As we discussed earlier, Frame Relay is working on the Datalink Layer of the OSI Model. So, it takes data packets from the upper layer of the OSI model, encapsulates the data into the Frame Relay frame, and then passes the frame to the physical layer of the OSI Model for delivery on the network. Frame Relay encapsulation is essential to understand because it is helpful to know how Frame Relay works. It encapsulates data for sending over the network, as shown in Figure 1.
The Frame Relay encapsulation process accepts a packet from a network layer protocol, such as IPv4 or IPv6. The process makes changes to an address field that contains the DLCI and a checksum.
Flag fields are added to indicate the beginning and end of the frame. The start and end of the frame are marked with the same values. The start and end of the frame are either represented as the hexadecimal number 7E or as the binary number 01111110. Frame Relay encapsulates the packet and passes the frame to the physical layer for transport.
The CPE encapsulates each Layer 3 packet inside a Frame Relay header and trailer before sending it across the Virtual Circuit (VC). LAPF defines the header and trailer. The Frame Relay header address field specifically contains the following fields, as shown in the figure:
DLCI—DLCI is the most important field in the Frame Relay header. DLCI represents the virtual connection between the DTE device and the switch. All virtual connections (VCs) multiplexed on to the physical channel are represented by a unique DLCI. I earlier said that DLCI values are local significance, so they are unique only to the physical channel on which they exist. Thus, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.
C/R– C/R bit follows the most significant DLCI byte in the Address field.
EA—EA stands for Extended Address. If the value of this field is 1, the current byte is determined to be the last DLCI octet. Even if current Frame Relay implementations all use a two-octet DLCI, this capability does allow longer DLCIs in the future. The last bit of each byte of the Address field indicates the EA.
Congestion Control—Congestion Control has three bits: the Forward Explicit Congestion Notification (FECN), Backward Explicit Congestion Notification (BECN), and Discard Eligible (DE) bits.
The frame relay frame is forwarded to physical layer of the OSI model for transmission. Physical Layer is typically EIA/TIA-232, 449, V.35, or X.21. The Frame Relay frame is a member of the HDLC frame type; therefore, it is enclosed with flag fields.
The 8-bit flag uses the bit pattern 01111110. The FCS determines whether any errors in the Layer 2 address field occurred during transmission. The sending node checks and calculates FCS before sending it. At the far-away end, a second FCS value is calculated and compared to the FCS in the frame. If both values are matched, the frame is processed. If there is dissimilarity, the frame is discarded. Frame Relay never notifies the source when if a frame is discarded. Error control is left to the upper layers of the OSI model.
Frame Relay is a multiplexing technique that transmits only one frame at a time. Multiple virtual circuits can be used on a single physical line. The Frame Relay Access Device (FRAD), or router connected to the Frame Relay network, may have multiple virtual circuits linking it to different endpoints.
Multiple VCs on a single physical line is distinguished because each Virtual Circuit has its own DLCI. I have mentioned in the previous article that the DLCI has local significance and may be different at each end of a VC.
Multiple Virtual Circuits reduces the number of equipment and network also reduces the network complexity. Using Multiple Virtual Circuits makes the network very cost-effective. With this configuration, each endpoint required a single access line and a single interface.
The figure-1 below illustrates the example of Multiple Virtual Circuits of the central office with its remote locations. Central Office is connected to the network using two VCs and each VC is assigned a DLCI.
As I mentioned earlier that Frame Relay decreases the network cost because customers only pay for the bandwidth they use and for the port they are using. When the customer needs more ports, they pay for more bandwidth, but they do not pay for equipment because all the ports in Frame Relay are virtual.
We have already discussed Frame Relay in the previous lessons. What is Frame Relay? Exclusive Introduction that it provides connection-oriented data link layer communication using a Frame Relay virtual circuit. Frame Relay Virtual Circuits (VCs) are a logical connection between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN). The circuits are virtual because there is no direct electrical connection from one side to another side.
