SIP-H.323 interworking with Dynamic Payload Types

The SIP and H.323 Protocols use Internet multimedia signaling over IP, and both use the Real-Time Transport Protocol (RTP) for transferring realtime audio/video data. The interworking function (IWF) provides a means of converting translation and signaling protocols and session descriptions between SIP and H.323. However, SIP and H.323 provide different mechanisms when exchanging payload types for media during IWF calls. Therefore, the International Telecomunications Union (ITU) modified the ITU H.245 recommendations in H.245 v16 to include a new “Dynamic Payload Type Replacement” capability that resolves this payload type conflict. This new capability provides a way for an H.323 endpoint to specify the payload type of a media stream for which the endpoint is willing to receive through the OLCacknowledgment (OLC-ACK) message in an audio/video call flow.

The Oracle® Enterprise Session Border Controller supports this new “Dynamic Payload Type Replacement” capability by ensuring interworking of SIP and H.323 when audio/video call flows use dynamic payload types. The Oracle® Enterprise Session Border Controller checks for the presence of this capability in the incoming TCS request. If it finds this capability in the TCS request, it sends an Open Logic Channel Acknowledgement (OLC-ACK) response with the payload type it is willing to receive.

Note:

The Oracle® Enterprise Session Border Controller always returns an OLC-ACK with a dynamic payload type value that it received in the incoming Session Description Protocol (SDP) from the SIP endpoint.

For devices that don't support the H.245 v16 recommendations, the Terminal Capability Set (TCS) request from the H.323 endpoint does not have the "Dynamic Payload Type Replacement" capability present. Therefore, the Oracle® Enterprise Session Border Controller rewrites the payload type within the RTP packets when these packets traverse the Oracle® Enterprise Session Border Controller. When devices in a session negotiate different payload types between SIP and H.323 packets, the RTP streams that they receive, always have the expected payload type in the RTP header.

Note:

The Oracle® Enterprise Session Border Controller always maps the payload type on the RTP stream received from the H.323 endpoint, and sends it to the SIP endpoint for both audio and video. The Oracle® Enterprise Session Border Controller does not support mapping of payload types in audio streams with 2833 DTMF packets.

Figure 1a and 1b below shows the call flow from an H.323 Endpoint B to a SIP Endpoint A, and from a SIP Endpoint A to an H.323 Endpoint B, respectively. These illustrations show the negotiation of different dynamic payload types for the video streams but the Codec negotiated is the same. The Oracle® Enterprise Session Border Controller dynamically replaces the payload type in the RTP header of the video stream received from the H.323 endpoint.

Figure 1a Endpoint B calling Endpoint A (H.323 endpoint does not have “Dynamic Payload Type Replacement” Capability)

The H.323 Endpoint B is not H.245 v16 compliant, and hence payload type replacement needs to be done in the RTP packets.

Figure 1b Endpoint A calling Endpoint B (H.323 endpoint does not have Dynamic Payload Type Replacement Capability)

The H.323 Endpoint B is not H.245 v16 compliant, and therefore payload type replacement needs to be done in the RTP packets.

There is no concept of H.245 compliance for the SIP Endpoint A.

Figure 2a shows the call flow of SIP Endpoint A calling an H.323 Endpoint B using slow start. The Oracle Communications Session Delivery Manager modifies the dynamic payload type in the OLC-ACK based on payload type received in the incoming SDP OFFER in the "INVITE" message.

Figure 2a Endpoint A calling Endpoint B (H.323 endpoint has TCS with Dynamic Payload Type Replacement Capability)

The H.323 Endpoint B is H.245 v16 compliant.

There is no concept of H.245 compliance for the SIP Endpoint A.

Figure 2b shows the call flow an H.323 Endpoint B using slow start, calling a SIP Endpoint A. The Oracle Communications Session Delivery Manager modifies the dynamic payload type in the OLC-ACK based on payload type received in the incoming SDP ANSWER in the "200 OK" message.

Figure 2b Endpoint B calling Endpoint A (H.323 endpoint here has TCS without “Dynamic Payload Type Replacement” Capability)