Network Working Group W. Lei
Internet-Draft W. Zhang
Intended Status: Experimental S. Liu
Expires: July 24, 2017 Northeastern University
January 24, 2017
Multipath Real-Time Transport Protocol
Based on Application-Level Relay (MPRTP-AR)
draft-leiwm-avtcore-mprtp-ar-07
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 multimedia 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 MPTS-AR.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 24, 2017.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. MPRTP-AR User Agent Behavior . . . . . . . . . . . . . . . . . 5
4.1 Flow Partitioning . . . . . . . . . . . . . . . . . . . . . 5
4.2 Subflow Packaging . . . . . . . . . . . . . . . . . . . . . 6
4.3 Subflow and Flow Recombination . . . . . . . . . . . . . . . 6
4.4 Subflow Reporting . . . . . . . . . . . . . . . . . . . . . 6
4.5 Flow 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 Subflow Sender Report . . . . . . . . . . . . . 10
5.2.2 MPRTP-AR Subflow Receiver Report . . . . . . . . . . . . 12
5.2.3 MPRTP-AR keep-alive packet . . . . . . . . . . . . . . . 14
5.2.4 MPRTP-AR Flow Recombination Report . . . . . . . . . . . 14
6. SDP Considerations . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Signaling MPTP Capability in SDP . . . . . . . . . . . . . . 16
6.2 An Offer/Answer Example . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 18
8.2 Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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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 multimedia
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 MPTS-AR.
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 RFC 2119 [RFC2119].
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
provided by normal 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 normal RTP including
payload type identification, sequence numbering, timestamping and so
on.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP-based multimedia Applications |
| (VoIP, video conference, streaming, ...) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^
| Application 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 transport statistics. So, payload content
of MPTC sub-layer includes both 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 individual MPRTP-AR data packet.
RTCP packets may be treated the exact same as RTP packets which are
distributed across multiple paths. In addition, as described in RTP
specification, RTCP traffic is limited to a small fraction of the
session bandwidth, which is recommended no more than five percent.
So, RTCP packets may also be treated differently from RTP packets and
delivered 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 keep-alive packets are used to keep relay paths alive
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actively by user agent. The user agent sender generates MPRTP-AR
keep-alive packets periodically for both active paths and non-active
paths and sends them along the associated path. The user agent
receiver does nothing when receiving MPRTP-AR keep-alive packets.
MPRTP-AR report packets are used to monitor the transport quality of
each active path and multiple concurrent paths. MPRTP-AR-aware user
agent MUST generate individual MPRTP-AR report packets for per
subflow. The user agent sender generates MPRTP-AR Subflow Sender
Report (SSR) packets for each subflow and sends them along the
associated active path. The user agent receiver generates a MPRTP-AR
Subflow Receiver Report (SRR) packet when receiving a MPRTP-AR SSR
packet and sends it along the default path. In addition, the user
agent receiver optionally generates MPRTP-AR Flow Recombination
Report (FRR) packets for the whole flow and send them to the user
agent sender along the default path. The user agent sender may modify
its strategies of flow partitioning and scheduling based on the
transport quality feedback in MPRTP-AR SRR and FRR packets.
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. Considering the
characteristics of multimedia data, flow partitioning may be done
based on a number of factors, such as 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. MPTC sub-layer does not care what media type (such as
audio and video) the payload data is and what coding method the
payload data uses. MPTC sub-layer dispatches the formatted RTP/RTCP
packets passed from upper RTP sub-layer to multiple subflows
evenly based on the local information maintained, such as 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
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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 and adverse impact of different transport qualities among
multiple paths on the final quality of media data received in the
receiving end.
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 flow sequence number increments by one
for each assigned RTP packet from upper application. The initial
value of flow the sequence number SHOULD be random.
The assigned RTCP packets are also formatted into MPRTP-AR data
packets except for the difference that the SSSN and FSN are fixed to
zero.
4.3 Subflow and Flow 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, MPTC 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. MPTC sub-layer has no need to restore firstly the order of
each subflow using path identifier and SSSN in MPTP data packet
headers before passing the encapsulated RTP/RTCP packets up to RTP
sub-layer. The RTP sub-layer follows the normal process defined in
RTP specification to recombine the original data flow.
4.4 Subflow Reporting
User agent generates MPRTP-AR report packets for per subflow to
monitor the quality of path delivery. The user agent sender generates
MPRTP-AR Subflow Sender Report (SSR) packets for each subflow and
sends them along the associated active path. The user agent receiver
generates a MPRTP-AR Subflow Receiver Report (SRR) packets when
receiving a MPRTP-AR SSR and sends it along the default path.
The user agent sender calculates the transmission interval of
MPRTP-AR SSR packets according to some strategy. The user agent
sender may generate MPRTP-AR SSR packets at a constant rate. In this
case, it is recommended that the default interval of MPRTP-AR SSR
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packets be one second. The user agent sender may also generate
MPRTP-AR SSR packets 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%, which
makes reference to the recommended value in RTP specification.
Reception quality feedback is useful for the user agent sender who
may modify its strategies of flow partitioning and scheduling based
on the estimated transport qualities of multiple paths.
Cumulative counts are used in both the MPRTP-AR SSR and SRR packets
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
transport quality of the associated path. The difference between the
two reports with a longer time interval can be used to estimate the
long-term transport quality of the associated path. The NTP timestamp
is also 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 SRR packets. 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
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
SRR packets include 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.
