AVT Working Group V. Singh
Internet-Draft T. Karkkainen
Intended status: Experimental J. Ott
Expires: August 29, 2011 S. Ahsan
Aalto University
L. Eggert
Nokia
February 25, 2011
Multipath RTP (MPRTP)
draft-singh-avtcore-mprtp-00
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 August 29, 2011.
Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
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
4.1. Relationship of MPRTP with Session Signaling . . . . . . . 8
5. Example Media Flow diagrams . . . . . . . . . . . . . . . . . 8
5.1. Streaming use-case . . . . . . . . . . . . . . . . . . . . 8
5.2. Conversational use-case . . . . . . . . . . . . . . . . . 9
5.3. Challenges with Multipath Interface Discovery . . . . . . 10
6. MPRTP Functional blocks . . . . . . . . . . . . . . . . . . . 10
7. Available mechanisms within the functional blocks . . . . . . 11
7.1. Session Setup . . . . . . . . . . . . . . . . . . . . . . 11
7.2. Expanding RTP . . . . . . . . . . . . . . . . . . . . . . 11
7.3. Adding New Interfaces . . . . . . . . . . . . . . . . . . 11
7.4. Expanding RTCP . . . . . . . . . . . . . . . . . . . . . . 12
7.5. Checking and Failure Handling . . . . . . . . . . . . . . 12
8. MPRTP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1.1. Subflow or Interface Advertisement . . . . . . . . . . 14
8.1.2. Path selection . . . . . . . . . . . . . . . . . . . . 14
8.1.3. Opening subflows . . . . . . . . . . . . . . . . . . . 15
8.2. RTP Transmission . . . . . . . . . . . . . . . . . . . . . 15
8.3. Playout Considerations at the Receiver . . . . . . . . . . 15
8.4. Flow specific RTCP Statistics and RTCP Aggregation . . . . 15
8.5. RTCP Transmission . . . . . . . . . . . . . . . . . . . . 16
9. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. MPRTP RTP Header Extension . . . . . . . . . . . . . . . . 16
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9.1.1. MPRTP RTP header extension for a subflow . . . . . . . 16
9.2. MPRTP RTCP Header Extension . . . . . . . . . . . . . . . 18
9.2.1. MPRTP RTCP header extension for flow specific SR/RR . 18
9.2.2. MPRTP RTCP header extension for Interface
advertisement . . . . . . . . . . . . . . . . . . . . 18
9.2.3. Interface Address Advertisement block . . . . . . . . 19
10. SDP Considerations . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Increased Throughput . . . . . . . . . . . . . . . . . . . 21
10.2. MPRTP using preloaded ICE . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
13. Security Considerations . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 often there is 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 transmit
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 [9], MPTCP MPTCP [10] and SHIM6 [11]. 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 connection.
o Interface: A 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: A 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.
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
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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 one RTP stream over multiple paths
can reduce the bandwidth usage, compared to transmitting the same
stream along a single path. This reduces the impact on other
traffic.
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.
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o Improved Reliability: MPRTP SHOULD be able to send redundant 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 [12] 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
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 sub-flows +--------------+
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
peer, which may, for example, be the peer's multiple interfaces 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.
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.
4.1. Relationship of MPRTP with Session Signaling
Session signaling (e.g., SIP [13], RTSP [14]) SHOULD be done over a
failover-capable or multipath-capable transport for e.g., SCTP [9] or
MPTCP [10] instead of TCP or UDP.
5. Example Media Flow diagrams
There may be many complex technical scenarios for MPRTP, however,
this memo only considers the following two scenarios: 1) an
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 standard RTCP Receiver Report (RR) for the overall
session and receiver statistics for each subflow. Host B distributes
the packets into 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
----------------------- ----------
Address A1 Address A2 Address B1
----------------------- ----------
| Session Setup |
|------------------------------------->| connection for the
|<-------------------------------------| peers may be "preloaded"
| | | (e.g., with ICE)
| | |
| (RTP data B1->A1, B1->A2) |
|<=====================================|
| |<========================|
| | |
Note: there maybe 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. All interfaces
that receive data send back RTCP receiver statistics for each path.
The peers balance their multimedia stream over multiple links based
on the reception statistics from its peer and its own volume of
traffic. Figure 4 describes an example of a bidirectional flow.
