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- 18 THE TCP/IP PROTOCOL SUITE
Now that we have an appreciation for the evolution of the Internet and the
TCP/IP protocol suite, let us turn our attention to the structure of the
protocol suite. However, since the TCP/IP protocol suite has a layered
structure, we will ®rst examine the ISO Reference Model and the subdivision
of its second layer by the Institute of Electrical and Electronic Engineers
(IEEE) to provide a standardized frame of reference.
2.3 THE ISO REFERENCE MODEL
The International Organization for Standardization is an agency of the United
Nations headquartered in Geneva, Switzerland. The ISO is tasked with the
development of worldwide standards to facilitate the international exchange
of goods and services. The membership of the ISO consists of the national
standards organization of most countries, with over 100 countries participat-
ing in its work. One of the most notable achievements of the ISO in the ®eld of
data communications was its development of the seven-layer Open Systems
Interconnection (OSI) Reference Model. This model de®nes the communica-
tions process as a set of seven layers, with speci®c functions isolated and
associated with each layer.
Figure 2.2 illustrates the seven layers of the ISO Reference Model. Each
layer covers lower layer processes, effectively isolating them from higher layer
functions. In this way, each layer performs a set of functions necessary to
provide a set of services to the layer above it. Layer isolation permits the
characteristics of a given layer to change without impacting the remainder of
the model, provided that the supporting services remain the same. This
layering was developed as a mechanism to enable users to mix and match
OSI-conforming communications products to tailor their communications
systems to satisfy a particular networking requirement. Although OSI-
conforming communications products never gained a signi®cant degree of
acceptance, the OSI Reference Model provides a framework for comparing
Figure 2.2 The International Organization for Standardization (ISO) Open System
Interconnection (OSI) Reference Model
- 2.3 THE ISO REFERENCE MODEL 19
and contrasting the features and structure of other protocol suites. In
addition, by understanding the structure of the model and the subdivision of
its second layer by the IEEE, we can also obtain an appreciation of the
capabilities and limitations of other protocol suites as well as the manner by
which those suites support data ¯ow from source to destination.
2.3.1 Layers of the OSI Reference Model
With the exception of layers 1 and 7, each layer in the ISO Reference Model is
bounded by the layers above and below it. Layer 1, the physical layer, which
is responsible for moving bits in electrical or optical form, can be considered
to be bound below by the interconnecting medium over which transmission
¯ows. In comparison, layer 7 is the upper layer and has no upper boundary.
Within each layer is a group of functions that can be viewed as providing a set
of de®ned services to the layer that bounds it from above, resulting in layer n
using the services of layer n-1. To obtain an appreciation of the manner in
which the ISO's Reference Model operates, let us turn our attention to each of
the layers in the model.
Layer 1: the physical layer
At the lowest or most basic layer, the physical layer represents a set of rules
that speci®es the electrical, optical, and physical connection between devices
and the transmission medium. Typically, the physical layer can include the
coding method by which data is placed onto the medium as well as the
cabling interface to include the operation of different pins on the cabling
connection.
Layer 2: the data link layer
The data link layer de®nes how a device gains access to the medium speci®ed
by the physical layer as well as the data formats to include framing, error
control procedures, and other link control activities. The data format
speci®cation includes procedures employed to correct transmission errors,
thus, layer 2 becomes responsible for the reliable delivery of information.
At the data link layer information is grouped into entities referred to as
frames. As a minimum, each frame contains control information that enables
the receiver to synchronize itself to an incoming frame, addressing
information that identi®es the source and destination, a ®eld containing
the actual information being transmitted from source to destination, and a
®eld used for verifying the integrity of the data.
One important characteristic of data link protocols is the fact that they do
not have network addresses and as such are non-routable. As we will note
later in this chapter, Ethernet, Token-Ring, and FDDI represent examples of
data link protocols.
- 20 THE TCP/IP PROTOCOL SUITE
Because the development of OSI layers was originally targeted towards
wide area networking, its applicability to local area networks required a
degree of modi®cation. Under IEEE 802 standards, the data link layer was
subdivided into two sublayers: Logical Link Control (LLC) and Media Access
Control (MAC). The LLC layer is responsible for generating and interpreting
commands that control the ¯ow of data and perform recover operations in the
event of errors. In comparison, the MAC layer is responsible for providing
access to the local area network, which enables a station on the network to
transmit information. Later in this chapter we will discuss the subdivision in
additional detail.
Layer 3: the network layer
The third layer in the ISO Reference Model is the network layer. As its name
implies, this layer is responsible for arranging a logical connection through a
network to include the selection and management of a route for the ¯ow of
information between source and destination based upon the available paths
in a network. Services provided by this layer are associated with the
movement of data packets through a network, including addressing, routing,
switching, sequencing, and ¯ow control procedures. In a complex network,
the source and destination may not be directly connected by a single path,
but instead require a path to be established that consists of many subpaths.
