RFC 3497
RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) 292M Video
okay bestie, RFC 3497 is about getting uncompressed HDTV video moving over RTP, so it's basically the protocol that carries live video over IP 🎬✨
Let's start with the title block 🎀 It's RFC 3497, called "RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) 292M Video". The authors are L. Gharai, C. Perkins (USC/ISI), G. Goncher (Tektronix), and A. Mankin (Bell Labs, Lucent Corporation). It's a Standards Track document, dated March 2003. It doesn't list anything it obsoletes or updates 💅
Status of this Memo 🌸
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice 🦋
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract 🧚
okay so this memo sets up an RTP payload format for carrying uncompressed High Definition Television (HDTV), as defined by the Society of Motion Picture and Television Engineers (SMPTE) standard SMPTE 292M 🎥 SMPTE is the big standards body for the motion imaging industry, and SMPTE 292M is the one that defines a bit-serial digital interface for local area HDTV transport. iconic 👑
1. Introduction ✨
so the serial digital interface, SMPTE 292M [1], is basically the universal way to swap uncompressed HDTV between different video gear (cameras, encoders, VTRs, etc.) 📼 SMPTE 292M says the source data has to be in 10 bit words, and the total data rate has to be either 1.485 Gbps or 1.485/1.001 Gbps 🔥
a dedicated serial connection makes sense in a studio, but it's way more useful to use the huge availability of high bandwidth IP connectivity for efficient wide area delivery of SMPTE 292M content 🌈 so this memo defines an RTP payload format for SMPTE 292M format video. main character energy 💅
ngl there's a catch 💀 SMPTE 292M streams have a constant high bit rate and aren't congestion controlled. so this payload format should be tightly controlled and limited to private networks, or to networks that provide resource reservation and enhanced quality of service. we dig into this more in section 9 🫶
this memo only covers the transfer of uncompressed HDTV. compressed HDTV is a subset of MPEG-2 [9], which is fully described in document A/53 [10] of the Advanced Television Standards Committee. the ATSC also adopted the MPEG-2 transport system (ISO/IEC 13818-1) [11]. so RFC 2250 [12] already sufficiently describes transport for compressed HDTV over RTP 🎀
2. Overview of SMPTE 292M 🌷
okay this section is the vibe check on how a SMPTE 292M line is built 🔍
a SMPTE 292M television line has two interleaved streams. one has the luminance (Y) samples, and the other has the chrominance (CrCb) values. chrominance is horizontally sub-sampled (4:2:2 coding), so the two streams end up the same length (see Figure 3 of SMPTE 292M [1]) 🍓 they also share the same structure. each stream is split into four parts (figure 1): (1) start of active video timing reference (SAV); (2) digital active line; (3) end of active video timing reference (EAV); and (4) digital line blanking. a SMPTE 292M line can also carry horizontal ancillary data (H-ANC) or vertical ancillary data (V-ANC) instead of the blanking level. likewise, ancillary data can be sent instead of a digital active line 🧚
the EAV and SAV are each made of three 10 bit words, with constant values of 0x3FF 0x000 0x000, plus one more word (called XYZ in figure 2) that carries a bunch of flags 🚩 there's an F flag, which says which field (1 or 2) the line is carrying, and a V flag, which says whether field blanking is happening. Table 1 shows the code values in SAV and EAV 💖 after EAV come two words, LN0 and LN1 (Table 2), which hold the 11 bit line number for the SMPTE 292M line. the Cyclic Redundancy Check (CRC) is also a two word value, shown as CR0 and CR1 in figure 2 ✨
here's the line layout (Figure 1) 👀
+------------+-----------------------+-----+---------------------+
| | Digital Line Blanking | | Digital Active Line |
| EAV+LN+CRC | (Blanking level or | SAV | (Active Picture or |
| | Ancillary Data) | | Ancillary Data) |
+------------+-----------------------+-----+---------------------+
Figure 1. The SMPTE 292M line format.and the timing reference format (Figure 2) 🌈
0 20 40 60 80 0 20 40
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|3FF| 0 | 0 |XYZ|LN1|LN2|CR0|CR1| |3FF| 0 | 0 |XYZ|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
<---- EAV -----> <- LN-> <- CRC-> <----- SAV ----->
Figure 2. Timing reference format.the bit-level breakdown of the timing reference codes (Table 1) 🔥
+---------------------------------------------------------+
| (MSB) (LSB) |
| Word 9 8 7 6 5 4 3 2 1 0 |
+---------------------------------------------------------+
| 3FF 1 1 1 1 1 1 1 1 1 1 |
| 000 0 0 0 0 0 0 0 0 0 0 |
| 000 0 0 0 0 0 0 0 0 0 0 |
| XYZ 1 F V H P P P P P P |
+---------------------------------------------------------+
| NOTES: |
| F=0 during field 1; F=1 during field 2. |
| V=0 elsewhere; V=1 during field blanking. |
| H=0 in SAV; H=1 in EAV. |
| MSB=most significant bit; LSB=least significant bit.|
| P= protected bits defined in Table 2 of SMPTE 292M |
+---------------------------------------------------------+
Table 1: Timing reference codes.and the line number data (Table 2) 🦋
+---------------------------------------------------------+
| (MSB) (LSB) |
| Word 9 8 7 6 5 4 3 2 1 0 |
+---------------------------------------------------------+
| LN0 R L6 L5 L4 L3 L2 L1 L0 R R |
