Introduction

This section gives specific details on the configuration and operation of the Broadcast scan task.

The Broadcast scan task is strictly an output only scan task. The scan task will send out IP multicast/unicast (see uni-multi-mode) messages in the following format:

• first two bytes contain the type
  
• the next two bytes contain the RTAP data element type of the data that follows
  
• the next two bytes contain the number of elements that follow
  
• the next two bytes contain the number of data bytes that follow
  
• data bytes
  
• the last two bytes contain the CRC of the whole message

The byte ordering is such that the most significant byte is first.

IP multicasting provides a mechanism for sending a single datagram to a group of systems. Only systems that have joined the multicast group will receive the datagram.

Multicast datagrams are transmitted and delivered with the same "best effort" reliability as regular unicast IP datagrams. There is no guarantee that the datagram will arrive intact at all members of the multicast group or in the same order as the datagrams were sent.

Membership in a multicast group is dynamic. Systems can join and leave groups at any time.

A Class D IP address in the range 224.0.0.0 to 239.255.255.255 is a "multicast address". It is also known as a "host group address", since datagrams with a multicast destination address can be received by all hosts that have joined the group that an address represents. The address 224.0.0.0 is reserved, and 224.0.0.1 are assigned to the permanent group of all IP hosts.

Multicast addresses provide a limited form of broadcasting, but without the problems and limitations of traditional broadcasts.

Sending datagrams to a multicast address is analogous to transmitting radio signals on a particular frequency. Just as you must tune a radio receiver to the particular frequency, you must join a multicast group in order to receive the multicast datagrams. All that is required between sender and receiver is a properly configured multicast router(s).

IP unicasting provides a mechanism for sending a single datagram to a single system.

The CRC (Cyclic Redundancy Check) is calculated using the following polynomial with the initial 16 bit value set to all 1's:

x^16 + x^15 + x^2 + 1

The resulting message including the CRC using the above polynomial at the receiver will give a zero remainder if no errors have occurred.

The following symbol indicates a reference to the specified section in section 4 of the RTAP/Plus Integration Manual §.