Communications - July 2012

universally misclassified as congestion (a completely different and much rarer pathology). These theories usually assume Poisson arrival processes, which are, by definition, uncorrelated. The arrivals of a closed-loop, reliable transport process such as TCP are completely correlated, resulting in an arrival and departure rate equality that theorists have dismissed as unnatural and wildly improbable. Since normal cures for congestion such as usage limits or usage-based billing have no effect on bufferbloat but annoy customers and discourage network use, addressing the real problem would be prudent.

The bufferbloat problem, making the window match the pipe size, is hard to address. Window sizes are chosen by senders while queues manifest at bottleneck gateways. It is difficult for senders to compute a window size, since both terms—bottleneck bandwidth and RTT—change constantly as connections come and go, paths change because of rerouting, layers 1 and 2 protocols adapt bandwidth for changing physical conditions, and so on. Since queues can be directly measured at the bottleneck, the most promising approach is to detect the problem there, then signal senders to reduce their windows (via TCP congestion-control mechanisms).

Reliable detection is difficult, however. The big queue at the start of Figure 3 is necessary for connections to get started, so queue magnitude contains no information about excess queue. The fact that the problem lies in the flat part of Figure 3 has caused some researchers to look at the time the buffer is occupied as an indicator.

Figure 4 shows the queue vs. time
for a TCP receiver that sends one
ack per window rather than one per
packet. This is a legal, though brittle,
operating mode used by some com-
mercial operating systems with very
high per-packet overhead to reduce
the number of acks generated so as to
perform credibly in benchmark tests.
This kind of queue length variation is
also seen in Web traffic—a superposi-
tion of mostly small transfers that are
essentially all startup transient. Since
this ack policy means a full window of
packets is always delivered as a back-
to-back burst, the initial turn-on tran-
sient repeats every RTT and the buffer
is always occupied. The window ex-
actly fills the pipe, however, and there
is no excess queue, so any attempt to
reduce this queue will result in poor
utilization of the bottleneck. Thus, oc-
cupancy time contains no information
about excess queue.

Controlled Delay management

In early 1998, we set out to understand why the original RED was difficult to configure and came up with a new RED algorithm (described in a talk and an unpublished paper10, 12) that needed only one parameter: the queue’s output bandwidth or average departure rate. Despite improvements in performance, issues remained—almost anything worked for long-lived TCP transfers and almost nothing worked for bursty traffic. In the decade since, many researchers have made strides in AQM, but no one has produced an AQM that has the following characteristics: