AVT B. VerSteeg
Internet-Draft A. Begen
Intended status: Standards Track Cisco
Expires: May 22, 2011 T. VanCaenegem
Alcatel-Lucent
Z. Vax
Microsoft Corporation
November 18, 2010
Unicast-Based Rapid Acquisition of Multicast RTP Sessions
draft-ietf-avt-rapid-acquisition-for-rtp-17
Abstract
When an RTP receiver joins a multicast session, it may need to
acquire and parse certain Reference Information before it can process
any data sent in the multicast session. Depending on the join time,
length of the Reference Information repetition (or appearance)
interval, size of the Reference Information as well as the
application and transport properties, the time lag before an RTP
receiver can usefully consume the multicast data, which we refer to
as the Acquisition Delay, varies and can be large. This is an
undesirable phenomenon for receivers that frequently switch among
different multicast sessions, such as video broadcasts.
In this document, we describe a method using the existing RTP and RTP
Control Protocol (RTCP) machinery that reduces the acquisition delay.
In this method, an auxiliary unicast RTP session carrying the
Reference Information to the receiver precedes/accompanies the
multicast stream. This unicast RTP flow can be transmitted at a
faster than natural bitrate to further accelerate the acquisition.
The motivating use case for this capability is multicast applications
that carry real-time compressed audio and video. However, this
method can also be used in other types of multicast applications
where the acquisition delay is long enough to be a problem.
Status of this Memo
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Acquisition of RTP Streams vs. RTP Sessions . . . . . . . 7
1.2. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 7
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Elements of Delay in Multicast Applications . . . . . . . . . 9
5. Protocol Design Considerations and Their Effect on
Resource Management for Rapid Acquisition . . . . . . . . . . 10
6. Rapid Acquisition of Multicast RTP Sessions (RAMS) . . . . . . 13
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Message Flows . . . . . . . . . . . . . . . . . . . . . . 13
6.3. Synchronization of Primary Multicast Streams . . . . . . . 24
6.4. Burst Shaping and Congestion Control in RAMS . . . . . . . 24
6.5. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 27
7. Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 28
7.1. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 29
7.1.1. Vendor-Neutral Extensions . . . . . . . . . . . . . . 30
7.1.2. Private Extensions . . . . . . . . . . . . . . . . . . 30
7.2. RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 31
7.3. RAMS Information . . . . . . . . . . . . . . . . . . . . . 34
7.3.1. Response Code Definitions . . . . . . . . . . . . . . 37
7.4. RAMS Termination . . . . . . . . . . . . . . . . . . . . . 38
8. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 40
8.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 40
8.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 40
8.3. Example and Discussion . . . . . . . . . . . . . . . . . . 41
9. NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 44
10. Security Considerations . . . . . . . . . . . . . . . . . . . 45
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
11.1. Registration of SDP Attributes . . . . . . . . . . . . . . 48
11.2. Registration of SDP Attribute Values . . . . . . . . . . . 48
11.3. Registration of FMT Values . . . . . . . . . . . . . . . . 48
11.4. SFMT Values for RAMS Messages Registry . . . . . . . . . . 49
11.5. RAMS TLV Space Registry . . . . . . . . . . . . . . . . . 49
11.6. RAMS Response Code Space Registry . . . . . . . . . . . . 50
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 53
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
14.1. Normative References . . . . . . . . . . . . . . . . . . . 53
14.2. Informative References . . . . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56
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1. Introduction
Most multicast flows carry a stream of inter-related data. Receivers
need to acquire certain information to start processing any data sent
in the multicast session. This document refers to this information
as Reference Information. The Reference Information is
conventionally sent periodically in the multicast session (although
its content can change over time) and usually consists of items such
as a description of the schema for the rest of the data, references
to which data to process, encryption information including keys, as
well as any other information required to process the data in the
multicast stream [IC2009].
Real-time multicast applications require receivers to buffer data.
Receivers may have to buffer data to smooth out the network jitter,
to allow loss-repair methods such as Forward Error Correction and
retransmission to recover the missing packets, and to satisfy the
data processing requirements of the application layer.
When a receiver joins a multicast session, it has no control over
what point in the flow is currently being transmitted. Sometimes the
receiver might join the session right before the Reference
Information is sent in the session. In this case, the required
waiting time is usually minimal. Other times, the receiver might
join the session right after the Reference Information has been
transmitted. In this case, the receiver has to wait for the
Reference Information to appear again in the flow before it can start
processing any multicast data. In some other cases, the Reference
Information is not contiguous in the flow but dispersed over a large
period, which forces the receiver to wait for the whole Reference
Information to arrive before starting to process the rest of the
data.
The net effect of waiting for the Reference Information and waiting
for various buffers to fill up is that receivers can experience
significantly large delays in data processing. In this document, we
refer to the difference between the time an RTP receiver wants to
join the multicast session and the time the RTP receiver acquires all
the necessary Reference Information as the Acquisition Delay. The
acquisition delay might not be the same for different receivers; it
usually varies depending on the join time, length of the Reference
Information repetition (or appearance) interval, size of the
Reference Information as well as the application and transport
properties.
The varying nature of the acquisition delay adversely affects the
receivers that frequently switch among multicast sessions. While
this problem equally applies to both any-source (ASM) and source-
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specific (SSM) multicast applications, in this specification we
address it for the SSM-based multicast applications by describing a
method that uses the fundamental tools offered by the existing RTP
and RTCP protocols [RFC3550]. In this method, either the multicast
source (or the distribution source in an SSM session) retains the
Reference Information for a period after its transmission, or an
intermediary network element (that we refer to as Retransmission
Server) joins the multicast session and continuously caches the
Reference Information as it is sent in the session and acts as a
feedback target (See [RFC5760]) for the session. When an RTP
receiver wishes to join the same multicast session, instead of simply
issuing a Source Filtering Group Management Protocol (SFGMP) Join
message, it sends a request to the feedback target for the session
and asks for the Reference Information. The retransmission server
starts a new unicast RTP (retransmission) session and sends the
Reference Information to the RTP receiver over that session. If
there is residual bandwidth, the retransmission server might burst
the Reference Information faster than its natural rate. As soon as
the receiver acquires the Reference Information, it can join the
multicast session and start processing the multicast data. A
simplified network diagram showing this method through an
intermediary network element is depicted in Figure 1.
This method potentially reduces the acquisition delay. We refer to
this method as Unicast-based Rapid Acquisition of Multicast RTP
Sessions. A primary use case for this method is to reduce the
channel-change times in IPTV networks where compressed video streams
are multicast in different SSM sessions and viewers randomly join
these sessions.
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-----------------------
+--->| Intermediary |
| | Network Element |
| ...|(Retransmission Server)|
| : -----------------------
| :
| v
----------- ---------- ----------
| Multicast | | |---------->| Joining |
| Source |------->| Router |..........>| RTP |
| | | | | Receiver |
----------- ---------- ----------
|
| ----------
+---------------->| Existing |
| RTP |
| Receiver |
----------
-------> Multicast RTP Flow
.......> Unicast RTP Flow
Figure 1: Rapid acquisition through an intermediary network element
A principle design goal in this solution is to use the existing tools
in the RTP/RTCP protocol family. This improves the versatility of
the existing implementations, and promotes faster deployment and
better interoperability. To this effect, we use the unicast
retransmission support of RTP [RFC4588] and the capabilities of RTCP
to handle the signaling needed to accomplish the acquisition.
A reasonable effort has been made in this specification to design a
solution that reliably works in both engineered and best-effort
networks. However, a proper congestion control combined with the
desired behavior of this solution is difficult to achieve. Rather,
this solution has been designed based on an assumption that the
retransmission server and the RTP receivers have some knowledge about
where the bottleneck between them is. This assumption does not
generally hold unless both the retransmission server and the RTP
receivers are in the same edge network. Thus, this solution should
not be used across any best-effort path of the Internet.
Furthermore, this solution should only be used in networks that are
already carrying non-congestion-responsive multicast traffic and have
throttling mechanisms in the retransmission servers to ensure the
(unicast) burst traffic is a known constant upper-bound multiplier on
the multicast load.
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1.1. Acquisition of RTP Streams vs. RTP Sessions
In this memo we describe a protocol that handles the rapid
acquisition of a single multicast RTP session (called primary
multicast RTP session) carrying one or more RTP streams (called
primary multicast streams). If desired, multiple instances of this
protocol may be run in parallel to acquire multiple RTP sessions
simultaneously.
When an RTP receiver requests the Reference Information from the
retransmission server, it can opt to rapidly acquire a specific
subset of the available RTP streams in the primary multicast RTP
session. Alternatively, the RTP receiver can request the rapid
acquisition of all of the RTP streams in that RTP session.
Regardless of how many RTP streams are requested by the RTP receiver
or how many will be actually sent by the retransmission server, only
one unicast RTP session will be established by the retransmission
server. This unicast RTP session is separate from the associated
primary multicast RTP session. As a result, there are always two
different RTP sessions in a single instance of the rapid acquisition
protocol: (i) the primary multicast RTP session with its associated
unicast feedback and (ii) the unicast RTP session.
If the RTP receiver wants to rapidly acquire multiple RTP sessions
simultaneously, separate unicast RTP sessions will be established for
each of them.
1.2. Outline
In the rest of this specification, we have the following outline: In
Section 4, we describe the delay components in generic multicast
applications. Section 5 presents an overview of the protocol design
considerations for rapid acquisition. We provide the protocol
details of the rapid acquisition method in Section 6 and Section 7.
Section 8 and Section 9 discuss the SDP signaling issues with
examples and NAT-related issues, respectively. Finally, Section 10
discusses the security considerations.
Section 3 provides a list of the definitions frequently used in this
document.
2. Requirements Notation
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 [RFC2119].
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3. Definitions
This document uses the following acronyms and definitions frequently:
(Primary) SSM (or Multicast) Session: The multicast session to which
RTP receivers can join at a random point in time. A primary SSM
session can carry multiple RTP streams.
Primary Multicast RTP Session: The multicast RTP session an RTP
receiver is interested in acquiring rapidly. From the RTP receiver's
viewpoint, the primary multicast RTP session has one associated
unicast RTCP feedback stream to a Feedback Target, in addition to the
primary multicast RTP stream(s).
Primary Multicast (RTP) Streams: The RTP stream(s) carried in the
primary multicast RTP session.
Source Filtering Group Management Protocol (SFGMP): Following the
definition in [RFC4604], SFGMP refers to the Internet Group
Management Protocol (IGMP) version 3 [RFC3376] and the Multicast
Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and
IPv6 networks, respectively. However, the rapid acquisition method
introduced in this document does not depend on a specific version of
either of these group management protocols. In the remainder of this
document, SFGMP will refer to any group management protocol that has
Join and Leave functionalities.
Feedback Target (FT): Unicast RTCP feedback target as defined in
[RFC5760]. FT_Ap denotes a specific feedback target running on a
particular address and port.
Retransmission (or Burst) Packet: An RTP packet that is formatted as
defined in Section 4 of [RFC4588]. The payload of a retransmission
or burst packet comprises the retransmission payload header followed
by the payload of the original RTP packet.
Reference Information: The set of certain media content and metadata
information that is sufficient for an RTP receiver to start usefully
consuming a media stream. The meaning, format and size of this
information are specific to the application and are out of scope of
this document.
Preamble Information: A more compact form of the whole or a subset
of the Reference Information transmitted out-of-band.
(Unicast) Burst (or Retransmission) RTP Session: The unicast RTP
session used to send one or more unicast burst RTP streams and their
associated RTCP messages. The terms "burst RTP session" and
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"retransmission RTP session" can be used interchangeably.
(Unicast) Burst (Stream): A unicast stream of RTP retransmission
packets that enable an RTP receiver to rapidly acquire the Reference
Information associated with a primary multicast stream. Each burst
stream is identified by its Synchronization Source (SSRC) identifier
that is unique in the primary multicast RTP session. Following the
session-multiplexing guidelines in [RFC4588], each unicast burst
stream will use the same SSRC and Canonical Name (CNAME) as its
primary multicast RTP stream.
