On Nov 13, 2012, at 21:30 , joel jaeggli <joe...@bogus.com> wrote: > On 11/13/12 9:20 PM, Mikael Abrahamsson wrote: >> >> Why do you believe we need coordination between service providers to permit >> multihomed services to work well? I thought the whole idea was to handle >> multiple upstream prefixes and make sure everything is routed to the correct >> ISP? > > If coordination is required, it's not going to work.
Let's put on our thinking caps. Say I have a HOMENET with two provider-provisioned border gateways, each from different providers. Let's call them ALFA Broadband and BRAVO Networks. Say that ALFA delegates 2001:db8:aaaa::/64 to me and BRAVO delegates 2001:db8:bbbb::/64 to me (yes, they could delegate shorter prefixes, but let's say I have only one link to number, so the prefixes above are the ones that each gateway will be advertising in Router Advertisement messages on my HOMENET link). When they're both up and running, each router is a default gateways on my link. Each host gets two prefixes on the link, i.e. 2001:db8:aaaa::/64 and 2001:db8:bbbb::/64 and a default router list comprising both the gateways for ALFA and BRAVO. Given how the hosts in the field today behave, only one of the routers will be forwarding outbound packets. Which is fine. Let's say my gateway from ALFA is the default router all my hosts always pick, i.e. it's at the front of the default router list. Now let's say a host on my network chooses to connect to a remote destination where source address selection rules say that it should pick an address from the BRAVO prefix delegation. The SYN packet with source address 2001:db8:bbbb::XXXXX goes to the ALFA router. What does it do with that? - Does it forward the packet? If so, then the return path will be asymmetric, i.e. it will come back through my BRAVO gateway. Asymmetric paths are the reason 6to4 anycast is broken. I thought we learned our lesson about that. Maybe not all of us did, but I bet we soon will if we keep going this road. - Does it recognize the 2001:db8:bbbb::/64 prefix and redirect to the BRAVO router? If so, then how does it know to do that? Coordination, because routers don't process Router Advertisements, so the ALFA gateway won't have reason to know about the BRAVO prefix unless it makes an exception to the standard rules. I'm not optimistic that the players involved will have any incentive to adopt protocols that enable that happen. This all sounds very squirrelly to me, and the incentives to do it seem completely missing in action. (By the way, how do you redirect a whole prefix? You don't. How do you cancel a redirection? You don't. How do we fix this? By getting 6MAN to revise Router Advertisements and rolling out new host implementations everywhere. Good luck with that. Oh but wait: maybe, the ALFA router doesn't redirect, but it forwards instead. Path asymmetry again— just not as badly asymmetric as it would otherwise be, i.e. asymmetry is confined to the local link.) Maybe I'm missing something, but I think this is really an intractable problem, given what I know about it. -- james woodyatt <j...@apple.com> core os networking _______________________________________________ homenet mailing list homenet@ietf.org https://www.ietf.org/mailman/listinfo/homenet
