DICOM PS3.22 2021e - Real-Time Communication

6.2 Transport

6.2.1 RTP Header

All Essences shall be transported with RTP according to [SMPTE ST 2110-10], which requires that each Flow is described by an SDP object which specifies its content as well as connection details enabling the receiver to join the session. In addition to mandatory information specified in [SMPTE ST 2110-10], for Audio and Video Essence, the SDP may also include the following information:

  • PTP Sync Timestamp

  • PTP Origin Timestamp

  • Source Identifier

  • Flow Identifier

Note

This information is the best way for associating multiple Flows originating from the same device. The presence of such information in the SDP implies that it is contained in the RTP Extended Header present in the first IP packet of a Grain (video frame, audio sample, metadata set). It makes it possible to automatically associate and temporarily synchronize two Flows based on their content.

By definition, all the Flows according to [SMPTE ST 2110-10] are synchronized by means of a common reference to the Universal Time, using PTP, with precision on the order of nanoseconds.

The RTP Header, for video and audio Flows, shall follow [SMPTE ST 2110-20] and [SMPTE ST 2110-30], respectively.

The RTP Header, for DICOM Metadata Flows, shall follow [SMPTE ST 2110-10]. The clock rate shall be identical to the one defined in the referenced audio or video Flow. The following additional constraints apply:

extension (X) : 1 bit

Shall be set to 1.

payload type (PT)

The value of payload type is selected from the range 96-127. It is recommended to avoid numbers frequently used for audio (97) and video (96), and for example use 104 for DICOM Metadata Essence. The value shall be associated to the media type "application" and the subtype "dicom" in the SDP. E.g., (DICOM Metadata on port 12345):

m=application12345 RTP/AVP104

a=rtpmap:104 dicom/90000

For the DICOM Metadata Essence, the RTP Header Extension defined by NMOS shall be present, including the following information:

  • PTP Sync Timestamp

  • PTP Origin Timestamp

  • Source Identifier

  • Flow Identifier

The "defined by profile" part of the RTP Header Extension shall be set to 0xBEDE identifying that the one-byte header extension form is used, as specified in [RFC 5285].

DICOM PS3.22 2021e - Real-Time Communication