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draft-ietf-avtcore-mprtp.xml
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<rfc category="exp" docName="draft-ietf-avtcore-mprtp-03" ipr='trust200902'>
<!-- category values: std, bcp, info, exp, and historic ipr
values: full3667, noModification3667, noDerivatives3667 you can add the
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<front>
<title abbrev="Multipath RTP">Multipath RTP (MPRTP)</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<author fullname="Varun Singh" initials="V." surname="Singh">
<organization abbrev="callstats.io">Nemu Dialogue Systems Oy</organization>
<address>
<postal>
<street>Runeberginkatu 4c A 4 </street>
<city>Helsinki</city>
<code>00100</code>
<country>Finland</country>
</postal>
<email>[email protected]</email>
<uri>http://www.callstats.io/</uri>
</address>
</author>
<author fullname="Teemu Karkkainen" initials="T" surname="Karkkainen">
<organization>Technical University of Munich</organization>
<address>
<postal>
<street>Faculty of Informatics</street>
<street>Boltzmannstrasse 3</street>
<city>Garching bei Muenchen</city>
<region>DE</region>
<code>85748</code>
<country>Germany</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author initials="J." surname="Ott" fullname="Joerg Ott">
<organization>Technical University of Munich</organization>
<address>
<postal>
<street>Faculty of Informatics</street>
<street>Boltzmannstrasse 3</street>
<city>Garching bei Muenchen</city>
<region>DE</region>
<code>85748</code>
<country>Germany</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author fullname="Saba Ahsan" initials="S" surname="Ahsan">
<organization>Aalto University</organization>
<address>
<postal>
<street>School of Electrical Engineering</street>
<street>Otakaari 5 A</street>
<city>Espoo</city>
<region>FIN</region>
<code>02150</code>
<country>Finland</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author initials="L." surname="Eggert" fullname="Lars Eggert">
<organization abbrev="NetApp"> NetApp </organization>
<address>
<postal>
<street>Sonnenallee 1</street>
<code>85551</code> <city>Kirchheim</city>
<country>Germany</country>
</postal>
<phone>+49 151 12055791</phone>
<email>[email protected]</email>
<uri> http://eggert.org/ </uri>
</address>
</author>
<date />
<!-- <area>RAI</area> <workgroup>AVT Working
Group</workgroup> -->
<area>Internet Engineering Task Force</area>
<workgroup>AVT Core Working Group</workgroup>
<keyword>RTP</keyword>
<keyword>RTCP</keyword>
<keyword>multipath</keyword>
<keyword>streaming</keyword>
<abstract>
<t>The Real-time Transport Protocol (RTP) is used to deliver real-time
content and, along with the RTP Control Protocol (RTCP), forms the
control channel between the sender and receiver. However, RTP and RTCP
assume a single delivery path between the sender and receiver and make
decisions based on the measured characteristics of this single path.
Increasingly, endpoints are becoming multi-homed, which means that they
are connected via multiple Internet paths. Network utilization can be
improved when endpoints use multiple parallel paths for communication.
The resulting increase in reliability and throughput can also enhance
the user experience. This document extends the Real-time Transport
Protocol (RTP) so that a single session can take advantage of the
availability of multiple paths between two endpoints.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Multi-homed endpoints are becoming common in today's Internet, e.g.,
devices that support multiple wireless access technologies such as 3G and
Wireless LAN. This means that there is often more than one network path
available between two endpoints. Transport protocols, such as RTP, have
not been designed to take advantage of the availability of multiple
concurrent paths and therefore cannot benefit from the increased capacity
and reliability that can be achieved by pooling their respective
capacities.</t>
<t>Multipath RTP (MPRTP) is an extension to <xref target="RFC3550">RTP</xref>
which splits a single <xref target="RFC7656">RTP Stream</xref> (At any given
point in time, an RTP stream can have one and only one SSRC) into
multiple subflows that are transmitted over different network interfaces paths
(i.e., 2- out of the 5-tuples are different). In effect, this pools the
resource capacity of multiple paths. Multipath RTCP (MPRTCP) is an extension
to RTCP, it is used along with MPRTP to report sender and receiver
characteristics for each subflow.</t>
<t>Other IETF transport protocols that are capable of using multiple
paths include <xref target="RFC4960">SCTP</xref>, <xref
target="RFC6182">MPTCP</xref> and <xref target="RFC5533">SHIM6</xref>.
