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Operational Considerations for Streaming Media
draft-ietf-mops-streaming-opcons-10

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draft-ietf-mops-streaming-opcons-10
MOPS                                                          J. Holland
Internet-Draft                                 Akamai Technologies, Inc.
Intended status: Informational                                  A. Begen
Expires: 23 October 2022                                 Networked Media
                                                              S. Dawkins
                                                     Tencent America LLC
                                                           21 April 2022

             Operational Considerations for Streaming Media
                  draft-ietf-mops-streaming-opcons-10

Abstract

   This document provides an overview of operational networking issues
   that pertain to quality of experience when streaming video and other
   high-bitrate media over the Internet.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   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."

   This Internet-Draft will expire on 23 October 2022.

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/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notes for Contributors and Reviewers  . . . . . . . . . .   4
       1.1.1.  Venues for Contribution and Discussion  . . . . . . .   4
   2.  Our Focus on Streaming Video  . . . . . . . . . . . . . . . .   5
   3.  Bandwidth Provisioning  . . . . . . . . . . . . . . . . . . .   6
     3.1.  Scaling Requirements for Media Delivery . . . . . . . . .   6
       3.1.1.  Video Bitrates  . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  Virtual Reality Bitrates  . . . . . . . . . . . . . .   6
     3.2.  Path Bandwidth Constraints  . . . . . . . . . . . . . . .   7
       3.2.1.  Recognizing Changes from an Expected Baseline . . . .   8
     3.3.  Path Requirements . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Caching Systems . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Predictable Usage Profiles  . . . . . . . . . . . . . . .  11
     3.6.  Unpredictable Usage Profiles  . . . . . . . . . . . . . .  11
     3.7.  Extremely Unpredictable Usage Profiles  . . . . . . . . .  12
   4.  Latency Considerations  . . . . . . . . . . . . . . . . . . .  14
     4.1.  Ultra Low-Latency . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Low-Latency Live  . . . . . . . . . . . . . . . . . . . .  15
     4.3.  Non-Low-Latency Live  . . . . . . . . . . . . . . . . . .  16
     4.4.  On-Demand . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Adaptive Encoding, Adaptive Delivery, and Measurement
           Collection  . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  Adaptive Encoding . . . . . . . . . . . . . . . . . . . .  18
     5.3.  Adaptive Segmented Delivery . . . . . . . . . . . . . . .  18
     5.4.  Advertising . . . . . . . . . . . . . . . . . . . . . . .  18
     5.5.  Bitrate Detection Challenges  . . . . . . . . . . . . . .  20
       5.5.1.  Idle Time between Segments  . . . . . . . . . . . . .  21
       5.5.2.  Head-of-Line Blocking . . . . . . . . . . . . . . . .  21
       5.5.3.  Wide and Rapid Variation in Path Capacity . . . . . .  22
     5.6.  Measurement Collection  . . . . . . . . . . . . . . . . .  22
   6.  Evolution of Transport Protocols and Transport Protocol
           Behaviors . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.1.  UDP and Its Behavior  . . . . . . . . . . . . . . . . . .  24
     6.2.  TCP and Its Behavior  . . . . . . . . . . . . . . . . . .  25
     6.3.  QUIC and Its Behavior . . . . . . . . . . . . . . . . . .  26
   7.  Streaming Encrypted Media . . . . . . . . . . . . . . . . . .  28
     7.1.  General Considerations for Media Encryption . . . . . . .  29
     7.2.  Considerations for "Hop-by-Hop" Media Encryption  . . . .  30
     7.3.  Considerations for "End-to-End" Media Encryption  . . . .  32
   8.  Further Reading and References  . . . . . . . . . . . . . . .  32
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  33
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  33
   12. Informative References  . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

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1.  Introduction

   This document examines networking and transport protocol issues as
   they relate to quality of experience (QOE) in Internet media
   delivery, especially focusing on capturing characteristics of
   streaming video delivery that have surprised network designers or
   transport experts who lack specific video expertise, since streaming
   media highlights key differences between common assumptions in
   existing networking practices and observations of video delivery
   issues encountered when streaming media over those existing networks.

   This document specifically focuses on streaming applications and
   defines streaming as follows:

   *  Streaming is transmission of a continuous media from a server to a
      client and its simultaneous consumption by the client.

   *  Here, "continuous media" refers to media and associated streams
      such as video, audio, metadata, etc.  In this definition, the
      critical term is "simultaneous", as it is not considered streaming
      if one downloads a video file and plays it after the download is
      completed, which would be called download-and-play.

   This has two implications.

   *  First, the server's transmission rate must (loosely or tightly)
      match to client's consumption rate in order to provide
      uninterrupted playback.  That is, the client must not run out of
      data (buffer underrun) or accept more data than it can buffer
      before playback (buffer overrun) as any excess media that cannot
      be buffered is simply discarded.

   *  Second, the client's consumption rate is limited not only by
      bandwidth availability,but also media availability.  The client
      cannot fetch media that is not available from a server yet.

   This document contains

   *  A short description of streaming video characteristics in
      Section 2, to set the stage for the rest of the document,

   *  General guidance on bandwidth provisioning (Section 3) and latency
      considerations (Section 4) for streaming video delivery,

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   *  A description of adaptive encoding and adaptive delivery
      techniques in common use for streaming video, along with a
      description of the challenges media senders face in detecting the
      bitrate available between the media sender and media receiver, and
      collection of measurements by a third party for use in analytics
      (Section 5),

   *  A description of existing transport protocols used for video
      streaming, and the issues encountered when using those protocols,
      along with a description of the QUIC transport protocol [RFC9000]
      that we expect to be used for streaming media (Section 6),

   *  A description of implications when streaming encrypted media
      (Section 7), and

   *  A number of useful pointers for further reading on this rapidly
      changing subject (Section 8).

   Making specific recommendations on operational practices aimed at
   mitigating the issues described in this document is out of scope,
   though some existing mitigations are mentioned in passing.  The
   intent is to provide a point of reference for future solution
   proposals to use in describing how new technologies address or avoid
   existing observed problems.

1.1.  Notes for Contributors and Reviewers

   Note to RFC Editor: Please remove this section and its subsections
   before publication.

   This section is to provide references to make it easier to review the
   development and discussion on the draft so far.

1.1.1.  Venues for Contribution and Discussion

   This document is in the Github repository at:

   https://github.com/ietf-wg-mops/draft-ietf-mops-streaming-opcons
   (https://github.com/ietf-wg-mops/draft-ietf-mops-streaming-opcons)

   Readers are welcome to open issues and send pull requests for this
   document.

   Substantial discussion of this document should take place on the MOPS
   working group mailing list (mops@ietf.org).

   *  Join: https://www.ietf.org/mailman/listinfo/mops
      (https://www.ietf.org/mailman/listinfo/mops)

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   *  Search: https://mailarchive.ietf.org/arch/browse/mops/
      (https://mailarchive.ietf.org/arch/browse/mops/)

2.  Our Focus on Streaming Video

   As the internet has grown, an increasingly large share of the traffic
   delivered to end users has become video.  The most recent available
   estimates found that 75% of the total traffic to end users was video
   in 2019.  At that time, the share of traffic that was video had been
   growing for years and was projected to continue growing (Appendix D
   of [CVNI]).

   A substantial part of this growth is due to increased use of
   streaming video, although the amount of video traffic in real-time
   communications (for example, online videoconferencing) has also grown
   significantly.  While both streaming video and videoconferencing have
   real-time delivery and latency requirements, these requirements vary
   from one application to another.  For additional discussion of
   latency requirements, see Section 4.

   In many contexts, video traffic can be handled transparently as
   generic application-level traffic.  However, as the volume of video
   traffic continues to grow, it is becoming increasingly important to
   consider the effects of network design decisions on application-level
   performance, with considerations for the impact on video delivery.

   Much of the focus of this document is on reliable media using HTTP.
   HTTP is widely used because

   *  support for HTTP is widely available in a wide range of operating
      systems,

   *  HTTP is also used in a wide variety of other applications,

   *  HTTP has been demonstrated to provide acceptable performance over
      the open Internet,

   *  HTTP includes state of the art standardized security mechanisms,
      and

   *  HTTP can make use of already-deployed caching infrastructure such
      as CDNs (Content Delivery Networks), local proxies, and browser
      caches.

   Various HTTP versions have been used for media delivery.  HTTP/1.0,
   HTTP/1.1 and HTTP/2 are carried over TCP, and TCP's transport
   behavior is described in Section 6.2.  HTTP/3 is carried over QUIC,
   and QUIC's transport behavior is described in Section 6.3.

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   Unreliable media delivery using RTP and other UDP-based protocols is
   also discussed in Section 4.1, Section 6.1, and Section 7.2, but it
   is difficult to give general guidance for these applications.  For
   instance, when loss occurs, the most appropriate response may depend
   on the type of codec being used.

3.  Bandwidth Provisioning

3.1.  Scaling Requirements for Media Delivery

3.1.1.  Video Bitrates

   Video bitrate selection depends on many variables including the
   resolution (height and width), frame rate, color depth, codec,
   encoding parameters, scene complexity and amount of motion.
   Generally speaking, as the resolution, frame rate, color depth, scene
   complexity and amount of motion increase, the encoding bitrate
   increases.  As newer codecs with better compression tools are used,
   the encoding bitrate decreases.  Similarly, a multi-pass encoding
   generally produces better quality output compared to single-pass
   encoding at the same bitrate, or delivers the same quality at a lower
   bitrate.

   Here are a few common resolutions used for video content, with
   typical ranges of bitrates for the two most popular video codecs
   [Encodings].

