Internet Engineering Task Force (IETF) R. Krishna
Request for Comments: 9699
Category: Informational A. Rahman
ISSN: 2070-1721 Ericsson
December 2024
Use Case for an Extended Reality Application on Edge Computing
Infrastructure
Abstract
This document explores the issues involved in the use of edge
computing resources to operationalize a media use case that involves
an Extended Reality (XR) application. In particular, this document
discusses an XR application that can run on devices having different
form factors (such as different physical sizes and shapes) and needs
edge computing resources to mitigate the effect of problems such as
the need to support interactive communication requiring low latency,
limited battery power, and heat dissipation from those devices. This
document also discusses the expected behavior of XR applications,
which can be used to manage traffic, and the service requirements for
XR applications to be able to run on the network. Network operators
who are interested in providing edge computing resources to
operationalize the requirements of such applications are the intended
audience for this document.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9699.
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Table of Contents
1. Introduction
2. Use Case
2.1. Processing of Scenes
2.2. Generation of Images
3. Technical Challenges and Solutions
4. XR Network Traffic
4.1. Traffic Workload
4.2. Traffic Performance Metrics
5. Conclusion
6. IANA Considerations
7. Security Considerations
8. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Extended Reality (XR) is a term that includes Augmented Reality (AR),
Virtual Reality (VR), and Mixed Reality (MR) [XR]. AR combines the
real and virtual, is interactive, and is aligned to the physical
world of the user [AUGMENTED_2]. On the other hand, VR places the
user inside a virtual environment generated by a computer
[AUGMENTED]. MR merges the real and virtual along a continuum that
connects a completely real environment at one end to a completely
virtual environment at the other end. In this continuum, all
combinations of the real and virtual are captured [AUGMENTED].
XR applications have several requirements for the network and the
mobile devices running these applications. Some XR applications
(such as AR applications) require real-time processing of video
streams to recognize specific objects. This processing is then used
to overlay information on the video being displayed to the user. In
addition, other XR applications (such as AR and VR applications) also
require generation of new video frames to be played to the user.
Both the real-time processing of video streams and the generation of
overlay information are computationally intensive tasks that generate
heat [DEV_HEAT_1] [DEV_HEAT_2] and drain battery power [BATT_DRAIN]
on the mobile device running the XR application. Consequently, in
order to run applications with XR characteristics on mobile devices,
computationally intensive tasks need to be offloaded to resources
provided by edge computing.
Edge computing is an emerging paradigm where, for the purpose of this
document, computing resources and storage are made available in close
network proximity at the edge of the Internet to mobile devices and
sensors [EDGE_1] [EDGE_2]. A computing resource or storage is in
close network proximity to a mobile device or sensor if there is a
short and high-capacity network path to it such that the latency and
bandwidth requirements of applications running on those mobile
devices or sensors can be met. These edge computing devices use
cloud technologies that enable them to support offloaded XR
applications. In particular, cloud implementation techniques
[EDGE_3] such as the following can be deployed:
Disaggregation: Using Software-Defined Networking (SDN) to break
vertically integrated systems into independent components. These
components can have open interfaces that are standard, well
documented, and non-proprietary.
Virtualization: Being able to run multiple independent copies of
those components, such as SDN Controller applications and Virtual
Network Functions, on a common hardware platform.
Commoditization: Being able to elastically scale those virtual
components across commodity hardware as the workload dictates.
Such techniques enable XR applications that require low latency and
high bandwidth to be delivered by proximate edge devices. This is
because the disaggregated components can run on proximate edge
devices rather than on a remote cloud several hops away and deliver
low-latency, high-bandwidth service to offloaded applications
[EDGE_2].
This document discusses the issues involved when edge computing
resources are offered by network operators to operationalize the
requirements of XR applications running on devices with various form
factors. For the purpose of this document, a network operator is any
organization or individual that manages or operates the computing
resources or storage in close network proximity to a mobile device or
sensor. Examples of form factors include the following: 1) head-
mounted displays (HMDs), such as optical see-through HMDs and video
see-through HMDs, 2) hand-held displays, and 3) smartphones with
video cameras and location-sensing capabilities using systems such as
a global navigation satellite system (GNSS). These devices have
limited battery capacity and dissipate heat when running. Also, as
the user of these devices moves around as they run the XR
application, the wireless latency and bandwidth available to the
devices fluctuates, and the communication link itself might fail. As
a result, algorithms such as those based on Adaptive Bitrate (ABR)
techniques that base their policy on heuristics or models of
deployment perform sub-optimally in such dynamic environments
[ABR_1]. In addition, network operators can expect that the
parameters that characterize the expected behavior of XR applications
are heavy-tailed. Heaviness of tails is defined as the difference
from the normal distribution in the proportion of the values that
fall a long way from the mean [HEAVY_TAIL_3]. Such workloads require
appropriate resource management policies to be used on the edge. The
service requirements of XR applications are also challenging when
compared to current video applications. In particular, several
Quality-of-Experience (QoE) factors such as motion sickness are
unique to XR applications and must be considered when
operationalizing a network. This document examines these issues with
the use case presented in the following section.
