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>
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@ -66,15 +66,15 @@ static inline u64 rfc3390_initial_rate(struct sock *sk)
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}
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/**
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* ccid3_update_send_interval - Calculate new t_ipi = s / X_inst
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* This respects the granularity of X_inst (64 * bytes/second).
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* ccid3_update_send_interval - Calculate new t_ipi = s / X
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* This respects the granularity of X (64 * bytes/second) and enforces the
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* scaled minimum of s * 64 / t_mbi = `s' bytes/second as per RFC 3448/4342.
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*/
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static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
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{
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if (unlikely(hctx->x <= hctx->s))
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hctx->x = hctx->s;
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hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x);
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ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hctx->t_ipi,
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hctx->s, (unsigned)(hctx->x >> 6));
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}
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static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now)
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@ -115,7 +115,6 @@ static void ccid3_hc_tx_update_x(struct sock *sk, ktime_t *stamp)
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if (hctx->p > 0) {
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hctx->x = min(((u64)hctx->x_calc) << 6, min_rate);
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hctx->x = max(hctx->x, (((u64)hctx->s) << 6) / TFRC_T_MBI);
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} else if (ktime_us_delta(now, hctx->t_ld) - (s64)hctx->rtt >= 0) {
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@ -197,8 +196,9 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
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if (hctx->t_rto == 0 || hctx->p == 0) {
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/* halve send rate directly */
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hctx->x = max(hctx->x / 2, (((u64)hctx->s) << 6) / TFRC_T_MBI);
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hctx->x /= 2;
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ccid3_update_send_interval(hctx);
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} else {
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/*
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* Modify the cached value of X_recv
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@ -213,9 +213,7 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
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BUG_ON(hctx->p && !hctx->x_calc);
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if (hctx->x_calc > (hctx->x_recv >> 5))
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hctx->x_recv =
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max(hctx->x_recv / 2,
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(((__u64)hctx->s) << 6) / (2 * TFRC_T_MBI));
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hctx->x_recv /= 2;
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else {
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hctx->x_recv = hctx->x_calc;
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hctx->x_recv <<= 4;
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