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Re: OMB: IPv6 by June 2008

  • From: Joe Abley
  • Date: Thu Jul 07 11:12:06 2005

On 2005-07-07, at 10:23, Andre Oppermann wrote:

It was about a spot in the global routing table. No matter if one gets
PA or PI they get a routing table entry in the DFZ. There is no way around
it other than to make the routing protocols more scaleable.
With the hole-punching/CIDR abuse multihoming that is widely used in IPv4, a slot in the DFZ gets burned each time an end site adds a provider, regardless of whether they are using PA or PI addresses. This slot represents state information for the multi-homed site which answers the question "how else can this set of addresses be reached?"

The shim6 approach shifts this state from the DFZ to the endpoints which are exchanging unicast traffic. The endpoints exchange a set of possible locators through a protocol element within the IP layer and handle locator migration transparently to the transport layer above. Hence the question "how else can this particular remote address be reached" is answered using information on the host, not information in the network.

With shim6 an end site can multi-home using one PA prefix per provider, without taking up additional slots in the DFZ. Hosts within the site are given multiple addresses (locators), and the layer-3 shim handles any change of locator needed for traffic exchanged between any two hosts.

If one (or both) of the hosts exchanging traffic don't support shim6, then the traffic is exchanged without transport-layer stability across re-homing events (and, potentially, without any optimisation as to the choice of endpoint addresses for the session).

So, the shim6 future of multihoming looks like this:

1. ISPs multi-home exactly as people are used to doing today, using PI prefixes, and taking up a slot in the DFZ per transit provider. Everybody is familiar with this already. There is no change for ISPs in this picture.

2. Multi-homed end sites obtain one PA prefix per upstream ISP, and hosts within those end-sites are assigned multiple addresses (in some automated, secure and controllable fashion). There are no additional slots burned in the DFZ by end site multi-homing. Hosts obtain transport-layer reliability across re-homing events using shim6, rather than relying on the network to take care of it.