2. Use Case
This use case involves an XR application running on a mobile device.
Consider a group of tourists who are taking a tour around the
historical site of the Tower of London. As they move around the site
and within the historical buildings, they can watch and listen to
historical scenes in 3D that are generated by the XR application and
then overlaid by their XR headsets onto their real-world view. The
headset continuously updates their view as they move around.
The XR application first processes the scene that the walking tourist
is watching in real time and identifies objects that will be targeted
for overlay of high-resolution videos. It then generates high-
resolution 3D images of historical scenes related to the perspective
of the tourist in real time. These generated video images are then
overlaid on the view of the real world as seen by the tourist.
This processing of scenes and generation of high-resolution images
are discussed in greater detail below.
2.1. Processing of Scenes
The task of processing a scene can be broken down into a pipeline of
three consecutive subtasks: tracking, acquisition of a model of the
real world, and registration [AUGMENTED].
Tracking: The XR application that runs on the mobile device needs to
track the six-dimensional pose (translational in the three
perpendicular axes and rotational about those three axes) of the
user's head, eyes, and objects that are in view [AUGMENTED]. This
requires tracking natural features (for example, points or edges
of objects) that are then used in the next stage of the pipeline.
Acquisition of a model of the real world: The tracked natural
features are used to develop a model of the real world. One of
the ways this is done is to develop a model based on an annotated
point cloud (a set of points in space that are annotated with
descriptors) that is then stored in a database. To ensure that
this database can be scaled up, techniques such as combining
client-side simultaneous tracking and mapping with server-side
localization are used to construct a model of the real world
[SLAM_1] [SLAM_2] [SLAM_3] [SLAM_4]. Another model that can be
built is based on a polygon mesh and texture mapping technique.
The polygon mesh encodes a 3D object's shape, which is expressed
as a collection of small flat surfaces that are polygons. In
texture mapping, color patterns are mapped onto an object's
surface. A third modeling technique uses a 2D lightfield that
describes the intensity or color of the light rays arriving at a
single point from arbitrary directions. Such a 2D lightfield is
stored as a two-dimensional table. Assuming distant light
sources, the single point is approximately valid for small scenes.
For larger scenes, many 3D positions are additionally stored,
making the table 5D. A set of all such points (either a 2D or 5D
lightfield) can then be used to construct a model of the real
world [AUGMENTED].
Registration: The coordinate systems, brightness, and color of
virtual and real objects need to be aligned with each other; this
process is called "registration" [REG]. Once the natural features
are tracked as discussed above, virtual objects are geometrically
aligned with those features by geometric registration. This is
followed by resolving occlusion that can occur between virtual and
real objects [OCCL_1] [OCCL_2]. The XR application also applies
photometric registration [PHOTO_REG] by aligning brightness and
color between the virtual and real objects. Additionally,
algorithms that calculate global illumination of both the virtual
and real objects [GLB_ILLUM_1] [GLB_ILLUM_2] are executed.
Various algorithms are also required to deal with artifacts
generated by lens distortion [LENS_DIST], blur [BLUR], noise
[NOISE], etc.
2.2. Generation of Images
The XR application must generate a high-quality video that has the
properties described above and overlay the video on the XR device's
display. This step is called "situated visualization". A situated
visualization is a visualization in which the virtual objects that
need to be seen by the XR user are overlaid correctly on the real
world. This entails dealing with registration errors that may arise,
ensuring that there is no visual interference [VIS_INTERFERE], and
finally maintaining temporal coherence by adapting to the movement of
user's eyes and head.
3. Technical Challenges and Solutions
As discussed in Section 2, the components of XR applications perform
tasks that are computationally intensive, such as real-time
generation and processing of high-quality video content. This
section discusses the challenges such applications can face as a
consequence and offers some solutions.
