1. Roadmap WAN Basics Introduction to Point-to-Point protocol (PPP)
Three Types of WAN Services
Introduction to Point-to-Point protocol (PPP)
Basic Components of PPP
Frame Formats of PPP
General Operation of PPP

2. Point-to-Point Links Introduction
If two devices are directly connected to each other, then they are said to be in point-to-point configuration
Point-to-point protocol is a data link layer protocol which is used to control the communication over such links

3. Point-to-Point Link

4. An Overview of Wide-Area Services
Central Office
(CO) CO
Basic Telephone Service
Time-Division Multiplexed Circuits
(56/64 kbps or T1/E1) B C A E
X.25/Frame Relay
Networks
Call Setup
(SS7 or other)
X.25 / Frame Relay
A simplified look inside the WAN cloud
The router also uses a WAN central office

5. WAN Basics Three Major types of WAN Services are: Leased Line
Circuit Switching/dial lines
Packet Switching

6. WAN Connection Types Term Description Leased lines
A dedicated, always on circuit between two end points. The service provider just passes a constant rate bit stream. Generally is more expensive than packet switching today
Circuit switched/dial
Provide dedicated bandwidth between two points, but only for the duration of the call. Typically used as a cheaper alternative to leased lines, particularly when connectivity is not needed all the time. Also useful for backup when a leased line or packet-switched service fails
Packet-switched
Provides virtual circuits between pairs of sites, with contracted traffic rates for each VC. Each site’s physical connectivity consists of a leased line from the site to a device in the provider’s network. Generally cheaper than leased lines.

7. PPP over the Internet
Today, million of Internet users who need to connect their home PCs to the server of an ISP use PPP
The Internet needs a PPP for a variety of purposes, including router-to-router traffic (leased line) and home-user to ISP (dial-up) traffic.
Majority users have a traditional modem, DSL modem or a cable modem which connects them to the Internet through a Telephone line or a TV cable connection.
These lines provide a physical link, but to control and manage the transfer of data, there is a need of PPP
For both connections, router-to-router (leased line connections) and home-user to ISP (dial-up connections) some data link protocol is required on the line for:
Framing or encapsulation
Error control
and other data link layer functions

8. WAN Data-Link Protocols for Point-to-Point Links
Leased line
Circuit-switched
Packet-switched

9. Definition - PPP
PPP is defined in RFC 1661 and further elaborated in later RFCs.
It originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links
PPP also established a standard for
assigning and managing IP addresses
asynchronous and synchronous encapsulation
network protocol multiplexing
link configuration & link quality testing
error detection, option negotiation

10. PPP Components Main components of PPP include: Encapsulation/Framing
Link Control Protocol (LCP)
Network Control Protocol (NCP)
These components help PPP to define the frame formats to be exchanged, how the 2 devices will negotiate with each other to establish a link and for exchanging the data, how the link is configured and terminated.

11. 1. Encapsulation
It is a method for encapsulating datagrams of network layer over serial links
A framing mechanism defines the boundaries of the start and end of the PPP frame
It also helps in error detection

12. 2. Link Control Protocol
LCP is used to establish, configure and test the data link connections
Its main purpose is:
to bring the lines up
test them
negotiate options
and bringing them gracefully down again when they are no longer needed
It supports synchronous and asynchronous circuits
LCP configuration options include:
Maximum frame size or MTU (maximum transmission unit)
Authentication protocol specifications (if any)
Options to skip control and address fields of PPP frame
PPP negotiates configuration parameters at the start of the connection using LCP.

