On Shared Risk Link Group Optimization (Invited)

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Oct 29, 2013 (4 years and 11 days ago)

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1


Abstract


Shared Risk Link Group
s

(SRLG
s
) ha
ve

been investigated during the past 10 years due to
optical network innovation and deployment. Dense
Wavelength Division Multiplexing (DWDM)
technology
has
significantly increased
the
data
transmission capability per fiber. S
ervice provider
s
build their

ove
rlay networks
,

such as IP networks,
on
top of optical networks and all optical networks are
built over some comb
ination of DWDM equipment
and

fibers.
If there is a single DWDM system outage
or fiber outage, the set of overlay
network
links
dependent on the

failed resource

would
all
fail at the
same time. The set of links
failed by
a common
resource
outage
is called a shared risk link group
(SRLG).

SRLG
s

have
been designed and implemented
in
many
network planning tools and some network
routing protocols.
A s
ingle
SRLG

represents one
potential outage (or failure mode) and
a large service
provider’s network could easily
contain tens
of
thousands of
failure mode
s. The
greater
the
number
of
SRLG
s
, the more difficult
it is to attain good
performance from planning
tools (like routers) whose
computations are dependent on the number of
SRLGs
. For some routing protocols
using
SRLG
information, the situation becomes even worse
because a routing protocol may have space
constraints to hold
a
limited number of SRLGs.
These

issues create a

challenge to optimize the SRLG
calculation
s

such that the SRLG
-
related functions
are not impacted or
the impa
cts
to
the

SRLG
-
related
function
s are

limited.
T
his paper takes a closer look

at the SRLG

optimization issue and proposes

algorith
m
s

for

how to reduce the
number

of SRLGs
for different applications
.


Index Terms

Shared Risk Link Group,
Optimization, Algorithm, Case Study.

I.

I
NTRODUCTION

ost service provider overlay networks are built

on
top of optical networks and all

optical
networks
are built over some combination of DWDM equipment
and/or fibers. If there is a single DW
DM system
outage or fiber cut
, the set of over
lay links routed over
the
failure risk (the DWDM system or fiber)

would fail
simultaneously
. Th
is

set of
simultan
eously
failed
links is called a shared risk link group (SRLG).
SRLGs are
typically

represented by
identifiers

that
are associated with all of the links in the set.

These
IDs are used by
network planning tools or network
routing protocols

to represent the d
ifferent
ways that
the network can fail, i.e., the failure modes.
The
SRLG IDs can also be
viewed as

a representation of a
network’s diversity, since two links that are not
simultaneously failed by an SRLG ID can be
considered to be diverse under that part
icular mode of
failure
.


For example, an IP link (
part of an
overlay
network)

between
two routers may
have

multiple
SRLG IDs
associated with it
and a single SRLG ID
could be
shared by

multiple IP links
. Thus the SR
LG
i
nformation
for each IP

link describes a list of SRLG

ID
s to which the link belongs. An SRLG can also
represent
a
potenti
al node outage
,

such as
a
total or
partial router outage or

outages due to

router

maintenance or software u
pgrade procedure
s

in the IP
network
.

To manage the
total number of SRLG IDs, the
lower
-
layer topology information is often consolidated.
For example, for the purpose of restoration planning,
a long path of fiber cables that do not bifurcate at
intermediate locations can be aggregated into a single
SRLG ID
(i.e., the path does not encounter a splice
location where some fibers are spliced into a different
cable or end at an fiber patch panel in a central
office).
Conversely
, for a given SRLG
ID, we can list
all the IP

links that route over that SRLG. Each
SRL
G ID is unique within a network routing domain.

For diverse routing and protection purposes, IGP
(interior gateway protocol) routing protocols in both
the
standardized specification and
in
commercial
products would support the

assignment of

SRLG
informatio
n

to the network links
. IETF RFC 4203 [1]
specifies an

SRLG as a sub
-
TLV (type, length, value)
of the link TLV. The value is an unordered list of
integer SRLG IDs that the link belongs to.

On Shared Risk Link Group
Optimization

(Invited)

Guangzhi Li, Dongmei Wang, Timothy Gallivan, and Robert Doverspike

AT&T Labs, New Jersey, USA,
{guangzhi.li,dongmei.wang,timothy.gallivan,rdoverspike}@att.com

M

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2


Figure
1
: A

Sample of

Overlay Networks


Figure 1 shows a typical service provider
’s

overly
networks.
The bottom of the network layers is the
fiber network, different DWDM sub
-
networks,
including early point
-
to
-
point DWDM sub
-
networks,
large reconfigurable optical add
/drop multiplexing
(ROADM) sub
-
networks
and

recent
colorless/directionless coherent optical sub
-
networks.

In order to better use optical network capacity and
support relative
ly

lower
-
speed private line services,
service providers
may
build an electronic
al
ly
-
switched
optical network layer, including SONET/SDH ring
networks, intelligent optical switching networks, or
optical transport networks.

As more and more
services

and
applications are IP based, the IP network
is necessary for a service provider and it
can be
built
on top of the electronic
ally
-
switched optical network
layer and/or on top of the DWDM optical network
layer.

A link in the IP network is a circuit in the
optical networks.

Of course there are digital cross
-
connect network
s

on top of
the
electr
onic
ally
-
switched
optical network to

support very low rate services.

Thus for all wired services, the data transmission
must ultimately

go down to the fibers. If a fiber is c
ut,
or an optical device fails
,
a large number
of upper
-
layer services
may
go down

simultaneously. In order
to support protection/restoration at the upper layer
overlay networks, we need to know which overlay
network links share a common failure

risk and

which
overlay network links
are unlikely to
fail at the same
time. This information

is provided
by associating
SRLG ID
s (
called bundle ID
s

in some products
) with
the overlay network links
.

In the AT&T Intell
igent Optical Switch (IOS)
network
, the equipment node is the Ciena Core
Director. The nodes are connected by “lines” (SONET
OC
-
48
s

or OC
-
192
s
) and multiple lines between the
same switch pair are aggregated into “links”. The
Ciena OSRP

(Optical Switching and Routing Protocol)

protocol defines a list of SRLG bundle IDs for each
OSRP link [2
,3
]. From the point of view of the Core
Direct
or and its Element Management System, these

bundle IDs are simply integers

and constraints
associated with each link, i.e., they have no actual
topological graph model for lower layer networks.
Each bundle ID
may
represent a portion of the
underlying fiber

path

or it may represent some other
failure risk (like a
n intra
-
office

tie
cable or some piece
of equipment)
.

IEFT RFC 4203 does not specify a maximum length
for the list of SRLGs per link, but some comme
rcial
products

implement
maxima. For example, the C
iena
OSRP routing protocol enforces a maximum list
length
for

the
number of bundle IDs per link [3
].
However, in reality, there are links
whose SRLGs

ex
ceed the maximum

number
. A
s mentioned above, if
each SRLG represents the smallest unit of an
individual
fiber span (i.e., cable between two cable
splice locations
1
), the number of SRLG IDs could
easily
exceed 4
0,000 IDs in a large carrier
’s

fiber
network
. One solution is to combine multiple fiber
spans (without bifurcation)
in
to one super fiber span
to reduc
e the number of SRLGs.


Figure
2
: Overlay Network with Fiber

S
pans

Figure 2 shows
an
example of one
simple
overlay

1
This fiber span definition is over
-
simplified to make it clear.

A fiber
span can encompass multiple fiber cables and may not have splice
points at
its ends.


Two cables travelling diversely could converge in
a man
-
hole cover (no splice) and travel together for some distance
before separating again.

In such a case, a single fiber span would
contain both cables for the distance that they travel togethe
r.

That is,
fiber spans are really defined in terms of physical proximity of
multiple cables.


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3

network with associated fiber

spans. In this example,
link AB routes over span 1, s
pan2, span3; link BC
over span4; link CD routes

over
span5, s
pan6; link AD
routes over span1, span12, span11, span10, span9,
span8, and span7; link AG

routes over span1, span12,
span11, span10, span13, span14, span15, and span16;
link D
E
routes over span17
; link GE routes over
span16, span15, span14, span13, span9, span24,
span23; link EF

routes over span20, span21, span22;
link G
F
routes
over span1
6
, sp
an18, span19. There
are
a
total

of

24

spans. If each span

is one SRLG,
there are total
24
SRLGs. Both lin
k AD and link GE

have 7 SRLGs

and link AG has 8 SRLGs
. If we
combine multiple spans into
one
super span, we could
combine spa
n2, span3 into span2
-
3; span5, span6 into
span5
-
6;
span10, span11
, span12 into span
10
-
11
-
12;
span 7, span8 into span7
-
8
;
span13,
sp
an14, span15

into span
13
-
14
-
15; span18, span19 into span
18
-
19;
and span20, span21
, span22

into span20
-
21
-
22,

span23, span24 into span23
-
24.
After combination, w
e
can re
duce SRLG number from
24
into 13

without
impacting the
information carried by the

SRLG
s
.