The connection is logical, and data moves from end to end without a direct electrical circuit. With Virtual Circuits (VCs), Frame Relay shares the bandwidth among multiple users, and any single site can communicate with any other single site without using multiple dedicated physical lines.
Bidirectional communication is possible on a single physical line due to the data-link connection identifier (DLCI). Using DLCI, several virtual circuits can be multiplexed into a single physical circuit for transmission across the network. There are two types of VCs:
Switched Virtual Circuits (SVC): A switched virtual circuit (SVC) provides a temporary connection between two different network nodes until a data transfer session is completed, after which the connection is terminated. Svc has different states e.g. CALL SETUP, DATA TRANSFER, IDLE, and CALL TERMINATION.
Permanent Virtual Circuits (PVCs): A permanent virtual circuit (PVC) provides the user with a dedicated virtual circuit. Still, this permanent virtual circuit ( PVC) is part of a shared pool of circuit resources that support multiple users as they require the connections. PVCs are more commonly implemented than SVCs.
Frame Relay creates a Virtual Circuit by storing input-port to output-port mapping in the memory of each switch and thus links one switch to another until a continuous path from one end of the circuit to the other is identified. A Virtual Circuit can pass through any intermediate devices (switches) within the Frame Relay network.
Virtual Circuits (VCs) provide a bidirectional communication path from one device to another. VCs are known by DLCIs, as shown in Figure 1. Due to its local implication, the DLCI value is not unique in the Frame relay network. A DLCI identifies a VC to the equipment at an endpoint. So, a DLCI has no significance further than the single link. Devices connected by a VC may use a different DLCI value to refer to the same connection.
As we know, DLCIs are locally significant, so the same address can be used in several locations while referring to different connections. Local addressing also prevents customers from running out of DLCIs as the network grows.
In Frame Relay Network, the frames moves across the network, Frame Relay labels each VC with a DLCI number. The DLCI number is stored in the address field of every frame transmitted to tell the network how the frame should be routed. Usually, DLCIs 0 to 15 and 1,008 to 1,023 are reserved. So, the Frame Relay service providers typically assign DLCIs from 16 to 1,007.
Challenge Handshake Authentication Protocol (CHAP) periodically checks the character of the far off hub utilizing a three-way handshake. The hostname on one switch must match the username the other switch has designed. The passwords should likewise coordinate. The password value is variable and changes unpredictably while the link exists.
When the PPP link establishment phase is complete, the local router sends a challenge message to the remote node containing Challenge Handshake Authentication Protocol (CHAP) user name and a hash value that is based on the Challenge Handshake Authentication Protocol (CHAP)Password.
The remote router compares the local routers username and password in its local database and calculates hash value with the value sent from local router. The remote node then responds with a calculated value using a one way hash function, usually with Message Digest 5 (MD5) based on the password and challenge message. The figure1 illustrates the CHAP 3 way handshake.
The local router checks the reaction against its own calculation of the likely hash value. In case of value match, the initiating node acknowledges the authentication; otherwise, the initiating node immediately terminates the connection.
Challenge Handshake Authentication Protocol (CHAP) provides better protection then PAP because it protects devices from playback attacks using a variable challenge value that is unique and unpredictable. The challenge and resulted hash value are unique and random. The use of repetitive challenges limits the time of vulnerability to any single attack. The local router or a third-party authentication server is in control of the frequency and timing of the challenges.
We have learnt in the previous section that CHAP periodically identifies the remote node using a three-way handshake. The hostname on one router should match the username the other router has configured. The passwords also required to match. This occurs on initial link establishment and can be repeated any time after the link has been established. The commands for configuring CHAP on R1 are the following:
Router R-1 (Local)
Router>enable
Router#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname R1
R1(config)#username R2 password Cisco
R1(config)# interface serial0/0/0
R1(config-if)#no shut
R1(config-if)# ip address 192.168.10.1 255.255.255.252
PPP support different option that can be configured optionally. One of these options is authentication. The authentication option provides a method to negotiate the use of a particular protocol for Authentication. By default, authentication is not required. Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) can be used for PPP links authentication which is defined in RFC 1334
Password Authentication Protocol is a basic two-way process used to authenticate a user’s password. The username and password are sent in plain text without any encryption. If it is accepted, the connection is allowed. As an authentication protocol, PAP is the least secure because the password is sent in plain text.
