RFC2250 - RTP Payload Format for MPEG1/MPEG2 Video

Network Working Group D. Hoffman

Request for Comments: 2250 G. Fernando

Obsoletes: 2038 Sun Microsystems, Inc.

Category: Standards Track V. Goyal

Precept Software, Inc.

M. Civanlar

AT&T Labs - Research

January 1998

RTP Payload Format for MPEG1/MPEG2 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.

Abstract

This memo describes a packetization scheme for MPEG video and audio

streams. The scheme proposed can be used to transport sUCh a video

or audio flow over the transport protocols supported by RTP. Two

approaches are described. The first is designed to support maximum

interoperability with MPEG System environments. The second is

designed to provide maximum compatibility with other RTP-encapsulated

media streams and future conference control work of the IETF.

This memo is a revision of RFC2038, an Internet standards track

protocol. In this revision, the packet loss resilience mechanisms in

Section 3.4 were extended to include additional picture header

information required for MPEG2. A new section on security

considerations for this payload type is added.

1. Introduction

ISO/IEC JTC1/SC29 WG11 (also referred to as the MPEG committee) has

defined the MPEG1 standard (ISO/IEC 11172)[1] and the MPEG2 standard

(ISO/IEC 13818)[2]. This memo describes a packetization scheme to

transport MPEG video and audio streams using the Real-time Transport

Protocol (RTP), version 2 [3, 4].

The MPEG1 specification is defined in three parts: System, Video and

Audio. It is designed primarily for CD-ROM-based applications, and

is optimized for approximately 1.5 Mbits/sec combined data rates. The

video and audio portions of the specification describe the basic

format of the video or audio stream. These formats define the

Elementary Streams (ES). The MPEG1 System specification defines an

encapsulation of the ES that contains Presentation Time Stamps (PTS),

Decoding Time Stamps and System Clock references, and performs

multiplexing of MPEG1 compressed video and audio ES's with user data.

The MPEG2 specification is structured in a similar way. However, it

hasn't been restricted only to CD-ROM applications. The MPEG2 System

specification defines two system stream formats: the MPEG2 Transport

Stream (MTS) and the MPEG2 Program Stream (MPS). The MTS is tailored

for communicating or storing one or more programs of MPEG2 compressed

data and also other data in relatively error-prone environments. The

MPS is tailored for relatively error-free environments.

We seek to achieve interoperability among 4 types of end-systems in

the following specification. The 4 types are:

1. Transmitting Interworking Unit (TIU)

Receives MPEG information from a native MTS system for

distribution over packet networks using a native RTP-based

system layer (such as an IP-based internetwork). Examples:

real-time encoder, MTS satellite link to Internet, video

server with MTS-encoded source material.

2. Receiving Interworking Unit (RIU)

Receives MPEG information in real time from an RTP-based

network for forwarding to a native MTS environment.

Examples: Internet-based video server to MTS-based cable

distribution plant.

3. Transmitting Internet End-System (TAES)

Transmits MPEG information generated or stored within the

internet end-system itself, or received from internet-based

computer networks. Example: video server.

4. Receiving Internet End-System (RAES)

Receives MPEG information over an RTP-based internet for

consumption at the internet end-system or forwarding to

traditional computer network. Example: desktop PC or

workstation viewing training video.

Each of the 2 types of transmitters must work with each of the 2

types of receivers. Because it is probable that the TAES, and

certain that the RAES, will be based on existing and planned

internet-connected computers, it is highly desirable for the

interoperable protocol to be based on RTP.

Because of the range of applications that might employ MPEG streams,

we propose to define two payload formats.

Much interest in the MPEG community is in the use of one of the MPEG

System encodings, and hence, in Section 2 we propose encapsulations

of MPEG1 System streams and MPEG2 Transport and Program Streams with

RTP. This profile supports the full semantics of MPEG System and

offers basic interoperability among all four end-system types.

