Handle ECN handshake in simultaneous open

[rscheff]

Summary

While addessing the simultaneous open TCP options, and then
testing this with ECN found that negotiation to fail.

As normally SYN,ACK processing is done in the syncache, adding
a special case for simultaneous open ECN handling.

Test Plan

extended the syn-rcvd-via-syn-sent-ipv4.pkt script to include ECN
handshake, as well as a data segment to validate that ECT0 marks
are sent.

--ip_version=ipv4
--tolerance_usecs=50000

// Ensure that all relevant sysctl variables have their default variables.
 0.00 `sysctl -w net.inet.tcp.rfc1323=1`
+0.00 `sysctl -w net.inet.tcp.sack.enable=1`
+0.00 `sysctl -w net.inet.tcp.ecn.enable=1`
+0.00 `sysctl -w net.inet.tcp.rexmit_slop=200`
+0.00 `sysctl -w net.inet.tcp.rexmit_drop_options=0`
+0.00 `sysctl -w kern.ipc.maxsockbuf=2097152`
// Flush host cache.
+0.00 `sysctl -w net.inet.tcp.hostcache.purgenow=1`
// Create a TCP endpoint in the SYN-RCVD state.
+0.00 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.00 fcntl(3, F_GETFL) = 0x02 (flags O_RDWR)
+0.00 fcntl(3, F_SETFL, O_RDWR | O_NONBLOCK) = 0
+0.00 getsockopt(3, SOL_SOCKET, SO_RCVBUF, [65536], [4]) = 0
+0.00 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0.00 > SEW  0:0(0) win 65535 <mss 1460,nop,wscale 6,sackOK,TS val 100 ecr 0>
+0.10 < SEW  0:0(0) win 32767 <mss 1460>
+0.00 > SE. 0:0(0) ack 1 win 65535 <mss 1460>
// Verify that there are no errors pending at the socket layer.
+0.10 getsockopt(3, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
// Now it is in the SYN-RCVD state.
+0.00 %{ assert tcpi_state == TCPI_SYN_RECEIVED }%

+0.01 <[noecn] . 1:1(0) ack 1 win 65535
// Set Socket back to blocking state
+0.00 fcntl(3, F_SETFL, O_RDWR) = 0
//Verify that data segment is sent with ECT0
+0.00 write (3, ..., 10) = 10
+0.00 >[ect0] P. 1:11(10) ack 1
// Tear it down.
+0.00 close(3) = 0

[rscheff]

Note: BBR does not have ECN support currently.

[rscheff]

• update to include only ECN diff against master

[rgrimes]

Approving as mentor, only a visual code read was done.