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QuicR - Media Delivery Protocol over QUIC

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Cullen Fluffy Jennings , Suhas Nandakumar
Last updated 2022-03-04
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Network Working Group                                        C. Jennings
Internet-Draft                                                     cisco
Intended status: Informational                             S. Nandakumar
Expires: 5 September 2022                                          Cisco
                                                            4 March 2022

               QuicR - Media Delivery Protocol over QUIC


   This specification outlines the design for a media delivery protocol
   over QUIC.  It aims at supporting multiple application classes with
   varying latency requirements including ultra low latency applications
   such as interactive communication and gaming.  It is based on a
   publish/subscribe metaphor where entities publish and subscribe to
   data that is sent through, and received from, relays in the cloud.
   The information subscribed to is named such that this forms an
   overlay information centric network.  The relays allow for efficient
   large scale deployments.

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
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   This Internet-Draft will expire on 5 September 2022.

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   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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  QuicR . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Contributing  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Advantages of QuicR . . . . . . . . . . . . . . . . . . . . .   5
   5.  QuicR architecture  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  QuicR Delivery Network Architecture with Origin as the only
           Relay Function. . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  QuicR Delivery Network Architecture . . . . . . . . . . .   7
   6.  Names and Named Objects . . . . . . . . . . . . . . . . . . .   8
     6.1.  Objects Groups  . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Named Objects . . . . . . . . . . . . . . . . . . . . . .   9
     6.3.  Name Hashes . . . . . . . . . . . . . . . . . . . . . . .  10
     6.4.  Wildcarding with Names  . . . . . . . . . . . . . . . . .  11
   7.  Objects . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Relays  . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  QuicR Usage Design Patterns . . . . . . . . . . . . . . . . .  12
     9.1.  QuicR Manifest Objects  . . . . . . . . . . . . . . . . .  12
     9.2.  QuicR Video Objects . . . . . . . . . . . . . . . . . . .  13
       9.2.1.  RUSH over QuicR . . . . . . . . . . . . . . . . . . .  13
       9.2.2.  Warp over QuicR . . . . . . . . . . . . . . . . . . .  14
     9.3.  QuicR Audio Objects . . . . . . . . . . . . . . . . . . .  14
     9.4.  QuicR Game Moves Objects  . . . . . . . . . . . . . . . .  14
     9.5.  Messaging Objects . . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   11. Protocol Design Considerations  . . . . . . . . . . . . . . .  15
     11.1.  HTTP/3 . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.2.  QUIC Streams and Datagrams . . . . . . . . . . . . . . .  16
     11.3.  QUIC Congestion Control  . . . . . . . . . . . . . . . .  16
     11.4.  Why not RTP  . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Recently new usecases have emerged requiring higher scalability of
   delivery for interactive realtime applications and much lower latency
   for streaming applications and a combination thereof.  On one side
   are usecases such as normal web conferences wanting to distribute out
   to millions of viewers and allow viewers to instantly move to being a
   presenter.  On the other side are usescases such as streaming a

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   soccer game to millions of people including people in the stadium
   watching the game live.  Viewers watching an e-sports event want to
   be able to comment with mininal latency to ensure the interactivity
   aspects between what different viewers are seeing is preserved.  All
   of these usescases push towards latencies that are in the order of
   100ms over the natural latency the network causes.

   Interactive realtime applications, such as web conferencing systems,
   require ultra low latency (< 150ms).  Such applications create their
   own application specific delivery network over which latency
   requirements can be met.  Realtime transport protocols such as RTP
   over UDP provide the basic elements needed for realtime
   communication, both contribution and distribution, while leaving
   aspects such as resiliency and congestion control to be provided by
   each application.  On the other hand, media streaming applications
   are much more tolerant to latency and require highly scalable media
   distribution.  Such applications leverage existing CDN networks, used
   for optimizing web delivery, to distribute media.  Streaming
   protocols such as HLS and MPEG-DASH operates on top of HTTP and gets
   transport-level resiliency and congestion control provided by TCP.

