MSYNC
draft-bichot-msync-05
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draft-bichot-msync-05
Internet-Draft S. Bale
Intended Status: Informational R. Brebion
Expires: January 30, 2023 G. Bichot
Broadpeak
July 29, 2022
MSYNC
draft-bichot-msync-05
Abstract
This document describes the Multicast Synchronization (MSYNC)
Protocol that aims at transferring video media objects over IP
multicast operating preferably RTP. Although generic, MSYNC has been
primarily designed for transporting HTTP adaptive streaming (HAS)
objects including manifest/playlists and media segments (e.g. MP4,
CMAF) according to an HAS protocol such as Apple HLS or MPEG DASH
between a multicast server and a multicast gateway.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright and License Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. MSYNC Protocol . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. MSYNC Packet Format . . . . . . . . . . . . . . . . . . . . 6
3.2. Object Info Packet . . . . . . . . . . . . . . . . . . . . 7
3.3. Object Data Packet . . . . . . . . . . . . . . . . . . . . 9
3.4. Object HTTP Header Packet . . . . . . . . . . . . . . . . . 9
3.5. Object Data-part Packet . . . . . . . . . . . . . . . . . . 10
3.6. Maximum Size Of A MSYNC Packet . . . . . . . . . . . . . . 11
3.7. Sending MSYNC Objects Over IP/Transport Multicast
Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.8. HAS Protocol Dependency . . . . . . . . . . . . . . . . . . 13
3.8.1. Object Info Packet . . . . . . . . . . . . . . . . . . 13
3.8.1.1. Media Sequence . . . . . . . . . . . . . . . . . . 13
3.8.1.2. Object URI . . . . . . . . . . . . . . . . . . . . 14
3.8.2. Sending Rules . . . . . . . . . . . . . . . . . . . . . 15
3.9. RTP As The Transport Multicast Session Protocol . . . . . . 15
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Normative References . . . . . . . . . . . . . . . . . . . 17
5.2. Informative References . . . . . . . . . . . . . . . . . . 18
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1 Introduction
MSYNC relies preferably on RTP that makes it particularly suited for
transitioning IPTV legacy (MPEG2 TS/RTP) to the HAS ecosystem. MSYNC
is simple (no flow control, no forward error correction) although
reliable, flexible and extensible; it has been experimented and
deployed over IPTV infrastructure (xDSL, cable, fiber) and
satellite.
1.1 Terminology
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].
1.2 Definitions
manifest: A file gathering the configuration for conducting a
streaming session; corresponds to a play list as defined by HLS
[RFC8216]. During a HAS streaming session, a manifest or
playlist can be modified.
media chunk: A piece of a media segment of a fixed duration as
specified in [MPEGCMAF].
media segment: A piece of a media sub-stream of a fixed duration
(e.g. 2s) as specified in [MPEGCMAF].
init segment: A piece of a media sub-stream used to initialize the
decoder as specified in [MPEGCMAF].
media: A digitalized piece of video, audio, subtitle, image, ....
media stream: Gathers one or more media sub-streams.
media sub-stream: A version of a media encoded in a particular bit-
rate, format and resolution; also called representation or
variant stream.
variant stream : A media sub-stream as defined by HLS [RFC8216];
corresponds to a representation as defined by [MPEGDASH].
representation: A media sub-stream as defined by [MPEGDASH];
Corresponds to a variant stream as defined by HLS [RFC8216].
HTTP Adaptive Streaming (HAS) session: Transport one or more media
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streams (e.g. one video, two audios, One subtitle) according to
HTTP. A HAS session is triggered by a player downloading first a
manifest file(s), then init and/or media segments (belonging to
possibly different sub-streams according to the selected
representation) and possibly more manifest files according to
the HAS protocol.
MSYNC object: As part of a HAS session carried over MSYNC, an MSYSNC
object can be an addressable HAS entity like an init segment, a
media segment (or fragment, or chunk), a manifest. An MSYNC
object can also be a non-addressable transport entity like a
part of a segment (an HTTP2 frame or an HTTP 1.1 CTE block). As
part of the control channel, an MSYNC object may transport some
control plane information (for the receiver as e.g. the
multicast gateway configuration). An MSYNC object is typically
associated with metadata (aka info), data and possibly an HTTP
header.
MSYNC packet: The transport unit of MSYNC. Several MSYNC packets mays
be used to transport an MSYNC object.
transport multicast session: Operating a transport protocol that is
(possibly based on) UDP over IP multicast. A session is
identified by the transport (UDP) port number, the source IP
address and the IP multicast address.
