TODO Working Group M.P. Sharabayko
Internet-Draft M.A. Sharabayko
Intended status: Informational Haivision Network Video, GmbH
Expires: 29 January 2022 28 July 2021
Tunnelling SRT over QUIC
draft-sharabayko-srt-over-quic-00
Abstract
This document presents an approach to tunnelling SRT live streams
over QUIC datagrams.
QUIC [RFC9000] is a UDP-based transport protocol providing TLS
encryption, stream multiplexing, and connection migration. It was
designed to become a faster alternative to the TCP protocol
[RFC7323].
An Unreliable Datagram Extension to QUIC [QUIC-DATAGRAM] adds support
for sending and receiving unreliable datagrams over a QUIC
connection, but transfers the responsibility for multiplexing
different kinds of datagrams, or flows of datagrams, to an
application protocol.
SRT [SRTRFC] is a UDP-based transport protocol. Essentially, it can
operate over any unreliable datagram transport. SRT provides loss
recovery and stream multiplexing mechanisms. In its live streaming
configuration SRT provides an end-to-end latency-aware mechanism for
packet loss recovery. If SRT fails to recover a packet loss within a
specified latency, then the packet is dropped to avoid blocking
playback of further packets.
The Datagram Extension to QUIC could be used as an underlying
transport instead of UDP. This way QUIC would provide TLS-level
security, connection migration, and potentially multi-path support.
It would be easier for existing network facilities to process, route,
and load balance the unified QUIC traffic. SRT on its side would
provide end-to-end latency tracking and latency-aware loss recovery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. SRT for Low Latency . . . . . . . . . . . . . . . . . . . 3
1.2. QUIC for Universal Transport . . . . . . . . . . . . . . 3
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 4
3. Use Cases for SRT over QUIC . . . . . . . . . . . . . . . . . 4
4. Tunnelling SRT over QUIC . . . . . . . . . . . . . . . . . . 4
4.1. Overhead . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Packet Integrity . . . . . . . . . . . . . . . . . . . . 6
4.3. Connection Establishment . . . . . . . . . . . . . . . . 6
4.4. Bidirectional Transmission . . . . . . . . . . . . . . . 7
4.5. Congestion Control . . . . . . . . . . . . . . . . . . . 7
4.6. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.7. Connection Mitigation . . . . . . . . . . . . . . . . . . 8
4.8. Datagram vs H3 Datagram . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . . 8
Informative References . . . . . . . . . . . . . . . . . . . . 9
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
1.1. SRT for Low Latency
The Secure Reliable Transport (SRT) protocol [SRTRFC] is a
connection-based transport protocol that enables the secure, reliable
transport of data across unpredictable networks, such as the
Internet. While any data type can be transferred via SRT, it is
ideal for low latency (sub-second) video streaming. SRT enables high
contribution bitrates over long distance connections.
To achieve low latency streaming, SRT addresses timing issues. The
characteristics of a stream from a source network can be notably
changed by transmission over the public Internet, introducing delays,
jitter, and packet loss. This, in turn, leads to problems with
decoding, as audio and video decoders do not receive packets at the
expected pace. The use of large buffers helps, but latency is
increased. SRT includes a mechanism to keep a constant end-to-end
latency, thus recreating the signal characteristics on the receiver
side, and reducing the need for buffering.
SRT employs a listener (server) / caller (client) model. The data
flow is bi-directional and independent of the connection initiation -
either the sender or receiver can operate as listener or caller to
initiate a connection.
The SRT protocol provides an internal multiplexing mechanism,
allowing multiple SRT connections to share the same UDP port,
providing access control functionality to identify the caller on the
listener side. This mechanism is exactly what QUIC DATAGRAM
describes as a responsibility of the application protocol.
Supporting forward error correction (FEC) and selective packet
retransmission (ARQ), SRT provides the flexibility to use either of
the two mechanisms or both combined, allowing for use cases ranging
from the lowest possible latency to the highest possible reliability.
SRT also allows fast file transfers, and adds support for AES
encryption.
1.2. QUIC for Universal Transport
The QUIC transport protocol [RFC9000] is a connection-based transport
protocol built on top of UDP. It provides a workflow similar to that
of TCP, but for modern fast networks.
