Questions: PPP: Point-to-Point Protocol

Question 1

Two routers connect via a serial leased line. Before IP traffic can flow, they must agree on the maximum frame size and whether to use compression. Which PPP component handles this negotiation?

• A. NCP (Network Control Protocol), which negotiates all parameters including link-layer options
• B. CHAP, which authenticates both sides before any other negotiation can begin
• C. LCP (Link Control Protocol), which negotiates link-layer parameters like MRU and compression
• D. HDLC framing, which embeds frame size preferences in the 0x7E flag bytes

Answer: (C)

LCP (Link Control Protocol) is specifically responsible for link-layer parameter negotiation — it is the 'handshake' phase where both sides agree on maximum receive unit (MRU), compression settings, authentication method, and other link properties. NCP (Network Control Protocol) comes after LCP and negotiates which network-layer protocols to run (e.g., IPCP for IPv4). CHAP is one authentication method that LCP may negotiate, not a separate negotiation phase. HDLC-like framing provides byte boundaries but carries no negotiation semantics.

Question 2

Why is CHAP more secure than PAP for PPP authentication?

• A. CHAP uses longer minimum password lengths than PAP allows
• B. CHAP encrypts the entire PPP session with a shared key, while PAP only protects the authentication exchange
• C. CHAP uses a challenge-response mechanism where a hash of the shared secret is sent instead of the password itself
• D. CHAP operates at the network layer (Layer 3), making it harder to intercept than PAP's link-layer operation

Answer: (C)

CHAP's security advantage is that the password is never transmitted. Instead, the authenticator sends a random challenge, and the peer responds with a hash (typically MD5) of the challenge combined with the shared secret. An eavesdropper who captures the exchange sees only the challenge and the hash, not the password. PAP sends credentials in plaintext — anyone monitoring the line gets the password directly. CHAP also periodically re-challenges during the session, so capturing one exchange does not give permanent access.

Question 3

PPP is an obsolete protocol used primarily during the dial-up modem era and has no relevance to modern networking infrastructure.

• T. True
• F. False

Answer: False

PPP remains widely deployed in modern networks through PPPoE (PPP over Ethernet), which encapsulates PPP frames inside Ethernet frames. DSL providers use PPPoE to authenticate subscribers and assign IP addresses over broadband connections. PPP is also used on serial leased lines between routers and on some wireless backhaul links. Its clean separation of framing, link negotiation (LCP), and network-layer negotiation (NCP) makes it a versatile and still-relevant protocol wherever a point-to-point connection needs more structure than raw bit delivery.

Question 4

LCP negotiates link-layer parameters (such as frame size and compression), while NCPs negotiate which network-layer protocols (such as IP) will be carried over the PPP link.

• T. True
• F. False

Answer: True

This is the core architectural design of PPP. LCP runs first and establishes the link parameters that both sides agree to use — without successful LCP negotiation, no data flows. After LCP completes, one or more NCPs run to bring up network-layer protocols: IPCP configures IPv4, IPv6CP configures IPv6, and legacy NCPs handled IPX or AppleTalk. This layered negotiation approach is what makes PPP flexible enough to carry any network-layer protocol over any point-to-point serial link.

Question 5

What three problems does PPP solve that a raw serial byte stream cannot handle, and why is each necessary for reliable network communication?

Model answer: First, framing: a raw serial stream is a continuous sequence of bytes with no boundaries. PPP uses flag bytes (0x7E) and byte-stuffing to delimit frames so the receiver knows where each packet begins and ends. Without framing, the receiver cannot tell which bytes belong together. Second, link parameter negotiation via LCP: before data flows, both sides must agree on maximum frame size, compression, and authentication method. Mismatched parameters would cause silent failures. Third, network-layer protocol negotiation via NCP: the link may carry IP, IPv6, or other protocols. NCPs allow both sides to agree on which protocols to activate and configure them (e.g., IPCP assigns IP addresses). Together these three phases make PPP a complete, interoperable link-layer protocol.

Raw serial lines provide only a bit pipe — they transmit whatever bytes they receive with no interpretation. PPP adds the minimal structure needed for reliable, negotiated, authenticated communication between exactly two devices: boundaries (framing), rules (LCP), and payload type (NCP). This is why PPP persists in modern infrastructure despite being designed in the dial-up era.