QuicR - Media Delivery Protocol over QUIC
draft-jennings-moq-quicr-arch-00
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| Authors | Cullen Fluffy Jennings , Suhas Nandakumar | ||
| Last updated | 2022-03-04 | ||
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draft-jennings-moq-quicr-arch-00
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
draft-jennings-moq-quicr-arch-00
Abstract
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 September 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
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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
networks.
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: "https://github.com/fluffy/draft-jennings-
moq-arch"
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
experience.
* 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
protocol.
* 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
backend.
* 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
Function.
+-------------+
|Relay |
+----------------> |Origin:tw.com|-----+
| +-------------+ |
| ^ |
|pub:tw.com/ch22/3/1 | |
| | |
| sub:tw.com/ch22/*| |
| | |pub:tw.com/ch22/3/1
| | v
+-----------+ +----------+
| Publisher | |Subscriber|
+-----------+ +----------+
The above picture shows QuicR delivery network for an hypothetical
streaming architecture rooted at the Origin Relay (for the domain
tw.com). 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
components:
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:
sfu.webex.com:5004.
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
acme.meeting.com 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.
quicr://acme.meeting.com/meeting123/alice/cam5/HiRes/15/17
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
Relays.
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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.
"quicr://acme.meeting.com/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
request.
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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
application.
* 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
encrypted.
* 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
example].
* 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
on.
"quicr://rush-ingest-server/session1/track12/video-id/group1/
object10"
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:
"quicr://twitch.com/channel-fluffy/video-quality-id/group12/object0"
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
object.
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
follow.
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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
SDP".
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
cisco
Canada
Email: fluffy@iii.ca
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Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
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