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The synchronization source (SSRC) is a 32-bit number that uniquely identifies media streams within an RTP session and is included in SDP signaling when establishing or modifying media sessions. The WebRTC specification requires that the SSRC value in an RTP stream match the SSRC sent in the SDP. However some endpoints, such as PSTN gateways, are not capable of generating SSRC values so they are not present in the SDP. Other endpoints change the SSRC during call hold/resume scenarios. 

To address these potential interoperability issues, you can configure the SBC to generate an SSRC and related attributes to ensure that the required values are present in calls toward the Kandy Link WebRTC gateway. When enabled, the SBC generates an SSRC value for audio or video streams and includes it in SDP signaling and RTP/RTCP streams. You can also configure the SBC to update SSRC values in call hold/resume scenarios. When configured to modify the SSRC, after the call resumes, the SBC generates a new SSRC value and includes both the previous and new SSRC values in SDP signaling. Note that the SBC only makes mid-call modifications to the SSRC if it is also configured to generate the SSRC.

When the SBC generates an SSRC, it also generates and signals the following related media attributes that are needed in WebRTC deployments:

  • cname
  • msid
  • msid-semantic
  • mslabel
  • label

Command Syntax

To avoid possible interoperability issue in deployments with Kandy Link WebRTC gateways, the Packet Service profile assigned to the trunk group facing the Kandy network should enable the flags that generate and modify SSRC values. The syntax for these flags is shown below.

% set profiles media packetServiceProfile <profile name> flags
      generateAndSignalSSRCAndCname <disabled | enabled>
      allowMidCallSSRCModification <disabled | enabled>
Note

The SBC only allows mid-call modifications to the SSRC if it is generating the SSRC. Therefore, the first parameter, generateAndSignalSSRCAndCname, must be enabled for the second parameter, allowMidCallSSRCModification, to become available.

Command Parameters

ParameterDescription

generateAndSignalSSRCAndCname

Enable this flag to generate an SSRC value and associated attributes and include them in SDP signaling and RTP/RTCP streams. Options are:

  • disabled (default)
  • enabled

Note: This flag takes precedence over the Packet Service Profile ssrcRandomize flag.

allowMidCallSSRCModification

Enable this flag so that in call hold/resume scenarios the SBC modifies the SSRC and associated attributes after the call resumes. The SBC sends both the previous and updated SSRC in SDP signaling and includes the new SSRC iin RTP/RTCP streams. Options are:

  • disabled (default)
  • enabled

Note: You must enable the generateAndSignalSSRCAndCname flag before you can enable this flag.

Note: If you enable the IP Signaling profile common IP attributes flag minimizeRelayingOfMediaChangesFromOtherCallLegAll, you must also enable the relayDataPathModeChangeFromOtherCallLeg flag to have the SSRC modification processing take effect.

Configuration Example

Use the following commands to configure flags in the DEFAULT Packet Service profile to enable generating SSRC values and allow modification of SSRC values in call hold/resume scenarios. You must enable the generateAndSignalSSRCAndCname flag first.

% set profiles media packetServiceProfile DEFAULT flags generateAndSignalSSRCAndCname enabled
% set profiles media packetServiceProfile DEFAULT flags allowMidCallSSRCModification enabled
% commit