Network Working Group Q. Wu
Internet-Draft F. Xia
Intended status: Standards Track R. Even
Expires: April 12, 2011 Huawei
October 9, 2010
RTCP Report Extension for Feedback Suppression
draft-wu-avt-retransmission-supression-rtp-03
Abstract
This document specifies an extension to the RTCP feedback messages
defined in the Audio-Visual Profile with Feedback (AVPF). This
extension allows an intermediate node in the network (such as an RTP
translator) inform downstream receivers that sending a feedback
message concerning a specified set of RTP messages may be
unnecessary, or even harmful. For example, in a source-specific
multicast session with large fan-out, packet loss close to the media
or distribution source of the session is detected as a loss by a
large number of receivers. The RTCP NACK messages used to request
retransmission of the missing packets are all addressed to the media
sender, or a designated feedback target. This may result in a
phenomenon known variously as a "NACK implosion" or "feedback storm".
The Feedback Suppression message defined herein is used to notify
receivers that packet loss was detected and that the sender of the
report will either proactively recover, or no recovery is possible.
Receivers respond to receipt of a feedback suppression message by not
sending a feedback message (e.g. a NACK) that they otherwise would,
This in turn reduces load on both the feedback target and on the
network. This document registers two new RTCP messages for Feedback
Suppression.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on April 12, 2011.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7
4. RTCP Receiver Feedback Report Extension . . . . . . . . . . . 9
4.1. Transport Layer Feedback Message . . . . . . . . . . . . . 9
4.1.1. NACK implosion Suppression Summary report . . . . . . 9
4.2. Payload Specific Feedback Message . . . . . . . . . . . . 10
4.2.1. FIR implosion Suppression Summary report . . . . . . . 10
5. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Source Specific Multicast (SSM) use case . . . . . . . . . 12
6.1.1. Simple Feedback Model . . . . . . . . . . . . . . . . 12
6.1.2. Distribution Source Feedback Summary Model . . . . . . 14
6.2. RTP transport translator use case . . . . . . . . . . . . 15
6.3. Multipoint Control Unit (MCU) use case . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Example scenarios for Retransmission Storm
Suppression . . . . . . . . . . . . . . . . . . . . . 19
A.1. Scenario 1: One or more media sender,One distribution
source . . . . . . . . . . . . . . . . . . . . . . . . . . 19
A.2. Scenario 2:One media sender, Two distribution sources
in cascade . . . . . . . . . . . . . . . . . . . . . . . . 20
A.3. Scenario 3:One media sender, Two distribution sources
in parallel . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Applicability of Feedback Storm Suppression . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
RTCP feedback messages [RFC4585]allow the receivers in an RTP session
to report events and ask for action from the sender (or a delegated
feedback target). There are cases where multiple receivers may
initiate the same, or an equivalent message towards the same sender.
When the receiver count is large, this behavior may overload the
sender or the network or both. One common case is receivers
utilizing RTP retransmission as a packet loss recovery technique in a
real-time application such as streaming video or
audio.[RFC4588]Feedback is accomplished using the RTCP NACK message
which conveys the RTP sequence number(s) of the lost packet(s). This
information can then be used by the sender to retransmit the missing
RTP packets using the RTP retransmission payload format[RFC4585].
However, in topologies utilizing transport translators (Topo-Trn-
Translator) or large-scale multicast transmission (Topo-Multicast) as
defined in [RFC5117], packet loss can occur on either an upstream
link or a downstream aggregate link of the intermediate network
element (e.g., Retransmission server, Distribution Source). Where
there are many receivers, this may result in a NACK implosion towards
the sender, i.e., large number of NACK requests to the same multicast
sender for retransmission of the same RTP packets. This phenomenon
goes by a number of alternate names, such as the "NACK storm"
terminology of [DVB-IPTV]. In an attempt to increase its robustness
against the loss of a NACK message or of retransmission packets, a
receiver may send multiple NACKs for the same detected packet loss,
which may aggravate the NACK implosion.
Another use case involves video Fast Update requests. A storm of
these feedback messages can occur in conversational multimedia
scenarios like Topo-Video-switch-MCU [RFC5117]. In this scenario,
packet loss may happen on an upstream link of an intermediate network
element such as a Multipoint Control Unit(MCU). Receivers missing
the packets issue fast update requests (i.e., Full Intra Request(FIR)
described in [RFC5104]), which results in an implosion of FIR
requests from receivers to the same media sender.
