[Bug 275798] panic: sackhint bytes rtx >= 0

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275798

--- Comment #6 from Richard Scheffenegger <rscheff@freebsd.org> ---
can you please provide the core (w/ kernel & kernel.debug)? The bblog data will
have captured the evolution how we ended up in this state - but it currently
requires the core to undig all that information manually.

And I spotted an oversight in the tcp_output error path, but that's not the
issue here... 

But the problem happens quite early in the affected TCP session:

iss = 331585966

recover = 14492 
fack = 14492
una = 10936
max = 14492

in frame 6, the incoming tcp header option sack appears to cover:
11171-12164(993)
in frame 5, the sack block seems to cover the entire range from una to max
(10936-14492) -  but this is probably after processing it in full.

The sack_bytes_rexmit is -235, or the delta between snd_una and the left edge
of the to_sack block...

I wonder if this is a DSACK block on a cumulative ack, after the holes have
been retransmitted, ending up miscalculating this.. (So with reordering of
packet delivery or ACKs carrying SACK information).

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