However, these protocols are not suitable for real-time
communications.</t>
<section title="Requirements Language">
<t>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 <xref target="RFC2119"
/>.</t>
</section>
<section title="Terminology">
<t><list style="symbols">
<t>Endpoint: corresponds to a single host either initiating or
terminating a <xref target="RFC7656">communication session
</xref>.</t>
<t>Interface: logical or physical component that is capable of
acquiring a unique IP address.</t>
<t>Path: sequence of links between a sender and a receiver.
Typically, defined by a set of source and destination
addresses. In the context of <xref target="RFC7656">RTP
taxonomy</xref> it best matches a media transports.</t>
<!-- VS: confirm the taxonomy -->
<t>Multipath RTP Stream: An RTP Stream that makes use of one or more
paths, i.e., simultaneously makes use of multiple media
transports to send and receive from a particular RTP Stream.
In cases, where a single path is available, an MPRTP Stream will
appear to be an RTP Stream.
</t>
<t>MPRTP Subflow: The RTP packets from a single RTP Stream that are
sent or received over a particular media transport are identified
as a subflow. Therefore, an MPRTP Stream has several subflows,
each subflow is associated to a particular media transport.</t>
<!-- Each subflow has a distinct subflow identifier, and their
own subflow sequence numbers. -->
<t>Subflow Identifier: within a communication session, the subflow
identifier denotes the RTP packets belonging to a RTP Stream sent
over a particular media transport. There are two design choices:
<list style="symbols">
<t>Each media transport at an endpoint chooses a unique
identifier for itself, and the Subflow RTP Streams use the
chosen identifier.</t>
<t>The RTP Stream chooses distinct identifiers for each
Subflow RTP stream </t>
</list>
</t>
<!-- An RTP Stream is split into multiple subflows, each
subflow is then sent over different media transports, i.e.,
flow of RTP packets along a specific path, i.e., a
subset of the packets belonging to an RTP stream. The combination
of all RTP subflows forms the complete RTP stream. Typically, a
subflow would map to a unique path, i.e., each combination of IP
addresses and port pairs (5-tuple, including protocol) is a unique
subflow. -->
</list></t>
</section>
<section title="Use-cases">
<t>
The primary use-case for MPRTP is transporting high bit-rate streaming
multimedia content between endpoints, where at least one is multi-homed.
Such endpoints could be residential IPTV devices that connect to the
Internet through two different Internet service providers (ISPs), or
mobile devices that connect to the Internet through 3G and WLAN
interfaces. By allowing a single RTP Stream to use multiple paths for
transmission, the following gains can be achieved:
<list style="symbols">
<t>Higher quality: Pooling the resource capacity of multiple Internet
paths allows higher bit-rate and higher quality codecs to be used.
From the application perspective, the available bandwidth between the
two endpoints increases.</t>
<t>Load balancing: Transmitting a single RTP stream (or several
RTP streams) over multiple paths reduces the bandwidth usage on
each path, which in turn reduces the impact of the media stream(s)
on other traffic on those paths.</t>
<t>Fault tolerance: When multiple paths are used in conjunction with
redundancy mechanisms (FEC, re-transmissions, etc.), outages on one
path have less impact on the overall perceived quality of the media
stream.</t>
</list>
</t>
<t> A secondary use-case for MPRTP is transporting Voice over IP (VoIP)
calls to a device with multiple interfaces. Again, such an endpoint
could be a mobile device with multiple wireless interfaces. In this
case, little is to be gained from resource pooling, i.e., higher
capacity or load balancing, because a single path should be easily
capable of handling the required load. However, using multiple
concurrent subflows can improve fault tolerance, because traffic can
shift between the subflows when path outages occur. This results in very
fast transport-layer handovers that do not require support from
signaling. </t>
<!--<t>JO: There may be other scenarios such as, High Quality Audio
</t>-->
</section>
</section>
<section title="Goals">
<t>This section outlines the basic goals that multipath RTP aims to
meet. These are broadly classified as Functional goals and Compatibility
goals.</t>
<section title="Functional goals">
<t>Allow an unicast RTP Stream to be split into multiple subflows in
order to be carried over multiple paths. This may prove beneficial in
case of video streaming.