As these feedback storms propagate (e.g., NACK implosion or Fast
update implosion), the network may be permeated with more and more
feedback traffic, resulting in a positive feedback loop as the
network is also saturated with media traffic. RTCP feedback storms
pose a substantial risk of increasing network congestion, and in
extreme cases to congestion collapse on the control channel (e.g.
RTCP feedback), the data channel (i.e. RTP restransmission), or
both.
In order to mitigate these behaviors, the current text in [RFC5760]
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allows the distribution source to filter out the NACK messages and
[DVB-IPTV] suggests sending a NACK message from sender to the
receiver. However NACK is defined as a receiver report sent from a
receiver to the sender and therefore exhibits a semantic mismatch
when used as a suppression indication from the sender (or
intermediary) to the receiver. This document instead specifies a
newly message for this function. It further is more precise in the
intended uses and less likely to be confusing to receivers. It tells
receivers explicitly that feedback for a particular packet loss is
not needed and can provide an early indication before the receiver
reacts to the loss and invokes its packet loss repair machinery.
The Feedback Suppression message asks a receiver to not send feedback
messages for particular packets (indicated by their RTP sequence
numbers) independent of whether the receiver detected the packet loss
or detected a need for a decoder refresh point).
In order to detect packet loss before the receivers perceive it, one
or more intermediate nodes are placed between the media sender and
receiver (e.g., Distribution server, MCU, RTP translator). These
intermediaries monitor for packet loss upstream of themselves by
checking RTP sequence numbers, just as receivers do. Upon detecting
(or suspecting) an upstream loss, the intermediary may either
initiate its own feedback toward the source to provoke a
retransmission, send Feedback Suppression message towards the
receivers as defined in this specification, or both.
Instead of using specialized intermediaries, another possibility is
to instantiate one or more RTP receivers upstream of the loss region
to act as immediate reporters as described in[DVB-IPTV]. These
intermediate nodes need to take into account such factors as the
tolerable application delay, the network dynamics, and the media
type. When the packet loss is detected upstream of the intermediary
and additional latency is tolerable, the intermediate node may itself
send a feedback message asking for retransmission of the suspected
lost packet or ask for the correct decoder refresh point. Because it
has already provided the necessary feedback toward the source, the
intermediate node can be reasonably certain that it will help the
situation by sending a Feedback Suppression message to all the
relevant receivers, thereby indicating that the receivers should not
themselves transmit feedback messages. When the sender receives the
request from the intermediate node, the sender resends the missing
packets to the receivers by using the RTP retransmission payload
format [RFC4588]or resends a new refresh point for FIR Initiator
[RFC5104], depending on the type of feedback it received.
RTCP Feedback Storm Suppression follows the same semantic model as
RTCP NACK - it conveys the packet receipt/loss events at the sequence
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number level and considers missing packets as unrepaired. But unlike
RTCP NACK, the Feedback Suppression messages can be generated at
intermediate nodes who are not RTP receivers and sent to the
corresponding receivers. Intermediaries downsteam of an intermediary
detecting loss obviously SHOULD NOT initiate their own additional
feedback suppression messages for the same packet sequence numbers.
They may either simply forward the Feedback Suppression message
received from upstream, or augment (or replace) it with a feedback
suppression message that reflects the loss pattern they have
themselves seen.
Since feedback suppression interacts strongly with repair timing, it
has to work together with feedback and retransmission to not
adversely impact the repair of lost source packets. In some cases
where the loss was detected and repair initiated much closer to the
source, the delay for the receiver to recover from packet loss can be
reduced through the combination of intermediary feedback to the
source and feedback suppression downstream. In all (properly
operating) cases, the risk of increasing network congestion is
decreased. A receiver may still have sent a Feedback message before
receiving a feedback suppression message, but further feedback
messages for those sequence numbers will be suppressed by this
technique.
This document registers two new RTCP Feedback messages for Feedback
Suppression. Applications that are employing one or more loss-repair
methods MAY use Feedback Suppression together with their existing
loss-repair methods either for every packet they expect to receive,
or for an application-specific subset of the RTP packets in a
session. In other words, receivers MAY ignore Feedback Suppression
messages, but SHOULD react to them unless they have good reason to
still send feedback messages despite having been requested to
suppress them.
2. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Intermediate Source:
One intermediate node with logical function which is used to
process or foward packet at the RTP layer. The intermediate
Source is located between media sender and receivers. The
examples of intermediate source are Distribution server, MCU, RTP
translator.
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Upstream RTP Client:
An RTP Client located in the upstream from the intermediate source
as described in [DVB-IPTV]. This Client is able to detect
upstream packet loss impacting all the RTP receivers serviced by
the intermediate source and receiving SSM service.
Immediate reporting RTP Client:
An RTP Client located on a downstream aggregate link from the
intermediate source as described in [DVB-IPTV]. This Client is
able to detect downstream packet loss on the aggregate link or
other nodes upstream of the aggregat link, thus impacting all the
RTP receivers serviced by the intermediate source and receiving
SSM service.
Loss Reporter:
The Loss Reporter is one logical function which is used to detect
the packet loss at the RTP layer and report it to the intermediate
source. The function of the loss reporter may be collocated with
or integrated into the same entity. In this case, for a session
defined as having a intermediate source A, on ports n for the RTP
channel and k for the RTCP channel, the unicast RTCP Feedback
Target is identified by an IP address of intermediate source A on
port k. The loss reporter MAY also be implemented in one or more
entities different from the intermediate source. In this case,
the loss reporter may be an upstream RTP client or immediate
reporting RTP client located on a downstream aggregate link of the
intermediate source.
3. Protocol Overview
In order to avoid the forms of NACK implosion described in section 1,
the loss reporter is introduced. The loss reporter detects and
reports packet loss, on both the upstream link and the downstream
aggregate link. When upstream link or downstream aggregate link
packet loss occurs, the Loss reporter may inform intermediate source
of the detected packet loss using Feedback Suppression messages. In
response, the intermediate source either forwards packet loss
suppression report received from loss reporter or creates a Feedback
Suppression report and sends it to all the RTP receivers, over the
multicast channel. This loss suppression report tells the receivers
that the lost packet will either be forthcoming from intermediate
source via retransmission, or it irretrievably lost such that there
is nothing to be gained by the receiver sending a NACK to the media
sender. The intermediate source then can (optionally) ask for
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retransmission of the lost packets from the media sender on behalf of
all the RTP receivers. Upon receiving the lost packet via the RTP
retransmission payload format, the intermediate source forwards the
retransmitted packet to all the receivers.
When the loss reporter(s) are part of a feedback target collocated
with the intermediate source, redistribution of the feedback
suppression report is trivial. In such cases, the loss reporter
function in the feedback target detects packet loss coming from an
upstream link and informs the collocated intermediate source. Also
the loss reporter may detect packet loss occurring within
intermediate source itself and report to intermediate source using
the same method. When the loss reporter(s) are physically and(or)
topologically distinct from intermediate source, each loss reporter
MUST create a packet loss report using the similar format as
conventional RTCP NACK packets at the RTP layer and send it to the
intermediate source . The loss reporters may be upstream client or
downstream immediate reporter who is dedicated to detect and report
packet loss.
The intermediate source must be able to communicate with all group
members in order for either mechanism to be effective at suppressing
feedback. The general architecture is displayed below in Figure 1.
The Intermediate Source must be able to communicate with all group
members in order for either mechanism to be effective at suppressing
feedback. The general architecture is displayed below in Figure 1
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+--------+ +------------+ Source-specific
|Media | | | Multicast
|Sender 1|<------->| | +----------------> R(1)
+--------+ |Intermediate| |
| SOURCE | +--+
+--------+ | | | |
|Media |<------->| | | +-----------> R(2)
|Sender 2| | Feedback |->+ +---- :
+--------+ |+ Target --+| | +------> R(n-1)
: || +---+ || | |
: || | R| || +--+--> R(n)
|| | E| ||
+--------+ || |L P| ||
|Media | || |O O| ||
|Sender M|<---- -->|| |S R| ||
+--------+ || |S T| ||
|| | E| ||
|| | R| ||
|| +---+ ||
|+----------+|
+------------+
Transport of packet loss informationfrom the Loss Reporter to the
Feedback Target is via unicast RTCP feedback if the two are not
co-located.
