dcsctp: Bump rto_min to 220 ms

The minimum RTO time shouldn't be lower than the delayed ack timeout
of the peer to avoid sending retransmissions before the peer has
actually intended to reply.

In usrsctp, the default delayed ack timeout is 200ms and configurable
using the `sctp_delayed_sack_time_default` option. In dcsctp, it's
min(RTO/2, 200ms), to avoid this issue.

Bug: webrtc:12614
Change-Id: Ie84c331334af660d66b1a7d90d20f5cf7e2a5103
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/219100
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34026}

net/dcsctp/public/dcsctp_options.h

diff --git a/net/dcsctp/public/dcsctp_options.h b/net/dcsctp/public/dcsctp_options.h
index 08893fd..4f5b50c 100644
--- a/net/dcsctp/public/dcsctp_options.h
+++ b/net/dcsctp/public/dcsctp_options.h
@@ -95,8 +95,9 @@
   // Maximum RTO value.
   DurationMs rto_max = DurationMs(800);
 
-  // Minimum RTO value.
-  DurationMs rto_min = DurationMs(120);
+  // Minimum RTO value. This must be larger than an expected peer delayed ack
+  // timeout.
+  DurationMs rto_min = DurationMs(220);
 
   // T1-init timeout.
   DurationMs t1_init_timeout = DurationMs(1000);