Retransmission Server (RS): The RTP/RTCP endpoint that can generate
the retransmission packets and the burst streams. The RS may also
generate other non-retransmission packets to aid rapid acquisition.
4. Elements of Delay in Multicast Applications
In a source-specific (SSM) multicast delivery system, there are three
major elements that contribute to the acquisition delay when an RTP
receiver switches from one multicast session to another one. These
are:
o Multicast switching delay
o Reference Information latency
o Buffering delays
Multicast switching delay is the delay that is experienced to leave
the current multicast session (if any) and join the new multicast
session. In typical systems, the multicast join and leave operations
are handled by a group management protocol. For example, the
receivers and routers participating in a multicast session can use
the Internet Group Management Protocol (IGMP) version 3 [RFC3376] or
the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810].
In either of these protocols, when a receiver wants to join a
multicast session, it sends a message to its upstream router and the
routing infrastructure sets up the multicast forwarding state to
deliver the packets of the multicast session to the new receiver.
Depending on the proximity of the upstream router, the current state
of the multicast tree, the load on the system and the protocol
implementation, the join times vary. Current systems provide join
latencies usually less than 200 milliseconds (ms). If the receiver
had been participating in another multicast session before joining
the new session, it needs to send a Leave message to its upstream
router to leave the session. In common multicast routing protocols,
the leave times are usually smaller than the join times, however, it
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is possible that the Leave and Join messages might get lost, in which
case the multicast switching delay inevitably increases.
Reference Information latency is the time it takes the receiver to
acquire the Reference Information. It is highly dependent on the
proximity of the actual time the receiver joined the session to the
next time the Reference Information will be sent to the receivers in
the session, whether the Reference Information is sent contiguously
or not, and the size of the Reference Information. For some
multicast flows, there is a little or no interdependency in the data,
in which case the Reference Information latency will be nil or
negligible. For other multicast flows, there is a high degree of
interdependency. One example of interest is the multicast flows that
carry compressed audio/video. For these flows, the Reference
Information latency can become quite large and be a major contributor
to the overall delay.
The buffering component of the overall acquisition delay is driven by
the way the application layer processes the payload. In many
multicast applications, an unreliable transport protocol such as UDP
[RFC0768] is often used to transmit the data packets, and the
reliability, if needed, is usually addressed through other means such
as Forward Error Correction (e.g., [RFC6015]) and retransmission.
These loss-repair methods require buffering at the receiver side to
function properly. In many applications, it is also often necessary
to de-jitter the incoming data packets before feeding them to the
application. The de-jittering process also increases the buffering
delays. Besides these network-related buffering delays, there are
also specific buffering needs that are required by the individual
applications. For example, standard video decoders typically require
an amount, sometimes up to a few seconds, of coded video data to be
available in the pre-decoding buffers prior to starting to decode the
video bitstream.
5. Protocol Design Considerations and Their Effect on Resource
Management for Rapid Acquisition
This section is for informational purposes and does not contain
requirements for implementations.
Rapid acquisition is an optimization of a system that is expected to
continue to work correctly and properly whether or not the
optimization is effective, or even fails due to lost control and
feedback messages, congestion, or other problems. This is
fundamental to the overall design requirements surrounding the
protocol definition and to the resource management schemes to be
employed together with the protocol (e.g., QoS machinery, server load
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management, etc). In particular, the system needs to operate within
a number of constraints:
o First, a rapid acquisition operation must fail gracefully. The
user experience must be not significantly worse for trying and
failing to complete rapid acquisition compared to simply joining
the multicast session.
o Second, providing the rapid acquisition optimizations must not
cause collateral damage to either the multicast session being
joined, or other multicast sessions sharing resources with the
rapid acquisition operation. In particular, the rapid acquisition
operation must avoid interference with the multicast session that
might be simultaneously being received by other hosts. In
addition, it must also avoid interference with other multicast and
non-multicast sessions sharing the same network resources. These
properties are possible, but are usually difficult to achieve.
One challenge is the existence of multiple bandwidth bottlenecks
between the receiver and the server(s) in the network providing the
rapid acquisition service. In commercial IPTV deployments, for
example, bottlenecks are often present in the aggregation network
connecting the IPTV servers to the network edge, the access links
(e.g., DSL, DOCSIS) and in the home network of the subscribers. Some
of these links might serve only a single subscriber, limiting
congestion impact to the traffic of only that subscriber, but others
can be shared links carrying multicast sessions of many subscribers.
Also note that the state of these links can vary over time. The
receiver might have knowledge of a portion of this network, or might
have partial knowledge of the entire network. The methods employed
by the devices to acquire this network state information is out of
scope for this document. The receiver should be able to signal the
server with the bandwidth that it believes it can handle. The server
also needs to be able to rate limit the flow in order to stay within
the performance envelope that it knows about. Both the server and
receiver need to be able to inform the other of changes in the
requested and delivered rates. However, the protocol must be robust
in the presence of packet loss, so this signaling must include the
appropriate default behaviors.
A second challenge is that for some uses (e.g., high-bitrate video)
the unicast burst bitrate is high while the flow duration of the
unicast burst is short. This is because the purpose of the unicast
burst is to allow the RTP receiver to join the multicast quickly and
thereby limit the overall resources consumed by the burst. Such
high-bitrate, short-duration flows are not amenable to conventional
admission control techniques. For example, end-to-end per-flow
signaled admission control techniques such as RSVP have too much
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latency and control channel overhead to be a good fit for rapid
acquisition. Similarly, using a TCP (or TCP-like) approach with a
3-way handshake and slow-start to avoid inducing congestion would
defeat the purpose of attempting rapid acquisition in the first place
by introducing many round-trip times (RTT) of delay.
These observations lead to certain unavoidable requirements and goals
for a rapid acquisition protocol. These are:
o The protocol must be designed to allow a deterministic upper bound
on the extra bandwidth used (compared to just joining the
multicast session). A reasonable size bound is e*B, where B is
the nominal bandwidth of the primary multicast streams, and e is
an excess-bandwidth coefficient. The total duration of the
unicast burst must have a reasonable bound; long unicast bursts
devolve to the bandwidth profile of multi-unicast for the whole
system.
o The scheme should minimize (or better eliminate) the overlap of
the unicast burst and the primary multicast stream. This
minimizes the window during which congestion could be induced on a
bottleneck link compared to just carrying the multicast or unicast
packets alone.
o The scheme must minimize (or better eliminate) any gap between the
unicast burst and the primary multicast stream, which has to be
repaired later, or in the absence of repair, will result in loss
being experienced by the application.
In addition to the above, there are some other protocol design issues
to be considered. First, there is at least one RTT of "slop" in the
control loop. In starting a rapid acquisition burst, this manifests
as the time between the client requesting the unicast burst and the
burst description and/or the first unicast burst packets arriving at
the receiver. For managing and terminating the unicast burst, there
are two possible approaches for the control loop: The receiver can
adapt to the unicast burst as received, converge based on observation
and explicitly terminate the unicast burst with a second control loop
exchange (which takes a minimum of one RTT, just as starting the
unicast burst does). Alternatively, the server generating the
unicast burst can pre-compute the burst parameters based on the
information in the initial request and tell the receiver the burst
duration.
The protocol described in the next section allows either method of
controlling the rapid acquisition unicast burst.
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6. Rapid Acquisition of Multicast RTP Sessions (RAMS)
We start this section with an overview of the rapid acquisition of
multicast sessions (RAMS) method.
6.1. Overview
[RFC5760] specifies an extension to the RTP Control Protocol (RTCP)
to use unicast feedback in an SSM session. It defines an
architecture that introduces the concept of Distribution Source,
which - in an SSM context - distributes the RTP data and
redistributes RTCP information to all RTP receivers. This RTCP
information is retrieved from the Feedback Target, to which RTCP
unicast feedback traffic is sent. One or more entities different
from the Distribution Source MAY implement the feedback target
functionality, and different RTP receivers MAY use different feedback
targets.
This document builds further on these concepts to reduce the
acquisition delay when an RTP receiver joins a multicast session at a
random point in time by introducing the concept of the Burst Source
and new RTCP feedback messages. The Burst Source has a cache where
the most recent packets from the primary multicast RTP session are
continuously stored. When an RTP receiver wants to receive a primary
multicast stream, it can first request a unicast burst from the Burst
Source before it joins the SSM session. In this burst, the packets
are formatted as RTP retransmission packets [RFC4588] and carry
Reference Information. This information allows the RTP receiver to
start usefully consuming the RTP packets sent in the primary
multicast RTP session.
Using an accelerated bitrate (as compared to the nominal bitrate of
the primary multicast stream) for the unicast burst implies that at a
certain point in time, the payload transmitted in the unicast burst
is going to be the same as the payload in the associated multicast
stream, i.e., the unicast burst will catch up with the primary
multicast stream. At this point, the RTP receiver no longer needs to
receive the unicast burst and can join the SSM session. This method
is referred to as the Rapid Acquisition of Multicast Sessions (RAMS).
This document defines extensions to [RFC4585] for an RTP receiver to
request a unicast burst as well as for additional control messaging
that can be leveraged during the acquisition process.
6.2. Message Flows
Figure 2 shows the main entities involved in rapid acquisition and
the message flows. They are
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o Multicast Source
o Feedback Target (FT)
o Burst/Retransmission Source (BRS)
o RTP Receiver (RTP_Rx)
----------- --------------
| |------------------------------------>| |
| |.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->| |
| | | |
| Multicast | ---------------- | |
| Source | | Retransmission | | |
| |-------->| Server (RS) | | |
| |.-.-.-.->| | | |
| | | ------------ | | |
----------- | | Feedback | |<.=.=.=.=.| |
| | Target (FT)| |<~~~~~~~~~| RTP Receiver |
PRIMARY MULTICAST | ------------ | | (RTP_Rx) |
RTP SESSION with | | | |
UNICAST FEEDBACK | | | |
| | | |
- - - - - - - - - - - |- - - - - - - - |- - - - - |- - - - - - - |- -
| | | |
UNICAST BURST | ------------ | | |
(or RETRANSMISSION) | | Burst and | |<~~~~~~~~>| |
RTP SESSION | | Retrans. | |.........>| |
| |Source (BRS)| |<.=.=.=.=>| |
| ------------ | | |
| | | |
---------------- --------------
-------> Multicast RTP Flow
.-.-.-.> Multicast RTCP Flow
.=.=.=.> Unicast RTCP Reports
~~~~~~~> Unicast RTCP Feedback Messages
.......> Unicast RTP Flow
Figure 2: Flow diagram for unicast-based rapid acquisition
The feedback target (FT) is the entity as defined in [RFC5760], to
which the RTP_Rx sends its RTCP feedback messages indicating packet
loss by means of an RTCP NACK message or indicating RTP_Rx's desire
to rapidly acquire the primary multicast RTP session by means of an
RTCP feedback message defined in this document. While the Burst/
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Retransmission Source (BRS) is responsible for responding to these
messages and for further RTCP interaction with the RTP_Rx in the case
of a rapid acquisition process, it is assumed in the remainder of the
document that these two logical entities (FT and BRS) are combined in
a single physical entity and they share state. In the remainder of
the text, the term Retransmission Server (RS) is used whenever
appropriate, to refer to this single physical entity.
The FT is involved in the primary multicast RTP session and receives
unicast feedback for that session as in [RFC5760]. The BRS is
involved in the unicast burst (or retransmission) RTP session and
transmits the unicast burst and retransmission packets formatted as
RTP retransmission packets [RFC4588] in a single separate unicast RTP
session to each RTP_Rx. In the unicast burst RTP session, as in any
other RTP session, the BRS and RTP_Rx regularly send the periodic
sender and receiver reports, respectively.
