RTP/RTCP extension for RTP Splicing Notification
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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|Authors||Jinwei Xia , Rachel Huang , Deng Lingli|
|Replaced by||draft-ietf-avtext-splicing-notification, RFC 8286|
|Stream||Stream state||(No stream defined)|
|RFC Editor Note||(None)|
|IESG||IESG state||I-D Exists|
|Send notices to||(None)|
AVTEXT Working Group J. Xia INTERNET-DRAFT R. Huang Intended Status: Standards Track Huawei Expires: April 20, 2014 L. Deng China Mobile October 17, 2013 RTP/RTCP extension for RTP Splicing Notification draft-xia-avtext-splicing-notification-02 Abstract Content splicing is a process that replaces the content of a main multimedia stream with other multimedia content, and delivers the substitutive multimedia content to the receivers for a period of time. The RTP mixer is designed to handle RTP splicing in [RFC6828], but how the RTP mixer knows when to start and end the splicing is still unspecified. This memo defines two RTP/RTCP extensions to indicate the splicing related information to the RTP mixer: an RTP header extension that conveys the information in-band and an RTCP packet that conveys the information out-of-band. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html <> Expires <Expiry Date> [Page 1] INTERNET DRAFT <Document Title> <Issue Date> Copyright and License Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. 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Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Overview of RTP Splicing Notification . . . . . . . . . . . . . 4 3 Conveying Splicing Interval in RTP/RTCP extensions . . . . . . 5 3.1 RTP Header Extention . . . . . . . . . . . . . . . . . . . . 5 3.2 RTCP Splicing Notification Message . . . . . . . . . . . . . 6 4 Reduing Splicing Latency . . . . . . . . . . . . . . . . . . . 7 5 Failure Cases . . . . . . . . . . . . . . . . . . . . . . . . . 7 6 SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . . 8 7 Security Considerations . . . . . . . . . . . . . . . . . . . . 9 8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 8.1 RTCP Control Packet Types . . . . . . . . . . . . . . . . . 9 8.2 RTP Compact Header Extensions . . . . . . . . . . . . . . . 10 9 Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10.1 Normative References . . . . . . . . . . . . . . . . . . . 10 10.2 Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 <> Expires <Expiry Date> [Page 2] INTERNET DRAFT <Document Title> <Issue Date> 1 Introduction Splicing is a process that replaces some multimedia content with other multimedia content and delivers the substitutive multimedia content to the receivers for a period of time. In some predictable splicing cases, e.g., advertisement insertion, the splicing duration MUST be inside of the specific, pre-designated time slot. Certain timing information about when to start and end the splicing must be first acquired by the mixer to start the splicing. This document refers to this information as Splicing Interval. [SCTE35] provides a method that encapsulates the Splicing Interval inside the MPEG2-TS layer in cable TV systems. But in RTP splicing scenario described in [RFC6828], the mixer has to decode the RTP packets, search and solve the Splicing Interval inside the payloads. The need for such processing enhances the workload of the mixer and limits the size of RTP sessions the mixer can support. The document defines an RTP header extension [RFC5285] through which the main RTP sender can provide the Splicing Interval by including it in the RTP packets. Nevertheless, the Splicing Interval conveyed in the RTP header extension might not reach the mixer successfully, any splicing un- aware middlebox on the path between the RTP sender and the mixer might strip the RTP header extension. To increase robustness against above case, the document also defines a new RTCP packet type in a complementary fashion to carry the Splicing Interval to the mixer even though RTCP is inherently unreliable too. 1.1 Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Most terminology defined in "Content Splicing for RTP Sessions" [RFC6828] applies to this document except the following one. Splicing Interval: A set of certain metadata that allows the mixer to know when to start and end the RTP splicing. The information consists of a couple of NTP-format timestamps on the splicing in point and on the splicing out point. <> Expires <Expiry Date> [Page 3] INTERNET DRAFT <Document Title> <Issue Date> 2 Overview of RTP Splicing Notification According to RTP