Network Working Group S. Hurst Internet-Draft BBC Research & Development Intended status: Informational 30 October 2020 Expires: 3 May 2021 QRT: QUIC RTP Tunnelling draft-hurst-quic-rtp-tunnelling-00 Abstract QUIC is a UDP-based transport protocol for stream-orientated, congestion-controlled, secure, multiplexed data transfer. RTP carries real-time data between endpoints, and the accompanying control protocol RTCP allows monitoring and control of the transfer of such data. With RTP and RTCP being agnostic to the underlying transport protocol, it is possible to multiplex both the RTP and associated RTCP flows into a single QUIC connection to take advantage of QUIC features such as low-latency setup and strong TLS-based security. 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 3 May 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components Hurst Expires 3 May 2021 [Page 1] Internet-Draft QRT October 2020 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. Conventions and Definitions . . . . . . . . . . . . . . . 3 1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 2. Use Cases for an RTP Mapping over QUIC . . . . . . . . . . . 4 2.1. Live Event Contribution Feed . . . . . . . . . . . . . . 4 2.2. Audio and Video Conference via a Central Server . . . . . 5 3. QRT Sessions . . . . . . . . . . . . . . . . . . . . . . . . 5 4. RTP Sessions . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. QRT Flow Identifier . . . . . . . . . . . . . . . . . . . 6 4.2. RTCP Mapping . . . . . . . . . . . . . . . . . . . . . . 7 4.2.1. Restricted RTCP Packet Types . . . . . . . . . . . . 7 5. Loss Recovery and Retransmission . . . . . . . . . . . . . . 8 6. Using the Session Description Protocol to Advertise QRT Sessions . . . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Using the Session Description Protocol to Advertise QRT Sessions using RTP Retransmission . . . . . . . . . . . . 9 7. Exposing Round-Trip Time to RTP applications . . . . . . . . 10 8. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 10 8.1. Draft Version Identification . . . . . . . . . . . . . . 10 9. Security Considerations . . . . . . . . . . . . . . . . . . . 11 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10.1. Registration of Protocol Identification String . . . . . 11 10.2. Registration of SDP Protocol Identifier . . . . . . . . 11 10.3. Registration of SDP Attribute Field . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 11.1. Normative References . . . . . . . . . . . . . . . . . . 12 11.2. Informative References . . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction The Real-time Transport Protocol (RTP) [RFC3550] provides end-to-end network transport functions suitable for applications transmitting data, such as audio and video, over multicast or unicast network services for the purposes of telephony, video streaming, conferencing and other real-time applications. The QUIC transport protocol is a UDP-based stream-orientated and encrypted transport protocol aimed at offering improvements over the common combination of TCP and TLS for web applications. Compared with TCP+TLS, QUIC offers much reduced connection set-up times, Hurst Expires 3 May 2021 [Page 2] Internet-Draft QRT October 2020 improved stream multiplexing aware congestion control, and the ability to perform connection migration. QUIC offers two modes of data transfer: * Reliable transfer using STREAM frames, as specified in [QUIC-TRANSPORT], [QUIC-RECOVERY], etc. * Unreliable transfer using DATAGRAM extension frames, as specified in [QUIC-DATAGRAM]. RTP has traditionally been run over UDP or DTLS to achieve timely but unreliable data transfer. For use cases such as real-time audio and video transmission, the underlying media codecs can be considered in part fault-tolerant to an unreliable transport mechanism, with missing data from the stream resulting in glitches in the media presentation, such as missing video frames or gaps in audio playback. By purposely using an unreliable transport mechanism, applications can minimise the added latency that would otherwise result from managing the large packet reception buffers needed to account for network reordering or transport protocol retransmission. 1.1. Conventions 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. Packet and frame diagrams in this document use the format described in [QUIC-TRANSPORT]. 