Address Autoconfiguration BOF (ADDRCONF) Reported by Susan Thomson/Bell Communications Research A BOF was held in Toronto to discuss the charter of a proposed working group for dealing with IPv6 autoconfiguration issues. The meeting was co-chaired by Dave Katz and Sue Thomson. Scope and Timeline Sue Thomson discussed the scope and timeline of the proposed working group. The intention is that the working group be short-lived with the minimal purpose of specifying a host address autoconfiguration protocol for use with IPv6. The timeline must follow the aggressive schedule of IPv6, i.e., protocol specifications should be available as drafts for review in September, and be ready to be submitted as Proposed Standards in December. It is recognized that there is more to host autoconfiguration than just address configuration and that attention needs to be paid to easing configuration of routers as well. However, address assignment is the minimal configuration requirement (it is conceivable that some hosts may not need more than this), and it was argued that autoconfiguration of other information should be considered outside of the scope of this working group, at least for now. There is also a need for cooperation with other working groups, such as IPv6 (system discovery) and DNS (which is investigating extensions to enable autoregistration of addresses). Architecture Dave Katz presented a straw man architecture for address autoconfiguration. He started off with some new taxonomy. In particular, it was noted that an autoconfiguration mechanism requiring minimal administration and state need not be ``serverless.'' A ``stateless server'' (a server that maintains no non-volatile state) can also be used to provide automatic address configuration. The term ``semi-autonomous'' (rather than serverless) was used to refer to a host that forms an address from information known to the host, such as a unique host ID, and information learned from an outside source such as a network prefix advertised by a neighboring router. The goal is to provide a single mechanism that enables host address autoconfiguration with or without administrative control in a range of networks, from small routerless networks to large globally attached networks. The basic requirements are that the mechanism must eliminate the need for manual host configuration and support both initial address assignment when bootstrapping and readdressing (when, for example, a host's site changes provider and provider-rooted addresses are in use). The mechanism should be deterministic to the extent feasible so that the same address is assigned to a host whenever possible. The basic elements of the presented straw man architecture is as follows: automatic address configuration is performed by a server. A stateless server meets requirements for fully automatic operation and a state-based server for tight administrative control. The argument against allowing ``semi-autonomous'' operation as described above was that hosts should be ignorant about routing and addressing structure in order to avoid host dependencies on the routing paradigm, should it ever change. The exception to this design principle is that a host may form an address usable only on the directly-connected subnetwork autonomously by concatenating a well-known prefix with the host cookie. This address would be used during the bootstrap process to acquire an address with wider scope from a server and when no autoconfiguration servers are available. A stateless server is normally co-located with a router. The presentation viewed stateless autoconfiguration as an integral part of router functionality; there was an objection to doing this. The basic function of the server is to hand an address to a host for some period of time, given a host identifier (cookie) and optionally a hint regarding what the address should look like, e.g. an IPv4 address if an IPv4-compatible address is needed for transition purposes. It was initially proposed that the lifetime be fixed to a maximum of 65535 seconds (approximately 18 hours). However, there was a strong consensus that this limit was too small. On receiving an address request from a host, a server forms an address from the host cookie and the topological information known to the router and returns this to the host with the specified expiration time. To enable tight administrative control, a stateless server may be configured to forward address requests to a state-based server. It was noted by various people that this mode of operation is similar to that of DHCP, but different to that of the initial SIPP address autoconfiguration proposal which did away with relay agents by having hosts automatically configure an address with intra-subscriber domain scope. The SIPP proposal, however, did not allow tight administrative control on a per subnetwork basis which now appears to be requirement. There was some discussion about which layer of the protocol stack would be used to transport address autoconfiguration messages. Three alternatives were discussed: UDP (compatible with DHCP, but requires hosts to know a well-known port and is not necessarily appropriate for a bootstrapping function), data link layer, in ICMP (may make most architectural sense, but does mean that address configuration will be a different protocol from that used for other configuration information such as is now provided by DHCP). There appeared to be consensus that ICMP should be used, with data link layer being the least preferred. During the presentation, there were many questions, many trying to anticipate what was to come. Other pertinent points not mentioned above include: o The mechanisms presented assume a host has an identifier that is unique at least within the subnetwork. We need to consider point-to-point links where hosts have no cookie. o Security concerns must be addressed. It was pointed out that addresses are not secret and can be typed in, so address configuration cannot be the basis for security. Some sort of ``smart card'' device to authenticate a host/user is probably needed. Next Steps Sue Thomson agreed to get the BOF minutes done, and to draft a charter for a proposed working group on host address autoconfiguration. Dave Katz volunteered to take a first cut at writing drafts of the address configuration architecture and protocol specification.