RTP multicast session: A transport multicast session based on RTP as
defined in [RFC3550].
IP multicast session: A session gathering transport multicast
sessions having the same source IP address and destination
multicast IP address.
MSYNC channel: The set of transport multicast sessions carrying a HAS
session as a set of MSYNC objects.
MSYNC control channel: the transport multicast session carrying
control plane MSYNC objects.
2. Overview
MSYNC is a simple protocol typically used between a multicast server
(the MSYNC sender) and a multicast gateway (the MSYNC receiver). The
multicast server gets ingested with a unicast HAS session conforming
to a HAS protocol as e.g. MPEG DASH [MPEGDASH] or HLS [RFC8216] and
sends the HAS session elements over a broadcast/multicast link
according to MSYNC supporting [possibly RTP/] UDP/IP multicast up to
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the multicast gateway(s) that serve the HAS player(s) in unicast
conforming to the same HAS protocol. MSYNC can serve simultaneously
multiple gateways conforming to one or several HAS protocols and
formats.
The Multicast server is configured in order to get the unicast HAS
feeds. Considering one among several possible ingest methods (e.g.
HTTP GET), for each ingested feed, the multicast server behaves as a
sort of player, reading the manifest, discovering the available
representations and downloading concurrently media segments of all
(or a subset) of the available representations. Finally the multicast
server is configured for sending all those HAS session elements over
[possibly RTP/]UDP/IP multicast according to a certain UDP flow
arrangement (for example all the objects related to each video
representation are sent over a separate multicast transport session
(multicast IP address + port number) whereas all audio
representations are sent over the same transport multicast session.
The multicast gateway is configured accordingly in order to be
attached to the transport multicast sessions (in particular, it has
to subscribe to the corresponding multicast IP group address). Note
that the multicast gateway might not be capable of being attached to
all the concurrent transport multicast sessions in the same time per
bandwidth restriction (e.g. ADSL). In that case, the multicast
gateway attaches to the transport multicast session corresponding to
the player's request (and detaches from the other previous one).
At any time, the multicast gateway can detect corrupted and lost
packets and attempt to repair using a repair protocol. This is
possible thanks to the RTP protocol if used as the transport layer
over UDP .
The multicast gateway receives the MSYNC objects and is ready to
serve it (e.g. feeds a local cache). Whenever a HAS request is sent
by a media player and received by the multicast gateway, the latter
reads first its local cache. In case of cache hit, it returns the
object. In case of cache miss, the multicast gateway can possibly
retrieve the requested object from the associated CDN (or a dedicated
server) over an unicast interface (if existing) through operating
HTTP conventionally and forwards back to the media player the object
once retrieved.
With MSYNC deployed over a multicast link/network, the end user media
player gets basically the HAS content in full transparency (i.e. the
player is absolutely unaware of getting the content through MSYNC or
not).
Note that nothing precludes the multicast gateway to be co-located
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with the media player and therefore embedded in the end-user
terminal.
Note that nothing precludes application dependent features in the
multicast server and/or the multicast gateway that may adapt/modify
the content delivered to the end-user player.
3. MSYNC Protocol
3.1. MSYNC Packet Format
The MSYNC packet has the following format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | packet type | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-header |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| data |
| .... |
version: 8 bits
version of the MSYNC protocol = 0x3
packet type: 8 bits
Defines the MSYNC packet type. The sub-header and the associated
data (if any) are dependent on the packet type. The following
types are defined.
0x01: object info
0x02: object info redundancy packet
0x03: object data
0x04: Reserved
0x05: object http header
0x06: object data-part as a piece of an object data for
transporting e.g. an MPEG CMAF chunk, an HTTP 1.1 chunk or yet
an HTTP2 frame.
object identifier: 16 bits
The field identifies the object being transferred. All MSYNC
packets associated with the same object carry the same object
identifier in their MSYNC packet header.
sub-header: series of N x 32 bits
The packet sub-header is linked to the packet type. The details of
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each packet type is given in the next sections.
data: series of D x 8 bits
This field is optional and is present depending on the packet
type. D is bounded by the maximum size of a transport multicast
session protocol packet size and the MTU (Maximum Transfer Unit)
otherwise as depicted in 3.6.
3.2. Object Info Packet
The Object info packet is used to transport the meta-data
associated with an object. It permits to characterize the object
in term of e.g. size and type. The object information is carried
over one object info packet only. The object info packet is
typically sent along with the object data it describes.
