Re: [PWE3] GAL above and below the PW label

[Sriganesh Kini <[e-mail]>]

See inline

On Thu, Mar 31, 2011 at 4:40 AM, Curtis Villamizar <[e-mail]> wrote:
>
> Sriganesh, Sasha, Loa, Dave,
>
> Would you please not top post.  See inline.
>
> Curtis
>
> In message <[e-mail]>
> Sriganesh Kini writes:
>>
>> Sasha, that issue with VCCV Type-3 (without CW) is not addressed by
>> the draft. Practically, is that a major issue ? Especially setting TTL
>> > 1 for SS PW is possible I guess but would be some kind of serious
>> operational oversight. Besides such a packet would fail some check
>> along the path.
>
> End to end OAM should set TTL>1 and not use VCCV Type-3.  The T-PE may
> not have reliable information to set TTL, so in MS-PW it might be
> worth discouraging VCCV Type-3 except for use as a traceroute
> capability or to address S-PEs by node distance along the path.

If there is indeed a concern of TTL exceeding number of segments (with
Type-3 and no CW), the same concern would apply when tracing. So
tracing would be impossible.

>
>> Also, what I heard at the mic regarding impl results is that it
>> doesn't make sense to deprecate what is already deployed.
>
> Yes.  But VCCV Type-3 with MS-PW is not already deployed.

I am not aware of the survey addressing this.

>
>> Note that CC Type 1 prevents ECMP (when FAT-PW is not available).
>> Using GAL below PW also requires ECMP based on label hashing to ignore
>> reserved labels.
>
> You don't want ECMP to hash on PW payload.  With CW and without
> fat-pw, you don't get ECMP on a PW but you do get ECMP on an LSP
> containing many PW.

When the PW payload is IP, the hashing it for ECMP is useful (If the
payload is not IP then use CW). FAT-PW may not be present in all
deployments.

>
>> 2011/3/31 Alexander Vainshtein <[e-mail]>:
>> > Sriganesh, Curtis and all,
>> > IMHO and FWIW I think that the only problem with VCCV Type 3 is the
>> > risk of OAM packets leaking out of T-PE towards the AC (if you
>> > accidentally set the TTL value too high). To the best of my
>> > understanding the offset proposal does not solve that - or did I miss
>> > something?
>> >
>> > One way to solve it is to use VCCV Type 1 (clean where applicable).
>> > Another way would be by using GAL below the PW label (possible but
>> > messy). If the industry moves towards deprecating Ethernet PWs
>> > without CW,  it becomes completely unnecessary.
>> >
>> > My 2c,
>> >     Sasha
>
> This is the context suggesting a potential problem with VCCV Type 3.
>
>> >> -----Original Message-----
>> >> From: [e-mail] [mailto:[e-mail]] On Behalf Of
>> >> Sriganesh Kini
>> >> Sent: Thursday, March 31, 2011 10:09 AM
>> >> To: [e-mail]
>> >> Cc: [e-mail]
>> >> Subject: Re: [PWE3] GAL above and below the PW label
>> >>
>> >> All,
>> >>
>> >> Just an FYI that a proposal for fate-sharing PW OAM and data given in
>> >> draft-kini-pwe3-inband-cc-offset
>> >>
>> >> Just to summarize the proposal.
>> >> 1. It does not require GAL in PW. TTL expiry is used to alert the S/T-
>> >> PE.
>> >> 2. It is mainly applicable when CW is not used in the PW.
>> >> 3. It uses a fixed offset (negotiated between PW endpoints) after the
>> >> label stack before starting the OAM msg.
>> >> 4. The bytes between the label-stack and the fixed offset is referred
>> >> to as a pseudoflow header and is filled with byte-values (by the PE)
>> >> that represent the flow for which OAM is desired. This helps PW OAM
>> >> and data to fate-share even when the intermediate node looks beyond
>> >> label stack to do multipath forwarding decisions.
>> >>
>> >>
>> >> On Thu, Mar 31, 2011 at 12:03 AM, Curtis Villamizar <[e-mail]>
>> >> wrote:
>> >> >
>> >> > Loa, Dave, Sasha,
>> >> >
>> >> > I've snipped from various posts on the thread that Sasha started.
>> >>  See
>> >> > inline.
>> >> >
>> >> > In message <[e-mail]>
>> >> > Loa Andersson writes:
>> >> >>
>> >> >> All,
>> >> >>
>> >> >> missed a nuance in Sasha subject line.
>> >> >>
>> >> >> We have two issues
>> >> >>
>> >> >> - where the GAL is placed relative to the PW label, I  believe it is
>> >> >>    necessary to have the GAL below the PW label.
>> >> >>
>> >> >> - whether the the GAL label needs to be bottom of stack or not, it
>> >> >>    figure that this is really a discussion if it is possible to have
>> >> >>    a FAT label below the GAL or not. I'm not sure  about my
>> >> preferences
>> >> >>    but I think it is possible.
>> >> >>
>> >> >> /Loa
>> >> >
>> >> > I agree with Loa's summary.  Not putting GAL at the bottom may
>> >> confuse
>> >> > some LSR, but putting it above the PW label is likely to be even more
>> >> > problematic.
>> >> >
>> >> >
>> >> > In message
>> >> <[e-mail]
>> >> .se>
>> >> > David Allan I writes:
>> >> >>
>> >> >> It does become a new behavior, GAL with S=3D0. However the
>> >> combination of G=
>> >> >> AL being top label at the S-PE and TTL being encoded in the PW label
>> >> means =
>> >> >> that fate sharing is broken at every S-PE. Life only gets a little
>> >> more str=
>> >> >> ange if there is a FAT label as well...
>> >> >>
>> >> >> That being said, GAL as bottom label is broken in any ECMP
>> >> environment, whi=
>> >> >> ch is why GAL is a TP construct.
>> >> >>
>> >> >> my 2 cents
>> >> >> D
>> >> >
>> >> > Dave,
>> >> >
>> >> > If GAL is broken for ECMP, which it is, then all TP OAM is broken.
>> >> > If all it takes to fix it is simple then lets just fix it.
>> >> >
>> >> > This is what we'd have to do.
>> >> >
>> >> >  1) Relax the requirement that GAL be at the bottom
>> >> >
>> >> >  2) Have the ingress insert GAL in the stack immediately below the
>> >> >     label for which the measurement is made, keeping the rest of the
>> >> >     label stack in place.
>> >> >
>> >> > The only thing the midpoing LSR at a multipath (LAG, link bundle for
>> >> > MPLS) has to do is skip over the GAL when hashing, as if the GAL
>> >> > wasn't there.  That will yield the same hash value and it will
>> >> > preserve fate-sharing across multipath.
>> >> >
>> >> > I prefer that we fix things rather than complain that they are
>> >> broken.
>> >> >
>> >> > Curtis
>> >> > _______________________________________________
>> >> > pwe3 mailing list
>> >> > [e-mail]
>> >> > https://www.ietf.org/mailman/listinfo/pwe3
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> - Sri
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>> >
>>
>>
>>
>> --
>> - Sri
>>
>>
>
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-- 
- Sri