Splicing draft [RFC6828], a mixer is designed to do splicing on the RTP layer, but it cannot insert the substitutive content randomly but only do that at the reserved time slots set by the main RTP sender. This implies the mixer must first know the Splicing Interval from the main RTP sender before splicing starts. When a new splicing is forthcoming, the main RTP sender MUST send the Splicing Interval to the mixer. Usually, the Splicing Interval SHOULD be sent more than once to against the possible packet loss. To enable the mixer to get the substitutive content before the splicing starts, the main RTP sender MUST send the Splicing Interval far enough in advance. Alternatively, the main RTP sender can estimate when to send the Splicing Interval based on the round-trip time (RTT) following the mechanisms in section 6.4.1 of [RFC3550] when the mixer sends RTCP RR to the main sender. The substitutive sender also needs to learn the Splicing Interval from the main RTP sender in advance, and thus estimates when to transfer the substitutive content to the mixer. The Splicing Interval could be transmitted from the main RTP sender to the substitutive content using some out-of-band mechanisms, the details how to achieve that are beyond the scope of this memo. To ensure the Splicing Interval is valid to the main RTP sender and the substitutive RTP sender, the two senders MUST share a common reference clock, so the mixer can achieve accurate splicing. In this document, the main RTP sender uses a couple of NTP-format timestamps, derived from the common reference clock, to indicate when to start and end the splicing to the mixer: the timestamp of the first substitutive RTP packet on the splicing in point, and the timestamp of the first main RTP packet on the splicing out point. When the substitutive RTP sender gets the Splicing Interval, it must prepare the substitutive stream. The RTP timestamp of the first substitutive RTP packet that would be presented on the receivers MUST correspond to the same time instant as the former NTP timestamp in the Splicing Interval. To enable mixer to know the first substitutive RTP packet it begins to output, the substitutive RTP sender MUST enable the mixer to know above RTP timestamp in advance, e.g., from prior receipt of RTCP SR message. When the splicing will end, the RTP timestamp of the first main RTP packet that would be presented on the receivers MUST correspond to the same time instant as the latter NTP timestamp in the Splicing Interval. <> Expires <Expiry Date> [Page 4] INTERNET DRAFT <Document Title> <Issue Date> 3 Conveying Splicing Interval in RTP/RTCP extensions This memo defines two backwards compatible RTP extensions to convey the Splicing Interval to the mixer: an RTP header extension and an RTCP splicing notification message. 3.1 RTP Header Extention The RTP header extension mechanism defined in [RFC5285] can be adapted to carry the Splicing Interval consisting of a couple of NTP- format timestamps. One variant is defined for this header extension. It carries the 7 octets splicing-out NTP timestamp (lower 24-bit part of the Seconds of a NTP-format timestamp and the 32 bits of the Fraction of a NTP- format timestamp as defined in [RFC5905]), followed by the 8 octets splicing-in NTP timestamp (64-bit NTP-format timestamp as defined in [RFC5905]). The top 8 bits of the splicing-out NTP timestamp are referred from the top 8 bits of the splicing-in NTP timestamp, under the consumption that the splicing-out time is after the splicing-in time, and the splicing interval is less than 2^25 seconds, this order allows full resolution for splicing-in NTP timestamp while keeping 4 octets alignment. The format is shown in Figures 1. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0xBE | 0xDE | length=4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+E | ID | L=15 | OUT NTP timestamp format - Seconds (bit 8-31) |x +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+t | OUT NTP timestamp format - Fraction (bit 0-31) |e +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+n | IN NTP timestamp format - Seconds (bit 0-31) |s +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+i | IN NTP timestamp format - Fraction (bit 0-31) |o +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+n Figure 1: Sample hybrid NTP Encoding Using the One-Byte Header Format Note that the inclusion of an RTP header extension will reduce the efficiency of RTP header compression. It is RECOMMENDED that the main sender begins to insert the RTP header extensions into a number of RTP packets in advance of the splicing starting, while leaving the <> Expires <Expiry Date> [Page 5] INTERNET DRAFT <Document Title> <Issue Date> remain RTP packets unmarked. After the mixer intercepts the RTP header extension and derives the Splicing Interval, it will generate its own stream and could not include the RTP header extension in outgoing packets to reduce header overhead. Furthermore, whether the in-band NTP-format timestamps are included or not, RTCP splicing notification message in next section MUST be sent to provide robustness in the case of any splicing-unaware middlebox that might strip RTP header extensions. 