A few days ago, I sent you email saying the following:
Ladan:
The draft is not clear on the when feedback is sent back in instances
where the loss intervals are at most 2RTTs and the loss event rate is
computed as K/N? For TFRC, once a packet is lost in a new RTT, the
loss event rate changes, feedback is sent immediately and the
remaining losses within that RTT are ignored. However for TFRC-SP,
each loss modifies the loss event rate, therefore, should feedback be
sent immediately and on every loss (since p changes with every loss)
or should it wait until the end of the RTT?
Sally:
Good question! Feedback should be sent when the loss is first
detected, and every
succeeding RTT. I added a section to the draft discussing this, and it
is appended to
the end of this email. Feedback would be welcome.
Since then, I have made a change to TFRC-SP to deal with possible
changes in the
loss event rate with short intervals. So the text attached to the
earlier email has
been replaced by the following.
Many thanks for the feedback.
- Sally
http://www.icir.org/floyd/
Added to Section 3:
Section 5.4 of RFC 3448 specifies that the calculation of the
average loss interval includes the most recent loss interval only
if this increases the calculated average loss interval. However,
for TFRC-SP, including the most recent loss interval can increase
the calculated average loss interval too much if the most recent
loss interval ends up being a short loss interval with multiple
packet drops. Therefore, TFRC-SP adds the restriction that the
calculation of the average loss interval can include the most
recent loss interval only if more than two round-trip times have
passed since the beginning of that loss interval.
...
4.6. The Calculated Loss Interval Length for Short Loss Intervals
For a TFRC-SP receiver, the guidelines from Section 6 of RFC 3448
govern when the receiver should send feedback messages. In
particular, from [RFC3448], "a feedback packet should ... be sent
whenever a new loss event is detected without waiting for the end of
an RTT". In addition, feedback packets are sent at least once per
RTT.
We note that in a connection with a short loss interval (less that
two round-trip times) and multiple packet losses per loss interval,
the loss interval length calculated for the most recent loss
interval can decrease from one round-trip time to the next, as
multiple packet losses are detected for that loss interval. As an
example, when the TFRC-SP receiver sends a feedback packet the
current loss interval might contain N packets, with only one loss,
giving a calculated loss interval length for that interval of N
packets. When the TFRC-SP receiver sends a feedback packet one
round-trip time later, K additional lost or marked packets might
have been detected, giving a calculated loss interval length for
that interval of only (N+K)/(K+1) packets. For K=N/2, this could
lead to a change in the calculated loss interval length from N to
close to 2 packets. To prevent unnecessary oscillations in the
average loss interval, Section 3 specifies that the current loss
interval can be included in the calculation of the average loss
interval only if the current loss interval is longer than two round-
trip times. For a loss interval longer than two round-trip times,
the detection of new losses for the loss interval will not
*decrease* the calculated loss interval length for that loss
interval.
