Internet DRAFT - draft-engelbart-rtp-over-quic
draft-engelbart-rtp-over-quic
AVTCORE J. Ott
Internet-Draft M. Engelbart
Intended status: Standards Track Technical University Munich
Expires: 26 December 2022 24 June 2022
RTP over QUIC
draft-engelbart-rtp-over-quic-04
Abstract
This document specifies a minimal mapping for encapsulating RTP and
RTCP packets within QUIC. It also discusses how to leverage state
from the QUIC implementation in the endpoints to reduce the exchange
of RTCP packets and how to implement congestion control.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/mengelbart/rtp-over-quic-draft.
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 26 December 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.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5
5. Connection Establishment and ALPN . . . . . . . . . . . . . . 5
5.1. Draft version identification . . . . . . . . . . . . . . 6
6. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. QUIC Streams . . . . . . . . . . . . . . . . . . . . . . 7
6.2. QUIC Datagrams . . . . . . . . . . . . . . . . . . . . . 8
7. RTCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Transport Layer Feedback . . . . . . . . . . . . . . . . 9
7.2. Application Layer Repair and other Control Messages . . . 11
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 12
8.1. Congestion Control at the QUIC layer . . . . . . . . . . 12
8.2. Congestion Control at the Application Layer . . . . . . . 13
8.3. Shared QUIC connections . . . . . . . . . . . . . . . . . 14
9. API Considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. Information to be exported from QUIC . . . . . . . . . . 14
9.2. Functions to be exposed by QUIC . . . . . . . . . . . . . 15
10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Flow Identifier . . . . . . . . . . . . . . . . . . . . 16
10.2. Impact of Connection Migration . . . . . . . . . . . . . 16
10.3. 0-RTT considerations . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12.1. Registration of a RTP over QUIC Identification String . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Normative References . . . . . . . . . . . . . . . . . . 17
13.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Experimental Results . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
The Real-time Transport Protocol (RTP) [RFC3550] is generally used to
carry real-time media for conversational media sessions, such as
video conferences, across the Internet. Since RTP requires real-time
delivery and is tolerant to packet losses, the default underlying
transport protocol has been UDP, recently with DTLS on top to secure
the media exchange and occasionally TCP (and possibly TLS) as a
fallback. With the advent of QUIC [RFC9000] and, most notably, its
unreliable DATAGRAM extension [RFC9221], another secure transport
protocol becomes available. QUIC and its DATAGRAMs combine desirable
properties for real-time traffic (e.g., no unnecessary
retransmissions, avoiding head-of-line blocking) with a secure end-
to-end transport that is also expected to work well through NATs and
firewalls.
Moreover, with QUIC's multiplexing capabilities, reliable and
unreliable transport connections as, e.g., needed for WebRTC, can be
established with only a single port used at either end of the
connection. This document defines a mapping of how to carry RTP over
QUIC. The focus is on RTP and RTCP packet mapping and on reducing
the amount of RTCP traffic by leveraging state information readily
available within a QUIC endpoint. This document also describes
different options for implementing congestion control for RTP over
QUIC.
The scope of this document is limited to unicast RTP/RTCP.
This document does not cover signaling for session setup. Signaling
for RTP over QUIC can be defined in separate documents such as
[I-D.draft-dawkins-avtcore-sdp-rtp-quic] does for SDP.
Note that this draft is similar in spirit to but differs in numerous
ways from [I-D.draft-hurst-quic-rtp-tunnelling].
2. Terminology and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are used:
Datagram: Datagrams exist in UDP as well as in QUICs unreliable
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datagram extension. If not explicitly noted differently, the term
datagram in this document refers to a QUIC Datagram as defined in
[RFC9221].
Endpoint: A QUIC server or client that participates in an RTP over
QUIC session.
Frame: A QUIC frame as defined in [RFC9000].
Media Encoder: An entity that is used by an application to produce a
stream of encoded media, which can be packetized in RTP packets to
be transmitted over QUIC.
Receiver: An endpoint that receives media in RTP packets and may
send or receive RTCP packets.
Sender: An endpoint that sends media in RTP packets and may send or
receive RTCP packets.
Packet diagrams in this document use the format defined in
Section 1.3 of [RFC9000] to illustrate the order and size of fields.
