RTCP XR Report Block for Measurement Identity
draft-ietf-avt-rtcp-xr-meas-identity-01.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
    4.1.  New RTCP XR Block Type value
    4.2.  Contact information for registration
5.  Security Considerations
6.  Changes since last version
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Authors' Addresses

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 sequence number of the first packet of the RTP session,
• the extended sequence numbers of 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 | resv  |      block length = 7         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    SSRC of stream source                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        sub-identifier                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            reserved           |         first seq num         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           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.

resv: 4 bits

These bits are reserved. They MUST be set to zero by senders. They MUST be ignored by receivers.

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 7.

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.

reserved: 16 bits

These bits are reserved. They MUST be ignored by receivers. They MUST be set to zero by senders.

first seq num: 16 bits

The RTP sequence number of the first received RTP packet of the session, used to determine the number of packets contributing to cumulative measurements.

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.

No additional SDP signaling is defined for this block. Instead, the need for this block SHOULD be inferred from a request in SDP signalling for a block type (such as [DISCARD] (Hunt, G., “RTCP XR Report Block for Discard metric Reporting,” February 2009.)) which depends on it.

4.  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].

4.1.  New RTCP XR Block Type value

This document assigns the block type value NMI in the IANA "RTCP XR Block Type Registry" to the "Measurement Identity Block".

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

4.2.  Contact information for registration

The contact information for the registration is:

Geoff Hunt (geoff.hunt@bt.com)

Orion 2 PP3, Adastral Park, Martlesham Heath, Ipswich IP5 3RE, United Kingdom

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.  Changes since last version

Expanded and clarified IANA Considerations section

Changed to remove explicit SDP signalling for this block - need for block is implicit if a metrics block is requested which depends on this block.

Modified block structure to send first sequence number without extension, rather than extend to 32-bit number with leading 16 bits set to 0. These 16 bits are now reserved. Addresses Colin Perkins' comment of 15-Nov-2008

Removed "forwarding count" field following Colin Perkins' request for a use case. The field had been intended for use by translators to establish the "distance" (as a count of forwarding systems) to the point at which a measurement was made. However a recommendation to increment the count could have forced forwarding translator devices to parse the packet on a slow (CPU) path, possibly compromising RTCP measurement of round-trip delay [RFC3550] (Schulzrinne, H., “RTP: A Transport Protocol for Real-Time Applications,” July 2003.).

7.  References

7.1. Normative References

7.2. Informative References

Authors' Addresses