Internet DRAFT - draft-ietf-avtcore-mprtp
draft-ietf-avtcore-mprtp
AVT Core Working Group V. Singh
Internet-Draft callstats.io
Intended status: Experimental T. Karkkainen
Expires: September 22, 2016 J. Ott
Technical University of Munich
S. Ahsan
Aalto University
L. Eggert
NetApp
March 21, 2016
Multipath RTP (MPRTP)
draft-ietf-avtcore-mprtp-02
Abstract
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 22, 2016.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 40
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Use-cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Functional goals . . . . . . . . . . . . . . . . . . . . 5
2.2. Compatibility goals . . . . . . . . . . . . . . . . . . . 6
3. RTP Topologies . . . . . . . . . . . . . . . . . . . . . . . 6
4. MPRTP Architecture . . . . . . . . . . . . . . . . . . . . . 6
5. Example Media Flow Diagrams . . . . . . . . . . . . . . . . . 8
5.1. Streaming use-case . . . . . . . . . . . . . . . . . . . 8
5.2. Conversational use-case . . . . . . . . . . . . . . . . . 9
6. MPRTP Functional Blocks . . . . . . . . . . . . . . . . . . . 10
7. Available Mechanisms within the Functional Blocks . . . . . . 11
7.1. Session Setup . . . . . . . . . . . . . . . . . . . . . . 11
7.1.1. Connectivity Checks . . . . . . . . . . . . . . . . . 11
7.1.2. Adding New or Updating Interfaces . . . . . . . . . . 11
7.1.3. In-band vs. Out-of-band Signaling . . . . . . . . . . 12
7.2. Expanding RTP . . . . . . . . . . . . . . . . . . . . . . 13
7.3. Expanding RTCP . . . . . . . . . . . . . . . . . . . . . 13
7.4. Failure Handling and Teardown . . . . . . . . . . . . . . 14
8. MPRTP Protocol . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1.1. Gather or Discovering Candidates . . . . . . . . . . 15
8.1.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . 15
8.1.3. Choosing between In-band (in RTCP) and Out-of-band
(in SDP) Interface Advertisement . . . . . . . . . . 16
8.1.4. In-band Interface Advertisement . . . . . . . . . . . 16
8.1.5. Subflow ID Assignment . . . . . . . . . . . . . . . . 17
8.1.6. Active and Passive Subflows . . . . . . . . . . . . . 17
8.2. RTP Transmission . . . . . . . . . . . . . . . . . . . . 17
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8.3. Playout Considerations at the Receiver . . . . . . . . . 18
8.4. Subflow-specific RTCP Statistics and RTCP Aggregation . . 18
8.5. RTCP Transmission . . . . . . . . . . . . . . . . . . . . 19
9. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. RTP Header Extension for MPRTP . . . . . . . . . . . . . 19
9.1.1. MPRTP RTP Extension for a Subflow . . . . . . . . . . 20
9.2. RTCP Extension for MPRTP (MPRTCP) . . . . . . . . . . . . 21
9.2.1. MPRTCP Extension for Subflow Reporting . . . . . . . 23
9.2.1.1. MPRTCP for Subflow-specific SR, RR and XR . . . . 25
9.3. MPRTCP Extension for Interface advertisement . . . . . . 27
10. RTCP Timing reconsiderations for MPRTCP . . . . . . . . . . . 28
11. SDP Considerations . . . . . . . . . . . . . . . . . . . . . 29
11.1. Signaling MPRTP Header Extension in SDP . . . . . . . . 29
11.2. Signaling MPRTP capability in SDP . . . . . . . . . . . 29
11.3. MPRTP with ICE . . . . . . . . . . . . . . . . . . . . . 30
11.4. Increased Throughput . . . . . . . . . . . . . . . . . . 30
11.5. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . 31
11.5.1. In-band Signaling Example . . . . . . . . . . . . . 31
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
12.1. MPRTP Header Extension . . . . . . . . . . . . . . . . . 32
12.2. MPRTCP Packet Type . . . . . . . . . . . . . . . . . . . 32
12.3. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 33
12.3.1. "mprtp" attribute . . . . . . . . . . . . . . . . . 33
13. Security Considerations . . . . . . . . . . . . . . . . . . . 34
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
15.1. Normative References . . . . . . . . . . . . . . . . . . 35
15.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Interoperating with Legacy Applications . . . . . . 37
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 38
B.1. Changes in draft-ietf-avtcore-mprtp-01, and -02 . . . . . 38
B.2. Changes in draft-ietf-avtcore-mprtp-00 . . . . . . . . . 38
B.3. Changes in draft-singh-avtcore-mprtp-10 . . . . . . . . . 38
B.4. Changes in draft-singh-avtcore-mprtp-09 . . . . . . . . . 38
B.5. Changes in draft-singh-avtcore-mprtp-08 . . . . . . . . . 38
B.6. Changes in draft-singh-avtcore-mprtp-06 and -07 . . . . . 38
B.7. Changes in draft-singh-avtcore-mprtp-05 . . . . . . . . . 38
B.8. Changes in draft-singh-avtcore-mprtp-04 . . . . . . . . . 39
B.9. Changes in draft-singh-avtcore-mprtp-03 . . . . . . . . . 39
B.10. Changes in draft-singh-avtcore-mprtp-02 . . . . . . . . . 39
B.11. Changes in draft-singh-avtcore-mprtp-01 . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
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
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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.