The
number of SRLGs per link is al
so reduced, for
example, link AD has only 4

SRLGs, and

link AG

only
has 4

SRLGs.

SRLG

has been proposed as a fundamental

concept
for failure management

in networks [4].
In general,

SRLG information is maintained

manually by the
network

operator with the knowl
edge of the physical
fiber routes

of the

network. In some cases, SRLG auto

discovery schemes can also

be used
[5].

SRLG is an
important component in survivable network design.

In order to provide the failure
-
independent protection
to a

c
ustomer demand

from any single SRLG failure,

a pair of

SRLG
-
disjoint paths
should be determi
ned.
That is, the links along service

path
must

not share
any common SRLGs with t
he links along the
restoration path.

According to rese
arch work [6],
finding

a pair of SRLG disjoint paths is
an NP
-
complete
problem. Thus some network management tools may need to
rely on integer linear programing to find SRLG diverse paths.
During the past decade, almost all of research papers related
on SR
LGs are focusing on either restoration capacity planning
[7,8,9], or
distributed signaling [10,11], or
SRLG

diverse path
algorithms [
12,13
,14], or trap avoidance algorithm [15]. To
the best of our knowledge, we have not seen any research
work focusing on h
ow to optimize SRLG size in a network,
which is our objective of this paper [16].


The paper is organized
as

follow
s
:
In
section II
,

we
describe

several

SRLG
-
related functions. Our
objective is to
reduce

the number of SRLGs without
impacting those SRLG
-
related functions. Then we
present SRLG optimization algorithms in section III.
Section IV proves the correctness of our algorithm and
discusses why our optimization does not impact the
SRLG
-
related functions. In section V, we present a
few case studies of

implementation
s

of SRLG
optimization algorithms in network operation
applications. Section VI is our conclusion.

II.

SRLG

FUNCTIONS

SRLG
s

were
invented for diverse routing, including
protection and restoration. If two paths do not share a
common SRLG, these t
wo paths won’t fail
simultaneously by a single SRLG failure. In practical
network management, we classify SRLG functions
into two classes

completely diverse routing and
maximally diverse routing
.

Complete
ly

Diverse Routing
is useful in a number of
differen
t situations. Following
are a few examples.
In
a network with IGP supporting SRLG information,
each node has the view of the entire network,
including the list of SRLG IDs in each link. Then each
node or the element management system is able to
provide

a

Fast R
eroute

Computation
.


I
n
link
-
based

MPLS FRR (multi
-
protocol label switching fast
reroute), each backup LSP (label switched path) is a
list

of
links that are divers
ely routed from a given
link. In node
-
based FRR, a backup LSP

is also a list of
links,
but further depends on the next hop of the
primary LSP at each node along its path (the backup
LSP skips the next node). Each node in the LSP is
required to create an SRLG diverse backup LSP to its
next
-
next hop node except the last two nodes. The
second l
ast node is required to create a SRLG diverse
backup LSP to the last node. During any outage, the
right upstream node detects the outage and it
switches the LSP traffic to the SRLG
-
diverse backup
LSP immediately. In any single
-
SRLG outage, such a
scheme pr
ovides the fastest r
ecovery to the failed
LSPs.
Fully
-
diverse routing is also used in end
-
to
-
end
Protection/Restoration Path Computation
s.

I
n
end
-
to
-
end protection/restoration scheme
s
, each source node
or the element management system needs to find one
ser
vice path and one protection/restoration path for
each demand, and the two
end
-
to
-
end
paths have to be
SRLG diverse.
A
Diverse Routes Computation

may be
necessary when
a customer want
s

to create his own
overlay network by provisioning several mutual
divers
e LSPs. Since finding two SRLG diverse paths
is NP
-
complete problem, the network will use either
a

heuristic algorithm or integer linear programming to
find the diverse routing paths. Either way, we do not
want any two

paths to share any SRLG
s
.

Fully
-
diver
se routing plays a useful role in
Maxim
um

Restoration C
apacity

C
alculation
s
.