         +============+================+============+============+
         | Name       | Width x Height | H.264      | H.265      |
         +============+================+============+============+
         | DVD        | 720 x 480      | 1.0 Mbps   | 0.5 Mbps   |
         +------------+----------------+------------+------------+
         | 720p (1K)  | 1280 x 720     | 3-4.5 Mbps | 2-4 Mbps   |
         +------------+----------------+------------+------------+
         | 1080p (2K) | 1920 x 1080    | 6-8 Mbps   | 4.5-7 Mbps |
         +------------+----------------+------------+------------+
         | 2160p (4k) | 3840 x 2160    | N/A        | 10-20 Mbps |
         +------------+----------------+------------+------------+

                                  Table 1

3.1.2.  Virtual Reality Bitrates

   The bitrates given in Section 3.1.1 describe video streams that
   provide the user with a single, fixed, point of view - so, the user
   has no "degrees of freedom", and the user sees all of the video image
   that is available.

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   Even basic virtual reality (360-degree) videos that allow users to
   look around freely (referred to as "three degrees of freedom", or
   3DoF) require substantially larger bitrates when they are captured
   and encoded as such videos require multiple fields of view of the
   scene.  Yet, due to smart delivery methods such as viewport-based or
   tiled-based streaming, we do not need to send the whole scene to the
   user.  Instead, the user needs only the portion corresponding to its
   viewpoint at any given time ([Survey360o]).

   In more immersive applications, where limited user movement ("three
   degrees of freedom plus", or 3DoF+) or full user movement ("six
   degrees of freedom", or 6DoF) is allowed, the required bitrate grows
   even further.  In this case, immersive content is typically referred
   to as volumetric media.  One way to represent the volumetric media is
   to use point clouds, where streaming a single object may easily
   require a bitrate of 30 Mbps or higher.  Refer to [MPEGI] and [PCC]
   for more details.

3.2.  Path Bandwidth Constraints

   Even when the bandwidth requirements for video streams along a path
   are well understood, additional analysis is required to understand
   the contraints on bandwidth at various points in the network.  This
   analysis is necessary because media servers may react to bandwith
   constraints using two independent feedback loops:

   *  Media servers often respond to application-level feedback from the
      media player that indicates a bottleneck link somewhere along the
      path, by adjusting the amount of media that the media server will
      send to the media player in a given timeframe.  This is described
      in greater detail in Section 5.

   *  Media servers also typically implement transport protocols with
      capacity-seeking congestion controllers that probe for bandwidth,
      and adjust the sending rate based on transport mechanisms.  This
      is described in greater detail in Section 6.

   The result is that these two (potentially competing) "helpful"
   mechanisms each respond to the same bottleneck with no coordination
   between themselves, so that each is unaware of actions taken by the
   other, and this can result in QOE for users that is significantly
   lower than what could have been achieved.

   In one example, if a media server overestimates the available
   bandwidth to the media player,

   *  the transport protocol detects loss due to congestion, and reduces
      its sending window size per round trip,

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   *  the media server adapts to application-level feedback from the
      media player, and reduces its own sending rate,

   *  the transport protocol sends media at the new, lower rate, and
      confirms that this new, lower rate is "safe", because no
      transport-level loss is occuring, but

   *  because the media server continues to send at the new, lower rate,
      the transport protocol's maximum sending rate is now limited by
      the amount of information the media server queues for
      transmission, so

   *  the transport protocol can't probe for available path bandwidth by
      sending at a higher rate.

   In order to avoid these types of situations, which can potentially
   affect all the users whose streaming media traverses a bottleneck
   link, there are several possible mitigations that streaming operators
   can use, but the first step toward mitigating a problem is knowing
   when that problem occurs.

3.2.1.  Recognizing Changes from an Expected Baseline

   There are many reasons why path characteristics might change
   suddenly, for example,

   *  "cross traffic" that traverses part of the path, especially if
      this traffic is "inelastic", and does not, itself, respond to
      indications of path congestion.

   *  routing changes, which can happen in normal operation, especially
      if the new path now includes path segments that are more heavily
      loaded, offer lower total bandwidth, or simply cover more
      distance.

   In order to recognize that a path carrying streaming media is "not
   behaving the way it normally does", having an expected baseline that
   describes "the way it normally does" is fundamental.  Analytics that
   aid in that recognition can be more or less sophisticated, and can be
   as simple as noticing that the apparent round trip times for media
   traffic carried over TCP transport on some paths are suddenly and
   significantly longer than usual.  Passive monitors can detect changes
   in the elapsed time between the acknowledgements for specific TCP
   segments from a TCP receiver, since TCP octet sequence numbers and
   acknowledgements for those sequence numbers are "carried in the
   clear", even if the TCP payload itself is encrypted.  See Section 6.2
   for more information.

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   As transport protocols evolve to encrypt their transport header
   fields, one side effect of increasing encryption is that the kind of
   passive monitoring, or even "performance enhancement" ([RFC3135])
   that was possible with the older transport protocols (UDP, described
   in Section 6.1 and TCP, described in Section 6.2) is no longer
   possible with newer transport protocols such as QUIC (described in
   Section 6.3).  The IETF has specified a "latency spin bit" mechanism
   in Section 17.4 of [RFC9000] to allow passive latency monitoring from
   observation points on the network path throughout the duration of a
   connection, but currently chartered work in the IETF is focusing on
   end-point monitoring and reporting, rather than on passive
   monitoring.

   One example is the "qlog" mechanism [I-D.ietf-quic-qlog-main-schema],
   a protocol-agnostic mechanism used to provide better visibility for
   encrypted protocols such as QUIC ([I-D.ietf-quic-qlog-quic-events])
   and for HTTP/3 ([I-D.ietf-quic-qlog-h3-events]).

3.3.  Path Requirements

   The bitrate requirements in Section 3.1 are per end-user actively
   consuming a media feed, so in the worst case, the bitrate demands can
   be multiplied by the number of simultaneous users to find the
   bandwidth requirements for a router on the delivery path with that
   number of users downstream.  For example, at a node with 10,000
   downstream users simultaneously consuming video streams,
   approximately 80 Gbps might be necessary in order for all of them to
   get typical content at 1080p resolution.

   However, when there is some overlap in the feeds being consumed by
   end users, it is sometimes possible to reduce the bandwidth
   provisioning requirements for the network by performing some kind of
   replication within the network.  This can be achieved via object
   caching with delivery of replicated objects over individual
   connections, and/or by packet-level replication using multicast.

   To the extent that replication of popular content can be performed,
   bandwidth requirements at peering or ingest points can be reduced to
   as low as a per-feed requirement instead of a per-user requirement.

3.4.  Caching Systems

   When demand for content is relatively predictable, and especially
   when that content is relatively static, caching content close to
   requesters, and pre-loading caches to respond quickly to initial
   requests is often useful (for example, HTTP/1.1 caching is described
   in [I-D.ietf-httpbis-cache]).  This is subject to the usual
   considerations for caching - for example, how much data must be

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   cached to make a significant difference to the requester, and how the
   benefits of caching and pre-loading caches balances against the costs
   of tracking "stale" content in caches and refreshing that content.

   It is worth noting that not all high-demand content is "live"
   content.  One relevant example is when popular streaming content can
   be staged close to a significant number of requesters, as can happen
   when a new episode of a popular show is released.  This content may
   be largely stable, so low-cost to maintain in multiple places
   throughout the Internet.  This can reduce demands for high end-to-end
   bandwidth without having to use mechanisms like multicast.

   Caching and pre-loading can also reduce exposure to peering point
   congestion, since less traffic crosses the peering point exchanges if
   the caches are placed in peer networks, especially when the content
   can be pre-loaded during off-peak hours, and especially if the
   transfer can make use of "Lower-Effort Per-Hop Behavior (LE PHB) for
   Differentiated Services" [RFC8622], "Low Extra Delay Background
   Transport (LEDBAT)" [RFC6817], or similar mechanisms.

   All of this depends, of course, on the ability of a content provider
   to predict usage and provision bandwidth, caching, and other
   mechanisms to meet the needs of users.  In some cases (Section 3.5),
   this is relatively routine, but in other cases, it is more difficult
   (Section 3.6, Section 3.7).

   And as with other parts of the ecosystem, new technology brings new
   challenges.  For example, with the emergence of ultra-low-latency
   streaming, responses have to start streaming to the end user while
   still being transmitted to the cache, and while the cache does not
   yet know the size of the object.  Some of the popular caching systems
   were designed around cache footprint and had deeply ingrained
   assumptions about knowing the size of objects that are being stored,
   so the change in design requirements in long-established systems
   caused some errors in production.  Incidents occurred where a
   transmission error in the connection from the upstream source to the
   cache could result in the cache holding a truncated segment and
   transmitting it to the end user's device.  In this case, players
   rendering the stream often had the video freeze until the player was
   reset.  In some cases the truncated object was even cached that way
   and served later to other players as well, causing continued stalls
   at the same spot in the video for all players playing the segment
   delivered from that cache node.

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3.5.  Predictable Usage Profiles

   Historical data shows that users consume more videos and at a higher
   bit rate than they did in the past on their connected devices.
   Improvements in the codecs that help with reducing the encoding
   bitrates with better compression algorithms could not have offset the
   increase in the demand for the higher quality video (higher
   resolution, higher frame rate, better color gamut, better dynamic
   range, etc.).  In particular, mobile data usage has shown a large
   jump over the years due to increased consumption of entertainment as
   well as conversational video.