<list style="symbols">
<t>Increased Throughput: Cumulative capacity of the two paths may
meet the requirements of the multimedia session. Therefore, MPRTP
MUST support concurrent use of the multiple paths. <!-- (Note:
should this be a function of bandwidth-delay product) possibility
of streaming future data, i.e. send current data along a low delay
path while future data along a high delay, such that data along
the two paths arrive relatively at the time of playback. --> </t>
<t>Improved Reliability: MPRTP SHOULD be able to send redundant
packets or re-transmit packets along any available path to
increase reliability.</t>
</list>
The protocol SHOULD be able to open new subflows for an existing
multimedia session when new paths appear and MUST be able to close
subflows when paths disappear.</t>
</section>
<section title="Compatibility goals">
<t>Multipath RTP (MPRTP) MUST be backwards compatible; an MPRTP Stream
needs to fall back to be compatible with legacy RTP stacks if MPRTP
support is not successfully negotiated.
<list style="symbols">
<t>Application Compatibility: MPRTP service model MUST be
backwards compatible with existing RTP applications, i.e., an
MPRTP stack MUST be able to work with legacy RTP applications and
not require changes to them. Therefore, the basic RTP APIs MUST
remain unchanged, but an MPRTP stack MAY provide extended APIs so
that the application can configure any additional features
provided by the MPRTP stack.</t>
<t>Network Compatibility: individual MPRTP subflows MUST themselves
be flows of well-formed RTP packets, so that they are able to
traverse NATs and firewalls. This MUST be the case even when
interfaces appear after session initiation.
<xref target="RFC5245">Interactive Connectivity Establishment
(ICE)</xref> MAY be used for discovering new interfaces or
performing connectivity checks.</t>
</list>
</t>
</section>
</section>
<!--
<section title="Comparison to TCP, SCTP, and MPTCP">
Teemu: Can we get rid of this? Especially the TCP stuff seems irrelevant.
<t>TCP is a transport protocol and runs over IP, TCP has a strong
feedback loop provides services such as reliability and congestion
control. RTP is an application layer protocols and runs on top of UDP.
RTP is capable of running over multicast network and has a loose
feedback loop using RTCP. Due to this loose binding RTP/RTCP provides
limited services for Congestion Control, Reliability etc <xref
target="RFC4585"></xref> <xref target="RFC3611"></xref>.</t>
<t>While SCTP supports multihoming and multipath functions, it is
typically used as a failover mechanism and cannot be used to make
concurrent data transfer over multiple interfaces. However, <xref
target="I-D.ietf-mptcp-architecture">MPTCP</xref> describes an extension
to TCP for multipath concurrent data transfer.</t>
<t>(...)</t>
<t>However, MPRTP provides aggregate path information for each subflow
that may be used to adapt to the link characteristics.</t>
</section>
-->
<section title="RTP Topologies">
<t><xref target="RFC5117">RFC 5117</xref> describes a number of scenarios
using mixers and translators in single-party (point-to-point), and
multi-party (point-to-multipoint) scenarios. <xref target="RFC3550">RFC
3550</xref> (Section 2.3 and 7.x) discuss in detail the impact of mixers
and translators on RTP and RTCP packets. MPRTP assumes that if a mixer or
translator exists in the network, then either all of the multiple paths or
none of the multiple paths go via this component. </t>
</section>
<section title="MPRTP Architecture">
<t>In a typical scenario, an RTP Stream uses a single path. In an MPRTP
scenario, an RTP Stream uses multiple subflows, those subflows may
each use a different path. <xref target="fig-mprtp-streaming" /> shows the
difference. </t>
<!-- <section title="Operation overview"> -->
<figure anchor="fig-mprtp-streaming" title="Comparison between traditional RTP
streaming and MPRTP">
<artwork><![CDATA[
+--------------+ Signaling +--------------+
| |------------------------------------>| |
| Client |<------------------------------------| Server |
| | Single RTP Stream | |
+--------------+ +--------------+
+--------------+ Signaling +--------------+
| |------------------------------------>| |
| Client |<------------------------------------| Server |
| |<------------------------------------| |
+--------------+ MPRTP subflows +--------------+
]]></artwork>
</figure>
<!-- </section> -->
<figure anchor="fig-mprtp-archit" title="MPRTP Architecture">
<artwork><![CDATA[
+-----------------------+ +-------------------------------+
| Application | | Application |
+-----------------------+ +-------------------------------+
| | | RTP |
+ RTP + +- - - - - - - -+- - - - - - - -+
| | | MPRTP subflow | MPRTP subflow |
+-----------------------+ +---------------+---------------+
| UDP | | UDP | UDP |
+-----------------------+ +---------------+---------------+
| IP | | IP | IP |
+-----------------------+ +---------------+---------------+
]]></artwork>
</figure>
<t><xref target="fig-mprtp-archit" /> illustrates the differences between
the standard RTP stack and the MPRTP stack. MPRTP receives a normal RTP
Stream from the application and splits it into multiple RTP subflows.