The figure below illustrates the process of PAP. R1 sends its PAP user name and password to R2. R2 evaluate R1’s user name and password against its local database. If it matches, it accepts the connection and accepts the message. If not, it rejects the connection and sends a reject message.
The command “ppp authentication pap” is used for PPP authentication. When the command is entered, the username and password are sent as one LCP data packet. After PPP completes the link establishment phase, the remote node constantly sends a username with password across the link until the receiving node acknowledges the connection or terminates the connection.
The receiving node checked the username-password using an authentication server that either allows or denies the connection. If the connection is accepted, it sends the connection acknowledgement message otherwise it returned the reject message. Password Authentication Protocol authenticates only during establishing a connection, it never re-authenticate after connection establishment. This is a network vulnerability to the attacker; they can attack using this area.
Password Authentication Protocol can be used where a large installed base of client applications that do not support CHAP. Password Authentication Protocol can also be important between different vendor equipment, where implementations of CHAP are not compatible. It is also used where a plaintext password is necessary to simulate a login at the remote host
Configuring PAP Authentication
Now we know the basic information about PAP. We know that PAP is a two-way authentication protocol. Both routers authenticate and are authenticated. The PAP username and password that each router sends must match those configured with the username name password password command of the other router.
PAP only did authentication during initial link establishment. The hostname on one router must match the username the other router has configured for PPP. The passwords must also match. Specify the username and password parameters, The PAP configuration for the above topology is the following:-
Router R-1 (Remote Site)
Router>enable
Router#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname R1
R1(config)#username R2 password ccna12345
R1(config)# interface serial0/0/0
R1(config-if)#no shut
R1(config-if)# ip address 192.168.10.1 255.255.255.252
Remember that HDLC is the default encapsulation protocol for Cisco routers. If PPP configuration is not done on Cisco routers, the default encapsulation will be used. To set PPP as the encapsulation method used by a serial interface we need PPP configuration on the serial interfaces. The PPP configuration for the below topology is following.
Router1
Router1#configure terminal
Router1(config)# interface serial 0/0/0
Router1(config-if)# encapsulation ppp
Router2
Router2#configure terminal
Router2(config)# interface serial 0/0/0
Router2(config-if)# encapsulation ppp
The encapsulation ppp interface command has no arguments. The figure billow illustrates that Router1 and Router2 have been configured with both an IPv4 and an IPv6 address on the serial interfaces. PPP is a Layer 2 encapsulation that supports different Layer 3 protocols.
Compression Configuration
The command syntax for compression configuration is “compress”. Compression can be configured after enabling PPP encapsulation on the serial interface of a Cisco router. This option invokes a software compression process; therefore it can affect system performance. If the traffic already contains compressed files, such as .zip, .tar, etc, never use this option. The command for configuration is as under:
Two keywords are available for “compress” command. With predictor keyword, the predictor algorithms are used. While stac is used for LSZ algorithms.
Link Quality PPP Configuration (LQM)
Quality is also the option of the LCP phase. LCP tests the link to verify whether the link quality is sufficient to use Layer 3 protocols or not. After configuring this option, the percentages are calculated for both incoming and outgoing directions. If the link quality percentage is not maintained, the link is supposed to be of poor quality and is taken down. The “ppp quality percentage” command is used to ensure that the link meets the quality requirement set; otherwise, the link closes down. The percentage range is from 1 to 100. The LQM configuration for the above network is as under:
PPP support different option than can be configured optionally. Authentication using PAP or CHAP, Compression using either Stacker or Predictor and Multilink that combines two or more channels to increase the WAN bandwidth. To negotiate these PPP options, the LCP link-establishment frames contain option information in the data field of the LCP frame, as shown in Figure 1. If LCP frame has no configuration option, then the default value for that configuration option is assumed. This phase is complete when a configuration acknowledgement frame has been exchanged. The configurable LCP options for specific WAN connection may include the following LCP options:
Authentication
This 16 bits long field with a code value of 0x03. This Configuration Option provides a method to negotiate the use of a particular protocol for Authentication. By default, authentication is not required. Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) can be used for authentication.
