Multipath RTP (MPRTP)
draft-singh-avtcore-mprtp-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Varun Singh , Teemu Karkkainen , Joerg Ott , Saba Ahsan , Lars Eggert | ||
| Last updated | 2011-10-31 (Latest revision 2011-07-08) | ||
| Replaces | draft-singh-avt-mprtp | ||
| Replaced by | draft-ietf-avtcore-mprtp | ||
| RFC stream | (None) | ||
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draft-singh-avtcore-mprtp-03
AVT Core Working Group V. Singh
Internet-Draft T. Karkkainen
Intended status: Experimental J. Ott
Expires: May 3, 2012 S. Ahsan
Aalto University
L. Eggert
Nokia
October 31, 2011
Multipath RTP (MPRTP)
draft-singh-avtcore-mprtp-03
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
This Internet-Draft is submitted in full conformance with the
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 May 3, 2012.
Copyright Notice
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Copyright (c) 2011 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
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Use-cases . . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . 11
7.2. Expanding RTP . . . . . . . . . . . . . . . . . . . . . . 13
7.3. Expanding RTCP . . . . . . . . . . . . . . . . . . . . . . 13
7.4. Failure Handling and Teardown . . . . . . . . . . . . . . 13
8. MPRTP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1.1. Gather or Discovering Candidates . . . . . . . . . . . 15
8.1.2. RTCP Subflow or Interface advertisement . . . . . . . 15
8.1.3. Connectivity Checks . . . . . . . . . . . . . . . . . 15
8.1.4. Active and Passive Subflows . . . . . . . . . . . . . 16
8.1.5. Middlebox Considerations . . . . . . . . . . . . . . . 16
8.2. RTP Transmission . . . . . . . . . . . . . . . . . . . . . 16
8.3. Playout Considerations at the Receiver . . . . . . . . . . 17
8.4. Subflow-specific RTCP Statistics and RTCP Aggregation . . 17
8.5. RTCP Transmission . . . . . . . . . . . . . . . . . . . . 17
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9. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. RTP Header Extension for MPRTP . . . . . . . . . . . . . . 18
9.1.1. MPRTP RTP Extension for a Subflow . . . . . . . . . . 19
9.1.2. MPRTP RTP Extension for Connectivity Checks . . . . . 20
9.1.3. MPRTP RTP Extension for Keep-alive Packets . . . . . . 20
9.2. RTCP Extension for MPRTP (MPRTCP) . . . . . . . . . . . . 20
9.2.1. MPRTCP Extension for Subflow Reporting . . . . . . . . 22
9.2.1.1. MPRTCP for Subflow-specific SR, RR and XR . . . . 23
9.3. MPRTCP Extension for Interface advertisement . . . . . . . 25
9.3.1. Interface Advertisement block . . . . . . . . . . . . 26
10. RTCP Timing reconsiderations for MPRTCP . . . . . . . . . . . 27
11. SDP Considerations . . . . . . . . . . . . . . . . . . . . . . 27
11.1. Signaling MPRTP Header Extension in SDP . . . . . . . . . 27
11.2. Signaling MPRTP capability in SDP . . . . . . . . . . . . 28
11.3. MPRTP Interface Advertisement in SDP (out-of-band
signaling) . . . . . . . . . . . . . . . . . . . . . . . . 28
11.3.1. "interface" attribute . . . . . . . . . . . . . . . . 28
11.3.2. Example . . . . . . . . . . . . . . . . . . . . . . . 29
11.4. MPRTP with ICE . . . . . . . . . . . . . . . . . . . . . . 29
11.5. Increased Throughput . . . . . . . . . . . . . . . . . . . 30
11.6. Offer/Answer Examples . . . . . . . . . . . . . . . . . . 30
11.6.1. In-band signaling . . . . . . . . . . . . . . . . . . 30
11.6.2. Out-of-band signaling . . . . . . . . . . . . . . . . 31
11.6.2.1. Without ICE . . . . . . . . . . . . . . . . . . . 31
11.6.2.2. With ICE . . . . . . . . . . . . . . . . . . . . . 32
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
12.1. MPRTP Header Extension . . . . . . . . . . . . . . . . . . 33
12.2. MPRTCP Packet Type . . . . . . . . . . . . . . . . . . . . 34
12.3. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 34
13. Security Considerations . . . . . . . . . . . . . . . . . . . 34
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
15.1. Normative References . . . . . . . . . . . . . . . . . . . 35
15.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Interoperating with Legacy Applications . . . . . . . 36
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 36
B.1. changes in draft-singh-avtcore-mprtp-03 . . . . . . . . . 36
B.2. changes in draft-singh-avtcore-mprtp-02 . . . . . . . . . 37
B.3. changes in draft-singh-avtcore-mprtp-01 . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
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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
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 [1] 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 [10], MPTCP [11] and SHIM6 [12]. However, these
protocols are not suitable for realtime 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 [2].
