Network Working Group L. Gharai

Network Working Group L. Gharai
Request for Comments: 3497 C. Perkins
Category: Standards Track USC/ISI
G. Goncher
Tektronix
A. Mankin
Bell Labs, Lucent Corporation
March 2003

RTP Payload Format for
Society of Motion Picture and Television Engineers (SMPTE) 292M Video

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

This memo specifies an RTP payload format for encapsulating
uncompressed High Definition Television (HDTV) as defined by the
Society of Motion Picture and Television Engineers (SMPTE) standard,
SMPTE 292M. SMPTE is the main standardizing body in the motion
imaging industry and the SMPTE 292M standard defines a bit-serial
digital interface for local area HDTV transport.

1. Introduction

The serial digital interface, SMPTE 292M [1], defines a universal
medium of interchange for uncompressed High Definition Television
(HDTV) between various types of video equipment (cameras, encoders,
VTRs, etc.). SMPTE 292M stipulates that the source data be in 10 bit
words and the total data rate be either 1.485 Gbps or 1.485/1.001
Gbps.

The use of a dedicated serial interconnect is appropriate in a studio
environment, but it is desirable to leverage the widespread
availability of high bandwidth IP connectivity to allow efficient
wide area delivery of SMPTE 292M content. Accordingly, this memo
defines an RTP payload format for SMPTE 292M format video.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

It is to be noted that SMPTE 292M streams have a constant high bit
rate and are not congestion controlled. Accordingly, use of this
payload format should be tightly controlled and limited to private
networks or those networks that provide resource reservation and
enhanced quality of service. This is discussed further in section 9.

This memo only addresses 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 has also adopted the MPEG-2 transport system (ISO/IEC
13818-1) [11]. Therefore RFC 2250 [12] sufficiently describes
transport for compressed HDTV over RTP.

2. Overview of SMPTE 292M

A SMPTE 292M television line comprises two interleaved streams, one
containing the luminance (Y) samples, the other chrominance (CrCb)
values. Since chrominance is horizontally sub-sampled (4:2:2 coding)
the lengths of the two streams match (see Figure 3 of SMPTE 292M
[1]). In addition to being the same length the streams also have
identical structures: each stream is divided 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 may also carry horizontal
ancillary data (H-ANC) or vertical ancillary data (V-ANC) instead of
the blanking level; Likewise, ancillary data may be transported
instead of a digital active line.

The EAV and SAV are made up of three 10 bit words, with constant
values of 0x3FF 0x000 0x000 and an additional word (designated as XYZ
in figure 2), carrying a number of flags. This includes an F flag
which designates which field (1 or 2) the line is transporting and
also a V flag which indicates field blanking. Table 1, further
displays the code values in SAV and EAV. After EAV, are two words,
LN0 and LN1 (Table 2), that carry 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.

+------------+-----------------------+-----+---------------------+
| | 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.

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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.

+---------------------------------------------------------+
| (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.

+---------------------------------------------------------+
| (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 source format documents. Currently, source video
formats transfered by SMPTE 292M include SMPTE 260M, 295M, 274M and
296M [5-8]. In this memo, we specify how to transfer SMPTE 292M over
RTP, irrespective of the source format.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

3. Conventions Used in this Document

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

Each SMPTE 292M data line is packetized into one or more RTP packets.
This includes all timing signals, blanking levels, active lines
and/or ancillary data. Start of active video (SAV) and end of active
video (EAV+LN+CRC) signals MUST NOT be fragmented across packets, as
the SMPTE 292M decoder uses them to detect the start of scan lines.

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, thereby extending the sequence number to 32 bits
and enabling RTP to accommodate HDTV's high data rates. At 1.485
Gbps, with packet sizes of at least one thousand octets, 32 bits
allows for an approximate 6 hour period before the sequence number
wraps around. Given the same assumptions, the standard 16 bit RTP
sequence number wraps around in less than a second (336
milliseconds), which is clearly not sufficient for the purpose of
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 allows the receiver to reconstruct the timing of the
SMPTE 292M stream, without knowledge of 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 provides more information at the
application level and adds a level of resiliency, in case the packet
containing the EAV is lost.

The bit length of both timing signals, SAV and EAV+LN+CRC, are
multiples of 8 bits, 40 bits and 80 bits, respectively, and therefore
are naturally octet aligned.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

For the video content, it is desirable for the video to both octet
align when packetized and also adhere to the principles of
application level framing, also known as ALF [13]. For YCrCb video,
the ALF principle translates into not fragmenting related luminance
and chrominance values across packets. For example, with the 4:2:0
color subsampling, a 4 pixel group is represented by 6 values, Y1 Y2
Y3 Y4 Cr Cb, and video content should be packetized such that these
values are not fragmented across 2 packets. However, with 10 bit
words, this is a 60 bit value which is not octet aligned. To be both
octet aligned, and adhere to ALF, an ALF unit must represent 2 groups
of 4 Pixels, thereby becoming octet aligned on a 15 octet boundary.
This length is referred to as the pixel group or pgroup, and it is
conveyed in the SDP parameters. Table 3 displays the pgroup value
for various color samplings. Typical source formats use 4:2:2
sampling, and require a pgroup of 5 octets, 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 is applicable to instances where the source
format is not 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.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

5. RTP Packetization

The standard RTP header is followed by a 4 octet payload header, and
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 payload

5.1. The RTP Header

The following fields of the RTP fixed header are used for SMPTE 292M
encapsulation (the other fields in the RTP header 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, and is set to 1
for the last packet of the video frame and is otherwise set to 0
for all other packets.

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Sequence Number (low bits): 16 bits
The low order bits for RTP sequence counter. The standard 16 bit
RTP sequence number is augmented with another 16 bits in the
payload header in order 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 else where.

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 cannot
accurately represent the octet count and number of packets lost since
the beginning of transmission, as defined in RFC 1889. Therefore,
for network monitoring purposes or any other application that
requires 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.

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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 <[email protected]>
IETF AVT working group.

Intended usage: COMMON

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

Author/Change controller:
Ladan Gharai <[email protected]>

8. Mapping to SDP Parameters

Parameters are mapped to SDP [14] as follows:

m=video 30000 RTP/AVP 111
a=rtpmap:111 SMPTE292M/148500000
a=fmtp:111 pgroup=5

In 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

RTP sessions using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [3] and any appropriate RTP profile (e.g., [4]).

This payload format does not exhibit any significant non-uniformity
in the receiver side computational complexity for packet processing
to cause a potential denial-of-service threat for intended receivers.

The bandwidth of this payload format is high enough (1.485 Gbps
without the RTP overhead) to cause potential for denial-of-service if
transmitted 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.

If QoS enhanced service is used, RTP receivers SHOULD monitor packet
loss to ensure that the service that was requested is actually being
delivered. If it is not, then they SHOULD assume that they are
receiving best-effort service and behave accordingly.

If best-effort service is being used, RTP receivers MUST monitor
packet loss to ensure that 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 if the loss rate is
unacceptably high.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

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.

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

[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: [email protected]

Colin Perkins
USC/ISI
3811 Fairfax Dr.
Arlington VA 22203

EMail: [email protected]

Allison Mankin
Bell Labs, Lucent Corporation

EMail: [email protected]

Gary Goncher
Tektronix, Inc.
P.O. Box 500, M/S 50-480
Beaverton, OR 97077

EMail: [email protected]

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RFC 3497 RTP Payload Format for SMPTE 292M Video March 2003

14. Full Copyright Statement

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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