GLOBAL KNOWLEDGE NETWORK ä CERTIFICATION PRESS Cisco Certified Network Associate Exam Router and Switching Study Guide

Chapter 4

TCP/IP Protocol Suite

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Certification Objectives

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Remote Procedure Calls

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Sockets

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Transport Layer Interface

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NetBIOS

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Transport Layer

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TCP

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Flow Control

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Sliding Windows

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UDP

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UDP Flooding

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IP Helper Addresses

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Internet Protocol (IP)

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Connectionless, Unreliable Delivery Service

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Packet Fragmentation and Reassembly

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Routing Functions

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Address Resolution Protocol (ARP)

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Reverse Address Resolution Protocol (RARP)

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Inverse Address Resolution Protocol (InARP)

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Internet Control Message Protocol (ICMP)

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ICMP Echo Reply (Ping)

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ICMP Redirects

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ICMP Source Quench

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UNIX

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From the Classroom

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TFTP and Syslog

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32-bit Windows

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Certification Objectives

- Application Layer Services
- Presentation and Session Layer Services
- Detailed Protocol Structure
- Network Layer
- Operating System Commands

{Answer to Self Test Question #21}

The TCP/IP protocol suite is not a static entity. Rather, it is a dynamically changing collection of internetworking protocols that continually push the state of the art in internetworking. Because of its academic origins, much of the significant development work in major universities is focused on TCP/IP. Researchers from around the world are continually developing and proposing extensions or enhancements to the TCP/IP protocol suite.

As a de facto standard, the TCP/IP protocol suite is not owned by any particular vendor, but is supported by everyone. For internetworks consisting of heterogeneous computer systems that need to communicate with each other, TCP/IP stands out as the common denominator across all types of platforms.

{Answer to Self Test Question #3} The TCP/IP protocol suite has four major layers, which can roughly correspond to the seven layers of the OSI reference model. Figure 4-1 shows the general structure of the TCP/IP protocol suite as it compares to the ISO/OSI Reference model. Explanations of each protocol component are presented in the following sections.

Figure 1 Four-layered TCP/IP model

Application Layer Services

The TCP/IP protocol suite includes several common application programs. {Answer to Self Test Question #2}Applications function at the OSI reference model layer 7, or layer 4 of the TCP/IP suite. It is important for anyone aiming for the CCIE certification to be familiar with the applications listed in Table 4-1 at the indicated level. Each application uses either UDP (unreliable) or TCP (reliable) as the transport mechanism. {Answer to Self Test Question #1}

TCP/IP Service | Transport protocol and port # | Function | Level of Familiarity required for CCIE
Telnet | TCP 23 | Remote terminal | Extremely
FTP | TCP 21 TCP 20 | File transfer | Medium
TFTP | UDP 69 | File transfer | Extremely
NFS |  | File services | None
SMTP | TCP 25 | e-mail | Need to know transports and port # for firewalls
DNS | TCP 53 | Name server | Need to know transports and port # for firewalls
HTTP | TCP 80 | Hypertext www | Need to know transports and port # for firewalls
HTTPS | TCP 443 | Encrypted Web traffic | Need to know transports and port # for firewalls

Table 1 TCP/IP Services

Presentation and Session Layer Services

{Answer to Self Test Question #4}

The TCP/IP protocol model does not have a formal presentation or session layer in general, although Sun Microsystems has offered eXternal Data Representation (XDR) as a presentation layer service along with their original Remote Procedure Call (RPC) design work. This RPC development work has been continued by the Open Software Foundation (OSF) as part of the Distributed Computing Environment (DCE) initiative.

Remote Procedure Calls

Remote procedure calls are a method for executing programs (here called "procedures") on other network nodes, such that they appear to be executing locally.

{Answer to Self Test Question #6}Remote procedure calls (RPCs) do not fit in well with the OSI Reference Model. As a result, RPCs can only be identified as existing somewhere between the application and transport layer. {Answer to Self Test Question #5}An RPC can be viewed as a connectionless session. Part of the RPC function is to abstract the application layer from some of the networking details. {Answer to Self Test Question #23}In other words, one of the goals of RPC is transparency, in that they try to make remote calls look like local ones.

A common RPC implementation across heterogeneous computer platforms significantly enhances portability of applications between different systems.

{Answer to Self Test Question #24}An example of a standard RPC mechanism is the Distributed Computing Environment (DCE) RPC. DEC, IBM, Hewlett-Packard and Microsoft all conform to the DCE RPC model. The next generation of object-oriented middleware is being built on top of the DCE RPC model.

Sockets

{Answer to Self Test Question #7}

Berkeley Sockets is a session layer Application Programming Interface (API). Sockets are built into the BSD UNIX operating system kernel. Windows sockets (WinSock) runs on computer systems that use the Microsoft TCP/IP-32 stack. WinSock is a version of the popular BSD socket implementation. {Answer to Self Test Question #8} Sockets allow applications to share the TCP/IP connection to the network. {Answer to Self Test Question #9}, {Answer to Self Test Question #25}WinSock is also a session layer API. The WinSock specification is not owned by Microsoft, and was created and developed through the cooperation of several software vendors. It was developed in both 16-bit and 32-bit versions. The 16-bit version was not included as part of Microsoft Windows 3.x. WinSock specifically addresses Internet connections. It is typically implemented as a DLL with additional programs, such as a dialer program, to initiate an Internet connection. However, WinSock does not provide the utilities, just the sockets API that the utilities access.

