Playback delay in p2p streaming systems with random packet forwarding Viktoria Fodor and Ilias Chatzidrossos Laboratory for Communication Networks School of Electrical Engineering KTH, Royal Institute of Technology

18-Sep-08 FMN P2P Multimedia Streaming Peer-to-peer system  peers contribute with transmission bandwidth and processing power  system transmission capacity scales as the number of peers increases Peer-to-peer live streaming  newly generated content has to be propagated to all peers with low delay Different from offline content distribution  strict delay requirements

18-Sep-08 FMN Context of this work We propose streaming algorithms for mesh based streaming systems Build an analytic framework for performance evaluation Verify the validity of our model Derive playback delay – playout continuity charactersitics

18-Sep-08 FMN Mesh based overlays (I) Peers are organized in a mesh (grid) There is minimal overhead in maintaining the overlay Each peer has a set of neighboring peers that it communicates and exchanges data with Each data chunk in a mesh overlay goes down a spanning tree to reach all peers. That tree is different for every packet

18-Sep-08 FMN Mesh based overlays (II) Different forwarding schemes  Push: a peer decides which data to send to which neighbor  Pull: a peer explicitly asks for specific data from a neighbor  Hybrid: mixture of the above schemes How do peers know whether some of their neighbors have a specific packet or not?

18-Sep-08 FMN Buffer contents and buffer maps All peers have a buffer to absorb variations in packet delivery times Any of the packets that a peer has in its buffer could be potentially sent to some of its neighbors Data exchange between neighbors is based on information that they have on each others buffer contents A buffer map is a compact representation of a peer’s buffer, suitable for sending to other peers

18-Sep-08 FMN Push scheduling algorithms Random scheduling:  Peer constructs the list of neighbors that are missing at least one packet that itself has  Chooses randomly one of them to forward to.  Chooses randomly one missing packet to send Priority Scheduling:  Peer selection same as in the previous case.  Once the neighbor is chosen, the ”oldest” missing packet is sent

18-Sep-08 FMN System description No playback lag among peers  At any point in time peers have the same limits for their buffers Time is slotted  Length of a time-slot equal to a packet duration time  All transmissions occur within a time-slot Synchronous and Asynchronous schemes Static Overlay  Streaming server Upload capacity = m * streaming rate  N peers Upload capacity = streaming rate Download capacity unconstrained

18-Sep-08 FMN Data propagation At time-slot i, root node forwards packet i to m randomly chosen peers Each peer forwards one packet to one of its neighbors at each time-slot based on the algorithm used Buffer map exchanges among neighboring peers occur at every time-slot  Forwarding decision based on perfect knowledge After B time-slots, peers start playing out the content they have received  Buffer size = Playback delay

18-Sep-08 FMN Model skeleton Transmission trees are different for each packet The path that a packet follows depends on the local decisions at the peers Peers having a large amount of neighbors generate per packet distribution trees that are very different The position of the peers in the distribution trees is statistically the same

18-Sep-08 FMN Model parameters Number of peers: N Root capacity: m Number of neighbors of a peer: d Buffer size of peers: B Buffer contents of peer α at time i:

18-Sep-08 FMN Mathematical model (I) Denote by P i j the probability that an arbitrary peer is in possession of packet j by the end of time-slot i Probability that a packet j will be successfully played out A peer is in possession of a packet at the end of a time slot i, if it already had that packet at time-slot i-1 or if it did not have it but received it by some neighbor during slot i. Probability that an arbitrary neighbor sends packet j during time-slot i

18-Sep-08 FMN Mathematical model (II) We consider an arbitrary peer r that does not have packet j and a neighbor thereof, s, that has it We define the events And we get that The factor that differentiates the two considered schemes

18-Sep-08 FMN Model validation For small values of d, the dispersion of the measured probabilities around the mean is big whereas as d increases this dispersion becomes smaller and smaller

18-Sep-08 FMN Playout probability and number of neighbors Discrepancy between model and simulations for small values of d For d > 8, the model gives a very good match with the simulations, verifying our assumption of statistical independence For d > 10, the playout probability seems to be insensitive to the increase of d

18-Sep-08 FMN Playout probability and delay Minimum delay for optimal tree Random Scheduling

18-Sep-08 FMN Playout probability and delay Priority scheduling Minimum delay for optimal tree

18-Sep-08 FMN Scalability Increase of the minimum playback delay is logarithmic in N for both forwarding schedules

18-Sep-08 FMN Conclusions We have proposed a general model to study the playback delay in p2p streaming networks We have proved the validity of the model via simulations The random forwarding proves to be efficient in delivering data to a large amount of peers at a relatively low delay Priority scheduling performs poorly even at high playback delays and thus should not be used

Playback delay in p2p streaming systems with random packet forwarding Viktoria Fodor and Ilias Chatzidrossos Laboratory for Communication Networks School of Electrical Engineering KTH, Royal Institute of Technology