| XR Block Working Group | V. Singh, Ed. |
| Internet-Draft | J. Ott |
| Intended status: Standards Track | Aalto University |
| Expires: February 02, 2014 | I. Curcio |
| Nokia Research Center | |
| August 01, 2013 |
RTP Control Protocol (RTCP) Extended Reports (XR) for Bytes Discarded Metric
draft-singh-xrblock-rtcp-xr-bytes-discarded-metric-00
The RTP Control Protocol (RTCP) is used in conjunction with the Real- time Transport Protocol (RTP) in to provide a variety of short-term and long-term reception statistics. The available reporting may include aggregate information across longer periods of time as well as individual packet reporting. This document specifies a report computing the bytes discarded from the de-jitter buffer after successful reception.
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RTP [RFC3550] provides a transport for real-time media flows such as audio and video together with the RTP control protocol (RTCP) which provides periodic feedback about the media streams received in a specific duration. In addition, RTCP can be used for timely feedback about individual events to report (e.g., packet loss) [RFC4585]. Both long-term and short-term feedback enable a media sender to adapt its media transmission and/or encoding dynamically to the observed path characteristics.
RFC3611 [RFC3611] defines RTCP Extended Reports as a detailed reporting framework to provide more than just the coarse Receiver Report (RR) statistics. The detailed reporting may enable a media sender to react more appropriately to the observed networking conditions as these can be characterized better, although at the expense of extra overhead.
In addition to lost packets, RFC3611 defines the notion of "discarded" packets: packets that were received but dropped from the de-jitter buffer because they were either too early (for buffering) or too late (for playout). The "discard rate" metric is part of the VoIP metrics report block even though it is not just applicable to audio: it is specified as the fraction of discarded packets since the beginning of the session. See section 4.7.1 of RFC3611 [RFC3611]. The discard metric is believed to be applicable to a large class of RTP applications which use a de-jitter buffer RFC5481 [RFC5481].
Recently proposed extensions to the Extended Reports (XR) reporting suggest enhancing this discard metric:
However, none of these metrics allow a receiver to report precisely the number of bytes that were discarded. While this information could in theory be derived from high-frequency reporting on the number of discarded packets [I-D.ietf-xrblock-rtcp-xr-discard] or from the Discard RLE report [I-D.ietf-xrblock-rtcp-xr-discard-rle-metrics], these two mechanisms do not appear feasible: The former would require an unduly high amount of reporting which still might not be sufficient due to the non-deterministic scheduling of RTCP packets. The latter incurs significant complexity (by storing a map of sequence numbers and packet sizes) and reporting overhead.
An XR block is defined in this document to indicate the number of bytes discarded, per interval or for the duration of the session, similar to other XR report blocks.
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 BCP 14, RFC 2119 [RFC2119].
The terminology defined in RTP [RFC3550] and in the extensions for XR reporting [RFC3611] applies.
The XR Bytes Discarded report block uses the following format which follows the model of the framework for performance metric development [RFC6390].
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=BDR | I |E|reserved | block length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| number of bytes discarded |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: XR Bytes Discarded Report Block
Block Type (BT, 8 bits): A Bytes Discarded Packets Report Block is identified by the constant BDR.
[Note to RFC Editor: please replace BDR with the IANA provided RTCP XR block type for this block. Please remove this note prior to publication as an RFC.]
The Interval Metric flag (I) (2 bits) is used to indicate whether the discard metric is Interval, or a Cumulative metric, that is, whether the reported value applies to the most recent measurement interval duration between successive reports (I=10, the Interval Duration) or to the accumulation period characteristic of cumulative measurements (I=11, the Cumulative Duration). Since the bytes discarded are not measured at a particular time instance but over one or several reporting intervals, the metric MUST NOT be reported as a Sampled Metric (I=01). In addition, the value I=00 is reserved and MUST NOT be sent, and MUST be discarded when received.
The 'E' bit is introduced to distinguish between packets discarded due to early arrival and those discarded due to late arrival. The 'E' bit is set to '1' if it reports bytes discarded due to early arrival and is set to '0' if it reports bytes discarded due to late arrival. If a duplicate packet is received and discarded, these duplicate packets are ignored and not reported. In case both early and late discarded packets shall be reported, two Bytes Discarded report blocks MUST be included.
reserved (5 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 receiver.
block length (16 bits) MUST be set to 2, in accordance with the definition of this field in [RFC3611]. The block MUST be discarded if the block length is set to a different value.