Host A Host B
----------------------- -----------------------
Address A1 Address A2 Address B1 Address B2
----------------------- -----------------------
| | | |
| Session Setup | | connection for
|----------------------------------->| | the peers may
|<-----------------------------------| | be "preloaded"
| | | | (e.g., with ICE)
| | | |
| (RTP data B1<->A1, B2<->A2) | |
|<===================================| |
| |<===================================|
|===================================>| |
| |===================================>|
| | | |
Note: the address pairs may have other permutations,
and they maybe symmetric or asymmetric combinations.
Figure 4: Bidirectional flow
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5.3. Challenges with Multipath Interface Discovery
For some applications, where the user expects immediate playback,
e.g., High Definition Media Streaming or IPTV, it may not be possible
to perform connectivity checks within the given time bound. In these
cases, connectivity checks MAY need to be done ahead of time.
[Open Issue: ICE or any other system would have to be aware of the
endpoint's interfaces ahead of time].
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: Multipath session setup is an upgrade or add-on to
a typical RTP session. 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.
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 signaled. This can be done at session setup
and/or during the session. When discovering and receiving new
interfaces, the MPRTP layer needs to select address and port
pairs.
4. Expanding RTCP: MPRTP MUST recombine RTCP reports from each path
to re-create a single RTCP message to maintain backward
compatibility with legacy applications.
5. Maintenance and Failure Handling: In a multi-homed endpoint
interfaces may appear and disappear. If this happens at the
sender, it has to re-adjust the load on the available links. On
the other hand, if this occurs on 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
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receiver has to explicitly signal the disappearance of an
interface, or the sender has to detect it. What happens if the
interface that setup the session disappears? does the control
channel also failover? re-start the session?
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 [15] in SIP [13], RTSP
[14]) or in-band signaling (e.g., RTP/RTCP header extension).
Furthermore, ICE [3] may be used for discovering and performing
connectivity checks during session setup.
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 flow identifier to associate the characteristics to a
path. The RTP header extension for MPRTP is shown in Section 9).
7.3. Adding New Interfaces
When interfaces appear and disappear mid-session, ICE [3] may be used
for discovering interfaces and performing connectivity checks.
However, MPRTP may require a capability re-negotiation (using SDP) to
include all these new interfaces. This method is referred to as out-
of-band multipath advertisement.
Alternatively, when new interfaces appear the interface
advertisements may be done in-band using RTP/RTCP extensions. The
peers perform connectivity checks (see Figure 5 for more details).
If the connectivity packets are received by the peers, then
multimedia data can flow between the new address, port pairs.
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7.4. Expanding RTCP
To provide accurate per path information an MPRTP host MUST send
(SR/RR) report for each unique subflow along with the overall overall
session RTCP report. Therefore, the additional sub flow reporting
affects the RTCP bandwidth and the RTCP reporting interval for each
subflow. 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 maybe 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.
7.5. Checking and Failure Handling
[Note: If the original interface that setup the session disappears
then does the session signaling failover to another interface? Can
we recommend that SIP/RTSP be run over MPTCP, SCTP].
8. MPRTP Protocol
To enable a quick start of a multimedia session, a multipath session
MUST be upgraded from a single path session. Therefore, no explicit
changes are needed in multimedia session setup and the session can be
setup as before.
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Host A Host B
----------------------- -----------------------
Address A1 Address A2 Address B1 Address 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 standard single path multimedia session to multipath
session.
(1) The first two interactions between the hosts describes the
standard session setup. This may be SIP or RTSP.
(2) Following the setup, like in a conventional RTP scenario, host
B using interface B1 starts to stream data to host A at interface
A1.
(3) Host B is an MPRTP-capable media sender and becomes aware of
another interface B2.
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(4) Host B advertises the multiple interface addresses using an
RTP header extensions.
(5) Host A is an MPRTP-capable media receiver and becomes aware of
another interface A2. It advertises the multiple interface
addresses using an RTCP RR extension.
Side note, if an MPRTP-capable host has only one interface even
then it SHOULD advertise its single interface.
(6) Each host receives information about the additional interfaces
and performs the connectivity tests (not shown in figure). If the
paths are reachable then the host starts to stream the multimedia
content using the additional paths.
