Audio/Video Transport Working G. Hunt
Group P. Arden
Internet-Draft BT
Intended status: Informational May 18, 2010
Expires: November 19, 2010
Monitoring Architectures for RTP
draft-hunt-avt-monarch-01.txt
Abstract
This memo proposes an architecture for extending RTCP with a new RTCP
XR (RFC3611) block type to report new metrics regarding media
transmission or reception quality, as proposed in
draft-ietf-avt-rtcp-guidelines (work in progress [replace with RFC
number]). This memo suggests that a new block should contain a
single metric or a small number of metrics relevant to a single
parameter of interest or concern, rather than containing a number of
metrics which attempt to provide full coverage of all those
parameters of concern to a specific application. Applications may
then "mix and match" to create a set of blocks which covers their set
of concerns. Where possible, a specific block should be designed to
be re-usable across more than one application, for example, for all
of voice, streaming audio and video.
Status of this Memo
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This Internet-Draft will expire on November 19, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Using small blocks . . . . . . . . . . . . . . . . . . . . . . 5
4. The identity block . . . . . . . . . . . . . . . . . . . . . . 6
5. An example of a metric block . . . . . . . . . . . . . . . . . 10
6. Application to translators . . . . . . . . . . . . . . . . . . 11
7. Application to RFC 5117 topologies . . . . . . . . . . . . . . 12
8. Expanding the RTCP XR block namespace . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. Informative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Any proliferation of metrics for transport and application quality
monitoring has been identified as a potential problem for RTP/RTCP
interoperability. Different applications layered on RTP may have
some monitoring requirements in common, which should be satisfied by
a common design. The objective here is to define an extensible
framework and a small number of re-usable metrics to reduce
implementation costs and to maximise inter-operability. Work-in-
progress on [GUIDELINES] has stated that, where RTCP is to be
extended with a new metric, the preferred mechanism is by the
addition of a new RTCP XR [RFC3611] block. This memo assumes that
any requirement for a new metric to be transported in RTCP will use a
new RTCP XR block.
[GUIDELINES] provides advice on when and how new metrics should be
introduced, including recommending that metrics are based on existing
standards whenever possible.
Section 3 describes the key proposal of this memo, the use of small
metrics blocks each of which addresses a single parameter of interest
which may be "mixed and matched", rather than providing a large block
to address all the parameters which might be of interest to a broad
class of applications (for example, all VoIP applications).
Section 4 describes an optimisation to avoid repetition of
identification information, which becomes desirable when small blocks
are used.
Section 5 provides an example of the application of these principles
to a specific case, that of a metric block to report packet delay
variation.
Section 6 draws attention to the guidance in [RFC3550] concerning
RTCP and translators.
Section 7 discusses the potential application of RTCP XR metrics
blocks to the conferencing topologies discussed in [RFC5117].
Section 8 consists (in this draft) only of an "Editor's note" asking
whether the limited namespace available for RTCP XR blocks is a
concern, and if so whether it would be desirable to work on a
standardised means to expand it.
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2. Requirements notation
This memo is informative and as such contains no normative
requirements.
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3. Using small blocks
Different applications using RTP for media transport certainly have
differing requirements for metrics transported in RTCP to support
their operation. For many applications, the basic metrics for
transport impairments provided in RTCP SR and RR packets [RFC3550]
(together with source identification provided in RTCP SDES packets)
are sufficient. For other applications additional metrics may be
required or at least sufficiently useful to justify the overheads,
both of processing in endpoints and of increased session bandwidth.
For example an IPTV application using Forward Error Correction (FEC)
might use either a metric of post-repair loss or a metric giving
detailed information about pre-repair loss bursts to optimise payload
bandwidth and the strength of FEC required for changing network
conditions. However there are many metrics available. It is likely
that different applications or classes of applications will wish to
use different metrics. Any one application is likely to require
metrics for more than one parameter but if this is the case,
different applications will almost certainly require different
combinations of metrics. If larger blocks are defined containing
multiple metrics to address the needs of each application, it becomes
likely that many different such larger blocks are defined, which
becomes a danger to interoperability.