Transport Layer Interface

{Answer to Self Test Question #10}, {Answer to Self Test Question #26}

The transport layer interface (TLI) is a System V API that was introduced in Release 3.0 in 1986. The TLI is not a part of the UNIX kernel, and relies on library routines. TLI ensures that the transport layer will retain its independence from the session, presentation, and application layer services.

NetBIOS

{Answer to Self Test Question #11}

The NetBIOS (Network Basic Input/Output System) encountered in Microsoft environments is not a protocol, but a session layer API. NetBIOS over TCP/IP is described in RFCs 1001/2, and can use either UDP or TCP as the underlying (routable) transport protocol.

Applications that are written to the NetBIOS API generally need more information than a DNS can provide in order to communicate with remote NetBIOS applications across a router.

{Answer to Self Test Question #27}Microsoft has developed a proprietary NetBIOS name server, called the Windows Internet Name Server (WINS), which provides the capability to dynamically register NetBIOS computer names and provide NetBIOS name resolution across logical IP subnets (router hops).

A good understanding of NetBIOS is critical in any network that uses Microsoft LAN Manager or Windows NT as a network operating system.

{Answer to Self Test Question #12}It should be noted that in the Microsoft model, NetBIOS, can bind to TCP/IP, IPX, or NetBEUI. Since NetBEUI does not have a network layer, it is not a routable protocol. NetBIOS, on the other hand, can be routed if WINS (or a static LMHOSTS file) is used, and if NetBIOS is bound to TCP/IP. Using NetBIOS over IPX is difficult to implement, since routers must be configured to forward IPX broadcasts to each of the internetwork segments. NetBIOS is not a strategic API for Microsoft. Their strategic direction is Network OLE and distributed objects, which are all built on top of the DCE RPC. In the meantime, NetBIOS will be around for a long while, so all one can really do is to migrate NetBIOS applications to use TCP/IP as the underlying transport, and away from NetBEUI (NetBIOS Extended User Interface), which forces bridging due to its inability to be routed.

Detailed Protocol Structure

The TCP/IP protocol suite is a four-layer model. Even though it has four layers, it contains services that map to the entire seven layers of the OSI reference model. This is demonstrated by the incorporation of presentation- and session-type services such as TLI or RPCs.

{Answer to Self Test Question #13}Where the OSI model has the upper layers (application, presentation, and session) separated, the TCP/IP model contains them in one layer. And where the OSI model has separated the physical hardware layer from the data link framing layer, the TCP/IP model lumps them together as well. Figure 4-2 illustrates the relationships among OSI reference model layer 1-7 entities, relative to the TCP/IP protocol model.

{Answer to Self Test Question #28}

Figure 2 The TCP/IP protocol model structure

Transport Layer

{Answer to Self Test Question #29}

The transport layer is the single most important layer with respect to connectivity to other systems. Its function is to provide the reliable transport of data between two systems, regardless of the underlying networks in between. In the TCP/IP protocol suite, there are two transport layer protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Both are important and are used extensively by higher-layer applications.

Both TCP and UDP use the concepts of ports and sockets. Ports and sockets are important concepts when establishing or identifying a connection between two networked computer systems.

{Answer to Self Test Question #15}The source and destination ports are used to identify the points where upper-layer source and destination processes access TCP services. {Answer to Self Test Question #14}A socket is an IP address plus a port.

- sending socket = source IP address + source port number
- receiving socket = destination IP address + destination port number

Knowledge of which port is used by a given TCP/IP application layer service is important when designing IP access filters. Table 4-2 illustrates some of the more common TCP/IP applications and their associated port numbers.

Service | Name | Protocol | Port
DNS | Domain Name Service | TCP, UDP | 53
SMTP | Simple Mail Transport Protocol | TCP | 25
FTP-Data | File Transfer Protocol – Data | TCP | 20 & >1023
FTP | File Transfer Protocol | TCP | 21
Telnet |  | TCP | 23
NTP | Network Time Protocol | TCP, UDP | 123
NNTP | Network News Transport Protocol | TCP | 119
HTTP | Hypertext Transport Protocol | TCP | 80
X-Windows |  | TCP | 6000-6100

Table 2 Common TCP/IP Services and Transports

TCP

{Answer to Self Test Question #30} , {Answer to Self Test Question #31}

The Transmission Control Protocol is defined in RFC 793 and defines a reliable, connection-oriented full-duplex byte stream for a user process. It moves data in a continuous, unstructured byte stream. The TCP header, diagrammed in Figure 4-3, contains 11 fields plus a variable-length data field.

Figure 3 TCP header

The TCP protocol provides the following major services:

- Reliable data transfer
- Connection-oriented virtual circuit
- Buffered transfer
- Resequencing
- Multiplexing
- Efficient, full-duplex transmission
- Flow control

Flow Control

TCP identifies each octet sent (Remember that TCP is stream oriented) using a sequence number. {Answer to Self Test Question #32} The sequence number is used for purposes of acknowledgment, in the reordering of received octets, and also for the rejection of duplicates. Octets that may need to be retransmitted are contained in buffer space on the sending side until an ACK is received.