Compression
Compression option is used to Increases the effective throughput on PPP links. Increasing throughput is possible due to reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at the receiving side. Stacker and predictor protocols are used for this purpose.
Magic Number (Error Detection)
The Magic Number field is used for detecting links that are in a looped-back condition. It is responsible to ensure a reliable, and loop-free data link. Until the Magic-Number Configuration Option has been successfully negotiated, the Magic-Number must be transmitted as zero. Magic numbers are randomly generated number used to identify one end of a point-to-point connection. Each side of the link negotiates its magic number. If both sides discover that the magic numbers they are negotiating are the same, both side attempts to change its magic number. If the magic number remains the same after changing, the session terminates because of the loopback that is found.
PPP Callback
PPP callback option is used to improve security the in PPP links. Cisco router can act as a callback client or a callback server in the LCP option. The call back occurs at the beginning of a new PPP session. LCP includes the callback option and its settings. The client makes the opening call, requests for server call back, and terminates its call. The callback router answers the call and makes the return call to the client based on its configuration statements. The command is PPP callback [accept | request]. RFC-1570 further describes PPP callback.
Multilink
The multilink option is used for load balancing over the router interfaces that PPP uses. It is also referred to as MP, MPPP, and MLP. It provides a method for spreading traffic across multiple physical WAN links. Multilink indicates to its peers that it is capable of combining multiple physical links into a single bundle. A system indicates to its peer that it is prepared to do multilink by sending the multilink option as part of the initial LCP option negotiation.
When a PPP link has been initiated, the LCP passes control to the appropriate Network Control Protocol (NCP). NCP is a set of protocols that form part of PPP. PPP is a data link layer protocol that can transmit multiprotocol data between two point-to-point.
PPP’s modular model allows LCP to set up the link and then transfer the details of a network protocol to a particular NCP. Each network protocol has a corresponding NCP and corresponding RFC. For example, IPv4, IP6, IPX, and many others have corresponding NCPs. These NCPs use the same packet as the LCPs.
After the LCP configuration and authentication in the basic link, the inappropriate Network Control Protocol is called on to complete the particular configuration of the network layer protocol. After the network layer protocol’s successful Network Control Protocol configuration, the network protocol is in the open state on the established LCP link. From there, PPP can carry the corresponding network-layer protocol packets.
Internet Protocol Control Protocol (IPCP)
When LCP has established the link, the routers exchange IPCP messages for option negotiation. IPCP is responsible for configuring, enabling, and disabling the IPv4 modules on both ends of the PPP link. The same responsibilities are allocated to IPV6CP for IPv6 addresses. IPCP negotiates two options:
Compression—The compression protocol allows devices to negotiate an algorithm to compress TCP and IP headers and save bandwidth. For compression, the Van Jacobson TCP/IP header compression is used. The Van Jacobson TCP/IP header compression compresses the size of the TCP/IP headers to 3 bytes. Which is the important improvement on slow serial lines, particularly for interactive traffic?This compression method is similar in concept to the Protocol-Field-Compression and Address-and-Control-Field-Compression (ACFC) options in LCP.
IP-Address-IP address allows the initiating device to send the Configure-Request. To specify an IP address, it needs to be used for routing IP over the PPP link or to request that the other device supply it with one. This is most commonly used for dial-up networking links before the advent of broadband technologies such as DSL and cable modem services.