   This document outlines, QuicR, a unified architecture and protocol
   for data delivery that enables a wide range of realtime applications
   with different resiliency and latency needs without compromising the
   scalability and cost effectiveness associated with content delivery

1.1.  QuicR

   The architecture defines and uses QuicR, a delivery protocol that is
   based on a publish/subscribe metaphor where client endpoints publish
   and subscribe to named objects that is sent to, and received from
   relays, that forms an overlay delivery network similar to what CDN
   provides today.  The subscribe messages allow subscription to a name
   that includes a wildcard to match multiple published names, so a
   single subscribe can allow a client to receive publishes for a wide
   class of named objects.  Objects are named such that it is unique for
   the relay/delivery network and scoped to an application.

   QuicrR provides services based on application requirements (with the
   support of underlying transport, where necessary) such as estimation
   of available bandwidth, fragmentation and reassembly, resiliency,
   congestion control and prioritization of data delivery based on data
   lifetime and importance of data.  It is designed to be NAT and
   firewall traversal friendly and can be fronted with load balancers.

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   The Relays are arranged in a logical tree (as shown below) where, for
   a given application, there is an origin Relay at root of the tree
   that controls the namespace.  Publish messages are sent towards the
   root of the tree and down the path of any subscribers to that named
   object.  QuicR is designed to make it easy to implement relays so
   that fail over could happen between relays with minimal impact to the
   clients and relays can redirect a client to a different relay.

                 │            │
                 │            ▼
                 │       ┌────────┐
                 │   ▬ ▬▶│Relay-0 │ ◀▬▬ ▬▬ ▬▮
             pub │  ▮    │ Origin ├┐        ▮
                 │  ▮    └────────┘│        ▮
                 │  ▮ sub          │        ▮ sub
                 │  ▮          pub │        ▮
                 │  ▮              │        ▮
            ┌───────▮┐ ◀▬▮         │  ┌─────▮──┐
        ┌──▶│ Relay-1│   ▮         └─▶│ Relay-2│◀▮▮
        │   └─────┬──┘   ▮             ▲──┬────┤  ▮
    pub │         │      ▮ sub     sub ▮  │    │  ▮ sub
        │      pub│      ▮             ▮  │pub ▼  ▮
       ┌┴─────┐   │ ┌────▮─┐     ┌─────▮┐ │   ┌───▮──┐
       │Alice │   └▶│ Bob  │     │ Carl │◀┘   │Derek │
       └──────┘     └──────┘     └──────┘     └──────┘

                       Figure 1: QuicR Delivery Tree

   The design supports sending media and other named objects between a
   set of participants in a game or video call with under a hundred
   milliseconds of latency and meets the needs of conferencing systems.
   The design can also be used for large scale streaming to millions of
   participants with latency ranging from a few seconds to under a
   hundred milliseconds based on applications needs.  It can also be
   used as low latency publish/subscribe system for real time systems
   such as messaging, gaming, and IoT.

2.  Contributing

   All significant discussion of development of this protocol is in the
   GitHub issue tracker at: "

   QuicR is pronounced something close to “ (U+201C)quicker” (U+201D)
   but with more of a pirate "arrrr" at the end.

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3.  Terminology

   *  Relay Function: Functionality of the QuicR architecture, that
      implements store and forward behavior at the minimum.  Such a
      function typically receives subscriptions and publishes data to
      the other endpoints that have subscribed to the named data.  Such
      functions may cache the data as well for optimizing the delivery

   *  Relay: Server component (physical/logical) in the cloud that
      implements the Relay Function.

   *  Publisher: An endpoint that sends named objects to a Relay. [ also
      referred to as producer of the named object]

   *  Subscriber: An endpoint that subscribes and receives the named
      objects.  Relays can act as subscribers to other relays.
      Subscribers can also be referred to as consumers.

   *  Client/QuicR Client: An endpoint that acts as a Publisher,
      Subscriber, or both.  May also implement a Relay Function in
      certain contexts.

   *  Named Object: Application level chunk of Data that has a unique
      Name, a limited lifetime, priority and is transported via QuicR

   *  Origin server: Component managing the QuicR namespace for a
      specific application and is responsible for establishing trust
      between clients and relays.  Origin servers can implement other
      QuicR functions.

