dccp: make implementation of Syn-RTT symmetric

This patch is thanks to Andre Noll who reported the issue and helped testing.

The Syn-RTT sampled during the initial handshake currently only works for
the client sending the DCCP-Request. TFRC penalizes the absence of an RTT
sample with a very slow initial speed (1 packet per second), which delays
slow-start significantly, resulting in sluggish performance.

This patch mirrors the "Syn RTT" principle by adding a timestamp also onto
the DCCP-Response, producing an RTT sample  when the (Data)Ack completing
the handshake arrives.

Also changed the documentation to 'TFRC' since Syn RTTs are also used by CCID-4.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 2 deletions

diff --git a/net/dccp/input.c b/net/dccp/input.c
index 6beb6a7d6fba..10c957a88f4f 100644
--- a/net/dccp/input.c
+++ b/net/dccp/input.c
@@ -430,7 +430,7 @@ static int dccp_rcv_request_sent_state_process(struct sock *sk,
                 if (dccp_parse_options(sk, NULL, skb))
                         return 1;
 
-                /* Obtain usec RTT sample from SYN exchange (used by CCID 3) */
+                /* Obtain usec RTT sample from SYN exchange (used by TFRC). */
                 if (likely(dp->dccps_options_received.dccpor_timestamp_echo))
                         dp->dccps_syn_rtt = dccp_sample_rtt(sk, 10 * (tstamp -
                             dp->dccps_options_received.dccpor_timestamp_echo));
@@ -535,6 +535,8 @@ static int dccp_rcv_respond_partopen_state_process(struct sock *sk,
                                                    const struct dccp_hdr *dh,
                                                    const unsigned len)
 {
+        struct dccp_sock *dp = dccp_sk(sk);
+        u32 sample = dp->dccps_options_received.dccpor_timestamp_echo;
         int queued = 0;
 
         switch (dh->dccph_type) {
@@ -559,7 +561,14 @@ static int dccp_rcv_respond_partopen_state_process(struct sock *sk,
                 if (sk->sk_state == DCCP_PARTOPEN)
                         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
 
-                dccp_sk(sk)->dccps_osr = DCCP_SKB_CB(skb)->dccpd_seq;
+                /* Obtain usec RTT sample from SYN exchange (used by TFRC). */
+                if (likely(sample)) {
+                        long delta = dccp_timestamp() - sample;
+
+                        dp->dccps_syn_rtt = dccp_sample_rtt(sk, 10 * delta);
+                }
+
+                dp->dccps_osr = DCCP_SKB_CB(skb)->dccpd_seq;
                 dccp_set_state(sk, DCCP_OPEN);
 
                 if (dh->dccph_type == DCCP_PKT_DATAACK ||