Internet-Draft | HESP | November 2021 |
Speelmans | Expires 24 May 2022 | [Page] |
This document describes a protocol for delivering multimedia data, enabling ultra-low latency and fast channel change over HTTP networks. It specifies the data format of the files and the actions to be taken by the server (sender) and the clients (receivers) of the streams. It describes version 1 of this protocol.¶
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Viewers are more demanding than ever, calling for a streaming protocol that combines ultra-low latency, fast zapping and cost-effective scalability.¶
HESP is an HTTP-based streaming approach that works with standard contribution feeds from (live) productions, standard encoders, albeit specifically configured, a HESP compliant packager, regular CDNs and an HESP compliant player.¶
HESP offers sub-second latency, near real-time interactivity, fast startup and channel change times and cost-effective scalability up to millions of viewers.¶
The purpose of this document is to facilitate interoperability between HESP implementations by describing the media transmission protocol.¶
This document describes version 1 of the protocol.¶
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 [RFC2119].¶
This section contains an overview of the HESP protocol and its building blocks.¶
HESP follows a regular approach to online video streaming. The content is ingested and transcoded in different qualities. Each quality requires two streams. The encoded streams are packaged by an HESP packager and made available via an origin server. An HESP player requests the stream using HTTP requests. A CDN can be used.¶
_________ ________ __________ ________ _____ ________ | | | | | | | | | | | | | Input |\| xcoder |\| packager |\| origin |\| CDN |\| HESP | | Streams |/| |/| |/| |/| |/| player | |_________| |________| |__________| |________| |_____| |________|
HESP is based on using two streams for each track, the Initialization Stream and the Continuation Stream. The encoder MUST ensure that the corresponding media data of both streams are issued with synchronized presentation timestamps. The packager MUST ensure this data remains in sync.¶
The Initialization Stream consists of Initialization Packets. Initialization Packets MUST be individually addressable. An Initialization Packet MUST contain an independent media sample, the reference to the segment and the position in the segment where the Continuation Stream can start. Since the contained samples are independent, playback can start with any Initialization Packet.¶
The Continuation Stream can start playback immediately after an Initialization Packet, allowing for very fast channel start and switch times. This mechanism puts referencing limitations to the Continuation Stream. When a reference is made from the Initialization Stream to a Continuation Stream, all data needed to render the sample with the subsequent Presentation timestamp MUST be available at the reference offset. In addition, the samples in the Initialization Stream and the samples in the Continuation Stream MUST be aligned. That is, the corresponding media samples of both streams MUST have the same PTS and MUST be made available at the same time. The Continuation Stream is addressed using byte-range requests. The Continuation Stream SHOULD be published in chunks in order to reduce end-to-end latency.¶
The player receives the Initialization Packet, initializes the decoder, puts the media data in the decoder buffer, and requests the subsequent media data from the Continuation Stream (using HTTP Range requests, starting by the range offset given by the Initialization Packet.)¶
time sequence T1 T2 T3 T4 T5 T6 T7 T8 +----+----+----+----+----+----+----+----+ initialization stream | I1 | I2 | I3 | I4 | I5 | I6 | I7 | I8 | +----+----+----+----+----+----+----+----+ +----+----+----+----+----+----+----+----+ continuation stream | C1 | C2 | C3 | C4 | C5 | C6 | C7 | C8 | +----+----+----+----+----+----+----+----+ +----+----+----+----+----+----+ playback buffer | I3 | C4 | C5 | C6 | C7 | C8 | +----+----+----+----+----+----+
The object model follows the CMAF media object model [CMAF].¶
A Track is used to contain media samples (audio or video) or metadata. It consists of a Continuation Stream and (except for metadata Tracks) an Initialization Stream.¶
The Initialization Stream consists of Initialization Packets, where each such packet MUST contain a CMAF Header and possibly a CMAF Fragment. Each Initialization Packet MUST be individually addressable through Sequence Numbers. It is further explained in Section 4.¶
The Continuation Stream consists of Continuation Segments. These Segments MUST consist of CMAF Chunks that can be sent individually to clients. It is further explained in Section 5.¶
A Switching Set groups together Tracks that contain the same content but with different encoding parameters (e.g., different resolution or different bitrate). A client is able to seamlessly switch between Tracks of a Switching Set as a result. If multiple Switching Sets contain differing content but are aligned in their timings (e.g., multiple view perspectives on the same performance or different languages of the same audio), we can consider them Aligned Switching Sets.¶
A Selection Set groups together multiple Switching Sets of the same media type. HESP currently allows 3 Selection Sets: audio, video and metadata Selection Sets.¶
A Presentation contains the Selection Sets for a given period of time. Multiple Presentations together form a continuous timeline of media content, even though each Presentation might have different content, encodings or timestamps. It can be an advertisement, or a part of a show, or the first half of a game,... The Presentation is the lowest granularity inside a Manifest. All Tracks of a Presentation MUST have media data for the full duration of the Presentation.¶
The Manifest informs clients of the aforementioned data structure and must be retrieved before any media data. The format of the Manifest is detailed in Section 3.¶
A new Manifest is not needed to request a new Segment. Segment addressing can happen automatically within a Presentation for an efficient and continuous delivery of the Continuation Stream. A Manifest is only obligated to be updated before the start of a new Presentation. This gives the opportunity for low frequency Manifest updates, though this update rate can be freely configured.¶
Since Presentations can be of unknown duration, a different mechanism is used to signal a new Presentation to a client. To that extent, in-band metadata events are introduced in the Continuation Stream. Such an event can signal the client to retrieve a new Manifest, when information about an upcoming Presentation becomes available.¶
+---------+ +---------+ | Player | | Origin | +---------+ +---------+ | | _____________|_____________________________________________|_____ ! LOOP / all presentations | ! !_______/ | | ! ! | | ! ! | request Manifest file | ! ! |-------------------------------------------->| ! ! | | ! ! | Return Manifest | ! ! |<--------------------------------------------| ! ! | | ! ! | parse Manifest | ! ! |--------------- | ! ! | | | ! ! |<-------------- | ! ! | | ! ! | | ! ! | Request Initialization Packet | ! ! |-------------------------------------------->| ! ! | | ! ! | Return Initialization Packet | ! ! |<--------------------------------------------| ! ! | | ! ! | Initialize decode pipeline | ! ! |--------------------------- | ! ! | | | ! ! |<-------------------------- | ! ! | | ! ! | Parse position information | ! ! | of the corresponding data | ! ! | in the Continuation Stream | ! ! |-------------------------- | ! ! | | | ! ! |<------------------------- | ! ! | | ! ! | | ! ! | Request Continuation Stream: | ! ! | (Segment n) byte-range [now, -) | ! ! |-------------------------------------------->| ! ! | | ! ! | Return CMAF Fragments until the | ! ! | end of the Segment | ! ! |<--------------------------------------------| ! ! __________|_____________________________________________|__ ! ! ! LOOP / all Segments of the Presentation | ! ! ! !_______/ | | ! ! ! ! | | ! ! ! ! | Request Continuation Stream: | ! ! ! ! | (Segment n+i) without byte-range | ! ! ! ! |-------------------------------------------->| ! ! ! ! | | ! ! ! ! | Return CMAF Fragments until the | ! ! ! ! | end of the Segment | ! ! ! ! |<--------------------------------------------| ! ! ! ! | | ! ! ! !__________|_____________________________________________|__! ! ! | | ! !_____________|_____________________________________________|_____! | ! | !