In TCP, efficient transmission over the network, and flow control between senders and receivers, is achieved by using a variable sliding window mechanism. It is important to understand a simple acknowledgment protocol before the reasoning behind sliding windows (and then variable sliding windows) becomes apparent. Figure 4-4 illustrates the basics of an acknowledgment protocol.

Figure 4 Sliding window messages

Sliding Windows

{Answer to Self Test Question #16}

A mechanism is needed to make stream transmission efficient. The general approach is to use a sliding window. The idea is to make better use of the available network bandwidth by filling up the pipe at all times. The weakness with the simple positive acknowledgment protocol in the previous section is that there is often a significant amount of latency between packets. Latency is the lag time between the initiation of the packet transfer and the completion of the packet transfer at the destination.

With sliding windows, several packets can be sent out at once. As acknowledgments come back, new packets are sent out. In this manner, more of the available bandwidth is used on an ongoing basis. A well-tuned sliding window protocol can keep the network completely saturated with packets and obtain substantially higher throughput.

It should be noted that TCP operates in full-duplex mode, so both the sender and receiver may be transferring data simultaneously over the same connection, one in each direction. The implication of this is that each end system maintains two windows per TCP connection, one for sending and the other for receiving. TCP uses a variable sliding window size as an enhancement to a fixed sliding window size. This "window advertisement" provides a dynamic method for TCP to tune itself between end systems, based on such things as availability of buffers.

{Answer to Self Test Question #33}

It should be noted that variable sliding windows solve the end-to-end flow control problem, but have no concept of the intervening network. If an intermediate node such as a router becomes congested, there is no mechanism defined that can notify TCP. If a particular TCP implementation is reactive to timeouts and retransmits, this can severely exacerbate network congestion. Instead, most TCP implementations use congestion avoidance techniques. Congestion avoidance techniques are beyond the scope of the CCIE training. It should be noted, however, that congested gateways do send ICMP source quench messages to transmitting nodes requesting them to throttle back. This is discussed in the section on ICMP. ACKs (Acknowledgments) are embedded in return packets if there are packets queued to be sent. If there are no packets queued to be sent, an explicit ACK packet is sent.

UDP

UDP is a much simpler protocol than TCP, and is useful in situations where the reliability mechanisms of TCP are not necessary.

{Answer to Self Test Question #17}UDP provides a connectionless, "unreliable" datagram service. {Answer to Self Test Question #34} The UDP header has only four fields: source port, destination port, length, and UDP checksum. The source and destination port fields serve the same functions as they do in the TCP header. The length field specifies the length of the UDP header and data, and the checksum field allows packet integrity checking. The UDP checksum is optional.

UDP Flooding

{Answer to Self Test Question #36}

UDP flooding uses the IEEE 802.1d spanning-tree algorithm to forward packets in a controlled manner. Transparent bridging is enabled on each router interface for the sole purpose of building the spanning tree. The spanning tree prevents loops by stopping a broadcast from being forwarded out an interface on which the broadcast was received. The spanning tree also prevents packet duplication by placing certain interfaces in the blocked state (so that no packets are forwarded) and other interfaces in the forwarding state (so that packets that need to be forwarded are forwarded).

To enable UDP flooding, the router must be running software that supports transparent bridging, and bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured for an interface, the interface will receive broadcasts, but the router will not forward those broadcasts and will not use that interface as a destination for sending broadcasts received on a different interface.

When configured for UDP flooding, the router uses the destination address specified by the IP broadcast-address command on the output interface to assign a destination address to a flooded UDP datagram.

{Answer to Self Test Question #35}Thus, the destination address might change as the datagram propagates through the network. The source address, however, does not change.

IP Helper Addresses

The second approach that can be used to forward UDP broadcasts is the IP helper address. This is the default setting. In order to configure this, use the ip forward-protocol UDP global configuration command. By default, the ip forward-protocol UDP command enables forwarding for the following protocol services:

- TFTP
- DNS
- Time
- NetBIOS Name Server
- NetBIOS Datagram Server
- Boot Protocol
- TACACS

{Answer to Self Test Question #37}

IP helper addresses are a form of static addressing. When using IP helper addresses, it is necessary to specify the IP helper address on every interface receiving broadcasts that need to be forwarded.

Network Layer

The network layer deals primarily with addressing. The four main protocols in the TCP/IP protocol suite at this layer, which maps to OSI Reference Model Layer 3, are:

- IP
Internet Protocol
- ICMP
Internet Control Message Protocol
- ARP
Address Resolution Protocol
- RARP
Reverse Address Resolution Protocol
- InARP
Inverse Address Resolution Protocol

IP is the protocol where network and node addresses are set. ICMP is a sub-protocol of IP, and handles the control messages at the network layer. ARP, RARP and InARP all resolve IP addresses in different ways, depending on what addressing resources the node is aware of.

Internet Protocol (IP)

The Internet Protocol is defined in RFC 791 and exists at the network layer of the OSI Reference Model.

{Answer to Self Test Question #18}IP can be thought of as a delivery mechanism that moves packets from one host to another. Because it handles the delivery, it also supplies the addressing. IP provides three major functions:

- Connectionless, unreliable delivery service
- Packet fragmentation and reassembly
- Routing functions

Connectionless, Unreliable Delivery Service

The IP protocol is said to provide a connectionless, unreliable delivery service. What this means is that as a whole, the TCP/IP protocol has been designed to operate at different levels in a hierarchy.

