Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 672)

IPv6 Introduction
Software Reference for x310 Series Switches
26.6 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
Basic IPv6 header structure
The headers contain information necessary to move the packet across the internet. They
must be able to cope with missing and duplicated packets as well as possible
fragmentation (and reassembly) of the original packet.
IPv6 headers are twice as long as IPv4 headers (40 bytes instead of 20 bytes) and contain
four times the address space size (128 bits instead of 32 bits).
They no longer contain the header length, identification, flags, fragment offset, and
header checksum fields. Some of these options are placed in extension headers. The Time
To Live field is replaced with a hop limit, and the IPv4 Type of Service field is replaced with
a Differentiated Services field. The Differentiated Services field contains the DSCP bits,
used in a Quality of Service (QoS) regime. The following table explains IPv4 header fields
that changed in IPv6.
Extension headers IPv6 implements many of the less commonly used fields in the IPv4 header (or their
equivalents) as extension headers, which are placed after the basic IPv6 header. The
length of each header must be a multiple of 8 bytes.
The first extension header is identified by the Next Header field in the basic IPv6 header.
Any subsequent extension headers are identified by an 8-bit “Next Header” value at the
beginning of the preceding extension header.
IPv6 nodes that originate packets are required to place extension headers in a specific
order:
1. The basic IPv6 header. This must come immediately before the extension headers.
2. The Hop-by-Hop header. This specifies options that must be examined by every node
in the routing path.
3. A Destination Options header. This is used to specify options to be processed by the
first destination or final destination. The destination options header is the only
extension header that may be present more than once in the IPv6 packet.
4. The Routing header. This enables a static path to be specified for the packet, if the
dynamically-determined path is undesirable.
5. The Fragment header. This indicates that the source node has fragmented the packet,
and contains information about the fragmentation.
Changed Field Description
Type of Service The type of service that a connection should receive is
indicated in IPv6 by the Flow Label field in the IPv6 header.
Fragmentation
information (the
Identification field, the
Flags field and the
Fragment Offset field) In most cases fragmentation does not occur in IPv6. If it
does, packets are fragmented at their source and not en
route. Therefore, the fragmentation information is
contained in an extension header to reduce the size of the
basic IPv6 header.
Header Checksum This option has not been provided in IPv6. This is because
transport protocols implement checksums and because of
the availability of the IPsec authentication header (AH) in
IPv6.
Options Extension headers handle all the optional values associated
with IPv6 packets. The biggest advantage of this scheme is
that the size of the basic IP header is a constant.