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, 1 insertions, 0 deletions

diff --git a/src/lib/pb/ipcp_config.proto b/src/lib/pb/ipcp_config.proto
index eac4da37..a111b6d8 100644
--- a/src/lib/pb/ipcp_config.proto
+++ b/src/lib/pb/ipcp_config.proto
@@ -42,6 +42,7 @@ message dt_config_msg {
         required uint32 eid_size            = 2;
         required uint32 max_ttl             = 3;
         required routing_config_msg routing = 4;
+        optional uint32 max_rtt             = 5;
 }
 
 message dir_dht_config_msg {