One document matched: draft-ghani-optical-rings-00.txt
Internet Draft N. Ghani
Expiration: July 2001 Sorrento Networks
J. Fu
Sorrento Networks
D. Guo
Sorrento Networks
X. Liu
Sorrento Networks
Z. Zhang
Sorrento Networks
Architectural Framework for Automatic Protection
Provisioning In Dynamic Optical Rings
draft-ghani-optical-rings-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
Given the large installed base of ring fiber-plants and the extensive
experience operators have gained in operating SONET (SDH) ring networks,
optical rings are becoming increasingly important. As such, optical
rings will play a crucial role in the migration from existing TDM-based
SONET (SDH) architectures to more dynamic lightpath provisioning
paradigms. To date, various optical ring concepts have been tabled,
proposing multi-services support and mirroring the fast protection
switching capabilities of existing SONET (SDH) rings. Nevertheless,
the emerging IP-based MPL(ambda)S framework for optical networks is
largely based upon (optical) mesh routing concepts. Clearly, there is
a strong need to formalize a more comprehensive architectural framework
Ghani et. al. [Page 1]
for dynamic optical rings and ensure their integration within the
emerging MPL(ambda)S architecture. Along these lines, the various
optical ring schemes are summarized and associated MPL(ambda)S
interworking concerns detailed.
Table of Contents
1 Introduction ................................................ 3
2 Review of SONET Ring Architectures .......................... 5
2.1 Uni-directional Path-Switched Rings (UPSR) ............. 5
2.2 Bi-directional Line-Switched Rings (BLSR) .............. 5
3 Framework for Optical Rings ................................. 6
3.1 Dedicated Path Protection Rings (DPRING) ............... 8
3.2 Shared Protection Rings (SPRING) ...................... 10
3.2.1 OMS-Shared Protection Ring (OMS-SPRING)............ 10
3.2.2 OCh-Shared Protection Ring (OCh-SPRING)............ 12
3.3 Signaling Channel Considerations ...................... 16
3.4 Fault Detection and Isolation ......................... 16
4 MPL(ambda)S Interworking ................................... 18
4.1 Channel Provisioning ................................. 19
4.1.1 Signaling Extensions .............................. 19
4.1.2 Resource and State Dissemination .................. 22
4.2 Protection Signaling .................................. 23
4.2.1 Direct Interworking ............................... 24
4.2.2 O-APS Protocol .................................... 27
4.2.3 Multi-Layer Escalation Strategies ................. 28
4.3 Advanced Evolutions ................................... 30
5 Security Considerations .................................... 31
6 References ................................................. 31
7 Authors Information ........................................ 34
8 Full Copyright Notice ...................................... 35
List of Acronyms
ADM: Add-drop multiplexer
APS: Automatic protection switching
AS: Autonomous system (routing domain)
BER: Bit error rate
BLSR: Bi-directional line-switched ring
BPSR: Bi-directional path-switched ring
COPS: Common open policy service
CR-LDP: Constraint-routing label distribution protocol
DPRING: Dedicated protection ring
DWDM: Dense wavelength division multiplexing
EXC: Electronic cross-connect (electronic cross-point switch)
FIS: Failure indication signal
FRS: Failure recovery signal
GMPLS: Generalized multi-protocol label switching
IED: Integrated edge device
IGP: Interior gateway protocol
ILM: Incoming label map
LMP: Link management protocol
LOF: Loss of framing
LOS: Loss of signal
Ghani et. al. [Page 2]
LSA: Link state attribute
LSP: Label switched path
LSR: Label switch router (also lambda switch router)
MEMS: Micro-electro-mechanical systems
MPLS: Multi-protocol label switching
NHLFE: Next-hop label forwarding entry
NMS: Network management system
NNI: Network-to-network interface
O-ADM: Optical add-drop multiplexer
O-BLSR: Optical bi-directional line-switched rings
O-BPSR: Optical bi-directional path-switched rings
OCh: Optical channel (e.g., lightpath entity)
OMS: Optical multiplex section (e.g., fiber span)
OSC: Optical supervisory channel
OSPF: Open shortest path first protocol
OXC: Optical cross-connect switch
PDH: Plesiochronous digital hierarchy
PML: Protection merge LSR
PMTG: Protected MPLS traffic group
