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Re: [RRG] A Late Response to Questions on Six/One Router




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. However
if 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



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