| LN1 R R R R L10 L9 L8 L7 R R |
+---------------------------------------------------------+
| NOTES: |
| LN0 - L10 - line number in binary code. |
| R = reserved, set to "0". |
+---------------------------------------------------------+
Table 2: Line number data.the number of words and the format for active lines and line blanking is defined by the source format documents. right now, source video formats carried by SMPTE 292M include SMPTE 260M, 295M, 274M and 296M [5-8]. in this memo, we spell out how to transfer SMPTE 292M over RTP, no matter which source format it is 🫶
3. Conventions Used in this Document 💜
okay, the key words are the ones that tell you how strict a requirement is, so here they are. 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 BCP 14, RFC 2119 [2] 🎀
4. Payload Design 🎀
okay so each SMPTE 292M data line gets packetized into one or more RTP packets 🦋 and that includes literally everything: all the timing signals, blanking levels, active lines and/or ancillary data. the big rule is that Start of active video (SAV) and end of active video (EAV+LN+CRC) signals MUST NOT be fragmented across packets, because the SMPTE 292M decoder uses them to detect the start of scan lines. no splitting these, periodt 💅
The standard RTP header is followed by a 4 octet payload header. All information in the payload header pertains to the first data sample in the packet. The end of a video frame (the packet containing the last sample before the EAV) is marked by the M bit in the RTP header. ✨
The payload header contains a 16 bit extension to the standard 16 bit RTP sequence number, so the sequence number gets stretched to 32 bits and RTP can handle HDTV's high data rates 🔥 At 1.485 Gbps, with packet sizes of at least one thousand octets, 32 bits lasts about 6 hours before the sequence number wraps around. Under the same assumptions, the standard 16 bit RTP sequence number wraps in less than a second (336 milliseconds), which is clearly not enough for detecting loss and out of order packets 😭
A 148.5 MHz (or 148.5/1.001 MHz) time-stamp is used as the RTP timestamp 🕰️ This lets the receiver rebuild the timing of the SMPTE 292M stream without knowing the exact type of source format (e.g., SMPTE 274M or SMPTE 296M). With this timestamp, the location of the first sample of each packet can be uniquely identified in the SMPTE 292M stream. At 148.5 MHz, the 32 bit timestamp wraps around in 21 seconds 🌸
The payload header also carries the 11 bit line number from the SMPTE 292M timing signals 🧚 This gives more info at the application level and adds a layer of resiliency, in case the packet containing the EAV gets lost.
The bit length of both timing signals, SAV and EAV+LN+CRC, are multiples of 8 bits, 40 bits and 80 bits, respectively, so they're naturally octet aligned 🫶
For the video content, it's desirable for the video to both octet align when packetized and also follow the principles of application level framing, also known as ALF [13]. For YCrCb video, the ALF principle means not fragmenting related luminance and chrominance values across packets 💜 For example, with 4:2:0 color subsampling, a 4 pixel group is represented by 6 values, Y1 Y2 Y3 Y4 Cr Cb, and the video content should be packetized so these values aren't fragmented across 2 packets. BUT with 10 bit words, that's a 60 bit value which isn't octet aligned 💀 To be both octet aligned and ALF-compliant, an ALF unit must represent 2 groups of 4 pixels, which makes it octet aligned on a 15 octet boundary. This length is called the pixel group or pgroup, and it's conveyed in the SDP parameters. Table 3 shows the pgroup value for various color samplings. Typical source formats use 4:2:2 sampling and require a pgroup of 5 octets; the other values are included for completeness 🍓
The contents of the Digital Active Line SHOULD NOT be fragmented within a pgroup. A pgroup of 1 indicates that data may be split at any octet boundary (this applies when the source format isn't known). The SAV and EAV+LN+CRC fields MUST NOT be fragmented 🙈
+-------------------------------------------------------+
| Color 10 bit |
|Subsampling Pixels words aligned on octet# pgroup|
+-----------+-------+--------+-------------------+------+
| 4:2:0 | 4 | 6*10 | 2*60/8 = 15 | 15 |
+-----------+-------+--------+-------------------+------+
| 4:2:2 | 2 | 4*10 | 40/8 = 5 | 5 |
+-----------+-------+--------+-------------------+------+
| 4:4:4 | 1 | 3*10 | 4*30/8 = 15 | 15 |
+-----------+-------+--------+-------------------+------+
Table 3. Color subsampling and pgroups.5. RTP Packetization 📦
The standard RTP header is followed by a 4 octet payload header and then the payload data, as shown in Figure 3 💖
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 |P|X| CC |M| PT | sequence# (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ssrc |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sequence# (high bits) |F|V| Z | line no |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: SMPTE 292M data :
: :
| |
+---------------------------------------------------------------+
Figure 3: RTP Packet showing SMPTE 292M headers and payload5.1. The RTP Header 🌈
These fields of the RTP fixed header are used for SMPTE 292M encapsulation (the other RTP header fields are used in their usual manner):
- Payload Type (PT): 7 bits 🎫 A dynamically allocated payload type field that designates the payload as SMPTE 292M.
