[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>

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 05513f3..aa5440e 100644 (file)
--- 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 dbb8844..27cb20a 100644 (file)
--- 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