Multipath RTP (MPRTP) is an OPTIONAL extension to RTP [RFC3550] that
allows splitting a single RTP stream into multiple subflows that are
transmitted over different paths. 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 per-path sender and
receiver characteristics.
Other IETF transport protocols that are capable of using multiple
paths include SCTP [RFC4960], MPTCP [RFC6182] and SHIM6 [RFC5533].
However, these protocols are not suitable for real-time
communications.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology
o Endpoint: host either initiating or terminating an RTP flow.
o Interface: logical or physical component that is capable of
acquiring a unique IP address.
o Path: sequence of links between a sender and a receiver.
Typically, defined by a set of source and destination addresses.
o Subflow: 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.
1.3. Use-cases
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
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through 3G and WLAN interfaces. By allowing RTP to use multiple
paths for transmission, the following gains can be achieved:
o 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.
o Load balancing: Transmitting an RTP stream over multiple paths
reduces the bandwidth usage on a single path, which in turn
reduces the impact of the media stream on other traffic on that
path.
o 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
stream.
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.
2. Goals
This section outlines the basic goals that multipath RTP aims to
meet. These are broadly classified as Functional goals and
Compatibility goals.
2.1. Functional goals
Allow unicast RTP session to be split into multiple subflows in order
to be carried over multiple paths. This may prove beneficial in case
of video streaming.
o 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.
o Improved Reliability: MPRTP SHOULD be able to send redundant
packets or re-transmit packets along any available path to
increase reliability.
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The protocol SHOULD be able to open new subflows for an existing
session when new paths appear and MUST be able to close subflows when
paths disappear.
2.2. Compatibility goals
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.
o 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.
o Network Compatibility: individual RTP subflows MUST themselves be
well-formed RTP flows, so that they are able to traverse NATs and
firewalls. This MUST be the case even when interfaces appear
after session initiation. Interactive Connectivity Establishment
(ICE) [RFC5245] MAY be used for discovering new interfaces or
performing connectivity checks.
3. RTP Topologies
RFC 5117 [RFC5117] describes a number of scenarios using mixers and
translators in single-party (point-to-point), and multi-party (point-
to-multipoint) scenarios. RFC 3550 [RFC3550] (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.
4. MPRTP Architecture
In a typical scenario, an RTP session uses a single path. In an
MPRTP scenario, an RTP session uses multiple subflows that each use a
different path. Figure 1 shows the difference.
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+--------------+ Signaling +--------------+
| |------------------------------------>| |
| Client |<------------------------------------| Server |
| | Single RTP flow | |
+--------------+ +--------------+
+--------------+ Signaling +--------------+
| |------------------------------------>| |
| Client |<------------------------------------| Server |
| |<------------------------------------| |
+--------------+ MPRTP subflows +--------------+
Figure 1: Comparison between traditional RTP streaming and MPRTP
+-----------------------+ +-------------------------------+
| Application | | Application |
+-----------------------+ +-------------------------------+
| | | MPRTP |
+ RTP + +- - - - - - - -+- - - - - - - -+
| | | RTP subflow | RTP subflow |
+-----------------------+ +---------------+---------------+
| UDP | | UDP | UDP |
+-----------------------+ +---------------+---------------+
| IP | | IP | IP |
+-----------------------+ +---------------+---------------+
Figure 2: MPRTP Architecture
Figure 2 illustrates the differences between the standard RTP stack
and the MPRTP stack. MPRTP receives a normal RTP session 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, well-formed RTP flow. At the
receiver, the subflows are combined to recreate the original RTP
session. The MPRTP layer performs the following functions:
o 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. MPRTP also selects
interfaces to send and receive data. Furthermore, it manages the
port and IP address pair bindings for each subflow.
o Packet Scheduling: the layer splits a single RTP flow into
multiple subflows and sends them across multiple interfaces
(paths). The splitting MAY BE done using different path
characteristics.
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o Subflow recombination: the layer creates the original stream by
recombining the independent subflows. Therefore, the multipath
subflows appear as a single RTP stream to applications.
5. Example Media Flow Diagrams
There may be many complex technical scenarios for MPRTP, however,
this memo only considers the following two scenarios: 1) a
unidirectional media flow that represents the streaming use-case, and
2) a bidirectional media flow that represents a conversational use-
case.
5.1. Streaming use-case
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. Figure 3 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 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.
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".
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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.
Figure 3: Unidirectional media flow
5.2. Conversational use-case
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. Figure 4 describes an example of a bidirectional
flow.
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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.
Figure 4: Bidirectional flow
6. MPRTP Functional Blocks
This section describes some of the functional blocks needed for
MPRTP. We then investigate each block and consider available
mechanisms in the next section.
1. Session Setup: Interfaces may appear or disappear at anytime
during the session. To preserve backward compatibility with
legacy applications, a multipath session MUST look like a bundle
of individual RTP sessions. A multipath session may be upgraded
from a typical single path session, as and when new interfaces
appear or disappear. However, it is also possible to setup a
multipath session from the beginning, if the interfaces are
available at the start of the multimedia session.
2. Expanding RTP: For a multipath session, each subflow MUST look
like an independent RTP flow, so that individual RTCP messages
can be generated per subflow. Furthermore, MPRTP splits the
single multimedia stream into multiple subflows based on path
characteristics (e.g. RTT, loss-rate, receiver rate, bandwidth-
delay product etc.) and dynamically adjusts the load on each
link.
3. Adding Interfaces: Interfaces on the host need to be regularly
discovered and advertised. This can be done at session setup
and/or during the session. Discovering interfaces is outside the
scope of this document.
