The MPLS WG Archive

["Joel M. Halpern" <joel@stevecrocker.com>]

The basic question is how much complication an implementation wishes 
to undertake to allow for the mismatch.
Note that such mismatches are rare.

As a local matter, an implementation can chose to create the FTN 
entry for the exact match, and keep track of the relationship between 
that entry and the actual routing entry.  Eric has argued that such 
is undesirable complication.  I would tend to agree with him in the 
abstract.  What you choose to do in your implementation may depend 
upon your structures and on what problems you confront in the 
field.  Creating the local entries and using them is legal.  Such 
entries would not affect routing advertisements.  They probably don't 
even effect label advertisements.

Yours,
Joel M. Halpern

At 04:45 PM 4/11/2006, Dutta, Pranjal wrote:
>Thanks for the explanation on this.
>
>Let me elaborate a scenario based on the rules that you mentioned.
>
>For the procedure of label_map_received we have to apply both the rules
>1) and 2) right. For rule one, If Ru gets a label from Rd and Rd is NOT
>the next hop for the FEC. How Ru decides that the FEC is not next hop
>are
>a) FEC exactly matches a route entry but next hop is another LSR or b)
>As per your rule one FEC matches LPM to a shorter prefix P, and Rd is
>not next hop for the matched prefix P. Lets say we hit issue b) and due
>to liberal retention at Ru it retains the label but will not be used for
>forwarding.
>
>Now let's say that routing at Ru changes and now Rd becomes next hop for
>Prefix P. So now Ru should introduce an entry for the FEC into
>forwarding table and install the NHLFE entry right? But that violates
>your Rule 2 then
>as the exact match condition should be met. If we apply rule 2 now, then
>we shouldn't use the label from Rd although Rd has become the next hop
>for Prefix P. Please correct me if I am wrong in this.
>
>Regards,
>Pranjal
>
>-----Original Message-----
>From: Joel M. Halpern [mailto:joel@stevecrocker.com]
>Sent: Tuesday, 11 April, 2006 7:53 PM
>To: Dutta, Pranjal
>Cc: mpls@ietf.org
>Subject: Re: [mpls] One LDP Implementation specific question of receive
>label mapping for prefix FECs
>
>I think that there may be a better way to explain this.  There are
>several different factors at work:
>
>1) A received LDP label MUST NOT be used if the neighbor sending the
>label is not the next hop for the FEC.  This condition does not
>require an exact match in the forwarding table.
>2) A Received label MUST NOT be used for packets which do not match
>the received FEC.  This condition requires an exact match in the
>forwarding table.
>
>This does allow an implementation to create an exact match entry in
>the forwarding table to select packets that use the label, even if
>routing provided a shorter prefix.  This does allow one to use /32
>entries without injecting those /32s into the actual routing
>protocol.  However, the most common uses of .LDP (even with /32s)
>will result in the entries being in the forwarding table anyway.
>
>Another way to look at the condition above is that condition 1 is a
>restriction on the use of a received label advertisement.  LPM check
>is suitable.  Condition 2 is a restriction on the contents of the
>Forwarding-Table-to-Next-hop-label-forwarding-entry that one
>creates.  That must be an exact match.
>
>Yours,
>Joel M. Halpern
>
>At 07:02 AM 4/11/2006, Eric W Gray wrote:
> >The implementation MUST do an exact match, as downstream LSRs will
> >forward based
> >only on the label.  Because they do not (typically) perform a route
> >look-up on the label
> >encapsulated inner IP packet, there is no reason to expect a
> >downstream LSR to be able
> >to separate a flow based on longest match when the routes further
> >downstream are distinct
> >for "longer matches".
> >
> >Dutta, Pranjal wrote:
> >
> >>
> >>Hi Eric,
> >>           As per RFC 3036 Label mapping receive procedures, when Ru
> >>received a label mapping from Rd for a FEC x, Ru need to find an
>"exact"
> >>match of the FEC x(IPv4/V6 prefix) in its route table. In
>Inter-IGP-area
> >>or inter-AS case the routes from Rd would be summarized to Ru and in
> >>such a case Ru MAY not find the exact match for such FEC x. In terms
>of
> >>RFC 3036 then Ru MAY not find exact match for the FEC received from Rd
> >>in its route table and no further action at Ru. My question was why Ru
> >>can't we do a longest prefix match for the FEC x as LPM means it can
>be
> >>reached by a downstream Rd?
> >>
> >>Thanks,
> >>Pranjal
> >


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