Point-to-Point Protocol

Semester 4, Chapter 4

PPP and Data Links

PPP operates at the Data Link layer. Components of PPP include: A method for encapsulating packets (datagrams) over serial

links The Link Control Protocol (LCP) to establish., maintain, test and

terminate the data-link connection The Network Control Protocol (NCP) to allow simultaneous

encapsulation of multiple network layer protocols across the same data-link which is refered to as protocol Multiplexing

At the physical layer, PPP can be used across synchronous (e.g., ISDN, leased lines) and asynchronous (e.g., modem dialup) data links.

PPP Layer Functions

Figure shows that PPP is similar to an HDLC frame.

The Protocol field contains the Layer 3 protocol ID.

Figure 1Figure 1

Figure 2Figure 2

PPP Layer Functions

PhysicalPhysicalSynchronous or asynchronous Synchronous or asynchronous

physical media (Router to physical media (Router to Router, or Host to Network)Router, or Host to Network)

Data LinkData Link

High-Level Data Link Control High-Level Data Link Control (HDLC)(HDLC)

Network Control Protocol Network Control Protocol (NCP)(NCP)

(specific to ea. Network layer (specific to ea. Network layer protocol ex. IPCP, IPXCP)protocol ex. IPCP, IPXCP)

Link Control Protocol (LCP) Link Control Protocol (LCP) (Authentication, Compression, MLP, (Authentication, Compression, MLP,

and Call Back)and Call Back)

PPP Session Establishment

Phases of PPP Establishment

PPP goes through four distinct phases to provide communications over point-to-point links1. Link establishment & configuration negotiation

2. Link quality determination

3. Network-layer protocol configuration negotiation

4. Link termination

1. Link Establishment & Configuration Negotiation Link establishment occurs at the data link layer with each PPP

device sending LCP packets. The Link Control Protocol packets contain a configuration field for

options such as... MTU compression link authentication MLP Call back

LCP must establish the link before any network layer protocols can be exchanged

This phase is completed when a configuration acknowledgement has been sent and received.

2. Link-Quality Determination PPP provides optional testing to determine

whether the link is good enough to bring up network layer protocols.

In addition, if authentication is required it occurs during this phase:PAP (Password Authentication Protocol)CHAP (Challenge Handshake Authentication Protocol)

Authentication occurs before the network layer configuration phase begins.

3. Network-Layer Negotiation

Once LCP finishes the link-quality phase, network layer protocols can be configured by the appropriate NCP

NCPs are sent for each protocol (e.g., IP, IPX, AppleTalk)

If LCP terminates the link, it informs NCP so it can take the appropriate action

To view the status of LCP and NCP, use the show interfaces command

4. Link Termination

LCP can terminate the link at any time. Reasons include:Requested by user (closing internet

connection)Loss of carrier at the physical layer

Enabling PPP

You enable PPP encapsulation on an interface by using the following command: Router(config-if)#encapsulation ppp

Thus, if you want dial-in hosts on terminal line 2 to use PPP, you would enter the following commands: RTA(config)#interface async 2RTA(config-if)#encapsulation ppp

Note that the encapsulation command is issued in interface configuration mode, not line configuration mode.

Dialup PPP vs. Dialup EXEC Sessions EXEC Sessions: No IP

addressing or PPP encapsulation is needed for this type of connection. Data is sent as asynchronous characters.

Dialup PPP: a remote host can dial in to an access server and send a Layer 3 protocol packet encapsulated by PPP. This type of connection allows the remote user to access network resources such as file servers and mail servers

You can also configure the router's asynchronous interface to automatically select between PPP data sessions and EXEC sessions.

Async Interface Commands Enabling this feature requires two steps. First, you must configure the

asynchronous interface(s) with the async mode interactive command in interface configuration mode. This command configures the router so that it allows the remote host to choose either a PPP session or an EXEC session. The following example shows how to configure interface async 1: RTA(config)#interface async 1

RTA(config-if)#encapsulation pppRTA(config-if)#async mode interactive

Second, you must configure the corresponding terminal line(s) with the autoselect ppp command in line configuration mode. To complete the example configuration, you would enter the following commands: RTA(config)#line 1

RTA(config-line)#autoselect ppp during-login The autoselect command permits the access server to allow an appropriate

process to start automatically when a starting character is received. If the start character is a return character, then the access server starts an EXEC session. On the other hand, if the access server recognizes the start character as PPP, SLIP, or ARAP, it will begin a session for whichever protocol it detects . So, if an end user is using a program that sends a PPP frame which has a flag character 7E in hexadecimal (or 01111110 in binary) format, the access server will automatically start a PPP session.

