The MPLS WG Archive

[Alia Atlas <aatlas@avici.com>]

Shahram,

I do not believe that the "path-specific method" is a good 
solution.  Unfortunately, it is one of the methods which is currently 
available.

Alia

At 02:33 PM 8/12/2002 -0700, Shahram Davari wrote:
>Alia,
>
>Sundara and I also presented a number of other problems with 
>"path-specific method".
>Having so many problems, I am wondering why do we have it in the first place?
>Why not have the Sender_template only?
>
>-Shahram
>
>
>
>
> > -----Original Message-----
> > From: Alia Atlas [mailto:aatlas@avici.com]
> > Sent: Monday, August 12, 2002 4:24 PM
> > To: Sundara Murugan
> > Cc: Shahram Davari; mpls@UU.net
> > Subject: RE: fast-reroute merging
> >
> >
> > The idea of merging detour Path messages with different EROs
> > but the same
> > egress interface is based upon the "path-specific method".  With that
> > method, it is required because there is no way to determine
> > from the RESV
> > which of the two (or more) detour Path messages the RESV
> > belonged to.  This
> > is because both detour Path messages have the same
> > SENDER_TEMPLATE; the
> > Path messages can be distinguished by the Detour object and the EROs.
> >
> > However, doing such merging has very bad consequences.
> > First, there are
> > cases where a detour LSP which was up is temporarily
> > unavailable.  Second,
> > it is not possible to guarantee protection against an SRLG failure.
> >
> > The solution is to use the "sender-template-specific" method,
> > where the
> > sender address in the SENDER_TEMPLATE is set to an address
> > belonging to the
> > PLR instead of being left as an address belonging to the ingress.
> >
> > I presented about the problems of merging Path messages with
> > different EROs
> > in Salt Lake City.  The draft I did then,
> > draft-atlas-rsvp-local-protect-interop-02.txt, describes the
> > issues in
> > detail; it has expired, because the solution was incorporated
> > into the
> > merged fast-reroute draft.
> >
> > Alia
> >
> >
> > At 12:02 PM 8/12/2002 -0700, Sundara Murugan wrote:
> > >Shahram,
> > >
> > >Let us assume that detours from B and C don't merge at H.
> > But these two
> > >LSPs share the same link between H-I. The at I there will be
> > 2 detour LSP
> > >(B to D and another from C to E).
> > >
> > >If the detour LSPs use SE filter, then the RESV message from
> > I to H can
> > >specify only one filter (Session + sender of all the detours
> > will be the
> > >same) and only one Label. Which label does I select?
> > >
> > >-sundara
> > >
> > >-----Original Message-----
> > >From: Shahram Davari [mailto:Shahram_Davari@pmc-sierra.com]
> > >Sent: Monday, August 12, 2002 11:45 AM
> > >To: Sundara Murugan; mpls@UU.net
> > >Subject: RE: fast-reroute merging
> > >
> > >
> > >Sundara,
> > >
> > >I think the shortest ERO is only a suggested example. and
> > not a standard rule.
> > >
> > >
> > >
> > >                 G----H----I--\
> > >                 |    |    |   \
> > >            A----B----C----D----E---F
> > >
> > >
> > >I think no merging should happen in the G, H and I LSRs. The
> > reason is that
> > >may be B has calculated the route B-G-H-I-D-E-F assuming some BW
> > >restrictions, etc.
> > >We don't want H to change that to B-G-H-I-E-F.
> > >
> > >Also since each PLR does an independent ERO calculation, it
> > is quite possible
> > >for C to compute computes the path as being
> > C-H-I-D-J-K-M-N-... Clearly
> > >C's detour should
> > >not be merged with B's detour here.
> > >
> > >If we have rule changed to "for merging the path messages
> > should have the
> > >same ERO",
> > >then we won't have such a problem.
> > >
> > >
> > >-Shahram
> > >
> > > > -----Original Message-----
> > > > From: Sundara Murugan [mailto:smurugan@riverstonenet.com]
> > > > Sent: Monday, August 12, 2002 2:22 PM
> > > > To: Shahram Davari; mpls@UU.net
> > > > Subject: RE: fast-reroute merging
> > > >
> > > >
> > > > Shahram,
> > > >
> > > > Section 5.3.1 says that the MP selects the shorter ERO path
> > > > length. This may not be correct.
> > > >
> > > > If there are other routers between I and E, then the ERO
> > > > lenght of the detour from C could be more than the ERO lenght
> > > > of the detour from B. If you go by the rule, then H will
> > > > merge the detours from B & C and establish a detour to D.
> > > > Then the detour from C will not avoid its next-hop D. So
> > > > there won't be a detour from C.
> > > >
> > > > So the MP has to select the detour which merges with the main
> > > > LSP that is closer to the egress router.
> > > >
> > > > -sundara
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: Shahram Davari [mailto:Shahram_Davari@pmc-sierra.com]
> > > > Sent: Monday, August 12, 2002 10:57 AM
> > > > To: Sundara Murugan; mpls@UU.net
> > > > Subject: RE: fast-reroute merging
> > > >
> > > >
> > > > Sundara,
> > > >
> > > > This is exactly the same question that I asked in another email:
> > > >
> > > > "Since as stated in section 7, each PLR has option to apply
> > > > its own constraints, then each PLR  may reach to a different
> > > > backup-path ERO for the same flow. Assuming that the EROs
> > > > have a single link (the downstream link of an MP LSP) in
> > > > common, isn't it wrong for an MP LSR to merge those Path
> > messages?"
> > > >
> > > >
> > > > I think the solution is to require the same ERO for merging.
> > > >
> > > > Yours,
> > > > -Shahram
> > > >
> > > >
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Sundara Murugan [mailto:smurugan@riverstonenet.com]
> > > > > Sent: Monday, August 12, 2002 1:36 PM
> > > > > To: mpls@UU.net
> > > > > Subject: fast-reroute merging
> > > > >
> > > > >
> > > > > Section 5.3.1 in draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt
> > > > > explains merging procedure. In the fig below if there is a
> > > > > link between I and F then D will have a detour D-I-F. In this
> > > > > case how router I (the MP) handles detour LSPs from B, C and D?
> > > > >
> > > > > -sundara
> > > > >
> > > > > ------
> > > > >
> > > > > 5.3.1. An Example on Path Message Merging
> > > > >
> > > > > Consider the following example:
> > > > >
> > > > >
> > > > >                 G----H----I--\
> > > > >                 |    |    |   \
> > > > >            A----B----C----D----E---F
> > > > >
> > > > >
> > > > >    The protected LSP is A-B-C-D-E-F. After running CSPF, let
> > > > > the detour
> > > > >    ERO from B be B-G-H-I-D-E-F, and the detour ERO from C be
> > > > > C-H-I-E-F.
> > > > >
> > > > >    H will receive Path messages that have the same SESSION and
> > > > >
> > > > >
> > > > >
> > > >
> >
> >