3. Scope
RTP over QUIC mostly defines an application usage of QUIC
[I-D.draft-ietf-quic-applicability]. As a baseline, the
specification does not expect more than a standard QUIC
implementation as defined in [RFC8999], [RFC9000], [RFC9001], and
[RFC9002]. Nevertheless, the specification can benefit from QUIC
extesions such as QUIC datagrams [RFC9221] as described below.
Moreover, this document describes how a QUIC implementation and its
API can be extended to improve efficiency of the protocol operation.
On top of QUIC, this document defines an encapsulation of RTP and
RTCP packets.
The scope of this document is limited to carrying RTP over QUIC. It
does not attempt to enhance QUIC for real-time media or define a
replacement or evolution of RTP. Such new media transport protocols
may be covered elsewhere, e.g., in the MOQ WG.
Protocols for negotiating connection setup and the associated
parameters are defined separately, e.g., in
[I-D.draft-dawkins-avtcore-sdp-rtp-quic].
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4. Protocol Overview
This document introduces a mapping of the Real-time Transport
Protocol (RTP) to the QUIC transport protocol. RTP over QUIC allows
the use of QUIC streams and unreliable QUIC datagrams to transport
real-time data, and thus, the QUIC implementation MUST support QUICs
unreliable datagram extension, if RTP packets should be sent over
QUIC datagrams. Since datagram frames cannot be fragmented, the QUIC
implementation MUST also provide a way to query the maximum datagram
size so that an application can create RTP packets that always fit
into a QUIC datagram frame.
[RFC3550] specifies that RTP sessions need to be transmitted on
different transport addresses to allow multiplexing between them.
RTP over QUIC uses a different approach to leverage the advantages of
QUIC connections without managing a separate QUIC connection per RTP
session. QUIC does not provide demultiplexing between different
flows on datagrams but suggests that an application implement a
demultiplexing mechanism if required. An example of such a mechanism
are flow identifiers prepended to each datagram frame as described in
Section 2.1 of [I-D.draft-ietf-masque-h3-datagram]. RTP over QUIC
uses a flow identifier to replace the network address and port number
to multiplex many RTP sessions over the same QUIC connection.
A congestion controller can be plugged in to adapt the media bitrate
to the available bandwidth. This document does not mandate any
congestion control algorithm. Some examples include Network-Assisted
Dynamic Adaptation (NADA) [RFC8698] and Self-Clocked Rate Adaptation
for Multimedia (SCReAM) [RFC8298]. These congestion control
algorithms require some feedback about the network's performance to
calculate target bitrates. Traditionally this feedback is generated
at the receiver and sent back to the sender via RTCP. Since QUIC
also collects some metrics about the network's performance, these
metrics can be used to generate the required feedback at the sender-
side and provide it to the congestion controller to avoid the
additional overhead of the RTCP stream.
5. Connection Establishment and ALPN
QUIC requires the use of ALPN [RFC7301] tokens during connection
setup. RTP over QUIC uses "rtp-mux-quic" as ALPN token in the TLS
handshake (see also Section 12.
Note that the use of a given RTP profile is not reflected in the ALPN
token even though it could be considered part of the application
usage. This is simply because different RTP sessions, which may use
different RTP profiles, may be carried within the same QUIC
connection.
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*Editor's note:* "rtp-mux-quic" indicates that RTP and other
protocols may be multiplexed on the same QUIC connection using a
flow identifier as described in Section 6. Applications are
responsible for negotiation of protocols in use by appropriate use
of a signaling protocol such as SDP.
*Editor's note:* This implies that applications cannot use RTP
over QUIC as specified in this document over WebTransport.
5.1. Draft version identification
*RFC Editor's note:* Please remove this section prior to
publication of a final version of this document.
RTP over QUIC uses the token "rtp-mux-quic" to identify itself in
ALPN.
Only implementations of the final, published RFC can identify
themselves as "rtp-mux-quic". Until such an RFC exists,
implementations MUST NOT identify themselves using this string.
Implementations of draft versions of the protocol MUST add the string
"-" and the corresponding draft number to the identifier. For
example, draft-engelbart-rtp-over-quic-04 is identified using the
string "rtp-mux-quic-04".
Non-compatible experiments that are based on these draft versions
MUST append the string "-" and an experiment name to the identifier.
6. Encapsulation
QUIC supports two transport methods: reliable streams [RFC9000] and
unreliable datagrams [RFC9221]. This document specifies a mapping of
RTP to both of the transport modes. The encapsulation format for RTP
over QUIC is described in Figure 1.
