Explanation:

65-2
The mode of operation is a description of the conditions under
which an AIS functions, based on the sensitivity of data processed
and the clearance levels and authorizations of the users. Four modes
of operation are defined:
Dedicated ModE. An AIS is operating in the dedicated mode
when each user with direct or indirect individual access to the
AIS, its peripherals, remote terminals, or remote hosts has all of
the following:
A. Avalid personnel clearance for all information on the system
B. Formal access approval for, and has signed nondisclosure
agreements for all the information stored and/or processed
(including all compartments, subcompartments, and/or
special access programs)
C. A valid need-to-know for all information contained within
the system
System-High ModE. An AIS is operating in the system-high mode
when each user with direct or indirect access to the AIS, its
peripherals, remote terminals, or remote hosts has all of the
following:
A. A valid personnel clearance for all information on the AIS
B. Formal access approval for, and has signed nondisclosure
agreements for all the information stored and/or processed
(including all compartments, subcompartments, and/or
special access programs)
C. A valid need-to-know for some of the information contained
within the AIS
Compartmented ModE. An AIS is operating in the
compartmented mode when each user with direct or indirect
access to the AIS, its peripherals, remote terminals, or remote
hosts has all of the following:
A. A valid personnel clearance for the most restricted
information processed in the AIS
B. Formal access approval for, and has signed nondisclosure
agreements for that information to which he/she is to have
access
C. A valid need-to-know for that information to which he/she
is to have access
Multilevel ModE. An AIS is operating in the multilevel mode
when all the following statements are satisfied concerning the
users with direct or indirect access to the AIS, its peripherals,
remote terminals, or remote hosts:
A. Some do not have a valid personnel clearance for all the
information processed in the AIS.
B. All have the proper clearance and have the appropriate
formal access approval for that information to which he/she
is to have access.
C. All have a valid need-to-know for that information to which
they are to have access.
Source: DoD 5200.28-STD Department of Defense Trusted Computer
System Evaluation Criteria.

The most important reason for identifying threats is to know from what do the assets
need protection and what is the likelihood that a threat will occur. Threats cannot be eliminated,
but can be anticipated, and safeguards put in place to minimize their impact.
Operations Security provides audit and monitoring for mechanisms, tools and facilities which
permit the identification of security events and documentation of subsequent corrective actions.
Source: State of Nebraska - Information Security Systems (ISS) Security Officer Instruction Guide.

The Smurf Attack is a denial-of-service attack in which large numbers of
Internet Control Message Protocol (ICMP) packets with the intended victim's spoofed source IP are broadcast to a computer network using an IP Broadcast address.
Most devices on a network will, in their default settings, respond to this by sending a reply to the source IP address. If the number of machines on the network that receive and respond to these packets is very large, the victim's computer will be flooded with traffic.
This can slow down the victim's computer to the point where it becomes impossible to work on.
The name Smurf comes from the file "smurf.c", the source code of the attack program, which was released in 1997 by TFreak.
The author describes the attack as:
The `smurf' attack is quite simple. It has a list of broadcast addresses which it stores into an array, and sends a spoofed icmp echo request to each of those addresses in series and starts again. The result is a devistating attack upon the spoofed ip with, depending on the amount of broadcast addresses used, many, many computers responding to the echo request.
Mitigation:
- Best method for mitigating this threat is to control access to the physical network infrastructure. If the attacker can't send the attack, this attack will obviously not work.
- Currently the preferred method for controlling access to the network is by using 802.1X -
Certificate security.
- Also, modern operating systems don't usually permit a PING to a broadcast address and just returns an error message if you try.
The following answers are incorrect:
- Fraggle Attack: Close but not quite right. A Fraggle attack uses UDP rather than the ICMP that Smurf Attack uses.
- LAND Attack: Sorry, not correct. A LAND attack is simply a series of packets sent to the target where the source and destination IP Addresses are the same as the victim.
- Replay Attack: This isn't an attack that takes advantage of a system vulnerability so it isn't the correct answer.
The following reference(s) was used to create this question:
http://en.wikipedia.org/wiki/Smurf_attack
and
http://searchsecurity.techtarget.com/answer/What-is-a-land-attack
and
http://www.phreak.org/archives/exploits/denial/smurf.c