Re: [mpls] working group last call on draft-ietf-mpls-tp-mip-mep-map

["Adrian Farrel" <[e-mail]>]

<co-author mode>

Hi Shahram,

I am worried about the precedent of a comment like this during WG last call.
While comments that improve the document or point out fundamental flaws are
welcome whenever they arrive, points with the flavour "I wouldn't have done it
like this" that arrive this late in the process don't feel very constructive.
But I will leave the chair to worry about process and try to address the
technical points...

> Identifying whether to terminate an OAM packet and process it in In-MIP vs.
Out-
> MIP requires line rate lookup, otherwise the OAM packet will not take the same
> path as data packets.  Therefore any MIP identifier that is proposed in this
draft
> requires one extra lookup and therefore adds significantly to cost.

If I am not wrong, this is a feature of an out-MIP. If you decide to implement
out-MIPs, and if you want the OAM to follow exactly the same path as the data,
then it is a requirement that the out interface inspects the packets (at line
rate) to determine whether they are OAM and targeted at the interface.

We cannot change that aspect. All we can do is aim to make the lookup as easy as
possible.

> Perhaps a
> similar method to Ethernet MDL/MEL (Maintenance Domain Level) may be used
> that requires only 3 bits and achieves the same result.

Perhaps it could.
But before going there, why is the lookup in the current version of the I-D
arduous?

Presumably you do not propose making any change to the way In-MIPs are currently
identified, so the lookups being done at line rate today on the incoming
interfaces will not be changed. If you are proposing such a change, then the
discussion is outside the scope of this I-D and becomes a much wider question
for the working group.

This leaves me with the trade-off of enabling a *simpler* lookup on the outgoing
interfaces versus doing identical lookups on both interfaces. My assumption was
that if the incoming interface can do the lookup at line rate, it is not hard to
perform the same lookup on the outgoing interface. Furthermore, there is a
reduction in complexity by having fewer things to look up. 

Another possibility is that the full lookup could be done on the incoming
interface and the packet marked for easy interception on the outgoing interface.
The concern with this approach is that the packet would no longer be being
forwarded exactly as data because it would be being modified in flight.
Furthermore, in the case of P2MP, it is not enough to flag the packet as a local
Out-MIP and further identifier-based lookup is needed.

Some of these issues were raised and discussed as the I-D progressed, and some
of the alternative solutions were tracked with their pros and cons in Appendix A
of the I-D (look at revision -03).

Thanks,
Adrian
> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]] On Behalf Of
> Adrian Farrel
> Sent: Monday, November 19, 2012 8:45 AM
> To: 't.petch'; 'Loa Andersson'; [e-mail]
> Cc: [e-mail]; [e-mail];
draft-ietf-mpls-tp-mip-
> [e-mail]
> Subject: Re: [mpls] working group last call on draft-ietf-mpls-tp-mip-mep-map
> 
> Yeah, it's a boring draft. Did you expect me to co-author anything else?
> 
> The point was that when I started the I-D lots of people were saying "it's
> complex" and "it can't be done" and "it won't be backward compatible".
> 
> So the I-D says "here it is"
> 
> A (sorry not to offer you excitement)
> 
> > -----Original Message-----
> > From: t.petch [mailto:[e-mail]]
> > Sent: 19 November 2012 12:38
> > To: Loa Andersson; [e-mail]
> > Cc: [e-mail]; [e-mail]; MPLS-TP ad hoc
team;
> > [e-mail]
> > Subject: Re: [mpls] working group last call on
draft-ietf-mpls-tp-mip-mep-map
> >
> > After getting to section 6 and its features (requirements!), I find
> > myself underwhelmed; is that it?  Well, I suppose so, it is
> > Informational and not Standards Track.
> >
> > Meanwhile, I suggest some editorial issues.
> >
> > Title
> > Handling MPLS-TP OAM Packets Targeted at Internal MIPs
> > [Handling MPLS-TP OAM Packets Targeted at Interface MIPs
> > seems a more informative statement unless and until you get to the
> > definition of Internal in s3; and s6, which is the crux of the document
> > says
> > The preferred solution to per-interface MIP message handling is
> >    presented in this section]
> >
> > s1
> > two (or more) MIPs per node on both sides of the forwarding engine.
> > [two on both sides sounds like four in total to me; suggest 'one on each
> > side of the forwarding engine']
> >
> > s4
> >    o  CV between a MEP and a MIP
> > [expand CV on first use]
> >
> > s5
> > In-band OAM messages are sent using the G-ACh [RFC5586] for MPLS-TP
> >    LSPs and MPLS-TP PWs, respectively.
> > ['respectively' suggests to me that there should be two precedents, not
> > just RFC5586; the second paragraph specifies RFC5586 for LSPs,
> > RFC6423/RFC4385 for PWs, in which case, strike this sentence as
> > redundant]
> >
> > s6
> > The appendix of this document contains a
> >    few solutions that the authors have discarded which have been left in
> >    the document for informational purposes.
> > [not any more they haven't!]
> >
> > The node itself is addresses
> > [The node itself is addressed]
> >
> > The identification information indside
> > [The identification information inside ]
> >
> > MIP identifiers are not know
> > [MIP identifiers are not known]
> >
> > reserved MIP address
> > [reserved MIP addressses or a reserved MIP address]
> >
> > Tom Petch
> >
> >
> > ----- Original Message -----
> > From: "Loa Andersson" <[e-mail]>
> > To: <[e-mail]>
> > Cc: <[e-mail]>; <[e-mail]>; "MPLS-TP ad
> > hoc team" <[e-mail]>;
> > <[e-mail]>
> > Sent: Wednesday, November 14, 2012 3:16 PM
> >
> > > Working Group,
> > >
> > > This is to start a 2 week working group last call on
> > > draft-ietf-mpls-tp-mip-mep-map.
> > >
> > > Please send your comments to the mpls working group mailing
> > > list ([e-mail]).
> > >
> > > Please send both technical comments, and if you are happy with the
> > > document as is also indications of support.
> > >
> > > This working group last call will end on November 28.
> > >
> > > /Loa
> > > for the wg co-chairs
> > >
> 
> 
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