Arguing against XML, Alan DeKok says: "Does that extensibility have to
exist in DNS records? I think
that's the point of contention, here."
It seems to me that as we invent extensions, the DNS records need to
either contain the extension data, or contain some kind of information
that points us to the extension data outside of DNS.
It also seems reasonable that as extensions are invented, if the amount
of data that needs to be communicated is small, it should probably go
in-line in the DNS record. If the amount of data is large, it probably
shouldn't. This would argue that extensions that define flags and small
constants should certainly go in the DNS record, and extensions that
carry certificates as data almost certainly should not go in DNS. In
between, we have things like data containing email addresses (probably
in DNS), and data containing public keys (possibly in DNS, but probably
not).
The point of using XML is to have confidence that we have sufficient
syntactic flexibility to describe the probable future extensions,
whether they contain data in-line or they contain pointers to external
data.
I also believe that, for years to come, a single DNS record (that fits
in a UDP packet) should be sufficient to handle the email policy needs
of everyone except the largest organizations on the planet. And the
indirection capabilities (as defined) will also these largest
organizations to use a small number of DNS records.
In summary, the requirements that drove the current design include:
1. It MUST be possible for organizations to publish email policy records
without installing any new software. (This pushes us toward TXT
records.)
2. It MUST be possible to extend the kinds of policy information that
get published in the future, without breaking previously deployed
interpreters. (This pushes us toward XML, plus requiring interpreters to
ignore tags in namespaces they don't understand.)
3. It MUST be possible for EVERY organization who chooses to, to publish
their policy data in such a fashion that fallback to DNS over TCP is not
required. (This pushes us toward short XML tags, indirection elements,
and eliminating the boilerplate namespace declarations from the records
themselves.)
I would humbly suggest that people who don't like XML, or don't like TXT
records, or don't like something else, either need to argue that the
above requirements aren't appropriate, or they need to explain why their
favorite alternative does a better job than the current design of
meeting the above requirements.
-- Jim Lyon
mailto:JimLyon(_at_)Microsoft(_dot_)Com
Internet commerce will never really take off until you can buy something
online without getting spammed by the vendor.