Thus, routing of data through the network onto the correct paths is an
important feature of this layer.
Several protocols represent commonly used layer 3 protocols. Those
protocols include the X.25 packet protocol, which governs the ¯ow of
information within a packet network, Novell's Internet Packet Exchange
(IPX), and the Internet Protocol (IP).
Layer 4: the transport layer
The fourth layer in the ISO's Reference Model is the transport layer. This layer
is responsible for guaranteeing that the transfer of information occurs
correctly after a route has been established by the network layer protocol.
Thus, the primary function of this layer is to control the communications
session between nodes once a path has been established by the network
control layer. Error control, sequence checking, and other end-to-end data
reliability factors are the primary concern of this layer. In addition, to support
the transfer of different types of data between source and destination, this
layer is also responsible for multiplexing and de-multiplexing data streams
between upper layer application processes.
Although most transport layer protocols provide an end-to-end reliability
mechanism, this is an optional feature associated with this layer. Similarly,
although most transport layer protocols are connection-oriented, requiring
the destination to acknowledge its ability to receive data prior to a
transmission session being established, this is also an optional feature.
- 2.3 THE ISO REFERENCE MODEL 21
Instead of operating as a connection-oriented protocol, a transport layer
protocol can operate on what is referred to as a best-effort basis. This means
that the protocol will initiate transmission without knowing if the destination
is ready to receive data or even if it is powered on and operational. Although
this method of operation may appear awkward, the originator will set a timer
that decrements in value. If no response is received to the initial packet ¯ow
by the time the timer expires, the originator will assume that the destination
is not reachable and terminate the session. The use of a connectionless
protocol avoids the relatively long handshaking process associated with some
connection-oriented transport layer protocols. Examples of transport layer
protocols include Novell's Sequenced Packet Exchange (SPX) as well as the
Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
TCP is a connection-oriented, error-free delivery protocol. In comparison,
UDP is a connectionless, best effort protocol.
Layer 5: the session layer
The ®fth layer in the OSI Reference Model is the session layer. This layer
provides a set of rules for establishing and terminating data streams between
nodes in a network. The services that the session layer can provide include
establishing and terminating node connections, ¯ow control, dialogue
control, and end-to-end data control.
Layer 6: the presentation layer
The sixth layer in the ISO's OSI Reference Model is the presentation layer.
This layer is primarily responsible for formatting, data transformation, and
syntax-related operations. One of the primary functions of this layer that is
both visible and probably overlooked as we take it for granted is the
conversion of transmitted data at the receiver into a display format for a
receiving device. Concerning the receiving device, different presentation
layers reside on different devices, since the manner in which data is
displayed on a PC would more than likely differ from the manner in which
data is displayed on a dumb terminal. Other functions that can be performed
by the presentation layer include encryption/decryption and compression/
decompression.
Layer 7: the application layer
The seventh and top layer of the OSI Reference Model is the application layer.
This layer can be viewed as functioning as a window through which the
application gains access to all of the services provided by the seven-layer
model. Examples of functions that can be performed at the application layer
include ®le transfer, electronic mail transmission, and remote terminal
access.
- 22 THE TCP/IP PROTOCOL SUITE
While the ®rst four layers in the Reference Model are fairly well de®ned, the
functions associated with the upper three layers can vary considerably,
based upon the application, the type of data transported, and the manner in
which the attributes of the display of a device are used for the presentation of
information. As we will note later in this chapter, such popular Internet
protocols as the File Transfer Protocol (FTP), Telnet, and the HyperText
Transport Protocol (HTTP) represent a blend of layer 5 through layer 7 functions.
2.3.2 Data ¯ow
As data ¯ows within an ISO network each layer appends appropriate heading
information to frames of information ¯owing within the network while
removing the heading information added by a lower layer. In this manner,
layer n interacts with layer n-1 as data ¯ows through an ISO network.
Figure 2.3 illustrates the appending and removal of frame header
information as data ¯ows through a network constructed according to the
ISO Reference Model. Since each higher level removes the header appended
by a lower level, the frame traversing the network arrives in its original form
at its destination.
2.3.3 Layer subdivision
Prior to examining the major components of the TCP/IP protocol suite, a
discussion of layer subdivision resulting from the efforts of the Institute of
Electrical and Electronic Engineers (IEEE) is in order. The IEEE is
responsible for developing LAN standards in the USA, and its efforts are
commonly incorporated by the American National Standards Institute (ANSI)
into US standards, either as is or with slight modi®cation.
