Merge pull request #157 from matttbe/dss_drop_after_data_fallback

mptcp: check fallback if opt dropped after first data

gtests/net/mptcp/dss/dss_drop_after_data_fallback.pkt

@@ -0,0 +1,30 @@
+--tolerance_usecs=100000
+`../common/defaults.sh`
+
+0.0   socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 3
++0    setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+
++0    bind(3, ..., ...) = 0
++0    listen(3, 1) = 0
+
+// 3WHS is OK
++0.0    <  S   0:0(0)                    win 65535  <mss 1460, sackOK, nop, nop, nop, wscale 6, mpcapable v1 flags[flag_h] nokey>
++0.0    >  S.  0:0(0)           ack 1               <mss 1460, nop, nop, sackOK, nop, wscale 8, mpcapable v1 flags[flag_h] key[skey]>
++0.1    <   .  1:1(0)           ack 1    win 2048                                              <mpcapable v1 flags[flag_h] key[ckey=2, skey]>
++0    accept(3, ..., ...) = 4
+
+// Data from the client with valid MPTCP options (no DATA_ACK: normal)
++0.1    <  P.  1:501(500)       ack 1    win 2048                                              <mpcapable v1 flags[flag_h] key[skey, ckey] mpcdatalen 500, nop, nop>
+// From here, the MPTCP options will be dropped by a middlebox
++0.0    >   .  1:1(0)           ack 501                                                        <dss dack8=501 dll=0 nocs>
++0.1   read(4, ..., 500) = 500
++0     write(4, ..., 100) = 100
+
+// The server replies with data, still thinking MPTCP is being used
++0.0    >  P.  1:101(100)       ack 501                                                        <dss dack8=501 dsn8=1 ssn=1 dll=100 nocs, nop, nop>
+// But the client already did a fallback to TCP, because the two previous packets have been received without MPTCP options
++0.1    <   .  501:501(0)       ack 101  win 2048
+
++0.0    <  P.  501:601(100)     ack 101  win 2048
+// The server should fallback to TCP, not reset: it didn't get a DATA_ACK, nor data for more than the initial window
++0.0    >   .  101:101(0)       ack 601