When a path has been partly calculated offline,
the ingress router may explicitly complete the
route calculation and instantiate the LSP by use
of signalling. In this case the resulting LSP is
termed a strict ER-LSP.
For the parts of the route that have not been cal-
culated offline, the ingress router may also use
abstract nodes in the explicit route representa-
tion. This permits local flexibility in fulfilling
the request for a constraint-based route. The
resulting LSP is termed a loose ER-LSP. In this
case, the question of route pinning should be
considered. Route pinning is applicable to seg-
ments of the ER-LSP that are loosely routed, and
should be applied if it is undesirable to change
the path for the loosely routed segments of the
LSP.
If global optimisation of network resources is
required, the LSP path selection must take place
offline. Online path selection calculates one LSP
at a time, and the order in which the LSPs are
calculated determines the resulting set of physi-
cal paths in the network. This will probably not
result in optimal network resource utilisation.
An offline path selection tool is able to simulta-
neously examine the requirements for each LSP
and the resource constraints of each link. A
global calculation may be performed, and the
output of this calculation is a set of LSPs that
optimises resource utilisation for the network as
a whole. After completion of the offline calcula-
tion, the LSPs may be instantiated in any order.
4.2.4 Generic Traffic Trunk Attributes
A traffic trunk is defined as an aggregation of
traffic flows of the same class that are placed
inside an LSP. This abstract representation of
traffic allows for specific characteristics to be
associated with traffic aggregates. Traffic trunks
are objects that can be routed, and traffic trunk
characteristics may put constraints on the path
of the LSP into which it is placed.
A number of generic traffic trunk attributes have
been defined in [RFC2702]. Some of these
attributes are applicable to LSP path selection,
and are described below.
Traffic parametersindicate the resource require-
ments for the traffic trunk, as they define the
characteristics of the FEC to be transported
through the LSP. These characteristics may
include peak rates, average rates, permissible
burst size, etc. For the purpose of path selection,
or bandwidth allocation in general, the traffic
parameters can be used to calculate a single
value for the LSP bandwidth requirements.
Resource class affinityattributes associated with
a traffic trunk can be used to specify the class of
resources that are to be explicitly included or
excluded from the path of the traffic trunk, i.e.
from the LSP.The priorityattribute defines the relative impor-
tance of LSPs. Priorities can be used to deter-
mine the order in which path selection is done
for LSPs. Priorities are also important if pre-
emption (see below) is permitted. They can be
used to define a partial order on a set of LSPs,
and pre-emptive policies may be actualised
according to this. [CR-LDP] defines two priority
parameters, namely setupPriority and holding-
Priority.The pre-emptionattribute determines whether a
specific LSP can pre-empt another LSP from a
given path, and whether another LSP can pre-
empt a specific LSP. Pre-emption means rerout-
ing existing LSPs to reallocate resources to a
new path. Pre-emption can be used to assure
that relatively favourable paths always can be
selected for high priority LSPs. Setup and hold-
ing priorities are used to rank existing LSPs and
the new LSP to determine if the new LSP can
pre-empt an existing one.A path preference ruleattribute should be asso-
ciated with administratively specified ER-LSPs.
This is a binary attribute with values “manda-
tory” and “non-mandatory”. If the ER-LSP path
is defined as “mandatory”, then that path must
be used. If the specified path for some reason
cannot be instantiated, the LSP instantiation pro-
cess fails. If the LSP instantiation process suc-
ceeds, the LSP is implicitly pinned. On the other
hand, “non-mandatory” paths are used if feasi-
ble. If not, an alternate path can be chosen
instead by the ingress router.4.2.5 Generic Resource Attributes
A number of generic resource attributes have
been defined in [RFC2702]. Some of these
attributes are applicable to path selection, and
are described below.The maximum allocation multiplierof a resource
is an administratively configurable attribute that
determines the proportion of the resource avail-
able for allocation to LSPs. The attribute is most
applicable to link bandwidth, but can also be
applied to buffer resources on LSRs. The rela-
tionship between maximum bandwidth and max-
imum reservable bandwidth of a link represents
the maximum allocation multiplier concept.