Subject: RE: [mpls] Problem about the collision between multicast and unicastlabel

From: "Fil Dickinson" <[email-masked]>
Date: Thu, 8 Jun 2006 12:19:18 +1000

I agree (based on your example) we need a mechanism to distiguish
Unicast labels from Multicast, but are there not better ways to do it
than "Ethertype" such as defined label ranges or something related to
the problem rather than a clever use of an unrelated field?
I do not have an answer, but perhaps others have thought of this. 

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] 
Sent: Wednesday, 7 June 2006 8:32 PM
To: [email-masked]
Subject: [mpls] Problem about the collision between multicast and
unicastlabel

   In multicast, all downstream can be allocated the same label value in
order to avoiding branch LSR traffic replication on a LAN for
point-to-multipoint (P2MP)LSPs. 
   The series documents of mpls upstream label are already made for wg
docs. 
   Through upstream label assignment or downstream label assignment
(based on upstream label suggestion),  all next-hop downstream routers
will receive and process the packet with the same label.

   But if unicast label is allocated before multicast label, it is
possible that the same label will be allocated on the same LSR for
unicast LSP and multicast LSP on the same link. 
   The unicast LSP and the multicast LSP will use the same incoming
label on the same interface. 

For example:
 
                                D 
                               / 
                              /   
                         A---B---C---E 
                              \      
                               \     
                                F 

   There already exists one unicast LSP (A-B-C-E). And the incoming
label on LSR C is equal to K.
   There also exists one multicast tree (A-B, B-D, B-F). And the
incoming label on LSR D and LSR F is equal to K. LSR D and LSR F are the
next hop downstream router of LSR B, and their incoming labels must  be
same. 
   Subsequently, LSR C wants to join the multicast tree. LSR C is also
the next hop downstream router of LSR B. The incoming multicast label on
LSR C must be equal to K, just like LSR D and LSR F on the same
multicast tree.
   So there will appear the same incoming labels on LSR C if we don't
distinguish between multicast and unicast.

------------------------------------------------------------------------
----------------------------------------
   In order to distinguish between multicast and unicast LSPs with the
same label value, the labels can be differentiated by the use of
different Ethertypes that indicate whether the labeled packet that is
carried in a layer 2 frame is for unicast or multicast transfer. 
   But if LSR does not support these procedures, it can't discern the
labeled packet and must ignore it. 
   On the other hand, the process of layer 2 frame is complex.

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