When the Protocol Id field is 0 (PR_BCAST) and the Address Type field is 0 (AD_CALL), the packet contains the sender's amateur callsign and one or more DUAL Link addresses that are to be associated with this callsign. The format of the packet is as follows:
Figure : DUAL Station Identification Packet
The packet has no destination address; it is implicit that the packet is to be received by all stations, and hence the destination address is non-existent (or more strictly, it has a length of zero). The Checksum field is 2 octets in length and is calculated as per the checksum in PR_IP.
Between the Callsign and the CRC are one or more blocks which give link addresses associated with the callsign. Each block begins with two octets. The first gives the size in octets of the link address in the block. The second octet gives a Protocol-Id/Address-type pair that identifies the format of the given link address.
As an example, on a Class-C subnet where the operator VK1XWT uses the IP address 220.127.116.11, a PR_BCAST/AD_CALL would look like:
Figure : Example Station Identification Packet
The Size field in each block is required, as an implementation of DUAL may not be able to recognise a given link address. Therefore it needs to know the size of the given link address so that it can skip it and parse any further link addresses. The drawback of this field is that it limits link addresses to 256 octets in size.