HP ProCurve 7102dl Reference Manual page 1400

SROS Command Line Interface Reference Guide
010 Immediate Servicing
001 Priority Traffic
000 Routine Data
The IP Precedence values provide network routers with information about what kind of traffic is contained
in the IP packet. Based on the IP Precedence values, some networks (when supported) can offer special
handling to certain packets. In addition, providing IP Precedence values to critical traffic (such as route
information) ensures that critical packets will always be delivered regardless of network congestion. This
traffic is often critical to network and internetwork operation. In general, the higher the IP Precedence
value, the more important the traffic and the better handling it should receive in the network. It is important
to remember that not all equipment in the public IP network will be configured to recognize and handle IP
Precedence values. While it is a good idea to set the values for critical traffic, it does not guarantee special
handling.
In addition to the precedence values, RFC791 specifies bits for delay, throughput, and reliability to help
balance the needs of particular traffic types when traveling on the IP network infrastructure. When these
bits are set to 0, they are handled with normal operation. When set to 1, each bit specifies premium
handling for that parameter. For example, a 1 in the delay position indicates that the traffic is delay
sensitive and care should be taken to minimize delay. A 1 in the throughput position indicates that the
traffic has higher bandwidth requirements that should be met. A 1 in the reliability position indicates that
the traffic is sensitive to delivery issues and care should be taken to ensure proper delivery with all packets
of this type. These extra bits are rarely used because they are quite difficult to balance the cost and
benefits of each parameter (especially when more than one bit is set to 1).
The Differentiated Services (DiffServ or DS) model was created in RFC2474 and 2475 to build on the
original Type of Service field by creating a six-bit sequence (combining the precedence value with the
delay, throughput, and reliability bits). This six-bit sequence increased the number of available values from
8 to 64. The DiffServ model introduced a new concept to QoS in the IP network environment: per-hop
behaviors (PHBs). The PHB premise is that equipment using the DiffServ model have an agreed upon set
of rules (PHB types) for handling certain network traffic. Though the RFC explicitly defines what each PHB
should be capable of, it does not restrict vendor-specific implementation of the PHBs. Each vendor is free
to decide how their network product implements the various defined PHBs.
According to RFC2474, the DS field contains the following bits:
* The previously unused bits in the DS field are now used for congestion control and are not discussed in this document.
Equipment following the DiffServ model (DS-compliant nodes) must use the entire six-bit DSCP value to
determine the appropriate PHB. The PHBs are defined as the following:
• Default PHB
• Class Selector PHB
• Assured Forwarding PHB (RFC2597)
• Expedited Forwarding PHB (RFC2598)
5991-2114
0 1 2 3
Differentiated Service Code Point
© Copyright 2007 Hewlett-Packard Development Company, L.P.
Quality of Service (QoS) Map Commands
4 5 6 7
Unused*
1398