Sean has built a site-to-site VPN architecture between the head office and the branch office...

 In the context of VPNs, when both the header and payload of traffic are encapsulated, it indicates the use of Tunnel Mode. This mode is typically employed in site-to-site VPNs where the entire IP packet is wrapped with a new IP header. Tunnel Mode is designed to secure traffic between different networks over the internet, making it suitable for connecting multiple sites of an organization. Unlike Transport Mode, which only encrypts the payload and leaves the original IP header intact, Tunnel Mode encrypts the entire IP packet and adds a new header, which allows for the secure passage of the traffic through untrusted networks.

References: The explanation provided aligns with standard VPN implementations and the principles outlined in network security documents and study guides related to Certified Network Defender (CND) objectives.