[DCCP]: 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@xxxxxxxxxxxxxx> --- net/dccp/ccids/ccid3.c | 21 +++++---------------- net/dccp/ccids/ccid3.h | 2 -- 2 files changed, 5 insertions(+), 18 deletions(-) --- a/net/dccp/ccids/ccid3.c +++ b/net/dccp/ccids/ccid3.c @@ -137,8 +137,7 @@ static void ccid3_hc_tx_update_x(struct 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); @@ -231,12 +230,9 @@ static void ccid3_hc_tx_no_feedback_time * 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 || - hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv) + if (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)); else @@ -449,15 +445,8 @@ static void ccid3_hc_tx_packet_recv(stru 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); --- a/net/dccp/ccids/ccid3.h +++ b/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, - To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html