As a result of performing computationally intensive tasks on XR
devices such as XR glasses, excessive heat is generated by the
chipsets that are involved in the computation [DEV_HEAT_1]
[DEV_HEAT_2]. Additionally, the battery on such devices discharges
quickly when running such applications [BATT_DRAIN].
A solution to problem of heat dissipation and battery drainage is to
offload the processing and video generation tasks to the remote
cloud. However, running such tasks on the cloud is not feasible as
the end-to-end delays must be within the order of a few milliseconds.
Additionally, such applications require high bandwidth and low jitter
to provide a high QoE to the user. In order to achieve such hard
timing constraints, computationally intensive tasks can be offloaded
to edge devices.
Another requirement for our use case and similar applications, such
as 360-degree streaming (streaming of video that represents a view in
every direction in 3D space), is that the display on the XR device
should synchronize the visual input with the way the user is moving
their head. This synchronization is necessary to avoid motion
sickness that results from a time lag between when the user moves
their head and when the appropriate video scene is rendered. This
time lag is often called "motion-to-photon delay". Studies have
shown that this delay can be at most 20 ms and preferably between
7-15 ms in order to avoid motion sickness [PER_SENSE] [XR] [OCCL_3].
Out of these 20 ms, display techniques including the refresh rate of
write displays and pixel switching take 12-13 ms [OCCL_3] [CLOUD].
This leaves 7-8 ms for the processing of motion sensor inputs,
graphic rendering, and round-trip time (RTT) between the XR device
and the edge. The use of predictive techniques to mask latencies has
been considered as a mitigating strategy to reduce motion sickness
[PREDICT]. In addition, edge devices that are proximate to the user
might be used to offload these computationally intensive tasks.
Towards this end, a 3GPP study suggests an Ultra-Reliable Low Latency
of 0.1 to 1 ms for communication between an edge server and User
Equipment (UE) [URLLC].
Note that the edge device providing the computation and storage is
itself limited in such resources compared to the cloud. For example,
a sudden surge in demand from a large group of tourists can overwhelm
the device. This will result in a degraded user experience as their
XR device experiences delays in receiving the video frames. In order
to deal with this problem, the client XR applications will need to
use ABR algorithms that choose bitrate policies tailored in a fine-
grained manner to the resource demands and play back the videos with
appropriate QoE metrics as the user moves around with the group of
tourists.
However, the heavy-tailed nature of several operational parameters
(e.g., buffer occupancy, throughput, client-server latency, and
variable transmission times) makes prediction-based adaptation by ABR
algorithms sub-optimal [ABR_2]. This is because with such
distributions, the law of large numbers (how long it takes for the
sample mean to stabilize) works too slowly [HEAVY_TAIL_2] and the
mean of sample does not equal the mean of distribution
[HEAVY_TAIL_2]; as a result, standard deviation and variance are
unsuitable as metrics for such operational parameters [HEAVY_TAIL_1].
Other subtle issues with these distributions include the "expectation
paradox" [HEAVY_TAIL_1] (the longer the wait for an event, the longer
a further need to wait) and the mismatch between the size and count
of events [HEAVY_TAIL_1]. These issues make designing an algorithm
for adaptation error-prone and challenging. In addition, edge
devices and communication links may fail, and logical communication
relationships between various software components change frequently
as the user moves around with their XR device [UBICOMP].
4. XR Network Traffic
4.1. Traffic Workload
As discussed in Sections 1 and 3, the parameters that capture the
characteristics of XR application behavior are heavy-tailed.
Examples of such parameters include the distribution of arrival times
between XR application invocations, the amount of data transferred,
and the inter-arrival times of packets within a session. As a
result, any traffic model based on such parameters is also heavy-
tailed. Using these models to predict performance under alternative
resource allocations by the network operator is challenging. For
example, both uplink and downlink traffic to a user device has
parameters such as volume of XR data, burst time, and idle time that
are heavy-tailed.
Table 1 below shows various streaming video applications and their
associated throughput requirements [METRICS_1]. Since our use case
envisages a 6 degrees of freedom (6DoF) video or point cloud, the
table indicates that it will require 200 to 1000 Mbps of bandwidth.
Also, the table shows that XR applications, such as the one in our
use case, transmit a larger amount of data per unit time as compared
to regular video applications. As a result, issues arising from
heavy-tailed parameters, such as long-range dependent traffic
[METRICS_2] and self-similar traffic [METRICS_3], would be
experienced at timescales of milliseconds and microseconds rather
than hours or seconds. Additionally, burstiness at the timescale of
tens of milliseconds due to the multi-fractal spectrum of traffic
will be experienced [METRICS_4]. Long-range dependent traffic can
have long bursts, and various traffic parameters from widely
separated times can show correlation [HEAVY_TAIL_1]. Self-similar
traffic contains bursts at a wide range of timescales [HEAVY_TAIL_1].