Though the manifest file is shared, video, audio and metadata (subtitles) media data MUST be distributed separately. They each require separate requests from the client.¶
The first step for playback of an HESP stream is to fetch a Manifest. It contains information on the available Tracks and how to request content from every Track's Initialization and Continuation Streams.¶
A distinction is made between Manifest Timestamps and Media Timestamps in the definitions below. Media Timestamps MUST represent Presentation timestamps as they are given by the media data itself. Manifest Timestamps MUST define those same Media Timestamps, but with an additional offset such that the timestamps of all Tracks of a Presentation are aligned. This offset MUST be given by the Manifest for each Track or Switching Set.¶
For example, consider the first Segment of a Track with Media Timestamps starting at 0 seconds. This Track is part of the second Presentation of an HESP stream. The first Presentation has run for a significant amount of time and the second Presentation follows immediately after the first Presentation ends. In this case, the Manifest Timestamps of the second Presentation must succeed the Manifest Timestamps of the first Presentation. As a result, the Media Timestamps and Manifest Timestamps of the second Presentation now differ. An offset must be given by the Manifest in order to inform clients about this difference between timestamp types for each Track of the second Presentation.¶
An example of this distinction is given in Appendix A.1.2.¶
Each Initialization Packet belonging to a Track is given a Sequence Number. This is a simple identifier used to retrieve specific Initialization Packets by the client. The Sequence Number is a positive integer that MUST be increased by 1 for each subsequent Initialization Packet of the same Track.¶
To calculate the Sequence Number of a chosen Initialization Packet, the Manifest provides the client multiple values. Each active Presentation MUST contain both its current Manifest Timestamp and, for each Track, the Sequence Number of the latest Initialization Packet and the (constant) frame rate.¶
It is given that:¶
As a result, it is possible to derive Sequence Numbers for arbitrary Initialization Packets. By taking the latest Sequence Number of a Track, its associated Manifest Timestamp, and the frame rate, it is possible to calculate the number of frames between the given Initialization Packet and the required Initialization Packet in the past or future. This amount can then be subtracted for that latest Sequence Number in order to derive the Sequence Number of the desired Initialization Packet.¶
For example, suppose the latest Sequence Number would be 103 at Manifest Timestamp 00:00:04.120
for a Track with a frame rate of 25 fps. To find the Sequence Number at Manifest Timestamp 00:00:01.360
, the difference between both frames should be calculated. This is 2.760 seconds, or 2.760 * 25 = 69 frames. This means that the Sequence Number for the provided Manifest Timestamp is 103 - 69 = 34.¶
The Manifest MUST be formatted as a JSON file. All data types used in the information below MUST satisfy the JavaScript Object Notation specification [RFC8259]. With regards to the definition of number
s within the JSON specification, their range and precision are implementation-dependent. It should be noted that for interoperability, number
s SHOULD be representable using IEEE 754 double-precision numbers.¶
Some additional data types are introduced on top of the JSON specification to narrow the allowed values of some fields further.¶
An integer
(or int
) is a subset of the number
type defined in the JSON specification, limited to integer numbers. Concretely, following Section 6 of the JSON specification, a number
MUST NOT have a fraction part to be considered an integer
. Note that as mentioned above, integer
s SHOULD be representable using IEEE 754 double-precision numbers. As a result, integer
s SHOULD be within the [(-2**53)+1, (2**53)-1] range to make sure implementations agree on the exact numeric value.¶
An unsigned integer
(or uint
) is a subset of the integer
type, only allowing for nonnegative integer numbers. Concretely, following Section 6 of the JSON specification, an integer
MUST NOT contain a minus to be considered an unsigned integer
.¶
An enumeration defines a list of constant strings, where a valid value of this type MUST equal one of these strings. The type is written as Enum(x, y, z)
, where x
, y
and z
are the constant strings allowed to be used.¶
A DateTime MUST be formatted as a string in the format defined by ISO 8601 [ISO8601]:¶
YYYY-MM-DDThh:mm:ss.mmmTZD where YYYY = four-digit year MM = two-digit month DD = two-digit day of the month hh = two digits of the hour (00 through 23) mm = two digits of a minute (00 through 59) ss = two digits of a second (00 through 59) mmm = three digits of a millisecond (000 through 999) TZD = time zone designator (Z or +hh:mm or -hh:mm)¶
This data type represents the root of the Manifest. The structure consists of general playback information and a collection of Presentations.¶
The table below gives the possible fields. The "Required?" column indicates if a field is REQUIRED (Y) or OPTIONAL (N).¶
Attribute | Type | Req? | Description |
---|---|---|---|
availabilityDuration | ScaledValue | Y | The amount of time (in seconds) that live content MUST be kept available to be retrieved by a client. It is used to define an interval [live - n, live], where live is the current live point and n is availabilityDuration , where content MUST be available for clients of a live stream. For VOD streams, this value MUST be ignored. |
creationDate | DateTime | Y | The timestamp of the moment that this specific Manifest was created by the packager (in local packager time.) |
fallbackPollRate | uint | Y | The number of seconds a player SHOULD wait to poll a new Manifest, if it hasn't requested any since retrieving the current Manifest. |
manifestVersion | Enum("1.1.0") | Y | The version number of the Manifest. |
presentations | Presentation Type[] |
Y | A list of all Presentations that are currently known. |
streamType | Enum("live", "vod") | Y | Indicates whether the stream is a live stream (live ) which does not have a known ending, or a video on demand stream (vod ) with a known ending and duration. |
activePresentation | string | N | The identifier of the currently active Presentation. This value MUST be available if streamType equals live , but MUST be ignored if streamType equals vod . |
contentBaseUrl | string | N | The base URL for all content requests relating to this Manifest. See Section 3.4 for more information. |
timeSource | TimeSource | N | A reference to a time server with which the packager is synced. This value MUST be ignored if streamType equals vod . |
A TimeSource is used to sync the packager and player internal clocks to make requests for data only once it is available.¶
Attribute | Type | Required | Description |
---|---|---|---|
scheme | string | Y | A universally unique identifier (UUID) [UUID] that defines the format of the time server response. |
url | string | Y | The URL to be queried for a time server response. |
In order to avoid rounding issues introduced through floating-point numbers, this structure defines two integers. The total value is calculated by dividing value
over scale
.¶
Attribute | Type | Required | Description |
---|---|---|---|
value | int | Y | The defined integer value. |
scale | uint | N | If not defined, the scale SHALL equal 1. |
This is the type definition of a Presentation.¶
Attribute | Type | Req? | Description |
---|---|---|---|
id | string | Y | The unique identifier for this Presentation. It MUST be unique over all Presentations of this Manifest. It MUST NOT change over Manifest updates. |
timeBounds | TimeBounds | Y | The time boundaries of this Presentation, in Manifest Time. The start time MUST be announced at least 2 seconds before the Presentation is active. The end time of an active Presentation MUST be available at least 2 seconds before the actual end of that Presentation. |
audio | AudioSwitchingSetType[] | N | The audio Selection Set. It contains all audio Switching Sets of this Presentation. If not defined, this value SHALL equal an empty list. |
baseUrl | string | N | The base URL of this Presentation. It is part of the content base URLs for all Switching Sets belonging to this Presentation. See Section 3.4 for more information. |
currentTime | ScaledValue | N | The most recent composition timestamp of any Track contained by this Presentation when this Manifest is created. It SHOULD be specified in Manifest Time, not in Media Time (as that could have different offsets from Track to Track.) If the Presentation is active, the current time MUST be defined. If the Presentation is not active, this value MUST be ignored. |
events | PresentationEventType[] | N | List of all currently available Presentation Events related to this Presentation. If not defined, this value SHALL equal an empty list. |
metadata | MetadataSwitchingSetType[] | N | The metadata Selection Set. It contains all metadata Switching Sets of this Presentation. If not defined, this value SHALL equal an empty list. |
video | VideoSwitchingSetType[] | N | The video Selection Set. It contains all video Switching Sets of this Presentation. If not defined, this value SHALL equal an empty list. |
A TimeBounds structure denotes a time interval with a start and end time. A start or end time may be undefined. The desired behavior in such a situation depends on the specific usage of the structure.¶
