deploying and managing ip over wdm networks - Index of

34 Deploying and Managing IP over WDM Networks
the mapping between the packet headers and the path that will be followed up
to the final end is done just once, when the packet enters into the network. The
path to which a given packet is assigned is encoded into a label that is virtually
transported with it as it traverses the network. As a consequence, there is no
longer the need to go up to the network packet headers to decide where to send
a packet arriving at a router. Instead the label is checked against a routing table
that will specify the next hop in the path, similar to the way that ATM routing
tables deal with virtual paths. The routers entrusted to insert and remove the
MPLS labels are the edge routers of the MPLS network cloud; the routers that
are able to take decisions based on such MPLS labels are called label switching
routers (LSRs). The path that is assigned to a packet to cross the network is
called a label-switched path (LSP). Moreover, at the path establishment the
routers inside the MPLS network cloud need to inform themselves about the
mapping between the labels and the LSP. This is carried out through a protocol
called label distribution protocol (LDP). More detailed information can be
found in [3] and [4].
Therefore, the network is characterized by end-to-end LSPs with QoS
constraints that support specific traffic flows defined by forwarding equivalence
classes (FECs). These FECs are classifiers based on several characteristics, such as
origin and destination IP addresses or class of service (CoS) identifiers, which
allow the mapping of the ingress traffic to specific LSPs. These LSPs use
constraint-based routing, which can be statically defined as explicit routes by the
configuration management application (a hop-by-hop list) or calculated by the
routing protocol, like open shortest path first (OSPF) or intermediate systemintermediate
system (IS-IS), with TE extensions. This means that the routing
calculation for an MPLS-enabled network can be done off-line by a management
application and then downloaded to the network devices, with possible
rerouting triggered by the control plane in case of failures. The connectivity
matrix can also be built by the network itself using the aforementioned TEenabled
routing protocols and the TE signaling mechanisms. In either case, this
model permits the compilation of an end-to-end traffic matrix that enables the
service provider to keep track of network usage on a flow-by-flow or client-byclient
basis (or any other chosen criteria). This is a great aid for the network
management, especially for monitoring SLAs.
3.2.1 MPLS-Based Applications
Currently there are three main applications making use of MPLS in the core of
large ISP networks:
1. Traffic engineering implementation;
2. Differentiated services support;