| XR Block Working Group | J. Ott |
| Internet-Draft | V. Singh |
| Intended status: Standards Track | Aalto University |
| Expires: June 22, 2013 | I. Curcio |
| Nokia Research Center | |
| December 19, 2012 |
RTP Control Protocol (RTCP) Extended Reports (XR) for Run Length Encoding (RLE) of Discarded Packets
draft-ietf-xrblock-rtcp-xr-discard-rle-metrics-05.txt
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 per-packet report metric capturing individual packets discarded from the jitter buffer after successful reception.
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 June 22, 2013.
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 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 RR statistics. The detailed reporting may enable a sender to react more appropriately to the observed networking conditions as these can be characterized better, although at the expense of extra overhead.
Among many other report blocks, RFC3611 specifies the Loss RLE block which reports runs of packets received and lost with the granularity of individual packets. This can help both error recovery and path loss characterization. In addition to lost packets, RFC3611 defines the notion of "discarded" packets: packets that were received but dropped from the 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].
Recently proposed extensions to the XR reporting suggest enhancing this discard metric:
However, none of these metrics allow a receiver to report precisely which packets 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 gap/burst report [I-D.ietf-xrblock-rtcp-xr-burst-gap-discard], 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 incur significant complexity and reporting overhead and might still not deliver the desired accuracy.
This document defines a discard report block following the idea of the run-length encoding applied for lost and received packets in [RFC3611].
Complementary to or instead of the indication which packets were discarded, an XR block is defined 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] and indicate requirement levels for compliant implementations.
The terminology defined in RTP [RFC3550] and in the extensions for XR reporting [RFC3611] applies.
The XR Discard RLE report block uses the same format as specified for the loss and duplicate report blocks in [RFC3611]. Figure 1 recaps the packet format. The fields "BT", "T", "block length", "SSRC of source", "begin_seq", and "end_seq" SHALL have the same semantics and representation as defined in [RFC3611]. The "chunks" encoding the run length SHALL have the same representation as in RFC3611, but encode discarded packets.
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=DRLE |rsvd |E| T | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: XR Discard Report Block
Block Type (BT, 8 bits): A Run-length encoded Discarded Packets Report Block is identified by the constant DRLE.
[Note to RFC Editor: please replace DRLE with the IANA provided RTCP XR block type for this block. Please remove this note prior to publication as an RFC.]
rsvd (3 bits): These reserved bits SHOULD be set to zero by receivers and MUST be ignored by senders.
The 'E' bit is introduced to distinguish between packets discarded due to early arrival and those discarded due to late arrival. The 'E' bit MUST be set to '1' if the chunks represent packets discarded due to too early arrival and MUST be set to '0' otherwise.
In case both early and late discarded packets shall be reported, two Discard RLE report blocks MUST be included; their sequence number range MAY overlap, but individual packets MUST only be reported as either early or late and not appear marked in both. Packets reported in neither are considered to be properly received and not discarded.
Discard RLE Report Blocks SHOULD be sent in conjunction with an RTCP RR as a compound RTCP packet.
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 2: 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).
The 'E' bit is introduced to distinguish between packets discarded due to early arrival and those discarded due to late arrival. The 'E' bit MUST be set to '1' if it reports bytes discarded due to early arrival and MUST be set to '0' if it reports bytes discarded due to late arrival. In case both early and late discarded packets shall be reported, two Bytes Discarded report blocks MUST be included.
These reserved bits (5 bits) SHOULD be set to zero by receivers and MUST be ignored by senders.
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.
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 RR as a compound RTCP packet.
This section describes the behavior of the reporting (= receiver) node and the media sender.
Transmission of RTCP XR Discard RLE Reports is up to the discretion of the receiver, as is the reporting granularity. However, it is RECOMMENDED that the receiver signals all discarded packets using the method defined in this document. If all packets over a reporting period were lost, the receiver MAY use the Discard Report Block [I-D.ietf-xrblock-rtcp-xr-discard] instead. In case of limited available reporting bandwidth, it is up to the receiver whether or not to include RTCP XR Discard RLE reports.
The receiver MAY send the Discard RLE Reports as part of the regularly scheduled RTCP packets as per RFC3550. It MAY also include Discard RLE Reports in immediate or early feedback packets as per RFC4585.
The media sender MUST be prepared to operate without receiving any Discard RLE reports. If Discard RLE reports are generated by the receiver, the sender cannot rely on all these reports being received, nor can the sender rely on a regular generation pattern from the receiver side.
However, if the sender receives any RTCP reports but no Discard RLE report blocks and is aware that the receiver supports Discard RLE report blocks, it MAY assume that no packets were discarded at the receiver.
The 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 two report blocks 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 RLE discard metric and bytes discarded metric.
rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)]
CRLF ; defined in [RFC3611]
xr-format =/ xr-discard-rle
/ xr-discard-bytes
xr-discard-rle = "discard-rle"
xr-discard-bytes = "discard-bytes"
The parameter 'discard-rle' MUST be used to indicate support for the Discard RLE Report Block defined in Section 3, the parameter 'discard-bytes' to indicate support for the Bytes Discarded Report Block defined in Section 4
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 security considerations of [RFC3550], [RFC3611], and [RFC4585] apply. Since this document offers only a more precise reporting for an already existing metric, no further security implications are foreseen.
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 two new values: DRLE and BDR.
[Note to RFC Editor: please replace DRLE and 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 two new parameters for the Session Description Protocol (SDP), "discard-rle" and "discard-bytes", in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".
Joerg Ott (jo@comnet.tkk.fi)
Aalto University Comnet, Otakaari 5A, 02150 Espoo, Finland.
Thanks to Qin Wu, Colin Perkins, Dan Romascanu, Roni Even and Dan Wing for providing valuable feedback on earlier versions of this draft
| [I-D.ietf-xrblock-rtcp-xr-discard] | Clark, A, Zorn, G and W Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count metric Reporting", Internet-Draft draft-ietf-xrblock-rtcp-xr-discard-09, October 2012. |
| [I-D.ietf-xrblock-rtcp-xr-burst-gap-discard] | Clark, A, Huang, R and W Wu, "RTP Control Protocol(RTCP) Extended Report (XR) Block for Discard Count metric Reporting", Internet-Draft draft-ietf-xrblock-rtcp-xr-burst-gap-discard-06, October 2012. |
Note to the RFC-Editor: please remove this section prior to publication as an RFC.