Computer Network & Network Security System — Data Link Layer, NEC licence examination syllabus (Nepal Engineering Council).
PPP — Point to Point Protocol
The quiet protocol that used to connect entire households to the internet, one phone line at a time.
PPP is designed for a direct link between exactly two devices — historically famous as the protocol behind dial-up internet connections.
🔗 Connection Phases
LCPLink Control Protocol — establishes, configures, and tests the data-link connection itself.
NCPNetwork Control Protocol — configures the specific network-layer protocol (like IP) that will actually run over that link.
PPP's frame format is deliberately simple: Flag – Address – Control – Protocol – Data – FCS – Flag.
💡 One-liner: "What's the order of PPP connection phases?" → Link establishment (LCP) → authentication (optional) → network-layer configuration (NCP) → data transfer → termination.
Why the phases run in that order
The sequence is not arbitrary, and seeing why makes it impossible to misremember.
1. Link establishment (LCP)
Agree the link's own parameters — frame size, authentication
protocol, compression. Nothing else can happen first, because
the two ends do not yet agree on how to talk at all.
2. Authentication (optional)
Prove identity, using whatever LCP just agreed on. Must come
BEFORE network configuration: no point assigning an IP address
to someone who turns out not to be a subscriber.
3. Network configuration (NCP)
One NCP instance per network protocol — IPCP for IPv4. This is
where the dial-up user is assigned an IP address.
4. Data transfer
5. Termination
The order is a dependency chain: you cannot authenticate before agreeing how to communicate, and you should not hand out network resources before knowing who is asking. A question giving the phases out of order is testing exactly that reasoning.
Why PPP outlived dial-up
The modems are gone, but PPP is not — because it solved a general problem rather than a dial-up one.
🔌 What PPP provides that raw links do not
AuthenticationA point-to-point link has no inherent way to identify who connected. PPP supplies it, which is why any subscriber service wants it.
Dynamic addressingNCP assigns a network address at connection time rather than requiring pre-configuration.
Protocol multiplexingThe Protocol field lets one link carry IPv4, IPv6 and others simultaneously.
Link negotiationThe two ends agree parameters instead of both needing manual setup.
Those needs did not disappear with dial-up, so PPP was carried forward — PPPoE over Ethernet for DSL, and PPP inside tunnelling protocols for VPNs. The link changed; the requirements did not.
💡 PPP uses the same bit stuffing and flag-delimited frame as HDLC, covered in the Data Link Protocol topic. Its frame is Flag–Address–Control–Protocol–Data–FCS–Flag, where the extra Protocol field is what makes multiplexing possible. Recognising the shared framing means there is less to learn than it appears.
Syllabus points
Frame format; connection phases
LCP and NCP
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