Module 8 Wide Area Network WAN PointtoPoint Connection

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Module 8Wide Area Network (WAN)Point-to-Point
Connection
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• Textbook sections
• LG 5.4.1 HDLC Data Link Control
• LG 5.4.2 Point-to-Point Protocol
• Topics
• WAN Options
• High-level Data Link Control (HDLC)
• Overview
• System Set-up Station Type, Configuration, Mode
• Frame Format Fields, Bit stuffing Frame Types,
  Frame Exchange
• Point-to-Point Protocol (PPP)
• Overview
• Framing
• Link Control Protocol (LCP)
• Authentication
• Network Control Protocol (NCP)
• A Typical Scenario

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1. WAN Options
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2. High-level Data Link Control (HDLC) - Overview
Point-to-Point Link
Note HDLC From Cisco's perspective, HDLC is the
default choice of CISCO routers Interface
Operating System (IOS). HDLC requires Cisco's
router on each end. PPP The point-to-point
protocol is the de facto standard for remote
dial-in connections to the IP networks.
Virtually all dial-in connections to the Internet
use PPP.
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2. High-level Data Link Control (HDLC) - Overview

• Description
• HDLC is a widely accepted international protocol
  governing information transfer that was developed
  by the International Standard Organization (ISO).
  
• HDLC is a bit-oriented, synchronous protocol that
  applies to the Data Link layer of the OSI model.
  
• Under the HDLC protocol, data is transmitted in
  units called frames, each of which can contain a
  variable amount of data.
• Applicable to point-to-point communication lines
  as well as to connections over a network where
  messages follow the same path

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2. High-level Data Link Control (HDLC) - Overview

• Highlights
• HDLC is an ISO standard bit-oriented data-link
  protocol that encapsulates data on synchronous
  serial data links.
• HDLC does not have the type field to indicate
  layer 3 protocol
• HDLC does not inherently support multiple
  protocols on a single link because it does not
  have a standard way to indicate which protocol it
  is carrying.
• Ethernet frames have a type field to indicate
  which layer 3 protocol is used to transport data.
  The lack of a protocol field limits standard HDLC

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2. High-level Data Link Control (HDLC) - Overview

• Highlights
• Ciscos proprietary version of HDLC
• The Cisco HDLC frame uses a proprietary type
  field that act as a protocol field, which make it
  possible for multiple network-layer protocols to
  share the same serial link
• This implementation is proprietary and can only
  be used between devices that can interpret the
  Cisco frame type
• Ciscos HDLC is a point-to-point protocol that
  can be used on leased lines between two devices
  supporting Cisco proprietary HDLC encapsulation.
  If communicating with a non-Cisco device,
  synchronous PPP is a more viable option

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HDLC Frame Formats
Standard HDLC supports only single protocol
environments
Ciscos HDLC has a proprietary data field to
support multi-protocol environments. Ciscos
HSDLC is proprietary. It wont communicate with
any other vendors HDLC implementation. But do
not give Cisco grief for it. Everyones HDLC
implementation is proprietary.
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2. High-level Data Link Control (HDLC) - System
Set-up

• Station type
• Primary
• A primary station is the device in either a
  point-to-point or multipoint line configuration
  that sends commands to the secondary stations.
• Secondary
• A secondary station issues responses.
• Combined
• A combined station can both command and respond.
• Configuration
• Unbalanced
• also called a master/slave configuration
• Only one device is primary station and the others
  are secondary station
• Can be either point-to-point or multipoint
• Balanced
• Both stations in a point-to-point topology are of
  the combined type
• Both stations are linked by a single line that
  can be controlled by either station.
• HDLC does not support a balanced multipoint
  configuration. This necessitated the invention
  of media access protocols for LANs

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LG Figure 5.34 HDLC configuration
Unbalanced Point-to-point link
Commands
Primary
Secondary
Responses
Unbalanced Multipoint link
Commands
Primary
Responses
Secondary
Secondary
Secondary
Balanced Point-to-point link between Combined
Stations
Commands
Secondary
Primary
Responses
Primary
Secondary
Commands
Responses
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2. High-level Data Link Control (HDLC) - System
Set-up

• Mode
• Normal response mode (NRM)
• Refers to the standard primary-secondary
  relationship.
• In this mode, a secondary station must have
  permissions from the primary station before
  transmitting.
• Asynchronous balanced mode (ABM)
• In this mode, all stations are equal and
  therefore only combined stations connected in
  point-to-point are used
• Either combined station may initiate transmission
  with the other combined station without
  permission.