4.  Advantages of QuicR

   As its evident, QuicR and its architecture uses similar concepts and
   delivery mechanisms to those used by streaming standards such as HLS
   and MPEG-DASH.  Specifically the use of a CDN-like delivery network,
   the use of named objects and the receiver-triggered media/data
   delivery.  However there are fundamental characteristics that QuicR
   provides to enable ultra low latency delivery for interactive
   applications such as conferencing and gaming.

   *  To support low latency the granularity of the delivered objects,
      in terms of time duration, need to be quite small making it
      complicated for clients to request each object individually.
      QuicR uses a publish and subscription semantic along with a
      wildcard name to simplify and speed object delivery for low
      latency applications.  For latency-tolerant applications, larger

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      granularity of data, aka group of objects, can be individually
      requested and delivered without instantiating state in the

   *  Some realtime applications operating in ultra low latency mode
      require objects delivered as and when they are available without
      having to wait for previous objects delayed due to network loss or
      out of order network delivery.  QuicR supports Quic datagrams
      based object delivery for this purpose.  Note that QuicR also uses
      Quic stream for delivery of objects that are latency-tolerant.

   *  QuicR supports resiliency mechanisms that are more suitable for
      realtime delivery such as FEC and selective retransmission.

   *  QUIC's current congestion control algorithms need to be evaluated
      for efficacy in low latency interactive real-time contexts,
      specifically mechanisms such as slow start, multiplicative
      decrease and queue buildup drainage during BBR probing.  Based on
      the results of the evaluation work, QuicR can select the
      congestion control algorithm suitable for the application's class.

   *  Published objects in QuicR have associated max-age that specifies
      the validity of such objects. max-age influences relay's drop
      decisions and can also be used by the underlying QUIC transport to
      cease retransmissions associated with the named object.

   *  Unlike streaming architectures where media contribution and media
      distribution are treated differently, QuicR can be used for both
      object contribution/publishing and distribution/subscribing as the
      split does not exist for interactive communications.

   *  QuicR supports "aggregation of subscriptions" to the named objects
      where the subscriptions are aggregated at the relay functions and
      allows "short-circuited" delivery of published objects when there
      is a match at a given relay function.

   *  QuicR allows publishers to associate a priority with objects.
      Priorities can help the delivery network and the subscribers to
      make decisions about resiliency, latency, drops etc.  Priorities
      can used to set relative importance between different qualities
      for layered video encoding, for example.

   *  QuicR is designed so that objects are encrypted end-to-end and
      will pass transparently through the delivery network.  Any
      information required by the delivery network, e.g priorities, will
      be included as part of the metadata that is accessible to the
      delivery network for further processing as appropriate.

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5.  QuicR architecture

   A typical media delivery architecture based on QuicR enables delivery
   tree allowing :

   *  Publishing entities to publish named data

   *  Subscribers to express interest in the named objects

   *  Delivery tree made up of one or more Relays to allow the flow of
      the named objects.

   In the following subsections, 2 common QuicR delivery tree
   architectures are non-normatively discussed

5.1.  QuicR Delivery Network Architecture with Origin as the only Relay

                          |Relay        |
       +----------------> ||-----+
       |                  +-------------+     |
       |                                ^     |
       |             |     |
       |                                |     |
       |     *|     |
       |                                |     |
       |                                |     v
   +-----------+                      +----------+
   | Publisher |                      |Subscriber|
   +-----------+                      +----------+

   The above picture shows QuicR delivery network for an hypothetical
   streaming architecture rooted at the Origin Relay (for the domain  In this architecture, the media contribution is done by
   publishing named objects corresponding to channel-22 to the ingest
   server at the Origin Relay.  Media consumption happens via subscribes
   sent to the Origin Relay to the wildcarded name (ch22/*) for all
   media streams happening over the named channel-22.  The media
   published either by the source publisher or the Relay (as Publisher)
   might be encoded into multiple qualities.