Each subflow is then sent along a different path to the receiver. To the
network, each subflow appears as an independent flow of well-formed
RTP packets. At the receiver, the subflows are combined to recreate the
original RTP Stream. The MPRTP layer performs the following functions:
<list style="symbols">
<t>Path Management: The layer is aware of alternate paths to the other
host, which may, for example, be the peer's multiple interfaces. This
enables the endpoint to transmit differently marked packets along
separate paths.
<!-- detects the host's multiple interfaces and advertises them as
they appear and disappear.--> MPRTP also selects interfaces to send
and receive data. Furthermore, it manages the port and IP address pair
bindings for each subflow.
<!--
Path Management: The layer is aware of alternate paths to the peer,
which may, for example, be the peer's multiple interfaces or routing
labels for an intermediate router to send differently marked packets
along separate paths. (The entire draft so far talked about identifying
paths by IP address/interface. This route label stuff is unclear even for
MPTCP. Suggest to remove it. - Lars)
MPRTP also selects interfaces or marks packets with different
routing labels to send and receive data. Furthermore, it manages
the port and IP address pair bindings for each interface. (Does it?
Isn't the IP stack doing that? - Lars) -->
</t>
<t>Packet Scheduling: the layer splits a single RTP Stream into
multiple subflows and sends them across multiple interfaces
(paths). The splitting MAY BE done using different path
characteristics.</t>
<t>Subflow recombination: the layer creates the original RTP
stream by recombining the independent subflows associated with
the RTP Stream. Therefore, the subflows are eventually presented
as a single RTP stream to the applications.</t>
</list>
</t>
<!--change from ind-02-draft [Christer/MMUSIC]:The text says that, for SIP,
one SHOULD use SCTP or MPTCP instead of UDP or TCP, but there is no
justification. Also, isn't such statement is outside the scope of the
document?
<section title="Relationship of MPRTP with Session Signaling">
<t> Session signaling (e.g., <xref target="RFC3261">SIP</xref>, <xref
target="I-D.ietf-mmusic-rfc2326bis">RTSP</xref>) SHOULD be done over a
failover-capable or multipath-capable transport for e.g., <xref
target="RFC4960">SCTP</xref> or <xref target="RFC6182">MPTCP</xref>
instead of TCP or UDP. </t>
</section>
-->
</section>
<!-- NOTE: find the drawback/pain of using ICE!! -->
<section title="Example Media Flow Diagrams">
<t>There may be many complex technical scenarios for MPRTP, however, this
memo only considers the following two scenarios: 1) a unidirectional media
stream that represents the streaming use-case, and 2) a bidirectional media
stream that represents a conversational use-case.</t>
<section title="Streaming use-case">
<t>In the unidirectional scenario, the receiver (client) initiates a
multimedia session with the sender (server). The receiver or the sender
may have multiple interfaces and both endpoints are MPRTP-capable. <xref
target="fig-mprtp-unidir" /> shows this scenario. In this case, host A has
multiple interfaces. Host B performs connectivity checks on host A's
multiple interfaces. If the interfaces are reachable, then host B streams
multimedia data along multiple paths to host A. Moreover, host B also
Sends RTCP Sender Reports (SR) for the overall session and for each subflow
and host A responds with a normal RTCP Receiver Report (RR) for the overall
session as well as the receiver statistics for each subflow. Host B
distributes the packets across the subflows based on the individually measured
path characteristics.</t>
<t>Alternatively, to reduce media startup time, host B may start streaming
multimedia data to host A's initiating interface and then perform
connectivity checks for the other interfaces. This method of updating a
single path session to a multipath session is called "multipath session
upgrade". </t>
<figure anchor="fig-mprtp-unidir" title="Unidirectional media flow">
<artwork><![CDATA[
Host A Host B
----------------------- ----------
Interface A1 Interface A2 Interface B1
----------------------- ----------
| |
|------------------------------------->| Session setup with
|<-------------------------------------| multiple interfaces
| | |
| | |
| (RTP data B1->A1, B1->A2) |
|<=====================================|
| |<========================|
| | |
Note: there may be more scenarios.
