This project implements HyStart++ (as described in RFC 9406) within the TCP CUBIC congestion control algorithm in the Linux kernel (i.e., in tcp_cubic.c).
While HyStart++ is designed to replace its predecessor HyStart, we have currently retained both algorithms, allowing users to choose between them.
To ensure clarity, maintainability, and ease of review, all modifications in tcp_cubic.c have been made without altering or removing any existing statements. Instead, the necessary changes have been introduced by adding new lines of code. This approach preserves the original structure and logic of the current version while making it easier to track modifications and maintain compatibility with future updates to the Linux kernel. To achieve this, a new module parameter, hystartpp, has been defined, along with a few additional variables instead of reusing similar ones from HyStart. For example, rather than reusing HyStart's curr_rtt to track the minimum RTT in the current round, we introduced hspp_current_round_minrtt. In this implementation, HyStart++ overrides HyStart, meaning it controls the slow-start phase whenever the hystartpp module parameter is set to a nonzero value, regardless of the hystart setting.
The implementation includes RFC9406_Lnnn markers, where nnn is a three-digit number referencing the corresponding line in RFC 9406. For example, {RFC9406_L227} points to a concept, instruction, or code located on line 227 of the RFC see here.
To provide clarity, we included a block diagram that introduces the new procedures and code added to tcp_cubic.c and illustrates their interactions.
The patch, and the modified tcp_cubic.c can be found in the implementation directory within the project.
We have conducted preliminary tests to evaluate the performance of HyStart++ in TCP CUBIC. The results, along with discussions and analysis, can be found here.
- Selecting the Algorithm
By default, HyStart is enabled. To switch between HyStart and HyStart++, use the following commands:
Enable HyStart++ (disabling HyStart automatically):
echo 1 > /sys/module/tcp_cubic/parameters/hystartpp
Revert to HyStart (disable HyStart++):
echo 0 > /sys/module/tcp_cubic/parameters/hystartpp
- Running a Test
After enabling the desired algorithm, you can conduct network performance tests. By default, the source port used for TCP data transfer is 80. If your test uses a different source port, you can set it using the following command:
echo port_number > /sys/module/tcp_cubic/parameters/hystartpp_source_port
This setting enables logging of information in the /var/log/kern.log file on Linux.
Foe example, the following log entry:
t 874877310 c 22 i 21 f 0 r 100949 a 1 d 18824 l 0
is from /var/log/kern.log, where:
- t represents the timestamp in microseconds.
- c represents the size of cwnd.
- i represents the number of packets in flight.
- f represents the flag (see the patch).
- r represents the smoothed RTT in microseconds.
- a represents the number of packets acknowledged by the received ACK.
- d represents the amount of data delivered so far.
- l represents the number of packet losses since the start of data transfer.
For questions, feedback, or issues related to this project, feel free to reach out to Mahdi Arghavani via email: ma.arghavani[at]yahoo.com