Re: [quicwg/base-drafts] Question on ECN validation (#3733)

[Lars Eggert <[e-mail]>]

So let me tell you what my code is doing now to verify incoming ACK-ECNs, and then you tell me if this is correct:

I maintain ECN reference counter vectors (`ecn-ref`) per pn space. These hold the last counts received in an ACK-ECN from the peer.

I could new ACKs for ECN-marked packets that I send in a `new_ack`vector. (I only send ECN_ECT0 packets though, so this isn't really a vector, just a variable.)

When I receive a new ACK-ECN with ECN counts (`ecn_cnt`), I compute the `ecn_inc` vector as `ecn_ref - ecn_cnt`.

I then do the verification per Section 13.4.2.2:

* (First bullet in 13.4.2.2 is easy, omitted here)

* Second bullet in 13.4.2.2: `ecn_inc[ECN_ECT0] + ecn_inc[ECN_ECT1] + ecn_inc[ECN_CE] <  new_ack[ECN_ECT0]` => verification failed

* Third bullet in 13.4.2.2: `ecn_inc[ECN_ECT0] + ecn_cnt[ECN_CE] < new_ack[ECN_ECT0]` => verification failed. (Since I only send ECN_ECT0, I can omit looking at ECN_ACT1, I believe.)

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