- Timestamp: 32 bits ⏱️ For a SMPTE 292M transport stream at 1.485 Gbps (or 1.485/1.001 Gbps), the timestamp field contains a 148.5 MHz (or 148.5/1.001 MHz) timestamp, respectively. This allows for a unique timestamp for each 10 bit word.
- Marker bit (M): 1 bit 🚩 The Marker bit denotes the end of a video frame. It's set to 1 for the last packet of the video frame and set to 0 for all other packets.
- Sequence Number (low bits): 16 bits 🔢 The low order bits for the RTP sequence counter. The standard 16 bit RTP sequence number is augmented with another 16 bits in the payload header to accommodate the 1.485 Gbps data rate of SMPTE 292M.
5.2. Payload Header 🎀
- Sequence Number (high bits): 16 bits 🔢 The high order bits for the 32 bit RTP sequence counter, in network byte order.
- F: 1 bit 👯 The F bit as defined in the SMPTE 292M timing signals (see Table 1). F=1 identifies field 2 and F=0 identifies field 1.
- V: 1 bit 🫧 The V bit as defined in the SMPTE 292M timing signals (see Table 1). V=1 during field blanking, and V=0 elsewhere.
- Z: 2 bits 🌙 SHOULD be set to zero by the sender and MUST be ignored by receivers.
- Line No: 11 bits 📏 The line number of the source data format, extracted from the SMPTE 292M stream (see Table 2). The line number MUST correspond to the line number of the first 10 bit word in the packet.
6. RTCP Considerations 📊
RFC 1889 should be used as specified in RFC 1889 [3], which specifies two limits on the RTCP packet rate: RTCP bandwidth should be limited to 5% of the data rate, and the minimum for the average of the randomized intervals between RTCP packets should be 5 seconds. Considering the high data rate of this payload format, the minimum interval is the governing factor in this case 🏃♀️
It should be noted that the sender's octet count in SR packets wraps around in 23 seconds, and that the cumulative number of packets lost wraps around in 93 seconds. This means these two fields can't accurately represent the octet count and number of packets lost since the beginning of transmission, as defined in RFC 1889 😭 So for network monitoring, or any other application that needs the sender's octet count and the cumulative number of packets lost since the beginning of transmission, the application itself must keep track of the number of rollovers of these fields via a counter 🧮
7. IANA Considerations 📝
This document defines a new RTP payload format and associated MIME type, SMPTE292M. The MIME registration form for SMPTE 292M video is enclosed below 💌
MIME media type name: video
MIME subtype name: SMPTE292M
Required parameters: rate
The RTP timestamp clock rate. The clock runs at either 148500000
Hz or 148500000/1.001 Hz. If the latter rate is used a timestamp
of 148351648 MUST be used, and receivers MUST interpret this as
148500000/1.001 Hz.
Optional parameters: pgroup
The RECOMMENDED grouping for aligning 10 bit words and octets.
Defaults to 1 octet, if not present.
Encoding considerations: SMPTE292M video can be transmitted with RTP
as specified in RFC 3497.
Security considerations: see RFC 3497 section 9.
Interoperability considerations: NONE
Published specification: SMPTE292M
RFC 3497
Applications which use this media type:
Video communication.
Additional information: None
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Ladan Gharai <ladan@isi.edu>
IETF AVT working group.
