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
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -632,8 +632,16 @@
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;