diff options
| -rw-r--r-- | net/dccp/ccids/ccid3.c | 40 | ||||
| -rw-r--r-- | net/dccp/ccids/ccid3.h | 6 | ||||
| -rw-r--r-- | net/dccp/ccids/lib/tfrc.h | 15 |
3 files changed, 59 insertions, 2 deletions
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c index 7cd76c6c790..06cfdad84a6 100644 --- 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, 0644); MODULE_PARM_DESC(ccid3_debug, "Enable debug messages"); diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h index 342235c57bf..af6e1bf937d 100644 --- 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; diff --git a/net/dccp/ccids/lib/tfrc.h b/net/dccp/ccids/lib/tfrc.h index bb47146ac7d..ede12f53de5 100644 --- 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 */ |