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.
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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
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.
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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.
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) [3] MAY be used for discovering new interfaces or performing
connectivity checks.
3. RTP Topologies
RFC 5117 [13] describes a number of scenarios using mixers and
translators in single-party (point-to-point), and multi-party (point-
to-multipoint) scenarios. RFC 3550 [1] (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
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MPRTP scenario, an RTP session uses multiple subflows that each use a
different path. Figure 1 shows the difference.
+--------------+ 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. So that it is able to send 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.
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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.
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 and 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.
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.
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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. Multipath session may be upgraded
from a typical single path session, as and when new interfaces
appear or disappear. However, it should be 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.
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).
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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., SDP [14] in SIP [15], RTSP
[16]) or in-band signaling (e.g., RTP/RTCP header extension, STUN
messages).
[[Comment.1: Using SIP over SCTP, MPTCP instead of UDP/TCP are out of
scope of the document. --MMUSIC Review]]
7.1.1. Connectivity Checks
The endpoint SHOULD be capable of performing connectivity checks
(e.g., using ICE [3]). 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
method to advertise 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
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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.
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.
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
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would look like.
[[Comment.2: Some have suggested STUN may be suitable for doing in-
band interface advertisement. This is still under consideration, but
depends on implementation challenges as many legacy systems don't
implement STUN and many RTP systems ignore STUN messages. --Editor]]
7.2. Expanding RTP
RTCP [1] 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 [1], 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.
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 the subflows for failure
(errors, unreachability, congestion) and decide not to use the under-
performing subflows.
If an interface disappears, the endpoint MUST send an updated
interface advertisement without the interface and release the the
associated subflows.
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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
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.
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(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 it MUST advertise it to the other peers.
The endpoint may also use ICE [3] to gather additional candidates.
8.1.2. RTCP Subflow or Interface advertisement
To advertise the multiple interfaces, an MPRTP-capable endpoint MUST
add the MPRTP Interface Advertisement defined in Figure 13 with the
RTCP Sender Report (SR). Each unique address is 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.
8.1.3. Connectivity Checks
After MPRTP support has been negotiated and interface advertisements
have been exchanged, the endpoint MAY initiate connectivity checks to
determine which pairs of interfaces offer valid paths between the
sender and the receiver. Each combination of sender and receiver IP
addresses and port pairs (5-tuple) is a unique subflow. An endpoint
MUST associate a Subflow ID to each unique subflow.
To initiate a connectivity check, the endpoints send an RTP packet
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using the appropriate MPRTP extension header (See Figure 7),
associated Subflow ID and no RTP payload. The receiving endpoint
replies to the connectivity check with an RTCP packet with the
appropriate packet type (See Figure 10) and Subflow ID. After the
endpoint receives the reply, the path is considered a valid candidate
for sending data. An endpoint MAY choose to do any number of
connectivity checks for any interface pairs at any point in a
session.
[[Comment.3: Open Issue: How should the endpoint adjust the RTCP
Reporting interval/schedule the RTCP packet on receiving a
connectivity check containing a new Subflow ID? One option is send
immediately as defined in RFC4585. Another option is the RTCP timing
defined in RFC6263. --Editor]]
8.1.4. 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 packet are called passive subflows.
An endpoint should send MPRTP keep alive packets (see Section 9.1.3)
on the subflows where no media is sent to keep the NAT bindings
alive.
[[Comment.4: Open Issue: How to differentiate between Passive and
Active connections? Editor: Active paths get "regular flow" of media
packets while passive paths are for failover of active paths.
--Editor]]
[[Comment.5: Open Issue: How to keep a passive connection alive, if
not actively used? Alternatively, what is the maximum timeout? keep-
alive for ICE/NAT bindings should not be less than 15 seconds
[RFC5245]. --Editor]]
8.1.5. Middlebox Considerations
TBD.
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
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the media packet to a specific subflow (see Figure 8). The MPRTP
layer SHOULD associate an RTP packet with a subflow based on a
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).
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 [1], 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.
