Certification Objectives *

Remote Procedure Calls *

Sockets *

Transport Layer Interface *

NetBIOS *

Transport Layer *

TCP *

Flow Control *

Sliding Windows *

UDP *

UDP Flooding *

IP Helper Addresses *

Internet Protocol (IP) *

Connectionless, Unreliable Delivery Service *

Packet Fragmentation and Reassembly *

Routing Functions *

Address Resolution Protocol (ARP) *

Reverse Address Resolution Protocol (RARP) *

Inverse Address Resolution Protocol (InARP) *

Internet Control Message Protocol (ICMP) *

ICMP Echo Reply (Ping) *

ICMP Redirects *

ICMP Source Quench *

UNIX *

From the Classroom *

TFTP and Syslog *

32-bit Windows *

• 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.

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}

{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 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.

{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.

{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.

{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.

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 #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.

{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.

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.

{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.

{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 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.

{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).

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.

{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.

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:

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.

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.

• 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

{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.

{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.

{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.

{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.

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.

{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.

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.

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.

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.

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.

{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.

1. 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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2. 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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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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16. 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

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