1.2. Definitions * Endpoint: host capable of being a participant in a QRT session. * QRT session: A QUIC connection carrying one or more RTP sessions, each with or without an accompanying RTCP channel. * Client: The endpoint which initiates the QUIC connection. * Server: The endpoint which accepts the incoming QUIC connection. Hurst Expires 3 May 2021 [Page 3] Internet-Draft QRT October 2020 2. Use Cases for an RTP Mapping over QUIC The following sections describe some possible use cases for an RTP and RTCP mapping over QUIC, hereafter QRT. The examples were chosen to illustrate some basic concepts, and are neither an exhaustive list of possible use cases nor a limitation on what QRT may be used for. 2.1. Live Event Contribution Feed A news organisation wishes to provide a two-way link to a live event for distribution as part of an item in a news programme hosted in a studio with a news anchor. The single camera remote production crew will include a camera operator, sound technician and the reporter. In order to deliver this experience, the following media flows are required: * A high-quality video feed from the remote camera to the news organisation's gallery; * One or more audio feeds for microphones at the event, including an ambient microphone attached to the camera, a lapel microphone for the reporter, and a handheld microphone to conduct interviews, all synchronized; * A video feed of the programme output from the gallery, after mixing for local monitoring and for use on a comfort monitor; * An audio feed from the anchor in the studio to the reporter; * A two-way audio feed from the gallery to the remote production crew for talkback communication; * A tally light feed for the remote camera. These media flows may be realised as a group of RTP sessions, some of which must be synchronised together. The talkback streams do not require any tight synchronisation with other streams in the group, whereas the camera video feed and various microphone feeds need to be tightly synchronised together. At the event, a production machine running a software package that includes a QRT client has two connections to the Internet; a high- speed fibre link and a bonded cellular network link for backup. In order to prevent a bad actor on the network path being able to tamper with the contribution, all communication between the news organisation's gallery and the remote production need to be encrypted. Because all the data is flowing between the same two Hurst Expires 3 May 2021 [Page 4] Internet-Draft QRT October 2020 endpoints, only a single QRT session is required, and the various RTP sessions that are encapsulated by the QRT session are (de)multiplexed at each end. During the live contribution, an accident cuts the fibre connection to the remote production crew. Using the QUIC connection migration mechanism presented in Section 9 of [QUIC-TRANSPORT], the QRT session migrates from the fibre link onto the backup cellular link. This preserves the state of the RTP sessions across a network migration event, and all sessions continue. 2.2. Audio and Video Conference via a Central Server A teleconference is taking place across multiple sites using a centralised server. All participants connect to this single server, and the server acts as an RTP mixer to reduce the number of RTP sessions being sent to all participants, as well as re-encoding the streams for efficiency reasons. One participant of this conference has connected via mobile phone. However, when the participant enters the range of a previously- associated WiFi network, the mobile phone switches its network connection across to this new network. The QRT session can then migrate across, and the participant is able to continue the call with minimal interruption. 3. QRT Sessions A QRT session is defined as a QUIC connection which carries one or more RTP sessions (including any associated RTCP flows) using "DATAGRAM" frames, as specified in Section 4. Those RTP sessions may be part of one or more RTP multimedia sessions, and a multimedia session may be comprised of RTP sessions carried in one or more QRT sessions. A QRT session inherits the standard QUIC handshake as specified in [QUIC-TRANSPORT], and all communications between endpoints are secured as specified in [QUIC-TLS]. 