dccp ccid-3: Preventing Oscillations

This implements [RFC 3448, 4.5], which performs congestion avoidance behaviour
by reducing the transmit rate as the queueing delay (measured in terms of
long-term RTT) increases.

Oscillation can be turned on/off via a module option (do_osc_prev) and via sysfs
(using mode 0644), the default is off.

Overflow analysis:
------------------
 * oscillation prevention is done after update_x(), so that t_ipi <= 64000;
 * hence the multiplication "t_ipi * sqrt(R_sample)" needs 64 bits;
 * done using u64 for sqrt_sample and explicit typecast of t_ipi;
 * the divisor, R_sqmean, is non-zero because oscillation prevention is first
   called when receiving the second feedback packet, and tfrc_scaled_rtt() > 0.

A detailed discussion of the algorithm (with plots) is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid3/sender_notes/oscillation_prevention/

The algorithm has negative side effects:
  * when allowing to decrease t_ipi (leads to a large RTT) and
  * when using it during slow-start;
both uses are therefore disabled.

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

1 files changed, 4 insertions, 2 deletions

diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h
index 342235c57bf3..af6e1bf937d9 100644
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -47,8 +47,8 @@
 /* Two seconds as per RFC 3448 4.2 */
 #define TFRC_INITIAL_TIMEOUT	   (2 * USEC_PER_SEC)
 
-/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
-#define TFRC_T_MBI		   64
+/* Maximum backoff interval t_mbi (RFC 3448, 4.3) */
+#define TFRC_T_MBI		   (64 * USEC_PER_SEC)
 
 /*
  * The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are
@@ -76,6 +76,7 @@ enum ccid3_options {
  * @x_recv - Receive rate    in 64 * bytes per second
  * @x_calc - Calculated rate in bytes per second
  * @rtt - Estimate of current round trip time in usecs
+ * @r_sqmean - Estimate of long-term RTT (RFC 3448, 4.5)
  * @p - Current loss event rate (0-1) scaled by 1000000
  * @s - Packet size in bytes
  * @t_rto - Nofeedback Timer setting in usecs
@@ -94,6 +95,7 @@ struct ccid3_hc_tx_sock {
 	u64				x_recv;
 	u32				x_calc;
 	u32				rtt;
+	u16				r_sqmean;
 	u32				p;
 	u32				t_rto;
 	u32				t_ipi;