8.1.1. Subflow or Interface Advertisement
To advertise the multiple interfaces, an MPRTP-capable endpoint MUST
add the MPRTP Interface Advertisement defined in Figure 8 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
its own set of interfaces.
If the sender and receiver have only one interface, then the
endpoints MUST respond with the default IP, RTP port and RTCP port
addresses. If an endpoint receives an RTCP report without the MPRTP
Interface Advertisement, then the endpoint MUST assume that the other
endpoint is not MPRTP capable.
8.1.2. Path selection
After MPRTP support has been discovered and interface advertisements
have been exchanged, the sender MUST initiate connectivity checks to
determine which interface pairs offer valid paths between the sender
and the receiver. Each combination of IP addresses and port pairs
(5-tuple) is a unique subflow. An endpoint MUST associate a Flow ID
to each unique subflow. To initiate a connectivity check, the
endpoints send an RTP packet using the appropriate MPRTP extension
header (See Figure 7), associated Flow ID and no RTP payload. The
receiving endpoint replies to each connectivity check with an RTCP
packet with the appropriate packet type (See Figure 9) and Flow 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.
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[Open Issue: How should the endpoint adjust the RTCP Reporting
interval/schedule the RTCP packet on receiving a connectivity check
containing a new FlowID? Editor: One option is send immediately as
defined in [4].]
8.1.3. Opening subflows
The sender MAY open any number of subflows from the set of candidate
subflows after performing connectivity checks. To use the subflow,
the sender simply starts sending the RTP packets with an MPRTP
extension shown in Figure 6. The MPRTP extension carries a mapping
of a subflow packet to the aggregate flow. Namely, sequence numbers
and timestamps associated with the subflow.
[Open Issue: How to differentiate between Passive and Active
connections?]
[Open Issue: How to keep a passive connection alive, if not actively
used?]
8.2. RTP Transmission
The MPRTP layer SHOULD associate an RTP packet to 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, an MPRTP sender can change its scheduling
strategy during an ongoing session. The MPRTP sender MUST also
populate the flow specific fields described in the MPRTP extension
header (see Section 9.1).
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.
By calculating optimum skew across all paths, the receiver can
compensate for the jitter by modifying the playout delay (adaptive
playout) of the received RTP packets.
8.4. Flow specific RTCP Statistics and RTCP Aggregation
Aggregate RTCP provides the overall media statistics and follows the
standard RTCP defined in RFC3550 [1]. However, flow specific RTCP
provides the per path media statistics because the aggregate RTCP
report may not provide sufficient per path information to an MPRTP
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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 [5] to transmit the aggregate and flow-specific RTCP reports.
Also, each subflow and the aggregate RTCP report MUST follow the
timing rules defined in [4].
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, a peer 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
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 sub-section we define the protocol structures described in
the previous sections.
9.1. MPRTP RTP Header Extension
The MPRTP header extension is used to 1) distribute a single RTP
stream over multiple subflows, 2) advertise the endpoint's multiple
interface addresses, and 3) perform connectivity checks on the
advertised interfaces.
9.1.1. MPRTP RTP header 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|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 0x10 | 0x00 | length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP H-Ext ID | length | MPR_Type=0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow ID | Flow specific Sequence Number |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: MPRTP header for subflow
RTP H-Ext ID and length: 8-bits each
It conforms to the 2-byte RTP header extension defined in [6].
RTP H-Ext=TBD
The 8-bit length field is the length of extension data in bytes
not including the RTP H-Ext ID and length fields.
MPR_Type: 16-bits
The MPR_Type field corresponds to the type of RTP packet.
Namely:
+---------------+---------------------------------------------------+
| MPR_Type | Use |
| Value | |
+---------------+---------------------------------------------------+
| 0x00 | Subflow RTP Header. For this case the Length is |
| | set to 7 |
| 0x01 | Connectivity Check. For this case the length is |
| | set to 0 |
| TBD | Keep Alive Packet. |
+---------------+---------------------------------------------------+
Figure 7: RTP header extension values for MPRTP (MPR_Type)
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Flow ID: Identifier of the subflow. Every RTP packet belonging to
the same subflow carries the same unique flow identifier.