PSL: Protection switch LSR
RNT: Reverse notification tree
RPR: Resilient packet ring
RSVP: Resource reservation protocol
RWA: Routing and wavelength assignment
SDH: Synchronous digital hierarchy
SHR: Self-healing ring
SNC: Sub-network connection
SPRING: Shared protection ring
SONET: Synchronous optical network
SRLG: Shared risk link group
TCP: Transport control protocol
TDM: Time division multiplexing
TLV: Type length value (field)
UNI: User network interface
VPOR: Virtual private optical ring
UPSR: Uni-directional path-switched ring
WDM: Wavelength division multiplexing
WRS: Wavelength-routing switch
1. Introduction
Many networks today are based upon fiber-ring architectures, as
evidenced by the proliferation of multi-level SONET (SDH) rings,
especially in metropolitan and regional areas. For example, at the
access-side, smaller OC-3 (155 Mb/s) tributary rings are used to
aggregate and groom traffic from enterprise customers. These rings
are then connected to larger granularity OC-12 (622 Mb/s) and
possibly OC-48 (2.5 Gb/s) rings spanning larger metropolitan
distances. Metropolitan rings are then used to feed into even larger
regional (and possibly long-haul) fiber-ring topologies with increased
bit rates, such as OC-192 (10 Gb/s). As a result, ring architectures
will clearly play a major role in the evolution of optical network
architectures.
Ghani et. al. [Page 3]
Given this large, entrenched base of ring topologies, currently many
operators are planning for a migration to equivalent dynamic optical
ring architectures. These optical wavelength routing rings, commonly
referred to as optical add-drop ring multiplexer (O-ADM) rings, will
form the mainstay architecture for most metro and regional networks and
will help operators ease their transition to future optical (mesh)
networks. Since many operators have significant experience in deploying
and maintaining TDM rings (i.e., SONET/SDH), such optical analogs of
time-division multiplexing (TDM) ring switching are of great
transitional value. Here, wavelength channels (as opposed to TDM
circuits) undergo bypass, add, or drop operations at O-ADM network
elements [MARCENAC]. O-ADM rings will allow operators to immediately
leverage their current fiber topologies and avoid lengthy fiber-
expansion costs (i.e., associated with deploying mesh networks).
Furthermore, ring architectures are well-known for their inherently
fast protection switching capabilities, and perhaps, this is the main
reason for the widescale acceptance of SONET technology (i.e., TDM
rings). Network operators have become well-accustomed to the fast,
timely recovery capabilities provided by SONET automatic protection
switching (APS) schemes, such as uni-directional path switched rings
(UPSR) and bi-directional line switched rings (BLSR) [GR1230]. These
architectures can achieve service recovery within 50 ms after a fault
event, via detailed electronic frame monitoring and fast (protection)
switchover signaling provisions.
Meanwhile, recently there have also been significant developments in
extending the ubiquitous multi-protocol label switching (MPLS) framework
to the optical networking domain, namely "IP over optical" via
MPL(ambda)S [AWDUCHE],[GHANI1],[RAJAGOPALAN] and more recently,
generalized MPLS (GMPLS) [XU]. Nevertheless, given its origins from
(mesh) IP packet routing networks, this framework as it stands today,
is largely geared to support dynamic optical mesh networks. Conversely,
no standards exist for O-ADM rings and most offerings do not provide
dynamic channel routing (add-drop) capabilities, relying instead upon
proprietary, static solutions. Now given the abundance and strategic
importance of ring fiber-plants (as detailed above), it is crucial to
extend the existing MPL(ambda)S framework to provision dynamic optical
ring networks. Although some may state that rings are special cases of
meshes (technically speaking), the various intricacies of ring networks
require special attention in the MPL(ambda)S framework. As most long-
haul optical networks continue to migrate towards mesh-based
MPL(ambda)S setups, along with increasingly MPLS-based "client" router
networks, intermediate metro/regional networks (largely ring-based)
must also evolve to a similar "IP-based" architecture. Such a uniform
provisioning framework will permit true optical services provisioning
across all network/geographic domains.
