1. Multipath RTP Applying Multipath Communication to Real time Applications
By: Saba Ahsan
Supervisor: Prof. Jörg Ott
Conducted at Comnet, Aalto University of Science & Technology

2. Contents Motivation Problem Statement Background MPRTP Protocol
Goals
Architecture
MPRTP Header Extension
Implementation – RAMP-UP
Receiver Jitter Buffer
RAMP-UP Sender
Testing and Results
Conclusion

3. Motivation
Most transport protocols select a single path for communication flow between two end hosts, even when multiple paths exist. Such flows are unable to fully utilize the available resources.
Multihomed clients have more than one network interface. Multipath capability refers to the simultaneous use of multiple paths through the network, which may significantly improve performance and reliability.
In real-time communication this could would improve the end-user experience by enhancing the QoS.
Bandwidth-hungry applications such as video streaming and IP-TV can benefit from the increased, combined throughput available to multihomed clients
retransmission of lost data is often uncharacteristic of real-time traffic because of time constraints; multipath senders can avoid lossy paths or send redundant data over multiple paths
session-based real-time communication can benefit from the redundancy by implementing failover in case of network failures

4. Problem Statement
Design a solution for the transport of real-time data while simultaneously using multiple paths on multihomed clients.
Design of Multipath RTP (MPRTP) an extension of RTP protocol, with multipath capabilities.
Design, implementation and testing of an MPRTP based solution for video streaming to a single receiver, when multiple paths exist between the sender and receiver.

5. Background Real-Time Protocol (RTP)
RTP is an end-to-end protocol designed for transporting real-time traffic such as voice and video, over multicast and unicast.
It uses RTCP (Real-time Control Protocol) for monitoring the transmission quality, however it does not guarantee QoS.
It is independent of the transport and network layers.
A signalling protocol such as SIP or SDP is used for managing RTP sessions.
It does not care about congestion control or fair usage like TCP.

6. MPRTP Protocol
Internet Draft :

7. MPRTP Goals Increased Throughput Improved Reliability Compatibility
Concurrent use of paths such that the combined available capacity is higher than the capacity of any individual path
Improved Reliability
MPRTP should be able to transmit redundant streams on different paths for reliability and support fallback in case of path failures for robustness.
Compatibility
Application Compatibility: MPRTP stack must be capable of working with legacy RTP applications.
Network Compatibility: MPRTP subflows should appear as RTP flows and be able to traverse through NATs and Firewalls.

8. Architecture Each path represents an MPRTP subflow.
Like RTP, MPRTP can work with different transport protocols.
Application
MPRTP
RTP …
UDP/TCP IP
Physical

9. MPRTP Specification Internet Path Management
Path awareness and management of port+IP pair bindings. MPRTP is designed to use in-band signaling for path advertisements and/or connectivity checks. Interface discovery may be done using ICE.
Packet Scheduling
Splitting of data into multiple subflows across different paths.
Subflow recombination
Recombining the subflows, so that it appears as a single stream to the application
MPRTP Sender
MPRTP Receiver
Internet
subflow 1
subflow 2
subflow 3
MPRTP Flow
Gather characteristics of different paths and schedule packets accordingly
Reorder the data correctly and hand over to application, send reports about quality

10. MPRTP Header Extension for RTP
RTP sequence numbers used for packet reordering of the stream
Flow-specific sequence numbers increase monotonically for each path, independent of other paths. 1 2 3 4 5 6 7 8 9 10 20 30
V=2 P X CC M PT
Sequence number
Timestamp
Synchronization source (SSRC) identifier
Contributing source (CSRC) identifiers ……
RTP H-Ext ID
length
MPR_Type
Flow ID
Flow specific sequence number
RTP payload …….

11. RTP Adaptation for Multiple Paths – Using Percentage distribution
MPRTP Implementation
RAMP-UP
RTP Adaptation for Multiple Paths – Using Percentage distribution

12. RAMP-UP Receiver Ordering is based on overall sequence number.
Packets are inserted into the jitter buffer as soon as they arrive.
Playout starts after a predefined latency period. We use 2 seconds for our testing.
Late packets are discarded 9 5 8 6 4 7 3 2 1
Time
Path 1
Path 2
Reordering in Jitter Buffer

13. RAMP-UP Sender (1)
RAMP-UP sender is designed for video streaming across multiple paths.
We assume that the bitrate of the video is higher than the bitrate of any of the available paths and hence it is necessary to use more than one path.
A common bottleneck may exist. However, if it is common to all available paths, then congestion/losses can not be avoided.
The sender is not capable of reducing video bitrate.
We utilize a non-aggressive approach, which implies that we do not put more traffic on a path unless necessary. This in turn implies that the full capacity of certain paths may never be known.

14. RAMP-UP Sender (2)
RAMP-UP uses percentage distribution on the paths. Percentages are assigned according to measured characteristics. This approach eliminates flapping and ensures equal distribution of traffic on all paths if video rate is increased.
Measurements are based on data gathered by RTCP reports.
All packets of a frame are sent on the same path.
Initially, equal percentage is assigned to each path.
Path 1 Queue
To Receiver
Sender’s Buffer
Decision based on percentage level
Path 2 Queue

15. RAMP-UP Sender (3)
Using RTCP reports, the sender is able to calculate the bitrate observed on each path (TBi)
The observed bitrate depends on the amount of traffic being sent on the path, hence the sender would only update bitrate values if they are higher than what was previously recorded, except if the ratio of lost packets (Li) is greater than 0 in the RTCP RR.
Average packet size (Si) is calculated for each interval during which the bitrate was measured.
If a path has continuous losses, it is considered congested, and in this case the observed bitrate is stored in another variable called CBi .
The sender assigns a percentage of traffic to each path using TBi values if path is not congested, and CBi value if path is congested.
Congestion condition is cleared if losses don’t appear for a predefined amount of time ( in our testing we use 25 seconds)

16. RAMP-UP Sender (4) Sender Receiver HSN = 1000 HSN2 = 2300, t2
t2 -t1
HSN = 2300
RTP
RTCP

17. Testing & Results

18. Test Setup
The test environment consists of virtual machines running on a single physical machine.
Network properties are emulated using Network Emulator (NetEm)
MPRTP Sender
MPRTP Receiver
Router1
Router2
Router3
Virtual Environment

19. Results: Bitrate of data being sent on each path as measured by the sender when three paths are available. Path capacities are changed during simulation.
Bitrate (kpbs)
Time (s)

20. Results: Percentages assigned to the paths over time Total Packets lost = 1.6%, Frames lost = 6%, BER = 1.8%
Assigned Ratios
Time (s)

21. Conclusion
It is possible to achieve higher bitrates using multiple paths, which may help streaming higher quality videos.
The extra paths may be used to avoid losses due to temporary congestion on any of the paths.
Video streaming, is just one of the many applications of MPRTP.
MPRTP protocol is still in its infancy.
RAMP-UP focuses on video streaming only. The scheduling algorithm can be improved further. Multiple streams (voice/video, lip-sync) and rate-control mechanism can be incorporated.
Many research opportunities arise from this study
MPRTP for mobile environments, 3G, GPRS & WLAN interfaces
MPRTP for voice fallback