The unicast burst is started by an RTCP feedback message that is
defined in this document based on the common packet format provided
in [RFC4585]. An RTP retransmission is triggered by an RTCP NACK
message defined in [RFC4585]. Both of these messages are sent to the
FT of the primary multicast RTP session, and can start the unicast
burst/retransmission RTP session.
In the extended RTP profile for RTCP-based feedback (RTP/AVPF), there
are certain rules that apply to scheduling of both of these messages
sent to the FT in the primary multicast RTP session, and these are
detailed in Section 3.5 of [RFC4585]. One of the rules states that
in a multi-party session such as the SSM sessions we are considering
in this specification, an RTP_Rx cannot send an RTCP feedback message
for a minimum of one second period after joining the session (i.e.,
Tmin=1.0 second). While this rule has the goal of avoiding problems
associated with flash crowds in typical multi-party sessions, it
defeats the purpose of rapid acquisition. Furthermore, when RTP
receivers delay their messages requesting a burst by a deterministic
or even a random amount, it still does not make a difference since
such messages are not related and their timings are independent from
each other. Thus, in this specification we initialize Tmin to zero
and allow the RTP receivers to send a burst request message right at
the beginning. For the subsequent messages (e.g., updated requests)
during rapid acquisition, the timing rules of [RFC4585] still apply.
Figure 3 depicts an example of messaging flow for rapid acquisition.
The RTCP feedback messages are explained below. The optional
messages are indicated in parentheses and they might or might not be
present during rapid acquisition. In a scenario where rapid
acquisition is performed by a feedback target co-located with the
media sender, the same method (with the identical message flows)
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still applies.
-------------------------
| Retransmission Server |
----------- | ------ ------------ | -------- ------------
| Multicast | | | FT | | Burst/Ret. | | | | | RTP |
| Source | | | | | Source | | | Router | | Receiver |
| | | ------ ------------ | | | | (RTP_Rx) |
----------- | | | | -------- ------------
| ------------------------- | |
| | | | |
|-- RTP Multicast ---------->--------------->| |
|-. RTCP Multicast -.-.-.-.->-.-.-.-.-.-.-.->| |
| | | | |
| | |********************************|
| | |* PORT MAPPING SETUP *|
| | |********************************|
| | | | |
| |<~~~~~~~~~~~~~~~~~~~~~~~~~ RTCP RAMS-R ~~~|
| | | | |
| | |********************************|
| | |* UNICAST SESSION ESTABLISHED *|
| | |********************************|
| | | | |
| | |~~~ RTCP RAMS-I ~~~~~~~~~~~~~~~>|
| | | | |
| | |... Unicast RTP Burst .........>|
| | | | |
| |<~~~~~~~~~~~~~~~~~~~~~~~~ (RTCP RAMS-R) ~~|
| | | | |
| | |~~ (RTCP RAMS-I) ~~~~~~~~~~~~~~>|
| | | | |
| | | | |
| | | |<= SFGMP Join ==|
| | | | |
|-- RTP Multicast ------------------------------------------->|
|-. RTCP Multicast -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.>|
| | | | |
| | | | |
| | |<~~~~~~~~~~~~~~~ RTCP RAMS-T ~~~|
| | | | |
: : : : :
| | |<.=.= Unicast RTCP Reports .=.=>|
: : : (for the unicast session) :
| | | | |
-------> Multicast RTP Flow
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.-.-.-.> Multicast RTCP Flow
.=.=.=.> Unicast RTCP Reports
~~~~~~~> Unicast RTCP Feedback Messages
=======> SFGMP Messages
.......> Unicast RTP Flow
Figure 3: Message flows for unicast-based rapid acquisition
This document defines the expected behaviors of the RS and RTP_Rx.
It is instructive to consider independently operating implementations
on the RS and RTP_Rx that request the burst, describe the burst,
start the burst, join the multicast session and stop the burst.
These implementations send messages to each other, but provisions are
needed for the cases where the control messages get lost, or re-
ordered, or are not being delivered to their destinations.
The following steps describe rapid acquisition in detail:
1. Port Mapping Setup: For the primary multicast RTP session, the
RTP and RTCP destination ports are declaratively specified
(Refer to Section 8 for examples in SDP). However, the RTP_Rx
needs to choose its RTP and RTCP receive ports for the unicast
burst RTP session. Since this unicast session is established
after the RTP_Rx has sent a RAMS-Request (RAMS-R) message as
unicast feedback in the primary multicast RTP session, the
RTP_Rx MUST first setup the port mappings between the unicast
and multicast sessions and send this mapping information to the
FT along with the RAMS-R message so that the BRS knows how to
communicate with the RTP_Rx.
The details of this setup procedure are explained in
[I-D.ietf-avt-ports-for-ucast-mcast-rtp]. Other NAT-related
issues are left to Section 9 to keep the present discussion
focused on the RAMS message flows.
2. Request: the RTP_Rx sends a rapid acquisition request (RAMS-R)
for the primary multicast RTP session to the unicast feedback
target of that session. The request contains the SSRC
identifier of the RTP_Rx (aka SSRC of packet sender) and can
contain the media sender SSRC identifier(s) of the primary
multicast stream(s) of interest (aka SSRC of media source). The
RAMS-R message can contain parameters that constrain the burst,
such as the buffer and bandwidth limits.
Before joining the SSM session, the RTP_Rx learns the addresses
for the multicast source, group and RS by out-of-band means. If
the RTP_Rx desires to rapidly acquire only a subset of the
primary multicast streams available in the primary multicast RTP
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session, the RTP_Rx MUST also acquire the SSRC identifiers for
the desired RTP streams out-of-band. Based on this information,
the RTP_Rx populates the desired SSRC(s) in the RAMS-R message.
When the FT successfully receives the RAMS-R message, the BRS
responds to it by accepting or rejecting the request.
Immediately before the BRS sends any RTP or RTCP packet(s)
described below, it establishes the unicast burst RTP session.
3. Response: The BRS sends RAMS-Information (RAMS-I) message(s) to
the RTP_Rx to convey the status for the burst(s) requested by
the RTP_Rx.
If the primary multicast RTP session associated with the FT_Ap
(a specific feedback target running on a particular address and
port) on which the RAMS-R message was received contains only a
single primary multicast stream, the BRS SHALL always use the
SSRC of the RTP stream associated with the FT_Ap in the RAMS-I
message(s) regardless of the media sender SSRC requested in the
RAMS-R message. In such cases the 'ssrc' attribute MAY be
omitted from the media description. If the requested SSRC and
the actual media sender SSRC do not match, the BRS MUST
explicitly populate the correct media sender SSRC in the initial
RAMS-I message (See Section 7.3).
The FT_Ap could also be associated with an RTP session that
carries two or more primary multicast streams. If the RTP_Rx
wants to issue a collective request to receive the whole primary
multicast RTP session, it does not need the 'ssrc' attributes to
be described in the media description.
If the FT_Ap is associated with two or more RTP sessions,
RTP_Rx's request will be ambiguous. To avoid any ambiguity,
each FT_Ap MUST be only associated with a single RTP session.
If the RTP_Rx is willing to rapidly acquire only a subset of the
primary multicast streams, the RTP_Rx MUST list all the media
sender SSRC(s) denoting the stream(s) it wishes to acquire in
the RAMS-R message. Upon receiving such a message, the BRS MAY
accept the request for all or a subset of the media sender
SSRC(s) that matched the RTP stream(s) it serves. The BRS MUST
reject all other requests with an appropriate response code.
* Reject Responses: The BRS MUST take into account any
limitations that may have been specified by the RTP_Rx in the
RAMS-R message when making a decision regarding the request.
If the RTP_Rx has requested to acquire the whole primary
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multicast RTP session but the BRS cannot provide a rapid
acquisition service for any of the primary multicast streams,
the BRS MUST reject the request via a single RAMS-I message
with a collective reject response code, which is defined as
510 in Section 11.6, and whose media sender SSRC field is set
to one of SSRCs served by this FT_Ap. Upon receiving this
RAMS-I message, the RTP_Rx abandons the rapid acquisition
attempt and can immediately join the multicast session by
sending an SFGMP Join message towards its upstream multicast
router.
In all other cases, the BRS MUST send a separate RAMS-I
message with the appropriate 5xx-level response code (as
defined in Section 11.6) for each primary multicast stream
that has been requested by the RTP_Rx but cannot be served by
the BRS. There could be multiple reasons why the BRS has
rejected the request, however, the BRS chooses the most
appropriate response code to inform the RTP_Rx.
Upon receiving a reject response indicating a transient
problem such as insufficient BRS or network resources, if the
RTP_Rx wants to retry sending the same request, the RTP_Rx
MUST follow the RTCP timer rules of [RFC4585] to allow the
transient problems go away. However, if the reject response
indicates a non-transient problem (such as the ones reported
by response codes 504, 505 and 506), the RTP_Rx MUST NOT
attempt a retry. Different response codes have different
scopes. Refer to Section 7.3.1 for details.
The BRS can employ a policing mechanism against the broken
RTP_Rx implementations that are not following these rules.
See Section 10 for details.
* Accept Responses: The BRS MUST send at least one separate
RAMS-I message with the appropriate response code (either
zero indicating a private response or response code 200
indicating success as listed in Section 11.6) for each
primary multicast stream that has been requested by the
RTP_Rx and will be served by the BRS. Such RAMS-I messages
comprise fields that can be used to describe the individual
unicast burst streams. When there is a RAMS-R request for
multiple primary multicast streams, the BRS MUST include all
the individual RAMS-I messages corresponding to that RAMS-R
request in the same compound RTCP packet if these messages
fit in the same packet. Note that if the BRS is sending only
the preamble information but not the whole unicast burst
stream, it will not accept the request, but will send a
response code 511 instead as defined in Section 11.6.
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The RAMS-I message carries the RTP sequence number of the
first packet transmitted in the respective RTP stream to
allow the RTP_Rx to detect any missing initial packet(s).
When the BRS accepts a request for a primary multicast
stream, this field MUST always be populated in the RAMS-I
message(s) sent for this particular primary multicast stream.
It is RECOMMENDED that the BRS sends a RAMS-I message at the
start of the burst so that the RTP_Rx can quickly detect any
missing initial packet(s).
It is possible that the RAMS-I message for a primary multicast
stream can get delayed or lost, and the RTP_Rx can start
receiving RTP packets before receiving a RAMS-I message. An
RTP_Rx implementation MUST NOT assume it will quickly receive
the initial RAMS-I message. For redundancy purposes, it is
RECOMMENDED that the BRS repeats the RAMS-I messages multiple
times as long as it follows the RTCP timer rules defined in
[RFC4585].
4. Unicast Burst: For the primary multicast stream(s) for which
the request is accepted, the BRS starts sending the unicast
burst(s) that comprises one or more RTP retransmission packets
sent in the unicast burst RTP session. In addition, in some
applications the BRS can send preamble information data to the
RTP_Rx in addition to the requested burst to prime the media
decoder(s). However, for the BRS to send the preamble
information in a particular format, explicit signaling from the
RTP_Rx is required. In other words, the BRS MUST NOT send
preamble information in a particular format unless the RTP_Rx
has signaled support for that format in the RAMS-R message
through a public or private extension as defined in Section 7.1.
The format of this preamble data is RTP-payload specific and not
specified here.
5. Updated Request: The RTP_Rx MAY send an updated RAMS-R message
(as unicast feedback in the primary multicast RTP session) with
a different value for one or more fields of an earlier RAMS-R
message. The BRS MUST be able to detect whether a burst is
already planned for or being transmitted to this particular
RTP_Rx for this particular media sender SSRC. If there is an
existing burst planned for or being transmitted, the newly
arriving RAMS-R is an updated request; otherwise it is a new
request. It is also possible that the RTP_Rx has sent an
improperly formatted RAMS-R message or used an invalid value in
the RAMS-R message. If notified by the BRS using a 4xx-level
response code (as defined in Section 11.6) and only after
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following the timing rules of [RFC4585], the RTP_Rx MAY re-send
its corrected request.
