This paper appears in:  Computer Communications and Networks, 2006. ICCCN 2006.

Proceedings.15th International Conference on 

指導教授 : 許子衡

報告者 : 黃群凱 1

With geocast, messages can be sent to all mobile and stationary hosts currently located in a geographic target area.

Geocast messages can be addressed either by geometric figures, or by symbolic names.

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Symbolic addressing is an important alternative to geometric addressing.

We need a complex location model including geometric descriptions of every symbolically addressable location.

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Forwarding decisions are made based on comparisons of symbolic or geometric target and service areas.

we propose a heuristics to improve hierarchical routing by adding shortcuts to the routing hierarchy.

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Since multiple a priori unknown senders may send messages to a given location, either a source-based tree or shared tree protocol can be applied, which both have their limitations.

In the worst case, each message may cause a new tree to be established in the network.

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we will propose an optimization of our approach which sends the first messages of a sender over the overlay and then switches to a Source-Specific Multicast (SSM [8]) protocol.

The combination of our overlay network and SSM reduces the overhead to set up source-based trees.

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The three components of our architecture are hosts, message servers, and routers

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Geocast Routers (GR) are responsible for forwarding geocast messages from the sender to the GMSs whose service areas overlap with the target area of the message.

GRs are arranged in an overlay network and exchange messages using the UDP service offered by the underlying IP-based Internet infrastructure.

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For two locations 11 and 12 it holds 11 < 12, if 12 spatially contains 11.

11 is called a descendant of location 12, and 12 is an ancestor of 11.

A direct descendant of location I is called a child location and a direct ancestor a parent location.

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Ancestors(l), children(l) and parent(l) denote the set of ancestor locations, child locations, and the parent location, respectively.

Locations of the location model are used to define host positions, and service areas of GRs and GMSs.

A benefit of using an overlay network is that our location address space is not restricted by the length limitation of IP addresses.

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Overlay Network Nodes◦ GRs constitute the nodes of the overlay network.

Each location I is associated with one designated Geocast Router, say r.

Overlay Network Links

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This reduces the load of by-passed GRs and leads to shorter message paths.

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Consider for instance the New York City GR (r new) that wants to join the overlay network. The following steps are executed to integrate r new into the overlay network:

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Our approach uses three phases to forward messages from the sending host to all hosts in the target area, t, of the message.◦ The message is forwarded to the designated GR,

rt, of the target area.◦ The message is distributed among all routers in

the target area by forwarding it down the router hierarchy starting at rt.

◦ These GMSs finally forward the message to the hosts in their access networks that are located in the target area.

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Forwarding to Access Networks in Target Area

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Optimized Message Forwarding◦ The basic idea is to start delivering messages to

GMSs via the overlay network and then switch to layer 3 multicast.

◦ Such an optimization is especially useful if several messages are sent frequently to the same target area rather than only single messages that are sent sporadically.

◦ The class of source-specific multicast (SSM [15]) protocols is well suited for our requirements.

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The whole topology consists of the backbones of 8 major Internet service providers in the USA.

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We presented a geocast protocol for the efficient distribution of symbolically addressed geocast messages.

Our approach leads to short message paths and low overlay network router load and thus high scalability.

In future work, we are going to investigate how to further improve routing in the overlay network.

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