Internet DRAFT - draft-dong-priority-rtp-packet
draft-dong-priority-rtp-packet
Independent Submission L. Dong
Internet-Draft R. Li
Intended status: Informational Futurewei Technologies Inc.
Expires: 11 January 2023 S. Clayman
University College London
M. Sayit
Ege University
10 July 2022
Discarding Priority of RTP Video Packets
draft-dong-priority-rtp-packet-02
Abstract
This document illustrates that significance difference or discarding
priority might exist among RTP packets which encapsulate video
streaming data with the existing modern video codecs, i.e., H.264/
AVC, SVC, H.265/HEVC and H.266/VVC.
The document overviews the RTP NALU header format for the existing
modern video codecs. Each contains at least one field that indicates
the RTP packet's relative significance within the video stream. With
the dominance of video traffic in the Internet, selectively dropping
RTP packets from competing video streams according to their
significances or discarding priorities could be a complementary
mechanism when dealing with network congestion. The document
proposes the Differentiated Services Code Point (DSCP) value mapping
to the RTP packet discarding priority carried in the RTP NALU header.
The document also proposes a new Hop-by-Hop Extension Header (HbH-EH)
with a value that is copied from the discarding priority of the RTP
packet, if the 6-bit DSCP value is not long enough for the mapping.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 11 January 2023.
Copyright 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 3
3. Packet Level Priority . . . . . . . . . . . . . . . . . . . . 4
3.1. Packet Level Priority Difference in H.264 RTP Packets . . 4
3.2. Packet Level Priority Difference in SVC RTP Packets . . . 6
3.3. Packet Level Priority Difference in H.265 RTP Packets . . 6
3.4. Packet Level Priority Difference in H.266 RTP Packets . . 8
4. Implementation of Priority-Based Discarding of RTP Video
Packets . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The modern video codecs, e.g., H.264/AVC [H.264], SVC [H.264], H.265/
HEVC [H.265], and H.266/VVC [ISO23090-3] [VVC]use the NAL-unit-based
syntax structure. The NAL unit structure provides convenient
packetization/framing of video data to be transmitted in packet-based
systems using transport protocols such as RTP [RFC3550]. The
transport layer can identify the boundaries among adjacent NAL units
without use of start code. Therefore, the overhead for these start
codes can be eliminated. Depending on the characteristics of the NAL
unit(s) encapsulated in a RTP packet, the priority/importance of RTP
packets from the same video streaming flow could differ from each
other. In the following, we firstly overview how the priority
information is carried in RTP packets for H.264/AVC, SVC, H.265/HEVC,
and H.266/VVC by referring to [RFC6184] [RFC6190] [RFC7798] [RTP.VVC]
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respectively. Next we discuss how to make the network layer aware of
and utilize such priority information for selective packet dropping
when network congestion happens and outgoing buffer overflows.
2. Terms and Abbreviations
The terms and abbreviations used in this document are listed below.
* AF: Assured Forwarding
* AP: Aggregation Packet
* AVC: Advanced Video Coding
* DF: Default Forwarding
* DSCP: Differentiated Services Code Point
* EF: Expedited Forwarding
* HDTV: High Definition Television
* HEVC: High Efficiency Video Coding
* HbH-EH: Hop-by-Hop Extension Header
* IDR: Instantaneous Decoding Refresh
* FU: Fragmentation Unit
* MANE: Media Aware Network Element
* MTAP: Multi-Time Aggregation Packet
* NAL: Network Abstract Layer
* PACI: PAyload Content Information
* PHB: Per Hop Behavior
* QoE: Quality of Experience
* QoS: Quality of Service
* RTP: Real Time Protocol
* STAP: Signal-Time Aggregation Packet
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* SNR: Signal-to-Noise Ratio
* SVC: Scalable Video Coding
* VCL: Video Coding Layer
The above terminology is defined in greater details in the remainder
of this document.
3. Packet Level Priority
For different versions of video encoding schemes, the RTP packet
payload format has been and is being standardized. Within a video
flow, the importance or discarding priority can differ among
different RTP packets, depending on the NAL unit(s) encapsulated in
the RTP packets. In the following, we give a brief overview of such
property, which is shown in different versions of video encoders.