The 'number of bytes discarded' is a 32-bit unsigned integer value indicating the total number of bytes discarded. Bytes discarded corresponds to the RTP payload size of every RTP packet that is discarded (due to early or late arrival). Hence, the bytes discarded ignores the size of any RTP header extensions and the size of the padding bits. Also the discarded packet is associated to the interval in which it was discarded and not when it was expected.
If Interval Metric flag (I=11) is set, the value in the field indicates the number of bytes discarded from the start of the session, if Interval Metric flag (I=01) is set, it indicates the number of bytes discarded since the last RTCP XR Byte Discarded Block was received.
If the XR block follows a measurement identity block [RFC6776] in the same RTCP compound packet then the cumulative (I=11) or the interval (I=10) for this report block corresponds to the values of the "measurement duration" in the measurement information block.
If the receiver sends the Bytes Discarded Report Block without the measurement identity block then the discard block MUST be sent in conjunction with an RTCP Receiver Report (RR) as a compound RTCP packet.
This section describes the behavior of the reporting node (= media receiver) and the media sender.
Transmission of RTCP XR Bytes Discarded Report is up to the discretion of the media receiver, as is the reporting granularity. However, it is RECOMMENDED that the media receiver signals all discarded packets using the method defined in this document. If all packets over a reporting period were discarded, the media receiver MAY use the Discard Report Block [I-D.ietf-xrblock-rtcp-xr-discard] instead.
The media receiver MAY send the Bytes Discard Reports as part of the regularly scheduled RTCP packets as per RFC3550. It MAY also include Bytes Discard Reports in immediate or early feedback packets as per RFC4585.
The media sender MUST be prepared to operate without receiving any Bytes Discarded reports. If Bytes Discarded reports are generated by the media receiver, the media sender cannot rely on all these reports being received, nor can the media sender rely on a regular generation pattern from the media receiver.
However, if the media sender receives any RTCP reports but no Bytes Discard report blocks and is aware that the media receiver supports Bytes Discard report blocks, it MAY assume that no packets were discarded at the media receiver.
The media sender SHOULD accept the Bytes Discarded Report Block only if it is received in a compound RTCP receiver report or if it is preceded by a measurement identity block [RFC6776]. Under all other circumstances it MUST ignore the block.
A participant of a media session MAY use SDP to signal its support for the report block specified in this document or use them without any prior signaling (see section 5 of [RFC3611]).
For signaling in SDP, the RTCP XR attribute as defined in [RFC3611] MUST be used. The SDP [RFC4566] attribute 'xr-format' defined in RFC3611 is augmented as described in the following to indicate the bytes discarded metric.
rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)]
CRLF ; defined in [RFC3611]
xr-format =/ xr-discard-bytes
xr-discard-bytes = "discard-bytes"
The parameter 'discard-bytes' to indicate support for the Bytes Discarded Report Block defined in Section 3.
When SDP is used in Offer/Answer context, the mechanism defined in [RFC3611] for unilateral "rtcp-xr" attribute parameters applies (see section 5.2 of [RFC3611]).
The Bytes Discarded block does not provide per-packet statistics, hence the risk to confidentiality documented in Section 7, paragraph 3 of [RFC3611] does not apply. In some situations, returning very detailed error information (e.g., over-range measurement or measurement unavailable) using this report block can provide an attacker with insight into the security processing. Implementers should consider the guidance in [I-D.ietf-avt-srtp-not-mandatory] for using appropriate security mechanisms, i.e., where security is a concern, the implementation should apply encryption and authentication to the report block. For example this can be achieved by using the AVPF profile together with the Secure RTP profile as defined in [RFC3711]; an appropriate combination of the two profiles (an "SAVPF") is specified in [RFC5124]. However, other mechanisms also exist (documented in [I-D.ietf-avtcore-rtp-security-options]) and might be more suitable.
Additionally, The security considerations of [RFC3550], [RFC3611], and [RFC4585] apply.
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 extends the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry" by a new value: BDR (Bytes Discarded Report).
[Note to RFC Editor: please replace BDR with the IANA provided RTCP XR block type for this block here and in the diagrams above. Please remove this note prior to publication as an RFC.]
This document registers a new parameters for the Session Description Protocol (SDP), "discard-bytes" in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".
Varun Singh (varun.singh@iki.fi)
Aalto University Comnet, Otakaari 5A, 02150 Espoo, Finland.
The authors would like to thank Alan Clark, Roni Even, Sam Hartman, Colin Perkins, Dan Romascanu, Dan Wing, and Qin Wu for providing valuable feedback on earlier versions of this draft.
RFC EDITOR NOTE: please change XXXX in [RFCXXXX] by the new RFC number, when assigned.
Note to the RFC-Editor: please remove this section prior to publication as an RFC.