{Answer to Self Test Question #39} The term unreliable is used when describing IP because the IP protocol is a best-effort service, and does not guarantee delivery. Reliable delivery is handled at the transport layer by TCP. Connection-oriented activities are handled by TCP as well. The function of IP is to provide the delivery mechanism for getting packets to and from the transport layer protocols.

Packet Fragmentation and Reassembly

A further function of the IP protocol is to achieve efficiency by enforcing an upper limit to the datagram size, based on the maximum transmission unit (MTU). Since one of the major goals of network design is to hide the underlying network technology, IP chooses a convenient datagram size and then proceeds to divide larger datagrams into fragments.

{Answer to Self Test Question #40} This process is called fragmentation, and usually occurs on a router somewhere between the source and destination end points of the datagram. Fragmentation is performed in a manner whereby the fragment is sized just right to fit into a single frame for shipment over the network. The fragments are then reassembled at the final destination.

Routing Functions

The IP protocol is best known for its routing functions. Please refer to Chapter 5 for a detailed discussion of IP routing.

Address Resolution Protocol (ARP)

Computers on the same physical network can communicate only if they know each other’s physical MAC address. Local physical addresses are just that, local. There are many different physical networks, including Ethernet, Token Ring and FDDI. As a way to abstract addressing and to provide routing capability, the IP protocol is used instead of local physical addresses. With IP, a unique IP address is bound to each and every unique MAC address. The IP address abstraction is necessary for several reasons.

- It makes addressing logically independent of the physical hardware
- It allows the use of multicast addresses to selected physical MAC addresses

{Answer to Self Test Question #41} Administrators don't have to physically manage the MAC address of each NIC
- Replacing the NIC does not change the IP address, only the physical address
- It separates the routing function from the physical/data link layer
- Packets are routed based on destination network, not on destination host

The protocol used to map IP addresses to MAC addresses on broadcast networks is called the Address Resolution Protocol. ARP is a dynamic protocol that is easy to maintain, and has the added benefit of being efficient as well. ARP is a component of the physical (local) network delivery, and is not related to routing or internetworking per se. Figure 4-5 illustrates the ARP broadcast process.

Figure 5 ARP broadcast process

{Answer to Self Test Question #43}

Broadcasts are expensive, so each computer system on the network maintains a cache of all the IP address-to-MAC address mappings that it has learned. When sending to an IP address, a host will always check its ARP cache to see if it already knows the physical address it wants to send to. {Answer to Self Test Question #42} A host's ARP cache is good for a certain time period, and then times out.

Reverse Address Resolution Protocol (RARP)

{Answer to Self Test Question #38}, {Answer to Self Test Question #44}

The Reverse Address Resolution Protocol is used by systems that know their hardware MAC address but do not yet know their IP address. Examples include X-Windows Stations and diskless workstations. With RARP, a system broadcasts its hardware address. It is then up to a RARP server on the network to recognize that it has been configured with the requester’s IP address. The server then contacts the originator directly and informs it of its IP address.

Inverse Address Resolution Protocol (InARP)

{Answer to Self Test Question #45}

The Inverse ARP protocol is generally used in nonbroadcast networks such as Frame Relay. The goal is to dynamically associate a remote data-link connection identifier (DLCI) with an IP address. A router can dynamically determine a remote DLCI by querying the Frame Relay switch. Once it knows the DLCI "hardware address", it is then possible to use InARP to obtain the IP address of the remote site.

Internet Control Message Protocol (ICMP)

{Answer to Self Test Question #19}

The Internet Control Message Protocol is defined in RFC 792. ICMP messages are contained within IP datagrams. This ensures that the ICMP message will be able to find its way to the appropriate host within a group of subnets. Table 4-3 lists the ICMP message types.

Type Field | Message Type
0 | Echo Reply
3 | Destination Unreachable
4 | Source Quench
5 | Redirect
8 | Echo Request
11 | Time Exceeded for a Datagram
12 | Parameter Problem on a Datagram
13 | Timestamp Request
14 | Timestamp Reply
15 | Information Request
16 | Information Reply
17 | Address Mask Request
18 | Address Mask Reply

Table 3 ICMP Message Types

ICMP Echo Reply (Ping)

The most frequently used ICMP message is popularly implemented in a program called ping. It provides feedback to the sender on the state of IP connectivity and is often used as a debugging tool. {Answer to Self Test Question #46} Ping makes use of the ICMP ECHO REQUEST and ECHO REPLY parameters.

ICMP Redirects

{Answer to Self Test Question #47}

The ICMP redirect message is sent by a gateway to the host, and instructs the host to use a different route when the router detects that its route is not as optimal as that of another router on the same network segment. If the gateway detects a better route for the IP datagram, it will send the host a redirect message with the address of the preferred gateway. TCP/IP will then send all traffic to this new IP address for another subnet.

ICMP Source Quench

IP provides a very basic form of flow control with the ICMP source quench message. The source quench message informs the originating host that the gateway or receiving host is being overrun and can’t keep up with the traffic. The originating host then lowers the rate at which it sends datagrams to the receiving host, until it stops receiving "source quench" messages. After some time, the originating host may then gradually increase the rate at which it sends out datagrams.

Operating System Commands

In order to access and use protocol services, the implementations of the TCP/IP protocol suite on various operating systems include protocol commands. Some of the most common commands included are FTP and PING. FTP is an extremely useful utility. It provides a way to transfer files from a remote node to the local workstation.

