Router Is Connected Via A Ppp Computer Science Essay

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After both a successful issuing of an IP address through Dynamic Host configuration protocol which is also known as DHCP and also a successful RADIUS authentication have been established. The next part is a process of communications in which will take place between both the routers which is to create that part of the connection. The creation of the link between both of the routers is done by the use of PPP also known as point to point protocol. Point to point protocol is a data link later or a layer 2 protocol in reference to the OSI model and is also widely recognized and used to create a link layer connection between two nodes.

The Physical Layer

Layer 1 or the physical layer is in control in terms of the transmission of data over network communications. It operates with data in the form of bits which are sent from the physical layer of the sending source and then received at the physical layer of the destination. (Basic Networking Concepts., 2006)

As specified within the brief, the routers are connected with a v.34 modem connection. The v.34 connection is at the physical layer or layer 1 of the OSI mode. The maximum connection speed that is possible between two v.34 modems is approximately 28,800 bits per second and it will be connected using a telephone connection (POTS)

Point to Point Protocol

Point to point protocol is widely used as it was developed to be used with lots of network layer protocols such as Internet protocol (IP), Internet packet exchange (IPX) and apple talk. A huge advantage to point to point is that it can be used over mainly all media types which are available. Link control protocol, which is also known as LCP is a necessary part of the point to point protocol. LCP is necessary because firstly after the point to point link has been established LCP then handles the configurations of the link.

LCP has key roles when working within link configurations which are as follow:

Setting up and negotiating the parameters

Link maintenance in terms of managing the open link

Link termination - The process of closing an existing link when it is no longer needed or if the underlying physical connection closes. (lcp link control protocol, 2005)

Within this project, it is outlined that challenge and handshake protocol is to be used for authentication. This being said, Link control protocol is also for the process for selecting optional authentication.

The Data Link Layer

The data link layer provides the building blocks for communication across a variety of physical media. It connects upper layer processes to the physical layer and also places data on and receives data from the network. The data link layer is sub divided into two layers which are the media access control layer and the logical link control layer. In order for data to be sent across the network, network layer packets have to be encapsulated into frames. The frames are given both header and trailer information in order to create a protocol data unit (PDU).

At the data link layer or layer 2 of the OSI model, point to point protocol will use a link control protocol as specified before. It will be used primarily to check the identity of the router which is linked and then it will either accept or decline the other router. LCP will then choose what size the packet should be for the upcoming transmissions. Finally, it will then search to ensure that there are no errors within the configuration.

Challenge Handshake Authentication Protocol (CHAP)

As specified before, the link will be using chap through point to point protocol. Firstly a basic link control protocol connection would be created by one of the routers which is more commonly known as the initiator. Eventually the other router which authenticates the link would then take presidence over that link. This type of authentication is known as the 'three-way handshake.

There are three steps in regards to the three way handshake. First is the challenge in which is when the chap authenticator will create a PDU named "challenge", it will then send it to the initiator. After this, the peer will then send back a response PDU which will contain the value of which has been calculated from the challenge and shared "secret". This process is done with a one way hash function. Finally the authenticator will then check the response and either send back a success or a failure PDU.

The image below clearly illustrates an example of this.

(, 2007)

The network layer

The basic function of the network layer is to provide an end to end communications capability to the transport layer which lies above it. The network layer is the lowest layer within the OSI model that deals with the end to end transmission.

At the network layer or layer 3 of the OSI mode PPP will provide a collection of network control protocols (NCPS's). Once the link has officially been established and after the authentication has been verified, PPP then sends network control packets with the aims to both discover and also configure the network layer protocols which are to be used. After configuration has then taken place then step is for the datagrams from the network layer protocols to be sent over the created link. Network control protocol also supports the negotiation of the IP control protocol also known as ICIP which starts the negotiation of IP addresses between both parties and as the setup provided is an IP based network, IPCP would be used.


When talking about point to point encapsulation, it uses a protocol named HDCL also known as high level data link control. The purpose for this protocol is for it to encapsulate datagrams which are in use from the payload of the point to point frames.

The following table provides the fields in which are included within the point to point header and trailer:


Flag - By default it is set to 0x7E (bit sequence 011111110) this indicates the start and also the end of the PPP frame. When successive Point to point frames are sent then only a single flag character is used.


Address - Within a high level data link control environment, the Address field is normally used to address the frame to its destination. However, on a point to point link, the destination will not need to be addressed. This means for point to point that the address field is set to 0xFF, which is a broadcast address. If both of the PPP peers agree to allow the address and control compression during link control protocol negotiation, the address field is not sent.


Control - In high level data link control environments, the control field is used for layer 2 acknowledgments and sequencing. PPP does not provide a reliable link to link data tranfer. This means for PPP frames, the control field will be set 0X03 by default which will indicate the unnumbered information of UI iframe. If both of the PPP peers agree to allow the control and control compression during Link control protocol negotiation, the address is not sent.


Protocol ID -The 2 byte protocol ID will identify what protocol the PPP payload will be using. If both of the PPP peers agree to perform a protocol field compression during the link control protocol negotiation, the protocol ID field is set to use one byte for protocol Ids and these will be ranged between 0X00-00 to 0X00-FF


The frame check sequence -This is a 16 bit checksum that will be used to check the data for errors at bit level in the PPP frame. If the peer receiving the frame calculation of the frame check sequence does not match the frame check sequence inside the PPP frame, then the PPP frame will be silently discarded.


Routing Information Protocol Version 2

The distance vector routing protocol which is specified to be used between the two routers is routing information protocol version 2 (RIPv2) and is a layer 3 or a network layer protocol in reference to the OSI model. This protocol will provide for both routers to create route communication between one another. Ripv2 uses a method called "Routing by rumour" of which is shares routing information between routers. In other terms it could be described as it sends its full complete routing table every thirty seconds to the other router, and all other routers which are using Ripv2 do the same thing, this being said the route is decided from this exchange. Ripv2 however does have its drawbacks which include that it does not support any network which have anything more than fifteen on the hop-count. This results in an infinite amount and is not supported.

Interesting Traffic

Interesting traffic is basically traffic in which the routers thinks is important, from this all other traffic is classed as "uninteresting". Whenever interesting traffic enters the router with the destination for a remote network, the router establishes a call to the remote network and sends the data. Interesting traffic is configured on the router with the dialer-list command. The dialer list command is then associated with a protocol and then either permitted, denied, or matched to an access list. (Syngress, 2003)

Routing Table

(Solomon, 2011) discusses routing tables well by saying that each entry into a routing table specifies only one particular route, path or link. All routing tables contain at least on which is the default route. It also contains the information of the routers default gateway which is typically the gateway address of the next hop address of the service provider.

When the router sends the datagram, it chooses which physical address to use. However, if the packet is supposed to be sent to a totally different address on another subnet, it would then use the routing table to send the datagram to a gateway. As the routing table has the IP address of the date, ARP will then use the IP to form the physical address of the gateway. The datagram will then be sent between the routers until it finally reaches the router in which is connected to the subnet that is needed.

Point to Point Phases

After all of the point to point phases have been completed, which include the physical layer connection being established, then the authentication being successful which would have a successful LCP configuration negotiation. It would then move on to the network layer protocol and for the link to be open it would have to have a successful NCP configuration negotiatiom.

The image below illustrates the stages and structure of the point to point phases