This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
CoS
|
197
information. Many vendors call ToS IP precedence. 802.1p and ToS tend to be used
together when TCP/IP is the networking protocol.
If your VoIP network will be more than 70% data-to-voice and
unlikely to reach capacity, packet prioritization techniques like LAN-
oriented 802.1p and its WAN cousin DiffServ are adequate.
802.1p is a standard that benefits a single data link as opposed to a group of them
using a single CoS policy. So it tends to be a feature of Ethernet switches, as opposed
to IP routers, which instead support ToS. Since 3 bits are allocated for classification,
802.1p allows for eight classes of service. The names associated with these classes are
arbitrary to the network they are used on, but Table 9-4 lists the suggested, generic
names.
The eight classes’ individual per-hop behaviors are administrator-defined, so you
could define class 5 for voice traffic (which is the prevailing wisdom). This would
give voice traffic higher precedence on the local data link. Each hop along the path of
the traffic would have to support the same per-hop precedence behavior in order to
make the QoS policy consistent across a routed WAN.
Like many low-layer standards, 802.1p is a product of the IEEE.
Project 9.1, later in the chapter, describes how to check all the routers
along a certain call path to see if they support 802.1p.
DiffServ
DiffServ, defined in RFC 2474, is a CoS standard that uses ToS tags in a more elabo-
rate manner than 802.1p. While 802.1p tends to be used in switched Ethernet
Table 9-4. Suggested 802.1p classes
Number Name
0 Routine
1 Priority
2 Immediate
3 Flash
4 Flash-override
5 Critical
6 Internet
7 Network
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
198
|
Chapter 9: Quality of Service
environments, DiffServ is used to support routed point-to-point WANs. DiffServ is
an abbreviation for Differentiated Services.
When a packet reaches the edge of the network, either from an endpoint or from a
remote network, DiffServ tags that packet’s ToS header based on the priority estab-
lished for that packet by policy. Once admitted into a DiffServ-equipped WAN,
however, all subsequent router hops must enforce the priority set by the edge router
that admitted the packet.
So, while DiffServ is considered a per-hop behavior CoS system like 802.1p, all Diff-
Serv routers in the network core must uphold the prioritization decision that occurs
at the edge. That is to say, core routers don’t reorder or prioritize the packets, but
they do forward them using a precedence policy that was established at the edge.
Since a majority of bottlenecks occur on the edge of the network, on
access links, rather than within the core network where there’s usu-
ally tons of bandwidth, DiffServ may be the only Quality-of-Service
standard necessary. Its policy decision points are always at the edge of
the network.
In a nutshell, DiffServ furthers the concept of 802.1p so that priority policy can be
established at the edge—once for the entire network—rather than on each individ-
ual hop. What neither of these protocols does, though, is ensure that the network is
never overrun—that’s a key difference between CoS (what 802.1p and DiffServ are)
and QoS.
Policy servers
Common Open Policy Service, or COPS, is a way of storing and querying central-
ized policy information on the network. DiffServ can use COPS to obtain its march-
ing orders for how to handle traffic coming into the network. In a COPS scheme, a
centralized server called the policy server contains a policy record of traffic shaping
and prioritization preferences that DiffServ or another CoS/QoS mechanism can
retrieve. COPS itself doesn’t enforce the prioritization—that’s the job of DiffServ,
which runs on routers; COPS just provides a way of maintaining a centralized record
of the traffic policy. COPS is a product of the IETF Resource Allocation Protocol
working group. Another IETF recommendation, LDAP (Lightweight Directory
Access Protocol), can also be used as the basis of a policy server.
DSCP classes
DiffServ Code Points (DSCP) are IP packet headers DiffServ associates with different
levels of importance. Since they’re 6 bits in length, DSCPs can be used to define quite
a wide scale of possible service levels. Most implementations support only 3 bits,

Get Switching to VoIP now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.