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Date:      Sun, 01 Jul 2007 02:02:50 +0200
From:      cpghost <>
To:        Modulok <>
Cc:, chloe K <>
Subject:   Re: MTU question
Message-ID:  <>
In-Reply-To: <>
References:  <> <>

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Modulok wrote:
> On 6/30/07, chloe K <> wrote:
>> Hi all
>>   ls our network provider insists to set the mtu to 1600? but I can 
>> only set
>> the freebsd as 1500.
>>   ls there any network issue?
>>   thank you
> A value of 1600 is not standard compliant. A value of 1500 is the
> largest MTU for standard ethernet, as stated in RFC 894 - "A Standard
> for the Transmission of IP Datagrams over Ethernet Networks", and RFC
> 1191 - "Path MTU discovery." Perhaps your provider is confused?
> RFC 894:
> "...the maximum length of an IP datagram sent over an Ethernet is 1500 
> octets."

That's correct, but...

perhaps they're not using Ethernet? Some layer 2 technologies also
support larger frames, e.g. 4k. However, any router can (and generally
will) fragment an IP packet into many IP packets so that they fit into
smaller MTUs (provided the don't fragment bit is not set). Those packets
will then be routed independently of each other, and usually reassembled
at the end point by the TCP stack of the receiving machine.

> As far as overriding this to a higher value, you may be restricted in
> doing so by your hardware, as stated in the ifconfig(8) manpage,
> "...Not all interfaces support setting the MTU, and some interfaces
> have range restrictions."
> -Modulok-

There are also some other reasons to avoid jumbograms and stick
to Ethernet's 1500 MTU (or less, as specified by the ISP, often if they
tunnel their traffic to their upstream provider).


Cordula's Web.

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