The object identifier corresponds to the object identifier of the
object data packets or the object data-part packets the object
info packet relates to.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | 0x1 or 0x2 | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| number of MSYNC packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object CRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object type | Reserved | mtype | object URI size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| media sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| object URI |
: :
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
object size: 32 bits
The number of bytes that compose the object payload transported
with a MSYNC object data packet (Section 3.3) or MSYNC object
data-part packet (Section 3.5). The size may be 0 indicating that
there is no corresponding object's payload transmission foreseen
(i.e. no expected MSYNC data or MSYNC data-part packets) . In case
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of a super object transmission (Section 3.5), If the object URI of
an object info with an object size set to 0 matches the super
object URI then it MUST be interpreted as the end of the super
object transmission (Section 3.8.1.2).
number of MSYNC packets: 32 bits
Number of MSYNC packets that compose the transported object. If
the object size is null (set to 0) then the number of MSYNC
packets MUST be null (set to 0).
object CRC: 32 bits
A CRC applied to the object data payload for corruption detection.
mtype: 4 bits
The manifest (playlist) type, the MSYNC INFO is compatible with.
The field can take the following values.
0x00: Not Applicable
0x01: MPEG Dash as specified in [MPEGDASH].
0x02: HLS as specified in [RFC8216].
0x03-0xF: Reserved
object URI size: 12bits
The size in bytes of the object URI field. The value MUST
guarantee that the MSYNC info packet size is not greater than the
network MTU.
object type : 8 bits
Defines the type of MSYNC data object associated with this MSYNC
info packet
0x00: Reserved
0x01: media manifest (playlist)
0x02: Reserved
0x03: media data or data-part: Transport stream (MPEG2-TS)
0x04: media data or data-part: MPEG4 (CMAF)
0x05: control: control plane information (e.g. multicast gateway
configuration)
0x06-0xFF: Reserved
media sequence: 32 bits
It is a sequence number associated with the MSYNC object data
(segment or manifest). It is dependent on the mtype value. It is
used to synchronize unicast and multicast receptions in the
multicast gateway. The values and rules are detailed in the
section 3.8 dedicated to the HAS protocol dependencies. The
default value is 0x00.
object URI: Quotient(object URI size/32) bits
This the path name associated with the object. It MAY corresponds
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to a storage/Cache path. There SHOULD be a direct relationship
between this URI and the URL associated with the addressable
object (e.g. HAS segment or CMAF chunk and/or a manifest). The
rules are detailed in the section 3.8 dedicated to the HAS
protocol dependencies.
3.3. Object Data Packet
This MSYNC packet carries part or all of the the object's data
payload. The type of data and the way to process the object's data
packets is function of the associated object's info packet. Object
payload is transported through a series of object data packets.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | 0x3 | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
object offset: 32 bits
The index from which the MSYNC object data packet payload is to be
written in order to compose the object data at the receiver side
(i.e. the multicast gateway). The first data packet of an object
has an offset equal to 0.
data: N x 8bits
The size N is not declared; it is bounded by the maximum size of
the under-laying transport multicast session packet (e.g. RTP) as
depicted in Section 3.6. The total size (number of bytes) of the
object data is indicated in the associated object info (field
object size).
3.4. Object HTTP Header Packet
The HTTP header packet carries part or all of an HTTP header
related to the object (data) to be sent. There is at most one HTTP
header per object that can be repeated.
The object identifier is the same than the one present in the
object data packets or object data-part packets it relates to.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | 0x5 | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header size | header offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
header size: 16 bits
An object HTTP header can be transported over one or several
under-laying transport packets. This field indicates the total
size of the HTTP header in bytes and it is indicated in each the
HTTP header's packet.
header offset: 16 bits
The index from which this HTTP header MSYNC packet payload data is
to be written in order to complement the HTTP header at the
receiver side (i.e the multicast gateway). The first packet of the
HTTP header has an offset equal to 0.
data: N x 8bits
The size N is not declared; it is bounded by either the header
size field value or by the maximum size of the under-laying
transport packet(e.g. RTP)as depicted in Section 3.6.
3.5. Object Data-part Packet
This MSYNC packet carries part or all of the media data-part
object payload. The type of data and the way to process the
object's data-part packets is function of the associated info
packet. Object payload is transported through a series of object
data-part packets. The data-part is used when the object
corresponds to a "part" (a block) of a super object for which the
size is unknown (a super object may correspond to a stream or a
media segment not yet complete and for which the size is therefore
unknown).
