Specification of Christian's Congestion Control Code (C4)
draft-huitema-ccwg-c4-spec-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Christian Huitema , Suhas Nandakumar , Cullen Fluffy Jennings | ||
| Last updated | 2025-10-19 | ||
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draft-huitema-ccwg-c4-spec-00
Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Experimental S. Nandakumar
Expires: 22 April 2026 C. Jennings
Cisco
19 October 2025
Specification of Christian's Congestion Control Code (C4)
draft-huitema-ccwg-c4-spec-00
Abstract
Christian's Congestion Control Code is a new congestion control
algorithm designed to support Real-Time applications such as Media
over QUIC. It is designed to drive towards low delays, with good
support for the "application limited" behavior frequently found when
using variable rate encoding, and with fast reaction to congestion to
avoid the "priority inversion" happening when congestion control
overestimates the available capacity. The design emphasizes
simplicity and avoids making too many assumptions about the "model"
of the network.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 April 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. C4 variables . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Nominal rate . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Nominal max RTT . . . . . . . . . . . . . . . . . . . . . 4
3.3. Global variables . . . . . . . . . . . . . . . . . . . . 5
3.4. Per era variables . . . . . . . . . . . . . . . . . . . . 6
4. States and Transition . . . . . . . . . . . . . . . . . . . . 6
4.1. Setting pacing rate, congestion window and quantum . . . 7
4.2. Initial state . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Recovery state . . . . . . . . . . . . . . . . . . . . . 9
4.3.1. Restarting Initial if High Jitter . . . . . . . . . . 10
4.4. Cruising state {#c4-cruising } . . . . . . . . . . . . . 10
4.5. Pushing state . . . . . . . . . . . . . . . . . . . . . . 10
5. Handling of congestion signals . . . . . . . . . . . . . . . 10
5.1. Variable Sensitivity . . . . . . . . . . . . . . . . . . 11
5.2. Detecting Excessive Delays . . . . . . . . . . . . . . . 11
5.3. Detecting Excessive Losses . . . . . . . . . . . . . . . 12
5.3.1. Do not react to Probe Time Out . . . . . . . . . . . 12
5.4. Detecting Excessive CE Marks . . . . . . . . . . . . . . 12
5.5. Rate Reduction on Congestion . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Christian's Congestion Control Code (C4) is a congestion control
algorithm designed to support Real-Time multimedia applications,
specifically multimedia applications using QUIC [RFC9000] and the
Media over QUIC transport [I-D.ietf-moq-transport].
The two main variables describing the state of a flow are the
"nominal rate" (see Section 3.1) and the "nominal max RTT" (see
Section 3.2). C4 organizes the management of the flow through a
series of states: Initial, during which the first assessment of
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nominal-rate and nominal max RTT are obtained, Recovery in which a
flow is stabilized after the Initial or Pushing phase, Cruising
during which a flow uses the nominal rate, and Pushing during which
the flow tries to discover if more resource is available -- see
Section 4.
C4 divides the duration of the connection in a set of "eras", each
corresponding to a packet round trip. Transitions between protocol
states typically happen at the end of an era, except if the
transition is forced by a congestion event.
C4 assumes that the transport stack is capable of signaling events
such as acknowledgements, RTT measurements, ECN signals or the
detection of packet losses. It also assumes that the congestion
algorithm controls the transport stack by setting the congestion
window (CWND) and the pacing rate (see Section 5).
C4 introduces the concept of "sensitivity" (see Section 5.1) to
ensure that flows using a large amount of bandwidth are more
"sensitive" to congestion signals than flows using fewer bandwidth,
and thus that multiple flows sharing a common bottleneck are driven
to share the resource evenly.
2. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. C4 variables
In addition to the nomnal rate and the nominal max RTT, C4 maintains
a set a variables per flow (see Section 3.3) and per era (see
Section 3.4).
3.1. Nominal rate
The nominal rate is an estimate of the bandwidth available to the
flow. On initialization, the nominal rate is set to zero, and
default values are used when setting the pacing rate and CWND for the
flow.
C4 evaluates the nominal rate after acknowledgements are received
using the number of bytes acknowledged since the packet was sent
(bytes_acknowledged) and the time delay it took to process these
packets.
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That delay is normally set to the difference between the time at
which the acknowledged packet was sent (time_sent), and the current
time (current_time). However, that difference may sometimes be
severely underestimated because of delay jitter and ACK compression.
We also compute a "send delay" as the difference between the send
time of the acknowledged packet and the send time of the oldest
"delivered" packet.
delay_estimate = max (current_time - time_sent, send_delay)
rate_estimate = bytes_acknowledged /delay_estimate
If we are not in a congestion situation, we update the nominal rate:
if not congested and nominal_rate > rate_estimate:
nominal_rate = rate_estimate
The data rate measurements can only cause increases in the nominal
rate. The nominal rate is reduced following congestion events, as
specified in Section 5.
The "congested" condition is defined as being in the recovery state
and having either entered that state due to a congestion event, or
having received a congestion event after entering recovery.
Updating the nominal rate in these conditions would cause a
congestion bounce: the nominal rate is reduced because of a
congestion event, C4 enters recovery, but then packets sent at the
previous rate are received during recovery, generating a new estimate
and resetting the nominal rate to a value close to the one that
caused congestion.
3.2. Nominal max RTT
The nominal max RTT is an estimate of the maximum RTT that can occur
on the path in the absence of queues. The RTT samples observed for
the flow are the sum of four components:
* the latency of the path
* the jitter introduced by processes like link layer contention or
link layer retransmission
* queuing delays caused by competing applications
* queuing delays introduced by C4 itself.
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C4's goal is to obtain a estimate of the combination of path latency
and maximum jitter. This is done by only taking measurements when C4
is sending data at a rate not higher than the nominal transmission
rate, as happens for example in the recovery and cruising states.
These measurements will happen during the following era. C4 captures
them by recording the max RTT for packets sent in that era. C4 will
also progressively reduce the value of the nominal max RTT over time,
to account for changes in network conditions.
# on end of era
if alpha_previous <= 1.0:
if era_min_rtt < running_min_rtt:
running_min_rtt = era_min_rtt
else:
running_min_rtt =
(7*running_min_rtt + era_min_rtt)/8
if era_max_rtt > running_min_rtt + MAX_JITTER:
# cap RTT increases to MAX_JITTER, i.e., 250ms
era_max_rtt = running_min_rtt + MAX_JITTER
if era_max_rtt > nominal_max_rtt:
nominal_max_rtt = era_max_rtt
else:
nominal_max_rtt =
(7*nominal_max_rtt + era_max_rtt)/8
The decrease over time is tuned so that jitter events will be
remembered for several of the cruising-pushing-recovery cycles, which
is enough time for the next jitter event to happen, at least on Wi-Fi
networks.
3.3. Global variables
In addition to the nominal rate and nominal MAX RTT, C4 maintains a
set of variables tracking the evolution of the flow:
* running min RTT, an approximation of the min RTT for the flow,
* number of eras without increase (see Section 4.2),
* number of successful pushes,
* current state of the algorithm, which can be Initial, Recovery,
Cruising or Pushing.
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3.4. Per era variables
C4 keeps variables per era:
era_sequence; /* sequence number of first packet sent in this era */
alpha_current; /* coefficient alpha used in the current state */
alpha_previous; /* coefficient alpha used in the previous era */
era_max_rtt; /* max RTT observed during this era */
era_min_rtt; /* min RTT observed during this era */
These variables are initialized at the beginning of the era.
4. States and Transition
The state machine for C4 has the following states:
* "Initial": the initial state, during which the CWND is set to
twice the "nominal_CWND". The connection exits startup if the
"nominal_cwnd" does not increase for 3 consecutive round trips.
When the connection exits startup, it enters "recovery".
* "Recovery": the connection enters that state after "Initial",
"pushing", or a congestion detection in a "cruising" state. It
remains in that state for at least one roundtrip, until the first
packet sent in "recovery" is acknowledged. Once that happens, the
connection goes back to "startup" if the last 3 pushing attemps
have resulted in increases of "nominal rate", or if it detects
high jitter and the previous initial was not run in these
conditions (see ). It enters "cruising" otherwise.
* "Cruising": the connection is sending using the "nominal_rate" and
"nominal_max_rtt" value. If congestion is detected, the
connection exits cruising and enters "recovery" after lowering the
value of "nominal_cwnd". Otherwise, the connection will remain in
"cruising" state until at least 4 RTT and the connection is not
"app limited". At that point, it enters "pushing".
* "Pushing": the connection is using a rate and CWND 25% larger than
"nominal_rate" and "nominal_CWND". It remains in that state for
one round trip, i.e., until the first packet send while "pushing"
is acknowledged. At that point, it enters the "recovery" state.
These transitions are summarized in the following state diagram.
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Start
|
v
+<-----------------------+
| |
v |
+----------+ |
| Initial | |
+----|-----+ |
| |
v |
+------------+ |
+--+---------->| Recovery | |
^ ^ +----|---|---+ |
| | | | First High Jitter |
| | | | or Rapid Increase |
| | | +------------------->+
| | |
| | v
| | +----------+
| | | Cruising |
| | +-|--|-----+
| | Congestion | |
| +-------------+ |
| |
| v
| +----------+
| | Pushing |
| +----|-----+
| |
+<------------------+
4.1. Setting pacing rate, congestion window and quantum
If the nominal rate or the nominal max RTT are not yet assessed, C4
sets pacing rate, congestion window and pacing quantum to initial
values:
* pacing rate: set to the data rate of the outgoing interface,
* congestion window: set to the equivalent of 10 packets,
* congestion quantum: set to zero.
If the nominal rate or the nominal max RTT are both assessed, C4 sets
pacing rate, and congestion window to values that depends on these
variables and on a coefficient alpha_current:
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pacing_rate = alpha_current_ * nominal_rate
cwnd = max (pacing_rate * nominal_max_rtt, 2*MTU)
quantum = max ( min (cwnd / 4, 64KB), 2*MTU)
The coefficient alpha for the different states is:
+==========+==============+===========================+
| state | alpha | comments |
+==========+==============+===========================+
| Initial | 2 | |
+----------+--------------+---------------------------+
| Recovery | 15/16 | |
+----------+--------------+---------------------------+
| Cruising | 1 | |
+----------+--------------+---------------------------+
| Pushing | 5/4 or 17/16 | see Section 4.5 for rules |
| | | on choosing 5/4 or 17/16 |
+----------+--------------+---------------------------+
Table 1
4.2. Initial state
When the flow is initialized, it enters the Initial state, during
which it does a first assessment of the "nominal rate" and "nominal
max RTT". The coefficient alpha_current is set to 2. The "nominal
rate" and "nominal max RTT" are initialized to zero, which will cause
pacing rate and CWND to be set default initial values. The nominal
max RTT will be set to the first assessed RTT value, but is not
otherwise changed during the initial phase. The nominal rate is
updated after receiving acknowledgements, see {#nominal-rate}.
C4 will exit the Initial state and enter Recovery if the nominal rate
does not increase for 3 consecutive eras, omitting the eras for which
the transmission was "application limited".
C4 exit the Initial if receiving a congestion signal and the
following conditions are true:
1- If the signal is due to "delay", C4 will only exit the initial
state if the nominal_rate did not increase in the last 2 eras.
2- If the signal is due to "loss", C4 will only exit the initial
state if more than 20 packets have been received.