]]></artwork>
</figure>
</section>
<section title="Conversational use-case">
<t>In the bidirectional scenario, multimedia data flows in both
directions. The two hosts exchange their lists of interfaces with each
other and perform connectivity checks. Communication begins after each
host finds suitable address, port pairs. Interfaces that receive data send
back RTCP receiver statistics for that path (based on the 5-tuple). The
hosts balance their multimedia stream across multiple paths based on the
per path reception statistics and its own volume of traffic. <xref
target="fig-mprtp-bidir" /> describes an example of a bidirectional
flow.</t>
<figure anchor="fig-mprtp-bidir" title="Bidirectional flow">
<artwork>
<![CDATA[
Host A Host B
--------------------------- ---------------------------
Interface A1 Interface A2 Interface B1 Interface B2
--------------------------- ---------------------------
| | | |
| | | |Session setup
|----------------------------------->| |with multiple
|<-----------------------------------| |interfaces
| | | |
| | | |
| (RTP data B1<->A1, B2<->A2) | |
|<===================================| |
| |<===================================|
|===================================>| |
| |===================================>|
| | | |
Note: the address pairs may have other permutations,
and they may be symmetric or asymmetric combinations.
]]></artwork>
</figure>
</section>
</section>
<!--
[Christer/MMUSIC]:
ED_5_3-1: I don't think this section belongs to the example call flows.
ED_5_3-2: The text talks about "ahead of time". Ahead of what time?
<section title="Challenges with Multipath Interface Discovery">
<t>For some applications, where the user expects immediate playback, e.g.,
High Definition Media Streaming or IPTV, it may not be possible to perform
connectivity checks within the given time bound. In these cases, connectivity
checks MAY need to be done ahead of time.</t>
<t>[Open Issue: ICE or any other system would have to be aware of the
endpoint's interfaces ahead of time].</t>
</section>
-->
<section title="MPRTP Functional Blocks">
<t>This section describes some of the functional blocks needed for MPRTP. We
then investigate each block and consider available mechanisms in the next
section.
<list style="numbers">
<t>Session Setup: Interfaces may appear or disappear at anytime during
the communication session. To preserve backward compatibility with legacy
applications, a MPRTP session MUST look like a bundle of individual
RTP sessions. A MPRTP session may start using a single path, later start
using multiple paths as and when new interfaces appear or disappear.
However, it is also possible to setup a MPRTP session from the beginning,
if multiple interfaces are available at the start of the multimedia session.
</t>
<t>Expanding RTP: For a MPRTP Stream, each subflow MUST look like an
independent flow of RTP packets, so that individual RTCP messages can be
generated for each subflow. Consequently, MPRTP may split the RTP Stream
into multiple subflows based on path characteristics (e.g. RTT, fractional
losses, receiver rate, bandwidth-delay product, etc.), i.e.,
dynamically adjusts the load on each subflow.</t>
<t>Adding Interfaces: Interfaces on the host need to be regularly
discovered and advertised. This can be done when the MPRTP session is set up
and/or during the communication session. Discovering interfaces is
outside the scope of this document.
<!--When discovering and receiving new interfaces, the
MPRTP layer needs to select address and port pairs.--></t>
<!--Ralf -03: An endpoint must be capable of advertising changes in its set of
usable interfaces, which may be the result of an interface appearing or
disappearing. -->
<t>Expanding RTCP: MPRTP MUST provide per path RTCP reports for
monitoring the quality of the path, for load balancing, or for
congestion control, etc. To maintain backward compatibility with
legacy applications, the receiver MUST also send aggregate RTCP
reports along with the per-path reports.</t>
<t>Maintenance and Failure Handling: In a multi-homed endpoint
interfaces may appear and disappear. If this occurs at the sender, it
has to re-adjust the load on the available paths. On the other hand,
if this occurs at the receiver, then the multimedia data transmitted
by the sender to those interfaces is lost. This data may be
re-transmitted along a different path i.e., to a different interface
on the receiver. Furthermore, the endpoint has to either explicitly
signal the disappearance of an interface, or the other endpoint has to
detect it (by lack of media packets, RTCP feedback, or keep-alive
packets).</t>
<t>Teardown: The MPRTP layer releases the occupied ports on the
interfaces.</t>
</list>
</t>
</section>
<section title="Available Mechanisms within the Functional Blocks">
<t>This section discusses some of the possible alternatives for each
functional block mentioned in the previous section.</t>
<section title="Session Setup">
<t>MPRTP session can be set up in many possible ways e.g., during
handshake, or upgraded mid-session. The capability exchange may be
done using out-of-band signaling (e.g., <xref target="RFC3264"
>Session Description Protocol (SDP)</xref> in <xref
target="RFC3261">Session Initiation Protocol (SIP)</xref>, <xref
target="I-D.ietf-mmusic-rfc2326bis">Real-Time Streaming Protocol
(RTSP)</xref>) or in-band signaling (e.g., RTP/RTCP header extension,
Session Traversal Utilities for NAT (STUN) messages).</t>
<t>[Editor's Note: , with continuous ICE Nomination.]</t>
<!-- <t><cref anchor="note-sip-mp" source="MMUSIC Review">Using SIP over
SCTP, MPTCP instead of UDP/TCP are out of scope of the
document.</cref></t>
-->
<section title="Connectivity Checks">
<t>The endpoint SHOULD be capable of performing connectivity
checks (e.g., using <xref target="RFC5245">ICE</xref>). If the IP
addresses of the endpoints are reachable (e.g., globally
addressable, same network etc) then connectivity checks are not
needed.