After the Network Control Protocol process is completed, the link goes into the open state, and LCP gets the link in the maintenance phase. Link traffic contains possible LCP, NCP, and network-layer protocol packet combinations. After the data transfer is completed, the Network Control Protocol terminates the protocol link, and the LCP terminates the PPP connection. The different NCPs are;
When two devices are first turned on, there is no physical layer connection between them. Once the connection is made, the link can continue to establish a PPP session. There are three phases of establishing a PPP session:
Phase 1: Link establishment and configuration negotiation
Before PPP exchanges any network layer datagrams, such as IP, the LCP must first perform the basic setup of the link. The sending device sends an LCP configuration request to a receiving device. Suppose sending device is A and receiving device is B. The B receive the configuration request message over the physical link, containing the parameters wishes to use.
If device B agrees with the parameters, it replies with an acknowledgement (Ack) otherwise sends negative acknowledgement (NACK) message. In the case of NACK, sending device tries different parameters with a new request. This phase is complete when the negotiation between both is successful. After the successful completion of Phase 1, the status goes to LCP open and proceed to the authentication phase.
Phase 2: Link quality determination
Link Quality determination is the second phase of the PPP session establishment. It as an optional phase in the PPP session. The LCP check the link quality to decide that the link is sufficient to bring up network-layer protocols. The LCP can delay data sending of network layer protocol information until this phase is complete.
After the link quality determination, the specific configuration of the appropriate network layer protocol is performed by invoking the suitable NCP, such as IPCP, IPXCP etc. If the LCP terminate the link, it informs the network-layer protocols so that they can take appropriate action. More than one NCP can be run on a particular PPP link, and each NCP can be terminate independently when it is no longer needed. The PPP link remains configured for communications until explicit LCP or NCP frames terminate the link, or until some outside event occurs.
LCP Operation
Link Control Protocol (LCP) is the main part of the PPP session. It is responsible for controlling the links in Point-to-Point Protocol (PPP) suite. It is implemented at Data Link Layer in the OSI model. LCP is used to automatically agree upon the encapsulation format, sizes of packets, looped-back link, and common misconfiguration errors. It is also responsible for connection termination. LCP operation uses three classes of LCP frames to accomplish the work of each of the LCP phases:
Link-establishment frames
Link-maintenance frames
Link-termination frames
Link Establishment
Link establishment is the first phase of LCP operation that must complete successfully, before exchanging any network layer packets. During link establishment, the LCP opens the connection and negotiates the configuration parameters. The initiating device starts link establishment by sending a Configure-Request frame to the responder. The frame contains a variable number of configuration options needed to set up on the link. The initiator includes the options for how it wants the link created, including protocol or authentication parameters.
The responder processes the request. If the options are not suitable or not recognized the responder sends a Configure-NAK or Configure-Reject In case if the negotiation fails, the initiator must restart the process with new options. If the options are suitable and acceptable, the responder replies with a Configure-Ack message and the process moves on to the next stage. Next stage is NCP operation. When NCP has completed configurations, authentication if configured, the line is available for data transfer. During the data exchange, LCP transitions into the link maintenance phase. Figure 1 illustrates the Link Establishment phase of LCP.
Link Maintenance
When the link is in the maintenance phase, LCP can test the link using test and feedback messages as shown in Figure 2.
Test Messages Frame. Echo-Request, Echo-Reply, and Discard-Request can be used for testing the link.
Feedback Messages Frame. Code-Reject and Protocol-Rejectframe types provide link feedback when one device receives an invalid frame due to unrecognized LCP code or a bad protocol identifier. Code-Reject packet is sent in response. The sending device will resend the packet.
Link Termination
The link remains open until the LCP terminates the link. If the LCP terminates the link before NCP, the NCP session is also terminated. NCP only terminates the network layer and NCP links. PPP can terminate the link at any time. PPP link termination can occur because of the loss of the carrier including authentication failure, link quality failure, the expiration of an idle-period timer, or the administrative closing of the link. For closing the link, LCP exchanged the Terminate packets.
The device that starts the shutdown process sends a Terminate-Request message. The other device replies with a Terminate-Ack. When the link is closing, PPP informs the network-layer protocols so that they may take suitable action.
