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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Datagram Encapsulation and Formatting

Previous Topic/Section
IP Datagram General Format
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1
2
3
Next Page
IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly
Next Topic/Section

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.



Previous Topic/Section
IP Datagram General Format
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2
3
Next Page
IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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