XPost: comp.os.linux.networking
From: grahn+nntp@snipabacken.se
["Followup-To:" header set to comp.protocols.tcp-ip.]
On Mon, 2010-09-27, Morten Reistad wrote:
> In article ,
> Mark Hobley wrote:
>>My LAN is ethernet based. However, for connectivity to the internet, I have
>>an Edimax 3g-6200n router, which connects to the internet via a 3g mobile
>>broadband stick.
>>
>>Can a 3g mobile broadband stick route the 1500 byte packets arriving from
>>the LAN without fragmentation, or should I reduce the size of the MTU for
>>outbound traffic? If so, what should I reduce it to? (My internet service
>>provider is Three Mobile, if that matters.)
>
> You expect us to answer this? You are sitting on the network. Fire
> up wireshark, or even tcpdump, and send som max-sized ping packets
> to somewhere, and then you can tell us.
Yeah.
I can add that there's most likely at least one bottleneck: at one
point your packets go over GTP over UDP over IP, on Ethernet.
*Someone* will get hurt if you use 1500-octet packets -- possibly the
operator.
>>My next question is ...
>>
>>Will Linux allow me to configure two MTU sizes on a single network interface,
>>depending on whether or not the destination address is local or remote?
>>
>>If I need to reduce the MTU for outbound traffic, can I keep using a 1500
>>byte MTU for LAN traffic?
>
> No. You can use a setsocopt to set tcp segment sizes on sockets. I would
> like to see this as a sysctl variable. One mtu for all. The linux box
> will ACCEPT bigger muts though.
You've mentioned this several times recently. What about PMTU
discovery -- doesn't that work for you? It seems to work well enough
(or never kick in) for me.
/Jorgen
--
// Jorgen Grahn O o .
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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