2 cissp ® Official Study Guide Eighth Edition

Application Layer
The 
Application layer (layer 7)
is responsible for interfacing user applications, network 
services, or the operating system with the protocol stack. It allows applications to com-
municate with the protocol stack. The Application layer determines whether a remote com-
munication partner is available and accessible. It also ensures that sufficient resources are 
available to support the requested communications.

---

TCP/IP Model 
451
The application is not located within this layer; rather, the protocols and services 
required to transmit files, exchange messages, connect to remote terminals, and so on are 
found here. Numerous application-specific protocols are found within this layer, such as the 
following:
■
Hypertext Transfer Protocol (HTTP)
■
File Transfer Protocol (FTP)
■
Line Print Daemon (LPD)
■
Simple Mail Transfer Protocol (SMTP)
■
Telnet
■
Trivial File Transfer Protocol (TFTP)
■
Electronic Data Interchange (EDI)
■
Post Office Protocol version 3 (POP3)
■
Internet Message Access Protocol (IMAP)
■
Simple Network Management Protocol (SNMP)
■
Network News Transport Protocol (NNTP)
■
Secure Remote Procedure Call (S-RPC)
■
Secure Electronic Transaction (SET)
There is a network device (or service) that works at the Application layer, namely, the 
gateway. However, an Application layer gateway is a specific type of component. It serves 
as a protocol translation tool. For example, an IP-to-IPX gateway takes inbound commu-
nications from TCP/IP and translates them over to IPX/SPX for outbound transmission. 
Application layer firewalls also operate at this layer. Other networking devices or filtering 
software may observe or modify traffic at this layer.
TCP/IP Model
The TCP/IP model (also called the DARPA or the 
DOD model
) consists of only four lay-
ers, as opposed to the OSI Reference Model’s seven. The four layers of the TCP/IP model 
are Application (also known as Process), Transport (also known as Host-to-Host), Internet 
(sometimes Internetworking), and Link (although Network Interface and sometimes 
Network Access are used). Figure 11.5 shows how they compare to the seven layers of the 
OSI model. The TCP/IP protocol suite was developed before the OSI Reference Model was 
created. The designers of the OSI Reference Model took care to ensure that the TCP/IP pro-
tocol suite fit their model because of its established deployment in networking.

---

452
Chapter 11 
■
Secure Network Architecture and Securing Network Components
F I g u r e 11. 5
Comparing the OSI model with the TCP/IP model
Application
Presentation
Session
Transport
Network
Data Link
Transport
Internet
Link
Physical
Application