During the early development of LAN standards, the IEEE recognized that it
would be desirable to subdivide the data link layer. The result of this
subdivision was the creation of Logical Link Control (LLC) and Media Access
Control (MAC) sublayers. The MAC sublayer, which resides at the bottom of
the portion of the data link layer that was subdivided, de®nes the manner by
which a station gains access to a LAN. Examples of MAC methods include
Ethernet's Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
scheme and Token-Ring's free token acquisition method. Above the MAC
layer, which differs for each type of LAN, is the LLC layer. The LLC layer,
which is common for each IEEE network, is used for controlling the
establishment, maintenance, and termination of logical connections between
stations on a network.
Addressing
Access to an IEEE network is accomplished through the MAC layer. Frames
placed on an IEEE network include two address-related ®elds: destination
and source address. Each address normally represents a 6-byte address
burnt into read-only memory (ROM) on the network adapter card of the frame
- 2.3 THE ISO REFERENCE MODEL
Figure 2.3 Data ¯ow within an ISO Reference Model network
23
- 24 THE TCP/IP PROTOCOL SUITE
originator (source address) and the frame recipient (destination address). The
®rst three bytes of the 6-byte network adapter card address are assigned by
the IEEE to an adapter card manufacturer, and represent the manufacturer
identi®cation (ID) portion of the address. The next three bytes are used by the
adapter card manufacturer to uniquely identify each adapter card that it
manufactures. If the manufacturer is so successful that it runs out of its
allocated 3-byte sequence of numbers, it will request another manufacturer
ID from the IEEE and use that ID for producing a new series of network
adapter cards.
Figure 2.4 illustrates the general format of an IEEE Mac address. When
used as a source address, a bit composition of all binary 1s represents a
broadcast address and results in each station copying the contents of the
frame off the network. Depending upon the type of LAN, the setting of different
bits within the 6-byte source MAC address can be used to identify different
groups. Then, each workstation associated with the group identi®er would
copy the frame off the network. If the frame's destination address is neither a
broadcast nor a group address, it will only be copied off the network by the
station whose adapter address matches the destination address in the frame.
Universally vs. locally administered addresses
Two types of addresses can be associated with stations on an IEEE network:
universally administered and locally administered. When a burnt-in ROM
address is used, it is referred to as a universally administered address, as it is
uniquely assigned by the IEEE. In comparison, a second type of address
results from the effort of a LAN administrator or network manager to override
the universally administered address. This second type of MAC address
results from the creation of a batch ®le statement being used to set a locally
generated address that overrides the burnt-in ROM address. Because this
address is developed locally, it is referred to as a locally administered
address. Note that, regardless of the type of MAC address, it is a layer 2
address that is 48 bits in length. Because TCP/IP addresses are 32 bits in
length (IPv4) and represent both a network address and a host address on a
network, a translation process is required to associate a layer 3 IP address to
a layer 2 MAC address. Later in this book we will examine the address
resolution process that performs the required translation.
2.4 THE TCP/IP PROTOCOL SUITE
In the previous section we have an overview of the functions of the seven
layers in the ISO Reference Model to provide a frame of reference when
examining the TCP/IP protocol suite. In actuality, TCP/IP represents one of
the earliest developed layered protocol suites and preceded the development
of the ISO's OSI Reference Model by approximately 20 years. Although it
predates the OSI Reference Model, we can obtain an appreciation of the
protocol suite by comparing it with that model.
- 2.4 THE TCP/IP PROTOCOL SUITE 25
Figure 2.4 The IEEE MAC address format
2.4.1 Comparison with the ISO Reference Model
Similar to the ISO Reference Model, the TCP/IP protocol suite is subdivided
into distinct layers, commencing at the network layer. Although the protocol
suite does not include equivalents to the lower two layers of the ISO Reference
Model, it does provide a mechanism to translate addressing from the network
layer of the reference model to MAC addresses used by LANs at the lower
portion of the data link layer. This enables the TCP/IP protocol suite to use
the physical layer supported by different LANs.
A second key difference between the ISO Reference Model and the TCP/IP
protocol suite occurs at the top of the suite. TCP/IP applications can be
considered to represent the equivalent of layers 5 through 7 of the OSI
Reference Model. Based upon the preceding, Figure 2.5 provides a general
comparison of the TCP/IP protocol suite with the ISO Reference Model. Note
that, as previously mentioned, the TCP/IP protocol suite commences at the
equivalent of layer 3 of the ISO Reference Model. Thus, the dashed lines
surrounding Ethernet, Token-Ring, and FDDI layer 2 protocols and their
physical layers indicate that they are not actually part of the TCP/IP protocol
suite. Instead, the Address Resolution Protocol (ARP), which can be viewed as
a facility of the Internet Protocol (IP), provides the translation mechanism
that enables IP addressed packets to be correctly delivered to workstations
that use MAC addresses. In fact, the TCP/IP protocol suite can also run over
ATM, with a special type of address resolution used to resolve IP to ATM
addresses. Thus, address resolution enables the TCP/IP protocol suite to be
transported by other protocols and use the physical layer speci®ed by those
protocols.
