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Hi,
I’ve been asked to reach out to this mailing list, which, after much ado
(temporarily unreactive mailman) I hope I can finally do.
The goal is to gather some feedback around the support for tcpdump to properly
decode the upcoming (WGLC) Accurate ECN signalling enhancement, which is part
of the L4S (low loss, low latency, scalable) effort, ultimately culminating in
a variant of TCP called TCP Prague:
Here is the change to tcpdump – adding the additional “A” TCP header flag bit;
Since the ACK bit has traditionally been mapped to “.”, and the refurbished
former Nonce bit #9 in the 12-bit TCP header flags field has been named “AE”
(Accurate ecn Enable), the code changes to other tools such as Wireshark and
Packetdrill are using the mapping to the character “A” (or decode Accurate ECN
stateful into the ACE counter value of 0..7)
https://github.com/the-tcpdump-group/tcpdump/pull/999
This change to the parser in libpcap allows access to all 12 bits when using
the sample from the man page like this
tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'
to also include the ‘tcp-ae’ flag:
https://github.com/the-tcpdump-group/libpcap/pull/1210
If you are interested in learning more about L4S:
https://datatracker.ietf.org/doc/rfc9330/
Accurate ECN requirements:
https://datatracker.ietf.org/doc/rfc7560/
Accurate ECN signaling protocol:
https://datatracker.ietf.org/doc/draft-ietf-tcpm-accurate-ecn
The adjacent patch to packetdrill for decoding and encoding AccECN (stateful):
https://github.com/google/packetdrill/commit/77ab15806a56de226345fbe1910139829e431fe0
And the adjacent patch to wireshark – including the TCP Option of Accurate ECN
https://gitlab.com/wireshark/wireshark/-/merge_requests/7816/diffs?commit_id=ecefcf880121961c03d2ec05ad378c82e951e0a2
Richard Scheffenegger
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