Tech Note #36: Introduction to the Alarm Clock Protocol (ACP)
1998 Bionic Buffalo Corporation; All Rights Reserved.
12 June 1998
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The following diagram illustrates this example for the case when the request is not cancelled:
10-second Alarm Single request
Note that this diagram shows the flow of message segments. There may also be other message
segments within each message.
Notice or notification messages may optionally be broadcast, rather than directed at specific
machines. This may be more efficient when multiple machines are listening together for the
same notification (of events, alarms, or lifetime expirations).
Certain requests may be broadcast to all available servers. This may be used to discover the
The Alarm Clock Protocol can sit above most transport protocols, including TCP/IP, UDP/IP,
and message passing operating systems. If the transport mechanism does not allow the server to
determine the origin of a message, then tagged fields within the message may contain this
information so the server can respond to the client.
Two well-known addresses are required to implement ACP. Clients use one well-known
address for two purposes:
clients have a known destination for request messages when using datagrams
clients have a known destination to establish a connection with a server (in fact, this is
simply a specialized form of the first use)
Servers use the second well-known address when notification messages are to be broadcast
simultaneously to multiple clients.