INTERNET-DRAFT Q. Wei
Intended Status: Standard Track R. Huang
Expires: May 2, 2017 H. Zheng
Huawei
October 29, 2016
RTP Payload Format for HTTP Adaptive Streaming
draft-wei-payload-has-over-rtp-01
Abstract
whis document introduces a new RTP payload format for encapsulating
the HTTP Adaptive Streaming (HAS) data into RTP, so that current RTP
schemes can be leveraged into OTT video delivery services. For
example, operators can easily deliver OTT live content through
multicast eliminating the impact of live content consumption peaks.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Existing Technologies . . . . . . . . . . . . . . . . . . . . 3
3.1 HTTP Adaptive Streaming . . . . . . . . . . . . . . . . . . 3
3.2 Multicast Adaptive Bit Rate (Multicast-ABR) . . . . . . . . 4
4. HAS Over RTP Use Scenarios . . . . . . . . . . . . . . . . . . 5
5. Overview of HTTP Adaptive Streaming over RTP . . . . . . . . . 5
6. HTTP Adaptive Streaming Payload . . . . . . . . . . . . . . . 7
6.1 RTP Payload Format for HAS Content . . . . . . . . . . . . 7
6.2 Use Existing RTP Payload Format for HAS Content . . . . . . 10
6.3 Manifest file and Initial Information Consideration . . . . 10
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 11
7.1 Media Type Definition . . . . . . . . . . . . . . . . . . . 11
7.2 SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . 12
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1 Normative References . . . . . . . . . . . . . . . . . . . 13
12.2 Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Video consumption has exploded over the last few years as more and
more consumers are watching live Over-the-Top (OTT) content on
smartphones, tablets, PCs and other IP connected devices. Since OTT
video services rely on HTTP adaptive streaming (HAS) technology,
e.g., DASH and HTTP Live Streaming (HLS), to deliver content, so
every time a user requests a piece of content, a stream is sent
throughout the entire network. If a significant number of users are
requesting content, the operator's bandwidth is drained. It is
usually difficult for operators to predict the popularity of live
video content, especially for some major sporting events. For such an
event, millions of users will be watching the content simultaneously,
and causing peak traffic in the network. Even when Content Delivery
Network (CDN) is used to help distribute the traffic load over
network edge nodes, it might still not be sufficient. Since CDNs
cannot be deployed close enough to the users, its scalability is in
question. Furthermore, using CDN may also incur a very high expense,
especially for the new video services like 4K live, or VR live. All
of this leads to a poor Quality of Experience (QoE) for the users,
and a high cost for the service providers. The most effective
solution is to use multicast technology, even for OTT live content
delivery. Through multicast technology, operators can stream live
content only once in their network, regardless of the number of
viewers watching, which eliminates the impact of live content
consumption peaks.
This document introduces a new RTP payload format for encapsulating
the HAS data into RTP, so that current RTP schemes can be leveraged
into OTT video delivery services. For example, operators can easily
deliver OTT live content through multicast to eliminating the impact
of live content consumption peaks.
2. 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 RFC 2119 [RFC2119].
3. Existing Technologies
3.1 HTTP Adaptive Streaming
HTTP adaptive streaming has become a popular approach in video
commercial deployments. The multimedia content is captured, divided
into small segments, and stored on an HTTP server. The consuming user
first obtains the manifest file, e.g., the Media Presentation
Description (MPD), which describes a manifest of the available
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segments information, corresponding bitrates, their URL addresses,
and other characteristics. Based on this, the consuming user selects
the appropriate encoded alternative and starts streaming of the
content by fetching the segments using HTTP GET requests in unicast.
MPEG developed the specification, known as MPEG Dynamic Adaptive
Streaming over HTTP (DASH) [DASH], to standardize the MPD and the
segment formats. Other private mechanisms like Apple's HTTP Live
Streaming (HLS) [HLS] are also popular.
HAS is a typical client pull model. All the manifest files, HAS
segments, and etc., are pulled from the HTTP server one after another
by the clients issuing HTTP requests.
HTTP adaptive streaming is very efficient for the usage of Video on
Demand (VOD). However, when delivering live content simultaneously to
millions of users, this becomes quite a problem. The peak bandwidth
in video consumption is simply too much for an operator to handle
since each viewer counts as a separate unicast session. As live OTT
multi-screen video consumption shows no signs of slowing down, a
traditional unicast delivery method is becoming too expensive in
terms of bandwidth and investments that must be made to maintain the
network. Partnering with a CDN provider only helps optimize the
traffic on the backbone for known content. Additional infrastructure
investment is still required at the edge of the network to absorb the
load, but is too costly of an undertaking and would only be a
temporary solution, as there would always be a need for more servers
when live OTT consumption increases.
