Multipath Real-Time Transport Protocol Based on Application-Level Relay (MPRTP-AR)
draft-leiwm-avtcore-mprtp-ar-00
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| Authors | Lei Weimin , Wei Zhang , Shaowei Liu | ||
| Last updated | 2013-07-29 | ||
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draft-leiwm-avtcore-mprtp-ar-00
AVT Core Working Group W. Lei
Internet-Draft W. Zhang
Intended status: Experimental S. Liu
Expires: January 28, 2014 Northeastern University
July 29, 2013
Multipath Real-Time Transport Protocol
Based on Application-Level Relay (MPRTP-AR)
draft-leiwm-avtcore-mprtp-ar-00
Abstract
Currently, most multimedia applications utilize a combination of
real-time transport protocol (RTP) and user datagram protocol (UDP).
Application programs at the source end format payload data into RTP
packets using RTP specifications and dispatch them using unreliable
UDP along a single path. Multipath transport is an important way to
improve the efficiency of data delivery. In order to apply the
framework of multipath transport system based on application-level
relay (MPTS-AR) to RTP-based multipath applications, this document
defines a multipath real-time transport protocol based on
application-level relay (MPRTP-AR), which is a concrete
application-specific multipath transport protocol (MPTP). Packet
formats and packet types of MPRTP-AR follow the common rules
specified in MPTP profile. Based on MPRTP-AR, RTP-based multimedia
applications can make full use of the advantages brought by the
multipath transport system based on application-level relay.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 28, 2014.
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Copyright Notice
Copyright (c) 2013 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 (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your
rights and restrictions with respect to this document. Code
Components extracted from this document must include Simplified
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Provisions and are provided without warranty as described in the
Simplified BSD License.
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Table of Contents
1. Introduction ................................................. 4
2. Terminology .................................................. 4
3. Overview ..................................................... 4
4. MPRTP-AR User Agent Behavior ................................. 6
4.1 Flow partitioning ........................................ 6
4.2 Subflow Packaging ........................................ 6
4.3 Subflow Recombination .................................... 7
4.4 Subflow Reporting ........................................ 7
5. MPRTP-AR Packet Format ....................................... 8
5.1 MPRTP-AR Data Packet ..................................... 8
5.1.1 MPRTP-AR Data Packet for RTP packet ................ 8
5.1.2 MPRTP-AR Data Packet for RTCP packet ............... 9
5.2 MPRTP-AR Control Packet ................................. 10
5.2.1 MPRTP-AR Sender Report ............................ 10
5.2.2 MPRTP-AR Receiver Report .......................... 12
5.2.3 MPRTP-AR Keep-alive Packet ........................ 14
6. SDP Considerations ........................................... 14
6.1 Signaling MPTP Capability in SDP ......................... 14
6.2 Offer/Answer ............................................. 15
6.2.1 An Example ......................................... 15
7. Security Considerations ...................................... 16
8. References ................................................... 17
8.1 Normative References ..................................... 17
8.2 Informative References ................................... 17
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1. Introduction
Currently, most multimedia applications utilize a combination of
real-time transport protocol (RTP) [1] and user datagram protocol
(UDP). Application programs at the source end format payload data
into RTP packets using RTP specifications and dispatch them using
unreliable UDP along a single path. Multipath transport is an
important way to improve the efficiency of data delivery. In order to
apply the framework of multipath transport system based on
application-level relay (MPTS-AR) [12] to RTP-based multipath
applications, this document defines a multipath real-time transport
protocol based on application-level relay (MPRTP-AR), which is a
concrete application-specific multipath transport protocol (MPTP).
Packet formats and packet types of MPRTP-AR follow the common rules
specified in MPTP profile [12]. Based on MPRTP-AR, RTP-based
multimedia applications can make full use of the advantages brought
by the multipath transport system based on application-level relay.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC2119 [2].
3. Overview
The protocol stack architecture of the MPRTP-AR is shown in figure 1.
