where a keypress within the last polling interval would not be noticed.
Add a new function, conisshift(), which is used to detect whether a shift key
is pressed. Use this via awaitkey() to allow interrupting the boot by holding
down shift (similar to LILO).
This allows setting the timeout to 0 and still being able to use the boot
program.
and it could detect even older time stamps. (Really, to be 100% correct, there
should be a timer that clears these out -- but it probably doesn't matter in
the real world.)
* t_partialacks<0 means we are not in fast recovery.
* t_partialacks==0 means we are in fast recovery, but we have not received
any partial acks yet.
* t_partialacks>0 means we are in fast recovery, and we have received
partial acks.
This is used to implement 2 changes in RFC 3782:
* We keep the notion that we are in fast recovery separate from t_dupacks, so
it is not reset due to out-of-order acks. (This affects both the Reno and
NewReno cases.)
* We only reset the retransmit timer on the first partial ack -- preventing us
from possibly taking one RTO per segment once fast recovery is initiated.
As before, it is hard to measure any difference between Reno and NewReno in the
real-world cases that I've tested.
1) If an echoed RFC 1323 time stamp appears to be later than the current time,
ignore it and fall back to old-style RTT calculation. This prevents ending
up with a negative RTT and panicking later.
2) Fix NewReno. This involves a few changes:
a) Implement the send_high variable in RFC 2582. Our implementation is
subtly different; it is one *past* the last sequence number transmitted
rather than being equal to it. This simplifies some logic and makes
the code smaller. Additional logic was required to prevent sequence
number wraparound problems; this is not mentioned in RFC 2582.
b) Make sure we reset t_dupacks on new acks, but *not* on a partial ack.
All of the new ack code is pushed out into tcp_newreno(). (Later this
will probably be a pluggable function.) Thus t_dupacks keeps track of
whether we're in fast recovery all the time, with Reno or NewReno, which
keeps some logic simpler.
c) We do not need to update snd_recover when we're not in fast recovery.
See tech-net for an explanation of this.
d) In the gratuitous fast retransmit prevention case, do not send a packet.
RFC 2582 specifically says that we should "do nothing".
e) Do not inflate the congestion window on a partial ack. (This is done by
testing t_dupacks to see whether we're still in fast recovery.)
This brings the performance of NewReno back up to the same as Reno in a few
random test cases (e.g. transferring peer-to-peer over my wireless network).
I have not concocted a good test case for the behavior specific to NewReno.
