The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

Takao Okamawari wrote:
> 
> 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...)

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 ..."
> 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.

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.

> 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?)

Yes.

-- David