{Answer to Self Test Question #20} PING is used to verify that a network node is accessible, by issuing an ICMP echo command to the address specified.

UNIX

One of the first operating systems to include TCP/IP, was BSD UNIX. The UNIX commands are integral to the UNIX operating system now.

Even before the popularity of the Internet, TCP/IP was ported to other operating systems. But after Internet use became widespread, TCP/IP became fairly standard. Table 4-4 lists some UNIX commands that access TCP/IP protocol suite services. Note that some of these commands require switches in order to work properly. For example, ARP will reveal the ARP table by using the ARP –A command.

Command | Usage
ARP | View the ARP table on the local computer to detect invalid entries.
HOSTNAME | Print the name of the current host. The HOSTNAME command can also be used to set the hostname on UNIX systems if logged in as root.
NETSTAT | Display protocol statistics and the state of current TCP/IP connections. The NETSTAT command is of particular use when determining the name of each physical interface that is configured on a given system.
IFCONFIG | Used to set the IP address, broadcast address and netmask. May also be used to view the configuration of an interface determined from the NETSTAT -R command.
PING | The PING command is used to verify whether IP is configured correctly, and that a remote TCP/IP system can be reached.

Table 4 UNIX TCP/IP Commands

From the Classroom

TFTP and Syslog

Anyone who works with Cisco routers will need to get comfortable with using TFTP and syslog servers. You will need to use TFTP to back up and possibly to load your configurations and software images, especially if you’re not in an environment where you have a Cisco-specific management tool such as CiscoWorks. Fortunately, setting up these services is fairly easy, even if you’re not a UNIX guru.

First of all, you need to find the software and set it up. All flavors of UNIX come with TFTP and syslog daemons, but if you’re setting up a small lab and don’t want to invest in a UNIX box, there are versions available for most Intel-based operating systems, including MS-DOS, Windows 95, and Windows NT. There’s even a TFTP server application that runs as an NLM on a Novell NetWare server. Most of the large shareware sites on the Internet have several versions available for you to choose from. Make sure your TFTP server software will support multiple clients simultaneously, if this is a requirement for you. Not all of them do.

The main requirement for the TFTP server software is that there be a network (LAN) connection with a functioning IP stack on the box where you install it, and disk space available on the box with global read and write permissions. TFTP has no provision for authenticating clients, so if you are installing a TFTP server in a production network you will have some security concerns. UNIX versions of TFTP often require that a file be created on the server before it can be written to, and you will need to follow the file-naming conventions of whatever operating system the server software is running on.

The syslog server can be a great help to you if you are using the router’s debug facility to troubleshoot a problem. Debug can be a dangerous thing for your router, because every character output from the router to the console screen requires a CPU interrupt, and some debug commands produce so much output they can use 100 percent of your router’s CPU cycles, effectively shutting it down. It takes far less processing power for the router to put each debug message in a packet and forward it to a syslog server elsewhere on the network. This is a nicer way to look at debug output as well, because it is stored in a file that can be scrolled, output to a spreadsheet, and sorted, or manipulated in whatever way you like. And if you need to look at debug output from several routers at once, this is the only way to do it.

Again, a search on the Internet can lead you to an inexpensive or free syslog application if you don’t happen to have a UNIX box. Just be aware when you are configuring the router that by default the lowest level of console messages that will be forwarded to the syslog server is "informational." You will need to change that default on the router by using the global configuration command, logging trap debug. Don’t forget to turn off the debug output to the console at the same time you enable logging to your syslog server.

—Pamela Forsyth, CCIE, CCSI, CNX

32-bit Windows

TCP/IP commands were incorporated as part of 32-bit Microsoft operating systems, such as Windows NT, Windows 95, and Windows 98. Some of the utilities available are listed in Table 4-5.

Command | Included in Windows NT | Included in Windows 95 | Included in Windows 98
Ipconfig /all | Yes | No | Yes
Winipcfg | No | Yes | Yes
FTP | Yes | Yes | Yes
arp | Yes | Yes | Yes
ping | Yes | Yes | Yes
telnet | Yes | Yes | Yes

Table 5 Windows TCP/IP Commands

The two commands IPCONFIG and WINIPCFG view the same type of IP settings information, and are similar to the UNIX IFCONFIG command used for viewing. However, IPCONFIG is a text-based command, while WINIPCFG has a graphical interface.

Certification Summary

Application layer services are user processes occurring at the OSI reference model layer 7, which correlates to the fourth layer of the TCP/IP protocol suite. CCIEs must be familiar with the usage, the transport protocol, and the port numbers for several TCP/IP applications, such as Telnet, TFTP and FTP.

The OSI reference model presentation and session layer services correlate to the application layer of the TCP/IP model somewhat. However, there are some TCP/IP services that compare to the session and presentation layer functions. These include remote-procedure calls (RPCs), sockets, transport layer interface (TLI), and NetBIOS over TCP/IP.

RPCs are akin to a connectionless session. An RPC abstracts the application layer from the network layer so that remote calls act like local ones.

Sockets are a session layer software interface that allows a wide variety of applications to use and share an Internet connection. WinSock, a version of sockets implemented in Windows, is a dynamic link library (DLL) with some supporting programs, such as a dialer program that initiates the connection.

The transport layer interface (TLI) is a UNIX service that ensures that the transport layer retains its independence from the upper-layer protocols.

