26 Mar
2026
26 Mar
'26
1:53 p.m.
Hi Matthew,
On 26/03/2026 14.19, Matthew Wild wrote:
On Thu, 26 Mar 2026 at 13:02, Stephen Paul Weber
<singpolyma(a)singpolyma.net> wrote:
Handling unknown child elements in unknown namespaces is certainly a
requirement for XMPP implementations. But for a specific implemented
namespace defined in a XEP, it is absolutely valid for implementations
to reject elements which were not defined in the namespace's schema.
We even have error conditions explicitly defined for use in such
cases.
I am personally not one of the people who feel most strongly about
this, and there have been times in the past where I personally would
have preferred to sneak in some elements to established namespaces (if
the disruption would be lower than that caused by a namespace change).
However it is a position that we have taken as a community since the
namespace versioning was introduced, that generally schemas do not
change after publication (I can imagine exceptions for very young
unimplemented experimental XEPs). This is because some applications do
choose the strict validation option (sometimes this is for technical
reasons, because it ensures they can accurately translate XML to
custom data structures).
That many implementations will happily ignore extra elements just adds
to the problems around this particular change, given the nature of
these elements being user opt-ins (if any clients are sending them
today, they are 100% being ignored). It's actually a perfect example
of why namespace versioning was introduced.
I am sorry, but this is not how I perceive the situation.
I think we both agree that implementations are supposed to ignore
unknown elements. However, if I understood you correctly, you limit this
to unknown elements in an different namespace from the parent element.
I, on the other hand, believe this is also true for unknown elements in
the same namespace.
In general, we want it to be fine for recipients to ignore unknown
elements (and probably attributes). This allows us to evolve a protocol
as long as possible without namespace bumps.
Obviously, this only works if the new element (or attribute) can be
safely ignored by the recipient without breaking functionality. For
example, hints that the recipient could process, but doesn't have to.
I am not sure why we should artificially limit ourselves here and create
an extra burden due to additional namespace bumps if we don't have to?
And that's me saying this. The person who also argues for strict
verification because it eventually increases the robustness of the
overall ecosystem. For example, rejecting "foo" as an the value of an
attribute declared to be xs:int, as it is clearly a violation of the
specification.
But why should we reject something if we don't know what it is supposed
to be, especially if there are obvious drawbacks associated with such an
approach?
- Flow
Generally, adding previously undefined elements is considered to be a
breaking change that requires a new namespace, especially in a
protocol that is already deployed.
Adding new elements seems like the least breaking change? Unless they're
marked as manadatory, any implementation needs to be able to handle unknown
child elements in any position they don't expect anyway.