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Module 3 – PPP

June 10th, 2009 Tom GT

Index

Serial Point-to-Point Links

TDM
Time division multiplexing interleaves data so protocols do not have to wait for another protocol to finish before they can transmit. It is a similar concept to Hyper-Threading in CPUs.
Demarcation Point
The demarcation point or demarc is where the responsibility of the link goes to the customer, rather than the link provider.

In the US, this is before the DCE device (CSU/DSU), where the local loop ends. In most other countries, the DCE is provided by the telco, in the form of an NTU. NTUs allow the telco to manage and troubleshoot the local loop.
DTE and DCE
The DTE is the customer device, which is commonly a router, though there are many other devices it could be, including a computer or fax machine.

The DCE is the device which changes the DTE’s data into a form suitable for the WAN link.

There are 4 main specifications defined in DTE/DCE standards:

Physical – number of pins and connector form factor
Electrical – voltage levels
Functional – assigns functions to different signals
Procedural – sequence of data transmission

When two DTE devices are connected together, such as in a lab, a null modem cable must be used. For synchronous links, one of the DTEs must have a clock rate set, to emulate a DCE.

A DTE will usually have a DB-60 or smart-serial connection on it, and the cable will have the connection specified by the telco on the other end.
HDLC
HDLC is the main signaling standard used by WAN links. Below is a list of some of the derivatives of HDLC in use:

Link Access Procedure, Balanced for X.25
Link Access Procedure on the D channel for ISDN
Link Access Procedure for Modems and PPP for modems
Link Access Procedure for Frame Relay

HDLC has three types of frames:

Information frames – for data
Supervisory frames – request/response for when piggybacking is not used
Unnumbered frames – control, such as connection setup

Enabling HDLC on an Interface
Router(config-if)# encapsulation hdlc
Troubleshooting a serial interface
By using show interface [type] [no.], details of the interface will be shown, so the incorrect parameter can be found.

PPP Authentication

PPP layered architecture

PPP is made from two sub-protocols:

Link Control Protocol – establishes a point-to-point link
Network Control Protocol – configures the network layer protocols

LCP is used for the following:

Authentication - authenticates the connection, using either PAP or CHAP
Compression – sets up compression on the link which is then decompressed when it reaches the other end of the link
Error detection – recognizing errors
Multilink – IOS 11.1 and later supports multilink, which provides load balancing
PPP Callback – increases security by requesting the other device calls back to make the connection

NCP has a different protocol to control each network layer protocol, e.g IPCP is used for IP connections.

PPP uses the following fields:

Flag – 01111110 is used to mark the beginning and end of a frame
Address – a broadcast address (11111111) is used, as PPP doesn’t use individual station addresses
Protocol – specifies the type of data being carried
Data - between 0 and 1500 bytes of data
FCS - checksum used to detect errors

Establishing a PPP connection

Link-establishment phase – LCP frames are used to configure the link. Details such as MTU are confirmed during this phase. The phase is completed with a configuration acknowledgement frame
Authentication phase (optional) – if configured, the link is authenticated before the network layer link is initialized
Network layer protocol phase – configures network layer protocols

PPP Authentication Protocols

There are two authentication protocols used by PPP, Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). CHAP is the preferred protocol, as it is more secure.

PAP

PAP sends the username and password repeatedly until the other node acknowledges it as correct or terminates the session. PAP is not very secure as the username and password are sent in plain text.

CHAP

CHAP uses a 3 way handshake, starting off with a challenge from the central site router. The remote router then uses the password and the challenge and sends back the checksum, if the checksum is correct, the connection is allowed, if it fails, the connection is dropped. There is a limit on the number of tries from the remote router, to reduce the risk even further.

To set up the hostname and password for CHAP:

Router(config-if)# ppp chap hostname [hostname]

Router(config-if)# ppp chap password [password]