[PLUG] Cannot set mtu in profile.d on Ubuntu 8.10

Parag Shah adaptives at gmail.com
Mon Apr 19 15:33:00 IST 2010

Hi Abhijeet,

On Tue, Apr 13, 2010 at 12:39 PM, Abhijit Bhopatkar
<bain at devslashzero.com>wrote:

> The cvs will have a huge bulk traffic going in and out. The main
> problem with MTU is not at server or client side, but is usually in
> the middle, the routers. The problem is that some routers on the
> network are not able to handle large packets very well. They start
> building up huge chains of backlog packets and eventually cause TCP
> retransmissions. The problem keeps aggravating very quickly and within
> seconds the network traffic for that particular connection starts
> grounding to halt. Addition of VPN on top of CVS might just be tipping
> the sizes beyond the trigger point for your network setup. The problem
> is typically intense when a cheap router is being used to share a
> broadband connection on LAN.
> This is just a guess, all the network guru's thrash me  if i am wrong :p
> The Network manager might be resetting the interfaces, you can add
> scripts in  /etc/NetworkManager/dispatcher.d instead to set the MTU on
> connection UP or DOWN for every interface.
> Take a look at 01ifupdown script as an example you should add 02MTU or
> something like that.
> BTW you could also use the if-updown scripts as being invoked from
> 01ifupdown. I never used them so don't know how those work.
> _______________________________________
> Pune GNU/Linux Users Group Mailing List

Thanks for your suggestions.

In the context of what you mentioned about the chain of backlog packets
building up at the router:

If a router cannot handle the size of a packet it receives, will it drop the
packet, or will it try to fragment it?
If I understand the above passage correctly, what you are suggesting is, if
a router cannot handle large packets, it is going to cause re-transmissions
and very soon the network will become sluggish. In this case will that
router become a bottleneck for the entire LAN connected through it? The
reason I ask this is because, if I am able to determine that the router is
not becoming a bottleneck for the entire network, then would it be safe to
say that the router is probably not responsible for mis-handling large

Thanks... your suggestion for creating a script
'/etc/NetworkManager/dispatcher/d02mtu' worked very well.

Thanks & Regards
Parag Shah

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