Differential D56535

tcp: Align PRR implementation with RFC 9937
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Authored by mohnishhemanthkumar_gmail.com on Apr 20 2026, 5:46 PM.

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Reviewers

rscheff

Group Reviewers

transport

Commits

rG506e3e30a43c: tcp: Align PRR implementation with RFC 9937

Summary

The main changes implemented in tcp_do_prr_ack() (in tcp_input.c) are:

Early return when no new data is delivered (del_data = 0). (RFC 9937 6.2)
Switching from PRR-CRB to PRR-SSRB only when both SND.UNA advances and no further loss is indicated. (RFC 9937 4)
Accounting for sequence ranges SACKed before entering recovery in RecoverFS calculation. (RFC 9937 4)
Force a fast retransmit upon entering recovery when prr_out is 0 AND SndCnt is 0. (RFC 9937 4)
Set cwnd to ssthresh post recovery. (RFC 9937 4)

This work was contributed by a team from the National Institute of Technology Karnataka (NITK), under the guidance of Dr. Mohit P. Tahiliani. Team members: @mohnishhemanthkumar_gmail.com, @nikhilkottoli2005_gmail.com , @manishagarwal428728_gmail.com , and @ppal61679_gmail.com .

Test Plan

packetdrill_prr.pkt2 KBDownload

Detailed Report: https://docs.google.com/document/d/16OeBZ8Zp0D5umq_asm8Dl6grINuLwn1I8baoncuhTF0/edit?tab=t.0

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rG FreeBSD src repository

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mohnishhemanthkumar_gmail.com created this revision.Apr 20 2026, 5:46 PM

mohnishhemanthkumar_gmail.com created this object with edit policy "mohnishhemanthkumar_gmail.com (Mohnish Hemanth Kumar)".

Herald added a reviewer: transport. · View Herald TranscriptApr 20 2026, 5:46 PM

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mohnishhemanthkumar_gmail.com requested review of this revision.Apr 20 2026, 5:46 PM

mohnishhemanthkumar_gmail.com attached a referenced file: F153364136: packetdrill_prr.pkt. (Show Details)

mohnishhemanthkumar_gmail.com edited the summary of this revision. (Show Details)Apr 20 2026, 5:50 PM

mohnishhemanthkumar_gmail.com retitled this revision from Align PRR implementation with RFC9937 to tcp: Align PRR implementation with RFC 9937.Apr 20 2026, 6:43 PM

Thanks,
I'll discuss this with all the transport committers in the next call.

First reading looks good, have to check the packetdrill differences too.

rscheff mentioned this in D58319: minor rearrangement of declarations and white space clean-up of diff. no functional change.Sat, Jul 18, 8:36 AM

tuexen added a subscriber: tuexen.Sat, Jul 18, 8:39 AM

This revision was not accepted when it landed; it landed in state Needs Review.Sat, Jul 18, 9:25 AM

Closed by commit rG506e3e30a43c: tcp: Align PRR implementation with RFC 9937 (authored by rscheff). · Explain Why

This revision was automatically updated to reflect the committed changes.

rscheff added a commit: rG506e3e30a43c: tcp: Align PRR implementation with RFC 9937.

@tuexen is currently looking to create standard compliant packetdrill scripts to check various test cases.

Reviewed this together with @tuexen; for some reason, this diff can not be edited to have additional reviewers or minor changes.
Accepting this on behalf of @rscheff and @tuexen.

The commit will have the declarations moved up to the beginning of the functions, and some minor white-space fixes - no functional changes.

mohnishhemanthkumar_gmail.com edited the summary of this revision. (Show Details)Fri, Jul 24, 12:37 PM

mohnishhemanthkumar_gmail.com added subscribers: • manishagarwal428728_gmail.com, nikhilkottoli2005_gmail.com, • ppal61679_gmail.com.