These boundaries are inclusive at the start time and exclusive at the end time. I.e., if two TimeBounds need to be continuous, then the end time of the first one must equal the start time of the second.¶
Attribute | Type | Required | Description |
---|---|---|---|
startTime | uint | N | This value denotes the start time in seconds when divided by the timescale. |
endTime | uint | N | This value denotes the end time in seconds when divided by the timescale. |
scale | uint | N | If the timescale is not defined, it SHALL equal 1. |
This is the type definition of an audio Switching Set.¶
Attribute | Type | Req? | Description |
---|---|---|---|
id | string | Y | The unique identifier for this Switching Set. It MUST be unique within its Presentation. |
language | string | Y | The language of all audio Tracks of this Switching Set. It MUST be specified here by its ISO 639-2 [ISO6392] code. |
tracks | AudioTrackType[] | Y | The collection of all Tracks belonging to this Switching Set. |
alignId | string | N | A unique identifier that SHOULD be set by all Switching Sets that are Aligned Switching Sets with each other. |
baseUrl | string | N | The base URL of this Switching Set. It is part of the content base URLs for all Tracks belonging to this Switching Set. See Section 3.4 for more information. |
channels | uint | N | The audio channel configuration of all audio Tracks belonging to this Switching Set. It is defined as the total number of audio channels. This value is used for ABR selection by the player. |
codecs | string | N | The definition of the codec(s) necessary to render the content of all Tracks belonging to this Switching Set. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Switching Set. The pattern MUST include the {segmentId} string; see Section 3.4 for more information. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
initializationPattern | string | N | The URL pattern used to request Initialization Packets belonging to this Switching Set. The pattern MUST include the {initId} string; see Section 3.4 for more information. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
label | string | N | A human-readable name for this Switching Set. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by Segments of this Switching Set. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
mimeType | string | N | The MIME type of all content belonging to this Switching Set. It MUST be a valid audio MIME type. If not set, the MIME type of all content of this Switching Set SHALL equal audio/mp4 . |
protection | SwitchingSet Protection |
N | The information related to content protection for all Tracks belonging to this Switching Set. |
sampleRate | uint | N | The sample rate (in Hz) of all audio Tracks belonging to this Switching Set. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
samplesPerFrame | uint | N | The number of audio samples in one frame. If set, this MUST apply to each Track belonging to this Switching Set that does not define this value itself. If neither the Track nor the Switching Set has this value set, it SHALL equal 1024. |
The following fields are defined on the SwitchingSetProtection structure:¶
Attribute | Type | Req? | Description |
---|---|---|---|
type
|
Enum("cenc", "cbcs") | Y | The protection scheme used to encrypt this Switching Set. |
systems
|
SwitchingSetProtectionSystem[] | Y | Metadata about the DRM systems that can be used to playback this Switching Set. This list MUST contain at least one entry. |
More information on content protection can be found in Section 7.¶
The following fields are defined on the SwitchingSetProtectionSystem structure:¶
Attribute | Type | Required | Description |
---|---|---|---|
pssh
|
string | N | A Base 64 encoded ProtectionSystemSpecificHeaderBox . If it is not defined in the Manifest, then such a box MUST be available in the Initialization Stream of each Track belonging to this Switching Set. |
schemeId
|
string | Y | A UUID [UUID] that identifies the protection system used to protect this Switching Set. |
Additional attributes MAY be defined by specific protection systems such as license acquisition URLs, authorization-related URLs, specific initialization data (a default key ID),... These attributes SHOULD be defined on this SwitchingSetProtectionSystem structure; however, they will not be defined here. Depending on the scheme ID that is given here, a client SHOULD verify that these additional attributes exist.¶
This is the type definition of a video Switching Set.¶
Attribute | Type | Req? | Description |
---|---|---|---|
id | string | Y | The unique identifier for this Switching Set. It MUST be unique within its Presentation. |
tracks | VideoTrackType[] | Y | The collection of all Tracks belonging to this Switching Set. |
alignId | string | N | A unique identifier that SHOULD be set by all Switching Sets that are Aligned Switching Sets with each other. |
baseUrl | string | N | The base URL of this Switching Set. It is part of the content base URLs for all Tracks belonging to this Switching Set. See Section 3.4 for more information. |
codecs | string | N | The definition of the codec(s) necessary to render the content of all Tracks belonging to this Switching Set. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Switching Set. The pattern MUST include the {segmentId} string; see Section 3.4 for more information. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
frameRate | ScaledValue | N | The frame rate of all video Tracks belonging to this Switching Set. If this is not defined, then every Track MUST set its frame rate separately. |
initializationPattern | string | N | The URL pattern used to request Initialization Packets belonging to this Switching Set. The pattern MUST include the {initId} string; see Section 3.4 for more information. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
label | string | N | A human-readable name for this Switching Set. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by Segments of this Switching Set. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
mimeType | string | N | The MIME type of all content belonging to this Switching Set. It MUST be a valid video MIME type. If not set, the MIME type of all content of this Switching Set SHALL equal video/mp4 . |
protection | SwitchingSet Protection |
N | The information related to content protection for all Tracks belonging to this Switching Set. |
This is the type definition of a metadata Switching Set.¶
Attribute | Type | Req? | Description |
---|---|---|---|
id | string | Y | The unique identifier for this Switching Set. It MUST be unique within its Presentation. |
mimeType | string | Y | The MIME type of all content belonging to this Switching Set. |
tracks | MetadataTrackType[] | Y | The collection of all Tracks belonging to this Switching Set. |
schemeId | string | Y | An identifier that denotes the type of metadata contained by this metadata Switching Set. Client behavior may differ depending on the vendor-specific identifier set here. |
alignId | string | N | A unique identifier that SHOULD be set by all Switching Sets that are Aligned Switching Sets with each other. |
baseUrl | string | N | The base URL of this Switching Set. It is part of the content base URLs for all Tracks belonging to this Switching Set. See Section 3.4 for more information. |
codecs | string | N | The definition of the codec(s) necessary to render the content of all Tracks belonging to this Switching Set. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. If this Switching Set does not define it, then it must be defined on all its Tracks separately. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Switching Set. The pattern MUST include the {segmentId} string; see Section 3.4 for more information. If this is not defined, it MUST be defined on all Tracks of this Switching Set separately. |
label | string | N | A human-readable name for this Switching Set. |
language | string | N | The language of all Tracks of the metadata Switching Set. It MUST be specified here by its ISO 639-2 [ISO6392] code. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by Segments of this Switching Set. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
This is the type definition of an audio Track.¶
Attribute | Type | Req? | Description |
---|---|---|---|
bandwidth | uint | Y | The peak bitrate of this Track. It is denoted in bits per second. The measured bitrates of all Segments of the Track MUST NOT exceed this value. This value is used to aid in ABR decisions by the player. |
id | string | Y | The unique identifier for this Track. It MUST be unique within its Switching Set. |
segments | SegmentType[] | Y | The metadata of the Segments contained by this Track at the moment of Manifest creation. If a Segment duration is set in the Track of this Segment, then only the active Segment MUST be shown here. If a segment duration is not set, then each Segment MUST be announced by the Manifest before it is active. More information about Segment availability is given in Section 5.2. |
activeSegment | uint | N | The identifier of the Segment that is currently active. If this Track's Presentation is currently active, or if the Track's Presentation was previously active but contains Segments that can still be requested, then this value MUST equal the identifier of the most recent Continuation Segment that is available for retrieval. If the Track's Presentation has not yet been active (and as a result, no Continuation Segments are available yet), then this value MUST be ignored by the client. |
