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and C3. Thus, the logical overlay connections C1-C2, C2-C3, and C3-C4 are
all ine cient.
In order to discover these ine cient connections, each peer periodically
floods some detector messages to their neighbors, which in turn forward them
to other nodes, and so on. These detector messages have a controlled scope.
For example, their TTL values are set to 2 or 3. On receipt of such a detec-
tor message, the TTL value is decremented. More importantly, each message
is time-stamped. Thus, when multiple messages originating from the same
source are received, the transmission time (i.e., the cost) of the messages can
be compared by checking their respective time-stamp values. This is done to-
gether with a “probing” action, which works by having a peer send a detector
message directly to another peer. The cost of this message is also then noted
and compared with the cost of the regular detector messages. Accordingly,
some existing connections could be terminated if the newly probed connec-
tion generates a lower cost. This heuristic protocol is illustrated in Figure 4.5.
C1
C2
C1
C2
C1
C2
X
cutting
C3
C4
C3
C4
C3
C4
inefficient
overlay topology
cutting
C1
C2
C1
C2
C1
C2
X
C3
C4
C3
C4
C3
C4
X
cutting
C1
C2
C3
C4
efficient
overlay topology
FIGURE 4.5: An example of location-aware topology matching (LTM) [Liu
et al., 2005b].
Zhang et al. [Zhang et al., 2005a] proposed an interesting scheme for con-
structing low-diameter overlay networks with power law topologies. In their
system, each peer is uniquely identified by a 4-tuple: (IP address, port, network
coordinate, capacity). Here, network coordinates are measured using mecha-
nisms such as Vivaldi [Dabek et al., 2004]. Such coordinates are useful for
computing the physical “distances” between two peers. Just like many other
P2P systems, a new peer i obtains a list of existing peers from a designated
bootstrapping server. Specifically, the bootstrapping server selects a list of
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