RTCP XR Report Block for Measurement Identity
draft-ietf-avt-rtcp-xr-meas-identity-00.txt

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Abstract

This document defines an RTCP XR Report Block carrying parameters which identify a measurement, to which one or more other RTCP XR Report Blocks may refer.

Table of Contents

1.  Introduction
    1.1.  Measurement Identity Report Block
    1.2.  RTCP and RTCP XR Reports
    1.3.  Performance Metrics Framework
    1.4.  Applicability
2.  Measurement Identity Block
    2.1.  Report Block Structure
    2.2.  Definition of Fields in Measurement Identity Report Block
3.  SDP Signaling
4.  IANA Considerations
5.  Security Considerations
6.  References
    6.1.  Normative References
    6.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

1.1.  Measurement Identity Report Block

This draft defines a new block type to augment those defined in [RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.) for use in a range of RTP applications. This block type does not itself contain any measurement results (metrics). However, this new block type provides information relevant to a measurement reported in one or more other block types, including

• a tag or key by which other blocks (containing metrics information) may refer to this block
• the SSRC of the measured stream,
• a field for incorporation of an application-specific auxiliary identifier,
• the extended sequence numbers of the first packet of the RTP session, the first packet of the current measurement interval, and the last packet included in the measurement,
• the duration of the most recent measurement interval and
• the duration of the interval applicable to cumulative measurements (which may be the duration of the RTP session to date).

The method for calculation of the extended RTP sequence number is provide in [RFC3550] (Schulzrinne, H., “RTP: A Transport Protocol for Real-Time Applications,” July 2003.).

This block is intended to provide a single copy of the information necessary to relate measurement data in other blocks to the stream, and measurement period, to which they refer. Commonly, multiple other small blocks contain measurement data for the same stream and period, and it would be a large overhead if all of these blocks carried duplicated data for measurement identification. Other blocks make a reference to this block (by tag).

A Measurement Identity block is associated with the set of RTCP XR metrics blocks which share its tag value. There MAY be several such sets in an RTCP packet, up to a limit of 8 arising from the use of 3-bit tags. There MAY also be RTCP XR blocks in the packet which are not associated with a Measurement Identity block, for example blocks which were defined before the Measurement Identity mechanism was introduced by this document.

1.2.  RTCP and RTCP XR Reports

The use of RTCP for reporting is defined in [RFC3550] (Schulzrinne, H., “RTP: A Transport Protocol for Real-Time Applications,” July 2003.). [RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.) defined an extensible structure for reporting using an RTCP Extended Report (XR). This draft defines a new Extended Report block that MUST be used as defined in [RFC3550] (Schulzrinne, H., “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) and [RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.).

1.3.  Performance Metrics Framework

The Performance Metrics Framework [PMOLFRAME] (Clark, A., “Framework for Performance Metric Development,” July 2008.) provides guidance on the definition and specification of performance metrics. Metrics described in this draft either reference external definitions or define metrics generally in accordance with the guidelines in [PMOLFRAME] (Clark, A., “Framework for Performance Metric Development,” July 2008.).

1.4.  Applicability

This block provides identification information for members of a family of RTCP XR metrics blocks which are designed to use it. To use the mechanism defined here, a metrics block must be in the same RTCP packet as the Measurement Identity block and must refer to the Measurement Identity block via the 3-bit tag field defined below.

2.  Measurement Identity Block

2.1.  Report Block Structure

       0               1               2               3
       0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     BT=NMI    |0| tag |  fwd  |      block length = 7         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    SSRC of stream source                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        sub-identifier                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           extended first sequence number (cumulative)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           extended first sequence number of interval          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 extended last sequence number                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Measurement Duration (Cumulative) (ms)               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Measurement Duration (Interval) (ms)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.  Definition of Fields in Measurement Identity Report Block

Bits shown as '0' in the figure SHOULD be set to zero.

block type (BT): 8 bits

A Measurement Identity Report Block is identified by the constant NMI.

