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[Xen-users] Re: [Xen-devel] Ethernet MTU

To: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Subject: [Xen-users] Re: [Xen-devel] Ethernet MTU
From: "Molle Bestefich" <molle.bestefich@xxxxxxxxx>
Date: Wed, 16 Aug 2006 12:28:03 +0200
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, Sylvain Coutant <sco@xxxxxxxxxx>
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James Harper wrote:
If this is a problem, it must be a new one - I've trunked a virtual
interface from dom0 to domU before without any hassles (except for a
memory leak which kills dom0 after a few days)

Fiddling with the MTU sizes has been mentioned as a workaround for
VLAN problems on xen-users countless times, AFAIR.

Quoting Cisco:

"The IP protocol was designed for use on a wide variety of
transmission links. Although the maximum length of an IP datagram is
64K, most transmission links enforce a smaller maximum packet length
limit, called a MTU. The value of the MTU depends on the type of the
transmission link. The design of IP accommodates MTU differences by
allowing routers to fragment IP datagrams as necessary. The receiving
station is responsible for reassembling the fragments back into the
original full size IP datagram.

IP fragmentation involves breaking a datagram into a number of pieces
that can be reassembled later. The IP source, destination,
identification, total length, and fragment offset fields, along with
the "more fragments" and "don't fragment" flags in the IP header, are
used for IP fragmentation and reassembly. For more information about
the mechanics of IP fragmentation and reassembly, please see RFC 791:

... just in case anyone wanted to read half a RFC on the mailing list :-)

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