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C H A P T E R
Network Models
and Protocols
EXAM OBJECTIVES
1.1 Layered Network Models
1.2 The Layers of the TCP/IP 5-Layer Model
1.3 Network Protocols
1.4 Peer-to-Peer Communication
1.5 TCP/IP Protocols by Name and Function
fter completing this chapter, you will be
able to meet the following Network
Administration Exam objectives:
•Identify the purpose of each layer in the TCP/IP 5-layer model. •Describe the functionality of each of the following Network Protocols: TCP, UDP, IP, and ICMP. •Describe the relationship between the following Network Protocols: TCP, UDP, IP, and ICMP. •Describe peer-to-peer communication.
To help you meet these objectives, this chapter
covers the following topics:
•layered network models •the layers of the TCP/IP 5-layer model •network protocols •peer-to-peer communications •TCP/IP protocols by name and function
A
2 Chapter 1 | Network Models and Protocols
This chapter first introduces layered network models and then describes the services provided by each layer of the model. We then briefly describe, in the context of a protocol stack, the network protocols that pro- vide the services to upper layer protocols or applications at each layer. You will learn about the features of the most important network proto- cols, TCP/UDP/IP and ICMP, and this information will serve as the foundation for later chapters that cover these protocols in greater detail. This gradual or phased introduction of the important network protocols will allow you to understand the basics of each protocol before we explore their more complex aspects. Network protocols are modular by design and function at specific layers of a hierarchical protocol stack. Each layer in the hierarchy pro- vides services to the layer above it and uses the services of the layer beneath it. There are instances in which nonadjacent layers communi- cate directly, but these are exceptions to the rule. Through this hierarchy, each layer provides an abstraction to the layer just above it. This abstraction is desirable, as upper layers need not know how their data is routed across the Internet, or over which net- work their data will travel. To understand how applications such as sendmail , telnet , and ftp interface with the Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols, we must examine how the protocols com- municate with each other and how they offer a service to applications. Each protocol was designed to offer a service to another protocol or application and will be explored in that context. We can best view the protocols as an ordered stack of modules based on a set of hierarchical relationships. The hierarchy is of funda- mental importance because it explains and exposes not only the relation- ships among the interacting protocols but also the properties of each protocol, revealing why a particular protocol is able meet the require- ments of a particular application. There are many protocol families and models. This book explores two models, which are covered by Sun course SA-389, Solaris Operating Environment TCP/IP Network Administration: •the OSI/ISO 7-layer reference model •the TCP/IP Sun/DoD 5-layer model
1.1 Layered Network Models
4 Chapter 1 | Network Models and Protocols
•Chronologically, the OSI model was created long after the TCP/ IP family of protocols. The model is to some degree an ideal , as it does not pertain to any spe- cific protocol family, but rather provides a framework within which network protocol designers and hardware manufacturers may work as they strive to produce modular products. We next outline the TCP/IP model and compare and contrast it with the OSI model.
The TCP/IP 5-Layer Model
The Department of Defense (DoD) TCP/IP 5-layer model was created in 1969. Table 1.2 shows the layers of this model and the service pro- vided by each layer.
With this model, aimed specifically at TCP/IP conventions, we can identify the protocols at each layer, as shown in Table 1.3. The most important points to note about the TCP/IP model are: •The TCP/IP protocols were developed and funded by the USA DoD for purposes of research and experimentation. •The TCP/IP model was conceived in 1969. •The TCP/IP model accommodates only the TCP/IP protocols. •The TCP/IP model has only five layers.
Table 1.2 Layers of the TCP/IP Model and Purpose of Each Layer LAYER (NUMBER ) PURPOSE
Application (5) Reserved for applications and protocols Transport (4) Provides end-to-end delivery service for layer 5 applications and protocols Internet (3) Provides a network routing service to upper layers Network (2) Provides a framing service to the Internet layer Physical (1) Provides an electrical signal bit transmission service to the network
Layered Network Models 5
Benefits of Using Network Models: A Summary
It is beneficial to consider the organization of any network model because the network model •reveals the hierarchical, modular nature of network protocol design and implementation. •enables us to think in terms of each protocol performing a given function or service at a specific layer. •visually reveals a host’s protocol stack as implemented in the kernel. •reveals the order of the protocol stack. The striking differences between the models are shown in Table 1.4.