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4. 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.
5. 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 links. 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).
6. Teardown: The MPRTP layer releases the occupied ports on the
interfaces.
7. Available Mechanisms within the Functional Blocks
This section discusses some of the possible alternatives for each
functional block mentioned in the previous section.
7.1. Session Setup
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., Session Description Protocol
(SDP) [RFC3264] in Session Initiation Protocol (SIP) [RFC3261], Real-
Time Streaming Protocol (RTSP) [I-D.ietf-mmusic-rfc2326bis]) or in-
band signaling (e.g., RTP/RTCP header extension, Session Traversal
Utilities for NAT (STUN) messages).
7.1.1. Connectivity Checks
The endpoint SHOULD be capable of performing connectivity checks
(e.g., using ICE [RFC5245]). If the IP addresses of the endpoints
are reachable (e.g., globally addressable, same network etc) then
connectivity checks are not needed.
7.1.2. Adding New or Updating Interfaces
Interfaces can appear and disappear during a 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
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method to advertise or notify the other endpoint about the updated
set of usable interfaces.
7.1.3. In-band vs. Out-of-band Signaling
MTRTP 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.
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 [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.
Examples for in-band signaling include RTCP extensions as noted above
or suitable extensions to STUN [I-D.wing-mmusic-ice-mobility].
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.
Examples for out-of-band signaling are dedicated extensions to SDP;
those may be combined with ICE.
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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.
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).
In its present version, this document explores both options to
provide a broad understanding of how the corresponding mechanisms
would look like.
7.2. Expanding RTP
RTCP [RFC3550] 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
[RFC3550], the MPRTP sender MUST add subflow-specific sequence
numbers to help calculate fractional losses, jitter, RTT, playout
time, etc., for each path, and a subflow identifier to associate the
characteristics with a path. The RTP header extension for MPRTP is
shown in Section 9.1.
7.3. Expanding RTCP
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.)
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.
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7.4. Failure Handling and Teardown
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.
If an interface disappears, the endpoint MUST send an updated
interface advertisement without the interface and release the the
associated subflows.
8. MPRTP Protocol
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
Figure 5: MPRTP New Interface
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8.1. Overview
The bullet points explain the different steps shown in Figure 5 for
upgrading a single path multimedia session to multipath session.
(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.
(2) When the RTP session has been established, host B streams
media using its interface B1 to host A at interface A1.
(3) Host B supports sending media using MPRTP and becomes aware of
an additional interface B2.
(4) Host B advertises the multiple interface addresses.
(5) Host A supports receiving media using MPRTP and becomes aware
of an additional interface A2.
Side note, even if an MPRTP-capable host has only one interface,
it MUST respond to the advertisement with its single interface.
(6) Each host receives information about the additional interfaces
and the appropriate endpoints starts to stream the multimedia
content using the additional paths.
If needed, each endpoint will need to independently perform
connectivity checks (not shown in figure) and ascertain
reachability before using the paths.
8.1.1. Gather or Discovering Candidates
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 ICE [RFC5245] to gather additional
candidates.
8.1.2. NAT Traversal
After gathering their interface candidates, the endpoints decide
internally if they wish to perform connectivity checks.
[note-iceornot]
If the endpoint chooses to perform connectivity checks then it MUST
first advertise the gathered candidates as ICE candidates in SDP
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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.
Alternatively, if the endpoint supports mobility extensions for ICE
[I-D.wing-mmusic-ice-mobility], 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.
If an endpoint supports Interface selection via PCP Flow Extension
[I-D.reddy-mmusic-ice-best-interface-pcp], 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.
[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 [ACM-MPRTP].]
8.1.3. Choosing between In-band (in RTCP) and Out-of-band (in SDP)
Interface Advertisement
If there is no media flowing at the moment and the application wants
to use the interfaces from the start of the session, it should
advertise them in SDP (See [I-D.singh-mmusic-mprtp-sdp-extension]).
Alternatively, the endpoint can setup the session as a single path
media session and upgrade the session to multipath by advertising the
session in-band (See Section 8.1.4 or
[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 out-of-band advertisement, or vice versa then the session is
setup using out-of-band mechanisms.
8.1.4. In-band Interface Advertisement
To advertise the multiple interfaces in RTCP, an MPRTP-capable
endpoint MUST add the MPRTP Interface Advertisement defined in
Figure 13 with the RTCP Sender Report (SR). Each unique address is
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encapsulated in an Interface Advertisement block and contains the IP
address, RTP and RTCP port addresses. The Interface Advertisement
blocks are ordered based on a decreasing priority level. On
receiving the MPRTP Interface Advertisement, an MPRTP-capable
receiver MUST respond with the set of interfaces (subset or all
available) it wants to use.
If the sender and receiver have only one interface, then the
endpoints MUST indicate the negotiated single path IP, RTP port and
RTCP port addresses.
8.1.5. Subflow ID Assignment
After interface advertisements have been exchanged, the endpoint MUST
associate a Subflow ID to each unique subflow. Each combination of
sender and receiver IP addresses and port pairs (5-tuple) is a unique
subflow. If the connectivity checks have been performed then the
endpoint MUST only use the subflows for which the connectivity checks
have succeeded.
8.1.6. Active and Passive Subflows
To send and receive data an endpoint MAY use any number of subflows
from the set of available subflows. The subflows that carry media
data are called active subflows, while those subflows that don't send
any media packets (fallback paths) are called passive subflows.