4. RTP Sessions QRT allows multiple RTP sessions to be carried in a single QRT session. Each RTP session is operated independently of all the others, and individually discriminated by an QRT Flow Identifier, as described below in Section 4.1. Hurst Expires 3 May 2021 [Page 5] Internet-Draft QRT October 2020 RTP and RTCP packets are carried in QUIC "DATAGRAM" frames, as described in [QUIC-DATAGRAM]. QUIC allows multiple QUIC frames to be carried within a single QUIC packet, so multiple RTP/RTCP packets for one (or more) RTP sessions may therefore be carried in a single QUIC packet, subject to the network path MTU. If multiple RTP packets are to be carried within a single QUIC packet, then all but the final "DATAGRAM" frame must specify the length of the datagram, since the RTP packet header does not provide its own length field. [QUIC-DATAGRAM] specifies that if a "DATAGRAM" frame is received without a Length field, then this "DATAGRAM" frame extends to the end of the QUIC packet. 4.1. QRT Flow Identifier [RFC3550] specifies that RTP sessions are distinguished by pairs of transport addresses. However, since QUIC allows for connections to migrate between transport address associations, and because we wish to multiplex multiple RTP session flows over a single QRT session, this profile of RTP amends this statement and instead introduces a flow identifier to distinguish between RTP sessions. The QRT Flow Identifier is a 62-bit unsigned integer between 0 and 2^62 - 1. This specification does not mandate a means by which QRT Flow Identifiers are allocated for use within QRT sessions. An example mapping for this is discussed in Section 6 below. Implementations SHOULD allocate flow identifiers that make the most efficient use of the variable length integer packing mechanism, by not using flow identifiers greater than can be expressed in the smallest variable length integer field until all available flow identifiers have been used. The flow of packets belonging to an RTP session is identified using an RTP Session Flow Identifier header carried in the "DATAGRAM" frame payload before each RTP/RTCP packet. This flow identifier is encoded as a variable-length integer, as defined in [QUIC-TRANSPORT]. QRT Datagram Payload { QRT Flow Identifier (i), RTP/RTCP Packet (..) } Figure 1: QRT Datagram Payload Similar to QUIC stream IDs, the least significant bit (0x1) of the QRT Flow Identifier distinguishes between an RTP and an RTCP packet flow. "DATAGRAM" frames which carry RTP packet flows set this bit to 0, and "DATAGRAM" frames which carry RTCP packet flows set this bit to 1. As a consequence, RTP packet flows have even numbered QRT Flow Hurst Expires 3 May 2021 [Page 6] Internet-Draft QRT October 2020 Identifiers, and RTCP packet flows have odd-numbered QRT Flow Identifiers. Carriage of RTCP packets is discussed further in Section 4.2. +=======================+=====================================+ | Least significant bit | Flow identifier category | +=======================+=====================================+ | 0x0 | RTP packet flow for an RTP session | +-----------------------+-------------------------------------+ | 0x1 | RTCP packet flow for an RTP session | +-----------------------+-------------------------------------+ Table 1: RTP session flow identifer categories *Author's Note:* The author welcomes comments on whether a state model of RTP session flows would be beneficial. Currently, once an RTP session has been used by an endpoint, it is then considered an extant RTP session and implementations would have to keep any resources allocated to that RTP session until the QRT session is complete. In addition, how should endpoints react to receiving packets for unknown QRT flow identifiers? 4.2. RTCP Mapping An RTP session may have RTCP packet flows associated with it. These flows are carried with different QRT Flow Identifiers, as described in Section 4.1. The QRT Flow Identifier of the RTCP packet flow is always the value of the RTP packet flow QRT Flow Identifier + 1. For example, for an RTP packet flow using flow identifier 18, the RTCP packet flow would use flow identifier 19. Since RTCP packets contain a length field in their header, implementations MAY combine several RTCP packets pertaining to the same RTP session into a single "DATAGRAM" frame. Alternatively, implementations MAY choose to carry these RTCP packets each in their own "DATAGRAM" frame. 