Figure 1: System Architecture
In this figure, we assume the intermediate source is separated from a
particular media sender and the loss reporter is part of feedback
target collocated with intermediate source. The communication
between the media sender and the intermediate source is compliant
with the methods described in [RFC5760]. Configuration information
also follows [RFC5760], with the following additional considerations:
o The Loss Reporters know the addresses of their respectively
responsible Feedback Targets.
4. RTCP Receiver Feedback Report Extension
4.1. Transport Layer Feedback Message
4.1.1. NACK implosion Suppression Summary report
The NACK implosion Suppression message is an extension to the RTCP
feedback report and identified by RTCP packet type value PT=RTPFB and
FMT=TBD.
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The FCI field MUST contain one or more NACK Suppression Early
Indication (NSEI) entries. Each entry applies to a different media
sender, identified by its SSRC.
The Feedback Control Information (FCI) for NSEI uses the similar
format of message Types defined in the section 4.3.1.1 of [RFC5104].
The format is shown in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | BLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Message Format for the NSEI report
SSRC (32 bits):
The SSRC value of the media sender that is requested to send the
lost packet.
Packet ID (PID): 16 bits
The PID field is used to specify a lost packet. The PID field
refers to the RTP sequence number of the lost packet.
bitmask of following lost packets (BLP): 16 bits
The BLP allows for reporting losses of any of the 16 RTP packets
immediately following the RTP packet indicated by the PID. The
BLP's definition is identical to that given in [RFC4585].
4.2. Payload Specific Feedback Message
4.2.1. FIR implosion Suppression Summary report
The FIR implosion Suppression message is an extension to the RTCP
receiver feedback report and identified by RTCP packet type value
PT=PSFB and FMT=TBD.
The FCI field MUST contain one or more FIR suppression Early
Indication (FSEI) entries. Each entry applies to a different media
sender, identified by its SSRC.
The Feedback Control Information (FCI) for FSEI uses the similar
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format of message Types defined in the section 4.3.1.1 of [RFC5104].
The format is shown in Figure 3.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq nr. | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Message Format for the FSEI report
SSRC (32 bits):
The SSRC value of the media sender that is requested to send a
decoder refresh point.
Seq nr:8bits Command sequence number. The sequence number space is
unique for each pairing of the SSRC of command source and the SSRC
of the command target. The sequence number SHALL be increased by
1 modulo 256 for each new command.
Reserved: 24 bits
All bits SHALL be set to 0 by the sender and SHALL be ignored on
reception.
5. SDP Signaling
A new feedback value "fss" needs to be defined for the Feedback Storm
Suppression message to be used with Session Description Protocol
(SDP)[RFC4566] using the Augmented Backus-Naur Form (ABNF)[RFC4585].
The "fss" feedback value SHOULD be used with parameters that indicate
the feedback suppression supported. In this document, we define two
such parameters, namely:
o "fsei" denotes support of fir suppression early indication (fsei).
o "nsei" denotes support of NACK suppression early indication.
In the ABNF for rtcp-fb-val defined in [RFC4585], there is a
placeholder called rtcp-fb-id to define new feedback types. "fss" is
defined as a new feedback type in this document, and the ABNF for the
parameters for fss is defined here (please refer to section 4.2 of
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[RFC4585] for complete ABNF syntax).
rtcp-fb-val =/ "fss" rtcp-fb-fss-param
rtcp-fb-fss-param = SP "nsei";nack suppression early indication
/ SP "fsei";fir suppression early indication
/ SP token [SP byte-string]
; for future commands/indications
byte-string = <as defined in section 4.2 of [RFC4585] >
Refer to Section 4.2 of [RFC4585] for a detailed description and the
full syntax of the "rtcp-fb" attribute.
6. Example Use Cases
The operation of feedback suppression is similar for all types of RTP
sessions and topologies [RFC5117], however the exact messages used
and the scenarios in which suppression is employed differ for various
use cases. The following sections outline the intended use cases for
feedback suppression and give an overview of the particular
mechanisms.
6.1. Source Specific Multicast (SSM) use case
In SSM RTP sessions as described in [RFC5760], one or more Media
Senders send RTP packets to a Distribution Source. The Distribution
Source relays the RTP packets to the receivers using a source-
specific multicast group.
As outlined in the [RFC5760], there are two Unicast Feedback models
that may be used for reporting, - the Simple Feedback model and the
Distribution Source Feedback Summary Model. The RTCP Feedback
Suppression report extension specified in the section 4 of this
document will work in both Feedback models. Details of operation in
each are specified below.