Now that we have an appreciation for the general relationship between the
TCP/IP protocol stack and the ISO's Open System Interconnection Reference
Model, let's turn our attention to the actual layers of the protocol suite.
The network layer
The Internet Protocol (IP) represents the network layer protocol employed by
the TCP/IP protocol suite. IP packets are formed by the addition of an IP
header to the layer 4 protocol data entity, which is either the Transport
Control Protocol (TCP) or the User Datagram Protocol (UDP).
IP headers contain 32-bit source and destination addresses that are
normally subdivided to denote a network address and host address on the
- 26 THE TCP/IP PROTOCOL SUITE
Figure 2.5 Comparing the TCP/IP Protocol Suite with the ISO Reference Model
network. In actuality, the host address is really an interface on the network,
since a host can have multiple interfaces, with each having a distinct
address. However, over the years the terms host address and interface
address have been used synonymously Ð although this is not technically
correct. In Chapter 3 we will examine the IP header in detail.
ICMP
The Internet Control Message Protocol (ICMP) represents a diagnostic testing
and error reporting mechanism that enables devices to generate various
types of status and error reporting messages. Two of the more popularly
employed ICMP messages are the Echo Request and Echo Response packets
generated by the Ping application.
Although Figure 2.5 indicates that ICMP is a layer 3 protocol, from a
technical perspective an ICMP message is formed by the addition of an IP
header to an ICMP message with the Type ®eld within the IP header set to
indicate it is transporting an ICMP message. When we examine IP in Chapter
3, we will also turn our attention to the Internet Message Protocol.
The transport layer
The designers of the TCP/IP protocol suite recognized that two different types
of data delivery transport protocols would be required. This resulted in two
transport protocols supported by the protocol suite.
TCP TCP is a reliable, connection-oriented protocol used to transport appli-
cations that require reliable delivery and for which actual data should not be
- 2.4 THE TCP/IP PROTOCOL SUITE 27
exchanged until a session is established. From Figure 2.5 you will note that FTP,
Telnet, SMTP, and HT TP are transported by TCP.
Because TCP is a connection-oriented protocol, this means that actual data
will not be transferred until a connection is established. While this makes
sense when you are transmitting a ®le or Web pages, it also delays actual
data transfer.
UDP A second transport protocol supported by the TCP/IP protocol suite is
UDP. UDP represents a connectionless protocol that operates on a best e¡ort
basis. This means that instead of waiting for con¢rmation that a destination is
available, UDP will commence actual data transfer, leaving it to the application
to determine if a response was received. Examples of applications that use UDP
include SNMP, NFS, and BOOTP.
The use of UDP and TCP results in the pre®x of an appropriate header to
application data. When TCP is used as the transport layer protocol, the TCP
header and application data are referred to as a TCP segment. When UDP is
used as the transport layer protocol, the UDP header and application data
transported by UDP is referred to as a UDP datagram.
Port numbers BecauseTCP and UDP were designed to transport multiple types
of application data between a source and the same or di¡erent destinations, a
mechanism was needed to distinguish one type of application from another.
This mechanism is obtained by port number ¢elds contained in TCP and UDP
headers and explains how a Web server can also support FTP and other appli-
cations. In Chapter 4 we will turn our attention to the composition of TCP/IP
transport protocol headers and the use of di¡erent port numbers.
2.4.3 Application data delivery
In concluding this chapter we will examine the use of TCP/IP and LAN
headers to facilitate the delivery of application data from a host on one
Figure 2.6 LAN delivery of TCP/IP application data
- 28 THE TCP/IP PROTOCOL SUITE
network to a host on another network. Figure 2.6 illustrates the manner by
which a LAN frame containing TCP/IP application data is formed. The LAN
frame header uses a MAC destination address to direct the frame to a router.
The router removes the LAN header and trailer and uses a Wide Area Network
(WAN) protocol to transport the IP datagram. At the destination network
another router receives the inbound packet, removes the WAN header and
trailer, and encapsulates the IP datagram into a LAN frame for delivery to the
appropriate IP address. However, since LAN frames use MAC addresses while
TCP/IP applications use IP addresses, the router will either check its memory
to determine if it previously discovered the MAC address associated with the
destination IP address or use the Address Resolution Protocol (ARP) to
discover the MAC address. Once the destination MAC address is known, the
router can complete the formation of the LAN frame and transmit it onto the
network for delivery to the appropriate device.
