On Sat, Apr 4, 2009 at 7:24 PM, Tony Li <[email protected]> wrote: >>> That would make an MPLS tag an "address". Somehow I don't think so. >> >> Why not? What characteristics of what you think of as an address are >> missing from the typical use of an MPLS tag? > > Well, for one thing, an MPLS label is relative. Label value X is only > meaningful pairwise between a pair of routers along the path. The same > value X between any different pair of routers will imply a separate LSP and > thus a different path entirely.
Tony, Maybe I misunderstand how MPLS is used with respect to Internet routing. Please correct: As I understand it, an IPv4 packet arrives at [A], an MPLS encoding router. The router looks up the destination in its routing table and determines that [B], a border router to another system, should deal with the packet next. So, [A] tags the packet with "555" which means "send to router B," and then passes the packet router C which appears to currently have the shortest path to [B]. Router C knows that tag 555 means pass the packet to [B], so it sends it to router D which appears to currently have the shortest path to [B]. Rinse and repeat until it gets to [B] which knows that it owns tag 555 which means decapsulate the packet back to IPv4 and route it using the BGP table. In that use case, tag 555 is most certainly a locator within the scope of the MPLS system. I'll concede that MPLS tags are general purpose and can be programmed with other functions besides that of a locator. In theory, you could define an MPLS tag to mean "beep the speaker and then pop the stack." In practice, MPLS is used to define locators with an AS-interior scope so that only the AS border routers need to carry the BGP table and make BGP routing decisions. Or have I misrepresented how MPLS is being used as it pertains to Internet routing? Regards, Bill Herrin -- William D. Herrin ................ [email protected] [email protected] 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004 _______________________________________________ rrg mailing list [email protected] http://www.irtf.org/mailman/listinfo/rrg
