Re: [tcpinc] TCP's treatment of data in SYN packets

[Kyle Rose <[e-mail]>]

> > But I have read elsewhere that the server might queue it up and wait for
> the 3WHS to complete (though if it is conformant, it will ACK only a single
> byte for the SYN and have the blob retransmitted by the client anyway).
>
> We did some tests with this when during TFO development. AFAIK Linux,
> *BSD/iOS, Windows all return a SYN that acks only the ISN but not the
> data. None of them queues the data hence the active side needs to
> retransmit the data. But my knowledge is 5 years old.
>

Which naturally makes me curious if this behavior has changed since TFO was
implemented. What sort of tools do/did you have to do this kind of probing?


> FWIW I and other TFO developers in other stacks (cc'd) have all found
> that some middle-boxes will crash or hang on receiving SYN-data
> (regardless of TFO cookie option or not). For instance, I found a bug
> in Linux netfliter that the conntrack module assumes SYN never carries
> data, so the internal sequence was only incremented by one on SYN-data
> packe, and led to rejecting every packet afterward. Others have found
> FW blocking the IP completely (see last meetings material by Praveen).
> We were talking about putting a TFO-bis on these issues.
>

This is really good information: thanks! I think if tcpinc decides to
pursue early data, we'll want to learn as much as possible from your
experience.

Kyle