mailing list archives
Re: I'm missing 2 bytes (GRE implementation)
From: "Stefan Fouant" <sfouant () shortestpathfirst net>
Date: Tue, 09 Aug 2011 18:23:52 -0700
Everything from checksums, keys, and sequence numbers is optional. The only required fields IIRC amount to 2 bytes of
overhead. Sounds like they both interpret what should be included in the GRE header slightly differently.
GPG Key ID: 0xB4C956EC
Sent from my HTC EVO.
----- Reply message -----
From: "Franck Martin" <fmartin () linkedin com>
Date: Tue, Aug 9, 2011 5:57 pm
Subject: I'm missing 2 bytes (GRE implementation)
To: "nanog () nanog org" <nanog () nanog org>
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)?