I'm missing 2 bytes (GRE implementation)

[Franck Martin <fmartin () linkedin com>]

I'm using a GRE IPv4 tunnel between a cisco and linux machines

I did some packet capture, and saw that my MTU was 1418, but the cisco was sending TCP packet with a MSS of 1380. This 
created a bunch of issues. When I told the cisco box to use a MSS of 1378 everything starting to work fine.

So why Cisco is off by 2 Bytes?

Does the GRE implementation on Linux uses 2 extra bytes compared to Cisco (or vice versa)?