[CCID3]: Use initial RTT sample from SYN exchange

The patch follows the following recommendation made in an erratum to RFC 4342:

  "Senders MAY additionally make use of other available RTT measurements,
   including those from the initial Request-Response packet exchange."

It implements larger initial windows with regard to this inital RTT measurement,
using the mechanism suggested in draft-ietf-dccp-rfc3448bis, section 4.2.

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>

net/dccp/ccids/ccid3.c

@@ -334,19 +334,30 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
 				usecs_to_jiffies(TFRC_INITIAL_TIMEOUT)));
 		hctx->ccid3hctx_last_win_count	 = 0;
 		hctx->ccid3hctx_t_last_win_count = now;
-		ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);
-
-		/* Set initial sending rate X/s to 1pps (X is scaled by 2^6) */
-		hctx->ccid3hctx_x = hctx->ccid3hctx_s = skb->len;
-		hctx->ccid3hctx_x <<= 6;
-
-		/* First timeout, according to [RFC 3448, 4.2], is 1 second */
-		hctx->ccid3hctx_t_ipi = USEC_PER_SEC;
-		/* Initial delta: minimum of 0.5 sec and t_gran/2 */
-		hctx->ccid3hctx_delta = TFRC_OPSYS_HALF_TIME_GRAN;
 
 		/* Set t_0 for initial packet */
 		hctx->ccid3hctx_t_nom = now;
+
+		hctx->ccid3hctx_s = skb->len;
+
+		/*
+		 * Use initial RTT sample when available: recommended by erratum
+		 * to RFC 4342. This implements the initialisation procedure of
+		 * draft rfc3448bis, section 4.2. Remember, X is scaled by 2^6.
+		 */
+		if (dp->dccps_syn_rtt) {
+			ccid3_pr_debug("SYN RTT = %uus\n", dp->dccps_syn_rtt);
+			hctx->ccid3hctx_rtt  = dp->dccps_syn_rtt;
+			hctx->ccid3hctx_x    = rfc3390_initial_rate(sk);
+			hctx->ccid3hctx_t_ld = now;
+		} else {
+			/* Sender does not have RTT sample: X = MSS/second */
+			hctx->ccid3hctx_x = dp->dccps_mss_cache;
+			hctx->ccid3hctx_x <<= 6;
+		}
+		ccid3_update_send_interval(hctx);
+
+		ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);
 		break;
 	case TFRC_SSTATE_NO_FBACK:
 	case TFRC_SSTATE_FBACK: