| Internet-Draft | C4 Tests | July 2026 |
| Huitema, et al. | Expires 20 January 2027 | [Page] |
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 was validated by series of simulations, and also by initial deployments in control networks. We describe here these simulations and tests.¶
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Christian's Congestion Control Code (C4) is a new 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 design was validated by series of simulations, and also by initial deployments in control networks. We describe here these simulations (see Section 2), the simulation results for each of the test cases (see Section 3), and the live networking tests (see Section 4).¶
We test the design by running a series of simulations, which cover:¶
reaction to network events¶
competition with other congestion control algorithms¶
handling of high jitter environments¶
handling of multimedia applications¶
handling of ECN¶
We are running the tests using the picoquic network simulator [Picoquic_ns]. The simulator embeds the picoquic implementation of QUIC [Picoquic]. Picoquic itself comes with support for a variety of congestion control protocols, including Cubic and BBR. We added an implementation of C4.¶
That implementation is designed so that the same code can be used in execution over the network and in simulations, the main difference being a replacement of the socket API by a simulation API. When running in simulation, the code runs in "virtual time", with a virtual clock driven by simulation events such as arrival and departure of packets from simulated queues. With the virtual clock mechanism, we can simulate in a fraction of a second a connection that would last 10 seconds in "real time".¶
The first series of simulation test how C4 behaves in simple scenarios when it is the sole user of a link. The list of test includes:¶
a 20Mbps connection,¶
a 200Mbps connection,¶
a geostationary satellite connection,¶
a sudden increase in path capacity, i.e. "low and up"¶
a sudden decrese in path capacity followed by a return to normal, i.e. "drop and back"¶
a sudden drop to 0 of path capacity for 2 seconds, i.e. a "black hole"¶
a sudden increase in path latency, from "short" to "long"¶
This scenario simulates a 10MB download over a 20 Mbps link, with an 80ms RTT, and a bottlneck buffer capacity corresponding to 1 BDP.¶
In a typical simulation, we see a initial phase complete in less than 800ms, followed by a recovery phase in which the transmission rate stabilizes to the line rate. After that, the RTT remains very close to the path RTT, except for periodic small bumps during the "push" transitions.¶
This scenario simulates a 20MB download over a 200 Mbps link, with a 40ms RTT, and a bottleneck buffer capacity corresponding to 1 BDP.¶
This short test shows that the initial phase correctly discover the path capacity, and that the transmission operates at the expected rate after that.¶
This scenario simulates a 4MB download over a 1.5 Mbps link, with a 40ms RTT, and a bottleneck buffer capacity corresponding to 1.25 BDP.¶
This short test shows that the initial phase correctly discover the path capacity, and that the transmission operates at the expected rate after that.¶
This scenario simulates a 1MB download over a 512 kbps link, with a 34ms RTT, and a bottleneck buffer capacity corresponding to 50ms of transmission.¶
This short test shows that the initial phase correctly discover the path capacity, and that the transmission operates at the expected rate after that.¶
The "low and up" scenario simulates a sudden increase in the capacity of the path. At the beginning of the simulation, the simulated bandwidth is set at 5 Mbps. It increases to 10 Mbps after 2.5 seconds. The RTT remains constant at 100ms.¶
The goal of the test is to verify that C4 promptly discovers the increase in bandwidth, and increases the transmission rate.¶
The "drop and back" scenario simulates a sudden decrease in the capacity of the path, followed by return to normal. At the beginning of the simulation, the simulated bandwidth is set at 10 Mbps. It decreases to 5 Mbps after 1.5 second, then returns to 10 Mbps after 2 seconds. The RTT remains constant at 100ms.¶
The goal of the test is to verify that C4 adapts promptly to changes in the available bandwidth on a path.¶
The "black hole" scenario simulates a sudden decrease in the capacity of the path, followed by return to normal. At the beginning of the simulation, the simulated bandwidth is set at . After 2 seconds, the path capacity is set to 0, and is restored to normal 2 seconds later. The RTT remains constant at 70ms.¶
The goal of the test is to verify that C4 recovers promptly after a short suspension of the path.¶
The "short and long" scenario simulates a sudden increase in the latency of the path. At the beginning of the simulation, the simulated RTT is set at 30ms. After 1 second, the latency increases to 100ms. The data rate remains constant at 100ms.¶
The goal of the test is to verify that C4 react properly exercises the "slow down" mechanism to discover the new RTT.¶
This scenario simulates a 100MB download over a 250 Mbps link, with a 600ms RTT, and a bottleneck buffer capacity corresponding to 1 BDP, i.e., simulating a geostationary satellite connection. The scenario also tests the support for careful resume [RFC9959] by setting the remembered CWND to 18750000 bytes and the remembered RTT to 600.123ms.¶
In accordance with [RFC9743], we evaluate competition between C4 connections, or between C4 and Cubic or BBR. We design a series of tests, each correponding to a competition scenario between a "main" connection and a "background" connection. For each test, we run the test using either C4, Cubic or BBR for the "main" connection. The test scenario specifies the algorithm managing the background connection, as well as scenario details.¶
we design series of tests of multiple competing flows all using C4. We want to test different conditions, such as data rate and latency, and also different scenarios, such as testing whether the "background" connection starts at the same time, before or after the "main" connection.¶
We test that the bandwidth is shared reasonably by testing the completion time of a download, and setting the target value so it can only be achieved if the main connection gets "about half" of the bandwidth.¶
Our first test simulates a main connection starting at the same time as a background C4 connection. The path has a 20Mbps data rate and 80ms RTT. The background connection tries to download 10MB, the main connection downloads 5MB.¶
The "background first" test simulates a main connection competing with the background C4 connection that started 0.5 seconds before the main connection. The path has a 20Mbps data rate and 80ms RTT. The background connection tries to download 10MB, the main connection downloads 5MB.¶
The "background last" simulates a main connections competing with the background connection that starts 0.5 seconds after the main connection. The path has a 50Mbps data rate and 30ms RTT. The background connection tries to download 20MB, the main connection downloads 10MB.¶
The long connection test simulates a main connections starting at the same time as the background. The path has a 20Mbps data rate and 80ms RTT. The background connection tries to download 30MB, the main connection downloads 20MB.¶
There are three variants of that test, depending on the background connection algorithm: C4 (vs_c4_lg), Cubic (vs_cubic_lg) or BBR (vs_bbr_lg).¶
The long "background last" test simulates a main connections competing with the background connection starting 1 second after it. The path has a 10Mbps data rate and 70ms RTT. The background connection tries to download 15MB, the main connection downloads 10MB.¶
There are three variants of that test, depending on the background connection algorithm: C4 (vs_c4_lg2), Cubic (vs_cubic_lg2) or BBR (vs_bbr_lg2).¶
In the design of C4, we have been paying special attention to "bad Wi-Fi" environments, in which the usual delays of a few milliseconds could spike to 50 or even 200ms. We spent a lot of time trying to understand what causes such spikes. Our main hypothesis is that this happens when multiple nearby Wi-Fi networks operate on the same frequency or "channel", which causes collisons due to the hidden node problem. This causes collisions and losses, to which Wi-Fi responses involves two leves of exponential back-off.¶
We built a model to simulate this jitter by combining two generators:¶
A random value r between 0 and 1 ms to model collision avoidance,¶
A Poisson arrival model with lambda=1 providing the number N1 of short scale 1ms intervals to account for collision defferal and retry,¶
A Poisson arrival arrival model with lambda = 12, and an interval length of 7.5ms to account for Wi-Fi packet restransmission.¶
We combine these generators models by using a coefficient "x" that indicates the general degree of collisions and repetitions:¶
For a fraction (1-x) of the packets, we set the number N2 to 0.¶
For a fraction (x) of the packets, we compute N2 from the Poisson arrival model with lambda = 12, and an interval length of 7.5ms.¶
The latency for a single sample will be: ~~~ latency = N11ms + N27.5ms if N1 >= 1: latency -= r ~~~ The coefficient x is derived from the target average jitter value. If the target is 1ms or less, we set x to zero. If it is higher than 91ms, we set x to 1. If it is in between, we set: ~~~ x = (average_jitter - 1ms)/90ms ~~~ We have been using this simulation of jitter to test our implementation of multiple congestion control algorithms.¶
The "bad Wi-Fi" test simulates a connection experiencing a high level of jitter. The average jitter is set to 7ms, which implies multiple spikes of 100 to 200ms every second. The data rate is set to 10Mbps, and the base RTT before jitter is set to 2ms, i.e., simulating a local server.¶
The "Wi-Fi fade" trial simulates varying conditions. The connection starts with a data rate of 20Mbps, an 80ms latency, and Wi-Fi jitter with average 1ms. After 1 second, the data rate drops to 2Mbps and the jitter average increases to 12ms. After another 2 seconds, data rate and jitter return to the original condition.¶
The "Wi-Fi suspension" test simulates a connection experiencing multiple "suspensions". For every 1.8 second of a 2 second interval, the data rate is set to 20Mbps, and the base RTT before jitter is set to 10ms. For the last 200ms of these intervals, the data rate is set to 0. This model was developed before we got a better understanding of the Wi-Fi jitter. It is obsolete, but we kept it as a test case anyhow.¶
The "compete over bad Wi-Fi" test simulates a main connection using a "bad Wi-Fi" path and competing on the same path with a background connection, with the main connection starting 1 second after the background connection. The path has a 10Mbps data rate and 2ms RTT, plus Wi-Fi jitter set to 7ms average -- the same jitter characteristics as in the "bad Wi-Fi" test (see Section 2.3.1). The background connection tries to download 10MB, the main connection downloads 4MB.¶
There are three variants of that test, depending on the background connection algorithm: C4 (wifi_bad_c4), Cubic (wifi_bad_cubic) or BBR (wifi_bad_bbr).¶
To evaluate the handling of ECN, we run a series of tests in which the bottleneck queue is managed by the "duaQ" adaptie queue management algorithm (AQM) specified for L4S [RFC9743]¶
The "ECN" test simulates a 20 Mbps link, with an 80ms RTT, and a bottleneck buffer capacity corresponding to 1 BDP.¶
When using C4 we set the ECT1 marking, signaling support of L4S. We do not set these markings when using Cubic or BBR.¶
The "compete over ECN" tests simulates a main connection competing against a background connection, using the same network path characteristics as the "ECN" test (see Section 2.4.1).¶
There are three variants of this test, with the background connection using either C4 (ecn_c4), Cubic (ecn_cubic) or BBR (ecn_bbr).¶
C4 is specifically designed to properly handle multimedia applications. We test that function by running simulations of a call including:¶
a simulated audio stream sending 80 bytes simulated audio segments every 20 ms.¶
a simulated compressed video stream, sending 30 frames per second, organized as groups of 30 frames each starting with a 37500 bytes simulated I-Frame followed by 149 3750 bytes P-frames.¶
a simulated less compressed video stream, sending 30 frames per second, organized as groups of 30 frames each starting with a 62500 bytes simulated I-Frame followed by 149 6250 bytes P-frames.¶
The simulation sends each simulated audio segment as QUIC datagram, with QUIC priority 2, and each group of frames as a separate QUIC stream with priority 4 for the compressed stream, and a priority 6 for the less compressed stream.¶
If the frames delivered on the less compressed stream fall are delivered more than 250ms later than the expected time, the receiver sends a "STOP SENDING" request on the QUIC stream to cancel it; transmission will restart with the next group of frame, simulating a plausible "simulcast" behavior.¶
The simulator collects statistics on the delivery of media frame, which are summarized as average and maximum frame delivery delay. For each test, the simulation specifies an expected average and an expected maximum delay, as well as a "start measurement" time, typically set long enough to start after the initial "startup" phase. The test passes if the average and max value for the simulated audio and for the simulated compressed video measured after the start time are below the specified values.¶
The "media" test verifies simulates the handling of media on a 100 Mbps connection with a 30ms RTT. The test lasts for 5 video groups of frames, i.e. 5 seconds. The measurements start 200ms after the start of the connection.¶
The "media10" test verifies the handling of media on a 10 Mbps connection with a 40ms RTT. The test lasts for 5 video groups of frames, i.e. 5 seconds. The measurements start 200ms after the start of the connection.¶
The "media600" media checks that media performance does not degrade over time, simulating a 100Mbps connection with a 30ms RTT. The test lasts for 20 video groups of frames, i.e. 20 seconds. The measurements start 200ms after the start of the connection.¶
The "media_short_long" media test verifies that media performance does not degrade over time, simulating a 100Mbps connection with a 30ms RTT, that changes to a 100ms RTT after 1 second. The test lasts for 10 video groups of frames, i.e. 10 seconds. The measurements start 5 seconds after the start of the connection.¶
The "bad Wi-Fi" media test verifies that media performance does not degrade too much on a connection that has the kind of jitter discussed in Section 2.3. The connection has the characteristics similar to the "bad Wi-Fi" scenario described in Section 2.3.1. The average jitter is set to 7ms, which implies multiple spikes of 100 to 200ms every second. The data rate is set to 20Mbps, and the base RTT before jitter is set to 2ms, i.e., simulating a local server. The test lasts for 5 video groups of frames, i.e. 5 seconds. The measurements start 200ms after the start of the connection.¶
The "fading Wi-Fi" media test verifies that media performance does not degrade too much on a connection that hast characteristics similar to the "fading Wi-Fi" scenario described in Section 2.3.2. The connection starts with a data rate of 20Mbps, 40ms RTT, and Wi-Fi jitter with average 1ms. After 1 second, the data rate drops to 2Mbps and the jitter average increases to 12ms. The test lasts for 5 video groups of frames, i.e. 5 seconds. The measurements start 200ms after the start of the connection.¶
The "varying Wi-Fi" media test verifies that media performance does not degrade too much on a connection experiences suspensions as discussed in Section 2.3.3. For every 1.8 second of a 2 second interval, the data rate is set to 20Mbps, and the base RTT before jitter is set to 10ms. For the last 200ms of these intervals, the data rate is set to 0. The test lasts for 5 video groups of frames, i.e. 5 seconds. The measurements start 200ms after the start of the connection.¶
The "varying Wi-Fi" media test verifies that media works as expected on a path managed using ECN/L4S. The set up is similar to the "ECN" test discussed in Section 2.4.¶
Simulations include random events, such as network jitter or the precise timing of packet arrivals and departure. Minute changes in starting conditions can have cascading effects. To get reliable results, we run each test 100 times. The simulator produces a log of each test execution (in QLOG format), and a summary of each test results, including the completion time for each test, and for tests checking media the average and max frame delivery time.¶
We present here a summary of the results, including the average and the 90th percentile of the completion time for each test. For media tests, we also report the average frame delivery time and the 90th percentile of the max frame delivery time.¶
We run these tests for C4, Cubic and BBR, and present the results for these 3 congestion control algorithms in a set of tables. All times are expressed in microseconds, and for all results lower time values are considered better.¶
Here are the statistics for the network events test cases.¶
| average time for network events tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| alone | 4502913 | 4689260 | 4472465 | 4642195 |
| alone_200 | 1115776 | 1221630 | 1145722 | 1161980 |
| alone_1_5M | 21504710 | 21717251 | 21514264 | 21660915 |
| alone_512k | 16173870 | 16211371 | 16183314 | 16213861 |
| low_and_up | 7569237 | 7506849 | 8035433 | 7762235 |
| drop_and_back | 7554195 | 7625693 | 7629764 | 7697371 |
| blackhole | 5591981 | 5811316 | 5695660 | 5628028 |
| short_long | 17536781 | 42331541 | 21368101 | 17537092 |
| satellite | 6807127 | 7492539 | 6704246 | 6807111 |
| top 90% time for network events tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| alone | 4564480 | 4698415 | 4518852 | 4835141 |
| alone_200 | 1181668 | 1222012 | 1148423 | 1186067 |
| alone_1_5M | 21511156 | 21718512 | 21552321 | 21661024 |
| alone_512k | 16173974 | 16217210 | 16208261 | 16215577 |
| low_and_up | 7570221 | 7511647 | 8071920 | 7764215 |
| drop_and_back | 7579428 | 7630825 | 7632455 | 7698289 |