An endpoint MUST multiplex the subflow specific RTP and RTCP packets
on the same port to keep the NAT bindings alive. This is inline with
the recommendation made in RFC6263[RFC6263]. Moreover, if an
endpoint uses ICE, multiplexing RTP and RTCP reduces the number of
components from 2 to 1 per media stream. If no MPRTP or MPRTCP
packets are received on a particular subflow at a receiver, the
receiver SHOULD remove the subflow from active and passive lists and
not send any MPRTCP reports for that subflow. It may keep the
bindings in a dead-pool, so that if the 5-tuple or subflow reappears,
it can quickly reallocate it based on past history.
8.2. RTP Transmission
If both endpoints are MPRTP-capable and if they want to use their
multiple interfaces for sending the media stream then they MUST use
the MPRTP header extensions. They MAY use normal RTP with legacy
endpoints (see Appendix A).
An MPRTP endpoint sends RTP packets with an MPRTP extension that maps
the media packet to a specific subflow (see Figure 8). The MPRTP
layer SHOULD associate an RTP packet with a subflow based on a
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scheduling strategy. The scheduling strategy may either choose to
augment the paths to create higher throughput or use the alternate
paths for enhancing resilience or error-repair. Due to the changes
in path characteristics, the endpoint should be able change its
scheduling strategy during an ongoing session. The MPRTP sender MUST
also populate the subflow specific fields described in the MPRTP
extension header (see Section 9.1.1).
[ACM-MPRTP] describes and evaluates a scheduling algorithm for video
over multiple interfaces. The video is encoded at a single target
bit rate and is evaluated in various network scenarios, as discussed
in [I-D.ietf-rmcat-eval-criteria].
8.3. Playout Considerations at the Receiver
A media receiver, irrespective of MPRTP support or not, should be
able to playback the media stream because the received RTP packets
are compliant to [RFC3550], i.e., a non-MPRTP receiver will ignore
the MPRTP header and still be able to playback the RTP packets.
However, the variation of jitter and loss per path may affect proper
playout. The receiver can compensate for the jitter by modifying the
playout delay (i.e., by calculating skew across all paths) of the
received RTP packets. For example, an adaptive playout algorithm is
discussed in [ACM-MPRTP].
8.4. Subflow-specific RTCP Statistics and RTCP Aggregation
Aggregate RTCP provides the overall media statistics and follows the
normal RTCP defined in RFC3550 [RFC3550]. However, subflow specific
RTCP provides the per path media statistics because the aggregate
RTCP report may not provide sufficient per path information to an
MPRTP scheduler. Specifically, the scheduler should be aware of each
path's RTT and loss-rate, which an aggregate RTCP cannot provide.
The aggregate RTCP report (i.e., the regularly scheduled RTCP report)
MUST be sent compounded as described in [RFC5506], however, the
subflow-specific RTCP reports MAY be sent non-compounded. Further,
each subflow and the aggregate RTCP report MUST follow the timing
rules defined in [RFC4585].
The RTCP reporting interval is locally implemented and the scheduling
of the RTCP reports may depend on the the behavior of each path. For
instance, the RTCP interval may be different for a passive path than
an active path to keep port bindings alive. Additionally, an
endpoint may decide to share the RTCP reporting bit rate equally
across all its paths or schedule based on the receiver rate on each
path.
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8.5. RTCP Transmission
The sender sends an RTCP SR on each active path. For each SR the
receiver gets, it echoes one back to the same IP address-port pair
that sent the SR. The receiver tries to choose the symmetric path
and if the routing is symmetric then the per-path RTT calculations
will work out correctly. However, even if the paths are not
symmetric, the sender would at maximum, under-estimate the RTT of the
path by a factor of half of the actual path RTT.
9. Packet Formats
In this section we define the protocol structures described in the
previous sections.
9.1. RTP Header Extension for MPRTP
The MPRTP header extension is used to distribute a single RTP stream
over multiple subflows.
The header conforms to the one-byte RTP header extension defined in
[RFC5285]. The header extension contains a 16-bit length field that
counts the number of 32-bit words in the extension, excluding the
four-octet extension header (therefore zero is a valid length, see
Section 5.3.1 of [RFC3550] for details).
The RTP header for each subflow is defined below:
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|1| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 0xBE | 0xDE | length=N-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | LEN | MPID |LENGTH | MPRTP header data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Generic MPRTP header extension
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MPID:
The MPID field corresponds to the type of MPRTP packet.
Namely:
+---------------+--------------------------------------------------+
| MPID ID | Use |
| Value | |
+---------------+--------------------------------------------------+
| 0x0 | Subflow RTP Header. For this case the Length is |
| | set to 4 |
+---------------+--------------------------------------------------+
Figure 7: RTP header extension values for MPRTP (H-Ext ID)
length
The 4-bit length field is the length of extension data in bytes
not including the H-Ext ID and length fields. The value zero
indicates there is no data following.
MPRTP header data
Carries the MPID specific data as described in the following
sub-sections.
9.1.1. MPRTP RTP Extension for a Subflow
The RTP header for each subflow is defined below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|1| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 0xBE | 0xDE | length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | LEN=4 | 0x0 | LEN=4 | Subflow ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific Seq Number | Pad (0) | Pad (0) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: MPRTP header for subflow
MP ID = 0x0
Indicates that the MPRTP header extension carries subflow
specific information.
length = 4
Subflow ID: Identifier of the subflow. Every RTP packet belonging
to the same subflow carries the same unique subflow identifier.
