is implemented as an overlay network composed of servers that store triggers and forward packets.
To maintain this overlay network and to route packets in , we use the Chord lookup protocol . Chord assumes a circular identifier space of integers , where follows . Every server has an identifier in this space, and all trigger identifiers belong to the same identifier space. The server with identifier is responsible for all identifiers in the interval , where is the identifier of the node preceding on the identifier circle. Figure 2(a) shows an identifier circle for . There are five servers in the system with identifiers 5, 16, 24, 36, and 50, respectively. All identifiers in the range (5, 16] are mapped on server 16, identifiers in (17, 24] are mapped on server 24, and so on.
When a trigger is inserted, it is stored at the node responsible for . When a packet is sent to , it is routed by to the node responsible for its ; there it is matched against (any) triggers for that and forwarded (using IP) to all hosts interested in packets sent to that identifier. Chord ensures that the server responsible for an identifier is found after visiting at most other servers irrespective of the starting server ( represents the total number of servers in the system). To achieve this, Chord requires each node to maintain only routing state. Chord allows servers to leave and join dynamically, and it is highly robust against failures. For more details refer to . Figure 2(b) shows an example in which trigger is inserted at node 36 (i.e., the node that maps , and thus is responsible for identifier 30). Packet is forwarded to server 30, matched against trigger , and then forwarded via IP to .
Note that packets are not stored in ; they are only forwarded. End hosts must periodically refresh their triggers in . Hosts need only know one node to use the infrastructure. This can be done through a static configuration file, or by a DNS lookup assuming is associated with a DNS domain name. In Figure 2(b), the sender knows only server 16, and the receiver knows only server 5.