[DCCP] ccid3: Shift window counter computation

 This puts the window counter computation [RFC 4342, 8.1] into a separate
 function which is called whenever a new packet is ready for immediate
 transmission in ccid3_hc_tx_send_packet.

Justification:

 The window counter update was previously computed after the packet was sent. This has
 two drawbacks, both fixed by this patch:
   1) re-compute another timestamp almost directly after the packet was sent (expensive),
   2) the CCVal for the window counter is needed at the instant the packet is sent.

Further details:

 The initialisation of the window counter is left in the state NO_SENT, as before.
 The algorithm will do nothing if either RTT is initialised to 0 (which is ok) or if
 the RTT value remains below 4 microseconds (which is almost pathological).

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, 29 insertions, 20 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 122a716eb877..1a2ffa1d8794 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -161,6 +161,33 @@ static inline void ccid3_hc_tx_update_s(struct ccid3_hc_tx_sock *hctx, int len)
          */
 }
 
+/*
+ *      Update Window Counter using the algorithm from [RFC 4342, 8.1].
+ *      The algorithm is not applicable if RTT < 4 microseconds.
+ */
+static inline void ccid3_hc_tx_update_win_count(struct ccid3_hc_tx_sock *hctx,
+                                                struct timeval *now)
+{
+        suseconds_t delta;
+        u32 quarter_rtts;
+
+        if (unlikely(hctx->ccid3hctx_rtt < 4))  /* avoid divide-by-zero */
+                return;
+
+        delta = timeval_delta(now, &hctx->ccid3hctx_t_last_win_count);
+        DCCP_BUG_ON(delta < 0);
+
+        quarter_rtts = (u32)delta / (hctx->ccid3hctx_rtt / 4);
+
+        if (quarter_rtts > 0) {
+                hctx->ccid3hctx_t_last_win_count = *now;
+                hctx->ccid3hctx_last_win_count  += min_t(u32, quarter_rtts, 5);
+                hctx->ccid3hctx_last_win_count  &= 0xF;         /* mod 16 */
+
+                ccid3_pr_debug("now at %#X\n", hctx->ccid3hctx_last_win_count);
+        }
+}
+
 static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
 {
         struct sock *sk = (struct sock *)data;
@@ -333,6 +360,8 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
                  */
                 if (delay - (suseconds_t)hctx->ccid3hctx_delta >= 0)
                         return delay / 1000L;
+
+                ccid3_hc_tx_update_win_count(hctx, &now);
                 break;
         case TFRC_SSTATE_TERM:
                 DCCP_BUG("Illegal %s state TERM, sk=%p", dccp_role(sk), sk);
@@ -353,7 +382,6 @@ static void ccid3_hc_tx_packet_sent(struct sock *sk, int more, unsigned int len)
         const struct dccp_sock *dp = dccp_sk(sk);
         struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
         struct timeval now;
-        suseconds_t quarter_rtt;
         struct dccp_tx_hist_entry *packet;
 
         BUG_ON(hctx == NULL);
@@ -373,25 +401,6 @@ static void ccid3_hc_tx_packet_sent(struct sock *sk, int more, unsigned int len)
         }
         packet->dccphtx_tstamp = now;
         packet->dccphtx_seqno  = dp->dccps_gss;
-        /*
-         * Check if win_count have changed
-         * Algorithm in "8.1. Window Counter Value" in RFC 4342.
-         */
-        quarter_rtt = timeval_delta(&now, &hctx->ccid3hctx_t_last_win_count);
-        if (likely(hctx->ccid3hctx_rtt > 8))
-                quarter_rtt /= hctx->ccid3hctx_rtt / 4;
-
-        if (quarter_rtt > 0) {
-                hctx->ccid3hctx_t_last_win_count = now;
-                hctx->ccid3hctx_last_win_count   = (hctx->ccid3hctx_last_win_count +
-                                                    min_t(unsigned long, quarter_rtt, 5)) % 16;
-                ccid3_pr_debug("%s, sk=%p, window changed from "
-                               "%u to %u!\n",
-                               dccp_role(sk), sk,
-                               packet->dccphtx_ccval,
-                               hctx->ccid3hctx_last_win_count);
-        }
-
         hctx->ccid3hctx_idle = 0;
         packet->dccphtx_rtt  = hctx->ccid3hctx_rtt;
         packet->dccphtx_sent = 1;