13. Protocol Field of PPP Frame

14. LCP packet encapsulated in a frame

15. LCP Phase: Common Options
Default
Maximum receive unit
1500
Authentication protocol
None
Protocol field compression
Off
Address and control field compression

16. Link Control Protocol (LCP)
LCP is responsible for establishing, maintaining, configuring, and terminating links.
It also provides negotiation mechanisms to set options between the two endpoints. Both endpoints of the link must reach an agreement about the options before the link can be established.
When PPP is carrying a LCP packet, it is either in establishing or terminating state
No user data is exchanged during this state
All LCP packets are carried in the data field of the PPP frame

17. LCP packets and their codes
Packet Type
Description
0116
Configure-request
Contains the list of proposed options and their values
0216
Configure-ack
Accepts all options proposed
0316
Configure-nak
Announces that some options are not acceptable
0416
Configure-reject
Announces that some options are not recognized
0516
Terminate-request
Requests to shut down the line
0616
Terminate-ack
Accepts the shut down request
0716
Code-reject
Announces an unknown code
0816
Protocol-reject
Announces an unknown protocol
0916
Echo-request
A type of hello message to check if the other end is alive
0A16
Echo-reply
The response to the echo-request message
0B16
Discard-request
A request to discard the packet

18. PPP Authentication…

19. Authentication Protocols
PPP use two authentication protocols
Password Authentication Protocol (PAP)
Challenge Handshake Authentication Protocol (CHAP)
PAP sends the username and password in clear text.
The user who wants to access a system sends username and password
The system checks the validity of the username and password and either accepts or denies connection.
For those systems that require greater security, PAP is not enough; a third party with access to the link can easily pick up the password and access the system resources.

20. Password Authentication Protocol (PAP)

21. Selecting a PPP Authentication Protocol

22. Challenge Handshake Authentication Protocol (CHAP)
It is a three handshaking authentication protocol that provides greater security that PAP.
In this method the password is kept secret, it is never sent online.
The system sends to the user a challenge packet containing a challenge value, usually a few byte.
The user applies a predefined function (Message Digest-5, one way hash function) that takes the challenge value and the user’s own password and creates a result. The user sends the result in the response packet to the system

23. CHAP… Challenge Handshake Authentication Protocol (CHAP)…
The system does the same. It applies the same function to the password of the user (known to the system) and the challenge value to create a result. If the result created the same as the result in the response packet, access is granted; otherwise it is denied
CHAP is more secure than PAP, especially if the system continuously changes the challenge value. Even if the intruder learns the challenge value and the result, the password is still secret.

24. CHAP…

25. Selecting a PPP Authentication Protocol

26. 3. Network Control Protocol
NCP allows the encapsulation of data coming from the network layer protocols into PPP frame
PPP is designed to allow simultaneous use of multiple network layer protocols
PPP provides a way to negotiate network layer options independent to the network layer protocol to be used

27. Network Control Protocol (NCP)
After the link is established and authentication (if any) is successful, the connection goes to the networking state.
In this state, PPP uses another protocol called Network Control Protocol (NCP).
NCP is a set of control protocols to allow the encapsulation of data coming from network layer protocols into PPP frame.
PPP requires two parties to negotiate not only at the data link layer, but also at the network layer.
Before user data can be sent, a connection must be established at this level.
The set of packets that establish and terminate a network layer connection for IP packets is called IP control protocol (IPCP).

28. PPP- A data link protocol with network layer services
Layering PPP Elements
PPP- A data link protocol with network layer services

30. PPP Frame Format

31. PPP Frame Format (2) Flag Address One byte field
It identifies the start and end of the PPP frame
Contains the binary sequence:
Address
As the PPP is used for point-to-point connections, not for multipoint connections like LANs, it does not assign individual station addresses
Uses the standard broadcast address value to avoid data link layer addressing (MAC)
It uses the fixed binary sequence:

32. One may wonder.?
As both the address and control fields an currently take only fixed values, one wonders why these fields are even defined in the first place?
The PPP specifications as mentioned in RFC 1662 states that other values “may be defined at a later time”
Although None has been defined to date

33. PPP Frame Format (3) Control protocol One byte field
It specifies that there is no flow and error control
It uses the fixed binary sequence:
protocol
Two byte field
It tells the PPP receiver the upper layer protocol to which the received encapsulated data belongs
Whether the PPP frame carries user data or other information
Codes are defined to identify different protocols

34. Protocol Field Codes Protocol Field Code Protocol Description C02116
LCP
C02316
PAP (for authentication)
C22316
CHAP (for authentication)
802116
NCP
IPCP (IP Datagram for Data)

