MOPS

Internet Engineering Task Force (IETF)                        R. Krishna
Internet-Draft
Intended status:
Request for Comments: 9699
Category: Informational                                        A. Rahman
Expires: 21 December 2024
ISSN: 2070-1721                                                 Ericsson
                                                            19 June
                                                           December 2024

 Media Operations Use Case for an Extended Reality Application on Edge
                        Computing Infrastructure
                     draft-ietf-mops-ar-use-case-18

Abstract

   This document explores the issues involved in the use of Edge
   Computing resources to operationalize media use cases that involve
   Extended Reality (XR) applications.  In particular, this document
   discusses those XR applications that run on devices having different form
   factors (such as different physical sizes and shapes) and need Edge
   computing resources to mitigate the effect of problems such as a the
   need to support interactive communication requiring low latency,
   limited battery power, and heat dissipation from those devices.  The
   intended audience for this document are network
   Network operators who are interested in providing edge computing
   resources to operationalize the requirements of such applications. applications are
   the intended audience for this document.  This document also
   discusses the expected behavior of XR applications applications, which can be used
   to manage the
   traffic.  In addition, the document discusses traffic, and the service requirements of for XR applications
   to be able to run on the network.

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   This Internet-Draft will expire on 21 December 2024.
   https://www.rfc-editor.org/info/rfc9699.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Use Case  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Processing of Scenes  . . . . . . . . . . . . . . . . . .   5
     2.2.  Generation of Images  . . . . . . . . . . . . . . . . . .   6
   3.  Technical Challenges and Solutions  . . . . . . . . . . . . .   6
   4.  XR Network Traffic  . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Traffic Workload  . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Traffic Performance Metrics . . . . . . . . . . . . . . .   9
   5.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Acknowledgements
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Extended Reality (XR) is a term that includes Augmented Reality (AR),
   Virtual Reality (VR) (VR), and Mixed Reality (MR) [XR].  AR combines the
   real and virtual, is interactive 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
   [AUGMENTED].  MR merges the real and virtual world 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 will bring have several requirements for the network and the
   mobile devices running these applications.  Some XR applications such
   as AR require a real-time processing of video streams to recognize
   specific objects.  This is then used to overlay information on the
   video being displayed to the user.  In addition, XR applications such
   as AR and VR will 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_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 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_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 follows following can be deployed:

   *  Disaggregation (using SDN

   Disaggregation:  Using Software-Defined Networking (SDN) to break
      vertically integrated systems into independent components- these components.  These
      components can have open interfaces which that are standard, well documented
      documented, and not
      proprietary),

   *  Virtualization (being non-proprietary.

   Virtualization:  Being able to run multiple independent copies of
      those components components, such as SDN Controller apps, applications and Virtual
      Network
      Functions Functions, on a common hardware platform).

   *  Commoditization (being platform.

   Commoditization:  Being able to elastically scale those virtual
      components across commodity hardware as the workload dictates). dictates.

   Such techniques enable XR applications requiring low-latency 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
   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.  A network operator for  For the purposes purpose of this document document, a network operator is any
   organization or individual that manages or operates the compute computing
   resources or storage in close network proximity to a mobile device or
   sensors.
   sensor.  Examples of form factors include Head Mounted Displays
   (HMD) head-mounted displays
   (HMDs), such as Optical-see through optical see-through HMDs and video-see-through HMDs video see-through HMDs,
   and
   Hand-held hand-held displays.  Smart phones  Smartphones with video cameras and location location-
   sensing capabilities using systems such as a global navigation
   satellite system (GNSS) are another example of such devices.  These
   devices have limited battery capacity and dissipate heat when
   running.  Besides  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 fluctuates, and the communication link itself might fail.
   As a result, algorithms such as those based on adaptive-bit-
   rate 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 the current video applications.  In particular particular, several
   Quality of Experience
   Quality-of-Experience (QoE) factors such as motion sickness are
   unique to XR applications and must be considered when
   operationalizing a network.  This document motivates these issues
   with a use-case use case that is presented in the following sections. section.

2.  Use Case

   A

   This use case is now described that involves an application with characteristics of an XR
   systems' characteristics.
   system.  Consider a group of tourists who are being
   conducted in 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 then continuously updates their view as they move around.

   The XR application first processes the scene that the walking tourist
   is watching in real-time 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. real time.  These generated video images are then
   overlaid on the view of the real-world real world as seen by the tourist.

   This processing of scenes and generation of high-resolution images is
   now
   are discussed in greater detail. detail below.

