[PATCH 8/10]: Larger initial windows

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

[CCID 3]: Larger initial windows

This implements the larger-initial-windows feature for CCID 3, as described
in section 5 of RFC 4342. When the first feedback packet arrives, the sender
can send up to 2..4 packets per RTT, instead of just one. 

The patch further
 * reduces the number of timestamping calls by passing the timestamp value
   (which is computed in one of the calling functions anyway) as argument

 * renames one constant with a very long name into one which is shorter and 
   resembles the one in RFC 3448 (t_mbi)

 * simplifies some of the min_t/max_t cases where both `x', `y' have the same
   type

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
---
 net/dccp/ccids/ccid3.c |   75 +++++++++++++++++++++++++++----------------------
 net/dccp/ccids/ccid3.h |    4 +-
 2 files changed, 44 insertions(+), 35 deletions(-)

--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -129,35 +129,42 @@ static inline void ccid3_update_send_tim
  * If X has changed, we also update the scheduled send time t_now,
  * the inter-packet interval t_ipi, and the delta value.
  */ 
-static void ccid3_hc_tx_update_x(struct sock *sk)
+static void ccid3_hc_tx_update_x(struct sock *sk, struct timeval *now)
+
 {
 	struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
 	const __u32 old_x = hctx->ccid3hctx_x;
 
 	DCCP_BUG_ON(hctx->ccid3hctx_rtt = 0);
 
-	/* To avoid large error in calcX */
-	if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
-		hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
-						     hctx->ccid3hctx_rtt,
-						     hctx->ccid3hctx_p);
-		hctx->ccid3hctx_x = max_t(u32, min_t(u32, hctx->ccid3hctx_x_calc,
-							  2 * hctx->ccid3hctx_x_recv),
-					       (hctx->ccid3hctx_s /
-					        TFRC_MAX_BACK_OFF_TIME));
-	} else {
-		struct timeval now;
+	/* Compute X after the first feedback packet has arrived */
+	if (hctx->ccid3hctx_state = TFRC_SSTATE_NO_FBACK) {
+		/* This implements `Larger Initial Windows' [RFC 4342, sec. 5]
+		 * We deviate in that we use `s' instead of `MSS'.           */
+		u16 w_init = max(     4 * hctx->ccid3hctx_s,
+				 max( 2 * hctx->ccid3hctx_s, 4380));
+		hctx->ccid3hctx_x    = usecs_div(w_init, hctx->ccid3hctx_rtt);
+		hctx->ccid3hctx_t_ld = *now;
+
+ 	/* To avoid large error in calcX */
+	} else if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
+ 		hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
+ 						     hctx->ccid3hctx_rtt,
+ 						     hctx->ccid3hctx_p);
+		hctx->ccid3hctx_x = max_t(u32, min(hctx->ccid3hctx_x_calc,
+						   hctx->ccid3hctx_x_recv * 2),
+					       hctx->ccid3hctx_s / TFRC_t_mbi );
+
+	} else if (timeval_delta(now, &hctx->ccid3hctx_t_ld) >+							  hctx->ccid3hctx_rtt) {
+		hctx->ccid3hctx_x = max(min(hctx->ccid3hctx_x_recv,
+					    hctx->ccid3hctx_x      ) * 2,
+					usecs_div(hctx->ccid3hctx_s,
+					       	  hctx->ccid3hctx_rtt)   );
+		hctx->ccid3hctx_t_ld = *now;
+	} else
+		ccid3_pr_debug("Not changing X\n");
 
-		dccp_timestamp(sk, &now);
-	       	if (timeval_delta(&now, &hctx->ccid3hctx_t_ld) >-		    hctx->ccid3hctx_rtt) {
-			hctx->ccid3hctx_x = max_t(u32, min_t(u32, hctx->ccid3hctx_x_recv,
-								  hctx->ccid3hctx_x) * 2,
-						       usecs_div(hctx->ccid3hctx_s,
-							       	 hctx->ccid3hctx_rtt));
-			hctx->ccid3hctx_t_ld = now;
-		}
-	}
 	if(hctx->ccid3hctx_x != old_x)
 		ccid3_update_send_time(hctx);
 }
@@ -185,6 +192,7 @@ static void ccid3_hc_tx_no_feedback_time
 	struct sock *sk = (struct sock *)data;
 	struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
 	unsigned long next_tmout = USEC_PER_SEC / 5;
+	struct timeval now;
 