Multi-fractal spectrum bursts for traffic summarize the statistical
distribution of local scaling exponents found in a traffic trace
[HEAVY_TAIL_1]. The operational consequence of XR traffic having
characteristics such as long-range dependency and self-similarity is
that the edge servers to which multiple XR devices are connected
wirelessly could face long bursts of traffic [METRICS_2] [METRICS_3].
In addition, multi-fractal spectrum burstiness at the scale of
milliseconds could induce jitter contributing to motion sickness
[METRICS_4]. This is because bursty traffic combined with variable
queueing delays leads to large delay jitter [METRICS_4]. The
operators of edge servers will need to run a "managed edge cloud
service" [METRICS_5] to deal with the above problems.
Functionalities that such a managed edge cloud service could
operationally provide include dynamic placement of XR servers,
mobility support, and energy management [METRICS_6]. Providing
support for edge servers in techniques such as those described in
[RFC8939], [RFC9023], and [RFC9450] could guarantee performance of XR
applications. For example, these techniques could be used for the
link between the XR device and the edge as well as within the managed
edge cloud service. Another option for network operators could be to
deploy equipment that supports differentiated services [RFC2475] or
per-connection Quality-of-Service (QoS) guarantees using RSVP
[RFC2210].
Thus, the provisioning of edge servers (in terms of the number of
servers, the topology, the placement of servers, the assignment of
link capacity, CPUs, and Graphics Processing Units (GPUs)) should be
performed with the above factors in mind.
+===============================================+============+
| Application | Throughput |
| | Required |
+===============================================+============+
| Real-world objects annotated with text and | 1 Mbps |
| images for workflow assistance (e.g., repair) | |
+-----------------------------------------------+------------+
| Video conferencing | 2 Mbps |
+-----------------------------------------------+------------+
| 3D model and data visualization | 2 to 20 |
| | Mbps |
+-----------------------------------------------+------------+
| Two-way 3D telepresence | 5 to 25 |
| | Mbps |
+-----------------------------------------------+------------+
| Current-Gen 360-degree video (4K) | 10 to 50 |
| | Mbps |
+-----------------------------------------------+------------+
| Next-Gen 360-degree video (8K, 90+ frames per | 50 to 200 |
| second, high dynamic range, stereoscopic) | Mbps |
+-----------------------------------------------+------------+
| 6DoF video or point cloud | 200 to |
| | 1000 Mbps |
+-----------------------------------------------+------------+
Table 1: Throughput Requirements for Streaming Video
Applications
4.2. Traffic Performance Metrics
The performance requirements for XR traffic have characteristics that
need to be considered when operationalizing a network. These
characteristics are discussed in this section.
The bandwidth requirements of XR applications are substantially
higher than those of video-based applications.
The latency requirements of XR applications have been studied
recently [XR_TRAFFIC]. The following characteristics were
identified:
* The uploading of data from an XR device to a remote server for
processing dominates the end-to-end latency.
* A lack of visual features in the grid environment can cause
increased latencies as the XR device uploads additional visual
data for processing to the remote server.
* XR applications tend to have large bursts that are separated by
significant time gaps.
Additionally, XR applications interact with each other on a timescale
of an RTT propagation, and this must be considered when
operationalizing a network.
Table 2 shows a taxonomy of applications with their associated
required response times and bandwidths (this data is from Table V in
[METRICS_6]). Response times can be defined as the time interval
between the end of a request submission and the end of the
corresponding response from a system. If the XR device offloads a
task to an edge server, the response time of the server is the RTT
from when a data packet is sent from the XR device until a response
is received. Note that the required response time provides an upper
bound for the sum of the time taken by computational tasks (such as
processing of scenes and generation of images) and the RTT. This
response time depends only on the QoS required by an application.
The response time is therefore independent of the underlying
technology of the network and the time taken by the computational
tasks.
Our use case requires a response time of 20 ms at most and preferably
between 7-15 ms, as discussed earlier. This requirement for response
time is similar to the first two entries in Table 2. Additionally,
the required bandwidth for our use case is 200 to 1000 Mbps (see
Section 4.1). Since our use case envisages multiple users running
the XR application on their devices and connecting to the edge server
that is closest to them, these latency and bandwidth connections will
grow linearly with the number of users. The operators should match
the network provisioning to the maximum number of tourists that can
be supported by a link to an edge server.