To avoid this pitfall, this memo proposes the use of small RTCP XR
metrics blocks each containing a very small number of individual
metrics characterising only one parameter of interest to an
application running over RTP. For example, at the RTP transport
layer, the parameter of interest might be packet delay variation, and
specifically the metric "IPDV" defined by [Y1540]. See Section 5 for
architectural considerations for a metrics block, using as an example
a metrics block to report packet delay variation.
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4. The identity block
Any measurement must be identified. However if metrics are delivered
in small blocks there is a danger of inefficiency arising from
repeating this information in a number of metrics blocks within the
same RTCP packet, in cases where the same identification information
applies to multiple metrics blocks.
An instance of a metric must be identified using information which is
likely to include most of the following:
o the node at which it was measured,
o the source of the measured stream (for example, its CNAME),
o the SSRC of the measured stream,
o the sequence number of the first packet of the RTP session,
o the extended sequence numbers of the first packet of the current
measurement interval, and the last packet included in the
measurement,
o the duration of the most recent measurement interval and
o the duration of the interval applicable to cumulative measurements
(which may be the duration of the RTP session to date).
[Editor's note: this set of information overlaps with, but is more
extensive than, that in the union of similar information in RTCP RR
packets. Should we assume that RR information is always present if
XR is sent, and that measurement intervals are exactly coincident?
If so, state assumption and remove overlaps. What were the design
considerations which led to the additional information *not* being
present in RRs? The reason for the additional information here is
the perceived difficulty of "locating" the *start* of the RTP session
(sequence number of 1st packet, duration of interval applicable to
cumulative measurements) using only RR. Is this a misconception? It
leads to redundant information in this design because equivalent
information is provided multiple times, once in *every*
identification packet. Though this ensures immunity to packet loss,
the design is ugly and the overhead is not completely trivial.]
This section proposes an approach to minimise the inefficiency of
providing this identification information, assuming that an
architecture based on small blocks means that a typical RTCP packet
will contain more than one metrics block needing the same
identification. The choice of identification information to be
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provided is discussed in [IDENTITY] (work in progress).
The approach is to define a stand-alone block containing only
identification information, and to tag this identification block with
a number which is unique within the scope of the containing RTCP XR
packet. The "containing RTCP XR packet" is defined here as the RTCP
XR header with PT=XR=207 defined in Section 2 of [RFC3611] and the
associated payload defined by the length field of this RTCP XR
header. The RTCP XR header itself includes the SSRC of the node at
which all of the contained metrics were measured, hence this SSRC
need not be repeated in the stand-alone identification block. A
single containing RTCP XR packet may contain multiple identification
blocks limited by the range of the tag field. Typically there will
be one identification block per monitored source SSRC, but the use of
more than one identification block for a single monitored source SSRC
within a single containing RTCP XR packet is not ruled out.
There will be zero or more metrics blocks dependent on each
identification block. The dependence of an instance of a metrics
block on an identification block is established by the metrics
block's having the same numeric value of the tag field as its
identification block (in the same containing RTCP XR packet).
Figure 1 below illustrates this principle using as an example an RTCP
XR packet containing four metrics blocks, reporting on streams from
two sources. The measurement identity information is provided in two
blocks with Block Type NMI, and tag values 0 and 1 respectively.
Note: in this example, RTCP XR block type values for four proposed
new block types (work in progress) are given as NMI, NPDV, NBGL and
NDEL. These represent numeric block type codepoints to be allocated
by IANA at the conclusion of the work.
Each of these two identity blocks will specify the SSRC of one of the
monitored streams, as well as information about the span of the
measurement. There are two metrics blocks with tag=0 indicating
their association with the measurement identity block which also has
tag=0. These are the two blocks following the identity block with
tag=0, though this positioning is not mandatory. There are also two
metrics blocks with tag=1 indicating their association with the
measurement identity block which also has tag=1, and these are the
two blocks following the identity block with tag=1.
[Editor's note: if we mandated that metrics blocks associated with an
identity block must always follow the identity block we could save
the tag field and possibly simplify processing. Is this preferable
to cross-referencing with a numeric tag?]
