2 cissp ® Official Study Guide Eighth Edition

Specialized Protocols
Some WAN connection technologies require additional specialized protocols to support 
various types of specialized systems or devices. Three of these protocols are SDLC, HDLC, 
and HSSI:
Synchronous Data Link Control (SDLC)

Synchronous Data Link Control is used on 

permanent physical connections of dedicated leased lines to provide connectivity for main-
frames, such as IBM Systems Network Architecture (SNA) systems. SDLC uses polling, 
operates at OSI layer 2 (the Data Link layer), and is a bit-oriented synchronous protocol.

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Miscellaneous Security Control Characteristics 
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High-Level Data Link Control (HDLC)

High-Level Data Link Control is a refined version of 

SDLC designed specifically for serial synchronous connections. HDLC supports full-duplex 
communications and supports both point-to-point and multipoint connections. HDLC, like 
SDLC, uses polling and operates at OSI layer 2 (the Data Link layer). HDLC offers flow 
control and includes error detection and correction.
Dial-Up Encapsulation Protocols
The Point-to-Point Protocol (PPP) is an encapsulation protocol designed to support the 
transmission of IP traffic over dial-up or point-to-point links. PPP allows for multivendor 
interoperability of WAN devices supporting serial links. All dial-up and most point-to-
point connections are serial in nature (as opposed to parallel). PPP includes a wide range of 
communication services, including the assignment and management of IP addresses, man-
agement of synchronous communications, standardized encapsulation, multiplexing, link 
configuration, link quality testing, error detection, and feature or option negotiation
(such as compression).
PPP was originally designed to support CHAP and PAP for authentication. However, 
recent versions of PPP also support MS-CHAP, EAP, and SPAP. PPP can also be used to sup-
port Internetwork Packet Exchange (IPX) and DECnet protocols. PPP is an internet standard 
documented in RFC 1661. It replaced the Serial Line Internet Protocol (SLIP). SLIP offered 
no authentication, supported only half-duplex communications, had no error-detection capa-
bilities, and required manual link establishment and teardown.
Miscellaneous Security Control 
Characteristics
When you’re selecting or deploying security controls for network communications, you 
need to evaluate numerous characteristics in light of your circumstances, capabilities, and 
security policy. We discuss these issues in the following sections.
Transparency
Just as the name implies, 
transparency
is the characteristic of a service, security control, or 
access mechanism that ensures that it is unseen by users. Transparency is often a desirable 
feature for security controls. The more transparent a security mechanism is, the less likely 
a user will be able to circumvent it or even be aware that it exists. With transparency, there 
is a lack of direct evidence that a feature, service, or restriction exists, and its impact on 
performance is minimal.
In some cases, transparency may need to function more as a configurable feature than
as a permanent aspect of operation, such as when an administrator is troubleshooting, 
evaluating, or tuning a system’s configurations.

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