MPRTP-AR is divided into two sub-layers: RTP sub-layer and multipath
transport control (MPTC) sub-layer. RTP sub-layer is fully compatible
with the existing RTP specifications and provides upper applications
with the same application programming interfaces (APIs) as those by
RTP. MPTC sub-layer provides essential support for multipath
transport, including path management, flow partitioning, subflow
packaging, subflow recombination, subflow reporting and so on. At the
user agent sender, RTP sub-layer first formats the data received from
upper application into RTP packets and then passes them to lower MPTC
sub-layer. MPTC sub-layer further formats the RTP packets into
MPRTP-AR data packets. At the user agent receiver, MPTC sub-layer
extracts the RTP/RTCP packet by removing the fixed header fields of
MPRTP-AR data packet from the received packet and passes it directly
to upper RTP sub-layer. RTP sub-layer follows the normal process
defined in RTP specification to restore original data flow. This
design decision is to maximize backwards compatibility with existing
RTP applications. Moreover, MPRTP-AR can make full use of real-time
transport functions provided by RTP including payload type
identification, sequence numbering and timestamping.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Applications |
| (VoIP, video conference, streaming, ...) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^
| Applicaiton Programming
| Interfaces (APIs)
V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MPRTP-AR | RTP sub-layer |
layer +- - - - - - - - - - - - - - - - - - - - - - - - -+
| MPTC sub-layer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. The protocol stack architecture of MPRTP-AR
As defined in MPTP profile, MPRTP-AR packets are divided into two
types: MPRTP-AR data packets and MPRTP-AR control packets. MPRTP-AR
control packets include MPRTP-AR keep-alive packets and MPRTP-AR
report packets.
Besides RTP packets, RTP sub-layer generates real-time transport
control (RTCP) packets following the normal RTCP defined in [1] to
provide the overall media statistics. Payload content of MPTC
sub-layer includes RTP packets and RTCP packets. In MPTC sub-layer,
each RTP/RTCP packet is treated as an independent piece of payload
data and packaged into an MPRTP-AR data packet. RTCP packets may be
treated the exact same as RTP packets which are distributed across
multiple paths. RTCP packets may also be transferred along any one of
active paths, such as the default path or the best path among all
active paths based on quality of delivery.
When RTP and RTCP packets are multiplexed, the RTCP packet type field
occupies the same position in the packet as the combination of the
RTP marker (M) bit and the RTP payload type (PT). This field is used
to distinguish RTP and RTCP packets. It is RECOMMENDED that RTP
sub-layer follows the guidelines in the RTP/AVP profile [3] for the
choice of RTP payload type values, with the additional restriction in
[4].
MPRTP-AR report packets are used to monitor the transport quality of
the path. MPRTP-AR-aware user agent MUST generate individual MPRTP-AR
report packets for per subflow. The user agent sender generates
MPRTP-AR Sender Reports (SR) for each subflow and sends them along
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the associated active path. The user agent receiver generates
MPRTP-AR Receiver Reports (RR) when receiving a MPRTP-AR SR and sends
them along the default path. The user agent sender may modify its
strategies of flow partitioning and scheduling based on the feedback
in MPRTP-AR RRs.
4. MPRTP-AR User Agent Behavior
In addition to user agent behaviors defined in [12], MPRTP-AR user
agent needs to follow the following behaviors to support multipath
transport for RTP-based multimedia applications.
4.1 Flow Partitioning
If multiple paths are used concurrently, the original multimedia
stream should be divided into several substreams. The partition may
be done based on media type, encoding scheme and path
characteristics. Flow partitioning methods can be divided into
coding-aware partitioning methods and coding-unaware partitioning
methods. Coding-unaware partitioning methods can be performed in MPTC
sub-layer. The formatted RTP/RTCP packets are dispatched balancedly
to subflows based on the delivery quality of the associated active
paths.
For multimedia applications, a multistream coder using layered
coding, multiple description coding or object-oriented coding can
produce multiple compressed media flows. In this case, coding-aware
partitioning methods could be performed in RTP sub-layer. In layered
coding, a flow is either the base layer or one of the enhancement
layers; in multiple description coding, a flow typically consists of
packets from a description; in object-oriented coding, various
objects are coded individually and placed in so-called elementary
steams. Each coding flow corresponds to a separate subflow, or
several coding flows are multiplexed into one subflow. Data
participant in this way can effectively reduce the correlations among
subflows.