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TODO: Add more details to the current section. Write about lower
connection times, faster delivery, ARQ, CC, etc.
2. Terms and Definitions
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.
SRT: The Secure Reliable Transport protocol.
3. Use Cases for SRT over QUIC
SRT itself is very close to QUIC, and provides similar transport
mechanisms. However, the main focus of SRT is on low-latency live
contribution and distribution. SRT is widely used by various
broadcasters to enable low-latency streaming of live events. It is
also used in various mobile and IoT devices to get low-latency
feedback and live feeds.
QUIC is supported by CDN companies. Many facilities know how to
handle and route QUIC traffic. QUIC provides certain security
advantages (TLS, encrypting headers so that traffic is not
distinguishable).
SRT tunnelled over QUIC allows managing live delivery mechanisms
(preserving end-to-end latency and dropping "too late" data).
4. Tunnelling SRT over QUIC
The QUIC DATAGRAM frame is structured as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Length (i)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Datagram Data (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: QUIC DATAGRAM Frame Format
Length. A variable-length integer specifying the length of the
datagram in bytes.
Datagram Data. The bytes of the datagram to be delivered.
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The structure of the SRT packet is shown in Figure 2. For SRT over
QUIC tunnelling the full SRT packet is placed inside the Datagram
Data field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| (Field meaning depends on the packet type) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (Field meaning depends on the packet type) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Socket ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Packet Contents |
| (depends on the packet type) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SRT Packet Structure
F: 1 bit. Packet Type Flag. A control packet has this flag set to
"1". A data packet has this flag set to "0".
Timestamp: 32 bits. The timestamp of the packet, in microseconds.
The value is relative to the time the SRT connection was
established. Depending on the transmission mode, the field stores
the packet send time or the packet origin time.
Destination Socket ID: 32 bits. A fixed-width field providing the
SRT socket ID to which a packet should be dispatched. The field
may have the special value "0" when the packet is a connection
request.
Packet Contents. Packet Contents as per packet type.
4.1. Overhead
An SRT data packet has a 16-byte header, which adds to the payload of
a QUIC packet.
For example, let us consider a payload size of 1128 bytes (six
188-byte MPEG-TS packets). For a 20 Mbps stream, knowing that each
data packet gets an additional 16 bytes of overhead, SRT would
provide an overhead of only ~280 kbits/s (or 1.4%).
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Increasing the size of the payload, e.g., to 1316 bytes (seven
188-byte MPEG-TS packets), SRT overhead at 20 Mbps would be ~240
kbits/s (or 1.2%).
An SRT receiver also sends a full ACK packet every 10 ms. The size
of the ACK packet is 44 bytes. This traffic goes in the opposite
direction: from the payload receiver to the payload sender. The
payload sender responds to every ACK packet with a corresponding
16-byte ACKACK packet. This gives an additional 1600 bytes per
second, which may be considered negligible.
4.2. Packet Integrity
SRT does not provide mechanisms to verify the integrity of packets,
nor to distinguish a packet from a continuous data stream. SRT
assumes that the underlying transport protocol delivers a single and
undamaged packet to SRT. Therefore, the underlying transport MUST
provide a mechanism for SRT to send and receive exactly one packet.
One SRT packet MUST be sent over exactly one QUIC Datagram frame.
4.3. Connection Establishment
QUIC has a fast and secure crypto handshake based on TLS. A client
connects to a server, and it can verify the server based on its
certificate. A new client connection takes two times the RTT to be
established. If a client connects to a known server, then it can try
to establish a faster 0-RTT connection.
Once a QUIC connection is established, QUIC datagrams can be sent in
both directions. SRT can use this QUIC datagram tunnelling to
establish one or many connections on its own. Each SRT connection
would take two times the RTT for handshaking.
The first handshake round of SRT is intended to get an initial
response from the server, identifying handshake procedure version and
getting a cookie from the server (listener) to mitigate potential DoS
attacks. Apart from that, no viable data is exchanged during this
first "induction" handshake phase.
If an SRT connection is established over an established and verified
QUIC connection, the SRT connection time could be reduced to a single
RTT interval if only the "conclusion" handshaking phase is performed.
