I'm not sure how it is in violation of guideline.

There isn’t a guideline. It’s just how strategy is designed. ndnSIM 1.0’s strategy can control almost everything. NFD’s strategy is designed to only have control over Interest forwarding.

> Let me try to cite a scenario:
> If the forwarding strategy finds queue on a particular outgoing data face is congested and it wants to defer the forwarding to a later time (say next data packet arrival for same face or a timer which ever occurs first).

Strategy currently has no way to stop a Data from being forwarded onto a congested link.
As explained in #3797-37 <https://redmine.named-data.net/issues/3797#note-37>, a Data may satisfy multiple Interests, and those Interests may belong to multiple strategies. Therefore, not a single strategy has the authority to control how the Data is forwarded.
However, if Interest digest  <https://redmine.named-data.net/issues/3333>is adopted, each Data satisfies only one Interest, and then strategy could have control over Data forwarding.

Yours, Junxiao
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