4.2 Subflow Packaging
For each subflow, the assigned RTP packets are treated as payload
data and formatted into MPRTP-AR data packets. The initial SSSN is
randomly generated when the subflow is first established and the SSSN
in subsequent subflow MPRTP-AR data packets for RTP packets is
monotonically increasing.
The assigned RTCP packets are also formatted into MPRTP-AR data
packets except for the difference that the SSSN is fixed to zero.
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4.3 Subflow Recombination
The user agent receiver recombines the original data flow according
to MPRTP-AR data packets received from multiple paths. After
receiving a MPRTP-AR data packet, MPTCP sub-layer first extracts the
RTP/RTCP packet by removing the fixed header fields of MPTP data
packet from the received packet and passes it directly to upper RTP
sub-layer. The RTP sub-layer follows the normal process defined in
RTP specification.
4.4 Subflow Reporting
User agent generates MPRTP-AR reports for per subflow to monitor the
quality of path delivery. The user agent sender generates MPRTP-AR
Sender Reports (SR) for each subflow and sends them along the
associated active path. The user agent receiver generates MPRTP-AR
Receiver Reports (RR) when receiving a MPRTP-AR SR and sends them
along the default path.
The user agent sender calculates the transmission interval of MPTP
SRs according to some strategy. The user agent sender may generate
MPRTP-AR SRs at a constant rate. In this case, it is recommended that
the default interval of subflow MPRTP-AR SRs be one second. The user
agent sender may also generate MPRTP-AR SRs at a variable rate. For
example, the report traffic is limited to a small fraction of the
associated subflow data traffic. In this case, it is recommended that
the fraction of the subflow data traffic added for subflow report be
fixed at 5%.
Reception quality feedback is useful for the user agent sender who
may modify its strategies of flow partitioning and scheduling based
on the feedback.
Cumulative counts are used in both the MPRTP-AR SRs and RRs so that
differences may be calculated between any two reports to make
measurements over both short and long time periods, and to provide
resilience against the loss of a report. The difference between the
last two reports received can be used to estimate the recent quality
of the distribution. The NTP timestamp is included so that rates may
be calculated from these differences over the interval between two
reports.
An example calculation is the packet loss rate over the interval
between two MPRTP-AR RRs. The difference in the cumulative number of
packets lost gives the number lost during that interval.
The difference in the highest SSSNs received gives the number of
packets expected during the interval. The ratio of these two is the
packet loss fraction over the interval. This ratio provides a
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short-term packet loss measurement if the two reports are
consecutive. The loss rate per second can be obtained by dividing the
loss fraction by the difference in NTP timestamps, expressed in
seconds. The number of packets received is the number of packets
expected minus the number lost.
In addition to the cumulative counts which allow both long-term and
short-term measurements using differences between reports, MPRTP-AR
RRs includes an interarrival jitter field which provides short-term
measurement of network congestion from a single report. Packet loss
tracks persistent congestion while the jitter measure tracks
transient congestion. The jitter measure may indicate congestion
before it leads to packet loss. The interarrival jitter field is only
a snapshot of the jitter at the time of a report and is not intended
to be taken quantitatively. Rather, it is intended for comparison
across a number of reports.
5. MPRTP-AR Packet Format
Packet types and packet formats of MPRTP-AR follow the common rules
specified in MPTP profile. As defined in MPTP profile, MPRTP-AR
packets are divided into two types: MPRTP-AR data packets and
MPRTP-AR control packets. MPRTP-AR control packets include MPRTP-AR
keep-alive packets and MPRTP-AR report packets.
5.1 MPRTP-AR Data Packet
As stated in section 3, the RTP packets and RTCP packets are packaged
into MPRTP-AR data packets intact. Each RTP packet and RTCP packet
corresponds to a MPRTP-AR data packet.