Flow-Specific Sequence Number (FSSN): Sequence of the packet in
the subflow. Each subflow has its own strictly monotonically
increasing sequence number space.
9.2. RTCP Extension for MPRTP (MPRTCP)
The MPRTP RTCP header extension is used to 1) provide RTCP feedback
per subflow to determine the characteristics of each path, and 2)
advertise each interface.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=MPRTCP=211 | encaps_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type | block length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MPRTCP Reports |
| ... |
| ... |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 9: Generic RTCP Extension for MPRTP (MPRTCP) [appended to
normal SR/RR]
MPRTCP: 8 bits
Contains the constant 211 to identify this as an Multipath RTCP
packet.
encaps_length: 16 bits
As described for the RTCP packet (see Section 6.4.1 of the RTP
specification [RFC3550]), the length of this is in 32-bit words
minus one, including the header and any padding.
The encaps_length covers all MPRTCP_Type report blocks included
within this report.
MPRTCP_Type: 8-bits
The MPRTCP_Type field corresponds to the type of MPRTP RTCP
packet. Namely:
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+---------------+--------------------------------------------------+
| MPRTCP_Type | Use |
| Value | |
+---------------+--------------------------------------------------+
| 0 | Subflow Specific Report |
| 1 | Interface Advertisement (IPv4 Address) |
| 2 | Interface Advertisement (IPv4 Address) |
| 3 | Interface Advertisement (DNS Address) |
+---------------+--------------------------------------------------+
Figure 10: RTP header extension values for MPRTP (MPRTCP_Type)
block length: 8-bits
The 8-bit length field is the length of the encapsulated MPRTCP
reports in 32-bit word length (i.e., length * 4 bytes)
including the MPRTCP_Type and length fields. The value zero is
invalid and the report block MUST be ignored.
MPRTCP Reports: variable size
Defined later in 9.2.1 and 9.3.
9.2.1. MPRTCP Extension for Subflow Reporting
When sending a report for a specific subflow the sender or receiver
MUST add only the reports associated with that 5-tuple. Each subflow
is reported independently using the following MPRTCP Feedback header.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=MPRTCP=211 | encaps_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0 | block length | Subflow ID #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific reports |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0 | block length | Subflow ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific reports |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: MPRTCP Subflow Reporting Header
MPRTCP_Type: 0
The value indicates that the encapsulated block is a subflow
report.
block length: 8-bits
The 8-bit length field is the length of the encapsulated subflow-
specific reports in 32-bit word length not including the
MPRTCP_Type and length fields.
Subflow ID: 16 bits
Subflow identifier is the value associated with the subflow the
endpoint is reporting about. If it is a sender it MUST use the
Subflow ID associated with the 5-tuple. If it is a receiver it
MUST use the Subflow ID received in the Subflow-specific Sender
Report.
Subflow-specific reports: variable
Subflow-specific report contains all the reports associated with
the Subflow ID. For a sender, it MUST include the Subflow-
specific Sender Report (SSR). For a receiver, it MUST include
Subflow-specific Receiver Report (SRR). Additionally, if the
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receiver supports subflow-specific extension reports then it MUST
append them to the SRR.
9.2.1.1. MPRTCP for Subflow-specific SR, RR and XR
[note-rtcp-reuse]
Example:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=MPRTCP=211 | encaps_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0 | block length | Subflow ID #1 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|V=2|P| RC | PT=SR=200 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| subflow's packet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| subflow's octet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0 | block length | Subflow ID #2 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|V=2|P| RC | PT=RR=201 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| fraction lost | cumulative number of packets lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| inter-arrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Subflow specific extension reports |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Example of reusing RTCP SR and RR inside an MPRTCP header
(Bi-directional use-case, in case of uni-directional flow the subflow
will only send an SR or RR).
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9.3. MPRTCP Extension for Interface advertisement
This sub-section defines the RTCP header extension for in-band
interface advertisement by the receiver. The interface advertisement
block describes a method to represent IPv4, IPv6 and generic DNS-type
addresses in a block format. It is based on the sub-reporting block
in [RFC5760]. The interface advertisement SHOULD immediately follow
the Receiver Report. If the Receiver Report is not present, then it
MUST be appended to the Sender Report.
The endpoint MUST advertise the interfaces it wants to use whenever
an interface appears or disappears and also when it receives an
Interface Advertisement.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=MPRTCP=211 | encaps_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type | block length | RTP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type | block length | RTP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type | block length | RTP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address #.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type | block length | RTP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address #m |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 13: MPRTP Interface Advertisement. (appended to SR/RR)
MPRTCP_Type: 8 bits
The MPRTCP_Type corresponds to the type of interface address.
Namely:
1: IPv4 address
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2: IPv6 address
3: DNS name
block length: 8 bits
The length of the Interface Advertisement block in 32-bit word
lengths.
For an IPv4 address, this should be 2 (8 octets including =
2 + 2 + 4).
For an IPv6 address, this should be 5 (20 octets = 2 +2 +
16).
For a DNS name, the length field indicates the number of
word lengths making up the address string plus 1 (32-bit
word for the 2 byte port number and 2 byte header).
RTP Port: 2 octets
The port number to which the sender sends RTP data. A port
number of 0 is invalid and MUST NOT be used.
Interface Address: 4 octets (IPv4), 16 octets (IPv6), or n octets
(DNS name)
The address to which receivers send feedback reports. For IPv4
and IPv6, fixed-length address fields are used. A DNS name is
an arbitrary-length string. The string MAY contain
Internationalizing Domain Names in Applications (IDNA) domain
names and MUST be UTF-8 [RFC3629] encoded.
