Users Guide
The router redirect functionality in the neighbor discovery protocol (NDP) is similar to IPv4 router redirect messages. NDP uses
ICMPv6 redirect messages (Type 137) to inform nodes that a better router exists on the link.
IPv6 Headers
The IPv6 header has a xed length of 40 bytes. This xed length provides 16 bytes each for source and destination information and
eight bytes for general header information.
The IPv6 header includes the following elds:
• Version (4 bits)
• Trac Class (8 bits)
• Payload Length (16 bits)
• Next Header (8 bits)
• Hop Limit (8 bits)
• Source Address (128 bits)
• Destination Address (128 bits)
IPv6 provides for extension headers. Extension headers are used only if necessary. There can be no extension headers, one extension
header or more than one extension header in an IPv6 packet. Extension headers are dened in the Next Header eld of the
preceding IPv6 header.
IPv6 Header Fields
The 40 bytes of the IPv6 header are ordered, as shown in the following illustration.
Figure 58. IPv6 Header Fields
Version (4 bits)
The Version eld always contains the number 6, referring to the packet’s IP version.
Trac Class (8 bits)
The Trac Class eld deals with any data that needs special handling. These bits dene the packet priority and are dened by the
packet Source.
Sending and forwarding routers use this eld to identify dierent IPv6 classes and priorities. Routers understand the priority settings
and handle them appropriately during conditions of congestion.
Flow Label (20 bits)
To manage real-time data trac, the Flow Label eld identies packets requiring special treatment.
The sending router can label sequences of IPv6 packets so that forwarding routers can process packets within the same ow
without needing to reprocess each packet’s header separately.
IPv6 Routing
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