IPv6: Ignore PTB packets with an MTU < 1280
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Authored by vangyzen on Aug 8 2025, 10:10 PM.

Details

Reviewers

glebius
kib
kp
markj
tuexen

Group Reviewers

transport

Commits

rGa9f5b6883709: IPv6: Ignore PTB packets with an MTU < 1280

Summary

RFC 2460 section 5 paragraph 7 allowed a Packet Too Big message
to report a Next-Hop MTU less than 1280 in support of 6-to-4 routers.
A node receiving such a message was required to add a Fragment
Header to outgoing packets, even though they were not fragmented.

Almost 20 years later, RFC 8200 was published. It obsoletes RFC 2460
and removes that paragraph. UNH IOL Intact was updated to test for
compliance with the new standard.

Test Plan

Test cases v6LC_4_1_06a and 06b failed before this change, saying:

DUT processed PTB and sent a fragmented echo reply

Remove code supporting that obsolete paragraph. Those two test cases
now pass:

DUT did not process PTB and sent un-fragmented echo reply

All PMTU test cases pass except v6LC_4_1_08. It fails because we
ignore the MTU in RAs.

Diff Detail

Repository

rG FreeBSD src repository

Lint

Lint Passed

Unit

No Test Coverage

Build Status

Buildable 66137
Build 63020: arc lint + arc unit

Event Timeline

vangyzen created this revision.Aug 8 2025, 10:10 PM

Herald added a reviewer: transport. · View Herald TranscriptAug 8 2025, 10:10 PM

Herald added subscribers: melifaro, dab, badger and 2 others. · View Herald Transcript

vangyzen requested review of this revision.Aug 8 2025, 10:10 PM

Harbormaster completed remote builds in B66126: Diff 160087.Aug 8 2025, 10:10 PM

I've only quickly scrolled through but I believe this is correct; There's further cleanup in there (as a result) which I would probably have preferred to come separately but it's fine this way too.

I agree with bz: I would have preferred one functional change and a follow-up change cleaning up things without a functional change.

sys/netinet6/icmp6.c
1134	Right now validated is not used, but I have a change in mind which would use it. But I can bring this back (as a boolean), when I actually need it.
sys/netinet6/ip6_output.c
1553–1554	The same is true for SCTP.

SCTP handles PMTU changes, too

Harbormaster completed remote builds in B66137: Diff 160131.Aug 9 2025, 6:52 PM

Thanks for the feedback about cleanup. In hindsight, yes, the cleanup did go a little farther than I intended. I could split it into two commits, but I'd have to re-test, twice. I'll try to be more restrained next time. It was just so satisfying.

tuexen accepted this revision.Aug 9 2025, 7:21 PM

This revision is now accepted and ready to land.Aug 9 2025, 7:21 PM

Closed by commit rGa9f5b6883709: IPv6: Ignore PTB packets with an MTU < 1280 (authored by vangyzen). · Explain WhyAug 12 2025, 11:24 PM

This revision was automatically updated to reflect the committed changes.

vangyzen added a commit: rGa9f5b6883709: IPv6: Ignore PTB packets with an MTU < 1280.

meta mentioned this in D51866: if_gif: Use the MTU on the gif interface for the outgoing packets.Aug 20 2025, 8:41 AM

meta mentioned this in D52594: if_gif: Use the MTU on the gif interface for the outgoing packets.Sep 17 2025, 3:46 PM