MSYNC
draft-bichot-msync-08
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draft-bichot-msync-08
Internet-Draft S. Bale
Intended Status: Informational R. Brebion
Expires: August 15, 2023 G. Bichot
Broadpeak
February 11, 2023
MSYNC
draft-bichot-msync-08
Abstract
This document describes the Multicast Synchronization (MSYNC)
Protocol that aims at transferring video media objects over IP
multicast. Although generic, MSYNC has been primarily designed for
transporting HTTP adaptive streaming (HAS) objects including
manifest/playlists and media segments (e.g. CMAF) according to an HAS
protocol such as Apple HLS or MPEG DASH between a multicast sender
and a multicast receiver.
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) 2023 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
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. MSYNC Protocol . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. MSYNC Packet Format . . . . . . . . . . . . . . . . . . . . 8
3.2. Object Info Packet . . . . . . . . . . . . . . . . . . . . 9
3.3. Object Data Packet . . . . . . . . . . . . . . . . . . . . 11
3.4. Object HTTP Header Packet . . . . . . . . . . . . . . . . . 12
3.5. Object Data-part Packet . . . . . . . . . . . . . . . . . . 13
3.6. Maximum Size of an MSYNC Packet . . . . . . . . . . . . . . 14
3.7. Sending an receiving MSYNC Objects Over Transport/IP
Multicast Sessions . . . . . . . . . . . . . . . . . . . . 14
3.8. HAS Protocol Dependency . . . . . . . . . . . . . . . . . . 16
3.8.1. Object Info Packet . . . . . . . . . . . . . . . . . . 16
3.8.1.1. Media Sequence . . . . . . . . . . . . . . . . . . 16
3.8.1.2. Object URI . . . . . . . . . . . . . . . . . . . . 17
3.8.2. Sending Rules . . . . . . . . . . . . . . . . . . . . . 18
3.9. RTP As The Transport Multicast Session Protocol . . . . . . 18
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . . 21
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Normative References . . . . . . . . . . . . . . . . . . . 21
6.2. Informative References . . . . . . . . . . . . . . . . . . 22
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1 Introduction
Transporting media content over multicast is known to be very
effective for saving network resources (bandwidth). Multicast is
available and used by Internet service providers for providing IPTV
services. The IPTV technology relies essentially on MPEG Transport
Stream (MPEG TS) format, UDP transport and IP multicast while the
HTTP adaptive bit-rate streaming (HAS), a unicast "Over The Top"
technology relies on HTTP /TCP, new container formats (as MP4/CMAF)
and signaling protocols (Apple HLS, MPEG DASH). With the
generalization of HAS streaming there is a need to operate multicast
in order to achieve the same level of bandwidth efficiency provided
by an IPTV service. MSYNC allows transporting HTTP based ABR flows
over multicast relying on IP/UDP and optionally RTP that makes it
particularly suited for transitioning IPTV legacy (MPEG2 TS) to the
HAS ecosystem. MSYNC is simple (no flow control, no forward error
correction) although reliable (enable easy coupling with a unicast
based repair protocols) and extensible; it has been experimented and
deployed over various IPTV infrastructures (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
HTTP Adaptive Streaming (HAS) session: Transport one or more media
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.
init segment: A piece of a media sub-stream used to initialize the
decoder as specified in [MPEGCMAF].
IP multicast session: A session gathering transport multicast
sessions having the same source IP address and destination
multicast IP address.
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
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playlist can be modified.
media: A digitalized piece of video, audio, subtitle, image, ....
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].
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.
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. As part of the control channel, an
MSYNC object may transport some control plane information (as
e.g. the MSYNC receiver configuration).
MSYNC object: As part of a HAS session carried over MSYNC, an MSYSNC
object can be an addressable HAS entity like an initialization
segment, a media segment (or fragment, or chunk), a manifest (or
playlist). 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). An MSYNC object is sent by an MSYNC sender
over a transport multicast session and receives by an MSYNC
receiver.