| blackhole | 5592061 | 5815444 | 5699327 | 5628156 |
| short_long | 17538429 | 43394841 | 21541922 | 17538424 |
| satellite | 6807174 | 7834142 | 6704247 | 6807137 |
Here the statistics for the compete test cases.¶
| average time for compete tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| vs_bbr | 2817663 | 4501471 | 2853022 | 2964582 |
| vs_c4 | 4361871 | 6813101 | 7891899 | 4490594 |
| vs_cubic | 3428960 | 6974953 | 5348004 | 3484869 |
| after_c4 | 6563029 | 6846566 | 7208456 | 5239798 |
| before_c4 | 2640670 | 4281776 | 3105136 | 2699206 |
| vs_c4_lg | 21026786 | 32250064 | 23618741 | 21067859 |
| vs_c4_lg2 | 20979188 | 21139542 | 21818194 | 21102894 |
| vs_bbr_lg | 15612556 | 21098503 | 15562778 | 16742530 |
| vs_bbr_lg2 | 16449739 | 18711270 | 21520837 | 20600335 |
| vs_cubic_lg | 17902039 | 21430554 | 20902893 | 17578391 |
| vs_cubic_lg2 | 17080952 | 15533300 | 20672401 | 16969990 |
| top 90% time for compete tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| vs_bbr | 2824981 | 4580449 | 2877804 | 2983881 |
| vs_c4 | 4453585 | 6843240 | 8424848 | 4864821 |
| vs_cubic | 3761300 | 7089722 | 5580459 | 3555684 |
| after_c4 | 6742984 | 6991485 | 7494092 | 6102901 |
| before_c4 | 2734698 | 5404668 | 4163561 | 3001428 |
| vs_c4_lg | 21139706 | 39556964 | 25105527 | 21141447 |
| vs_c4_lg2 | 21046812 | 21379953 | 22272580 | 21174182 |
| vs_bbr_lg | 15808681 | 21131562 | 15839671 | 16936214 |
| vs_bbr_lg2 | 16522592 | 18954745 | 22323666 | 21138531 |
| vs_cubic_lg | 20251838 | 21760143 | 21120555 | 18440982 |
| vs_cubic_lg2 | 17419617 | 15706948 | 20930258 | 17548782 |
Here the statistics for the wifi test cases.¶
| average time for wifi tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| wifi_bad | 4059372 | 5601202 | 4076699 | 4144883 |
| wifi_fade | 5065021 | 5403001 | 5341227 | 5203858 |
| wifi_suspension | 4564740 | 4615871 | 4600118 | 4563252 |
| wifi_bad_bbr | 7582895 | 7280777 | 6837401 | 7581238 |
| wifi_bad_c4 | 8750784 | 9650917 | 8426742 | 9347050 |
| wifi_bad_cubic | 8618719 | 8731338 | 10397119 | 8407363 |
| top 90% time for wifi tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| wifi_bad | 4643322 | 7615210 | 4475581 | 4806788 |
| wifi_fade | 5335174 | 5599818 | 5550898 | 5480744 |
| wifi_suspension | 4574165 | 4616328 | 4602178 | 4573648 |
| wifi_bad_bbr | 12112441 | 11626769 | 12533043 | 11985779 |
| wifi_bad_c4 | 11690859 | 12288047 | 12435459 | 12401707 |
| wifi_bad_cubic | 11961135 | 12011172 | 13905062 | 11723366 |
Here the statistics for the ecn test cases.¶
| average time for ecn tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| ecn | 4465878 | 4670054 | 4460773 | 4494003 |
| ecn_c4 | 12286476 | 17269928 | 13977479 | 11422019 |
| ecn_cubic | 8362141 | 9701695 | 13356991 | 8235549 |
| ecn_bbr | 13079389 | 13246715 | 16900370 | 13083701 |
| top 90% time for ecn tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| ecn | 4466761 | 4671124 | 4457939 | 4494072 |
| ecn_c4 | 13033989 | 17698371 | 14561797 | 12383356 |
| ecn_cubic | 9108260 | 10561707 | 13961159 | 8720974 |
| ecn_bbr | 13342537 | 13372125 | 17458084 | 13345131 |
Here the statistics for the media test cases.¶
| average av_latency for media tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| media | 33511 | 33427 | 33512 | 33511 |
| media10 | 45378 | 44991 | 47755 | 45204 |
| media_600fr | 33625 | 33545 | 33629 | 33624 |
| media_short_long | 100794 | 134059 | 100766 | 101036 |
| media_wb | 80894 | 85353 | 84391 | 77485 |
| media_wf | 82547 | 86474 | 83914 | 82971 |
| media_ws | 22941 | 21645 | 22495 | 22854 |
| media_ecn | 34413 | 34481 | 34716 | 34408 |
| top 90% max_latency for media tests | c4 | bbr | cubic | c4_2026_07_05 |
|---|---|---|---|---|
| media | 43453 | 43453 | 43453 | 43453 |
| media10 | 71128 | 71128 | 92163 | 71128 |
| media_600fr | 43453 | 43453 | 43453 | 43453 |
| media_short_long | 111153 | 334491 | 109180 | 117984 |
| media_wb | 270847 | 304794 | 274677 | 269770 |
| media_wf | 279458 | 365839 | 298720 | 298762 |
| media_ws | 197821 | 195521 | 197821 | 197821 |
| media_ecn | 47975 | 50996 | 50996 | 49700 |
We need real life tests as well.¶
Loopback tests were performed on Windows, downloading 10GB of data over a loopback connection. They showed picoquic using C4 achieving a data rate of 3Gbps, slightly more than the 2.9Gbps achieved when using Cubic or the 2.6 Gbps achieved when using BBR.¶
To do. Write down.¶
This documentation of protocol testing does not have any particular security considerations.¶
We did not include specific security oriented tests in this document.¶
This document has no IANA actions.¶
TODO acknowledge.¶