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2. High-level Data Link Control (HDLC) - Frame
Format

• Frame format
• Each frame in HDLC can contain up to six fields
• In multiple frame transmissions, the ending flag
  of one frame can double as the beginning of the
  next frame

Flag
Address
FCS
Flag
Control
Data
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2. High-level Data Link Control (HDLC) - Frame
Format

• Frame format
• Flag field
• The flag field of an HDLC frame is an eight-bit
  sequence with a bit pattern 01111110.
• Bit stuffing
• Used to prevent the occurrence of the flag inside
  the HDLC frame
• The transmitter examines the contents inside the
  frame and inserts an extra 0 after each instance
  of five consecutive 1s
• When receiver find five 1s followed by a 0, the 0
  is decided to be a stuffing bit, and is removed.
  Five 1s followed by 10 indicate the presence of a
  flag.
• Address field
• Contains the address of the secondary station
  that is either the originator or destination of
  the frame
• If a primary station creates a frame it contains
  a to address. If a secondary creates the frame,
  it contains a from address

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Bit Un-stuffing Algorithm
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2. High-level Data Link Control (HDLC) - Frame
Format

• Control field
• Figure 5.36 Control field format
• Control fields differ depending on frame type
• Frame types
• Information frame (I-frame) when the first bit
  of the control bit is 0
• Supervisory frame (S-frame) when the first two
  bits are 10
• Unnumbered frame (U-frame) when the first two
  bits are 11
• Information frame (I-frame)
• N(S) fields provides the send sequence number
• N(R) field is used to piggyback acknowledgement
  the to indicate the next frame that is expected
  at the given station
• Supervisory frame (S-frame)
• Used for acknowledge, flow control, and error
  control whenever piggybacking that information on
  an-I frame is either impossible or inappropriate
  (such as when the station either has no data of
  its own to send, or needs to send a command or
  response other than an acknowledge)

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2. High-level Data Link Control (HDLC) - Frame
Format
LG Figure 5.36 Control field format
Information Frame
1
5
2-4
6-8
N(R)
0
N(S)
P/F
Supervisory Frame
N(R)
1
0
S
S
P/F
Unnumbered Frame
1
1
M
M
M
M
P/F
M
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2. High-level Data Link Control (HDLC) - Frame
Format

• Frame format
• Supervisory frame (S-frame)
• Receive ready (RR) frame (a value of SS 00) to
  acknowledge frames when no I-frame are available
  to piggyback the acknowledgement.
• Reject (REJ) frame (a value of SS 01) to send
  a negative acknowledgment by the receiver. A REJ
  frame indicates that an error has been detected
  and that the transmitter should go back and
  retransmit frames from N(R) onwards (for
  Go-Back-N)
• Receive not ready (RNR) (a value of SS 10) The
  RNR frame acknowledge all frame up to N(R) -1 and
  informs the transmitter that the receiver has
  temporary problems, that is, no buffers, and will
  not accept any more frames.
• Selective reject (SREJ) (a value of SS11) SREJ
  indicates to the transmitter that it should
  retransmit the frame indicated in the N(R)
  subfield (for Selective Repeat ARQ).

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2. High-level Data Link Control (HDLC) - Frame
Format

• Unnumbered frame (U-frame)
• Used to exchange session management and control
  information between connected devices. Unlike
  S-frame, U-frame contain an information field.
  It is used for system management information
  rather than user data.
• Set asynchronous balanced mode (SABM) frame
  indicates that the sender wishes to set up an
  asynchronous balanced mode connection
• Set normal response mode (SNRM) frame indicates
  that the sender wishes to set up a normal
  response mode connection
• Unnumbered acknowledgment (UA) frame acknowledge
  frames during call setup and call releases.
• Disconnect (DISC) frame indicates that a station
  wishes to terminate a connection

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2. High-level Data Link Control (HDLC) - Frame
Format

• P/F bit
• It has meaning only when it is set (bit 1)
• It means poll (the primary selects a secondary to
  receive data or polls secondaries to see if they
  have data to send)
• It means final (the final packet) when the frame
  is sent by a secondary to a primary.

