The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

Hi Rahul,

> A couple of comments:
> 
> 1. Maybe there is a need to enhance the semantics of the RSVP_HOP object
> to work over unnumbered links. The ip address of the previous hop
> could be set to the router id of the previous hop. 

In our first pass at this draft, the ERO subobject included both a
router ID (of the PHOP) and the interface index.  We then decided
that just the interface index is sufficient; see below.

> 2. This is more of a clarification. Consider:
> A----------B-----------C
>  Ia     Ib          Ic
> 
> The link from B to C is unnumbered. My understanding from the draft is 
> that B will generate the interface id for Ic. (and C for Ib) Is that correct? 

B will indeed generate the interface id Ic, but C has nothing to do
with Ib (as I see your picture).

> The ero could be <Ib, Ic>. Since B generated the id for Ic, it will know the
> outgoing interface to send the path msg to C.

> C will receive the ero as <Ic>. Now C needs to check the ero to see if
> it belongs to the first subobject i.e. Ic. How should this check be done?
> This goes back to (1) above.

I assume that C can tell which LSR it got the path msg from (i.e., B).
C then looks to see in its TE link state database if B is advertising
a link to C with index Ic.  (I will add this to the next rev.)

One could be more paranoid and add a router ID to the unnumbered ERO
subobject.  Then the check is: a) does the router ID of the LSR sending
the path msg match the router ID in the subobject? AND b) if so, is
that LSR advertising a link to me with the index in the subobject (Ic)?
This approach is slightly more likely to catch errors because there are
two checks.  Suppose for example A sent the first Path message to D (by
mistake) AND D didn't catch this error.  Suppose also that D has an
unnumbered link to C with index Ic (same as B's).

Now, when C gets the path msg from D (without the router ID), it will
erroneously accept it.  However, for this to happen: A made a mistake;
D missed the mistake; and D had the same ifindex to C as B did.

Thanks for your comments!

Kireeti.