Section 6.1 and Section 6.2 explain the specifics of mapping of RTP
to QUIC streams and QUIC datagrams respectively.
Payload {
Flow Identifier (i),
RTP/RTCP Packet (..)
}
Figure 1: RTP over QUIC Payload Format
Flow Identifier: Flow identifier to demultiplex different data flows
on the same QUIC connection.
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RTP/RTCP Packet: The RTP/RTCP packet to transmit.
For multiplexing different RTP and other data streams on the same
QUIC connection, each RTP/RTCP packet is prefixed with a flow
identifier. A flow identifier is a QUIC variable-length integer
which must be unique per stream.
RTP and RTCP packets of a single RTP session MAY be sent using the
same flow identifier (following the procedures defined in [RFC5761],
or they MAY be sent using different flow identifiers. The respective
mode of operation MUST be indicated using the appropriate signaling.
RTP and RTCP packets of different RTP sessions MUST be sent using
different flow identifiers.
Differentiating RTP/RTCP packets of different RTP sessions from non-
RTP/RTCP datagrams is the responsibility of the application by means
of appropriate use of flow identifiers and the corresponding
signaling.
This specification defines two ways of carrying RTP packets in QUIC:
1) using reliable QUIC streams and 2) using unreliable QUIC
DATAGRAMs. Every RTP session MUST choose exactly one way of carrying
RTP and RTCP packets, different RTP sessions MAY choose different
ways.
6.1. QUIC Streams
An application MUST open a new QUIC stream for each Application Data
Unit (ADU). Each ADU MUST be encapsulated in a single RTP packet and
the application MUST not send more than one RTP packet per stream.
Opening a new stream for each packet adds implicit framing to RTP
packets, allows to receive packets without strict ordering and gives
an application the possibility to cancel certain packets.
Large RTP packets sent on a stream will be fragmented in smaller QUIC
frames, that are transmitted reliably and in order, such that a
receiving application can read a complete packet from the stream. No
retransmission has to be implemented by the application, since QUIC
frames that are lost in transit are retransmitted by the QUIC
connection. If it is known to either the sender or the receiver,
that a packet, which was not yet successfully and completely
transmitted, is no longer needed, either side can close the stream.
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*Editor's Note:* We considered adding a framing like the one
described in [RFC4571] to send multiple RTP packets on one stream,
but we don't think it is worth the additional overhead only to
reduce the number of streams. Moreover, putting multiple ADUs
into a single stream would also require defining policies when to
use the same (and which) stream and when to open a new one.
*Editor's Note:* Note, however, that using a single frame per
stream in a single RTP packet may cause interworking issues when a
translator wants to forward packets received via RTP-over-QUIC to
an endpoint as UDP packets because the received ADUs may exceed
the MTU size or even maximum UDP packet size.
6.2. QUIC Datagrams
RTP packets can be sent in QUIC datagrams. QUIC datagrams are an
extension to QUIC described in [RFC9221]. QUIC datagrams preserve
frame boundaries, thus a single RTP packet can be mapped to a single
QUIC datagram, without the need for an additional framing. Senders
SHOULD consider the header overhead associated with QUIC datagrams
and ensure that the RTP/RTCP packets, including their payloads, QUIC,
and IP headers, will fit into path MTU.
If an application wishes to retransmit lost RTP packets, the
retransmission has to be implemented by the application by sending a
new datagram for the RTP packet, because QUIC datagrams are not
retransmitted on loss (see also Section 7.1 for loss signaling).
7. RTCP
The RTP Control Protocol (RTCP) allows RTP senders and receivers to
exchange control information to monitor connection statistics and to
identify and synchronize streams. Many of the statistics contained
in RTCP packets overlap with the connection statistics collected by a
QUIC connection. To avoid using up bandwidth for duplicated control
information, the information SHOULD only be sent at one protocol
layer. QUIC relies on certain control frames to be sent.
In general, applications MAY send RTCP without any restrictions.
This document specifies a baseline for replacing some of the RTCP
packet types by mapping the contents to QUIC connection statistics.
Future documents can extend this mapping for other RTCP format types.
It is RECOMMENDED to expose relevant information from the QUIC layer
to the application instead of exchanging addtional RTCP packets,
where applicable.
