Fr-Soltanzadeh left a comment (kamailio/kamailio#4170)
@miconda
Thank you for the feedback. I agree that my initial approach—only hiding up to
the last Record-Route (RR) header—was incorrect, as it doesn’t account for
scenarios where the Topoh proxy (e.g., P-CSCF) is not the last entity in the RR
chain.
However, I believe there’s a misunderstanding about the specific issue in the
IMS topology I’m working with. Let me clarify the problem more precisely.
Scenario
In an IMS setup, due to spiraling behavior (e.g., S-CSCF -> I-CSCF -> S-CSCF),
the same proxy (like P-CSCF) may appear multiple times in the Record-Route
list. For example, in the 180 Ringing or 200 OK responses sent from the core
network back to the UE, the RR headers might look like:
Record-Route: PCSCF
Record-Route: SCSCF
Record-Route: SCSCF
Record-Route: PCSCF
Here, P-CSCF (which is running Topoh) appears twice—once at the beginning and
once at the end. However, the current Topoh implementation works by scanning
RRs top-down and stops at the first occurrence of its own IP. That means in
this example, none of the RR headers are hidden, because the first occurrence
of its IP is at the top of the list.
In this case, Topoh fails to hide any RR headers, which results in exposing
internal IPs (like those of S-CSCF). This breaks the topology hiding
requirement expected in IMS.
Suggested Behavior
Instead of stopping at the first match of its own IP in the RR headers, Topoh
should:
* First, scan all Record-Route headers to find the last occurrence of its own
IP.
* Then, hide all Record-Route headers up to that position.
This ensures that:
All internal hop headers before and including the last instance of the Topoh
proxy are hidden.
Any legitimate Record-Route headers after the Topoh proxy (e.g., from a
downstream proxy or UE) remain intact.
I'd appreciate your thoughts on whether this approach aligns with the original
design goals of Topoh, especially with respect to spiraling.
--
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