You are adding a dial-up remote access server to a TCP/IP network. The data link protocol must support secure authentication for users. Which protocol should you choose for the dial-up link?

Introduction / Context:
Dial-up remote access historically relied on data link protocols that framed IP packets over serial lines. Security requirements include authenticating the user before granting network access. Different protocols offer different levels of built-in authentication support.

Given Data / Assumptions:

• Transport is a serial dial-up line terminating on a remote access server.
• IP is the network protocol in use.
• Secure authentication (for example, CHAP) is required.

Concept / Approach:
PPP (Point-to-Point Protocol) encapsulates network protocols over serial links and includes Link Control Protocol (LCP) for negotiating options and Authentication Control Protocols supporting PAP, CHAP, MS-CHAP, and EAP. SLIP predates PPP and has no standard authentication or configuration negotiation. DLC is an IBM protocol not suited for IP dial-up authentication, and PPTP is a VPN tunneling method that itself commonly runs over PPP sessions.

Step-by-Step Solution:

Verification / Alternative check:
Historical RAS configurations on Windows/Unix default to PPP with CHAP/MS-CHAP. Packet traces show PPP LCP authentication exchanges prior to IPCP (IP Control Protocol) address assignment.

Why Other Options Are Wrong:

• SLIP: No authentication or option negotiation.
• PPTP: A tunneling protocol that typically encapsulates PPP; not the base serial link protocol.
• DLC: Not designed for IP dial-up and lacks the needed authentication frameworks.

Common Pitfalls:
Confusing application/VPN protocols with link protocols. Ensure strong authentication (for example, MS-CHAPv2/EAP) and consider encryption at higher layers if needed.

Final Answer:
PPP