dccp ccid-3: Preventing Oscillations (a3cbdde8) · Commits · 戴 / test

net/dccp/ccids/ccid3.c

+40 −0

Original line number	Diff line number	Diff line
		@@ -49,6 +49,8 @@ static int ccid3_debug;
		/*
		* Transmitter Half-Connection Routines
		*/
		/* Oscillation Prevention/Reduction: recommended by rfc3448bis, on by default */
		static int do_osc_prev = true;

		/*
		* Compute the initial sending rate X_init in the manner of RFC 3390:
		@@ -296,6 +298,9 @@ static int ccid3_hc_tx_send_packet(struct sock sk, struct sk_buff skb)
		hctx->s = ccid3_hc_tx_measure_packet_size(sk, skb->len);
		ccid3_update_send_interval(hctx);

		/* Seed value for Oscillation Prevention (sec. 4.5) */
		hctx->r_sqmean = tfrc_scaled_sqrt(hctx->rtt);

		} else {
		delay = ktime_us_delta(hctx->t_nom, now);
		ccid3_pr_debug("delay=%ld\n", (long)delay);
		@@ -400,6 +405,38 @@ done_computing_x:
		hctx->s, hctx->p, hctx->x_calc,
		(unsigned)(hctx->x_recv >> 6),
		(unsigned)(hctx->x >> 6));
		/*
		* Oscillation Reduction (RFC 3448, 4.5) - modifying t_ipi according to
		* RTT changes, multiplying by X/X_inst = sqrt(R_sample)/R_sqmean. This
		* can be useful if few connections share a link, avoiding that buffer
		* fill levels (RTT) oscillate as a result of frequent adjustments to X.
		* A useful presentation with background information is in
		* Joerg Widmer, "Equation-Based Congestion Control",
		* MSc Thesis, University of Mannheim, Germany, 2000
		* (sec. 3.6.4), who calls this ISM ("Inter-packet Space Modulation").
		*/
		if (do_osc_prev) {
		r_sample = tfrc_scaled_sqrt(r_sample);
		/*
		* The modulation can work in both ways: increase/decrease t_ipi
		* according to long-term increases/decreases of the RTT. The
		* former is a useful measure, since it works against queue
		* build-up. The latter temporarily increases the sending rate,
		* so that buffers fill up more quickly. This in turn causes
		* the RTT to increase, so that either later reduction becomes
		* necessary or the RTT stays at a very high level. Decreasing
		* t_ipi is therefore not supported.
		* Furthermore, during the initial slow-start phase the RTT
		* naturally increases, where using the algorithm would cause
		* delays. Hence it is disabled during the initial slow-start.
		*/
		if (r_sample > hctx->r_sqmean && hctx->p > 0)
		hctx->t_ipi = div_u64((u64)hctx->t_ipi * (u64)r_sample,
		hctx->r_sqmean);
		hctx->t_ipi = min_t(u32, hctx->t_ipi, TFRC_T_MBI);
		/* update R_sqmean _after_ computing the modulation factor */
		hctx->r_sqmean = tfrc_ewma(hctx->r_sqmean, r_sample, 9);
		}

		/* unschedule no feedback timer */
		sk_stop_timer(sk, &hctx->no_feedback_timer);
		@@ -749,6 +786,9 @@ static struct ccid_operations ccid3 = {
		.ccid_hc_tx_getsockopt = ccid3_hc_tx_getsockopt,
		};

		module_param(do_osc_prev, bool, 0644);
		MODULE_PARM_DESC(do_osc_prev, "Use Oscillation Prevention (RFC 3448, 4.5)");

		#ifdef CONFIG_IP_DCCP_CCID3_DEBUG
		module_param(ccid3_debug, bool, 0644);
		MODULE_PARM_DESC(ccid3_debug, "Enable debug messages");

net/dccp/ccids/ccid3.h

+4 −2

Original line number	Diff line number	Diff line
		@@ -47,8 +47,8 @@
		/* Two seconds as per RFC 3448 4.2 */
		#define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC)

		/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
		#define TFRC_T_MBI 64
		/* Maximum backoff interval t_mbi (RFC 3448, 4.3) */
		#define TFRC_T_MBI (64 * USEC_PER_SEC)

		/*
		* The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are
		@@ -76,6 +76,7 @@ enum ccid3_options {
		* @x_recv - Receive rate in 64 * bytes per second
		* @x_calc - Calculated rate in bytes per second
		* @rtt - Estimate of current round trip time in usecs
		* @r_sqmean - Estimate of long-term RTT (RFC 3448, 4.5)
		* @p - Current loss event rate (0-1) scaled by 1000000
		* @s - Packet size in bytes
		* @t_rto - Nofeedback Timer setting in usecs
		@@ -94,6 +95,7 @@ struct ccid3_hc_tx_sock {
		u64 x_recv;
		u32 x_calc;
		u32 rtt;
		u16 r_sqmean;
		u32 p;
		u32 t_rto;
		u32 t_ipi;

net/dccp/ccids/lib/tfrc.h

+15 −0

Original line number	Diff line number	Diff line
		@@ -47,6 +47,21 @@ static inline u32 scaled_div32(u64 a, u64 b)
		return result;
		}

		/**
		* tfrc_scaled_sqrt - Compute scaled integer sqrt(x) for 0 < x < 2^22-1
		* Uses scaling to improve accuracy of the integer approximation of sqrt(). The
		* scaling factor of 2^10 limits the maximum @sample to 4e6; this is okay for
		* clamped RTT samples (dccp_sample_rtt).
		* Should best be used for expressions of type sqrt(x)/sqrt(y), since then the
		* scaling factor is neutralised. For this purpose, it avoids returning zero.
		*/
		static inline u16 tfrc_scaled_sqrt(const u32 sample)
		{
		const unsigned long non_zero_sample = sample ? : 1;

		return int_sqrt(non_zero_sample << 10);
		}

		/**
		* tfrc_ewma - Exponentially weighted moving average
		* @weight: Weight to be used as damping factor, in units of 1/10

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