Dedicated Mode VS. Interactive Mode

Assigning An IP address to The Async Interface and To The Remote User RTA(config)#interface async 1

RTA(config-if)#ip address 10.1.1.1 255.255.255.0

LCP options1. Authentication

Authentication Authentication, with PAP or

CHAP, is used as a security measure with PPP. Authentication allows the dial-up target to identify that any given dialup client is a valid client with a preassigned username and password. 

If chosen, occurs during the link-quality determination phase.

Requires that the calling side of the link Provide authentication information.

The two authentication options supported by PPP are: PAP (Password Authentication

Protocol) CHAP (Challenge Handshake

Authentication Protocol)

PAP Is Not As Good As CHAP Password Authecntication

Protocol (PAP): Passwords are sent across the link in

clear text The remote node is in control of the

frequency and timing of the login attempts.

Challenge Handshake Authentication Protocol (CHAP): Encryption and Hashing The access server is in charge of the

frequency and timing of the login attempts

Authentication is done upon initial link establishment and may be repeated any time after the link has been established.

Configuring PAP Mutual Authentication On each router, define the user name and password to except from the remote router. Enable PPP and PAP on the interface.

Lab-A(config)#username Lab-B password class

Lab-A(config-if)#encap pppLab-A(config-if)#ppp authentication papLab-A(config-if)#ppp pap sent-username Lab-A password cisco---------Lab-B(config)#username Lab-A password cisco

Lab-B(config-if)#encap pppLab-B(config-if)#ppp authentication papLab-B(config-if)#ppp pap sent-username Lab-B password class

Configuring CHAP

Lab-A(config)#username Lab-B password cisco

Lab-A(config-if)#encap pppLab-A(config-if)#ppp authentication chap

---------Lab-B(config)#username Lab-A password cisco

Lab-B(config-if)#encap pppLab-B(config-if)#ppp authentication chap

Verifying Authentication To verify that you have PAP or CHAP configured

correctly, use the debug features of Cisco’s IOS.Close all telnet sessions first to return to the original

consoled router. In Privileged Exec. Mode, enter the command…

Lab-A#debug ppp authenticationGo to the ppp interface. Shut it down and then bring it

back up. You should see PAP or CHAP info come across the link as it comes back up and the routers authenticate each other.

LCP options2. Compression

Data Compression

PPP can also maximize performance by using data compression, which may provide higher data throughput across low-speed links

Data Compression Compression is an option that is negotiated by LCP. So, if the

party you are calling is not configured for compression, no compression will take place.

Typically, you should only configure compression on low-speed links because the router compresses data using software, which requires router CPU time and memory. Some algorithms are more memory-intensive; others are more CPU-intensive. In either case, the router's ability to route packets is impaired by the drain on its resources. 

If you frequently transfer already compressed data, such as graphics and video, you need to consider whether you want to set up compression. Trying to compress already compressed data can take longer than transferring the data without compression. Ideally, you can attain a 2:1 or 3:1 compression ratio for information that was not previously compressed. Expect an average of 1.6:1 compression for mixed compressed and uncompressed source data. The ratio for compressed data is 1:1.

Compression Types

Configuring Compression

Verifying Compression

ReviewPPP Main Components

PPP

EncapsulationSynchronous and asynchronousPPP provides error detection

Link Control Protocol

Establishes, configures, test, and terminate the data link connection.

Authenticates the identity of a the peer on the link.

Performs data compression Negotiates Options

Network Control Protocol

Opens, configures, and terminates network layer protocol.

Internet Protocol Control Protocol (IPCP) is a sub protocol of NCP.

IPCP configures IP at the network layer and can negotiate IP address such as using DHCP.

Multilink Protocol (MLP) links the LCP and NCP layers.