- Managing TCP/IP Networks: Techniques, Tools and
Security Considerations. Gilbert Held
Copyright & 2000 John Wiley & Sons Ltd
Print ISBN 0-471-80003-1 Online ISBN 0-470-84156-7
3
THE INTERNET
PROTOCOL
In this chapter we continue to acquire a foundation of knowledge concerning
the TCP/IP protocol suite by focusing attention upon the network layer in the
suite. The Internet Protocol (IP) represents both the network layer protocol in
the TCP/IP protocol suite as well as the data delivery mechanism that
enables packets to be routed from source to destination.
We will ®rst examine the composition of the ®elds within the IP header. This
will include a detailed examination of IP addressing, since many network-
related problems can be traced to this area. Because Internet Control
Message Protocol (ICMP) messages are transported via IP, we will also
examine the ICMP in this chapter. Once this has been accomplished, we will
conclude this chapter by turning our attention to the evolving replacement of
the present version of the IP, IPv4. That replacement is IPv6, which is
sometimes referred to as the next generation IP or IPng.
3.1 THE IPv4 HEADER
As noted above, the current version of the IP is version 4. Therefore, we will
commence our examination of the network layer of the TCP/IP protocol suite
by turning our attention to the IPv4 header.
The ®elds in the IPv4 header are illustrated in Figure 3.1. In examining that
illustration note that the header contains a minimum of 20 octets of data.
Also note that the width of each ®eld is shown in Figure 3.1 with respect to a
32-bit word.
In this chapter and succeeding chapters we will use the term octet to
reference the width of different header ®elds. The term octet was employed by
standards organizations to explicitly reference 8 bits operated upon as an
entity at a time when computers were manufactured with different numbers
of bits per byte. To alleviate potential confusion when referencing a group of 8
bits, standards organizations turned to the term octet. Today essentially all
computers use 8-bit bytes, and the terms byte and octet are commonly used
synonymously. To obtain an appreciation for the functions performed by the
- 30 THE INTERNET PROTOCOL
Figure 3.1 The IPv4 header
IPv4 header, let us turn our attention to reviewing the functions of each of the
®elds in the header.
3.1.1 Vers ®eld
The Vers ®eld consists of four bits that identify the version of the IP used to
create the datagram. The current version of the IP is 4 and the next
generation of the IP is assigned version number 6. As we will note later in this
chapter, the Vers ®eld retains its meaning in both the IPv4 and IPv6 headers.
3.1.2 Hlen and Total Length ®elds
The second and fourth ®elds in the IPv4 header indicate the length of the
header and the total length of the datagram, respectively. The Hlen ®eld
indicates the length of the IPv4 header in 32-bit words. In comparison, the
Total Length ®eld indicates the total length of the datagram to include its
header and higher layer information, such as a following TCP or UDP header
and application data following either of those headers. Because the Total
Length ®eld consists of 16 bits, an IP datagram can be up to 216, or 65 535
octets in length.
3.1.3 Type of Service ®eld
The Type of Service (TOS) ®eld is 1 octet or 8 bits in length. The purpose of
this ®eld is to denote the importance of the datagram (precedence), delay,
throughput, and reliability requested by the originator.
Figure 3.2 illustrates the assignment of bit positions within the TOS ®eld.
Because the TOS ®eld provides a mechanism to de®ne priorities for the
routing of IP datagrams, it would appear that the TOS ®eld could be used to
provide a Quality of Service (QoS) for IP. Applications can set the appropriate
values in the TOS ®eld to indicate the type of routing path they would like.
For example, a ®le transfer would probably request normal delay, high
throughput, and normal reliability. In comparison, a real time video
- 3.1 THE IPv4 HEADER 31
Figure 3.2 The Type of Service ®eld
application would probably select low delay, high throughput, and high
reliability. While this concept appears to provide a QoS, this is not the case,
as it does not provide a mechanism to reserve bandwidth. For example, 10
stations, each requiring 512 Kbps, could all de®ne an immediate priority for
¯owing through a router connected on a T1 circuit operating at 1.544 Mbps.
Another problem associated with the TOS ®eld is the fact that many routers
ignore its settings. This is due to the fact that, to support the TOS ®eld, a
router would have to construct and maintain multiple routing tables, which
in the era of relatively slow processors when the Internet evolved was not an
attractive option with router manufacturers. Thus, although this ®eld
provides a precedence de®nition capability, its use on a public network can
be limited. Recognizing this limitation, plans were being developed to reuse
the TOS ®eld as a mechanism to differentiate services requested when a data
stream enters a network. This action resulted in a proposal to rename the
TOS byte as a Diff Service ®eld, and an RFC was being developed to de®ne its
use when this book was written.