All data-part packets belonging to the same data part object have
the same object identifier that is the same one present in the
object info packet and HTTP header (if any) packets the data-part
object relates too.
All data-part objects composing a super object have a different
object identifier. The way to link data-part objects with a super
object is thanks to the object info packet (object URI) as
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explained in Section 3.8.1.2.
The end of super-object transmission is signaled with an object
info packet having both the object size and the number of MSYNC
packets set to 0 and having the object URI matching the object URI
of the already received parts according to Section 3.8.1.2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | 0x6 | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| super object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
object offset: 32 bits
The index from which the data-part packet payload is to be written
in order to compose the object data-part at the receiver side
(i.e. the multicast gateway). The first packet of the data-part
has an offset equal to 0.
super object offset: 32 bits
The index from which the object part-data packet payload is to be
written in order to compose the super object data at the receiver
side (i.e. the multicast gateway). The first data-part object
composing a super object has the super object offset equal to 0.
The super object offset is the same for all object data-part
packets composing the same object data-part.
data: N x 8bits
The size N is not declared; it is bounded by the maximum size of
the under-laying transport packet (e.g. RTP) as depicted in the
section 3.6. The total size (number of bytes) of the object data
is indicated in the associated object info (field object size).
3.6. Maximum Size Of A MSYNC Packet
An MSYNC packet is composed with a header part and a data part for
which the size is bounded by the transport multicast session
packet. In case there is no particular restriction as with RTP
and/or UDP (which authorize up to 65235 bytes), then the maximum
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size is linked to the path MTU (Maximum Transfer Unit) as the
largest transfer unit supported between the source (the multicast
server) and the destination (the multicast gateway) without
fragmentation. An MSYNC packet MUST fit within a link layer
packet.
For Ethernet, as an example, the MTU is typically 1500 bytes,
assuming a 20 bytes IPv4 header, a 8 bytes UDP header and the 8
bytes MSYNC object data packet header, it gives an MTU of 1464
bytes for the MSYNC object data packet. Operating RTP, the MSYNC
object data MTU is decreased by 12 bytes (= 1452 bytes).
3.7. Sending MSYNC Objects Over IP/Transport Multicast Sessions
The following considerations are linked to the multicast server
configuration.
Per MSYNC channel, the way to map MSYNC objects related to a media
stream with an IP or transport multicast session is not
constrained. The arrangement is chosen function of the network
architecture and capacity. For example, in xDSL, the capacity
dedicated to multicast is limited which may drive to an
arrangement where each sub-stream/representation of a HAS
session/MSYNC channel matches with one dedicated IP multicast
session. The MSYNC receiver switches to the IP transport session
corresponding to the sub-stream/representation it should serve to
the user terminal/player. Alternatively, one would like to have
one IP multicast session (with possibly multiple transport
multicast sessions, each having a different destination port
number) for the entire HAS session/MSYNC channel that is an
arrangement a la "IPTV", less bandwidth efficient but where only
one multicast IP address is allocated per HAS session/MSYNC
channel.
Considering a satellite network, as all transport multicast
sessions are carried simultaneously, all arrangements may make
sense.
Regarding the mapping with a transport multicast session, the
triplet: source IP address, destination multicast IP address and
destination transport port number is the discriminator. It is
recommended to carry media sub-streams and the control channel in
separate transport multicast channels. It facilitates potential
error correction procedures.
The following granularity is possible:
- One IP multicast session per media (audio or video or
subtitle) sub-stream (representation); each transport multicast
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session having a different destination multicast IP address.
- One transport multicast session for the MSYNC control channel.
- It is perfectly possible to send all the MSYNC packets in only
one transport multicast session.
For each MSYNC object (see object type in 3.2) to be sent, the
sender MUST send the following MSYNC packets in the specified
order: one object info packet, zero or more object info redundant
packets, zero or more HTTP header packets and one or more object
data packets or object data-part packets.
When the MSYNC object is a media data-part object of size null
(used to signal the end of the transmission of a super object)
then only one object info packet MUST be sent.
3.8. HAS Protocol Dependency
A certain number of MSYNC packet header fields have a dependency
on the HAS protocol and therefore on the manifest type. Similarly
the sending rules may also depend from the HAS protocol.
