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Date: Tue, 29 Jul 2008 11:05:51 +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 6/7] dccp ccid-3: Update the computation of X_recv
This updates the computation of X_recv with regard to Errata 610/611 for
RFC 4342 and draft rfc3448bis-06, ensuring that at least an interval of 1
RTT is used to compute X_recv.
That change is wrapped into a new function, ccid3_hc_rx_x_recv().
Further changes:
----------------
* feedback is not sent when no data packets arrived (bytes_recv == 0), as per
rfc3448bis-06, 6.2;
* take the timestamp for the feedback /after/ dccp_send_ack() returns, to avoid
taking the transmission time into account (in case layer-2 is busy);
* clearer handling of failure in ccid3_first_li().
Signed-off-by: Gerrit Renker <gerrit@....abdn.ac.uk>
---
net/dccp/ccids/ccid3.c | 64 +++++++++++++----------------------
net/dccp/ccids/lib/packet_history.c | 30 ++++++++++++++++
net/dccp/ccids/lib/packet_history.h | 13 ++++++-
3 files changed, 66 insertions(+), 41 deletions(-)
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -533,9 +533,6 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
enum ccid3_fback_type fbtype)
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
- struct dccp_sock *dp = dccp_sk(sk);
- ktime_t now = ktime_get_real();
- s64 delta = 0;
switch (fbtype) {
case CCID3_FBACK_INITIAL:
@@ -565,42 +562,33 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
/*
* When parameters change (new loss or p > p_prev), we do not
* have a reliable estimate for R_m of [RFC 3448, 6.2] and so
- * need to reuse the previous value of X_recv. However, when
- * X_recv was 0 (due to early loss), this would kill X down to
- * s/t_mbi (i.e. one packet in 64 seconds).
- * To avoid such drastic reduction, we approximate X_recv as
- * the number of bytes since last feedback.
- * This is a safe fallback, since X is bounded above by X_calc.
+ * always check whether at least RTT time units were covered.
*/
- if (hcrx->x_recv > 0)
- break;
- /* fall through */
+ hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
+ break;
case CCID3_FBACK_PERIODIC:
/*
- * FIXME: check if delta is less than or equal to 1 RTT using
- * the receiver RTT sample. This is described in Errata 610/611
- * of RFC 4342 which reference section 6.2 of RFC 3448.
+ * Step (2) of rfc3448bis-06, 6.2:
+ * - if no data packets have been received, just restart timer
+ * - if data packets have been received, re-compute X_recv
*/
- delta = ktime_us_delta(now, hcrx->tstamp_last_feedback);
- if (delta <= 0)
- DCCP_BUG("delta (%ld) <= 0", (long)delta);
- else
- hcrx->x_recv = scaled_div32(hcrx->hist.bytes_recvd, delta);
+ if (hcrx->hist.bytes_recvd == 0)
+ goto prepare_for_next_time;
+ hcrx->x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
break;
default:
return;
}
- ccid3_pr_debug("Interval %ldusec, X_recv=%u, 1/p=%u\n",
- (long)delta, hcrx->x_recv, hcrx->p_inverse);
+ ccid3_pr_debug("X_recv=%u, 1/p=%u\n", hcrx->x_recv, hcrx->p_inverse);
- hcrx->tstamp_last_feedback = now;
- hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;
- hcrx->hist.bytes_recvd = 0;
- hcrx->feedback = fbtype;
-
- dp->dccps_hc_rx_insert_options = 1;
+ dccp_sk(sk)->dccps_hc_rx_insert_options = 1;
dccp_send_ack(sk);
+
+prepare_for_next_time:
+ tfrc_rx_hist_restart_byte_counter(&hcrx->hist);
+ hcrx->last_counter = dccp_hdr(skb)->dccph_ccval;
+ hcrx->feedback = fbtype;
}
static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
@@ -639,7 +627,7 @@ static u32 ccid3_first_li(struct sock *sk)
{
struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
- rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p, delta;
+ rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p;
u64 fval;
/*
@@ -650,16 +638,9 @@ static u32 ccid3_first_li(struct sock *sk)
if (unlikely(hcrx->feedback == CCID3_FBACK_NONE))
return 5;
- delta = ktime_to_us(net_timedelta(hcrx->tstamp_last_feedback));
- x_recv = scaled_div32(hcrx->hist.bytes_recvd, delta);
- if (x_recv == 0) { /* would also trigger divide-by-zero */
- DCCP_WARN("X_recv==0\n");
- if (hcrx->x_recv == 0) {
- DCCP_BUG("stored value of X_recv is zero");
- return ~0U;
- }
- x_recv = hcrx->x_recv;
- }
+ x_recv = tfrc_rx_hist_x_recv(&hcrx->hist, hcrx->x_recv);
+ if (x_recv == 0)
+ goto failed;
fval = scaled_div32(scaled_div(s, rtt), x_recv);
p = tfrc_calc_x_reverse_lookup(fval);
@@ -667,7 +648,10 @@ static u32 ccid3_first_li(struct sock *sk)
ccid3_pr_debug("%s(%p), receive rate=%u bytes/s, implied "
"loss rate=%u\n", dccp_role(sk), sk, x_recv, p);
- return p == 0 ? ~0U : scaled_div(1, p);
+ if (p > 0)
+ return scaled_div(1, p);
+failed:
+ return UINT_MAX;
}
static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
--- a/net/dccp/ccids/lib/packet_history.h
+++ b/net/dccp/ccids/lib/packet_history.h
@@ -93,7 +93,8 @@ struct tfrc_rx_hist_entry {
* @rtt_sample_prev: Used during RTT sampling, points to candidate entry
* @rtt_estimate: Receiver RTT estimate
* @packet_size: Packet size in bytes (as per RFC 3448, 3.1)
- * @bytes_recvd: Number of bytes received since last sending feedback
+ * @bytes_recvd: Number of bytes received since @bytes_start
+ * @bytes_start: Start time for counting @bytes_recvd
*/
struct tfrc_rx_hist {
struct tfrc_rx_hist_entry *ring[TFRC_NDUPACK + 1];
@@ -105,6 +106,7 @@ struct tfrc_rx_hist {
/* Receiver sampling of application payload lengths */
u32 packet_size,
bytes_recvd;
+ ktime_t bytes_start;
};
/**
@@ -169,6 +171,15 @@ static inline u32 tfrc_rx_hist_rtt(const struct tfrc_rx_hist *h)
return h->rtt_estimate;
}
+static inline void tfrc_rx_hist_restart_byte_counter(struct tfrc_rx_hist *h)
+{
+ h->bytes_recvd = 0;
+ h->bytes_start = ktime_get_real();
+}
+
+extern u32 tfrc_rx_hist_x_recv(struct tfrc_rx_hist *h, const u32 last_x_recv);
+
+
extern void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
const struct sk_buff *skb, const u64 ndp);
--- a/net/dccp/ccids/lib/packet_history.c
+++ b/net/dccp/ccids/lib/packet_history.c
@@ -385,6 +385,36 @@ int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
}
EXPORT_SYMBOL_GPL(tfrc_rx_handle_loss);
+/* Compute the sending rate X_recv measured between feedback intervals */
+u32 tfrc_rx_hist_x_recv(struct tfrc_rx_hist *h, const u32 last_x_recv)
+{
+ u64 bytes = h->bytes_recvd, last_rtt = h->rtt_estimate;
+ s64 delta = ktime_to_us(net_timedelta(h->bytes_start));
+
+ WARN_ON(delta <= 0);
+ /*
+ * Ensure that the sampling interval for X_recv is at least one RTT,
+ * by extending the sampling interval backwards in time, over the last
+ * R_(m-1) seconds, as per rfc3448bis-06, 6.2.
+ * To reduce noise (e.g. when the RTT changes often), this is only
+ * done when delta is smaller than RTT/2.
+ */
+ if (last_x_recv > 0 && delta < last_rtt/2) {
+ tfrc_pr_debug("delta < RTT ==> %ld us < %u us\n",
+ (long)delta, (unsigned)last_rtt);
+
+ delta = (bytes ? delta : 0) + last_rtt;
+ bytes += div_u64((u64)last_x_recv * last_rtt, USEC_PER_SEC);
+ }
+
+ if (unlikely(bytes == 0)) {
+ DCCP_WARN("X_recv == 0, using old value of %u\n", last_x_recv);
+ return last_x_recv;
+ }
+ return scaled_div32(bytes, delta);
+}
+EXPORT_SYMBOL_GPL(tfrc_rx_hist_x_recv);
+
void tfrc_rx_hist_purge(struct tfrc_rx_hist *h)
{
int i;
--
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