In particular, the MPL(ambda)S framework must address ring channel
provisioning and protection switching functions. Undoubtedly, optical
(ring) solutions must provide equivalent, or improved, capabilities in
order to replace TDM rings in a timely manner. Since each fiber (or
wavelength) in an optical network can now carry a much higher degree of
multiplexed traffic, APS capabilities are even more crucial. This
report details an architectural framework for O-ADM rings, representing
a logical, structured evolution (expansion) from existing SONET (TDM)
Ghani et. al. [Page 4]
ring paradigms. A brief overview of existing SONET ring schemes is
first given, and subsequently, optical ring equivalents of SONET
protection schemes are presented. The detailed interworkings of the
proposed optical ring architectures with the emerging, ubiquitous
MPL(ambda)S framework are then detailed and key areas identified for
future work.
2. Review of SONET Ring Architectures
SONET (SDH) ring architectures have emerged to dominate the transport
landscape. Termed also as self-healing rings (SHR), perhaps their
defining characteristic is stringent recovery timescales. Namely,
SONET (and SDH) standards stipulate a service recovery time of 50 ms
after the fault condition (i.e., including detection, guard time,
switching time, ring propagation delays, and re-synchronization).
These values are derived from frame synchronization at the lowest
frame speed, namely DS1 (1.5 Mb/s). A very brief outline of the
SONET ring framework is given here. However, this summary is only
intended to serve as a background reference, and interested readers
are referred to the specifications for complete details [ANSI],[ITU],
[GR1230]. As will be seen, these existing architectures will form
the basis for much of the counterpart optical ring frameworks.
2.1 Uni-directional Path-Switched Ring (UPSR)
The UPSR concept is designed for channel level protection in two-fiber
rings. Although two-fiber BLSR architectures also exist, termed BLSR/2,
the UPSR architecture is significantly less complex. UPSR rings
dedicate one fiber for working TDM channels (timeslots) and the other
for corresponding protection channels (counter-propagating directions).
Traffic is permanently bridged at the head-end and sent along both
fibers, namely 1+1 protection. UPSR working traffic travels in the
clockwise direction and protection traffic travels in the counter-
clockwise direction. This implies that bi-directional connections
will consume resources on all working and protection fibers,
restricting ring throughput to that of a single fiber. Clearly, UPSR
rings represent simpler designs and do not require any notification or
switchover signaling mechanisms between ring nodes, i.e., receiver nodes
perform channel switchovers. As such, they are resource inefficient
since they do not re-use fiber capacity (both spatially and between
working/protection paths). Moreover, span (i.e., fiber) protection is
undefined for UPSR rings, and such rings are typically most efficient
in access rings where traffic patterns are concentrated around
collector hubs.
2.2 Bi-Directional Line Switched Ring (BLSR)
BLSR rings are designed to protect at the line (i.e., fiber) level, and
there are two possible variants, namely two-fiber (BLSR/2) and four-
fiber (BLSR/4) rings. The BLSR/2 concept is designed to overcome the
spatial reuse limitations associated with two-fiber UPSR rings and only
provides path (i.e., line) protection. Specifically, the BLSR/2
scheme divides the capacity timeslots within each fiber evenly between
same-direction working and protection channels (with working channels
Ghani et. al. [Page 5]
on a given fiber being protected by protection channels on the other
fiber). Therefore bi-directional connections between nodes will now
traverse the same intermediate nodes but on differing fibers. This
allows for sharing loads away from saturated spans and increases the
level of spatial re-use (sharing), a major advantage over two-fiber
UPSR rings. Protection slots for working channels are pre-assigned
based upon a fixed odd/even numbering scheme, and in case of a fiber
cut, all affected timeslots are looped back in the opposite direction
of the ring. This is commonly termed "loop-back" line/span protection
and avoids any per-channel processing. However, loop-back protection
increases the distance and transmission delay of the restored channels
(nearly doubling path lengths in the worst case). More importantly,
since BLSR rings perform line switching at intermediate nodes, more
complex active signaling functionality is required. Further bandwidth
utilization improvements can also be made here by allowing lower-
priority traffic to traverse on idle protection spans.
