HP-UX IPQoS A.01.00 Administrator's Guide (October 2005)

Introduction
Overview
Chapter 124
Router and Host Behaviors with DSCP Marking An IP QoS-capable system, such as an
HP-UX IPQoS host, marks the field with a DSCP value. A DS-aware router then applies the
appropriate forwarding behavior associated with the DSCP value to the packet.
The DSCP is used for prioritizing transmission bandwidth. For example, when a router
becomes congested, it uses the DSCP values of queued packets to decide which ones to drop, if
necessary.
Routers can also use the DSCP to re-create the VLAN priority tag. Routers at the boundaries
between administrative domains, such as between ISPs, can convert tags to different values.
Minimally, the values roughly correspond to the same values in the other domain.
Unlike VLAN tags, DSCP markers do not add to the traffic, because the DSCP marker is
carried in an existing field of the IP packet.
Summary
In the DiffServ model, each traffic-generating source cooperates in two ways.
The first is traffic conditioning. Traffic conditioning reduces load peaks and consequent
queueing delays. It assures that when the source node generates data faster than the adapter
can send it, the most important traffic goes out first. HP-UX IPQoS policies use the
reservation bandwidth and maximum allowed bandwidth policy attributes to help with this
“send now or later” type of bandwidth management.
The second is marking, where sources place a VLAN or DSCP priority tag in their packets so
that infrastructure equipment can decide which packets should be sent first and which should
be dropped first.
Transmission priority (reserved and maximum bandwidth allocation) is of limited use as a
control. Since the transmission priority isn’t carried with the packet, its usefulness ends at
the first router. VLAN marking propagates through the first-hop switch. Marking the DS
field, which is carried in the packet, can have wider significance.