First In, First Out - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

Setting Up Quality of Service
Overview
7-10
Only 13 DSCP values have actually been standardized. Individual network
administrators define in more detail which set of DSCP values match to a
specific PHB. This allows them to use DiffServ with the QoS policies already
implemented in a network.
Remember, a PHB simply defines the standard of service that a node should
provide for traffic marked for that PHB. DiffServ does not dictate how the
node will actually grant such service. Such specifications are left to individual
programmers and network administrators.
On the ProCurve Secure Router, you can mark packets with a DSCP, but you
cannot define PHB. You can configure the ProCurve Secure Router to:
mark a packet with a DSCP for use in the network to which it will be
forwarded
read a packet's DSCP and map it to an IP precedence value to use for WFQ
read a packet's DSCP value and assign it to a class for CBWFQ
read a packet's DSCP value and assign it to a low-latency queue

First In, First Out

The most basic type of service for packets is First In, First Out (FIFO). A router
gives all packets the same best-effort service, forwarding the first packet that
arrives on an interface first. (See Figure 7-1.)
When a router uses switched processing to route packets, it forces a packet
to wait in the queue until it has completed other CPU processes.
Routers can use fast caching to speed processing for packets that travel often-
used routes. When a packet arrives on a fast-cache interface, the router
interrupts its other processes to look up a route for the packet in the fast-cache
table. This table contains the forwarding interfaces for the destinations of the
most recently served packets. If the router does not find a match, it queues
the packet as usual. If it does find a match, however, it forwards the packet
immediately. Fast caching reduces delay and improves QoS. (Fast caching is
enabled by default on the ProCurve Secure Router. For more information
about configuring fast caching, see "Fast Caching" on page 11-12 in the Basic
Management and Configuration Guide.)