<!--If the connectivity checks are successful, then
multimedia data can flow between the new address, port
pairs.--></t>
</section>
<section title="Adding New or Updating Interfaces">
<!-- <t> When interfaces appear and disappear mid-session, ICE
<xref target="RFC5245" /> may be used for discovering interfaces
and performing connectivity checks. However, MPRTP may require a
capability re-negotiation (e.g. using SDP) to include all these
new interfaces. This method is referred to as out-of-band
multipath advertisement.</t>
<t>Alternatively, when new interfaces appear, the interface
advertisements may be done in-band using RTP/RTCP extensions. The
endpoints perform connectivity checks (see <xref
target="fig-mprtp-new-subflow" /> for more details).</t> -->
<t> Interfaces can appear and disappear during a MPRTP session, the
endpoint MUST be capable of advertising the changes in its set of
usable interfaces. Additionally, the application or OS may define
a policy on when and/or what interfaces are usable. However, MPRTP
requires a method to advertise or notify the other endpoint about
the updated set of usable interfaces.</t>
<!--<t>Interface Advertisements can be done by sending an updated
offer using SDP (see <xref
target="mprtp-sdp-ice-mprtp-interface-attribute" />) or in-band using
RTP/RTCP extensions (see <xref target="sec-mprtp-pkt-format-rtcp-ia"
/>).</t> -->
</section>
<section title="In-band vs. Out-of-band Signaling">
<t> MPRTP nodes will generally use a signaling protocol to establish
their MPRTP session. With the existence of such a signaling
relationship, two alternatives become available to exchange
information about the available interfaces on each side for extending
RTP sessions to MPRTP and for modifying MPRTP sessions: in-band and
out-of-band signaling. </t>
<t> In-band signaling refers to using mechanisms of RTP/RTCP itself to
communicate interface addresses, e.g., a dedicated RTCP extensions
along the lines of the one defined to communicate information about
the feedback target for RTP over SSM <xref target="RFC5760" />.
In-band signaling does not rely on the availability of a separate
signaling connection and the information flows along the same path as
the media streams, thus minimizing dependencies. Moreover, if the
media channel is secured (e.g., by means of SRTP), the signaling is
implicitly protected as well if SRTCP encryption and authentication
are chosen. In-band signaling is also expected to take a direct path
to the peer, avoiding any signaling overlay-induced indirections and
associated processing overheads in signaling elements -- avoiding such
may be especially worthwhile if frequent updates may occur as in the
case of mobile users. Finally, RTCP is usually sent sufficiently
frequently (in point-to-point settings) to provide enough
opportunities for transmission and (in case of loss) retransmission of
the corresponding RTCP packets. </t>
<t> Examples for in-band signaling include RTCP extensions as noted
above or suitable extensions to <xref
target="I-D.wing-mmusic-ice-mobility">STUN</xref>. </t>
<t> Out-of-band signaling refers to using a separate signaling
connection (via SIP, RTSP, or HTTP) to exchange interface information,
e.g., expressed in SDP. Clear benefits are that signaling occurs at
setup time anyway and that experience and SDP syntax (and procedures)
are available that can be re-used or easily adapted to provide the
necessary capabilities. In contrast to RTCP, SDP offers a reliable
communication channel so that no separate retransmissions logic is
necessary. In SDP, especially when combined with ICE, connectivity
check mechanisms including sophisticated rules are readily available.
