Subject: Label Abort Procedure

From: R S Srinath <[email-masked]>
Date: Wed, 27 Oct 1999 14:00:46 +0530

Consider the following configuration of LSR's

A --------- B --------- C

1. Consider that LSR A has requested the LSP setup for
     a FEC F and it has resulted in successful setup of
    the LSP which traverses the LSR's A, B and C.

2.

After the LSP has been setup, Assuming that VC
merging is not supported in the entire network
configuration above , LSR A requests for the setup
of another LSP for same FEC F ( this could be in
response to the Label Request from any LSR's in
upstream of A ).

When this second request is received by B, it forwards
the request to an LSR D ( not shown in the figure but is
directly connected to B ), If before receiving a Label
Mapping from D the link between B and D fails,
then B will need to terminate this label request
notifying the upstrea peer A, what is the notification
sent in this case. ? Or would a Label Release
be required. In case of Label Release, such a message
will not have the Label TLV but have only the FEC, which
interprets to " release all the label associated with the
FEC specified ", as a result the LSP setup earlier as in
Setp 1. above would also be released. Certainly this is
not desirable.

Note : Assume that in case of Link Failure there
              pending LSP is not alternate routed but is
              released.

Any inputs  ?

Regards

R S Srinath

---

From: Michelle Burns <[email-masked]>
Date: Wed, 27 Oct 1999 14:00:04 +0200

LSR B should reply to LSR A with a Notfication (No Route).
 
On reception of this Notfication, LSR A could either:
a) do local repair and re-send the Label Request message 
at a deferred time interval to LSR B.
b) not re-issue the Label Request and assume that 
LSR B will provide the label mapping.

In the meantime, a routing table change could occur and 
a Label Abort may be sent to LSR B and a Label Request 
may be sent to the new next hop.

c) propogate the Notifcation (No route) and let the ingress 
LSR take care of the LSP re-establishment.

This leads me to another question about the propogation 
of Notfication Messages. There is no mention in the LDP 
spec of propogating Notification Messages. It is however 
mentioned in the CR LDP spec (with a reference to the LDP spec):

4.3.2.3 Notification Message

   If an LSR receives a Notification Message for a CR-LSP, it SHOULD
   release any resources that it possibly had reserved for the CR-LSP.
   In addition, on receiving a Notification Message from a Downstream
   LSR that is associated with a Label Request from an upstream LSR,
   the local LSR MUST propagate the Notification message using the
   procedures in [1].

Any comments?

Michelle