Fix TCP Fast Open for TCP RACK

[tuexen]

Summary

TCP Fast Open does not work currently in combination with TCP RACK. This patch fixes the RACK specific bugs for TCP Fast Open:

• Fix a bug where the SYN handling during established state was applied to a front state.
• Move a check for retransmission after the timer handling which was suppressing timer based retransmissions.
• Fix an off-by one byte in the sequence number of retransmissions.
• Apply fixes corresponding to r336934.

Test Plan

Run the following packetdrill scripts:

passive-connection-setup-using-fo-two-sends-ipv4.pkt2 KBDownload

passive-connection-setup-using-fo-syn-ack-rtx-01-ipv4.pkt2 KBDownload

passive-connection-setup-using-fo-syn-ack-rtx-00-ipv4.pkt2 KBDownload

passive-connection-setup-using-fo-rcv-rst-ipv4.pkt2 KBDownload

passive-connection-setup-using-fo-ipv6.pkt2 KBDownload

passive-connection-setup-using-fo-ipv4.pkt2 KBDownload

active-connection-setup-using-fo-two-sends-ipv4.pkt3 KBDownload

active-connection-setup-using-fo-syn-rtx-ipv4.pkt3 KBDownload

active-connection-setup-using-fo-rcv-rst-ipv4.pkt3 KBDownload

active-connection-setup-using-fo-ipv6.pkt3 KBDownload

active-connection-setup-using-fo-ipv4.pkt3 KBDownload

[rrs]

sys/netinet/tcp_stacks/rack.c

5403 ↗

(On Diff #47317)

So no challenge ack is now sent. Is that going to cause a
problem with respect to RFC5961?

[tuexen]

sys/netinet/tcp_stacks/rack.c

5403 ↗

(On Diff #47317)

RFC 5961 requires this "in the synchronized" states. rfc793 states on page 32, for example:

If the receiving TCP is in a  non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user.

The code taken out is in rack_do_syn_recv(), so this is not in a synchronized state as required by RFC 5961.

So I think it is fine to take it out.