The BRS determines the identity of the requesting RTP_Rx by
looking at its canonical name identifier (CNAME item in the SDES
source description). Thus, the RTP_Rx MUST choose a method that
ensures it uses a unique CNAME identifier. Different such ways
are provided in [I-D.ietf-avt-rtp-cnames]. In addition to one
or more fields with updated values, an updated RAMS-R message
may also include the fields whose values did not change.
Upon receiving an updated request, the BRS can use the updated
values for sending/shaping the burst, or refine the values and
use the refined values for sending/shaping the burst.
Subsequently, the BRS MAY send an updated RAMS-I message in the
unicast burst RTP session to indicate the changes it made.
It is an implementation-dependent decision for an RTP_RX whether
and when to send an updated request. However, note that the
updated request messages can get delayed and arrive at the BRS
after the initial unicast burst was completed. In this case,
the updated request message can trigger a new unicast burst and
by then if the RTP_Rx has already started receiving multicast
data, a congestion is likely to occur. In this case, the RTP_Rx
can detect this only after a delay and then it can try to
terminate the new burst. However, in the mean time, the RTP_Rx
can experience packet loss or other problems. This and other
similar corner cases SHOULD be carefully examined if updated
requests are to be used.
6. Updated Response: The BRS can send more than one RAMS-I
messages in the unicast burst RTP session, e.g., to update the
value of one or more fields in an earlier RAMS-I message. The
updated RAMS-I messages might or might not be a direct response
to a RAMS-R message. The BRS can also send updated RAMS-I
messages to signal the RTP_Rx in real time to join the SSM
session, when the BRS had already sent an initial RAMS-I
message, e.g., at the start of the burst. The RTP_Rx depends on
the BRS to learn the join time, which can be conveyed by the
first RAMS-I message, or can be sent/revised in a later RAMS-I
message. If the BRS is not capable of determining the join time
in the initial RAMS-I message, the BRS MUST send another RAMS-I
message (with the join time information) later.
7. Multicast Join Signaling: The RAMS-I message allows the BRS to
signal explicitly when the RTP_Rx needs to send the SFGMP Join
message. The RTP_Rx SHOULD use this information from the most
recent RAMS-I message unless it has more accurate information.
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If the request is accepted, this information MUST be conveyed in
at least one RAMS-I message and its value MAY be updated by
subsequent RAMS-I messages.
There can be missing packets if the RTP_Rx joins the multicast
session too early or too late. For example, if the RTP_Rx
starts receiving the primary multicast stream while it is still
receiving the unicast burst at a high excess bitrate, this can
result in an increased risk of packet loss. Or, if the RTP_Rx
joins the multicast session some time after the unicast burst is
finished, there can be a gap between the burst and multicast
data (a number of RTP packets might be missing). In both cases,
the RTP_Rx can issue retransmission requests (via RTCP NACK
messages sent as unicast feedback in the primary multicast RTP
session) [RFC4585] to the FT entity of the RS to fill the gap.
The BRS might or might not respond to such requests. When it
responds and the response causes significant changes in one or
more values reported earlier to the RTP_Rx, an updated RAMS-I
SHOULD be sent to the RTP_Rx.
8. Multicast Receive: After the join, the RTP_Rx starts receiving
the primary multicast stream(s).
9. Terminate: The BRS can know when it needs to ultimately stop
the unicast burst based on its parameters. However, the RTP_Rx
may need to ask the BRS to terminate the burst prematurely or at
a specific sequence number. For this purpose, the RTP_Rx uses
the RAMS-Termination (RAMS-T) message sent as RTCP feedback in
the unicast burst RTP session. A separate RAMS-T message is
sent for each primary multicast stream served by the BRS unless
an RTCP BYE message has been sent in the unicast burst RTP
session as described in Step 10. For the burst requests that
were rejected by the BRS, there is no need to send a RAMS-T
message.
If the RTP_Rx wants to terminate a burst prematurely, it MUST
send a RAMS-T message for the SSRC of the primary multicast
stream it wishes to terminate. This message is sent in the
unicast burst RTP session. Upon receiving this message, the BRS
MUST terminate the unicast burst. If the RTP_Rx requested to
acquire the entire primary multicast RTP session but wants to
terminate this request before it learns the individual media
sender SSRC(s) via RAMS-I message(s) or RTP packets, the RTP_Rx
cannot use RAMS-T message(s) and thus MUST send an RTCP BYE
message in the unicast burst RTP session to terminate the
request.
Otherwise, the default behavior for the RTP_Rx is to send a
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RAMS-T message in the unicast burst RTP session immediately
after it joins the multicast session and has started receiving
multicast packets. In that case, the RTP_Rx MUST send a RAMS-T
message with the sequence number of the first RTP packet
received in the primary multicast stream. Then, the BRS MUST
terminate the respective burst after it sends the unicast burst
packet whose Original Sequence Number (OSN) field in the RTP
retransmission payload header matches this number minus one. If
the BRS has already sent that unicast burst packet when the
RAMS-T message arrives, the BRS MUST terminate the respective
burst immediately.
If an RTCP BYE message has not been issued yet as described in
Step 10, the RTP_Rx MUST send at least one RAMS-T message for
each primary multicast stream served by the BRS. The RAMS-T
message(s) MUST be sent to the BRS in the unicast burst RTP
session. Against the possibility of a message loss, it is
RECOMMENDED that the RTP_Rx repeats the RAMS-T messages multiple
times as long as it follows the RTCP timer rules defined in
[RFC4585].
The binding between RAMS-T and ongoing bursts is achieved
through RTP_Rx's CNAME identifier, and packet sender and media
sender SSRCs. Choosing a globally unique CNAME makes sure that
the RAMS-T messages are processed correctly.
10. Terminate with RTCP BYE: When the RTP_Rx is receiving one or
more burst streams, if the RTP_Rx becomes no longer interested
in acquiring any of the primary multicast streams, the RTP_Rx
SHALL issue an RTCP BYE message for the unicast burst RTP
session and another RTCP BYE message for the primary multicast
RTP session. These RTCP BYE messages are sent to the BRS and FT
logical entities, respectively.
Upon receiving an RTCP BYE message, the BRS MUST terminate the
rapid acquisition operation, and cease transmitting any further
burst packets and retransmission packets. If support for
[RFC5506] has been signaled, the RTCP BYE message MAY be sent in
a reduced-size RTCP packet. Otherwise, Section 6.1 of [RFC3550]
mandates the RTCP BYE message always to be sent with a sender or
receiver report in a compound RTCP packet. If no data has been
received, an empty receiver report MUST be still included. With
the information contained in the receiver report, the RS can
figure out how many duplicate RTP packets have been delivered to
the RTP_Rx (Note that this will be an upper-bound estimate as
one or more packets might have been lost during the burst
transmission). The impact of duplicate packets and measures
that can be taken to minimize the impact of receiving duplicate
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packets will be addressed in Section 6.4.
Since an RTCP BYE message issued for the unicast burst RTP
session terminates that session and ceases transmitting any
further packets in that session, there is no need for sending
explicit RAMS-T messages, which would only terminate their
respective bursts.
For the purpose of gathering detailed information about RTP_Rx's
rapid acquisition experience, [I-D.ietf-avt-multicast-acq-rtcp-xr]
defines an RTCP Extended Report (XR) Block. This report is designed
to be payload-independent, thus, it can be used by any multicast
application that supports rapid acquisition.
6.3. Synchronization of Primary Multicast Streams
When an RTP_RX acquires multiple primary multicast streams, it might
need to synchronize them for playout. This synchronization is
achieved by the help of the RTCP sender reports [RFC3550]. If the
playout will start before the RTP_Rx has joined the multicast
session, the RTP_Rx needs to receive the information reflecting the
synchronization among the primary multicast streams early enough so
that it can play out the media in a synchronized fashion.
The suggested approach is to use the RTP header extension mechanism
[RFC5285] and convey the synchronization information in a header
extension as defined in [RFC6051]. Per [RFC4588] "if the original
RTP header carried an RTP header extension, the retransmission packet
SHOULD carry the same header extension." Thus, as long as the
multicast source emits a header extension with the synchronization
information frequently enough, there is no additional task that needs
to be carried out by the BRS. The synchronization information will
be sent to the RTP_Rx along with the burst packets. The frequent
header extensions sent in the primary multicast RTP sessions also
allow rapid synchronization of the RTP streams for the RTP receivers
that do not support RAMS or that directly join the multicast session
without running RAMS. Thus, in RAMS applications, it is RECOMMENDED
that the multicast sources frequently send synchronization
information by using header extensions following the rules presented
in [RFC6051]. The regular sender reports are still sent in the
unicast session by following the rules of [RFC3550].
6.4. Burst Shaping and Congestion Control in RAMS
This section provides informative guidelines about how the BRS can
shape the transmission of the unicast burst and how congestion can be
dealt within the RAMS process. When the RTP_Rx detects that the
unicast burst is causing severe congestion, it can prefer to send a
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RAMS-T message immediately to stop the unicast burst.
A higher bitrate for the unicast burst naturally conveys the
Reference Information and media content to the RTP_Rx faster. This
way, the RTP_Rx can start consuming the data sooner, which results in
a faster acquisition. A higher bitrate also represents a better
utilization of the BRS resources. As the burst may continue until it
catches up with the primary multicast stream, the higher the bursting
bitrate, the less data the BRS needs to transmit. However, a higher
bitrate for the burst also increases the chances for congestion-
caused packet loss. Thus, as discussed in Section 5, there has to be
an upper bound on the bandwidth used by the burst.
When the BRS transmits the unicast burst, it is expected to take into
account all available information to prevent any packet loss that
might take place during the bursting as a result of buffer overflow
on the path between the RS and RTP_Rx and at the RTP_Rx itself. The
bursting bitrate can be determined by taking into account the
following information, when available:
a. Information obtained via the RAMS-R message, such as Max RAMS
Buffer Fill Requirement and/or Max Receive Bitrate (See
Section 7.2).
b. Information obtained via RTCP receiver reports provided by the
RTP_Rx in the retransmission session, allowing in-session bitrate
adaptations for the burst. When these receiver reports indicate
packet loss, this can indicate a certain congestion state in the
path from the RS to the RTP_Rx.
c. Information obtained via RTCP NACKs provided by the RTP_Rx in the
primary multicast RTP session, allowing in-session bitrate
adaptations for the burst. Such RTCP NACKs are transmitted by
the RTP_Rx in response to packet loss detection in the burst.
NACKs can indicate a certain congestion state on the path from
the RS to RTP_Rx.
d. There can be other feedback received from the RTP_Rx, e.g., in
the form of ECN-CE markings [I-D.ietf-avt-ecn-for-rtp] that can
influence in-session bitrate adaptation.
e. Information obtained via updated RAMS-R messages, allowing in-
session bitrate adaptations, if supported by the BRS.
f. Transport protocol-specific information. For example, when DCCP
is used to transport the RTP burst, the ACKs from the DCCP client
can be leveraged by the BRS / DCCP server for burst shaping and
congestion control.
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g. Pre-configured settings for each RTP_Rx or a set of RTP_Rxs that
indicate the upper-bound bursting bitrates for which no packet
loss will occur as a result of congestion along the path of the
RS to RTP_Rx. For example, in managed IPTV networks, where the
bottleneck bandwidth along the end-to-end path is known and where
the network between the RS and this link is provisioned and
dimensioned to carry the burst streams, the bursting bitrate does
not exceed the provisioned value. These settings can also be
dynamically adapted using application-aware knowledge.
The BRS chooses the initial burst bitrate as follows:
o When using RAMS in environments as described in (g), the BRS MUST
transmit the burst packets at an initial bitrate higher than the
nominal bitrate, but within the engineered or reserved bandwidth
limit.
o When the BRS cannot determine a reliable bitrate value for the
unicast burst (using a through g), it is desirable that the BRS
chooses an appropriate initial bitrate not above the nominal
bitrate and increases it gradually unless a congestion is
detected.
