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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>
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Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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Slow but steady.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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2022 was a rather slow year...
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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Growing pains.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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The ugent email addresses are shut down, updated to Ouroboros mail
addresses.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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Happy New Year, Ouroboros!
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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The ECN marking function should be able to use the packet QoS to allow
prioritizing traffic under congestion. Not yet implemented in MB-ECN.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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The previous value of the ECN field should be passed to the congestion
notification function.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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The RIB will now show some stats for the flow allocator, including
congestion avoidance statistics. This is needed before decoupling the
data transfer component and the flow allocator as some current stats
show in DT will move to FA.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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This adds congestion avoidance policies to the unicast IPCP. The
default policy is a multi-bit explicit congestion avoidance algorithm
based on data-center TCP congestion avoidance (DCTCP) to relay
information about the maximum queue depth that packets experienced to
the receiver. There's also a "nop" policy to disable congestion
avoidance for testing and benchmarking purposes.
The (initial) API for congestion avoidance policies is:
void * (* ctx_create)(void);
void (* ctx_destroy)(void * ctx);
These calls create / and or destroy a context for congestion control
for a specific flow. Thread-safety of the context is the
responsability of the flow allocator (operations on the ctx should be
performed under a lock).
ca_wnd_t (* ctx_update_snd)(void * ctx,
size_t len);
This is the sender call to update the context, and should be called
for every packet that is sent on the flow. The len parameter in this
API is the packet length, which allows calculating the bandwidth. It
returns an opaque union type that is used for the call to check/wait
if the congestion window is open or closed (and allowing to release
locks before waiting).
bool (* ctx_update_rcv)(void * ctx,
size_t len,
uint8_t ecn,
uint16_t * ece);
This is the call to update the flow congestion context on the receiver
side. It should be called for every received packet. It gets the ecn
value from the packet and its length, and returns the ECE (explicit
congestion experienced) value to be sent to the sender in case of
congestion. The boolean returned signals whether or not a congestion
update needs to be sent.
void (* ctx_update_ece)(void * ctx,
uint16_t ece);
This is the call for the sending side top update the context when it
receives an ECE update from the receiver.
void (* wnd_wait)(ca_wnd_t wnd);
This is a (blocking) call that waits for the congestion window to
clear. It should be stateless (to avoid waiting under locks). This may
change later on if passing the context is needed for different algorithms.
uint8_t (* calc_ecn)(int fd,
size_t len);
This is the call that intermediate IPCPs(routers) should use to update
the ECN field on passing packets.
The multi-bit ECN policy bases the value for the ECN field on the
depth of the rbuff queue packets will be sent on. I created another
call to grab the queue depth as fccntl is write-locking the
application. We can further optimize this to avoid most locking on the
rbuff.
Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>
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