8.4. Subflow-specific RTCP Statistics and RTCP Aggregation
Aggregate RTCP provides the overall media statistics and follows the
normal RTCP defined in RFC3550 [1]. 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 sender/receiver MUST use non-compound RTCP reports defined in
RFC5506 [4] to transmit the aggregate and subflow-specific RTCP
reports. Also, each subflow and the aggregate RTCP report MUST
follow the timing rules defined in [5].
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.
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
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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 1) distribute a single RTP
stream over multiple subflows, 2) perform connectivity checks on the
advertised interfaces, and 3) keep-alive passive interfaces (paths).
The header conforms to the one-byte RTP header extension defined in
[6]. 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 [1] 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 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| ID | LEN | MPID |LENGTH | MPRTP header data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Generic MPRTP header extension
MPID:
The MPID field corresponds to the type of MPRTP packet.
Namely:
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+---------------+--------------------------------------------------+
| MPID ID | Use |
| Value | |
+---------------+--------------------------------------------------+
| 0x00 | Subflow RTP Header. For this case the Length is |
| | set to 6 |
| 0x01 | Connectivity Check. For this case the length is |
| | set to 0 |
| TBD | Keep Alive Packet. |
+---------------+--------------------------------------------------+
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:
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 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| ID | LEN=4 | 0x00 | LEN=4 | Subflow ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific Seq Number | Pad (0) | Pad (0) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: MPRTP header for subflow
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MP ID = 0x00
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.
9.1.2. MPRTP RTP Extension for Connectivity Checks
[[Comment.6: Open Issue: What sequence number to use for the RTP
session? Alternative 1: An MPRTP receiver MUST NOT give the packet
with MPID=0x01 to the decoder and ignore these packets from RTCP
calculation. Alternative 2: Instead of sending an RTP packet the
sender transmits a modified STUN packet. --Editor]]
9.1.3. MPRTP RTP Extension for Keep-alive Packets
[[Comment.7: RTCP guidelines for keep alive packet [RFC6263]
recommends multiplexing RTP and RTCP. If so, we can do the same and
remove the keep alive from the list. Alternatively, the endpoint can
send zero payload RTP packet with the correct RTP sequence number,
RTP timestamp and monotonically increasing the subflow specific
sequence number each time [RFC6263 Sec 4.6].] --Editor]]
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, 2)
advertise each interface and perform connectivity check on the other
endpoint's interfaces, and 3) to keep alive a passive connection.
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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 | 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.
length: 16 bits
As described for the RTCP packet (see Section 6.4.1 of the RTP
specification [1]), the length of this is in 32-bit words minus
one, including the header and any padding.
MPRTCP_Type: 16-bits
The MPRTCP_Type field corresponds to the type of MPRTP RTCP
packet. Namely:
+---------------+--------------------------------------------------+
| MPRTCP_Type | Use |
| Value | |
+---------------+--------------------------------------------------+
| 0x00 | Subflow Specific Report |
| 0x01 | Connectivity Check. For this case the length is |
| | set to 0 |
| 0x02 | Interface Advertisement |
| TBD | Keep Alive Packet. |
+---------------+--------------------------------------------------+
Figure 10: RTP header extension values for MPRTP (MPRTCP_Type)
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block length: 16-bits
The 16-bit length field is the length of the encapsulated
MPRTCP reports in 32-bit word length not including the
MPRTCP_Type and length fields. The value zero indicates there
is no data following.
MPRTCP Reports: variable size
Defined later in 9.2.1 and 9.3.1.
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.
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 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0x02 | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow ID #1 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific reports |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0x02 | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow ID #2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow-specific reports |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: MPRTCP Subflow Reporting Header
MPRTCP_Type: 0x02
block length: 16-bits
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The 16-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
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
[[Comment.8: 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. --Editor]]
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 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0x02 | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow ID #1 | reserved |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|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=0x02 | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subflow ID #2 | reserved |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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).
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
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 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRTCP_Type=0x02 | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #1 Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #2 Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #... Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #m Advertisement block |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 13: MPRTP Interface Advertisement. (appended to SR/RR)
MPRTCP_Type: 0x00
block length: 16-bits
The 16-bit length field is the length of the encapsulated
interface advertisement blocks in 32-bit word length not
including the MPRTCP_Type and length fields.
Interface Advertisement block: variable size
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Defined later in 9.3.1.
9.3.1. Interface Advertisement block
This 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 [7].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type={0,1,2} | Length | Subflow ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTCP Port | RTCP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
+ +
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Interface Advertisement block during path discovery
Type: 8 bits
The Type corresponds to the type of address. Namely:
0: IPv4 address
1: IPv6 address
2: DNS name
Length: 8 bits
The length of the Interface Advertisement block in bytes.
