13. Appendix I - Changes from IGMPv1

13. Appendix I - Changes from IGMPv1

The IGMPv1 "Version" and "Type" fields are combined into a single "Type" field.

A new IGMP Type is assigned to Version 2 Membership Report messages, so a router may tell the difference between an IGMPv1 and IGMPv2 host report.

A new IGMP Type is created for the IGMPv2 Leave Group message.

The Membership Query message is changed so that a previously unused field contains a new value, the Max Response Time.

The IGMPv2 spec now specifies a querier election mechanism. In IGMPv1, the querier election was left up to the multicast routing protocol, and different protocols used different mechanisms. This could result in more than one querier per network, so the election mechanism has been standardized in IGMPv2. However, this means that care must be taken when an IGMPv2 router is trying to coexist with an IGMPv1 router that uses a different querier election mechanism. In particular, it means that an IGMPv2 router must be able to act as an IGMPv1 router on a particular network if configured to do so. The actions required include:

• Set the Max Response Time field to 0 in all queries.

• Ignore Leave Group messages.

The IGMPv2 spec relaxes the requirements on validity-checking for Membership Queries and Membership Reports. When upgrading an implementation, be sure to remove any checks that do not belong.

The IGMPv2 spec requires the presence of the IP Router Alert option [RFC 2113] in all packets described in this memo.