This paper appears in: Computer Communications and Networks, ICCCN 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. 2

 Symbolic addressing is an important alternative to geometric addressing.  We need a complex location model including geometric descriptions of every symbolically addressable location. 3

 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. 4

 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. 5

 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. 6

 The three components of our architecture are hosts, message servers, and routers 7

 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. 8

 For two locations 1 1 and 1 2 it holds 1 1 < 1 2, if 1 2 spatially contains 1 1.  1 1 is called a descendant of location 1 2, and 1 2 is an ancestor of 1 1.  A direct descendant of location I is called a child location and a direct ancestor a parent location. 9

 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. 10

 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 11

 This reduces the load of by-passed GRs and leads to shorter message paths. 12

 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: 13

 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. 14

 Forwarding to Access Networks in Target Area 15

 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. 16

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 The whole topology consists of the backbones of 8 major Internet service providers in the USA. 18

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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. 20