Which layer of the DoD TCP/IP model controls the communication flow between hosts?
Correct Answer: B
Whereas the host-to-host layer (equivalent to the OSI's transport layer) provides
end-to-end data delivery service, flow control, to the application layer.
The four layers in the DoD model, from top to bottom, are:
The Application Layer contains protocols that implement user-level functions, such as mail
delivery, file transfer and remote login. The Host-to-Host Layer handles connection rendez vous, flow control, retransmission of lost data, and other generic data flow management between hosts. The mutually exclusive TCP and UDP protocols are this layer's most important members. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol. The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network The OSI model organizes communication services into seven groups called layers. The layers are as follows:
Layer 7, The Application Layer: The application layer serves as a window for users and application processes to access network services. It handles issues such as network transparency, resource allocation, etc. This layer is not an application in itself, although some applications may perform application layer functions. Layer 6, The Presentation Layer: The presentation layer serves as the data translator for a network. It is usually a part of an operating system and converts incoming and outgoing data from one presentation format to another. This layer is also known as syntax layer. Layer 5, The Session Layer: The session layer establishes a communication session between processes running on different communication entities in a network and can support a message-mode data transfer. It deals with session and connection coordination. Layer 4, The Transport Layer: The transport layer ensures that messages are delivered in the order in which they are sent and that there is no loss or duplication. It ensures complete data transfer. This layer provides an additional connection below the Session layer and assists with managing some data flow control between hosts. Data is divided into packets on the sending node, and the receiving node's Transport layer reassembles the message from packets. This layer is also responsible for error checking to guarantee error-free data delivery, and requests a retransmission if necessary. It is also responsible for sending acknowledgments of successful transmissions back to the sending host. A number of protocols run at the Transport layer, including TCP, UDP, Sequenced Packet Exchange (SPX), and NWLink. Layer 3, The Network Layer: The network layer controls the operation of the subnet. It determines the physical path that data takes on the basis of network conditions, priority of service, and other factors. The network layer is responsible for routing and forwarding data packets. Layer 2, The Data-Link Layer: The data-link layer is responsible for error free transfer of data frames. This layer provides synchronization for the physical layer. ARP and RARP would be found at this layer. Layer 1, The Physical Layer: The physical layer is responsible for packaging and transmitting data on the physical media. This layer conveys the bit stream through a network at the electrical and mechanical level.
See a great flash animation on the subject at:
http://www.maris.com/content/applets/flash/comp/fa0301.swf
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten
Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and
Network Security (page 85).
Also: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002,
chapter 7: Telecommunications and Network Security (page 344).