PPP IOS Commands

User Control

Configuring the router to provide a prompt for entering encapsulation information and an IP address,

Or, you can take the prompt away and assign everything by the router

Async Mode Dedicated

The exec Prompt does not appear The interface will use either SLIP or PPP

encapsulation as configured by the network engineer.

Example: Interface Async 4Async mode dedicated Encapsulation SLIP

Async Mode Interactive

Allows SLIP and PPP EXEC commands for the user.

Example: Interface Async 6Async mode interactive

Providing IP address

Peer default IP addressThe server router gives an IP address to the

client.Example:

Interface async 1 Peer default IP-address 172.16.42.26

Pooling Local A set of IP addresses is defined in a local

database Simplest mechanism for assigning IP addresses. Suitable when there is only one access server

providing access to the network. Example:

(config)# IP address-Pool Local (config)# IP local pool Sales 172.16.80.1 172.16.80.16 (config)#Int async 1 (config-if) Peer default IP-address pool Sales

ISDN example-Address pool

(config)# username bill password bailey (config)# IP local-pool isdnpool 192.1.170.2

192.1.170.9 (config)# IP address-pool local (config)# Int bri0

(config-if) encapsulation PPP (config-if) IP address 192.1.170.1 255.255.255.0 (config-if) per default IP address pool isdnpool (config-if) PPP authentication chap

Pool-DHCP

A pool of IP addresses is defined inside a centralized IP address server, called DHCP server.

This central database can serve addresses to several different access servers at the same time

You can enable DHCP address pooling on an access server by performing the following commands: Specify that the access server uses the DHCP client-proxy on all

asynchronous interfaces by using the command: (config)#IP address-pool dhcp-proxy-client.

Specify at least one and up to ten IP addresses of DHCP servers. (config)#IP DHCP-server 192,168.5.5

Configure the appropriate interfaces using the command: (config-if)# peer default ip-address dhcp

Example DHCP Pooling

(config)# IP address-pool dhcp-proxy-client

(config)# Interface group-async 1 (config-if) encapsulation PPP (config-if) Peer default IP-address dhcp

Group Asynchronous Interfaces

Gather asynchronous interfaces into a group interface and configure only the group interface to eliminate manual configuration duplication.

Example: (config)# Interface group-async 0 (config)# Group-range 2 7 (config)# Interface group-async 0 (config-if)# encapsulation PPP (config-if)# member 1 async default IP address 172.30.1.1

Link Control Protocol Options

Authentication Callback Compression Multilink

PPP Callback

Why?: Minimizing cost Centralized billing

Process that occur during a PPP callback connection:

1. Initiation of a call by a client. The client requests callback as one of the options during the LCP negotiation phase.

2. Callback request is acknowledged by the server, and the server checks its configurations to see if the call is allowed.

PPP Callback

Process that occur during a PPP callback connection:

3. User authentication occurs, and the client username is used in the dialer map command to identify the dial string to be used in the return call.

4. If the authentication is successful but there is no callback option, the call continues but the client pays for the call; otherwise, the call is disconnected by the server.

PPP Callback

Process that occur during a PPP callback connection:

5. Client is called by the server using the dial string.

6. Authentication occurs again.

7. The connection continues.

Example-async PPP Callback

Router1 (Callback server)(config)# username callman callback-

dialstring 5551234 password cisco(config)# Int async 7(config-if) PPP call back accept

PCMust support RFC 1570 for PPP callback.

Multilink PPP (MLP)

It allows you to combine channels into a multilink bundle so that data could be sent at a higher rates.

Example: Interface bri0 IP address 1.2.3.4 255.255.255.0 Encapsulation PPP PPP authentication chap PPP multilink Dialer map IP 1.0.0.5 name mlpPeer 5554444 Dialer load threshold 128 either

PPP compression Data compression:

Stacker: More CPU intensive, and less memory intensive

Predictor: Less CPU intensive and more memory intensive

(config-if) PPP compress predictor TCP header compression

(config-if) IP TCP header-compression passive

Verifying and troubleshooting

Debug PPP CHAP: If the remote host passed the authentication

the message “ remote passed CHAP authentication”. Or “failed CHAP authentication with remote”

Debug PPP negotiation:Check for address negotiation (IPCP)