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IPv6 Datagram Overview and General Structure
(Page 1 of 2)
The method by which IPv6 encapsulates
data received from higher-layer protocols for transmission across the
internetwork is basically the same as that
used by IPv4. The data received from the
transport or higher layers is made the payload of an IPv6 datagram,
which has one or more headers that control the delivery of the message.
These headers provide information to routers to enable them to move
the datagram across the network, and also to hosts so they can tell
which datagrams they are intended to receive.
Overview of Major Changes to Datagram Structure and Fields in IPv6
While the basic use of datagrams
hasn't changed since IPv4, many modifications were made to their structure
and format when IPv6 was created. This was done partly out of necessity:
IPv6 addresses are different than IPv4 addresses and IP addresses go
in the datagram header. The increase in the size of IP addresses from
32 bits to 128 bits adds a whopping extra 192 bits, or 24 bytes of information
to the header. This in turn led to an effort to remove fields that weren't
strictly necessary, to compensate for the necessary increase in size.
However, changes were also made to IPv6 datagrams to add features to
them and to make them better suit the needs of modern internetworking.
The following is a list of the most
significant overall changes to datagrams in IPv6:
- Multiple Header Structure: Rather than
a single header that contains all fields for the datagram (possibly
including options), the IPv6 datagram supports a main header
and then extension headers for additional information when needed.
- Streamlined Header Format: Several fields
have been removed from the main header to reduce its size and increase
efficiency. Only the fields that are truly required for pretty much
all datagrams remain in the main header; others are put
into extension headers and used as needed. Some were removed because
they were no longer needed, such as the Internet Header Length
field; the IPv6 header is of fixed length. This is discussed more thoroughly
topic on the main IPv6 header.
- Renamed Fields: Some fields have been
renamed to better reflect their actual use in modern networks.
- Greater Flexibility: The extension
headers allow a great deal of extra information
to accompany datagrams when needed. Options
are also supported in IPv6.
- Elimination of Checksum Calculation: In
IPv6, a checksum is no longer computed on the header. This saves both
the calculation time spent by every device that packages IP datagrams
(hosts and routers) and the space the checksum field took up in the
- Improved Quality of Service Support: A
new field, the Flow Label, is defined to help support the prioritization
Key Concept: IPv6 datagrams use a general structure that begins with a mandatory main header 40 bytes in length, followed by optional extension headers and then a variable-length Data area. This structure was created to allow the main header to be streamlined while allowing devices to add extra information to datagrams when needed.
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Table Of Contents - Contact Us
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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