In both cases, during the burst transmission the BRS MUST
continuously monitor for packet losses as a result of congestion by
means of one or more of the mechanisms described in (b,c,d,e,f).
When the BRS relies on RTCP receiver reports, sufficient bandwidth
needs to be provided to RTP Rx for RTCP transmission in the unicast
burst RTP session. To achieve a reasonable fast adaptation against
congestion, it is recommended that the RTP_Rx sends a receiver report
at least once every two RTTs between the RS and RTP_Rx. Although the
specific heuristics and algorithms that deduce a congestion state and
how subsequently the BRS acts are outside the scope of this
specification, the following two methods are the best practices:
o Upon detection of a significant amount of packet loss, which the
BRS attributes to congestion, the BRS decreases the burst bitrate.
The rate by which the BRS increases and decreases the bitrate for
the burst can be determined by a TCP-friendly bitrate adaptation
algorithm for RTP over UDP , or in the case of (f) by the
congestion control algorithms defined in DCCP [RFC5762].
o If the congestion is persistent and the BRS has to reduce the
burst bitrate to a point where the RTP Rx buffer might underrun or
the burst will consume too many resources, the BRS terminates the
burst and transmits a RAMS-I message to RTP Rx with the
appropriate response code. It is then up to RTP Rx to decide when
to join the multicast session.
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Even though there is no congestion experienced during the burst,
congestion may anyway arise near the end of the burst as the RTP_Rx
eventually needs to join the multicast session. During this brief
period both the burst packets and the multicast packets can be
simultaneously received by the RTP_Rx, thus enhancing the risk of
congestion.
Since the BRS signals the RTP_Rx when the BRS expects the RTP_Rx to
send the SFGMP Join message, the BRS can have a rough estimate of
when the RTP_Rx will start receiving multicast packets in the SSM
session. The BRS can keep on sending burst packets but reduces the
bitrate accordingly at the appropriate instant by taking the bitrate
of the whole SSM session into account. If the BRS ceases
transmitting the burst packets before the burst catches up, any gap
resulting from this imperfect switch-over by the RTP_Rx can be later
repaired by requesting retransmissions for the missing packets from
the RS. The retransmissions can be shaped by the BRS to make sure
that they do not cause collateral loss in the primary multicast RTP
session and the unicast burst RTP session.
6.5. Failure Cases
In the following, we examine the implications of losing the RAMS-R,
RAMS-I or RAMS-T messages and other failure cases.
When the RTP_Rx sends a RAMS-R message to initiate a rapid
acquisition but the message gets lost and the FT does not receive it,
the RTP_Rx will get neither a RAMS-I message, nor a unicast burst.
In this case, the RTP_Rx MAY resend the request when it is eligible
to do so based on the RTCP timer rules defined in [RFC4585]. Or,
after a reasonable amount of time, the RTP_Rx can time out (based on
the previous observed response times) and immediately join the SSM
session.
In the case the RTP_Rx starts receiving a unicast burst but it does
not receive a corresponding RAMS-I message within a reasonable amount
of time, the RTP_Rx can either discard the burst data or decide not
to interrupt the unicast burst, and be prepared to join the SSM
session at an appropriate time it determines or as indicated in a
subsequent RAMS-I message (if available). If the network is subject
to packet loss, it is RECOMMENDED that the BRS repeats the RAMS-I
messages multiple times based on the RTCP timer rules defined in
[RFC4585].
In the failure cases where the RAMS-R message is lost and the RTP_Rx
gives up, or the RAMS-I message is lost, the RTP_Rx MUST still
terminate the burst(s) it requested by following the rules described
in Section 6.2.
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In the case a RAMS-T message sent by the RTP_Rx does not reach its
destination, the BRS can continue sending burst packets even though
the RTP_Rx no longer needs them. If an RTP_Rx is receiving burst
packets it no longer needs after sending a RAMS-T message, it is
RECOMMENDED that the RTP_Rx repeats the RAMS-T message multiple times
based on the RTCP timer rules defined in [RFC4585]. The BRS MUST be
provisioned to terminate the burst when it can no longer send the
burst packets faster than it receives the primary multicast stream
packets.
Section 6.3.5 of [RFC3550] explains the rules pertaining to timing
out an SSRC. When the BRS accepts to serve the requested burst(s)
and establishes the retransmission session, the BRS needs to check
the liveness of the RTP_Rx via the RTCP messages and reports the
RTP_Rx sends. The default rules explained in [RFC3550] apply in RAMS
as well.
7. Encoding of the Signaling Protocol in RTCP
This section defines the formats of the RTCP transport-layer feedback
messages that are exchanged between the retransmission server (RS)
and RTP receiver (RTP_Rx) during rapid acquisition. These messages
are referred to as the RAMS Messages. They are payload-independent
and MUST be used by all RTP-based multicast applications that support
rapid acquisition regardless of the payload they carry.
Payload-specific feedback messages are not defined in this document.
However, further optional payload-independent and payload-specific
information can be included in the exchange.
The common packet format for the RTCP feedback messages is defined in
Section 6.1 of [RFC4585] and is also sketched in Figure 4.
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| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 4: The common packet format for the RTCP feedback messages
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Each feedback message has a fixed-length field for version, padding,
feedback message type (FMT), packet type (PT), length, SSRC of packet
sender, SSRC of media source as well as a variable-length field for
feedback control information (FCI).
In the RAMS messages, the PT field is set to RTPFB (205) and the FMT
field is set to RAMS (6). Individual RAMS messages are identified by
a sub-field called Sub Feedback Message Type (SFMT). Any Reserved
field SHALL be set to zero and ignored.
Depending on the specific scenario and timeliness/importance of a
RAMS message, it can be desirable to send it in a reduced-size RTCP
packet [RFC5506]. However, unless support for [RFC5506] has been
signaled, compound RTCP packets MUST be used by following [RFC3550]
rules.
Following the rules specified in [RFC3550], all integer fields in the
messages defined below are carried in network-byte order, that is,
most significant byte (octet) first, also known as big-endian.
Unless otherwise stated, numeric constants are in decimal (base 10).
7.1. Extensions
To improve the functionality of the RAMS method in certain
applications, it can be desirable to define new fields in the RAMS
Request, Information and Termination messages. Such fields MUST be
encoded as Type-Length-Value (TLV) elements as described below and
sketched in Figure 5:
o Type: A single-octet identifier that defines the type of the
parameter represented in this TLV element.
o Length: A two-octet field that indicates the length (in octets)
of the TLV element excluding the Type and Length fields, and the
8-bit Reserved field between them. This length does not include
any padding that is required for alignment.
o Value: Variable-size set of octets that contains the specific
value for the parameter.
In the extensions, the Reserved field SHALL be set to zero and
ignored. If a TLV element does not fall on a 32-bit boundary, the
last word MUST be padded to the boundary using further bits set to
zero.
An RTP_Rx or BRS MAY include vendor-neutral and private extensions in
any RAMS message. The RTP_Rx or BRS MUST place such extensions after
the mandatory fields and mandatory TLV elements (if any), and MAY
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place such extensions in any order. The RTP_Rx or BRS MUST NOT
include multiple TLV elements with the same Type value in a RAMS
message.
The support for extensions (unless specified otherwise) is OPTIONAL.
An RTP_Rx or BRS receiving a vendor-neutral or private extension that
it does not understand MUST ignore that extension.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Structure of a TLV element
7.1.1. Vendor-Neutral Extensions
If the goal in defining new TLV elements is to extend the
functionality in a vendor-neutral manner, they MUST be registered
with IANA through the guidelines provided in Section 11.5.
The current document defines several vendor-neutral extensions in the
subsequent sections.
7.1.2. Private Extensions
It is desirable to allow vendors to use private extensions in a TLV
format. For interoperability, such extensions must not collide with
each other.
A certain range of TLV Types (between - and including - 128 and 254 )
is reserved for private extensions (Refer to Section 11.5). IANA
management for these extensions is unnecessary and they are the
responsibility of individual vendors.
The structure that MUST be used for the private extensions is
depicted in Figure 6. Here, the enterprise numbers are used from
http://www.iana.org/assignments/enterprise-numbers. This will ensure
the uniqueness of the private extensions and avoid any collision.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Structure of a private extension
7.2. RAMS Request
The RAMS Request (RAMS-R) message is identified by SFMT=1. This
message is sent as unicast feedback in the primary multicast RTP
session by the RTP_Rx to request rapid acquisition of a primary
multicast RTP session, or one or more primary multicast streams
belonging to the same primary multicast RTP session. In the RAMS-R
message, the RTP_Rx MUST set both the packet sender SSRC and media
sender SSRC fields to its own SSRC since the media sender SSRC may
not be known. The RTP_Rx MUST provide explicit signaling as
described below to request stream(s). This minimizes the chances of
accidentally requesting a wrong primary multicast stream.
The FCI field MUST contain only one RAMS Request. The FCI field has
the structure depicted in Figure 7.
The semantics of the RAMS-R message is independent of the payload
type carried in the primary multicast RTP session.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Requested Media Sender SSRC(s) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: FCI field syntax for the RAMS Request message
o Requested Media Sender SSRC(s): Mandatory TLV element that lists
the media sender SSRC(s) requested by the RTP_Rx. The BRS MUST
ignore the media sender SSRC specified in the header of the RAMS-R
message.
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If the Length field is set to zero (i.e., no media sender SSRC is
listed), it means that the RTP_Rx is requesting to rapidly acquire
the entire primary multicast RTP session. Otherwise, the RTP_Rx
lists the individual media sender SSRCs in this TLV element and
sets the Length field of the TLV element to 4*n, where n is the
number of SSRC entries.
Type: 1
o Min RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the minimum milliseconds of data that the RTP_Rx
desires to have in its buffer before allowing the data to be
consumed by the application.
The RTP_Rx can have knowledge of its buffering requirements.
These requirements can be application and/or device specific. For
instance, the RTP_Rx might need to have a certain amount of data
in its application buffer to handle transmission jitter and/or to
be able to support error-control methods. If the BRS is told the
minimum buffering requirement of the receiver, the BRS can tailor
the burst(s) more precisely, e.g., by choosing an appropriate
starting point. The methods used by the RTP_Rx to determine this
value are application specific, and thus, out of the scope of this
document.
If specified, the amount of backfill that will be provided by the
unicast bursts and any payload-specific information MUST NOT be
smaller than the specified value. Otherwise, the backfill will
not be able to build up the desired level of buffer at the RTP_Rx
and can cause buffer underruns.
Type: 2
o Max RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the maximum milliseconds of data that the RTP_Rx can
buffer without losing the data due to buffer overflow.
The RTP_Rx can have knowledge of its buffering requirements.
These requirements can be application or device specific. For
instance, one particular RTP_Rx might have more physical memory
than another RTP_Rx, and thus, can buffer more data. If the BRS
knows the buffering ability of the receiver, the BRS can tailor
the burst(s) more precisely. The methods used by the receiver to
determine this value are application specific, and thus, out of
scope.
If specified, the amount of backfill that will be provided by the
unicast bursts and any payload-specific information MUST NOT be
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larger than this value. Otherwise, the backfill may cause buffer
overflows at the RTP_Rx.
Type: 3
o Max Receive Bitrate (64 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) at which the RTP_Rx can
process the aggregation of the unicast burst(s) and any payload-
specific information that will be provided by the BRS. The limits
can include local receiver limits as well as network limits that
are known to the receiver.
If specified, the total bitrate of the unicast burst(s) plus any
payload-specific information MUST NOT be larger than this value.
Otherwise, congestion and packet loss are more likely to occur.
Type: 4
o Preamble-only Allowed (0 bits): Optional TLV element that
indicates that the RTP_Rx is willing to receive only the preamble
information for the desired primary multicast stream(s) in case
the BRS cannot send the unicast burst stream(s) (In such cases,
the BRS will not accept the request, but will send a response code
511 in the RAMS-I message as defined in Section 11.6). Note that
the RTP_Rx signals the particular preamble format(s) it supports
through a public or a private extension in the same RAMS-R
message.
