Subject: [arch-d] Tunnel MTU link adaptation and the Reserved Fragmentation (RF) bit

From: "Templin, Fred L" <[email-masked]>
Date: Mon, 7 May 2007 12:04:44 -0700

IPv6-in-(foo)-in-IPv4 tunneling mechanisms (e.g., LISP,
IPvLX, ISATAP, Teredo, 6to4, etc.) are currently limited
to sending packets only slightly larger than the IPv6
minMTU of 1280 bytes due to operational aspects of the
IPv4 Internet. A solution to this limitation is given in
'draft-templin-linkadapt', which specifies a Sub-IP layer
mechanism that supports larger MTUs (up to 9KB or larger)
through link adaptation at a layer below IPv6 fragmentation
but above IPv4 fragmentation.

A key requirement of the spec is that both the ITR and ETR
be made aware that the link adaptation scheme is being used,
and previous versions of the spec did this by requiring the
ITR to set the "Reserved Fragmentation (RF)" bit in the IPv4
headers of tunneled packets. The current document version
(-05) has gone away from this approach and now requires a
prior negotiation between the ITR and ETR.

This decision has some important ramifications; first, it
sets the use or non-use of the link adaptation on a per-
tunnel basis rather than on a per-flow or per-packet basis.
This means that all flows and all packets that traverse the
tunnel would either use the scheme or not. Secondly, it
requires that there be an explicit prior negotiation between
the ITR and ETR as to whether the scheme is enabled. Finally,
it does not allow for an adpative "revert to classical IPv4
fragmentation" option after the tunnel is established.

Use of the RF bit has some associated uncertainties, however.
For example, it is not clear as to whether the state of the
RF bit would be preserved across all paths since RFC791 says
that it is "reserved, must be zero" but does not say what
network middleboxes should do with a packet that has RF=1.
The other consideration is that there are other proposals
that would like to make use of this bit.

So, can we have some discussion on the use or non-use of
the RF bit for link adaptation over IPv6-in-(foo)-in-IPv4
tunnels?

Fred
[email-masked]

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