+===================+==============+==========+=====================+
| Application | Required | Expected | Possible |
| | Response | Data | Implementations/ |
| | Time | Capacity | Examples |
+===================+==============+==========+=====================+
| Mobile XR-based | Less than 10 | Greater | Assisting |
| remote assistance | milliseconds | than 7.5 | maintenance |
| with uncompressed | | Gbps | technicians, |
| 4K (1920x1080 | | | Industry 4.0 |
| pixels) 120 fps | | | remote |
| HDR 10-bit real- | | | maintenance, |
| time video stream | | | remote assistance |
| | | | in robotics |
| | | | industry |
+-------------------+--------------+----------+---------------------+
| Indoor and | Less than 20 | 50 to | Guidance in theme |
| localized outdoor | milliseconds | 200 Mbps | parks, shopping |
| navigation | | | malls, |
| | | | archaeological |
| | | | sites, and |
| | | | museums |
+-------------------+--------------+----------+---------------------+
| Cloud-based | Less than 50 | 50 to | Google Live View, |
| mobile XR | milliseconds | 100 Mbps | XR-enhanced |
| applications | | | Google Translate |
+-------------------+--------------+----------+---------------------+
Table 2: Traffic Performance Metrics of Selected XR Applications
5. Conclusion
In order to operationalize a use case such as the one presented in
this document, a network operator could dimension their network to
provide a short and high-capacity network path from the edge
computing resources or storage to the mobile devices running the XR
application. This is required to ensure a response time of 20 ms at
most and preferably between 7-15 ms. Additionally, a bandwidth of
200 to 1000 Mbps is required by such applications. To deal with the
characteristics of XR traffic as discussed in this document, network
operators could deploy a managed edge cloud service that
operationally provides dynamic placement of XR servers, mobility
support, and energy management. Although the use case is technically
feasible, economic viability is an important factor that must be
considered.
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
The security issues for the presented use case are similar to those
described in [DIST], [NIST1], [CWE], and [NIST2]. This document does
not introduce any new security issues.
8. Informative References
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[BATT_DRAIN]
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"Distributed Systems: Concepts and Design", Addison-
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[EDGE_1] Satyanarayanan, M., "The Emergence of Edge Computing",
Computer, vol. 50, no. 1, pp. 30-39,
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[EDGE_2] Satyanarayanan, M., Klas, G., Silva, M., and S. Mangiante,
"The Seminal Role of Edge-Native Applications", 2019 IEEE
International Conference on Edge Computing (EDGE), pp.
33-40, DOI 10.1109/EDGE.2019.00022, 2019,
<https://ieeexplore.ieee.org/document/8812200>.
[EDGE_3] Peterson, L. and O. Sunay, "5G Mobile Networks: A Systems
Approach", Synthesis Lectures on Network Systems,
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[GLB_ILLUM_1]
Kan, P. and H. Kaufmann, "Differential Irradiance Caching
for fast high-quality light transport between virtual and
real worlds", 2013 IEEE International Symposium on Mixed
and Augmented Reality (ISMAR), pp. 133-141,
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Franke, T., "Delta Voxel Cone Tracing", 2014 IEEE
International Symposium on Mixed and Augmented Reality
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[HEAVY_TAIL_1]
Crovella, M. and B. Krishnamurthy, "Internet Measurement:
Infrastructure, Traffic and Applications", John Wiley and
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[HEAVY_TAIL_2]
Taleb, N., "Statistical Consequences of Fat Tails: Real
World Preasymptotics, Epistemology, and Applications",
Revised Edition, STEM Academic Press, 2022,
<https://arxiv.org/pdf/2001.10488>.
[HEAVY_TAIL_3]
Ehrenberg, A., "A Primer in Data Reduction: An
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Acknowledgements
Many thanks to Spencer Dawkins, Rohit Abhishek, Jake Holland, Kiran
Makhijani, Ali Begen, Cullen Jennings, Stephan Wenger, Eric Vyncke,
Wesley Eddy, Paul Kyzivat, Jim Guichard, Roman Danyliw, Warren
Kumari, and Zaheduzzaman Sarker for providing helpful feedback,
suggestions, and comments.
Authors' Addresses
Renan Krishna
United Kingdom
Email: renan.krishna@gmail.com
Akbar Rahman
Ericsson
349 Terry Fox Drive
Ottawa Ontario K2K 2V6
Canada
Email: Akbar.Rahman@ericsson.com