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In the example, the block types of the metrics blocks associated with
tag=0 are BT=NPDV (a PDV metrics block) and BT=NBGL (a burst and gap
loss metrics block). The block types of the metrics blocks
associated with tag=1 are BT=NPDV (a second PDV metrics block) and
BT=NDEL (a delay metrics block). This illustrates that:
o multiple instances of the same metrics block may occur within a
containing RTCP XR packet, associated with different
identification information, and
o differing measurements may be made, and reported, for the
different streams arriving at an RTP system.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=XR=207 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of RTCP XR packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NMI |0|tag=0| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of stream source 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...measurement identity information, source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NPDV |I|tag=0|pdvtyp | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...PDV information for source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NBGL |I|tag=0| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...burst-gap-loss information for source 1... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NMI |0|tag=1| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of stream source 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...measurement identity information, source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NPDV |I|tag=1|pdvtyp | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...PDV information for source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=NDEL |I|tag=1| resv | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. ...delay information for source 2... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTCP XR block with identity blocks
This approach of separating the identification information is more
costly than providing identification in each metrics block if only a
single metrics block is sent in an RTCP packet, but becomes
beneficial as soon as more than one metrics block shares common
identification.
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5. An example of a metric block
This section uses the example of an existing proposed metrics block
to illustrate the application of the principles set out in Section 3.
The example [PDV] (work in progress) is a block to convey information
about packet delay variation (PDV) only, consistent with the
principle that a metrics block should address only one parameter of
interest. One simple metric of PDV is available in the RTCP RR
packet as the "jit" field. There are other PDV metrics which may be
more useful to certain applications. Two such metrics are the IPDV
metric ([Y1540], [RFC3393]) and the MAPDV2 metric [G1020]. Use of
these metrics is consistent with the principle in Section 5 of
[GUIDELINES] that metrics should usually be defined elsewhere, so
that RTCP standards define only the transport of the metric rather
than its nature. The purpose of this section is to illustrate the
architecure using the example of [PDV] (work in progress) rather than
to document the design of the PDV metrics block or to provide a
tutorial on PDV in general.
Given the availability of at least three metrics for PDV, there are
design options for the allocation of metrics to RTCP XR blocks:
o provide an RTCP XR block per metric
o provide a single RTCP XR block which contains all three metrics
o provide a single RTCP block to convey any one of the three
metrics, together with a identifier to inform the receiving RTP
system of the specific metric being conveyed
In choosing between these options, extensibility is important,
because additional metrics of PDV may well be standardised and
require inclusion in this framework. The first option is extensible
but only by use of additional RTCP XR blocks, which may consume the
limited namespace for RTCP XR blocks at an unacceptable rate. The
second option is not extensible, so could be rejected on that basis,
but in any case a single application is quite unlikely to require
transport of more than one metric for PDV. Hence the third option
was chosen. This implies the creation of a subsidiary namespace to
enumerate the PDV metrics which may be transported by this block, as
discussed further in [PDV] (work in progress).
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6. Application to translators
Section 7.2 of [RFC3550] describes processing of RTCP by translators.
RTCP XR is within the scope of the recommendations of [RFC3550].
Some RTCP XR metrics blocks may usefully be measured at, and reported
by, translators. As described in [RFC3550] this creates a
requirement for the translator to allocate an SSRC for itself so that
it may populate the SSRC in the RTCP XR packet header (although the
translator is not a Synchronisation Source in the sense of
originating RTP media packets). It must also supply this SSRC and
the corresponding CNAME in RTCP SDES packets.
In RTP sessions where one or more translators generate any RTCP
traffic towards their next-neighbour RTP system, other translators in
the session have a choice as to whether they forward a translator's
RTCP packets. Forwarding may provide additional information to other
RTP systems in the connection but increases RTCP bandwidth and may in
some cases present a security risk. RTP translators may have
forwarding behaviour based on local policy, which might differ
between different interfaces of the same translator.
[Editor's note: for bidirectional unicast, an RTP system may usually
detect RTCP from a translator by noting that the sending SSRC is not
present in any RTP media packet. However even for bidirectional
unicast there is a possibility of a source sending RTCP before it has
sent any RTP media (leading to transient mis-categorisation of an RTP
end system or RTP mixer as a translator), and for multicast sessions
- or unidirectional/streaming unicast - there is a possibility of a
receive-only end system being permanently mis-categorised as a
translator. Is there a need for a translator to declare itself
explicitly? Needs further thought.]
