Looks like the code for checking duplicate packets
was changed in 1.1.3 or 1.1.4. The new code has a
bug, because it initialises gre.seq_recv to 0, which
implies that the server has already received packet
0, and expects packet 1 next.
However, according to RFC 2637, the first packet in
the connection is always number 0. Therefore, poptop
receives and discards this packet, thinking that it
has already received it.