activeSequenceNumber | uint | N | The active Sequence Number of this Track. It MUST denote the Sequence Number of the Initialization Packet of this Track that was most recently published at the time of the creation of this Manifest. If this Track's Presentation was previously active or is currently active, then the most recent Initialization Packet's Sequence Number MUST be shown here. If the Track's Presentation has not yet been active (i.e., no Initialization Packets are yet available), then the client MUST ignore this value. |
averageBandwidth | uint | N | The average bitrate of this Track, denoted in bits per second. It is expected that over a duration of 10 minutes, the average bitrate of this Track SHALL be within 5% of the given value. This value is used to aid in ABR decisions by the player. |
baseUrl | string | N | The base URL of this Track. It is part of the content base URLs used to request this Track's Initialization Packets and Continuation Segments. See Section 3.4 for more information. |
channels | integer | N | The audio channel configuration of this audio Track, defined as the total number of audio channels in the media data. This is used for ABR selection by the player. |
codecs | string | N | The definition of the codec(s) necessary to render the content of this Track. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Track. The pattern needs to include the {segmentId} string; see Section 3.4 for more information. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
label | string | N | A human-readable name for this Track. |
initializationPattern | string | N | The URL pattern used to request Initialization Packets belonging to this media Track. The pattern needs to include the {initId} string; see Section 3.4 for more information. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by segments of this Track. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
sampleRate | uint | N | The sample rate (in Hz) of this audio Track. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
samplesPerFrame | uint | N | The number of audio samples in one frame of this audio Track. If neither the Track nor the Switching Set has this value set, it SHALL equal 1024. |
segmentDuration | ScaledValue | N | The duration (in seconds) of each Segment contained by this Track. If this value is set, then every segmentDuration seconds, a new Segment SHOULD be available. More information about the availability is given in Section 5.2. If not set, then each Segment MUST be individually defined by the segments field. |
For any attributes that also exist on AudioSwitchingSetType, if both AudioSwitchingSetType and AudioTrackType have a value for this attribute, then only the value set by AudioTrackType must be considered (except for the identifier and label.)¶
This is the type definition of a video Track.¶
Attribute | Type | Req? | Description |
---|---|---|---|
bandwidth | uint | Y | The peak bitrate of this Track. It is denoted in bits per second. The measured bitrates of all Segments of the Track MUST NOT exceed this value. This value is used to aid in ABR decisions by the player. |
id | string | Y | The unique identifier for this Track. It MUST be unique within its Switching Set. |
resolution | Resolution | Y | The resolution of this video Track. |
segments | SegmentType[] | Y | The metadata of the Segments contained by this Track at the moment of Manifest creation. If a Segment duration is set in the Track of this Segment, then only the active Segment MUST be shown here. If a segment duration is not set, then each Segment MUST be announced by the Manifest before it is active. More information about Segment availability is given in Section 5.2. |
activeSegment | uint | N | The identifier of the Segment that is currently active. If this Track's Presentation is currently active, or if the Track's Presentation was previously active but contains Segments that can still be requested, then this value MUST equal the identifier of the most recent Continuation Segment that is available for retrieval. If the Track's Presentation has not yet been active (and as a result, no Continuation Segments are available yet), then this value MUST be ignored by the client. |
activeSequenceNumber | uint | N | The active Sequence Number of this Track. It MUST denote the Sequence Number of the Initialization Packet of this Track that was most recently published at the time of the creation of this Manifest. If this Track's Presentation was previously active or is currently active, then the most recent Initialization Packet's Sequence Number MUST be shown here. If the Track's Presentation has not yet been active (i.e., no Initialization Packets are yet available), then the client MUST ignore this value. |
averageBandwidth | uint | N | The average bitrate of this Track, denoted in bits per second. It is expected that over a duration of 10 minutes, the average bitrate of this Track SHALL be within 5% of the given value. This value is used to aid in ABR decisions by the player. |
baseUrl | string | N | The base URL of this Track. It is part of the content base URLs used to request this Track's Initialization Packets and Continuation Segments. See Section 3.4 for more information. |
codecs | string | N | The definition of the codec(s) necessary to render the content of this Track. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Track. The pattern needs to include the {segmentId} string; see Section 3.4 for more information. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
frameRate | ScaledValue | N | The frame rate of this video Track. If it is not defined by the Switching Set, then it must be defined here. |
label | string | N | A human-readable name for this Track. |
initializationPattern | string | N | The URL pattern used to request Initialization Packets belonging to this media Track. The pattern needs to include the {initId} string; see Section 3.4 for more information. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by segments of this Track. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
segmentDuration | ScaledValue | N | The duration (in seconds) of each Segment contained by this Track. If this value is set, then every segmentDuration seconds, a new Segment SHOULD be available. More information about the availability is given in Section 5.2. If not set, then each Segment MUST be individually defined by the segments field. |
For any attributes that also exist on VideoSwitchingSetType, if both VideoSwitchingSetType and VideoTrackType have a value for this attribute, then only the value set by VideoTrackType must be considered (except for the identifier and label.)¶
A Resolution contains the following elements:¶
Attribute | Type | Required | Description |
---|---|---|---|
width | uint | Y | The width of the picture. |
height | uint | Y | The height of the picture. |
sarWidth | uint | N | The width of the sample aspect ratio of the resolution. If it is not set, it SHALL equal 1. |
sarHeight | uint | N | The height of the sample aspect ratio of the resolution. If it is not set, it SHALL equal 1. |
The display aspect ratio belonging to this Resolution can be calculated by using this sample aspect ratio width and height, and the picture width and height as follows:¶
dar = (darWidth, darHeight) darWidth = sarWidth * width darHeight = sarHeight * height¶
This is the type definition of a metadata Track.¶
Attribute | Type | Req? | Description |
---|---|---|---|
id | string | Y | The unique identifier for this Track. It MUST be unique within its Switching Set. |
segments | SegmentType[] | Y | The metadata of the Segments that are contained by this Track at the moment of Manifest creation. If a Segment duration is set in the Track of this Segment, then only the active Segment MUST be shown here. If a segment duration is not set, then each Segment MUST be announced by the Manifest before it is active. More information about Segment availability is given in Section 5.2. |
activeSegment | uint | N | The identifier of the Segment that is currently active. If this Track's Presentation is currently active, or if the Track's Presentation was previously active but contains Segments that can still be requested, then this value MUST equal the identifier of the most recent Continuation Segment that is available for retrieval. If the Track's Presentation has not yet been active (and as a result, no Continuation Segments are available yet), then this value MUST be ignored by the client. |
averageBandwidth | uint | N | The average bitrate of this Track, denoted in bits per second. It is expected that over a duration of 10 minutes, the average bitrate of this Track SHALL be within 5% of the given value. This value is used to aid in ABR decisions by the player. |
bandwidth | uint | N | The peak bitrate of this Track. It is denoted in bits per second. The measured bitrates of all Segments of the Track MUST NOT exceed this value. This value is used to aid in ABR decisions by the player. |
baseUrl | string | N | The base URL of this Track. It is part of the content base URLs used to request this Track's Continuation Segments. See Section 3.4 for more information. |
codecs | string | N | The definition of the codec(s) necessary to render the content of this Track. It MUST follow the ISO File Format Name Space as defined by [RFC6381]. |
continuationPattern | string | N | The URL pattern used to request Continuation Segments belonging to this Track. The pattern needs to include the {segmentId} string; see Section 3.4 for more information. If this is not defined here, then it MUST be defined by the Track's Switching Set. |
label | string | N | A human-readable name for this Track. |
