Re: [Quickfix-developers] Missing FIX messages in Initiator?

[Joerg T. <Joe...@ma...>]

Edde wrote:
> By MTU you mean, Maximum Transmission Unit? (I had to look it up on Google ;-))

Sorry, I was a bit in a hurry. Probably you also read some explanations about MTU...

> So, does this mean that the VPN connection can be limited to a certain
> MTU and if a unit arrives with a bigger size the VPN turns into a
> bottleneck?

My knowledge is rather vague here, any network export may correct me if I tell nonsense...

An IP packet crosses different networks (ethernet, ADSL, ATM etc etc) and every network 
has a restriction known as MTU. There are is also something like MTU discovery to get MTU 
which fits all crossing nets. Otherwise, some intermediate routers may have to fragment 
the packet (and later re-assemble it). Nowadays, in a world of firewalls and security 
concerns this does not seem to work very well. E.g. if the network admin disables all ICMP 
messages (MTU discovery is an ICMP message).

A VPN is a virtual network on top of a physical network. I.e. all packets are wrapped into 
network headers (and trailers) for this virtual network. But the physical space for 
payload (the MTU) stays the same, so the MTU possible over the virtual network is physical 
MTU - virtual header size - virtual trailer size.

If your normal MTU is 1500 (ethernet), and your VPN wrapper creates e.g. 200 bytes of 
overhead per packet, setting the MTU to 1300 (1500-200) may cure the kind of problems 
described.

Actually this is a hack to workaround bad router and network implementations somewhere on 
the way through the internet, but it works for us.

Cheers, Jörg

-- 
Joerg Thoennes
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