5.2.  QuicR Delivery Network Architecture

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              +----> |Realay-O| <----------------+
              |      +--------+                  |
              |       ^       |                  |sub:alice-low
    pub:alice-hi      |       pub:alice-hi       |sub:alice-hi
    pub:alice-low     |       pub:alice-low      |
              | sub:alice-low |                  |
              |       |       |                  |
             +---------+      |  +------------------------+
     +------>| Relay-A |      +->|    Relay-B             |
     |       +---------+         +------------------------+
     |           |  ^              |    ^          |    ^
    pub1:alice-hi|  |              |    |          |    |
    pub2:alice-low  |              |    |          |    |
     |           |  |              |    |          |    |
     |          pub:alice-low     pub:alice-hi,low pub:alice-hi,low
     |           |  |              |    |          |    |
     |           | sub:alice-low   |   sub:alice*  |   sub:alice*
     |           v  |              v    |          v    |
     +------+    +---+              +----+         +-----+
     | Alice|    |Bob|              |Carl|         |Derek|
     +------+    +---+              +----+         +-----+

   The above picture shows QuicR media delivery tree formed with
   multiple relays in the network.  The example has 4 participants with
   Alice being the publisher and rest being the subscribers.  Alice's is
   capable of publishing video streams at 2 qualities identified by
   their appropriate names.  Bob subscribes to a low resolution video
   feed from alice, where as Carl/Derek carryout wildcard subscribes to
   all the qualities of video feed published by Alice.  All the
   subscribes are sent to the Origin Relay and are saved at the on-path
   Relays, this allowing for "short-circuited" delivery of published
   data at the relays.  In the above example, Bob gets Alice's published
   data directly from Relay-A instead of hairpinning from the Origin
   Relay.  Carl and Derek, however get their video stream relayed from
   Alice via Origin Relay and Relay-B.

6.  Names and Named Objects

   Names are basic elements with in the QuicR architecture and they
   uniquely identify objects.  Named objects can be cached in relays in
   a way CDNs cache resources and thus can obtain similar benefits such
   caching mechanisms would offer.

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6.1.  Objects Groups

   Objects with in QuicR belong to a group.  A group (a.k.a group of
   objects) represent an independent composition of set of objects,
   where there exists dependency relationship between the objects within
   the group.  Groups, thus can be independently consumable by the
   subscriber applications.

   A typical example would be a group of pictures/video frames or group
   of audio samples that represent synchronization point in the video
   conferencing example.

   Latency-tolerant applications can request individual group of objects
   allowing delivery of objects without instantiation of persistent
   state within the delivery network.  This is important for the
   preservation of the scalability of delivery networks at levels
   similar to what is currently available when streaming protocols such
   as HLS/HTTP are used.

6.2.  Named Objects

   The names of each object in QuicR is composed of the following

   1.  Domain Component

   2.  Application Component

   3.  Group ID Component

   4.  Object ID Component

   Domain component uniquely identifies a given application domain.
   This is like a HTTP Origin and uniquely identifies the application
   and a root relay function.  This is a DNS domain name or IP address
   combined with a UDP port number mapped to into the domain.  Example:

   Application component is scoped under a given Domain.  This component
   identifies aspects specific to a given application instance hosted
   under a given domain (e.g.meeting identifier, which movie or channel,
   media type or media quality identifier).

   Inside each Application Component, there is a set of groups.  Each
   group is identified by a monotonically increasing integer.  Inside of
   each Group, each object is identified by another monotonically
   increasing integer inside that group.  The groupID and objectID start
   at 0 and are limited to 16 bits long.

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   Example: In the example below, the domain component identifies domain, the application component identifies an
   instance of a meeting under this domain, say "meeting123", and high
   resolution camera stream from the user "alice".  It also identifies
   the object 17 under group 15.


6.3.  Name Hashes

   All Names need to hash or map down to 128 bits.  This allows for:

   *  compact representation for efficient transmission and storage,

   *  cache friendly datatypes ( like Keys in CDN caches) for storage
      and lookup purposes and,

   *  enable rapid data lookup at the relays based on partial as well as
      whole names ( wildcard support ).

