The discussion of M/O bits caused me to think about the meaning and
specification of host behavior for M/O bits and for SLAAC. In
particular, I'm trying to understand the degree of control over host
behavior specified in both cases.
I'll focus here on what I can understand about SLAAC, because we've been
discussing the M/O bits in a separate thread.
One part of the specification in section 5.5 of RFC 2462 seems pretty
clear:
5.5. Creation of Global and Site-Local Addresses
[...]
Creation of global and site-local addresses and configuration of
other parameters as described in this section SHOULD be locally
configurable. However, the processing described below MUST be enabled
by default.
I read this to mean that the information in the RAs is advisory.
In section 5.5.3 of RFC 2462, the following text seems to imply that the
"autonomous address-configuration flag" (A bit) controls whether a host
forms a SLAAC address for a prefix:
a) If the Autonomous flag is not set, silently ignore the
Prefix Information
option.
But, there is no RFC 2119 language, and the earlier text allows for
local host configuration, so it's not clear if a host is compliant with
RFC 2462 if it forms a SLAAC address from a prefix advertised with the
A-bit not set.
Later in the same list of behaviors:
d) If the prefix advertised does not match the prefix of an address
already in the list, and the Valid Lifetime is not 0, form an
address (and add it to the list) [...]
This text also does not contain RFC 2119 language, so it's not clear
that a host is required to form an SLAAC address from a prefix
advertised with the A-bit set.
While open to interpretation, I read the text about SLAAC addresses to
be advisory, which would give roughly the same control over host
behavior as interpreting the M/O bits as indicating the availability of
DHCP services, but not requiring specific behavior on the part of hosts.
- Ralph
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