dccp tfrc/ccid-3: Computing Loss Rate from Loss Event Rate
This adds a function to take care of the following cases occurring in the computation of the Loss Rate p: * 1/(2^32-1) is mapped into 0% as per RFC 4342, 8.5; * 1/0 is mapped into the maximum of 100%; * we want to avoid that p = 1/x is rounded down to 0 when x is very large, since this means accidentally re-entering slow-start (indicated by p==0). In the last case, the minimum-resolution value of p is returned. Furthermore, a bug in ccid3_hc_rx_getsockopt is fixed (1/0 was mapped into ~0U), which now allows to consistently print the scaled p-values as printf("Loss Event Rate = %u.%04u %%\n", rx_info.tfrcrx_p / 10000, rx_info.tfrcrx_p % 10000); Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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@ -410,10 +410,10 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
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/* Update loss event rate (which is scaled by 1e6) */
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pinv = opt_recv->ccid3or_loss_event_rate;
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if (pinv == ~0U || pinv == 0) /* see RFC 4342, 8.5 */
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if (pinv == 0)
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hctx->p = 0;
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else /* can not exceed 100% */
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hctx->p = scaled_div(1, pinv);
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else
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hctx->p = tfrc_invert_loss_event_rate(pinv);
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/*
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* Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3
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@ -854,8 +854,7 @@ static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
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return -EINVAL;
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rx_info.tfrcrx_x_recv = hcrx->x_recv;
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rx_info.tfrcrx_rtt = hcrx->rtt;
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rx_info.tfrcrx_p = hcrx->p_inverse == 0 ? ~0U :
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scaled_div(1, hcrx->p_inverse);
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rx_info.tfrcrx_p = tfrc_invert_loss_event_rate(hcrx->p_inverse);
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len = sizeof(rx_info);
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val = &rx_info;
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break;
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@ -58,6 +58,7 @@ static inline u32 tfrc_ewma(const u32 avg, const u32 newval, const u8 weight)
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extern u32 tfrc_calc_x(u16 s, u32 R, u32 p);
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extern u32 tfrc_calc_x_reverse_lookup(u32 fvalue);
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extern u32 tfrc_invert_loss_event_rate(u32 loss_event_rate);
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extern int tfrc_tx_packet_history_init(void);
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extern void tfrc_tx_packet_history_exit(void);
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@ -658,7 +658,6 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
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result = scaled_div(s, R);
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return scaled_div32(result, f);
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}
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EXPORT_SYMBOL_GPL(tfrc_calc_x);
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/**
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@ -693,5 +692,19 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
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index = tfrc_binsearch(fvalue, 0);
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return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
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}
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EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);
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/**
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* tfrc_invert_loss_event_rate - Compute p so that 10^6 corresponds to 100%
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* When @loss_event_rate is large, there is a chance that p is truncated to 0.
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* To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
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*/
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u32 tfrc_invert_loss_event_rate(u32 loss_event_rate)
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{
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if (loss_event_rate == UINT_MAX) /* see RFC 4342, 8.5 */
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return 0;
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if (unlikely(loss_event_rate == 0)) /* map 1/0 into 100% */
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return 1000000;
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return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P);
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}
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EXPORT_SYMBOL_GPL(tfrc_invert_loss_event_rate);
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