3.6.  Unpredictable Usage Profiles

   Although TCP/IP has been used with a number of widely used
   applications that have symmetric bandwidth requirements (similar
   bandwidth requirements in each direction between endpoints), many
   widely-used Internet applications operate in client-server roles,
   with asymmetric bandwidth requirements.  A common example might be an
   HTTP GET operation, where a client sends a relatively small HTTP GET
   request for a resource to an HTTP server, and often receives a
   significantly larger response carrying the requested resource.  When
   HTTP is commonly used to stream movie-length video, the ratio between
   response size and request size can become arbitrarily large.

   For this reason, operators may pay more attention to downstream
   bandwidth utilization when planning and managing capacity.  In
   addition, operators have been able to deploy access networks for end
   users using underlying technologies that are inherently asymmetric,
   favoring downstream bandwidth (e.g.  ADSL, cellular technologies,
   most IEEE 802.11 variants), assuming that users will need less
   upstream bandwidth than downstream bandwidth.  This strategy usually
   works, except when it fails because application bandwidth usage
   patterns have changed in ways that were not predicted.

   One example of this type of change was when peer-to-peer file sharing
   applications gained popularity in the early 2000s.  To take one well-
   documented case ([RFC5594]), the Bittorrent application created
   "swarms" of hosts, uploading and downloading files to each other,
   rather than communicating with a server.  Bittorrent favored peers
   who uploaded as much as they downloaded, so that new Bittorrent users
   had an incentive to significantly increase their upstream bandwidth
   utilization.

   The combination of the large volume of "torrents" and the peer-to-
   peer characteristic of swarm transfers meant that end user hosts were
   suddenly uploading higher volumes of traffic to more destinations
   than was the case before Bittorrent.  This caused at least one large

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   Internet service provider (ISP) to attempt to "throttle" these
   transfers in order to to mitigate the load that these hosts placed on
   their network.  These efforts were met by increased use of encryption
   in Bittorrent, and complaints to regulators calling for regulatory
   action.

   The BitTorrent case study is just one example, but the example is
   included here to make it clear that unpredicted and unpredictable
   massive traffic spikes may not be the result of natural disasters,
   but they can still have significant impacts.

   Especially as end users increase use of video-based social networking
   applications, it will be helpful for access network providers to
   watch for increasing numbers of end users uploading significant
   amounts of content.

3.7.  Extremely Unpredictable Usage Profiles

   The causes of unpredictable usage described in Section 3.6 were more
   or less the result of human choices, but we were reminded during a
   post-IETF 107 meeting that humans are not always in control, and
   forces of nature can cause enormous fluctuations in traffic patterns.

   In his talk, Sanjay Mishra [Mishra] reported that after the CoViD-19
   pandemic broke out in early 2020,

   *  Comcast's streaming and web video consumption rose by 38%, with
      their reported peak traffic up 32% overall between March 1 to
      March 30,

   *  AT&T reported a 28% jump in core network traffic (single day in
      April, as compared to pre stay-at-home daily average traffic),
      with video accounting for nearly half of all mobile network
      traffic, while social networking and web browsing remained the
      highest percentage (almost a quarter each) of overall mobility
      traffic, and

   *  Verizon reported similar trends with video traffic up 36% over an
      average day (pre COVID-19)}.

   We note that other operators saw similar spikes during this time
   period.  Craig Labowitz [Labovitz] reported

   *  Weekday peak traffic increases over 45%-50% from pre-lockdown
      levels,

   *  A 30% increase in upstream traffic over their pre-pandemic levels,
      and

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   *  A steady increase in the overall volume of DDoS traffic, with
      amounts exceeding the pre-pandemic levels by 40%. (He attributed
      this increase to the significant rise in gaming-related DDoS
      attacks ([LabovitzDDoS]), as gaming usage also increased.)

   Subsequently, the Internet Architecture Board (IAB) held a COVID-19
   Network Impacts Workshop [IABcovid] in November 2020.  Given a larger
   number of reports and more time to reflect, the following
   observations from the draft workshop report are worth considering.

   *  Participants describing different types of networks reported
      different kinds of impacts, but all types of networks saw impacts.

   *  Mobile networks saw traffic reductions and residential networks
      saw significant increases.

   *  Reported traffic increases from ISPs and Internet Exchange Points
      (IXP) over just a few weeks were as big as the traffic growth over
      the course of a typical year, representing a 15-20% surge in
      growth to land at a new normal that was much higher than
      anticipated.

   *  At DE-CIX Frankfurt, the world's largest Internet Exchange Point
      in terms of data throughput, the year 2020 has seen the largest
      increase in peak traffic within a single year since the IXP was
      founded in 1995.

   *  The usage pattern changed significantly as work-from-home and
      videoconferencing usage peaked during normal work hours, which
      would have typically been off-peak hours with adults at work and
      children at school.  One might expect that the peak would have had
      more impact on networks if it had happened during typical evening
      peak hours for video streaming applications.

   *  The increase in daytime bandwidth consumption reflected both
      significant increases in "essential" applications such as
      videoconferencing and virtual private networks (VPN), and
      entertainment applications as people watched videos or played
      games.

   *  At the IXP level, it was observed that physical link utilization
      increased.  This phenomenon could probably be explained by a
      higher level of uncacheable traffic such as videoconferencing and
      VPNs from residential users as they stopped commuting and switched
      to work-at-home.

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4.  Latency Considerations

   Streaming media latency refers to the "glass-to-glass" time duration,
   which is the delay between the real-life occurrence of an event and
   the streamed media being appropriately displayed on an end user's
   device.  Note that this is different from the network latency
   (defined as the time for a packet to cross a network from one end to
   another end) because it includes video encoding/decoding and
   buffering time, and for most cases also ingest to an intermediate
   service such as a CDN or other video distribution service, rather
   than a direct connection to an end user.

   Streaming media can be usefully categorized according to the
   application's latency requirements into a few rough categories:

   *  ultra low-latency (less than 1 second)

   *  low-latency live (less than 10 seconds)

   *  non-low-latency live (10 seconds to a few minutes)

   *  on-demand (hours or more)

4.1.  Ultra Low-Latency

   Ultra low-latency delivery of media is defined here as having a
   glass-to-glass delay target under one second.

   Some media content providers aim to achieve this level of latency for
   live media events.  This introduces new challenges relative to less-
   restricted levels of latency requirements because this latency is the
   same scale as commonly observed end-to-end network latency variation
   (for example, due to effects such as bufferbloat ([CoDel]), Wi-Fi
   error correction, or packet reordering).  These effects can make it
   difficult to achieve this level of latency for the general case, and
   may require tradeoffs in relatively frequent user-visible media
   artifacts.  However, for controlled environments or targeted networks
   that provide mitigations against such effects, this level of latency
   is potentially achievable with the right provisioning.

   Applications requiring ultra low latency for media delivery are
   usually tightly constrained on the available choices for media
   transport technologies, and sometimes may need to operate in
   controlled environments to reliably achieve their latency and quality
   goals.

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   Most applications operating over IP networks and requiring latency
   this low use the Real-time Transport Protocol (RTP) [RFC3550] or
   WebRTC [RFC8825], which uses RTP for the media transport as well as
   several other protocols necessary for safe operation in browsers.

   Worth noting is that many applications for ultra low-latency delivery
   do not need to scale to more than a few users at a time, which
   simplifies many delivery considerations relative to other use cases.

   Recommended reading for applications adopting an RTP-based approach
   also includes [RFC7656].  For increasing the robustness of the
   playback by implementing adaptive playout methods, refer to [RFC4733]
   and [RFC6843].

   Applications with further-specialized latency requirements are out of
   scope for this document.

4.2.  Low-Latency Live

   Low-latency live delivery of media is defined here as having a glass-
   to-glass delay target under 10 seconds.

   This level of latency is targeted to have a user experience similar
   to traditional broadcast TV delivery.  A frequently cited problem
   with failing to achieve this level of latency for live sporting
   events is the user experience failure from having crowds within
   earshot of one another who react audibly to an important play, or
   from users who learn of an event in the match via some other channel,
   for example social media, before it has happened on the screen
   showing the sporting event.

   Applications requiring low-latency live media delivery are generally
   feasible at scale with some restrictions.  This typically requires
   the use of a premium service dedicated to the delivery of live video,
   and some tradeoffs may be necessary relative to what is feasible in a
   higher latency service.  The tradeoffs may include higher costs, or
   delivering a lower quality video, or reduced flexibility for adaptive
   bitrates, or reduced flexibility for available resolutions so that
   fewer devices can receive an encoding tuned for their display.  Low-
   latency live delivery is also more susceptible to user-visible
   disruptions due to transient network conditions than higher latency
   services.

   Implementation of a low-latency live video service can be achieved
   with the use of low-latency extensions of HLS (called LL-HLS)
   [I-D.draft-pantos-hls-rfc8216bis] and DASH (called LL-DASH)
   [LL-DASH].  These extensions use the Common Media Application Format
   (CMAF) standard [MPEG-CMAF] that allows the media to be packaged into

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   and transmitted in units smaller than segments, which are called
   chunks in CMAF language.  This way, the latency can be decoupled from
   the duration of the media segments.  Without a CMAF-like packaging,
   lower latencies can only be achieved by using very short segment
   durations.  However, shorter segments means more frequent intra-coded
   frames and that is detrimental to video encoding quality.  CMAF
   allows us to still use longer segments (improving encoding quality)
   without penalizing latency.

   While a LL-HLS client retrieves each chunk with a separate HTTP GET
   request, a LL-DASH client uses the chunked transfer encoding feature
   of the HTTP [CMAF-CTE] which allows the LL-DASH client to fetch all
   the chunks belonging to a segment with a single GET request.  An HTTP
   server can transmit the CMAF chunks to the LL-DASH client as they
   arrive from the encoder/packager.  A detailed comparison of LL-HLS
   and LL-DASH is given in [MMSP20].

