ppp protocols

8/9/2019 Ppp Protocols

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PPP and Data Links

PPP operates at the Data Link layer. Components ofPPP include:

A method for encapsulating packets datagrams! o"er serial links#he Link Control Protocol LCP! to esta$lish., maintain, test andterminate the data-link connection#he %et&ork Control Protocol %CP! to allo& simultaneousencapsulation of multiple net&ork layer protocols across thesame data-link &hich is refered to as protocol 'ultiple(ing

At the physical layer, PPP can $e used acrosssynchronous e.g., )SD%, leased lines! andasynchronous e.g., modem dialup! data links.


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PPP Layer *unctions


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*igure sho&s thatPPP is similar to an+DLC frame.#he Protocol fieldcontains the Layer protocol )D.

Figure 1Figure 1

Figure 2Figure 2


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PPP Layer *unctions

PhysicalPhysicalSynchronous or asynchronousSynchronous or asynchronous

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

Data Link Data Link

High-Le el Data Link !ontrolHigh-Le el Data Link !ontrol

(HDL!)(HDL!)

Network !ontrol ProtocolNetwork !ontrol Protocol(N!P)(N!P)

(speci"c to ea# Network layer(speci"c to ea# Network layerprotocol e$# %P!P, %P&!P)protocol e$# %P!P, %P&!P)

Link !ontrol Protocol (L!P)Link !ontrol Protocol (L!P)('uthentication, !ompression, LP,('uthentication, !ompression, LP,

and !all ack)and !all ack)


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PPP Sessionsta$lishment


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Phases of PPP sta$lishment

PPP goes through four distinct phases topro"ide communications o"er point-to-point

links. Link esta$lishment / configuration negotiation

0. Link 1uality determination. %et&ork-layer protocol configuration negotiation

4. Link termination


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. Link sta$lishment /

Configuration %egotiationLink esta$lishment occurs at the data link layer &ith each PPPde"ice sending LCP packets.#he Link Control Protocol packets contain a configuration field for

options such as...'#2compressionlink authentication'LPCall $ack

LCP must esta$lish the link $efore any net&ork layer protocols can$e e(changed#his phase is completed &hen a configuration ackno&ledgementhas $een sent and recei"ed.


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0. Link-3uality DeterminationPPP pro"ides optional testing to determine &hetherthe link is good enough to $ring up net&ork layerprotocols.

)n addition, if authentication is re1uired it occursduring this phase:PAP Pass&ord Authentication Protocol!C+AP Challenge +andshake Authentication Protocol!

Authentication occurs $efore the net&ork layerconfiguration phase $egins.


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. %et&ork-Layer %egotiation

nce LCP finishes the link-1uality phase, net&orklayer protocols can $e configured $y the appropriate%CP%CPs are sent for each protocol e.g., )P, )P5,

Apple#alk!)f LCP terminates the link, it informs %CP so it can

take the appropriate action#o "ie& the status of LCP and %CP, use the showinterfaces command


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4. Link #ermination

LCP can terminate the link at any time.6easons include:

6e1uested $y user closing internetconnection!Loss of carrier at the physical layer


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na$ling PPP

7ou ena$le PPP encapsulation on an interface $y usingthe follo&ing command:

Router(config-if)#encapsulation ppp

#hus, if you &ant dial-in hosts on terminal line 0 to usePPP, you &ould enter the follo&ing commands:

RTA(config)#interface async 2RTA(config-if)#encapsulation ppp

%ote that the encapsulation command is issued ininterface configuration mode, not line configurationmode.


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Dialup PPP "s. Dialup 5 C

Sessions5 C Sessions: %o )P

addressing or PPP encapsulationis needed for this type ofconnection. Data is sent asasynchronous characters.Dialup PPP: a remote host candial in to an access ser"er andsend a Layer protocol packetencapsulated $y PPP. #his type ofconnection allo&s the remote userto access net&ork resources suchas file ser"ers and mail ser"ers7ou can also configure the router8sasynchronous interface toautomatically select $et&een PPPdata sessions and 5 Csessions.