3.1.4 Identi®cation ®eld
The Identi®cation ®eld is two octets or 16 bits in length. This ®eld is used to
identify each fragmented datagram and is one of three ®elds that govern
fragmentation. The other two ®elds that govern fragmentation are the Flags
®eld and the Fragmentation Offset ®eld.
IP fragmentation results when data ¯ow between networks encounters
different size maximum transmission units (MTUs). The MTU is commonly
set when a device driver initializes an interface and represents the payload
portion of a frame, i.e., the frame length less frame overhead. Most protocol
stacks support MTUs up to 64KÀ1 octets (65 535). Another MTU is a per
route MTU, which represents the MTU that can be used without causing
fragmentation from source to destination. Per route MTUs are usually
- 32 THE INTERNET PROTOCOL
Table 3.1 Flag ®eld bit values
Bit 0: Reserved (set to 0)
Bit 1: 0 = may fragment, 1 = don't fragment
Bit 2: 0 = last fragment, 1 = more fragment(s) follow
maintained as a value in a host's routing table and are set either by manual
con®guration or via a discovery process. When a route has interfaces with
different MTUs and a large datagram must be transferred via an interface
with a smaller MTU, the routing entity will either fragment the packet or drop
it. As we will note in the next section, if the DON'T_FRAGMENT bit is set in
the ¯ag ®eld the router will drop the datagram. This will result in the router
generating an ICMP `Destination Unreachable±Fragmentation Needed'
message to the originator, which will cause the MTU discovery algorithm to
select a smaller MTU for the path and subsequent transmissions.
3.1.5 Flags ®eld
This 3-bit ®eld indicates how fragmentation will occur. Bit 0 is reserved and
set to zero, while the values of bits 1 and 2 de®ne whether or not
fragmentation can occur and if the present fragment is the last fragment or
if one or more fragments follow. Table 3.1 lists the values associated with the
three bits in the Flags ®eld.
3.1.6 Fragment Offset ®eld
The third ®eld in the IPv4 header that is involved with fragmentation is the
Fragment Offset ®eld. This ®eld is 13 bits in length and indicates where the
fragment belongs in the complete message. The actual value placed in this
®eld is an integer that corresponds to a unit of 8 octets and provides an offset
in 64-bit units.
IP fragmentation places the burden of effort upon the receiving station and
the routing entity. When a station receives an IP fragment, it must fully
reassemble the complete IP datagram prior to being able to extract the TCP
segment, resulting in a requirement for additional buffer memory and CPU
processing power at the receiver. In doing so it use the values in the Fragment
Offset ®eld in each datagram fragment to correctly reassemble the complete
datagram. Because the dropping of any fragment in the original datagram
requires the original datagram to be present, most vendor TCP/IP protocol
stacks set the DON'T_FRAGMENT bit in the Flag ®eld. As mentioned above,
setting that bit causes oversized IP datagrams to be dropped and results in an
ICMP `Destination Unreachable±Fragmentation Needed' message trans-
mitted to the originator. This action results in the MTU discovery algorithm
selecting a smaller MTU for the path and using that MTU for subsequent
transmissions.
- 3.1 THE IPv4 HEADER 33
3.1.7 Time-to-Live ®eld
The Time-to-Live (TTL) ®eld is one octet in length. This ®eld contains a value
that represents the maximum amount of time a datagram can live. The use of
this ®eld prevents a mis-addressed or mis-routed datagram from endlessly
wandering the Internet or a private IP network.
The value placed in the TTL ®eld can represent router hops or seconds,
with a maximum value for either being 255. Because an exact time is dif®cult
to measure and requires synchronized clocks, this ®eld is primarily used as a
hop count ®eld. That is, routers decrement the value in the ®eld each time a
datagram ¯ows between networks. When the value of this ®eld reaches zero,
the datagram is sent to the great bit bucket in the sky. The current
recommended default time-to-live value for IP is 64.
3.1.8 Protocol ®eld
The purpose of the Protocol ®eld is to identify the higher layer protocol being
transported within an IP datagram. By examining the value of this ®eld,
networking devices can determine if they have to look further into the
datagram or should simply forward the datagram towards its destination. For
example, a router that receives an IP datagram whose Protocol ®eld value is 6
and which indicates the higher layer protocol is TCP would simply forward
the datagram towards its destination.
The 8-bit positions in the Protocol ®eld enables up to 256 protocols to be
uniquely de®ned. Table 3.2 lists the current assignment of Internet protocol
numbers. Although TCP and UDP by far represent the vast majority of upper
layer protocol transmission, other protocols can also be transported that
govern the operation of networks, such as the Exterior Gateway Protocol
(EGP) and Interior Gateway Protocol (IGP) that govern the interconnection of
autonomous networks. In examining the entries in Table 3.2 note that a large
block of numbers are currently unassigned. Also note that the evolving IPv6
uses a Next Header ®eld in place of the Protocol ®eld but uses the values
contained in the table.
