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.  The 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@erg.abdn.ac.uk>

3 files changed, 66 insertions, 41 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 8e64d9665a21..f2f9514dbad2 100644
--- 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)
diff --git a/net/dccp/ccids/lib/packet_history.c b/net/dccp/ccids/lib/packet_history.c
index 5c4ded1cf422..547ad098ea6a 100644
--- 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;
diff --git a/net/dccp/ccids/lib/packet_history.h b/net/dccp/ccids/lib/packet_history.h
index ba5832bbc348..6552be63cb0a 100644
--- 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);