NetBIOS is a session layer API that can run over TCP or UDP transport protocols. NetBIOS is not a protocol, although NetBEUI is.

The TCP/IP protocol suite is a four-layer model, but it contains the same services that the OSI reference model yields.

{Answer to Self Test Question #22}

The transport layer consists of two protocols: TCP and UDP. Upper-layer services use either one or the other, and are mapped to a port number in the protocol.

TCP (Transmission Control Protocol) is a connection-oriented, reliable transport. It uses a sliding window method of transporting data in order to control the flow of data. TCP manages the following: reliable data transfer, connection-oriented virtual circuit, buffered transfer, resequencing of data, multiplexing, full-duplex transmission and flow control.

UDP (User Datagram Protocol) is a connectionless transport layer protocol in the TCP/IP protocol stack. UDP exchanges datagrams without acknowledgments or guaranteed delivery, which requires other protocols to handle error recovery and retransmission. UDP is defined in RFC 768.

The network layer handles addressing. The protocols in this layer are: ICMP, IP, ARP, InARP, and RARP. IP assigns the network and node address to the node. ICMP sends control messages using the IP address. ARP, InARP, and RARP all handle resolving the IP address of a node with the MAC address, depending on the information that the node already has.

There are a number of operating system commands that access TCP/IP functions. These commands include FTP, PING, and ARP. The commands enable the user to access the TCP/IP network connection and control various aspects of it.

Two-Minute Drill

- For internetworks consisting of heterogeneous computer systems that need to communicate with each other, TCP/IP stands out as the common denominator across all types of platforms.
- The TCP/IP protocol suite has four major layers, which can roughly correspond to the seven layers of the OSI reference model.
- Applications function at the OSI reference model layer 7, or layer 4 of the TCP/IP suite.
- The TCP/IP protocol model does not have a formal presentation or session layer in general
- Remote procedure calls are a method for executing programs (here called "procedures") on other network nodes, such that they appear to be executing locally.
- Berkeley Sockets is a session layer Application Programming Interface (API).
- Windows sockets (WinSock) runs on computer systems that use the Microsoft TCP/IP-32 stack.
- TLI ensures that the transport layer will retain its independence from the session, presentation, and application layer services.
- The NetBIOS (Network Basic Input/Output System) encountered in Microsoft environments is not a protocol, but a session layer API.
- It should be noted that in the Microsoft model, NetBIOS, can bind to TCP/IP, IPX, or NetBEUI.
- The transport layer’s function is to provide the reliable transport of data between two systems, regardless of the underlying networks in between.
- The Transmission Control Protocol is defined in RFC 793 and defines a reliable, connection-oriented full-duplex byte stream for a user process.
- In TCP, efficient transmission over the network, and flow control between senders and receivers, is achieved by using a variable sliding window mechanism.
- A well-tuned sliding window protocol can keep the network completely saturated with packets and obtain substantially higher throughput.
- UDP provides a connectionless, "unreliable" datagram service.
- UDP flooding uses the IEEE 802.1d spanning-tree algorithm to forward packets in a controlled manner.
- The second approach that can be used to forward UDP broadcasts is the IP helper address.
- The network layer deals primarily with addressing.
- IP can be thought of as a delivery mechanism that moves packets from one host to another.
- The protocol used to map IP addresses to MAC addresses on broadcast networks is called the Address Resolution Protocol (ARP).
- The Reverse Address Resolution Protocol is used by systems that know their hardware MAC address but do not yet know their IP address.
- The Inverse ARP protocol is generally used in nonbroadcast networks such as Frame Relay. The goal is to dynamically associate a remote data-link connection identifier (DLCI) with an IP address.
- ICMP messages are contained within IP datagrams. This ensures that the ICMP message will be able to find its way to the appropriate host within a group of subnets.
- Some of the most common protocol commands included are FTP and PING.
- One of the first operating systems to include TCP/IP, was BSD UNIX.
- TCP/IP commands were incorporated as part of 32-bit Microsoft operating systems, such as Windows NT, Windows 95, and Windows 98.

Self Test

Match the following application layer services to their corresponding transport layer protocol port.

1. SMTP 1. TCP/23
2. FTP 2. TCP/25
3. TFTP 3. TCP/80
4. Telnet 4. TCP/21
5. HTTP 5. UDP/69
6. DNS 6. TCP/53

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A-2, B-4, C-5, D-1, E-3, F-6

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- Which OSI reference model layer does Telnet function at?

1. Transport
2. Network
3. Session
4. Application

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D. Telnet is an application layer service.

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- How many layers does the TCP/IP protocol suite have, compared to the OSI reference model?

1. TCP model has 4, OSI model has 6
2. TCP model has 7, OSI model has 8
3. TCP model has 4, OSI model has 7
4. TCP model has 7, OSI model has 4

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C.

The TCP/IP protocol suite has four layers, while the OSI reference model has seven.

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- The TCP/IP protocol suite has a formal session layer that includes NetBIOS, RPCs, and TLI functions

1. True
2. False

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B.

False. The TCP/IP protocol suite does not have a formal session layer.

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- What is the function of RPCs?

1. To move files from remote PCs to a local PC
2. To make remote function calls transparent, so they appear to be local
3. To initialize a program on a remote PC
4. To send a procedure that is local to a remote node for processing elsewhere

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B.

The function of RPCs is to transparently access remote procedures, making them appear local.

