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, 5 insertions, 0 deletions

diff --git a/src/irmd/configfile.c b/src/irmd/configfile.c
index 5967e490..e2e1e554 100644
--- a/src/irmd/configfile.c
+++ b/src/irmd/configfile.c
@@ -477,6 +477,7 @@ static int toml_dt(toml_table_t *     table,
         toml_datum_t addr;
         toml_datum_t eid;
         toml_datum_t ttl;
+        toml_datum_t max_rtt;
 
         addr = toml_int_in(table, "addr_size");
         if (addr.ok)
@@ -490,6 +491,10 @@ static int toml_dt(toml_table_t *     table,
         if (ttl.ok)
                 conf->max_ttl = ttl.u.i;
 
+        max_rtt = toml_int_in(table, "max_rtt");
+        if (max_rtt.ok)
+                conf->max_rtt = max_rtt.u.i;
+
         if (toml_routing(table, conf) < 0) {
                 log_err("Invalid routing option.");
                 return -1;