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addrconf-minutes-95jun.txt
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1995-10-18
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INTERIM_MEETING_REPORT_
Reported by Susan Thomson/Bellcore
Minutes of the Address Autoconfiguration Working Group (ADDRCONF)
The ADDRCONF Working Group held an interim meeting at Sun Microsystems
in Menlo Park, California on 13-14 June 1995.
Specification Overview and Discussion
Sue Thomson presented an overview of the latest specification,
particularly how it ties in with the latest Neighbor Discovery draft,
and raised open issues.
Addrconf information is advertised in Router Advertisements. In
particular, there are flags in the fixed part of the Router
Advertisement that indicate whether stateless or stateful mode is in use
for address and other configuration parameters. The Prefix Information
extension (which is also used by prefix discovery) contains the
necessary information for obtaining and timing out prefixes, and
addresses formed from those prefixes.
On receiving Router Advertisements with a Prefix extension, the host
updates its address list, adding any new addresses formed from new
prefixes advertised, and resetting the lifetime of addresses formed from
prefixes that are being readvertised.
In the latest draft, two lifetimes are advertised per prefix: one to
specify when addresses change from being valid to being deprecated, and
one to specify when addresses changed from being deprecated to being
invalid. The intention of the two lifetimes is to allow a graceful
transition period during renumbering. That is, the deprecation time
warns the host that an address is going to become invalid (the
invalidation time must be no less than the deprecation time) and that
the host should start using another (presumably longer lasting) address
in new communications to minimise the risk of breaking communications
when the old times out. The idea is to make the deprecation period long
enough so that most, if not all, communications have switched over to
using the new address by the time the old one becomes invalid.
There was concern about application impact of deprecated addresses.
However, all the specification intends to specify is some mechanism that
enables an application (perhaps with the help of the network layer) to
choose an address at the start of a communication that is likely to last
for the duration of that communication, so that the problem of
renumbering during a communication is avoided to the extent possible.
There was a suggestion that protocols should be designed so that
communications can survive address changes and so that deprecation is
not needed. However, it was argued that this is not a problem that is
likely to be solved soon and that the above is a reasonable compromise
at this time.
There was some discussion about how much to specify with respect to
default source address preferences. It was clear that this is a
slippery slope and that things quickly become complicated. The
resolution was to simply specify that valid global/site-local addresses
are preferred over deprecated addresses in choosing a source address for
new communications, assuming there are other valid global/site-local
addresses to choose from. It should be noted that there will always be
at least one valid address to choose from, the link-local address.
However, the rules should allow a deprecated address to be used if only
the link-local is valid and the destination is (possibly) off-link.
It is possible for hosts to get address and other information, such as
MTU and hop limit, using both stateless and stateful mode. It is also
possible that both stateless and stateful mode are configured to be on
at the same time. There was some discussion about how much to specify
with respect to the advertisement of conflicting information. (It is
also possible for multiple routers to hand out conflicting information,
e.g., the same prefix could be advertised with different lifetimes.)
The implication of the current specification is that 1) the host accepts
the union of all information received from all sources and 2) if
conflicting information is received from different sources, the host
will just update its state with the latest information received, i.e.,
there are no special rules for giving preference to one or the other.
It was decided that this should be left as is, but such events may be
logged. It was also decided that in the Neighbor Discovery (ND)
document the router advertisement should be sent to the ``all-nodes''
multicast address, rather than the ``all-hosts'' address so that routers
could discover and log that they had been configured to hand out
conflicting information.
Hosts are expected to attempt stateful autoconfiguration when no routers
are detected on the link. It was suggested that we only do this on
startup, since the need to do this at other times is rare.
Default lifetimes for prefix advertisements were not specified in the
current specification, the intention being to force system
administrators to configure the lifetimes specifically since the
lifetimes are expected to be environment-dependent. It was suggested
that defaults should be set, but that they be very conservative, e.g., a
deprecation lifetime of several days (longer than a long weekend) and an
invalidation time of infinity. The value of infinity needs to be
specified in the specification.
There was agreement as to the transition between stateless and stateful
configuration as outlined in the specification.
An overview of the duplicate address detection algorithm was presented.
It was agreed that there would be one change to the soliciting node part
of the algorithm when the node is validating its address. Since
inhibiting local loopback on sending out multicast packets is not always
supported, it was agreed that the neighbor solicitation to detect a
duplicate address must always be looped back. A duplicate address would
then be detected if more than solicitation is received by a host. It
was also noted that the IPv6 multicast document needs to be written and
the local delivery requirements clarified.
It was also agreed that if the stateful mode is being used to acquire
addresses and these addresses are not the same as those that would be
configured using the stateless mode, then the duplicate address
detection algorithm should be applied to these addresses too.
It is now defined that hosts should randomise their delay both before
embarking on the duplicate detection algorithm (addrconf specification)
and when sending a Router Solicitation (ND specification). If a host
does both on initialisation, dithering is only necessary once and should
be specified as such. This needs to be cleaned up.
There was some discussion about how much space should be available for
storing prefixes and addresses. There was a strong suggestion that
hosts should be able to accept all advertised prefixes, and complain if
not.
The issue of whether addrconf information should be defined as part of
Router Advertisements or advertised separately was discussed. It was
noted that addrconf processing needs to be done as often as prefix
processing and needs similar information so why not advertise together.
Also, that Router Advertisements have been designed to advertise a range
of information, not only that needed for router discovery, e.g., it is
used to advertise other configuration information such as MTU and hop
limit. There is, however, a demultiplexing issue -- folks felt that
many implementations did this already for ICMP messages, and for those
implementations that did not, new software needs to be written in any
case, so this should not present a problem. The consensus of the group
was to maintain the current approach, with further discussion to be done
on the mailing list.
In addition, there were several suggestions to add clarity to the
document:
o Provide more rationale for the concept of deprecated addresses.
o Change the name or acronym of the Administered flag (M flag) in
Router Advertisement. The alternative suggestion was ``Managed.''
o Make interface initialisation procedure more explicit. At the
moment, the various functions that need to be done are not spelled
out in order in one place.
o Clarify the relationship between the network and upper layers in
choosing source addresses in new communications.
o Need to clarify use of ``hardware address'' vs. use of ``MAC
address'' as tokens. The link-specific part of the document will
be moved to ``IP-over-foo'' documents, yet to be written.