   │ Domain      │ Application   │ GroupID       │ ObjectID    │
   │ Component   │ Component     │ Component     │ Component   │
       48 bits        48 bits         16 bits       16 bits

                            Figure 2: QuicR Name

   This is done by hashing the origin to first 48 bits.  Any relay that
   forms an connection to an new origin needs to ensure this does not
   collide with an existing origin.  The application component is mapped
   to the next 48 bits and it is the responsibility of the application
   to ensure there are no collisions within a given origin.  Finally the
   group ID and object ID each map to 16 bits.

   Design Note: It is possible to let each application define the size
   of these boundaries as well as sub boundaries inside the application
   component but for sake of simplicity it is described as fixed
   boundaries for now.

   Wildcard search simply turns into a bitmask at the appropriate bit
   location of the hashed name.

   The hash names are key part of the design for allowing small objects
   without adding lots of overhead and for efficient implementation of

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6.4.  Wildcarding with Names

   QuicR allows subscribers to request for media based on wildcard'ed
   names.  Wildcarding enables subscribes to be made as aggregates
   instead of at the object level granularity.  Wildcard names are
   formed by skipping the right most segments of names.

   For example, in an web conferencing use case, the client may
   subscribe to just the origin and meetingId to get all the media for a
   particular conference as indicated by the example below.  The example
   matches all the named objects published as part of meeting123.


   When subscribing, there is an option to tell the relay to one of:

   A.  Deliver any new objects it receives that matches the name

   B.  Deliver any new objects it receives and in addition send any
   previous objects it has received that are in the same group that
   matches the name.

   C.  Wait until an object that has a objectID that matches the name is
   received then start sending any objects that match the name.

7.  Objects

   Once a named object is created, the content inside the named object
   can never be changed.  Objects have an expiry time after which they
   should be discarded by caches.  Objects have an priority that the
   relays and clients can use to sequence the sending order.  The data
   inside an object is end-to-end encrypted whose keys are not available
   to Relay(s).

8.  Relays

   The Relays receive subscriptions and intent to publish request and
   forward them towards the origin.  This may send the messages directly
   to the Origin Relay or possibly traverse another Relay.  Replies to
   theses message follow the reverse direction of the request and when
   the Origin gives the OK to a subscription or intent to publish, the
   Relay allows the subscription or future publishes to the Names in the

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   Subscription received are aggregated.  When a relay receives a
   publish request with data, it will forward it both towards the Origin
   and to any clients or relays that have a matching subscription.  This
   "short circuit" of distribution by a relay before the data has even
   reached the Origin servers provides significant latency reduction for
   nearby client.

   The Relay keeps an outgoing queue of objects to be sent to the each
   subscriber and objects are sent in priority order.

   Relays MAY cache some of the information for short period of time and
   the time cached may depend on the Origin.

9.  QuicR Usage Design Patterns

   This section explains design patters that can be use to build
   applications on top of QuicR.

9.1.  QuicR Manifest Objects

   Names can optionally be discovered via manifests.  In such cases, the
   role of the manifest is to identify the names as well as aspects
   pertaining to the associated data in a given usage context of the

   *  Typically a manifest identifies the domain and application aspects
      for the set of names that can be published.

   *  The content of Manifest is application defined and end-to-end

   *  The manifest is owned by the application's origin server and are
      accessed as a protected resources by the authorized QuicR clients.

   *  The QuicR protocol treats Manifests as a named object, thus
      allowing for clients to subscribe for the purposes of
      bootstrapping into the session as well as to follow manifest
      changes during a session [ new members joining a conference for

   *  The manifest has well known name on the Origin server.

   Also to note, a given application might provide non QuicR mechanisms
   to retrieve the manifest.

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9.2.  QuicR Video Objects

   Most video applications would use the application component to
   identity the video stream, as well as the encoding point such as
   resolution and bitrate.  Each independently decodable set of frames
   would go in a single group, and each frame inside that group would go
   in a separate named object inside the group.  This allows an
   application to receive a given encoding of the video by subscribing
   just to the application component of the Name with a wildcard for
   group and object IDs.

   This also allows a subscription to get all the frames in the current
   group if it joins lates, or wait until the next group before starting
   to get data, based on the subscription options.  Changing to a
   different bitrate or resolution would use a new subscription to the
   appropriate Name.

