An important term is Behaviour Aggregate
(BA). This is the aggregation of all packets
with the same DSCP crossing a given link in a
particular direction [RFC2475]. The set of BAs
sharing an ordering constraint is called an
Ordered Aggregate (OA). For example, all pack-
ets belonging to a given AF class and crossing
a given link in a particular direction share an
ordering constraint. This is because the AF defi-
nition states that AF packets of the same micro-
flow belonging to the same AF class must not be
reordered regardless of their drop precedence.
The classification of packets in BAs is based on
the Service Level Agreement (SLA) between the
parties (e.g. customer and provider). Of interest
in this context is the technical part of the agree-
ment, called Service Level Specification (SLS).
It contains, among other things, details on how
much traffic of different types the customer can
initiate and the quality he can expect. From such
information the network operator must assure
that his network is able to carry all customer cre-
ated traffic within the contracted limits with a
satisfactory quality.
A Per Hop Behaviour Scheduling Class (PSC)
is the set of PHBs that are applied to the BAs
belonging to an OA [mpls-diff-ext]. For exam-
ple, the PHBs that are associated with a given
AF class constitute a PSC.
Multi Protocol Label Switching
(MPLS)
At the IP layer (layer 3) a router makes forward-
ing decisions for a packet based on information
in the IP header. The analysis of the packet
header is performed and a routing algorithm is
executed in each router. This can be viewed as a
two-step process. First the packets are classified
into a set of Forwarding Equivalence Classes
(FECs). Then each FEC is mapped to a next hop.
An FEC is a group of packets that shall be for-
warded over the same path with the same for-
warding treatment.
With Multi-Protocol Label Switching (MPLS)
the classification of packets into FECs is only
performed at the ingress to the MPLS domain.
The packet is then mapped to a Label Switched
Path (LSP) by encapsulation of an MPLS
header. The LSP is identified locally by the
header, or more correctly by the label field in
the header. Based on the value of the label the
packet is mapped to the next hop. In successive
routers within the MPLS domain the label is
swapped (therefore it can have only local signifi-
cance) and the packet is mapped to the next hop.
The MPLS architecture is described in [RFC3031]
whereas support of DiffServ over MPLS net-
works is described in [mpls-diff-ext].
Class of Service
To be able to design and manage a network for
carrying services with different quality require-
ments, it is necessary to define a set of Classes
of Service (CoS) as seen from a network point of
view. This set should on the one hand reflect dif-
ferent service and customer requirements, and on
the other hand the possibility of the network to
provide differentiated service levels. The users
must be able to see the difference between the
different choices, not only in price but also in
service levels.In [Johnsen 1999] a CoS is defined as ‘a cate-
gory based on type of users, type of applications,
or some other criteria that QoS systems can use
to provide differentiated classes of service. The
characteristics of the CoS may be appropriate
for high throughput traffic, for traffic with a
requirement for low latency or simply for Best
Effort. The QoS experienced for a particular
flow will be dependent on the number and type
of other traffic flows admitted to its class.’This wide definition opens up for included
parameters like packet loss, latency, throughput,
as well as survivability aspects in defining the
different classes. But it could be discussed
whether CoS should be used in a relative sense
and the term QoS classes should be used when
classes are differentiated with quantitative
requirements.In an MPLS context CoS is used in relation to
the CoS-field, which is three bits in the MPLS
header. This field can either be used to differen-
tiate between different CoS within an LSP (E-
LSP) or to identify colouring in case the LSP is
dedicated to one CoS (L-LSP). The classifica-
tion into CoS is left for the network operators
to decide.In a DiffServ context the term ‘traffic class’ is
sometimes used for traffic that shares a common
set of QoS requirements. Such a class could be
characterised by using a standardised PHB group
[diffserv-new-terms] like EF, one of the AF
classes or Best Effort (BE), in each node. Such
classes are also often referred to as DiffServ
classes. CoS in the first definition above could
in addition add criteria like resilience so that a
DiffServ class could contain many CoS.An associated term in DiffServ is Per-Domain
Behaviour (PDB). This is defined in [diffserv-
pdb-def] as “the expected treatment that an iden-
tifiable or target group of packets will receive
from ‘edge to edge’ of a DS domain”. A particu-
lar PHB (or, if applicable, list of PHBs) and traf-
fic conditioning requirements are associated
with each PDB.