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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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glebius
imp
melifaro

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Reviewers
rscheff
Group Reviewers
transport
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)
Test Plan

packetdrill_prr.pkt2 KBDownload

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

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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
Herald added subscribers: glebius, melifaro, imp. · View Herald Transcript
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
rscheff added a comment.Apr 21 2026, 10:28 AM

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.

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sys/
netinet/
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sys/netinet/tcp_input.c

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