T
o enable rapid
restoration,
some intelligent networks pre
-
calculate

the restoration path
for each service path and store

the
restoration
path in
the
so
urce node of the service
path [3
]. Once an outage occurs, the source node
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4

detects (or is notified of) the outage and starts the
restoration process using the pre
-
calculated SRLG
-
diverse restoration path. To reduce required
restoration
capacity, the network is usually designed
to c
onsider only single SRLG outages and thus
restoration paths could share restoration capacity
over non
-
simultaneous SRLG outages. This is called
shared mesh restoration

[10
]. Network
managemen
t
can use the fully
-
diverse restoration
paths

to calculate the
ma
ximum

restoration cap
acity
required on

each link

for any single SRLG failure.


Maximal Diverse Routing

is an option

when
it is
impossible to find complete
ly

diverse
routing
paths for
a demand set in the network
.

In maximally
-
diverse
routing, a

shared SRLG
penalty is minimized, where
each SRLG is associated with
some

penalty number.
For example, each fiber span has distance and the
distan
ce could be the penalty number

when routes
overlap on that span.

E
ach

overlapping

node may also
be
associate
d

with
some

positive penalty number.

III.

O
PTIMIZATION ALGORITH
M
S

One may define any potential outage as one SRLG,
such as a city, a building, a switch or cross
-
connect
component, a conduit, a fiber span, etc. Although
there are SRLGs in upper layers, such as router
common

equipment outages, the most common SRLG
represents the potential outage of some sort of fiber
spans, or spans for simplicity. As mentioned
previously, in this case we need to explore methods to
consolidate the size of the SRLG set for large carriers,
but
without affecting the network restoration metrics
or network SRLG functions
.
In this paper, we mainly

consider how to optimize SRLGs from spans

only and
the optimization algorithms provided in this paper
can be easily extended to other type of SRLGs.

Our
span
-
based
SRLG
s

have
following properties:

(1) e
ach
spa
n is included in only one SRLG; (2) two links
sharing no SRLG

are span diverse (not necessarily
node diverse)
; (3) two links sharing a SRLG are not
span diverse, and t
hey share all o
f the spans in the

shared SRLG; (4) SRLGs

are the smallest possible set
of quantities that fully rep
resent the diversity of
overlay

network
; (5) spans comprising a given SRLG
are not
required

to be geographically adjacent, which
is more general than the situation in Figure

2.

The diagram in Figure 3

illustrates the
general
r
elationship among links of an overlay netw
ork (such
as the IP layer or intelligent

network
layer), SRLGs,
and spans. Each circle represents an overlay link and
contains the spans over which it routes. By examining
the areas of overlap, for the purposes of restoration
calculation of this overlay network, we could group
the
1
6

fiber spans into 7 S
RLGs. For example, the
link associated with the red bubble route (the top
circ
le) over SRLGs 1, 2, 3 and 4,

the blue route (the
right cir
cle) over SRLGs, 3, 4, 6, and 7, the green
route (the left circle) over SRLGs, 2, 3, 5, and 6.


Figure
3
: Example of Grouping Spans into SRLGs

Thus, for network capacity design, we
independently consider the failure of each SRLG and
how rerouting is accomplished. For example, the
outage of SRLG
-
2 represents an outage of either fiber
-
span 6 or 10.
Next, we will formally describe how to
combine spans
2

into SRLGs for a specific network
G(V,E)
, where V is the set of overlay network nodes
and E is the set of overlay network links (which we
will refer to simply as
links
). Assume we know the
specific fibe
r routes of each link. Then each link
l
has
a list of fiber spans that it routes over, denoted as
F
l

,
and each fiber span
f

has a list of links that route over
it (called the
dependent

set of links), denoted as
L
f
.
Now we take a close
look at

following
cases:



The

dependent set of fiber span
x

equals the
dependent set of another fiber span
y
,
L
x
= L
y
: in
this case, we can combine these two fiber spans
into one single SRLG because if two upper
-
layer

links are diverse from one fiber span

x
, the same
two
upper
-
layer links

must
also
be diverse from
another fiber span

y
.

Actually we can combine all
fiber spans with the same dependent set into one
single SRLG. This is ex
actly what we showed in
Figure 3
.



The dependent set of fiber span
x i
s a subset of the
dependent set of fiber span
y
: if two upper
-
layer
links do not share span y,
the same two upper
-
layer links

must not share span x. For the first
three SRLG related functions in section II of
complete diverse routing, we can drop fiber spa
n
x

and keep span y as one SRLG. However for
maximal diverse routing, we need to keep both

2

We use span consolidation as one example. Other failure modes
can be consolidated similarly.