mediaTimeOffset | ScaledValue | N | The offset to be added to Manifest Timestamps to calculate Media Timestamps contained by segments of this Track. If neither the Track nor the Switching Set has this value set, it SHALL equal 0. |
segmentDuration | ScaledValue | N | The duration (in seconds) of each Segment contained by this Track. If this value is set, then every segmentDuration seconds, a new Segment SHOULD be available. More information about the availability is given in Section 5.2. If not set, then each Segment MUST be individually defined by the segments field. |
This is the type definition of a Segment.¶
Attribute | Type | Required | Description |
---|---|---|---|
id | uint | Y | The unique identifier for this Segment within the Track. This identifier MUST be incremented by 1 for every new Segment of the same Track. |
timeBounds | TimeBounds | N | The time boundaries of this Segment. If this Segment's Track does not have a constant segment duration, then this value MUST be set, and the Segment's duration MUST be available at least 2 seconds before the actual end of that Segment. |
Timed metadata events can be embedded in the Manifest by using Presentation Events. Each PresentationEventType contains the following elements:¶
Attribute | Type | Req? | Description |
---|---|---|---|
data | string | Y | The event payload. |
id | string | Y | An unique identifier for this event. It MUST be unique within this Presentation's events. |
timeBounds | PresentationEventTimeBounds | Y | The time boundaries for the event, during which the event SHALL be active. |
encoding | Enum("identity", "base64", "json") | N | The content encoding of the event data. identity signifies that the encoding is plaintext. base64 signifies Base64 encoding. json signifies that the payload given by data is a valid JSON document. If this JSON document is not valid, this event MUST be ignored. If not set, the encoding SHALL default to identity . |
The time boundaries of a PresentationEventType are defined as follows:¶
Attribute | Type | Required | Description |
---|---|---|---|
startTimeOffset | uint | N | The scaled start time offset of the event, defined in seconds divided by the timescale. The actual start time of the event can be calculated by dividing this value by the scale and adding the resulting value to the start time of the Presentation. It is defined as seconds divided by the timescale. If it is not set, it SHALL equal 0. |
duration | uint | N | The scaled duration of the event, defined in seconds divided by the timescale. If it is not set, it SHALL equal 0. |
scale | uint | N | The timescale used for these time bounds. If it is not set, it SHALL be equal to 1. |
The Manifest SHOULD be available through an HTTP GET request. The URL of this Manifest can be personalized by the user.¶
Request path | Method | Summary |
---|---|---|
(chosen by the stream publisher) | GET | Retrieve the stream Manifest. |
Success | |
---|---|
Status code | 200 |
Content-Type | JSON (MIME type MUST equal application/vnd.theo.hesp+json ) |
Description | When the given Manifest exists on the server, the Manifest data is returned. |
Status code | Description |
---|---|
404 | The Manifest is not available for the given URL. |
If an error occurs, the player SHOULD attempt to retry the request. In case of consecutive unsuccessful requests, the media player SHOULD assume the content is unavailable and cease playback.¶
Several elements of the Manifest contain a baseUrl
attribute. These attributes are used to construct the URLs of media content (and metadata.)¶
A content request URL for a specific Track SHALL be constructed by applying relative resolution (as explained in Section 5.2 of RFC 3986 [RFC3986]) to each defined baseUrl
or contentBaseUrl
attribute relating to that Track, starting from the root of the Manifest. The URL of the Manifest SHALL be used as a base for the first resolution, after which the target URL of the previous resolution SHALL be used as the base URL of the next resolution.¶
This means that the content request URL for a Track's Initialization Stream is constructed as follows (in pseudocode):¶
T = manifestUrl if isDefined(Manifest.contentBaseUrl): T = resolve(Manifest.contentBaseUrl, T) if isDefined(Presentation.baseUrl): T = resolve(Presentation.baseUrl, T) if isDefined(SwitchingSet.baseUrl): T = resolve(SwitchingSet.baseUrl, T) if isDefined(Track.baseUrl): T = resolve(Track.baseUrl, T) contentRequestURL = resolve(initializationPattern, T)¶
where initializationPattern
is the attribute given by either SwitchingSet or Track (with Track taking precedence if given by both) and resolve(R, B)
is relative resolution, where R is a URI reference and B is a base URI.¶
The content request URL for the Track's Continuation Stream can be constructed in the same manner if the initializationPattern
is replaced with the continuationPattern
. Both URLs MUST be unique for each unique Track.¶
An example of content addressing is given in Appendix A.1.3.¶
The content request URL MUST include the {initId}
pattern for Initialization Stream URLs and the {segmentId}
pattern for Continuation Stream URLs. These patterns are replaced in the actual content requests: {initId}
is replaced with the requested Sequence Number of the Initialization Stream, and {segmentId}
is replaced with the requested segment identifier of the Continuation Stream.¶
In order to add leading zeros to the identifier, the following can be added to the patterns initId
and segmentId
: :0(n)d
where (n)
is the minimal amount of characters the resulting identifier has (if the identifier is already longer, it will not be altered.)¶
For example, for a continuation segment with identifier 100, a following content request URL https://www.example.com/s-1/content-{segmentId:06d}.mp4
will resolve to https://www.example.com/s-1/content-000100.mp4
. While the content request URL https://www.example.com/s-1/content-{segmentId:02d}.mp4
resolves to https://www.example.com/s-1/content-100.mp4
¶
Further details about these requests are given in Section 4 and Section 5.¶
A full Manifest example, together with information on content addressing and timing information, is given by Appendix A.1.¶
The player uses the information in the HESP Manifest to fetch an Initialization Packet.¶
The Initialization Stream is a stream containing only independent samples. This stream is not regularly used by a single client nor completely streamed. The purpose of the Initialization Stream is to make available, upon request of the client, the separate independent frames packaged in an Initialization Packet. This way, a client has more control over the specific position of the stream where it wants to initiate playback. For example, it allows for more granularity when starting playback at the live edge, seeking a specific point in time, or switching to an alternative Track.¶
Additionally, the Initialization Packet contains information on the position of the following frame in the Continuation Stream to achieve regular media playback.¶
A video Initialization Packet MUST contain:¶
A CMAF header, i.e., all information required to initialize the media decoder, stored in ISO Base Media File Format [ISOBMFF] boxes as specified by CMAF [CMAF]. The minimal required contents are defined in Section 4.2.3¶
A CMAF segment, i.e., an independent media sample and timing information related to this sample, stored in ISOBMFF [ISOBMFF] boxes as specified by CMAF [CMAF].¶
An audio Initialization Packet MUST contain:¶
A CMAF header, i.e., all information required to initialize the media decoder, stored in ISO Base Media File Format [ISOBMFF] boxes as specified by CMAF [CMAF]. The minimal required contents are defined below.¶
An additional in-band metadata event containing extra HESP information, included in the aforementioned CMAF segment. The definition of such an event is denoted below.¶
It MAY also contain an independent (audio) media sample, as is defined by the video Initialization Packet. However, it is heavily discouraged as it leads to more storage costs for the provider without any significant advantage.¶
A CMAF header is defined in ISO/IEC 23000-19:2020 [CMAF] as: "a sequence of CMAF constrained ISOBMFF boxes that do not reference any media samples (3.3.15), but are associated with a CMAF track (3.2.1) and necessary for the decoding of its CMAF fragments (3.1.1)." As such, the header MUST NOT contain any samples.¶
This means that for the ISO Base Media File Format, at least the following boxes MUST be given:¶
In order to pass information about the Continuation Stream, an in-band event in the form of an emsg
box must be added to the Initialization Packet. The format of this box is defined at Section 6.2.1.1. The message_data
field MUST contain the following two values (see Table 31):¶
the Continuation Segment index (index
), which defines the index of the Continuation Segment containing the next media sample.¶
the Continuation Segment byte offset (offset
), which defines the position of the next media sample in the Continuation Segment. This field is OPTIONAL if the next media sample is located at the start of the segment, in which case a byte-range request is not needed.¶
The message_data
field of the emsg
box could for example look like this:¶
{"index":1,"offset":1234}
¶
Section 3.4 defines how to create the correct URLs and how to request Initialization Packets. The request content URL is used together with the desired Sequence Number to retrieve an Initialization Packet.¶
Additionally, {initId}
can also be replaced with the string now
. If this URL is requested, the most recently available Initialization Packet MUST be returned.¶