Four-fiber BLSR rings extend upon the BLSR/2 concepts by providing
added span switching capabilities. In BLSR/4 rings, two fibers are
used for working traffic and two for protection traffic (counter-
propagating pairs, one in each direction). Again, working traffic
can be carried in both directions (clockwise, counter-clockwise),
and this minimizes spatial resource utilization for bi-directional
connection setups. Line protection is used when both working and
protection fibers are cut, looping traffic around the long-side path.
If, however, only the working fiber is cut, less disruptive switching
can be performed at the fiber level. Here, all failed channels are
switched to the corresponding protection fiber going in the same
direction (and lower-priority channels pre-empted). Overall, the
BLSR/4 ring capacity is twice that of the BLSR/2 ring, and the four-
fiber variant can handle more failures. Also, it should be noted
that both two- and four-fiber rings provide node failure recovery
for pass-through traffic. Essentially, all channels on all fibers
traversing the failed node are line switched away from the node.
As mentioned above, BLSR rings (unlike UPSR rings) require a
protection signaling mechanism. Since protection channels can be
shared, each node must have global state, and this requires state
signaling over both spans (directions) of the ring. This is achieved
via an automatic protection switching (APS) protocol running on the
"embedded" K1/K2 bytes in the SONET frame overhead (commonly also
termed as the K1/K2 byte protocol) [GR1230]. This protocol uses a
4-bit node identifier (in the K1 byte) and hence only allows up to
16 nodes per ring. Additional bits are designated to identify the
type of function requested (e.g., bi-directional or unidirectional
switching) and the fault condition (i.e., channel state). Control
nodes performing the switchover functions utilize frame-persistency
checks to avoid premature actions and discard any invalid message
codes. Further details can be found in [GR1230].
3. Framework for Optical Rings
Due to the inherent transparency of optical switching technologies,
optical rings can develop significantly on existing TDM SONET rings.
Ghani et. al. [Page 6]
Namely, the concept of "transparent" optical rings is envisioned,
permitting a full range of protocols/bit-streams being carried in their
native format, e.g., ATM, IP, ESCON, Gigabit Ethernet, etc.
Fundamentally optical ring network elements must perform the optical
"equivalents" of TDM ADM channel operations: pass-through, add, drop,
and fast APS functions [ARIJIS],[MARCENAC]. Expectedly, "active"
operations are implied here, otherwise the principles of dynamic
wavelength provisioning, and hence MPL(ambda)S, are largely
inapplicable. Specifically, TDM circuit/timeslots are now replaced by
wavelength-based lightpath entities. These requirements can be met by
either using optical add-drop multiplexer (O-ADM) or optical cross-
connect (OXC)/wavelength routing switch (WRS) devices. The latter types
are well-suited to larger inter-ring connection applications, which
require added (mesh) spatial switching capabilities.
_ +------------+ _
Demux / |---<o>-->| |---<o>-->| \ Mux
W-E in >----+ |---<o>-->| |---<o>-->| +----> W-E out
\_|---<o>-->| Lambda |---<o>-->|_/
_ | pass-thru/ | _
Mux / |<--<o>---| protection |<--<o>---| \ Demux
E-W out <----+ |<--<o>---| |<--<o>---| +----< E-W in
\_|<--<o>---| |<--<o>---|_/
+------------+
| | | ^ ^ ^ -<o>- Optional O-E conv.
