[AusNOG] GRE Tunnel MTU suggestions

[Ben Dale]

Hi Joseph

MSS is the size of the payload inside a TCP packet, so basically the IP MTU - TCP Header = MSS

Assuming Cisco gear (L2 headers not counted in MTU calculations), then the back of the napkin maths gets you:

IP MTU 1500 Bytes = [ 20 bytes IP Header ] + [ 1480 Payload ]
GRE IP MTU = [ 1480 bytes Payload ] - [ 4 bytes GRE ] - [ 20 bytes Inner IP Header ] = 1456 bytes  
MSS = [ 1456 bytes GRE IP MTU ] - [ 20 bytes TCP Header ] = 1436 bytes

Cheers,

Ben


On 1 Jul 2014, at 4:56 pm, Joseph Goldman <joe at apcs.com.au> wrote:

> Hi List,
> 
> Setting up a GRE tunnel for a customer and would appreciate a bit of input.
> 
> I can successfully push 1500byte packets with df-bit set between the 2 endpoints (1501 fails), so it is a full mtu of 1500.
> 
> I'd like to set an ip mtu on the tunnel and an ip tcp adjust-mss. Obviously I can't use 1500 as we have to account for GRE, so I'd like to know the best suggestions for an MTU, and if its worth setting the MSS at the same size as the MTU or if I should lower the MSS adjust and if so by how much?
> 
> Note: IPSec is not used on top, just GRE.
> 
> Thanks,
> Joe
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