2 cissp ® Official Study Guide Eighth Edition

OSI Model
TCP/IP Model
The TCP/IP model’s Application layer corresponds to layers 5, 6, and 7 of the OSI 
model. The TCP/IP model’s Transport layer corresponds to layer 4 from the OSI model. 
The TCP/IP model’s internet layer corresponds to layer 3 from the OSI model. The TCP/IP 
model’s Link layer corresponds to layers 1 and 2 from the OSI model. 
It has become common practice (through confusion, misunderstanding, and probably 
laziness) to also call the TCP/IP model layers by their OSI model layer equivalent names. 
The TCP/IP model’s Application layer is already using a name borrowed from the OSI, 
so that one is a snap. The TCP/IP model’s Host-to-Host layer is sometimes called the 
Transport layer (the OSI model’s fourth layer). The TCP/IP model’s internet layer is some-
times called the Network layer (the OSI model’s third layer). And the TCP/IP model’s Link 
layer is sometimes called the Data Link or the Network Access layer (the OSI model’s sec-
ond layer). 
Since the TCP/IP model layer names and the OSI model layer names can 
be used interchangeably, it is important to know which model is being 
addressed in various contexts. Unless informed otherwise, always assume 
that the OSI model provides the basis for discussion because it’s the most 
widely used network reference model.
TCP/IP Protocol Suite Overview 
The most widely used protocol suite is TCP/IP, but it is not just a single protocol; rather, it 
is a protocol stack comprising dozens of individual protocols (see Figure 11.6 ). TCP/IP is 
a platform-independent protocol based on open standards. However, this is both a benefi t 
and a drawback. TCP/IP can be found in just about every available operating system, but 
it consumes a signifi cant amount of resources and is relatively easy to hack into because it 
was designed for ease of use rather than for security. 

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TCP/IP Model 
453
F I g u r e 11. 6
The four layers of TCP/IP and its component protocols
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internet
Link
FTP
TFTP
TCP
ICMP
Ethernet
Telnet
SMTP
UDP
IGMP
Fast
Ethernet
SNMP
NFS
Token Ring
LPD
X Window
IP
FDDI
TCP/IP can be secured using virtual private network (VPN) links between systems. VPN 
links are encrypted to add privacy, confidentiality, and authentication and to maintain data 
integrity. Protocols used to establish VPNs are 
Point-to-Point Tunneling Protocol (PPTP)
, 
Layer 2 Tunneling Protocol (L2TP)
, 
Secure Shell (SSH)
, 
OpenVPN
(SSL/TLS VPNs), and 
Internet Protocol Security (IPSec)
. Another method to provide protocol-level security is to 
employ TCP wrappers. A 
TCP wrapper
is an application that can serve as a basic firewall 
by restricting access to ports and resources based on user IDs or system IDs. Using TCP 
wrappers is a form of port-based access control.