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Async )nterface Commandsna$ling this feature re1uires t&o steps. *irst, you must configure the

asynchronous interface s! &ith the async mode interactive commandin interface configuration mode. #his command configures the router so thatit allo&s the remote host to choose either a PPP session or an 5 Csession. #he follo&ing e(ample sho&s ho& to configure interface async :

RTA(config)#interface async 1RTA(config-if)#encapsulation pppRTA(config-if)#async mode interactive

Second, you must configure the corresponding terminal line s! &ith theautoselect ppp command in line configuration mode. #o complete thee(ample configuration, you &ould enter the follo&ing commands:

RTA(config)# line 1 RTA(config-line)# autoselect ppp during-login

#he autoselect command permits the access ser"er to allo& an appropriateprocess to start automatically &hen a starting character is recei"ed. )f thestart character is a return character, then the access ser"er starts an 5 Csession. n the other hand, if the access ser"er recogni9es the startcharacter as PPP, SL)P, or A6AP, it &ill $egin a session for &hiche"erprotocol it detects . So, if an end user is using a program that sends a PPPframe &hich has a flag character in he(adecimal or ; ; in $inary!format, the access ser"er &ill automatically start a PPP session.


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Dedicated 'ode


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Assigning An )P address to #he Async

)nterface and #o #he 6emote 2ser RTA(config)# interface async 1RTA(config-if)# ip address 10.1.1.1 255.255.255.0


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LCP options. Authentication


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AuthenticationAuthentication , &ith PAP orC+AP, is used as a securitymeasure &ith PPP. Authenticationallo&s the dial-up target to identifythat any gi"en dialup client is a"alid client &ith a preassignedusername and pass&ord.)f chosen, occurs during the link-1uality determination phase.6e1uires that the calling side ofthe link Pro"ide authenticationinformation.#he t&o authentication options

supported $y PPP are:PAP Pass&ord AuthenticationProtocol!C+AP Challenge +andshake

Authentication Protocol!


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PAP )s %ot As =ood As C+APPass&ord Authecntication Protocol

PAP!:Pass&ords are sent across the link inclear te(t#he remote node is in control of thefre1uency and timing of the loginattempts.

Challenge +andshake Authentication Protocol C+AP!:

ncryption and +ashing#he access ser"er is in charge of thefre1uency and timing of the loginattempts

Authentication is done upon initial linkesta$lishment and may $e repeated

any time after the link has $eenesta$lished.


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Configuring PAP'utual Authentication

n each router, define the user name and pass&ord to e(cept from the remote router.na$le PPP and PAP on the interface.

La -A(config)#username La -! password class

La -A(config-if)#encap pppLa -A(config-if)#ppp authentication pap

La -A(config-if)#ppp pap sent-username La -A password cisco---------La -!(config)#username La -A password cisco

La -!(config-if)#encap pppLa -!(config-if)#ppp authentication pap

La -!(config-if)#ppp pap sent-username La -! password class


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Configuring C+AP

La -A(config)#username La -! password cisco

La -A(config-if)#encap pppLa -A(config-if)#ppp authentication chap

---------La -!(config)#username La -A password cisco

La -!(config-if)#encap pppLa -!(config-if)#ppp authentication chap


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s ) S.

Close all telnet sessions first to return to the originalconsoled router.

)n Pri"ileged (ec. 'ode, enter the command?La -A#de ug ppp authentication=o to the ppp interface. Shut it do&n and then $ring it $ackup. 7ou should see PAP or C+AP info come across the linkas it comes $ack up and the routers authenticate each other.


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LCP options0. Compression


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Data Compression

PPP can also ma(imi9e performance $y usingdata compression, &hich may pro"ide higherdata throughput across lo&-speed links


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Data CompressionCompression is an option that is negotiated $y LCP. So, if theparty you are calling is not configured for compression, nocompression &ill take place.#ypically, you should only configure compression on low-speedlinks $ecause the router compresses data using soft&are, &hichre1uires router CP2 time and memory. Some algorithms are morememory-intensi"e@ others are more CP2-intensi"e. )n either case,the router8s a$ility to route packets is impaired $y the drain on itsresources.)f you fre1uently transfer already compressed data, such asgraphics and "ideo, you need to consider &hether you &ant to setup compression. #rying to compress already compressed data can

take longer than transferring the data &ithout compression. )deally,you can attain a 2:1 or 3:1 compression ratio for information that&as not pre"iously compressed. (pect an a"erage of 1.6:1 compression for mixed compressed and uncompressed sourcedata. #he ratio for compressed data is : .


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Compression #ypes


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Configuring Compression


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6e"ie&

PPP 'ainComponents


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PPP

ncapsulationSynchronous and asynchronous

PPP pro"ides error detection


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Link Control Protocol

sta$lishes, configures, test, andterminate the data link connection.

Authenticates the identity of a the peer onthe link.Performs data compression%egotiates ptions


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%et&ork Control Protocol

pens, configures, and terminates net&orklayer protocol.

)nternet Protocol Control Protocol )PCP! is asu$ protocol of %CP.)PCP configures )P at the net&ork layer andcan negotiate )P address such as using D+CP.

'ultilink Protocol 'LP! links the LCP and %CPlayers.


