Discuss whether this affects the max_ack_delay used in PTO

[ianswett]

If it doesn't change it, we should say so. If it allows it to be reduced, then we need to understand what the limits are(ie: it can't be lowered to 1us presumably?).

Admittedly, the way we're using max_ack_delay should be safe to assuming a 0 value, since the SRTT will accrue the excess.