[TFRC]: Ringbuffer to track loss interval history

A ringbuffer-based implementation of loss interval history is easier to
maintain, allocate, and update.

The `swap' routine to keep the RX history sorted is due to and was written
by Arnaldo Carvalho de Melo, simplifying an earlier macro-based variant.

Details:
 * access to the Loss Interval Records via macro wrappers (with safety checks);
 * simplified, on-demand allocation of entries (no extra memory consumption on
   lossless links); cache allocation is local to the module / exported as service;
 * provision of RFC-compliant algorithm to re-compute average loss interval;
 * provision of comprehensive, new loss detection algorithm
 	- support for all cases of loss, including re-ordered/duplicate packets;
 	- waiting for NDUPACK=3 packets to fill the hole;
	- updating loss records when a late-arriving packet fills a hole.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 214 insertions, 4 deletions

diff --git a/net/dccp/ccids/lib/packet_history.c b/net/dccp/ccids/lib/packet_history.c
index dd2cf2d6b8f..5b10a1ecf13 100644
--- a/net/dccp/ccids/lib/packet_history.c
+++ b/net/dccp/ccids/lib/packet_history.c
@@ -151,11 +151,10 @@ void tfrc_rx_packet_history_exit(void)
 	}
 }
 
-void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
-			     const struct sk_buff *skb,
-			     const u32 ndp)
+static inline void tfrc_rx_hist_entry_from_skb(struct tfrc_rx_hist_entry *entry,
+					       const struct sk_buff *skb,
+					       const u32 ndp)
 {
-	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);
 	const struct dccp_hdr *dh = dccp_hdr(skb);
 
 	entry->tfrchrx_seqno = DCCP_SKB_CB(skb)->dccpd_seq;
@@ -164,6 +163,15 @@ void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
 	entry->tfrchrx_ndp   = ndp;
 	entry->tfrchrx_tstamp = ktime_get_real();
 }
+
+void tfrc_rx_hist_add_packet(struct tfrc_rx_hist *h,
+			     const struct sk_buff *skb,
+			     const u32 ndp)
+{
+	struct tfrc_rx_hist_entry *entry = tfrc_rx_hist_last_rcv(h);
+
+	tfrc_rx_hist_entry_from_skb(entry, skb, ndp);
+}
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_add_packet);
 
 /* has the packet contained in skb been seen before? */
@@ -209,6 +217,208 @@ int tfrc_rx_hist_new_loss_indicated(struct tfrc_rx_hist *h,
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_new_loss_indicated);
 