6.1.1. Simple Feedback Model
In the simple Feedback Model, the Loss reporter(s) are disjoint from
the distribution source. In this case, an upstream client or
immediate reporting receiver may be chosen as the loss reporter.
Also in this model, the distribution source includes support for
retransmission as part of the offered SDP and expects such support
from the Media Sender.
As one dedicated receiver for packet loss reporting, the Loss
reporter MUST listen on the corresponding RTP session for data. When
the Loss reporter observes that a sequence of RTP packets from a
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Media Sender contains gaps (by checking the sequence number of
packets), the Loss reporter MUST use the same packet types as
traditional RTCP feedback described in[RFC3550]and create one new
RTCP Feedback Report with information on the RTP sequence number of
the lost packets and suppression early indication event. When a
receiver is eligible to transmit, it MUST send this Report packet to
the distribution source via unicast feedback.
The Distribution Source (unicast Feedback Target) MUST listen for
unicast RTCP data sent to the RTCP port. Upon receiving the unicast
RTCP Feedback Report packet from the loss reporter, the Distribution
Source MUST forward it to the group on the multicast RTCP session.
Every RTCP packet from each Loss reporter MUST be reflected
individually. If the loss reporter is part of group, the
Distribution source MUST filter this packet out and not forward it
back to this loss reporter.
If there are multiple loss reporters looking at the same RTP stream,
then the loss may be identified by more than one and those detecting
the loss will all send requests for the same packet loss. In this
case, the distribution source MUST filter the duplicated packet loss
request out and only forward one copy of the RTCP Feedback report
packet from the first loss reporter to the group impacted by packet
loss.
This unicast RTCP Feedback Report lets the receivers know that
feedback for this packet loss is not needed and should not be sent to
the media sender(s). If the Media Sender(s) are part of the SSM
group for RTCP packet reflection, the Distribution Source MUST filter
this packet out. If the Media Sender(s) are not part of the SSM
group for RTCP packets, the Distribution Source MUST not forward this
RTCP packets received from the receivers to the Media Sender(s).
When the receiver receives the RTCP packet, if the receiver
understands the message it will not send feedback (e.g., NACK) for
the missing packets reported in the message and will accept a
retransmission packet (if forthcoming) transmitted from the
Distribution Source. If it did not understand the Feedback
Suppression report the receiver may of course still send feedback
messages to the specified media sender. When the distribution source
receives a feedback message reporting loss from one or more receivers
after it has already detected packet loss or gotten a NACK feedback
message from loss reporter, the distribution source MUST filter them
out until the Retransmission stream is ready in the Distribution
Source.
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6.1.2. Distribution Source Feedback Summary Model
In the distribution source feedback summary model, the distribution
source will include the support for retransmission as part of the
offered SDP and will expect such support from the Media Sender, also
the Loss reporter instance may be integrated in the distribution
source or may be separated from the distribution source. In some
cases, several loss reporter instances for the same session can exist
at the same time, e.g., one loss reporter instance (loss reporter A)
is implemented in the upstream client A, one loss reporter instance
(loss reporter B) is implemented in the upstream client B, another
loss reporter instance for the same session (loss reporter C) is part
of feedback target within the distribution source. In this section,
we focus on this generic case to discuss the distribution Source
Feedback Summary Model.
The Loss reporter A and the Loss reporter B MUST listen on the RTP
channel for data. When the Loss reporter observes RTP packets from a
Media Sender are not consecutive by checking the sequence number of
packets, the loss reporter generates NACK message described
in[RFC4585] or generates the new RTCP Feedback Report packet
described in the section 6, and then send either of them to the
distribution source via unicast feedback.
The Distribution Source (unicast Feedback Target) MUST listen for
unicast RTCP data sent to the RTCP port. Upon receiving the unicast
RTCP Feedback Report packet from the loss reporter, the distribution
source needs to filter them out, i.e., identify these unicast RTCP
packets coming from the Dedicated receivers (i.e.,Loss Reporter A and
Loss Reporter B)based on the IP address of loss reporters or
dedicated RTCP port for loss report, then summarize the information
received from all the RTCP Feedback Reports generated by the
Dedicated receivers together with the information generated by the
loss reporter integrated in the feedback target and then create the
summary report to include all these information. In order to reduce
the processing load at the distribution source, the individual
instance of Loss Reporter may provide preliminary summarization
report.
