CCNA 4 Chapter 4

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CCNA 4 Chapter 4

• Point-to-Point Protocol
• (PPP)

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OVERVIEW

• WAN connections are controlled by protocols that
  perform the same basic functions as Layer 2 LAN
  protocols (such as Ethernet)
• WAN protocols establish a data path and provide
  for flow control procedures

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PPP Overview

• PPP is the most widely used and popular WAN
  protocol
• Point-to-Point Protocol operates at the lower
  layers of OSI model (data-link layer primarily)
• PPP is a successor to (SLIP) Serial Line Internet
  Protocol
• PPP provides router-to-router and host-to-network
  connections over synchronous and asynchronous
  circuits
• synchronous circuit - signal transmitted with
  precise clocking. Signals having the same
  frequency, with individual characters
  encapsulated in control bits.
• asynchronous circuit - signal transmitted without
  precise clocking.

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Features of PPP

• PPP offers
• Control of data link setup
• Assignment management of IP addresses
• Network protocol multiplexing
• Link configuration link quality testing
• Error detection
• Negotiation options for capabilities such as
  network-layer address negotiation and data
  compression negotiation

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PPP Three Main Components

• A method for encapsulating datagrams over serial
  links.
• Uses High Level Data Link Control (HDLC) as a
  basis for encapsulating datagrams over
  point-to-point links.
• A Link Control Protocol (LCP) to establish,
  configure, and test the data-link connection
• A family of Network Control Programs (NCPs), for
  establishing configuring different network
  layer protocols
• PPP allows for simultaneous use of multiple
  network layer protocols (IP, IPX AppleTalk)

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PPP Frame Formats/Fields

• Flag - Indicates the beginning and end of a frame
• Address - Consists of the standard broadcast
  address, which is the binary sequence 11111111.
  PPP does not assign individual station address
• Control - 1 byte, a connectionless link service
  similar to that of Logical Link Control (LLC)
• Protocol - 2 bytes that identify the protocol
  encapsulated in the data field of the frame
• Data - 0 or more bytes that contain the datagram
  for the protocol specified in the Protocol field.
  Default maximum length is 1500 bytes
• FCS - Normally 16 bits (2 bytes). Refers to the
  extra characters added to a frame for error
  control purposes

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PPP Session Establishment
Four Phases to PPP Session Establishment/Terminati
on

• Link establishment and configuration negotiation
• Link-quality determination (Optional
  authentication phase)
• Network-layer protocol configuration negotiation
• Link Termination

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LCP FRAMES

• LCP frames are used to perform the 4 phases of
  session establishment and termination.
• Classes of LCP frames
• Link establishment frames used to establish and
  configure a link
• Link maintenance frames used to manage and
  debug a link
• Link termination frames used to terminate a link

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Phase 1 Link Establishment Configuration
Negotiation

• In this phase, an originating PPP node sends LCP
  frames to configure establish the data link.
• LCP packets contain a configuration option field
  that allows devices to negotiate the use of
  options
• Options include MTU, compression of certain PPP
  fields, link authentication protocol
• If a configuration option is not included in an
  LCP packet, the default value is assumed.
• LCP must open the connection and negotiate the
  configuration parameters before any network layer
  datagrams can be exchanged (a configuration
  acknowledgement frame is then sent)

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Phase 2 LinkQuality Determination

• PPP performs optional Layer-2 authentication in
  this phase
• The link is tested to determine whether the link
  quality is sufficient to bring up the
  network-layer protocol
• LCP can delay transmission of network-layer
  protocol information until this phase is
  completed
• PPP supports 2 authentication protocols PAP
  (Password Authentication Protocol) CHAP
  (Challenge Handshake Authentication Protocol)
• CHAP is the preferred method

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Phase 3 Network-Layer Protocol Configuration
Negotiation

• During this phase, originating PPP node sends NCP
  frames to choose configure network-layer
  protocols
• When each of the chosen network-layer protocols
  has been configured, datagrams from each network
  layer protocol can be sent over the link
• If LCP closes the link, it informs the network
  layer protocols
• show interfaces command checks the LCP NCP
  states once PPP is configured

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Phase 4 Link Termination

• In PPP, the link remains configured for
  communications until LCP or NCP frames close the
  link or until some external event occurs
• LCP can terminate the link at any time (at the
  request of the user)
• Termination can also occur because of a physical
  event (loss of carrier or time out)

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PPP Authentication - PAP

• Password Authentication Protocol
• Supported only on PPP lines
• Allows PPP peers to authenticate one another
• The calling side of the link must enter
  authentication information (this ensures that the
  user has the network administrators permission to
  make the call)
• PAP provides a simple method for a remote node to
  establish its identity through a 2-way handshake
• The PPP link establishment phase is completed
• A username/password pair is repeatedly sent by
  the remote node across the link until
  authentication is acknowledged or the connection
  is terminated

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PPP Authentication - PAP

• Disadvantages of using PAP
• PAP is not a strong authentication protocol
• Passwords are sent across the link in clear text.
• No protection from repeated trial and error
  attacks.
• The remote node is in control of the frequency
  and timing of the login attempts

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PPP Authentication - CHAP

• Challenge Handshake Authentication Protocol
• Security feature supported on lines using PPP
  encapsulation that prevents unauthorized access
• CHAP is used to periodically verify the identity
  of the remote node using a three-way handshake
• Verification is done upon initial link
  establishment and can be repeated any time after

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PPP Authentication - CHAP

• After PPP link establishment phase is complete,
  the host sends a challenge message to the remote
  router
• Remote router responds with a value
• The host checks the response against its own
  value
• If the values match authentication is
  acknowledged
• If values dont match connection is terminated

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CCNA 4 Chapter 4
PPP

• THE END