| A/V Transport Payloads Working Group | T. Edwards |
| Internet-Draft | FOX |
| Intended status: Standards Track | June 15, 2017 |
| Expires: December 17, 2017 |
RTP Payload for SMPTE ST 291 Ancillary Data
draft-ietf-payload-rtp-ancillary-10
This memo describes a real-time transport protocol (RTP) payload format for the Society of Motion Picture and Television Engineers (SMPTE) Ancillary data, as defined by SMPTE ST 291-1. SMPTE Ancillary data is generally used along with professional video formats to carry a range of ancillary data types, including time code, Closed Captioning, and the Active Format Description (AFD).
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This memo describes a real-time transport protocol (RTP) payload format for the Society of Motion Picture and Television Engineers (SMPTE) Ancillary data (ANC), as defined by SMPTE ST 291-1. ANC data is transmitted in the ancillary space of serial digital video interfaces, the space outside of the active video region of images intended for users to view. Ancillary space roughly corresponds to vertical and horizontal blanking periods of cathode ray tube type displays. ANC can carry a range of data types, including time code, Closed Captioning, and the Active Format Description (AFD).
ANC is generally associated with the carriage of metadata within the bit stream 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, 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, it is identified by a registered data identification word.
This memo describes an RTP payload that supports carriage of ANC data packets with origin from any location within any SMPTE defined SDI signal, or even if the ANC packets did not originate in an SDI signal. Sufficient information is provided to enable the ANC data 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). An optional Media Type parameter allows for signaling of carriage of one or more types of ANC data as specified by Data Identification (DID) or Secondary Data Identification (SDID) words. Another optional Media Type parameter allows for the identification of the Video Payload ID (VPID) Code of the source interface of ANC data packets.
Note that the ancillary data flag (ADF) word is not specifically carried in this RTP payload. The ADF might be specified in a document defining an interconnecting digital video interface, otherwise a default ADF is specified by SMPTE ST 291-1.
This ANC payload can be used by itself, or used along with a range of RTP video formats. In particular, it has been 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.
An example of 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 | F | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Line_Number | Horizontal_Offset |S| StreamNum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DID | SDID | Data_Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
User_Data_Words...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum_Word | word_align |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Line_Number | Horizontal_Offset |S| StreamNum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DID | SDID | Data_Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
User_Data_Words...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum_Word |word_align |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SMPTE Ancillary Data RTP Packet Format
In this example, two ANC data packets are present. The first has four 10-bit User Data Words, and the second has five 10-bit User Data Words.
The term "network byte order" in the payload format SHALL refer to the Data Transmission Order as defined in Appendix B of RFC 791.
RTP packet header fields SHALL be interpreted as per RFC 3550, with the following specifics:
The ANC RTP payload header fields are defined as:
For each ANC data packet in the payload, the following ANC data packet header fields MUST be present:
An ANC data packet with the header fields Line_Number of 0x7FF and Horizontal_Offset of 0xFFF SHALL be considered to be carried without any specific location within the field or frame.
For each ANC data packet in the payload, immediately after the ANC data packet header fields, the following data fields MUST be present, with the fields DID, SDID, Data_Count, User_Data_Words, and Checksum_Word representing the 10-bit words carried in the ANC data packet, as per SMPTE ST 291-1:
At the end of each ANC data packet in the payload:
When reconstructing an SDI signal based on this payload, it is important to place ANC data packets into the locations indicated by the ANC payload header fields C, Line_Number and Horizontal_Offset, and also to follow the requirements of SMPTE ST 291-1 Section 7 "Ancillary Data Space Formatting (Component or Composite Interface)", which include rules on the placement of initial ANC data into allowed spaces as well as the contiguity of ANC data packet sequences within those spaces in order to assure that the resulting ANC data packets in the SDI signal are valid. The optional Media Type parameter VPID_Code can inform receivers of the type of originating SDI interface. For multi-stream originating interfaces, the StreamNum field can provide information regarding which stream an ANC data packet can be placed in to match the ANC data location in the originating SDI interface.
Senders of this payload SHOULD transmit available ANC data packets as soon as practical to reduce end-to-end latency, especially if receivers will be embedding the received ANC data packet into an SDI signal emission. One millisecond is a reasonable upper bound for the amount of time between when an ANC data packet becomes available to a sender and the emission of an RTP payload containing that ANC data packet.
ANC data packets with headers that specify specific location within a field or frame SHOULD be sent in raster scan order, both in terms of packing position within an RTP packet and in terms of transmission time of RTP packets.
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 6838.
Note that the Media Type Definition is in the "video" tree due to the expected use of SMPTE ST 291 Ancillary Data along with video formats.
