t header contains the source MAC address, the IPv6 source address is unspecified, as at this point, the node does not have any IPv6 address available for use [46]. The reason why this message is sent, even before any IPv6 address is available, is to ensure successful operation of DAD (Section 3.4.7). Specifically, this message is needed to inform MLD aware switches, possibly existing in the access network, that a node exists that listens for particular addresses. In some networks the MLD-snooping switches may not forward multicast packets at all, unless they have seen an MLD report being sent. Next we will look at the details of the whole packet. For the subsequent packets we will not repeat the description of the IPv6 header, but only look at what is new in each message. Starting with the IPv6 header itself, at the 'Internet Protocol Version 6' line we can see contents for the fields shown in Figure 3.8. IP-version field is set to '6' to indicate that this packet is IPv6. For this packet, Traffic Class and Flow Label are set to default, to zero. Payload Length indicates that there are 36 bytes of data after the main IPv6 header. The Next Header field indicates the presence of a Hop-by-hop option, and more on that later. The MLD packets' Hop Limit is always one, as shown also in this example. The MLD packets need not travel any further than the first-hop routers. As said before, the IPv6 source address has been set to unspecified because at this point the node does not have any source address available. As required by the MLDv2 RFC 3810 Section 5, the Hop-by-hop option header (see Section 3.2.2) includes a Router Alert option [45]. The length field of the Hop-by-hop option contains zero, which in this case is specified to mean eight bytes (see RFC 2460 Section 4.3 [21]). The Router Alert then indicates to the router that this IPv6 packet contains an MLD message Section ([31] Section 2.1). One PadN padding option (Section 3.2.2), of two bytes in length, is included to make the whole Hop-by-hop option eight bytes l