3.8.1. Object Info Packet
3.8.1.1. Media Sequence
The media sequence is used by the multicast gateway to synchronize
the MSYNC (i.e. multicast) reception with unicast reception. The
multicast gateway may operate jointly MSYNC and unicast retrieval
of HAS objects. This is useful in some occasions like processing a
new streaming session request (i.e. a manifest request after a
channel switch) or in the case of segment repair. The multicast
gateway may attempt to retrieve a manifest object or segment(s)
through a unicast mean (e.g. a CDN server or a repair server) in
order to speed up the start of the session or to repair damaged
object(s). Consequently, the multicast gateway needs to understand
the freshness of the HAS object received through multicast with
regard to unicast.
If no unicast reception is used jointly with MSYNC in the
multicast gateway (e.g. like in one way delivery only), the
default value MAY be used: 0x00
HLS master playlist: 0x00
HLS variant playlist; MUST contain the value of EXT-X-MEDIA-SEQUENCE
added with the position in the playlist of the last segment
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transmitted.
HLS segment: MUST contain the value of EXT-X-MEDIA-SEQUENCE added
with the position of the segment in the playlist.
DASH manifest: MUST contain $time$/scale or $Number$ corresponding to
the last segment transmitted or under transmission (and possibly
received partially) and declared by the manifest.
DASH segment: MUST contain the $time$/scale or $Number$ value
3.8.1.2. Object URI
In the context of HTTP adaptive streaming, The object URI is a URI
reference.
if the object is a HAS addressable entity (e.g. a segment or a
CMAF chunk), the object URI MUST match (be a sub-string) with the
URL announced in the corresponding manifest/playlist.
Examples:
- The object URI: /tvChannel1/Q1/S_2 matches with the segment's
URL that is computed from the associated manifest/playlist:
".../tvChannel1/Q1/S_2.mp4"
- The object URI /tvChannel11/Q1/S_2_3 matches with the CMAF
chunk URL that is computed from the associated
manifest/playlist: ".../tvChannel11/Q1/S_2_3.mp4".
If the object is a non-addressable HAS entity (e.g. a HTTP 1.1 CTE
block), the object URI is composed with a sub-string (that MUST
match with the URL announced in the corresponding manifest) and a
suffix composed with the hash sign/character (#) and the block
number).
Example:
- The object URI of the 3rd HTTP CTE block of the segment S_2:
tvChannel11/Q1/S_2.mp4#2 matches with the segment's request URL
that terminates with ".../tvChannel1/Q1/S_2.mp4"
The block number of an object URI attached to a media data-part
object MUST be incremented for each subsequent transmission.
When all the MSYNC data-part packets for all the media data-part
objects (e.g. CTE blocks) composing a super object (e.g. a media
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segment) have been sent, the MSYNC sender MUST signal the end of
the MSYNC super object transmission through sending an MSYNC
object info packet with the object size set to zero (0) . In
addition, the object URI MUST contain the URI reference of the
next block (never transmitted). see 3.2.
Example:
- The object URI of the object info packet associated with the
1st HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#0
- The object URI of the object info packet associated with the
2nd HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#1
- The object URI of the object info packet associated with the
3rd HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#2
- The object URI of the object info packet associated with the
4st HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#3
- The object URI of the object info packet associated with the
5st HTTP CTE block (of size null) signaling the end of the super
object (i.e. segment) transmission: tvChannel11/Q1/S_2.m4s_4
3.8.2. Sending Rules
Whenever a manifest (playlist) has to be sent:
- The manifest (playlist) object MUST be sent within (duplicated
in) all the transport multicast sessions related to the
transmission of the video segment objects the manifest/playlist
refers to.
- It MUST reference addressable objects (segment or CMAF chunk)
that have already been sent or for which the transmission has
started.
3.9. RTP As The Transport Multicast Session Protocol
RTP [RFC3550] MAY be used as part of the transport multicast
session protocol according to the following.
- The payload type (PT) value is not specified. It may
correspond to one of the values specified in [RFC3551]. Its
value should be communicated to the MSYNC receiver through a
separate unspecified offline mean.
- Each RTP multicast session MUST operate a unique different
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SSRC number [RFC3550]. This allows packet retransmission (if
used) on the RTP transport multicast session basis.
- RTCP usage is not required.
Packet retransmission MAY be used in association with the RTP
multicast session for packet loss recovery. If this is the case
then the RTP Repair client and RTP repair server MUST be compliant
with [RFC4585], [RFC4588], [RFC5506] and [RFC5761] according to
the following.
- The RTP Repair client (i.e. the MSYNC Receiver) submits
transport layer feedback (FB) messages in NACK mode (Generic
NACK) to the RTP Repair Server according to [RFC5506] and
[RFC4585].
