[CCID3]: Wrap computation of RFC3390-initial rate into separate function

The CCID 3 and TFRC specs (RFC 4342, RFC 3448, draft-3448bis) make frequent
reference to the computation of the RFC-3390 initial sending rate:

  1. Initial sending rate when RTT is known (RFC 4342, p. 6)
  2. Response to Idle/Application-Limited periods (RFC 4342, 5.1)

This warrants putting the code into its own function, for later code reuse.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 7 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index dc6e41883315..8d33a09608e9 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -82,6 +82,21 @@ static void ccid3_hc_tx_set_state(struct sock *sk,
 }
 
 /*
+ * Compute the initial sending rate X_init according to RFC 3390:
+ *      w_init   =    min(4 * MSS, max(2 * MSS, 4380 bytes))
+ *      X_init   =    w_init / RTT
+ * For consistency with other parts of the code, X_init is scaled by 2^6.
+ */
+static inline u64 rfc3390_initial_rate(struct sock *sk)
+{
+        const struct dccp_sock *dp = dccp_sk(sk);
+        const __u32 w_init = min(4 * dp->dccps_mss_cache,
+                                 max(2 * dp->dccps_mss_cache, 4380U));
+
+        return scaled_div(w_init << 6, ccid3_hc_tx_sk(sk)->ccid3hctx_rtt);
+}
+
+/*
  * Recalculate t_ipi and delta (should be called whenever X changes)
  */
 static inline void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
@@ -469,20 +484,16 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                         /*
                          * Larger Initial Windows [RFC 4342, sec. 5]
                          */
-                        __u32 w_init = min(4 * dp->dccps_mss_cache,
-                                           max(2 * dp->dccps_mss_cache, 4380U));
                         hctx->ccid3hctx_rtt  = r_sample;
-                        hctx->ccid3hctx_x    = scaled_div(w_init << 6, r_sample);
+                        hctx->ccid3hctx_x    = rfc3390_initial_rate(sk);
                         hctx->ccid3hctx_t_ld = now;
 
                         ccid3_update_send_interval(hctx);
 
-                        ccid3_pr_debug("%s(%p), s=%u, MSS=%u, w_init=%u, "
+                        ccid3_pr_debug("%s(%p), s=%u, MSS=%u, "
                                        "R_sample=%dus, X=%u\n", dccp_role(sk),
                                        sk, hctx->ccid3hctx_s,
-                                       dp->dccps_mss_cache,
-                                       w_init,
-                                       (int)r_sample,
+                                       dp->dccps_mss_cache, (int)r_sample,
                                        (unsigned)(hctx->ccid3hctx_x >> 6));
 
                         ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);