While SDP is not inherently protected, suitable security may need to
be applied anyway to the basic session setup. </t>
<t> Examples for out-of-band signaling are dedicated extensions to
SDP; those may be combined with ICE. </t>
<t> Both types of mechanisms have their pros and cons for middleboxes.
With in-band signaling, control packets take the same path as the
media packets and they can be directly inspected by middleboxes so
that the elements operating on the signaling channel do not need to
understand new SDP. With out-of-band signaling, only the middleboxes
processing the signaling need to be modified and those on the data
forwarding path can remain untouched. </t>
<t> Overall, it may appear sensible to provide a combination of both
mechanisms: out-of-band signaling for session setup and initial
interface negotiation and in-band signaling to deal with frequent
changes in interface state (and for the potential case, albeit rather
theoretical case of MPRTP communication without a signaling channel).
</t>
<t> In its present version, this document explores both options to
provide a broad understanding of how the corresponding mechanisms
would look like. </t>
</section>
</section>
<section title="Expanding RTP">
<t><xref target="RFC3550">RTCP</xref> is generated per media session.
However, with MPRTP, the media sender spreads the RTP load across
several interfaces. The media sender SHOULD make the path selection,
load balancing and fault tolerance decisions based on the
characteristics of each path. Therefore, apart from normal RTP
sequence numbers defined in <xref target="RFC3550" />, the MPRTP
sender MUST add the following within the SSRC scope, subflow-specific
sequence numbers to help calculate fractional losses, jitter, RTT,
playout time, etc., for each subflow, and a subflow identifier to
associate the characteristics with a path. The RTP header extension
for MPRTP is shown in <xref target="sec-mprtp-pkt-format-hdrext" />. </t>
</section>
<section title="Expanding RTCP">
<t> To provide accurate per path information an MPRTP endpoint MUST
send (SR/RR) report for each unique subflow along with the overall
session RTCP report. Therefore, the additional subflow reporting
affects the RTCP bandwidth and the RTCP reporting interval. RTCP
report scheduling for each subflow may cause a problem for RTCP
recombination and reconstruction in cases when 1) RTCP for a subflow
is lost, and 2) RTCP for a subflow arrives later than the other
subflows. (There may be other cases as well.)</t>
<t>The sender distributes the media across different paths using the
per path RTCP reports. However, this document doesn't cover algorithms
for congestion control or load balancing.</t>
</section>
<!--
[Christer/MMUSIC]:
I don't think it's within the scope of this draft to make any recommendations
regarding SIP/SDP transport.
<section title="Checking and Failure Handling">
<cref anchor="Note 1" source="Editor">If the original interface that
setup the session disappears then does the session signaling failover
to another interface? Can we recommend that SIP/RTSP be run over
MPTCP, SCTP].</cref>
</section>-->
<section title="Failure Handling and Teardown">
<t>An MPRTP endpoint MUST keep alive subflows that have been
negotiated but no media is sent on them. Moreover, using the
information in the subflow reports, a sender can monitor an active
subflow for failure (errors, unreachability, congestion) and decide
not to use (make the active subflow passive), or teardown the
subflow.</t>
<t>If an interface disappears, the endpoint MUST send an updated
interface advertisement without the interface and release the
associated subflows.</t>
</section>
</section>
<section title="MPRTP Protocol" anchor="sec-mprtp-proto">
<!-- [Comment MMUSIC/Christer: keep generic].
<t>To enable a quick start of a multimedia session, a multipath session MUST
be upgraded from a single path session. Therefore, no explicit changes are
needed in multimedia session setup and the session can be setup as before.</t>
-->
<!-- <section title="Connection Initiation">
<t>The multipath discovery and transmission begins after establishing
a single path RTP session.</t>
</section> -->
<figure anchor="fig-mprtp-new-subflow" title="MPRTP New Interface">
<artwork><![CDATA[
Host A Host B
----------------------- -----------------------
Interface A1 Interface A2 Interface B1 Interface B2
----------------------- -----------------------
| | | |
| | (1) | |
|--------------------------------------->| |
|<---------------------------------------| |
| | (2) | |
|<=======================================| |
|<=======================================| (3) |
| | (4) | |
|<- - - - - - - - - - - - - - - - - - - -| |
|<- - - - - - - - - - - - - - - - - - - -| |
|<- - - - - - - - - - - - - - - - - - - -| |
| | (5) | |
|- - - - - - - - - - - - - - - - - - - ->| |
|<=======================================| (6) |
|<=======================================| |
| |<========================================|
|<=======================================| |
| |<========================================|
Key:
| Interface
---> Signaling Protocol
<=== RTP Packets
- -> RTCP Packet
]]></artwork>
</figure>
<section title="Overview" anchor="sec-mprtp-proto-overview">
<!--due to changes in connections. new interfaces can appear, old ones
can disappear.-->
<t>
The bullet points explain the different steps shown in <xref
target="fig-mprtp-new-subflow" /> for upgrading a single path
multimedia session to multipath session.