[Note to RFC Editor: please replace NMI with the IANA provided RTCP XR block type for this block.]

tag: 3 bits

This field is a tag or key which identifies this Measurement Identity block within the scope of an RTCP packet. If an RTCP packet contains more than one Measurement Identity block, each Measurement Identity block MUST have a unique tag field to enable metrics blocks in the same RTCP packet to refer unambiguously to the correct Measurement Identity block. The 3-bit field allows up to 8 Measurement Identity blocks in each RTCP packet. If additional metrics must be sent at a given time, and they require more than 8 blocks of Measurement Identity information, then the metrics must be sent in multiple RTCP packets.

Forwarding count (fwd): 4 bits

These bits provide a count of the number of times the block, and those blocks which refer to it have been forwarded. They MUST be set to zero by the RTP system which creates the Measurement Identity block. The Forwarding count SHOULD be incremented by any RTP system which forwards the block, provided that the operation does not cause the Forwarding count to wrap from 0xF to 0x0. Then, values of the Forwarding count less than 0xF are unambiguous. An RTP system which receives a block with Forwarding count set to 0xF may deduce that the block has been forwarded 15 or more times.

block length: 16 bits

The length of this report block in 32-bit words minus one. For the Measurement Identity block, the block length is equal to 5.

SSRC of source: 32 bits

The SSRC [RFC3550] (Schulzrinne, H., “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) of the source of the stream being reported. Note that the SSRC of the reporting RTP system (the originator of the report block) is present in the RTCP XR header defined in Section 2 of [RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.).

sub-identifier: 32 bits

An additional identifier which is useful in the context of a specific application, e.g. an MPEG-2 transport identifier [MPEG2] (ISO/IEC, “Standard 13818-1,” December 2000.). Where the identifier is less than 32 bits, the identifier SHOULD be mapped into the most significant bits of the field. If no additional identifier is provided, all bits of the field MUST be set to zero. This field MUST be ignored by applications which are not configured to make use of it.

extended first sequence number (cumulative): 32 bits

The extended RTP sequence number of the first received RTP packet relevant to cumulative measurements. Usually this will be the extended sequence number of the first packet of the RTP session.

extended first sequence number of interval: 32 bits

The extended RTP sequence number of the first received RTP packet of the current measurement interval.

extended last sequence number: 32 bits

The extended RTP sequence number of the last received RTP packet which contributed to this measurement.

Measurement Duration (Cumulative) (ms): 32 bits

The duration in ms of the reporting interval applicable to Cumulative reports which use this Measurement Identity block.

Measurement Duration (Interval) (ms): 32 bits

The duration in ms of the reporting interval applicable to Interval reports which use this Measurement Identity block.

3.  SDP Signaling

[RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.) defines the use of SDP (Session Description Protocol) [RFC4566] (Handley, M., “SDP: Session Description Protocol,” July 2006.) for signaling the use of XR blocks. XR blocks MAY be used without prior signaling.

This section augments the SDP [RFC4566] (Handley, M., “SDP: Session Description Protocol,” July 2006.) attribute "rtcp-xr" defined in [RFC3611] (Friedman, T., “RTP Control Protocol Extended Reports (RTCP XR),” November 2003.) by providing a "xr-format" to signal the use of the report block defined in this document.

The need for this block could be inferred from a request in SDP signalling for a block type (such as [BASICLOSS] (Hunt, G., “RTCP XR Block for Measurement Identity,” October 2008.)) which depends on it. However to remove ambiguity, if SDP is used to signal a desire to receive a block type which depends on the Measurement Identity block, then the Measurement Identity block itself MUST be signalled explicitly in SDP as described here.

rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF

(defined in RFC3611)

xr-format = xr-format / xr-mi-block

xr-mi-block = "xr-mi"

4.  IANA Considerations

This document creates a new block type within the IANA "RTCP XR Block Type Registry" called the Measurement Identity Block, and a new parameter xr-mi within the "RTCP XR SDP Parameters Registry".

5.  Security Considerations

RTCP reports can contain sensitive information since they can provide information about the nature and duration of a session established between two or more endpoints.

6.  References

6.1. Normative References

6.2. Informative References

Authors' Addresses

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