4.2.1. Restricted RTCP Packet Types *Author's Note:* I have specifically avoided calling this section "Prohibited RTCP packet types" for the time being, so as to not unnecessarily exclude the carriage of these packet types for the purposes of experimentation. Similarly, most statements below use SHOULD NOT instead of MUST NOT. The author welcomes comments on whether the document should prohibit the sending of some or all of these packet types. Hurst Expires 3 May 2021 [Page 7] Internet-Draft QRT October 2020 In order to reduce duplication, the following RTCP packet types SHOULD NOT be sent in a QRT session: * The "Generic NACK" packet. [RFC4585] states that Generic NACK feedback SHOULD NOT be used if the underlying transport protocol is capable of providing similar feedback information to the sender. Since all "DATAGRAM" frames are ACK-eliciting, QUIC already fulfils this requirement. * The "Loss RLE" Extended Report (XR) packet defined in [RFC3611] contains information that should already be known to both ends of the QUIC connection by means of the loss detection mechanism specified in [QUIC-RECOVERY]. * The "Port Mapping" packet type defined in [RFC6284] is used to negotiate UDP port pairs for the carriage of RTP and RTCP packets to peers. This does not apply in a QRT session, because the QUIC endpoints manage the UDP port association(s) for the QUIC connection as a whole. 5. Loss Recovery and Retransmission *Author's Note:* Do we want to mandate (make a MUST) doing session-multiplexing instead of SSRC-multiplexing for RTP retransmission? [RFC4588] specifies two schemes to support retransmission in the case of RTP packet loss. Since QRT natively supports RTP session multiplexing on a single QUIC connection, endpoints choosing to implement retransmission SHOULD do so using the session-multiplexing scheme. The selection of a new QRT Flow Identifier to use for the retransmission RTP session is implementation-specific. Section 6.1 specifies how the mapping between original and retransmission RTP sessions is expressed using the Session Description Protocol (SDP). 6. Using the Session Description Protocol to Advertise QRT Sessions [RFC4566] describes a format for advertising multimedia sessions, which is used by protocols such as [RFC3261]. This specification introduces a new SDP value attribute ""qrtflow"" as a means of assigning QRT Flow Identifiers to RTP and RTCP packet flows. Its formatting in SDP is described by the following ABNF [RFC5234]: Hurst Expires 3 May 2021 [Page 8] Internet-Draft QRT October 2020 qrtflow-attribute = "a=qrtflow:" qrt-flow-id qrt-flow-id = 1*DIGIT ; unsigned 62-bit integer Per Section 4.1 the value of the "qrt-flow-id" is required to be an even number. (The odd-numbered RTCP flow associated with the RTP session is not explicitly signalled in the SDP object.) The example in Figure 2 below shows a hypothetical QRT server advertising an endpoint to use for live contribution. It instructs a prospective client to send a VC2-encoded video stream and a Vorbis- encoded audio stream on two separate RTP sessions. In addition, it uses the SDP grouping framework described in [RFC5888] to ensure lip synchronisation between both of those RTP sessions. v=0 o=gfreeman 1594130940 1594135167 IN IP6 qrt.example.org s=Live Event Contribution c=IN IP6 2001:db8::7361:6d68 t=1594130980 1594388466 a=group:LS 1 2 m=video 443 RTP/QRT 96 a=qrtflow:0 a=rtpmap:96 vc2 a=mid:1 a=sendonly m=audio 443 RTP/QRT 97 a=qrtflow:2 a=rtpmap:97 vorbis a=mid:2 a=sendonly Figure 2: SDP object describing a QRT session Since the value of a QRT Flow Identifier for an associated RTCP flow is specified in Section 4.2, SDP advertisements containing the "a=qrtflow:" attribute MUST NOT contain an instance of the "a=rtcp:" attribute as defined in [RFC3605]. 6.1. Using the Session Description Protocol to Advertise QRT Sessions using RTP Retransmission The example in Figure 3 below shows a hypothetical QRT session advertisement for a bidirectional RTP session carrying an MPEG-2 Transport Stream in each direction on QRT Flow Identifier 0, and a corresponding pair of retransmission flows on QRT Flow Identifier 2. Hurst Expires 3 May 2021 [Page 9] Internet-Draft QRT October 2020 v=0 o=gfreeman 1594130940 1594135167 IN IP6 qrt.example.org s=Live Event Contribution c=IN IP6 2001:db8::4242:4351:5254 t=1594130980 1594388466 m=video 443 RTP/QRT 33 a=qrtflow:0 m=video 443 RTP/QRT 96 a=rtpmap:97 rtx/90000 a=fmtp:96 apt=33;rtx-time=4000 a=qrtflow:2 Figure 3: SDP object describing a QRT session with RTP retransmission 7. Exposing Round-Trip Time to RTP applications Section 5 of [QUIC-RECOVERY] specifies a mechanism for QUIC endpoints to estimate the rount-trip time (RTT) of a connection. QRT implementations SHOULD expose the values of "min_rtt", "smoothed_rtt" and "rttvar" for each network path to the RTP layer, and they MAY use these values either alone or in combination with RTCP messages to discern the round-trip time of the QRT session. *Author's Note:* The author welcomes comments on how appropriate these QUIC RTT measurements are to the RTP layer. 