[DCCP] ccid3: Deprecate TFRC_SMALLEST_P

 This patch deprecates the existing use of an arbitrary value TFRC_SMALLEST_P
 for low-threshold values of p. This avoids masking low-resolution errors.
 Instead, the code now checks against real boundaries (implemented by preceding
 patch) and provides warnings whenever a real value falls below the threshold.

 If such messages are observed, it is a better solution to take this as an
 indication that the lookup table needs to be re-engineered.

Changelog:
----------
 This patch
   * makes handling all TFRC resolution errors local to the TFRC library

   * removes unnecessary test whether X_calc is 'infinity' due to p==0 -- this
     condition is already caught by tfrc_calc_x()

   * removes setting ccid3hctx_p = TFRC_SMALLEST_P in ccid3_hc_tx_packet_recv
     since this is now done by the TFRC library

   * updates BUG_ON test in ccid3_hc_tx_no_feedback_timer to take into account
     that p now is either 0 (and then X_calc is irrelevant), or it is > 0; since
     the handling of TFRC_SMALLEST_P is now taken care of in the tfrc library

Justification:
--------------
 The TFRC code uses a lookup table which has a bounded resolution.
 The lowest possible value of the loss event rate `p' which can be
 resolved is currently 0.0001.  Substituting this lower threshold for
 p when p is less than 0.0001 results in a huge, exponentially-growing
 error.  The error can be computed by the following formula:

    (f(0.0001) - f(p))/f(p) * 100      for p < 0.0001

 Currently the solution is to use an (arbitrary) value
     TFRC_SMALLEST_P  =   40 * 1E-6   =   0.00004
 and to consider all values below this value as `virtually zero'.  Due to
 the exponentially growing resolution error, this is not a good idea, since
 it hides the fact that the table can not resolve practically occurring cases.
 Already at p == TFRC_SMALLEST_P, the error is as high as 58.19%!

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/dccp/ccids/ccid3.c

@@ -137,8 +137,7 @@ static void ccid3_hc_tx_update_x(struct sock *sk, struct timeval *now)
 	struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
 	const __u32 old_x = hctx->ccid3hctx_x;
 
-	/* To avoid large error in calcX */
-	if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
+	if (hctx->ccid3hctx_p > 0) {
 		hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
 						     hctx->ccid3hctx_rtt,
 						     hctx->ccid3hctx_p);
@@ -226,16 +225,14 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
 				       ccid3_tx_state_name(hctx->ccid3hctx_state));
 			/* Halve sending rate */
 
-			/*  If (X_calc > 2 * X_recv)
+			/*  If (p == 0 || X_calc > 2 * X_recv)
 			 *    X_recv = max(X_recv / 2, s / (2 * t_mbi));
 			 *  Else
 			 *    X_recv = X_calc / 4;
 			 */
-			BUG_ON(hctx->ccid3hctx_p >= TFRC_SMALLEST_P &&
-			       hctx->ccid3hctx_x_calc == 0);
+			BUG_ON(hctx->ccid3hctx_p && !hctx->ccid3hctx_x_calc);
 
-			/* check also if p is zero -> x_calc is infinity? */
-			if (hctx->ccid3hctx_p < TFRC_SMALLEST_P ||
+			if (hctx->ccid3hctx_p  == 0 ||
 			    hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv)
 				hctx->ccid3hctx_x_recv = max_t(u32, hctx->ccid3hctx_x_recv / 2,
 								    hctx->ccid3hctx_s / (2 * TFRC_T_MBI));
@@ -449,15 +446,8 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 		pinv = opt_recv->ccid3or_loss_event_rate;
 		if (pinv == ~0U || pinv == 0)
 			hctx->ccid3hctx_p = 0;
-		else {
-			hctx->ccid3hctx_p = 1000000 / pinv;
-
-			if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
-				hctx->ccid3hctx_p = TFRC_SMALLEST_P;
-				ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
-					       dccp_role(sk), sk);
-			}
-		}
+		else
+ 			hctx->ccid3hctx_p = 1000000 / pinv;
 
 		dccp_timestamp(sk, &now);
 

net/dccp/ccids/ccid3.h

@@ -51,8 +51,6 @@
 /* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
 #define TFRC_T_MBI		   64
 
-#define TFRC_SMALLEST_P		   40
-
 enum ccid3_options {
 	TFRC_OPT_LOSS_EVENT_RATE = 192,
 	TFRC_OPT_LOSS_INTERVALS	 = 193,