This TCP segment is sent immediately by the stack. The ``SYN`` flag is set and the dot next to the ``S`` character indicates that the ACK flag is also set. The SYN+ACK segment does not contain any data but its acknowledgment number is set to 1 (relative to the initial sequence number). For outgoing packets, packetdrill_ does not verify the value of the advertised window. In this line, it also accepts any TCP options (``<...>``).