MSYNC packet: The transport unit of MSYNC. Several MSYNC packets MAY
be used to transport an MSYNC object.
MSYNC receiver. The MSYNC end point that receives MSYNC objects over
multicast according to MSYNC. It is typically part of a
multicast gateway that receives MSYNC objects relying on the
MSYNC receiver and reconstructs/serves in unicast the original
HAS session on demand (i.e. based on HAS player requests).
MSYNC sender. The MSYNC end point that sends MSYNC objects over
multicast according to MSYNC. It is typically part of a
multicast server that acquire HAS session payload and send it
over multicast as MSYNC objects relying on the MSYNC sender.
MSYNC super object. When an object to be transmitted is composed of
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parts delivered on the fly when available in such a way the size
of an object to be transmitted is unknown in advance, it is
called a super object. A super object may correspond to a stream
or a media segment not yet completely generated/received and for
which the size is therefore unknown.
representation: A media sub-stream as defined by [MPEGDASH];
Corresponds to a variant stream as defined by HLS [RFC8216].
RTP multicast session: A transport multicast session based on RTP as
defined in [RFC3550].
transport multicast session: Operating a transport protocol that is
(possibly based on) UDP over IP multicast. A transport multicast
session is identified by the transport (UDP) port number, the
source IP address and the IP multicast address.
variant stream : A media sub-stream as defined by HLS [RFC8216];
corresponds to a representation as defined by [MPEGDASH].
2. Overview
MSYNC is a simple protocol typically used between a multicast server
(hosting the MSYNC sender) and a multicast gateway (hosting the MSYNC
receiver) as represented in the figure 1 below. The multicast server
acquires HAS session elements in unicast conforming to a HAS protocol
as e.g. MPEG DASH [MPEGDASH] or HLS [RFC8216] and sends those HAS
session elements over a multicast link according to MSYNC supporting
[possibly RTP/] UDP/IP multicast up to the multicast gateways. A
multicast gateway listens the corresponding multicast flows and serve
the HAS player(s) in unicast conforming to the same HAS protocol.
MSYNC permits a sender to serve simultaneously multiple receivers
conforming to one or several HAS protocols and formats (e.g. assuming
one shared multicast network, one sender could served some receivers
with MPEG DASH compliant content and some other receivers with HLS
compliant content).
The Multicast server is configured (by e.g. the ISP operating the
multicast network) in order to acquire HAS content from a Content
Distribution Network (CDN) over a unicast link. Considering one among
several possible content ingest methods (e.g. HTTP GET), for each HAS
session, the multicast server behaves as a sort of HAS 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
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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 (by the same ISP having
configured the multicast server) for being aware of the same UDP flow
arrangement in order to listen and to receive the right transport
multicast sessions (in particular the MSYNC receiver has to subscribe
to the multicast IP group addresses associated with that UDP flow
arrangement. 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/or lost
packets and attempt to repair using a repair protocol. This is
possible interacting with the HAS CDN or thanks to the RTP protocol
if used as the transport layer over UDP (see section 3.9).
The multicast gateway receives the MSYNC objects and is ready to
serve it (e.g. acts as 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 HAS player the object
once retrieved.
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Unicast server Multicast server
+-------- + + -------------------- +
| HAS | < -- unicast --> | HAS | MSYNC |
| CDN | | Ingest | Sender |
+ ------- + + ---------------------+
^ ^ |
| | |
unicast - - - - -unicast - - - - - multicast
| | |
v v V
+-------- + + -------------------- +
| HAS | < -- unicast --> | HAS | MSYNC |
| Player | | Server |Receiver |
+ ------- + + ---------------------+
End-user Multicast gateway
terminal
Figure 1: example of MSYNC deployment
The figure 1 shows a typical MSYNC deployment where a HAS player may
interacts with an HAS server in a unicast way over e.g. Internet and
may also interact with a multicast gateway over e.g. a local network
(LAN) according to the same HAS protocol. With MSYNC deployed over a
multicast link/network, the HAS 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 MSYNC receiver or even the multicast
gateway to be co-located with the media player and therefore embedded
in the end-user terminal as shown in the figure 2.
