Re: [L2tpext] Suresh Krishnan's Discuss on draft-ietf-l2tpext-keyed-ipv6-tunnel-07: (with DISCUSS)

Giles Heron <> Wed, 08 February 2017 17:08 UTC

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From: Giles Heron <>
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Date: Wed, 8 Feb 2017 17:07:56 +0000
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Subject: Re: [L2tpext] Suresh Krishnan's Discuss on draft-ietf-l2tpext-keyed-ipv6-tunnel-07: (with DISCUSS)
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Sorry for the delay in responding.

At any rate I don’t think this is an issue.  As far as I can tell the mention of RFC2473 in section 4.1.4 of RFC3931 pertains to an IPv6 payload, not to IPv6 transport.  Our draft doesn’t discuss payload fragmentation - as we’ve generally assumed systems are acting as LACs rather than as LNSes (i.e. just forwarding at layer 2, not routing between a L2 circuit and a “home network”), though there’s nothing to stop an implementation that routes into a keyed IPv6 tunnel from fragmenting IPv4 packets before forwarding them into the tunnel, or performing L2TPv3 fragmentation for IPv4 or IPv6 packets.

In terms of preference the draft is pretty clear that it’s:
(1) ensure MTU is sufficient
(2) use L2TPv3 fragmentation
(3) NOT RECOMMENDED - use IPv6 fragmentation.

In case 3 (as in case 2) the ingress router will fragment and the egress router will reassemble.

or have I missed something?


> On 3 Nov 2016, at 04:04, Suresh Krishnan <>; wrote:
> Suresh Krishnan has entered the following ballot position for
> draft-ietf-l2tpext-keyed-ipv6-tunnel-07: Discuss
> When responding, please keep the subject line intact and reply to all
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> introductory paragraph, however.)
> Please refer to
> for more information about IESG DISCUSS and COMMENT positions.
> The document, along with other ballot positions, can be found here:
> ----------------------------------------------------------------------
> ----------------------------------------------------------------------
> * Section 5
> I am having a hard time seeing how fragmentation is expected to work 
>   It is NOT RECOMMENDED for routers implementing this specification to
>   enable IPv6 fragmentation (as defined in section 4.5 of RFC2460) for
>   keyed IP tunnels.  IP fragmentation issues for L2TPv3 are discussed
>   in section 4.1.4 of RFC3931.
> And that specific section of RFC3931 recommends using RFC2473 to tunnel
> the packets which again ends up using the RFC2460 fragment header that
> this draft is trying to forbid.
> So, can you please clarify exactly what happens when the size of the
> packet to be tunneled exceeds the MTU?
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