ipcpd: Rework congestion avoidance

Congestion avoidance is a property of the layer, orthogonal to ARQ
and to flow control: FRCP retransmits and lets the peer pace the
sender, per flow, end-to-end; the IPCP paces path aggregates. Each
signal means one thing: a loss triggers a retransmission, a mark
means congestion, the peer window means a slow receiver. Every flow
is paced by the same rate law whatever its QoS, so a greedy raw
sender shares a bottleneck fairly with a reliable stream.

The unit of control is the (destination address, QoS cube)
aggregate: all flows toward that destination share one controller
and one rate; a start-time fair-queuing pacer divides the rate
across them by deadline instead of blocking the send path, and a
new flow rides the aggregate's estimates at its current rate, with
no probing of its own. Slow start runs once per aggregate.

The congestion signal is a multi-bit magnitude: forwarders mark
packets with their standing queue depth, MAX-combined across hops,
so a packet carries the deepest queue on its path. The receiver
feeds back a time-integral mean over a window that adapts to the
flow's byte rate, measuring a slow flow with the same fidelity as a
fast one. The sender runs AIMD scaled by elapsed wall-clock time,
which makes the steady-state allocation RTT-independent.

The PCI gains one byte: the path capacity as a quarter-log2 code.
Forwarders estimate their egress rate from busy-period drain and
MIN-stamp the byte, the receiver returns the window minimum with
its feedback, and the sender scales its rate floor and additive
slope to the bottleneck (C / 32). A deep cut implies a backlogged
bottleneck and a backlogged bottleneck advertises its capacity, so
the scaled floor is live exactly when recovery needs it: the probe
heals a halving in seconds at any link rate, and the floor bounds
the deepest hole to a factor 32 below the bottleneck.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

1 files changed, 13 insertions, 7 deletions

diff --git a/src/ipcpd/unicast/ca/ops.h b/src/ipcpd/unicast/ca/ops.h
index 6d2ddf1d..a86c1b3b 100644
--- a/src/ipcpd/unicast/ca/ops.h
+++ b/src/ipcpd/unicast/ca/ops.h
@@ -30,24 +30,30 @@ struct ca_ops {
 
         void     (* ctx_destroy)(void * ctx);
 
-        ca_wnd_t (* ctx_update_snd)(void * ctx,
-                                    size_t len);
+        time_t   (* ctx_update_snd)(void *     ctx,
+                                    size_t     len,
+                                    uint8_t    lecn,
+                                    uint64_t * ftag);
 
         bool     (* ctx_update_rcv)(void *     ctx,
                                     size_t     len,
                                     uint8_t    ecn,
-                                    uint16_t * ece);
+                                    uint8_t    cap,
+                                    uint16_t * ece,
+                                    uint8_t *  fcap);
 
         void     (* ctx_update_ece)(void *   ctx,
-                                    uint16_t ece);
-
-        void     (* wnd_wait)(ca_wnd_t wnd);
+                                    uint16_t ece,
+                                    uint8_t  cap);
 
-        int      (* calc_ecn)(int       fd,
+        int      (* calc_ecn)(size_t    queued,
                               uint8_t * ecn,
                               qoscube_t qc,
                               size_t    len);
 
+        /* True if calc_ecn inspects the queue; gates the lookup. */
+        bool        marks_ecn;
+
         /* Optional, can be NULL */
         ssize_t  (* print_stats)(void * ctx,
                                  char * buf,