(AGIE) Using Internet Protocols - Aviation Committees - AEEC - AMC

4.0 AGIE FUNCTIONAL SPECIFICATIONARINC PROJECT PAPER 830 – Page 43Pub-Sub is based on pre-defined topics. Senders publish messages that include aspecial topic attribute in their message header. Destination clients determine whichtopics they wish to subscribe to. This determination can be at any time duringsystem operation. The Pub-Sub service maintains a list of subscribers for each topicpublished and clients notify the Pub-Sub service when a change in subscriptionstatus occurs. The pub-sub service also ensures the messages are sent to eachavailable destination in the subscriber list for the matching topic.The list of topics available for subscription is static and well bounded but shall onlybe changed via admin configuration updates.Topics are named by string values and AGIE does not prescribe a particular formatfor topic strings. Topics may be further defined by substring matches usingwildcards and dot notation. For example, the topic “NA Weather Turbulence Data”maps exactly to a topic of same string, “NA Weather Turbulence Data.” Moreovertopic names are not case sensitive and therefore this topic example also maps to“na weather turbulence data”. Use of wildcards is permitted. For example, the string“NA Weather Turbulence*” also maps to the same topic. Refer to the treatment ofname and address parsing in Section 4.5 for further details.In order to minimize admin processing and allow longevity to topics and names,complex parsing algorithms above and beyond the basic matches defined in thenaming and addressing section may be implemented, but are not defined as such inthis standard.Clients subscribe to messages via the standard message service interface. XMLmessage field parameters map to a predefined coordination message. See Section6.0 for details. The subscription and publication topic strings may include thewildcards and dot notation. Clients may unsubscribe using the same coordinationmessage, but with the unsubscribe request.The message delivery from publisher to the Pub-Sub service is via a “push” only tothe Primary. It is assumed, though not required, that most publishers will be on theground-side as is the Primary. Message delivery to the destination may be by pushor pull (notify).4.4.4 Priorities and Flow ControlThis section describes how AGIE controls message processing priorities andperforms flow control. Prioritizing message traffic is a key aspect of AGIE systemfunctionality and its interfaces with the underlying network(s). While on one hand itis desirable for AGIE to be able to completely and deterministically manage allmessage prioritization aspects, it is also clear that there are many variables in theunderlying networks that make this not possible. Often AGIE priority control is nomore than attempting to influence the underlying implementations.AGIE is datalink agnostic. AGIE transmits data over any IP datalink. Datalinks havedifferent prioritization capabilities. Some datalinks have no prioritization and arebased on equal and fair treatment of all users. Other datalinks have moderateprioritization by IP class of service, and lastly there are datalinks that provide a highdegree of prioritization (usually with a higher cost) via separated services. AGIE isdesigned to function efficiently over any or all of these links.AGIE servers are defined logically not physically. Therefore, specifying hardcomputing priorities is not possible. It is not known if the AGIE servers execute on areal-time kernel or a best effort Operating System. AGIE priorities are defined not as