Ole, Consider the topology below. See https://www.ietf.org/mail-archive/web/rtgwg/current/msg05472.html for a more detailed description of the topology. For H31 to send a packet to destination B3, H31 must choose a source address from within subnet A3x. ----------------- LAN-X LAN-Y ,-----. / \ +--+ +--+ ,' `. : : +--+R7+-----+---+R1+-+ ISP 1 ---+-- : | +--+ | +--+ `. ,' : : H31-+ H21-+ `-----' : The Internet : | | ,-----. : : H32-+ +--+ H22-+ +--+ ,' `. : : +--+R8+-----+---+R2+-+ ISP 2 ) : : ++-+ | +- + `. ,' : :--B0 | | `--+--' : : | | | : : | | B2 : : | | : : | | ,-----. : : | | +--+ ,' `. : : | +---+R3+-+ ISP 3 ---+--- : | +--+ `. ,' : : | `--+--' : : | | : : | B3 : : | : : | ,-----. : : | +--+ ,' `. : : +-----------+R4|-+ ISP 4 ---+--- : +--+ `. ,' : : `--+--' : : | \ / B4 -----------------
For this example, we assume that the R1-R4 originate the following (D,S) routes in the IGP. R1 originates a route for (D=::/0, S=A1). R2 originates a route for (D=B2, S=A2). R3 originates routes for (D=::/0, S=A3) and (D=B3, S=A3). R4 originates routes for (D=::/0, S=A4) and (D=B4, S=A4). R7 and R8 receive these routes via the IGP. With the existing mechanisms in Neighbor Discovery Router Advertisements, R7 and R8 can advertise the following PIOs and RIOs. PIOs = A4x, A2x, A1x, A3x RIOs = B2, B3, B4, B1 I have intentionally changed the order of the prefixes in the set of PIOs and RIOs to emphasize that there is no required ordering or relationship between prefixes in PIOs and RIOs. With only this information, I do not see how H31 can correctly choose a source address in A3x when it needs to send a packet with destination address B3. If this analysis is correct, then it seems like a mechanism like draft-sarikaya-6man-sadr-ra-03 is needed. Thanks, Chris -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Tuesday, April 12, 2016 12:25 PM To: Chris Bowers <[email protected]> Cc: David Lamparter <[email protected]>; [email protected] Subject: Re: Implications of default-only SADR (was: Re: multi-homing for provider-assigned IPv6 addresses) Chris, > Do you think that Neighbor Discovery needs to be extended to support the > advertisement of (S,D) routes? No, I'm hoping we can avoid that. > Currently ND Router Advertisements allow the advertisement of source prefixes > in PIOs and destination prefixes in RIOs, but I don't see any existing > mechanism to associate a particular source prefix with a particular > destination prefix. draft-sarikaya-6man-sadr-ra-03 offers one way to > associate a source prefix with a destination prefix in Router Advertisements. > > If not, what mechanism do you think should be used to communicate (S,D) > routes to the host? In the general case explicit S,D routes to hosts are not needed. There are two cases, first when a host is connected to two uncoordinated routers (e.g. ISP A, ISP B), in that case draft-ietf-6man-multi-homed-host clarifies that a host should associate source prefix with the default from router discovery. I.e. an implicit instead of explicit S,D route. In the case, e.g. homenet it is assumed that all routers are part of the same IGP, and if host picks 'wrong' router, the only consequence is an extra hop. If a host really really wants to participate in routing, it can of course run a routing protocol. Do you see anything missing? Best regards, Ole
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