RTP Payload for SMPTE ST 291 Ancillary Data

This memo describes an RTP Payload format for Society of Motion Picture and Television Engineers (SMPTE) Ancillary data (ANC), as defined by SMPTE ST 291-1. ANC can carry a range of data types, including time code, KLV metadata, Closed Captioning, and the Active Format Description (AFD). ANC is generally associated with the carriage of metadata within the bit stream multiplex of a Serial Digital Interface (SDI) such as SMPTE ST 259, the standard definition (SD) Serial Digital Interface (with ANC data inserted as per SMPTE ST 125), or SMPTE ST 292-1, the 1.5 Gb/s Serial Digital Interface for high definition (HD) television applications. ANC data packet payload definitions for a specific application are specified by a SMPTE Standard, Recommended Practice, or Registered Disclosure Document, or by a document generated by another organization, a company, or an individual (an Entity). When a payload format is registered with SMPTE, an application document describing the payload format is required, and the registered ancillary data packet is identified by a registered data identification word. This RTP payload supports ANC data packets regardless of whether they originate from an SD or HD interface, or if the ANC data packet is from the vertical ancillary space (VANC) or the horizontal ancillary space (HANC), or if the ANC packet is located in the luma (Y) or color-difference (C) channel. Sufficient information is provided to enable the ANC packets at the output of the decoder to be restored to their "original" locations in the serial digital video signal raster (if that is desired). This payload could be used by itself, or used along with a range of RTP video formats. In particular, it has been specifically designed so that it could be used along with RFC 4175 "RTP Payload Format for Uncompressed Video" or RFC 5371 "RTP Payload Format for JPEG 2000 Video Streams." The data model in this document for the ANC data RTP payload is based on the data model of SMPTE ST 2038, which standardizes the carriage of ANC data packets in an MPEG-2 Transport Stream.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The format of an RTP packet containing SMPTE ST 291 Ancillary Data is shown below:

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Sequence Number | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ANC_Count |C| Line_Number | Horizontal_Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DID | SDID | Data_Count | R | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | User_Data_Words... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum_Word |octet_align| (next ANC data packet)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RTP packet header fields SHALL be interpreted as per RFC 3550, with the following specifics: The timestamp field is interpreted in a similar fashion to RFC 4175: For progressive scan video, the timestamp SHALL denote the sampling instant of the frame to which the ancillary data in the RTP packet belongs. Packets MUST NOT include ANC data from multiple frames, and all packets with ANC data belonging to the same frame MUST have the same timestamp. For interlaced video, the timestamp SHALL denote the sampling instant of the field to which the ancillary data in the RTP packet belongs. Packets MUST NOT include ANC data from multiple fields, and all packets belonging to the same field MUST have the same timestamp. A 90-kHz timestamp SHOULD be used in both cases. If the sampling instant does not correspond to an integer value of the clock, the value SHALL be truncated to the next lowest integer, with no ambiguity. The marker bit set to "1" SHALL indicate the last RTP packet containing ANC data for a frame (for progressive scan video) or the last RTP packet containing ANC data for a field (for interlaced video).

The ANC RTP payload header fields are defined as: The high order bits of the extended 32-bit sequence number, in network byte order. This is the same as the Extended Sequence Number field in RFC 4175. Number of octets of the ANC RTP payload, beginning with the "C" bit of the first ANC packet data. This field is the count of the total number of ANC data packets carried in the RTP payload. A single ANC RTP packet payload SHALL NOT carry more than 255 ANC data packets. And for each ANC data packet in the payload, the following header fields MUST be present: For HD signals, this flag, when set to "1", indicates that the ANC data corresponds to the color-difference channel (C). When set to "0", this flag indicates that the ANC data corresponds to the luma (Y) channel. For SD signals, this flag SHALL be set to "0". This field contains the line number (as defined in ITU-R BT.1700 for SD video or ITU-R BT.1120 for HD video) that corresponds to the location of the ANC data packet. The lines that are available to convey ANC data are as defined in the applicable sample structure specification (e.g., SMPTE 274M, SMPTE ST 296, ITU-R BT.656) and may be further restricted per SMPTE RP 168. This field defines the location of the ANC packet relative to the start of active video (SAV). 0 means that the Ancillary Data Flag (ADF) of the ANC packet begins immediately following SAV. For HD, this shall be in units of luma sample numbers as specified by the defining document of the particular image (e.g., SMPTE 274M for 1920 x 1080 active images, or SMPTE ST 296 for 1280 x 720 progressive active images). For SD, this is in units of (27MHz) multiplexed word numbers, as specified in SMPTE ST 125. It should be noted that HANC space in the digital blanking area will generally have higher luma sample numbers than any samples in the active digital line. The fields DID, SDID, Data_Count, User_Data_Words, and Checksum_Word represent the 10-bit words carried in the ANC data packet, as per SMPTE ST 291: Data Identification Word Secondary Data Identification Word. Used only for a "Type 2" ANC data packet. Note that in a "Type 1" ANC data packet, this word will actually carry the Data Block Number (DBN). The lower 8 bits of Data_Count, corresponding to bits b7 (MSB) through b0 (LSB) of the 10-bit Data_Count word, contain the actual count of 10-bit words in User_Data_Words. Bit b8 is the even parity for bits b7 through b0, and bit b9 is the inverse (logical NOT) of bit b8. User_Data_Words (UDW) are used to convey information of a type as identified by the DID word or the DID and SDID words. The number of 10-bit words in the UDW is defined by the Data_Count field. The Checksum_Word can be used to determine the validity of the ANC data packet from the DID word through the UDW. The lower 8 bits of Checksum_Word, corresponding to bits b8 (MSB) through b0 (LSB) of the 10-bit data count word, contain the actual checksum value. Bit b9 is the inverse (logical NOT) of bit b8. The checksum value is equal to the nine least significant bits of the sum of the nine least significant bits of the DID word, the SDID word, the Data_Count word, and all User_Data_Words in the ANC data packet. The checksum is initialized to zero before calculation, and any end carry resulting from the checksum calculation is ignored. Octet align contains enough "0" bits as needed to complete the last octet of an ANC packet's data in the RTP payload. This ensures that the next ANC packet's data in the RTP payload begins octet-aligned despite ANC packets being made up of 10-bit words. If an ANC data packet in the RTP payload ends aligned with an octet, there is no need to add any octet alignment bits.