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5

span x and span y and cannot drop the SRLG
associated with span x. For example, in Figure 3,
for complete diverse routing, we may only need to
maintain SRLG 3 per li
nk and drop all other
SRLGs. However for maximal diverse routing, we
need to maintain all SRLGs for a single link,
shared by two links, and shared by all three links,
i.e., all the 7 SRLGs as well as the SRLG penalty
associated with each SRLG overlap, say
the total
fiber span distance of each SRLG.

The above observations can be
easily
cast

into two

algorithm
s

to quickly compute a reduced set of SLRGs
for each network link that are adequate to accurately
perform the
complete diverse routing and maximal
div
erse routing functions.

Input:
network G(V,E) and

span route of each link

Output: SRLG info for each link

Algorithm

1
: for complete diverse routing

[1]


read network

[2]


find the list of
spans for each link
F
l

[3]


for each
span

f
, find the list of links over it
L
f

and

set
span[f].remove = 0

[4]


for i=0 to n
-
1
, where n is the number of

spans

[5]


for j=i+1 to n

[6]


if
L
i

=
L
j
, mark span[j].removed = 1

[7]


else if
L
i



L
j
, mark span[i].removed = 1

[8]


else if
L
j



L
i
, mark span[j].
removed = 1

[9]


end j

[10]

end i

[11]

for each span
f
, associate
SRLG[
s
f
]

with span
f

if
span
[
f
].remove = 0

[12]

for each link
l
, define reduced SRLG set
S
l
=



[13]

for each span
f

in
F
l


[14]

if span
[
f
].remove = 0
, add
SRLG[
s
f
]

to
S
l

[15]

end for each span
f

[16]

end for each link
l

[17]

for each link
l
, report SRLG info of
S
l
.


Algorithm

2
: for maximal diverse routing

[1]

read network

[2]

find the list of
spans for each link
F
l

[3]

for each
span

f
, find the list of links over it
L
f
,


record span[f].length, and

set
span[f].remove = 0

[4]

for i=0 to n
-
1
, where n is the number of

spans

[5]


for j=i+1 to n

if
L
i

=
L
j
,

mark span[j].removed = 1
, and
record span[i].length += span[j].length

[6]


end j

[7]

end i

[8]

for each span
f
, associate
SRLG[
s
f
]

with span
f

if
span
[f].remove = 0

and
set
SRLG[
s
f
].penalty

=
span[f].length

[9]

for each link
l
, define reduced SRLG set
S
l
=



[10]

for each span
f

in
F
l


[11]


if span
[
f
].remove = 0
, add
SRLG[
s
f
]

to
S
l

[12]

end

for each span
f

[13]
end for each link
l

[14]
for

each link
l
, report SRLG info of
S
l
.

IV.

A
LGORITHM CORRECTNESS

In above SRLG optimization algorithm

1
, we mark
following two
types of fiber spans as removed: (1) if
two
spans have exactly the same set of
dependent
links, we combine them to
gether and leave onl
y one
span to represent them. Basically the network
separates the set of spans into different groups based
on their
dependent

links. Each group is assigned one
SRLG ID. This

idea is illustrated in Figure 2 and
Figure 3
; (2) if the
dependent

links of one gr
oup is a
subset of
the dependent

links of another group, we
can drop the first group and only keep the second
group. The reason is that when two
upper
-
layer

links
are diverse, they must be diverse on all SRLG groups,
i.e., they do not share any common SRLG
. If two
upper
-
layer

links are diverse on the second group,
they must be diverse on the first group. So we are safe
to drop the first group for
complete
ly
-
diverse routing.
In Figure 3
, SRLG1 only supports link1, SRLG2
supports both link1 and link2, while S
RLG3 supports
link1, link2, and link3. In this case, we can drop
SRLG1 and SRLG2, and only keep SRLG3. Similarly
we can also drop SRLG4, SRLG5, SRLG6, and SRLG7
without losing essential information about link
diversity.

Thus it is easy to verify the algorithm
correctness for
complete
ly
-
diverse routing
function
s
listed in section II.

Next
we look at the
maximal restoration capacity
calculation

function
. In a shared mesh restoration
scheme [4], we define a matrix called
failneed[
s
][l]
,
where
s

is the SRLG index and
l

is the link index.
Matrix
failneed[
s
][l]

maintains the restoration
capacity needed in link
l

if
SRLG
s

fails. The maximal
restoration capacity is defined as:
R[l] = max
s

failneed[s
][l]

over all SRLGs.
For any

two SRLG
s
1

and
s
2
, assume
L
s
1



L
s
2
. If we can prove
failneed[s
1][l]


failneed[s
2][l]

for any link
l
,
then

we
are safe to drop SRLG
s
1

without impacting the
calculation of
R[l]
.

For

any outage
s

and a set of path
s

P, we define
P
s

=


{p

P, p∩L
s



}
,

i.e.,
the subset of paths routed
over
SRLG
s
.

For any path
p

P
, we define
Kp

as the
set of links of
p
,
Cp

as the capacity of
p
, and
p*

as the
pre
-
calculated complete
ly
-
diverse restoration path of
p
. We further define
V
s

=

{l: l

p*, p

P
s
}
. Then we
have
failneed[s
][l]
=


p


Ps
, l


p*

Cp
, i.e,

failneed[s
][l]

is
the sum
of the bandwidth of all demands

failed by
s

whose

restoration paths use link
l
.

Assume
L
s
1



L
s
2
,
then for any
p

P
s
1
, p∩L
s
1



, we have
p∩L
s
2



. So we
have
P
s
1



P
s
2
. Thus for any
l

V
s
1
, we have
failneed[s
1][l]

failneed[s
2][l]
. For any
l

V
s
1
,
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failneed[s
1]l]=0
. So for any link
l
, we have
failneed[s
1][l]


failneed[s
2][l]

when
L
s
1



L
s
2
.
According to the definition of maximal restoration
capacity calculation formula, we can drop SRLG
s
1

without impacting the maximal restoration capacity
calculation for any link.

Algorithm 1 can be used for complete
ly
-
diverse
routing, but cannot be used for maximal
ly
-
diverse
routing.
For examp
le, if we apply algorithm 1 to

the
network of Figure 2, we have link AB with SRLG1;
link BC with SRLG4; link CD with SRLG5; link AD
with SRLG1, SRLG9; l
ink AG with SRLG1, SRLG
16;
link GE with SRLG
16, SRLG9; link DE with SRLG17;
link GF with SRLG16
; and link EF with SRLG20.
If
we need t
o find two maximal
ly
-
diverse routes between
A and G, the solution could be AG, and AD
-
DE
-
GE,
which only share common SRLG1 and SRLG16. In
fact, these two routes also share SRLG10
-
11
-
12 and
SRLG13
-
14
-
15. These two routes are not maximal
diverse routes. On t
he other hand, if we apply
algorithm 2 to the same network, we only combine
spans with same set of dependent set and accumulate
the combined distance. In this way, we do not lose any
failure related information. The maximal diverse
routes between A and G w
ill be route AG and route
AB
-
BC
-
CD
-
DE
-
EF
-
FG, which only share common
SRLG1 and SRLG16.

V.

C
ASE STUDIES

We have used
the
SRLG o
ptimization process
es
described above

in many
of AT&T’s

internal
management tools. In this case study

section
, we
describe a
DWDM ne
twork planning tool with
complete
ly
-
diverse routing and maximal
ly
-
diverse
routing
using

SRLG optimization
.

We also describe

a
process

based on SRLG optimization

to drop extra
SRLGs due to protocol limitations

for
the i
ntelligent

optical
network
.

AT&T
’s

DWDM layer is a highly heterogeneous
network, including DWDM systems from early point
-
to
-
point DWDM systems to recent reconfigurable
optical add/drop multiplexing (ROADM) systems;
from 2.5Gbps per wavelength system
s

to 40Gbps per
wavelength system
s;
from
optical switching to
electronic switching
;
from wavelength routing to de
-
multiplexing sub
-
channel routing
;
et al. In order to
help planners make cost
-
effective
service provisioning

decisions
, we built a web
-
based interactive tool with

integrate
d optimizati
on algorithms to route

high
speed circuits over heterogeneous networks through a
graphic user interface. The tool provides visualization
of the heterogeneous DWDM ne
twork and the status
of its network

links (sub
-
network type, distance,
maximum and availabl
e wavelengths number,
available 10G and 40G channels,
deployment date
,
and

other related information). It can show the whole
network or any particular set of sub
-
networks filtered
with the sub
-
network types and/or the supported
maximum speeds. It can also
predict and highlight
hot
-
spot links, i.e., DWDM links that will be
exhausted by a specified future date based on
historical circuit requests and wavelength usage
information.