Table 1.3 Layers of the TCP/IP Model and Entities That Function at Each Layer
LAYER (NUMBER ) NETWORK^ COMPONENTAT THIS LAYER^ THAT^ OPERATES
Application (5) HTTP, FTP, telnet, SMTP, NTP, POP, IMAP, and others Transport (4) TCP/UDP Internet (3) IP, ICMP, ARP, RARP Network (2) Data Link: Ethernet, Token Ring, FDDI, ATM, and others Physical (1) Coaxial, fiberoptic, twisted pair
Table 1.4 Differences Between the OSI 7-Layer and TCP/IP 5-Layer Models OSI MODEL TCP/IP MODEL
Devised 1983 Devised 1969 Created by ISO Created by USA DoD Multiple vendors/multiple protocols/ ISO protocols
TCP/IP protocol family
Seven layers Five layers Generic networking model TCP/IP-specific model
The Layers of the TCP/IP 5-Layer Model 7
•Internet Message Access Protocol (IMAP) •Simple Mail Transport Protocol (SMTP) •File Transfer Protocol (FTP)
- telnet and rlogin •Domain Name Service (DNS) •Network File System (NFS) •Network Information Service (NIS) and Network Information Service Plus (NIS+) Application layer protocols and applications are unable to deliver their own data across a network or Internet unassisted and so need to be encapsulated in a Transport protocol such as TCP or User Datagram Pro- tocol (UDP). The Transport layer provides this transport service (also known as a delivery service) to the Application layer. The Transport layer protocol in turn relies on the Internet Protocol (IP) to provide an end- to-end routing service. You will visit these concepts again when IP (Chapter 5), routing (Chapter 6), TCP/UDP (Chapter 7), and the Client-Server model (Chapter 8) are investigated in greater detail. Focus on the following important points that summarize features of the Application layer. Chapter 8 examines how these applications inter- act with Transport layer protocols.
E XAM NOTES
KEY LEARNING POINTS
- Many protocols function at the Application layer.
- HTTP, SMTP, POP, FTP, and DNS are the most heavily used Application layer protocols on the Internet.
- Web browsers, mail clients, news readers, ftp, telnet, sendmail are just a few of the thousands of layer 5 applications.
- The Application layer uses the Transport layer for delivery.
- Users often interact directly with Application layer programs.
- The Application layer (5) is at the top of the stack.
8 Chapter 1 | Network Models and Protocols
Transport Layer (4)
TCP and UDP are the only two protocols that function at the Transport layer (4). They encapsulate or carry the layer 5 protocols and offer an end-to-end transport service. They accept data from a client network application on a client host and deliver it to the server application on the server host that is providing the client with the service. The client and the servers are usually on different systems and therefore need a network to connect them. Data travels between the client and server across one or more networks. For example, a telnet client on the client host needs to reach the in.telnetd server daemon running on the server host. The telnet client process uses the TCP Transport layer protocol to connect to the in.tel- netd server process (which usually exists on a different system). Other Application layer protocols use the UDP transport protocol, which offers a nonguaranteed transport service, trading guaranteed delivery for speed and minimized overhead. DNS queries, for example, use UDP, as speed is of the essence, and failure considerations are not so critical. The choice of using either TCP or UDP at the Transport layer is made by the network programmer and is based on the type of service required. Some well-known network protocols that function at the Application layer use both TCP and UDP for different functions— DNS, for example, which is fully explored in Chapter 11. Some applica- tions that were originally designed to use UDP, such as Sun’s NFS that allows file sharing between systems, have switched to using TCP. Ver- sion 2 of NFS used UDP, but version 3 uses TCP. Focus on the following important points before proceeding to the next section on the Internet layer.
E XAM NOTES
KEY LEARNING POINTS
- Most Solaris applications use TCP and UDP for end-to-end delivery.
- TCP and UDP are transport protocols.
- TCP and UDP are encapsulated in IP.
- TCP and UDP are not able to route their own data and so use IP to perform this task.
10 Chapter 1 | Network Models and Protocols
layer where Ethernet resides. Chapter 4 examines ARP further. Chapter 5 covers IPv4 in detail and Chapter 13 explores IPv6.
E XAM NOTES
KEY LEARNING POINTS
- IP makes the routing decision and routes IP datagrams.
- IP routes datagram(s) to the destination IP address in the IP header.
- IP datagrams typically carry (encapsulate) TCP/UDP application data.
- IP functions at the Internet layer.
- IP’s header is a minimum of 20 bytes and can be larger.
- IP’s unit of data is an IP datagram.
Network Interface Layer (2)
Ethernet functions at this layer. There are alternatives to Ethernet at this layer, but Ethernet is dominant in terms of UNIX LANS and is the only LAN examined in detail in this book. Ethernet frames or encapsulates the data it is carrying, prior to transmitting its packet onto the Ethernet bus. Network Interface layer technologies have Maximum Transmission Units (MTUs) of various sizes. (The MTU determines the maximum number of bytes that can be transmitted in a single frame, not including the headers.) As Table 1.5 shows, MTU size varies according to the Net- work Interface layer technology.
Table 1.5 Network Layer MTUs NETWORK TYPE MTU (BYTES )
Hyperchannel 65, 16 MB/second Token Ring (IBM) 17, 4 Mbits/second Token Ring (IEEE 802.5) 4, Fiber Distributed Data Interface (FDDI) 4, Ethernet 1, IEEE 802.3/802.2 1, Point to Point (low delay) 296
Network Protocols 11
Ethernet’s MTU is especially significant because it determines whether an IP datagram carried by Ethernet needs fragmenting. See Chapter 5 for further information.
E XAM NOTES
KEY LEARNING POINTS
- The Network Interface layer MTU determines the maximum number of bytes carried in a single frame.
- Ethernet is a Network Interface layer technology.
- Network Interface layer technologies have MTUs of various sizes.
Physical Layer (1)
This layer at the base of the stack identifies the LAN transmission media, typically fiberoptic or wire-based media. This layer concerns the transmission of bits across a physical media with very little interpretation of the data. Chapter 3 examines the copper wire and fiberoptic-based media used by Ethernet, FDDI, and ATM in greater detail. So far, we have examined the network protocols in the context of a network model. The ensuing discussion of the most important network protocols reveals their features and provides a foundation for chapters that follow.
This section is a brief introduction to TCP, UDP, IP, and ICMP. First, we briefly examine the two transport protocols, TCP and UDP, and then take a cursory look at IP and ICMP.
1.3 Network Protocols
Network Protocols 13
telnet, ftp , and sendmail , all of which require a guaranteed delivery ser- vice and therefore use TCP rather than UDP.
E XAM NOTES
KEY LEARNING POINTS
- TCP is a connection-oriented protocol.
- The segment is the TCP unit of data.
- TCP is a Transport layer protocol.
- TCP is encapsulated and routed by IP.
- TCP is said to offer a guaranteed, reliable, connection-oriented service that creates significant overhead.
Essentially, if data security must be guaranteed, TCP is a better choice than UDP, although more expensive in terms of both the amount of control data transmitted and the control overhead incurred. Chapter 7 examines the TCP protocol in greater detail.
User Datagram Protocol (UDP)
UDP is a connectionless transport protocol, which means that no con- nection is established at the Transport layer prior to data being sent between client and server applications. Some applications—for example, a router propagating routing information every 30 seconds—can afford to lose an occasional data packet. Routing clients that miss the occasional routing table update do not usually suffer adverse effects. The Routing Information Protocol (RIP) uses UDP. DNS queries also use UDP, as speed is more important than reliability for this application. UDP applications tend to send small packets that can be transported in a single UDP datagram. UDP has an optional checksum error check, which introduces minimal overhead and is usually turned off. To check the UDP checksum variable ( udp_do_checksum ) under Solaris 8 use ndd :
ndd –get /dev/udp udp_do_checksum
0
14 Chapter 1 | Network Models and Protocols
A value of 0 means false; that is, disable the UDP checksum feature. A value of 1, which means true, indicates that the UDP protocol check- sum feature is enabled. To enable the UDP checksum feature, issue the following command:
ndd –set /dev/udp udp_do_checksum 1
To check the current value of udp_do_checksum :
ndd –get /dev/udp udp_do_checksum
1
E XAM NOTES
KEY LEARNING POINTS
- UDP is a connectionless protocol.
- UDP is a transport protocol.
- UDP is encapsulated and routed in IP.
- UDP is not guaranteed.
- UDP does minimal error checking.
- UDP has a simple checksum for error checking.
- UDP’s header is only eight bytes.
Chapter 7 looks at the UDP protocol in greater detail.
Internet Protocol (IPv4)
IPv4 is a connectionless protocol like UDP. Unlike UDP, however, IP is not a transport protocol but instead offers a datagram routing service to the Transport layer (a datagram is the unit of data for the IP layer). The Transport layer protocols, TCP and UDP, use IP to route their client data between application client and server hosts. It is worth stressing, therefore, that IP routes data between hosts, and in effect, between the Transport layers on end-to-end hosts, but not between clients and server processes. The Transport layer protocols, TCP and UDP, transport the
16 Chapter 1 | Network Models and Protocols
E XAM NOTES
KEY LEARNING POINTS
- ICMP is a connectionless protocol.
- ICMP is an Internet layer protocol.
- ICMP is unusual in that it is encapsulated in IP but is not a transport protocol.
- ICMP assists IP with error detection and correction.
- ICMP generates two types of messages: error and query.
