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I'm missing 2 bytes (GRE implementation)

Subject: I'm missing 2 bytes (GRE implementation)
From: fmartin at linkedin.com (Franck Martin)
Date: Wed, 10 Aug 2011 00:57:44 +0000

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)?

Follow-Ups:
- I'm missing 2 bytes (GRE implementation)
  - From: dr at cluenet.de (Daniel Roesen)

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