[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,
-
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