4.3.  Non-Low-Latency Live

   Non-low-latency live delivery of media is defined here as a live
   stream that does not have a latency target shorter than 10 seconds.

   This level of latency is the historically common case for segmented
   video delivery using HLS [RFC8216] and DASH [MPEG-DASH].  This level
   of latency is often considered adequate for content like news or pre-
   recorded content.  This level of latency is also sometimes achieved
   as a fallback state when some part of the delivery system or the
   client-side players do not have the necessary support for the
   features necessary to support low-latency live streaming.

   This level of latency can typically be achieved at scale with
   commodity CDN services for HTTP(s) delivery, and in some cases the
   increased time window can allow for production of a wider range of
   encoding options relative to the requirements for a lower latency
   service without the need for increasing the hardware footprint, which
   can allow for wider device interoperability.

4.4.  On-Demand

   On-Demand media streaming refers to playback of pre-recorded media
   based on a user's action.  In some cases on-demand media is produced
   as a by-product of a live media production, using the same segments
   as the live event, but freezing the manifest after the live event has
   finished.  In other cases, on-demand media is constructed out of pre-
   recorded assets with no streaming necessarily involved during the
   production of the on-demand content.

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   On-demand media generally is not subject to latency concerns, but
   other timing-related considerations can still be as important or even
   more important to the user experience than the same considerations
   with live events.  These considerations include the startup time, the
   stability of the media stream's playback quality, and avoidance of
   stalls and video artifacts during the playback under all but the most
   severe network conditions.

   In some applications, optimizations are available to on-demand video
   that are not always available to live events, such as pre-loading the
   first segment for a startup time that doesn't have to wait for a
   network download to begin.

5.  Adaptive Encoding, Adaptive Delivery, and Measurement Collection

5.1.  Overview

   A simple model of video playback can be described as a video stream
   consumer, a buffer, and a transport mechanism that fills the buffer.
   The consumption rate is fairly static and is represented by the
   content bitrate.  The size of the buffer is also commonly a fixed
   size.  The fill process needs to be at least fast enough to ensure
   that the buffer is never empty, however it also can have significant
   complexity when things like personalization or ad workflows are
   introduced.

   The challenges in filling the buffer in a timely way fall into two
   broad categories: 1. content selection and 2. content variation.
   Content selection comprises all of the steps needed to determine
   which content variation to offer the client.  Content variation is
   the number of content options that exist at any given selection
   point.  A common example, easily visualized, is Adaptive BitRate
   (ABR), described in more detail below.  The mechanism used to select
   the bitrate is part of the content selection, and the content
   variation are all of the different bitrate renditions.

   ABR is a sort of application-level response strategy in which the
   streaming client attempts to detect the available bandwidth of the
   network path by observing the successful application-layer download
   speed, then chooses a bitrate for each of the video, audio, subtitles
   and metadata (among the limited number of available options) that
   fits within that bandwidth, typically adjusting as changes in
   available bandwidth occur in the network or changes in capabilities
   occur during the playback (such as available memory, CPU, display
   size, etc.).

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5.2.  Adaptive Encoding

   Media servers can provide media streams at various bitrates because
   the media has been encoded at various bitrates.  This is a so-called
   "ladder" of bitrates, that can be offered to media players as part of
   the manifest that describes the media being requested by the media
   player, so that the media player can select among the available
   bitrate choices.

   The media server may also choose to alter which bitrates are made
   available to players by adding or removing bitrate options from the
   ladder delivered to the player in subsequent manifests built and sent
   to the player.  This way, both the player, through its selection of
   bitrate to request from the manifest, and the server, through its
   construction of the bitrates offered in the manifest, are able to
   affect network utilization.

5.3.  Adaptive Segmented Delivery

   ABR playback is commonly implemented by streaming clients using HLS
   [RFC8216] or DASH [MPEG-DASH] to perform a reliable segmented
   delivery of media over HTTP.  Different implementations use different
   strategies [ABRSurvey], often relying on proprietary algorithms
   (called rate adaptation or bitrate selection algorithms) to perform
   available bandwidth estimation/prediction and the bitrate selection.

   Many systems will do an initial probe or a very simple throughput
   speed test at the start of a video playback.  This is done to get a
   rough sense of the highest video bitrate in the ABR ladder that the
   network between the server and player will likely be able to provide
   under initial network conditions.  After the initial testing, clients
   tend to rely upon passive network observations and will make use of
   player side statistics such as buffer fill rates to monitor and
   respond to changing network conditions.

   The choice of bitrate occurs within the context of optimizing for one
   or more metrics monitored by the client, such as highest achievable
   video quality or lowest chances for a rebuffering event (playback
   stall).

5.4.  Advertising

   A variety of business models exist for producers of streaming media.
   Some content providers derive the majority of the revenue associated
   with streaming media directly from consumer subscriptions or one-time
   purchases.  Others derive the majority of their streaming media
   associated revenue from advertising.  Many content providers derive
   income from a mix of these and other sources of funding.  The

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   inclusion of advertising alongside or interspersed with streaming
   media content is therefore common in today's media landscape.

   Some commonly used forms of advertising can introduce potential user
   experience issues for a media stream.  This section provides a very
   brief overview of a complex and evolving space, but a complete
   coverage of the potential issues is out of scope for this document.

   The same techniques used to allow a media player to switch between
   renditions of different bitrates at segment or chunk boundaries can
   also be used to enable the dynamic insertion of advertisements
   (herafter referred to as "ads").

   Ads may be inserted either with Client Side Ad Insertion (CSAI) or
   Server Side Ad Insertion (SSAI).  In CSAI, the ABR manifest will
   generally include links to an external ad server for some segments of
   the media stream, while in SSAI the server will remain the same
   during advertisements, but will include media segments that contain
   the advertising.  In SSAI, the media segments may or may not be
   sourced from an external ad server like with CSAI.

   In general, the more targeted the ad request is, the more requests
   the ad service needs to be able to handle concurrently.  If
   connectivity is poor to the ad service, this can cause rebuffering
   even if the underlying video assets (both content and ads) are able
   to be accessed quickly.  The less targeted, the more likely the ad
   requests can be consolidated and can leverage the same caching
   techniques as the video content.

   In some cases, especially with SSAI, advertising space in a stream is
   reserved for a specific advertiser and can be integrated with the
   video so that the segments share the same encoding properties such as
   bitrate, dynamic range, and resolution.  However, in many cases ad
   servers integrate with a Supply Side Platform (SSP) that offers
   advertising space in real-time auctions via an Ad Exchange, with bids
   for the advertising space coming from Demand Side Platforms (DSPs)
   that collect money from advertisers for delivering the
   advertisements.  Most such Ad Exchanges use application-level
   protocol specifications published by the Interactive Advertising
   Bureau [IAB-ADS], an industry trade organization.

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   This ecosystem balances several competing objectives, and integrating
   with it naively can produce surprising user experience results.  For
   example, ad server provisioning and/or the bitrate of the ad segments
   might be different from that of the main video, either of which can
   sometimes result in video stalls.  For another example, since the
   inserted ads are often produced independently they might have a
   different base volume level than the main video, which can make for a
   jarring user experience.

   Additionally, this market historically has had incidents of ad fraud
   (misreporting of ad delivery to end users for financial gain).  As a
   mitigation for concerns driven by those incidents, some SSPs have
   required the use of players with features like reporting of ad
   delivery, or providing information that can be used for user
   tracking.  Some of these and other measures have raised privacy
   concerns for end users.

   In general this is a rapidly developing space with many
   considerations, and media streaming operators engaged in advertising
   may need to research these and other concerns to find solutions that
   meet their user experience, user privacy, and financial goals.  For
   further reading on mitigations, [BAP] has published some standards
   and best practices based on user experience research.

5.5.  Bitrate Detection Challenges

   This kind of bandwidth-measurement system can experience trouble in
   several ways that are affected by networking and transport protocol
   issues.  Because adaptive application-level response strategies are
   often using rates as observed by the application layer, there are
   sometimes inscrutable transport-level protocol behaviors that can
   produce surprising measurement values when the application-level
   feedback loop is interacting with a transport-level feedback loop.

   A few specific examples of surprising phenomena that affect bitrate
   detection measurements are described in the following subsections.
   As these examples will demonstrate, it is common to encounter cases
   that can deliver application level measurements that are too low, too
   high, and (possibly) correct but varying more quickly than a lab-
   tested selection algorithm might expect.

   These effects and others that cause transport behavior to diverge
   from lab modeling can sometimes have a significant impact on bitrate
   selection and on user QOE, especially where players use naive
   measurement strategies and selection algorithms that don't account
   for the likelihood of bandwidth measurements that diverge from the
   true path capacity.

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5.5.1.  Idle Time between Segments

   When the bitrate selection is chosen substantially below the
   available capacity of the network path, the response to a segment
   request will typically complete in much less absolute time than the
   duration of the requested segment, leaving significant idle time
   between segment downloads.  This can have a few surprising
   consequences:

   *  TCP slow-start when restarting after idle requires multiple RTTs
      to re-establish a throughput at the network's available capacity.
      When the active transmission time for segments is substantially
      shorter than the time between segments, leaving an idle gap
      between segments that triggers a restart of TCP slow-start, the
      estimate of the successful download speed coming from the
      application-visible receive rate on the socket can thus end up
      much lower than the actual available network capacity.  This in
      turn can prevent a shift to the most appropriate bitrate.
      [RFC7661] provides some mitigations for this effect at the TCP
      transport layer, for senders who anticipate a high incidence of
      this problem.

