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| <h1 id="pubsub-admin-udp-multicast">PUBSUB-Admin UDP Multicast</h1> |
| <hr> |
| <h2 id="description">Description</h2> |
| <p>This description is particular for the UDP-Multicast PUB-SUB.</p> |
| <p>The UDP multicast pubsub admin is used to transfer user data transparent via UDP multicast. UDP packets can contain approximately<br> |
| 64kB . To overcome this limit the admin has a protocol on top of UDP which fragments the data to be send and these<br> |
| fragments are reassembled at the reception side.</p> |
| <h3 id="ip-addresses">IP Addresses</h3> |
| <p>To use UDP-multicast 2 IP adresses are needed:</p> |
| <ol> |
| <li>IP address which is bound to an (ethernet) interface</li> |
| <li>The multicast address (in the range 224.X.X.X - 239.X.X.X)</li> |
| </ol> |
| <p>When the PubSubAdmin starts it determines the bound IP address. This is done in the order:</p> |
| <ol> |
| <li>The first IP number bound to the interface which is set by the “PSA_INTERFACE” property</li> |
| <li>The interfaces are iterated and the first IP number found is used. (typically this is 127.0.0.1 (localhost)</li> |
| </ol> |
| <p>The Multicass IP address is determined in the order:</p> |
| <ol> |
| <li>If the <code>PSA_IP</code> property is defined, this IP will be used as multicast.</li> |
| <li>If the <code>PSA_MC_PREFIX</code> property, is defined, this property is used as the first 2 numbers of the multicast address extended with the last 2 numbers of the bound IP.</li> |
| <li>If the <code>PSA_MC_PREFIX</code> property is not defined <code>224.100</code> is used.</li> |
| </ol> |
| <h3 id="discovery">Discovery</h3> |
| <p>When a publisher request for a topic a TopicSender is created by a ServiceFactory. This TopicSender uses the multicast address as described above with a random chosen portnumber. The combination of the multicast-IP address with the portnumber and protocol(udp) is the endpoint.<br> |
| This endpoint is published by the PubSubDiscovery within its topic in ETCD (i.e. udp://224.100.10.20:40123).</p> |
| <p>A subscriber, interested in the topic, is informed by the the ToplogyManager that there is a new endpoint. The TopicReceiver at the subscriber side creates a listening socket based on this endpoint.</p> |
| <p>Now a data-connection is created and data send by the publisher will be received by the subscriber.</p> |
| <hr> |
| <h2 id="properties">Properties</h2> |
| <table border="1"> |
| <tr><th>Property</th><th>Description</th></tr> |
| <tr><td>PSA_INTERFACE</td><td>Interface which has to be used for multicast communication</td></tr> |
| <tr><td>PSA_IP</td><td>Multicast IP address used by the bundle</td></tr> |
| <tr><td>PSA_MC_PREFIX</td><td>First 2 digits of the MC IP address </td></tr> |
| </table> |
| <hr> |
| <h2 id="shortcomings">Shortcomings</h2> |
| <ol> |
| <li>Per topic a random portnr is used for creating an endpoint. It is theoretical possible that for 2 topic the same endpoint is created.</li> |
| <li>For every message a 32 bit random message ID is generated to discriminate segments of different messages which could be sent at the same time. It is theoretically possible that there are 2 equal message ID’s at the same time. But since the mesage ID is valid only during the transmission of a message (maximum some milliseconds with large messages) this is not very plausible.</li> |
| <li>When sending large messages, these messages are segmented and sent after each other. This could cause UDP-buffer overflows in the kernel. A solution could be to add a delay between sending of the segements but this will introduce extra latency.</li> |
| <li>A Hash is created, using the message definition, to identify the message type. When 2 messages generate the same hash something will terribly go wrong. A check should be added to prevent this (or another way to identify the message type). This problem is also valid for the other admins.</li> |
| </ol> |
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