<list style="empty">
<t>(1) The first two interactions between the hosts represents the
establishment of a normal RTP session. This may performed e.g.
using SIP or RTSP.</t>
<t>(2) When the RTP session has been established, host B streams
media using its interface B1 to host A at interface A1.</t>
<t>(3) Host B supports sending media using MPRTP and becomes aware
of an additional interface B2.</t>
<t>(4) Host B advertises the multiple interface addresses.</t>
<t>(5) Host A supports receiving media using MPRTP and becomes
aware of an additional interface A2.</t>
<t>Side note, even if an MPRTP-capable host has only one
interface, it MUST respond to the advertisement with its single
interface.</t>
<t>(6) Each host receives information about the additional
interfaces and the appropriate endpoints starts to stream the
multimedia content using the additional paths.</t>
<t>If needed, each endpoint will need to independently perform
connectivity checks (not shown in figure) and ascertain
reachability before using the paths.</t>
</list> </t>
<section title="Gather or Discovering Candidates"
anchor="sec-mprtp-gather-candidates">
<t>The endpoint periodically polls the operating system or is
notified when an additional interface appears. If the endpoint wants
to use the additional interface for MPRTP it MUST advertise it to
the other peers. The endpoint may also use <xref
target="RFC5245">ICE</xref> to gather additional candidates.</t>
</section>
<section title="NAT Traversal" anchor="sec-mprtp-nat-traversal">
<t>After gathering their interface candidates, the endpoints decide
internally if they wish to perform connectivity checks.</t>
<t><cref anchor="note-iceornot" source="Editor">Legacy applications
do not require ICE for session establishment, therefore, MPRTP
should not require it as well.</cref></t>
<t>If the endpoint chooses to perform connectivity checks then it
MUST first advertise the gathered candidates as ICE candidates in
SDP during session setup and let ICE perform the connectivity
checks. As soon as a sufficient number of connectivity checks
succeed, the endpoint can use the successful candidates to advertise
its MPRTP interface candidates.</t>
<t>Alternatively, if the endpoint supports <xref
target="I-D.wing-mmusic-ice-mobility">mobility extensions for
ICE</xref>, it can let the ICE agent gather and perform the
connectivity checks. When the connectivity checks succeed, the ICE
agent should notify the MPRTP layer of these new paths (5-tuples),
these new paths are then used by MPRTP to send media packets
depending on the scheduling algorithm.
</t>
<t>
If an endpoint supports <xref
target="I-D.reddy-mmusic-ice-best-interface-pcp">Interface selection
via PCP Flow Extension</xref>, it will receive notifications when new
interfaces become available and additionally when the link quality of a
currently available interface changes. The application can advertise
and perform connectivity checks with the new interface and in the
case of changes in link quality pass the information to the scheduling
algorithm for better application performance.
</t>
<t>[Editors note: The interaction between the RTP agent and ICE
agent is needed for implementing a scheduling algorithm or
congestion control. See details of a scheduling algorithm in
<xref target="ACM-MPRTP" />.]</t>
</section>
<section title="Choosing between In-band (in RTCP) and Out-of-band
(in SDP) Interface Advertisement" anchor="sec-mprtp-subflow-advert">
<t>If there is no media flowing at the moment and the application
wants to use the interfaces from the start of the communication
session, it should advertise them in SDP (See <xref
target="I-D.singh-mmusic-mprtp-sdp-extension" />). Alternatively,
the endpoint can setup the session as a single path communication
session and upgrade the session to multipath by advertising the session
in-band (See <xref target="sec-mprtp-rtcp-subflow-advert" /> or <xref
target="I-D.wing-mmusic-ice-mobility" />).
Moreover, if an interface appears and disappears, the endpoint
SHOULD prefer to advertise it in-band because the endpoint would
not have to wait for a response from the other endpoint before
starting to use the interface. However, if there is a conflict
between an in-band and out-of-band advertisement, i.e., the
endpoint receives an in-band advertisement while it has a pending