When operating only among internet-based end-systems (i.e., TAES and

RAES) a payload format that provides greater compatibility with the

Internet architecture is desired, deferring some of the system issues

to other protocols being defined in the Internet community (such as

the MMUSIC WG). In Section 3 we propose an encapsulation of

compressed video and audio data (referred to in MPEG documentation as

"Elementary Streams" (ES)) complying with either MPEG1 or MPEG2.

Here, neither of the System standards of MPEG1 or MPEG2 are utilized.

The ES's are directly encapsulated with RTP.

Throughout this specification, we make extensive use of MPEG

terminology. The reader should consult the primary MPEG references

for definitive descriptions of this terminology.

2. Encapsulation of MPEG System and Transport Streams

Each RTP packet will contain a timestamp derived from the sender's

90KHz clock reference. This clock is synchronized to the system

stream Program Clock Reference (PCR) or System Clock Reference (SCR)

and represents the target transmission time of the first byte of the

packet payload. The RTP timestamp will not be passed to the MPEG

decoder. This use of the timestamp is somewhat different than

normally is the case in RTP, in that it is not considered to be the

media display or presentation timestamp. The primary purposes of the

RTP timestamp will be to estimate and reduce any network-induced

jitter and to synchronize relative time drift between the transmitter

and receiver.

For MPEG2 Transport Streams the RTP payload will contain an integral

number of MPEG transport packets. To avoid end system

inefficiencies, data from multiple small MTS packets (normally fixed

in size at 188 bytes) are aggregated into a single RTP packet. The

number of transport packets contained is computed by dividing RTP

payload length by the length of an MTS packet (188).

For MPEG2 Program streams and MPEG1 system streams there are no

packetization restrictions; these streams are treated as a packetized

stream of bytes.

2.1 RTP header usage

The RTP header fields are used as follows:

Payload Type: Distinct payload types should be assigned for

MPEG1 System Streams, MPEG2 Program Streams and MPEG2

Transport Streams. See [4] for payload type assignments.

M bit: Set to 1 whenever the timestamp is discontinuous

(such as might happen when a sender switches from one data

source to another). This allows the receiver and any

intervening RTP mixers or translators that are synchronizing

to the flow to ignore the difference between this timestamp

and any previous timestamp in their clock phase detectors.

timestamp: 32 bit 90K Hz timestamp representing the target

transmission time for the first byte of the packet.

3. Encapsulation of MPEG Elementary Streams

The following ES types may be encapsulated directly in RTP:

(a) MPEG1 Video (ISO/IEC 11172-2) (b) MPEG2 Video (ISO/IEC

13818-2) (c) MPEG1 Audio (ISO/IEC 11172-3) (d) MPEG2 Audio

(ISO/IEC 13818-3)

A distinct RTP payload type is assigned to MPEG1/MPEG2 Video and

MPEG1/MPEG2 Audio, respectively. Further indication as to whether the

data is MPEG1 or MPEG2 need not be provided in the RTP or MPEG-

specific headers of this encapsulation, as this information is

available in the ES headers.

Presentation Time Stamps (PTS) of 32 bits with an accuracy of 90 kHz

shall be carried in the fixed RTP header. All packets that make up a

audio or video frame shall have the same time stamp.

3.1 MPEG Video elementary streams

MPEG1 Video can be distinguished from MPEG2 Video at the video

sequence header, i.e. for MPEG2 Video a sequence_header() is followed

by sequence_extension(). The particular profile and level of MPEG2

Video (MAIN_Profile@MAIN_Level, HIGH_Profile@HIGH_Level, etc) are

determined by the profile_and_level_indicator field of the

sequence_extension header of MPEG2 Video.

The MPEG bit-stream semantics were designed for relatively error-free

environments, and there is significant amount of dependency (both

temporal and spatial) within the stream such that loss of some data

make other uncorrupted data useless. The format as defined in this

encapsulation uses application layer framing information plus

additional information in the RTP stream-specific header to allow for

certain recovery mechanisms. Appendix 1 suggests several recovery

strategies based on the properties of this encapsulation.