 	bh_lock_sock(sk);
 	if (sock_owned_by_user(sk)) {
@@ -200,10 +208,8 @@ static void ccid3_hc_tx_no_feedback_time
 	case TFRC_SSTATE_NO_FBACK:
 		/* Halve send rate */
 		hctx->ccid3hctx_x /= 2;
-		if (hctx->ccid3hctx_x < (hctx->ccid3hctx_s /
-					 TFRC_MAX_BACK_OFF_TIME))
-			hctx->ccid3hctx_x = (hctx->ccid3hctx_s /
-					     TFRC_MAX_BACK_OFF_TIME);
+		if (hctx->ccid3hctx_x < (hctx->ccid3hctx_s / TFRC_t_mbi))
+			hctx->ccid3hctx_x = hctx->ccid3hctx_s / TFRC_t_mbi;
 
 		ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %d "
 			       "bytes/s\n",
@@ -240,12 +246,13 @@ static void ccid3_hc_tx_no_feedback_time
 			if (hctx->ccid3hctx_p < TFRC_SMALLEST_P ||
 			    hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv)
 				hctx->ccid3hctx_x_recv = max_t(u32, hctx->ccid3hctx_x_recv / 2,
-								    hctx->ccid3hctx_s / (2 * TFRC_MAX_BACK_OFF_TIME));
+								    hctx->ccid3hctx_s / (2 * TFRC_t_mbi));
 			else
 				hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc / 4;
 
 			/* Update sending rate */
-			ccid3_hc_tx_update_x(sk);
+			dccp_timestamp(sk, &now);
+			ccid3_hc_tx_update_x(sk, &now);
 		}
 		/*
 		 * Schedule no feedback timer to expire in
@@ -488,10 +495,9 @@ static void ccid3_hc_tx_packet_recv(stru
 		 *
 		 * q is a constant, RFC 3448 recomments 0.9
 		 */
-		if (hctx->ccid3hctx_state = TFRC_SSTATE_NO_FBACK) {
-			ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
+		if (hctx->ccid3hctx_state = TFRC_SSTATE_NO_FBACK)
 			hctx->ccid3hctx_rtt = r_sample;
-		} else
+		else
 			hctx->ccid3hctx_rtt = (hctx->ccid3hctx_rtt * 9) / 10 +
 					      r_sample / 10;
 
@@ -508,7 +514,11 @@ static void ccid3_hc_tx_packet_recv(stru
 		sk_stop_timer(sk, &hctx->ccid3hctx_no_feedback_timer);
 
 		/* Update sending rate (and likely t_ipi, t_nom, and delta) */
-		ccid3_hc_tx_update_x(sk);
+		ccid3_hc_tx_update_x(sk, &now);
+
+		/* Update the state if necessary */
+		if (hctx->ccid3hctx_state = TFRC_SSTATE_NO_FBACK)
+			ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
 
 		/* remove all packets older than the one acked from history */
 		dccp_tx_hist_purge_older(ccid3_tx_hist,
@@ -519,7 +529,6 @@ static void ccid3_hc_tx_packet_recv(stru
 		 */
 		sk->sk_write_space(sk);
 
-
 		/* Update timeout interval. We use the alternative variant of
 		 * [RFC 3448, 3.1] which sets the upper bound of t_rto to one
 		 * second, as it is suggested for TCP (see RFC 2988, 2.4). */
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -48,8 +48,8 @@
 /* In usecs - half the scheduling granularity as per RFC3448 4.6 */
 #define TFRC_OPSYS_HALF_TIME_GRAN  (USEC_PER_SEC / (2 * HZ))
 
-/* In seconds */
-#define TFRC_MAX_BACK_OFF_TIME	   64
+/* Parameter t_mbi from [RFC 3448, 4.3]: In seconds */
+#define TFRC_t_mbi		   64
 
 #define TFRC_SMALLEST_P		   40