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, 12 insertions, 1 deletions

diff --git a/src/ipcpd/unicast/ca.h b/src/ipcpd/unicast/ca.h
index d73d35f5..fc6de445 100644
--- a/src/ipcpd/unicast/ca.h
+++ b/src/ipcpd/unicast/ca.h
@@ -29,7 +29,11 @@
 #include <stdbool.h>
 #include <sys/types.h>
 
-int      ca_init(enum pol_cong_avoid ca);
+/* Buffer a policy's ca_print_stats output must fit in. */
+#define CA_STATS_STRLEN 2048
+
+int      ca_init(enum pol_cong_avoid ca,
+                 uint32_t            rtt_ms);
 
 void     ca_fini(void);
 
@@ -56,6 +60,13 @@ void     ca_ctx_update_ece(void *   ctx,
                            uint16_t ece,
                            uint8_t  cap);
 
+bool     ca_ctx_hb_due(void *   ctx,
+                       uint64_t now);
+
+void     ca_ctx_rtt(void *   ctx,
+                    uint64_t now,
+                    uint64_t rtt);
+
 int      ca_calc_ecn(size_t    queued,
                      uint8_t * ecn,
                      qoscube_t qc,