   *  Mobile flow-bandwidth spectrum and timing mapping can be impacted
      by idle time in some networks.  The carrier capacity assigned to a
      link can vary with activity.  Depending on the idle time
      characteristics, this can result in a lower available bitrate than
      would be achievable with a steadier transmission in the same
      network.

   Some receiver-side ABR algorithms such as [ELASTIC] are designed to
   try to avoid this effect.

   Another way to mitigate this effect is by the help of two
   simultaneous TCP connections, as explained in [MMSys11] for Microsoft
   Smooth Streaming.  In some cases, the system-level TCP slow-start
   restart can also be disabled, for example as described in
   [OReilly-HPBN].

5.5.2.  Head-of-Line Blocking

   In the event of a lost packet on a TCP connection with SACK support
   (a common case for segmented delivery in practice), loss of a packet
   can provide a confusing bandwidth signal to the receiving
   application.  Because of the sliding window in TCP, many packets may
   be accepted by the receiver without being available to the
   application until the missing packet arrives.  Upon arrival of the
   one missing packet after retransmit, the receiver will suddenly get
   access to a lot of data at the same time.

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   To a receiver measuring bytes received per unit time at the
   application layer, and interpreting it as an estimate of the
   available network bandwidth, this appears as a high jitter in the
   goodput measurement, presenting as a stall, followed by a sudden leap
   that can far exceed the actual capacity of the transport path from
   the server when the hole in the received data is filled by a later
   retransmission.

   It is worth noting that more modern transport protocols such as QUIC
   have mitigation of head-of-line blocking as a protocol design goal.
   See Section 6.3 for more details.

5.5.3.  Wide and Rapid Variation in Path Capacity

   As many end devices have moved to wireless connectivity for the final
   hop (Wi-Fi, 5G, or LTE), new problems in bandwidth detction have
   emerged from radio interference and signal strength effects.

   Each of these technologies can experience sudden changes in capacity
   as the end user device moves from place to place and encounters new
   sources of interference.  Microwave ovens, for example, can cause a
   throughput degradation of more than a factor of 2 while active
   [Micro]. 5G and LTE likewise can easily see rate variation by a
   factor of 2 or more over a span of seconds as users move around.

   These swings in actual transport capacity can result in user
   experience issues that can be exacerbated by insufficiently
   responsive ABR algorithms.

5.6.  Measurement Collection

   Media players use measurements to guide their segment-by-segment
   adaptive streaming requests, but may also provide measurements to
   streaming media providers.

   In turn, providers may base analytics on these measurements, to guide
   decisions such as whether adaptive encoding bitrates in use are the
   best ones to provide to media players, or whether current media
   content caching is providing the best experience for viewers.

   To that effect, the Consumer Technology Association (CTA) who owns
   the Web Application Video Ecosystem (WAVE) project has published two
   important specifications.

   *  CTA-2066: Streaming Quality of Experience Events, Properties and
      Metrics

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   [CTA-2066] specifies a set of media player events, properties, QOE
   metrics and associated terminology for representing streaming media
   QOE across systems, media players and analytics vendors.  While all
   these events, properties, metrics and associated terminology is used
   across a number of proprietary analytics and measurement solutions,
   they were used in slightly (or vastly) different ways that led to
   interoperability issues.  CTA-2066 attempts to address this issue by
   defining a common terminology as well as how each metric should be
   computed for consistent reporting.

   *  CTA-5004: Common Media Client Data (CMCD)

   Many assume that the CDNs have a holistic view into the health and
   performance of the streaming clients.  However, this is not the case.
   The CDNs produce millions of log lines per second across hundreds of
   thousands of clients and they have no concept of a "session" as a
   client would have, so CDNs are decoupled from the metrics the clients
   generate and report.  A CDN cannot tell which request belongs to
   which playback session, the duration of any media object, the
   bitrate, or whether any of the clients have stalled and are
   rebuffering or are about to stall and will rebuffer.  The consequence
   of this decoupling is that a CDN cannot prioritize delivery for when
   the client needs it most, prefetch content, or trigger alerts when
   the network itself may be underperforming.  One approach to couple
   the CDN to the playback sessions is for the clients to communicate
   standardized media-relevant information to the CDNs while they are
   fetching data.  [CTA-5004] was developed exactly for this purpose.

6.  Evolution of Transport Protocols and Transport Protocol Behaviors

   Because networking resources are shared between users, a good place
   to start our discussion is how contention between users, and
   mechanisms to resolve that contention in ways that are "fair" between
   users, impact streaming media users.  These topics are closely tied
   to transport protocol behaviors.

   As noted in Section 5, ABR response strategies such as HLS [RFC8216]
   or DASH [MPEG-DASH] are attempting to respond to changing path
   characteristics, and underlying transport protocols are also
   attempting to respond to changing path characteristics.

   For most of the history of the Internet, these transport protocols,
   described in Section 6.1 and Section 6.2, have had relatively
   consistent behaviors that have changed slowly, if at all, over time.
   Newly standardized transport protocols like QUIC [RFC9000] can behave
   differently from existing transport protocols, and these behaviors
   may evolve over time more rapidly than currently-used transport
   protocols.

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   For this reason, we have included a description of how the path
   characteristics that streaming media providers may see are likely to
   evolve over time.

6.1.  UDP and Its Behavior

   For most of the history of the Internet, we have trusted UDP-based
   applications to limit their impact on other users.  One of the
   strategies used was to use UDP for simple query-response application
   protocols, such as DNS, which is often used to send a single-packet
   request to look up the IP address for a DNS name, and return a
   single-packet response containing the IP address.  Although it is
   possible to saturate a path between a DNS client and DNS server with
   DNS requests, in practice, that was rare enough that DNS included few
   mechanisms to resolve contention between DNS users and other users
   (whether they are also using DNS, or using other application
   protocols that share the same pathways).

   In recent times, the usage of UDP-based applications that were not
   simple query-response protocols has grown substantially, and since
   UDP does not provide any feedback mechanism to senders to help limit
   impacts on other users, application-level protocols such as RTP
   [RFC3550] have been responsible for the decisions that TCP-based
   applications have delegated to TCP - what to send, how much to send,
   and when to send it.  Because UDP itself has no transport-layer
   feedback mechanisms, UDP-based applications that send and receive
   substantial amounts of information are expected to provide their own
   feedback mechanisms, and to respond to the feedback the application
   receives.  This expectation is most recently codified in Best Current
   Practice [RFC8085].

   In contrast to adaptive segmented delivery over a reliable tansport
   as described in Section 5.3, some applications deliver streaming
   media using an unreliable transport, and rely on a variety of
   approaches, including:

   *  raw MPEG Transport Stream ("MPEG-TS")-formatted video [MPEG-TS]
      over UDP, which makes no attempt to account for reordering or loss
      in the transport,

   *  RTP [RFC3550], which can notice loss and repair some limited
      reordering,

   *  SCTP [RFC4960], which can use partial reliability [RFC3758] to
      recover from some loss, but can abandon recovery to limit head-of-
      line blocking, and

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   *  SRT [SRT], which can use forward error correction and time-bound
      retransmission to recover from loss within certain limits, but can
      abandon recovery to limit head-of-line blocking.

   Under congestion and loss, approaches like the above generally
   experiences transient video artifacts more often and delay of
   playback effects less often, as compared with reliable segment
   transport.  Often one of the key goals of using a UDP-based transport
   that allows some unreliability is to reduce latency and better
   support applications like videoconferencing, or for other live-action
   video with interactive components, such as some sporting events.

   Congestion avoidance strategies for deployments using unreliable
   transport protocols vary widely in practice, ranging from being
   entirely unresponsive to congestion, to using feedback signaling to
   change encoder settings (as in [RFC5762]), to using fewer enhancement
   layers (as in [RFC6190]), to using proprietary methods to detect QOE
   issues and turn off video in order to allow less bandwidth-intensive
   media such as audio to be delivered.

   RTP relies on RTCP Sender and Receiver Reports [RFC3550] as its own
   feedback mechanism, and even includes Circuit Breakers for Unicast
   RTP Sessions [RFC8083] for situations when normal RTP congestion
   control has not been able to react sufficiently to RTP flows sending
   at rates that result in sustained packet loss.

   The notion of "Circuit Breakers" has also been applied to other UDP
   applications in [RFC8084], such as tunneling packets over UDP that
   are potentially not congestion-controlled (for example,
   "Encapsulating MPLS in UDP", as described in [RFC7510]).  If
   streaming media is carried in tunnels encapsulated in UDP, these
   media streams may encounter "tripped circuit breakers", with
   resulting user-visible impacts.

6.2.  TCP and Its Behavior

   For most of the history of the Internet, we have trusted TCP to limit
   the impact of applications that sent a significant number of packets,
   in either or both directions, on other users.  Although early
   versions of TCP were not particularly good at limiting this impact
   [RFC0793], the addition of Slow Start and Congestion Avoidance, as
   described in [RFC2001], were critical in allowing TCP-based
   applications to "use as much bandwidth as possible, but to avoid
   using more bandwidth than was possible".  Although dozens of RFCs
   have been written refining TCP decisions about what to send, how much
   to send, and when to send it, since 1988 [Jacobson-Karels] the
   signals available for TCP senders remained unchanged - end-to-end
   acknowledgements for packets that were successfully sent and

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   received, and packet timeouts for packets that were not.

   The success of the largely TCP-based Internet is evidence that the
   mechanisms TCP used to achieve equilibrium quickly, at a point where
   TCP senders do not interfere with other TCP senders for sustained
   periods of time, have been largely successful.  The Internet
   continued to work even when the specific mechanisms used to reach
   equilibrium changed over time.  Because TCP provides a common tool to
   avoid contention, as some TCP-based applications like FTP were
   largely replaced by other TCP-based applications like HTTP, the
   transport behavior remained consistent.

