Routing Information Exchange in Large-Scale WDM Networks
Yong Zhu, Admela Jukan and Mostafa Ammar
Georgia Institute of Technology, Atlanta, GA 30332, USA
{yongzhu, ajukan, ammar}@cc.gatech.edu
Abstract—In this paper, we focus on issues of routing information
exchange in large-scale WDM networks. Specifically, we present
two schemes, i.e., direct routing information exchange and
routing information broker. We then analyze and compare these
schemes based on their scalability, complexity and
communication overhead. Finally, on examples of metro alloptical networks interconnected over an all-optical WDM
backbone, we present the related numerical results based on
simulations.
I.
INTRODUCTION
In large-scale optical networks that generally include
multiple domains, such as backbone and metro networks based
on WDM, reliable routing information exchange among
different parts of the network is an important issue. For multidomain networks or for networks with multiple segments [1],
i.e., where an optical network is composed of a set of
networking segments, each representing a sub-network that
requires separate consideration for wavelength routing and
administration, this function involves both intra-segment and
inter-segment routing information exchange.
There is very little existing work addressing and evaluating
routing information exchange, in particular for multi-segment
networks. Pendarakis et al., considered routing information
exchange across the UNI and NNI [2]. In [3], OSPF was
extended to support carrying link state information for
Generalized Multi-Protocol Label Switching (GMPLS).
Wang et al., proposed OSPF extensions for metro/core interworking when routing a connection across multiple subnetworks [4]. However, little work has been done to evaluate
the performance of different routing information exchange
schemes.
In this paper, we first present two solutions for routing
information exchange: direct routing information exchange
and routing information broker, differing in the location where
the routing information is maintained and routing decision is
made. In combination with various information refreshing
approaches, e.g. periodical or event-driven, these schemes are
used to handle dynamic network conditions with low overhead.
Furthermore, they are adapted to multi-segment networks in a
hybrid way to allow gradual upgrading. We analyze and
compare these schemes based on their scalability, complexity
and communication overhead. Finally, we present related
numerical results on examples of metro all-optical networks
interconnected over an all-optical WDM backbone.
II.
ROUTING INFORMATION EXCHANGE ARCHITECTURE
A. Network model
The network we are using for evaluation is composed of
nodes and links, which are collaborating in the optical control
plane to provide routing information and to make the routing
decision. The network under control is modeled using the
multi-segment model [1], which is capable of representing the
network interconnections and capturing the following three
critical dimensions: i) heterogeneous, segment-specific
topologies and routing objectives; ii) global, intra-segment,
and local, inter-segment, traffic; iii) gateway adaptation on
segment boundaries. Clustering into segments can reflect any
consideration for wavelength routing such as administrative
domain, management policy, traffic aggregation, or vendor
specification. In this paper, we assume the existence of
multiple segments, e.g. metro segments interconnected over
the optical backbone (Fig. 1). The above model is generic,
since every network that is not segmented can be represented
as a segmented network with a single segment.
Metropolitan network
Metropolitan network
S4
S3
S1
Backbone network
Internal Link
Gateway Link
S2
S5
Metropolitan network
Metropolitan network
Fig. 1.
Metro/Backbone multi-segment network
Wavelength routing in optical networks, i.e., path selection
and wavelength allocation, requires the control plane to
provide the following routing information regarding current
optical network states:
• Topology: includes both connectivity within segments and
segment interconnections through gateways.
• Resource availability: number of wavelengths on links (i.e.,
wavelength capacity), wavelength continuity constraints.
• Gateway adaptation information: number of gateways,
their placements and corresponding adaptation properties
(wavelength conversion, waveband interchangeability).
• Administrative information: such as choices of segment
specific routing algorithms, gateway selection rules [1],
and protection/restoration considerations.
Based on how the routing information is maintained and
exchanged, we present two schemes, namely direct routing
information exchange and routing information broker.
B. Direct routing information exchange (DRIE)
In the Direct Routing Information Exchange (DRIE)
scheme, routing information is exchanged directly among
nodes, either through in-band or out-of-band channels. DRIE
can be accomplished through either flooding or hierarchical
distribution. In flooding based DRIE (Fig. 2.a), each node
builds a consistent view of the whole network by advertising its
local information and forwarding any received information to
all its neighbors, except for the node that the information
comes from. This involves excessive amount of information
exchange and does not scale well. A more scalable approach is
hierarchical information exchange (Fig. 2.b), where the routing
information is flooded within the local segment and only the
aggregated information (such as segment connectivity) is
distributed to other segments. The tradeoff of this approach is
that each node has detailed local information and only
aggregated information regarding other segments.
(a)
RIB have the flexibility of providing rich functionality in terms
of routing and signaling, while at the same time hiding
segment-specific implementations or administrative policies.
D. Hybrid routing information exchange
In a heterogeneous optical network, different segments can
have different properties such as traffic pattern, wavelength
conversion capability, and optical technology. Specifically,
each segment can have its own solutions of routing
information exchange, either direct exchange or through
brokers. Fig. 4 shows an example of hybrid architecture,
where segment A has a RIB and the other two segments
implement hierarchical based direct information exchange.
This flexibility is important such that it allows network
carriers a smooth upgrade.
(b)
A
Networking Segment
Gateway and
Gateway Link
Routing Information
Exchange
Fig. 2.
Direct routing information exchange: (a) flooding based DRIE (b)
hierarchical DRIE.
C. Routing information broker (RIB)
The idea of broker has been proposed in differentiated
services [5], where the bandwidth broker (BB) is an agent
responsible for allocating preferred service to users as
requested, and for configuring the network routers with the
correct forwarding behavior for the defined service. We use
this idea to overcome the problem of the DRIE scheme, where
the basic assumption is that all segments can directly
communicate with each other. This might be hard to achieve
among heterogeneous segments from different vendors. This
motivates us to consider the Routing Information Broker (RIB),
a separate entity sitting on top of the network collecting and
maintaining routing information and accomplishing a variety of
routing functions.
(a)
(b)
Networking
Segment
Gateway and
Gateway Link
RIB
Broker-Broker Routing Information
Interface
Exchange
Fig. 3.
Options of optical routing information broker: (a) central RIB (b)
segment specific RIB.
Two RIB architectures are possible: central broker (Fig. 3.a)
and segment-specific broker (Fig.3.b). The central RIB collects
all the routing information of the whole network through some
information exchange channels, maintains the routing
information database and makes the routing decision. On the
other hand, segment specific RIB architecture has multiple
brokers and each of them perform local wavelength routing
functions for a single segment, e.g. based on vendor-specific
implementations or administrative policies. Brokers also
exchange global information with each other. Segment specific
B
Fig. 4.
C
Hybrid routing information exchange.
E. Refreshing the routing informaiton
The state of optical network resources is not expected to be
static. It can be dynamic in case of link/node up/down,
gateway up/down, or based on resource availability change
(wavelength capacity, free wavelengths). Outdated routing
information tends to deteriorate wavelength routing
performances and may even result in a sub-optimal or
resource-inefficient routing decision. Keeping the routing
information up-to-date is therefore of paramount importance.
The refreshing of the routing information can be periodical or
event-driven. Periodical refreshing may be suitable when the
network information tends to be more static, in which case the
information can be relatively infrequently refreshed, to reduce
communication overhead. Another way is to trigger refreshing
by events such as changes in topology, wavelength capacity,
and wavelength utilization. For example, when wavelength
utilization reaches a threshold, the link is required to advertise
the current available wavelengths. Since only the link
information related to the change is refreshed, event-driven
refreshing has the advantage of smaller routing information
exchange overhead. A special event can be a connection
triggered refreshing, where some important connections (such
as long term, high bandwidth connections) will solicit the
network for the current routing information. In the eventdriven approach, the refreshing time depends on the traffic
pattern and the routing algorithm, i.e. bursty traffic will result
in more frequent refreshing of the routing information. The
periodical and event-driven approaches can be also combined
to achieve a more flexible solution in terms of low
communication overhead and accuracy. For example, using
infrequent periodical refreshing with event-driven updates can
provide quick responds to bursty local network state changes
and capture smoother fluctuations with low communication
overhead.
F. Interactions with wavelength routing
Routing in the optical control plane includes two critical
dimensions: routing information exchange and wavelength
routing (i.e., path selection and wavelength allocation). On the
one dimension, the routing algorithm performs the path
selection and wavelength allocation based on the available
routing information. On the other dimension, wavelength
routed networks need to provide up-to-date and consistent
routing information to the routing algorithm.
We have developed three wavelength routing algorithms for
multi-segment optical networks: end to end shortest path (E2E),
concatenated shortest path routing (CSR) and hierarchical
routing (HIR), differing in the way the global traffic is
accommodated and the routing information is maintained [1].
E2E routing assumes every node maintains full information of
the network and selects the end-to-end path using global
shortest path algorithm. In CSR routing, each segment decides
the route and allocates wavelengths only based on local
information. Gateways, on the other hand, make the decision
regarding to the next segment towards the destination based on
the segment interconnection information. HIR routing is
between E2E and CSR in the sense that all nodes maintain
local information and some inter-segment connectivity
information such that they can directly choose the right
gateway towards the next segment to the destination.
Routing Algorithm
E2E
CSR
HIR
PNNI
Flooding
DRIE
HIR DRIE
Central RIB
Segment
RIB
Routing Information Exchange Schemes
Fig. 5.
Relationship between routing algorithms and routing information
exchange scheme.
Both flooding based DRIE and central RIB schemes can
provide the routing algorithm full routing information of the
whole network, therefore, they can accommodate any routing
algorithms including E2E, CSR and HIR. On the other hand,
both hierarchical DRIE and segment specific routing schemes
can only provide detailed routing information regarding the
local segment and aggregated information regarding other
segments. This is sufficient for both CSR and HIR routing but
inadequate for end-to-end path selection and wavelength
allocation. PNNI-based routing is similar to the CSR routing
since it requires internal nodes to maintain local information
and peer group leaders maintain global information. Therefore,
PNNI can be accommodated by all the routing information
exchange schemes we proposed. Fig. 5 summarizes the
relationship of routing algorithms and routing information
exchange schemes.
Wavelength routing is based on the approximate
information (due to dynamic conditions) given by routing
information exchange schemes and the traffic pattern and
routing algorithms will affect the fluctuation of resource
utilization which will in turn trigger routing information
refreshing. Therefore, problems of routing information
exchange and the wavelength routing are coupled. In this
scenario, how to evaluate the performance of the routing
function as a whole is an interesting research issue.
III. ANALYSIS OF ROUTING INFORMATION EXCHANGE SCHEMES
To be able to quantitatively analyze how different routing
information exchange schemes will perform and scale, we
identify the following parameters related to the scalability,
complexity and communication overhead.
• Bandwidth requirements (BR): In all routing information
exchange schemes, the routing information is disseminated
in forms of Routing Information Advertisement packets
(RIA packets). Since the RIA packets are handled by
individual nodes (or brokers) and transmitted through links,
the amount of RIA packets determines requirements of
both the process power at individual nodes and bandwidth
on links in the control plane.
• Memory requirements (MR): All routing information
exchange schemes need to maintain databases regarding
current network states. The size of the database determines
the memory requirements at network nodes and brokers.
In the rest of this section, we will analyze each routing
information exchange scheme based on these issues. As
described previously, routing information can be refreshed
periodically or triggered by events. The bandwidth
requirement analysis here is for periodical refreshing. For the
event-driven refreshing, bandwidth requirements cannot be
estimated without the knowledge of the traffic pattern and
wavelength routing algorithms.
A. Analysis of the direct information exchange
In flooding based DRIE, since the routing information
regarding each optical link is flooded throughout the control
plane, any control plane link carries at least one RIA packets
for each optical link during any refreshing period. This
determines the lower bound of the link bandwidth requirement.
If a node has seen the incoming packet before, it silently
discards the packet. Therefore, there are at most two copies of
the same RIA packet carried by any single link, this refers to
the situation where both ends of the link forward the same RIA
packet at simultaneously. Assume the network under control
has a total of L links, the size of the RIA packet is S, and the
refreshing period is T, then the link bandwidth requirement can
be expressed as follows:
L⋅S
L⋅S
(1)
≤ B < 2⋅
T
R
T
The flooding based DRIE scheme results in synchronized
routing information database at each node. The size of such
database is given by:
(2)
L⋅R
R is the size of routing information record for a single link.
In hierarchical DRIE, detailed intra-segment information is
flooded within the local segment and inter-segment
information is exchanged among all segments in the network.
We differentiate between two kinds of links: internal link
(connecting nodes within the same segment) and gateway link
(connecting nodes from consecutive segments). Internal links
carry both intra-segment RIA packets and inter-segment RIA
packets and gateway links carry only inter-segment RIA
packets. For the kth segment, the bandwidth needed for intrasegment RIA is:
Lk ⋅ S k
Lk ⋅ S k
(3)
k
Intra
k
T Intra
≤ B Intra < 2 ⋅
Intra
k
T Intra
where Lk is the total number of links in the kth segment, SkIntra
and TkIntra are intra-segment RIA packet size and intra-segment
refreshing period of the kth segment respectively. The
bandwidth required for inter-segment advertisement is:
L ⋅S
LG ⋅ S Inter
(4)
≤B
< 2 ⋅ G Inter
R _ Inter
TInter
TInter
where SInter and TInter are inter-segment RIA packet size and
inter-segment refreshing period respectively, LG is the total
number of gateway links. Therefore, the bandwidth
requirement for an internal link in the kth segment is:
Lk ⋅ S k
Lk ⋅ S k
LG ⋅ S Inter (5)
LG ⋅ S Inter
k
Intra
k
T Intra
+
TInter
≤ BR < 2 ⋅ (
Intra
k
T Intra
+
TInter
)
And the bandwidth requirement on the gateway link is BR_Inter
given in (4).
In hierarchical DRIE, every node is only required to store
intra-segment information and aggregated inter-segment
information. Therefore, memory requirements for nodes in the
kth segment is:
(6)
Lk ⋅ R k + LG ⋅ R Inter
In which Rk is the routing information record size of the kth
segment and RInter is the inter-segment routing information
record size.
Comparing results of hierarchical DRIE and flooding based
DRIE, we can see that by using multiple segments (i. e., Lk<L),
hierarchical exchange can significantly reduce the bandwidth
requirements and memory requirements. The improvement
depends on the clustering granularity such that using small
segments can achieve low intra-segment communication
overhead and database size. However, this will increase the
total number of segments for the same network, which will in
turn increase inter-segment exchanges. Furthermore, small
segment means more and more connections will travel across
multiple segments and this will increase the blocking
probability [1][6]. Equations (1) and (3) also show that the
bandwidth requirements are inversely proportion to the
refreshing period, which is consistent with the refreshing
analysis in the previous section.
It should be noted that the information exchanged through
hierarchical DRIE can be tuned to accommodate different
routing algorithms, for example, in the CSR routing [1] and
PNNI routing, the inter-segment information only needs to be
maintained at the gateway or the peer group leader (PGL)
instead of having the inter-segment information disseminated
into the segment. Consequently, the second part of (5) should
be removed and (6) becomes the requirements for the
gateway/PGL.
B. Analysis of the routing information broker scheme
In the routing information broker scheme, the routing
information is exchanged through channels between the
network nodes and the broker. Bandwidth requirements on
these channels depend on the physical location of the broker
and the topology of the network. Therefore, instead of
evaluating the bandwidth requirement on a specific link, we
estimate the total bandwidth required to perform routing
information exchange.
In central RIB, all links periodically report their states to
the central broker. Therefore, only one RIA packet per link is
needed during any refreshing period and the total bandwidth
requirement to the broker is:
L⋅S
(7)
B =
R
T
Unlike the DRIE scheme, which has duplicated routing
information databases, a single database is maintained by the
broker. The size of the database at the central broker is the
same as that of the flooding based DRIE at individual node in
(2).
In segment specific RIB, routing information in each
segment is directly reported to the local broker and we assume
inter-segment information is flooded among brokers through
broker-to-broker channels. Therefore, there are bandwidth
requirements for both collecting local information and brokerto-broker communications, and is given by:
Q
Q
Lk ⋅ S k
Lk ⋅ S k
′ LG ⋅ S Inter
′ LG ⋅ S Inter (8)
∑
k =1
Intra
k
T Intra
+ LG ⋅
TInter
≤ BR < ∑
k =1
Intra
k
T Intra
+ 2 ⋅ LG ⋅
TInter
Where Q is the total number of segments and LG′ is the
number of gateway links in the control plane. Memory
requirement of the kth broker is the same as that of a single
node in the kth segment in the hierarchical DRIE scheme
given in (6).
Comparing above results with corresponding DRIE results,
we can see that the total bandwidth for RIB scheme is close to
the bandwidth on a single link in DRIE scheme. Furthermore,
RIB has memory requirements only on brokers, the number of
which is usually much smaller than the total number of nodes.
Therefore, we can significantly improve both bandwidth
requirements and memory requirements by using broker.
C. Effects of wavelength capacity
The existence of wavelengths differs the routing
information exchange in the WDM networks to IP networks.
Wavelength capacity affects the scalability of routing
information exchange in the following aspects. Associated
with each wavelength, there is a set of routing information,
such as service type, active connections carried by the
wavelength and wavelength conversion set. All of them need
to be advertised throughout the network. Therefore, it is
reasonable to assume that the size of the RIA packet for any
link is proportional to the wavelengths capacity at that link.
Also, under dynamic network conditions, information
refreshing can be triggered by the change of wavelength
utilization. Therefore, given the same data traffic load, higher
wavelength capacity can tolerant more changes and result in
less frequent refreshing.
IV.
EXPERIMENTS AND NUMERICAL RESULTS
We perform our simulation on a multi-segment network
where a 6 × 6 mesh-torus backbone connects 4 equal-sized
metro networks with ring topology (Fig. 1). In the following
experiments, we assume all segments have the same packet
size and refreshing period and measure the number of packets
instead of bandwidth to avoid selections of these parameters.
The first experiment illustrates the scalability of different
schemes by showing the relationship between the total number
of RIA packets and the network size. While fixing the
backbone network as well as gateway locations, we increase
the network size by increasing the metro segment size. As we
can see from Fig. 6, the number of packets goes up with the
increasing metro size in all four schemes. DRIE schemes
disseminate routing information through flooding so that they
generate much more packets than RIB schemes. Hierarchical
DRIE is more scalable than the flooding based DRIE since the
flooding is limited within the segment instead of the whole
network. Since there are only 4 segments, the segment specific
RIB requires slightly more packets than the central RIB for
inter-segment exchange. Fig. 6 also shows results for hybrid
information exchange where the backbone is assumed to use
RIB scheme (e.g. Hybrid 2 DRIE means 2 metro segments
perform DRIE and others use routing information broker). The
total number of packets for hybrid exchange is between that of
DRIE and RIB schemes.
considered together, both the backbone link change and metro
link change require the same number of RIA packets and this
number is increasing with the increasing metro size. However,
in hierarchical DRIE, the amount of RIA packets for backbone
link change is fixed regardless the size of metro segment. This
shows an advantage of the multi-segment model where effects
of state change is restricted within the local segment and does
not affect other segments. Compared with DRIE schemes, RIB
schemes are more scalable to the network change such that
only one packet is needed to report single link change to the
broker (not shown here).
The last experiment shows the effect of clustering (Fig. 8).
The simulation is performed on a 200-node bi-directional ring.
All nodes are clustered into a number of equal-sized segments
from 2 nodes/segment (100 segments) to 200 nodes/segment
(1 segment). Since both flooding based DRIE and central RIB
treat the multiple segments as a whole network, clustering
does not have any effect on them. In segment specific RIB
scheme, the total intra-segment bandwidth is fixed, but the
number of segments is decreasing with the increasing segment
size. Therefore, the total number of packets is decreasing due
to reduced inter-segment exchange. For hierarchical DRIE, as
we explained in previous section, increasing the segment size
will increase the intra-segment exchange but decrease intersegment exchange. Therefore, there is an optimal segment size
where the best tradeoff between intra-segment flooding and
inter-segment exchange can be achieved and the total number
of packets is minimized.
5
Fig. 6.
Total number of packets vs. metro segment size
1000
Flooding Metro Link Change
HIR
Metro Link Change
Flooding Backbone Link Change
HIR
Backbone Link Change
900
800
Total Number of Packets
10
4
10
Flooding DRIE
HIR
DRIE
Central RIB
Segment RIB
3
10
Num of RIA packets
700
2
10
600
500
40
60
200
100
0
20
40
60
80
100 120
Metro size
140
160
180
200
220
Number of RIA for single link change vs. metro segment size
To quickly respond to dynamic network conditions,
information regarding changes should be timely refreshed. Fig.
7 shows the number of packets to advertise the state change
(e.g. link up/down, wavelength capacity/utilization change) of
a single link (metro link or backbone link). To illustrate the
scalability, we plot results under increasing metro segment
sizes while keeping the backbone fixed. In flooding based
DRIE, where both the backbone and metro network are
80
100
120
Segment Size
140
160
180
200
Effects of segment granularity
V.
300
Fig. 7.
20
Fig. 8.
400
0
0
CONCLUSIONS
We have presented and compared two routing information
exchange schemes in this paper: direct routing information
exchange and routing information broker. To handle dynamic
network conditions, routing information needs to be refreshed
which can be either periodically or triggered by events. Both
analytical and simulation results showed that using the multisegment routing information exchange can reduce the
communication overhead and memory requirements.
Furthermore, using multiple segments allows isolation of
network changes and providing flexible segment specific
control functions. Finally, our simulation also gives the
guideline for choosing suitable segment size for a network to
reduce the routing information exchange overhead.
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