Initialization Packets MUST be available through a basic GET request. They MUST be able to be retrieved through two methods:¶
Request path | Method | Summary |
---|---|---|
Initialization content request URL {initId} is replaced with the string now
|
GET | Retrieve the most recent Initialization Packet from the given track. |
Initialization content request URL {initId} is replaced with a Sequence Number |
GET | Retrieve the Initialization Packet with the given Sequence Number from the given track. |
Success | |
---|---|
Status Code | 200 |
Content-Type | MUST match the mimeType given by the Stream's Switching Set (see Section 3 for more details) |
Description | Return the requested Initialization Packet when it exists on the media server. |
Status code | Description |
---|---|
404 | The Track does not exist on the media server, or the requested Initialization Packet does not exist on the media server. |
The client uses the information given by the Manifest and the Initialization Packet to request the Continuation Stream. This low latency stream is used for the bulk of media playback of HESP. The Continuation Stream is an independently playable CMAF stream.¶
The Continuation Stream is packaged as a regular CMAF stream, albeit with some encoding constraints, depending on the chosen profile (see Appendix C.)¶
Maximum Gain Profile¶
Compatibility Profile¶
Continuation Segments must together create a continuous stream of media data. There MUST NOT exist gaps in the time boundaries of subsequent Continuation Segments.¶
There exist two ways to signal Segment availability. Either the Track has a segmentDuration
set in the Manifest, which represents the constant duration of each Segment. In this case, a client can derive on its own when to start requesting a new Segment. A new Segment SHOULD be published every n seconds, where n equals segmentDuration
. This new Segment MUST become available within 100 milliseconds from that point. It is not allowed to drift, i.e., if a Segment is only published at n+100ms, then the next Segment MUST be available at 2n+100ms at the latest.¶
The duration of the last Segment of a Track MAY be shorter than this constant segmentDuration
. As a new Manifest must be retrieved near the end of a Presentation, a client should be able to start requesting content of the new Presentation in a timely manner, regardless of the length of this last Segment.¶
The other option is for the Track to specify all its Segments outright in the segments
field of the Manifest. As the Manifest does not get regularly retrieved, it is REQUIRED to signal a Manifest update (see Section 6.2.1.2) at the end of each Segment. It is recommended only to use this option in the "Maximum Gain Profile", as it can otherwise significantly increase the number of Manifest requests necessary.¶
To start playback of a Track, a client MUST request the Continuation Segments of the chosen Track(s) using the information given by (already obtained) the Manifest and Initialization Packet.¶
Requests for Content Segments should be made using HTTP GET requests. The first request, following an Initialization Packet, fetches the Segment indicated by the Initialization Packet. A byte-range header should be used to specify the range of data that needs to be requested. This information is also given by the Initialization Packet.¶
A client then automatically calculates the URL of the next Segment as it is indicated in the Manifest and requests the next Segment using a regular HTTP GET request.¶
Section 3.4 defines how to create the correct URLs and how to request a continuation segment. The request content URL is used together with the desired segment identifier to retrieve the Segment.¶
Suppose a client needs to request a partial Continuation Segment, for example, starting at the byte offset given by metadata of an Initialization Packet. In that case, it MUST use one or more HTTP Range [RFC7233] Requests. The start and end of the range of each request MUST be defined.¶
Often, the total length of the Continuation Segment will not yet be known at the time of the request. If a client cannot define an accurate end value for the HTTP Range of a request, then 2^53 - 1 (9007199254740991) SHOULD be used.¶
Request path | Method | Summary |
---|---|---|
Continuation content request URL {segmentId} is replaced with a segment identifier |
GET | Retrieve the Continuation Segment with the requested identifier from the given track. |
A distinction is made between how responses should be returned, depending on the version of the HTTP protocol used.¶
If an HTTP Range request is sent, then a 206 Partial Content response MUST be returned upon a successful response. The response MUST use Chunked Transfer Coding [RFC7230] to ensure timely delivery of media data.¶
Success | |
---|---|
Status Code | 206 |
Content-Type | MUST match the mimeType given by the Stream's Switching Set (see Section 3 for more details) |
Transfer-Encoding | chunked |
Description | The requested range of Segment data is returned from the server. Depending on the byte-range requested, the connection is kept open to retrieve live data. |
For requests without a Range header, a 200 OK response MUST be returned upon success.¶
Success | |
---|---|
Status Code | 200 |
Content-Type | MUST match the mimeType given by the Stream's Switching Set (see Section 3 for more details) |
Transfer-Encoding | chunked |
Description | The Segment data is returned from the server. Depending on the availability of the Segment, the connection is kept open to retrieve live data. |
HTTP/2 uses frame-based transmission and cannot use Chunked Transfer Coding. As such, this header is not given here.¶
Success | |
---|---|
Status code | 206 |
Content-Type | MUST match the mimeType given by the Stream's Switching Set (see Section 3 for more details) |
Description | The requested range of Segment data is returned from the server. Depending on the byte-range requested, the connection is kept open to retrieve live data. |
For requests without a Range header, a 200 OK response MUST be returned upon success.¶
Success | |
---|---|
Status code | 200 |
Content-Type | MUST match the mimeType given by the Stream's Switching Set (see Section 3 for more details) |
Description | The Segment data is returned from the server. Depending on the availability of the Segment, the connection is kept open to retrieve live data. |
Status code | Description |
---|---|
404 | The Track does not exist on the media server, or the requested Segment does not exist on the media server. |
416 | The requested byte-range could not be fulfilled. |
HESP supports timed metadata through two methods: metadata Tracks for continuous, often segmented metadata and metadata events for sporadic metadata updates.¶
Metadata Tracks function similarly to media Tracks, however, without the need for an Initialization Stream. The attributes for metadata Tracks in the Manifest can be found at Section 3.2.15. The addressing happens similarly to the addressing of Continuation segments:
a continuationPattern
is given for each metadata Track, either directly set on the Track or on the Switching Set of the Track. Each metadata Segment contains a numerical identifier that increments by one for each new Segment of the Track. This identifier, together with the addressing pattern, creates the URL used to retrieve the contents of the Segment.¶
The Manifest contains the duration of each metadata Segment, either stated individually per Segment or through the segmentDuration
attribute set on the metadata Track. It is possible that chunked encoding is used here to ensure that the contents are delivered as soon as possible, but this is not a requirement. This can be useful for subtitles alongside live content, for example.¶
Metadata events can be used to signal information that does not become available in regular intervals. These events can either be transmitted in-band, where it MUST be added to the video or audio Continuation Streams, or out-of-band, in which case details about the metadata event MUST be available in the Manifest.¶
In order to deliver events in-band, root-level Event Message (emsg
) boxes MUST be added to ongoing media Continuation Streams. The definition of an emsg
box can be found in the CMAF [CMAF] specification.¶
These boxes should be appended to each Track of a Presentation to ensure all viewers can receive such an event. It is recommended to use out-of-band events if the included data is significantly large.¶
The HESP specification defines a few in-band events that it leverages for client initialization and Manifest updates. The structure of these events is given below.¶
This event is appended to each Initialization Packet for the client to request the correct Continuation Segments. It is an 'emsg' box with the following REQUIRED values:¶
Attribute | Value |
---|---|
version
|
0 |
scheme_id_uri
|
"urn:theo:hesp:2020" |
value
|
"initdata" |
timescale
|
MUST match the timescale of the Initialization Packet in case of video or MUST equal 1 otherwise. |
presentation_time_delta
|
0 |
event_duration
|
MUST match the duration of the Initialization Packet in case of video or MUST equal 0 otherwise. |
id
|
can be freely set (and MUST be ignored by the player) |
message_data
|
MUST contain the data defined by Table 31, formatted as JSON |
Attribute | Type | Required | Description |
---|---|---|---|
index | integer | Y | The Continuation Segment index (see Section 4.2.4.) |
offset | integer | N | The Continuation Segment byte offset (see Section 4.2.4), it SHALL be 0 if not given here. |