8. Protocol Identifier The QRT protocol specified in this document is identified by the Application-Layer Protocol Negotiation (ALPN) [RFC7301] identifier "qrt". 8.1. Draft Version Identification *RFC Editor's Note:* Please remove this section prior to publication of a final version of this document. Only implementations of the final, published RFC can identify themselves as "qrt". Until such an RFC exists, implementations MUST NOT identify themselves using this string. Implementations of draft versions of the protocol MUST add the string "-h" and the corresponding draft number to the identifier. For example, draft- hurst-quic-rtp-tunnelling-00 is identified using the string "qrt- h00". Non-compatible experiments that are based on these draft versions MUST append the string "-" and an experiment name to the identifier. For example, an experimental implementation based on draft-hurst- Hurst Expires 3 May 2021 [Page 10] Internet-Draft QRT October 2020 quic-rtp-tunnelling-00 which uses extension features not registered with the appropriate IANA registry might identify itself as "qrt-h00- extension-foo". Note that any label MUST conform to the "token" syntax defined in Section 5.7.2 of [HTTP-SEMANTICS]. Experimenters are encouraged to coordinate their experiments. 9. Security Considerations Implementations of the protocol defined in this specification are subject to the security considerations discussed in [QUIC-TRANSPORT] and [QUIC-TLS]. 10. IANA Considerations 10.1. Registration of Protocol Identification String This document creates a new registration for the identification of the QUIC RTP Tunnelling protocol in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established by [RFC7301]. The "qrt" string identifies RTP sessions multiplexed and carried over a QUIC transport layer: Protocol: QUIC RTP Tunnelling Identification Sequence: 0x71 0x72 0x74 ("qrt") Specification: This document, Section 8 10.2. Registration of SDP Protocol Identifier This document creates a new registration for the SDP Protocol Identifier ("proto") "RTP/QRT" in the SDP Protocol Identifiers ("proto") registry established by [RFC4566]. The "RTP/QRT" string identifies a profile of RTP where sessions are multiplexed and carried over a QUIC transport layer: SDP Protocol Name: RTP/QRT Reference: This document, Section 6 10.3. Registration of SDP Attribute Field This document creates a new registration for the SDP Attribute Field ("att-field") "qrtflow" in the SDP Attribute Field registry established by [RFC4566]. Hurst Expires 3 May 2021 [Page 11] Internet-Draft QRT October 2020 SDP Attribute Field: "qrtflow" Reference: This document, Section 6 11. References 11.1. Normative References [HTTP-SEMANTICS] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf-httpbis-semantics-12, . [QUIC-DATAGRAM] Pauly, T., Ed., Kinnear, E., Ed., and D. Schinazi, Ed., "An Unreliable Datagram Extension to QUIC", Work in Progress, Internet-Draft, draft-ietf-quic-datagram-01, . [QUIC-RECOVERY] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", Work in Progress, Internet-Draft, draft-ietf-quic-recovery-32, . [QUIC-TRANSPORT] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", Work in Progress, Internet-Draft, draft-ietf-quic-transport-32, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, DOI 10.17487/RFC3605, October 2003, . Hurst Expires 3 May 2021 [Page 12] Internet-Draft QRT October 2020 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, . [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, DOI 10.17487/RFC4588, July 2006, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, DOI 10.17487/RFC5888, June 2010, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 11.2. Informative References [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using Transport Layer Security (TLS) to Secure QUIC", Work in Progress, Internet-Draft, draft-ietf-quic-tls-32, . [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, . [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, . Hurst Expires 3 May 2021 [Page 13] Internet-Draft QRT October 2020 [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, . [RFC6284] Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping between Unicast and Multicast RTP Sessions", RFC 6284, DOI 10.17487/RFC6284, June 2011, . Acknowledgments The author would like to thank Richard Bradbury, David Waring, Colin Perkins, Joerg Ott, and Lucas Pardue for their helpful comments on both the design and review of this document. Author's Address Sam Hurst BBC Research & Development Email: sam.hurst@bbc.co.uk Hurst Expires 3 May 2021 [Page 14]