dccp tfrc: Suppress unavoidable "below resolution" warning

In the congestion-avoidance phase a decay of p towards 0 is natural once fewer
losses are encountered. Hence the warning message "p is below resolution" is
not necessary, and thus turned into a debug message by this patch.

The TFRC_SMALLEST_P is needed since in theory p never actually reaches 0. When
no further losses are encountered, the loss interval I_0 grows in length,
causing p to decrease towards 0, causing X_calc = s/(RTT * f(p)) to increase.

With the given minimum-resolution this congestion avoidance phase stops at some
fixed value, an approximation formula has been added to the documentation.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index bc3dc2b..38239c4 100644 (file)
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -632,8 +632,16 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
 
        if (p <= TFRC_CALC_X_SPLIT)             {     /* 0.0000 < p <= 0.05   */
                if (p < TFRC_SMALLEST_P) {            /* 0.0000 < p <  0.0001 */
-                       DCCP_WARN("Value of p (%d) below resolution. "
-                                 "Substituting %d\n", p, TFRC_SMALLEST_P);
+                       /*
+                        * In the congestion-avoidance phase p decays towards 0
+                        * when there are no further losses, so this case is
+                        * natural. Truncating to p_min = 0.01% means that the
+                        * maximum achievable throughput is limited to about
+                        * X_calc_max = 122.4 * s/RTT (see RFC 3448, 3.1); e.g.
+                        * with s=1500 bytes, RTT=0.01 s: X_calc_max = 147 Mbps.
+                        */
+                       tfrc_pr_debug("Value of p (%d) below resolution. "
+                                     "Substituting %d\n", p, TFRC_SMALLEST_P);
                        index = 0;
                } else                                /* 0.0001 <= p <= 0.05  */
                        index =  p/TFRC_SMALLEST_P - 1;