Multicast server
+-------- + + -------------------- +
| HAS | < -- unicast --> | HAS | MSYNC |
| Server | | Player | Sender |
+ ------- + + ---------------------+
^ |
| |
unicast multicast
| |
v |
+ ----------------- + |
| HAS | MSYNC |<-------------------------
| Player |Receiver |
+ ------------------+
End-user terminal
Figure 2: MSYNC receiver in the terminal
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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 |
| .... |
Figure 3: MSYNC Packet
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 in a multicast
transport session. Considering one multicast transport session,
all MSYNC packets associated with the same object carry the same
object identifier in their MSYNC packet header. Whenever this
object ID change that means the transfer of the previous object is
finished.
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sub-header: series of N x 32 bits
The packet sub-header is linked to the packet type. The details of
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 | packet type | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| number of MSYNC packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object CRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object type | Reserved | mtype | object URI size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| media sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| object URI |
: :
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Object Info packet
packet type: 0x01 or 0x02
Redundant object INFO packets (packet type 02) MAY be sent in
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addition to the "main" object info packet according to section
3.7.
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 maximum size of an object (4.4
Gbytes) authorizes the transfer of a video segment of several tens
of seconds, 4K encoded.
The size may be 0 indicating that there is no corresponding
object's payload transmission foreseen (i.e. no expected MSYNC
data packet or MSYNC data-part packet) . In case 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.
object type : 8 bits
Defines the type of MSYNC data object associated with this MSYNC
info packet
0x00: Reserved for future use.
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
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
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network MTU.
media sequence: 32 bits
It is a sequence number associated with the MSYNC objects data and
data-part (for transporting a segment or a 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 * 8)/32) bits + 32 bits if
remainder ((object URI size * 8)/32) >0
This the path name associated with the object. It MAY corresponds
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.
The object URI is coded as a series of string characters.
Remaining non used bytes of the last 32 bits field MUST be filled
with the 0x00 value.
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 | packet type | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
Figure 5: Object Data packet
packet type: 0x03
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
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(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 object per data (or data-part) 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header size | header offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
Figure 6: Object HTTP Header packet
packet type: 0x05
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
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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
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 | packet type | object identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| super object offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
: :
: :
Figure 7: Object Data-part packet
packet type: 0x06
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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 an MSYNC Packet
An MSYNC packet MUST fit within the underlying protocol packet. As
detailed in section 3, an MSYNC packet is composed with a header
part and a data part for which the size is limited by the
transport multicast protocol. With RTP and/or UDP (which
authorize up to 65235 bytes), then the maximum size is linked to
the path MTU (Maximum Transfer Unit) as the largest transfer unit
supported between the source (the multicast sender) and the
destination (the multicast receiver) without fragmentation.
3.7. Sending an receiving MSYNC Objects Over Transport/IP Multicast
Sessions
The following considerations are linked to the MSYNC sender and
MSYNC receiver configuration. Note that the configuration
procedure (protocol and format) is out of the scope of that
document.
The mapping of MSYNC objects related to a HAS stream onto
transport/IP multicast sessions is not constrained by the MSYNC
protocol. The multicast IP flow arrangement is chosen (typically
by the ISP) function of the network architecture and capacity. For
example, with xDSL, the capacity dedicated to multicast is limited
which may drive to a IP multicast flow arrangement where each sub-
stream/representation matches with one dedicated IP multicast
session. The MSYNC receiver switches to the IP transport multicast
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session corresponding to the sub-stream/representation it should
serve to the end 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 IP multicast flow
arrangements may make sense.
Regarding the mapping onto 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 sessions because It permits the
deployment of different error correction (see section 3.9) or
content protection procedure (e.g. one ISP may decide to encrypt
the control channel).