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This section discusses what information can be exposed from the QUIC
connection layer to reduce the RTCP overhead and which type of RTCP
messages cannot be replaced by similar feedback from the transport
layer. The list of RTCP packets in this section is not exhaustive
and similar considerations SHOULD be taken into account before
exchanging any other type of RTCP control packets.
7.1. Transport Layer Feedback
This section explains how some of the RTCP packet types which are
used to signal reception statistics can be replaced by equivalent
statistics that are already collected by QUIC. The following list
explains how this mapping can be achieved for the individual fields
of different RTCP packet types.
QUIC Datagrams are ack-eliciting packets, which means, that an
acknowledgment is triggered when a datagram frame is received. Thus,
a sender can assume that an RTP packet arrived at the receiver or was
lost in transit, using the QUIC acknowledgments of QUIC Datagram
frames. In the following, an RTP packet is regarded as acknowledged,
when the QUIC Datagram frame that carried the RTP packet, was
acknowledged. For RTP packets that are sent over QUIC streams, an
RTP packet can be considered acknowledged, when all frames which
carried fragments of the RTP packet were acknowledged.
When QUIC Streams are used, the application should be aware that the
direct mapping proposed below may not reflect the real
characteristics of the network. RTP packet loss can seem lower than
actual packet loss due to QUIC's automatic retransmissions.
Similarly, timing information might be incorrect due to
retransmissions.
Some of the transport layer feedback that can be implemented in RTCP
contains information that is not included in QUIC by default, but can
be added via QUIC extensions. One important example are arrival
timestamps, which are not part of QUIC's default acknowledgment
frames, but can be added using [I-D.draft-smith-quic-receive-ts] or
[I-D.draft-huitema-quic-ts]. Another extension, that can improve the
precision of the feedback from QUIC is
[I-D.draft-ietf-quic-ack-frequency], which allows a sender to control
the delay of acknowledgments sent by the receiver.
* _Receiver Reports_ (PT=201, Name=RR, [RFC3550])
- _Fraction lost_: The fraction of lost packets can be directly
infered from QUIC's acknowledgments. The calculation SHOULD
include all packets up to the acknowledged RTP packet with the
highest RTP sequence number. Later packets SHOULD be ignored,
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since they may still be in flight, unless other QUIC packets
that were sent after the datagram frame, were already
acknowledged.
- _Cumulative lost_: Similar to the fraction of lost packets, the
cumulative loss can be infered from QUIC's acknowledgments
including all packets up to the latest acknowledged packet.
- _Highest Sequence Number received_: The highest sequence number
received is the sequence number of all RTP packets that were
acknowledged.
- Interarrival jitter: If QUIC acknowledgments carry timestamps
as described in one of the extensions referenced above, senders
can infer from QUIC acks the interarrival jitter from the
arrival timestamps.
- Last SR: Similar to RTP arrival times, the arrival time of RTCP
Sender Reports can be inferred from QUIC acknowledgments, if
they include timestamps.
- Delay since last SR: This field is not required when the
receiver reports are entirely replaced by QUIC feedback.
* _Negative Acknowledgments_ (PT=205, FMT=1, Name=Generic NACK,
[RFC4585])
- The generic negative acknowledgment packet contains information
about packets which the receiver considered lost.
Section 6.2.1. of [RFC4585] recommends to use this feature
only, if the underlying protocol cannot provide similar
feedback. QUIC does not provide negative acknowledgments, but
can detect lost packets through acknowledgments.
* _ECN Feedback_ (PT=205, FMT=8, Name=RTCP-ECN-FB, [RFC6679])
- ECN feedback packets report the count of observed ECN-CE marks.
[RFC6679] defines two RTCP reports, one packet type (with
PT=205 and FMT=8) and a new report block for the extended
reports which are listed below. QUIC supports ECN reporting
through acknowledgments. If the connection supports ECN, the
reporting of ECN counts SHOULD be done using QUIC
acknowledgments.
* _Congestion Control Feedback_ (PT=205, FMT=11, Name=CCFB,
[RFC8888])
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- RTP Congestion Control Feedback contains acknowledgments,
arrival timestamps and ECN notifications for each received
packet. Acknowledgments and ECNs can be infered from QUIC as
described above. Arrival timestamps can be added through
extended acknowledgment frames as described in
[I-D.draft-smith-quic-receive-ts] or
[I-D.draft-huitema-quic-ts].
