AVTEXT Working Group J. Xia
Internet-Draft R. Huang
Intended status: Standards Track Huawei
Expires: August 22, 2013 February 18, 2013
RTP/RTCP extension for RTP Splicing Notification
draft-xia-avtext-splicing-notification-00
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 in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 22, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of RTP Splicing Notification . . . . . . . . . . . . 4
4. Conveying Splicing Metadata in RTP/RTCP extensions . . . . . . 5
4.1. RTP Header Extention . . . . . . . . . . . . . . . . . . . 5
4.2. RTCP Splicing Notification Message . . . . . . . . . . . . 6
5. Reduing Splicing Latency . . . . . . . . . . . . . . . . . . . 7
6. Failure Cases . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Splicing is a process that replaces some multimedia content with
other multimedia content and delivers the substitutive multimedia
content to the receivers during specific, pre-designated time slot.
Certain timing information about when to start and end the splicing
must be first acquired by the splicer to start the splicing. This
document refers to this information as Splicing Metadata.
[SCTE35] provides a method that encapsulates the Splicing Metadata
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 Metadata 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 Metadata by including it
in the RTP packets.
Nevertheless, the Splicing Metadata 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 Metadata to the mixer even though RTCP is inherently
unreliable too.
2. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Most terminology defined in "Content Splicing for RTP Sessions"
[RFC6828] applies to this document except the following one:
Splicing Metadata
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.
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3. 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 Metadata from the main RTP sender before splicing starts.
When a new splicing is forthcoming, the main RTP sender MUST send the
Splicing Metadata to the mixer. Usually, the Splicing Metadata
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 Metadata
far enough in advance. Alternatively, the main RTP sender can
estimate when to send the Splicing Metadata 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 Metadata
from the main RTP sender in advance, and thus estimates when to
transfer the substitutive content to the mixer. The Splicing
Metadata 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 Metadata 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, followed by
the timestamp of the first main RTP packet on the splicing out point.
When the substitutive RTP sender gets the Splicing Metadata, 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 Metadata. 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
Metadata.
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4. Conveying Splicing Metadata in RTP/RTCP extensions
This memo defines two backwards compatible RTP extensions to convey
the Splicing Metadata to the mixer: an RTP header extension and an
RTCP splicing notification message.
4.1. RTP Header Extention
The RTP header extension mechanism defined in [RFC5285] can be
adapted to carry the Splicing Metadata consisting of a couple of NTP-
format timestamps.
One variant is defined for this header extension. The variant
carries the 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]. 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=6 | NTP timestamp format - Seconds (bit 8-31) |x
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+t
| NTP timestamp format - Fraction (bit 0-31) |e
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+n
| ID | L=6 | NTP timestamp format - Seconds (bit 8-31) |s
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+i
| NTP timestamp format - Fraction (bit 0-31) |o
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+n
Figure 1: Sample 56-Bit 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
remain RTP packets unmarked.
After the mixer intercepts the RTP header extension and derives the
Splicing Metadata, 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
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sent to provide robustness in the case of any splicing-unaware
middlebox that might strip RTP header extensions.
When SDP is used, the use of the RTP header extensions defined above
MUST be indicated as specified in [RFC5285]. Therefore, the
following URI MUST be used:
o The URI used for signalling the use of Variant B/56-bit NTP RTP
header extension in SDP is "urn:ietf:params:rtp-hdrext: splicing-
metadata-56".
4.2. RTCP Splicing Notification Message
Besides the RTP header extension, the main RTP sender includes the
Splicing Metadata in an RTCP splicing notification message.
The RTCP splicing notification message is a new RTCP packet type
defined with PT = 211. 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=211 | 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.
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SSRC: 32 bits
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]).
A basic assumption is that the main RTP sender and the mixers are in
an SSM group with the source being the main RTP sender. RTCP
splicing notification message from the main RTP sender is sent via
multicast to the mixers.
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].
5. 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
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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.
6. Failure Cases
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 Metadata reaches the mixers.
If the mixer does not get the Splicing Metadata 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].
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
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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. SDP Signaling
TBD
9. IANA considerations
TBD
10. Acknowledgments
TBD
11. References
11.1. 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.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "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.
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11.2. 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.
[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.
[SRTP-ENCR-HDR]
Lennox, J., "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)",
draft-ietf-avtcore-srtp-encrypted-header-ext-04 (work in
progress), January 2013.
[SCTE35] Society of Cable Telecommunications Engineers (SCTE),
"Digital Program Insertion Cueing Message for Cable",
2011.
Authors' Addresses
Jinwei Xia
Huawei
101 Software Avenue
Nanjing, Yuhua District 210012
China
Phone: +86-025-84565890
Email: xiajinwei@huawei.com
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Rachel Huang
Huawei
101 Software Avenue
Nanjing, Yuhua District 210012
China
Phone: +86-025-84565890
Email: rachel.huang@huawei.com
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