5.1.1 MPRTP-AR Data Packet for RTP packet
A MPRTP-AR data packet carrying a RTP packet consists of a fixed
eight-octet MPTP header and an intact RTP packet. An example is shown
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MPTP |V=1|1|P| AMT=1 |0|1|0|0| rsvd | SSSN |
header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
RTP |V=2|P|X| CC |M| PT | sequence number |
packet +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MPTP packet type (T) field is set to indicate that this packet is
a MPRTP-AR data packet.
The application-specific MPTP type (AMT) field is set to 1 to
indicate that this packet is a MPRTP-AR packet.
For RTP-based multimedia applications, latency and jitter are the
primary concerns, and occasional packet lost is acceptable. So the
delay bit in the type of service (TOS) field is set to indicate that
prompt delivery is important for this packet.
The initial SSSN is randomly generated when the subflow is first
established. The SSSN is increased by one for each subsequent
MPRTP-AR data packets carrying RTP packets of the same subflow.
5.1.2 MPRTP-AR Data Packet for RTCP packet
A MPRTP-AR data packet carrying a RTCP packet consists of a fixed
eight-octet MPTP header and an intact RTCP packet. An example is
shown 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MPTP |V=1|1|P| AMT=1 |1|0|0|1| rsvd | SSSN=0 |
header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
RTCP |V=2|P| RC | PT | length |
packet +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| .... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MPTP packet type (T) field is set to indicate that this packet
is a MPRTP-AR data packet.
The application-specific MPTP type (AMT) field is set to 1 to
indicate that this packet is a MPRTP-AR packet.
In a RTP session, RTCP packets have higher procedure and reliability
than RTP packets. So the precedence and reliability bits of the type
of service (TOS) field are set to indicate that the payload data in
this packet is more important and requires a higher level of effort
to ensure delivery.
The SSSN field in MPRTP-AR data packets carrying RTCP packets is
fixed to zero.
5.2 MPRTP-AR Control Packet
MPRTP-AR control packets include MPRTP-AR keep-alive packets and
MPRTP-AR report packets. MPRTP-AR report packets include MPRTP-AR
Sender Reports (SRs) and MPRTP-AR Receiver Reports (RRs).
5.2.1 MPRTP-AR Sender Report
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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=1|0|P| AMT=1 | CT=1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| subflow's packet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| subflow's octet count |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
MPRTP-AR SR summarizes the data transmissions of the associated
ubflow. The fields have the following meaning:
The MPTP packet type (T) field is set to zero to indicate that this
packet is a MPRTP-AR control packet.
The application-specific MPTP type (AMT) field is set to 1 to
indicate that this packet is a MPRTP-AR packet.
The MPTP control packet type field is set to 1 to indicate that this
packet is a MPRTP-AR SR.
NTP timestamp: 64 bits
Indicates the wallclock time when this report was sent so that it
may be used in combination with timestamps returned in receiver
reports to measure round-trip propagation to the user agent
receiver.
Wallclock time (absolute date and time) is represented using the
timestamp format of the Network Time Protocol (NTP), which is in
seconds relative to 0h UTC on 1 January 1900 [5]. The full
resolution NTP timestamp is a 64-bit unsigned fixed-point number
with the integer part in the first 32 bits and the fractional part
in the last 32 bits. An implementation is not required to run the
Network Time Protocol in order to use this MPTP extension. On a
system that has no notion of wallclock time but does have some
system-specific clock such as "system uptime", a user agent sender
MAY use that clock as a reference to calculate relative NTP
timestamps.
subflow's packet count: 32 bits
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The total number of MPRTP-AR data packets with non-zero SSSN value
transmitted within a subflow by the user agent sender since
starting transmission up until the time this MPRTP-AR SR was
generated.
subflow's octet count: 32 bits
The total number of payload octets (i.e., not including header or
padding) transmitted in MPRTP-AR data packets by the user agent
sender since starting transmission up until the time this MPRTP-AR
SR was generated. This field can be used to estimate the average
payload data rate.
5.2.2 MPRTP-AR Receiver Report
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=1|0|P| AMT=1 | CT=2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| highest SSSN received | cumulative num of packets lost|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
MPTP RR conveys statistics on the reception of MPTP data packets from
a single subflow. The fields have the following meaning:
The MPTP packet type (T) field is set to zero to indicate that this
packet is a MPRTP-AR control packet.