The tool also has the capability to generate fully
-
diverse and maximally
-
diverse

routes for groups of
circuits.

Since finding two SRLG
-
diverse routes is NP
-
hard

[6]
, we developed a built
-
in integer linear
programming (ILP) model to find multiple diverse
routes, including complete diverse routing and
maximal diverse routing. In order t
o speed up the ILP
processing time, we implemented algorithm

1 for
complete diverse routing and algorithm 2 for maximal
diverse routing. Working experience shows that our
optimized ILP

can complete most
task computations
within

1 minute
and provide visuali
zation of several
candidate paths for interactive navigation. The tool
has been deployed since 2009 and is actively used by
network planners for their daily
circuit planning
tasks.

AT&T has a large intelligent
optical switched
network

[3
]. The Ciena Core D
irector defines a list of
SRLG

IDs

(known as bundles in Ciena’s terminology)

for each link with a limited maximal number of
bundle

IDs.
This is typically less than the number of
fiber spans needed to describe the link's diversity. If
the number of real bundle IDs is larger than the
maximal number, the list of bundle IDs
must

be
truncated. In this case, which bundle ID should be
dropped bec
omes a critical question and bundle ID
optimization is required. Bundles are re
-
computed
periodically to keep current with ongoing changes in
the network link and fiber span data. When changes
are required, an attempt is made to minimize changes
to existin
g bundle IDs.

In the intelligent

optical switched

network
, we are
required to provide both
complete
diverse routing and
maximal dive
rse routing

if complete diverse routing is
unavailable
. Thu
s we cannot completely drop bundle
s
consisting of

subset link gro
u
ps

(group
i

if
L
i



L
j
).

I
nstead
we use a numeric num
ber to mea
sure the
importance of each bundle

related to the nature of the
link overlaps and its total mileage. We cons
ider three
factors for each bundle
: (1) the
mileage associated
with the bundle
; (2)

the number of s
imultaneous
outage
s

that the bundle

represents; (3) whet
her the
links failed by the bundle

are a subset

of those failed
by another bundle. When the number of bundles

e
xceeds the maximum allowed, bundle
s having lesser
importance are dropped
until the desired number is
achieved.

The
f
ollowing
formula is used to r
ank the
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7

importance of every bundle

based on the above three
criteria:

Importance(bundle) =
1000000*isSuperSet(bundle) +
10000*linkCount(bundle)

+ mi
leage(bundle
)


where isSuperSet(bun
dle
) = 1

if the links failed by the
bundle

are not a sub
-
set of any other bundle

failure
; it
i
s zero otherwise. linkCount(bundle
) = number of
links s
imultaneously failed by the bundle,
mileage(bundle
) = the sum of the mileages of the

fiber
spans in the bu
ndle.
This solution basically combines
algorithm 1 and algorithm 2 together and provides
similar results to complete diverse routing and
maximal diverse routing.

VI.

C
ONCLUSI
ON

In this paper, we studied the SRLG optimization
issue in detail. After considering
the relationship and
importance of individual SRLGs, we proposed
algorithm
s

on how to reduce the number of SRLGs

for
complete
ly
-
diverse routing and maximal
ly
-
diverse
routing
.


These SRLG
-
reduction strategies are useful
for improving the performance of netw
ork planning
tools that depend on SRLG information and in
selecting which SRLGs are most important when
network management systems impose a limit.

R
EFERENCES

[1]

K. Kompella ed., “OSPF extensions in support of Generalized
Multi
-
Protocol Lable Switching(GMPLS)”
, RFC 4203

[2]

B.

Ramamurthy et al, “CoreDirector CI system description”,
http://groups.geni.net/geni/wiki/Ciena%20Core%20Director%2
0switch%20component%20manager%20interface.

[3]

B.

Cortez, “The emerging intelligent optical network: now a
reality”, OFC 2002, WH1.

[4]

G. Ellinas
et al.
, “Routing and restoration architectures in mesh optical

networks,”
Opt. Network Mag.
, vol. 4, no. 1, pp. 91

106, 2003.

[5]

P. Sebos
et al.
, “Effectiveness of share
d risk link group auto
-
discovery
in optical networks,” in
OFC’02
, 2002, p.
Th05.