If this TLV element does not exist in the RAMS-R message, the BRS
MUST NOT respond to the RAMS-R message by sending the preamble
information only. Since this TLV element does not carry a Value
field, the Length field MUST be set to zero.
Type: 5
o Supported Enterprise Number(s): Optional TLV element that
indicates the enterprise number(s) that the RTP_Rx implementation
supports. Similar to the private extensions, the enterprise
numbers here are used from
http://www.iana.org/assignments/enterprise-numbers. This TLV
element, if exists, provides a hint information to the BRS about
which private extensions the RTP_Rx can potentially support. Note
that an RTP_Rx does not necessarily support all the private
extensions under a particular enterprise number. Unless the BRS
explicitly knows which private extensions an RTP_Rx supports
(through this or some out-of-band means), the BRS SHOULD NOT use
private extensions in the RAMS-I messages. The BRS SHOULD rather
use only vendor-neutral extensions. The Length field of this TLV
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element is set to 4*n, where n is the number of enterprise number
entries.
Type: 6
7.3. RAMS Information
The RAMS Information (RAMS-I) message is identified by SFMT=2. This
message is used to describe the unicast burst that will be sent for
rapid acquisition. It also includes other useful information for the
RTP_Rx as described below.
A separate RAMS-I message with the appropriate response code is sent
in the unicast burst RTP session by the BRS for each primary
multicast stream that has been requested by the RTP_Rx. In each of
these RAMS-I messages, the media sender SSRC and packet sender SSRC
fields are both set to the SSRC of the BRS, which equals the SSRC of
the respective primary multicast stream.
The FCI field MUST contain only one RAMS Information message. The
FCI field has the structure depicted in Figure 8.
The semantics of the RAMS-I message is independent of the payload
type carried in the primary multicast RTP session.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=2 | MSN | Response |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: FCI field syntax for the RAMS Information message
A RAMS-I message has the following fields:
o Message Sequence Number (MSN) (8 bits) : Mandatory field that
denotes the sequence number of the RAMS-I message for the
particular media sender SSRC specified in the message header. The
MSN value SHALL be set to zero when a new RAMS request is
received. During rapid acquisition, the same RAMS-I message MAY
be repeated for redundancy purposes without incrementing the MSN
value. If an updated RAMS-I message will be sent (either with a
new information or an updated information), the MSN value SHALL be
incremented by one. In the MSN field, the regular wrapping rules
apply. Note that if the RTP_Rx has sent an updated request, it
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MUST check every RAMS-I message entirely regardless of whether the
MSN value has changed or not.
o Response (16 bits): Mandatory field that denotes the response
code for this RAMS-I message. This document defines several
initial response codes in Section 7.3.1 and registers them with
IANA in Section 11.6. If a new vendor-neutral response code will
be defined, it MUST be registered with IANA through the guidelines
specified in Section 11.6. If the new response code is intended
to be used privately by a vendor, there is no need for IANA
management. Instead, the vendor MUST use the private extension
mechanism (Section 7.1.2) to convey its message and MUST indicate
this by putting zero in the Response field.
When the RTP_Rx receives a RAMS-I message with a response code
that it does not understand, the RTP_Rx MUST send a RAMS-T message
immediately to the BRS.
The following TLV elements have been defined for the RAMS-I messages:
o Media Sender SSRC (32 bits): Optional TLV element that specifies
the media sender SSRC of the unicast burst stream. If the FT_Ap
that received the RAMS-R message is associated with a single
primary multicast stream but the requested media sender SSRC does
not match the SSRC of the RTP stream associated with this FT_Ap,
the BRS includes this TLV element in the initial RAMS-I message to
let the RTP_Rx know that the media sender SSRC has changed. If
the two SSRCs match, there is no need to include this TLV element.
Type: 31
o RTP Seqnum of the First Packet (16 bits): TLV element that
specifies the RTP sequence number of the first packet that will be
sent in the respective unicast RTP stream. This allows the RTP_Rx
to know whether one or more packets sent by the BRS have been
dropped at the beginning of the stream. If the BRS accepts the
RAMS request, this element exists. If the BRS rejects the RAMS
request, this element SHALL NOT exist.
Type: 32
o Earliest Multicast Join Time (32 bits): TLV element that
specifies the delta time (in ms) between the arrival of the first
RTP packet in the unicast RTP stream (which could be a burst
packet or a payload-specific packet) and the earliest time instant
when an RTP_Rx MAY send an SFGMP Join message to join the
multicast session. A zero value in this field means that the
RTP_Rx MAY send the SFGMP Join message right away. If the RTP_Rx
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does not want to wait until the earliest multicast join time, it
MAY send a RAMS-T message and only after a reasonable amount of
time, it MAY join the SSM session.
If the RAMS request has been accepted, the BRS sends this field at
least once, so that the RTP_Rx knows when to join the multicast
session. If the burst request has been rejected as indicated in
the Response field, this field MUST be set to zero. In that case,
it is up to the RTP_Rx when or whether to join the multicast
session.
When the BRS serves two or more bursts and sends a separate RAMS-I
message for each burst, the join times specified in these RAMS-I
messages SHOULD correspond to more or less the same time instant,
and the RTP_Rx sends the SFGMP Join message based on the earliest
join time.
Type: 33
o Burst Duration (32 bits): Optional TLV element that denotes the
time the burst will last (in ms), i.e., the difference between the
expected transmission times of the first and the last burst
packets that the BRS is planning to send in the respective unicast
RTP stream. In the absence of additional stimulus, the BRS will
send a burst of this duration. However, the burst duration can be
modified by subsequent events, including changes in the primary
multicast stream and reception of RAMS-T messages.
The BRS MUST terminate the flow in the timeframe indicated by this
burst duration or the duration established by those subsequent
events, even if it does not get a RAMS-T or a BYE message from the
RTP_Rx. It is OPTIONAL to send this field in a RAMS-I message
when the burst request is accepted. If the burst request has been
rejected as indicated in the Response field, this field MAY be
omitted or set to zero.
Type: 34
o Max Transmit Bitrate (64 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) that will be used by the
BRS for the RTP stream associated with this RAMS-I message.
Type: 35
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7.3.1. Response Code Definitions
1xx-Level Response Codes: These response codes are sent for
informational purposes.
o 100: This is used when the BRS wants to update a value that was
sent earlier to the RTP_Rx.
2xx-Level Response Codes: These response codes are sent to indicate
success.
o 200: This is used when the server accepts the RAMS request.
o 201: This is used when the unicast burst has been completed and
the BRS wants to notify the RTP_Rx.
4xx-Level Response Codes: These response codes are sent to indicate
that the message sent by the RTP_Rx is either improperly formatted or
contains an invalid value. The rules the RTP_Rx needs to follow upon
receiving one of these response codes are outlined in Step 5 in
Section 6.2. The 4xx-level response codes are also used as status
codes in the Multicast Acquisition Report Block
[I-D.ietf-avt-multicast-acq-rtcp-xr] in order to collect RTP_Rx-based
error conditions.
o 400: This is used when the RAMS-R message is improperly
formatted.
o 401: This is used when the minimum RAMS buffer fill requirement
value indicated in the RAMS-R message is invalid.
o 402: This is used when the maximum RAMS buffer fill requirement
value indicated in the RAMS-R message is invalid.
o 403: This is used when the maximum receive bitrate value
indicated in the RAMS-R message is insufficient.
o 404: This is used when the RAMS-T message is improperly
formatted.
5xx-Level Response Codes: These response codes are sent to indicate
an error on the BRS side. The rules the RTP_Rx needs to follow upon
receiving one of these response codes are outlined in Step 3 in
Section 6.2 and Section 7.2. The 5xx-level response codes are also
used as status codes in the Multicast Acquisition Report Block
[I-D.ietf-avt-multicast-acq-rtcp-xr] in order to collect BRS-based
error conditions.
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o 500: This is used when the BRS has experienced an internal error
and cannot accept the RAMS request.
o 501: This is used when the BRS does not have enough bandwidth to
send the unicast burst stream.
o 502: This is used when the BRS terminates the unicast burst
stream due to network congestion.
o 503: This is used when the BRS does not have enough CPU resources
to send the unicast burst stream.
o 504: This is used when the BRS does not support sending a unicast
burst stream.
o 505: This is used when the requesting RTP_Rx is not eligible to
receive a unicast burst stream.
o 506: This is used when RAMS functionality is not enabled for the
requested multicast stream.
o 507: This is used when the BRS cannot find a valid starting point
for the unicast burst stream satisfying the RTP_Rx's requirements.
o 508: This is used when the BRS cannot find the essential
reference information for the requested multicast stream.
o 509: This is used when the BRS cannot match the requested SSRC to
any of the streams it is serving.
o 510: This is used when the BRS cannot serve the requested entire
session.
o 511: This is used when the BRS sends only the preamble
information but not the whole unicast burst stream.
o 512: This is used when the RAMS request is denied due to a policy
specified for the requested multicast stream, requesting RTP_Rx or
this particular BRS.
7.4. RAMS Termination
The RAMS Termination (RAMS-T) message is identified by SFMT=3.
The RAMS Termination is used to assist the BRS in determining when to
stop the burst. A separate RAMS-T message is sent in the unicast
burst RTP session by the RTP_Rx for each primary multicast stream
that has been served by the BRS. Each of these RAMS-T message's
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media sender SSRC field SHALL BE populated with the SSRC of the media
stream to be terminated. If the media sender SSRC populated in the
RAMS-T message does not match the SSRC of the burst served by the
BRS, BRS SHALL ignore the RAMS-T message.
If the RTP_Rx wants the BRS to stop a burst prematurely, it sends a
RAMS-T message as described below. Upon receiving this message, the
BRS stops the respective burst immediately. If the RTP_Rx wants the
BRS to terminate all of the bursts, it needs to send all of the
respective RAMS-T messages in a single compound RTCP packet.
The default behavior for the RTP_Rx is to send a RAMS-T message
immediately after it joined the multicast session and started
receiving multicast packets. In that case, the RTP_Rx includes the
sequence number of the first RTP packet received in the primary
multicast stream in the RAMS-T message. With this information, the
BRS can decide when to terminate the unicast burst.
The FCI field MUST contain only one RAMS Termination. The FCI field
has the structure depicted in Figure 9.
The semantics of the RAMS-T message is independent of the payload
type carried in the primary multicast RTP session.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: FCI field syntax for the RAMS Termination message
o Extended RTP Seqnum of First Multicast Packet (32 bits): Optional
TLV element that specifies the extended RTP sequence number of the
first packet received from the SSM session for a particular
primary multicast stream. The low 16 bits contain the sequence
number of the first packet received from the SSM session, and the
most significant 16 bits extend that sequence number with the
corresponding count of sequence number cycles, which can be
maintained according to the algorithm in Appendix A.1 of
[RFC3550].
Type: 61
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8. SDP Signaling
8.1. Definitions
The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585].
Here we add the following syntax to the 'rtcp-fb' attribute (the
feedback type and optional parameters are all case sensitive):
(In the following ABNF [RFC5234], rtcp-fb-nack-param is used as
defined in [RFC4566].)
rtcp-fb-nack-param =/ SP "rai"
The following parameter is defined in this document for use with
'nack':
o 'rai' stands for Rapid Acquisition Indication and indicates the
use of RAMS messages as defined in Section 7.
This document also defines a new media-level SDP attribute ('rams-
updates') that indicates whether the BRS supports updated request
messages or not. This attribute is used in a declarative manner and
no Offer/Answer Model behavior is specified. If the BRS supports
updated request messages and this attribute is included in the SDP
description, the RTP_Rx can send updated requests. The BRS might or
might not be able to accept value changes in every field in an
updated RAMS-R message. However, if the 'rams-updates' attribute is
not included in the SDP description, the RTP_Rx SHALL NOT send
updated requests. The RTP_Rx MAY still repeat its initial request
without changes, though.
