Re: [Gen-art] Review of draft-ietf-6man-rfc1981bis-04

Joe Touch <touch@isi.edu> Thu, 16 February 2017 21:43 UTC

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Subject: Re: [Gen-art] Review of draft-ietf-6man-rfc1981bis-04
To: Stewart Bryant <stewart.bryant@gmail.com>, "Templin, Fred L" <Fred.L.Templin@boeing.com>, Brian E Carpenter <brian.e.carpenter@gmail.com>, "gen-art@ietf.org" <gen-art@ietf.org>
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From: Joe Touch <touch@isi.edu>
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Date: Thu, 16 Feb 2017 13:42:42 -0800
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On 2/16/2017 1:29 PM, Stewart Bryant wrote:
>
>
> On 16/02/2017 18:49, Joe Touch wrote:
>>
>> On 2/16/2017 7:59 AM, Stewart Bryant wrote:
>>>
>>> On 14/02/2017 23:00, Templin, Fred L wrote:
>>>> Unless there is operational assurance of
>>>> some size X>1280, however, tunnels have to use fragmentation to
>>>> guarantee that - at a minimum - packets up to 1280 will get through.
>>> In that case there really needs to be a note about MPLS.
>> IMO, this doc shouldn't be discussing tunneling as any different from
>> any other link.
>>
>>> You can fragment into an IP tunnel, but not an MPLS tunnel, because
>>> you cannot fragment the payload as you can in IPv4 and you cannot
>>> fragment MPLS.
>> There's no such thing as an "MPLS tunnel";
>
> In the MPLS world an "MPLS tunnel" is a common name for an MPLS LSP that
> is used to carry some payload such as IP.

MPLS just adds the label tags; there's no length field.  It's only a
tunnel when it goes over some other encapsulation layer, e.g., IP or
Ethernet.

>
>> at best, it's "MPLS over X",
>> e.g., MPLS over ethernet. MPLS doesn't indicate a message length so
>> while it can't support fragmentation it would never need it either. It
>> would be the next layer down (e.g., Ethernet, ATM, etc.) that might have
>> needed fragmentation.  If that can't be supported, then the addition of
>> MPLS would just reduce the effective MTU of the MPLS-over-X link.
>
> It was of course IPv6 over MPLS that concerned me.

IPv6 over MPLS isn't yet a tunnel. It would be IPv6 over MPLS over IPv6,
presumably (IPv6 over IPv4 isn't guaranteed to work because IPv4 EMTU_R
is only 576, and for an IPv6 tunnel it would be required to be at least
1280+encaps).

An IPv6 packet can traverse an IPv6 over MPLS over Ethernet tunnel (with
a 1500B payload) as long as the MPLS headers are smaller than 1500-1280
bytes.

An IPv6 packet can traverse an IPv6 over MPLS over IPv6 tunnel as long
as the MPLS + added IPv6 headers are smaller than 1500-1280 bytes.
However, if the arriving (transit) packet is larger than
1280-MPLS-IPv6ecaps, the MPLS tunnel ingress would need to do source
fragmentation inside the tunnel (with the egress doing reassembly).

Again, all these cases are discussed in draft-ietf-intarea-tunnels but
none have relevance IMO to this doc.

Joe