3.2 RTCP Splicing Notification Message Besides the RTP header extension, the main RTP sender includes the Splicing Interval in an RTCP splicing notification message. The RTCP splicing notification message is a new RTCP packet type. It has a fix header followed by a couple of NTP-format timestamps: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|reserved | PT=TBA | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IN NTP Timestamp (most significant word) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IN NTP Timestamp (least significant word) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OUT NTP Timestamp (most significant word) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OUT NTP Timestamp (least significant word) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: RTCP Splicing Notification Message The RSI packet includes the following fields: Length: 16 bits As defined in [RFC3550], the length of the RTCP packet in 32-bit words minus one, including the header and any padding. SSRC: 32 bits <> Expires <Expiry Date> [Page 6] INTERNET DRAFT <Document Title> <Issue Date> The SSRC of the Main RTP Sender. Timestamp: 64 bits Indicates the wallclock time when this splicing starts and ends. The full-resolution NTP timestamp is used, which is a 64-bit, unsigned, fixed-point number with the integer part in the first 32 bits and the fractional part in the last 32 bits. This format is similar to RTCP Sender Report (Section 6.4.1 of [RFC3550]). The RTCP splicing notification message can be appended to RTCP SR the main RTP sender generates in compound RTCP packets, and hence follows the compound RTCP rules defined in Section 6.1 in [RFC3550]. If the use of non-compound RTCP [RFC5506] was previously negotiated between the sender and the mixer, the RTCP splicing notification message may be sent as non-compound RTCP packets. When the mixer intercepts the RTCP splicing notification message, it MAY NOT forward the message to the receivers in order to reduce RTCP bandwidth consumption or to avoid downstream receivers from detecting splicing defined in Section 4.5 in [RFC6828]. 4 Reduing Splicing Latency When splicing starts or ends, the mixer outputs the multimedia content from another sender to the receivers. Given that the receivers must first acquire certain information ([RFC6285] refers to this information as Reference Information) to start processing the multimedia data, either the main RTP sender or the substitutive sender SHOULD provide the Reference Information align with its multimedia content to reduce the delay caused by acquiring the Reference Information. The means by which the Reference Information is distributed to the receivers is out of scope of this memo. Another latency element is synchronization caused delay. The receivers must receive enough synchronization metadata prior to synchronizing the separate components of the multimedia streams when splicing starts or ends. Either the main RTP sender or the substitutive sender SHOULD send the synchronization metadata early enough so that the receivers can play out the multimedia in a synchronized fashion. The mechanisms defined in [RFC6051] are RECOMMENDED to be adopted to reduce the possible synchronization delay. 5 Failure Cases <> Expires <Expiry Date> [Page 7] INTERNET DRAFT <Document Title> <Issue Date> This section examines the implications of losing RTCP splicing notification message and other failure case, e.g., the RTP header extension is stripped on the path. Given there may be splicing un-aware middlebox on the path between the main RTP sender and the mixer, one heuristics will be used to verify whether or not the Splicing Interval reaches the mixers. If the mixer does not get the Splicing Interval when the splicing starts, it will still output the main content to the downstream receivers and forward the RTCP RR packets sent from downstream receivers to the main RTP sender. In such case, the main RTP sender can learn the splicing failed. In a similar manner, the substitutive sender can learn the splicing failed if it does not receive any RTCP RR packets from downstream receivers when the splicing starts. Upon the detection of a failure, the main RTP sender or the substitutive sender SHOULD check the path to the failed mixer, or fallback to the payload specific mechanisms, e.g., MPEG-TS splicing solution defined in [SCTE35]. 