However SRT does not currently provide this functionality, thus
appropriate modifications are required.
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4.4. Bidirectional Transmission
Both QUIC and SRT allow bidirectional transmission of a payload over
a single SRT connection. Even with a payload sent in one direction,
some control packets are still sent in the opposite direction over
the same connection.
4.5. Congestion Control
QUIC as a transport mechanism can apply congestion control. It is
worth noting, however, the specifics of live streaming compared to
file-based transmissions. The payload is not available right away,
therefore using regular bandwidth probing mechanisms to increase or
decrease the sending rate would not work.
The current sending rate is provided by SRT, which in turn receives
the payload from a live source and can add some overhead when
retransmission of lost packets is performed.
However, QUIC can use congestion control to detect congestion and
throughput bottlenecks, and prevent sending above a certain limit.
SRT MUST handle such cases by eventually dropping packets, which can
no longer be recovered.
It would make sense for a QUIC connection to provide this throughput
limitation value back to SRT. In that case SRT could use the number
to make clever and transport-aware retransmission decisions.
It is also possible for QUIC to not apply any congestion control,
relying on SRT. However, SRT does not reduce the sending rate below
the input rate. If the bitrate of the original stream already
exceeds network throughput, SRT would still try to deliver it,
maintaining congestion and eventually breaking the SRT connection.
It is worth mentioning that in a live streaming scenario it may be
beneficial to move congestion control mechanisms outside of the
protocol towards the encoder (payload producer), implementing a
network adaptive encoding based on the telemetry provided by the SRT
and QUIC protocols.
4.6. Pacing
SRT uses ACK/ACKACK packet pairs to measure RTT on a link, and to
track latency and clock drift. It also uses packet pair probing to
estimate connection bandwidth, although in live configurations such
estimates are informative only.
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Buffering and pacing of SRT packets by QUIC SHOULD be done with the
understanding that this would interfere with the corresponding SRT
mechanisms. Alternatively, SRT may implement a pacer on top of
QUIC's congestion control and probing mechanisms to abstract the
complexity associated with live streaming use cases.
4.7. Connection Mitigation
QUIC utilizes Connection UUID to distinguish between connections
(compared to the IP:Port scheme used by UDP and TCP). This enables
already established connections to be handed over seamlessly across
network interfaces without requiring a new handshake/negotiation.
SRT may expand on this to enable network bonded delivery workflows to
switch between optimal transports without a latency hit.
4.8. Datagram vs H3 Datagram
As an alternative to using the QUIC Datagram extension, it might be
possible to consider the H3 Datagram version in order to be
compatible with more existing load balancers.
5. Security Considerations
TODO
6. IANA Considerations
TODO
Acknowledgments
It is worth acknowledging the participation of the following people
in the project discussions: Ying Yin (Google), Ian Swett (Google),
Victor Vasiliev (Google), Kazuko Oku (Fastly), Marc Cymontkowski
(Haivision), Nikos Kyriopoulos (Haivision), Jake Weissman (Facebook),
Jordi Cenzano (Facebook), Alan Frindell (Facebook), Jeongseok Kim (SK
Telecom), Joonwoong Kim (SK Telecom).
Quicly library [QUICLY] from Fastly was chosen to provide a QUIC
datagram transport layer for SRT over QUIC PoC. We would like to
thank Kazuho Oku (Fastly) for his help.
References
Normative References
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[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/info/rfc2119>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[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/info/rfc8174>.
[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/info/rfc9000>.
Informative References
[QUIC-DATAGRAM]
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", n.d.,
<https://datatracker.ietf.org/doc/draft-ietf-quic-
datagram/>.
[QUICLY] "QUIC protocol implementation quicly for H2O server", July
2021, <https://github.com/h2o/quicly>.
[SRTRFC] Sharabayko, M., Sharabayko, M., Dube, J., Kim, J., and J.
Kim, "The SRT Protocol", December 2019,
<https://datatracker.ietf.org/doc/draft-sharabayko-srt/>.
Authors' Addresses
Maxim Sharabayko
Haivision Network Video, GmbH
Email: maxsharabayko@haivision.com
Maria Sharabayko
Haivision Network Video, GmbH
Email: msharabayko@haivision.com
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