   In recent times, the TCP goal of probing for available bandwidth, and
   "backing off" when a network path is saturated, has been supplanted
   by the goal of avoiding growing queues along network paths, which
   prevent TCP senders from reacting quickly when a network path is
   saturated.  Congestion control mechanisms such as COPA [COPA18] and
   BBR [I-D.cardwell-iccrg-bbr-congestion-control] make these decisions
   based on measured path delays, assuming that if the measured path
   delay is increasing, the sender is injecting packets onto the network
   path faster than the receiver can accept them, so the sender should
   adjust its sending rate accordingly.

   Although TCP behavior has changed over time, the common practice of
   implementing TCP as part of an operating system kernel has acted to
   limit how quickly TCP behavior can change.  Even with the widespread
   use of automated operating system update installation on many end-
   user systems, streaming media providers could have a reasonable
   expectation that they could understand TCP transport protocol
   behaviors, and that those behaviors would remain relatively stable in
   the short term.

6.3.  QUIC and Its Behavior

   The QUIC protocol, developed from a proprietary protocol into an IETF
   standards-track protocol [RFC9000], turns many of the statements made
   in Section 6.1 and Section 6.2 on their heads.

   Although QUIC provides an alternative to the TCP and UDP transport
   protocols, QUIC is itself encapsulated in UDP.  As noted elsewhere in
   Section 7.1, the QUIC protocol encrypts almost all of its transport
   parameters, and all of its payload, so any intermediaries that
   network operators may be using to troubleshoot HTTP streaming media
   performance issues, perform analytics, or even intercept exchanges in
   current applications will not work for QUIC-based applications
   without making changes to their networks.  Section 7 describes the
   implications of media encryption in more detail.

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   While QUIC is designed as a general-purpose transport protocol, and
   can carry different application-layer protocols, the current
   standardized mapping is for HTTP/3 [I-D.ietf-quic-http], which
   describes how QUIC transport features are used for HTTP.  The
   convention is for HTTP/3 to run over UDP port 443 [Port443] but this
   is not a strict requirement.

   When HTTP/3 is encapsulated in QUIC, which is then encapsulated in
   UDP, streaming operators (and network operators) might see UDP
   traffic patterns that are similar to HTTP(S) over TCP.  Since earlier
   versions of HTTP(S) rely on TCP, UDP ports may be blocked for any
   port numbers that are not commonly used, such as UDP 53 for DNS.
   Even when UDP ports are not blocked and HTTP/3 can flow, streaming
   operators (and network operators) may severely rate-limit this
   traffic because they do not expect to see legitimate high-bandwidth
   traffic such as streaming media over the UDP ports that HTTP/3 is
   using.

   As noted in Section 5.5.2, because TCP provides a reliable, in-order
   delivery service for applications, any packet loss for a TCP
   connection causes "head-of-line blocking", so that no TCP segments
   arriving after a packet is lost will be delivered to the receiving
   application until the lost packet is retransmitted, allowing in-order
   delivery to the application to continue.  As described in [RFC9000],
   QUIC connections can carry multiple streams, and when packet losses
   do occur, only the streams carried in the lost packet are delayed.

   A QUIC extension currently being specified ([I-D.ietf-quic-datagram])
   adds the capability for "unreliable" delivery, similar to the service
   provided by UDP, but these datagrams are still subject to the QUIC
   connection's congestion controller, providing some transport-level
   congestion avoidance measures, which UDP does not.

   As noted in Section 6.2, there is an increasing interest in transport
   protocol behaviors that respond to delay measurements, instead of
   responding to packet loss.  These behaviors may deliver improved user
   experience, but in some cases have not responded to sustained packet
   loss, which exhausts available buffers along the end-to-end path that
   may affect other users sharing that path.  The QUIC protocol provides
   a set of congestion control hooks that can be used for algorithm
   agility, and [RFC9002] defines a basic algorithm with transport
   behavior that is roughly similar to TCP NewReno [RFC6582].  However,
   QUIC senders can and do unilaterally choose to use different
   algorithms such as loss-based CUBIC [RFC8312], delay-based COPA or
   BBR, or even something completely different.

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   The Internet community does have experience with deploying new
   congestion controllers without melting the Internet.  As noted in
   [RFC8312], both the CUBIC congestion controller and its predecessor
   BIC have significantly different behavior from Reno-style congestion
   controllers such as TCP NewReno [RFC6582], but both CUBIC and BIC
   were added to the Linux kernel in order to allow experimentation and
   analysis, and both were then selected as the default TCP congestion
   controllers in Linux, and both were deployed globally.

   The point mentioned in Section 6.2 about TCP congestion controllers
   being implemented in operating system kernels is different with QUIC.
   Although QUIC can be implemented in operating system kernels, one of
   the design goals when this work was chartered was "QUIC is expected
   to support rapid, distributed development and testing of features",
   and to meet this expectation, many implementers have chosen to
   implement QUIC in user space, outside the operating system kernel,
   and to even distribute QUIC libraries with their own applications.
   It is worth noting that streaming operators using HTTP/3, carried
   over QUIC, can expect more frequent deployment of new congestion
   controller behavior than has been the case with HTTP/1 and HTTP/2,
   carried over TCP.

   It is worth considering that if TCP-based HTTP traffic and UDP-based
   HTTP/3 traffic are allowed to enter operator networks on roughly
   equal terms, questions of fairness and contention will be heavily
   dependent on interactions between the congestion controllers in use
   for TCP-based HTTP traffic and UDP-based HTTP/3 traffic.

7.  Streaming Encrypted Media

   "Encrypted Media" has at least three meanings:

   *  Media encrypted at the application layer, typically using some
      sort of Digital Rights Management (DRM) system, and typically
      remaining encrypted "at rest", when senders and receivers store
      it.

   *  Media encrypted by the sender at the transport layer, and
      remaining encrypted until it reaches the ultimate media consumer
      (in this document, referred to as "end-to-end media encryption").

   *  Media encrypted by the sender at the transport layer, and
      remaining encrypted until it reaches some intermediary that is
      _not_ the ultimate media consumer, but has credentials allowing
      decryption of the media content.  This intermediary may examine
      and even transform the media content in some way, before
      forwarding re-encrypted media content (in this document referred
      to as "hop-by-hop media encryption").

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   In this document, we will focus on media encrypted at the transport
   layer, whether encrypted "hop-by-hop" or "end-to-end".  Because media
   encrypted at the application layer will only be processed by
   application-level entities, this encryption does not have transport-
   layer implications.  Of course, both "hop-by-hop" and "end-to-end"
   encrypted transport may carry media that is, in addition, encrypted
   at the application layer.

   Each of these encryption strategies is intended to achieve a
   different goal.  For instance, application-level encryption may be
   used for business purposes, such as avoiding piracy or enforcing
   geographic restrictions on playback, while transport-layer encryption
   may be used to prevent media steam manipulation or to protect
   manifests.

   This document does not take a position on whether those goals are
   "valid" (whatever that might mean).

   Both "end-to-end" and "hop-by-hop" media encryption have specific
   implications for streaming operators.  These are described in
   Section 7.2 and Section 7.3.

7.1.  General Considerations for Media Encryption

   The use of strong encryption does provide confidentiality for
   encrypted streaming media, from the sender to either an intermediary
   or the ultimate media consumer, and this does prevent Deep Packet
   Inspection by any intermediary that does not possess credentials
   allowing decryption.  However, even encrypted content streams may be
   vulnerable to traffic analysis.  An intermediary that can identify an
   encrypted media stream without decrypting it, may be able to
   "fingerprint" the encrypted media stream of known content, and then
   match the targeted media stream against the fingerprints of known
   content.  This protection can be lessened if a media provider is
   repeatedly encrypting the same content.  [CODASPY17] is an example of
   what is possible when identifying HTTPS-protected videos over TCP
   transport, based either on the length of entire resources being
   transferred, or on characteristic packet patterns at the beginning of
   a resource being transferred.

   If traffic analysis is successful at identifying encrypted content
   and associating it with specific users, this breaks privacy as
   certainly as examining decrypted traffic.

   Because HTTPS has historically layered HTTP on top of TLS, which is
   in turn layered on top of TCP, intermediaries do have access to
   unencrypted TCP-level transport information, such as retransmissions,
   and some carriers exploited this information in attempts to improve

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   transport-layer performance [RFC3135].  The most recent standardized
   version of HTTPS, HTTP/3 [I-D.ietf-quic-http], uses the QUIC protocol
   [RFC9000] as its transport layer.  QUIC relies on the TLS 1.3 initial
   handshake [RFC8446] only for key exchange [RFC9001], and encrypts
   almost all transport parameters itself, with the exception of a few
   invariant header fields.  In the QUIC short header, the only
   transport-level parameter which is sent "in the clear" is the
   Destination Connection ID [RFC8999], and even in the QUIC long
   header, the only transport-level parameters sent "in the clear" are
   the Version, Destination Connection ID, and Source Connection ID.
   For these reasons, HTTP/3 is significantly more "opaque" than HTTPS
   with HTTP/1 or HTTP/2.

   [I-D.ietf-quic-manageability] discusses manageability of the QUIC
   transport protocol that is used to encapsulate HTTP/3, focusing on
   the implications of QUIC's design and wire image on network
   operations involving QUIC traffic.  It discusses what network
   operators can consider in some detail.

   More broadly, RFC 9065 [RFC9065], "Considerations around Transport
   Header Confidentiality, Network Operations, and the Evolution of
   Internet Transport Protocols" describes the impact of increased
   encryption of transport headers in general terms.