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- What does RPC stand for?

1. Remote personal computer
2. Reserved-programming call
3. Routed-procedure call
4. Remote-procedure call

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D.

RPC is the acronym for remote-procedure call.

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- What OSI reference model layer do sockets function at?

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data link
7. Physical

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C. Sockets function at the session layer.

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- What function do sockets perform?

1. They make remote functions appear local, transparent to the user
2. They transfer files to and from remote nodes
3. They make the transport layer independent
4. They allow multiple applications to share the same connection to the network

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D.

Sockets allow multiple applications to use the same TCP/IP connection.

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- What is WinSock?

1. A version of sockets for the Microsoft Windows platform
2. Sockets on BSD UNIX
3. A session layer API commonly considered to be its own protocol
4. A network layer service for Microsoft Windows

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A.

WinSock is a Microsoft Windows version of sockets

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- What does TLI do?

1. It makes remote functions appear local, transparent to the user
2. It transfers files to and from remote nodes
3. It makes the transport layer independent
4. It allows multiple applications to share the same connection to the network

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C.

The transport layer interface makes the transport layer independent from upper-layer services.

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- What OSI layer does NetBIOS function at?

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. DataLink
7. Physical

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C.

NetBIOS functions at the OSI model session layer.

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- Which protocols can NetBIOS bind to? (Select all that are applicable.)

1. Appletalk
2. IPX
3. IP
4. NetBEUI

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B, C, D.

NetBIOS can run over IPX, IP, and NetBEUI.

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- What layers do not exist in the TCP/IP model that are in the OSI model?

1. Application, presentation, and network
2. Presentation, session, and data link
3. Session, network, and physical
4. Presentation, data link, and physical

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B.

There are no formal presentation, session, or data link layers in the TCP/IP model.

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- What is a socket in the transport layer?

1. The socket is an IP address plus a port
2. An API that makes the transport layer independent
3. An API that allows multiple applications to share a network connection
4. A function that makes remote procedures appear to be local

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A.

A socket is an IP address plus a port

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- What is a port?

1. An API that makes the transport layer independent
2. An API that allows multiple applications to share a network connection
3. A function that makes remote procedures appear to be local
4. The point where upper-layer processes access transport layer services

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D.

A port is where upper-layer processes access the transport layer.

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- Which of the following services uses a process called windowing?

1. Reliable data transfer
2. Connection-oriented virtual circuit
3. Buffered transfer
4. Resequencing
5. Multiplexing
6. Efficient, full-duplex transmission
7. Flow control

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G.

Windowing is a form of flow control.

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17. What is UDP?

1. An API that makes the transport layer independent
2. A connectionless, unreliable transport protocol
3. An API that allows multiple applications to share a network connection
4. A function that makes remote procedures appear to be local

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B. UDP is a connectionless, unreliable transport protocol.

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18. What is IP?

1. It is the transport mechanism for upper layer services
2. It is the session layer API for making the transport layer independent
3. It is the network layer protocol that moves data from one node to another
4. It is the physical layer protocol for Internet connections

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C. IP (Internet Protocol) is the network layer protocol that moves data from one node to another.

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19. What is ICMP?

1. It is a network layer protocol that handles control messages
2. It is a network layer protocol that resolves addresses
3. It is a session layer API that makes remote procedures transparent to a user
4. It is a transport layer function for unreliable transport

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A. ICMP handles control messages at the network layer.

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20. Ping sends an ICMP echo command to an IP address in order to determine whether a network connection exists to that node.

1. True
2. False

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A. True. Ping sends an ICMP echo command to an IP address, and from the response, verifies that the address is reachable from the local host

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21. Which of the following best describes TCP/IP?

1. A static protocol
2. A proprietary protocol
3. A collection of internetworking protocols

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C. TCP/IP is a dynamically changing collection of internetworking protocols.

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22. UDP and TCP represent mechanisms used by which layer of the TCP/IP?

1. Data link layer
2. Physical layer
3. Presentation layer
4. Transport layer

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D. TCP and UDP represent transport mechanism in the TCP/IP protocol structure.

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23. RPCs provide which of the following?

1. Connection-oriented session
2. Transparency to make remote calls look local
3. Portability of applications between heterogeneous systems
4. A and C only
5. B and C only
6. A, B, and C

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E. RPCs provide transparency to make remote calls look local, and portability of applications between heterogeneous systems. RPC is a connectionless session.

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24. Distributed Computing Environment (DCE) is an example of:

1. OSI model
2. RPC implementation
3. Extreme Data Representation
4. A and B

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B. DCE is an implementation of RPC.

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25. Which statement is true of WinSock?

A. It is a MAC application

B. It represents a graphical user interface

C. It represents a network layer

5. It provides the means for sharing an Internet connection between multiple IP protocol suite utilities

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D. WinSock represents a session layer API allowing multiple applications to share an Internet connection. It is not a GUI.

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26. Which statement is true of TLI?

A. It is a layer in the OSI model

B. It is a layer in the TCP/IP model

C. It is a System V API

1. It is part of the UNIX Kernel

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C. TLI represents a System V Application Programming Interface.

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27. Which statement is true of Windows Name Server (WINS)?

A. It is a protocol

B. It provides capability for name resolution

C. It is a network layer

D. It is a proprietary name server

E. A, B, and D only

F. B and D only

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F. WINS is a Microsoft proprietary NetBIOS name server that provides capability for name resolution.