Flow specific Sequence Number: Sequence of the packet in the
subflow. Each subflow has its own strictly monotonically
increasing sequence number space.
9.2. MPRTP RTCP Header Extension
The MPRTP RTCP header extension is used 1) to provide RTCP feedback
per subflow to gauge the characteristics of each path, 2) to
advertise the multiple interface addresses for a media receiver, and
3) perform connectivity check on the new interfaces.
9.2.1. MPRTP RTCP header extension for flow specific SR/RR
TBD
9.2.2. MPRTP RTCP header extension for Interface advertisement
This sub-section defines the RTCP header extension for in-band
interface advertisement by the receiver, instead of relying on ICE or
in situations when the interface appears after SDP session
establishment.
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| RC | PT=MP_IA=2xx | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPRR_Type=0x00 | length | RESV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #1 Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #2 Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #... Advertisement block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface #m Advertisement block |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 8: MPRTP Interface Advertisement. (RTCP SR/RR header
extension)
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MP_IA: 8 bits
Indicates that it is a RTCP extension for interface
advertisement.
MPRR_Type: 16-bits
The MPRR_Type field corresponds to the type of MPRTP RTCP
packet. Namely:
+---------------+---------------------------------------------------+
| MPRR_Type | Use |
| Value | |
+---------------+---------------------------------------------------+
| 0x00 | Interface Advertisement |
| | |
| 0x01 | Connectivity Check. For this case the length is |
| | set to 0 |
| TBD | Keep Alive Packet. |
+---------------+---------------------------------------------------+
Figure 9: RTP header extension values for MPRTP (MPR_Type)
length: 8-bits
The 8-bit length field is the length of extension data in bytes
not including the MPRR_Type and length fields. The value zero
indicates there is no data following.
Interface Advertisement block: variable size
Defined later in the section.
9.2.3. Interface Address 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 RFC 5760 [7].
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type={0,1,2} | Length | RTP Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTCP Port | Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Interface Address 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
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.
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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 encoded [8].
10. SDP Considerations
The packet formats specified in this document define extensions for
RTP and RTCP. The use of MPRTP is left to the discretion of the
sender and receiver.
A participant of a media session MAY use SDP to signal that it
supports MPRTP. Not providing this information may/will make the
sender or receiver ignore the header extensions. However, MPRTP MAY
be used by either sender or receiver without prior signaling.
mprtp-attrib = "a=" "mprtp" [":"
mprtp-optional-parameter]
CRLF ; flag to enable MPRTP
The literal 'mprtp' MUST be used to indicate support for MPRTP.
Generally, senders and receivers SHOULD indicate this capability if
they support MPRTP and would like to use it in the specific media
session being signaled. However, it is possible for an MPRTP sender
to stream data using multiple paths to a non-MPRTP client.
Currently, there are no extensions defined for the literal 'mprtp'
but we provide the opportunity to extend it using the mprtp-optional-
parameter.
10.1. Increased Throughput
The MPRTP layer MAY choose to augment paths to increase throughput.
If the desired media rate exceeds the current media rate, the peers
MUST renegotiate the application specific ("b=AS:") [16] bandwidth.
10.2. MPRTP using preloaded ICE
TBD
11. Acknowledgements
Varun Singh, Saba Ahsan, and Teemu Karkkainen are supported by
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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.
12. IANA Considerations
This document defines a new SDP attribute, "mprtp", within the
existing IANA registry of SDP Parameters.
TBD.
13. Security Considerations
All drafts are required to have a security considerations section.
See RFC 3552 [17] for a guide.
14. References
14.1. 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] 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.
[5] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities and
Consequences", RFC 5506, April 2009.
[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
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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.
14.2. Informative References
[9] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
[10] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development",
draft-ietf-mptcp-architecture-05 (work in progress),
January 2011.
[11] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim
Protocol for IPv6", RFC 5533, June 2009.
[12] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[13] 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.
[14] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M.
Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)",
draft-ietf-mmusic-rfc2326bis-26 (work in progress),
November 2010.
[15] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[16] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[17] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", BCP 72, RFC 3552, July 2003.
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Authors' Addresses
Varun Singh
Aalto University
School of Science and Technology
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: varun@comnet.tkk.fi
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
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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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