4.5 Flow Reporting
The user agent receiver optionally generates MPRTP-AR Flow
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Recombination Report (FRR) packets for the whole recombined flow and
sends them to the user agent sender along the default path. Real-time
applications are generally latency- and loss rate-sensitive.
Therefore, the flow recombination reports include the cumulative
packet loss rate and interarrival jitter caused by out-of-order
packets which arrive at the receiver along different paths. The user
agent receiver calculates the transmission interval of MPRTP-AR FRR
packets according to some strategy. In this document, it is
recommended that the default interval of MPRTP-AR FRR packets be one
second.
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. For all the MPRTP-AR
packets, the application-specific MPTP type (AMT) field in the fixed
MPTP header is set to 1.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Sequence Number |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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 1 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.
The initial FSN is randomly generated when the multipath session is
first established. The FSN is increased by one for each RTP packet.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Sequence Number=0 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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 transmission requirements
of precedence 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 MPTP data packet is more
important and requires a higher level of effort to ensure delivery.
The SSSN field and FSN field in MPRTP-AR data packets carrying RTCP
packets are 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
Subflow Sender Report (SSR) packets, MPRTP-AR Subflow Receiver Report
(SRR) packets, and MPRTP-AR Flow Recombination Report (FRR) packets.
5.2.1 MPRTP-AR Subflow 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 SSR summarizes the data transmissions of the associated
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 whose packet formats
follow the rules specified in this document.
The MPTP control packet type field is set to 1 to indicate that this
packet is a MPRTP-AR SSR.
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 SSR packet
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
SSR packet was generated. This field can be used to estimate the
average payload data rate.
5.2.2 MPRTP-AR Subflow 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 SSR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SSR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
MPRTP-AR SRR 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 SRR.
The highest subflow-specific sequence number (SSSN) received: 16 bits
The highest subflow-specific sequence number received in an
MPRTP-AR data packet from a subflow.
Cumulative number of packets lost: 16 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 packets with non-zero SSSN value in a
subflow, 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 SRR is issued, the current value of J is
sampled.
Last SSR timestamp (LSR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp received as part
of the most recent MPRTP-AR SSR from a subflow. If no MPRTP-AR SSR
has been received yet, the field is set to zero.
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Delay since last SSR (DLSR): 32 bits
The delay, expressed in units of 1/65536 seconds, between
receiving the last MPRTP-AR SSR from a subflow and sending this
MPRTP-AR SRR. If no MPRTP-AR SSR 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 SRR is received, calculates the total round-trip time A-LSR
using the last SSR 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.
5.2.4 MPRTP-AR Flow Recombination 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=4 | Length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| highest FSN received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cumulative num of packets lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
MPRTP-AR FRR conveys statistics on the reception of MPTP data packets
of the whole recombined flow. 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 4 to indicate that this
packet is a MPRTP-AR FRR.
The highest Flow Sequence Number (FSN) received: 32 bits
The highest flow sequence number received in an MPRTP-AR data
packet of the whole flow.
Cumulative number of packets lost: 32 bits
The total number of MPRTP-AR data packets of a whole flow that
have 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 FSN of
MPTP data packets received less the initial FSN 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 packets with non-zero SSSN value in a flow,
measured in timestamp units and expressed as an unsigned integer.
Its calculation method is identical as the field with the same
name.
6. SDP Considerations
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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
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].
As recommended in [13] and [14], this example uses IPV6 addresses.
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 IP6 2001:db8:a0b:102::1
s=
c=IN IP6 2001:db8:a0b:102::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 2001:db8:a0b:103::1, 2001:db8:a0b:104::1 and
2001:db8:a0b:105::1 respectively.
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Modified Offer:
v=0
o=alice 2890866901 2890866902 IN IP6 2001:db8:a0b:102::1
s=
c=IN IP6 2001:db8:a0b:102::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 0x1a3b6c9d IP6/2001:db8:a0b:103::1/10000
a=relay-path:2 0x9i8u7y6t IP6/2001:db8:a0b:105::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 IP6 2001:db8:a0b:106::1
s=
c=IN IP6 2001:db8:a0b:106::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 IP6 2001:db8:a0b:106::1
s=
c=IN IP6 2001:db8:a0b:106::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 0x2w3e4r5t IP6/2001:db8:a0b:103::1/10000
a=relay-path:2 0x4r5t6y7u IP6/2001:db8:a0b:104::1/10000
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7. Security Considerations
TBD
All drafts are required to have a security considerations section.
See RFC 3552 [10] for a guide.
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.
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[12] Lei, W., Zhang, W., and S. Liu, "A Framework of Multipath
Transport System Based on Application-Level Relay",
draft-leiwm-tsvwg-mpts-ar-05 (work in progress), January 2016.
[13] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC3849, July 2004.
[14] Robachevsky, A., "Mandating use of IPv6 in examples",
draft-robachevsky-mandating-use-of-ipv6-examples-01 (work in
progress), April 2016.
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Authors' Addresses
Weimin Lei
Northeastern University
Department of Communication and Electronic Engineering
College of Computer Science and Engineering
Shenyang, China, 110819
P. R. China
Phone: +86-24-8368-3048
Email: leiweimin@mail.neu.edu.cn
Wei Zhang
Northeastern University
Department of Communication and Electronic Engineering
College of Computer Science and Engineering
Shenyang, China, 110819
P. R. China
Email: zhangwei1@mail.neu.edu.cn
Shaowei Liu
Northeastern University
IDepartment of Communication and Electronic Engineering
College of Computer Science and Engineering
Shenyang, China, 110819
P. R. China
Email: liu_nongfu@163.com
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