On Aug 15, 2008, Christian Vogt wrote:
better later than never: I have 3 questions on this writeup; Robin already brought up the first one in his long review comment (dated July 30, 2008 10:25:39 AM PDT), but I have not seen the rest been asked (or maybe I read Robin's long msg too quickly and missed). 1/ fig-4 in the above pdf file shows an example of backward compatibility packet exchanges between an upgraded and a legacy edge network, however note that both are single-homed in this example. If the legacy net L is multihomed, everything still works, as (I assume)L still has its prefix injected into the global routing table. Howeverif the upgraded network U is multihomed, the communication with L can only use the address of one of U's providers, right? (this would be a dis-incentive for U to do upgrading...)Hi Lixia, you are right in that a multi-homed upgraded edge network is reachable from a given legacy edge networks only via a single provider. That provider may differ depending on the legacy edge network, so all providers can still be used. Also, the provider may change over time, e.g., for fail-over. But such a fail-over requires that affected packet exchanges (with legacy edge networks) are re-established.
Lixia, elaborating on this a bit more...: The reason why only a single provider can be used for a given legacy edge network (at a given time) is this: For packets sent from the multi-homed upgraded edge network U to a legacy edge network L, the transit address that is seen by the recipient host in L must be uniquely determinable based on the corresponding edge address used within U. Provided that we want Six/One routers to be stateless, this is possible only with a one-to-one mapping between the PI prefixes used within U and the transit address prefixes that L can use. Consequently, if we use a single PI prefix within U, then only one transit address prefix can be reachable by L. OTOH, if upgraded edge networks use a separate PI prefix per provider, all providers can be used even for packet exchanges with legacy edge networks. Advantage: Better multi-homing capabilities for upgraded edge networks. Disadvantage: Multiple PI prefixes must be managed within upgraded edge networks. - Christian -- to unsubscribe send a message to rrg-request@psg.com with the word 'unsubscribe' in a single line as the message text body. archive: <http://psg.com/lists/rrg/> & ftp://psg.com/pub/lists/rrg