Subject: [PWE3] PW Label Withdraw procedures described in draft-ietf-pwe3-control-protocol-17.txt

From: "Florin Balus" <[email-masked]>
Date: Fri, 6 Jan 2006 10:30:49 -0500

Hi there,

I am sending this email as I noticed some different, non-compliant
behaviors on boxes from different vendors as we were performing some
interoperability testing: i.e. no Label Release messages sent back as a
result of a Label Withdraw for a L2FEC.

This is non-compliant with the text of
draft-ietf-mpls-rfc3036bis-03.txt:

"3.5.10.1. Label Withdraw Message Procedures
An LSR that receives a Label Withdraw message MUST respond with a Label
Release message."

So In the Label Withdraw procedures described in
draft-ietf-pwe3-control-protocol-17.txt maybe we should be more specific
about the need for the ingress PE to answer with a Label Release message
(as a MUST): i.e. copy the above text of
draft-ietf-mpls-rfc3036bis-03.txt just to make sure people follow it.

If the Label Release message not going out as a reply to a Label
Withdraw then we might end up with stranded labels, especially if a
large number of PWs go down for some reason - see also the text of
draft-ietf-mpls-rfc3036bis-03.txt:

"A.1.14. LSR decides to no longer label switch a FEC

Summary:
An LSR may unilaterally decide to no longer label switch a FEC for an
LDP peer. An LSR that does so MUST send a label withdraw message for the
FEC to the peer.
[..]
Notes:
1. The LSR may remove the label from forwarding/switching use as part of
this event or as part of processing the label release from the peer in
response to the label withdraw. If the LSR doesn't wait for the label
release message from the peer it SHOULD NOT reuse the label until it
receives the label release."

Also, as a second problem related to the PW Signaling procedures:
according to the above paragraph, the egress PE may not wait for the
Label Release anyhow. It might send a Label Mapping right away for some
reason.  What happens if the ingress PE didn't release yet the old
label-L2FEC mapping? We might end up at the ingress PE with 2 mappings
for the same L2FEC, one of which is no longer valid. How exactly do we
handle/signal this kind of error?

Regards,
Florin

Nortel - Data Network Engineering
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From: Luca Martini <[email-masked]>
Date: Fri, 06 Jan 2006 13:16:57 -0700

Florin,

This problem has arisen a few years ago. In using the PWE3 LDP control 
protocol there is an implied assumption that rfc3026 procedures MUST be 
followed as well. In the PWE3 control protocol RFC ( soon to be rfc ) we 
only mention the extensions to LDP and 
rfc3036/draft-ietf-mpls-rfc3036bis-03.txt.

It seems to me that there is a lot discussion about label release in the 
PWE3 LDP control draft......
It is pretty clear that you have encountered the same bug I did years 
ago .... it should be fixed by now ...
;-)

Luca