The application-specific MPTP type (AMT) field is set to 1 to
indicate that this packet is a MPRTP-AR packet.
The MPTP control packet type field is set to 2 to indicate that this
packet is a MPRTP-AR RR.
The highest subflow-specific sequence number (SSSN) received: 16 bits
The highest subflow-specific sequence number received in an
MPRTP-AR data packets from a subflow.
Cumulative number of packets lost: 24 bits
The total number of MPRTP-AR data packets from a subflow that have
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been lost since the beginning of reception. Note that a user agent
receiver cannot tell whether any packets were lost after the last
one received. This number is defined to be the number of packets
expected less the number of packets actually received. The number
of packets expected is defined to be the highest SSSN of MPTP data
packets received less the initial SSSN received. The number of
packets received includes any which are late. Thus, packets that
arrive late are not counted as lost.
Interarrival jitter: 32 bits
An estimate of the statistical variance of the interarrival time
of the MPRTP-AR data packet with non-zero SSSN value, measured in
timestamp units and expressed as an unsigned integer. The
interarrival jitter J is defined to be the mean deviation
(smoothed absolute value) of the difference D in packet spacing at
the user agent receiver compared to the user agent sender for a
pair of successive packets in a subflow. As shown in the equation
below, the difference D is also equivalent to the difference in
the "relative transit time" for the two successive packets; the
relative transit time is the difference between a packet's RTP
timestamp and the user agent receiver's clock at the time of
arrival, measured in the same units.
If Si is the RTP timestamp from packet i, and Ri is the time of
arrival in RTP timestamp units for packet i, then for two packets
i and j, D may be expressed as
D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si)
The interarrival jitter SHOULD be calculated continuously as each
MPTP data packet with non-zero SSSN value i is received from a
subflow, using this difference D for that packet and the previous
packet i-1 in order of arrival (not necessarily in sequence),
according to the formula
J(i) = J(i-1) + (|D(i-1,i)| - J(i-1))/16
This algorithm is the optimal first-order estimator and the gain
parameter 1/16 gives a good noise reduction ratio while
maintaining a reasonable rate of convergence [11].
Whenever a MPRTP-AR RR is issued, the current value of J is
sampled.
Last SR timestamp (LSR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp received as part
of the most recent MPRTP-AR SR from a subflow. If no MPRTP-AR SR
has been received yet, the field is set to zero.
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Delay since last SR (DLSR): 32 bits
The delay, expressed in units of 1/65536 seconds, between
receiving the last MPRTP-AR SR from a subflow and sending this
MPRTP-AR RR. If no MPRTP-AR SR has been received yet from this
subflow, the DLSR field is set to zero.
The user agent sender can compute the round-trip propagation delay to
the user agent receiver along a specific active path by doing the
following. The user agent sender records the time A when this
MPRTP-AR RR is received, calculates the total round-trip time A-LSR
using the last SR timestamp (LSR) field, and then subtracting this
field to leave the round-trip propagation delay as (A - LSR - DLSR).
5.2.3 MPRTP-AR keep-alive packet
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=1|0|P| AMT=1 | CT=3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
MPRTP-AR keep-alive packet is used to keep the active path alive. It
only contains a fixed eight-octet MPTP header.
The MPTP packet type (T) field is set to zero to indicate that this
packet is a MPRTP-AR control packet.
The application-specific MPTP type (AMT) field is set to 1 to
indicate that this packet is a MPRTP-AR packet.
The MPTP control packet type field is set to 3 to indicate that this
packet is a MPRTP-AR keep-alive packet.
6. SDP Considerations
6.1 Signaling MPTP Capability in SDP
This document defines a new mptp-name value for the mptp-relay
attribute: "mprtp-ar" for RTP-based multimedia application-specific
MPTP, i.e. MPRTP-AR.