The term peer-to-peer identifies two communicating entities, functioning or operating at the same layer of the stack, as peers. The peers are usually on different systems that are connected by one or more networks, as shown in Figure 1-1.
1.4 Peer-to-Peer Communication
Host 1
Application
Transport
Internet Network interface Hardware
Host 2
Application
Transport
Internet Network interface Hardware
Message or stream
Segment or datagram
Datagram
Frame
Signal
Figure 1–1 Peer relationships
TCP/IP Protocols by Name and Function 17
E XAM NOTES
KEY LEARNING POINTS
- The two ends of a communication are at the same layer of the stack.
- The peer entities may be application processes or protocols.
The following section identifies protocols by the Requests for Com- ments (RFCs) that formally describe them.
In this chapter we already looked briefly at some TCP/IP protocols. It is, however, worth summarizing each protocol formally in terms of its name and the RFC that describes it. Some of these protocols will be explored in later chapters as indicated in Table 1.6. The following list of RFCs and protocols is not definitive. Listed are the most useful RFCs, but not all RFCs that relate to the subject, as the list is extensive for the more complex protocols. The RFC column of the table lists the chapter of this book that covers the protocol where applicable. We work down the stack from the Application layer (5), again, taking the journey that application data takes. The Application layer has literally hundreds of applications and many protocols that the applications at this layer use. They are listed in Table 1.6 by relevant RFC, protocol name, and a brief description. Table 1.7 lists the important Transport layer protocols. Chapter 7 explores these in detail. Table 1.8 describes the protocols of the Internet layer. Chapter 4 cov- ers ARP/RARP, and Chapters 5 and 13 cover IPv4 and IPv6, respectively. Table 1.9 shows the Network layer protocols, with many technologies to choose from. Ethernet is by far the most important LAN technology to understand. Alternatives such as Token Ring and Token Bus exist, but they are far less important than Ethernet.
1.5 TCP/IP Protocols by Name and Function
TCP/IP Protocols by Name and Function 19
Table 1.7 Transport Layer (4) Protocol Descriptions RFC PROTOCOL DESCRIPTION
793 Chapter 7
TCP Transmission Control Protocol is a connection-oriented protocol that provides the full-duplex, stream service on which many application protocols depend. It is encapsulated in IP.
768 Chapter 7
UDP User Datagram Protocol provides a datagram delivery to application and is encapsulated in IP.
Table 1.8 Internet Layer (3) Protocol Descriptions RFC PROTOCOL DESCRIPTION
826 Chapter 4
ARP Address Resolution Protocol defines the method used to map a 32-bit IP address to a 48-bit Ethernet address.
903 Chapter 4
RARP Reverse Address Resolution Protocol is the reverse of ARP. It maps a 48-bit Ethernet address to a 32-bit IP address.
791, 919, Chapters 5 and 13
IP Internet Protocol determines the path a datagram must take, based on the destination host’s IP address.
792 Chapter 5
ICMP Internet Control Message Protocol communicates error messages and other controls within IP datagrams.
Table 1.9 Network Layer (2) Protocol Descriptions RFC PROTOCOL DESCRIPTION
1055 SLIP Serial Line IP encapsulates IP datagrams on serial lines.
1661
826, 894 Chapter 3
PPP
Ethernet
Point-to-Point Protocol transmits datagrams over serial point-to-point links. Ethernet is a broadcast-based, contention-bus LAN technology.
20 Chapter 1 | Network Models and Protocols
E XAM NOTES
KEY LEARNING POINTS
- The OSI/ISO 7-layer reference model is a generic network model.
- The DoD 5-layer model is specifically for the TCP/IP protocol family.
- TCP and UDP are transport protocols that provide a delivery service to the Application layer.
- IP is an Internet layer protocol that offers a datagram-based routing service to the Transport and Application layers.
SUMMARY
This chapter explored network models, specifically the OSI 7-layer model and the TCP/IP 5-layer model. Having contrasted the models, it should be apparent that OSI is a generic model in contrast to the TCP/ IP model, which is specifically aimed at the TCP/IP family of protocols. An examination of the protocol stack revealed a hierarchical approach to network management and a modular approach to protocol implementa- tion and design. The five layers of the TCP/IP model were examined and the main network protocols at each layer introduced. A discussion of peer-to-peer communications described how the communicating proto- cols or applications are considered peers when they are both functioning at the same layer in the stack. Finally, we provided a tabular summary of relevant protocols and appli- cations that function at various layers of the protocol stack, listed by RFC, protocol, description, and the chapter of this book that discusses them.
TEST YOURSELF
MULTIPLE CHOICE
1. Which of the following are layers of the OSI/ISO 7-layer reference model? Choose three. A. Session layer
