dccp ccid-3: Always perform receiver RTT sampling

This updates the CCID-3 receiver in part with regard to errata 610 and 611
(http://www.rfc-editor.org/errata_list.php), which change RFC 4342 to use the
Receive Rate as specified in rfc3448bis, requiring to constantly sample the
RTT (or use a sender RTT).

Doing this requires reusing the RX history structure after dealing with a loss.

The patch does not resolve how to compute X_recv if the interval is less
than 1 RTT. A FIXME has been added (and is resolved in subsequent patch).

Furthermore, since this is all TFRC-based functionality, the RTT estimation
is now also performed by the dccp_tfrc_lib module. This further simplifies
the CCID-3 code.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

2 files changed, 59 insertions, 18 deletions

diff --git a/net/dccp/ccids/lib/packet_history.c b/net/dccp/ccids/lib/packet_history.c
index ee34b4564242..e2e250aa5c89 100644
--- a/net/dccp/ccids/lib/packet_history.c
+++ b/net/dccp/ccids/lib/packet_history.c
@@ -151,14 +151,31 @@ int tfrc_rx_hist_duplicate(struct tfrc_rx_hist *h, struct sk_buff *skb)
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_duplicate);
 
+
+static void __tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
+{
+        struct tfrc_rx_hist_entry *tmp = h->ring[a];
+
+        h->ring[a] = h->ring[b];
+        h->ring[b] = tmp;
+}
+
 static void tfrc_rx_hist_swap(struct tfrc_rx_hist *h, const u8 a, const u8 b)
 {
-        const u8 idx_a = tfrc_rx_hist_index(h, a),
-                 idx_b = tfrc_rx_hist_index(h, b);
-        struct tfrc_rx_hist_entry *tmp = h->ring[idx_a];
+        __tfrc_rx_hist_swap(h, tfrc_rx_hist_index(h, a),
+                               tfrc_rx_hist_index(h, b));
+}
 
-        h->ring[idx_a] = h->ring[idx_b];
-        h->ring[idx_b] = tmp;
+/**
+ * tfrc_rx_hist_resume_rtt_sampling  -  Prepare RX history for RTT sampling
+ * This is called after loss detection has finished, when the history entry
+ * with the index of `loss_count' holds the highest-received sequence number.
+ * RTT sampling requires this information at ring[0] (tfrc_rx_hist_sample_rtt).
+ */
+static inline void tfrc_rx_hist_resume_rtt_sampling(struct tfrc_rx_hist *h)
+{
+        __tfrc_rx_hist_swap(h, 0, tfrc_rx_hist_index(h, h->loss_count));
+        h->loss_count = h->loss_start = 0;
 }
 
 /*
@@ -200,8 +217,7 @@ static void __one_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n2
 
                 if (dccp_loss_free(s2, s1, n1)) {
                         /* hole is filled: S0, S2, and S1 are consecutive */
-                        h->loss_count = 0;
-                        h->loss_start = tfrc_rx_hist_index(h, 1);
+                        tfrc_rx_hist_resume_rtt_sampling(h);
                 } else
                         /* gap between S2 and S1: just update loss_prev */
                         tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n2);
@@ -254,8 +270,7 @@ static int __two_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n3)
 
                         if (dccp_loss_free(s1, s2, n2)) {
                                 /* entire hole filled by S0, S3, S1, S2 */
-                                h->loss_start = tfrc_rx_hist_index(h, 2);
-                                h->loss_count = 0;
+                                tfrc_rx_hist_resume_rtt_sampling(h);
                         } else {
                                 /* gap remains between S1 and S2 */
                                 h->loss_start = tfrc_rx_hist_index(h, 1);
@@ -299,8 +314,7 @@ static void __three_after_loss(struct tfrc_rx_hist *h)
 
