Local Link Protection Scheme in IP Networks.

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Local Link Protection Scheme in IP Networks
Hui-Kai Su1and Cheng-Shong Wu2
1Dept. of Computer Science and Information Engineering
Nanhua University
No. 32, Chung Keng Li, Dalin, Chia-Yi, 622 Taiwan
hksu@mail.nhu.edu.tw
2Department of Electrical Engineering
National Chung-Cheng University
No. 160, San-Hsing, Min-Hsiung, Chia-Yi, 621 Taiwan
ieecsw@ccu.edu.tw
Abstract. In this paper, we proposed an IP Local Link-Protection
(IPLLP) scheme based on the characteristic of shortest-path routing
in IP networks. Our scheme working in an intra-area routing domain
provides a simple and efficient solution to improve IP network surviv-
ability without extra control protocols and enhanced routing protocols.
Because the backup next hops are predetermined in advance, the service
interrupted time can be limited to a few milliseconds. In the simulation
results, we observe that IP Local Link-Protection scheme can efficiently
improve network survivability in a small-scale and high-degree network.
Keyword: IP network survivability, link protection, fast reroute.
1 Introduction
Network availability becomes a more and more important QoS (Quality of Ser-
vice) parameter in IP networks. Certain services should not be interrupted re-
gardless of the scale, duration and type of failures. IP network has the ability
of routing restoration since the ARPANET was built, i.e. IP restoration. In
addition, many protection schemes have been implemented at lower layers in
IP networks, e.g. SONET APS (Automatic Protection Switching), MPLS Fast-
Reroute, etc. Since their backup paths are decided and set up for network failures
in advance, the service interrupted time can be limited to a few milliseconds.
Recently, the IP protection issue has been discussed since 2002. The precom-
putation scheme of second shortest paths is introduced in [1]. However, in the
practice, how to decide feasible backup routes efficiently, provide an efficient
fast reroute service and avoid routing loops was not discussed. The drafts of IP
Fast-Reroute (IPFRR) framework [2] and LFAP (Loop Free Alternate Paths)
scheme[3] were proposed by IETF Routing Area Working Group. Equal Cost
Multipath (ECMP) and LFAP offer the simplest repair paths, and it is antici-
pated that around 80% of failures can be repaired using these alone. However,
the ECMP scheme needs extra control protocols to negotiate which equal cost
Y. Shi et al. (Eds.): ICCS 2007, Part IV, LNCS 4490, pp. 797–800, 2007.
c ? Springer-Verlag Berlin Heidelberg 2007

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798H.-K. Su and C.-S. Wu
path is failed after a node or a link fails. Additionally, multi-hop repair paths
are considerably more complex, and extra control protocols or enhanced routing
protocols should be needed. It is anticipated that around 98% of failures can be
repaired.
In this paper, we proposed an IP Local Lode-Protection (IPLLP) scheme for
IP networks in an intra-area routing domain. Our scheme provides a simple and
efficient solution for IP network protection without extra control protocols and
enhanced routing protocols. According to characteristic of destination tree with
shortest-path routing, our scheme can prevent service disruption and packet
loss caused by the loops which normally occur during the re-convergence of the
network following a failure. The packets through the failure link can be locally
rerouted to the backup next hop as soon as the upstream adjacent nodes of the
failure node detect the failure. Because the backup next hop is predetermined,
the service interrupted time can be limited to within a few milliseconds.
2 Local Link Protection Scheme
A network topology G(N,L), where N and L denote the router set and the link
set respectively, is given. Note that G(N,L) can be deduced from the database of
link-state routing protocols. After calculating the primary next hop, every router
assumes that the connected link with their primary next hop to the destination
node d is failed. First, they calculate the destination tree Tdto node d according
to G(N,L). Second, they remove the connected link with their primary next hop
from the destination tree Td, divide the tree into two subtrees and then try to
repair the incomplete destination subtree to node d with the leaved tree as a
partial shortest-path destination treeˇTd. Because only the connected nodes of
the failed link can sense this failure in the first time, the traffic flows are delivered
along the partial shortest-path destination treeˇTdbefore IGP converges. There-
fore, if the adjacent nodes can cooperatively construct the partial shortest-path
destination tree based on the divided subtrees, their loop-free backup next hops
to the destination node d will be decided.
An example of IPLLP scheme is shown in Fig. 1. If a bidirectional link
{N3,N2} fails, N3 is responsible to predecide a feasible backup next hop for
this link failure to the destination node N0. The {N3,N2} is removed from the
topology, and TN0is divided into T?
and feasible next hop on T?
treeˇTN0, its backup next hop is existent; otherwise, the link failure cannot be
protected. N4is selected for link {N3,N2} failure to N0by N3in our example,
packet loss caused by the loops doesn’t arise during the IGP routing recovery.
N0and T?
N3. If N3can select a shortest-path
N0to construct a partial shortest-path destination
3 Simulation and Numerical Result
The goal of our simulations is to justify our IPLLP scheme. We observe the per-
formance in protectability. Protectability is defined as the ratio of the protectable
O-D pairs to the recoverable O-D pairs. For example, after IGP converges, the 6