* _Extended Reports_ (PT=207, Name=XR, [RFC3611])
- Extended Reports offer an extensible framework for a variety of
different control messages. Some of the standard report blocks
which can be implemented in extended reports such as loss RLE
or ECNs can be implemented in QUIC, too. For other report
blocks, it SHOULD be evaluated individually, if the contained
information can be transmitted using QUIC instead.
7.2. Application Layer Repair and other Control Messages
While the previous section presented some RTCP packet that can be
replaced by QUIC features, QUIC cannot replace all of the available
RTCP packet types. This mostly affects RTCP packet types which carry
control information that is to be interpreted by the application
layer instead of the transport itself.
_Sender Reports_ (PT=200, Name=SR, [RFC3550]) are similar to
_Receiver Reports_. They are sent by media senders and additionally
contain a NTP and a RTP timestamp and the number of packets and
octets transmitted by the sender. The timestamps can be used by a
receiver to synchronize streams. QUIC cannot provide a similar
control information, since it does not know about RTP timestamps. A
QUIC receiver can also not calculate the packet or octet counts,
since it does not know about lost datagrams. Thus, sender reports
are required in RTP over QUIC to synchronize streams at the receiver.
The sender reports SHOULD not contain any receiver report blocks, as
the information can be infered from the QUIC transport as explained
in the previous section.
Next to carrying transmission statistics, RTCP packets can contain
application layer control information, that cannot directly be mapped
to QUIC. This includes for example the _Source Description_ (PT=202,
Name=SDES), _Bye_ (PT=203, Name=BYE) and _Application_ (PT=204,
Name=APP) packet types from [RFC3550] or many of the payload specific
feedback messages (PT=206) defined in [RFC4585], which can for
example be used to control the codec behavior of the sender. Since
QUIC does not provide any kind of application layer control
messaging, these RTCP packet types SHOULD be used in the same way as
they would be used over any other transport protocol.
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8. Congestion Control
Like any other application on the internet, RTP over QUIC needs to
perform congestion control to avoid overloading the network.
QUIC is a congestion controlled transport protocol. Senders are
required to employ some form of congestion control. The default
congestion control specified for QUIC is an alogrithm similar to TCP
NewReno, but senders are free to choose any congestion control
algorithm as long as they follow the guidelines specified in
Section 3 of [RFC8085].
RTP does not specify a congestion controller, but provides feedback
formats for congestion control (e.g. [RFC8888]) as well as different
congestion control algorithms in separate RFCs (e.g. SCReAM
[RFC8298] and NADA [RFC8698]). The congestion control algorithms for
RTP are specifically tailored for real-time transmissions at low
latencies. The available congestion control algorithms for RTP
expose a target_bitrate that can be used to dynamically reconfigure
media codecs to produce media at a rate that can be sent in real-time
under the observed network conditions.
This section defines two architectures for congestion control and
bandwidth estimation for RTP over QUIC, but it does not mandate any
specific congestion control algorithm to use. The section also
discusses congestion control implications of using shared or multiple
separate QUIC connections to send and receive multiple independent
data streams.
It is assumed that the congestion controller in use provides a pacing
mechanism to determine when a packet can be sent to avoid bursts.
The currently proposed congestion control algorithms for real-time
communications provide such pacing mechanisms. The use of congestion
controllers which don't provide a pacing mechanism is out of scope of
this document.
*TODO:* Add considerations for bandwidth shares when a QUIC
connection is shared between RTP and non-RTP streams?
8.1. Congestion Control at the QUIC layer
If congestion control is to be applied at the transport layer, it is
RECOMMENDED to configure the QUIC Implementation to use a delay-based
real-time congestion control algorithm instead of a loss-based
algorithm. The currently available delay-based congestion control
algorithms depend on detailed arrival time feedback to estimate the
current one-way delay between sender and receiver. Since QUIC does
not provide arrival timestamps in its acknowledgments, the QUIC
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implementations of the sender and receiver MUST use an extension to
add this information to QUICs acknowledgment frames, e.g.
[I-D.draft-smith-quic-receive-ts].
If congestion control is done by the QUIC implementation, the
application needs a mechanism to query the currently available
bandwidth to adapt media codec configurations. The employed
congestion controller of the QUIC connection SHOULD expose such an
API to the application. If a current bandwidth estimation is not
available from the QUIC congestion controller, the sender can either
implement an alternative bandwidth estimation at the application
layer as described in Section 8.2 or a receiver can feedback the
observed bandwidth through RTCP, e.g., using
[I-D.draft-alvestrand-rmcat-remb].
