10.2. <del> A new group may be tied to an already existing group for the purpose of re-initializing the existing group, or to branch into a sub-group. Re-initializing an existing group may be used, for example, to restart the group with a different ciphersuite or protocol version. Branching may be used to bootstrap a new group consisting of a subset of current group members, based on the current group state. </del> <ins> A group may be reinitialized by creating a new group with the same membership and different parameters, and linking it to the old group via a resumption PSK.  The members of a group reinitialize it using the following steps: A member of the old group sends a ReInit proposal (see reinit) A member of the old group sends a Commit covering the ReInit proposal A member of the old group sends a Welcome message for the new group that matches the ReInit The "group_id", "version", and "cipher_suite" fields in the Welcome message MUST be the same as the corresponding fields in the ReInit proposal. The "epoch" in the Welcome message MUST be 1 The Welcome MUST specify a PreSharedKey of type "resumption" with usage "reinit".  The "group_id" must match the old group, and the "epoch" must indicate the epoch after the Commit covering the ReInit. The "psk_nonce" included in the "PreSharedKeyID" of the resumption PSK MUST be a randomly sampled nonce of length "KDF.Nh", for the KDF defined by the new grou's ciphersuite. Note that these three steps may be done by the same group member or different members.  For example, if a group member sends a commit with an inline ReInit proposal (steps 1 and 2), but then goes offline, another group member may send the corresponding Welcome. This flexibility avoids situations where a group gets stuck between steps 2 and 3. </ins> <del> In both cases, the "psk_nonce" included in the "PreSharedKeyID" object must be a randomly sampled nonce of length "KDF.Nh" to avoid key re-use. </del> <ins> Resumption PSKs with usage "reinit" MUST NOT be used in other contexts.  A PreSharedKey proposal with type "resumption" and usage "reinit" MUST be considered invalid. </ins> <del> 10.2.1. </del> <ins> 10.3. </ins> <del> If a client wants to create a subgroup of an existing group, they MAY choose to include a "PreSharedKeyID" in the "GroupSecrets" object of the Welcome message choosing the "psktype" "branch", the "group_id" of the group from which a subgroup is to be branched, as well as an epoch within the number of epochs for which a "resumption_secret" is kept. </del> <ins> A new group can be formed from a subset of an existing group's members, using the same parameters as the old group.  The creator of the group indicates this situation by including a PreSharedKey of type "resumption" with usage "branch" in the Welcome message that creates the branched subgroup. A client receiving a Welcome including a PreSharedKey of type "resumption" with usage "branch" MUST verify that the new group reflects a subgroup branched from the referenced group. The "version" and "ciphersuite" values in the Welcome MUST be the same as those used by the old group. Each KeyPackage in a leaf node of the new group's tree MUST be a leaf in the old group's tree at the epoch indicated in the PreSharedKey. In addition, to avoid key re-use, the "psk_nonce" included in the "PreSharedKeyID" object MUST be a randomly sampled nonce of length "KDF.Nh". Resumption PSKs with usage "branch" MUST NOT be used in other contexts.  A PreSharedKey proposal with type "resumption" and usage "branch" MUST be considered invalid. </ins> 11.