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PPP ) SCommands


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2ser Control

Configuring the router to pro"ide a promptfor entering encapsulation information and

an )P address,r, you can take the prompt a&ay and

assign e"erything $y the router


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Async 'ode Dedicated

#he e(ec Prompt does not appear #he interface &ill use either SL)P or PPP

encapsulation as configured $y the net&orkengineer.

(ample:)nterface Async 4

Async mode dedicatedncapsulation SL)P


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Async 'ode )nteracti"e

Allo&s SL)P and PPP 5 C commandsfor the user.

(ample:)nterface Async

Async mode interacti"e


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Pro"iding )P address

Peer default )P address#he ser"er router gi"es an )P address to the

client.(ample:

)nterface async Peer default )P-address 0. .40.0


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Pooling Local A set of )P addresses is defined in a local data$aseSimplest mechanism for assigning )P addresses.Suita$le &hen there is only one access ser"er

pro"iding access to the net&ork.(ample:

config!B )P address-Pool Localconfig!B )P local pool Sales 0. . ;. 0. . ;.config!B)nt async config-if! Peer default )P-address pool Sales


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)SD% e(ample-Address pool

config!B username $ill pass&ord $aileyconfig!B )P local-pool isdnpool 0. . ;.0

0. . ;.config!B )P address-pool localconfig!B )nt $ri;

config-if! encapsulation PPP

config-if! )P address 0. . ;. 0EE.0EE.0EE.;config-if! per default )P address pool isdnpoolconfig-if! PPP authentication chap


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Pool-D+CP

A pool of )P addresses is defined inside a centrali9ed )Paddress ser"er, called D+CP ser"er.#his central data$ase can ser"e addresses to se"eral

different access ser"ers at the same time7ou can ena$le D+CP address pooling on an accessser"er $y performing the follo&ing commands:

Specify that the access ser"er uses the D+CP client-pro(y on allasynchronous interfaces $y using the command:

config!B)P address-pool dhcp-pro(y-client.

Specify at least one and up to ten )P addresses of D+CP ser"ers.config!B)P D+CP-ser"er 0, .E.E

Configure the appropriate interfaces using the command:config-if!B peer default ip-address dhcp


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(ample D+CP Pooling

config!B )P address-pool dhcp-pro(y-client

config!B )nterface group-async config-if! encapsulation PPP

config-if! Peer default )P-address dhcp


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=roup Asynchronous )nterfaces

=ather asynchronous interfaces into a groupinterface and configure only the group interface toeliminate manual configuration duplication.

(ample:config!B )nterface group-async ;config!B =roup-range 0

config!B )nterface group-async ;config-if!B encapsulation PPPconfig-if!B mem$er async default )P address 0. ;. .


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Link Control Protocol ptions

AuthenticationCall$ackCompression'ultilink


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PPP Call$ack

FhyG:'inimi9ing costCentrali9ed $illing

Process that occur during a PPP call$ackconnection:. )nitiation of a call $y a client. #he client re1uests call$ack

as one of the options during the LCP negotiation phase.

0. Call$ack re1uest is ackno&ledged $y the ser"er, and theser"er checks its configurations to see if the call isallo&ed.


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PPP Call$ack

Process that occur during a PPP call$ackconnection:

. 2ser authentication occurs, and the clientusername is used in the dialer map command toidentify the dial string to $e used in the return call.

4. )f the authentication is successful $ut there is nocall$ack option, the call continues $ut the clientpays for the call@ other&ise, the call isdisconnected $y the ser"er.


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PPP Call$ack

Process that occur during a PPPcall$ack connection:

E. Client is called $y the ser"er using thedial string.

. Authentication occurs again.

. #he connection continues.


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(ample-async PPP Call$ack

6outer Call$ack ser"er!config!B username callman call$ack-

dialstring EEE 0 4 pass&ord ciscoconfig!B )nt async config-if! PPP call $ack accept

PC'ust support 6*C E ; for PPP call$ack.


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'ultilink PPP 'LP!

)t allo&s you to com$ine channels into a multilink$undle so that data could $e sent at a higher rates.

(ample:)nterface $ri;)P address .0. .4 0EE.0EE.0EE.;

ncapsulation PPPPPP authentication chap

PPP multilinkDialer map )P .;.;.E name mlpPeer EEE4444Dialer load threshold 0 either


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PPP compressionData compression:

Stacker: 'ore CP2 intensi"e, and lessmemory intensi"e

Predictor: Less CP2 intensi"e and morememory intensi"e

config-if! PPP compress predictor

#CP header compressionconfig-if! )P #CP header-compression passi"e


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ppp protocols

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