One week later suppose the current pair of IPSec SAs expires but the lifetime of IKE

SAs is valid. The IPSec modules would like to negotiate new IPSec SA keys without

stronger PFS$,$ but would keep the current IPSec algorithms and the current traffic

selectors TSi and TSr$.$ To this end, they need to exchange the two messages in Phase

$2.$ What are the formats of the two messages in Phase $2$ in this case?