| | | | | | (wavelength transponders,
v v v | | | possible SONET/digital
+------------+ wrappers monitoring)
---->| Wavelength |---->
From client nodes ---->| channel |----> To client nodes
---->| add/drop |---->
+------------+
Figure 1: Sample optical add-drop multiplexer (O-ADM) node (2-fiber)
A generic overview of a two-fiber O-ADM device is shown in Figure 1
and can easily be extended for four-fiber rings. Optical demux (mux)
devices split (combine) wavelength channels (wavelength bands) from
incoming (outgoing) fibers and connect to a wavelength channel (band)
add/drop/protection unit. This stage can be implemented using a
variety of techniques, such as optical switches (e.g., micro-electro-
mechanical systems (MEMS), bubble, thermo-optic, etc), electronic
cross-point switches (EXC), or simpler 2x1 switching devices. The add/
drop channels help to form the access stage of an O-ADM and this is
where signals are mapped/de-mapped) onto/from wavelength transmitters/
receivers. Optionally, O-E designs can perform edge SONET (or
digital wrappers) payload mapping at the access stage. Overall, O-ADM
devices can exhibit different levels of functionality. For example,
purely optical add-drop/switching fabrics are incapable of wavelength
conversion unlike designs based upon opto-electronic (O-E) conversion
techniques. Also, hybrid designs can offer partial wavelength conversion
for selected wavelengths and/or links (as illustrated in Figure 1). It
is important to define an O-APS framework that encompasses all (or as
many of) these possibilities.
Ghani et. al. [Page 7]
To date, the T1X1 forum has been most active with regards to proposals
for optical ring architectures, see [CHEN],[CVIJETIC1-2],[SOULLIERE].
Although this work is currently on the T1X1 living list and represents
a good starting point, detailed standards (comparable to SONET UPSR,
BLSR) are yet to emerge. Overall, optical ring proposals are classified
into two major types, namely dedicated protection rings (DPRING) and
shared protection rings (SPRING), and this delineation is re-used here
to define the conceptual framework. More specific provisioning
(signaling) requirements are treated in the subsequent sections.
Note that the terms channel and lightpath are used interchangeably
to represent wavelength circuits. Others have proposed further
classifications for lightpaths with or without wavelength conversion,
namely virtual wavelength path (VWP) and wavelength path (WP)
concepts, respectively. However, herein the term lightpath (channel)
is used to generically refer to both entities. Furthermore, the
prefix "O" is used to identify all optical ring concepts, in order to
clearly discern from existing TDM ring (SONET) schemes.
3.1 Dedicated Path Protection Rings (DPRING)
Dedicated protection rings are relatively simple in design are usually
associated with two-fiber uni-directional (path-switched) O-ADM rings,
O-UPSR/2. These rings can implement "edge-to-edge" wavelength channel
protection, and are therefore more commonly termed as optical channel
DPRING (OCh-DPRING) [ARIJIS]. Note that the term "edge-to-edge" is
chosen here, referring to a "sub-network" connection (SNC) entity, since
it most germane to a single ring (domain) and not necessarily a
complete "end-to-end" client connection, see [XUE]. Both the 1+1
(non-signaled) and 1:1 (signaled) protection switching paradigms can be
used herein. Each fiber in a OCh-DPRING carries wavelength channels in
counter-propagating directions, with one fiber each for working and
protection channels. The 1+1 OCh-DPRING solution is similar to SONET
UPSR rings, where bi-directional connections consume wavelength
resources on all fibers, i.e., head-end bridging. This OCh-DPRING
scheme is shown in Figure 2 for a uni-directional channel. Note that
an all-optical OCh-DPRING will likely require the same wavelength
value on the working and protection path (i.e., unless ingress traffic
bridging is done onto two separate wavelength transponders). Since
receiver-based switchovers can be done, no complex signaling protocols
are required for 1+1 optical protection. However, there is normally
an added power penalty, about 3 dB, when performing head-end bridging
[SOULLIERE].
Node A Node B
+--------+ +--------+
******************************************* |
| # | | * |
| # |------------------| * |
+------#-+ +------*-+
| # ** Working | *
| # ## Protection | *
| # (permanently | X<--Fiber
| # bridged) | * cut
| # | *
+------#-+ +------*-+
Ghani et. al. [Page 8]
| ############################*****> A-D
| | | |
| |------------------| |
+--------+ +--------+
Node C Node D
Figure 2: 1+1 wavelength path protection (2-fiber OCh-DPRING)
Additionally, signaled 1:1 protection can also be implemented in the
OCh-DPRING, essentially re-using protection wavelengths for lower-
priority traffic, i.e., head-end switching [SOULLIERE]. This requires
an optical APS signaling protocol (yet to be specified). Although 1:1
channel protection improves upon idle resource utilization, it still
has limited spatial wavelength re-use and is rather disruptive (i.e.,
full ring/path switch can affect many users, albeit lower pre-emptable
priority). The 1:1 OCh-DPRING structure is shown in Figure 3, where the
lower-priority lightpath C-D occupies a protection wavelength/span for
lightpath A-D. Conceivably, both 1+1 and 1:1 OCh-DPRING mechanisms can
coexist simultaneously on the same two-fiber ring, since they utilize
the same underlying fiber/wavelength plan.
