dccp ccid-2: prevent cwnd > Sequence Window

Add a check to prevent CCID-2 from increasing the cwnd greater than the
Sequence Window.

When the congestion window becomes bigger than the Sequence Window, CCID-2
will attempt to keep more data in the network than the DCCP Sequence Window
code considers possible. This results in the Sequence Window code issuing
a Sync, thereby inducing needless overhead. Further, if this occurs at the
sender, CCID-2 will never detect the problem because the Acks it receives
will indicate no losses. I have seen this cause a drop of 1/3rd in throughput
for a connection.

Also add code to adjust the Sequence Window to be about 5 times the number of
packets in the network (RFC 4340, 7.5.2) and to adjust the Ack Ratio so that
the remote Sequence Window will hold about 5 times the number of packets in
the network. This allows the congestion window to increase correctly without
being limited by the Sequence Window.

Signed-off-by: Samuel Jero <sj323707@ohio.edu>
Acked-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

2 files changed, 41 insertions, 15 deletions

diff --git a/net/dccp/ccids/ccid2.c b/net/dccp/ccids/ccid2.c
index b51cc92376da..9dbc4d88af16 100644
--- a/net/dccp/ccids/ccid2.c
+++ b/net/dccp/ccids/ccid2.c
@@ -85,7 +85,6 @@ static int ccid2_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
 
 static void ccid2_change_l_ack_ratio(struct sock *sk, u32 val)
 {
-        struct dccp_sock *dp = dccp_sk(sk);
         u32 max_ratio = DIV_ROUND_UP(ccid2_hc_tx_sk(sk)->tx_cwnd, 2);
 
         /*
@@ -98,14 +97,15 @@ static void ccid2_change_l_ack_ratio(struct sock *sk, u32 val)
                 DCCP_WARN("Limiting Ack Ratio (%u) to %u\n", val, max_ratio);
                 val = max_ratio;
         }
-        if (val > DCCPF_ACK_RATIO_MAX)
-                val = DCCPF_ACK_RATIO_MAX;
-
-        if (val == dp->dccps_l_ack_ratio)
-                return;
+        dccp_feat_signal_nn_change(sk, DCCPF_ACK_RATIO,
+                                   min_t(u32, val, DCCPF_ACK_RATIO_MAX));
+}
 
-        ccid2_pr_debug("changing local ack ratio to %u\n", val);
-        dp->dccps_l_ack_ratio = val;
+static void ccid2_change_l_seq_window(struct sock *sk, u64 val)
+{
+        dccp_feat_signal_nn_change(sk, DCCPF_SEQUENCE_WINDOW,
+                                   clamp_val(val, DCCPF_SEQ_WMIN,
+                                                  DCCPF_SEQ_WMAX));
 }
 
 static void ccid2_hc_tx_rto_expire(unsigned long data)
@@ -405,17 +405,37 @@ static void ccid2_new_ack(struct sock *sk, struct ccid2_seq *seqp,
                           unsigned int *maxincr)
 {
         struct ccid2_hc_tx_sock *hc = ccid2_hc_tx_sk(sk);
-
-        if (hc->tx_cwnd < hc->tx_ssthresh) {
-                if (*maxincr > 0 && ++hc->tx_packets_acked == 2) {
+        struct dccp_sock *dp = dccp_sk(sk);
+        int r_seq_used = hc->tx_cwnd / dp->dccps_l_ack_ratio;
+
+        if (hc->tx_cwnd < dp->dccps_l_seq_win &&
+            r_seq_used < dp->dccps_r_seq_win) {
+                if (hc->tx_cwnd < hc->tx_ssthresh) {
+                        if (*maxincr > 0 && ++hc->tx_packets_acked == 2) {
+                                hc->tx_cwnd += 1;
+                                *maxincr    -= 1;
+                                hc->tx_packets_acked = 0;
+                        }
+                } else if (++hc->tx_packets_acked >= hc->tx_cwnd) {
                         hc->tx_cwnd += 1;
-                        *maxincr    -= 1;
                         hc->tx_packets_acked = 0;
                 }
-        } else if (++hc->tx_packets_acked >= hc->tx_cwnd) {
-                        hc->tx_cwnd += 1;
-                        hc->tx_packets_acked = 0;
         }
+
+        /*
+         * Adjust the local sequence window and the ack ratio to allow about
+         * 5 times the number of packets in the network (RFC 4340 7.5.2)
+         */
+        if (r_seq_used * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_r_seq_win)
+                ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio * 2);
+        else if (r_seq_used * CCID2_WIN_CHANGE_FACTOR < dp->dccps_r_seq_win/2)
+                ccid2_change_l_ack_ratio(sk, dp->dccps_l_ack_ratio / 2 ? : 1U);
+
+        if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR >= dp->dccps_l_seq_win)
+                ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win * 2);
+        else if (hc->tx_cwnd * CCID2_WIN_CHANGE_FACTOR < dp->dccps_l_seq_win/2)
+                ccid2_change_l_seq_window(sk, dp->dccps_l_seq_win / 2);
+
         /*
          * FIXME: RTT is sampled several times per acknowledgment (for each
          * entry in the Ack Vector), instead of once per Ack (as in TCP SACK).
diff --git a/net/dccp/ccids/ccid2.h b/net/dccp/ccids/ccid2.h
index f585d330e1e5..18c97543e522 100644
--- a/net/dccp/ccids/ccid2.h
+++ b/net/dccp/ccids/ccid2.h
@@ -43,6 +43,12 @@ struct ccid2_seq {
 #define CCID2_SEQBUF_LEN 1024
 #define CCID2_SEQBUF_MAX 128
 
+/*
+ * Multiple of congestion window to keep the sequence window at
+ * (RFC 4340 7.5.2)
+ */
+#define CCID2_WIN_CHANGE_FACTOR 5
+
 /**
  * struct ccid2_hc_tx_sock - CCID2 TX half connection
  * @tx_{cwnd,ssthresh,pipe}: as per RFC 4341, section 5