2.1.  Processing of Scenes

   The task of processing a scene can be broken down into a pipeline of
   three consecutive subtasks namely subtasks: tracking, followed by an acquisition of a model of the
   real world, and finally 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 eyes, and the objects that are in view [AUGMENTED].  This
      requires tracking natural features (for example 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) based model that is then stored in a database.  To ensure that
      this database can be scaled up, techniques such as combining a
      client-side simultaneous tracking and mapping and a with server-side
      localization are used to construct a model of the real world [SLAM_1], [SLAM_2], [SLAM_3],
      [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 shape, which is expressed
      as a collection of small flat surfaces that are polygons.  In
      texture mapping, color patterns are mapped on to onto an object's
      surface.  A third modelling 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 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 and other; this
      process is called registration "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 the
      real objects [OCCL_1], [OCCL_1] [OCCL_2].  The XR application also applies
      photometric registration [PHOTO_REG] by aligning the 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_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]
      [NOISE], etc. are also required.

2.2.  Generation of Images

   The XR application must generate a high-quality video that has the
   properties described in the previous step and overlay the video on
   the XR device's display- a display.  This step is called situated visualization. "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 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 that are computationally intensive. content.  This
   section will
   discuss discusses the challenges such applications can face as a
   consequence.

   As a result of performing computationally intensive tasks on XR
   devices such as XR glasses, excessive heat is generated by the chip-
   sets
   chipsets that are involved in the computation [DEV_HEAT_1], [DEV_HEAT_1]
   [DEV_HEAT_2].  Additionally, the battery on such devices discharges
   quickly when running such applications [BATT_DRAIN].

   A solution to the problem of heat dissipation and battery drainage problem 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 applications, such
   as 360-degree streaming (streaming of video that represents a view in
   every direction in 3D space) 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 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. "motion-to-photon delay".  Studies have
   shown [PER_SENSE], [XR], [OCCL_3] that this delay can be at most 20ms 20 ms and preferably between 7-15ms
   7-15 ms in order to avoid the motion sickness
   problem. [PER_SENSE] [XR] [OCCL_3].
   Out of these 20ms, 20 ms, display techniques including the refresh rate of
   write displays and pixel switching take 12-13ms [OCCL_3], 12-13 ms [OCCL_3] [CLOUD].
   This leaves 7-8ms 7-8 ms for the processing of motion sensor inputs,
   graphic rendering, and round-trip-time 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 indicates an Ultra
   Reliable Ultra-Reliable Low
   Latency of 0.1ms 0.1 to 1ms 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.  So, for cloud.  For example,
   a sudden surge in demand from a large group of tourists can overwhelm that
   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 Adaptive Bit Rate (ABR) ABR algorithms that choose bit-rates bitrate policies tailored in a fine-grained fine-
   grained manner to the resource demands and playback 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
   makes prediction-based adaptation by ABR algorithms sub-optimal
   [ABR_2].  This is because with such distributions, the law of large
   numbers (how long does it take takes for the sample mean to stabilize) works
   too slowly
   [HEAVY_TAIL_2], [HEAVY_TAIL_2] and the mean of sample does not equal the
   mean of distribution [HEAVY_TAIL_2], and [HEAVY_TAIL_2]; as a result 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] where the (the
   longer the wait for an event, the longer a further need to wait wait) and
   the issue of mismatch between the size and count of events [HEAVY_TAIL_1].
   This makes designing an algorithm for adaptation error-prone and
   challenging.  Such operational parameters include but are not limited
   to buffer occupancy, throughput, client-server latency, and variable
   transmission times.  In addition, edge devices and communication
   links may fail 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 earlier, 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 invocation, 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 are themselves 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, it can
   be seen from the
   table indicates that it will require 200 to 1000Mbps 1000 Mbps of bandwidth.  As seen from the table,
   Also, the table shows that XR application applications, such as the one in our
   use case case, transmit a larger amount of data per unit time as compared
   to traditional video applications.  As a result, issues arising out
   of from
   heavy-tailed parameters parameters, such as long-range dependent traffic
   [METRICS_2],
   [METRICS_2] and self-similar traffic [METRICS_3], would be
   experienced at time scales timescales of milliseconds and microseconds rather
   than hours or seconds.  Additionally, burstiness at the time scale 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 bursts, and various traffic parameters from widely
   separated time times can show correlation [HEAVY_TAIL_1].  Self-similar
   traffic contains bursts at a wide range of time scales timescales [HEAVY_TAIL_1].
   Multi-fractal spectrum bursts for traffic summarizes summarize the statistical
   distribution of local scaling exponents found in a traffic trace
   [HEAVY_TAIL_1].  The operational consequences consequence of XR traffic having
   characteristics such as long-range dependency, 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_2] [METRICS_3].
   In addition, multi-fractal spectrum burstiness at the scale of milli-seconds
   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 "managed edge cloud service'
   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 support, and energy management [METRICS_6].  Providing Edge
   server support for the techniques being developed at the DETNET
   Working Group at in the IETF [RFC8939], [RFC9023], [RFC8939] [RFC9023] [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 the network operators could would be to deploy equipment that
   supports differentiated services [RFC2475] or per-connection quality-
   of-service Quality-
   of-Service (QoS) guarantees [RFC2210].