3.1. Packet Level Priority Difference in H.264 RTP Packets
The H.264 video codec [H.264] has a very broad application range that
covers all forms of digital compressed video, from low bitrate
Internet streaming applications to HDTV broadcast and digital cinema
applications with nearly lossless coding. The coded video data is
organized into NAL units, each of which contains an integer number of
bytes. The H.264/AVC specification adopts a byte stream format.
Each NAL unit has a prefix of a specific pattern of three bytes,
which is called a start code prefix. The boundaries of the NAL unit
can then easily be detected by searching the coded data for this
unique start code prefix pattern. A set of NAL units in a specified
form comprises as an access unit. The decoding of each access unit
results in one decoded picture.
The syntax and semantics of the NAL unit type octet are specified in
[H.264], includes the essential properties of the NAL unit type octet
in the NAL unit header. The RTP packet for H.264 video [RFC6184]
inherits the same NAL unit header. As shown in Figure 1, the 2 bits
NRI field (i.e., nal_ref_idc) indicates the relative importance/
transport priority of the NRI unit determined by the encoder. A
value of 00 indicates that the content of the NAL unit is not used to
reconstruct reference pictures for inter picture prediction. Such
NAL units can be discarded without risking the integrity of the
reference pictures. Values greater than 00 indicate that the
decoding of the NAL unit is required to maintain the integrity of the
reference pictures. The H.264 specification requires that the value
of NRI SHALL be equal to 0 for all NAL units having nal_unit_type
equal to 6, 9, 10, 11, or 12. For NAL units having nal_unit_type
equal to 7 or 8 (indicating a sequence parameter set or a picture
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parameter set, respectively), an H.264 encoder should set the value
of NRI to '11'. For coded slice NAL units of a primary coded picture
having nal_unit_type equal to 5 (indicating a coded slice belonging
to an IDR picture), an H.264 encoder sets the value of NRI to '11'.
Non-IDR coded slice is specified with '10' NRI value, coded slice
data partition A has '10' NRI value, while partition B and C have
'01' NRI value.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The Structure of the H.264 NAL Unit Header.
Figure 1
The 'Type' field indicates the payload format with three different
basic payload structures:
* Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The NRI field is associated with this single NAL unit.
* Aggregation Packet (AP): Packet type used to aggregate multiple
NAL units into a single RTP payload. This packet exists in four
versions, the Single-Time Aggregation Packet type A (STAP-A), the
Single-Time Aggregation Packet type B (STAP-B), Multi-Time
Aggregation Packet (MTAP) with 16-bit offset (MTAP16), and Multi-
Time Aggregation Packet (MTAP) with 24-bit offset (MTAP24). A NAL
unit header is followed by one or more NAL units in aggregation
packets. The value of NRI is the maximum of all the NAL units
carried in the aggregation packet.
* Fragmentation Unit (FU): Used to fragment a single NAL unit over
multiple RTP packets. It exists with two versions, FU-A and FU-B
respectively. Each FU packet has a FU indicator which has the
same format as above. The value of the NRI field is set according
to the value of the NRI field in the fragmented NAL unit, which
means all the FU packets belong to the same NAL unit have the same
NRI value.
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3.2. Packet Level Priority Difference in SVC RTP Packets
Scalable Video Coding (SVC) extension of the H.264/AVC video coding
standard is specified in Amendment 3 to ISO/IEC 14496 Part 10
[ISO_IEC14496-10] and equivalently in Annex G of ITU-T Rec. H.264
[H.264]. SVC defines a coded video representation in which a given
bitstream offers representations of the source material at different
levels of scalability: spatial (picture size), quality (or Signal-to-
Noise Ratio (SNR)), and temporal (pictures per second). Bitstream
components associated with a given level of spatial, quality, and
temporal fidelity are identified using corresponding parameters in
the bitstream: dependency_id, quality_id, and temporal_id. There are
three additional octets in the NAL unit header of SVC RTP packets
[RFC6190], which are shown in Figure 2.
+---------------+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|I| PRID |N| DID | QID | TID |U|D|O| RR|
+---------------+---------------+---------------+
Additional Octets in the SVC NAL Unit Header.