Node A Node B
+--------+ +--------+
******************************************* |
| # | | * |
| # | | * |
| # |------------------| * |
+------#-+ +------*-+
| # ** Working | *
| # ## Path | *
| # @@ Low Priority | X<--Fiber
| # (pre-emptable) | * cut
| # | *
+------#-+ +------*-+
| ############################*****> A-D
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@> C-D
| | | |
| |------------------| |
+--------+ +--------+
Node C Node D
Figure 3: 1:1 wavelength path protection (2-fiber OCh-DPRING)
Note that depending upon the O-ADM node's fault detection mechanism,
switchover signaling can be actuated using a variety of methods (see
Section 3.4, Section 4.2). For example, for translucent designs using
SONET framing, B1-byte (inband overhead) monitoring can be done to
detect progressive signal degradation. Alternatively, for transparent
optical rings, optical monitoring techniques (such as power or
signal-to-noise ratios) can be used to detect fiber (or wavelength)
faults. In summary, the OCh-DPRING scheme requires full (100%)
protection resource overhead and cannot achieve spatial re-use,
somewhat akin to SONET UPSR rings. Hence, the OCh-DPRING scheme is
Ghani et. al. [Page 9]
best suited for hubbed traffic demands, where wavelength counts (and
not spatial distributions) are the dominant factors.
3.2 Shared Protection Rings (SPRING)
Shared protection ring (SPRING) architectures are designed to improve
upon spatial resource utilization over UPSR designs. These rings are
derived from SONET BLSR rings, and are usually more complex, requiring
active signaling for fast recovery. Overall, two shared ring schemes
have been proposed, namely at the optical multiplex section (OMS) and
optical channel (OCh) levels, respectively. In all such schemes, bi-
directional connections between two endpoint nodes must traverse the
same set of intermediate nodes. Details are now presented (see also
[ARIJIS] for details).
3.2.1 Optical Multiplex Section-Shared Protection Rings (OMS-SPRING)
Fiber cuts are one of the most common faults in ring networks, and given
the increased multiplexing of DWDM systems, it is very desirable to also
protect at the fiber span (i.e., OMS) level. Since per-channel
protection switching can involve excessive signaling for large channel
counts, fiber (i.e., optical line) protection can be much more scalable.
Fiber protection basically provides an alternate fiber path between two
nodes experiencing a fiber cut, and usually also requires signaling
between the two end-points of fiber cut. Fiber protection is best
applied to "fiber-rich" four-fiber rings, although two-fiber schemes
are also possible. However, carefully note that line protection
requires fiber fault detection and isolation capabilities, unlike end-
to-end channel protection. A variety of OMS shared protection rings
are possible, termed OMS-SPRING, and details are presented.
Two-fiber OMS-SPRING line (fiber) protection schemes, termed herein as
O-BLSR/2, are very similar conceptually to SONET BLSR/2 designs. For
example, to permit resource sharing and (intra-fiber) coordination
between working/protection channels, these rings require a wavelength
numbering/assignment scheme to effect a grouping between working and
protection channels. This essentially creates two "virtual fibers" from
each physical fiber, albeit each with only half the number of
wavelengths. The rules for such a partitioning are somewhat similar to
those for timeslot partitioning in SONET BLSR/2 rings. Specifically,
each fiber has an equal number of working and protection wavelengths
traveling in the same direction, and the working wavelength group in a
given fiber corresponds to the protection wavelength group in the other
fiber. This implies opposing directions will be routed on the same
side (i.e., through common nodes) but use different fibers. For all-
optical rings, however, added wavelength numbering qualifications are
required to enforce the wavelength-continuity constraint. In
particular, the actual wavelength values within each group have to
match each other, thereby precluding the need for wavelength
conversion upon switchover events. For example, the first (W/2)
fiber wavelengths can be assigned for working channels, whereas the
last (W/2) wavelengths can be assigned for protection channels. This
condition can be relaxed for translucent O-ADM designs, where
wavelength values can also be interchanged upon protection switching.
