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Commit a3cbdde8 authored by Gerrit Renker's avatar Gerrit Renker
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dccp ccid-3: Preventing Oscillations 

This implements [RFC 3448, 4.5], which performs congestion avoidance behaviour
by reducing the transmit rate as the queueing delay (measured in terms of
long-term RTT) increases.

Oscillation can be turned on/off via a module option (do_osc_prev) and via sysfs
(using mode 0644), the default is off.

Overflow analysis:
------------------
 * oscillation prevention is done after update_x(), so that t_ipi <= 64000;
 * hence the multiplication "t_ipi * sqrt(R_sample)" needs 64 bits;
 * done using u64 for sqrt_sample and explicit typecast of t_ipi;
 * the divisor, R_sqmean, is non-zero because oscillation prevention is first
   called when receiving the second feedback packet, and tfrc_scaled_rtt() > 0.

A detailed discussion of the algorithm (with plots) is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid3/sender_notes/oscillation_prevention/



The algorithm has negative side effects:
  * when allowing to decrease t_ipi (leads to a large RTT) and
  * when using it during slow-start;
both uses are therefore disabled.

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 53ac9570

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 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb) 
			       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