Which TCP parameter is adjusted to reduce fragmentation risk over a GRE/IP tunnel?

Adjusting the Maximum Segment Size (MSS) is crucial for reducing fragmentation risk over a GRE/IP tunnel. MSS defines the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. When a GRE header and outer IP header are added to the original packet, the total size can exceed the interface's Maximum Transmission Unit (MTU). By lowering the MSS, the sender transmits smaller TCP segments, ensuring the encapsulated packet fits within the MTU without needing to be fragmented. MTU defines the maximum size of a packet that can be transmitted over a network medium, but directly adjusting it on the tunnel interface might not prevent fragmentation if the underlying physical interface has a smaller MTU. MRU (Maximum Receive Unit) is for receiving, not sending, and window size relates to flow control, not packet size relative to the MTU.