PPP

Point to Point Connection

PPP is a protocol that is able to handle authentication, compression, error detection, monitor link quality, and logically bundles multiple serial connections together to share the load.

 

1. Serial Communications

There are many different serial communication standards, each one using a different signaling method. There are three important serial communication standards affecting LAN-to-WAN connections:

 

    • RS-232 - Most serial ports on personal computers conform to the RS-232C or newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers. These types of peripheral devices for computers have been replaced by new and faster standards such as USB but many network devices use RJ-45 connectors that conform to the original RS-232 standard.

 

    • V.35 - Typically used for modem-to-multiplexer communication, this ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. In the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers. V.35 cables are high-speed serial assemblies designed to support higher data rates and connectivity between DTEs and DCEs over digital lines. There is more on DTEs and DCEs later in this section.

 

    • HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s. Engineers use HSSI to connect routers on LANs with WANs over high-speed lines, such as T3 lines. Engineers also use HSSI to provide high-speed connectivity between LANs, using Token Ring or Ethernet. HSSI is a DTE/DCE interface developed by Cisco Systems and T3 plus Networking to address the need for high-speed communication over WAN links.

 

With a leased line, despite the fact that customers are paying for dedicated services, and dedicated bandwidth is provided to the customer, the carrier still uses multiplexing technologies within the network. Multiplexing refers to a scheme that allows multiple logical signals to share a single physical channel. Two common types of multiplexing are time-division multiplexing (TDM) and statistical time-division multiplexing (STDM).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 　　　　　　　　　　　　      　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

 

 

 

　　　　

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Point to Point Encapsulation

The following are short descriptions of each type of WAN protocol:

    • HDLC - The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. HDLC is now the basis for synchronous PPP used by many servers to connect to a WAN, most commonly the Internet.

 

    • PPP - Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IPv4 and IPv6. PPP uses the HDLC encapsulation protocol, but also has built-in security mechanisms such as PAP and CHAP.

 

    • Serial Line Internet Protocol (SLIP) - A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.

 

    • X.25/Link Access Procedure, Balanced (LAPB) - An ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. X.25 is a predecessor to Frame Relay.

 

    • Frame Relay - An industry standard, switched, data link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25.

 

    • ATM - The international standard for cell relay in which devices send multiple service types, such as voice, video, or data, in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware; thereby, reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.

 

 

 

 

 

 

HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The current standard for HDLC is ISO 13239. HDLC was developed from the Synchronous Data Link Control (SDLC) standard proposed in the 1970s. HDLC provides both connection-oriented and connectionless service.

 

Cisco has developed an extension to the HLDC protocol to solve the inability to provide multiprotocol support. Although Cisco HLDC (also referred to as cHDLC) is proprietary, Cisco has allowed many other network equipment vendors to implement it. Cisco HDLC frames contain a field for identifying the network protocol being encapsulated. The figure compares standard HLDC to Cisco HLDC.

 

Flag

The flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110. Because there is a likelihood that this pattern occurs in the actual data, the sending HDLC system always inserts a 0 bit after every five consecutive 1s in the data field, so in practice the flag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. When frames are transmitted consecutively, the end flag of the first frame is used as the start flag of the next frame.

   

Address

The address field contains the HDLC address of the secondary station. This address can contain a specific address, a group address, or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary.

   

Control

The control field uses three different formats, depending on the type of HDLC frame used:

   

• Information (I) Frame - I-frames carry upper layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs flow and error control. The send sequence number refers to the number of the frame to be sent next. The receive sequence number provides the number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers. A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response.

   

• Supervisory (S) Frame - S-frames provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames. S-frames do not have an information field.

   

• Unnumbered (U) Frame - U-frames support control purposes and are not sequenced. Depending on the function of the U-frame, its control field is 1 or 2 bytes. Some U-frames have an information field.

   

Protocol

Only used in Cisco HDLC. This field specifies the protocol type encapsulated within the frame (e.g. 0x0800 for IP).

   

Data

The data field contains a Path Information Unit (PIU) or Exchange Identification (XID) information.

   

Frame Check Sequence (FCS)

The FCS precedes the ending flag delimiter and is usually a Cyclic Redundancy Check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If the result differs from the value in the original frame, an error is assumed.

 

 

PPP

 PPP encapsulation has been carefully designed to retain compatibility with most commonly used supporting hardware. PPP encapsulates data frames for transmission over Layer 2 physical links. PPP establishes a direct connection using serial cables, phone lines, trunk lines, cellular telephones, specialized radio links, or fiber-optic links.

