gnrc_sixlowpan_frag_sfr: add support for queue-based ECN

[miri64]

Contribution description

This provides two queue metrics set the ECN bit in a 6LoWPAN SFR RFRAG header:

• based on a netif's input queue, and
• based on SFR's internal frame queue (that holds frames waiting for the inter-frame gap)
  Both metrics can be combined, so that if either metric hits the threshold (default 1/2 of the queue size for both), the ECN bit is set.

Testing procedure

No tests provided, as this is not as easy to be reproducible locally. If I find a way, I will provide a test. For now, there is only the compile test like this:

USEMODULE="gnrc_sixlowpan_frag_sfr_ecn_if_in gnrc_sixlowpan_frag_sfr_ecn_if_out gnrc_sixlowpan_frag_sfr_ecn_fqueue" make -C tests/gnrc_sixlowpan_frag_sfr

Issues/PRs references

Requires #16174 (merged)

[miri64]

Added #16193 as additional dependency.

[miri64]

Rebased and squashed to current master.

[stale]

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[benpicco]

What does it mean if the ECN bit is set?

This needs a rebase btw 😉

[miri64]

What does it mean if the ECN bit is set?

It explicitly notifies the sender that a congestion happened (ECN = Explicit Congestion Notification) and thus amends loss-based congestion detection with hints, based on actual congestion. Here's some more info: https://en.wikipedia.org/wiki/Explicit_Congestion_Notification

[benpicco]

Would be great to also have this expanded in the doc somewhere 😇

[maribu]

Rebased on #18741 this would have passed Murdock

[miri64]

Let's merge it after feature freeze then. :-)