Since MPEG pictures can be large, they will normally be fragmented

into packets of size less than a typical LAN/WAN MTU. The following

fragmentation rules apply:

1. The MPEG Video_Sequence_Header, when present, will always

be at the beginning of an RTP payload.

2. An MPEG GOP_header, when present, will always be at the

beginning of the RTP payload, or will follow a

Video_Sequence_Header.

3. An MPEG Picture_Header, when present, will always be at the

beginning of a RTP payload, or will follow a GOP_header.

Each ES header must be completely contained within the packet.

Consequently, a minimum RTP payload size of 261 bytes must be

supported to contain the largest single header defined in the ES

(that is, the extension_data() header containing the

quant_matrix_extension()). Otherwise, there are no restrictions on

where headers may appear within packet payloads.

In MPEG, each picture is made up of one or more "slices," and a slice

is intended to be the unit of recovery from data loss or corruption.

An MPEG-compliant decoder will normally advance to the beginning of

next slice whenever an error is encountered in the stream. MPEG

slice begin and end bits are provided in the encapsulation header to

facilitate this.

The beginning of a slice must either be the first data in a packet

(after any MPEG ES headers) or must follow after some integral number

of slices in a packet. This requirement insures that the beginning

of the next slice after one with a missing packet can be found

without requiring that the receiver scan the packet contents. Slices

may be fragmented across packets as long as all the above rules are

met.

An implementation based on this encapsulation assumes that the

Video_Sequence_Header is repeated periodically in the MPEG bit-

stream. In practice (though not required by MPEG standard) this is

used to allow channel switching and to receive and start decoding a

continuously relayed MPEG bit-stream at arbitrary points in the media

stream. It is suggested that when playing back from an MPEG stream

from a file format (where the Video_Sequence_Header may only be

represented at the beginning of the stream) that the first

Video_Sequence_Header (preceded by an end-of-stream indicator) be

saved by the packetizer for periodic injection in to the network

stream.

3.2 MPEG Audio elementary streams

MPEG1 Audio can be distinguished from MPEG2 Audio from the MPEG

ancillary_data() header. For either MPEG1 or MPEG2 Audio, distinct

Presentation Time Stamps may be present for frames which correspond

to either 384 samples for Layer-I, or 1152 samples for Layer-II or

Layer-III. The actual number of bytes required to represent this

number of samples will vary depending on the encoder parameters.

Multiple audio frames may be encapsulated within one RTP packet. In

this case, an integral number of audio frames must be contained

within the packet and the fragmentation header defined in Section 3.5

shall be set to 0.

Also, if relatively short packets are to be used, one frame may be so

large that it may straddle multiple RTP packets. For example, for

Layer-II MPEG audio sampled at a rate of 44.1 KHz each frame would

represent a time slot of 26.1 msec. At this sampling rate if the

compressed bit-rate is 384 kbits/sec (i.e. 48 kBytes/sec) then the

average audio frame size would be 1.25 KBytes. If packets were to be

500 Bytes long, then each audio frame would straddle 3 RTP packets.

The audio fragmentation indicator header (See Section 3.5) shall be

present for an MPEG1/2 Audio payload type to provide for this

fragmentation.

3.3 RTP Fixed Header for MPEG ES encapsulation

The RTP header fields are used as follows:

Payload Type: Distinct payload types should be assigned

for video elementary streams and audio elementary streams.

See [4] for payload type assignments.

M bit: For video, set to 1 on packet containing MPEG frame

end code, 0 otherwise. For audio, set to 1 on first packet of

a "talk-spurt," 0 otherwise.

PT: MPEG video or audio stream ID.

timestamp: 32-bit 90K Hz timestamp representing presentation

time of MPEG picture or audio frame. Same for all packets

that make up a picture or audio frame. May not be

monotonically increasing in video stream if B pictures present

in stream. For packets that contain only a video sequence

and/or GOP header, the timestamp is that of the subsequent

picture.

