IP version 6 (IPv6) is a new version of the Internet Protocol,
designed as the successor to IP version 4 (IPv4) [RFC-791]. The
changes from IPv4 to IPv6 fall primarily into the following
categories:
o Expanded Addressing Capabilities
IPv6 increases the IP address size from 32 bits to 128 bits, to
support more levels of addressing hierarchy, a much greater
number of addressable nodes, and simpler auto-configuration of
addresses. The scalability of multicast routing is improved by
adding a "scope" field to multicast addresses. And a new type
of address called an "anycast address" is defined, used to send
a packet to any one of a group of nodes.
o Header Format Simplification
Some IPv4 header fields have been dropped or made optional, to
reduce the common-case processing cost of packet handling and
to limit the bandwidth cost of the IPv6 header.
o Improved Support for Extensions and Options
Changes in the way IP header options are encoded allows for
more efficient forwarding, less stringent limits on the length
of options, and greater flexibility for introducing new options
in the future.
o Flow Labeling Capability
A new capability is added to enable the labeling of packets
belonging to particular traffic "flows" for which the sender
requests special handling, such as non-default quality of
service or "real-time" service.
o Authentication and Privacy Capabilities
Extensions to support authentication, data integrity, and
(optional) data confidentiality are specified for IPv6.
2. IPv6 Header Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Header | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Traffic Class 8-bit traffic class field. See section 7.
Flow Label 20-bit flow label. See section 6.
Payload Length 16-bit unsigned integer. Length of the IPv6
payload, i.e., the rest of the packet following
this IPv6 header, in octets. (Note that any
extension headers [section 4] present are
considered part of the payload, i.e., included
in the length count.)
Next Header 8-bit selector. Identifies the type of header
immediately following the IPv6 header. Uses the
same values as the IPv4 Protocol field [RFC-1700
et seq.].
Hop Limit 8-bit unsigned integer. Decremented by 1 by
each node that forwards the packet. The packet
is discarded if Hop Limit is decremented to
zero.
Source Address 128-bit address of the originator of the packet.
Destination Address 128-bit address of the intended recipient of the
packet (possibly not the ultimate recipient, if
a Routing header is present).
3. IPv6 Extension Headers
In IPv6, optional internet-layer information is encoded in separate
headers that may be placed between the IPv6 header and the upper-
layer header in a packet. There are a small number of such extension
headers, each identified by a distinct Next Header value. As
illustrated in these examples, an IPv6 packet may carry zero, one, or
more extension headers, each identified by the Next Header field of
the preceding header:
+---------------+------------------------
| IPv6 header | TCP header + data
| |
| Next Header = |
| TCP |
+---------------+------------------------
+---------------+----------------+------------------------
| IPv6 header | Routing header | TCP header + data
| | |
| Next Header = | Next Header = |
| Routing | TCP |
+---------------+----------------+------------------------
+---------------+----------------+-----------------+-----------------
| IPv6 header | Routing header | Fragment header | fragment of TCP
| | | | header + data
| Next Header = | Next Header = | Next Header = |
| Routing | Fragment | TCP |
+---------------+----------------+-----------------+-----------------
by any node along a packet's delivery path, until the packet reaches
the node (or each of the set of nodes, in the case of multicast)
identified in the Destination Address field of the IPv6 header.
There, normal demultiplexing on the Next Header field of the IPv6
header invokes the module to process the first extension header, or
the upper-layer header if no extension header is present. The
contents and semantics of each extension header determine whether or
not to proceed to the next header. Therefore, extension headers must
be processed strictly in the order they appear in the packet; a
receiver must not, for example, scan through a packet looking for a
particular kind of extension header and process that header prior to
processing all preceding ones.
The exception referred to in the preceding paragraph is the Hop-by-
Hop Options header, which carries information that must be examined
and processed by every node along a packet's delivery path, including
the source and destination nodes. The Hop-by-Hop Options header,
when present, must immediately follow the IPv6 header. Its presence
is indicated by the value zero in the Next Header field of the IPv6
header.
If, as a result of processing a header, a node is required to proceed
to the next header but the Next Header value in the current header is
unrecognized by the node, it should discard the packet and send an
ICMP Parameter Problem message to the source of the packet, with an
ICMP Code value of 1 ("unrecognized Next Header type encountered")
and the ICMP Pointer field containing the offset of the unrecognized
value within the original packet. The same action should be taken if
a node encounters a Next Header value of zero in any header other
than an IPv6 header.
Each extension header is an integer multiple of 8 octets long, in
order to retain 8-octet alignment for subsequent headers. Multi-
octet fields within each extension header are aligned on their
natural boundaries, i.e., fields of width n octets are placed at an
integer multiple of n octets from the start of the header, for n = 1,
2, 4, or 8.
A full implementation of IPv6 includes implementation of the
following extension headers:
Hop-by-Hop Options
Routing (Type 0)
Fragment
Destination Options
Authentication.
Encapsulating Security Payload
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