the failing node cannot be used anymore. Hence, the local recovery path establishes
a (node-disjoint
4
) recovery path between every two ``neighbor'' nodes of the failing
node.
The other extreme is global recovery, in which the complete working path
between source and destination is bypassed by a recovery path. In other words,
the RHE and RTE will coincide with the source and destination of the working
path, respectively (Figure 1.20). In a preplanned recovery mechanism, the global
recovery path should be disjoint from the working path, which might impose some
additional constraints to compute both paths. For instance, if the mechanism aims
only at recovering from single-link failures, a recovery path that is link-disjoint
from the working path will be sufficient. If, on the other hand, the mechanism wants
to recover from single-node failures (or both single-link failures and single-node
failures), the recovery path must be node-disjoint
5
from the working path.
When comparing local and global recovery, several pros and cons arise:
.
In local recovery, the RHE and RTE are closer to the failure. Hence, these
nodes will typically detect the fault rather quickly, leading to a smaller
recovery time than for global recovery. In other words, local recovery is
usually much faster than global recovery, an important advantage in time-
sensitive applications.
.
An obvious drawback of local recovery is apparent from Figure 1.19: The
resulting route followed by the traffic after recovery is often longer than
needed. The main reason for this suboptimum result is that local recovery
does not consider other parts of the working path than the recovered
segment. Hence, the same traffic may cross a particular link twice. This
4
Except for the RHE and RTE, of course.
5
Except for the RHE and RTE, of course.
RTE
RTE
RHE
RHE
Working Paths
Recovery Paths
Figure 1.20 Global recovery for single-link (left) and single-node (right) failure.
Vasseur / Network Recovery
Final Proof
7.6.2004
12:18pm
page 33
1.5 Characteristics of Single-Layer Recovery Mechanisms
33