7.2.  Considerations for "Hop-by-Hop" Media Encryption

   Although the IETF has put considerable emphasis on end-to-end
   streaming media encryption, there are still important use cases that
   require the insertion of intermediaries.

   There are a variety of ways to involve intermediaries, and some are
   much more intrusive than others.

   From a content provider's perspective, a number of considerations are
   in play.  The first question is likely whether the content provider
   intends that intermediaries are explicitly addressed from endpoints,
   or whether the content provider is willing to allow intermediaries to
   "intercept" streaming content transparently, with no awareness or
   permission from either endpoint.

   If a content provider does not actively work to avoid interception by
   intermediaries, the effect will be indistinguishable from
   "impersonation attacks", and endpoints cannot be assumed of any level
   of privacy.

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   Assuming that a content provider does intend to allow intermediaries
   to participate in content streaming, and does intend to provide some
   level of privacy for endpoints, there are a number of possible tools,
   either already available or still being specified.  These include

   *  Server And Network assisted DASH [MPEG-DASH-SAND] - this
      specification introduces explicit messaging between DASH clients
      and network elements or between various network elements for the
      purpose of improving the efficiency of streaming sessions by
      providing information about real-time operational characteristics
      of networks, servers, proxies, caches, CDNs, as well as DASH
      client's performance and status.

   *  "Double Encryption Procedures for the Secure Real-Time Transport
      Protocol (SRTP)" [RFC8723] - this specification provides a
      cryptographic transform for the Secure Real-time Transport
      Protocol that provides both hop-by-hop and end-to-end security
      guarantees.

   *  Secure Media Frames [SFRAME] - [RFC8723] is closely tied to SRTP,
      and this close association impeded widespread deployment, because
      it could not be used for the most common media content delivery
      mechanisms.  A more recent proposal, Secure Media Frames [SFRAME],
      also provides both hop-by-hop and end-to-end security guarantees,
      but can be used with other transport protocols beyond SRTP.

   The choice of whether to involve intermediaries sometimes requires
   careful consideration.  As an example, when ABR manifests were
   commonly sent unencrypted some networks would modify manifests during
   peak hours by removing high-bitrate renditions in order to prevent
   players from choosing those renditions, thus reducing the overall
   bandwidth consumed for delivering these media streams and thereby
   improving the network load and the user experience for their
   customers.  Now that ubiquitous encryption typically prevents this
   kind of modification, in order to maintain the same level of network
   health and user experience across networks whose users would have
   benefitted from this intervention a media streaming operator
   sometimes needs to choose between adding intermediaries who are
   authorized to change the manifests or adding significant extra
   complexity to their service.

   Some resources that might inform other similar considerations are
   further discussed in [RFC8824] (for WebRTC) and
   [I-D.ietf-quic-manageability] (for HTTP/3 and QUIC).

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7.3.  Considerations for "End-to-End" Media Encryption

   "End-to-end" media encryption offers the potential of providing
   privacy for streaming media consumers, with the idea being that if an
   unauthorized intermediary can't decrypt streaming media, the
   intermediary can't use Deep Packet Inspection to examine HTTP request
   and response headers and identify the media content being streamed.

   "End-to-end" media encryption has become much more widespread in the
   years since the IETF issued "Pervasive Monitoring Is an Attack"
   [RFC7258] as a Best Current Practice, describing pervasive monitoring
   as a much greater threat than previously appreciated.  After the
   Snowden disclosures, many content providers made the decision to use
   HTTPS protection - HTTP over TLS - for most or all content being
   delivered as a routine practice, rather than in exceptional cases for
   content that was considered "sensitive".

   Unfortunately, as noted in [RFC7258], there is no way to prevent
   pervasive monitoring by an "attacker", while allowing monitoring by a
   more benign entity who "only" wants to use DPI to examine HTTP
   requests and responses in order to provide a better user experience.
   If a modern encrypted transport protocol is used for end-to-end media
   encryption, intermediary streaming operators are unable to examine
   transport and application protocol behavior.  As described in
   Section 7.2, only an intermediary streaming operator who is
   explicitly authorized to examine packet payloads, rather than
   intercepting packets and examining them without authorization, can
   continue these practices.

   [RFC7258] said that "The IETF will strive to produce specifications
   that mitigate pervasive monitoring attacks", so streaming operators
   should expect the IETF's direction toward preventing unauthorized
   monitoring of IETF protocols to continue for the forseeable future.

8.  Further Reading and References

   The Media Operations community maintains a list of references and
   resources for further reading at this location:

   *  https://github.com/ietf-wg-mops/draft-ietf-mops-streaming-
      opcons/blob/main/living-doc-mops-streaming-opcons.md
      (https://github.com/ietf-wg-mops/draft-ietf-mops-streaming-
      opcons/blob/main/living-doc-mops-streaming-opcons.md)

   Editor's note: The link above might or might not be changed during
   IESG Evaluation.  See https://github.com/ietf-wg-mops/draft-ietf-
   mops-streaming-opcons/issues/114 (https://github.com/ietf-wg-mops/
   draft-ietf-mops-streaming-opcons/issues/114) for updates.

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9.  IANA Considerations

   This document requires no actions from IANA.

10.  Security Considerations

   Security is an important matter for streaming media applications and
   it was briefly touched on in Section 7.1.  This document itself
   introduces no new security issues.

11.  Acknowledgments

   Thanks to Alexandre Gouaillard, Aaron Falk, Chris Lemmons, Dave Oran,
   Eric Vyncke, Glenn Deen, Kyle Rose, Leslie Daigle, Lucas Pardue, Mark
   Nottingham, Matt Stock, Mike English, Renan Krishna, Roni Even,
   Sanjay Mishra, and Will Law for very helpful suggestions, reviews and
   comments.

12.  Informative References

   [ABRSurvey]
              Taani, B., Begen, A. C., Timmerer, C., Zimmermann, R., and
              A. Bentaleb et al, "A Survey on Bitrate Adaptation Schemes
              for Streaming Media Over HTTP", IEEE Communications
              Surveys & Tutorials , 2019,
              <https://ieeexplore.ieee.org/abstract/document/8424813>.

   [BAP]      "The Coalition for Better Ads", n.d.,
              <https://www.betterads.org/>.

   [CMAF-CTE] Law, W., "Ultra-Low-Latency Streaming Using Chunked-
              Encoded and Chunked Transferred CMAF", October 2018,
              <https://www.akamai.com/us/en/multimedia/documents/white-
              paper/low-latency-streaming-cmaf-whitepaper.pdf>.

   [CODASPY17]
              Reed, A. and M. Kranch, "Identifying HTTPS-Protected
              Netflix Videos in Real-Time", ACM CODASPY , March 2017,
              <https://dl.acm.org/doi/10.1145/3029806.3029821>.

   [CoDel]    Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              Communications of the ACM, Volume 55, Issue 7, pp. 42-50 ,
              July 2012.

   [COPA18]   Arun, V. and H. Balakrishnan, "Copa: Practical Delay-Based
              Congestion Control for the Internet", USENIX NSDI , April
              2018, <https://web.mit.edu/copa/>.

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   [CTA-2066] Consumer Technology Association, "Streaming Quality of
              Experience Events, Properties and Metrics", March 2020,
              <https://shop.cta.tech/products/streaming-quality-of-
              experience-events-properties-and-metrics>.

   [CTA-5004] CTA, "Common Media Client Data (CMCD)", September 2020,
              <https://shop.cta.tech/products/web-application-video-
              ecosystem-common-media-client-data-cta-5004>.

   [CVNI]     "Cisco Visual Networking Index: Forecast and Trends,
              2017-2022 White Paper", 27 February 2019,
              <https://www.cisco.com/c/en/us/solutions/collateral/
              service-provider/visual-networking-index-vni/white-paper-
              c11-741490.html>.

   [ELASTIC]  De Cicco, L., Caldaralo, V., Palmisano, V., and S.
              Mascolo, "ELASTIC: A client-side controller for dynamic
              adaptive streaming over HTTP (DASH)", Packet Video
              Workshop , December 2013,
              <https://ieeexplore.ieee.org/document/6691442>.

   [Encodings]
              Apple, Inc, "HLS Authoring Specification for Apple
              Devices", June 2020,
              <https://developer.apple.com/documentation/
              http_live_streaming/
              hls_authoring_specification_for_apple_devices>.

   [I-D.cardwell-iccrg-bbr-congestion-control]
              Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V.
              Jacobson, "BBR Congestion Control", Work in Progress,
              Internet-Draft, draft-cardwell-iccrg-bbr-congestion-
              control-02, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-cardwell-
              iccrg-bbr-congestion-control-02>.

   [I-D.draft-pantos-hls-rfc8216bis]
              Pantos, R., "HTTP Live Streaming 2nd Edition", Work in
              Progress, Internet-Draft, draft-pantos-hls-rfc8216bis-10,
              8 November 2021, <https://datatracker.ietf.org/doc/html/
              draft-pantos-hls-rfc8216bis-10>.

   [I-D.ietf-httpbis-cache]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              cache-19>.

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   [I-D.ietf-quic-datagram]
              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-datagram-10, 4 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              datagram-10>.

   [I-D.ietf-quic-http]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [I-D.ietf-quic-manageability]
              Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-manageability-16, 6 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              manageability-16>.

   [I-D.ietf-quic-qlog-h3-events]
              Marx, R., Niccolini, L., and M. Seemann, "HTTP/3 and QPACK
              qlog event definitions", Work in Progress, Internet-Draft,
              draft-ietf-quic-qlog-h3-events-01, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              qlog-h3-events-01>.

