dccp ccid-2: Perform congestion-window validation

CCID-2's cwnd increases like TCP during slow-start, which has implications for
 * the local Sequence Window value (should be > cwnd),
 * the Ack Ratio value.
Hence an exponential growth, if it does not reflect the actual network
conditions, can quickly lead to instability.

This patch adds congestion-window validation (RFC2861) to CCID-2:
 * cwnd is constrained if the sender is application limited;
 * cwnd is reduced after a long idle period, as suggested in the '90 paper
   by Van Jacobson, in RFC 2581 (sec. 4.1);
 * cwnd is never reduced below the RFC 3390 initial window.

As marked in the comments, the code is actually almost a direct copy of the
TCP congestion-window-validation algorithms. By continuing this work, it may
in future be possible to use the TCP code (not possible at the moment).

The mechanism can be turned off using a module parameter. Sampling of the
currently-used window (moving-maximum) is however done constantly; this is
used to determine the expected window, which can be exploited to regulate
DCCP's Sequence Window value.

This patch also sets slow-start-after-idle (RFC 4341, 5.1), i.e. it behaves like
TCP when net.ipv4.tcp_slow_start_after_idle = 1.

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

1 files changed, 10 insertions, 0 deletions

diff --git a/net/dccp/ccids/ccid2.h b/net/dccp/ccids/ccid2.h
index da021ebce9af..f585d330e1e5 100644
--- a/net/dccp/ccids/ccid2.h
+++ b/net/dccp/ccids/ccid2.h
@@ -53,6 +53,10 @@ struct ccid2_seq {
  * @tx_rttvar:		     moving average/maximum of @mdev_max
  * @tx_rto:		     RTO value deriving from SRTT and RTTVAR (RFC 2988)
  * @tx_rtt_seq:		     to decay RTTVAR at most once per flight
+ * @tx_cwnd_used:	     actually used cwnd, W_used of RFC 2861
+ * @tx_expected_wnd:	     moving average of @tx_cwnd_used
+ * @tx_cwnd_stamp:	     to track idle periods in CWV
+ * @tx_lsndtime:	     last time (in jiffies) a data packet was sent
  * @tx_rpseq:		     last consecutive seqno
  * @tx_rpdupack:	     dupacks since rpseq
  * @tx_av_chunks:	     list of Ack Vectors received on current skb
@@ -76,6 +80,12 @@ struct ccid2_hc_tx_sock {
 	u64			tx_rtt_seq:48;
 	struct timer_list	tx_rtotimer;
 
+	/* Congestion Window validation (optional, RFC 2861) */
+	u32			tx_cwnd_used,
+				tx_expected_wnd,
+				tx_cwnd_stamp,
+				tx_lsndtime;
+
 	u64			tx_rpseq;
 	int			tx_rpdupack;
 	u32			tx_last_cong;