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The restriction on delay signals is meant to prevent spurious exit
due to delay jitter. The restriction on loss signals is meant to
ensure that enough packets have been received to properly assess the
loss rate.
4.3. Recovery state
The recovery state is entered from the Initial or Pushing state, or
from the Cruising state in case of congestion. The coefficient
alpha_current is set to 15/16. Because the multiplier is lower than
1, the new value of CWND may well be lower than the current number of
bytes in transit. C4 will wait until acknowledgements are received
and the number of bytes in transit is lower than CWND to send new
packets.
The Recovery ends when the first packet sent during that state is
acknowledged. That means that acknowledgement and congestion signals
received during recovery are the consequence of packets sent before.
C4 assumes that whatever corrective action is required by these
events will be taken prior to entering recovery, and that events
arriving during recovery are duplicate of the prior events and can be
ignored.
Rate increases are detected when acknowledgements received during
recovery reflect a successful "push" during the Pushing phase. The
prior "Pushing" is considered successful if it did not trigger any
congestion event, and if the data rate increases sufficiently between
the end of previous Recovery and the end of this one, with
sufficiently being defined as:
* Any increase if the prior pushing rate (alpha_prior) was 17/16 or
less,
* An increase of at least 1/4th of the expected increase otherwise,
for example an increase of 1/16th if alpha_previous was 5/4.
C4 re-enters "Initial" at the end of the recovery period if the
evaluation shows 3 successive rate increases without congestion, or
if high jitter requires restarting the Initial phase (see
Section 4.3.1. Otherwise, C4 enters cruising.
Reception of a congestion signal during the Initial phase does not
cause a change in the nominal_rate or nominal_max_RTT.
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4.3.1. Restarting Initial if High Jitter
The "nominal max RTT" is not updated during the Initial phase,
because doing so would prevent exiting Initial on high delay
detection. This can lead to underestimation of the "nominal rate" if
the flow is operating on a path with high jitter.
C4 will reenter the "initial" phase on the first time high jitter is
detected for the flow. The high jitter is detected after updating
the "nominal max RTT" at the end of the recovery era, if:
running_min_rtt < nominal_max_rtt*2/5
This will be done at most once per flow.
4.4. Cruising state {#c4-cruising }
The Cruising state is entered from the Recovery state. The
coefficient alpha_current is set to 1.
C4 will normally transition from Cruising state to Pushing state
after 4 eras. It will transition to Recovery before that if a
congestion signal is received.
4.5. Pushing state
The Pushing state is entered from the Cruising state. The
coefficient alpha_current is set to 5/4 if the previous pushing
attempt was successful (see Section 4.3), or 17/16 if it was not.
C4 exits the pushing state after one era, or if a congestion signal
is received before that. In an exception to standard congestion
processing, the reduction in nominal_rate and nominal_max_RTT are not
applied if the congestion signal is tied to a packet sent during the
Pushing state.
5. Handling of congestion signals
C4 responds to congestion events by reducing the nominal rate, and in
some condition also reducing the nominal max RTT. C4 monitors 3
types of congestion events:
1. Excessive increase of measured RTT,
2. Excessive rate of packet losses (but not mere Probe Time Out, see
Section 5.3.1),
3. Excessive rate of ECN/CE marks
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C4 monitors successive RTT measurements and compare them to a
reference value, defined as the sum of the "nominal max rtt" and a
"delay threshold". C4 monitors the arrival of packet losses computes
a "smoothed error rate", and compares it to a "loss threshold". When
the path supports ECN, C4 monitors the arrival of ECN marks and
computes a "smoothed CE rate", and compares it to a "CE threshold".
These coefficients depend on the sensitivity coefficient defined in
Section 5.1.
5.1. Variable Sensitivity
The three congestion detection thresholds are function of the
"sensitivity" coefficient, which increases with the nominal rate of
the flow. Flows operating at a low data rate have a low sensitivity
coefficient and reacts slower to congestion signals than flows
operating at a higher rate. If multiple flows share the same
bottleneck, the flows with higher data rate will detect congestion
signals and back off faster than flow operating at lower rate. This
will drive these flows towards sharing the available resource evenly.