35. PPP Frame Format (4) Protocol …
It indicates the protocol encapsulated in the data field of PPP frame e.g.
LCP
When PPP is carrying a LCP packet, it is either in establishing or terminating state
No user data is exchanged during this state
Authentication protocol
These protocols are used for authentication

36. PPP Frame Format (5) Protocol … NCP
It allows network layer data encapsulation into PPP frames
User data is exchanged during this state
Thus PPP is a multi-protocol framing method suitable for use over modems

37. PPP Frame Format (6) Data FCS Maximum 1500 bytes
Contains the datagrams for protocol specified in the protocol field
FCS
2 byte or 4 byte field
Used for error detection using CRC code

38. General Operation of PPP
Let us see how the three components of PPP fit together i.e.
Encapsulation
LCP
NCP

39. PPP – A scenario

40. A typical Scenario (1)
A typical scenario of a home user calling an ISP
P.C calls the ISP router via modem
After receiving the answer, the physical connection is established between two modems
PC sends the router a series of LCP packets in the data portion of PPP frames to configure and (optionally) test the data link connection
4. These LCP packets and their responses select the PPP parameters to be used
5. Then a series of NCP packets are used to configure network layer .
6. NCP frames are used to choose and configure one or more network layer protocols

41. A typical Scenario … (2)
7. Typically, a PC wants to run a TCP/IP protocol stack, so it needs an IP address
8. ISPs keep a block of IP addresses to be assigned dynamically to each newly attached PC for the duration of its login session
9. The NCP for IP assigns the IP address
10. At this point the PC is now an Internet host and can send and receive IP packets
11. The link will remain configure for communication until explicit LCP or NCP frames close the link or some external event (inactivity timer expires etc.)

42. A typical Scenario … (3)
12. When the user is finished, the NCP tears down the network layer connection, and frees up the IP address
13. Then LCP shut down the data link layer connection
14. Finally the computer tell the modem to hang-up the phone, releasing the physical connection.

43. Simplified PPP Phase Diagram

44. PPP Features/Services
Main services of PPP include:
Encapsulation/Framing
Error Detection and Correction
Support Multiple Network Layer Protocols
Authentication
Link Establishment, Termination, and Maintenance

45. PPP Services… Encapsulation/Framing
The PPP protocol take a network layer packet and
Encapsulate it within the PPP data link layer frame such that the receiver will be able to identify the start and end of the both data link frame and network layer packet within the frame.

46. PPP Services… Error Detection and Correction
All data link protocols like PPP and HDLC perform error detection.
All these protocols use a field in the trailer usually called frame check sequence (FCS),
FCS verifies whether bit errors occurred during transmission of the frame, if so frame is discarded.
Error recovery is the process that causes retransmission of lost or erred frames.
PPP can perform error correction but by default it is not enabled.

47. PPP Services… Support Multiple Network Layer Protocols Authentication
PPP supports multiple protocol suites like TCP/IP (IP), Novell’s (IPS), Appletalk etc running over same physical link at the same time.
PPP multiplex different network layer protocols over a single point-to-point connection
Authentication
PPP defines how two devices can authenticate each other.
PPP is designed for use over dial-up links where verification of user identify is necessary.
Authentication means validating the identity of a user who needs to access a set of resources
IT Internet Architecture and Protocols, PUCIT, University of the Punjab, Pakistan

48. PPP Services… Link Establishment, Termination, and Maintenance
PPP defines how two devices can negotiate the establishment of a link, the exchange of data, and termination of a link.

49. Phases of PPP Connection…
A PPP connection goes through different phases which are shown in transition state.
Idle State
Establishing State
Authenticating State
Networking State
Terminating State

50. Phases of PPP Connection…
Idle State
The idle state means that the link is not being used. There is no active carrier, and the link is quiet.
Establishing State
When one of the endpoints starts the communication, the connection goes into establishing state.
In this state, options are negotiated between two parties. If the negotiation is successful, the system goes to the authenticating state (if authentication is required) or directly to the networking state.
The Link Control Protocol packets are used for this purpose. Several packets may exchanged during this phase.

