D-Link xStack DGS-3426P, Руководство пользователя

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xStack DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch
Packet Format
As in IPv4, the IPv6 packet consists of the packet header and the payload, but the difference occurs in the packet header which has
been amended and improved for better packet flow and processing. The following will outline and detail the IPv6 enhancements
and parts of the IPv6 packet, with special attention to the packet header.
IPv6 Header
The IPv6 packet header has been modified and simplified from IPv4. The header length, identification, flags, fragment offset and
header checksum have all been removed in the IPv6 header due to lack of necessity or improvement to a better function of the
header. The minimum header length is now 20 bytes but may be increased to as much as 60 bytes, using 4-byte increment
extensions. The following picture is an example of an IPv6 packet header.
Eight fields make up the basic IPv6 packet header:
Version – This 4-bit field defines the packet version, which is IPv6 and is defined as the number 6.
Traffic Class – This 1-byte field replaces the Type of Service field used in IPv4 and is used to process real-time data and other
data requiring special packet management. This field defines the Class of Service priority of an IPv6 packet.
Flow Label – This 20-bit field is used to facilitate the handling of real-time traffic. Hosts sending data can place a flow label into
this field to identify a sequence of packets that have an identical set of options. In this way, router can process these packets more
efficiently once the flow class has been identified and the rest of the packet header no longer needs to be fully processed, just the
flow label and the source address. All flow label packets must have identical source and destination addresses.
Payload Length – Known as the datagram length in IPv4, this 16-bit field specifies the length of the IPv6 data carried after the
header of the packet. Extension headers are considered part of the payload and are included in the length specified here.
Next Header – This 8-bit field is used to identify the header immediately following the IPv6 header. When this field is set after
the hop by-hop header, it defines the extension header that will appear after the destination address. Each extension header must
be preceded by a Next Header field. Integers used to define extension headers in the next Header field use the same values as IPv4
(ex: 6=TCP, 17=UDP, etc.).
Hop Limit - Similar to the TTL field in IPv4, this 8-bit field defines the number of hops remaining after the packet has been
processed by a node, instead of the number of seconds left to live as on an IPv4 network. This field will decrement by one after
every node it passes and the packet will be discarded once this field reaches zero.
Source Address – This 16-byte field defines the IPv6 address of the source node sending the packet.
Destination Address – This 16-byte field defines the IPv6 address of the destination node receiving the packet. This may or may
not be the final destination node of this packet, depending on the routing header, if present.
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