3.1.9 Checksum ®eld
The tenth ®eld in the IPv4 header is the Checksum ®eld. This 16-bit or 2-octet
®eld protects the header and is also referred to as the Header Checksum ®eld.
3.1.10 Source and Destination address ®elds
Both the Source and Destination address ®elds are 32 bits in length. Each ®eld
contains an address that normally represents both a network address and a
host address on the network. Because it is extremely important to understand
IP addressing, this topic will be covered in detail in Section 3.2 below.
- 34 THE INTERNET PROTOCOL
Table 3.2 Assigned Internet Protocol numbers
Decimal Keyword Protocol
0 HOPOPT IPv6 Hop-by-Hop Option
1 ICMP Internet Control Message
2 IGMP Internet Group Management
3 GGP Gateway-to-Gateway
4 IP IP in IP (encapsulation)
5 ST Stream
6 TCP Transmission Control Protocol
7 CBT CBT
8 EGP Exterior Gateway Protocol
9 IGP any private interior gateway (used by Cisco for their IGRP)
10 BBN-RCC-MON BBN RCC Monitoring
11 NVP-II Network Voice Protocol Version 2
12 PUP PUP
13 ARGUS ARGUS
14 EMCON EMCON
15 XNET Cross Net Debugger
16 CHAOS Chaos
17 UDP User Datagram
18 MUX Multiplexing
19 DCN-MEAS DCN Measurement Subsystems
20 HMP Host Monitoring
21 PRM Packet Radio Measurement
22 XNS-IDP XEROX NS IDP
23 TRUNK-1 Trunk-1
24 TRUNK-2 Trunk-2
25 LEAF-1 Leaf-1
26 LEAF-2 Leaf-2
27 RDP Reliable Data Protocol
28 IRTP Internet Reliable Transaction
29 ISO-TP4 ISO Transport Protocol class 4
30 NETBLT Bulk Data Transfer Protocol
31 MFE-NSP MFE Network Services Protocol
32 MERIT-INP MERIT Internodal Protocol
33 SEP Sequential Exchange Protocol
34 3PC Third Party Connect Protocol
35 IDPR Inter-Domain Policy Routing Protocol
36 XTP XTP
37 DDP Datagram Delivery Protocol
38 IDPR-CMTP IDPR Control Message Transport Protocol
39 TP++ TP++ Transport Protocol
40 IL IL Transport Protocol
41 IPv6 Ipv6
42 SDRP Source Demand Routing Protocol
43 IPv6-Route Routing Header for IPv6
44 IPv6-Frag Fragment Header for IPv6
45 IDRP Inter-Domain Routing Protocol
46 RSVP Reservation Protocol
(Continued )
- 3.1 THE IPv4 HEADER 35
Table 3.2 Assigned Internet Protocol numbers (Continued )
Decimal Keyword Protocol
47 GRE General Routing Encapsulation
48 MHRP Mobile Host Routing Protocol
49 BNA BNA
50 ESP Encap Security Payload for IPv6
51 AH Authentication Header for IPv6
52 I-NLSP Integrated Net Layer Security
53 SWIPE IP with Encryption
54 NARP NBMA Address Resolution Protocol
55 MOBILE IP Mobility
56 TLSP Transport Layer Security Protocol (using Kryptonet key
management)
57 SKIP SKIP
58 IPv6-ICMP ICMP for IPv6
59 IPv6-NoNxt No Next Header for IPv6
60 IPv6-Opts Destination Options for IPv6
61 any host internal protocol
62 CFTP CFTP
63 any local network
64 SAT-EXPAK SATNET and Backroom EXPAK
65 KRYPTOLAN Kryptolan
66 RVD MIT Remote Virtual Disk Protocol
67 IPPC Internet Pluribus Packet Core
68 any distributed ®le system
69 SAT-MON SATNET Monitoring
70 VISA VISA Protocol
71 IPCV Internet Packet Core Utility
72 CPNX Computer Protocol Network Executive
73 CPHB computer Protocol Heart Beat
74 WSN Wang Span Network
75 PVP Packet Video Protocol
76 BR-SAT-MON Backroom SATNET Monitoring
77 SUN-ND SUN ND PROTOCOL±Temporary
78 WB-MON WIDEBAND Monitoring
79 WB-EXPAK WIDEBAND EXPAK
80 ISO-IP ISO Internet Protocol
81 VMTP VMTP
82 SECURE-VMTP SECURE-VMPT
83 VINES VINES
84 TTP TTP
85 NSFNET-IGP NSFNET-IGP
86 DGP Dissimilar Gateway Protocol
87 TCF TCF
88 EIGRP EIGRP
89 OSPFIGP OSPFIGP
90 Sprite-RPC Sprite RPC Protocol
91 LARP Locus Address Resolution Protocol
92 MTP Multicast Transport Protocol
(Continued )
- 36 THE INTERNET PROTOCOL
Table 3.2 Assigned Internet Protocol numbers (Continued )
Decimal Keyword Protocol
93 AX.25 AX.25 Frames
94 IPIP IP-within-IP Encapsulation Protocol
95 MICP Mobile Internetworking Control Protocol