*Editor's note:* An alternative to the hard requirement to use a
timestamp extension could be to use RTCP, but that would mean,
that an application has to negotiate RTCP congestion control
feedback which would then have to be passed to the QUIC congestion
controller.
*Editor's note:* How can a QUIC connection be shared with non-RTP
streams, when SCReAM/NADA/GCC is used as congestion controller?
Can these algorithms be adapted to allow different streams
including non-real-time streams? Do they even have to be adapted
or _should_ this just work?
8.2. Congestion Control at the Application Layer
If an application cannot access a bandwidth estimation from the QUIC
layer, or the QUIC implementation does not support a delay-based,
low-latency congestion control algorithm, it can alternatively
implement a bandwidth estimation algorithm at the application layer.
Calculating a bandwidth estimation at the application layer can be
done using the same bandwidth estimation algorithms as described in
Section 8 (NADA, SCReAM). The bandwidth estimation algorithm
typically needs some feedback on the transmission performance. This
feedback can be collected following the guidelines in Section 7.
If the application implements full congestion control rather than
just a bandwidth estimation at the application layer using a
congestion controller that satisfies the requirements of Section 7 of
[RFC9002], and the connection is only used to send real-time media
which is subject to the application layer congestion control, it is
RECOMMENDED to disable any other congestion control that is possibly
running at the QUIC layer. Disabling the additional congestion
controllers helps to avoid any interference between the different
congestion controllers.
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8.3. Shared QUIC connections
Two endpoints may want to establish channels to exchange more than
one type of data simultaneously. The channels can be intended to
carry real-time RTP data or other non-real-time data. This can be
realized in different ways. A straightforward solution is to
establish multiple QUIC connections, one for each channel. Or all
real-time channels are mapped to one QUIC connection, while a
separate QUIC connection is created for the non-real-time channels.
In both cases, the congestion controllers can be chosen to match the
demands of the respective channels and the different QUIC connections
will compete for the same resources in the network. No local
prioritization of data across the different (types of) channels would
be necessary.
Alternatively, (all or a subset of) real-time and non-real-time
channels are multiplexed onto a single, shared QUIC connection, which
can be done by using the flow identifier described in Section 6.
Applications multiplexing multiple streams in one connection SHOULD
implement some form of stream prioritization or bandwidth allocation.
9. API Considerations
The mapping described in the previous sections poses some interface
requirements on the QUIC implementation. Although a basic mapping
should work without any of these requirements most of the
optimizations regarding congestion control and RTCP mapping require
certain functionalities to be exposed to the application. The
following to sections contain a list of information that is required
by an application to implement different optimizations (Section 9.1)
and functions that a QUIC implementation SHOULD expose to an
application (Section 9.2).
Each item in the following list can be considered individually. Any
exposed information or function can be used by RTP over QUIC
regardless of whether the other items are available. Thus, RTP over
QUIC does not depend on the availability of all of the listed
features but can apply different optimizations depending on the
functionality exposed by the QUIC implementation.
9.1. Information to be exported from QUIC
This section provides a list of items that an application might want
to export from an underlying QUIC implementation. It is thus
RECOMMENDED that a QUIC implementation exports the listed items to
the application.
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* _Maximum Datagram Size_: The maximum datagram size that the QUIC
connection can transmit.
* _Datagram Acknowledgment and Loss_: Section 5.2 of [RFC9221]
allows QUIC implementations to notify the application that a QUIC
Datagram was acknowledged or that it believes a datagram was lost.
The exposed information SHOULD include enough information to allow
the application to maintain a mapping between the datagram that
was acknowledged/lost and the RTP packet that was sent in that
datagram.
* _Stream States_: The QUIC implementation SHOULD expose to a
sender, how much of the data that was sent on a stream was
successfully delivered and how much data is still outstanding to
be sent or retransmitted.
* _Arrival timestamps_: If the QUIC connection uses a timestamp
extension like [I-D.draft-smith-quic-receive-ts] or
[I-D.draft-huitema-quic-ts], the arrival timestamps or one-way
delays SHOULD be exposed to the application.
* _ECN_: If ECN marks are available, they SHOULD be exposed to the
application.
9.2. Functions to be exposed by QUIC
This sections lists functions that a QUIC implementation SHOULD
expose to an application to implement different features of the
mapping described in the previous sections of this document.
