A=ssrc attribute in SDP INVITE causes early media RTP issue with some carriers

Yeti-Switch
1

Hello,

We are running Yeti Switch 1.14 (SEMS 1.201.2 / yeti-module 1.14.47core201) and have identified an interoperability issue introduced in SEMS 1.169.0 when the a=ssrc attribute was added to SDP generation.

Problem:
Since upgrading to 1.14, one of our carriers (VOS3000-based) stopped sending early media RTP before the 200 OK. The same carrier with the same destination numbers works correctly on our Yeti 1.13 instance.

Evidence:

  • Yeti 1.14 INVITE SDP includes: a=ssrc:XXXXXXXXX cname:X.X.X.X
  • Yeti 1.13 INVITE SDP does NOT include a=ssrc
  • Carrier sends 183 Session Progress correctly in both cases
  • Carrier sends RTP early media only when a=ssrc is absent (1.13)
  • Pcap analysis on both legs confirmed 0 RTP packets before 200 OK on 1.14

Impact:
Downstream clients using platforms that depend on carrier early media RTP (such as VOS5000) experience NO-RBT (No Ring Back Tone) during the ringing phase, since no audio stream is received before the 200 OK.

Attempted workarounds:

  • Sdp Alines Filter Type: Blacklist with ssrc on the termination gateway — did not remove the attribute
  • Lua Script on gateway — feature not yet functional
  • Try Avoid Transcoding: Yes — did not remove a=ssrc

Request:
Could you add a per-gateway option to disable generation of the a=ssrc attribute in outgoing SDP? This would allow interoperability with carriers that don’t handle this attribute correctly.

Thank you!

2

Why do you think the SSRC attribute is causing the issue?

3

Hi Dimitry, thank you for the quick response.

To be honest, we are not 100% sure the a=ssrc is the root cause — it was our hypothesis based on it being the main SDP difference between our Yeti 1.13 and 1.14 instances.

What we confirmed is:

  • Yeti 1.14 sends a=ssrc in the INVITE SDP, Yeti 1.13 does not
  • The carrier (VOS3000-based) does not send early media RTP before 200 OK on 1.14
  • We captured pcaps on both legs confirming 0 RTP packets before 200 OK on 1.14
  • The same carrier appears to send early media on 1.13 (CDR shows Early Media Present: Yes)

We tried Sdp Alines Filter Type: Blacklist with ssrc but it did not remove the attribute from the outgoing INVITE. Is this filter supposed to work on outgoing SDP?

Could you help us identify if there are other SDP differences between 1.13 and 1.14 that could explain the carrier behavior?

Thank you!

4

So, you propose developing a feature just to test a hypothesis. We already have customers with interconnections to VOS3000, and they are not experiencing this issue.

  • Could you provide a PCAP trace first?
  • Have you tried escalating the issue to the carrier? What was their response?
  • Does Yeti violate any standards by sending the ssrc attribute?

Is this filter supposed to work on outgoing SDP?

Not in this case