why buffering?

Well, so far nobody provided a valid explanation for the necessity of
buffering in routers (and any other stochastically multiplexing devices).

The real reason for having buffers is the fact that information about
congestions takes some time to propagate. (In TCP/IP congestion are
detected by seeing lost packets).

If buffers are not sufficient to hold packets until TCP speakers see
congestion and slow down, the system becomes unstable. Buffers are,
essentially, inertial elements in the delayed negative-feedback control
loop. Insufficient inertia causes oscillations in such systems, which is
sometimes useful, but in case of networks leads to decreased througoutput
because the wire is utilized fully only at upswings and underutilized on
downswings (collapsed TCP windows aggravate the effect futher).

Consequently, the holding capacity of buffers should be sufficient to
bring the inertia of the system up to the reaction time of the negative
feedback (this is a simplification, of course). This reaction time is
about one round-trip time for end-to-end packets.

In real networks, the RTTs differ for different paths, so some
"characteristic" RTT is used. So, to hold packets until TCPs slow down a
router needs cRTT * BW bits of buffer memory (where BW is the speed of the
interface). This rule can be somewhat relaxed if congestion control loop
is engaged proactively before congestion occured (by using Random Early
Detection), but not much.

Even with properly sized buffers, sessions with longer RTTs suffer from
congestions disproportionately because TCPs on the ends never get enough
time to recover fully (i.e. to bring windows to large enough size to
maintain steady stream of packets), while small-RTT sessions recover
quickly, and, therefore, get bigger share of bandwidth. But I'm
digressing :slight_smile: