Shared bandwidth reservation of backup paths of multiple

International Journal Volume 1, NumberEngineering (IJCET), ISSN 0976 – 6367(Print),
International Journal of Computer Engineering and Technology
of Computer 1, May -June (2010), © IAEME
ISSN 0976 – 6375(Online)
and Technology (IJCET), ISSN 0976 – 6367(Print) IJCET
ISSN 0976 – 6375(Online) Volume 1 ©IAEME
Number 1, May - June (2010), pp. 92-102
© IAEME, http://www.iaeme.com/ijcet.html

SHARED BANDWIDTH RESERVATION OF BACKUP
PATHS OF MULTIPLE LSP AGAINST LINK AND NODE
FAILURES
P. Praba
Research Scholar
Anna University of Technology Coimbatore
Coimbatore -641 047

Dr. S. Balasubramanian
Research Supervisor
Anna University of Technology Coimbatore
Coimbatore -641 047

ABSTRACT
Resiliency has always been a main concern in the area of network engineering and
maintenance. The main task of the Multi Protocol Label Switching (MPLS) recovery
mechanism lies in controlling the traffic flow in the network and finding a backup path to
reroute the traffic in the event of network failure. The bandwidth reserved on each link
along the backup paths can be shared by all the affected service LSPs provided the
protected failure points are not expected to fail simultaneously. To support backup path
selection, some algorithms require extra information to be carried by routing and
signaling protocols. In this paper, the efficiency of distributed bandwidth management of
restoration backup path selection algorithms used for additional information distribution
and collection are analyzed.
INTRODUCTION
As service providers move all their applications to IP/MPLS backbone
networks, dynamic provisioning of bandwidth guaranteed paths with fast restoration
capability becomes more and more crucial. In recent times, MPLS has gained a lot of
attention due to increased restoration flexibility and high reliability for services. In MPLS
[1], the ingress LSR encapsulates the packet with labels and forwards the packets along

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International Journal of Computer Engineering and Technology (IJCET), ISSN 0976 – 6367(Print),
ISSN 0976 – 6375(Online) Volume 1, Number 1, May -June (2010), © IAEME

label switched paths (LSPs). These LSPs behave like virtual traffic trunks to carry the
flow aggregates belonging to same “forwarding equivalence classes”. The flow
aggregates when combined with explicit routing of bandwidth guaranteed LSPs enables
service providers to traffic engineer their networks [2] and to dynamically provision
bandwidth guaranteed paths. Fast restoration allows backup paths to be setup
simultaneously with the active path thus ensures quick restoration of a LSP upon failure
Recently there has been a great deal of work addressing restoration functionality
and schemes in IP/MPLS networks [3]–[6], [11]–[14], [16] as well as how to manage
restoration bandwidth among network and select optimized restoration paths [7]–[10].
However, all of them deal with path-based end-to-end LSP restoration and routing
protocol fast re-convergence [13], [14]. When MPLS restoration times must be
comparable to SONET restoration times, the MPLS local restoration [2] is a faster
alternative to path restoration. Local restorability means that upon a link or node failure,
the first node upstream from the failure must be able to switch the path to an alternate
preset outgoing link so that path continuity with bandwidth guarantees is restored by a
strictly local decision. Local restoration implies that to successfully route a path set-up
request, an active path and a bypass backup path for every link and node used by the
active path must be determined. The best related paper is [6], which deals with backup
bandwidth sharing for local restoration. Since the backup LSPs are pre-established, even
though they do not consume bandwidth before failure happens, the network has to reserve
enough restoration bandwidth to guarantee the LSP restoration for any failure. Without
bandwidth sharing among backup LSPs for different service LSPs, the network may need
to reserve much more bandwidth on its links than necessary
Internet Engineering Task Force (IETF) has published RFC 4090 [2] to address
the necessary signaling extensions for supporting MPLS fast reroute, and how to use path
merging technique to share the bandwidth on common links among backup paths of the
same service LSP. However, no solution is provided on how to share bandwidth among
backup paths of different service LSPs so far. The restoration models considered in this
paper is based on the observation that the restoration bandwidth can be shared by
multiple backup LSPs so long as their protected service LSP path segments are not
susceptible to simultaneous failure[6], [8], [10], [21], [22].

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This paper discusses the efficiency of distributed bandwidth management of
restoration path selection algorithms. This paper is organized as follows. Section II gives
an overview of the options and features of restoration models. Section III gives a review
of restoration path selection algorithms.
RESTORATION MODEL – AN OVERVIEW
A. Online Routing
Dynamic routing of LSP segment is handled by on-line algorithms that routes both
the active path and the backup path for each link or node and simultaneously it optimizes
network resource utilization. The objective of the algorithm is to compute an active path
and a backup path for each node or link in the active path. The connection setup fails if
sufficient bandwidth is not available to setup the active path or the backup path. Only
single link or node failure is considered in this paper.
B. Restoration modes
Two ways of restoration are end-to-end or path restoration and local restoration. In
end-to-end restoration, a disjoint backup path from the active path is provided from the
source to the destination for each request. When there is a link or node fails, the failure
information has to propagate back to the source which in turn switches all the demands to
the backup path as illustrated in Figure 1 and in Figure 2. This information propagation to
the source makes it unacceptable for many applications. In the case of local restoration,
the backup paths are setup locally and therefore failure information does not have to
propagate back to the source before connections are switched to the backup path.

S T

S T
Primary Path
Backup Path Fault Indication Signal

Figure 1Path Restoration Figure 2 Path Restoration on Link
failure

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Backup path for the failure
of node aand links i and j.

S a b c T
i j l
k

Backup path for the failure
of linkl

Figure 3 Backup Path for Single Element Failure
Restoration has to protect against single link or single element (node or link)
failure. The amount of resources needed to provide backup for the second mode will be
greater since a node failure results in multiple link failures. In the single link failure,
when a link fails, the two nodes that are at the end point of the link detect the failures,
and they immediately switch all the demands on to the backup path. In a single element
failure model, when a node fails assume that all the links incident on this node fails. This
is detected as in the link failure case and the demands are routed across the failed node.
The algorithms discussed in this paper can handle all single link and node failures and
does not provide a backup if the source or the destination of the traffic fails as illustrated
in Figure 3
C. Shared Reservation
A protected LSP in a MPLS network has an active path and bandwidth reserved
along the backup paths for each link or node along the active path. The bandwidth
reserved on each link along the backup paths can be shared across multiple backup paths
of the same LSP and multiple backup paths of the different LSPs provided the protected
failure points are not expected to fail simultaneously. This type of bandwidth sharing is
referred to as Inter-demand sharing and intra-demand sharing.

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The design objective is that the bandwidth reserved on the backup paths must be
sufficient to recover all affected service LSPs in the event of any link/node failure. Fig. 4
shows a simple example illustrating a shared reservation, which comes from [7]. The
figure shows a network with 6nodes and 7 links. Suppose LSP 1 request asking for one
unit of bandwidth from A to B arrives at node A. Node A selects A-B for the service path
and A-C-D-B for the backup path. When these LSPs are established, one unit of
bandwidth is allocated on link AB, and one unit of bandwidth is reserved on each of links
AC, CD, DB. Subsequently, another protected LSP 2 request asking one unit bandwidth
from E to F arrives at node E. Node E selects E-F for the service path and E-C-D-F for
the restoration path. In this example, when setting up the protected LSP, it is unnecessary
to reserve an additional unit of bandwidth on link CD because the two service paths AB
and EF are failure disjoint, i.e., they are not subject to simultaneous failure under the
single failure assumption. Thus, a total of five units of bandwidth are reserved to protect
both service paths against any single link failure whereas without shared reservations, a
total of six units of bandwidth would be needed. Sharing the reserved bandwidth among
the backup paths of different protected LSPs reduces the total reserved bandwidth
required.

LSP 1
A B

Backup Path

C D

Backup Path

E F
LSP 2

Figure 4 Shared Reservation Example

D. Partial and Complete Routing Information
In MPLS networks, a link state routing protocol, such as Open Shortest Path First
protocol (OSPF) or Intermediate System-Intermediate System protocol (IS-IS), is used to

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distribute network topology information to each node in a particular network routing
domain. Traditionally, these protocols have only propagated link up/down status among
the nodes. To support path computation in MPLS networks, both OSPF and IS-IS have
been extended to propagate additional information about each link, such as available
bandwidth, bandwidth in service, etc. [17], [18]. The following link state information is
flooded by the routing protocol, which is the so-called partial information from paper[8].
Note that these information are distributed via OSPF/IS-IS traffic engineering extensions.
1) Reserved bandwidth R[e]: This bandwidth is not used on unidirectional link
when the network is in a non-failure condition, but may be used by the restoration
process upon failure;
2) Available bandwidth A[e]: This bandwidth is free to be allocated to service
bandwidth or reserved bandwidth as new LSP requests arrive. Note that the total
bandwidth on unidirectional link e is R[e]+A[e]+S[e] where S[e] is total consumed
bandwidth for service paths on link. The three types of bandwidth share a common
bandwidth pool;
3) Administrative weight W[e]: This quantity is set by the network operator and may
be used as a measure of the heuristic “cost” to the network. A path with a smaller
total weight is typically preferable.
This information is used to select the service path for each LSP request. However, to
support backup path selection, some algorithms require extra information to be carried by
routing and signaling protocols [7], [10]. There are three possible scenarios based on the
information available to the routing algorithm to achieve maximum bandwidth sharing in
the backup paths.
The first scenario that we consider is what we call the no information scenario. In this
case, we assume that the only information that the routing algorithm has about the
network is the residual (available) bandwidth on each link. In this scenario, for each link
the amount of bandwidth utilized separately by the active and the backup paths is not
known. Only the total used bandwidth is known.
In the second scenario, we assume that that the routing algorithm has complete
information, i.e., it knows the routes for the active and backup paths of all the
connections currently in progress.

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In the third partial information scenario, the information available to the routing
algorithm is slightly more than that in the no information scenario. The additional
information in this scenario is that for each link instead of knowing only the total
bandwidth usage, we now separately know the total bandwidth used by active paths, and
the total bandwidth used by backup paths.
FAST RESTORATION ALGORITHMS
MPLS fast reroute is a relatively new scheme and it is still undergoing IETF
standardization process [2]. The internet draft [2] focus on the fast reroute scheme
definition and backup establishment signaling protocol, which leaves how to select the
backup paths and implementation open for vendor/carrier innovation. Much work has
been done on end-to-end and segment shared backup path selection, but few papers have
been published on MPLS fast reroute backup path selection. The basic idea of backup
path sharing and backup path control message carrying service path information has been
used before [3], [22], but both apply for end-to-end backup path sharing only. A practical
algorithm should maximize restoration bandwidth sharing among the backup paths of the
same or different service paths without scarifying performance and scalability.
Papers [6][8] classified the routing information model as minimal information,
partial information and complete information in end-to-end restoration. In [6], a few
algorithms to compute backup paths for link/node failure on the service path with the
goal of maximizing backup bandwidth sharing were presented.
The key idea of [6] is that restoration problems can be formulated as integer
programming problems and thus uses heuristic algorithm to solve it. The cost of backup
paths can be determined by solving shortest path problems, one for each link in the
network.Let be the cost of using link (u, v) on the backup path if link (i, j) is used in
the active path. Fij represent the total amount of bandwidth reserved for the demand that
use (i, j) on the active path and Guv represent the total amount of bandwidth reserved for
the demand that use (u, v) on the backup path. For a current demand of b units of
bandwidth is defined as

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0 if Fij + b Guv and
(i, j) (u,v)
= Fij + b – Guv if Fij + b > Guv and Ruv Fij + b – Guv and
(i, j) (u,v)
Otherwise

The backup for link (i, j) can start at node i but can end at any node on the path
from j to t. This case is handled by executing shortest path algorithm backwards starting
at the sink. To maintain intra domain sharing, each node maintains a m-vector that
gives the amount of bandwidth reserved for the current demand for backing up all the
links from the given node to the destination.Modifying the cost of the links in the
computation of the backup costs can be used to account for node failures. Cost of using
link (u, v) for partial information case is defined as

0 if Fij + b Guv and
(i, j) (u,v)
= Fij + b – Guv if Fij + b > Guv and Ruv Fij + b – Guv and
(i, j) (u,v)
Otherwise

However, their algorithms assume partial aggregated information and complete
information [6] available. Also papers [6], [8] find that complete information model is
able to achieve optimized backup selection but the complete information dissemination
may not be scalable. Thus, complete information model fits for centralized algorithm.
Partial information can be easily collected via current routing information extension.
Thus, partial information model fits for distributed algorithm.
A distributed algorithm for complete information model to MPLS fast reroute has
been proposed by [15]. The distributed bandwidth management method assumes the use
of RSVP-TE extensions [19] instead of routing protocol to collect extra information and
achieves restoration bandwidth sharing among the backup paths of different service paths
independent of the backup path selection algorithm. (CR-LDP) [20] also requires similar
extensions. The paper also designed internal node data structure and described how to
maintain those data structure at each node during LSP creation and deletion. This scheme
scales well due to two facts: (1) it does not require each LSR to maintain detailed
information on each LSP’s service path and restoration paths. Only two arrays, i.e.,

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and , are maintained at each node. is
maintained by the responsible node of each unidirectional link along a backup path to
keep track of the bandwidth on that has been reserved to protect against the failure ,
which will be updated during the signaling process of LSP creation and deletion.

is maintained by the responsible node of each failure along a service path
to keep track of the bandwidth on that has been reserved to protect against the failure
itself. Then the sharable restoration bandwidth on each unidirectional link would be

Where is a conflict matrix that represents the amount of traffic
that will be rerouted on unidirectional link when failure occurs. The first of is
disseminated via routing protocol exchange. To distribute and collect the second part
information, three new signaling messages: TELL, ASK and REPLY were added. Then
the algorithm will reset link weights based on and select the disjoint
shortest path as the restoration path, where is bandwidth requirement of the traffic
request. Similar idea has been applied in [7], [10], which assumed end-to-end shared
mesh restoration.
CONCLUSION
We analyzed the problem of fast local restoration that requires efficient
distributed bandwidth management to setup bypass path for every node or link traversed
by the service path. The objective of routing messages or signaling extensions is to
manage the bandwidth usage for each connection and to optimize the use of network
resources while protecting against single link or node failure. Approaches based on
efficient distributed routing and signaling protocols were compared. And we conclude
that significant cost savings could be achieved through sharing of bandwidth among
backup paths of different service LSPs
REFERENCES
[1] Rosen et al., “Multiprotocol Label Switching Architecture”, IETFRFC 3031, 2001.
[2] P. Pan, G. Swallow, and A. Atlas, “Fast Reroute Extensions to RSVP-TE for LSP
Tunnels”, IETF RFC 4090, 2005.

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[3] S. Kiniet al., “Shared Backup Label Switched Path Restoration”, IETFInternet draft,
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[4] V. Sharma and F. Hellstrand, “Framework for Multi-Protocol Label Switching
MPLS-Based Recovery”, IETF RFC 3469, 2003.
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