[DCCP]: Simplify calculation for reverse lookup of p
This simplifies the calculation of a value p for a given fval when the
first loss interval is computed (RFC 3448, 6.3.1). It makes use of the
two new functions scaled_div/scaled_div32 to provide overflow protection.
Additionally, protection against divide-by-zero is extended - in this
case the function will return the maximally possible value of p=100%.
Background:
-----------
The maximum fval, f(100%), is approximately 244, i.e. the scaled value of fval
should never exceed 244E6, which fits easily into u32. The problem is the scaling
by 10^6, since additionally R(TT) is in microseconds.
This is resolved by breaking the division into two stages: the first stage
computes fval=(s*10^6)/R, stores that into u64; the second stage computes
fval = (fval*10^6)/X_recv and complains if overflow is reached for u32.
This case is safe since the TFRC reverse-lookup routine then returns p=100%.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/ccids/ccid3.c | 39 ++++++++++++++++++++++-----------------
1 file changed, 22 insertions(+), 17 deletions(-)
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -785,12 +785,12 @@ static u32 ccid3_hc_rx_calc_first_li(str
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
struct dccp_rx_hist_entry *entry, *next, *tail = NULL;
- u32 rtt, delta, x_recv, fval, p, tmp2;
+ u32 rtt, delta, x_recv, p;
struct timeval tstamp = { 0, };
int interval = 0;
int win_count = 0;
int step = 0;
- u64 tmp1;
+ u64 fval;
list_for_each_entry_safe(entry, next, &hcrx->ccid3hcrx_hist,
dccphrx_node) {
@@ -834,30 +834,35 @@ found:
ccid3_pr_debug("%s, sk=%p, approximated RTT to %uus\n",
dccp_role(sk), sk, rtt);
- if (rtt == 0) {
- DCCP_WARN("RTT==0, setting to 1\n");
- rtt = 1;
+ /*
+ * Determine the length of the first loss interval via inverse lookup.
+ * Assume that X_recv can be computed by the throughput equation
+ * s
+ * X_recv = --------
+ * R * fval
+ * Find some p such that f(p) = fval; return 1/p [RFC 3448, 6.3.1].
+ */
+ if (rtt == 0) { /* would result in divide-by-zero */
+ DCCP_WARN("RTT==0, returning 1/p = 1\n");
+ return 1000000;
}
dccp_timestamp(sk, &tstamp);
delta = timeval_delta(&tstamp, &hcrx->ccid3hcrx_tstamp_last_feedback);
x_recv = scaled_div32(hcrx->ccid3hcrx_bytes_recv, delta);
- if (x_recv == 0)
- x_recv = hcrx->ccid3hcrx_x_recv;
-
- tmp1 = (u64)x_recv * (u64)rtt;
- do_div(tmp1,10000000);
- tmp2 = (u32)tmp1;
-
- if (!tmp2) {
- DCCP_CRIT("tmp2 = 0, x_recv = %u, rtt =%u\n", x_recv, rtt);
- return ~0;
+ if (x_recv == 0) { /* would also trigger divide-by-zero */
+ DCCP_WARN("X_recv==0\n");
+ if((x_recv = hcrx->ccid3hcrx_x_recv) == 0) {
+ DCCP_BUG("stored value of X_recv is zero");
+ return 1000000;
+ }
}
- fval = (hcrx->ccid3hcrx_s * 100000) / tmp2;
- /* do not alter order above or you will get overflow on 32 bit */
+ fval = scaled_div(hcrx->ccid3hcrx_s, rtt);
+ fval = scaled_div32(fval, x_recv);
p = tfrc_calc_x_reverse_lookup(fval);
+
ccid3_pr_debug("%s, sk=%p, receive rate=%u bytes/s, implied "
"loss rate=%u\n", dccp_role(sk), sk, x_recv, p);
-
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