An early offer INVITE is when the initial SIP Invite contains a Session Description Protocol (SDP) header. PSTN carriers by default always send early offer SIP Invites. The Customer can choose to send either an early offer or a delayed offer SIP Invite.
When an initial SIP Invite does not contain an SDP header, the invite is called a delayed offer INVITE. Delayed offer SIP Invites can be advantageous in some limited calling scenarios (i.e. when inviting multiple SIP endpoints to establish a call). In this scenario, the originator (specifically the SBC/PBX) can delay the selection of the codec until it knows which endpoint answered the call. With the Peeredge Orchestrator, customer-originated calls are generally always to a single endpoint (the Peeredge Orchestrator). As a result, delayed offer SIP Invites typically provide no advantages.
For a delayed offer SIP Invite, 46 Labs’ PSTN vendor Partner provides an SDP header in the first reliable SIP 18x response and/or the SIP 200 OK message. For 46 Labs’ PSTN carriers to provide a reliable SIP 18x response, the SIP Invite must contain the value 100rel in the Supported or Require headers. Without a reliable SIP 18x response with SDP, a delayed offer SIP Invite will not be able to establish an early media session. Early media sessions can provide network-based messages and tones (i.e. ringback tone) to the Customer endpoints (i.e. IP Phones).
Recommendations
46 Labs recommends using Early Offer SIP Invites.
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