dccp tfrc/ccid-3: computing the loss rate from the Loss Event Rate
This adds a function to take care of the following, separate 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 100%, the maximum; * 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, the minimum resolution value of p is now returned instead; * a bug in ccid3_hc_rx_getsockopt is fixed: 1/0 was mapped into ~0U. Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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@ -400,10 +400,10 @@ 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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hc->tx_p = 0;
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else /* can not exceed 100% */
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hc->tx_p = scaled_div(1, pinv);
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else
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hc->tx_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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@ -834,8 +834,7 @@ 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 = hc->rx_x_recv;
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rx_info.tfrcrx_rtt = hc->rx_rtt;
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rx_info.tfrcrx_p = hc->rx_pinv == 0 ? ~0U :
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scaled_div(1, hc->rx_pinv);
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rx_info.tfrcrx_p = tfrc_invert_loss_event_rate(hc->rx_pinv);
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len = sizeof(rx_info);
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val = &rx_info;
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break;
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@ -57,6 +57,7 @@ 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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@ -687,3 +687,17 @@ 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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/**
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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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