   The QUIC transport that QuicR is running on provides the congestion
   control but the application can see what objects are received and
   determine if it should change it's subscription to a different
   bitrate application component.

   Todays video is often encoded with I-frames at a fixed internal but
   this can result in pulsing video quality.  Future system may want to
   insert I-frames at each change of scene resulting in groups with a
   variable number of frames.  QuicR easily supports that.

9.2.1.  RUSH over QuicR

   RUSH is an application-level protocol for ingesting live video.  This
   section defines at a higher level how aspects of the RUSH protocol
   could be realized with QuicR.

   RUSH's video frame is equivalent to QuicR video object that
   represents an instance of encoder output.  For video ingestion, the
   RUSH publisher can assign the same groupID for all the frames
   generated between the I-Frame boundaries and the RUSH's frameID can
   be directly mapped to QuicR's object ID.  RUSH multistream mode can
   enabled by publishing each frame over QUIC Stream indicated via QuicR
   API, since QuicR supports both the QUIC Datagram and QUIC Stream
   modes of transport.

   The identifiers for the track and session forms the application
   component of the name.

   Below is an example that shows RUSH's video frame mapped to QuicR
   name for the session1, track 12 and video-id that maps to a given
   encoding.  The groupID and objectID follow the encoder output.  The

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   payload of the published message will be formed by the actual encoded
   data along with metadata such as PresentationTimeStamp (PTS) and so


9.2.2.  Warp over QuicR

   Warp is a segmented live video transport protocol.  Warp maps live
   media to QUIC streams based on the underlying media encoding.

   Conceptually, each Warp video media segment maps to QuicR groupID and
   frames within segment to QuicR objectID.  Warp video media segments
   are made up of I-Frames and zero or more related frames, which
   corresponds to QuicR group of objects.  QuicR named objects
   correspond to these frames mapped to these segments and are published
   individually.  For a given channel and video quality, a segment and
   its frames can be mapped to QuicR name as below:


   In this example, groupID 12 maps to Warp segmentId 12 and objectID 0
   corresponds to I-frame within that segment.

9.3.  QuicR Audio Objects

   Each small chuck of audio, such as 10 ms, can be its own QuicR

   Future sub 2 kbps audio codecs may take advantage of a rapidly
   updated model that are needed to decode the audio which could result
   in audio needing to use groups like video to ensure all the objects
   needed to decode some audio are in the same group.

9.4.  QuicR Game Moves Objects

   Some games send out a base set of state information then incremental
   deltas to this.  Each time a new base set is sent, a new group can be
   formed and each increment change as an Object in the group.  When new
   players join, they can subscribe to sync up to the latest state by
   subscribing to the current group and the incremental changes that

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9.5.  Messaging Objects

   Chat applications and messaging system can form a manifest
   representing the roster of the people in a given channel or talk
   room.  The manifest can provide information on the application
   component of the Quicr Name for user that are contributing messages.
   A subscription to each application such component enables reception
   of each new message.  Each message would be a single object.
   Typically QuicR would be used to get the recent messages and then a
   more traditional HTTP CDN approach could be used to retrieve copies
   of all the older objects.

10.  Security Considerations

   The links between Relay and other Relays or Clients can be encrypted,
   however, this does not protect the content from Relays.  To mitigate
   this all the objects needs to be end-to-end encrypted with a keying
   mechanism outside the scope of this protocol.  For may applications,
   simply getting the keys over HTTPS for a particular object/group from
   the origin server will be adequate.  For other applicants keying
   based on MLS may be more appropriate.  Many applications can leverage
   the existing key managed schemes used in HLS and DASH for DRM
   protected content.

   Relays reachable on the Internet are assumed to have a burstiness
   relationship with the Origin and the protocol provides a way to
   verify that any data moved is on behalf of a give Origin.

   Relays in a local network may choose to process content for any
   Origin but since only local users can access them, their is a way to
   mange which applications use them.

   Subscriptions need to be refreshed at least every 5 seconds to ensure
   liveness and consent for the client to continue receiving data.