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2. High-level Data Link Control (HDLC) - Frame
Exchange

• Frame Exchange
• Notation
• First entry indicates the contents of the
  address field
• Second entry type of frame, for example, I,
  RR,etc.
• For I-frame only
• Third entry Send sequence number N(S)
• Fourth entry Receive sequence number N(R)
• A P(poll) or F (final) at the end of indicates
  that the P/F bit is set

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2. High-level Data Link Control (HDLC) - Frame
Exchange
LG Figure 5.37 Exchange of frames for connection
establishment and release
Data transfer
SABM
UA
UA
DISC
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2. High-level Data Link Control (HDLC) - Frame
Exchange
LG Figure 5.38 Exchange of frames using normal
response mode
Secondaries B, C
Primary A
B, RR, 0, P
B, I, 0, 0
B, I, 1, 0
X
B, I, 2, 0,F
B, SREJ, 1
C, RR, 0, P
C, RR, 0, F
B, SREJ, 1,P
B, I, 1, 0
B, I, 3, 0
B, I, 4, 0, F
B, I, 0, 5
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2. High-level Data Link Control (HDLC) - Frame
Exchange
Figure 5.39 Exchange of frames using asynchronous
balanced mode
Combined Station A
Combined Station B
B, I, 0, 0
A, I, 0, 0
B, I, 1, 0
A, I, 1, 1
X
A, I, 2, 1
B, I, 2, 1
B, I, 3, 2
B, REJ, 1
B, I, 4, 3
A, I, 3, 1
B, I, 1, 3
B, I, 2, 4
B, RR, 2
B, I, 3, 4
B, RR, 3
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3. Point-to-Point Protocol (PPP) - Overview

• Provides a method for encapsulating IP packets
  over point-to-point links
• Used as a data link control to connect two
  routers or a personal computer to an Internet
  service provider (ISP)
• PPP provides four things
• Framing A framing method that unambiguously
  delineates the end of one frame and the start of
  the next one. The frame format also handles
  error detection
• LCP A link control protocol (LCP) for bringing
  lines up, testing them, negotiating options, and
  bringing them down again gracefully when they are
  no longer needed.

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3. Point-to-Point Protocol (PPP) - Overview

• Authentication require that the calling side of
  the link enter information to help ensure that
  the caller has the network administrators
  permission to make the call. Peer routers
  exchange authentication messages. Two
  alternatives are
• Password Authentication Protocol (PAP)
• Challenge Handshake Authentication Protocol
  (CHAP)
• NCP A way to negotiate network-layer options in
  a way that is independent of the network layer
  protocol to be used. The method chosen is to
  have a different network control protocol (NCP)
  for each network layer supported. For example,
• IPCP is used for IP
• IPXCP is used for IPX

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3. Point-to-Point Protocol (PPP) - Overview
PPP Layers
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Point-to-point protocol stack
Note It is important to understand that the PPP
protocol stack is specific at the Physical and
Data Link layer only. NCP is used to allow
communication of multiple Network layer protocols
by encapsulating the protocol across a PPP data
link.
OSI layer
Upper-layer protocols (such as IP, IPX, AppleTalk)
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Network Control Protocol (NCP) (specific to each
Network-layer protocol)
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Link Control Protocol (LCP)
A variation of High-level Data Link Control
Protocol (HDLC)
Physical layer (such as EIA/TIA-232, ISDN)
1
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3. Point-to-Point Protocol (PPP) - Framing

• Framing
• PPP protocol uses an HDLC-like frame format
• Frame format
• Flag field
• PPP frames consist of an integer number of bytes
• Byte insertion method that makes use of an escape
  character is applied
• Address field
• All 1s address field indicates that all stations
  are to accept the frame
• Control field
• Default to 00000011 because PPP is normally run
  in connectionless mode. This value indicates an
  unnumbered frame. In other words, PPP does not
  provide reliable transmission using sequence
  numbers and acknowledgements . In noisy
  environments, such as wireless networks, reliable
  transmission using numbered mode can be used (RFC
  1663)

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3. Point-to-Point Protocol (PPP) - Framing

• Protocol field
• PPP was designed to support multiple network
  protocols simultaneously
• Identify the network layer protocol of the packet
  contained in the information field
• Protocols starting with a 0 bit are network layer
  protocols such as IP, IPX, etc.
• Protocols start with a 1 bit are used to
  negotiate other protocols. These include LCP and
  a different NCP for each network layer protocol
  supported
• The default size of the field is 2 bytes, but it
  can be negotiated down to 1 byte using LCP.

