RFC 6285
Unicast-Based Rapid Acquisition of Multicast RTP Sessions
Pages: 56
Proposed Standard
Proposed Standard
Part 2 of 3 – Pages 12 to 40
6. Rapid Acquisition of Multicast RTP Sessions (RAMS)
We start this section with an overview of the Rapid Acquisition of Multicast RTP 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 RTP 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
As shown in Figure 2, the main entities involved in rapid acquisition and the message flows are: 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/ | |<~~~~~~~~>| | 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 As defined in [RFC5760], the feedback target (FT) is the entity 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/ 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 this 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/Audio-Visual Profile with Feedback (AVPF)), there are certain rules that apply to scheduling of both of these messages sent to the FT in the primary multicast RTP session; 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 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) 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 .-.-.-.> 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 reordered, 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 set up 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 [RFC6284]. 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
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 match 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
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 to 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.
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 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
following the timing rules of [RFC4585], the RTP_Rx MAY resend
its corrected request.
The BRS determines the identity of the requesting RTP_Rx by
looking at its canonical name identifier (CNAME item in the
source description (SDES)). Thus, the RTP_Rx MUST choose a
method that ensures it uses a unique CNAME identifier. Such
methods are provided in [RFC6222]. 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 meantime, 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 message
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.
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
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 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
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, [MULTICAST-ACQ] 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 with in the RAMS process. When the RTP_Rx detects that the unicast burst is causing severe congestion, it can prefer to send a 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 Explicit Congestion Notification - Congestion Experienced (ECN-CE) markings [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
Datagram Congestion Control Protocol (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.
(g) Preconfigured 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 for the BRS
to choose an appropriate initial bitrate not above the nominal
bitrate and increase 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) through (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 the BRS acts subsequently 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.
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 this section, 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 where RTP_Rx starts receiving a unicast burst but 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-I message is lost or the RAMS-R message is lost and the RTP_Rx gives up, the RTP_Rx MUST still terminate the burst(s) it requested by following the rules described in Section 6.2. In the case where 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 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, and 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
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 as listed on http://www.iana.org. This will ensure the uniqueness of the private extensions and avoid any collision. 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 Extension7.2. 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. 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 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
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 as listed on http://www.iana.org. This TLV
element, if exists, provides a hint 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 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 new information or 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 MUST check every RAMS-I message entirely, regardless of whether or not the MSN value has changed. 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 indicated in this element means
that the RTP_Rx MAY send the SFGMP Join message right away. If
the RTP_Rx does not want to wait until the earliest multicast join
time, it MAY send a RAMS-T message, and after a reasonable amount
of time, it MAY join the SSM session.
If the RAMS request has been accepted, the BRS sends this element
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 element MUST indicate a zero value. 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 element 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 element MAY be
omitted or indicate a zero value.
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
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 [MULTICAST-ACQ] 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. The 5xx-level response codes are also used as status
codes in the Multicast Acquisition Report Block [MULTICAST-ACQ] in
order to collect BRS-based error conditions.
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 that satisfies 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
for the requested multicast stream, the 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
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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