Why does 2nd HSS FE send Dia Error 3002 when DRA addesses the 2nd HSS FE Destination Host of the 1st HSS FE?

Question

3GPP TS 29.272 says in section 7.1.6/ Routing considerations the following sentence:
If an MME or SGSN knows the address/name of the HSS for a certain user, and the associated home network domain name, both the Destination-Realm and Destination-Host AVPs shall be present in the request......

In Diameter Base Protocol RFC 6733 you can read the following :
6.1.4. Processing Local Requests
A request is known to be for local consumption when one of the following conditions occurs:
o The Destination-Host AVP contains the local host’s identity;
o The Destination-Host AVP is not present, the Destination-Realm AVP contains a realm the server is configured to process locally, and the Diameter application is locally supported; or
o Both the Destination-Host and the Destination-Realm are not present.

Following erroneous Scenario:
1) The MME sends a Request to the Destination Host e.g. HSS_ABC01.xxx via the DRA. The Request contains Destination-Host and Destination-Realm…
2) But what happens when this HSS_ABC01.xxx is unavailable (out of order)? Would the DRA send/forward the Request from MME to a second HSS (front end)?, e.g. hssDEF01.xxx?
3) Would the 2nd HSS Frontend “hssDEF01.xxx” complain that the Destination Host does not match as DRA sent to 2nd HSS frontend the Request with Destination-Host = HSS_ABC01.xxx
-->should the DRA be smarter when the 1st HSS Frontend is unavailable ? my question is what can DRA do in such case when the original destination is out of order?

FYI:
DIAMETER_UNABLE_TO_DELIVER 3002
This error is given when Diameter cannot deliver the message to the destination, either because no host within the realm supporting the required application was available to process the request or because the Destination-Host AVP was given without the associated Destination-Realm AVP.

Answer 1

If the MME fills in the Destination-Host AVP, then Destination-Realm is not used in the forwarding decision. DRA will forward to the specified host, and if the host is not available (it is down or is not reachable), DRA would respond with 3002.

If destination-host is not filled in, then Destination-Realm is used to make the forwarding decision. In this case, if one server is not available, alternate servers will be tried by the DRA.

If you are implementing the MME, it may be a good idea to send with the destination-host filled in, and if the request comes back with 3002, you can send it without destination-host but with destination-realm filled in. We have seen this approach used especially for session based requests (Gx, Gy etc).

Many implementations use two-layered approach. If the base stack and the processing server application are running as separate processes, it is possible that the HSS itself may return 3002 if the application crashes (and stack is still running). We have seen this with a popular OCS solution.
rk

Comments

Thanks RK for helping hand as usual :)

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mate, thanks a lot for your answer. I am impressed!