Networking Cisco

Point-to-Point Protocol

Serial communication protocol for direct network connections with authentication and error detection capabilities

Point-to-Point Protocol (PTP)

  • Serial communication protocol used to establish direct connection between two network nodes over various physical media
  • Operates at Data Link Layer (Layer 2) and provides framing, error detection, and authentication capabilities
  • Successor to SLIP (Serial Line Internet Protocol) with enhanced features like authentication and compression
  • Commonly used for dial-up connections, DSL, and point-to-point WAN links

Key Components

  • Link Control Protocol (LCP) - Establishes, configures, and tests the data-link connection
  • Network Control Protocols (NCPs) - Configure different network layer protocols (IPCP for IPv4, IPV6CP for IPv6)
  • Authentication protocols - PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol)

PPP Frame Format

Field Size Purpose
Flag 1 byte Frame delimiter (0x7E)
Address 1 byte Always 0xFF (broadcast)
Control 1 byte Always 0x03 (unnumbered frame)
Protocol 2 bytes Identifies encapsulated protocol
Data Variable Payload (up to 1500 bytes default)
FCS 2 bytes Frame Check Sequence for error detection

PPP Session Establishment Phases

  1. Link Dead - Physical layer not ready
  2. Link Establishment - LCP negotiates connection parameters
  3. Authentication - Optional PAP or CHAP verification
  4. Network Layer Protocol - NCP configures network protocols
  5. Link Open - Data transfer occurs
  6. Link Termination - Connection closed gracefully

Authentication Methods

PAP (Password Authentication Protocol)

  • Two-way handshake with plaintext username/password transmission
  • Less secure but simpler to implement
  • Password sent in clear text (security risk)

CHAP (Challenge Handshake Authentication Protocol)

  • Three-way handshake using encrypted challenge/response
  • More secure than PAP - passwords never transmitted in clear text
  • Periodically re-authenticates during session (prevents replay attacks)

CHAP Process

  1. Server sends random challenge to client
  2. Client responds with hash of challenge + shared secret
  3. Server compares response with its own calculation

PPP Configuration Options

Feature Purpose Benefit
Compression Reduces data size Improves throughput on slow links
Multilink Combines multiple physical links Increases bandwidth and redundancy
Magic Number Loop detection Prevents data loops in misconfigured links
MRU Negotiation Maximum Receive Unit Optimizes frame size for link efficiency

Common Use Cases

  • Dial-up Internet - Traditional modem connections to ISPs
  • DSL connections - PPPoE (PPP over Ethernet) for broadband authentication
  • Serial WAN links - Direct router-to-router connections
  • VPN tunnels - PPTP (Point-to-Point Tunneling Protocol) implementations

Vocabulary

LCP (Link Control Protocol) - PPP component that handles link establishment, configuration, and maintenance

NCP (Network Control Protocol) - Family of protocols that configure network layer protocols over PPP links

PPPoE (PPP over Ethernet) - Encapsulates PPP frames in Ethernet frames, commonly used for DSL connections

MRU (Maximum Receive Unit) - Largest frame size a device can receive on a PPP link

Magic Number - Random number used in LCP to detect looped-back links and misconfigured connections

Notes

  • PPP is connection-oriented - requires session establishment before data transfer (unlike HDLC which is connectionless)
  • Default MTU for PPP is 1500 bytes, but can be negotiated during LCP phase
  • PPP supports error detection but not correction - corrupted frames are discarded
  • When configuring serial interfaces, PPP is often preferred over HDLC for multi-vendor environments (HDLC implementations vary by vendor)
  • CHAP is always preferred over PAP for security reasons in production networks
  • PPP can negotiate compression algorithms like Stacker or Predictor to improve performance on slow links
  • For CCNA exam: Focus on PPP vs HDLC differences, authentication methods, and basic configuration commands
  • Remember: PPP authentication is optional but highly recommended for security