Transcript EE 681 Fall 2000 Lecture 5

E E 681 - Module 16
Path-oriented Survivable Mesh Networks
W.D. Grover
TRLabs & University of Alberta
© Wayne D. Grover 2002, 2003
Three basic concepts about end-to-end path protection/restoration:
Shared Backup Path Protection
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–
–
–
each working path has a (single) fully-disjoint backup route pre-determined at pathprovisioning time.
When needed a protection path is cross-connected from spare channels along the backup
route.
like “1+1 APS with a shared backup”
same end-node activated reaction regardless of where failure occurs on working path.
(“True”) Path Restoration
–
–
adaptive, failure-specific, response to failure and network state.
for each failure scenario the set of affected end-nodes are simultaneously restored with an
MCMF-like response.
•
–
–
has cognizance of the “mutual capacity” issue and global (or self-organized) coordination.
Allows reuse of working capacity on surviving portion of failed paths.
Capacity design to assure 100% restorability to all defined scenarios.
GMPLS “mass redial”
–
–
–
completely ad-hoc reliance on mass independent re-provisioning attempts.
no co-ordination, self-organization, or other way to address mutual capacity
considerations
inherently unassured, best-efforts, unpredictable outcomes.
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Some initial notes on each:
Shared Backup Path Protection
–
–
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most dominant current paradigm for “survivable routing.”
preferred for router-centric control paradigm where “network is dumb, edges are smart.”
high dependency on conventional software, databases, abd current ideas of Internet like
global state dissemination, etc.
advantage in fully optical networks is that we don’t need rapid fault location.
(“True”) Path Restoration
–
–
–
–
–
theoretically most efficient possible scheme.
not currently popular with industry due to perceived complexity.
self-organizing distributed protocol for MCMF-like adaptive performance developed by us.
may return to importance in context of adaptive-second line of defence strategy for ultrahigh availability, or, for maximal recovery from arbitrary-attack failure scenarios (9/11 etc.)
true self-organization concepts currently too different from conventional software/
messaging paradigms for “distributed interaction.”
GMPLS “mass redial”
–
industry currently in two camps:
•
•
–
those that know this is a disaster waiting to happen if it is positioned as the only survivability
mechanism needed.
those that don’t understand the issue of mutual capacity yet, or just want to overprovision as much
capacity as needed to have a good chance of restoration
characterized in recent M.Sc. Thesis by G. Kaigala (and Globecom 2003 paper).
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Shared Backup Path Protection
•
“Protection” scheme (pre-determined fixed backup paths)
•
End-to-end path protection
•
Single protection path for each working path
•
Spare capacity can be shared between disjoint working paths
Working path
Failure scenario 1
• Restoration of path 1
Spare capacity
re-use
Failure scenario 2
Working path
• Restoration of path 2
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A slightly more general example of SBPP
Green-blue-yellow
protection sharing (x3)
Green-red
sharing (x2)
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Cost of Limiting the Maximum Sharing Relationships*
Average total
capacity increase for
F=1: 150.4%
Total wavelength-distance capacity
5.0
4.5
B-37
B-40
B-43
B-46
B-50
Average total
capacity increase for
F =2: 31.4%
4.0
3.5
Sharing Relationship
Limits of 1 and 2 yield total
capacity increases that
may be unacceptable.
Average total
capacity increase for
F =3: 5.1%
3.0
2.5
Sharing Limit of 3 may be
acceptable.
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
6
Sharing Limit
* Results for test network family with varying nodal degree based on a 25 nodes - 50 spans master network
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Qualitative Appreciation of SBPP vs Span Restoration
Characteristics
(relative to 1+1 APS)
Sharing of spare capacity
Multiple restoration paths
Localized response
Effect on
capacity efficiency
Effect on
availability
++
+
-
++
+
+++
---
Distributed/adaptive restoration
no effect
Sharing of spare capacity
++
+
Single backup path
End-to-end response
Non-adaptive restoration
no effect
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“True” path restoration: what we mean
•
The set of working paths severed by a span cut are restored by establishing
a set of replacement paths end-to-end, simultaneously, between each O-D
pair affected.
•
The replacement paths are formed on-demand using only shared spare
capacity (and possibly released working capacity (stub release).)
– There is no dedicated reservation of a 1-for-1 backup path for each working path.
•
Path restoration is equivalent to abandoning the damaged pre-failure paths
entirely and rapidly re-provisioning new paths end-to-end.
•
Path restoration distributes the impact of failures and the recovery effort
more widely over the network as a whole and therefore generally permits
greater efficiency in spare capacity design.
•
The capacity design and real-time restoration problems for path restoration
are considerably more complex than span-restoration
– the fall-back to each O-D pair creates a capacitated multi-commodity max-flow
problem.
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Comparative illustration of span versus path restoration
Pre-failure
3 service paths
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Comparative illustration of span versus path restoration
Failure occurs
All 3 service
paths are lost
until …
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First look at a span restoration reaction … (1)
Span restoration reaction
All 3 service
paths are lost
until …
failed working
links on failed
span are restored
by span
restoration
Note: example only, exact routes depend on working and spare capacities
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A span restoration reaction …(2)
Loopback /
backhaul
Loopback /
backhaul
This restoration path could stop here
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Now view a path restoration reaction...
Same failure occurs
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A path restoration reaction …with “stub release” (1)
Stub release
Path restoration action
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Notes about stub release in path restoration
•
Stub release is an option / issue which does not exist in span
restoration.
•
From a capacity design standpoint it is preferable to have stubrelease.
•
From an operational viewpoint stub release complicates things:
– a means of automatic signaling needed to rapidly release the surviving working
“stub” capacities,
•
AIS (Alarm inhibit signal) usually serves nicely for this, however
– after physical repair, the reversion process is more complex.
•
Ironically, without stub release, a reserve network capacitated to
support span-restoration may not be restorable under path
restoration !
• Class: Can you think why?
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Optimal Capacity Design for pathrestoration
Approach that follows is to first develop a “master formulation”
that can model joint / non-joint designs and cases with / without
stub release, then discuss modifications for each special case.
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Variations and options within the master formulation for path restoration
•
The “master formulation” for path-restoration allows for:
–
–
–
•
The master formulation requires as inputs:
–
–
–
•
modularity
joint optimization of working path routing
stub release or non-stub release
Note: in span restoration this is
for every span,
here it is for every OD pair.
point-to-point demand
a set of eligible distinct working routes for every (O-D) pair r
a set of eligible distinct restoration routes for every (O-D) pair r
for each failure scenario i .
It solves for:
–
–
–
the amount of working flow on each working route for each O-D pair
(working flows may be split over several routes)
the working, spare, (and module) capacity totals on each span
the composite restoration path-set for all affected demands in each failure scenario
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Parameters and variables in path-restorable capacity design
(in the master formulation)*
Input data
Cjm
Z
m
D
d
r
Cost of mth module
size on span j.
Capacity of mth
module size
= 1 if span j is in pth route
for relation r upon failure i
r
Q
r
Set of eligible working Q
routes for relation r
Encodes routes in
= 1 if span j is in qth route
for relation r
Design output variables
No. of operating
working and spare
links (channels)
on span j
wj
sj
Intermediate (internal)
variables
amount of demand on
relation r
Set of all spans between
mesh cross-connection points