   [I-D.ietf-quic-qlog-main-schema]
              Marx, R., Niccolini, L., and M. Seemann, "Main logging
              schema for qlog", Work in Progress, Internet-Draft, draft-
              ietf-quic-qlog-main-schema-02, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              qlog-main-schema-02>.

   [I-D.ietf-quic-qlog-quic-events]
              Marx, R., Niccolini, L., and M. Seemann, "QUIC event
              definitions for qlog", Work in Progress, Internet-Draft,
              draft-ietf-quic-qlog-quic-events-01, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              qlog-quic-events-01>.

   [IAB-ADS]  "IAB", n.d., <https://www.iab.com/>.

   [IABcovid] Arkko, J., Farrel, S., Kühlewind, M., and C. Perkins,
              "Report from the IAB COVID-19 Network Impacts Workshop
              2020", November 2020, <https://datatracker.ietf.org/doc/
              draft-iab-covid19-workshop/>.

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   [Jacobson-Karels]
              Jacobson, V. and M. Karels, "Congestion Avoidance and
              Control", November 1988,
              <https://ee.lbl.gov/papers/congavoid.pdf>.

   [Labovitz] Labovitz, C., "Network traffic insights in the time of
              COVID-19: April 9 update", April 2020,
              <https://www.nokia.com/blog/network-traffic-insights-time-
              covid-19-april-9-update/>.

   [LabovitzDDoS]
              Takahashi, D., "Why the game industry is still vulnerable
              to DDoS attacks", May 2018,
              <https://venturebeat.com/2018/05/13/why-the-game-industry-
              is-still-vulnerable-to-distributed-denial-of-service-
              attacks/>.

   [LL-DASH]  DASH-IF, "Low-latency Modes for DASH", March 2020,
              <https://dashif.org/docs/CR-Low-Latency-Live-r8.pdf>.

   [Micro]    Taher, T. M., Misurac, M. J., LoCicero, J. L., and D. R.
              Ucci, "Microwave Oven Signal Interference Mitigation For
              Wi-Fi Communication Systems", 2008 5th IEEE Consumer
              Communications and Networking Conference 5th IEEE, pp.
              67-68 , 2008.

   [Mishra]   Mishra, S. and J. Thibeault, "An update on Streaming Video
              Alliance", April 2020,
              <https://datatracker.ietf.org/meeting/interim-2020-mops-
              01/materials/slides-interim-2020-mops-01-sessa-april-
              15-2020-mops-interim-an-update-on-streaming-video-
              alliance>.

   [MMSP20]   Durak, K. and et al, "Evaluating the performance of
              Apple's low-latency HLS", IEEE MMSP , September 2020,
              <https://ieeexplore.ieee.org/document/9287117>.

   [MMSys11]  Akhshabi, S., Begen, A. C., and C. Dovrolis, "An
              experimental evaluation of rate-adaptation algorithms in
              adaptive streaming over HTTP", ACM MMSys , February 2011,
              <https://dl.acm.org/doi/10.1145/1943552.1943574>.

   [MPEG-CMAF]
              "ISO/IEC 23000-19:2020 Multimedia application format
              (MPEG-A) - Part 19: Common media application format (CMAF)
              for segmented media", March 2020,
              <https://www.iso.org/standard/79106.html>.

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   [MPEG-DASH]
              "ISO/IEC 23009-1:2019 Dynamic adaptive streaming over HTTP
              (DASH) - Part 1: Media presentation description and
              segment formats", December 2019,
              <https://www.iso.org/standard/79329.html>.

   [MPEG-DASH-SAND]
              "ISO/IEC 23009-5:2017 Dynamic adaptive streaming over HTTP
              (DASH) - Part 5: Server and network assisted DASH (SAND)",
              February 2017, <https://www.iso.org/standard/69079.html>.

   [MPEG-TS]  "H.222.0 : Information technology - Generic coding of
              moving pictures and associated audio information:
              Systems", 29 August 2018,
              <https://www.itu.int/rec/T-REC-H.222.0>.

   [MPEGI]    Boyce, J. M. and et al, "MPEG Immersive Video Coding
              Standard", Proceedings of the IEEE , n.d.,
              <https://ieeexplore.ieee.org/document/9374648>.

   [OReilly-HPBN]
              "High Performance Browser Networking (Chapter 2: Building
              Blocks of TCP)", May 2021,
              <https://hpbn.co/building-blocks-of-tcp/>.

   [PCC]      Schwarz, S. and et al, "Emerging MPEG Standards for Point
              Cloud Compression", IEEE Journal on Emerging and Selected
              Topics in Circuits and Systems , March 2019,
              <https://ieeexplore.ieee.org/document/8571288>.

   [Port443]  "Service Name and Transport Protocol Port Number
              Registry", April 2021, <https://www.iana.org/assignments/
              service-names-port-numbers/service-names-port-
              numbers.txt>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/rfc/rfc793>.

   [RFC2001]  Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
              Retransmit, and Fast Recovery Algorithms", RFC 2001,
              DOI 10.17487/RFC2001, January 1997,
              <https://www.rfc-editor.org/rfc/rfc2001>.

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   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,
              <https://www.rfc-editor.org/rfc/rfc3135>.

   [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/rfc/rfc3550>.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758,
              DOI 10.17487/RFC3758, May 2004,
              <https://www.rfc-editor.org/rfc/rfc3758>.

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              DOI 10.17487/RFC4733, December 2006,
              <https://www.rfc-editor.org/rfc/rfc4733>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/rfc/rfc4960>.

   [RFC5594]  Peterson, J. and A. Cooper, "Report from the IETF Workshop
              on Peer-to-Peer (P2P) Infrastructure, May 28, 2008",
              RFC 5594, DOI 10.17487/RFC5594, July 2009,
              <https://www.rfc-editor.org/rfc/rfc5594>.

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, DOI 10.17487/RFC5762, April
              2010, <https://www.rfc-editor.org/rfc/rfc5762>.

   [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/rfc/rfc6190>.

   [RFC6582]  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
              NewReno Modification to TCP's Fast Recovery Algorithm",
              RFC 6582, DOI 10.17487/RFC6582, April 2012,
              <https://www.rfc-editor.org/rfc/rfc6582>.

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   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <https://www.rfc-editor.org/rfc/rfc6817>.

   [RFC6843]  Clark, A., Gross, K., and Q. Wu, "RTP Control Protocol
              (RTCP) Extended Report (XR) Block for Delay Metric
              Reporting", RFC 6843, DOI 10.17487/RFC6843, January 2013,
              <https://www.rfc-editor.org/rfc/rfc6843>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/rfc/rfc7258>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,
              <https://www.rfc-editor.org/rfc/rfc7510>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <https://www.rfc-editor.org/rfc/rfc7656>.

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,
              <https://www.rfc-editor.org/rfc/rfc7661>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <https://www.rfc-editor.org/rfc/rfc8083>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
              <https://www.rfc-editor.org/rfc/rfc8084>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.

   [RFC8216]  Pantos, R., Ed. and W. May, "HTTP Live Streaming",
              RFC 8216, DOI 10.17487/RFC8216, August 2017,
              <https://www.rfc-editor.org/rfc/rfc8216>.

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   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8312>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC8622]  Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
              Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8622>.

   [RFC8723]  Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
              "Double Encryption Procedures for the Secure Real-Time
              Transport Protocol (SRTP)", RFC 8723,
              DOI 10.17487/RFC8723, April 2020,
              <https://www.rfc-editor.org/rfc/rfc8723>.

   [RFC8824]  Minaburo, A., Toutain, L., and R. Andreasen, "Static
              Context Header Compression (SCHC) for the Constrained
              Application Protocol (CoAP)", RFC 8824,
              DOI 10.17487/RFC8824, June 2021,
              <https://www.rfc-editor.org/rfc/rfc8824>.

   [RFC8825]  Alvestrand, H., "Overview: Real-Time Protocols for
              Browser-Based Applications", RFC 8825,
              DOI 10.17487/RFC8825, January 2021,
              <https://www.rfc-editor.org/rfc/rfc8825>.

   [RFC8999]  Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,
              <https://www.rfc-editor.org/rfc/rfc8999>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9001>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

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   [RFC9065]  Fairhurst, G. and C. Perkins, "Considerations around
              Transport Header Confidentiality, Network Operations, and
              the Evolution of Internet Transport Protocols", RFC 9065,
              DOI 10.17487/RFC9065, July 2021,
              <https://www.rfc-editor.org/rfc/rfc9065>.

   [SFRAME]   "Secure Media Frames Working Group (Home Page)", n.d.,
              <https://datatracker.ietf.org/doc/charter-ietf-sframe/>.

   [SRT]      Sharabayko, M., "Secure Reliable Transport (SRT) Protocol
              Overview", 15 April 2020,
              <https://datatracker.ietf.org/meeting/interim-2020-mops-
              01/materials/slides-interim-2020-mops-01-sessa-april-
              15-2020-mops-interim-an-update-on-streaming-video-
              alliance>.

   [Survey360o]
              Yaqoob, A., Bi, T., and G. Muntean, "A Survey on Adaptive
              360° Video Streaming: Solutions, Challenges and
              Opportunities", IEEE Communications Surveys & Tutorials ,
              July 2020, <https://ieeexplore.ieee.org/document/9133103>.

Authors' Addresses

   Jake Holland
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144,
   United States of America
   Email: jakeholland.net@gmail.com

   Ali Begen
   Networked Media
   Turkey
   Email: ali.begen@networked.media

   Spencer Dawkins
   Tencent America LLC
   United States of America
   Email: spencerdawkins.ietf@gmail.com

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