For an IPv4 address, this should be 9 (i.e., 5 octets for
the header and 4 octets for IPv4 address).
For an IPv6 address, this should be 21.
For a DNS name, the length field indicates the number of
octets making up the string plus the 5 byte header.
RTP Port: 2 octets
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The port number to which the sender sends RTP data. A port
number of 0 is invalid and MUST NOT be used.
RTCP Port: 2 octets
The port number to which receivers send feedback reports. A
port number of 0 is invalid and MUST NOT be used.
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 [8] encoded.
10. RTCP Timing reconsiderations for MPRTCP
MPRTP endpoints MUST conform to the timing rule imposed in [5], 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.
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:
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:
m=video 49170 RTP/AVP 98
a=extmap:1 urn:ietf:params:rtp-hdrext:mprtp
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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
(Section 9.3). Alternatively, advertise in SDP (Section 11.3).
11.3. MPRTP Interface Advertisement in SDP (out-of-band signaling)
If the endpoint is aware of its multiple interfaces and wants to use
them for MPRTP then it MAY use SDP to advertise these interfaces.
Alternatively, it MAY use in-band signaling to advertise its
interfaces, as defined in Section 9.3 (MPRTCP_Type=0x02). The
receiving endpoint MUST use the same mechanism to respond to an
interface advertisement. In particular, if an endpoint receives an
SDP offer, then it MUST respond to the offer in SDP.
11.3.1. "interface" attribute
The interface attribute is an optional media-level attribute and is
used to advertise an endpoint's interface address.
The syntax of the interface attribute is defined using the following
Augmented BNF, as defined in [9]. The definitions of unicast-
address, port, token, SP, and CRLF are according to RFC4566 [17].
mprtp-optional-parameter = mprtp-interface
; other optional parameters may be added later
mprtp-interface = "interface" ":" counter SP unicast-address
":" rtp_port "/" rtcp_port
*(SP interface-description-extension)
counter = 1*DIGIT
rtp_port = port ;port from RFC4566
rtcp_port = port ;port from RFC4566
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<mprtp-interface>: specifies one of the unicast IP address, the RTP
and RTCP port numbers of the endpoint. The unicast address with
lowest counter value MUST match the connection address ('c=' line).
Similarly, the RTP and RTCP ports MUST match with the RTP and RTCP
ports in the associated 'm=' line. The counter should start at 1 and
increment with each additional interface. Multiple interface lines
MUST be ordered in a decreasing priority level as is the case with
the Interface Advertisement blocks in in-band signaling (See
Figure 13).
<unicast-address>: is taken from RFC4566 [17]. It is one of the IP
addresses of the endpoint allows the use of IPv4 addresses, IPv6
addresses and Fully Qualified Domain Names (FQDN). An endpoint MUST
only include the IP address for which the connectivity checks have
succeeded.
<port>: is from RFC4566 [17]. It is the RTP or RTCP port associated
the unicast address.
<counter>: is a monotonically increasing positive integer starting at
1. The counter MUST reset for each media line. The counter value
for an interface, port should remain the same for the session.
The 'mprtp-interface' can be extended using the 'interface-
description-extension' parameter. An endpoint MUST ignore any
extensions it does not understand.
11.3.2. Example
The ABNF grammar is illustrated by means of an 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
a=mprtp interface:1 192.0.2.1:49170/49171 ;primary interface
a=mprtp interface:2 192.1.2.1:51372/51373 ;additional interface
11.4. MPRTP with ICE
If the endpoints intend to use ICE [3] for discovering interfaces and
running connectivity checks then the following two step procedure
MUST be followed:
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1. Advertise ICE candidates: in the initial OFFER the endpoints
exchange candidates, as defined in ICE [3]. Thereafter the
endpoints run connectivity checks
2. Advertise MPRTP interfaces: When a sufficient number of
connectivity checks succeed the endpoint MUST send an updated
offer containing the interfaces that they want to use for MPRTP.
Typically when a sufficient number of connectivity checks succeed the
endpoint choose the best ICE candidate as the default path. Since
more than one candidate may have succeeded the connectivity checks,
an MPRTP endpoint MAY advertise (some of) these as "mprtp-
interfaces".
11.5. 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") [17]
bandwidth.
11.6. Offer/Answer Examples
This section shows examples of SDP offer and answer for in-band and
out-of-band signaling.
11.6.1. In-band signaling
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=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=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).
11.6.2. Out-of-band signaling
If the multiple interfaces are included in an SDP offer then the
receiver MUST respond to the request with an SDP answer.
