Deployment Guide

IPv6 Header Fields
The 40 bytes of the IPv6 header are ordered, as shown in the following illustration.
Figure 57. IPv6 Header Fields
Version (4 bits)
The Version field always contains the number 6, referring to the packet’s IP version.
Traffic Class (8 bits)
The Traffic Class field deals with any data that needs special handling. These bits define the packet priority and are defined
by the packet Source. Sending and forwarding routers use this field to identify different IPv6 classes and priorities. Routers
understand the priority settings and handle them appropriately during conditions of congestion.
Flow Label (20 bits)
The Flow Label field identifies packets requiring special treatment in order to manage real-time data traffic.
The sending router can label sequences of IPv6 packets so that forwarding routers can process packets within the same flow
without needing to reprocess each packet’s header separately.
NOTE: All packets in the flow must have the same source and destination addresses.
Payload Length (16 bits)
The Payload Length field specifies the packet payload. This is the length of the data following the IPv6 header. IPv6 Payload
Length only includes the data following the header, not the header itself.
The Payload Length limit of 2 bytes requires that the maximum packet payload be 64 KB. However, the Jumbogram option type
Extension header supports larger packet sizes when required.
Next Header (8 bits)
The Next Header field identifies the next header’s type. If an Extension header is used, this field contains the type of Extension
header (as shown in the following table). If the next header is a transmission control protocol (TCP) or user datagram protocol
(UDP) header, the value in this field is the same as for IPv4. The Extension header is located between the IP header and the
TCP or UDP header.
The following lists the Next Header field values.
464
IPv6 Routing