6 SDP Signaling This document defines the URI for declaring this header extension in an extmap attribute to be "urn:ietf:params:rtp-hdrext:splicing- interval". This document also reuses the Flow Identification (FID) semantics defined in SDP Grouping Framework [RFC5888] to represent the relationship between the main RTP stream and the substitutive RTP stream. The next example shows how the "group" attribute used with FID semantics can indicate RTP splicing support on RTP sender. v=0 o=xia 1122334455 1122334466 IN IP4 splicing.example.com s=RTP Splicing Example t=0 0 a=group:FID 1 2 m=video 30000 RTP/AVP 100 i=Main RTP Stream c=IN IP4 18.104.22.168/127 a=rtpmap:100 MP2T/90000 a=extmap:1 urn:ietf:params:rtp-hdrext:splicing-interval <> Expires <Expiry Date> [Page 8] INTERNET DRAFT <Document Title> <Issue Date> a=mid: 1 m= video 30001 RTP/AVP 100 i=Substitutive RTP Stream c=IN IP4 22.214.171.124/127 a=sendonly a=mid: 2 Figure 3: Example SDP for a single-channel splicing scenario The mixer receiving the SDP message above receives one MPEG2-TS stream (payload 100) from the main RTP sender (with multicast destination address of 126.96.36.199) on port 30000, and/or receives another MPEG2-TS stream from the substitutive RTP sender (with multicast destination address of 188.8.131.52) on port 30001. But at a particular point in time, the mixer only selects one stream and output the content from the chosen stream to the downstream receivers. 7 Security Considerations The security considerations of the RTP specification [RFC3550], the general mechanism for RTP header extensions [RFC5285] and the security considerations of the RTP splicing specification [RFC6828] apply. The RTP header extension defined in Section 4.1 include two NTP- format timestamps. In the Secure Real-time Transport Protocol (SRTP)[RFC3711], RTP header extensions are authenticated but not encrypted. A malicious endpoint could choose to set the values in this header extension falsely, so as to falsely claim the splicing time. In scenarios where this is a concern, additional mechanisms MUST be used to protect the confidentiality of the header extension. This mechanism could be header extension encryption [SRTP-ENCR-HDR], or a lower-level security and authentication mechanism such as IPsec [RFC4301]. 8 IANA Considerations 8.1 RTCP Control Packet Types Based on the guidelines suggested in [RFC5226], a new RTCP packet format has been registered with the RTCP Control Packet Type (PT) Registry: <> Expires <Expiry Date> [Page 9] INTERNET DRAFT <Document Title> <Issue Date> Name: SNM Long name: Splicing Notification Message Value: TBA Reference: This document 8.2 RTP Compact Header Extensions The IANA has also registered a new RTP Compact Header Extension [RFC5285], according to the following: Extension URI: urn:ietf:params:rtp-hdrext:splicing-interval Description: Splicing Interval Contact: Jinwei Xia <email@example.com> Reference: This document 9 Acknowledges TBD 10 References 10.1 Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, July 2008. [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, June 2010. [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, <> Expires <Expiry Date> [Page 10] INTERNET DRAFT <Document Title> <Issue Date> "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP Flows", RFC 6051, November 2010. [RFC6828] Xia, J., "Content Splicing for RTP Sessions", RFC 6828, January 2013. 10.2 Informative References [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, April 2009. [RFC6285] Ver Steeg, B., Begen, A., Van Caenegem, T., and Z. Vax, "Unicast-Based Rapid Acquisition of Multicast RTP Sessions", RFC 6285, June 2011. [RFC6904] Lennox, J.,"Encryption of Header Extensions in the Secure Real-Time Transport Protocol (SRTP)", April 2013. [SCTE35] Society of Cable Telecommunications Engineers (SCTE), "Digital Program Insertion Cueing Message for Cable", 2011. Authors' Addresses Jinwei Xia Huawei Email: firstname.lastname@example.org Rachel Huang <> Expires <Expiry Date> [Page 11] INTERNET DRAFT <Document Title> <Issue Date> Huawei Email: email@example.com Lingli Deng China Mobile Email: firstname.lastname@example.org <> Expires <Expiry Date> [Page 12]