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>

1 files changed, 14 insertions, 29 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 04b1835..8e64d966 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -556,8 +556,8 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
 			 * would bring X down to s/t_mbi. That is why we return
 			 * X_recv according to rfc3448bis-06 for the moment.
 			 */
-			u32 rtt = hcrx->rtt ? : DCCP_FALLBACK_RTT,
-			    s	= tfrc_rx_hist_packet_size(&hcrx->hist);
+			u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
+			    rtt = tfrc_rx_hist_rtt(&hcrx->hist);
 
 			hcrx->x_recv = scaled_div32(s, 2 * rtt);
 			break;
@@ -576,6 +576,11 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk,
 			break;
 		/* fall through */
 	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.
+		 */
 		delta = ktime_us_delta(now, hcrx->tstamp_last_feedback);
 		if (delta <= 0)
 			DCCP_BUG("delta (%ld) <= 0", (long)delta);
@@ -633,8 +638,8 @@ static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
 static u32 ccid3_first_li(struct sock *sk)
 {
 	struct ccid3_hc_rx_sock *hcrx = ccid3_hc_rx_sk(sk);
-	u32 x_recv, p, delta,
-	    s = tfrc_rx_hist_packet_size(&hcrx->hist);
+	u32 s = tfrc_rx_hist_packet_size(&hcrx->hist),
+	    rtt = tfrc_rx_hist_rtt(&hcrx->hist), x_recv, p, delta;
 	u64 fval;
 
 	/*
@@ -645,11 +650,6 @@ static u32 ccid3_first_li(struct sock *sk)
 	if (unlikely(hcrx->feedback == CCID3_FBACK_NONE))
 		return 5;
 
-	if (hcrx->rtt == 0) {
-		DCCP_WARN("No RTT estimate available, using fallback RTT\n");
-		hcrx->rtt = DCCP_FALLBACK_RTT;
-	}
-
 	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 */
@@ -661,7 +661,7 @@ static u32 ccid3_first_li(struct sock *sk)
 		x_recv = hcrx->x_recv;
 	}
 
-	fval = scaled_div32(scaled_div(s, hcrx->rtt), x_recv);
+	fval = scaled_div32(scaled_div(s, rtt), x_recv);
 	p = tfrc_calc_x_reverse_lookup(fval);
 
 	ccid3_pr_debug("%s(%p), receive rate=%u bytes/s, implied "
@@ -696,25 +696,10 @@ static void ccid3_hc_rx_packet_recv(struct sock *sk, struct sk_buff *skb)
 		return; /* done receiving */
 
 	/*
-	 * Handle data packets: RTT sampling and monitoring p
-	 */
-	if (unlikely(!is_data_packet))
-		goto update_records;
-
-	if (!tfrc_lh_is_initialised(&hcrx->li_hist)) {
-		const u32 sample = tfrc_rx_hist_sample_rtt(&hcrx->hist, skb);
-		/*
-		 * Empty loss history: no loss so far, hence p stays 0.
-		 * Sample RTT values, since an RTT estimate is required for the
-		 * computation of p when the first loss occurs; RFC 3448, 6.3.1.
-		 */
-		if (sample != 0)
-			hcrx->rtt = tfrc_ewma(hcrx->rtt, sample, 9);
-	}
-	/*
 	 * Check if the periodic once-per-RTT feedback is due; RFC 4342, 10.3
 	 */
-	if (SUB16(dccp_hdr(skb)->dccph_ccval, hcrx->last_counter) > 3)
+	if (is_data_packet &&
+	    SUB16(dccp_hdr(skb)->dccph_ccval, hcrx->last_counter) > 3)
 		do_feedback = CCID3_FBACK_PERIODIC;
 
 update_records:
@@ -744,7 +729,7 @@ static void ccid3_hc_rx_exit(struct sock *sk)
 static void ccid3_hc_rx_get_info(struct sock *sk, struct tcp_info *info)
 {
 	info->tcpi_options  |= TCPI_OPT_TIMESTAMPS;
-	info->tcpi_rcv_rtt  = ccid3_hc_rx_sk(sk)->rtt;
+	info->tcpi_rcv_rtt  = tfrc_rx_hist_rtt(&ccid3_hc_rx_sk(sk)->hist);
 }
 
 static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
@@ -759,7 +744,7 @@ static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
 		if (len < sizeof(rx_info))
 			return -EINVAL;
 		rx_info.tfrcrx_x_recv = hcrx->x_recv;
-		rx_info.tfrcrx_rtt    = hcrx->rtt;
+		rx_info.tfrcrx_rtt    = tfrc_rx_hist_rtt(&hcrx->hist);
 		rx_info.tfrcrx_p      = tfrc_invert_loss_event_rate(hcrx->p_inverse);
 		len = sizeof(rx_info);
 		val = &rx_info;