LCP Packet
Figure 1 shows the fields in an LCP packet:
Code – This field is 8 bits long and identifies the type of LCP packet.
Identifier – The identifier is also 8 bits (one octet) long field in length and is used to match packet requests and replies.
Length – The length field is 16 bits long field in length and indicates the total length (including all fields) of the LCP packet.
Data – The data field is 0 or more bits in size as indicated by the length field. The format of this field is determined by the code field. Data field contains the information associated with the frame, which may be configuration options, frame information, or data.
Each LCP packet is a complete LCP message containing an LCP code, an identifier field, a length field indicating the size of the LCP packet and LCP packet type-specific data. Each LCP packet has a specific function in the exchange of configuration information depending on its packet type. The code field of the LCP packet defines the type of the packet. Different LCP packet types are:
Configure-Request – When the LCP Code field value is 0x01, it shows that the packet is the configure-request frame. This frame is used for opening or resetting a PPP connection. It contains a list of LCP option with changes to default values.
Configure-Ack – When the code field value is 0x02, it means that the frame is Configure-Ack field. This frame is sent when all of the value of the last configure-request received is recognized and acceptable. When both PPP peers send and receive Configure-Acks, the LCP negotiation is complete.
Configure-Nak – If the code field value is 0x03, it’s mean that the frame is Configure-NAK frame. This frame is sent When some LCP option is not acceptable and some option is acceptable. The frame includes the mismatching option and their acceptable values.
Configure-Reject – If the code field value is 0x04, the frame type is a Configure-Reject frame. This type of frame is sent when any LCP option is not recognized or accepted. This frame also includes the unrecognized or non-negotiable option.
Terminate-Request – if the code field value is 0x05, the frame is Terminate-Request frame. This frame is sent optionally for closing the PPP session.
Terminate-Ack – if the code field value is 0x06, the frame type is Terminate-Ack. This frame is sent in response to the Terminate-Request frame in PPP session.
Code-Reject – If the code field value is 0x07, the frame type is a Code-Reject frame. This frame is sent when the LCP code is unknown. The Code-Reject message also includes the rejected LCP packet.
Protocol-Reject – When a code field value is set to 0x08, the frame type is Protocol-Reject frame. It is sent when the PPP frame includes an unknown protocol ID. The Protocol-Reject message also includes the rejected LCP packet.
Echo-Request – If the code field value is 0x09, the frame type is Echo-Request. Echo-Request frame is sent optionally to test the PPP connection.
Echo-Reply – When a code field value is 0x0a, the frame type is Echo-Reply. This frame is sent in response to the Echo-Request frame. PPP Echo-Request and Echo-Reply frame are not related to the ICMP Echo-Request and Echo-Reply messages.
A layered architecture is a logical model that aids communication between interconnecting layers. We can map the Point-to-Point Protocol (PPP) layer model with the Open System Interconnection (OSI) model. Both Point-to-Point Protocol (PPP) and OSI models share the same physical layer although the Point-to-Point Protocol (PPP) distributes the functions of Link Control Protocol (LCP) and Network Control Protocol (NCP) differently.
We can configure various interfaces at the physical layer, including Asynchronous serial, Synchronous serial, HSSI, and ISDN. Point-to-Point Protocol (PPP) operates across any DTE/DCE interface including RS-232, RS-422, RS423 or V.35. The figure below illustrates the layered model of the Point-to-Point Protocol (PPP). LCP and NCP primarily work in the data-link layer and network layer. The LCP sets up the Point-to-Point Protocol (PPP) connection, including connection parameters. It is also responsible for connection termination. The NCPs handle higher-layer protocol configurations.
Visual representation of PPP components, including encapsulation, link control, and network control protocols.
