Question: In an established IMS multimedia session, if a UE (User Equipment) changes the codec for an existing media stream within the already reserved resources, what is the primary action taken by the CSCFs (Call/Session Control Functions) upon receiving the revised SDP (Session Description Protocol) in the INVITE message?
Option 1: Perform a new admission control decision to assess resource availability.
Option 2: Reject the INVITE message due to the change in media characteristics.
Option 3: Update the policy and charging control rules for the new codec.
Option 4: Recognize the SDP as a proper subset of the previously authorized one and take no further action.
Explanation: Since the change is within the resources already reserved, the CSCFs do not need to take any additional actions beyond recognizing that the new request does not exceed the previously authorized resource allocation. This ensures seamless adaptation to changes in media characteristics without unnecessary delays or potential rejections due to resource constraints already accounted for.