This RTP payload format is identified using the video/smpte291 media type, which is registered in accordance with RFC 4855, and using the template of RFC 4288. Note that the Media Type Definition is in the "video" tree due to the expected use of SMPTE ST 291 Ancillary Data with video formats.

Type name: video Subtype name: smpte291 Required parameters: Rate: RTP timestamp clock rate. Optional parameters: DID: Data identification word SDID: Secondary data identification word The presence of the DID and SDID parameters signal that all ancillary data packets of this stream are of a particular type, i.e., labeled with a particular DID and SDID. DID and SDID values are registered with SMPTE as per SMPTE ST 291-1. DID and SDID values should be specified in hexadecimal with a "0x" prefix (such as "0x61"). For example, EIA 608 Closed Caption data would be DID=0x61 and SDID=0x02. If DID and SDID are not specified, then the ancillary data stream may potentially contain ancillary data packets of any type. DID and SDID values can be found on the SMPTE Registry for Data Identification Word Assignments for Registered DIDs at: http://www.smpte-ra.org/S291/S291_reg.html Encoding considerations: This media type is framed and binary; see Section 4.8 of RFC 4288. Security considerations: See Section of [this RFC] Interoperability considerations: Data items in smpte291 can be very diverse. Receivers might only be capable of interpreting a subset of the possible data items. Some implementations may care about the location of the ANC data packets in the SDI raster, but other implementations may not care. Published specification: [this RFC] Applications that use this media type: Devices that stream real-time professional video, especially those that must interoperate with legacy serial digital interfaces (SDI). Additional Information: none Person & email address to contact for further information: T. Edwards <thomas.edwards@fox.com>, IETF Payload Working Group <payload@ietf.org> Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP RFC 3550. Transport within other framing protocols is not defined at this time. Author: T. Edwards <thomas.edwards@fox.com> Change controller: IETF Payload working group delegated from the IESG.

The mapping of the above defined payload format media type and its parameters SHALL be done according to Section 3 of RFC 4855. The type name ("video") goes in SDP "m=" as the media name. The subtype name ("smpte291") goes in SDP "a=rtpmap" as the encoding name. The optional DID and SDID parameters go in the SDP "a=fmtp" attribute as a semicolon-separated list of parameter=value pairs. A sample SDP mapping for ancillary data is as follows:

m=video 30000 RTP/AVP 112 a=rtpmap:112 smpte291/90000 a=fmtp:112 DID=0x61; SDID=0x02; In this example, a dynamic payload type 112 is used for ancillary data. The 90 kHz RTP timestamp rate is specified in the "a=rtpmap" line after the subtype. The RTP sampling clock is 90 kHz. In the "a=fmtp:" line, DID 0x61 and SDID 0x02 are specified (which are registered to EIA 608 Closed Caption Data by SMPTE).

When offering SMPTE ST 291 Ancillary data over RTP using the Session Description Protocol (SDP) in an Offer/Answer model or in a declarative manner (e.g., SDP in the Real-Time Streaming Protocol (RTSP) or the Session Announcement Protocol (SAP) ), the offerer could provide a list of streams available with specific DID & SDIDs, and the answerer could specify which streams with specific DID & SDIDs it would like to accept.

One media type (video/smpte291) has been defined and needs registration in the media types registry. See

RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification and any applicable RTP profile, e.g., AVP . To avoid potential buffer overflow attacks, receivers should take care to validate that the ANC packets in the RTP payload are of the appropriate length (using the Data_Count field) for the ANC data type specified by DID & SDID. Also the Checksum_Word should be checked against the ANC data packet to ensure that its data has not been damaged in transit. Some receivers will simply move the ANC data packet bits from the RTP payload into a serial digital interface (SDI). It may still be a good idea for these "re-embedders" to perform the above mentioned validity tests to avoid downstream SDI systems from becoming confused by bad ANC packets, which could be used for a denial of service attack. "Re-embedders" into SDI should also double check that the Line_Number and Horizontal_Offset leads to the ANC data packet being inserted into a legal area to carry ancillary data in the SDI video bit stream of the output video format.