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, 26 insertions, 1 deletions

diff --git a/src/ipcpd/unicast/ca.c b/src/ipcpd/unicast/ca.c
index d0ee2f73..f2a65fff 100644
--- a/src/ipcpd/unicast/ca.c
+++ b/src/ipcpd/unicast/ca.c
@@ -58,7 +58,8 @@ struct {
 #endif
 } ca;
 
-int ca_init(enum pol_cong_avoid pol)
+int ca_init(enum pol_cong_avoid pol,
+            uint32_t            rtt_ms)
 {
 #ifndef IPCP_CA_PER_FLOW
         size_t i;
@@ -71,6 +72,7 @@ int ca_init(enum pol_cong_avoid pol)
         case CA_MB_ECN:
                 log_dbg("Using multi-bit ECN.");
                 ca.ops = &mb_ecn_ca_ops;
+                mb_ecn_init(rtt_ms);
                 break;
         default:
                 return -1;
@@ -216,6 +218,29 @@ void ca_ctx_update_ece(void *   _ctx,
         return ca.ops->ctx_update_ece(ctx->pol, ece, cap);
 }
 
+bool ca_ctx_hb_due(void *   _ctx,
+                   uint64_t now)
+{
+        struct ca_ctx * ctx = _ctx;
+
+        if (ca.ops->ctx_hb_due == NULL)
+                return false;
+
+        return ca.ops->ctx_hb_due(ctx->pol, now);
+}
+
+void ca_ctx_rtt(void *   _ctx,
+                uint64_t now,
+                uint64_t rtt)
+{
+        struct ca_ctx * ctx = _ctx;
+
+        if (ca.ops->ctx_rtt == NULL)
+                return;
+
+        ca.ops->ctx_rtt(ctx->pol, now, rtt);
+}
+
 int  ca_calc_ecn(size_t    queued,
                  uint8_t * ecn,
                  qoscube_t qc,