3.4 MPEG Video-specific header

This header shall be attached to each RTP packet after the RTP fixed

header.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MBZ T TR NSBE P BFC FFC

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

AN FBV FFV

MBZ: Unused. Must be set to zero in current

specification. This space is reserved for future use.

T: MPEG-2 (Two) specific header extension present (1 bit).

Set to 1 when the MPEG-2 video-specific header extension (see

Section 3.4.1) follows this header. This extension may be

needed for improved error resilience; however, its inclusion

in an RTP packet is optional. (See Appendix 1.)

TR: Temporal-Reference (10 bits). The temporal reference of

the current picture within the current GOP. This value ranges

from 0-1023 and is constant for all RTP packets of a given

picture.

AN: Active N bit for error resilience (1 bit). Set to 1 when

the following bit (N) is used to signal changes in the

picture header information for MPEG-2 payloads. It must be

set to 0 for MPEG-1 payloads or when N bit is not used.

N: New picture header (1 bit). Used for MPEG-2 payloads when

the previous bit (AN) is set to 1. Otherwise, it must be set

to zero. Set to 1 when the information contained in the

previously transmitted Picture Headers can't be used to

reconstruct a header for the current picture. This happens

when the current picture is encoded using a different set of

parameters than the previous pictures of the same type. The N

bit must be constant for all RTP packets that belong to the

same picture so that receipt of any packet from a picture

allows detecting whether information necessary for

reconstruction was contained in that picture (N = 1) or a

previous one (N = 0).

S: Sequence-header-present (1 bit). Normally 0 and set to 1 at

the occurrence of each MPEG sequence header. Used to detect

presence of sequence header in RTP packet.

B: Beginning-of-slice (BS) (1 bit). Set when the start of the

packet payload is a slice start code, or when a slice start

code is preceded only by one or more of a

Video_Sequence_Header, GOP_header and/or Picture_Header.

E: End-of-slice (ES) (1 bit). Set when the last byte of the

payload is the end of an MPEG slice.

P: Picture-Type (3 bits). I (1), P (2), B (3) or D (4). This

value is constant for each RTP packet of a given picture.

Value 000B is forbidden and 101B - 111B are reserved to

support future extensions to the MPEG ES specification.

FBV: full_pel_backward_vector

BFC: backward_f_code

FFV: full_pel_forward_vector

FFC: forward_f_code

OBTained from the most recent picture header, and are

constant for each RTP packet of a given picture. For I frames

none of these values are present in the picture header and

they must be set to zero in the RTP header. For P frames

only the last two values are present and FBV and BFC must be

set to zero in the RTP header. For B frames all the four

values are present.

3.4.1 MPEG-2 Video-specific header extension

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

XEf_[0,0]f_[0,1]f_[1,0]f_[1,1] DC PSTPCQVARHGD

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

X: Unused (1 bit). Must be set to zero in current

specification. This space is reserved for future use.

E: Extensions present (1 bit). If set to 1, this header

extension, including the composite display extension when D =

1, will be followed by one or more of the following

extensions: quant matrix extension, picture display

extension, picture temporal scalable extension, picture

spatial scalable extension and copyright extension.

The first byte of these extensions data gives the length of

the extensions in 32 bit Words including the length field

itself. Zero padding bytes are used at the end if required to

align the extensions to 32 bit boundary.

Since they may not be vital in decoding of a picture, the

inclusion of any one of these extensions in an RTP packet is

optional even when the MPEG-2 video-specific header extension

is included in the packet (T = 1). (See Appendix 1.) If

present, they should be copied from the corresponding

extensions following the most recent MPEG-2 picture coding

extension and they remain constant for each RTP packet of a

given picture.