RTP and RTCP packets are multiplexed to transport. So the rtcp-mux
attribute MUST be used in Session Description Protocol (SDP) [8] to
indicate support for multiplexing of RTP and RTCP packets [4]. When
an endpoint receives an SDP containing "a=mptp-relay" but without
"a=rtcp-mux", the endpoint MUST infer that the peer, if as a user
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agent sender, supports multiplexing of RTP and RTCP packets.
A user agent sender MAY use multiple paths concurrently to increase
throughput. If the desired media rate exceeds the current media rate,
the user agent sender MUST renegotiate the application specific
("b=AS:xxx") [8] bandwidth.
6.2 An Offer/Answer Example
We take the usage scenario shown in Section 5.1 in [12] as an
example. Session Initiation Protocol (SIP) [7] and SDP is used to
negotiate a multipath session following the offer/answer model [9].
User A includes an initial SDP offer in the session invitation
message. The initial SDP offer is shown as following.
Initial Offer:
v=0
o=alice 2890866901 2890866902 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=video 39160 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=mptp-relay:mprtp-ar
a=rtcp-mux
When the invitation message is processed by the server system, two
candidate relay paths are assigned for the media flow from user B to
user A. The initial SDP offer in the session invitation message is
modified as shown below. The IP addresses of RTP relay-1/relay-2/
relay-3 are 192.0.3.1, 192.0.4.1 and 192.0.5.1 respectively.
Modified Offer:
v=0
o=alice 2890866901 2890866902 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=video 39160 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=mptp-relay:mprtp-ar
a=rtcp-mux
a=relay-path:1 1a3b6c9d0e2f6g1qazxsw23edcvfr45t IP4/192.0.3.1/
10000
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a=relay-path:2 0o9i8u7y6t5r4e3w2q0okm9ijn8uhb7y IP4/192.0.5.1/
10000
If user B accepts the invitation, it includes an initial SDP answer
in the session reply message. The initial SDP answer is shown as
following.
Initial Answer:
v=0
o=bob 2890866903 2890866904 IN IP4 192.0.6.1
s=
c=IN IP4 192.0.6.1
t=0 0
m=video 36120 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=mptp-relay:mprtp-ar
a=rtcp-mux
When the relay message is processed by the server system, two
candidate relay paths are assigned for the media flow from user A to
user B. The initial SDP answer in the session invitation message is
modified as shown below.
Modified Answer:
v=0
o=bob 2890866903 2890866904 IN IP4 192.0.6.1
s=
c=IN IP4 192.0.6.1
t=0 0
m=video 36120 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
a=mptp-relay:mprtp-ar
a=rtcp-mux
a=relay-path:1 2w3e4r5t6y7u8i9o0p1q2wsx3edc4rfv IP4/192.0.3.1/
10000
a=relay-path:2 4r5t6y7u8i9o0p1q2w3e4rfv5tgb6yhn IP4/192.0.4.1/
10000
7. Security Considerations
TBD
All drafts are required to have a security considerations section.
See RFC 3552 [10] for a guide.
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8. References
8.1 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] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[4] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[5] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC 1305, March 1992.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2 Informative References
[7] 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.
[8] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[9] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[10] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", BCP 72, RFC 3552, July 2003.
[11] Cadzow, J., Foundations of Digital Signal Processing and Data
Analysis New York, New York: Macmillan, 1987.
[12] Lei, W., Zhang, W, and Liu, S., "A Framework of Multipath
Transport System Based on Application-Level Relay",
draft-leiwm-tsvwg-mpts-ar-00 (work in progress), July 2013.
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Authors' Addresses
Weimin Lei
Northeastern University
Institute of Communication and Information System
College of Information Science and Engineering
Shenyang, China, 110819
P. R. China
Phone: +86-24-8368-3048
Email: leiweimin@ise.neu.edu.cn
Wei Zhang
Northeastern University
Institute of Communication and Information System
College of Information Science and Engineering
Shenyang, China, 110819
P. R. China
Email: zhangwei1@ise.neu.edu.cn
Shaowei Liu
Northeastern University
Institute of Communication and Information System
College of Information Science and Engineering
Shenyang, China, 110819
P. R. China
Email: liu_nongfu@163.com
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