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The TCP/IP Guide

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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 6 (IPv6) / IP Next Generation (IPng)
                     9  IPv6 Datagram Encapsulation and Formatting

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IPv6 Datagram Main Header Format
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 IPv6 Datagram Options
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IPv6 Datagram Extension Headers
(Page 5 of 6)

IPv6 Fragment Extension Header

The Fragment extension header is included in fragmented datagrams to provide the information necessary to allow the fragments to be reassembled. Its format can be found in Table 72 and Figure 108.


Table 72: IPv6 Fragment Extension Header Format

Field Name

Size (bytes)

Description

Next Header

1

Next Header: Contains the protocol number of the next header after the Fragment header. Used to link headers together as described above.

Reserved

1

Reserved: Not used; set to zeroes.

Fragment Offset

13/8 (13 bits)

Fragment Offset: Specifies the offset, or position, in the overall message where the data in this fragment goes. It is specified in units of 8 bytes (64 bits) and used in a manner very similar to the field of the same name in the IPv4 header.

Res

1/4 (2 bits)

Reserved: Not used; set to zeroes.

M Flag

1/8 (1 bit)

More Fragments Flag: Same as the flag of the same name in the IPv4 header—when set to 0, indicates the last fragment in a message; when set to 1, indicates that more fragments are yet to come in the fragmented message.

Identification

4

Identification: Same as the field of the same name in the IPv4 header, but expanded to 32 bits. It contains a specific value that is common to each of the fragments belonging to a particular message, to ensure that pieces from different fragmented messages are not mixed together.


Figure 108: IPv6 Fragment Extension Header Format

 

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