The MPLS WG Archive

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

"Lipovetsky, Sergey" wrote:
> 
> Here are comments on this thread.
> RFC2205 states "RSVP defines a "session" to be a data flow with a
> particular destination and transport-layer protocol." My
> understanding is that it is not different for RSVP-TE and a session
> object relates to the destination.

The SESSION object is not the same for RSVP and RSVP-TE.

In RSVP, the SESSION object consists of a 3-tuple consisting of
destination address, destination L4-protocol, and destination port
number.  In RSVP-TE, the SESSION object is a 3-tuple consisting of
egress address, tunnel ID and extended tunnel ID.

In either case, the data in the SESSION object is what uniquely
identifies the session, and nothing else does.

> TunnelId is a part of the session object and probably also relates
> to the destination. If this is true TunnelId should be locally
> unique to the egress node and a pair [egress address, TunnelId]
> should uniquely identify an RSVP session.

You're exchanging cause and effect here.  The data in the SESSION object
is what defines a session.  If two different LSPs are signaled with Path
messages containing identical SESSION objects, then they are in the same
session.  Period.  It doesn't matter what the user may or may not have
wanted.

Because tunnel ID can not be globally synchronized without using means
beyond the scope of RSVP-TE, the extended tunnel ID was created.  It can
either be zero, in the case where resource sharing among different
senders is desired, or it can be set to a globally unique value
(preferrably the sender's IP address) in the case where such resource
sharing is not desired.

> However, there is a problem with TunnelId allocation on the ingress
> node in this case.
> 
> The requirements to set Extended tunnel ID to a non-zero value to
> guarantee session uniqueness, when TunnelId is allocated by the
> ingress node prevents from merging LSPs having the same egress node
> but different ingress nodes.
> What is wrong here?

You are assuming that guaranteeing session uniqueness is something that
everybody always wants.  This is an invalid assumption.  There are
situations where it is desirable and there are situations where it is
not.

There are situations where an operator may explicitly want two different
LSPs from different senders to share resources.  The only way to signal
this is by using a zero value for extended tunnel ID.

-- David