Lesson 2:SLIP and PPP

The Serial Line Internet Protocol (SLIP) and the Point-to-Point Protocol (PPP) are also data-link layer protocols, but they are very different from Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). SLIP and PPP, which are part of the TCP/IP protocol suite, are not designed to connect systems to a LAN that uses a shared network medium. Instead they connect one system to another using a dedicated connection, such as a telephone line. For this reason, SLIP and PPP are called end-to-end protocols. Because the medium isn't shared, there is no contention and no need for a Media Access Control (MAC) mechanism, and because there are only two systems involved, there is no need to address the packets. As a result, these protocols are far simpler than Ethernet and Token Ring protocols. SLIP and PPP also do not include physical layer specifications; they operate strictly at the data-link layer. Another standard, such as the RS-232 specification, which defines the nature of the serial port that you use to connect a modem to your computer, provides the physical layer.

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After this lesson, you will be able to

• Describe the SLIP and PPP frame formats
• Diagram the PPP connection establishment process

Estimated lesson time: 20 minutes

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SLIP

SLIP is so simple it hardly deserves to be called a protocol. It is designed to transmit signals over a serial connection (which in most cases means a modem and a telephone line) and has very low control overhead, meaning that it doesn't add much information to the network layer data that it is transmitting. Compared to the 18 bytes that Ethernet adds to every packet, for example, SLIP adds only 1 byte. Of course, with only 1 byte of overhead, SLIP can't provide functions like error detection, network layer protocol identification, security, or anything else.

SLIP works by transmitting an IP datagram received from the network layer and following it with a single framing byte called an End Delimiter (see Figure 12.6).

Figure 12.6  SLIP uses an End Delimiter to mark the end of each data packet

This byte informs the receiving system when it has finished receiving the data portion of the packet. In some cases, the system surrounds the datagram with two End Delimiter fields, making it possible for the receiving system to easily ignore any line noise that occurs outside of the frame. Because of its limited capabilities, SLIP is rarely used today, having been replaced, for the most part, by PPP.

PPP

PPP is, in most cases, the protocol you use when you access the Internet by establishing a dial-up connection to an ISP. Many other WAN technologies use it as well. PPP is more complex than SLIP and is designed to provide a number of services that SLIP lacks. These include the ability of the systems to exchange IP addresses, carry data generated by multiple network layer protocols (which is called multiplexing), and support different authentication protocols. Still, PPP does all this using only a 5-byte header, which is larger than the SLIP header, but still less than half the size of the Ethernet frame.

The PPP Frame

The PPP frame is illustrated in Figure 12.7.

Figure 12.7  The PPP frame

The functions of the fields in the PPP frame are as follows:

• Flag (1 byte).  This field indicates the transmission of a packet is about to begin.
• Address (1 byte).  This field contains a value indicating that the packet is addressed to all recipients.
• Control (1 byte).  This field contains a code indicating that the frame contains an unnumbered information packet.
• Protocol (2 bytes).  This field identifies the protocol that generated the information found in the Data field.
• Data and Pad (up to 1500 bytes).  This field contains information generated by the protocol identified in the Protocol field, plus padding if necessary.
• Frame Check Sequence (2 or 4 bytes).  This field contains a checksum value that the receiving system will use for error detection.
• Flag (1 byte).  This field indicates that the transmission of the packet has been completed.

Establishing a PPP Connection

As small as it is, the PPP frame can't possibly provide all of the functions listed earlier. Instead, the protocol performs many of these functions by performing an elaborate connection establishment procedure when the two systems involved first communicate. This method is more efficient than increasing the size of the PPP header, because there's no need to include this additional information in every packet. For example, it's beneficial for the two communicating systems to know each other's IP addresses, but there's no need to include address fields in every packet header, as in Ethernet, because there are only two computers involved and they only have to identify themselves once. The same is true for functions like user authentication.

The PPP connection establishment procedure consists of the following phases that occur before the systems exchange any application data.

1. Link dead.  The two computers begin with no communication, until one of the two initiates a physical layer connection, such as running a program that causes the modem to dial.
2. Link establishment.  Once the physical layer connection is established, one computer generates a PPP frame containing a Link Control Protocol (LCP) request message. The computers use the LCP to negotiate the parameters they will employ during the rest of the PPP session. The message contains a list of options, such as the use of a specific authentication protocol, link quality protocol, header compression, network layer protocols, and so on. The receiving system can then acknowledge the use of these options or deny them and propose a list of its own. Eventually, the two systems agree on a list of options they have in common.
3. Authentication.  If the two systems have agreed to the use of a particular authentication protocol during the link establishment phase, they then exchange PPP frames containing messages specific to that protocol in the Data field. PPP computers commonly use the Password Authentication Protocol (PAP) or the Challenge Handshake Authentication Protocol (CHAP), but there are also other authentication protocols.
4. Link quality monitoring.  If the two computers have negotiated the use of a link quality monitoring protocol during the link establishment phase, the exchange of messages for that protocol occurs here.
5. Network layer protocol configuration.  For each of the network layer protocols that the computers have agreed to use, a separate exchange of Network Control Protocol (NCP) messages occurs at this point.
6. Link open.  Once the NCP negotiations are complete, the PPP connection is fully established and the exchange of packets containing network layer application data can commence.
7. Link termination.  When the two computers have finished communicating, they sever the PPP connection by exchanging LCP termination messages, after which the systems return to the link dead phase.

Point-to-Point Protocol over Ethernet

Point-to-Point Protocol over Ethernet (PPPoE) is a TCP/IP standard that defines a methodology for creating individual PPP connections between computers on an Ethernet LAN and external services connected to the LAN using a broadband device such as a cable or DSL modem. Broadband remote network access devices can easily support multiple computers, and Ethernet is the most common protocol used to network the computers together and connect them to the broadband device. However, a shared Ethernet LAN does not enable each computer to access remote services using individual parameters for functions such as access control and billing. The object of PPPoE is to blend the simplicity of connecting multiple computers to a remote network using an Ethernet LAN and broadband technology, while making it possible to establish a separate PPP connection between each computer and a given remote service, complete with all of the PPP components, such as LCP negotiation, authentication, and NCP configuration.

Exercise 1: PPP Connection Establishment

Place the following steps of the PPP connection establishment process in the correct order.

1. Link open
2. Link termination
3. Network layer protocol configuration
4. Authentication
5. Link quality monitoring
6. Link establishment
7. Link dead

Lesson Review

1. What is the name of the protocol that systems use to negotiate options during the PPP connection establishment procedure?
   1. CHAP
   2. LCP
   3. PAP
   4. NCP
2. How large is the End Delimiter field used during SLIP communications?
   1. 1 byte
   2. 2 bytes
   3. 5 bytes
   4. 18 bytes
3. Which of the following connection elements are configured by the Link Control Protocol? (Choose three.)
   1. The network layer protocols to be used during the connection
   2. The authentication protocol to be used during the establishment of the connection
   3. The application layer protocols to be used to generate the data transmitted during the connection
   4. The link quality protocol to be used during the establishment of the connection

Lesson Summary

• SLIP is a simple protocol that enables two systems connected through their serial ports to exchange messages with very little control overhead.
• PPP is a more complicated end-to-end protocol that enables two systems to negotiate the use of optional features such as authentication protocols and multiple network layer protocols.