8.2. Requirements
The use of SDP to describe the RAMS entities normatively requires the
support for:
o The SDP grouping framework and flow identification (FID) semantics
[RFC5888]
o The RTP/AVPF profile [RFC4585]
o The RTP retransmission payload format [RFC4588]
o The RTCP extensions for SSM sessions with unicast feedback
[RFC5760]
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o The 'multicast-rtcp' attribute [I-D.ietf-avt-rtcp-port-for-ssm]
o Multiplexing RTP and RTCP on a single port on both endpoints in
the unicast session [RFC5761]
o The 'portmapping-req' attribute
[I-D.ietf-avt-ports-for-ucast-mcast-rtp]
The support for the source-specific media attributes [RFC5576] can
also be needed when the 'ssrc' attribute is to be used for the media
descriptions.
8.3. Example and Discussion
This section provides a declarative SDP [RFC4566] example (for the
RTP_Rx side) for enabling rapid acquisition of multicast RTP
sessions.
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v=0
o=ali 1122334455 1122334466 IN IP4 rams.example.com
s=Rapid Acquisition Example
t=0 0
a=group:FID 1 2
a=rtcp-unicast:rsi
m=video 41000 RTP/AVPF 98
i=Primary Multicast Stream
c=IN IP4 233.252.0.2/255
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
a=rtpmap:98 MP2T/90000
a=multicast-rtcp:42000
a=rtcp:43000 IN IP4 192.0.2.1
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack rai
a=ssrc:123321 cname:iptv-ch32@rams.example.com
a=rams-updates
a=mid:1
m=video 51000 RTP/AVPF 99
i=Unicast Retransmission Stream (Ret. and Rapid Acq. Support)
c=IN IP4 192.0.2.1
a=sendonly
a=rtpmap:99 rtx/90000
a=rtcp-mux
a=rtcp:51500
a=fmtp:99 apt=98;rtx-time=5000
a=portmapping-req:55000
a=mid:2
Figure 10: Example SDP for a single-channel RAMS scenario
In this example SDP description, we have a primary multicast (source)
stream and a unicast retransmission stream. The source stream is
multicast from a distribution source (with a source IP address of
198.51.100.1) to the multicast destination address of 233.252.0.2 and
port 41000. The corresponding RTCP traffic is multicast to the same
multicast destination address at port 42000. Here, we are assuming
that the multicast RTP and RTCP ports are carefully chosen so that
different RTP and RTCP streams do not collide with each other.
The feedback target (FT_Ap) residing on the RS (with an address of
192.0.2.1) at port 43000 is declared with the "a=rtcp" line
[RFC3605]. The support for the conventional retransmission is
indicated through the "a=rtcp-fb:98 nack" line. The RTP receiver(s)
can report missing packets on the source stream to the feedback
target and request retransmissions. The SDP above includes the
"a=sendonly" line for the media description of the retransmission
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stream since the retransmissions are sent in only one direction.
The support for rapid acquisition is indicated through the "a=rtcp-
fb:98 nack rai" line. The parameter 'rtx-time' applies to both the
conventional retransmissions and rapid acquisition. However, when
rapid acquisition is enabled, the standard definition for the
parameter 'rtx-time' given in [RFC4588] is not followed. Instead,
when rapid acquisition support is enabled, 'rtx-time' specifies the
time in milliseconds that the BRS keeps an RTP packet in its cache
available for retransmission (measured from the time the packet was
received by the BRS, not from the time indicated in the packet
timestamp).
Once an RTP_Rx has acquired an SDP description, it can ask for rapid
acquisition before it joins a primary multicast RTP session. To do
so, it sends a RAMS-R message to the feedback target of that primary
multicast RTP session. If the FT_Ap is associated with only one RTP
stream, the RTP_Rx does not need to learn the SSRC of that stream via
an out-of-band method. If the BRS accepts the rapid acquisition
request, it will send a RAMS-I message with the correct SSRC
identifier. If the FT_Ap is associated with a multi-stream RTP
session and the RTP_Rx is willing to request rapid acquisition for
the entire session, the RTP_Rx again does not need to learn the SSRCs
via an out-of-band method. However, if the RTP_Rx intends to request
a particular subset of the primary multicast streams, it must learn
their SSRC identifiers and list them in the RAMS-R message. Since
this RTP_Rx has not yet received any RTP packets for the primary
multicast stream(s), the RTP_Rx must in this case learn the SSRC
value(s) from the 'ssrc' attribute of the media description
[RFC5576]. In addition to the SSRC value, the 'cname' source
attribute must also be present in the SDP description [RFC5576].
Listing the SSRC values for the primary multicast streams in the SDP
file does not create a problem in SSM sessions when an SSRC collision
occurs. This is because in SSM sessions, an RTP_Rx that observed an
SSRC collision with a media sender must change its own SSRC [RFC5760]
by following the rules defined in [RFC3550].
A feedback target that receives a RAMS-R message becomes aware that
the RTP_Rx wants to rapidly catch up with a primary multicast RTP
session. If the necessary conditions are satisfied (as outlined in
Section 7 of [RFC4585]) and available resources exist, the BRS can
react to the RAMS-R message by sending any transport-layer (and
optional payload-specific, when allowed) feedback message(s) and
starting the unicast burst.
In this section, we considered the simplest scenario where the
primary multicast RTP session carried only one stream and the RTP_Rx
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wanted to rapidly acquire this stream only. Best practices for
scenarios where the primary multicast RTP session carries two or more
streams or the RTP_Rx wants to acquire one or more streams from
multiple primary multicast RTP sessions at the same time are
presented in [I-D.begen-avt-rams-scenarios].
9. NAT Considerations
For a variety of reasons, one or more Network Address Port
Translators (NAPT - hereafter simply called NAT) can exist between
the RTP_Rx and RS. NATs have a variety of operating characteristics
for UDP traffic [RFC4787]. For a NAT to permit traffic from the BRS
to arrive at the RTP_Rx, the NAT(s) must first either:
a. See UDP (or DCCP) traffic sent from the RTP_Rx (which is on the
'inside' of the NAT) to the BRS (which is on the 'outside' of the
NAT). This traffic has the same transport address as the
subsequent response traffic, or;
b. Be configured to forward certain ports (e.g., using HTML
configuration or UPnP IGD [UPnP-IGD]). Details of this are out
of scope of this document.
For both (a) and (b), the RTP_Rx is responsible for maintaining the
NAT's state if it wants to receive traffic from the BRS on that port.
For (a), the RTP_Rx MUST send UDP traffic to keep the NAT binding
alive, at least every 30 seconds [RFC4787]. While (a) is more like
an automatic/dynamic configuration, (b) is more like a manual/static
configuration.
When the RTP_Rx sends a request (RAMS-R) message to the FT as unicast
feedback in the primary multicast RTP session, and the request is
received by the FT, a new unicast burst RTP session will be
established between the BRS and RTP_Rx.
While the FT and BRS ports on the RS are already signaled via out-of-
band means (e.g., SDP), the RTP_Rx needs to convey the RTP and RTCP
ports it wants to use on its side for the new session. Since there
are two RTP sessions (one multicast and one unicast) involved during
this process and one of them is established upon a feedback message
sent in the other one, this requires an explicit port mapping method.
Applications using RAMS MUST support the method presented in
[I-D.ietf-avt-ports-for-ucast-mcast-rtp] both on the RS and RTP_Rx
side to allow RTP receivers to use their desired ports and to support
RAMS behind NAT devices. The port mapping message exchange needs to
take place whenever the RTP_Rx intends to make use of the RAMS
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protocol for rapidly acquiring a specific multicast RTP session,
prior to joining the associated SSM session.
10. Security Considerations
Applications that are using RAMS make heavy use of the unicast
feedback mechanism described in [RFC5760], the payload format defined
in [RFC4588] and the port mapping solution specified in
[I-D.ietf-avt-ports-for-ucast-mcast-rtp]. Thus, these applications
are subject to the general security considerations discussed in those
documents. In particular, the threats and risks discussed in
[RFC5760] need to be considered carefully. In this section, we give
an overview of the guidelines and suggestions described in these
specifications from a RAMS perspective. We also discuss the security
considerations that explicitly apply to applications using RAMS.
First of all, much of the session description information is
available in the SDP descriptions that are distributed to the media
senders, retransmission servers and RTP receivers. Adequate security
measures are RECOMMENDED to ensure the integrity and authenticity of
the SDP descriptions so that transport addresses of the media
senders, distribution sources, feedback targets as well as other
session-specific information can be protected. See [RFC4566] for
details.
Compared to an RTCP NACK message that triggers one or more
retransmissions, a RAMS Request (RAMS-R) message can trigger a new
burst stream to be sent by the retransmission server. Depending on
the application-specific requirements and conditions existing at the
time of the RAMS-R reception by the retransmission server, the
resulting burst stream can potentially contain a large number of
retransmission packets. Since these packets are sent faster than the
nominal rate, RAMS consumes more resources on the retransmission
servers, RTP receivers and the network. In particular, this can make
denial-of-service (DoS) attacks more intense, and hence, more harmful
than attacks that target ordinary retransmission sessions.
As RAMS messages are sent as RTCP messages, following the suggestions
given in [RFC4588], counter-measures SHOULD be taken to prevent
tampered or spoofed RTCP packets. Tampered RAMS-R messages can
trigger inappropriate burst streams or alter the existing burst
streams in an inappropriate way. For example, if the Max Receive
Bitrate field is altered by a tampered RAMS-R message, the updated
burst can overflow the buffer at the receiver side, or oppositely,
can slow down the burst to the point that it becomes useless.
Tampered RAMS Termination (RAMS-T) messages can terminate valid burst
streams prematurely resulting in gaps in the received RTP packets.
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RAMS Information (RAMS-I) messages contain fields that are critical
for a successful rapid acquisition. Any tampered information in the
RAMS-I message can easily cause an RTP receiver to make wrong
decisions. Consequently, the RAMS operation can fail.
RTCP BYE messages are similar to RAMS-T messages in the sense that
they can be used to stop an existing burst. The CNAME of an RTP
receiver is used to bind the RTCP BYE message to an existing burst.
Thus, one should be careful if the CNAMEs are reasonably easy to
guess and off-path attacks can be performed. Also note that the
CNAMEs might be redistributed to all participants in the multicast
group (as in ASM or the simple feedback model of [RFC5760]).
The retransmission server has to consider if values indicated in a
RAMS-R message are reasonable. For example, a request demanding a
large value of many seconds in the Min RAMS Buffer Fill Requirement
element should, unless special uses cases exist, likely be rejected
since it is likely to be an attempt to prolong a DoS attack on the
retransmission server, RTP receiver and/or the network. The Max
Receive Bitrate could also be set to a very large value to try to get
the retransmission server to cause massive congestion by bursting at
a bitrate that will not be supported by the network. An RTP_Rx
should also consider if the values for the Earliest Multicast Join
Time and Burst Duration indicated by the retransmission server in a
RAMS-I message are reasonable. For example, if the burst packets
stop arriving and the join time is still significantly far into the
future, this could be a sign of a man-in-the-middle attack where the
RAMS-I message has been manipulated by an attacker.
A basic mitigation against DoS attacks introduced by an attacker
injecting tampered RAMS messages is source address validation
[RFC2827]. Also, most of the DoS attacks can be prevented by the
integrity and authenticity checks enabled by Secure RTP (SRTP)
[RFC3711]. However, an attack can still be started by legitimate
endpoints that send several valid RAMS-R messages to a particular
feedback target in a synchronized fashion and in a very short amount
of time. Since a RAMS operation can temporarily consume a large
amount of resources, a series of the RAMS-R messages can temporarily
overload the retransmission server. In these circumstances, the
retransmission server can, for example, reject incoming RAMS requests
until its resources become available again. One means to ameliorate
this threat is to apply a per-endpoint policing mechanism on the
incoming RAMS requests. A reasonable policing mechanism should
consider application-specific requirements and minimize false
negatives.
