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Re: What contains the announcement packet sent by a service ?
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Posted: Mar 14, 2002 10:16 AM
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> 1) Are the IP address and the port number the
> only informations contained inside the presence
> announcement packet ?
>
The presence announcement from discovering entities contains:
* Groups the device wants to join
* IP address and port number where device can be contacted
* Lookup services it has already heard back from
> 2) What is a multicast ? Do the service send
> that packet to all IP addresses ? I don't think so...
> How can the service determine to what IP address to
> send that presence announcement packet ?
>
Multicast uses "class D" IP addresses: 224.0.0.1 to 239.255.255.255. Receivers "subscribe" to the multicast group (IP address). Anyone can send to the group (not just subscribers). All subscribers are eligible to receive the packet. So the discovering entity just writes to a well-known IP multicast IP address, and lookup services listen to that IP address.
> 3) What can do the lookup service to determine
> that he dont have to contact the sender of the
> presence announcement packet ?
>
If a lookup service doesn't register any of the groups listed in a presence announcement, it doesn't contact the announcer. Likewise, if the lookup service is already listed in the announcement as having already been discovered, it doesn't contact the announcer.
> 4) Is it possible to have several lookup
> services at the same time ? What are the purposes of
> having several lookup services at the same time ?
>
Yup. The main idea is for redundancy in case of failure. If one lookup service crashes or becomes disconnected from the network, the others are still there and can keep connecting clients with services.
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