During the summary report creating, the Distribution Source MUST use
its own SSRC value for transmitting summarization information and
MUST perform proper SSRC collision detection and resolution.
In some case, the distribution source may receive RTCP NACK messages
from the receivers behind the Distribution Source before the
distribution source detects the packet loss which may cause potential
Feedback implosion. In such case, the distribution source may filter
them out if it already sent a packet loss request for the missing
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packet to the media sender. When the distribution source confirms
packet loss reported by the receiver, the distribution source
generates the summary report to include the packet loss information
from the corresponding receiver (e.g., upstream client or loss
reporter).
The distribution source may send this new RTCP summary report
described in the section 6 to the group on the multicast RTCP channel
and in the meanwhile sending a packet loss request to the media
sender.
If the loss reporter is part of group, the Distribution source MUST
not send the summary report back to this loss reporter.
If there are a couple of loss reporters looking at the same RTP
stream, then the loss may be identified by all and they will all send
requests for the same packet loss. In this case, the distribution
source MUST filter out the duplicated information from various loss
reporters and only append one copy of such information to the summary
report.
When the host receives the RTCP packet, if the host understands this
message it will not send packet loss request (e.g., NACK) for the
missing packets reported in the message and will accept a
retransmission stream transmitted from the Distribution Source. If
it did not understand this new message, the host may send packet loss
request(e.g., NACK messages) to the specified media sender. When the
distribution source receives the packet loss request from the hosts
after it has already detected packet loss or got packet loss report
from loss reporter, the distribution source MUST filter it out until
the Retransmission stream is ready in the Distribution Source.
6.2. RTP transport translator use case
A Transport Translator (Topo-Trn-Translator), as defined in [RFC5117]
is typically forwarding the RTP and RTCP traffic between RTP clients,
for example converting between multicast and unicast for domains that
do not support multicast. The translator can identify packet loss
from the upstream and send the Feedback Suppression message to the
unicast receivers. The translator can also serve as a loss reporter
on the multicast side as described in the SSM case.
6.3. Multipoint Control Unit (MCU) use case
In point to multipoint topologies using video switching MCU (Topo-
Video-switch-MCU) [RFC5117], the MCU typically forwards a single
media stream to each participant, selected from the available input
streams. The selection of the input stream is often based on voice
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activity in the audio-visual conference, but other conference
management mechanisms (like presentation mode or explicit floor
control) exist as well.
In this case the MCU may detect packet loss from the sender or may
decide to switch to a new source. In both cases the receiver may
lose synchronization with the video stream and may send a FIR
request. If the MCU itself can detect the mis-synchronization of the
video, the MCU can send the FIR suppression message to the receivers
and send a FIR request to the video source.
7. Security Considerations
The defined messages have certain properties that have security
implications. These must be addressed and taken into account by
users of this protocol.
Spoofed or maliciously created feedback messages of the type defined
in this specification can have the following implications:
Sending NACK Implosion Summary Suppression Indication with wrong
sequence number of lost packet that makes missing RTP packets can not
be compensated by retransmission mechanism.
Sending FIR Implosion Summary Suppression Indication with wrong
sequence number of lost packet that makes missing RTP packets can not
be compensated by update request mechanism.
To prevent these attacks, there is a need to apply authentication and
integrity protection of the feedback messages. This can be
accomplished against threats external to the current RTP session
using the RTP profile that combines Secure RTP [RFC3711] and AVPF
into SAVPF [RFC5124].
8. IANA Consideration
New feedback type and New parameters for RTCP FSS receiver feedback
report are subject to IANA registration. For general guidelines on
IANA considerations for RTCP feedback, refer to [RFC4585].
This document assigns one new feedback type value x in the RTCP
receiver feedback report registry to "Feedback Storm Suppression"
with the following registrations format:
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Name: FSS
Long Name: Feedback Storm Suppression
Value: TBD
Reference: This document.
This document also assigns the parameter value y in the RTCP FSS
receiver feedback report Registry to "NACK Suppression Early
Indication ", with the following registrations format:
Name: NSEI
Long name: NACK Suppression Early Indication
Value: TBD
Reference: this document.
This document also assigns the parameter value z in the RTCP FSS
receiver feedback report Registry to "FIR Suppression Early
Indication ", with the following registrations format:
Name: FSEI
Long name: FIR Suppression Early Indication
Value: TBD
Reference: this document.
