Service Manual

Version (4 bits)
The Version eld always contains the number 6, referring to the packet’s IP version.
Trac Class (8 bits)
The Trac Class eld deals with any data that needs special handling. These bits dene the packet priority and are dened by the
packet Source. Sending and forwarding routers use this eld to identify dierent IPv6 classes and priorities. Routers understand the
priority settings and handle them appropriately during conditions of congestion.
Flow Label (20 bits)
The Flow Label eld identies packets requiring special treatment in order to manage real-time data trac.
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.
NOTE: All packets in the ow must have the same source and destination addresses.
Payload Length (16 bits)
The Payload Length eld species 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 eld identies the next header’s type. If an Extension header is used, this eld 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 eld 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 eld values.
Value
Description
0 Hop-by-Hop option header
4 IPv4
6 TCP
8 Exterior Gateway Protocol (EGP)
41 IPv6
43 Routing header
44 Fragmentation header
50 Encrypted Security
51 Authentication header
59 No Next Header
60 Destinations option header
NOTE: This table is not a comprehensive list of Next Header eld values. For a complete and current listing, refer to the
Internet Assigned Numbers Authority (IANA) web page at .
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IPv6 Routing