___39___ is used to ensure the confidentiality, integrity and authenticity of the two end points in the private network. ___40___, an application-layer protocol, authenticates each peer in an IPSec transaction. IKE negotiates security policy, determining which algorithm may be used to set up the tunnel. It also handles the exchange of session keys used for that one transaction.
Networks that use ___41___ to secure data traffic can automatically authenticate devices by using by using ___42___, which verify the identities of the two users who are sending information back and forth. IPSec can be ideal way to secure data in large networks that require secure connections among many devices.
Users deploying IPSec can ___43___ their network infrastructure without affecting the applications on individual computer. The protocol suite is available as a software-only upgrade to the network infrastructure. This alows security to be implemented without costly changes to each computer. Most important, IPSec allows interoperability among different network devices, PCs and other computing systems.
43（）
A. secure
B. relation
C. script
D. strict