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7. Application to RFC 5117 topologies
An RTP system (end system, mixer or translator) which originates,
terminates or forwards RTCP XR blocks is expected to handle RTCP,
including RTCP XR, as specified in [RFC3550] for that class of RTP
systems. Provided this expectation is met, an RTP system using RTCP
XR is architecturally no different from an RTP system of the same
class (end system, mixer, or translator) which does not use RTCP XR.
This statement applies to the topologies investigated in [RFC5117],
where they use RTP end systems, RTP mixers and RTP translators as
these classes are defined in [RFC3550].
These topologies are specifically Topo-Point-to-Point, Topo-
Multicast, Topo-Translator (both variants, Topo-Trn-Translator and
Topo-Media-Translator, and combinations of the two), and Topo-Mixer.
The topologies based on systems which do not behave according to
[RFC3550], that is Topo-Video-Switch-MCU and Topo-RTCP-terminating-
MCU, suffer from the difficulties described in [RFC5117]. These
difficulties apply to systems sending, and expecting to receive, RTCP
XR blocks as much as to systems using other RTCP packet types. For
example, a participant RTP end system may send media to a video
switch MCU. If the media stream is not selected for forwarding by
the switch, neither RTCP RR packets nor RTCP XR blocks referring to
the end system's generated stream will be received at the RTP end
system. Strictly the RTP end system can only conclude that its RTP
has been lost in the network, though an RTP end system complying with
the robustness principle of [RFC1122] should survive with essential
functions unimpaired.
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8. Expanding the RTCP XR block namespace
[Editor's note: the RTCP XR block namespace is limited by the 8-bit
block type field in the RTCP XR header (Section 3 of [RFC3611]).
IESG have noted that this is potentially restrictive. It would be
possible to standardise an expansion mechanism, probably based on use
of a new field near the start of the variable-length "type-specific
block contents" field. Clearly this could apply only to new block
types, so might be standardised to apply to some subrange of the
current 8-bit range, for example the range 128 through 191 might be
used. At time of writing, block types 12 to 254 are unassigned and
255 is reserved for future expansion. Is there a consensus for, or
against, work to allow expansion? One potential use is through
hierarchical control, where one or a few codepoints at the top level
are given to other SDOs who may then define a number of metrics
distinguished by values in the (so far hypothetical) new field.]
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9. IANA Considerations
None.
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10. Security Considerations
This document itself contains no normative text and hence should not
give rise to any new security considerations, to be confirmed.
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11. Informative References
[G1020] ITU-T, "ITU-T Rec. G.1020, Performance parameter
definitions for quality of speech and other voiceband
applications utilizing IP networks", July 2006.
[GUIDELINES]
Ott, J., "Guidelines for Extending the RTP Control
Protocol (RTCP)", ID draft-ott-avt-rtcp-guidelines-03,
February 2010.
[IDENTITY]
Hunt, G., "RTCP XR Report Block for Measurement Identity",
ID draft-ietf-avt-rtcp-xr-meas-identity-02, May 2009.
[PDV] Hunt, G., "RTCP XR Report Block for Packet Delay Variation
Metric Reporting", ID draft-ietf-avt-rtcp-xr-pdv-03,
May 2009.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[RFC3393] Demichelis, C., "IP Packet Delay Variation Metric for IP
Performance Metrics (IPPM)", RFC 3393, November 2002.
[RFC3550] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.
[RFC3611] Friedman, T., "RTP Control Protocol Extended Reports (RTCP
XR)", RFC 3611, November 2003.
[RFC5117] Westerlund, M., "RTP Topologies", RFC 5117, January 2008.
[Y1540] ITU-T, "ITU-T Rec. Y.1540, IP packet transfer and
availability performance parameters", November 2007.
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Authors' Addresses
Geoff Hunt
BT
Orion 1 PP2
Adastral Park
Martlesham Heath
Ipswich, Suffolk IP5 3RE
United Kingdom
Phone: +44 1473 651704
Email: geoff.hunt@bt.com
Philip Arden
BT
Orion 3/7 PP4
Adastral Park
Martlesham Heath
Ipswich, Suffolk IP5 3RE
United Kingdom
Phone: +44 1473 644192
Email: philip.arden@bt.com
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