11.6.2.1. Without ICE
In this example, the offerer advertises two interfaces and the
answerer responds with a single interface description. The endpoint
MAY use one or both paths depending on the end-to-end characteristics
of each path.
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=mprtp interface:1 192.0.2.1:49170/49171
a=mprtp interface:2 192.1.2.1:51372/51373
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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=mprtp interface:1 192.0.2.2:4000/4001
11.6.2.2. With ICE
In this example, the endpoint first sends its ICE candidates in the
initial offer and the other endpoint answers with its ICE candidates.
Initial offer (with ICE candidates):
Offer:
v=0
o=alice 2890844526 2890844527 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=candidate:1 1 UDP 2130706431 192.0.2.1 49170 typ host
a=candidate:2 1 UDP 1694498815 192.1.2.1 51372 typ host
Answer:
v=0
o=bob 2890844528 2890844529 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=video 4000 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=candidate:1 1 UDP 2130706431 192.0.2.2 4000 typ host
Thereafter, each endpoint conducts ICE connectivity checks and when
sufficient number of connectivity checks succeed, the endpoint sends
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an updated offer. In the updated offer, the endpoint advertises its
multiple interfaces for MPRTP.
Updated offer (with MPRTP interfaces):
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=mprtp interface:1 192.0.2.1:49170/49171
a=mprtp interface:2 192.1.2.1:51372/51373
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=mprtp interface:1 192.0.2.2:4000/4001
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
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
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12.2. MPRTCP Packet Type
A new RTCP packet format has been registered with the RTCP Control
Packet type (PT) Registry:
Name: MPRTCP
Long name: Multipath RTCP
Value: 211
Reference: RFC XXXX.
12.3. SDP Attributes
This document defines a new SDP attribute, "mprtp", within the
existing IANA registry of SDP Parameters.
TBD.
13. Security Considerations
TBD
All drafts are required to have a security considerations section.
See RFC 3552 [18] 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 authors would also like acknowledge the contribution of Ralf
Globisch and Thomas Schierl for providing the input and text for the
MPRTP interface advertisement in SDP.
Thanks to Christer Holmberg, Miguel A. Garcia, and Roni Even for
providing valuable feedback on earlier versions of this draft
15. References
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15.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", RFC 5245, April 2010.
[4] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities and
Consequences", RFC 5506, April 2009.
[5] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control Protocol
(RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006.
[6] Singer, D. and H. Desineni, "A General Mechanism for RTP Header
Extensions", RFC 5285, July 2008.
[7] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast Sessions
with Unicast Feedback", RFC 5760, February 2010.
[8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[9] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
15.2. Informative References
[10] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
[11] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development",
RFC 6182, March 2011.
[12] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim
Protocol for IPv6", RFC 5533, June 2009.
[13] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
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[14] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[15] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[16] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M.
Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)",
draft-ietf-mmusic-rfc2326bis-28 (work in progress),
October 2011.
[17] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[18] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", BCP 72, RFC 3552, July 2003.
[19] Marjou, X. and A. Sollaud, "Application Mechanism for Keeping
Alive the NAT Mappings Associated with RTP / RTP Control
Protocol (RTCP) Flows", RFC 6263, June 2011.
Appendix A. Interoperating with Legacy Applications
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 and the receiver's de-jitter buffer will try to compensate for
the mismatch in per-path delay. However, the receiver can only send
the overall or aggregate RTCP report which may be insufficient for an
MPRTP sender to adequately schedule packets 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-singh-avtcore-mprtp-03
o Added this change log.
o Updated section 6, 7 and 8 based on comments from MMUSIC.
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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.2. 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.
B.3. changes in draft-singh-avtcore-mprtp-01
o Added MPRTP and MPRTCP protocol extensions and examples.
o WG changed from -avt to -avtcore.
Authors' Addresses
Varun Singh
Aalto University
School of Science and Technology
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: varun@comnet.tkk.fi
URI: http://www.netlab.tkk.fi/~varun/
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Teemu Karkkainen
Aalto University
School of Science and Technology
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: teemuk@comnet.tkk.fi
Joerg Ott
Aalto University
School of Science and Technology
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: jo@comnet.tkk.fi
Saba Ahsan
Aalto University
School of Science and Technology
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: sahsan@cc.hut.fi
Lars Eggert
Nokia Research Center
P.O. Box 407
Nokia Group 00045
Finland
Phone: +358 50 48 24461
Email: lars.eggert@nokia.com
URI: http://research.nokia.com/people/lars_eggert
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