* _Cancel Streams_: To allow an application to cancel
(re)transmission of packets that are no longer needed, the QUIC
implementation MUST expose a way to cancel the corresponding QUIC
streams.
* _Select Congestion Controller_: If congestion control is to be
implemented at the QUIC connection layer as described in
Section 8.1, the application must be able to choose an appropriate
congestion control algorithm.
* _Disable Congestion Controller_: If congestion control is to be
implemented at the application layer as described in Section 8.2,
and the application layer is trusted to apply adequate congestion
control, it is RECOMMENDEDto allow the application to disable QUIC
layer congestion control entirely.
10. Discussion
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10.1. Flow Identifier
[RFC9221] suggests to use flow identifiers to multiplex different
streams on QUIC Datagrams, which is implemented in Section 6, but it
is unclear how applications can combine RTP over QUIC with other data
streams using the same QUIC connections. If the non-RTP data streams
use the same flow identifies, too and the application can make sure,
that flow identifiers are unique, there should be no problem. Flow
identifiers could be problematic, if different specifications for RTP
and non-RTP data streams over QUIC mandate different incompatible
flow identifiers.
10.2. Impact of Connection Migration
RTP sessions are characterized by a continuous flow of packets into
either or both directions. A connection migration may lead to
pausing media transmission until reachability of the peer under the
new address is validated. This may lead to short breaks in media
delivery in the order of RTT and, if RTCP is used for RTT
measurements, may cause spikes in observed delays. Application layer
congestion control mechanisms (and also packet repair schemes such as
retransmissions) need to be prepared to cope with such spikes.
If a QUIC connection is established via a signaling channel, this
signaling may have involved Interactive Connectivity Establishment
(ICE) exchanges to determine and choose suitable (IP address, port
number) pairs for the QUIC connection. Subsequent address change
events may be noticed by QUIC via its connection migration handling
and/or at the ICE or other application layer, e.g., by noticing
changing IP addresses at the network interface. This may imply that
the two signaling and data "layers" get (temporarily) out of sync.
*Editor's Note:* It may be desirable that the API provides an
indication of connection migration event for either case.
10.3. 0-RTT considerations
For repeated connections between peers, the initiator of a QUIC
connection can use 0-RTT data for both QUIC streams and datagrams.
As such packets are subject to replay attacks, applications shall
carefully specify which data types and operations are allowed. 0-RTT
data may be beneficial for use with RTP over QUIC to reduce the risk
of media clipping, e.g., at the beginning of a conversation.
This specification defines carrying RTP on top of QUIC and thus does
not finally define what the actual application data are going to be.
RTP typically carries ephemeral media contents that is rendered and
possibly recorded but otherwise causes no side effects. Moreover,
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the amount of data that can be carried as 0-RTT data is rather
limited. But it is the responsibility of the respective application
to determine if 0-RTT data is permissible.
*Editor's Note:* Since the QUIC connection will often be created
in the context of an existing signaling relationship (e.g., using
WebRTC or SIP), specific 0-RTT keying material could be exchanged
to prevent replays across sessions. Within the same connection,
replayed media packets would be discarded as duplicates by the
receiver.
11. Security Considerations
RTP over QUIC is subject to the security considerations of RTP
described in Section 9 of [RFC3550] and the security considerations
of any RTP profile in use.
The security considerations for the QUIC protocol and datagram
extension described in Section 21 of [RFC9000], Section 9 of
[RFC9001], Section 8 of [RFC9002] and Section 6 of [RFC9221] also
apply to RTP over QUIC.
12. IANA Considerations
12.1. Registration of a RTP over QUIC Identification String
This document creates a new registration for the identification of
RTP over QUIC in the "TLS Application-Layer Protocol Negotiation
(ALPN) Protocol IDs" registry [RFC7301].
The "rtp-mux-quic" string identifies RTP over QUIC:
Protocol: RTP over QUIC
Identification Sequence: 0x72 0x74 0x70 0x2D 0x6F 0x76 0x65 0x72
0x2D 0x71 0x75 0x69 0x63 ("rtp-mux-quic")
Specification: This document
13. References
13.1. Normative References
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[I-D.draft-huitema-quic-ts]
Huitema, C., "Quic Timestamps For Measuring One-Way
Delays", Work in Progress, Internet-Draft, draft-huitema-
quic-ts-07, 6 March 2022,
<https://datatracker.ietf.org/doc/html/draft-huitema-quic-
ts-07>.