The sensitivity coefficient varies from 0 to 1, according to a simple
curve:
* set sensitivity to 0 if data rate is lower than 50000B/s
* linear interpolation between 0 and 0.92 for values between 50,000
and 1,000,000B/s.
* linear interpolation between 0.92 and 1 for values between
1,000,000 and 10,000,000B/s.
* set sensitivity to 1 if data rate is higher than 10,000,000B/s
The sensitivity index is then used to set the value of delay and loss
and CE thresholds.
5.2. Detecting Excessive Delays
The delay threshold is function of the nominal max RTT and the
sensitivity coefficient:
delay_fraction = 1/16 + (1 - sensitivity)*3/16
delay_threshold = min(25ms, delay_fraction*nominal_max_rtt)
A delay congestion signal is detected if:
rtt_sample > nominal_max_rtt + delay_threshold
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5.3. Detecting Excessive Losses
C4 maintains an average loss rate, updated for every packet as:
if packet_is_lost:
loss = 1
else:
loss = 0
smoothed_loss_rate = (loss + 15*smoothed_loss_rate)/16
The loss threshold is computed as:
loss_threshold = 0.02 + 0.50 * (1-sensitivity);
A loss is detected if the smoothed loss rate is larger than the
threshold. In that case, the coefficient beta is set to 1/4.
5.3.1. Do not react to Probe Time Out
QUIC normally detect losses by observing gaps in the sequences of
acknowledged packet. That's a robust signal. QUIC will also inject
"Probe time out" packets if the PTO timeout elapses before the last
sent packet has not been acknowledged. This is not a robust
congestion signal, because delay jitter may also cause PTO timeouts.
When testing in "high jitter" conditions, we realized that we should
not change the state of C4 for losses detected solely based on timer,
and only react to those losses that are detected by gaps in
acknowledgements.
5.4. Detecting Excessive CE Marks
TBD. The plan is to mimic the L4S specification.
5.5. Rate Reduction on Congestion
On entering recovery, C4 reduces the nominal_rate by the factor
"beta" corresponding to the congestion signal:
nominal_rate = (1-beta)*nominal_rate
The coefficient beta differs depending on the nature of the
congestion signal. For packet losses, it is set to 1/4, similar to
the value used in Cubic.
For delay based losses, it is proportional to the difference between
the measured RTT and the target RTT divided by the acceptable margin,
capped to 1/4:
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beta = min(1/4,
(rtt_sample - (nominal_max_rtt + delay_threshold)/
delay_threshod))
If the signal is an ECN/CE rate, this is still TBD. We could use a
proportional reduction coefficient in line with [RFC9331], but we
should use the sensitivity coefficient to modulate that signal.
6. Security Considerations
We do not believe that C4 introduce new security issues. Or maybe
there are, such as what happen if applications can be fooled in going
to fast and overwhelming the network, or going too slow and
underwhelming the application. Discuss!
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[I-D.ietf-moq-transport]
Nandakumar, S., Vasiliev, V., Swett, I., and A. Frindell,
"Media over QUIC Transport", Work in Progress, Internet-
Draft, draft-ietf-moq-transport-14, 2 September 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-moq-
transport-14>.
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[RFC9331] De Schepper, K. and B. Briscoe, Ed., "The Explicit
Congestion Notification (ECN) Protocol for Low Latency,
Low Loss, and Scalable Throughput (L4S)", RFC 9331,
DOI 10.17487/RFC9331, January 2023,
<https://www.rfc-editor.org/info/rfc9331>.
Acknowledgments
TODO acknowledge.
Authors' Addresses
Christian Huitema
Private Octopus Inc.
Email: huitema@huitema.net
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
Cullen Jennings
Cisco
Email: fluffy@iii.ca
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