Internet Protocol, Version 6 (IPv6)

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Internet Protocol, Version 6 (IPv6)

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:

  • 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.
  • 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.
  • 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.
  • 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.
  • Authentication and Privacy Capabilities: Extensions to support authentication, data integrity, and (optional) data confidentiality are specified for IPv6.

IPv6 Header Format

 Version 4-bit Internet Protocol version number = 6.
Traffic Class 8-bit traffic class field.
Flow Label20-bit flow label.
Payload Length16-bit unsigned integer.  Length of the IPv6 payload, i.e., the rest of the packet following this IPv6 header, in octets.
Next Header8-bit selector.  Identifies the type of header immediately following the IPv6 header.   Uses the same values as the IPv4 Protocol field
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 Address128-bit address of the originator of the packet.
Destination Address128-bit address of the intended recipient of the packet.


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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.

1.    Extension Header Order

When more than one extension header is used in the same packet, it is recommended that those headers appear in the following order:
IPv6 header
Hop-by-Hop Options header
Destination Options header
Routing header
Fragment header
Authentication header
Encapsulating Security Payload header
Destination Options header
upper-layer header

Each extension header should occur at most once, except for the Destination Options header which should occur at most twice (once before a Routing header and once before the upper-layer header).

If the upper-layer header is another IPv6 header (in the case of IPv6 being tunneled over or encapsulated in IPv6), it may be followed by its own extension headers, which are separately subject to the same ordering recommendations.

If and when other extension headers are defined, their ordering constraints relative to the above-listed headers must be specified.

IPv6 nodes must accept and attempt to process extension headers in any order and occurring any number of times in the same packet, except for the Hop-by-Hop Options header which is restricted to appear immediately after an IPv6 header only.  Nonetheless, it is strongly advised that sources of IPv6 packets adhere to the above-recommended order until and unless subsequent specifications revise that recommendation.

2.    Options

Two of the currently-defined extension headers :
1.    Hop-by-Hop Options header and
2.    Destination Options header
Which carry a variable number of type-length-value (TLV) encoded “options”,
The sequence of options within a header must be processed strictly in the order they appear in the header; a receiver must not, for example, scan through the header looking for a particular kind of option and process that option prior to processing all preceding ones.

3.    Hop-by-Hop Options Header

The Hop-by-Hop Options header is used to carry optional information that must be examined by every node along a packet’s delivery path. The Hop-by-Hop Options header is identified by a Next Header value of 0 in the IPv6 header.

4.    Routing Header

The Routing header is used by an IPv6 source to list one or more intermediate nodes to be “visited” on the way to a packet’s destination.

5.    Fragment Header

The Fragment header is used by an IPv6 source to send a packet larger than would fit in the path MTU to its destination.

6.    Destination Options Header

The Destination Options header is used to carry optional information that need be examined only by a packet’s destination node(s).  The Destination Options header is identified by a Next Header value of 60 in the immediately preceding header.

 7.    No Next Header

The value 59 in the Next Header field of an IPv6 header or any extension header indicates that there is nothing following that header.  If the Payload Length field of the IPv6 header indicates the presence of octets past the end of a header whose Next Header field contains 59, those octets must be ignored and passed on unchanged if the packet is forwarded.


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Packet Size

IPv6 requires that every link on the internet have an MTU of 1280 octets or greater.  On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.

Flow Labels

The 20-bit Flow Label field in the IPv6 header may be used by a source to label sequences of packets for which it requests special handling by the IPv6 routers, such as non-default quality of service or “real-time” service.  This aspect of IPv6 is, at the time of writing, still experimental and subject to change as the requirements for flow support on the Internet become clearer.  Hosts or routers that do not support the functions of the Flow Label field are required to set the field to zero when originating a packet, pass the field on unchanged when forwarding a packet, and ignore the field when receiving a packet.

Traffic Classes

The 8-bit Traffic Class field in the IPv6 header is available for use by originating nodes and/or forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets.

Upper-Layer Protocol Issues

1.    Upper-Layer Checksums

Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses.

The IPv6 version of ICMP [ICMPv6] includes the above pseudo-header in its checksum computation; this is a change from the IPv4 version of ICMP, which does not include a pseudo-header in its checksum.  The reason for the change is to protect ICMP from misdelivery or corruption of those fields of the IPv6 header on which it depends, which, unlike IPv4, are not covered by an internet-layer checksum. The Next Header field in the pseudo-header for ICMP contains the value 58, which identifies the IPv6 version of ICMP.

2.    Maximum Packet Lifetime

Unlike IPv4, IPv6 nodes are not required to enforce maximum packet lifetime.  That is the reason the IPv4 “Time to Live” field was renamed “Hop Limit” in IPv6.  In practice, very few, if any, IPv4 implementations conform to the requirement that they limit packet lifetime, so this is not a change in practice.  Any upper-layer protocol that relies on the internet layer (whether IPv4 or IPv6) to limit packet lifetime ought to be upgraded to provide its own mechanisms for detecting and discarding obsolete packets.

3.    Maximum Upper-Layer Payload Size

When computing the maximum payload size available for upper-layer data, an upper-layer protocol must take into account the larger size of the IPv6 header relative to the IPv4 header.

4.    Responding to Packets Carrying Routing Headers

When an upper-layer protocol sends one or more packets in response to a received packet that included a Routing header, the response packet(s) must not include a Routing header that was automatically derived by “reversing” the received Routing header UNLESS the integrity and authenticity of the received Source Address and Routing header have been verified (e.g., via the use of an Authentication header in the received packet).


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Books you may interested

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