[I-D.draft-ietf-quic-ack-frequency]
Iyengar, J. and I. Swett, "QUIC Acknowledgement
Frequency", Work in Progress, Internet-Draft, draft-ietf-
quic-ack-frequency-01, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
ack-frequency-01>.
[I-D.draft-smith-quic-receive-ts]
Smith, C. and I. Swett, "QUIC Extension for Reporting
Packet Receive Timestamps", Work in Progress, Internet-
Draft, draft-smith-quic-receive-ts-00, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-smith-quic-
receive-ts-00>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<https://www.rfc-editor.org/rfc/rfc3611>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/rfc/rfc4585>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<https://www.rfc-editor.org/rfc/rfc5761>.
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[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/rfc/rfc6679>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
2017, <https://www.rfc-editor.org/rfc/rfc8298>.
[RFC8698] Zhu, X., Pan, R., Ramalho, M., and S. Mena, "Network-
Assisted Dynamic Adaptation (NADA): A Unified Congestion
Control Scheme for Real-Time Media", RFC 8698,
DOI 10.17487/RFC8698, February 2020,
<https://www.rfc-editor.org/rfc/rfc8698>.
[RFC8888] Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
Control Protocol (RTCP) Feedback for Congestion Control",
RFC 8888, DOI 10.17487/RFC8888, January 2021,
<https://www.rfc-editor.org/rfc/rfc8888>.
[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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[RFC9221] Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", RFC 9221,
DOI 10.17487/RFC9221, March 2022,
<https://www.rfc-editor.org/rfc/rfc9221>.
13.2. Informative References
[I-D.draft-alvestrand-rmcat-remb]
Alvestrand, H., "RTCP message for Receiver Estimated
Maximum Bitrate", Work in Progress, Internet-Draft, draft-
alvestrand-rmcat-remb-03, 21 October 2013,
<https://datatracker.ietf.org/doc/html/draft-alvestrand-
rmcat-remb-03>.
[I-D.draft-dawkins-avtcore-sdp-rtp-quic]
Dawkins, S., "SDP Offer/Answer for RTP using QUIC as
Transport", Work in Progress, Internet-Draft, draft-
dawkins-avtcore-sdp-rtp-quic-00, 28 January 2022,
<https://datatracker.ietf.org/doc/html/draft-dawkins-
avtcore-sdp-rtp-quic-00>.
[I-D.draft-hurst-quic-rtp-tunnelling]
Hurst, S., "QRT: QUIC RTP Tunnelling", Work in Progress,
Internet-Draft, draft-hurst-quic-rtp-tunnelling-01, 28
January 2021, <https://datatracker.ietf.org/doc/html/
draft-hurst-quic-rtp-tunnelling-01>.
[I-D.draft-ietf-masque-h3-datagram]
Schinazi, D. and L. Pardue, "HTTP Datagrams and the
Capsule Protocol", Work in Progress, Internet-Draft,
draft-ietf-masque-h3-datagram-11, 17 June 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-masque-
h3-datagram-11>.
[I-D.draft-ietf-quic-applicability]
Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
Transport Protocol", Work in Progress, Internet-Draft,
draft-ietf-quic-applicability-16, 6 April 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
applicability-16>.
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
and RTP Control Protocol (RTCP) Packets over Connection-
Oriented Transport", RFC 4571, DOI 10.17487/RFC4571, July
2006, <https://www.rfc-editor.org/rfc/rfc4571>.
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[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>.
Appendix A. Experimental Results
An experimental implementation of the mapping described in this
document can be found on Github (https://github.com/mengelbart/rtp-
over-quic). The application implements the RTP over QUIC Datagrams
mapping and implements SCReAM congestion control at the application
layer. It can optionally disable the builtin QUIC congestion control
(NewReno). The endpoints only use RTCP for congestion control
feedback, which can optionally be disabled and replaced by the QUIC
connection statistics as described in Section 7.1.
Experimental results of the implementation can be found on Github
(https://github.com/mengelbart/rtp-over-quic-mininet), too.
Acknowledgments
The authors would like to thank Spencer Dawkins, Lucas Pardue and
David Schinazi for their valuable comments and suggestions
contributing to this document.
Authors' Addresses
Jörg Ott
Technical University Munich
Email: ott@in.tum.de
Mathis Engelbart
Technical University Munich
Email: mathis.engelbart@gmail.com
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