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, 18 insertions, 17 deletions

diff --git a/src/ipcpd/unicast/ca/nop.c b/src/ipcpd/unicast/ca/nop.c
index e5cacf66..e6965297 100644
--- a/src/ipcpd/unicast/ca/nop.c
+++ b/src/ipcpd/unicast/ca/nop.c
@@ -30,8 +30,8 @@ struct ca_ops nop_ca_ops = {
         .ctx_update_snd = nop_ctx_update_snd,
         .ctx_update_rcv = nop_ctx_update_rcv,
         .ctx_update_ece = nop_ctx_update_ece,
-        .wnd_wait       = nop_wnd_wait,
         .calc_ecn       = nop_calc_ecn,
+        .marks_ecn      = false,
         .print_stats    = NULL
 };
 
@@ -45,51 +45,52 @@ void nop_ctx_destroy(void * ctx)
         (void) ctx;
 }
 
-ca_wnd_t nop_ctx_update_snd(void * ctx,
-                            size_t len)
+time_t nop_ctx_update_snd(void *     ctx,
+                          size_t     len,
+                          uint8_t    lecn,
+                          uint64_t * ftag)
 {
-        ca_wnd_t wnd;
-
         (void) ctx;
         (void) len;
+        (void) lecn;
+        (void) ftag;
 
-        memset(&wnd, 0, sizeof(wnd));
-
-        return wnd;
-}
-
-void nop_wnd_wait(ca_wnd_t wnd)
-{
-        (void) wnd;
+        return 0;
 }
 
 bool nop_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;
         (void) len;
         (void) ecn;
+        (void) cap;
         (void) ece;
+        (void) fcap;
 
         return false;
 }
 
 void nop_ctx_update_ece(void *   ctx,
-                        uint16_t ece)
+                        uint16_t ece,
+                        uint8_t  cap)
 {
         (void) ctx;
         (void) ece;
+        (void) cap;
 }
 
 
-int nop_calc_ecn(int       fd,
+int nop_calc_ecn(size_t    queued,
                  uint8_t * ecn,
                  qoscube_t qc,
                  size_t    len)
 {
-        (void) fd;
+        (void) queued;
         (void) len;
         (void) ecn;
         (void) qc;