r, p
j
i ,
Set of eligible restoration
routes for relation r
upon failure i.
Set of all point to point
demand quantities, indexed
by r
S
r ,q
j
r
Pi
si0, j
r
xi
Stub release
quantity on span j
from failure i
Amount of demand lost
on relation r for
failure i
* some variables become pre-computable parameters in the variations that follow
No. of modules of
type m to install
on span j for min cost
m
nj
g r ,q
f ir ,
p
Working and
restoration
routing
solutions
N.B. “relation” = “OD pair”
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Orientation to the path restorable design context
(variable and parameters)
Xir
eligible restoration
routes for A-B after
the failure of span i,
Pir = 2
B
D
C
f ir,p
span i
working
route for A-B
A
gr,q
r = A-B
Qr = 1
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Master formulation for path-restorable capacity design
Minimize
m
m
C

n
 j j
Cost of modules of all sizes placed on all spans
m M jS
S. t.
(1)
r ,q
r
g

d

r D All demands must be routed
qQr
(2)
 
rD qQr
(3)
r ,q
j
 g r ,q  w j
r ,q
r ,q


g
 xi
r i
r
qQ
j  S
i  S; r D
Working capacity on spans must be adequate
Defines the amount of damaged working flow
for each relation under each failure scenario
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Master formulation for path-restorable capacity design (2)
(4)

r
pP i
(5)

fi
r, p
i  S; r D
r, p
p
2
 fir ,  sj  si0, j (i, j )  S
j
i j
 i ,
r
rD pP i
(6)
r
 xi


si0, j   rD
0


  jr ,q   ir ,q  g r ,q 
qQr



Restorability of working flows for
each relation
Spare capacity on spans must be
adequate (see note on stub release)
With stub release
Without stub release
M
(7)
s j  w j   n mj  Z m
j  S
Modularity of installed capacity
m 1
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Understanding how the formulation effects “stub-release”
Relation r
O
D
 jr ,q  1

