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>

1 files changed, 10 insertions, 2 deletions

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index bc3dc2b2a84..38239c4d5e1 100644
--- 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;