The following arrangement is possible:
- One IP multicast session per media (audio or video or
subtitle) sub-stream (representation); each transport multicast
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 over a
multicast transport multicast session , the MSYNC 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.
Detecting the end of an MSYNC data object (or super object)
transmission is done thanks to the object Info (see 3.2)
information. However entire packet loss is always possible in such
a way the end of an object (or super object) transmission cannot
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be detected without the usage of a timer. MSYNC does not mandate
any particular mechanism.; this is left to the implementer.The
MYNC receiver MAY arm a timer when the reception start (e.g. first
packet of a new object) and stop the timer after a certain time
(linked with the application) triggering the end of teh
transmission/reception of the object.
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 (an object Info Packet header field presented
in the section 3.2) is used by the multicast gateway to
synchronize the MSYNC (i.e. multicast) reception with unicast
reception. The multicast gateway may operate jointly
MSYNC/multicast and unicast for retrieving HAS elements as
indicated in section 2 and illustrated in figure 1. 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 of 0x00 MAY be used.
If unicast reception is used jointly with MSYNC then the media
sequence MUST be set depending on the object type (Info Packet
header field presented in the section 3.2.) as listed below.
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
transmitted.
HLS segment: MUST contain the value of EXT-X-MEDIA-SEQUENCE added
with the position of the segment in the playlist.
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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
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 section 3.2.
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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 has to be sent over MSYNC, the following
applies.
- The corresponding MSYNC object data MUST be sent over all the
transport multicast sessions related to the transmission of the
media segments the manifest 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) 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
SSRC number [RFC3550]. This allows packet retransmission (if
used) on the RTP transport multicast session basis.
- RTCP usage is not required.
Packet retransmission (see Figure 8 below) MAY be used in
association with the RTP multicast session for packet loss
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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 followings:
- The RTP Repair client (coupled to 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 located within the multicast server or it
MAY be hosted by 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].
Multicast server
+ ----------------- +
| HAS | MSYNC |
| Ingest | Sender |
+ ----------------- +
|
| + ------ +
multicast | RTP |
| ------->| Repair |
| | Server |
| + ------ +
V ^
+ ------------------------- + |
| HAS | MSYNC | RTP | <---
| | |Repair | unicast
| Server |Receiver |Client |
+ ------------------------- +
Multicast gateway
Figure 8: RTP repair
Note that instead of relying on "RTP retransmission", the MSYNC
receiver (i.e. the multicast gateway) could attempt to
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recover/repair damaged HAS elements (e.g. segments, manifest)
through HTTP (aka "HTTP repair") and byte-range requests. However
the latter method requires a CDN, relies on HTTP Byte-range
request for which the support is not harmonized and is less
reactive than operating RTCP ( UDP transactions over a dedicated
path are typically much quicker than HTTP/TCP transactions over
the unicast broadband data path).
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4. IANA Considerations
This document has no actions for IANA.
5. Security Considerations
MSYNC is exposed to the risks linked to the underlying transport
protocols: UDP and RTP. An attacker can spoof the source and
destination addresses, modify any MSYNC headers and, because MSYNC
applies to IP multicast, the MSYNC sender has no control about the
MSYNC receivers which may represent a non authorized party. The
multicast communication between the MSYNC sender (multicast
server) and the MSYNC receiver (the multicast gateway) SHOULD be
protected against confidentiality leaks, message tampering 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 encryption. Secure RTP (SRTP) [RFC3711] and
IPSec applied to multicast [RFC5374] are potential candidates for
providing such extensions.
6. References
6.1. 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>.
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[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>.
[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>.
6.2. 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>.
7. 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).
8. Change Log
- 08: Another round of editorial changes
- 07: Lots of editorial changes
- 06: Example in section 3.8.1.2. update the example for using the
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"#" character as the bloc number prefix instead of the "_"
character.
- 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
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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