At 09:59 AM 1/12/98 -0500, Phillip Vandry wrote:
>Net searches are not revealing the answer to the following question... is
>the GRE tunelling spec published?
>
See rfc 1701
-- Alex.
>Question: What happens when a GRE tunnelled packet requires fragmentation
>along the way?
>
>The default MTU for tunnel interfaces (and indeed for most interfaces) is
>1500, but if the tunnelled packets go out on, say, Ethernet, then the
>MTU would more correctly be 1500-(size of GRE and IP header).
>
>So when the tunnel finds that it needs to fragment packets along the way,
>I see two options at its disposal:
>
>(1) Behave as though the MTU of the tunnel interface was indeed lower
>than it is configured and fragment the packet before it enters the
>tunnel. Send back an error if the don't fragment bit is set (normal
>fragmentation processing).
>
>Advantages: Allows the sending host to perform path MTU discovery as normal.
>
>Disadvantages:
> - effective MTU may be lower than the interface's configured MTU, making
> the configured value untrustworthy
> - some passenger protocols may not support fragmentation or have higher
> minimum MTU values than IP.
> - Exposes the properties of the links used by the tunnel, which you
> might be trying to "hide" by using a tunnel
>
>To implement this: set IP_DF in the transport IP header
>
>(2) Honor the tunnel interface's configured MTU value and fragment the
>transport packets while leaving the passenger packets intact (the other
>tunnel endpoing reassembles)
>
>Advantages: Best transparency
>
>Disadvantages: Path MTU discovery cannot work and every large packet
>destined to the tunnel might be transmitted as two packets, nearly
>doubling the load and adding ~40 bytes overhead.
>
>To implement this: do not set IP_DF in the transport IP header
>
>In light of the answer to thie question (I suspect it is (2)), should I
>use a lower MTU on tunnel interfaces to prevent fragmentation, if the
>minimum MTU of the tunnel transport links is known? And if so, what is
>the exact size of the GRE/IP header to subtract from this minimum?
>
>-Phil
>
>
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