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SBC Sends SRTP To RTP Only Realms (Doc ID 3067207.1)

Last updated on JANUARY 14, 2025

Applies to:

Acme Packet 3900 - Version S-Cz9.2.0 and later
Information in this document applies to any platform.

Symptoms

The SBC is advertising SRTP to RTP-only realms, leading to a media issue in the second leg of a call.

When initiating a call from Teams, the first leg (Teams to SBC) proceeds without any issues. However, in the second leg (SBC to CUCM or an external carrier), the Session Description Protocol (SDP) contains the same information as sent by Teams. Specifically, the SBC is not inserting its own IP address and ports, which results in SRTP media being sent. Consequently, the CUCM or external carrier returns a 488 error due to the mismatched SDP.

Despite configuring the steering pool and ensuring that all relevant settings as per Oracle application notes for trunks with media bypass and LMO (Local Media Optimization) have been correctly applied, the issue persists .

 

Sample Flow:

+------------------+         +---------------------+         +----------------------+
|                  |         |                     |         |                      |
|     Teams        | -----> |         SBC          | -----> |     CUCM/Carrier     |
| (Sends INVITE    |         | (Receives INVITE     |         | (Fails to relay media |
|  with SDP &      |         |  with X-MS-MediaPath |         |  correctly)           |
|  X-MS-MediaPath) |         |  header)             |         |                      |
+------------------+         +---------------------+         +----------------------+
                                               |
                                               v
                          +---------------------------+
                          | SBC checks X-MS-MediaPath |
                          | in INVITE header          |
                          +---------------------------+
                                             |
                                             v
                          +---------------------------+
                          | SBC's hostname does not   |
                          | match X-MS-MediaPath      |
                          | (mismatch detected)       |
                          +---------------------------+
                                            |
                                            v
                          +---------------------------+
                          | SBC transparently passes  |
                          | the SDP without modifying |
                          | it (No IP/Port insertion) |
                          +---------------------------+
                                           |
                                           v
                          +---------------------------+
                          | Media flow failure,       |
                          | CUCM/Carrier returns 488  |
                          +---------------------------+

 

 

Cause

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In this Document
Symptoms
Cause
Solution


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