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, 8 insertions, 0 deletions

diff --git a/src/ipcpd/unicast/ca/ops.h b/src/ipcpd/unicast/ca/ops.h
index a86c1b3b..b01e8573 100644
--- a/src/ipcpd/unicast/ca/ops.h
+++ b/src/ipcpd/unicast/ca/ops.h
@@ -46,6 +46,14 @@ struct ca_ops {
                                     uint16_t ece,
                                     uint8_t  cap);
 
+        /* Optional, can be NULL: heartbeat pacing + RTT feedback. */
+        bool     (* ctx_hb_due)(void *   ctx,
+                                uint64_t now);
+
+        void     (* ctx_rtt)(void *   ctx,
+                             uint64_t now,
+                             uint64_t rtt);
+
         int      (* calc_ecn)(size_t    queued,
                               uint8_t * ecn,
                               qoscube_t qc,