ipcpd: Add flow overshoot protection to mb-ecntestingbe

The ECN marks were the only source of congestion information, and
going into congestion collapse (dropping a lot of ECN packets and
causing loss of signal) was unrecoverable.

The sender's slow-start ramp clock now tracks a measured RTT to avoid
overshooting into congestion collapse on the previously fixed (14ms)
doubling on long-delay (e.g. 100ms) links.

The flow allocator now carries a periodic heartbeat/ack to keep RTT
estimates up-to-date. The heartbeat also serves as a congestion
collapse detector: consecutive heartbeat losses will go into loss
recovery, halving the window and pausing Additive Increase until the
path is clear. The DT component now has a max_rtt config parameter that
serves as a hint for the layer RTT to optimize slow-start for
low-latency networks.

These two mechanisms follow TCP, however only during slow start up to
the first ECN signal and to recover from a total collapse until the
ECN signal returns. MB-ECN is still fully RTT-independent in the AI/PD
region. A shorter RTT path will just reach its equal fair share faster
than a longer RTT path.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/lib/protobuf.c b/src/lib/protobuf.c
index a824d357..6beae000 100644
--- a/src/lib/protobuf.c
+++ b/src/lib/protobuf.c
@@ -362,6 +362,8 @@ dt_config_msg_t * dt_config_s_to_msg(const struct dt_config * s)
         msg->addr_size    = s->addr_size;
         msg->eid_size     = s->eid_size;
         msg->max_ttl      = s->max_ttl;
+        msg->has_max_rtt  = true;
+        msg->max_rtt      = s->max_rtt;
         msg->routing      = routing_config_s_to_msg(&s->routing);
         if (msg->routing == NULL)
                 goto fail_routing;
@@ -381,6 +383,7 @@ struct dt_config dt_config_msg_to_s(const dt_config_msg_t * msg)
         s.addr_size    = msg->addr_size;
         s.eid_size     = msg->eid_size;
         s.max_ttl      = msg->max_ttl;
+        s.max_rtt      = msg->has_max_rtt ? msg->max_rtt : 200; /* ms       */
         s.routing      = routing_config_msg_to_s(msg->routing);
 
         return s;