- The RTP Repair server receives, processes and responds to the
feedback NACK messages (FB) according to [RFC4588]. The RTP
Repair server MAY be the MSYNC sender or it MAY be any
intermediate entity acting as a multicast RTP receiver (i.e.
capable of receiving the multicast RTP packets). In any case,
the RTP Repair server and the RTP Repair client MUST operate a
unicast interface.
- The Session-multiplexing scheme [RFC4588] MUST be applied: the
RTP retransmission (repair) stream MUST be sent on a different
RTP session than the original (multicast) RTP stream.
- The retransmission stream MUST support multiplexing the RTP
and RTCP traffic on a single port according to [RFC5761].
Note that instead of relying on "RTP retransmission", an MSYNC
receiver (i.e. the multicast gateway) could attempt to recover HAS
elements (segments, manifest) through HTTP (aka "HTTP repair").
However the latter method requires a CDN and is less reactive than
operating RTCP.
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4. IANA Considerations
This document has no actions for IANA.
5. Security Considerations
The multicast communication between the MSYNC sender (multicast
server) and the MSYNC receiver (the multicast gateway) SHOULD be
protected for confidentiality, message corruption and replay
attacks. The MSYNC protocol does not specify any security
mechanism. MSYNC relies on possibly content protection (Digital
Right Management) and on the underlying transport layer and
security extensions for providing message integrity/authentication
and replay. Secure RTP (SRTP) [RFC3711] and IPSec applied to
multicast [RFC5374] are potential candidates for providing such
extensions.
5. References
5.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC
3550, July 2003, <https://www.rfc-
editor.org/info/rfc3550>.
[MPEGDASH] "Information technology - Dynamic adaptive streaming over
HTTP (DASH) - Part1:Media presentation description and
segment formats",ISO/IEC23009-1
[MPEGCMAF] "Information technology - Multimedia application format
(MPEG-A) - Part 19:Common media application format (CMAF)
for segmented media"ISO/IEC 23000-19
[RFC5506] I. Johansson, M. Westerlund. "Support for Reduced-Size
Real-Time Transport Control Protocol(RTCP): Opportunities
and Consequences", RFC 5506, April 2009,
<https://www.rfc-editor.org/info/rfc5506>.
[RFC4585] J. Ott, S. Wenger, N. Sato, C. Burmeister, J. Rey.
"Extended RTP Profile for Real-time Transport Control
Protocol(RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006, <https://www.rfc-editor.org/info/rfc4585>.
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[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006, <https://www.rfc-editor.org/info/rfc4588>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010,
<https://www.rfc-editor.org/info/rfc5761>.
5.2. Informative References
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July
2003, <https://www.rfc-editor.org/info/rfc3551>.
[RFC3711] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman.
"The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004, <https://www.rfc-
editor.org/info/rfc3711>.
[RFC5374] B. Weis, G. Gross, D. Ignjatic. "Multicast Extensions to
the Security Architecture for the Internet Protocol", RFC
5374, November 2008, <https://www.rfc-
editor.org/info/rfc5374>.
[RFC8216] R. Pantos, Ed., W. May, "HTTP Live Streaming", RFC 8216,
August 2017, <https://www.rfc-editor.org/info/rfc8216>.
6. Acknowledgments
The authors will be ever grateful to their late colleague Arnaud
Leclerc who has been the initiator of that work.
The authors would like to thank the following people for their
feedback: Yann Barateau (Eutelsat).
7. Change Log
- 05: Updated section 3.9 adding reference (RFC4588) and details
for RTP retransmission. Updated/normalized references in section
5.1 and 5.2.
- 04: Added detection of super object transmission (Section 3.2
and Section 3.8.1.2); several adjustments regarding RFC style;
Section numbering correction.(Sections 3.9 and 3.10 are now
section 3.8 and 3.9 respectively).
Authors' Addresses
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Sophie Bale
Broadpeak
15 rue Claude Chappe
Zone des Champs Blancs
35510 Cesson-Sevigne
France
Email: sophie.bale@broadpeak.tv
Remy Brebion
Broadpeak
15 rue Claude Chappe
Zone des Champs Blancs
35510 Cesson-Sevigne
France
Email: remy.brebion@broadpeak.tv
Guillaume Bichot (Editor)
Broadpeak
15 rue Claude Chappe
Zone des Champs Blancs
35510 Cesson-Sevigne
France
Email: guillaume.bichot@broadpeak.tv
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