___57___ is used to ensure the confidentiality, integrity and authenticity of the two end points in the private network. ___58___, 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 ___59___ to secure data traffic can automatically authenticate devices by using by using ___60___, 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 ___61___ 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.

61（）

A. secure

B. relation

C. script

D. strict

钻井液在井内循环时，因钻井液流动产生一个附加压力，这个附加压力换算成密度再加上使用的钻井液密度称为（）。

A、静钻井液密度

B、当量密度

C、相对密度

D、重度