3.2 Multicast Adaptive Bit Rate (Multicast-ABR)
Operators are seeking ways to improve the quality of services
available, while also creating more balanced and effective delivery
of data to enhance the operators' cost-efficiency, and reduce wastage
across increasingly constrained bandwidths. Multicast-ABR, specified
in [CableLabs], is one of the innovations.
Multicast-ABR leverages HTTP streaming into multicast by keeping the
different alternatives in separate multicast groups, so that smart
network nodes or clients are able to select an appropriate rate by
joining the correct multicast and delivering these segments to
clients. And multicast-ABR uses NACK-Oriented Reliable Multicast
(NORM) [RFC5740] to deliver HTTP adaptive streaming data in
multicast.
Multicast-ABR is a low cost and easy to deploy solution that allows
operators to see multicast gains on all in-home devices leveraging
their TV Everywhere infrastructure. However, using NORM to convey
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HTTP adaptive streaming data has 3 shortcomings: Firstly, NORM has no
fast channel change (FCC) mechanisms, like [RFC6285], so that
changing different video resolutions may take some time and cause
video frame freeze. Secondly, some telecom operators only have IPTV
multicast platform, which may not support NORM protocol. Thirdly,
NORM is not aware of the media timing in a way that RTP is as RTP is
nature to handle multimedia.
Based on this, using RTP to deliver HTTP adaptive streaming data
could be an alternative.
4. HAS Over RTP Use Scenarios
Network operators running IPTV have already built up an RTP over IP
multicast infrastructure to deliver IPTV content. With the
introduction of HAS payload for RTP, network operators can reuse the
existing infrastructure for the delivery of OTT content using HAS.
The benefit is that it can greatly reduce the investment for IPTV
headend devices and simplify the whole architecture.
From the perspective of OTT content providers, HAS over RTP will
provide them with another means other than CDN for content delivery.
OTT content providers can deliver their in high-demand videos through
multicast, thus ensuring the Quality of Experience (QoE) for the end
users. This would not only help save the bandwidth for network
operators, but also provide them with more opportunities for revenue.
They can offer the multicast as a service to OTT providers.
5. Overview of HTTP Adaptive Streaming over RTP
Figure 1 shows the architecture for HTTP adaptive streaming over RTP,
which is similar to the ones defined in Multicast-ABR.
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+---------+
| Content |
| Source |
+---------+
| ----- Unicast
-------------------|
| | ***** Multicast
+-----------+ +------------+
| HAS | | Multicast |
| Consumer | | Gateway |
+-----------+ +------------+
*
*
*
****************************************
* *
* *
+------------+ +------------+
| Multicast | ................ | M2U |
| Consumer | +------------+
+------------+ |
|
|
+------------+
| Unicast |
| Consumer |
+------------+
Figure 1: HTTP Adaptive Streaming over RTP with Multicast
In the above figure, a HAS Consumer can access the Content Source
using HTTP as normal. This memo considers only the operation of the
Multicast Gateway, the packetisation of HAS content within Multicast
RTP, the operation of the Multicast Consumer that receives that
content, and the operation of the Unicast Consumer that receives the
content by Unicast RTP. A Multicast Gateway receives the unicast HAS
streams of various bitrates from the Content Source, and converts
each unicast stream to multicast to pass on a specific multicast
group. Multicast Consumers subscribe to a multicast group to receive
data from the specific bit-rate stream. Unicast Consumers don't
support multicast but unicast RTP. Therefore, an intermediate node
M2U (multicast-to-unicast) can be introduced to help terminate the
multicast and convert the stream back to unicast for further
delivery.
Multicast Gateway: It is responsible for converting the HAS
streams from unicast to multicast, and providing the multicast
service to its receivers. It will directly pass through the live
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content.
HAS Consumer: It is a standard HAS end point. It could be an
application, or just a user device.
Multicast Consumer: It is an end point supporting HAS RTP by
receiving the content in multicast.
Unicast Consumer: It is an end point supporting HAS RTP by
receiving the content in unicast.
M2U: It stands for multicast-to-unicast. It helps convert
multicast to unicast if the consumer doesn't support multicast.
HAS over RTP will be used throughout Multicast Server, M2U, Multicast
Consumer, and Unicast Consumer.
6. HTTP Adaptive Streaming Payload
This section specifies the format of the RTP payload of HTTP adaptive
streaming data. The structure of the payload is illustrated as Figure
2. This payload format uses the fields of the header in a manner
consistent with that specification.