Ghani et. al. [Page 10]
Node A Node B
+---------+ +---------+
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
*********************************************@ |
| | | *@ |
| oooooooooooooooooooooooooooooo*@ |
| o#############################*@ |
+------o#-+ +------*@-+
^ o# **,@@ Working ^ *@
| o# ##,oo Far-side | *@
| o# loop-back | XX<--Fiber
| o# protection | *@ cut
| o# | *@
+------o#-+ +------*@-+
| o#############################*@@@@@> B-D
| oooooooooooooooooooooooooooooooooooo> A-D
| | | |
| | | |
| |<------------------| |
+---------+ +---------+
Node C Node D
Figure 4: Loop-back span protection (O-BLSR/2)
Two-fiber OMS-SPRING (O-BLSR/2) protection is possible using "loop-
back" protection (akin to SONET BLSR/2 rings), namely, all failed fiber
wavelengths are re-routed on to the protection fiber route on the
counter-propagating side of the ring, see Figure 4. Again, wavelength
continuity concerns may arise for all-optical rings. As expected,
protection signaling is done on the far-side. Albeit an alternative,
optical loop-back protection, however, is not very attractive since it
increases the distance and transmission delay of the restored channels
(nearly doubling path lengths, as per SONET BLSR/2). Related analog
degradations will likely further hinder applicability in all-optical
rings. Furthermore, loop-back protection fully consumes protection
fiber resources and limits recovery to single fiber-cut faults at any
given time. As a result, it is unlikely that two-fiber OMS loop-back
schemes (O-BLSR/2) will see much favor in practical settings.
Node A Node B @
+-----------+ +----------@+
************************************************@|
| | | *@|
| |<--------------------| *@|
| | | *@|
| |-------------------->| *@|
| oooooooooooooooooooooooooooooooooooooooo*@|
| o#################################o#####*@|
+---o#------+ +---o#----*@+
^ o# ^ | ** Working 1 ^ o# ^ *@
| o# | | @@ Working 2 | o# | *@
| o# | | oo Protection 1 | o# | XX <--Fiber
| o# | | ## Protection 2 | o# | *@ cut
| o# | v | o# | *@
Ghani et. al. [Page 11]
+---o#------+ +---o#----*@+
| o#################################o#####*@@@@> B-D
| oooooooooooooooooooooooooooooooooooooooo*****> A-D
| |<--------------------| |
| | | |
| |-------------------->| |
| | | |
| |<--------------------| |
+-----------+ +-----------+
Node C Node D
Figure 5: Span protection, loop-back and near-side (O-BLSR/4)
Span switching is much more attractive for four-fiber rings since
protection fibers have the same wavelength directionality as working
fibers. By logically extending SONET BLSR/4 architectures, four-fiber
OMS-SPRING schemes can also be defined to protect against fiber cuts,
termed O-BLSR/4. Clearly, these rings can implement loop-back (i.e.,
far-side) span protection by simply re-routing all failed working
wavelengths on a fiber onto their associated, counter-propagating
protection fiber (like O-BLSR/2). This will extend the channel routes
as shown in Figure 5 (e.g., lightpath A-D protection via A-B-A-C-D,
lightpath B-D protection via B-A-C-D). Far-side loop-back switching is
especially attractive if all working-side fibers are cut (e.g., conduit
fault), but again suffers from increased analog degradations.
