AVT A. Begen
Internet-Draft Cisco Systems
Intended status: Standards Track March 3, 2009
Expires: September 4, 2009
Rapid Multicast Synchronization Report Block Type for RTCP XR
draft-begen-avt-rapid-sync-rtcp-xr-00
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Abstract
In most RTP-based multicast applications, the RTP source sends inter-
related data. Due to the dependency in the multicast data, randomly
joining RTP receivers may not be able to start usefully consuming the
data upon joining the multicast session, thus, they often experience
a random synchronization delay. In order to reduce this delay, an
auxiliary unicast RTP session that facilitates rapid synchronization
with the multicast session may be used between a retransmission
server and RTP receivers. Yet, due to various random components
pertaining to the RTP data and networking infrastructure, the
performance of rapid synchronization may vary. For quality reporting
and diagnostics, it is important to collect detailed information from
the RTP receivers about their rapid synchronization experiences.
This document addresses this issue by defining a new report block
type, called Rapid Multicast Synchronization Report Block, within the
framework of RTP Control Protocol (RTCP) Extended Reports (XR). This
document also defines the necessary signaling of the new report block
type in the Session Description Protocol (SDP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Rapid Multicast Synchronization Report Block . . . . . . . . . 6
4. Session Description Protocol Signaling . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
RTP Control Protocol (RTCP) is the out-of-band control protocol for
the applications that are using the Real-time Transport Protocol
(RTP) for media transport [RFC3550]. In addition to providing
minimal control functionality to RTP entities via sender and receiver
reports, RTCP also enables a basic level monitoring of RTP sessions.
More statistically detailed monitoring as well as application-
specific monitoring is usually achieved through the RTCP Extended
Reports (XR) [RFC3611].
In most RTP-based multicast applications such as the ones carrying
video content, the RTP source sends inter-related data. That is, the
RTP application may not be able to decode and present the data in an
RTP packet before decoding one or more earlier RTP packets and/or
before acquiring some certain information about the content itself.
Thus, RTP receivers that are randomly joining the multicast session
often experience a random synchronization delay. In order to reduce
this delay, [I-D.versteeg-avt-rapid-synchronization-for-rtp] proposes
an approach where an auxiliary unicast RTP session is established
between a retransmission server and the joining RTP receiver. Over
this unicast RTP session, the retransmission server provides all the
information the RTP receiver needs to rapidly synchronize with the
multicast session. However, depending on the variability in the IGMP
processing times [RFC3376], availability of network resources for
rapid synchronization and nature of the RTP data itself, not all RTP
receivers synchronize with the multicast session in the same amount
of time. The performance of rapid synchronization may vary not only
for different RTP receivers but also over time.
To increase the visibility of the service provider into its network,
to diagnose slow multicast synchronization issues and to collect the
synchronization experiences of the RTP receivers, this document
defines a new report block type, which is called Rapid Multicast
Synchronization Report Block, within the framework of RTCP XR. RTP
receivers that are benefiting from the rapid synchronization method
described in [I-D.versteeg-avt-rapid-synchronization-for-rtp] may use
this report every time they join a new multicast RTP session.
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2. Requirements Notation
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 [RFC2119].
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3. Rapid Multicast Synchronization Report Block
The report format is shown in Figure 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT | rsvd. | Status| Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of the Multicast Session |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Extended Seqnum of the First Received Multicast Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IGMP Join Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format for the Rapid Multicast Synchronization Report Block
BT (8 bits): Block type that identifies the block format. Rapid
Multicast Synchronization Report Block is identified by the
constant TBD.
rsvd. (4 bits): This field is reserved for future definition. In
the absence of such definition, the bits in this field MUST be set
to zero and MUST be ignored by the RTP receiver.
Status (4 bits): TBD.
Block Length (16 bits): The length of this report block,
including the header, in 32-bit words minus one. It MUST be set
to 3.
SSRC of the Multicast Session (32 bits): The SSRC of the
multicast RTP session that the RTP receiver has joined.
RTP Extended Seqnum of the First Received Multicast Packet (32
bits): The RTP extended sequence number of the first packet
received from the multicast session.
IGMP Join Time (32 bits): Greater of zero or the time difference
(in ms) between the instant IGMP Join message has been sent and
the instant the first RTP packet was received from the multicast
session.
The semantics of this report block is independent of the payload type
of the multicast RTP session.
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Editor's note: More fields can be defined in this XR report to give
more details about rapid synchronization such as:
o Delta time between the rapid synchronization request and the
arrival of the first unicast packet
o Delta time between the rapid synchronization request and the
arrival of the first multicast packet
o Duplicate packet information (duplication due to receiving the
same packet from both unicast and multicast session)
Details are TBD.
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4. Session Description Protocol Signaling
A new parameter is defined for the Rapid Multicast Synchronization
Report Block to be used with Session Description Protocol (SDP)
[RFC4566]. It has the following syntax within the 'rtcp-xr'
attribute:
rtcp-xr-attrib = "a=rtcp-xr:" [xr-format *(SP xr-format)] CRLF
xr-format = "multicast-join"
CRLF = %d13.10
Figure 2
Refer to Section 5.1 of [RFC3611] for a detailed description and the
full syntax of the "rtcp-xr" attribute.
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5. Security Considerations
The security considerations of [RFC3611] apply in this document as
well. Additional security considerations are TBD.
If desired, similar to other RTCP XR reports, the rapid multicast
synchronization reports MAY be protected by using SRTP and SRTCP
[RFC3711].
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6. IANA Considerations
New block types for RTCP XR are subject to IANA registration. For
general guidelines on IANA considerations for RTCP XR, refer to
[RFC3611].
This document (provisionally) assigns the block type value TBD in the
RTCP XR Block Type Registry to "Rapid Multicast Synchronization
Report Block." This document also registers the SDP [RFC4566]
parameter 'multicast-join' for the 'rtcp-xr' attribute in the RTCP XR
SDP Parameters Registry.
The contact information for the registrations is:
Ali Begen
abegen@cisco.com
170 West Tasman Drive
San Jose, CA 95134 USA
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7. Acknowledgments
TBC.
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8. References
8.1. 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.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[I-D.versteeg-avt-rapid-synchronization-for-rtp]
Steeg, B., Begen, A., and T. Caenegem, "Unicast-Based
Rapid Synchronization with RTP Multicast Sessions",
draft-versteeg-avt-rapid-synchronization-for-rtp-01 (work
in progress), November 2008.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
8.2. 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.
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Author's Address
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
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