Subject: RE: LDP Label WIthdraw doubt !!

From: Shahram Davari <[email-masked]>
Date: Thu, 28 Mar 2002 13:16:25 -0800

Manoj,

> -----Original Message-----
> From: Manoj Dutta [mailto:[email-masked]]
> Sent: Thursday, March 28, 2002 3:58 PM
> To: [email-masked]
> Subject: LDP Label WIthdraw doubt !!
> 
> 
> Hi,
> 
> I have a doubt regarding label wihdraw procedures described 
> in RFC 3036 
> section A.1.5.
> Consider the following topology :- 
> 
>      
>     A-------B---------C--------D
>             |                  |
>              -----D------------
> 
> 
>  Now, lets suppose that for a given FEC F,
> 
> a) C is B's nexthop for FEC F
> b) B is A's nexthop for FEC F
> 
> All the routers are using Downstream on demand with ordered 
> control. So, 
> A send request to B (for FEC F) and gets a label
> L (assuming B has a label from C and so on..). Now, for some 
> reason the 
> link between B and C goes down and consequently
> B withdraws label L from A. So, now A has no label for FEC F. 
> After some 
> time, B chooses D as the nexthop for FEC F and
> gets a label from D subsequently. But, A's nexthop still 
> points to B for 
> FEC F and so A never sends a request to B to get a label
> for FEC F.

After receiving the withdrawal message, "A" deletes its binding between
FEC F and label L. To get a new binding it starts sending label requests to B.
In other words since A is the ingress LER, it will never receive any request
from its upstream, therefore it is not considered an ordered control LSR, and should
behave like an independent control DoD LSR.

 This behaviour is due to LWd.5 in section A.1.5 in 
> the RFC. 
> Is there a reason for this check. So, basically A would send
> a request to B if it were in DOD Independent control. 
> According to me, 
> if two LDP peers are using DOD then, in case of a label 
> withdrawal the 
> upstream should always send a request to nexthop for theFEC 
> in question.

Only for the ingress LER this is true. For other LSRs, in case
of label withdrawal, they should also issue label withdrawal for
that FEC to their upstream peers.

-Shahram

> Does it make sense ??
> Thanks in advance
> Manoj