Which field in IGMPv2 specifies the time hosts have to respond to a query?

The correct answer is the MRT (Maximum Response Time) field. This field plays a crucial role in IGMPv2 (Internet Group Management Protocol Version 2) as it dictates the timeframe within which hosts must respond to a multicast membership query sent by a multicast router.

When a router issues a query, the MRT field indicates the maximum time the hosts have to reply, essentially allowing hosts to manage their responses and ensuring the router receives necessary membership information in a timely manner. This improves network efficiency by providing a structured approach to handling multicast group memberships and ensuring that the multicast traffic is delivered to the correct group of hosts.

In IGMPv2, this mechanism aids in reducing potential congestion that could occur if hosts did not have a defined period to respond. The specification of this time frame helps maintain a balance between timely responses and network resource utilization.

Thus, understanding the function of the MRT field is critical when working with multicast applications, as it fundamentally influences how members join or leave multicast groups based on the router's queries.