+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];
+
+	h->ring[idx_a] = h->ring[idx_b];
+	h->ring[idx_b] = tmp;
+}
+
+/*
+ * Private helper functions for loss detection.
+ *
+ * In the descriptions, `Si' refers to the sequence number of entry number i,
+ * whose NDP count is `Ni' (lower case is used for variables).
+ * Note: All __after_loss functions expect that a test against duplicates has
+ *       been performed already: the seqno of the skb must not be less than the
+ *       seqno of loss_prev; and it must not equal that of any valid hist_entry.
+ */
+static void __one_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n2)
+{
+	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
+	    s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
+	    s2 = DCCP_SKB_CB(skb)->dccpd_seq;
+	int n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp,
+	   d12 = dccp_delta_seqno(s1, s2), d2;
+
+	if (d12 > 0) {			/* S1  <  S2 */
+		h->loss_count = 2;
+		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n2);
+		return;
+	}
+
+	/* S0  <  S2  <  S1 */
+	d2 = dccp_delta_seqno(s0, s2);
+
+	if (d2 == 1 || n2 >= d2) {	/* S2 is direct successor of S0 */
+		int d21 = -d12;
+
+		if (d21 == 1 || n1 >= d21) {
+			/* hole is filled: S0, S2, and S1 are consecutive */
+			h->loss_count = 0;
+			h->loss_start = tfrc_rx_hist_index(h, 1);
+		} else
+			/* gap between S2 and S1: just update loss_prev */
+			tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n2);
+
+	} else {			/* hole between S0 and S2 */
+		/*
+		 * Reorder history to insert S2 between S0 and s1
+		 */
+		tfrc_rx_hist_swap(h, 0, 3);
+		h->loss_start = tfrc_rx_hist_index(h, 3);
+		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n2);
+		h->loss_count = 2;
+	}
+}
+
+/* return 1 if a new loss event has been identified */
+static int __two_after_loss(struct tfrc_rx_hist *h, struct sk_buff *skb, u32 n3)
+{
+	u64 s0 = tfrc_rx_hist_loss_prev(h)->tfrchrx_seqno,
+	    s1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_seqno,
+	    s2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_seqno,
+	    s3 = DCCP_SKB_CB(skb)->dccpd_seq;
+	int n1 = tfrc_rx_hist_entry(h, 1)->tfrchrx_ndp,
+	   d23 = dccp_delta_seqno(s2, s3), d13, d3, d31;
+
+	if (d23 > 0) {			/* S2  <  S3 */
+		h->loss_count = 3;
+		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 3), skb, n3);
+		return 1;
+	}
+
+	/* S3  <  S2 */
+	d13 = dccp_delta_seqno(s1, s3);
+
+	if (d13 > 0) {
+		/*
+		 * The sequence number order is S1, S3, S2
+		 * Reorder history to insert entry between S1 and S2
+		 */
+		tfrc_rx_hist_swap(h, 2, 3);
+		tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 2), skb, n3);
+		h->loss_count = 3;
+		return 1;
+	}
+
+	/* S0  <  S3  <  S1 */
+	d31 = -d13;
+	d3  = dccp_delta_seqno(s0, s3);
+
+	if (d3 == 1 || n3 >= d3) {	/* S3 is a successor of S0 */
+
+		if (d31 == 1 || n1 >= d31) {
+			/* hole between S0 and S1 filled by S3 */
+			int  d2 = dccp_delta_seqno(s1, s2),
+			     n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp;
+
+			if (d2 == 1 || n2 >= d2) {
+				/* entire hole filled by S0, S3, S1, S2 */
+				h->loss_start = tfrc_rx_hist_index(h, 2);
+				h->loss_count = 0;
+			} else {
+				/* gap remains between S1 and S2 */
+				h->loss_start = tfrc_rx_hist_index(h, 1);
+				h->loss_count = 1;
+			}
+
+		} else /* gap exists between S3 and S1, loss_count stays at 2 */
+			tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_loss_prev(h), skb, n3);
+
+		return 0;
+	}
+
+	/*
+	 * The remaining case: S3 is not a successor of S0.
+	 * Sequence order is S0, S3, S1, S2; reorder to insert between S0 and S1
+	 */
+	tfrc_rx_hist_swap(h, 0, 3);
+	h->loss_start = tfrc_rx_hist_index(h, 3);
+	tfrc_rx_hist_entry_from_skb(tfrc_rx_hist_entry(h, 1), skb, n3);
+	h->loss_count = 3;
+
+	return 1;
+}
+
+/* return the signed modulo-2^48 sequence number distance from entry e1 to e2 */
+static s64 tfrc_rx_hist_delta_seqno(struct tfrc_rx_hist *h, u8 e1, u8 e2)
+{
+	DCCP_BUG_ON(e1 > h->loss_count || e2 > h->loss_count);
+
+	return dccp_delta_seqno(tfrc_rx_hist_entry(h, e1)->tfrchrx_seqno,
+				tfrc_rx_hist_entry(h, e2)->tfrchrx_seqno);
+}
+
+/* recycle RX history records to continue loss detection if necessary */
+static void __three_after_loss(struct tfrc_rx_hist *h)
+{
+	/*
+	 * The distance between S0 and S1 is always greater than 1 and the NDP
+	 * count of S1 is smaller than this distance. Otherwise there would
+	 * have been no loss. Hence it is only necessary to see whether there
+	 * are further missing data packets between S1/S2 and S2/S3.
+	 */
+	int d2 = tfrc_rx_hist_delta_seqno(h, 1, 2),
+	    d3 = tfrc_rx_hist_delta_seqno(h, 2, 3),
+	    n2 = tfrc_rx_hist_entry(h, 2)->tfrchrx_ndp,
+	    n3 = tfrc_rx_hist_entry(h, 3)->tfrchrx_ndp;
+
+	if (d2 == 1 || n2 >= d2) {	/* S2 is successor to S1 */
+
+		if (d3 == 1 || n3 >= d3) {
+			/* S3 is successor of S2: entire hole is filled */
+			h->loss_start = tfrc_rx_hist_index(h, 3);
+			h->loss_count = 0;
+		} else {
+			/* gap between S2 and S3 */
+			h->loss_start = tfrc_rx_hist_index(h, 2);
+			h->loss_count = 1;
+		}
+
+	} else {			/* gap between S1 and S2 */
+		h->loss_start = tfrc_rx_hist_index(h, 1);
+		h->loss_count = 2;
+	}
+}
+
+/**
+ *  tfrc_rx_handle_loss  -  Loss detection and further processing
+ *  @h:		    The non-empty RX history object
+ *  @lh:	    Loss Intervals database to update
+ *  @skb:	    Currently received packet
+ *  @ndp:	    The NDP count belonging to @skb
+ *  @calc_first_li: Caller-dependent computation of first loss interval in @lh
+ *  @sk:	    Used by @calc_first_li (see tfrc_lh_interval_add)
+ *  Chooses action according to pending loss, updates LI database when a new
+ *  loss was detected, and does required post-processing. Returns 1 when caller
+ *  should send feedback, 0 otherwise.
+ */
+int tfrc_rx_handle_loss(struct tfrc_rx_hist *h,
+			struct tfrc_loss_hist *lh,
+			struct sk_buff *skb, u32 ndp,
+			u32 (*calc_first_li)(struct sock *), struct sock *sk)
+{
+	int is_new_loss = 0;
+
+	if (h->loss_count == 1) {
+		__one_after_loss(h, skb, ndp);
+	} else if (h->loss_count != 2) {
+		DCCP_BUG("invalid loss_count %d", h->loss_count);
+	} else if (__two_after_loss(h, skb, ndp)) {
+		/*
+		 * Update Loss Interval database and recycle RX records
+		 */
+		is_new_loss = tfrc_lh_interval_add(lh, h, calc_first_li, sk);
+		__three_after_loss(h);
+	}
+	return is_new_loss;
+}
+EXPORT_SYMBOL_GPL(tfrc_rx_handle_loss);
+
 int tfrc_rx_hist_alloc(struct tfrc_rx_hist *h)
 {
 	int i;