Figure 2
The priority of a NAL unit in SVC video stream can be further
specified by the priority_id field (PRID), which has 6 bits. A lower
value of PRID indicates a higher priority.
3.3. Packet Level Priority Difference in H.265 RTP Packets
The H.265/HEVC [H.265] significantly improves coding efficiency over
H.264. Similarly, H.265 also includes a Video Coding Layer (VCL),
which is often used to refer to the coding-tool features, and a
Network Abstraction Layer (NAL), which is often used to refer to the
systems and transport interface aspects of the codecs. HEVC includes
an improved support of temporal scalability over H.264, by inclusion
of the signaling of TemporalId in the NAL unit header. HEVC
maintains the NAL unit concept of H.264 with modifications. The RTP
packet for H.265/HEVC video [RFC7798] uses a two-byte NAL unit header
as shown in Figure 3.
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The 3 bits field TID specifies the temporal identifier of the NAL
unit plus 1. The value of TemporalId is equal to TID minus 1. The
TID value indicates (among other things) the relative importance of
an RTP packet. For example, because NAL units belonging to higher
temporal sub-layers are not used for the decoding of lower temporal
sub-layers. A lower value of TID indicates a higher importance.
More-important NAL units might need to be better protected against
transmission loss or packet dropping than less-important NAL units.
+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| Type | LayerId | TID |
+-------------+-----------------+
The Structure of the HEVC NAL Unit Header.
Figure 3
The type field indicates the different types of RTP packet payload
structures.
* Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The TID field is associated with this single NAL unit.
* Aggregation Packet (AP): Packet type used to aggregate multiple
NAL units into a single RTP payload. A payload header is followed
by one or more NAL units in aggregation packets. The value of TID
is set as the lowest value of TID of all the aggregated NAL units.
* Fragmentation Unit (FU): Used to fragment a single NAL unit over
multiple RTP packets. Each FU packet has a FU payload header
which has the same format as above. The value of the TID field is
set according to the value of the TID field in the fragmented NAL
unit, which means all the FU packets belong to the same NAL unit
have the same TID value.
* PAyload Content Information (PACI): Used to increase the payload
header efficiency. The value of TID is a copy of the TID field of
the PACI payload NAL unit or NAL-unit-like structure.
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3.4. Packet Level Priority Difference in H.266 RTP Packets
Versatile Video Coding (VVC) is formally published as both ITU-T
Recommendation H.266 [VVC] and ISO/IEC International Standard 23090-3
[ISO23090-3]. VVC is reported to provide significant coding
efficiency gains over H.265/HEVC, and other earlier video codecs.
The RTP payload format for H.266/VVC [RTP.VVC] allows for
packetization of one or more Network Abstraction Layer (NAL) units in
each RTP packet payload as well as fragmentation of a NAL unit into
multiple RTP packets.
VVC maintains the NAL unit concept of HEVC with modifications. VVC
uses a two-byte NAL unit header, as shown in Figure 4. The payload
of a NAL unit refers to the NAL unit excluding the NAL unit header.
+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|Z| LayerID | Type | TID |
+---------------+---------------+
The Structure of the VVC NAL Unit Header.
Figure 4
Similar to H.265, the TID value indicates (among other things) the
relative importance of an RTP packet, for example, because NAL units
belonging to higher temporal sublayers are not used for the decoding
of lower temporal sublayers. A lower value of TID indicates a higher
importance. More-important NAL units might need to be better
protected against transmission loss or packet dropping than less-
important NAL units.
The LayerID field is used to identify the layer a NAL unit belongs
to, wherein a layer may be, e.g., a spatial scalable layer, a quality
scalable layer, a layer containing a different view, etc. The
LayerID has integer values, where higher values designate components
that are higher in the hierarchy. Decoding of a particular component
requires the availability of all the components it depends upon,
either directly, or indirectly. So the NAL unit with lower LayerID
would be likely be used to predict the NAL units with higher LayerID,
therefore likely to be more important.
The type field indicates the different types of RTP packet payload
structures.
* Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The TID field is associated with this single NAL unit.
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* Aggregation Packet (AP): Packet type used to aggregate multiple
NAL units into a single RTP payload. A payload header is followed
by one or more NAL units in aggregation packets. The value of TID
is set as the lowest value of TID of all the aggregated NAL units.