+----------+------------+
|RTP Header|HAS Payload |
+----------+------------+
Figure 2: Packet Structure with RTP Header
There are two ways in which HAS content can be encoded in RTP
packets: The HAS Payload can be a new RTP Payload Format, specially
designed to carry HAS Content in RTP. Alternatively, an existing RTP
payload format can be used if the HAS Content uses a codec with an
existing format. We describe a new RTP payload format for HAS content
in section 6.1, and discuss using existing formats in Section 6.2.
6.1 RTP Payload Format for HAS Content
The format of RTP header is specified in [RFC3550] and is shown as
Figure 3 for convenience.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| [ contributing source (CSRC) identifiers ] |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: RTP Header Defined in [RFC3550]
The RTP header information to be set according to this RTP payload
format is set as followings:
Marker bit (M): 1 bits
The marker bit set "1" SHALL indicate the last RTP packet of the
media segment, carried in the current RTP stream. This is in line
with the normal use of the M bit in video formats to allow an
efficient playout buffer handling.
Payload Type (PT): 7 bits
The assignment of an RTP payload type for this new packet format
is outside of the scope of this document and will not be specified
here. The assignment of a payload type has to be performed either
through the profile used or in a dynamic way.
Sequence Number (SN): 16 bits
Set and used in accordance with [RFC3550].
Timestamp: 16 bits
The RTP timestamp is set to the sampling timestamp of the content.
The clock rate is specified dynamically through non-RTP means. If
no clock rate is signaled, 90 kHz MUST be used.
When a media segment is encapsulated into several RTP packets,
each of them shares the same timestamp.
The format of the HAS payload is illustrated in Figure 4.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| RSV | Length | [URL Length] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [ URL ...] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HAS data |
| |
| . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format for HTTP Adaptive Streaming Payload
Fragmentation (F): 1 bit
If the fragmentation is set, it indicates the received packet is a
part of a decodable fragment and can not be decoded correctly
until the whole decodable segment is received. The different parts
belong to the same decodable segment are ordered by their sequence
numbers, and share the same timestamp.
If the fragmentation is set, URL length field and URL field will
be omitted.
RSV : 7 bits
These bits are reserved for future use. They MUST be set to zero
by senders and ignored by receivers.
Length: 16 bits
The size of the RTP payload in bytes, excluding the RTP header but
including the payload header.
URL length: 8 bits
The size of the URL field in bytes, including the URL length.
Offset: 32 bits
The offset of this fragment in current decodable segment.
URL: bits defined by URL length.
This field indicates the URL of the content. Examples would be
"/PLTV/88888888/224/3221225484/3221225484.mpd", or
"Stream_1_1944000". It facilitates associating the content with
HTTP request so that receivers can easily turn it into HAS scheme
when receiving it. It is used to relate the RTP packet with
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corresponding HAS segment specified in the manifest.
Basically, it is required to split application data into RTP packets
so that each packet is usable, no matter what is lost. This is
possible when the multicast server has the ability and is authorized
to access the HAS content. However, when OTT content is encrypted to
the multicast server, the frame boundary that can be decoded
independently is hardly figured out. Accordingly, one fragment of the
HAS segment lost will lead to the whole segment undecodable, and the
receivers joining the multicast randomly cannot view the content
immediately. In this case, mechanisms like FEC [RFC5109], or
retransmission [RFC4585] MUST be used to alleviate packet losses. And
FCC [RFC6285] SHOULD be used to ensure endpoints who joins the
multicast randomly can view the content immediately.
Another possible way to do smart fragmenting is to extend the
manifest files, e.g. MPD, to allow the OTT content providers indicate
the fragmentation points where independently decodable application
data can be extracted. Thus, the multicast server bridging HAS into
RTP would fetch the extended manifest file, then use these hints to
determine how to fragment each segment into RTP packets. Since DASH
is the standard in MPEG, this method requires the work in MPEG to
specify the extended fragmentation points.
6.2 Use Existing RTP Payload Format for HAS Content
It is also possible to use existing RTP payload format to transport
HAS content. For example, if the HAS content is H.264, the multicast
gateway will parse the HAS segments to find the boundaries, extract
the NAL units, and generates RTP packets using H.264 RTP format
[RFC6184]. This approach has its advantage that the resulting RTP
stream will be more robust to packet loss, since it uses a loss
tolerant encoding, and will use a standard RTP payload format, that
many existing RTP clients can decode.
