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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)


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:


If you are interested in learning more about L4S:

Accurate ECN requirements:

Accurate ECN signaling protocol:

The adjacent patch to packetdrill for decoding and encoding AccECN (stateful):

And the adjacent patch to wireshark – including the TCP Option of Accurate ECN

Richard Scheffenegger 

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