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IP Datagram Options and Option Format
(Page 2 of 3)
IP Options
Table 59
lists the most common IPv4 options, showing the option class, option
number and length for each (a length of 1 indicating an option that
consists of only an Option Type field), and providing a brief description
of how each is used.
Table 59: Internet Protocol Version 4 (IPv4) Options
Option
Class
|
Option
Number
|
Length
(bytes)
|
Option
Name
|
Description
|
0
|
0
|
1
|
End Of Options
List
|
An option containing just a single
zero byte, used to mark the end of a list of options.
|
0
|
1
|
1
|
No
Operation
|
A dummy
option used as internal padding to align certain options
on a 32-bit boundary when required.
|
0
|
2
|
11
|
Security
|
An option provided for the military
to indicate the security classification of IP datagrams.
|
0
|
3
|
Variable
|
Loose
Source Route
|
One of two
options for source routing of IP datagrams. See below for an explanation.
|
0
|
7
|
Variable
|
Record Route
|
This option allows the route
used by a datagram to be recorded within the header for the datagram
itself. If a source device sends a datagram with this option in it,
each router that handles the datagram adds its IP address
to this option. The recipient can then extract the list of IP addresses
to see the route taken by the datagram.
Note that the length of this option is set by the originating device.
It cannot be enlarged as the datagram is routed, and if it fills
up before it arrives at its destination, only a partial route
will be recorded.
|
0
|
9
|
Variable
|
Strict
Source Route
|
One of two
options for source routing of IP datagrams. See below for an explanation.
|
2
|
4
|
Variable
|
Timestamp
|
This option is similar to the
Record Route option. However, instead of each device that handles
the datagram inserting its IP address into the option, it puts in a
timestamp, so the recipient can see how long it took for the datagram
to travel between routers.
As with the Record Route option, the length of this option is
set by the originating device and cannot be enlarged by intermediate
devices.
|
2
|
18
|
12
|
Traceroute
|
Used in the
enhanced implementation of the traceroute
utility, as described in RFC 1393. Also
see the
topic on the ICMP Traceroute messages.
|
Key Concept: Each IPv4 datagram has a 20-byte mandatory header, and may also include one or more options. Each option has its own field format, and most are variable in size. |
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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