6.2 TE and MPLS
An explicitly routed LSP is an LSP whose path
is established by means other than normal IP
routing. In one approach this requires among
other things a management system representa-
tion as described in [Henr01].
When utilising MPLS with Traffic Engineering,
a number of mapping relations is asked for, see
Figure 23:
- Mapping packets onto FECs. An FEC com-
poses a group of packets to be forwarded over
the same path with the same forwarding treat-
ment. In order to carry out this mapping fields
in the IP packet are examined. - Mapping FECs onto traffic trunks. A traffic
trunk is an aggregation of traffic flows of the
same class. A traffic trunk can again be routed
(placed inside an LSP, i.e. a traffic trunk is
only given for one LSP and not a sequence of
LSPs). - Mapping traffic trunks onto LSPs.
- Mapping LSPs onto links in the physical net-
work.
In several sources, the terms traffic trunk and
LSP are used synonymously. However, a funda-
mental difference between traffic trunk and LSP
can be observed; that is, a traffic trunk is an
abstract representation of traffic to which spe-
cific characteristics can be associated. An LSP
is a description of a path in the network through
which the traffic traverses.
Trunks having the same egress point may be
merged into a common tree. This may reduce the
number of trees significantly. Trunks can also be
aggregated by adding a new label to the stack for
each trunk (that is, bundling the trunks into a
single path/tunnel).
Designing an MPLS network “on top of” a phys-
ical network could be looked upon as relating
two graphs to each other;
- Physical graph, G= (V, E, c), is a capacitated
graph depicting the physical topology of the
network. Vis the set of nodes in the network
and Eis the set of links. For vto win V, (v, w)
represents the link in Ewhen vand ware
directly connected under G. cindicates the set
of capacity and other constraints associated
with Eand V. - MPLS graph, H= (U, F, d), where Uis a sub-
set of V representing the LSRs, that is the set
of LSRs that are end point of at least one LSP.
Fis the set of LSPs. For xand yin U, (x, y)
is in Fif there is an LSP going from xto y.
drepresents the set of demands and restric-
tions associated with F.The fundamental problem of designing an MPLS
network is to relate the two graphs such that an
objective function is optimised. This is add-
ressed in several accompanying papers in this
Telektronikkissue.One of the requirements from Traffic Engineer-
ing is to be able to reroute an LSP under a num-
ber of conditions (failure, better route available,
etc.). This should preferably be done without
disturbing the traffic flows; for example by
establishing the new LSP before the old/existing
LSP is released, which is called make-before-
break. In case the existing and new LSP compete
for the same resources, particular concerns have
to be made, also considered by the admission
control.As mentioned above, IP packets are classified at
the ingress of the MPLS domain into a number
of Forwarding Equivalence Classes (FECs). All
the packets in a given FEC are treated the same
way within the domain. Choosing the use of
FEC may depend on criteria like:- the user (derived from the source address,
interface, etc.); - the application type;
- the packet destination.
A traffic trunk is described by its ingress and
egress LSRs, the set of FECs which is mapped
onto it, and a set of attributes that gives its char-
acteristics. Two fundamental questions have to
be answered related to traffic trunks; i) how to
give the characteristics; and ii) how to relate
traffic trunks to the physical network (through
LSPs). This requires three capabilities:- Set of attributes characterising the traffic
trunks that give its characteristics; - Set of attributes related with resources that
constrain the placement of traffic trunks onto
the resources; - Mechanism for placing/maintaining traffic
trunks onto the set of resources.
For the last item, constraint-based routing could
be applied as described in [Feng01].Attributes characterising traffic trunks are
([RFC2702]):- Traffic parameter attributes. These are used
to describe the traffic flows (the FECs) trans-
ported in the traffic trunk. Relevant parame-