[DCCP] ccid3: 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

Commiter note: renamed TFRC_t_mbi to TFRC_T_MBI, to follow Linux coding style.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

2 files changed, 37 insertions, 33 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 05513f3df652..aa5440ee20ae 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -128,7 +128,8 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
  *          X = max(min(2 * X, 2 * X_recv), s / R);
  *          tld = now;
  */ 
-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;
@@ -138,23 +139,20 @@ static void ccid3_hc_tx_update_x(struct sock *sk)
                 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;
+                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);
 }
@@ -196,12 +194,9 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
         
         switch (hctx->ccid3hctx_state) {
         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);
+                /* RFC 3448, 4.4: Halve send rate directly */
+                hctx->ccid3hctx_x = min_t(u32, hctx->ccid3hctx_x / 2,
+                                               hctx->ccid3hctx_s / TFRC_T_MBI);
 
                 ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %d "
                                "bytes/s\n",
@@ -221,6 +216,8 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                 if (!hctx->ccid3hctx_idle ||
                     (hctx->ccid3hctx_x_recv >=
                      4 * usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_rtt))) {
+                        struct timeval now;
+
                         ccid3_pr_debug("%s, sk=%p, state=%s, not idle\n",
                                        dccp_role(sk), sk,
                                        ccid3_tx_state_name(hctx->ccid3hctx_state));
@@ -238,12 +235,13 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                         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
@@ -473,11 +471,21 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                  * q is a constant, RFC 3448 recomments 0.9
                  */
                 if (hctx->ccid3hctx_state == TFRC_SSTATE_NO_FBACK) {
+                        /* Use 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_rtt  = r_sample;
+                        hctx->ccid3hctx_x    = usecs_div(w_init, r_sample);
+                        hctx->ccid3hctx_t_ld = now;
+
+                        ccid3_update_send_time(hctx);
                         ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
-                        hctx->ccid3hctx_rtt = r_sample;
-                } else
+                } else {
                         hctx->ccid3hctx_rtt = (hctx->ccid3hctx_rtt * 9) / 10 +
                                               r_sample / 10;
+                        ccid3_hc_tx_update_x(sk, &now);
+                }
 
                 ccid3_pr_debug("%s, sk=%p, New RTT estimate=%uus, "
                                "r_sample=%us\n", dccp_role(sk), sk,
@@ -502,9 +510,6 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                 /* unschedule no feedback timer */
                 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);
-
                 /* remove all packets older than the one acked from history */
                 dccp_tx_hist_purge_older(ccid3_tx_hist,
                                          &hctx->ccid3hctx_hist, packet);
@@ -514,7 +519,6 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                  */
                 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). */
diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h
index dbb884426dfa..27cb20ae1da8 100644
--- 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]: backoff interval in seconds */
+#define TFRC_T_MBI                 64
 
 #define TFRC_SMALLEST_P            40