On Tue, 2006-09-05 at 16:35 +0100, Jenny Hopkins wrote:
On 05/09/06, Jonathan McDowell noodles@earth.li wrote:
I'd be checking MTUs everywhere, especially if he's on ADSL. Can he do other large transfers over the OpenVPN link to the same server?
He says (this is going to be awkward!) on our irc channel:
nickb I assumed that MTUs would affect throughput a lot but not the fact of it arriving. nickb Is this because of udp? nickb tcp will break up and reassemble packets
OpenVPN supports Path MTU Discovery which *should* find the maximum packet size which can be supported, "--mtu-disc yes" I decided I'd rather set a conservative Maximum Segment Size (-mss-fix 1250) to ensure that TCP running through the tunnel always worked.
Ideally you find the maximum packet size and make sure all packets are <= that value.
The packet header:data ratio is higher with smaller MTU's, but it is true that MTU can effect delivery or non-delivery. Not every place in a network will want to spend time fragmenting packets and may elect to drop a packet bigger than it's locally configured mtu.
Adam