96 SCC-SP Semaphore Communications Sec. Protocol
97 ETHERIP Ethernet-within-IP Encapsulation
98 ENCAP Encapsulation Header
99 any private encryption scheme
100 GMTP GMTP
101 IFMP Ipsilon Flow Management Protocol
102 PNNI PNNI over IP
103 PIM Protocol Independent Multicast
104 ARIS ARIS
105 SCPS SCPS
106 QNX QNX
107 A/N Active Networks
108 IPPCP IP Payload Compression Protocol
109 SNP Sitara Networks Protocol
110 Compaq-Peer Compaq Peer Protocol
111 IPX-in-IP IPX in IP
112 VRRP Virtual Router Redundancy Protocol
113 PGM PGM Reliable Transport protocol
114 any 0-hop protocol
115 L2TP Layer 2 Tunneling Protocol
116 DDX D-II Data Exchange (DDX)
117±254 Unassigned
255 Reserved
3.1.11 Options and Padding ®elds
The IP includes a provision for adding optional header ®elds. Such ®elds are
identi®ed by a value greater than zero in the ®eld. Table 3.3 indicates IP
Option ®eld values based upon the manner by which the Option ®eld is
subdivided. That subdivision includes a 1-bit copy ¯ag, a 2-bit class ®eld,
and a 5-bit option number. The value column in Table 3.3 indicates the value
of the 8-bit ®eld. IP options are commonly referred to by this value.
Options whose values are 0 and 1 are exactly 1 octet long, which is their
Type ®eld. All other options have their 1-octet Type ®eld followed by a 1-octet
length ®eld followed by one or more octets of option data. The optional
padding occurs when it becomes necessary to expand the header to fall on a
32-bit word boundary.
3.2 IP ADDRESSING
IP addressing provides the mechanism that enables packets to be routed
between networks as well as to be delivered to an appropriate host on a
- 3.2 IP ADDRESSING 37
Table 3.3 IP Option ®eld values
Copy Class Number Value Name
0 0 0 0 EOOL: End of Options List
0 0 1 1 NOP: No Operation
1 0 2 130 SEC: Security
1 0 3 131 LSR: Loose Source Route
0 2 4 68 TS: Time Stamp
1 0 5 133 E-SEC: Extended Security
1 0 6 134 CIPSO: Commercial Security
0 0 7 7 RR: Record Route
1 0 8 136 SID: Stream ID
1 0 9 137 SSR: Strict Source Route
0 0 10 10 ZSU: Experimental Measurement
0 0 11 11 MTUP: MTU Probe
0 0 12 12 TRUR: MTU Reply
1 2 13 205 FINN: Experimental Flow Control
1 0 14 142 VISA: Experimental Access Control
0 0 15 15 ENCODE
1 0 16 144 IMITD: IMI Traf®c Descriptor
1 0 17 145 EIP:
0 2 18 82 TR: Traceroute
1 0 19 147 ADDEXT: Address Extension
1 0 20 148 RTRALT: Router Alert
1 0 21 149 SDB: Selective Directed Broadcast
1 0 22 150 NSAPA: NSAP Addresses
destination network. As noted earlier in this chapter, there are two
versions of the IP. The current version in IPv4, while the next generation
IP, which is currently operated on an experimental portion of the Internet,
is IPv6. Because there are signi®cant differences in the method of
addressing used by each version of IP, we will cover both versions in
this section. First, we will focus our attention upon the 32-bit addressing
scheme employed under IPv4. Once we have obtained an appreciation for
the manner by which IPv4 addresses are formed and used, we will then
turn our attention to IPv6.
3.2.1 Overview
IPv4 uses 32-bit IP addresses to identify distinct device interfaces, such as
interfaces that connect routers, workstations, and gateways to networks, as
well as to route data to those devices. Each device interface in an IP network
must be assigned a unique IP address to enable it to receive communications
addressed to the interface. Normally workstations have a single interface in
the form of a LAN connection, which would be assigned an IP address.
However, routers typically have more than one interface, and some high
performance servers may have two network connections. In such instances
each device interface would have a separate IP address.
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