[DCCP] ccid3: Fix bug in calculation of send rate

The main object of this patch is the following bug:
 ==> In ccid3_hc_tx_packet_recv, the parameters p and X_recv were updated
     _after_ the send rate was calculated. This is clearly an error and is
     resolved by re-ordering statements.

In addition,
  * r_sample is converted from u32 to long to check whether the time difference
    was negative (it would otherwise be converted to a large u32 value)
  * protection against RTT=0 (this is possible) is provided in a further patch
  * t_elapsed is also converted to long, to match the type of r_sample
  * adds a a more debugging information regarding current send rates
  * various trivial comment/documentation updates

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>

1 files changed, 52 insertions, 37 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 22a07248c240..bd353044c547 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -121,12 +121,15 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
 /*
  * Update X by
  *    If (p > 0)
- *       x_calc = calcX(s, R, p);
+ *       X_calc = calcX(s, R, p);
  *       X = max(min(X_calc, 2 * X_recv), s / t_mbi);
  *    Else
  *       If (now - tld >= R)
  *          X = max(min(2 * X, 2 * X_recv), s / R);
  *          tld = now;
+ *
+ * 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, struct timeval *now)
 
@@ -413,10 +416,8 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
         struct dccp_tx_hist_entry *packet;
         struct timeval now;
         unsigned long t_nfb;
-        u32 t_elapsed;
         u32 pinv;
-        u32 x_recv;
-        u32 r_sample;
+        long r_sample, t_elapsed;
 
         BUG_ON(hctx == NULL);
 
@@ -427,31 +428,51 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 
         opt_recv = &hctx->ccid3hctx_options_received;
 
-        t_elapsed = dp->dccps_options_received.dccpor_elapsed_time * 10;
-        x_recv = opt_recv->ccid3or_receive_rate;
-        pinv = opt_recv->ccid3or_loss_event_rate;
-
         switch (hctx->ccid3hctx_state) {
         case TFRC_SSTATE_NO_FBACK:
         case TFRC_SSTATE_FBACK:
-                /* Calculate new round trip sample by
-                 * R_sample = (now - t_recvdata) - t_delay */
-                /* get t_recvdata from history */
+                /* get packet from history to look up t_recvdata */
                 packet = dccp_tx_hist_find_entry(&hctx->ccid3hctx_hist,
                                                  DCCP_SKB_CB(skb)->dccpd_ack_seq);
                 if (unlikely(packet == NULL)) {
-                        DCCP_WARN("%s, sk=%p, seqno %llu(%s) does't exist "
+                        DCCP_WARN("%s(%p), seqno %llu(%s) doesn't exist "
                                   "in history!\n",  dccp_role(sk), sk,
                             (unsigned long long)DCCP_SKB_CB(skb)->dccpd_ack_seq,
                                   dccp_packet_name(DCCP_SKB_CB(skb)->dccpd_type));
                         return;
                 }
 
-                /* Update RTT */
+                /* Update receive rate */
+                hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
+
+                /* Update loss event rate */
+                pinv = opt_recv->ccid3or_loss_event_rate;
+                if (pinv == ~0U || pinv == 0)
+                        hctx->ccid3hctx_p = 0;
+                else {
+                        hctx->ccid3hctx_p = 1000000 / pinv;
+
+                        if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
+                                hctx->ccid3hctx_p = TFRC_SMALLEST_P;
+                                ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
+                                               dccp_role(sk), sk);
+                        }
+                }
+
                 dccp_timestamp(sk, &now);
-                r_sample = timeval_delta(&now, &packet->dccphtx_tstamp);
-                if (unlikely(r_sample <= t_elapsed))
-                        DCCP_WARN("r_sample=%uus,t_elapsed=%uus\n",
+
+                /*
+                 * Calculate new round trip sample as per [RFC 3448, 4.3] by
+                 *      R_sample  =  (now - t_recvdata) - t_elapsed
+                 */
+                r_sample  = timeval_delta(&now, &packet->dccphtx_tstamp);
+                t_elapsed = dp->dccps_options_received.dccpor_elapsed_time * 10;
+
+                if (unlikely(r_sample <= 0)) {
+                        DCCP_WARN("WARNING: R_sample (%ld) <= 0!\n", r_sample);
+                        r_sample = 0;
+                } else if (unlikely(r_sample <= t_elapsed))
+                        DCCP_WARN("WARNING: r_sample=%ldus <= t_elapsed=%ldus\n",
                                   r_sample, t_elapsed);
                 else
                         r_sample -= t_elapsed;
@@ -474,31 +495,25 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                         hctx->ccid3hctx_t_ld = now;
 
                         ccid3_update_send_time(hctx);
-                        ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
-                } 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,
-                               hctx->ccid3hctx_rtt, r_sample);
+                        ccid3_pr_debug("%s(%p), s=%u, w_init=%u, "
+                                       "R_sample=%ldus, X=%u\n", dccp_role(sk),
+                                       sk, hctx->ccid3hctx_s, w_init, r_sample,
+                                       hctx->ccid3hctx_x);
 
-                /* Update receive rate */
-                hctx->ccid3hctx_x_recv = x_recv;/* X_recv in bytes per sec */
+                        ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
+                } else {
+                        hctx->ccid3hctx_rtt = (9 * hctx->ccid3hctx_rtt +
+                                                   (u32)r_sample        ) / 10;
 
-                /* Update loss event rate */
-                if (pinv == ~0 || pinv == 0)
-                        hctx->ccid3hctx_p = 0;
-                else {
-                        hctx->ccid3hctx_p = 1000000 / pinv;
+                        ccid3_hc_tx_update_x(sk, &now);
 
-                        if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
-                                hctx->ccid3hctx_p = TFRC_SMALLEST_P;
-                                ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
-                                               dccp_role(sk), sk);
-                        }
+                        ccid3_pr_debug("%s(%p), RTT=%uus (sample=%ldus), s=%u, "
+                                       "p=%u, X_calc=%u, X=%u\n", dccp_role(sk),
+                                       sk, hctx->ccid3hctx_rtt, r_sample,
+                                       hctx->ccid3hctx_s, hctx->ccid3hctx_p,
+                                       hctx->ccid3hctx_x_calc,
+                                       hctx->ccid3hctx_x);
                 }
 
                 /* unschedule no feedback timer */