Explanation
The TCP Maximum Segment Size (TCP MSS) defines the maximum amount of data that a host is willing to accept in a single TCP/IP datagram. This TCP/IP datagram might be fragmented at the IP layer. The MSS value is sent as a TCP header option only in TCP SYN segments. Each side of a TCP connection reports its MSS value to the other side. Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host.
TCP MSS takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints.
PMTUD was developed in order to avoid fragmentation in the path between the endpoints. It is

Explanation

Explanation
802.11r Fast Transition (FT) Roaming is an amendment to the 802.11 IEEE standards. It is a new concept for roaming. The initial handshake with the new AP occurs before client roams to the target AP. Therefor it is called Fast Transition. 802.11rprovides two methods of roaming:
+ Over-the-air: With this type of roaming, the client communicates directly with the target AP using IEEE
802.11authentication with the Fast Transition (FT) authentication algorithm. + Over-the-DS (distribution system): With this type of roaming, the client communicates with the target AP through the current AP. The communication between the client and the target AP is carried in FT action frames between the client and the current AP and is then sent through the controller.
But both of these methods do not deal with legacy clients.
The 802.11k allows 11k capable clients to request a neighbor report containing information about known neighbor APs that are candidates for roaming.
Reference: https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/80211r-ft/b-80211r-dg.html IEEE 802.11v is an amendment to the IEEE 802.11 standard which describes numerous enhancements to wireless network management. One such enhancement is Network assisted Power Savings which helps clients to improve the battery life by enabling them to sleep longer. Another enhancement is Network assisted Roaming which enables the WLAN to send requests to associated clients, advising the clients as to better APs to associate to. This is useful for both load balancing and in directing poorly connected clients.
Reference:
https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/config-guide/b_wl_16_10_cg/802-11v.pdf Cisco 802.11r supports three modes: + Pure mode: only allows 802.11r client to connect + Mixed mode:
allows both clients that do and do not support FT to connect + Adaptive mode: does not advertise the FT AKM at all, but will use FT when supported clients connect Therefore "Adaptive mode" is the best answer here.