Allowed host and realm naming for a diameter node

Question

I am trying to clarify the allowed naming convention for the host and realm of a diameter node. This relates to the values used in the Origin-Host AVP (AVP Code 264) and Origin-Realm AVP (AVP Code 296). I have reviewed the Diameter RFCs and cannot find a definitive answer to this issue.

The reason for asking this question is that I am in discussion with a vendor of a Diameter Routing Agent (DRA) which claims that the host of a diameter node has to be in the format host.realm.
(1) Example of the only allowed format according to the vendor:Origin-Realm: example.com
Origin-Host: node.example.com
I want to clarify if multiple subdomains are allowed to be added in the host without being present in the realm.(2) Example:Origin-Realm: example.com
Origin-Host: node.site1.example.com

According to the vendor, the example 2 is not allowed. To have the host as in example 2, the realm will have to be site1.example.com.
Could someone please clarify this naming issue or point me to the standard where this is defined.

Answer 1

The realm FQDN is always a final substring of the host name.
As you say, some vendors make further restrictions, smaller or bigger, on host and realm names.
Also, some standards specify a strict format for the host name (e.g. for an MME, see section 19.4.2.4. of 3GPP TS 23.003 ).