dccp ccid-3: Simplify computing and range-checking of t_ipi
This patch simplifies the computation of t_ipi, avoiding expensive computations
to enforce the minimum sending rate.
Both RFC 3448 and rfc3448bis (revision #06), as well as RFC 4342 sec 5., require
at various stages that at least one packet must be sent per t_mbi = 64 seconds.
This requires frequent divisions of the type X_min = s/t_mbi, which are later
converted back into an inter-packet-interval t_ipi_max = s/X_min = t_mbi.
The patch removes the expensive indirection; in the unlikely case of having
a sending rate less than one packet per 64 seconds, it also re-adjusts X.
The following cases document conformance with RFC 3448 / rfc3448bis-06:
1) Time until receiving the first feedback packet:
* if the sender has no initial RTT sample then X = s/1 Bps > s/t_mbi;
* if the sender has an initial RTT sample or when the first feedback
packet is received, X = W_init/R > s/t_mbi.
2) Slow-start (p == 0 and feedback packets come in):
* RFC 3448 (current code) enforces a minimum of s/R > s/t_mbi;
* rfc3448bis (future code) enforces an even higher minimum of W_init/R.
3) Congestion avoidance with no absence of feedback (p > 0):
* when X_calc or X_recv/2 are too low, the minimum of X_min = s/t_mbi
is enforced in update_x() when calling update_send_interval();
* update_send_interval() is, as before, only called when X changes
(i.e. either when increasing or decreasing, not when in equilibrium).
4) Reduction of X without prior feedback or during slow-start (p==0):
* both RFC 3448 and rfc3448bis here halve X directly;
* the associated constraint X >= s/t_mbi is nforced here by send_interval().
5) Reduction of X when p > 0:
* X is modified indirectly via X_recv (RFC 3448) or X_recv_set (rfc3448bis);
* in both cases, control goes back to section 4.3 (in both documents);
* since p > 0, both documents use X = max(min(...), s/t_mbi), which is
enforced in this patch by calling send_interval() from update_x().
I think that this analysis is exhaustive. Should I have forgotten a case,
the worst-case consideration arises when X sinks below s/t_mbi, and is then
increased back up to this minimum value. Even under this assumption, the
behaviour is correct, since all lower limits of X in RFC 3448 / rfc3448bis
are either equal to or greater than s/t_mbi.
Note on the condition X >= s/t_mbi <==> t_ipi = s/X <= t_mbi: since X is
scaled by 64, and all time units are in microseconds, the coded condition is:
t_ipi = s * 64 * 10^6 usec / X <= 64 * 10^6 usec
This simplifies to s / X <= 1 second <==> X * 1 second >= s > 0.
(A zero `s' is not allowed by the CCID-3 code).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index d77d3e6..7cd76c6 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -66,15 +66,15 @@
}
/**
- * ccid3_update_send_interval - Calculate new t_ipi = s / X_inst
- * This respects the granularity of X_inst (64 * bytes/second).
+ * ccid3_update_send_interval - Calculate new t_ipi = s / X
+ * This respects the granularity of X (64 * bytes/second) and enforces the
+ * scaled minimum of s * 64 / t_mbi = `s' bytes/second as per RFC 3448/4342.
*/
static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
{
+ if (unlikely(hctx->x <= hctx->s))
+ hctx->x = hctx->s;
hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x);
-
- ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hctx->t_ipi,
- hctx->s, (unsigned)(hctx->x >> 6));
}
static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now)
@@ -115,7 +115,6 @@
if (hctx->p > 0) {
hctx->x = min(((u64)hctx->x_calc) << 6, min_rate);
- hctx->x = max(hctx->x, (((u64)hctx->s) << 6) / TFRC_T_MBI);
} else if (ktime_us_delta(now, hctx->t_ld) - (s64)hctx->rtt >= 0) {
@@ -197,8 +196,9 @@
if (hctx->t_rto == 0 || hctx->p == 0) {
/* halve send rate directly */
- hctx->x = max(hctx->x / 2, (((u64)hctx->s) << 6) / TFRC_T_MBI);
+ hctx->x /= 2;
ccid3_update_send_interval(hctx);
+
} else {
/*
* Modify the cached value of X_recv
@@ -213,9 +213,7 @@
BUG_ON(hctx->p && !hctx->x_calc);
if (hctx->x_calc > (hctx->x_recv >> 5))
- hctx->x_recv =
- max(hctx->x_recv / 2,
- (((__u64)hctx->s) << 6) / (2 * TFRC_T_MBI));
+ hctx->x_recv /= 2;
else {
hctx->x_recv = hctx->x_calc;
hctx->x_recv <<= 4;