10. RTCP Timing reconsiderations for MPRTCP
MPRTP endpoints MUST conform to the timing rule imposed in [RFC4585],
i.e., the total RTCP rate between the participants MUST NOT exceed 5%
of the media rate. For each endpoint, a subflow MUST send the
aggregate and subflow-specific report. The endpoint SHOULD schedule
the RTCP reports for the active subflows based on the share of the
transmitted or received bit rate to the average media bit rate, this
method ensures fair sharing of the RTCP bandwidth. Alternatively,
the endpoint MAY schedule the reports in round-robin.
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11. SDP Considerations
11.1. Signaling MPRTP Header Extension in SDP
To indicate the use of the MPRTP header extensions (see Section 9) in
SDP, the sender MUST use the following URI in extmap:
urn:ietf:params:rtp-hdrext:mprtp. This is a media level parameter.
Legacy RTP (non-MPRTP) clients will ignore this header extension, but
can continue to parse and decode the packet (see Appendix A).
Example:
v=0
o=alice 2890844526 2890844527 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=extmap:1 urn:ietf:params:rtp-hdrext:mprtp
11.2. Signaling MPRTP capability in SDP
A participant of a media session MUST use SDP to indicate that it
supports MPRTP. Not providing this information will make the other
endpoint ignore the RTCP extensions.
mprtp-attrib = "a=" "mprtp" [
SP mprtp-optional-parameter]
CRLF ; flag to enable MPRTP
The endpoint MUST use 'a=mprtp', if it is able to send and receive
MPRTP packets. Generally, senders and receivers MUST indicate this
capability if they support MPRTP and would like to use it in the
specific media session being signaled. To exchange the additional
interfaces, the endpoint SHOULD use the in-band signaling (See
Section 9.3). Alternatively, advertise in SDP (See
[I-D.singh-mmusic-mprtp-sdp-extension]).
MPRTP endpoint multiplexes RTP and RTCP on a single port, sender MUST
indicate support by adding "a=rtcp-mux" in SDP [RFC5761]. If an
endpoint receives an SDP without "a=rtcp-mux" but contains "a=mprtp",
then the endpoint MUST infer support for multiplexing.
MPRTP endpoint uses Reduced-Size RTCP [RFC5506] for reporting path
characterisitcs per subflow (MPRTCP). An MPRTP endpoint MUST
indicate support by adding "a=rtcp-rsize" in SDP [RFC5761]. If an
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endpoint receives an "a=rtcp-rsize" but contains "a=mprtp", then the
endpoint MUST infer support for Reduced-Size RTCP.
[note-rtp-rtcp-mux]
11.3. MPRTP with ICE
If the endpoints intend to use ICE [RFC5245] for discovering
interfaces and running connectivity checks then the endpoint MUST
advertise its ICE candidates in the initial OFFER, as defined in ICE
[RFC5245]. Thereafter the endpoints run connectivity checks.
When an endpoint uses ICE's regular nomination [RFC5245] procedure,
it chooses the best ICE candidate as the default path. In the case
of an MPRTP endpoint, if more than one ICE candidate succeeded the
connectivity checks then an MPRTP endpoint MAY advertise (some of)
these in-band in RTCP as MPRTP interfaces.
When an endpoint uses ICE's aggressive nomination [RFC5245]
procedure, the selected candidate may change as more ICE checks
complete. Instead of sending updated offers as additional ICE
candidates appear (transience), the endpoint it MAY use in-band
signaling to advertise its interfaces, as defined in Section 9.3.
If the default interface disappears and the paths used for MPRTP are
different from the one in the c= and m= lines then the an alternate
interface for which the ICE checks were successful should be promoted
to the c= and m= lines in the updated offer.
When a new interface appears then the application/endpoint should
internally decide if it wishes to use it and sends an updated offer
with ICE candidates of the all its interfaces including the new
interface. The receiving endpoint responds to the offer with all its
ICE candidates in the answer and starts connectivity checks between
all its candidates and the offerer's ICE candidates. Similarly, the
initiating endpoint starts connectivity checks between the its
interfaces (incl. the new interface) and all the received ICE
candidates in the answer. If the connectivity checks succeed, the
initiating endpoint MAY use in-band signaling (See Section 9.3) to
advertise its new set of interfaces.
11.4. Increased Throughput
The MPRTP layer MAY choose to augment paths to increase throughput.
If the desired media rate exceeds the current media rate, the
endpoints MUST renegotiate the application specific ("b=AS:xxx")
[RFC4566] bandwidth.
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11.5. Offer/Answer
When SDP [RFC4566] is used to negotiate MPRTP sessions following the
offer/answer model [RFC3264], the "a=mprtp" attribute (see
Section 11.2) indicates the desire to use multiple interfaces to send
or receive media data. The initial SDP offer MUST include this
attribute at the media level. If the answerer wishes to also use
MPRTP, it MUST include a media-level "a=mprtp" attribute in the
answer. If the answer does not contain an "a=mprtp" attribute, the
offerer MUST NOT stream media over multiple paths and the offerer
MUST NOT advertise additional MPRTP interfaces in-band or out-of-
band.
When SDP is used in a declarative manner, the presence of an
"a=mprtp" attribute signals that the sender can send or receive media
data over multiple interfaces. The receiver SHOULD be capable to
stream media to the multiple interfaces and be prepared to receive
media from multiple interfaces.
The following sections shows examples of SDP offer and answer for in-
band and out-of-band signaling.
