Subject: change in next hop !!

From: Manoj Dutta <[email-masked]>
Date: Thu, 17 May 2001 23:07:12 -0700

Hi,

I have a few questions based on section A.1.7 RFC 3036.

1) Is is correct to assume that NH.1 - NH.5 also consider the case where
LSR has received and retained multiple label mappings from old next hop
??

3) Also NH.6 to NH.8 take into account the scenario where multiple
requests are pending.

Consider the following scenario :

LSR U1 and LSR U2 are upatream wrt LSR A and LSR D1 and LSR D2 are
downstream of LSR A.

                            LSR U1            LSRU2
                                |                       |
                                  \                   /
                                     \              /
                                       LSR A
                                     /            \
                              LSR  D1    LSR D2

Suppose LSR A is non merge capable and for a given FEC F,  LSR D1 is its
next hop.  D1 has distributed
labels L1 and L2 for FEC F which LSR A maps to lable L3 and L4 given to
LSR U1 and LSR U2 respectively.
Now, LSR D2 becomes the new next hop for LSR A (FEC F).
Cosider statement NH.11 :

a) Suppose, LSR A already has a label mapping  ,say L5, from LSR D2 for
FEC F. A recv mapping event will be generated. Now if LMp.18 of section
A.1.2  is executed, it contradicts Note 8 as we have sent labels, L3 and
L4, to more  than one peer (LU1 and LU2, in this case.) despite being a
non merge LSR. Even if we ignore this,  which of the upstream labels (L3
and L4) should we map to L5 .
Let say that we map L5 to L3 (LSR U1).  There is no way to let LSR U2
know that lable L4 is not mapped
to any downstream label now and if LSR A is using ordered control then
we have an exception here.

b) Now, lets take the case where LSR A does not have any mapping from
new hop LSR D2. Now, it might
send a single label request which again cannot be mapped to the two
upstream labels L3 and L4.

I would really appreciate any help in understanding this scenario.

Thanks in advance,
manoj

---

From: Eric Gray <[email-masked]>
Date: Fri, 18 May 2001 11:59:12 -0400

Manoj,

    When there is a next hop change for non merge-capable LSR ('A' in your
example) and specific FEC ('F' in your example), the LSR must do one of
the following for labels issued in association with this FEC to peer LSRs:

    a    the LSR might continue to use the label bindings for the old next hop;
        for many reasons, this may not be a good idea for very long (this
approach
        might be though of as the 'pretend it didn't happen yet approach).
    b    if operating in independent control mode, packets bearing these labels
        and having the bottom of stack bit set may be egressed by LSR-A (this
        approach is most applicable if the LSP is a hop-by-hop routed LSP);
        note that the fact that LSR-A is not merge capable may very well mean
        that it may not be able to provide egress for packets very well either.
    c    if operating in independent control mode, packets bearing these labels
        may be discarded until such time as a label binding exists for the new
        next hop; in this case (and if hop count was used in previous label
        mappings), a new label mapping (with the old label) should be sent
        with the unknown hop-count (and subsequently with a known hop count
        once a label binding has been established) for each label issued for F
        (this is analogous to behavior that would occur if the LSP were being
        established from scratch).
    d    if operating in ordered control mode, LSR-A would withdraw labels
        it has previously provided for FEC F to its peers - especially if it is
        not currently capable of establishing a new binding to a label issued
        by the new next hop; note that this will result in tearing down the LSP
        for FEC F fom each upstream peer all the way to however many ingress
        LSRs there are.

In short, LSR-A either continues to use the old LSP, removes labels and
forwards packets using IP forwarding, drops packets or arranges to stop
receiving packets with labels associated with FEC F until it has new labels
for the downstream leg of LSPs for the FEC.  Hope this makes the
behavior clearer...

--
Eric Gray