51. Phases of PPP Connection…
Authenticating State
The authenticating state is optional.
The two endpoints may decide, during the establishing state, not to go through this state. However, if they decide to proceed with authentication, they send several authentication packets.
If the result is successful, the connection goes to the networking state; otherwise it goes to the terminating state.
Networking State
The networking state is 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 endpoints wants to terminate the connection.

52. Phases of PPP Connection…
Terminating State
When the connection is in the terminating state, several packets are exchanged between the two endpoints for closing the link.

53. HDLC vs. PPP High Level Data Link Control Protocol
It is both point to point and multipoint protocol
It delivers data on synchronous links only
It provides error detection but does not provide error correction
It does not have protocol type field, so it does not support multiple protocols
Point to Point Protocol
It point to point protocol
It delivers data on both synchronous and asynchronous links
It provides both error detection and error correction
It has a protocol type field so it supports multiple protocols

54. SLIP vs. PPP Serial Line Internet Protocol Point-to-Point Protocol
Older protocol
Require no addressing
No authentication
No error checking
Supports only IP
No compression is supported in original Version
Point-to-Point Protocol
New protocol
Supports dynamic IP address assignment
LCP
Error checking
NCP , Carries multiple protocols: IP, IPX, AppleTalk, NetBIOS
Inherently supports compression

55. PPP Automatic Login
PPP provides two methods with which logins can be automated –
PAP (Password Authentication Protocol)
CHAP (Challenge-Handshake Authentication Protocol).
Both provide the means for your system to automatically send your login userid/password information to the remote system.

56. PPP LCP Configuration Options
Feature
How It Operates
Protocol
Require a password
PAP
Authentication
Perform Challenge Handshake
CHAP
Compress data at source; reproduce data at destination
Stacker or
Predictor
Compression
Purpose: The figure presents an overview of the most popular PPP features.
Emphasize: The table in the figure lists and describes the various LCP options.
PPP compression is offered in Cisco’s Compression Control Protocol (CCP).
RFC 1548 covers the Internet Engineering Task Force (IETF) approved PPP options in detail. RFC 1717 defines Multilink Protocol. RFC 1990, The PPP Multilink Protocol (MP), obsoletes RFC 1717.
Note: To further enhance security, Cisco IOS Release 11.1 offers callback over PPP. With this LCP option, a Cisco router can act as a callback client or as a callback server.
The client makes the initial DDR call requests that it be called back, and terminates its initial call. The callback server answers the initial call and makes the return call to the client based on its configuration statements.
This option is described in RFC 1570.
Reference: Students will only learn how to configure PAP and CHAP authentication in this course. To learn how to configure the other LCP options, students should attend the Building Cisco Remote Access (BCRAN) course.
Error Detection
Monitor data dropped on link
Quality
Magic Number
Avoid frame looping
Load balancing across multiple links
Multilink Protocol (MP)
Multilink

57. Sample pap/chap configuration
hostname RouterA
hostname RouterB
username RouterB password mustmat
username RouterA password mustmat
ppp encapsulation
ppp encapsulation
ppp authentication pap
ppp authentication pap
PSTN/ISDN
RouterA
RouterB
“RouterA, mustmat”
Accept

58. Verifying PPP P1R1#show interfaces s1
Serial1 is up, line protocol is up
Hardware is HD64570
Internet address is /24
MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 1/255
Encapsulation PPP, loopback not set, keepalive set (10 sec)
LCP Open
Open: IPCP, CDP, ATALKCP, IPXCP
Last input 00:00:04, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0 (size/max/drops); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/64/0 (size/threshold/drops)
Conversations 0/4 (active/max active)
Reserved Conversations 0/0 (allocated/max allocated)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
51938 packets input, bytes, 0 no buffer
--More--

59. References Computer Networks CCNA ICND Exam Study Guide
4th Edition, Chapter 3, Andrew S. Tanenbaum
CCNA ICND Exam Study Guide
9th Chapter, Cisco Press 2004
Computer Networking; A Top Down Approach Featuring the Internet
3rd Edition: Chapter 5, Jim Kurose and Keith Ross
Data Communications and Networking
3rd Edition, Behrouz A. Forouzan