@joelbax: I think RFC3261 mention the deprecation of `transport=tls` for `sips` scheme (ie., end to end secure transmission), but it is defined in the grammar (e.g., for hop-by-hop tls usage). See RFC3261 page 223:
``` transport-param = "transport=" ( "udp" / "tcp" / "sctp" / "tls" / other-transport)
```
- https://datatracker.ietf.org/doc/html/rfc3261