[6]

J. Hu, “Diverse routing in optical mesh networks,”
IEEE Trans.

Commun.
, vol. 51, pp. 489

494, 2003.

[7]

J. Doucette and W. D. Grover, “Capacity design studies of span
-
restorable

mesh transport networks with shared
-
risk link group (SRLG)
effects,”

in
Proc
. SPIE Opticomm 2002
, vol. 4874, 2002, pp. 25

38.

[8]

Y. Liu, D. Tipper, and P. Siripongwutikorn, “Approximating optimal

spare capacity allocation by successive survivable routing,” in
Proc.
INFOCOM’01
, 2001, pp. 699

708.

[9]

G. Li, B. Doverspike, and C. Kalmanek,

“Fiber span failure protection

in
mesh optical networks,”
Opt. Networks Mag.
, vol. 3, no. 3, May/June

2002.

[10]

G.

Li et al, “Efficient distributed restoration path selection for
shared mesh restoration”, IEEE
/ACM ToN, 11(5), October
2003,
pages 761
-
771.

[11]

C.
Qiao and D. Xu, “Distributed partial information management

(DPIM) schemes for survivable networks

Part I,” in
Proc. INFOCOM’

02
, June 2002, pp. 302

311.

[12]

R. Bhandari,
Survivable Networks: Algorithms for Diverse

Routing
.
Norwell, MA: Kluwer, 1999.

[13]

K. Lee an
d K. Siu, “An algorithmic framework for protection switching

WDM networks,” in
NFOEC’01
, July 2001, pp. 402

410.

[14]

M. Kodialam and T. V. Lakshman, “Dynamic routing of bandwidth

guaranteed tunnels with restoration,” in
Proc. INFOCOM’00
, 2000, pp.

902

911.

[15]

D. Xu et al, “Trap avoidance and protection schemes in
networks with shared risk link groups,” Journal of lightwave
technology, vol 21, no 11, Nov 2003.

[16]

G. Li et al
, “
On shared risk link group optimization,” OFC
2012, WH2
.


Guangzhi Li (M’00)

is a
princip
le member of technical
staff researcher at AT&T labs
-
research. He got his PhD and
MS in computer science from the
College of William and Mary.
His research interests include
IP
-
based control plane for
optical netw
orks, restoration

schemes and algorithms,
n
etwork simulation and
performance evaluation as well
as network related applications.
He holds about 30 patents and has published more than 90
research papers in journals and conferences.


Dongmei Wang

(M’00
)
received her PhD in physics

from the college of William and
Mary and joined AT&T
research lab at 2000. During
the past 12 years,

she has
worked on areas of network
design and optimization.
Recently she is leading an
important IPAG network
design for mobility access. She
holds about
20 patents and has
published about 60 journal and conference papers.



Timothy Gallivan

is a Senior
Network Engineer at AT&T. He
obtained a PhD in physics from the
University of Texas, Austin TX
USA
(
1989
) and an MS in
Electrical Engineering from the
Ge
orgia Institute of Technology,
Atlanta GA

USA

(1984).




Robert Doverspike

received his undergraduate
degree from the University of
Colorado and Masters and Ph.D.
degrees from Rensselaer
Polytechnic Institute (RPI).


He
began his career with Bell Labs
and, upon divestiture of the Bell
System, went to Bellcore (now
Telcordia).


La
ter, he returned to
AT&T Labs (Research) where he
is now Executive Director of
Network Evolution Research.


Dr. Doverspike has made extensive
contributions to the field of optimization of multi
-
layered
transmission and switching networks and pioneered the
concept of
packet transport in metro and long distance networks.


He also
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-
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8

pioneered work in spearheading the deployment of new
architectures for transport and IP networks, network restoration,
and integrated network management of IP
-
over
-
optical
-
layer
netw
orks.


He has over 1500 citations to his books and articles over
diverse areas/publications such as Telecommunications, Optical
Networking, Mathematical Programming, IEEE Magazine, IEEE
Communications Society, Operations Research, Applied Probability,
and
Network Management. Dr. Doverspike holds many professional
leadership positions and awards, such as INFORMS Fellow, IEEE
Fellow, member of Optical Society of America (OSA), co
-
founder of
the INFORMS Technical Section on Telecommunications, OFC
(Optical Fib
er Communications) Network Technologies and
Applications Committee, DRCN (Design of Reliable
Communications Networks) Steering Committee, and Associate
Editor of the Journal of Heuristics.