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, 9 insertions, 0 deletions

diff --git a/src/ipcpd/unicast/ca/mb-ecn.h b/src/ipcpd/unicast/ca/mb-ecn.h
index 7bf0b29f..781e25a8 100644
--- a/src/ipcpd/unicast/ca/mb-ecn.h
+++ b/src/ipcpd/unicast/ca/mb-ecn.h
@@ -25,6 +25,8 @@
 
 #include "ops.h"
 
+void     mb_ecn_init(uint32_t rtt_ms);
+
 void *   mb_ecn_ctx_create(void);
 
 void     mb_ecn_ctx_destroy(void * ctx);
@@ -45,6 +47,13 @@ void     mb_ecn_ctx_update_ece(void *   ctx,
                                uint16_t ece,
                                uint8_t  cap);
 
+bool     mb_ecn_ctx_hb_due(void *   ctx,
+                           uint64_t now);
+
+void     mb_ecn_ctx_rtt(void *   ctx,
+                        uint64_t now,
+                        uint64_t rtt);
+
 int      mb_ecn_calc_ecn(size_t    queued,
                          uint8_t * ecn,
                          qoscube_t qc,