                 if (dccp_loss_free(s2, s3, n3)) {
                         /* no gap between S2 and S3: entire hole is filled */
-                        h->loss_start = tfrc_rx_hist_index(h, 3);
-                        h->loss_count = 0;
+                        tfrc_rx_hist_resume_rtt_sampling(h);
                 } else {
                         /* gap between S2 and S3 */
                         h->loss_start = tfrc_rx_hist_index(h, 2);
@@ -340,6 +354,7 @@ int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
 
         if (h->loss_count == 0) {
                 __do_track_loss(h, skb, ndp);
+                tfrc_rx_hist_sample_rtt(h, skb);
         } else if (h->loss_count == 1) {
                 __one_after_loss(h, skb, ndp);
         } else if (h->loss_count != 2) {
@@ -435,11 +450,24 @@ static inline struct tfrc_rx_hist_entry *
  * Based on ideas presented in RFC 4342, 8.1. Returns 0 if it was not able
  * to compute a sample with given data - calling function should check this.
  */
-u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
+void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
 {
-        u32 sample = 0,
-            delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
-                            tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
+        u32 sample = 0, delta_v;
+
+        /*
+         * When not to sample:
+         * - on non-data packets
+         *   (RFC 4342, 8.1: CCVal only fully defined for data packets);
+         * - when no data packets have been received yet
+         *   (FIXME: using sampled packet size as indicator here);
+         * - as long as there are gaps in the sequence space (pending loss).
+         */
+        if (!dccp_data_packet(skb) || h->packet_size == 0 ||
+            tfrc_rx_hist_loss_pending(h))
+                return;
+
+        delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
+                        tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
 
         if (delta_v < 1 || delta_v > 4) {       /* unsuitable CCVal delta */
                 if (h->rtt_sample_prev == 2) {  /* previous candidate stored */
@@ -479,6 +507,6 @@ u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
         h->rtt_sample_prev = 0;        /* use current entry as next reference */
 keep_ref_for_next_time:
 
-        return sample;
+        h->rtt_estimate = tfrc_ewma(h->rtt_estimate, sample, 9);
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_sample_rtt);
diff --git a/net/dccp/ccids/lib/packet_history.h b/net/dccp/ccids/lib/packet_history.h
index b7c87a1a2720..ba5832bbc348 100644
--- a/net/dccp/ccids/lib/packet_history.h
+++ b/net/dccp/ccids/lib/packet_history.h
@@ -91,6 +91,7 @@ struct tfrc_rx_hist_entry {
  * @loss_count:         Number of entries in circular history
  * @loss_start:         Movable index (for loss detection)
  * @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
  */
@@ -98,7 +99,10 @@ struct tfrc_rx_hist {
         struct tfrc_rx_hist_entry *ring[TFRC_NDUPACK + 1];
         u8                        loss_count:2,
                                   loss_start:2;
+        /* Receiver RTT sampling */
 #define rtt_sample_prev           loss_start
+        u32                       rtt_estimate;
+        /* Receiver sampling of application payload lengths */
         u32                       packet_size,
                                   bytes_recvd;
 };
@@ -154,6 +158,15 @@ static inline u32 tfrc_rx_hist_packet_size(const struct tfrc_rx_hist *h)
                 return TCP_MIN_RCVMSS;
         }
         return h->packet_size;
+
+}
+static inline u32 tfrc_rx_hist_rtt(const struct tfrc_rx_hist *h)
+{
+        if (h->rtt_estimate == 0) {
+                DCCP_WARN("No RTT estimate available, using fallback RTT\n");
+                return  DCCP_FALLBACK_RTT;
+        }
+        return h->rtt_estimate;
 }
 
 extern void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
@@ -167,8 +180,8 @@ extern int  tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
                                 struct sk_buff *skb, const u64 ndp,
                                 u32 (*first_li)(struct sock *sk),
                                 struct sock *sk);
-extern u32 tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h,
-                                   const struct sk_buff *skb);
+extern void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h,
+                                    const struct sk_buff *skb);
 extern int  tfrc_rx_hist_init(struct tfrc_rx_hist *h, struct sock *sk);
 extern void tfrc_rx_hist_purge(struct tfrc_rx_hist *h);