IPv6の仕様（優先権）

   The 4-bit Priority field in the IPv6 header enables a source to
   identify the desired delivery priority of its packets, relative to
   other packets from the same source.  The Priority values are divided
   into two ranges:  Values 0 through 7 are used to specify the priority
   of traffic for which the source is providing congestion control,
   i.e., traffic that "backs off" in response to congestion, such as TCP
   traffic.  Values 8 through 15 are used to specify the priority of
   traffic that does not back off in response to congestion, e.g.,
   "real-time" packets being sent at a constant rate.

   For congestion-controlled traffic, the following Priority values are
   recommended for particular application categories:

         0 - uncharacterized traffic
         1 - "filler" traffic (e.g., netnews)
         2 - unattended data transfer (e.g., email)
         3 - (reserved)
         4 - attended bulk transfer (e.g., FTP, NFS)
         5 - (reserved)
         6 - interactive traffic (e.g., telnet, X)
         7 - internet control traffic (e.g., routing protocols, SNMP)

   For non-congestion-controlled traffic, the lowest Priority value (8)
   should be used for those packets that the sender is most willing to
   have discarded under conditions of congestion (e.g., high-fidelity
   video traffic), and the highest value (15) should be used for those
   packets that the sender is least willing to have discarded (e.g.,
   low-fidelity audio traffic).  There is no relative ordering implied
   between the congestion-controlled priorities and the non-congestion-
   controlled priorities.