The contact information for the registrations is:
Qin Wu
sunseawq@huawei.com
Site B, Floor 12F,Huihong Mansion, No.91,Baixia Rd.
Nanjing, JiangSu 210001 China
9. Acknowledgement
The authors would like to thank David R Oran, Bill Ver Steeg, Ingemar
Johansson S, Colin Perkins, Tom VAN CAENEGEM, WeeSan Lee for their
valuable comments and suggestions on this document.
10. References
10.1. Normative References
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
10.2. Informative References
[DVB-IPTV]
ETSI Standard, "Digital Video Broadcasting(DVB); Transport
of MPEG-2 TS Based DVB Services over IP Based Networks",
ETSI TS 102 034, V1.4.1 , August 2009.
[I-D.hunt-avt-monarch-01]
Hunt, G. and P. Arden, "Monitoring Architectures for RTP",
August 2008.
[I-D.ietf-pmol-metrics-framework-02]
Clark, A., "Framework for Performance Metric Development".
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Appendix A. Example scenarios for Retransmission Storm Suppression
Feedback Storm Suppression can be further extended when a distributed
content distribution network (CDN) are considered. In these cases,
several intermediate node and media senders may constitute
hierarchical model. In the distributed content distribution
environment, the Feedback Storm Suppression is used to suppress the
neighboring node from sending packet loss request for the missing
packets via unicast. How the neighboring node is discovered is
beyond scope of this document.
A.1. Scenario 1: One or more media sender,One distribution source
The general architecture for scenario 1 is displayed below in
Figure 4. In this architecture, one or more Media Senders send RTP
packets to the RTP Receivers through the same Distribution Source.
The Distribution Source relays the RTP packets to the receivers using
a source-specific multicast channel. In the reverse direction, the
receivers transmit RTCP packets via unicast to the distribution
source. The Distribution Source in turn relays RTCP packets to the
media sender and then transmits the RTCP packets back to the
receivers, using source-specific multicast. When packet loss happens
in the upstream link or downstream aggregate link of distribution
source, it may result in massive retransmission request for the same
RTP packets from all the receivers using RTCP NACK to the same
multicast sender. We refer to it as Retransmission Storm.
+-------+
|---->|RTP_Rx1|
+--------+ | +-------+
| | +--------------+ |
| | | | | +-------+
| Media |-------| Distribution |-------|---->|RTP_Rx2|
| | | Source | | +-------+
| Sender | | | | .
| | +--------------+ | .
| | | .
+--------+ | +-------+
|---->|RTP_Rxn|
+-------+
Figure 4: One media Sender, one Distribution Source
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A.2. Scenario 2:One media sender, Two distribution sources in cascade
+-------+
|---->|RTP_Rx1|
| +-------+
+------+ |
| | +------------+ +------------+ | +-------+
|Media |-+Distribution+--|Distribution+--|---->|RTP_Rx2|
|Sender| | Source1 | | Source2 | | +-------+
| | +------------+ +------------+ | .
+------+ | .
| .
| +-------+
|---->|RTP_Rxn|
+-------+
Figure 5: One media sender, Two distribution sources in cascade
The general architecture for scenario 2 is displayed below in
Figure 5. In this architecture, One media sender passes through two
distribution source in cascading and sends RTP packets to all the RTP
receivers. When packet loss happens in the upstream link or
downstream aggregate link of distribution source1, it may result in
massive retransmission request for the same RTP packets from all the
receivers using RTCP NACK to the same multicast sender. We refer to
it as Retransmission Storm. In this case, the distribution source 2
can be taken as one special RTP receiver located in the downstream
direction of distribution source 1.
A.3. Scenario 3:One media sender, Two distribution sources in parallel
The general architecture for scenario 3 is displayed below in
Figure 6. In this architecture, one media sender and two
Distribution source constitute one hierarchical tree model. In this
model, one Media Senders send RTP packets to all the RTP receivers
through two different path respectively. When packet loss happens in
the upstream link or downstream aggregate link of distribution
source, it may result in massive retransmission request for the same
RTP packets from all the receivers using RTCP NACK to the same
multicast sender. We refer to it as Retransmission Storm.
