TCP/IP Tutorial and Technical Overview - IBM Redbooks

different priorities, the DS field can be used to select which queue is liable for the
routing of this packet. The scale reaches from zero, for lowest priority, to seven
for highest priority. See Figure 8-14 as an example.
Incoming Traffic
Figure 8-14 DS routing example
Another example is a router that has a single queue with multiple drop priorities
for data packets. It uses the DS field to select the drop preference for the packets
in the queue. A value of zero means “it is most likely to drop this packet,” and
seven means “it is least likely to drop this packet.” Another possible constellation
is four queues with two levels of drop preference in each.
To make sure that the per-hop behaviors in each router are functionally
equivalent, certain common PHBs must be defined in future DS specifications to
avoid having the same DS field value causing different forwarding behaviors in
different routers of one DS domain. This means that in future DS specifications,
some unique PHB values must be defined that represent specific service
classes. All routers in one DS domain must know which service a packet with a
specific PHB should receive. The DiffServ Working Group will propose PHBs that
should be used to provide differentiated services. Some of these proposed PHBs
will be standardized; others might have widespread use, and still others might
remain experimental.
312 TCP/IP Tutorial and Technical Overview
Queue 7 (Highest Priority)
Queue 6
Queue 5
Queue 4
1 5 2 6 6 7 7 7
Queue 3
Queue 2
Queue 1
Router
Queue 0 (Lowest Priority)
Outgoing Traffic