tcp: improve the condition for detecting duplicate ACKs

[tuexen]

Summary

Take the condition of RFC 6675 into account. While there, remove stale comments.

Test Plan

Will provide some packetdrill tests.

[cc]

sys/netinet/tcp_input.c

2611–2612

Can wrap these two lines into a single line now.

2685–2686

Can wrap these two lines into a single line now.

2723–2725

Looks like dead code to me, as here should be "th->th_ack == tp->snd_una".

2806

* counter as per RFC 6675. The variable <== one extra space

I think a better read can be:
* t_dupacks as per RFC 6675. The variable

[tuexen]

Address comments from cc@ and add a condition as discussed with rscheff@.

[cc]

Thanks for the following improvements:

1. put the checks of "non-DUP-ACK" first
2. remove one tab indent
3. remove some dead code and the resume_partialack label
4. clear comments between line# [2597, 2633]
5. make sure advancing ACKs are counted as DUP-ACKs under the V_tcp_do_newsack toggle

Some minor clean-ups remain. I am good with this change.

sys/netinet/tcp_input.c

2797–2798

Do you mean this minor change for readability?

KASSERT(SEQ_GT(th->th_ack, tp->snd_una),
    ("%s: SEQ_LEQ(th_ack, snd_una)", __func__));

2802–2803

Can remove this extra KASSERT() now, as we have removed the resume_partialack label.

[tuexen]

Follow the last two suggestions from cc.

[tuexen]

Use the condition of RFC 6675 only if V_tcp_do_newsack is true.