The extension start code (32 bits) and the extension start

code ID (4 bits) are included. Therefore the extensions are

self identifying.

f_[0,0]: forward horizontal f_code (4 bits)

f_[0,1]: forward vertical f_code (4 bits)

f_[1,0]: backward horizontal f_code (4 bits)

f_[1,1]: backward vertical f_code (4 bits)

DC: intra_DC_precision (2 bits)

PS: picture_structure (2 bits)

T: top_field_first (1 bit)

P: frame_predicted_frame_dct (1 bit)

C: concealment_motion_vectors (1 bit)

Q: q_scale type (1 bit)

V: intra_vlc_format (1 bit)

A: alternate scan (1 bit)

R: repeat_first_field (1 bit)

H: chroma_420_type (1 bit)

G: progressive frame (1 bit)

D: composite_display_flag (1 bit). If set to 1, next 32 bits

following this one contains 12 zeros followed by 20 bits

of composite display information.

These values are copied from the most recent picture coding

extension and are constant for each RTP packet of a given

picture. Their meanings are as eXPlained in the MPEG-2 standard.

3.5 MPEG Audio-specific header

This header shall be attached to each RTP packet at the start of the

payload and after any RTP headers for an MPEG1/2 Audio payload type.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MBZ Frag_offset

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Frag_offset: Byte offset into the audio frame for the data

in this packet.

4. Security Considerations

RTP packets 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 (for example [4]).

This implies that confidentiality of the media streams is achieved by

encryption. Because the data compression used with this payload

format is applied end-to-end, encryption may be performed after

compression so there is no conflict between the two operations.

A potential denial-of-service threat exists for data encodings using

compression techniques that have non-uniform receiver-end

computational load. The attacker can inject pathological datagrams

into the stream which are complex to decode and cause the receiver to

be overloaded. However, this encoding does not exhibit any

significant non-uniformity.

As with any IP-based protocol, in some circumstances a receiver may

be overloaded simply by the receipt of too many packets, either

desired or undesired. Network-layer authentication may be used to

discard packets from undesired sources, but the processing cost of

the authentication itself may be too high. In a multicast

environment, pruning of specific sources may be implemented in future

versions of IGMP [5] and in multicast routing protocols to allow a

receiver to select which sources are allowed to reach it.

A security review of this payload format found no additional

considerations beyond those in the RTP specification.

Appendix 1. Error Recovery and Resynchronization Strategies.

The following error recovery and resynchronization strategies are

intended to be guidelines only. A compliant receiver is free to

employ alternative (or no) strategies.

When initially decoding an RTP-encapsulated MPEG Elementary Stream,

the receiver may discard all packets until the Sequence-header-

present bit is set to 1. At this point, sufficient state information

is contained in the stream to allow processing by an MPEG decoder.

Loss of packets containing the GOP_header and/or Picture_Header are

detected by an unexpected change in the Temporal-Reference and

Picture-Type values. Consider the following example GOP sequence:

In display order: 0B 1B 2I 3B 4B 5P 6B 7B 8P GOP_HDR 0B ...

In stream order: 2I 0B 1B 5P 3B 4B 8P 6B 7B GOP_HDR 2I ...

Consider also two counters:

ref_pic_temp (Reference Picture (I,P) Temporal Reference)

dep_pic_temp (Dependent Picture (B) Temporal Reference)

At each GOP beginning, set these counters to the temporal reference

value of the corresponding picture type. For our example GOP

sequence, ref_pic_temp = 2 and dep_pic_temp = 0. Keep incrementing

BOTH counters by unity with each following picture. Ref_pic_temp

should match the temporal references of the I and P frames, and

dep_pic_temp should match the temporal references of the B frames.

dep_pic_temp: - 0 1 2 3 4 5 6 7 8 9

In stream order: 2I 0B 1B 5P 3B 4B 8P 6B 7B GOP_H 2I 0B 1B ...

ref_pic_temp: 2 3 4 5 6 7 8 9 10 ^ 11

-------------------------- ^

Match Drop

Mismatch

in ref_pic_temp

The loss of a GOP header can be detected by matching the appropriate

counter (based on picture type) to the temporal reference value. A

mismatch indicates a lost GOP header. If desired, a GOP header can be

re-constructed using a "null" time_code, repeating the closed_gop

flag from previous GOP headers, and setting the broken_link flag to

The loss of a Picture_Header can also be detected by a mismatch in

the Temporal Reference contained in the RTP packet from the

appropriate dep_pic_temp or ref_pic_temp counters at the receiver.