* Fragmentation Unit (FU): Used to fragment a single NAL unit over
multiple RTP packets. Each FU packet has a FU payload header
which has the same format as above. The value of the TID field is
set according to the value of the TID field in the fragmented NAL
unit, which means all the FU packets belong to the same NAL unit
have the same TID value.
4. Implementation of Priority-Based Discarding of RTP Video Packets
Due to the explicit layering in the protocol stack, the upper layer
data or headers are transparent to the network layer. The priority
or importance associated with the NAL units encapsulated in RTP
packets is invisible to intermediate routers. The concept of media-
aware network element (MANE) was introduced in [RFC6184], which is a
network element, such as a middlebox or application layer gateway
that is capable of parsing certain aspects of the RTP payload headers
or the RTP payload and reacting to the contents. The concept of a
MANE goes beyond normal routers or gateways in that a MANE has to be
aware of the signaling (e.g., to learn about the payload type
mappings of the media streams) and that it has to be trusted when
working with Secure Real-time Transport Protocol (SRTP) [RFC3711].
The advantage of using MANEs is that they allow packets to be dropped
according to the needs of the media coding. For example, if a MANE
has to drop packets due to congestion on a certain link, it can
identify and remove those packets whose elimination produces the
least adverse effect on the user experience.
MANEs can access the field that indicates the importance of the NAL
unit, which was overviewed in the previous section. In summary:
* The two bits NRI field in H.264 and SVC NAL unit header.
* The 3 bits TID filed in H.265 and H.266 NAL unit header.
* The 6 bits PRID field in SVC NAL unit extension header, which
provides even finer granularity of priority differentiation for
NAL units in SVC.
* The 6 bits LayerID field in H.266 NAL unit payload header, which
provides even finer granularity of priority differentiation for
NAL units in VVC.
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MANE is an overlay network element that might be co-located with a
few routers, e.g., at network edge. So when network congestion
happens in other routers that is not deployed with MANE, the packet
dropping is subject to DiffServ classification [RFC2475]. DiffServ
uses a 6-bit differentiated services code point (DSCP) in the 8-bit
differentiated services field (DS field) in the IP header for packet
classification purposes. In theory, a network could have up to 64
different traffic classes by using the 64 available DSCP values.
However, the commonly defined per-hop behaviors only include 4
categories:
* Default Forwarding (DF) PHB, which is typically best-effort
traffic.
* Expedited Forwarding (EF) PHB, which is dedicated to low-loss,
low-latency traffic.
* Assured Forwarding (AF) PHB, which gives assurance of delivery
under prescribed conditions
* Class Selector PHBs, which maintain backward compatibility with
the IP precedence field.
We consider the two video types: interactive video and non-
interactive video. The video stream from both types could be encoded
according to H.264, SVC, H.265, H.266. For H.264 and SVC, the NAL
units have the NRI field to indicate the discarding priority of the
RTP packets. For H.265 and H.266, the NAL units have the TID field
to indicate the discarding priority of the RTP packets. The NRI
field is of 2 bits, and the TID field is of 3 bits, thus the DSCP
value can be mapped according to either the NRI value or the TID
value, as well as the video types. In general, the NAL units with
the same NRI value or the TID value in interactive video has higher
priority than in non-interactive video. The recommended DSCP values
for RTP packets according to NRI value and video type are shown in
Table 1. The recommended DSCP values for RTP packets according to
TID value and video type are shown in Table 2.These values are based
on the framework and recommended values in [RFC4594].