Intended usage: COMMON
Author/Change controller:
Ladan Gharai <ladan@isi.edu>8. Mapping to SDP Parameters 🗺️
Parameters are mapped to SDP [14] like this:
m=video 30000 RTP/AVP 111
a=rtpmap:111 SMPTE292M/148500000
a=fmtp:111 pgroup=5In this example, a dynamic payload type 111 is used for SMPTE292M. The RTP timestamp is 148500000 Hz, and the SDP parameter pgroup indicates that for video data after the SAV signal, it must be packetized in multiples of 5 octets 🎉
9. Security Considerations 🔐💅
okay so bestie, this is the security section and it is NOT the time to be casual about it 😭🫶 RTP sessions using this payload format are subject to the security considerations in the RTP spec [3] and any appropriate RTP profile (e.g., [4]). go read those too, they're giving important vibes 🎀
the payload format doesn't have any real non-uniformity in receiver-side computational complexity for packet processing, so it's not a denial-of-service threat for the intended receivers 🧚
BUT (and this is a big but 💀) the bandwidth is SO high, 1.485 Gbps without the RTP overhead, that it could cause denial-of-service if it gets sent onto most currently available Internet paths 🔥. Since congestion control is not possible for SMPTE 292M over RTP flows, use of the payload SHOULD be narrowly limited to suitably connected network endpoints, or to networks where QoS guarantees are available 🌈. literally a no-cap warning.
If QoS enhanced service is used, RTP receivers SHOULD monitor packet loss to make sure the service that was requested is actually being delivered 👑. If it isn't, they SHOULD assume they're getting best-effort service and behave accordingly 🦋.
If best-effort service is being used, RTP receivers MUST monitor packet loss to make sure the packet loss rate is within acceptable parameters, and MUST leave the session if the loss rate is too high 😭💜
The loss rate is considered acceptable if a TCP flow across the same network path, experiencing the same network conditions, would achieve an average throughput, measured on a reasonable timescale, that is not less than the RTP flow is achieving 💖. Since congestion control is not possible for SMPTE 292M flows, this condition can only be satisfied if receivers leave the session when the loss rate is unacceptably high ✨. iykyk, and now you do 🙈
10. Acknowledgments 🌸
We would like to thank David Richardson for his insightful comments and contributions to the document 💅. We would also like to thank Chuck Harrison for his input and for explaining the intricacies of SMPTE 292M 🫶.
11. Normative References 📚
[1] Society of Motion Picture and Television Engineers, Bit-Serial Digital Interface for High-Definition Television Systems, SMPTE 292M-1998.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996.
[4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, January 1996.
12. Informative References 🦋
[5] Society of Motion Picture and Television Engineers, Digital Representation and Bit-Parallel Interface - 1125/60 High-Definition Production System, SMPTE 260M-1999.
[6] Society of Motion Picture and Television Engineers, 1920x1080 50Hz, Scanning and Interface, SMPTE 295M-1997.
[7] Society of Motion Picture and Television Engineers, 1920x1080 Scanning and Analog and Parallel Digital Interfaces for Multiple Picture Rates, SMPTE 274M-1998.
[8] Society of Motion Picture and Television Engineers, 1280x720 Scanning, Analog and Digital Representation and Analog Interfaces, SMPTE 296M-1998.
[9] ISO/IEC International Standard 13818-2; "Generic coding of moving pictures and associated audio information: Video", 1996.
[10] ATSC Digital Television Standard Document A/53, September 1995, http://www.atsc.org
[11] ISO/IEC International Standard 13818-1; "Generic coding of moving pictures and associated audio information: Systems",1996.
[12] Hoffman, D., Fernando, G., Goyal, V. and M. Civanlar, "RTP Payload Format for MPEG1/MPEG2 Video", RFC 2250, January 1998.
[13] Clark, D. D., and Tennenhouse, D. L., "Architectural Considerations for a New Generation of Protocols", In Proceedings of SIGCOMM '90 (Philadelphia, PA, Sept. 1990), ACM.
[14] Handley, H. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.
13. Authors' Addresses 🎀
Ladan Gharai USC/ISI 3811 Fairfax Dr. Arlington VA 22203
EMail: ladan@isi.edu
Colin Perkins USC/ISI 3811 Fairfax Dr. Arlington VA 22203
EMail: csp@csperkins.org
Allison Mankin Bell Labs, Lucent Corporation
EMail: mankin@psg.com
Gary Goncher Tektronix, Inc. P.O. Box 500, M/S 50-480 Beaverton, OR 97077
EMail: Gary.Goncher@tek.com
14. Full Copyright Statement ⭐
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement 💜
Funding for the RFC Editor function is currently provided by the Internet Society. 🍓
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