The MPLS WG Archive

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

Apratim Mukherjee wrote:
> 
> If TunnelId is to distinguish between two sessions having the same
> egress, then TunnelId should have a globally significant value in a
> MPLS domain

And how do you propose to do this?  Require a global server that can
dish out unique tunnel IDs, perhaps?  Requiring that tunnel ID be
globally unique is not possible without adding so much overhead that
nobody will use it.

In RSVP-TE, _ALL_ of the values in a session object (Address, tunnel ID
and extended tunnel ID) are required to uniquely specify a session. 
Extended tunnel ID is required because there is no reasonable way to
ensure uniqueness of the tunnel ID field.

The only reason CR-LDP doesn't also have this problem is because CR-LDP
incorporates the sender address as part of the LSP ID.

> 1 ) Doubt here : Where does the TunnelId come from then ?

Tunnel ID is only locally unique to the ingress node.

> 2) If TunnelId HAS a globally significant value , then why do we
> need an Extended Tunnel Id ?

Your assumption is wrong.  It is not globally significant.  The extended
tunnel ID is required to make the entire session object globally unique.

You can't just blindly assume that extended tunnel ID will match the
sender address, because a zero value (meaning "don't care", needed for
sharing resources between two LSPs from different senders) is legal.

> [ If the administrator wants to tie a tunnel to an Ingress , all he
> has to do is to make sure that no other tunnel in the domain has
> the same TunnelId.]

Easier said than done.  The only way an administrator can guarantee this
is if every single LSP in the network is manually configured.  This is
not always going to be true.

> If TunnelId is locally assigned at a node , then Extended Tunnel Id
> MUST be set to the ingress IP address to be able to distinguish it
> from some tunnel being made at another ingress with same tunnel id.

Unless it is intended that this LSP will share resources with another
LSP terminating on the same node.

> ( possible since tunnel id is being locally decided ).
> However this means that one tunnel CANNOT have two lsps belonging
> to the same tunnel originating at two ingresses and exiting from
> the same egress.

There is no reason why you can not have this.  That is specifically what
the zero value for extended tunnel ID is meant for.

-- David