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+--------+
|---->|RTP_Rx11|
| +--------+
+--------------+ |
| | | +--------+
|--->| Distribution |----|---->|RTP_Rx12|
| | Source1 | | +--------+
| | | | .
+--------+ | +--------------+ | .
| | | | .
| | | | +--------+
| Media | | |---->|RTP_Rx1k|
| |---| +--------+
| Sender | | +--------+
| | | |---->|RTP_Rx21|
| | | | +--------+
+--------+ | +--------------+ |
| | | | +--------+
| | Distribution |----|---->|RTP_Rx22|
|--->| Source2 | | +--------+
| | | .
+--------------+ | .
| .
| +--------+
|---->|RTP_Rx2j|
+--------+
Figure 6: One Media Sender, more distribution sources
Appendix B. Applicability of Feedback Storm Suppression
This document defines new RTCP Receiver feedback Report, which we
refer to as Feedback Storm Suppression to deal with Retransmission
Storm mentioned above. Here we give two examples to show how this
new RTCP receiver feedback report is applied into three scenarios
described in Appendix A for Retransmission Storm Suppression.
Applicability of Retransmission Storm Suppression in Scenario 1
described in Figure 4 is shown in the Figure 7. In this figure, the
distribution source detect the packet loss before the receiver
perceive it and ask for retransmission of the missing packets from
the media sender, in the meanwhile, the distribution source transmits
the RTCP Retransmission Storm Suppression Indication back to the
receivers using source-specific multicast channel. In this way, the
delay for the receiver to recover from the packet loss can be reduced
and the risk of increasing network congestion can be mitigated.
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+------+ +--------------+ +--------+
|Media | | Distribution | | |
|Sender| | Source | | RTP_Rx |
+--+---+ +------+-------+ +---+----+
| | |
| | |
|------------------->|------RTP Multicast---->|
| | |
| | |
| +--------+----------+ |
| |Detect Packet Loss | |
| |and Identify the SN| |
| |of missing Packets | |
| +--------+----------+ |
|<-----RTCP NACK-----| |
| | |
| +--Multicast RTCP FSS--->|
| RTP Retransmission | |
|------------------->| |
| |------RTP Multicast---->|
| | Retransmission |
| | |
| | |
| | |
Figure 7: Applicability of Feedback Suppression Early Indication
Applicability of Feedback Storm Suppression in Scenario 2 or 3
described in Figure 5 and Figure 6 is shown in the Figure 8. The
procedure in the Figure 8 is similar to the one in the figure
Figure 7. The only difference is distribution source should not only
notify all the receiver behind itself not to send NACK but also
propagate the retransmission suppression indication to the
neighboring distribution sources. In this way, all the receivers
behind all the neighboring distribution source can avoid sending
massive retransmission request to the media sender.
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+------+ +-------+ +--------+ +-------+ +--------+
|Media | | | | RTP_Rx | | | | RTP_Rx |
|Sender| | DS1 | | (DS1) | | DS2 | | (DS2) |
+--+---+ +---+---+ +---+----+ +---+---+ +---+----+
| | | | |
| |RTP Multicast | | |
|----------->|------------->| | |
| | | | |
| | | |RTP Multicast|
|------------------------------------------->|------------>|
| | | | |
| +--------+------------+ | | |
| |Detect Packet Loss | | | |
| |and Identify the SN | | | |
| |of the missing Packets | | |
| +--------+------------+ | | |
| | | | |
|<-RTCP NACK-| Multicast RTCP RSSI | |
| |------------->| | |
| | | | |
| |-----Unicast RTCP RSSI-------->|Multicast RTCP FSS
| | | |------------>|
|RTP Retransmission | | |
|----------->| | | |
| | | | |
| | RTP Retransmission | |
|------------+--------------+--------------->| |
| | | | |
| | RTP Multicast| | RTP Multicast
| |Retransmission| |Retransmission
| |------------->| |------------>|
| | | | |
DS1: Distribution Source 1
DS2: Distribution Source 2
Figure 8: Applicability of Retransmission Suppression Early
Indication
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Authors' Addresses
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: sunseawq@huawei.com
Frank Xia
Huawei
1700 Alma Dr. Suite 500
Plano, TX 75075
USA
Phone: +1 972-509-5599
Email: xiayangsong@huawei.com
Roni Even
Huawei
14 David Hamelech
Tel Aviv 64953
Israel
Email: even.roni@huawei.com
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