For MPEG-1 payloads, after scanning to the next Beginning-of-slice

the Picture_Header is reconstructed from the P, TR, FBV, BFC, FFV and

FFC contained in that packet, and from stream-dependent default

values.

For MPEG-2, additional information is needed for the reconstruction.

This information is provided by the MPEG-2 video specific header

extension contained in that packet if the T bit is set to 1, or the

Picture Header for the current picture may be available from previous

packets belonging to the same picture. The transmitter's strategy for

inclusion of the MPEG-2 video specific header extension may depend

upon a number of factors. This header may not be needed when:

1. the information has been transmitted a sufficient number of

times in previous packets to assure reception with the desired

probability, or

2. the information is transmitted over a separate reliable

channel, or

3. expected loss rates are low enough that missed frames are not a

concern, or

4. conserving bandwidth is more important than error resilience,

etc.

If T=1 and E=0, there may be extensions present in the original video

bitstream that are not included in the current packet. The

transmitter may choose not to include extensions in a packet when

they are not necessary for decoding or if one of the cases listed

above for not including the MPEG-2 video specific header extension in

a packet applies only to the extension data.

If N=0, then the Picture Header from a previous picture of the same

type (I,P or B) may be used so long as at least one packet has been

received for every intervening picture of the same type and that the

N bit was 0 for each of those pictures. This may involve:

1. Saving the relevant picture header information that can be

obtained from the MPEG-2 video specific header extension or

directly from the video bitstream for each picture type,

2. Keeping validity indicators for this saved information based on

the received N bits and lost packets, and,

3. Updating the data whenever a packet with N=1 is received.

If the necessary information is not available from any of these

sources, data deletion until a new picture start code is advised.

Any time an RTP packet is lost (as indicated by a gap in the RTP

sequence number), the receiver may discard all packets until the

Beginning-of-slice bit is set. At this point, sufficient state

information is contained in the stream to allow processing by an MPEG

decoder starting at the next slice boundary (possibly after

reconstruction of the GOP_header and/or Picture_Header as described

above).

References

[1] ISO/IEC International Standard 11172; "Coding of moving pictures

and associated audio for digital storage media up to about 1,5

Mbits/s", November 1993.

[2] ISO/IEC International Standard 13818; "Generic coding of moving

pictures and associated audio information", November 1994.

[3] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,

"RTP: A Transport Protocol for Real-Time Applications", RFC1889,

January 1996.

[4] Schulzrinne, H., "RTP Profile for Audio and Video Conferences

with Minimal Control", RFC1890, January 1996.

[5] Deering, S., "Host Extensions for IP Multicasting", STD 5,

RFC1112, August 1989.

Authors' Addresses

Gerard Fernando

Sun Microsystems, Inc.

Mail-stop UMPK14-305

2550 Garcia Avenue

Mountain View, California 94043-1100

USA

Phone: +1 415-786-6373

EMail: gerard.fernando@eng.sun.com

Vivek Goyal

Precept Software, Inc.

1072 Arastradero Rd,

Palo Alto, CA 94304

USA

Phone: +1 415-845-5200

EMail: goyal@precept.com

Don Hoffman

Sun Microsystems, Inc.

Mail-stop UMPK14-305

2550 Garcia Avenue

Mountain View, California 94043-1100

USA

Phone: +1 503-297-1580

EMail: don.hoffman@eng.sun.com

M. Reha Civanlar

AT&T Labs - Research

100 Schutlz Drive, 3-213

Red Bank, NJ 07701-7033

USA

Phone: +1 732-345-3305

EMail: civanlar@research.att.com

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