TCP Fast Open Linux behaviour

Michael Tuexen tuexen at
Wed Jan 17 21:55:41 UTC 2018

Dear all,

at the last Telco the following question was raised:

When Linux is acting as TCP client using TCP fast open,
does it accept user data sent with the SYN-ACK.

I tested it with the Linux kernel 4.9.77, which is the current
LTS release and figured out that the data sent with the SYN-ACK
is accepted. See the attached .pcapng file.

The current Ubuntu 17.10 using the kernel 4.13.0 shows the
same behaviour.

Best regards

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