In addition to the DoS attacks, man-in-the-middle and replay attacks
will also be harmful. RAMS-R messages do not carry any information
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that allows the retransmission server to detect duplication or replay
attacks. Thus, the possibility of a replay attack using a captured
valid RAMS-R message exists unless a mitigation method such as Secure
RTCP (SRTCP) is used. Similarly, RAMS-T messages can be replayed.
The RAMS-I has sequence number that makes replay attacks less likely
but not impossible. Man-in-the-middle attacks that are capable of
capturing, injecting or modifying the RAMS messages can easily
accomplish the attacks described above. Thus, cryptographic
integrity and authentication is the only reliable protection. To
protect the RTCP messages from man-in-the-middle and replay attacks,
the RTP receivers and retransmission server SHOULD perform a DTLS-
SRTP handshake [RFC5764] over the RTCP channel. Because there is no
integrity-protected signaling channel between an RTP receiver and the
retransmission server, the retransmission server MUST maintain a list
of certificates owned by legitimate RTP receivers, or their
certificates MUST be signed by a trusted Certificate Authority. Once
the DTLS-SRTP security is established, non-SRTCP-protected messages
received from a particular RTP receiver are ignored by the
retransmission server. To reduce the impact of DTLS-SRTP overhead
when communicating with different feedback targets on the same
retransmission server, it is RECOMMENDED that RTP receivers and the
retransmission server both support TLS Session Resumption without
Server-side State [RFC5077]. To help scale SRTP to handle many RTP
receivers asking for retransmissions of identical data, implementors
may consider using the same SRTP key for SRTP data sent to the
receivers [I-D.ietf-avt-srtp-ekt] and be aware that such key sharing
allows those receivers to impersonate the sender, so source address
validation remains important.
[RFC4588] RECOMMENDS that the cryptography mechanisms are used for
the retransmission payload format to provide protection against known
plain-text attacks. As discussed in [RFC4588], the retransmission
payload format sets the timestamp field in the RTP header to the
media timestamp of the original packet and this does not compromise
the confidentiality. Furthermore, if cryptography is used to provide
security services on the original stream, then the same services,
with equivalent cryptographic strength, MUST be provided on the
retransmission stream per [RFC4588].
Finally, a retransmission server that has become subverted by an
attacker is almost impossible to protect against as such a server can
perform a large number of different actions to deny service to
receivers.
11. IANA Considerations
The following contact information shall be used for all registrations
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in this document:
Ali Begen
abegen@cisco.com
Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC.
11.1. Registration of SDP Attributes
This document registers a new attribute name in SDP.
SDP Attribute ("att-field"):
Attribute name: rams-updates
Long form: Support for Updated RAMS Request Messages
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: See this document
Reference: [RFCXXXX]
Values: None
11.2. Registration of SDP Attribute Values
This document registers a new value for the 'nack' attribute to be
used with the 'rtcp-fb' attribute in SDP. For more information about
the 'rtcp-fb' attribute, refer to Sections 4.2 and 6.2 of [RFC4585].
Value name: rai
Long name: Rapid Acquisition Indication
Usable with: nack
Reference: [RFCXXXX]
11.3. Registration of FMT Values
Within the RTPFB range, the following format (FMT) value is
registered:
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Name: RAMS
Long name: Rapid Acquisition of Multicast Sessions
Value: 6
Reference: [RFCXXXX]
11.4. SFMT Values for RAMS Messages Registry
This document creates a new sub-registry for the sub-feedback message
type (SFMT) values to be used with the FMT value registered for RAMS
messages. The registry is called the SFMT Values for RAMS Messages
Registry. This registry is to be managed by the IANA according to
the Specification Required policy of [RFC5226].
The length of the SFMT field in the RAMS messages is a single octet,
allowing 256 values. The registry is initialized with the following
entries:
Value Name Reference
----- -------------------------------------------------- -------------
0 Reserved [RFCXXXX]
1 RAMS Request [RFCXXXX]
2 RAMS Information [RFCXXXX]
3 RAMS Termination [RFCXXXX]
4-254 Assignable - Specification Required
255 Reserved [RFCXXXX]
The SFMT values 0 and 255 are reserved for future use.
Any registration for an unassigned SFMT value needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name, address, and email.
o A detailed description of what the new SFMT represents and how it
shall be interpreted.
New RAMS functionality is intended to be introduced by using the
extension mechanism within the existing RAMS message types not by
introducing new message types unless it is absolutely necessary.
11.5. RAMS TLV Space Registry
This document creates a new IANA TLV space registry for the RAMS
extensions. The registry is called the RAMS TLV Space Registry.
This registry is to be managed by the IANA according to the
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Specification Required policy of [RFC5226].
The length of the Type field in the TLV elements is a single octet,
allowing 256 values. The Type values 0 and 255 are reserved for
future use. The Type values between (and including) 128 and 254 are
reserved for private extensions.
The registry is initialized with the following entries:
Type Description Reference
---- -------------------------------------------------- -------------
0 Reserved [RFCXXXX]
1 Requested Media Sender SSRC(s) [RFCXXXX]
2 Min RAMS Buffer Fill Requirement [RFCXXXX]
3 Max RAMS Buffer Fill Requirement [RFCXXXX]
4 Max Receive Bitrate [RFCXXXX]
5 Request for Preamble Only [RFCXXXX]
6 Supported Enterprise Number(s) [RFCXXXX]
7-30 Assignable - Specification Required
31 Media Sender SSRC [RFCXXXX]
32 RTP Seqnum of the First Packet [RFCXXXX]
33 Earliest Multicast Join Time [RFCXXXX]
34 Burst Duration [RFCXXXX]
35 Max Transmit Bitrate [RFCXXXX]
36-60 Assignable - Specification Required
61 Extended RTP Seqnum of First Multicast Packet [RFCXXXX]
62-127 Assignable - Specification Required
128-254 No IANA Maintenance
255 Reserved [RFCXXXX]
Any registration for an unassigned Type value needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name, address, and email.
o A detailed description of what the new TLV element represents and
how it shall be interpreted.
11.6. RAMS Response Code Space Registry
This document creates a new IANA TLV space registry for the RAMS
response codes. The registry is called the RAMS Response Code Space
Registry. This registry is to be managed by the IANA according to
the Specification Required policy of [RFC5226].
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The length of the Response field is two octets, allowing 65536 codes.
However, the response codes have been classified and registered
following an HTTP-style code numbering in this document. New
response codes should be classified following the guidelines below:
Code Level Purpose
---------- ---------------
1xx Informational
2xx Success
3xx Redirection
4xx RTP Receiver (RTP_Rx) Error
5xx Burst/Retransmission Source (BRS) Error
The Response code 65535 is reserved for future use.
The registry is initialized with the following entries:
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Code Description Reference
----- -------------------------------------------------- -------------
0 A private response code is included in the message [RFCXXXX]
100 Parameter update for RAMS session [RFCXXXX]
200 RAMS request has been accepted [RFCXXXX]
201 Unicast burst has been completed [RFCXXXX]
400 Invalid RAMS-R message syntax [RFCXXXX]
401 Invalid min buffer requirement in RAMS-R message [RFCXXXX]
402 Invalid max buffer requirement in RAMS-R message [RFCXXXX]
403 Insufficient max bitrate requirement in RAMS-R
message [RFCXXXX]
404 Invalid RAMS-T message syntax [RFCXXXX]
500 An unspecified BRS internal error has occurred [RFCXXXX]
501 BRS has insufficient bandwidth to start RAMS
session [RFCXXXX]
502 Burst is terminated due to network congestion [RFCXXXX]
503 BRS has insufficient CPU cycles to start RAMS
session [RFCXXXX]
504 RAMS functionality is not available on BRS [RFCXXXX]
505 RAMS functionality is not available for RTP_Rx [RFCXXXX]
506 RAMS functionality is not available for
the requested multicast stream [RFCXXXX]
507 BRS has no valid starting point available for
the requested multicast stream [RFCXXXX]
508 BRS has no reference information available for
the requested multicast stream [RFCXXXX]
509 BRS has no RTP stream matching the requested SSRC [RFCXXXX]
510 RAMS request to acquire the entire session
has been denied [RFCXXXX]
511 Only the preamble information is sent [RFCXXXX]
512 RAMS request has been denied due to a policy [RFCXXXX]
Any registration for an unassigned Response code needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name, address, and email.
o A detailed description of what the new Response code describes and
how it shall be interpreted.
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12. Contributors
Dave Oran, Magnus Westerlund and Colin Perkins have contributed
significantly to this specification by providing text and solutions
to some of the issues raised during the development of this
specification.
13. Acknowledgments
The following individuals have reviewed the earlier versions of this
specification and provided helpful comments: Joerg Ott, Roni Even,
Dan Wing, Tony Faustini, Peilin Yang, Jeff Goldberg, Muriel
Deschanel, Orit Levin, Guy Hirson, Tom Taylor, Xavier Marjou, Ye-Kui
Wang, Zixuan Zou, Ingemar Johansson, Haibin Song, Ning Zong, Jonathan
Lennox, Jose Rey, Sean Sheedy and Keith Drage.
14. References
14.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, August 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
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"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[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.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[I-D.ietf-avt-rtcp-port-for-ssm]
Begen, A., "RTP Control Protocol (RTCP) Port for Source-
Specific Multicast (SSM) Sessions",
draft-ietf-avt-rtcp-port-for-ssm-03 (work in progress),
October 2010.
[I-D.ietf-avt-ports-for-ucast-mcast-rtp]
Begen, A. and B. Steeg, "Port Mapping Between Unicast and
Multicast RTP Sessions",
draft-ietf-avt-ports-for-ucast-mcast-rtp-02 (work in
progress), May 2010.
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[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
14.2. Informative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[I-D.begen-avt-rams-scenarios]
Begen, A., "Considerations for RAMS Scenarios",
draft-begen-avt-rams-scenarios-00 (work in progress),
October 2009.
[I-D.ietf-avt-rtp-cnames]
Begen, A., Perkins, C., and D. Wing, "Guidelines for
Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)", draft-ietf-avt-rtp-cnames-02 (work in
progress), November 2010.
[I-D.ietf-avt-multicast-acq-rtcp-xr]
Begen, A. and E. Friedrich, "Multicast Acquisition Report
Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", draft-ietf-avt-multicast-acq-rtcp-xr-01
(work in progress), May 2010.
[I-D.ietf-avt-ecn-for-rtp]
Westerlund, M., Johansson, I., Perkins, C., and K.
Carlberg, "Explicit Congestion Notification (ECN) for RTP
over UDP", draft-ietf-avt-ecn-for-rtp-03 (work in
progress), October 2010.
[RFC6015] Begen, A., "RTP Payload Format for 1-D Interleaved Parity
Forward Error Correction (FEC)", RFC 6015, October 2010.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
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(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5762] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", RFC 5762, April 2010.
[I-D.ietf-avt-srtp-ekt]
McGrew, D., Andreasen, F., Wing, D., and K. Fischer,
"Encrypted Key Transport for Secure RTP",
draft-ietf-avt-srtp-ekt-01 (work in progress), July 2010.
[UPnP-IGD]
Forum, UPnP., "Universal Plug and Play (UPnP) Internet
Gateway Device (IGD)", November 2001.
[IC2009] Begen, A., Glazebrook, N., and W. VerSteeg, "Reducing
Channel Change Times in IPTV with Real-Time Transport
Protocol (IEEE Internet Computing)", May 2009.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
Authors' Addresses
Bill VerSteeg
Cisco
5030 Sugarloaf Parkway
Lawrenceville, GA 30044
USA
Email: billvs@cisco.com
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
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Tom VanCaenegem
Alcatel-Lucent
Copernicuslaan 50
Antwerpen, 2018
Belgium
Email: Tom.Van_Caenegem@alcatel-lucent.be
Zeev Vax
Microsoft Corporation
1065 La Avenida
Mountain View, CA 94043
USA
Email: zeevvax@microsoft.com
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