SIP: Understanding Early Offer, Delayed Offer, and Early Media

As technology continues to evolve and communication systems become more complex, understanding Session Initiation Protocol (SIP) concepts is crucial for effective implementation and troubleshooting. In this article, we’ll delve into three fundamental SIP concepts: Early Offer, Delayed Offer, and Early Media. These concepts play a critical role in call establishment, re-INVITE, and media transmission.

Early Offer is a type of offer sent by a User Agent (UA) during the initial INVITE or re-INVITE process. This early offer helps to establish the best possible path for media transmission between two endpoints. By sending an Early Offer, the calling UA suggests its preferred set of parameters for the call, such as:

• Codecs: The specific audio and/or video encoding formats to be used during the call.
• Packet sizes: The size of packets transmitted during the call.
• Timing: The timing parameters for media transmission, such as packet inter arrival times.

Early Offers are particularly useful in scenarios where low latency is essential, like video conferencing or real-time communication. By sending an Early Offer in the call setup process, UAs can reduce the time it takes to establish a connection and improve overall user experience.

For example, consider a video conferencing application that uses H.264 video encoding and OPUS audio encoding. The calling UA might send an Early Offer suggesting these specific codecs to ensure that both endpoints are configured for optimal video quality.

So, if the INVITE contains SDP, that is an early offer.

Delayed Offer is an alternative approach in which the SDP offer is deferred until after the call setup phase. In this scenario, the initial INVITE message does not contain the SDP offer; instead, it carries a placeholder indicating that the offer will be provided later. Delayed offer is often used in scenarios where the callee wants to control the media negotiation process. It allows the callee to analyze the incoming call request and decide how to handle it before sending its own SDP answer.

Delayed Offers also allow UAs to adapt to changing network conditions and improve call quality by adjusting parameters such as:

• Bitrate: The rate at which media is transmitted.
• Packet size: The size of packets transmitted during the call.

This is particularly useful in scenarios where network congestion or QoS issues affect call establishment or media transmission. By sending a Delayed Offer, UAs can adjust their configuration to accommodate changing network conditions and ensure optimal call quality.

So, the INVITE will have no SDP. Once the far end answers with 200 OK, the callee will send its SDP in the ACK.

Early Media:

Early media refers to the capability of establishing media sessions, such as audio or video, before the actual call is answered. In other words, early media enables the initiation of media streams even while the call is in the ringing or early setup phase. This can be useful for providing audio announcements, call progress tones, or other media-related functionalities before the callee answers the call. With early media, the caller can provide important information or engage the callee with relevant media content even before the call is fully established. This can enhance the overall user experience and provide valuable feedback during the call setup process.

To summarize:

• Early offer involves including the SDP offer in the initial INVITE message, allowing the callee to make an informed decision about call parameters from the start.
• Delayed offer defers the SDP offer until after the call setup phase, enabling the callee to control the media negotiation process.
• Early media allows the establishment of media sessions before the call is answered, providing useful audio or video content during the call setup phase.

In this article, we’ve explored three fundamental SIP concepts: Early Offer, Delayed Offer, and Early Media. Understanding these concepts is crucial for effective implementation and troubleshooting of SIP-based communication systems. By mastering these concepts, you’ll be better equipped to optimize call establishment, re-INVITE, and media transmission, ultimately improving user experience and overall system performance.