Contents

1 What is the PPSP?

• The PPSP is an application-layer protocol for the real-time streaming of media content between (usually networked) processes (peers), as well as the delivery of content-on-demand.
• The PPSP comprises two different protocols:
  1. The PPSP-TP (PPSP Tracker Protocol): The tracker protocol handles the initial and periodic exchange of meta information between trackers and peers, such as which peers can provide the content. A descentralized tracking mechanism has also been developed.
  2. The PPSPP (PPS Peer Protocol): The peer protocol controls the exchange of the content between the peers.

2 Basic operation

• When a peer wants to receive streaming of a selected content (leech mode):
  1. Peer connects to a tracker and joins a swarm.
  2. Peer acquires a list of other peers in the swarm from the tracker.
  3. Peer exchanges its content availability with the peers on the obtained peer list (peer protocol).
  4. Peer identifies the peers with desired content.
  5. Peer requests content from the identified peers (peer protocol).

3 Transport protocols

• Usually, the UDP is used for transport in the PPSPP. In this case, each chunk is explicity acknowledged.
• The PPSP can use also the TCP.

4 Architecture

• The content is provided by seeder peers. Several seeder peers can be in the same swarm (content-on-demand).
• The peers meet each other asking a tracker (the tracker does not handle the content) or an other (connected) peers.
• Peers are “connected”¹ ramdomly, depending on the churn and the network behaviour.

5 Chunk delivery

• Peers annouce to the peers that are connected the chunks that it has using “HAVE” messages. In general, HAVE messages are piggybacked onto DATA (chunk) messages.
• Peers request chunks using “REQUEST” messges (pull scheme).
• Peers transmitt as much content as possible, depending on the requests of the others peers, although a peer can “choke” (avoid to send data) other peer for some reason (for example, because the upload capacity is exhausted).

6 Freeriding prevention

• Freeriding (how to deal with peers that only download content byt never upload) is optional (it is defined as an extension).
• Proposes to use the Tit-For-Tat Algorithm as used in BitTorrent (initially, a peer A tries to upload as much as possible to another peer B and if B does not recriprocate, A chokes B and try with a different randomly selected peer).

7 Content integrity

• Peers receive a cryptographic hash for earch chunk of data which can be used to check if the content has been modified in transit and allows every peer to directly detect when a malicious peer tries to distribute fake content.
• When a peer detects a malicious peer, it is placed on a blacklist and not served anymore.

8 Swarms (channels)

• Peers can listen to multiple swarms when they desire to receive several channels.