11.5.1. In-band Signaling Example
The following offer/answer shows that both the endpoints are MPRTP
capable and SHOULD use in-band signaling for interfaces
advertisements.
Offer:
v=0
o=alice 2890844526 2890844527 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=rtcp-mux
a=mprtp
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Answer:
v=0
o=bob 2890844528 2890844529 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=video 4000 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=rtcp-mux
a=mprtp
The endpoint MAY now use in-band RTCP signaling to advertise its
multiple interfaces. Alternatively, it MAY make another offer with
the interfaces in SDP (out-of-band signaling)
[I-D.singh-mmusic-mprtp-sdp-extension].
12. IANA Considerations
The following contact information shall be used for all registrations
in this document:
Contact: Varun Singh
mailto:varun.singh@iki.fi
tel:+358-9-470-24785
Note to the RFC-Editor: When publishing this document as an RFC,
please replace "RFC XXXX" with the actual RFC number of this document
and delete this sentence.
12.1. MPRTP Header Extension
This document defines a new extension URI to the RTP Compact Header
Extensions sub-registry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:mprtp
Description: Multipath RTP
Reference: RFC XXXX
12.2. MPRTCP Packet Type
A new RTCP packet format has been registered with the RTCP Control
Packet type (PT) Registry:
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Name: MPRTCP
Long name: Multipath RTCP
Value: 211
Reference: RFC XXXX.
This document defines a substructure for MPRTCP packets. A new sub-
registry has been set up for the sub-report block type (MPRTCP_Type)
values for the MPRTCP packet, with the following registrations
created initially:
Name: Subflow Specific Report
Long name: Multipath RTP Subflow Specific Report
Value: 0
Reference: RFC XXXX.
Name: IPv4 Address
Long name: IPv4 Interface Address
Value: 1
Reference: RFC XXXX.
Name: IPv6 Address
Long name: IPv6 Interface Address
Value: 2
Reference: RFC XXXX.
Name: DNS Name
Long name: DNS Name indicating Interface Address
Value: 3
Reference: RFC XXXX.
Further values may be registered on a first come, first served basis.
For each new registration, it is mandatory that a permanent, stable,
and publicly accessible document exists that specifies the semantics
of the registered parameter as well as the syntax and semantics of
the associated sub-report block. The general registration procedures
of [RFC4566] apply.
12.3. SDP Attributes
This document defines a new SDP attribute, "mprtp", within the
existing IANA registry of SDP Parameters.
12.3.1. "mprtp" attribute
o Attribute Name: MPRTP
o Long Form: Multipath RTP
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o Type of Attribute: media-level
o Charset Considerations: The attribute is not subject to the
charset attribute.
o Purpose: This attribute is used to indicate support for Multipath
RTP. It can also provide one of many possible interfaces for
communication. These interface addresses may have been validated
using ICE procedures.
o Appropriate Values: See Section 11.2 of RFC XXXX.
13. Security Considerations
TBD
All drafts are required to have a security considerations section.
See RFC 3552 [RFC3552] for a guide.
14. Acknowledgements
Varun Singh, Saba Ahsan, and Teemu Karkkainen are supported by
Trilogy (http://www.trilogy-project.org), a research project (ICT-
216372) partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the
author(s) only. The European Commission is not liable for any use
that may be made of the information in this document.
The work of Varun Singh and Joerg Ott from Aalto University has been
partially supported by the European Institute of Innovation and
Technology (EIT) ICT Labs activity RCLD 11882.
Lars Eggert has received funding from the European Union's Horizon
2020 research and innovation program 2014-2018 under grant agreement
No. 644866. This document reflects only the authors' views and the
European Commission is not responsible for any use that may be made
of the information it contains.
Thanks to Roni Even , Miguel A. Garcia , Ralf Globisch , Christer
Holmberg , and Frederic Maze for providing valuable feedback on
earlier versions of this draft.
15. References
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15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, DOI 10.17487/
RFC5760, February 2010,
<http://www.rfc-editor.org/info/rfc5760>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, DOI
10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
2009, <http://www.rfc-editor.org/info/rfc5506>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI
10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, DOI 10.17487/
RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
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15.2. Informative References
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, DOI
10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
<http://www.rfc-editor.org/info/rfc6182>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
June 2009, <http://www.rfc-editor.org/info/rfc5533>.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
DOI 10.17487/RFC5117, January 2008,
<http://www.rfc-editor.org/info/rfc5117>.
[I-D.ietf-mmusic-rfc2326bis]
Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, "Real Time Streaming Protocol 2.0
(RTSP)", draft-ietf-mmusic-rfc2326bis-40 (work in
progress), February 2014.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, DOI
10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
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[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263, DOI 10.17487/
RFC6263, June 2011,
<http://www.rfc-editor.org/info/rfc6263>.
[I-D.singh-mmusic-mprtp-sdp-extension]
Singh, V., Ott, J., Karkkainen, T., Globisch, R., and T.
Schierl, "Multipath RTP (MPRTP) attribute in Session
Description Protocol", draft-singh-mmusic-mprtp-sdp-
extension-04 (work in progress), September 2014.
[I-D.reddy-mmusic-ice-best-interface-pcp]
Reddy, T., Wing, D., Steeg, B., Penno, R., and V. Varun,
"Improving ICE Interface Selection Using Port Control
Protocol (PCP) Flow Extension", draft-reddy-mmusic-ice-
best-interface-pcp-00 (work in progress), October 2013.