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Local Link Protection Scheme in IP Networks799
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Fig.1. (a) The divided subtrees after link {N3,N2} failure occurs, and (b) the traffic
flows along the partial shortest-path destination tree to N0 with IPLLP scheme
failure paths can be repaired. However, by a protection scheme, if only 3 paths
can be protected, the protectability is equal to 0.5.
In our simulations, topologies are given. The shortest path algorithm (i.e.,
Dijkstra’s algorithm), our IPLLP and the LFAP are implemented in our pro-
gram. First, the routing tables of each node that contain primary next hops and
backup next hops are built. Second, all scenarios of each link failure are simu-
lated, and all O-D pairs are tested according to the present routing tables. Third,
the shortest path algorithm is performed to repair all scenarios of each link fail-
ure. Finally, the numbers of the protected paths and the recoverable paths are
collected, and the average protectability can be computed statistically.
Random flat topologies in our simulation are generated by BRITE topology
generator[4]. A new node connects to a candidate neighbor node using Waxman’s
probability function (α = 0.19 and β = 0.2), and the total node number |N|
and the average connectivity degree d are given. The connectivity degree means
the number of connected links in a node, and then a topology G(N,L) can be
generated. Additionally, all link costs in our topologies are constant and equal,
i.e., the same link bandwidth.
Fig. 2 shows the relationships between average network protectability and
network scalability while a link failure occurs in the random flat topologies
whose average connectivity degrees are equal to 4 and 6. We observe the per-
formances of our IPLLP scheme are better than the LFAP scheme, especially
in high connectivity-degree networks. Some ECMP next hops may be feasible
loop-free backup next hops, but they don’t satisfy the criteria of LFAP scheme.
Additionally, with the increase of network scalability, the protectabilities of both
schemes are decreased. Although many available paths can be found in a large
IP network, the IP network still limits the traffic to go through the few shortest
paths because of the destination-based routing. Thus, the performance of them
cannot achieve to that of other protection schemes in connection-oriented net-
works, e.g., MPLS network, SONET and optical network; however, the IPLLP
scheme can provide a simple and effective protection solution.

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0
0.2
0.4
0.6
0.8
1
Amount of nodes
Average network protectivity
d=4, IPLFRR
d=4, LFAP
d=6, IPLFRR
d=6, LFAP
Fig.2. The performance of link protection in random flat topologies
4 Conclusion
In this paper, we propose an IP Local Link-Protection (IPLLP) scheme in an
intra-area routing domain, which provides a simple and efficient solution for IP
network protection. Extra control protocols and enhanced routing protocols are
not needed if a conventional link-state routing protocol is used. In our simula-
tion, all of IPLLP scheme and LFAP scheme work well, and they are suitable for
a small-scale and high-degree intra-area network. In a small network, the compu-
tational complexity would not be the major factor to impact the performance of
IPLLP scheme. Thus, the alternative of computational complexity and network
scale may be considered between IPLLP scheme and LFAP scheme. Addition-
ally, based on the Destination SPT concept, our work may extend to protect
multiple failures by grouping failures and defining failure events. Its affected
area and feasible next hop can be decided in advance. Finally, we believe that
IPLLP scheme can give a good solution to IP network protection technology.
References
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2. Shand, M., Bryant, S.: IP fast reroute framework. IETF Draft (2006) draft-ietf-
rtgwg-ipfrr-framework-05.txt.
3. Atlas, A., Zinin, A.: Basic specification for IP fast-reroute: Loop-free alternates.
IETF Draft (2006) draft-ietf-rtgwg-ipfrr-spec-base-05.txt.
4. Medina, A., Lakhina, A., Matta, I., Byers, J.:
sal topology generation. In: Proc. IEEE Modeling, Analysis and Simulation of
Computer and Telecommunication Systems. (2001)
BRITE: an approach to univer-