dccp tfrc/ccid-3: Computing Loss Rate from Loss Event Rate

This adds a function to take care of the following cases occurring in the
computation of the Loss Rate p:

 * 1/(2^32-1) is mapped into 0% as per RFC 4342, 8.5;
 * 1/0        is mapped into the maximum of 100%;
 * we want to avoid that p = 1/x is rounded down to 0 when x is very large,
   since this means accidentally re-entering slow-start (indicated by p==0).

In the last case, the minimum-resolution value of p is returned.

Furthermore, a bug in ccid3_hc_rx_getsockopt is fixed (1/0 was mapped into ~0U),
which now allows to consistently print the scaled p-values as

        printf("Loss Event Rate = %u.%04u %%\n", rx_info.tfrcrx_p / 10000, 
                                                 rx_info.tfrcrx_p % 10000);

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

3 files changed, 20 insertions, 7 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 4c422fb2189..206204551f4 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -410,10 +410,10 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 
         /* Update loss event rate (which is scaled by 1e6) */
         pinv = opt_recv->ccid3or_loss_event_rate;
-        if (pinv == ~0U || pinv == 0)          /* see RFC 4342, 8.5   */
+        if (pinv == 0)
                 hctx->p = 0;
-        else                                   /* can not exceed 100% */
-                hctx->p = scaled_div(1, pinv);
+        else
+                hctx->p = tfrc_invert_loss_event_rate(pinv);
 
         /*
          * Update allowed sending rate X as per draft rfc3448bis-00, 4.2/3
@@ -854,8 +854,7 @@ static int ccid3_hc_rx_getsockopt(struct sock *sk, const int optname, int len,
                         return -EINVAL;
                 rx_info.tfrcrx_x_recv = hcrx->x_recv;
                 rx_info.tfrcrx_rtt    = hcrx->rtt;
-                rx_info.tfrcrx_p      = hcrx->p_inverse == 0 ? ~0U :
-                                           scaled_div(1, hcrx->p_inverse);
+                rx_info.tfrcrx_p      = tfrc_invert_loss_event_rate(hcrx->p_inverse);
                 len = sizeof(rx_info);
                 val = &rx_info;
                 break;
diff --git a/net/dccp/ccids/lib/tfrc.h b/net/dccp/ccids/lib/tfrc.h
index ed9857527ac..bb47146ac7d 100644
--- a/net/dccp/ccids/lib/tfrc.h
+++ b/net/dccp/ccids/lib/tfrc.h
@@ -58,6 +58,7 @@ static inline u32 tfrc_ewma(const u32 avg, const u32 newval, const u8 weight)
 
 extern u32  tfrc_calc_x(u16 s, u32 R, u32 p);
 extern u32  tfrc_calc_x_reverse_lookup(u32 fvalue);
+extern u32  tfrc_invert_loss_event_rate(u32 loss_event_rate);
 
 extern int  tfrc_tx_packet_history_init(void);
 extern void tfrc_tx_packet_history_exit(void);
diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 2f20a29cffe..bc3dc2b2a84 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -658,7 +658,6 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
         result = scaled_div(s, R);
         return scaled_div32(result, f);
 }
-
 EXPORT_SYMBOL_GPL(tfrc_calc_x);
 
 /**
@@ -693,5 +692,19 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
         index = tfrc_binsearch(fvalue, 0);
         return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
 }
-
 EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);
+
+/**
+ * tfrc_invert_loss_event_rate  -  Compute p so that 10^6 corresponds to 100%
+ * When @loss_event_rate is large, there is a chance that p is truncated to 0.
+ * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
+ */
+u32 tfrc_invert_loss_event_rate(u32 loss_event_rate)
+{
+        if (loss_event_rate == UINT_MAX)                /* see RFC 4342, 8.5 */
+                return 0;
+        if (unlikely(loss_event_rate == 0))             /* map 1/0 into 100% */
+                return 1000000;
+        return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P);
+}
+EXPORT_SYMBOL_GPL(tfrc_invert_loss_event_rate);