On Feb 23, 2013, at 3:16 AM, Michael Richardson <[email protected]> wrote:
> >>>>>> "Lorenzo" == Lorenzo Colitti <[email protected]> writes: >>> I.e. the "0123" is identical for the two prefixes? >>> > > Lorenzo> In the general case where the prefixes assigned by the > Lorenzo> operators are of different lengths, it cannot be. Right? > > True. > > If the ISP with the longest prefix is alive first, then the routers > pick subnet-id parts that fit into that. If that ISP has provided > enough subnets, then even when another ISP comes along, the "xx23" > part might remain stable for a long time. > > I think this is a human friendly feature that none of our protocols > should depend upon, but that nothing should forbid. Do you agree? > > -- > Michael Richardson <[email protected]>, Sandelman Software Works I think we have violent agreement here. The text we're discussing is <section anchor="allocation" title="Subnet prefix allocation and announcement"> <t>Each router advertising a <xref target="RFC5308">Reachability TLV </xref>, including a pseudonode on a LAN, when it receives the Autoconfiguration TLV Advertisement, calculates and announces a more specific prefix from the advertised autoconfiguration prefix in a Reachability TLV. This prefix is chosen at random, but may not collide with any prefix currently advertised within the network and therefore in the LSP database.</t> If you would like I can change This prefix is chosen at random, but may not collide with any prefix currently advertised within the network and therefore in the LSP database. to read In the absence of other considerations, this prefix is chosen at random. It MAY be derived from previous prefix allocation decisions, such as a prefix stored in nonvolatile memory, the prefix used by a previous pseudonode on the same LAN, or the subnet part of another prefix on the same interface. In any event, it MUST NOT collide with any prefix currently advertised within the network and therefore in the LSP database. BTW, a side-note on the issue of non-volatile memory. The OSPF autoconfig draft says that an allocated prefix MUST be stored in non-volatile memory and as a result survive a reboot. Speaking for myself, I don't see the need for that; I'm not opposed to someone doing it, but they now have to think about "what happens when" for various kinds of network changes. I think the principle might be one of "least surprise"; if a certain prefix is in use on a LAN and the DIS changes (perhaps the old one fails), the new DIS should use the same prefix as the old one, so that the hosts don't have to jump through hoops. That said, I don't see the argument around a whole-building power failure; unless there is a server being advertised in DNS, a randomly-selected new prefix will work just as well as the old one. _______________________________________________ homenet mailing list [email protected] https://www.ietf.org/mailman/listinfo/homenet