11.  Protocol Design Considerations

11.1.  HTTP/3

   It is tempting to base this on HTTP but there are a few high level
   architectural mismatches.  HTTP is largely designed for a stateless
   server in a client server architecture.  The whole concept of the
   PUB/SUB is that the relays are _not_ stateless and keep the
   subscription information and this is what allows for low latency and
   high throughput on the relays.

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   In todays CDN, the CDN nodes end up faking the credentials of the
   origin server and this limits how and where they can be a deployed.
   A design with explicitly designed relays that do not need to do this,
   while still assuming an end to end encrypted model so the relays did
   not have access to the content makes for a better design.

   It would be possible to start with something that looked like HTTP as
   the protocol between the relays with special conventions for
   wildcards in URLs of a GET and ways to stream non final responses for
   any responses perhaps using something like multipart MIME.  However,
   most of the existing code and logic for HTTP would not really be
   usable with the low latency streaming of data.  It is probably much
   simpler and more scalable to simply define a PUB/SUB protocol
   directly on top of QUIC.

11.2.  QUIC Streams and Datagrams

   There are pro and cons to mapping object transport on top of streams
   or on top of QUIC datagrams.  The working group would need to sort
   this out and consider the possibility of using both for different
   types of data and if there should be support for a semi-reliable
   transport of data.  Some objects, for example the manifest would
   always want to be received in a reliable way while other objects may
   have to be realtime.

11.3.  QUIC Congestion Control

   The basic idea in BBR of speeding up to probe then slowing down to
   drain the queue build up caused during probe can work fine with real
   time applications.  However the the current implementations in QUIC
   do not seem optimized for real time applications and have some times
   where the slow down causes too much jitter.  To not have playout
   drops, the jitter buffers adds latency to compensate for this.
   Probing for the RTT has been one of the phases that causes particular
   problems for this.  To reduce the latency of QUIC, this work should
   coordinate with the QUIC working group to have the QUIC working group
   develop congestion control optimizations for low latency use of QUIC.

11.4.  Why not RTP

   RTP has several desirable properties that optimize the transport of
   media over networks, including media payload formats explicitly
   designed for network packets, transport feedback, packet loss
   resilience mechanisms, multiplexing, and strong security.  It also
   has experimental congestion control (CC) algorithms explicitly
   designed for media delivery (RMCAT), without the issues described
   above in BBR.

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   However, these properties have less value in the context of QuicR for
   the following reasons.  QUIC adequately handles multiplexing,
   security, and transport feedback (except ack timestamps which require
   extensions proposed in drafts that have not yet been adopted by the
   QUIC WG).  QUIC lacks CC and resilience mechanisms optimized for
   media, but direct reuse of unaltered RTP mechanisms is not practical,
   so these aspects must be redesigned in the context of QUIC anyway,
   although they can leverage learnings from RTP.

   Finally, and most significantly, RTP media payload formats that were
   optimized for network packets are less useful in QuicR since a
   primary goal is to unify the streaming and real-time media delivery
   protocols.  Streaming protocols use "container" formats like CMAF,
   ISOBMFF, etc.  Codecs always first define their core "elementary"
   bitstream format, then define their container format binding, and
   finally define their RTP payload format binding.  These always
   differ.  The differences are not significant enough to justify
   supporting both, so QuicR only supports the container format binding.

   It is also interesting to observe that the use of RTP inadvertently
   leads to media description and negotiation using SDP.  Such
   complexity is justifiable when huge variation exists between clients'
   capabilities with very basic common lowest denominators.  Today, and
   while variations still exist, streamlining media capabilities into
   reasonable capability sets that are declared by publishers and
   subscribed to by subscribers is very feasible and is how the
   streaming applications do operate.  Simpler forms can and should be
   used for media declarations.  As a very wise guru once put it "RTP is
   an gateway drug to SDP and friends done't let friends try to debug

   In summary, the desirable aspects of RTP are absorbed into QUIC or
   QuicR layers rather than direct encapsulation of RTP.

Appendix A.  Acknowledgments

   Thanks to Nermeen Ismail, Mo Zanaty for contributions and suggestions
   to this specification.

Authors' Addresses

   Cullen Jennings


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   Suhas Nandakumar


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