ROUTING AND SWITCHING
Routing First-Step: IP header format
William R. Parkhurst
10.07.2004
Rating: -4.00- (out of 5)
|
The following is the fourth installment of a multi-part series on the fundamentals of routing. Each tip is excerpted from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.
[IMAGE]About the book
[IMAGE]
Routing First-Step explains the basics of Internet routing in language all of us can understand. This book takes you on a guided tour of routing, starting with systems you are familiar with: the postal system, the telephone system, and the interstate highway system. From there, you'll learn routing simply and easily. Whether you are looking to take your first step into a career in networking or are interested only in gaining knowledge of the technology, this book is for you!
Author William R. Parkhurst, Ph.D., CCIE, manages the CCIE Development group at Cisco Systems. The CCIE Development group is responsible for all new CCIE written qualification and laboratory exams. Prior to joining the CCIE team, Bill was a Consulting Systems Engineer supporting the Sprint Operation. He first became associated with Cisco Systems while a Professor of Electrical and Computer Engineering at Wichita State University. In conjunction with Cisco Systems, WSU established the first CCIE Preparation Laboratory.
IP header format
Unlike the post office, a router or computer cannot determine the size of a package
without additional information. A person can look at a letter or box and determine
how big it is, but a router cannot. Therefore, additional information is
required at the IP layer, in addit...
To continue reading for free, register below or login
To read more you must become a member of SearchNetworking.com
ion to the source and destination IP addresses. Figure
3-12 is a logical representation of the information that is used at the IP layer to
enable the delivery of electronic data. This information is called a header, and is
analogous to the addressing information on an envelope. A header contains the
information required to route data on the Internet, and has the same format regardless
of the type of data being sent. This is the same for an envelope where the
address format is the same regardless of the type of letter being sent.
Figure 3-12 - IP Header Format
[IMAGE]
The fields in the IP header and their descriptions are
Version - A 4-bit field that identifies the IP version being used. The current
version is 4, and this version is referred to as IPv4.
Length - A 4-bit field containing the length of the IP header in 32-bit increments.
The minimum length of an IP header is 20 bytes, or five 32-bit increments. The
maximum length of an IP header is 24 bytes, or six 32-bit increments.
Therefore, the header length field should contain either 5 or 6.
Type of Service (ToS) - The 8-bit ToS uses 3 bits for IP Precedence, 4 bits
for ToS with the last bit not being used. The 4-bit ToS field, although
defined, has never been used.
IP Precedence - A 3-bit field used to identify the level of service a packet
receives in the network.
Differentiated Services Code Point (DSCP) - A 6-bit field used to identify
the level of service a packet receives in the network. DSCP is a 3-bit expansion
of IP precedence with the elimination of the ToS bits.
Total Length - Specifies the length of the IP packet that includes the IP
header and the user data. The length field is 2 bytes, so the maximum size of
an IP packet is 216 – 1 or 65,535 bytes.
Identifier, Flags, and Fragment Offset - As an IP packet moves through
the Internet, it might need to cross a route that cannot handle the size of the
packet. The packet will be divided, or fragmented, into smaller packets and
reassembled later. These fields are used to fragment and reassemble packets.
Time to Live (TTL) - It is possible for an IP packet to roam aimlessly
around the Internet. If there is a routing problem or a routing loop, then you
don't want packets to be forwarded forever. A routing loop is when a packet
is continually routed through the same routers over and over. The TTL field
is initially set to a number and decremented by every router that is passed
through. When TTL reaches 0 the packet is discarded.
Protocol - In the layered protocol model, the layer that determines which
application the data is from or which application the data is for is indicated
using the Protocol field. This field does not identify the application, but identifies
a protocol that sits above the IP layer that is used for application identification.
Header Checksum - A value calculated based on the contents of the IP
header. Used to determine if any errors have been introduced during transmission.
Source IP Address - 32-bit IP address of the sender.
Destination IP Address - 32-bit IP address of the intended recipient.
Options and Padding - A field that varies in length from 0 to a multiple of
32-bits. If the option values are not a multiple of 32-bits, 0s are added or
padded to ensure this field contains a multiple of 32 bits.
The IP Precedence field can be used to prioritize IP traffic. (See Table 3-9.) This is
the same as the postal system having different classes of mail such as priority,
overnight, and 2-day delivery. Routers can choose to use this field to give preferential
treatment to certain types of IP traffic.
Table 3-9 - IP Precedence Values
[IMAGE]
[IMAGE][IMAGE]
[IMAGE]
[IMAGE]
The ToS bits were originally designed to influence the delivery of data based on
delay, throughput, reliability and cost. (See Table 3-10.) They are usually not used
and are therefore set to zero.
Table 3-10 - Type of Service Values
[IMAGE]
[IMAGE][IMAGE]
[IMAGE]
[IMAGE]
The IP Precedence field can have 8 or 23 possible values. Routers use two of these
values, 6 and 7, for routing protocol traffic. That leaves six values that can be used
to prioritize user traffic. Because the ToS bits are typically not used, the IP Precedence
field can be extended from 3 to 6 bits by using 3 bits from the ToS field.
(See Figure 3-13.)
Figure 3-13 - IP Header Type of Service (ToS) Field
[IMAGE]
[IMAGE][IMAGE]
[IMAGE]
[IMAGE]
This new field is called the Differentiated Services Code Point (DSCP). That gives
you 64 or 26 possible values that can be used to prioritize traffic. Although there
are 64 possible DSCP values, only 14 are used typically. (See Table 3-11 and the
explanation that follows.)
Table 3-11 Differentiated Services Code Point Values
[IMAGE]
[IMAGE][IMAGE]
[IMAGE]
[IMAGE]
Notice that the first 3 bits of the DSCP value are the 3 bits from the IP precedence.
An IP precedence of 000 maps into a DSCP value of 000 000, and both represent
best effort delivery. An IP precedence of 101 (Critical) maps into a DSCP value of
101 110 (High Priority or Expedited Forwarding). The remaining 4 IP precedence values are each mapped into 3 DSCP values. The additional 3-bit portion is used
to identify a drop probability within one of the four assured forwarding (AF)
classes.
This discussion of the contents of the IP header is meant as an overview. If you are
interested in learning more details regarding the IP header, refer to the references
at the end of this chapter. The important concept to take away from this discussion
is that the IP header contains the source and destination IP addresses. Routers use
the destination IP address to determine a route; therefore, the IP layer in the layered
model is the routing layer.
At this point, we could stop our discussion of the layered protocol model. This
book is about routing, and routing is the second or third layer depending on which
model is used. A router does not care what application sent the data, or how the
application is going to receive the data. The job of the router is to get the packet to
the proper destination. It is then the responsibility of the destination host to
deliver the data to the application. The incomplete layered model in Figure 3-8 is
sufficient for the remainder of this book. But, to be complete, let's go ahead and
finish the model.
All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.
|
|
|
|
|
|