[I-D.wing-mmusic-ice-mobility]
Wing, D., Reddy, T., Patil, P., and P. Martinsen,
"Mobility with ICE (MICE)", draft-wing-mmusic-ice-
mobility-07 (work in progress), June 2014.
[I-D.ietf-rmcat-eval-criteria]
Varun, V., Ott, J., and S. Holmer, "Evaluating Congestion
Control for Interactive Real-time Media", draft-ietf-
rmcat-eval-criteria-05 (work in progress), March 2016.
[ACM-MPRTP]
Singh, V., Ahsan, S., and J. Ott, "MPRTP: multipath
considerations for real-time media", in Proc. of ACM
Multimedia Systems, MMSys, 2013.
Appendix A. Interoperating with Legacy Applications
Some legacy endpoints may abort processing incoming packets, if they
are received from different source address. This may occur due to
the loop detection algorithm. Additionally, it is also possible for
the receiver to reset processing of the stream if the jump in packet
sequence numbers received over multiple interface is large. Both of
these errors are based on implementation-specific details.
An MPRTP sender can use its multiple interfaces to send media to a
legacy RTP client. The legacy receiver will ignore the subflow RTP
header extension and the receiver's de-jitter buffer will attempt to
compensate for any mismatch in per-path latency. However, the
receiver will only send the overall or aggregate RTCP report which
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may be insufficient for an MPRTP sender to adequately schedule
packets over multiple paths or detect if a path disappeared.
An MPRTP receiver can only use one of its interface when
communicating with a legacy sender.
Appendix B. Change Log
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
B.1. Changes in draft-ietf-avtcore-mprtp-01, and -02
o Keep-alive versions, document needs review.
o Updated authors' affiliations.
B.2. Changes in draft-ietf-avtcore-mprtp-00
o Submitted as a WG item.
B.3. Changes in draft-singh-avtcore-mprtp-10
o Editorial updates based on review comments.
o Renamed length to encaps_length.
B.4. Changes in draft-singh-avtcore-mprtp-09
o Editorial updates based on review comments.
o Clarified use of a=rtcp-rsize.
o Fixed bug in block length of interface advertisements.
B.5. Changes in draft-singh-avtcore-mprtp-08
o Added reference to use of PCP for discovering new interfaces.
B.6. Changes in draft-singh-avtcore-mprtp-06 and -07
o Added reference to Mobility ICE.
B.7. Changes in draft-singh-avtcore-mprtp-05
o SDP extensions moved to draft-singh-mmusic-mprtp-sdp-extension-00.
Kept only the basic 'a=mprtp' attribute in this document.
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o Cleaned up ICE procedures for advertising only using in-band
signaling.
B.8. Changes in draft-singh-avtcore-mprtp-04
o Fixed missing 0xBEDE header in MPRTP header format.
o Removed connectivity checks and keep-alives from in-band
signaling.
o MPRTP and MPRTCP are multiplexed on a single port.
o MPRTCP packet headers optimized.
o Made ICE optional
o Updated Sections: 7.1.2, 8.1.x, 11.2, 11.4, 11.6.
o Added how to use MPRTP in RTSP (Section 12).
o Updated IANA Considerations section.
B.9. Changes in draft-singh-avtcore-mprtp-03
o Added this change log.
o Updated section 6, 7 and 8 based on comments from MMUSIC.
o Updated section 11 (SDP) based on comments of MMUSIC.
o Updated SDP examples with ICE and non-ICE in out-of-band signaling
scenario.
o Added Appendix A on interop with legacy.
o Updated IANA Considerations section.
B.10. Changes in draft-singh-avtcore-mprtp-02
o MPRTCP protocol extensions use only one PT=210, instead of 210 and
211.
o RTP header uses 1-byte extension instead of 2-byte.
o Added section on RTCP Interval Calculations.
o Added "mprtp-interface" attribute in SDP considerations.
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B.11. Changes in draft-singh-avtcore-mprtp-01
o Added MPRTP and MPRTCP protocol extensions and examples.
o WG changed from -avt to -avtcore.
Editorial Comments
[note-iceornot] Editor: Legacy applications do not require ICE for
session establishment, therefore, MPRTP should not
require it as well.
[note-rtcp-reuse] Editor: inside the context of subflow specific reports
can we reuse the payload type code for Sender Report
(PT=200), Receiver Report (PT=201), Extension Report
(PT=207). Transport and Payload specific RTCP
messages are session specific and SHOULD be used as
before.
[note-rtp-rtcp-mux] Editor: If a=mprtp is indicated, does the endpoint
need to indicate a=rtcp-mux and a=rtcp-rsize?
because MPRTP mandates the use of RTP and RTCP
multiplexing, and Reduced-Size RTCP.
Authors' Addresses
Varun Singh
Nemu Dialogue Systems Oy
Runeberginkatu 4c A 4
Helsinki 00100
Finland
Email: varun.singh@iki.fi
URI: http://www.callstats.io/
Teemu Karkkainen
Technical University of Munich
Faculty of Informatics
Boltzmannstrasse 3
Garching bei Muenchen, DE 85748
Germany
Email: kaerkkae@in.tum.de
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Joerg Ott
Technical University of Munich
Faculty of Informatics
Boltzmannstrasse 3
Garching bei Muenchen, DE 85748
Germany
Email: ott@in.tum.de
Saba Ahsan
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: saba.ahsan@aalto.fi
Lars Eggert
NetApp
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 151 12055791
Email: lars@netapp.com
URI: http://eggert.org/
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