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>
diff --git a/net/dccp/ccids/ccid2.h b/net/dccp/ccids/ccid2.h
index da021eb..f585d33 100644
--- a/net/dccp/ccids/ccid2.h
+++ b/net/dccp/ccids/ccid2.h
@@ -53,6 +53,10 @@
* @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 @@
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;