However, it has several limitations: It is only possible when the
multicast server has the ability and is authorized to access the HAS
content; It will complicate the multicast gateway; And also the
multicast gateway is required to support multiple existing RTP
formats to enable flexibility since HAS content usually uses
container, e.g., MP4 [MPEG-4 Part 14], which can support multiple
encodings.
6.3 Manifest file and Initial Information Consideration
HAS comes with manifest files to describe the segments and initial
information to setup the session. Since these data needs reliable
ways to delivery, we do not consider transporting them in-band over
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RTP with HAS segments. In fact, the end devices may not need the
manifest files as HAS over RTP is a push model and allows some
packets to be dropped. When the manifest files and initial
information are needed, they can be acquired using out of band method
like HTTP or SDP. Especially when the receiver joins the multicast,
it's better to obtain the manifest or initial information by out of
band ways in advance.
7. Payload Format Parameters
This section specifies the media type and the parameters identifying
this RTP payload format.
7.1 Media Type Definition
This registration is done using the template defined in [RFC6838] and
following [RFC4855].
Type name: video
Subtype name: HAS
Required parameter:
has-type: This parameter indicates the HTTP adaptive streaming
protocol. The value of has-type MUST be in the range of 0 to 7,
inclusive. The detailed value can be seen as following.
HSPT=0: DASH
HSPT=1: Http Live Streaming (HLS)
HSPT=2-6: Reserved
HSPT=7: Profile-specific HTTP adaptive streaming
Optional parameters:
bitrate: This parameter can be used to signal receiver the bitrate
of the stream. How to use it is up to the receiver. The value of
this parameter is an integer in units of kilo-bits per second.
Encoding considerations: This media type is framed in RTP and
contains binary data; see Section 4.8 of [RFC6838].
Security considerations: See section 7 of RFCXXXX.
Published specification: N/A
Additional information: None
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File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information: Rachel
Huang (rachel.huang@huawei.com)
Intended usage: COMMON
Author: See Authors' Addresses section of RFCxxxx.
Change controller: IETF Audio/Video Transport Payloads working group
delegated from the IESG.
7.2 SDP Signaling
TBD.
Negotiation of the new RTP payload is required. Further details will
be provided in the next versions.
8. Congestion Control
Current DASH clients do congestion control individually. When using
multicast to transport HAS data, it is expected that multicast
receivers have the ability to dynamically join the corresponding
multicast group based on different network conditions. Multicast
receivers share the same stream in one multicast group, but HAS
receivers compete each other with different streams, which means the
congestion control mechanisms used in HAS don't work for multicast.
However, it is expected that HAS over RTP for multicast is deployed
in a managed network, hence the congestion can be well controlled.
In the future, when it is deployed on the Internet, a new congestion
control mechanism may be required to coordinate the multicast
receivers with same congestion problem, so that they can share the
stream to obtain the best quality they can have.
9. Security Considerations
TBD.
10. IANA Considerations
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TBD.
11. Acknowledgments
The authors would like to thank the following individuals who help to
review this document and provide very valuable comments: Colin
Perkins, Roni Even, Yuping Jiang.
12. References
12.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD
64, RFC 3550, July 2003.
[CableLabs] "IP Multicast Adaptive Bit Rate Architecture Technical
Report" http://www.cablelabs.com/wp-content/uploads/specdocs/OC-TR-
IP-MULTI-ARCH-V01-1411121.pdf
12.2 Informative References
[RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker
"NACK-Oriented Reliable Multicast (NORM) Transport Protocol", RFC
5740, November 2009.
[RFC6285] Steeg, B., Begen, A., Caenegem, T., and Z. Vax "Unicast-
Based Rapid Acquisition of Multicast RTP Sessions", RFC 6285, June
2011.
[DASH] ISO/IEC 23009-1:2014 Information technology -- Dynamic
adaptive streaming over HTTP (DASH) -- Part 1: Media presentation
description and segment format.
[HLS] Pantos, R. and W. May, "HTTP Live Streaming",
https://tools.ietf.org/html/draft-pantos-http-live-streaming-20,
September 2016.
[MPEG-4 Part 14] "Information technology - Coding of audio-visual
objects - Part 14:MP4 file format", ISO/IEC 14496-14, November 2003.
Authors' Addresses
Qikun Wei
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Huawei
Email: weiqikun@huawei.com
Rachel Huang
Huawei
Email: rachel.huang@huawei.com
Hui Zheng
Huawei
Email: marvin.zhenghui@huawei.com
Yong Xia
China SARFT
Email: xiayong@abs.ac.cn
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