IGMP version 1

Routers periodically send IGMP Host Membership Queries to the all-hosts group (224.0.0.1) with a TTL of 1 (once every minute or two). All multicast-capable hosts hear them, but dont answer immediately to avoid an IGMP Host Membership Report storm. Instead, they start a random delay timer for each group they belong to on the interface they received the query.Sooner or later, the timer expires in one of the hosts, and it sends an IGMP Host Membership Report (also with TTL 1) to the multicast address of the group being reported. As it is sent to the group, all hosts that joined the group -and which are currently waiting for their own timer to expire- receive it, too. Then, they stop their timers and dont generate any other report. Just one is generated -by the host that chose the smaller timeout-, and that is enough for the router. It only needs to know that there are members for that group in the subnet, not how many nor which.When no reports are received for a given group after a certain number of queries, the router assumes that no members are left, and thus it doesnt have to forward traffic for that group on that subnet. Note that in IGMPv1 there are no "Leave Group messages".When a host joins a new group, the kernel sends a report for that group, so that the respective process needs not to wait a minute or two until a new membership query is received. As you can see this IGMP packet is generated by the kernel as a response to theIP_ADD_MEMBERSHIP command, seen in section IP_ADD_MEMBERSHIP. Note the emphasis in the adjective "new": if a process issues anIP_ADD_MEMBERSHIP command for a group the host is already a member of, no IGMP packets are sent as we must already be receiving traffic for that group; instead, a counter for that groups use is incremented. IP_DROP_MEMBERSHIP generates no datagrams in IGMPv1.Host Membership Queries are identified by Type 0x11, and Host Membership Reports by Type 0x12.No reports are sent for the all-hosts group. Membership in this group is permanent.