Subject: Re: Sending mapping for REC-NEW-FEC

From: "Sreemoolanathan B Iyer" <[email-masked]>
Date: Thu, 27 Jan 2000 18:43:44 +0530

Hi Bob,

I also have a related question.

Bob Thomas wrote:
> > > We can imagine a case where sending a label mapping to 
the next > > > hop can be useful. If the current next hop is using 
conservative > > > mode, he will release the label, on receiving the 
mappnig. On the > > > other hand, if he is using request_never & 
liberal modes and if at a > > > later point of time the network 
topology changes drastically so that > > > we become his next 
hop for the FEC, the current next hop would > > > never be 
sending us a label_request at that time. If such a situation
> > > is possible, sending label mapping to the current next hop 
could be > > > useful. 
> > >
> 
> When LSRs use DU with liberal label retention an LSR can react
> to a change in next hop for a destination without interaction
> with other LSRs, assuming the new next hop had previously
> advertised the label for the destination.
> 

So we can make the assumption that in recognise new fec, the 
mapping is sent to the next hop also to take care of the DU-liberal 
situation. 

To quote from my earlier question, if L1 comes to know about a 
new FEC, and also that L2 is its next hop for that FEC, it will still 
send a label mapping to L2. So if at later point of time, L1 becomes 
the next hop of L2 for the same FEC, things will work properly 
since L2 will already be having a mapping for the FEC from L1.

Now, my doubt is, shouldn't the same be applicable to label 
mapping also ? According to  LMp 17, if we know the MsgSource 
to be the next hop, we send the mapping to all peers except the 
MsgSource. Again assuming DU & liberal retention, if L1 receives a 
label mapping from L2 which is its next hop, L1 would propagate it 
to all its peers except L2. 

Now, if later L1 becomes the next hop for L2 for the same FEC, 
wouldn't we run into trouble because L2 would never receive a label 
mapping from L1 ? 

Sree.

---

From: Bob Thomas <[email-masked]>
Date: Thu, 27 Jan 2000 09:13:15 -0500

Sree,

> wouldn't we run into trouble because L2 would never receive a label 

In DU mode label advertisement by an LSR is triggered by learning a
new route.  The procedures for independent and ordered control differ
in detail (see FEC.n, Section A.1.6).

If using independent control L1 would advertise the label when it
learned the route.  In this case, L2 would have L1's label should
L1 becomes L2's next hop for FEC.

For ordered control, if L2 is L1's next hop for the FEC, L1 would
delay advertising the label until it received a label from L2.  If L1
learned L2's label before it learned the route, it would retain L2's
label and advertise its label to L2 when it later learned the route.
If L1 learned the route before it learned the label from L2, the
LMp.17 loop as it currently stands would omit advertisement of L1's
label to L2.

There is some work in progress to suggest modification to the LMp.17
loop to deal with omissions relating to the non-merge case.  This
work will be expanded to address the issue you raise.

Thanks for pointing this out,
Bob