Link Control Protocol (LCP)
The Link Control Protocol (LCP) establishes, configures, and tests data-link connections. PPP sends and receive LCP packet before establishing communications over a point-to-point link. The LCP packet may be accepted or rejected based on a linked peer, packet size and other configurations. The LCP packet checks the identity of its linked peer, sets the packet size limits, and also check sfor common configuration errors. Once the LCP packet accepts the link, traffic can be forwarded on the network. If the LCP packet found the link is not working correctly, it terminates the link. The LCP packet has three different types.
Link configuration packets
Link termination packets
Link maintenance packets
The LCP provides automatic configuration of the interfaces at each end. The LCP also negotiates and sets up control options on the WAN data link, which are handled by the NCPs. When the link is established, PPP also uses the LCP to agree automatically on encapsulation formats such as authentication, compression, and error detection.
Network Control Protocol (NCP)
Network Control Protocol (NCP) was an early protocol implemented by ARPANET. The ARPANET is the world’s first operational packet-switching network that later evolved into what became the Internet. Network Control Protocol (NCP) is the part of Point − to − Point Protocol (PPP). It is a data link layer protocol used for transmitting multi-protocol data between two point-to-point devices. PPP uses a separate NCP for each network layer protocol. For example, IPv6 uses the IPv6 Control Protocol (IPv6CP) and IPv4 uses IPv4 Control Protocol (IPCP). Each NCP manages the particular needs required by its relevant network-layer protocols. Different components of NCP encapsulate and negotiate options for multiple network-layer protocols.
NCP allowed access and use to remote devices at remote locations and to transmit files between devices and computers. NCP is part of the protocol stack and enables application services such as email and file transfer. TCP/IP was replaced by NCP in the 1980s.
Point-to-Point Protocol (PPP) Frame
PPP is a byte-oriented protocol. A PPP frame contains six fields. Each PPP frame is composed of one or more bytes. The fields of a PPP frame are illustrated in the figure below:-
Flag – Flag field marks the beginning and the end of the Point-to-Point Protocol (PPP) frame. It flags the standard HDLC field containing 8 bits. The bits pattern of the flag field is 01111110.
Address – It is a bit field that contains the binary sequence. In case of broadcast, the sequence is set to 11111111, PPP does not assign individual station addresses.
Control – This is a one-byte long field with the binary sequence 00000011. The value 00000011 shows that the frame does not contain any sequence numbers. It provides a connectionless link service for a point-to-point link. So, for PPP, the Address field is set to 0xFF, the broadcast address. If both Point-to-Point Protocol (PPP) peers agree to do address and control field compression during the LCP negotiation, the Address field is not included.
Protocol – The protocol field is either one bit or two bits long and identifies the protocol encapsulated in the information field of the frame. If the field is 2-byte, the field specifies the protocol of the PPP payload. If both PPP peers agree for the compression of the protocol field during LCP negotiation, the Protocol field is reduced to one byte for the protocol identification. The up-to-date values of the Protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
Data/Payload – Data field is variable in length. It specifies the kind of packet in the data field, i.e., what is being carried in it. The data field can contain user data or other information. If the length is not negotiated using LCP during line setup, a default length of 1500 bytes is used.
Frame Check Sequence (FCS) – FSC stands for Frame Check Sequence. It is a 2-byte or 4-byte frame check sequence for error detection. The 2-byte frame is standard; however, LCPs can negotiate modifications to the standard PPP frame structure.
FAQs
What is Point-to-Point Protocol (PPP)?
PPP is a data link layer protocol used to establish a direct connection between two networking nodes.
What are the main components of PPP?
The main components of PPP include encapsulation, link control protocol (LCP), and network control protocols (NCPs).
How does PPP ensure secure communication?
PPP uses authentication protocols like Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) to ensure secure communication.
What is the role of the Link Control Protocol (LCP) in PPP?
LCP is responsible for establishing, configuring, testing, maintaining, and terminating links for transmission.
Can PPP support multiple network layer protocols?
Yes, PPP can support multiple network layer protocols by using different Network Control Protocols (NCPs) for each protocol.
I hope this covers everything! If there’s anything more you need, feel free to let me know.