r ,q
i
Route q
1
Working flow gr,q
Other span j
si0, j  
rD

qQr
r ,q
j
  ir ,q  g r ,q
Failure span i
Span j enjoys a stub release “credit” of spare capacity = g r,q for any
failure on span i such that:
( jr ,q  1)
( ir ,q  1)  true
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Typical result comparing span and path-restorable network designs
Total Network Capacity (Links)
5000
Span restorable
Path restorable
Path restorable
with stub release
“non joint”
4800
Spare capacity
optimization only
4600
• joint optimized
routing makes a
large
difference in
span restoration
Combined working
and spare capacity
optimization
“joint” designs
4400
• joint-span is about
as efficient as
non-joint path
4200
4000
1
2
3
4
Design Case
5
6
• joint design
adds relatively
little benefit to
path restoration
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Variations and options within the master formulation
•
•
•
If integer (“ideal”) but non-modular capacity is desired:
–
change objective function to cost-weighted sum of spares (and / or working, if joint)
–
drop set M (the family of modularities), variables
m
n j and constraint (7)
If non-joint design is desired:
–
drop (1), (2), (3), and (6)
–
–
pre-compute all x and w j as input parameters based on the pre-defined routing
i
0
pre-compute all stub-release quantities si , j according to (6)
r
If stub-release is not desired:
–
drop (6), i.e., set all
si0, j
=0
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A route-generating method for path restorable design formulation
•
For each OD pair relation:
– generate the set of “all distinct routes” between O-D with effectively unlimited
hop limit, e.g, H ~ 3/4 |S| and store in a (large) temporary routes file.
– sort the routes by increasing geographical length
– If joint formulation:
• take first N routes (a budgeted number) as the set of eligible working routes for the
formulation. (Do not remove from file).
– If non-joint formulation:
• take the single shortest route for the working paths between O-D
– For both joint or non-joint generate eligible restoration route-sets:
• Repeat (for each active O-D pair):
–
–
–
–
step out one span onto the shortest route, i
find first K routes in routes file which do not include span i as eligible restoration routes
remove chosen routes from file
go to next span along shortest route
Until last span in shortest route
• merge all routes found as eligible route-set for restoration of relation r.
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Route generating method for path restorable formulation (2)
•
What does this achieve ?
–
This procedure is effective in mediating the following trade-off in populating the routesets:
“All distinct routes”
Far too large DAT file sizes
for realistic AMPL / CPLEX runs
A single budgeted number
of distinct routes
Insufficient diversity / uniform
disjointness of route-sets found by
Depth First Search, infeasibilities
arising with even large route-set
budgets
project topic: write program to statistically sample the large eligible route space
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Observed tendency from taking budgeted number of distinct routes
of successive length found by DFS
( Leaf end
of DFS tree )
O
D
( Root end
of DFS tree )
Outright infeasibility
possible if all ‘budgeted’ routes neck
down onto one span in common
Boundary of a network graph
The preceding process for selecting routes stays
within a budget but avoids this problem by
repeatedly pushing out the grey envelope as it
proceeds in the direction across the network between
O-D nodes.
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Why ad-hoc replacement-route finding (“mass redial”)
is not an assured form of path restoration
• example shows same span-failure affecting 3 OD-pairs
• only spare capacity and restoration paths are shown for simplicity
Relation
Lost Capacity
A- D
B- D
G-D
Restoration Path
Unused Spare Capacity
A
F
G
F
G
E
B
C
H
E
X
X
• -incomplete
restoration
Failed Span
3
2
1
A
• ad-hoc set of
independent
replacement
paths
X
B
D
(a)
Restorability = 5/ 6= 83%
Possible restoration pathset
C
H
• collectively
co-ordinated
set of replacement
paths
• -complete
restoration
D
(b)
Restorability = 6/ 6= 100%
Preferred restoration pathset
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Other comments on path restoration
• (1) A path restorable network inherently provides a response to
node-failure and multiple span failures
- 100% restoration not guaranteed
- span restoration needs special extensions to the distributed
protocols to respond to these situations as gracefully
• (2) Path restoration also copes more gracefully with the multiple logical span
failures arising from nodal “bypass” situations.
same cable
terminated flows
express or
“bypass” flows
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Issue of nodal bypass and fault multiplication in span
restoration - not a problem for path restoration
B
physical picture =
(A-B)
span failure
A
C
logical picture
for span restoration
B
A
=
(A-B)
C
(A-C)
simultaneous dual
logical span failures
generic issue / phenomenon:
physical to logical layer fault multiplication
• for path restoration, however,
same set of end-node OD pair
failures arise in either case
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Other comments on path restoration
• Recent findings indicate that the capacity benefit of path restoration
(over span- restoration) may be considerably less than hoped for
in low degree networks.....
chain
consider:
- intra chain demands
higher degree mesh component
- demands that cross
only one mesh span
or chain
----- >
can do no better -( spare
capacity - wise) than
with span restoration )
chain
• for many demand pairs in a low degree network their path restoration solution is no
different than span restoration in the mesh that results from collapsing all degree-2 chains.
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