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+===========+===================+=======================+
| NRI Value | Interactive Video | Non-Interactive Video |
+===========+===================+=======================+
| 11 | AF41 | AF42 |
+-----------+-------------------+-----------------------+
| 10 | AF42 | AF43 |
+-----------+-------------------+-----------------------+
| 01 | AF31 | AF32 |
+-----------+-------------------+-----------------------+
| 00 | AF32 | AF33 |
+-----------+-------------------+-----------------------+
Table 1: Recommended DSCP Values for RTP Packets
According to NRI Value and Video Type (with H.264 or
SVC Encoder)
+===========+===================+=======================+
| TID Value | Interactive Video | Non-Interactive Video |
+===========+===================+=======================+
| 001 | AF41 | AF42 |
+-----------+-------------------+-----------------------+
| 010 | AF42 | AF43 |
+-----------+-------------------+-----------------------+
| 011 | AF31 | AF32 |
+-----------+-------------------+-----------------------+
| 100 | AF32 | AF33 |
+-----------+-------------------+-----------------------+
| 101 | AF21 | AF22 |
+-----------+-------------------+-----------------------+
| 110 | AF22 | AF23 |
+-----------+-------------------+-----------------------+
| 111 | AF11 | AF12 |
+-----------+-------------------+-----------------------+
Table 2: Recommended DSCP Values for RTP Packets
According to TID Value and Video Type (with H.265 or
H.266 Encoder)
Either the video host or the MANE at the DiffServ domain edge can do
the mapping and set up the DSCP value for each RTP packet. The
discarding precedence of the RTP packets can be determined when link
congestion happens.
Compared to H.265, SVC and H.266 employ additional scalability other
than the temporal scalability, namely spatial scalability and quality
scalability. Thus in the NAL extension header for SVC, there is an
additional field (i.e., PRID) used to indicate the importance of the
RTP packet at finer granularity. The PRID field occupies 6 bits
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additionally. In the NAL unit header for h.266, the LayerID is used
to identify the layer a NAL unit belongs to, wherein a layer may be,
e.g., a spatial scalable layer, a quality scalable layer, a layer
containing a different view, etc. The LayerID field provides the
importance information of the RTP packet at finer granularity as
well. The LayerID field occupies 6 bits additionally.
It is not feasible to use the DSCP mapping to indicate the additional
discarding precedence provided by the 6 bits PRID, and the 6 bits
LayerID. Thus, other solutions need to explored in the future if
discarding precedence at finer granularity is considered to be
supported.
5. IANA Considerations
This document requires no actions from IANA.
6. Security Considerations
This document introduces no new security issues.
7. Acknowledgements
8. Informative References
[H.264] ITU-T, "H.264 : Advanced Video Coding for Generic
Audiovisual Services", 2019,
<https://www.itu.int/rec/T-REC-H.264-201906-I/en>.
[H.265] "High efficiency video coding, ITU-T Recommendation
H.265", 2019, <http://handle.itu.int/11.1002/1000/14107>.
[ISO23090-3]
ISO/IEC 23090-3, "Information technology - Coded
representation of immersive media Part 3 Versatile Video
Coding", 2021, <https://www.iso.org/standard/73022.html>.
[ISO_IEC14496-10]
"ISO/IEC International Standard 14496-10", 2015.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998,
<https://datatracker.ietf.org/doc/html/rfc2475>.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "TConfiguration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<https://www.rfc-editor.org/info/rfc6184>.
[RFC6190] Wenger, S., Wang, Y.-K., Schierl, T., and A.
Eleftheriadis, "RTP Payload Format for Scalable Video
Coding", RFC 6190, DOI 10.17487/RFC6190, May 2011,
<https://www.rfc-editor.org/info/rfc6190>.
[RFC7798] Wang, Y.-K., Sanchez, Y., Schierl, T., Wenger, S., and M.
M. Hannuksela, "RTP Payload Format for High Efficiency
Video Coding (HEVC)", RFC 7798, DOI 10.17487/RFC7798,
March 2016, <https://www.rfc-editor.org/info/rfc7798>.
[RTP.VVC] Zhao, S., Black, D., Wnger, S., Sanchez, Y., and Y. Wang,
"RTP Payload Format for Versatile Video Coding (VVC)",
<https://www.ietf.org/archive/id/draft-ietf-avtcore-rtp-
vvc-14.html>.
[VVC] "Versatile Video Coding, ITU-T Recommendation H.266",
2020, <http://www.itu.int/rec/T-REC-H.266>.
Authors' Addresses
Lijun Dong
Futurewei Technologies Inc.
United States of America
Email: lijun.dong@futurewei.com
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Richard Li
Futurewei Technologies Inc.
United States of America
Email: richard.li@futurewei.com
Stuart Clayman
University College London
United Kingdom
Email: s.clayman@ucl.ac.uk
Muge Sayit
Ege University
Turkey
Email: mugefesci@gmail.com
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