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28. ICMP, IP, ARP, and RARP of the IP protocol suite map to:

1. OSI layers 1 and 2
2. OSI layer 5
3. OSI layer 3
4. OSI layer 2

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C. ICMP, IP, ARP, and RARP all map to OSI layer 3

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29. Which layer is most important in providing reliable data exchange between two systems?

1. Physical layer
2. Data link layer
3. Session layer
4. Transport layer

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D. The transport layer is the single most important layer in assuring reliable data transfer regardless of the underlying networks in between the TCP/IP protocol suite.

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30. Which of the following does TCP provide?

1. Unreliable data stream
2. Connectionless virtual circuit
3. Flow control
4. Structured byte stream movement\

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C. TCP provides a connection-oriented virtual circuit with reliable byte stream, where the byte stream is both continuous and unstructured.

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31. Of the following, which field is not a part of the TCP header?

1. Subnet mask
2. Sequence number
3. Data offset
4. Destination port

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A. The subnet mask is not a part of the TCP header.

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32. What is the sequence number in a TCP header used for?

1. Acknowledgments
2. Reordering of the octets received
3. Rejecting duplicate octets
4. All of the above

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D. The sequence number in a TCP header is used for acknowledgments, for reordering of the octets received, and for rejecting duplicate octets.

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33. Variable sliding windows provide an explicit mechanism for notifying TCP if an intermediate node (for example, a router) becomes congested.

1. True
2. False

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B. False.

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34. Which of the following parameters is not a part of UDP header?

1. Source port
2. Urgent pointer
3. Checksum
4. Length
5. Destination port

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B. The urgent pointer is not a part of the UDP header. The UDP header has only four parameters: source port, destination port, length, and UDP checksum.

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35. When a router has been configured for UDP flooding, the source address might change, but the destination address will not change as the datagram propagates through the network.

1. True
2. False

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B. False. The destination address might change, but the source address does not change.

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36. The spanning-tree algorithm allows:

1. Forwarding of packets with no control
2. Forwarding of broadcasts to an interface which already has received the broadcast
3. Prevention of duplication of forwarding of packets
4. A and B only

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C. Spanning tree forwards packets in a controlled manner.

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37. IP helper addresses are a form of ________ addressing and require the command specification of _________ on every interface receiving broadcasts that need to be forwarded.

1. Static / IP header address
2. Dynamic / IP forward-protocol UDP
3. Dynamic / IP forward-protocol TCP

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A. IP helper addresses are a form of static addressing and require specification of IP header address on every interface receiving broadcasts that need to be forwarded.

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38. Which of the following protocols provide address resolution?

1. ICMP
2. RARP
3. IP
4. UDP
5. TCP

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B. RARP is a Reverse Address Resolution Protocol.

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39. IP is described as an unreliable mechanism because it does not guarantee delivery.

1. True
2. False

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A. True. IP is described as an unreliable mechanism because it does not guarantee delivery.

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40. What does fragmentation in TCP/IP represent?

1. Segmenting of datagrams into 53-byte packets for ATM applications
2. Division of larger datagrams into convenient size packets
3. A process that occurs on a router
4. A, B, and C
5. A and C only
6. B and C only

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F. Fragmentation is a process that occurs on a router somewhere between the source and destination. This process segments the datagram into a convenient size to fit into a single frame for transport over the network.

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41. Which of the following statements is true of ARP?

1. It makes a MAC address logically independent of the physical hardware
2. It makes it necessary for the administrator to physically manage the MAC address of each NIC
3. It integrates routing function with the physical and data link layers
4. It routes packets based on destination host, not on destination network

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A. With ARP, an administrator does not have to physically manage the MAC address of each NIC. ARP separates routing function from the physical and data link layers. ARP routes packets based on destination network, and not destination host.

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42. A host’s ARP cache is good forever once it has been created.

1. True
2. False

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B. False. The host’s ARP cache is good only for a certain time period, and then it times out.

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43. ARP is a broadcast protocol, and ARP caching is used because broadcasts are expensive.

1. True
2. False

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A. True. ARP is a broadcast protocol, and ARP caching is used because broadcasts are expensive.

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44. Reverse Address Resolution Protocol (RARP) is termed "reverse" because:

1. It is used by the system that knows its IP address but does not know its MAC address
2. It is used by the system that knows its MAC address but does not know its IP address
3. It is used by the system that knows the destination’s IP address but does not know the destination’s MAC address
4. It is used by the system that knows the destination’s MAC address but does not know the destination’s IP address

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B. RARP is used by a system to resolve its IP address.

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45. Inverse Address Resolution Protocol (InARP) is generally used by:

1. Broadcast networks
2. Nonbroadcast networks
3. Both broadcast and nonbroadcast networks

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B. Inverse Address Resolution Protocol (InARP) is generally used by nonbroadcast networks such as Frame Relay.

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46. The PING command makes use of what ICMP parameter?

1. Redirect
2. Source quench
3. Echo reply
4. Destination unreachable

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C. The PING command makes use of the echo request and echo reply parameter.

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47. ICMP Redirect is sent by:

1. A host to the gateway
2. A gateway to the host
3. A router to another router
4. A router to a network

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B. ICMP Redirect is sent by a gateway to the host. It instructs the host to use a different route when the router detects that its route is not as optimal as that of another router.

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