[DCCP]: Avoid masking of low-resolution errors
This patch makes handling of all TFRC table lookup resolution errors local
to the TFRC library and removes such checks/warnings from calling code.
A warning message is printed out whenever the value of `p' is so small that
it falls below the resolution of the lookup table.
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. 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.
Furthermore, at p == TFRC_SMALLEST_P, the error is still as large as 58.1%!
The first half to resolve this problem has been implemented in the previous patch,
which will print warning messages whenever p is less than the given minimal resolution.
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 (which is trivial to do, the table
can be computed using a tiny C program).
Detailed Changelog:
-------------------
This patch therefore completes the protection against masking table-lookup resolution
errors by
* not substituting 0 if the value can not be resolved
* not using an arbitrary threshold such as TFRC_SMALLEST_P
* 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()
* remove setting ccid3hctx_p = TFRC_SMALLEST_P in ccid3_hc_tx_packet_recv since this
is now done by the TFRC library
* updated 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
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/ccids/ccid3.c | 18 ++++--------------
net/dccp/ccids/ccid3.h | 2 --
2 files changed, 4 insertions(+), 16 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
DCCP_BUG_ON(hctx->ccid3hctx_rtt == 0);
- /* 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);
@@ -230,12 +229,10 @@ static void ccid3_hc_tx_no_feedback_time
* Else
* X_recv = X_calc / 4;
*/
- BUG_ON(hctx->ccid3hctx_p >= TFRC_SMALLEST_P &&
+ BUG_ON(hctx->ccid3hctx_p > 0 &&
hctx->ccid3hctx_x_calc == 0);
- /* 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
@@ -457,16 +454,9 @@ static void ccid3_hc_tx_packet_recv(stru
/* Update loss event rate */
if (pinv == ~0 || pinv == 0)
hctx->ccid3hctx_p = 0;
- else {
+ 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);
- }
- }
-
/* Calculate new round trip sample by
* R_sample = (t_now - t_recvdata) - t_delay
* t_recvdata is taken from packet history */
--- 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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