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Message-Id: <1217340373-12620-6-git-send-email-gerrit@erg.abdn.ac.uk>
Date: Tue, 29 Jul 2008 15:06:12 +0100
From: Gerrit Renker <gerrit@....abdn.ac.uk>
To: dccp@...r.kernel.org
Cc: netdev@...r.kernel.org, Gerrit Renker <gerrit@....abdn.ac.uk>
Subject: [PATCH 5/6] dccp ccid-3: Preventing Oscillations
This implements (RFC 3448, 4.5), which performs congestion avoidance by reducing
the transmit rate as the queueing delay (measured in terms of long-term RTT
changes) 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/
Under the following conditions the algorithm will have 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: Gerrit Renker <gerrit@....abdn.ac.uk>
---
net/dccp/ccids/ccid3.c | 40 ++++++++++++++++++++++++++++++++++++++++
net/dccp/ccids/ccid3.h | 6 ++++--
net/dccp/ccids/lib/tfrc.h | 15 +++++++++++++++
3 files changed, 59 insertions(+), 2 deletions(-)
--- a/net/dccp/ccids/lib/tfrc.h
+++ b/net/dccp/ccids/lib/tfrc.h
@@ -48,6 +48,21 @@ static inline u32 scaled_div32(u64 a, u64 b)
}
/**
+ * 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
*/
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -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;
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -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, 0444);
MODULE_PARM_DESC(ccid3_debug, "Enable debug messages");
--
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