> > From: "Perry E. Metzger" <perry@piermont.com> > > > Identifier/locator separation has been a topic of conversation at > the > > IETF for at least the last decade if not longer. In spite of this > > continuous interest, an actual fruitful proposal has yet to arrive. > > As you seem to have forgotten since the last time I pointed this out to > you, > MobileIPv6 represents a fully-worked-out design which separates identity > from location. (The Home Address is still used as a rendezvous point, it > is > true.) Mobile IPv6 can indeed be used to solve part of the host multi-homing problem. A multi-homed host will have as many addresses as interfaces. At the start of any given TCP connection or UDP association, one of these addresses can be picked as the home address for the connection; when the need appears, binding updates can be used to move the connection or the association to different interface. We must however solve the double jump problem: if the two hosts "move" at the same time, the connection will only survive if at least one of the two "home addresses" remains reachable; this will not be true if the two "home interfaces" were to the same provider, and that provider fails. There are a few obvious solutions; we should pick one and standardize it. The part of host multi-homing that mobile IPv6 does not solve well is the selection of the destination address: in the absence of external knowledge, all destination addresses look equal, and one will be picked at random. The destination can use a binding update to redirect the connection to the preferred interface, but it can only do so if the non-preferred address is reachable; if it is not, we get a trial-and-error approach, which kind of work but is slow. There are two known solutions to this problem: a dynamic DNS update, and the establishment of a back-up tunnel. Both have issues, which would require more development than a single paragraph. It is also possible to treat site multi-homing as a variation of host multi-homing, assigning as many addresses to each host as there are providers to the site. I often hear the argument that this is unrealistic, that host software will not be able to handle multiple addresses, but if I run "ipconfig" on my desktop, I get a list of 14 addresses: 3ffe:8311:ffff:f282:8cc5:8c07:78bb:77ac 3ffe:8311:ffff:f282:69a3:a0b3:2a:d858 3ffe:8311:ffff:f282:a8e5:48c4:5a79:ec40 3ffe:8311:ffff:f282:a101:2c3f:c6bd:29b 3ffe:8311:ffff:f282:90e:f1a0:d2c7:36c0 3ffe:8311:ffff:f282:3198:8248:b889:cd74 3ffe:8311:ffff:f282:a12f:7a3a:5dbd:9eed fec0::f282:206:5bff:fe5e:7527%1 3ffe:8311:ffff:f282:206:5bff:fe5e:7527 fe80::206:5bff:fe5e:7527%4 2002:9d3b:8849::9d3b:8849 3ffe:2900:d005:f28b:0:5efe:157.59.136.73 fec0::f28b:0:5efe:157.59.136.73%1 fe80::5efe:157.59.136.73%2 Somehow, the system manages to handle these 14 addresses; it may not handle it perfectly, but we can certainly expect improvements over time. So, we have at least one conceptual solution: use a variation of Mobile IPv6 to improve host multi-homing; solve site-multi-homing by treating it as a variation of host-multi-homing. -- Christian Huitema