Early Offer versus Delayed Offer SIP Invites

An early offer INVITE is when the initial SIP Invite contains a Session Description Protocol (SDP) header. 46 Labs’ PSTN vendor partner always sends an early offer SIP Invite for all PSTN-originated calls to the Customer. The Customer can choose to send either a delayed offer or an early offer SIP Invite. 46 Labs recommends sending early offer SIP Invites.

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 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 SBC, customer-originated calls are generally sent to a single endpoint (the Peeredge SBC). 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 vendor Partner 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 IP Phone.

Recommendations

46 Labs recommends using Early Offer SIP Invites.