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Content-Aware P2P Video Streaming with Low Latency

By Pierpaolo Bacchichet, Jeonghun Noh, Eric Setton, Bernd Girod

2007 IEEE International Conference on In Multimedia and Expo, pp. 400-403.

Overlay Multicast Protocol

• Stanford P2P Multicast (SPPM) protocol
• Peer nodes in an overlay that consists of a set of multicast trees
• The source of the stream is the root of each distribution path and multiplexes video packets on different trees to ensure load balancing.
• Simple topology with eight peers and two distribution trees
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• Topology construction and management
• Join process consists of a "six-way" handshake
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1. JOIN : The new peer contacts the source of the video stream that replies with some setup information, such as the number of multicast trees and the video bit rate. Also, it sends a set of currently connected peers that can be pooled as candidate parents.
2. PROBE : For each tree, the new peer probes each candidate parent to obtain their current state and determine the best parent to which to connect. Each candidate parent replies providing the available bandwidth and it height in the distribution tree.
3. ATTACH : For each tree, one out of the set of candidate parents is selected and an actual connection is established, The parent is selected by minimizing the height of the distribution tree. Also, different parents are chosen as often as possible to exploit path diversity.

• Monitoring the connection
• Each peer periodically sends HELLO_REQ messages to the parents in the distribution trees which in turn reply with a HELLO_REP message.
• Peers can detect a parent disconnection and trigger the rejoin procedure for the affected tree.
• Parent peers record the arrival times of the HELLO_REQ messages received by each child, in order to detect children disconnections and release resources.

• Local retransmission
• See [7] for further details

• Loop prevention
• Each peer keeps the list of its ancestors to prevent sub-tree starvation
• A specific ANCESTOR_UPDATE packet allows updating of this list whenever a change in the topology occurs.
• Peers can easily reject attachment requests issued by an ancestor.
• This loop avoidance mechanism does not require global knowledge of the topology but only of a small subset of its nodes.
• Additional memory and processing power requirements are negligible.

• Content-aware scheduling
• Congestion-Distortion optimized CoDiO packet scheduler [7]
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• "importance" D(n) of each enqueued packet
• Like a frame dropping

System Architecture

• A real-time prototype of the SPPM protocol with H.264/AVC
• Maximum of 1300 bytes to reflect the MTU of a typical ethernet network
• UDP as a transport protocol to avoid the overhead involved in the usage of TCP
• Two different UDP ports are used for the control and video traffic respectively
• Packet types

• Software architecture of a SPPM peer
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• Control Agent
• Maintaining the connection to the system, Accepting connection requests from the children and Updating the routing tables accordingly.
• Monitoring the status of parents by sending HELLO_REQ messages and Triggering the reconnection process in case of a disconnection
• Video Agent
• Receiving the incoming video packets, Retrieving from the routing table th list of children to which the packet has to be relayed and then Storing it into the video queue
• Routing table
• Storing the list of children for each tree
• Queue Manager
• Implementing a common buffer to store packets that are to be relayed or displayed
• A garbage collector periodically discards packets that are expired, i.e. that have passed their playout deadline
• Relay Agent
• Periodically Extracting from the queue the next packet to be sent and Forwarding it to the appropriate child
• Playout Buffer
• Retrieving packets that are to be displayed from the video queue and Passing them to the video decoder

• Two modules of source of the video stream
• Video Packetizer
• Reading the H.264/AVC input video, either from a file or from a socket, and performs packet fragmentation whenever it is necessary.
• The maximum size of a video packet equal to 13-- bytes to reflect the MTU of an Ethernet
• Connected peers' database
• Maintaining an approximate list of connected peers.
• Each entry in the list is refreshed with periodical information received from the peers via PEER_UPDATE packets.
• If the source fails to receive such packets from a registered peer for a certain amount of time, it considers the peer as disconnected from the system and frees the entry in the table

• NAT Support
• When source receives a join request, it can detect the public address of the new peer and send this information to the set of potential candidate parents.
• The parents then transmit a dummy packet to the control port of the new peer.
• This creates a binding on the NAT device that will enable the forwarding of subsequent PROBE_REQ messages.

Experimental Results

• Environments
• Tested on Planet-Lab (http://www.planet-lab.org) and performed experiments using 100 nodes.
• CIF, 30 fps, H.264/AVC at 400 kbps, 34.5dB of PSNR, 30 minutes of video streaming
• One intra coded frame every half second, 15 frames in a GOP, 5 seconds playout deadline, 8 different multicast trees
• The peers were scheduled to randomly connect within the first minute and to stay connected to the system for the whole session.

• Video performance
• No retransmission : 30.1 dB, Retransmission : 31.8 dB, Retransmission and Prioritization : 33.2db
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• Scheme	0.5 to 1 sec.	1 to 2 sec.	More than 2 sec
  No retransmission	4262	1784	457
  Retransmission	742	283	127
  Retransmission & Prioritization	364	143	43

• Startup delay
• Average : 1 second, All the peers connected to the system within two seconds from the join request

• Protocol overhead
• Control packets are no more than 4% of the total traffic
• The overhead of the protocol is smaller than 8% of the total traffic
• include additional header information introduced for each video packet (i.e.: Sequence numbers and timestamps)

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