This event is used to signal the client that a new Manifest must be retrieved. This can occur for many reasons, such as before a Presentation change, availability of new out-of-band metadata, etc. It is an emsg
box with the following REQUIRED values:¶
Attribute | Value |
---|---|
version
|
0 |
scheme_id_uri
|
"urn:theo:hesp:2020" |
value
|
"manifestupdate" |
timescale
|
1 |
presentation_time_delta
|
0 |
event_duration
|
0 |
id
|
MAY be freely set (and MUST be ignored by the player) |
message_data
|
MUST contain the data defined by Table 33, formatted as JSON |
Attribute | Type | Required | Description |
---|---|---|---|
url | string | N | the URL of an alternative location of the Manifest |
If the event contains a URL, then this Manifest request MUST be made to this address. Further Manifest requests do not have this requirement.¶
Out-of-band events can be added to the Manifest through Presentation Events. The definition of a Presentation Event is given in Section 3.2.17. The contents of the event data are not predefined. It is possible to include arbitrary data as Base64 encoded data, and plaintext data can be included as-is. If needed, the data can be an URL that needs to be resolved separately, but this is up to the publisher of this data.¶
HESP has support for content protection. It allows DRM systems to be implemented through the common encryption standard.¶
As HESP has the requirement for media to be structured as ISOBMFF [ISOBMFF], common encryption [CENC] is to be used to encrypt that media content.¶
Common encryption specifies 4 protection schemes that may be used: cenc
, cbc1
, cens
and cbcs
. For HESP, either the AES-CBC subsample pattern encryption scheme (cbcs
) or the AES-CTR full sample pattern encryption scheme (cenc
) MUST be used.¶
The HESP Manifest has a SwitchingSetProtection
structure to set up common encryption for audio and video Switching Sets. The definition of this structure can be found in Chapter 3. As a result of this being set on a Switching Set level:¶
The SwitchingSetProtection
structure MAY contain a ProtectionSystemSpecificHeaderBox
(pssh
), which can also be contained by the Initialization Stream. Note that this box MUST be given by at least one of both options in order for a license request to be made. If both exist for a Switching Set, the pssh
box from the Initialization Stream MUST be disregarded. This allows for more straightforward alterations of license information after a stream has been created or published.¶
The media of a protected stream needs to contain certain ISOBMFF boxes to be compliant with CENC (and CMAF). HESP requires the same boxes to be present in media streams. A brief overview of these boxes is given below. More information on this requirement can be found in the CENC [CENC] and CMAF [CMAF] specifications.¶
Initialization Packets MUST contain the following boxes:¶
TrackEncryptionBox
(tenc
): This box contains default parameters regarding sample encryption.¶
SchemeTypeBox
(schm
): This box identifies the protection scheme. scheme_type
MUST be set to cbcs
or cenc
.¶
ProtectionSystemSpecificHeaderBox
(pssh
): If the Manifest does not contain a pssh
box that applies to this Track, then this box MUST be included in each Initialization Packet of each Track of the protected Switching Set. If the Manifest does contain a pssh
box that applies to this Track, then this box MUST be disregarded.¶
In order to signal that the Track is encrypted, the stream type MUST equal encv
for video Tracks and enca
for audio Tracks.¶
Additionally, video Initialization Packets contain a sample. If that sample is encrypted, then the same requirements for CMAF fragments of a Continuation Segment also apply here: a senc
box MUST be included, pssh
, saiz
and saio
boxes MAY be included. The following section contains more information about these requirements.¶
CMAF fragments of a Continuation Segment MUST contain the following box:¶
SampleEncryptionBox
(senc
): This box is used to store initialization vector data and information on subsample encryption. As each chunk in HESP must currently contain at most one sample, sample_count
SHALL always 1.¶
The following boxes MAY be included:¶
ProtectionSystemSpecificHeaderBox
(pssh
): If any updates need to be made to the underlying licensing system, a pssh
box MAY be included.¶
SampleAuxiliaryInformationSizesBox
(saiz
): This box is used to store the size of per-sample auxiliary information. It is only REQUIRED if such per-sample information exists.¶
SampleAuxiliaryInformationOffsetsBox
(saio
): This box is used to store the offsets of per-sample auxiliary information. It is only REQUIRED if such per-sample information exists.¶
Significant contributions to the design of this protocol were made by Egon Okerman, Samie Beheydt, and Johan Vounckx.¶
This memo requests that the following MIME type [RFC2046] be registered with the IANA:¶
Type name:
application
¶Subtype name:
vnd.theo.hesp+json
¶Required parameters: (none)¶
Optional parameters: (none)¶
Encoding considerations: encoded as text.¶
Security considerations: See Section 10.¶
Compression: this media type does not employ compression.¶
Interoperability considerations: There are no byte-ordering issues, since files are 7- or 8-bit text. Applications could encounter unrecognized tags, which SHOULD be ignored.¶
Since the protocol uses HTTP to transmit data, the regular HTTP security considerations apply. See section 15 of RFC 7230 [RFC7230].¶
Clients SHOULD take care when parsing files received from a server so that non-compliant files are rejected. Clients SHOULD range-check responses to prevent buffer overflows. See also the Security Considerations section of RFC 3986 [RFC3986]. Clients SHOULD load resources identified by URI lazily to avoid contributing to denial-of-service attacks.¶
HTTP requests often include session state ("cookies"), which may contain private user data. Implementations MUST follow cookie restriction and expiry rules specified by RFC 6265 [RFC6265]. See also the Security Considerations section of RFC 6265, and RFC 2964 [RFC2964].¶
Encryption keys are specified by URI. The delivery of these keys SHOULD be secured by a mechanism such as HTTP over TLS [RFC8446] (formerly SSL) in conjunction with a secure realm or a session cookie.¶
For the initial step, the client retrieves a Manifest.¶
The URL of the Manifest is given out of band to the client. The client sends out a GET request. In this case, let's suppose the Manifest is available at https://example.com/stream1/manifest.json
. The client then makes such a request:¶
GET /stream1/manifest.json HTTP/1.1 Host: example.com Accept: application/vnd.theo.hesp+json¶
The server responds with the following headers:¶
HTTP/1.1 200 OK Content-Type: application/vnd.theo.hesp+json; charset=utf-8 Content-Length: 6867 Date: Wed, 31 Mar 2021 08:00:00 GMT¶
and the following body:¶
{ "activePresentation":"1", "availabilityDuration":{ "value":1500 }, "creationDate":"2021-03-31T08:00:00.000Z", "fallbackPollRate":300, "manifestVersion":"1.1.0", "streamType":"live", "presentations":[ { "id":"0", "timeBounds":{ "startTime":0, "endTime":972000000, "scale":90000 }, "audio":[ { "id":"main-audio", "language":"eng", "baseUrl":"audio/", "channels":2, "codecs":"mp4a.40.2", "continuationPattern":"content-{segmentId}.mp4", "initializationPattern":"init-{initId}.mp4", "sampleRate":48000, "tracks":[ { "id":"96kbps", "activeSegment":1799, "activeSequenceNumber":506249, "averageBandwidth":96000, "bandwidth":96000, "baseUrl":"96k/", "segmentDuration":{ "value":540000, "scale":90000 }, "segments":[ { "id":1799, "timeBounds":{ "startTime":971460000, "scale":90000 } } ] } ] } ], "video":[ { "id":"main-video", "baseUrl":"video/", "frameRate":{ "value":25 }, "continuationPattern":"content-{segmentId}.mp4", "initializationPattern":"init-{initId}.mp4", "tracks":[ { "id":"720p", "activeSegment":1799, "activeSequenceNumber":269999, "bandwidth":3000000, "baseUrl":"720p/", "codecs":"avc1.4d001f", "resolution":{ "width":1280, "height":720 }, "segmentDuration":{ "value":540000, "scale":90000 }, "segments":[ { "id":1799, "timeBounds":{ "startTime":971460000, "scale":90000 } } ] } ] } ] }, { "id":"1", "timeBounds":{ "startTime":972000000, "scale":90000 }, "currentTime":{ "value":1080000000, "scale":90000 }, "baseUrl":"https://otherexample.com/s2/", "audio":[ { "id":"main-audio", "language":"eng", "baseUrl":"audio/", "channels":2, "codecs":"mp4a.40.2", "sampleRate":48000, "mediaTimeOffset":{ "value":-972000000, "scale":90000 }, "tracks":[ { "id":"128kbps", "activeSegment":200, "activeSequenceNumber":56250, "averageBandwidth":128000, "bandwidth":128000, "continuationPattern": "128k-content-{segmentId}.mp4", "initializationPattern": "128k-init-{initId}.mp4", "segmentDuration":{ "value":540000, "scale":90000 }, "segments":[ { "id":200, "timeBounds":{ "startTime":1080000000, "scale":90000 } } ] } ] } ], "video":[ { "id":"main-video", "baseUrl":"video/", "frameRate":{ "value":25 }, "tracks":[ { "id":"720p", "activeSegment":200, "activeSequenceNumber":29999, "bandwidth":3000000, "codecs":"avc1.4d001f", "continuationPattern": "720p-content-{segmentId}.mp4", "initializationPattern": "720p-init-{initId}.mp4", "resolution":{ "width":1280, "height":720 }, "segmentDuration":{ "value":540000, "scale":90000 }, "segments":[ { "id":200, "timeBounds":{ "startTime":1080000000, "scale":90000 } } ] }, { "id":"1080p", "activeSegment":200, "activeSequenceNumber":29999, "bandwidth":5000000, "codecs":"avc1.4d001f", "continuationPattern": "1080p-content-{segmentId}.mp4", "initializationPattern": "1080p-init-{initId}.mp4", "resolution":{ "width":1920, "height":1080 }, "segmentDuration":{ "value":540000, "scale":90000 }, "segments":[ { "id":200, "timeBounds":{ "startTime":1080000000, "scale":90000 } } ] } ] } ] } ] }¶
