Subject: Label allocation in RSVP-TE

From: "Takao Okamawari" <[email-masked]>
Date: Tue, 29 Jan 2002 13:42:51 -0000

Hi all, 

I have a question about label allocation in RSVP-TE.
When a label is drawn from a label space?
On receipt of PATH or on receipt of RESV? (And reason why...)

RFC3209, 4.1.1.1 says, 
"If no label is available, the node sends a PathErr
message ..."
Since PathErr is used to notify a failure of PATH message, 
this reminds me that a label is drawn from a label space
on receipt of the PATH.

On the other hand, Yakov Rekhter says in his book 
(MPLS technology and applications) that
"When an LSR wants to send a RESV message for a new RSVP flow,
the LSR allocates a label from its pool of free labels, 
creates an entry in its LFIB with the incoming label set to 
the allocated label, and sends out the RESV message..." (pp 153)
(Does it mean PathErr is sent when RESV has failed?)

Please let me know.

Takao

---

From: David Charlap <[email-masked]>
Date: Tue, 29 Jan 2002 10:54:53 -0500

> On receipt of PATH or on receipt of RESV? (And reason why...)

The inbound label (which is sent upstream to the PHOP) may be produced
at any time after receiving the Path message.  The outbound label (which
is received from the NHOP) obviously can not be allocated until after
the Resv arrives, since the label value is specified in that Resv
message.

> RFC3209, 4.1.1.1 says,
> "If no label is available, the node sends a PathErr message ..."

Not necessarily.  PathErr may also be generated in response to Resv
messages as well.  In RFC 3209, several resv-related conditions (like
failure to allocate labels, and admission control errors) result in
PathErr, and not ResvErr.  The names PathErr and ResvErr are really
misnomers - a PathErr is simply an error message that is sent towards
the ingress node, and a ResvErr is an error message that is sent towards
the egress node.

> (Does it mean PathErr is sent when RESV has failed?)

Yes.

-- David

---

From: Sachin Kalra <[email-masked]>
Date: Tue, 29 Jan 2002 11:08:08 -0500

Takao:

Label is drawn from Label Space on receipt of RESV message. The reason for 
doing this is:

At the time when LSP gets signaled, all LSRs (except Ingress LSR) decide 
upon the label they will accept for the LSP, creates entry in their LFIB 
and send that label to their UPSTREAM LSR. As the labels drawn have to be 
forwarded upstream, so they are drawn at the time when LSR receives RESV 
message (Resv message travels upstream)

This approach makes sure that each LSR gets to decide their own incoming 
label for each LSP.

Take Care.
Sachin Kalra

---

From: "Takao Okamawari" <[email-masked]>
Date: Wed, 30 Jan 2002 09:17:12 -0000

Hi David and Sachin, 

Which is the correct answer with regard to the inbound label?

David Charlap wrote:
> at any time after receiving the Path message.  

Sachin Kalra wrote:
>
> Label is drawn from Label Space on receipt of RESV message. 

Takao

---

From: David Charlap <[email-masked]>
Date: Wed, 30 Jan 2002 10:08:05 -0500

Takao Okamawari wrote:
>> The inbound label (which is sent upstream to the PHOP) may be
>> produced at any time after receiving the Path message.
> 
> Sachin Kalra wrote:
>>

It doesn't matter.  You are the one generating the label.

It must be installed in your forwarding hardware before you complete
your Resv processing.  If you want to choose the label then, you may. 
If you would rather choose it during Path processing, that is also OK. 
This is an implementation detail and is not specified by the standard.

-- David