> This should not be surprising. Given that the applications community > blindly assumes there is a single addressing scope, when they bump into > the reality of the deployed network there will be problems. Proclaiming > that scopes are bad for applications does not make the filtering that > causes scopes go away.
Bad, bad application developers. We should really punish them! :-) Seriously, application developers face some very real problems. The issues are uniqueness in space and uniqueness in time. The space dimension is experienced when a host is connected to several sites, e.g. home network and VPN. If the application receives a site local address from a local name service or a local discovery service, it must associate that address with a site identifier. The first stumbling block is that most applications don't have this concept, they just copy the address. The second stumbling block is that there are no readily available site identifiers -- you would need to manage a unique name space, which is precisely what you don't get from FFC0::/10. In the absence of site identifier, the routing of connections to the proper site is haphazard. The application is supposed to remember that the site local address was learned through a specific site, i.e. only use in site X the site local addresses learned from site X's DNS service. This become quickly very hairy, as the DNS does not in fact have a concept of site. Restricting hosts to handle exactly one site removes some of the problem. But you then bump into the time dimension. Hosts may become members of different sites at different times, e.g. a laptop moving from the office to the airport or to the home. Addresses learned in one site should not be used in the next, but for that you must have an easy way to tell that you have changed sites. Without a site identifier, that can be real hard. Tony, these problems are not theoretical. They come very much on top of the issues encountered by developers porting applications to IPv6. Using unambiguous addresses solves a lot of the problems: a multi homed site can route connections to the proper site, source address selection can work, and moves to another site can be detected. There are remaining issues, notably dealing with intermittent connectivity, but these issues are inherent to any mobility scenario. By the way, link local addresses tend to suffer from much of the same issue. However, it is easier for applications to deal with LL addresses, as there is a clear linkage between addresses and discovery protocols operating in a single link. -- Christian Huitema -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