Type name: video
Subtype name: smpte291
Required parameters:
Optional parameters:
Encoding considerations: This media type is framed and binary; see Section 4.8 of RFC 6838.
Security considerations: See Section 7 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 might care about the location of the ANC data packets in the SDI raster, but other implementations might not care.
Published specification: [this RFC]
Applications that use this media type: Devices that stream real-time professional video, especially those that interoperate with legacy serial digital interfaces (SDI).
Additional Information:
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: The IETF PAYLOAD working group, or other party as designated by 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.
DID and SDID values SHALL be specified in hexadecimal with a "0x" prefix (such as "0x61"). The ABNF as per RFC 5234 of the DID_SDID optional parameter SHALL be:
TwoHex = "0x" 1*2(HEXDIG)
DidSdid = "DID_SDID={" TwoHex "," TwoHex "}"
For example, EIA 608 Closed Caption data would be signalled with the parameter DID_SDID={0x61,0x02}. If a DID_SDID parameter is not specified, then the ancillary data stream might potentially contain ancillary data packets of any type.
Multiple DID_SDID parameters can be specified (separated by semicolons) to signal the presence of multiple types of ANC data in the stream. DID_SDID={0x61,0x02};DID_SDID={0x41,0x05}, for example, signals the presence of EIA 608 Closed Captions as well as AFD/Bar Data. Multiple DID_SDID parameters do not imply any particular ordering of the different types of ANC packets in the stream.
If the optional parameter VPID_Code is present, it SHALL be present only once in the semicolon-separated list, taking a single integer value.
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_SDID={0x61,0x02};DID_SDID={0x41,0x05};VPID_Code=132
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. In the "a=fmtp:" line, DID 0x61 and SDID 0x02 are specified (registered to EIA 608 Closed Caption Data by SMPTE), and also DID 0x41 and SDID 0x05 (registered to AFD/Bar Data). The VPID_Code is 132 (referring to SMPTE ST 292-1, 720-line video payloads on a 1.5 Gbps serial digital interface).
To associate an ANC RTP stream with other media streams, implementers can use the Lip Synchronization (LS) grouping defined in RFC 5888, which states that media streams whose corresponding "m" lines are grouped together using LS semantics are to be played back in a synchronized manner.
A sample SDP mapping for grouping ANC data with RFC 4175 video using LS semantics is as follows:
v=0
o=Al 123456 11 IN IP4 host.example.com
s=Professional Networked Media Test
i=A test of synchronized video and ANC data
t=0 0
a=group:LS V1 M1
m=video 50000 RTP/AVP 96
c=IN IP4 233.252.0.1/255
a=rtpmap:96 raw/90000
a=fmtp:96 sampling=YCbCr-4:2:2; width=1280; height=720; depth=10
a=mid:V1
m=video 50010 RTP/AVP 97
c=IN IP4 233.252.0.2/255
a=rtpmap:97 smpte291/90000
a=fmtp:97 DID_SDID={0x61,0x02};DID_SDID={0x41,0x05}
a=mid:M1
Receivers might wish to receive ANC data streams with specific DID_SDID parameters. Thus when offering ANC data streams using the Session Description Protocol (SDP) in an Offer/Answer model [RFC3264], the offeror MAY provide a list of ANC streams available with specific DID_SDID parameters in the fmtp line. The answerer MAY respond with all or a subset of the streams offered along with fmtp lines with all or a subset of the DID_SDID parameters offered. Or the answerer MAY reject the offer. There are no restrictions on updating DID_SDID parameters in a subsequent offer.
For declarative use of SDP, nothing specific is defined for this payload format. The configuration given by the SDP MUST be used when sending and/or receiving media in the session.
One media type (video/smpte291) has been defined and needs registration in the media types registry. See Section 3.1
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550], and in any applicable RTP profile such as RTP/AVP [RFC3551], RTP/AVPF [RFC4585] RTP/SAVP [RFC3711] or RTP/SAVPF [RFC5124]. However, as "Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202] discusses, it is not an RTP payload format's responsibility to discuss or mandate what solutions are used to meet the basic security goals like confidentiality, integrity and source authenticity for RTP in general. This responsibility lays on anyone using RTP in an application. They can find guidance on available security mechanisms and important considerations in Options for Securing RTP Sessions [RFC7201]. Applications SHOULD use one or more appropriate strong security mechanisms. The rest of this security consideration section discusses the security impacting properties of the payload format itself.
To avoid potential buffer overflow attacks, receivers SHOULD validate that the ANC data 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, but the Checksum_Word is unlikely to provide a payload integrity check in case of a directed attack.
Some receivers will simply move the ANC data packet bits from the RTP payload into a serial digital interface (SDI). It might 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 data 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.