In the above Manifest, we have two Presentations:¶
mediaTimeOffset
defined, so the Manifest Timestamps match the Media Timestamps.¶
mediaTimeOffset
defined. The Media Timestamps for this Presentation start at 00:00:00.000. That also means that for all the currently active Segments, the media data will contain a starting timestamp of 00:20:00.000.¶
The availabilityDuration
of the Manifest is 25 minutes. As only 20 minutes of the second Presentation have elapsed, some Segments of the first Presentation are still available. Therefore, the first Presentation must still be included in the Manifest. Once 5 minutes have passed, the first Presentation can be left out of the Manifest.¶
The client uses the Manifest to derive the content request URLs of each Track.¶
For the audio Track of the first Presentation, the following parts are found:¶
AudioSwitchingSet.baseUrl = "audio/" AudioTrack.baseUrl = "96k/" AudioSwitchingSet.initializationPattern = "init-{initId}.mp4" AudioSwitchingSet.continuationPattern = "content-{segmentId}.mp4"¶
The Manifest URL (https://example.com/stream1/manifest.json
) is used as the base for resolution, and then relative resolution is applied to each of the parts above.¶
T = resolve(AudioSwitchingSet.baseUrl, manifestUrl) = "https://example.com/stream1/audio/" T = resolve(AudioTrack.baseUrl, T) = "https://example.com/stream1/audio/96k/" initBaseUrl = resolve(AudioSwitchingSet.initializationPattern, T) = "https://example.com/stream1/audio/96k/init-{initId}.mp4" contBaseUrl = resolve(AudioSwitchingSet.continuationPattern, T) = "https://example.com/stream1/audio/96k/content-{segmentId}.mp4"¶
The final content request URLs are https://example.com/stream1/audio/96k/init-{initId}.mp4
and https://example.com/stream1/audio/96k/content-{segmentId}.mp4
for the Initialization Stream and Continuation Stream respectively.¶
For the audio Track of the second Presentation, the following parts are found:¶
Presentation.baseUrl = "https://otherexample.com/s2/" AudioSwitchingSet.baseUrl = "audio/" AudioTrack.initializationPattern = "128k-init-{initId}.mp4" AudioTrack.continuationPattern = "128k-content-{segmentId}.mp4"¶
The Manifest URL (https://example.com/stream1/manifest.json
) is used as the base for resolution, and then relative resolution is applied to each of the parts above.¶
T = resolve(Presentation.baseUrl, manifestUrl) = "https://otherexample.com/s2/" T = resolve(AudioSwitchingSet.baseUrl, T) = "https://otherexample.com/s2/audio/" initBaseUrl = resolve(AudioTrack.initializationPattern, T) = "https://otherexample.com/s2/audio/128k-init-{initId}.mp4" contBaseUrl = resolve(AudioTrack.continuationPattern, T) = "https://otherexample.com/s2/audio/128k-content-{segmentId}.mp4"¶
The final content request URLs are https://otherexample.com/s2/audio/128k-init-{initId}.mp4
and https://otherexample.com/s2/audio/128k-content-{segmentId}.mp4
for the Initialization Stream and Continuation Stream respectively.¶
In the second step, the client uses the Manifest to retrieve and then parse an Initialization Packet.¶
The client decides to retrieve the only audio Track of the active Presentation. The URL pattern for the Initialization Stream of this Track is https://otherexample.com/s2/audio/128k-init-{initId}.mp4
.¶
The client opts to retrieve the most recent Initialization Packet and sends out a request:¶
GET /s2/128k-init-now.mp4 HTTP/1.1 Host: otherexample.com Accept: */*¶
The server responds with the following headers¶
HTTP/1.1 200 OK Content-Type: audio/mp4 Content-Length: 742 Date: Wed, 31 Mar 2021 08:00:02 GMT¶
and a binary body containing the ISOBMFF media data of the audio Initialization Packet.¶
The client repeats this to retrieve one of the video Tracks.¶
The client parses the ISOBMFF boxes in the audio Initialization Packet. Once the emsg
box with scheme ID urn:theo:hesp:2020
and value initdata
is found, its message_data
field is parsed.¶
In this case, message_data
equals {"index":200,"offset":63275}
. The following media data for the chosen audio Track is available in the Continuation Segment with ID 200 and at a byte offset of 63275.¶
In the final step, the client uses the retrieved information to retrieve Continuation Segments and reaches playback.¶
The client retrieves the Continuation Segment of the chosen audio Track. The URL pattern for the Continuation Stream of this Track is https://otherexample.com/s2/audio/128k-content-{segmentId}.mp4
.¶
The client sends out a request for the segment with ID 200, starting at offset 63275:¶
GET /s2/128k-content-200.mp4 HTTP/1.1 Host: otherexample.com Accept: */* Range: bytes=63275-9007199254740991¶
As discussed in Section 5.3.2, a sufficiently large end byte value is chosen to ensure the entire range is retrieved.¶
The server responds with the following headers¶
HTTP/1.1 200 OK Content-Type: audio/mp4 Content-Range: bytes 63275-9007199254740991/9007199254677716 Transfer-Encoding: chunked Date: Wed, 31 Mar 2021 08:00:03 GMT¶
and a (chunked) binary body containing the ISOBMFF media data of the Continuation Segment.¶
The client repeats this for the chosen video Track and uses the retrieved media data to reach playback. The Manifest that was retrieved previously contains sufficient information to retrieve new Continuation Segments from this point on.¶
A Content Delivery Network (CDN) MAY be employed to increase the scalability and cacheability for delivering HESP.¶
While the HESP protocol uses HTTP/1.1 and delivery should be possible over most HTTP CDNs, care must be taken to ensure that the CDN has all the required features.¶
To correctly handle HESP, the CDN MUST either support HTTP/1.1 with chunked transfer encoding or support HTTP/2. It MUST also support Range Requests as well as the caching of the partial object responses. This ensures that HESP requests and responses pass correctly through the CDN and that responses can be cached for future use.¶
The CDN SHOULD support collapsing multiple HTTP/1.1 Range Requests with overlapping byte-ranges into a single request. This ensures that two requests with byte-ranges that are partially overlapping require only a single request to the media server for the overlapping part. This reduces the number of concurrent requests arriving on the media server.¶
In this annex, we describe two possible profiles for video Tracks of HESP streams. Both profiles place certain requirements on the underlying H.264 encoding of the HESP stream.¶
The goal of this profile is to allow the stream to reach the lowest latency and zapping times possible using the HESP protocol. In this profile, it should be ensured that an Initialization Packet exists for each frame of the Continuation Stream. As a result, a client can start playback, seek and switch between Tracks at any time position of the stream.¶
The Initialization Stream for Tracks of the Maximal Gain Profile must satisfy the following requirements:¶
The Continuation Stream for Tracks of the Maximal Gain Profile must satisfy the following requirements:¶
+----+----+----+----+----+----+----+----+----+----+ time positions | 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | +----+----+----+----+----+----+----+----+----+----+ +----+----+----+----+----+----+----+----+----+----+ initialization | IDR| IDR| IDR| IDR| IDR| IDR| IDR| IDR| IDR| IDR| stream | 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | +----+----+----+----+----+----+----+----+----+----+ | REF| REF| REF| REF| REF| REF| REF| REF| REF| REF| | a | b | c | d | e | f | g | h | i | j | +----+----+----+----+----+----+----+----+----+----+ +----+----+----+----+----+----+----+----+----+----+ continuation | I,P| I,P| I,P| I,P| I,P| I,P| I,P| I,P| I,P| I,P| stream | 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | +----+----+----+----+----+----+----+----+----+----+ | | | | | \___ position c | | | \___ position b | \___ position a
The goal of this profile is to allow media data from other HTTP-based adaptive bitrate protocols to be reused. This comes at the cost of some optimizations made by the previous profile. In this profile, it is not possible to start playback at any time position of the stream, as it is not possible for Initialization Packets to refer to every sample of a Continuation Segment.¶
The Initialization Stream for Tracks of the Compatibility Profile must satisfy the following requirements:¶
The Continuation Stream for Tracks of the Compatibility Profile must satisfy the following requirements:¶
Figure 5 depicts a part of H.264 output of 7 frames, sorted in decode order. The dependencies of each frame are shown with arrows.¶
______________________________________ | | | | V | | | +----+ +----+ +----+ +----+ +----+ +----+ +----+ |####| |%%%%| |%%%%| |%%%%| |@@@@| |@@@@| |@@@@| | I1 | | P4 | | B2 | | B3 | | P7 | | B5 | | B6 | +----+ +----+ +----+ +----+ +----+ +----+ +----+ ^ | ^ | | ^ | | |_____| | | | |______| | | | | | | | |________|_____| | |______________| | | |______________________|
A "sub-GOP" defines a set of B and P frames that only depend on one previous frame.¶
An Initialization Packet can be published if the subsequent media sample of the Continuation Stream depends on at most one previous frame.¶
For Figure 5, this is the case at the following positions:¶
A client requesting an Initialization Packet at other time positions must receive the most recent valid Initialization Packet. For example, that means that a request for an Initialization Packet at position 2 in Figure 5 must return the Initialization Packet at position 1.¶