Point-to-Point Protocol PPP Advantages & Operations

Point-to-Point Protocol PPP Advantages & Operations

Synchronous Point-to-Point Protocol used to connect to non-Cisco devices, control link quality, provide authentication or group links for sharing. PPP uses HDLC to encapsulate datagrams. This article is on what is PPP and Point-to-Point Protocol Advantages & Operationsis. Remember that HDLC is the default serial encapsulation method when connecting two Cisco routers. With an added protocol type field, the Cisco HDLC version is exclusive. Therefore, Cisco HDLC can only work with other Cisco devices. However, when there is a need to connect to a non-Cisco router, PPP encapsulation should be used.

WHAT IS PPP?

PPP (Point-to-Point Protocol) encapsulation was carefully designed to maintain compatibility with the most used hardware that supports it.
PPP encapsulates data frames to transmit them through physical Layer 2 links. PPP establishes a direct connection via serial cables, telephone lines, trunk lines, cell phones, specialized radio links or fiber optic links.
The Point-to-Point Protocol contains three main components:

• HDLC style framework to transport multiprotocol packets through point-to- point links .
• Extensible Link Control Protocol (LCP) to establish, configure and test the data link connection.
• Family of network control protocols (NCP) to establish and configure different network layer protocols. PPP allows the simultaneous use of several network layer protocols.

Some of the most common NCPs are the Internet Protocol Control Protocol (IPv4), the IPv6 Control Protocol, the AppleTalk Control Protocol, Novell IPX, the Cisco Systems Control Protocol, the SNA Control Protocol and the compression control protocol

ADVANTAGES OF PPP

PPP originally emerged as an encapsulation protocol to transport IPv4 traffic through point-to-point links. PPP provides a standard method for transporting multiprotocol packets through point-to-point links.
The use of PPP has many advantages, including the fact that it is not exclusive. PPP includes many features that are not available in HDLC:

• The link quality management feature, which is shown in the illustration, controls the link quality. If too many errors are detected, PPP deactivates the link.
• PPP supports PAP and CHAP authentication. This feature is explained and practiced later in another section.

PPP LAYER ARCHITECTURE

A layered architecture is a model, a design, or a logical plane that helps in the communication of the interconnecting layers.
In the illustration, the PPP layered architecture is compared with the open system interconnection (OSI) model.
PPP and OSI share the same physical layer, but PPP distributes the functions of LCP and NCP differently.
On the physical layer, you can configure PPP in a range of interfaces, including the following:

• Asynchronous serial
• Synchronous serial
• HSSI
• ISDN

PPP operates through any DTE / DCE interface (RS-232-C, RS-422, RS-423 or V.35). The only absolute requirement imposed by PPP is a full-duplex circuit , whether dedicated or switched, that can operate in synchronous or asynchronous serial bit mode, transparent to PPP link layer frames.
PPP does not impose any restrictions with respect to the transmission rate in addition to those imposed by the specific DTE / DCE interface used.
Most of the work that PPP does is carried out by LCP and NCPs in the data link and network layers. LCP configures the PPP connection and its parameters, the NCPs handle the upper layer protocol configurations, and LCP terminates the PPP connection.

PPP: LINK CONTROL PROTOCOL (LCP)

LCP works within the data link layer and plays a role in the establishment, configuration and testing of the data link connection. In addition, it establishes the point-to-point link.
LCP also negotiates and configures the control options in the WAN data link, managed by the NCPs.
LCP provides automatic configuration of the interfaces at each end, including the following:

• Handling different limits on package size
• Detection of common configuration errors
• Link termination
• Determining when a link works correctly or when it fails

Once the link is established, PPP also uses LCP to automatically agree on encapsulation formats, such as authentication, compression and error detection .

PPP: NETWORK CONTROL PROTOCOL (NCP)

PPP allows several network layer protocols to work on the same communication link. For each network layer protocol that is used, PPP uses a separate NCP. For example, IPv4 uses the IP control protocol (IPCP) and IPv6 uses the IPv6 control protocol (IPv6CP).
NCP protocols include functional fields that contain standardized codes to indicate the network layer protocol that PPP encapsulates.

PPP FRAME STRUCTURE

PPP frames consist of six fields.

The following descriptions summarize the fields of the PPP frames, which are shown in the illustration:

• Flag or Indicator : a single byte that indicates the beginning and end of a frame. The Signaling field is formed by the binary sequence 01111110. In successive PPP frames only a single flag character is used.
• Address : a single byte containing the binary sequence 11111111, the standard broadcast address. PPP does not assign addresses to individual stations.
• Control : a single byte formed by the binary sequence 00000011, which requires the transmission of user data in a non-sequential frame.
• Protocol : two bytes that identify the protocol encapsulated in the frame information field. The 2-byte Protocol field identifies the PPP content protocol.
• Data : zero or more bytes containing the datagram for the protocol specified in the Protocol field. To find the end of the information field, the sequence of the completion indicator must be searched and 2 bytes left for the FCS field.
• Frame Check Sequence (FCS) : typically 16 bits (2 bytes). Through a prior agreement, with the acceptance of PPP implementations, a 32-bit FCS (4 bytes) can be used for greater error detection.

LCP protocols can negotiate modifications to the structure of the standard PPP frame. However, modified frames always differ from standard frames.

SESSIONS FOR PPP

There are three phases of establishing a PPP session, as shown in the illustration:

1. Phase 1 , link establishment and configuration negotiation : Before PPP exchanges any network layer datagram (such as IP) LCP must first open the connection and negotiate the configuration options. This phase is completed when the receiving router sends a configuration acknowledgment frame back to the router that initiates the connection.
2. Phase 2 , link quality determination (optional) : LCP tests the link to determine if its quality is sufficient to activate network layer protocols. LCP may delay the transmission of network layer protocol information until this phase is completed.
3. Phase 3 , negotiation of the network layer protocol configuration : once LCP has completed the link quality determination phase, the corresponding NCP protocol can configure the network layer protocols separately, activate and deactivate them at any time . If LCP closes the link, it informs the network layer protocols so that they can take appropriate measures.

The link remains configured for communications until explicit LCP or NCP frames close the link, or until an external event occurs, for example, an inactivity timer expires or an administrator intervenes.
LCP can terminate the link at any time. This is usually done when one of the routers requests termination, but it can happen due to a physical event, such as the loss of a carrier or the expiration of an inactive period timer.