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though their small-scale experiment appears promising, the large-scale per-
formance is unknown. The time complexity of its topology-aware operation
would make it very difficult to be scalable for large-scale applications.
1.3.2 unstructured Peer-to-Peer Networks
In contrast with structured P2P networks, peers in unstructured P2P net-
works flood search requests to discover the media content.
Gnutella is a typical unstructured P2P network protocol for file sharing.
The peers in Gnutella network use ping-pong messages to discover hosts in
the network, and use query flooding to discover the content. The Time-To-
Live (TTL) field of packets is used in the query messages to limit the num-
ber of times each query message is forwarded. It is generally believed that
all nodes could be reached in six hops due to the famous study of the six
degrees of separation. The value of TTL is typically set to seven.
GnuStream [54] is a P2P streaming system build atop Gnutella. The
architecture of GnuStream consists of three layers: the Network Abstraction
Layer (NAL), the Control Streaming Layer (CSL), and the Media Player Layer
(MPL). The NAL is for locating, routing, and data retrieving from the net-
work. The CSL is for load balancing and network condition adaptation. The
MPL is in charge of playing back the video. GnuStream actually delegates
the content lookup service to Gnutella.
1.4 Application Layer Multicasts in Peer-to-Peer
Streaming Systems
As mentioned in an earlier section, multiple unicasts of the same content make
a server a bottleneck in the unicast systems. Therefore, if there is a flash crowd
of users who would like to watch the same video channel, we tend to arrange
these users into different topologies to take advantage of ALM for data deliv-
ery. For P2P live streaming systems, there are basically three different data
delivery methods: tree-push, mesh-pull, and push-pull [9-15,38-39,56]. In this
section, we present these three data delivery methods and classify a few P2P
live streaming systems based on these data delivery methods.
1.4.1 Tree-Push Methods
Most of the earliest ALM protocols employ tree-push methods to deliver
media data [32,33]. The media content is first delivered from the source node
to several intermediate nodes. The intermediate nodes then forward the data
to the remaining nodes. The content is actively pushed from the root of the
tree to all other nodes in the tree (see Figure 1.19).
