[DCCP]: Provide function for RTT sampling

A recurring problem, in particular in the CCID code, is that RTT samples
from packets with timestamp echo and elapsed time options need to be taken.

This service is provided via a new function dccp_sample_rtt in this patch.
Furthermore, to protect against `insane' RTT samples, the sampled value
is bounded between 100 microseconds and 4 seconds - for which u32 is sufficient.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 40 insertions, 0 deletions

diff --git a/net/dccp/input.c b/net/dccp/input.c
index a1900157e2d7..bd578c87b2e7 100644
--- a/net/dccp/input.c
+++ b/net/dccp/input.c
@@ -577,3 +577,43 @@ discard:
 }
 
 EXPORT_SYMBOL_GPL(dccp_rcv_state_process);
+
+/**
+ * dccp_sample_rtt  -  Sample RTT from packet exchange
+ *
+ * @sk:     connected dccp_sock
+ * @t_recv: receive timestamp of packet with timestamp echo
+ * @t_hist: packet history timestamp or NULL
+ */
+u32 dccp_sample_rtt(struct sock *sk, struct timeval *t_recv,
+                                     struct timeval *t_hist)
+{
+        struct dccp_sock *dp = dccp_sk(sk);
+        struct dccp_options_received *or = &dp->dccps_options_received;
+        suseconds_t delta;
+
+        if (t_hist == NULL) {
+                if (!or->dccpor_timestamp_echo) {
+                        DCCP_WARN("packet without timestamp echo\n");
+                        return DCCP_SANE_RTT_MAX;
+                }
+                timeval_sub_usecs(t_recv, or->dccpor_timestamp_echo * 10);
+                delta = timeval_usecs(t_recv);
+        } else
+                delta = timeval_delta(t_recv, t_hist);
+
+        delta -= or->dccpor_elapsed_time * 10;          /* either set or 0 */
+
+        if (unlikely(delta <= 0)) {
+                DCCP_WARN("unusable RTT sample %ld, using min\n", (long)delta);
+                return DCCP_SANE_RTT_MIN;
+        }
+        if (unlikely(delta - (suseconds_t)DCCP_SANE_RTT_MAX > 0)) {
+                DCCP_WARN("RTT sample %ld too large, using max\n", (long)delta);
+                return DCCP_SANE_RTT_MAX;
+        }
+
+        return delta;
+}
+
+EXPORT_SYMBOL_GPL(dccp_sample_rtt);