      +===============================================+============+
      | Application                                   | Throughput |
      |                                               | Required   |
      +===============================================+============+
      | Real-world objects annotated with text and    | 1 Mbps     |
      | images for workflow assistance (e.g. (e.g., repair) |            |
      +-----------------------------------------------+------------+
      | Video Conferencing conferencing                            | 2 Mbps     |
      +-----------------------------------------------+------------+
      | 3D Model model and Data Visualization data visualization               | 2 to 20    |
      |                                               | Mbps       |
      +-----------------------------------------------+------------+
      | Two-way 3D Telepresence telepresence                       | 5 to 25    |
      |                                               | Mbps       |
      +-----------------------------------------------+------------+
      | Current-Gen 360-degree video (4K)             | 10 to 50   |
      |                                               | Mbps       |
      +-----------------------------------------------+------------+
      | Next-Gen 360-degree video (8K, 90+ Frames- frames per | 50 to 200  |
      | per-second, High Dynamic Range, Stereoscopic) second, high dynamic range, stereoscopic)     | Mbps       |
      +-----------------------------------------------+------------+
      | 6 Degree of Freedom Video 6DoF video or Point Cloud point cloud                     | 200 to     |
      |                                               | 1000 Mbps  |
      +-----------------------------------------------+------------+

           Table 1: Throughput requirement Requirements for streaming video
                               applications Streaming Video
                               Applications

   Thus, the provisioning of edge servers in (in terms of the number of
   servers, the topology, where to place them, the placement of servers, the assignment of
   link capacity, CPUs CPUs, and GPUs Graphics Processing Units (GPUs)) should keep be
   performed with the above factors in mind.

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 now discussed. 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.:
   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 time
   scale timescale
   of a round-trip-time an RTT propagation, and this must be considered when
   operationalizing a network.

   The following

   Table 2 [METRICS_6] shows a taxonomy of applications with their
   associated required response times and bandwidths.  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 round-trip time 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 on for the sum of the time taken by
   computational tasks such (such as processing of scenes, scenes and generation of images
   images) and the round-trip time. RTT.  This response time depends only on the Quality of Service (QOS) 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 20ms 20 ms at most and preferably
   between 7-15ms 7-15 ms, as discussed earlier.  This requirement for response
   time is similar to the first two entries of in Table 2 below. 2.  Additionally,
   the required bandwidth for our use case as discussed in
   section 5.1, Table 1, is 200Mbps-1000Mbps. 200 to 1000 Mbps (see
   Section 4.1).  Since our use case envisages multiple users running
   the XR applications application on their
   devices, devices and connected connecting to an 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 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    | Theme Parks, Guidance in theme   |
   | localized outdoor | milliseconds | 200 Mbps | Shopping Malls, parks, shopping     |
   | navigation        |              |          | Archaeological malls,              |
   |                   |              |          | archaeological      |
   |                   |              | Sites, Museum          | sites, and          |
   |                   |              |          | guidance museums             |
   +-------------------+--------------+----------+---------------------+
   | Cloud-based       | Less than 50 | 50 to    | Google Live View,   |
   | Mobile 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 compute
   computing resources or storage to the mobile devices running the XR
   application.  This is required to ensure a response time of 20ms 20 ms at
   most and preferably between 7-15ms. 7-15 ms.  Additionally, a bandwidth of
   200 to 1000Mbps 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
   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 other
   streaming applications [DIST], [NIST1], [CWE], [DIST] [NIST1] [CWE] [NIST2].  This document itself introduces no
   does not introduce any new security issues.

9.

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Acknowledgements

   Many Thanks 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 very helpful feedback,
   suggestions
   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