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3. Point-to-Point Protocol (PPP) - Framing
LG Figure 5.40 PPP frame format
1 byte
1 byte
1 byte
1 or 2 byte
variable
2 or 4 byte
1 byte
flag
Address
Flag
Control
Protocol
Information
CRC
01111110
01111110
1111111
00000011
All stations are to accept the frame
Unnumbered frame
Specifies what kind of packet is contained in the
payload, e.g., LCP, NCP, IP, OSI CLNP, IPX For
example C02116 Link Control Protocol C02316
Password Authentication Protocol C22316
Challenge Handshake Authentication
Protocol 802116 IPCP
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Transition States
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3. Point-to-Point Protocol (PPP) - LCP

• Idle state
• The link is not being used. There is no active
  carrier and the line is quiet.
• Establishment state
• When one of the end points starts the
  communication, the connection goes into the
  establishment state.
• The Link Control Protocol (LCP) is used to
  establish the connection through an exchange of
  Configure packets
• All configuration Options are assumed to be at
  default values unless altered by the
  configuration exchange.
• If the negotiation is successful, the system goes
  to the authenticating state (if authentication is
  required) or directly to the networking state.
• Authenticating state (optional) Several
  authentication packets may be exchanged during
  this state

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3. Point-to-Point Protocol (PPP) - LCP

• Networking state
• This state is the heart of the transition states
• When a connection reaches this state, the
  exchange of user control and data packets can be
  started. The connection remains in this state
  until one of the end points wants to terminate
  the connection
• Terminating state
• Several packets are exchanged between the two
  ends for house cleaning and closing the link.

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3. Point-to-Point Protocol (PPP) - LCP

• Link Control Protocol (LCP)
• Defined in RFC 1661
• Functions
• To negotiate data link protocol during the
  ESTABLISHMENT phase.
• LCP is not actually concerned with the options
  themselves but with the mechanism for
  negotiation.
• LCP provides a way for the initiating process to
  make a proposal and for the responding process to
  accept or reject it, in whole or in part.
• To provide a way for the two processes to test
  the line quality.
• To taken down the lines when they are no longer
  needed

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3. Point-to-Point Protocol (PPP) - LCP

• Eleven types of LCP packets and their codes as
  defined in RFC 1661

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3. Point-to-Point Protocol (PPP) - LCP
LCP Packet Encapsulated in a Frame

• Note LCP packets can be used to
• Configure to negotiate the options between two
  ends
• Terminate the link between two ends
• Monitor and debug the link
• ID field This field holds a value used to match
  a request with the reply.
• One end point inserts a value in this field,
  which will be copied in the reply packets.

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3. Point-to-Point Protocol (PPP) - Authentication

• Password Authentication Protocol (PAP)
• PAP provides a simple method for a user node to
  establish its identity using a two-way handshake.
• PAP is done only upon initial link establishment
• After the PPP link establishment phase is
  complete, a username-and-password pair is
  repeatedly sent by the user node to the system
  node until authentication is acknowledged or the
  connection is terminated.
• PAP is not a strong authentication protocol
• Passwords are sent across the link in clear text.
  There is no protection from playback or repeated
  trial-and-error attacks. (A clear-text password,
  however, may be fine in environments that use
  token-type passwords that change with each
  authentication.)
• The user node is in control of the frequency and
  timing of the login attempts.

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3. Point-to-Point Protocol (PPP) - Authentication
PAP
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3. Point-to-Point Protocol (PPP) - Authentication
PAP Packets
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3. Point-to-Point Protocol (PPP) - Authentication

• Challenge Handshake Authentication Protocol
  (CHAP)
• CHAP is a stronger authentication method than
  PAP.
• CHAP is used at the startup of a link, and
  periodically, to verify the identity of the user
  node using a three-way handshake.
• CHAP is done upon initial link establishment and
  can be repeated any time after the link has been
  established.
• CHAP Authentication
• After the PPP link establishment phase is
  complete, the system sends a challenge message
  to the user node.
• The user node responds with a value calculated
  using a one-way hash function (typically MD5).
• The system node checks the response against its
  own calculation of the expected hash value. If
  the value match, the authentication is
  acknowledged. Otherwise, the connection is
  terminated immediately.

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3. Point-to-Point Protocol (PPP) - Authentication
CHAP
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3. Point-to-Point Protocol (PPP) - Authentication
CHAP Packets
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An Example of the states through which a PPP
connection goes to deliver some network layer
packets
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A Typical Scenario
LG Figure 5.41 PPP phase diagram
Idle state

• Home PC to Internet Service Provider
• 1. PC calls router via modem.
• 2. PC and router exchange LCP packets to
  negotiate PPP parameters.
• 3. Check on identities.
• 4. NCP packets exchanged to configure the
  network layer, e.g., TCP/IP ( requires IP address
  assignment).
• 5. Data transport, e.g. send/receive IP packets.
• 6. NCP used to tear down the network layer
  connection (free up IP address) LCP used to shut
  down data link layer connection.
• 7. Modem hangs up.

1. Carrier Detected
Dead
7. Carrier Dropped
failed
Establish
Terminate
2. Options Negotiated
6. Done
failed
Authenticate
5.
Open
3. Authentication Completed
4. NCP Configuration
Network
Network state