 

PPP contains three main components:

• HDLC-like framing for transporting multiprotocol packets over point-to-point links.
• Extensible Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.
• Family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. PPP allows the simultaneous use of multiple network layer protocols. Some of the more common NCPs are Internet Protocol (IPv4) Control Protocol, IPv6 Control Protocol, AppleTalk Control Protocol, Novell IPX Control Protocol, Cisco Systems Control Protocol, SNA Control Protocol, and Compression Control Protocol.

PPP includes many features not available in HDLC:

• The link quality management feature, as shown in the figure, monitors the quality of the link. If too many errors are detected, PPP takes the link down.
• PPP supports PAP and CHAP authentication.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Physical layer:

• Synchronous physical media, such as leased line service.
• Asynchronous physical media, such as those that use basic telephone service for modem dialup connections

 

Data Link Layer:

The LCP functions within the data link layer and has a role in establishing, configuring, and testing the data-link connection. The LCP establishes the point-to-point link. The LCP also negotiates and sets up control options on the WAN data link, which are handled by the NCPs.

 

Network Layer

NCPs include functional fields containing standardized codes to indicate the network layer protocol that PPP encapsulates. Figure 2 lists the PPP protocol field numbers. Each NCP manages the specific needs required by its respective network layer protocols. The various NCP components encapsulate and negotiate options for multiple network layer protocols.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A PPP frame consists of six fields. The following descriptions summarize the PPP frame fields illustrated in the figure:

 

• Flag - A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 01111110. In successive PPP frames, only a single Flag character is used.

 

• Address - A single byte that contains the binary sequence 11111111, the standard broadcast address. PPP does not assign individual station addresses.

 

• Control - A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. This provides a connectionless link service that does require the establishment of data links or links stations. On a point-to-point link, the destination node does not need to be addressed. Therefore, for PPP, the Address field is set to 0xFF, the broadcast address. If both PPP peers agree to perform address and control field compression during the LCP negotiation, the Address field is not included.
• Protocol - Two bytes that identify the protocol encapsulated in the information field of the frame. The 2-byte Protocol field identifies the protocol of the PPP payload. If both PPP peers agree to perform protocol field compression during LCP negotiation, the Protocol field is one byte for the protocol identification in the range 0x00-00 to 0x00-FF. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).

• Data - Zero or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length of the information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum information field length.
• Frame Check Sequence (FCS) - Normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection. If the receiver's calculation of the FCS does not match the FCS in the PPP frame, the PPP frame is silently discarded.

 

 

 

 

 

 

 

 

 

 

 

 

 

PPP Configuration

Enabling PPP on an Interface    

To set PPP as the encapsulation method used by a serial interface, use the encapsulation ppp.

 

The following example enables PPP encapsulation on interface serial 0/0/0:

    R3(config)# interface serial 0/0/0

    R3(config-if)# encapsulation ppp

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If the link quality percentage is not maintained, the link is deemed to be of poor quality and is taken down. Link Quality Monitoring (LQM) implements a time lag so that the link does not bounce up and down.

   

The following configuration example monitors the data dropped on the link and avoids frame looping:

   

R3(config)# interface serial 0/0/0

R3(config-if)# encapsulation ppp 

R3(config-if)# ppp quality 80

 

 

 

 

 

 

 

 

 

 

Multilink PPP (also referred to as MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple physical WAN links. Multilink PPP also provides packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inboundand outbound traffic.

 

Step 1. Create a multilink bundle.

The interface multilink number command creates the multilink interface.

In interface configuration mode, an IP address is assigned to the multilink interface.

The interface is enabled for multilink PPP.

The interface is assigned a multilink group number.

 

Step 2. Assign interfaces to the multilink bundle.

Each interface that is part of the multilink group:

Is enabled for PPP encapsulation.

Is enabled for multilink PPP.

Is bound to the multilink bundle using the multilink group number configured in Step 1.

 

Interface multilink 1

Ip address X.X.X.X X.X.X.X

Ppp multilink

Ppp multilink group 1

 

Int s 0/1/0

No ip address

Encapsulation ppp

Ppp multilink

Ppp multilink group 1

 

 

 

 

 

 

 

 

 

 

 

PPP Authentication RFC 1314

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PAP is a very basic two-way process. There is no encryption. The username and password are sent in plaintext. If it is accepted, the connection is allowed. CHAP is more secure than PAP. It involves a three-way exchange of a shared secret.

 

 

PAP Authentication Configuration

 

 

 

 

R1:

Username R2 password someone

Interface s0/0/0

Ip add 10.0.1.1 255.255.255.0

Encapsulation ppp

Ppp authentication pap

Ppp pap sent-username R1 password someone

 

R2:

Username R2 password someone

Interface s0/0/0

Ip add 10.0.1.1 255.255.255.0

Encapsulation ppp

Ppp authentication pap

Ppp pap sent-username R1 password someone