SAML attribute naming

[Peter Schober]

The page name even matches the subject from your post :)
https://wiki.shibboleth.net/confluence/display/SHIB2/AttributeNaming

* Eric Goodman <Eric.Goodman at ucop.edu> [2014-10-17 18:00]:
> I'm currently working with a vendor that is asking for attributes
> with names (not friendly names) such as "firstname", "lastname",
> etc.

That's legal, provided the name format matches ("basic").

It's also less than helpful because "name" or even "givenName" could
mean anything unless it's accompanied with a reference to an existing
standard, e.g. an IETF RFC.
URI (not URN specifically) naming avoids that by using globally unique
names, which also reference the source of the definition uniquely.
So both parties can be sure of the syntax and semantics via reference
to an existing, external standard.
-peter