RE: Last Call: <draft-ietf-6man-oversized-header-chain-08.txt> (Implications of Oversized IPv6 Header Chains) to Proposed Standard

["Templin, Fred L" <[e-mail]>]

Hi Brian,

Responding in a slightly re-arranged order:

> The problem is that you are asserting that middleboxes that a tunnel
> passes through are expected to examine the complete header chain of
> the encapsulated packet even if the encapsulated packet is a fragment.

Yes, but change "are expected to" to "should be able to".

> I think that's an unreasonable expectation. A reasonable expectation
> is that middleboxes should identify the encapsulated packet as
> a payload that they cannot analyse, and let it go (unless they
> have a policy setting to drop tunnelled packets, which is a
> different discussion).

But why? If headers beyond the first IPv6 encapsulation header are
available in the clear, the middlebox should be able to parse them
if it wants to. Wireshark already does exactly that - it keeps on
parsing beyond the first encapsulation header up to and including
the true ULP header. And, if Wireshark can do it, so can any other
middlebox that believes security would be improved by continuing
to parse the entire chain - whether or not there is a standard
saying it must not do so.

Parsing the additional headers beyond the first encapsulation header
provides defense-in-depth. Perimeter middleboxes can then weed out
the bad stuff without either allowing the bad stuff to penetrate more
deeply into the organization or dropping good stuff that should be
allowed through.

> Since the problem recurses as we tunnel tunnels, I don't see how any
> finite limit can solve the problem. 1280 itself is a pragmatic choice
> of "a bit shorter than 1500".

The 1280 is assuming that all links in the path have a 1500 MTU and
so RFC2460 allowed (1500 - 1280) = 220 bytes for additional IPv6
headers added by nested tunnels without incurring fragmentation.
I am asserting instead that we have to allow for paths that include
links with a 1280 MTU and so tunnels will have to fragment over
such paths.

That means that the first fragmenting tunnel would have room for
1240 in the first fragment, the second fragmenting tunnel would
have room for 1200 in the first fragment, etc. That is why I would
prefer that hosts limit the size of their header chains to 1024; so
that nested tunnels that fragment will still be highly likely to
have the entire header chain in the first fragment.
 
> I understood that to be the basis on which the WG reached consensus.

Maybe the WG didn't understand that such a consensus would make
tunnels less reliable and less secure.

Thanks - Fred
[e-mail]

>     Brian