Unlike two-fiber rings, four-fiber OMS-SPRING designs also enable more
interesting and less-disruptive "near-side" protection switching. This
form of protection switching is largely designed to protect against
fiber (and channel) failures, and not node failures. One of its main
purposes is to relegate protection signaling actions to the failed side
of the ring (i.e., working, near-side). In other words, the "dual-
directional" nature of fiber diversity of the four-fiber ring is
exploited to maintain the same edge-to-edge node route between working
and protection paths. Near-side protection switching is a generic
concept that can be applied on both the line and path levels. The line-
level case is illustrated for the O-BLSR/4 scheme in Figure 5, where
the failed wavelengths are routed to the same-direction protection fiber
on the near-side (only single direction shown for two working lightpaths
A-D, B-D, traversing the outer working fiber). Overall, near-side line
switching improves resource efficiencies since it does not disrupt
traffic along the whole (long-side) protection route, as per loop-back
techniques. However, near-side switching is less robust since it can
only protect against working fiber faults, and not those that may also
affect near-side protection fibers.
3.2.2 Optical Channel-Shared Protection Ring (OCh-SPRING)
Bi-directional SONET rings have only considered line protection since
channel protection will entail much more complicated tributary
extraction. However, for optical (ring) networks, per-wavelength
routing/processing capabilities are becoming commonplace. This enables
the bi-directional ring concept to be extended at the optical path level,
Ghani et. al. [Page 12]
termed OCh-SPRING, and consequently, optical bi-directional path switched
rings (BPSR) have also been proposed under the shared protection ring
framework. Again, two variants of the OCh-SPRING are possible, namely
for two- (O-BPSR/2) and four-fiber (O-BPSR/4) rings.
Node A Node B
+--------+ +--------+
******************************************* |
| # | | * |
D-A <@@@@@@+@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ * |
+-#----o-+ +-@----*-+
# o **,@@ Working @ *
# o ##,oo Far-side @ *
# o protection @ X <--Fiber
# o @ * cut
# o @ *
+-#----o-+ +-@----*-+
| # ooooooooooooooooooooooooooo@ * |
| # | | @ * |
| ################################+####*******> A-D
+--------+ +-@------+
@
Node C Node D
Figure 6: Bi-directional path protection schemes (O-BPSR/2)
The O-BPSR/2 proposal is based upon 1:1 protection (i.e., signaled
switchover) and utilizes the same wavelength plan as its line-switching
counterpart, O-BLSR/2. These schemes implement full bi-directional
edge-to-edge switching on the "far-side" of the ring, i.e., on the side
away from the fault. For example, lightpath A-D (D-A) re-routed from
A-B-D (D-B-A) to A-C-D (D-C-A), Figure 6. Again, depending upon the
translucency level,wavelength continuity may be required along all edge-
to-edge routes. Channel switchovers are performed for channels in both
directions, regardless of which one actually failed. This is more
beneficial in case of both working and protection fiber cuts on the
working side, e.g., conduit cuts. Lightpath faults are detected by
downstream nodes, which then effect switchover actions via expedited
upstream signaling along the far-side (albeit no standards are defined
yet). Clearly, far-side edge-to-edge path switching will be the most
disruptive, since (lower-priority) traffic and fast signaling are
required on the opposite side of the ring. However, far-side switching
can protect against intermediate node failures. It should, however,
be noted that signaling latencies will dictate maximum ring sizes (node
count limits) for all edge-to-edge ring switching schemes.
By utilizing the SPRING wavelength plan, O-BPSR/2 solutions also
significantly improve spatial resource sharing over their UPSR
counterparts, especially for "non-hubbed" traffic demands [ARIJIS].
Furthermore, differing levels of protection resource sharing can also
be allowed. For example, obviously, idle protection wavelengths can be
used to carry lower-priority pre-emptable traffic (termed 1:1 shared).
Furthermore, protection wavelengths (on the far-side) themselves can be
Ghani et. al. [Page 13]
shared between multiple working channels. This achieves a "1:N"
protection resource multiplexing effect for each wavelength (hop), and
not just the complete protection path. This feature improves resource
efficiency significantly, especially for all-optical rings without
wavelength conversion, and will yield reduced call setup blocking
probabilities. These multiple levels of protection/sharing (e.g.,
1:1 dedicated, 1:1 shared, 1:N shared, pre-emptable) will allow
operators to define more differing grades of service. Further
variations to the O-BPSR/2 framework are for future study.
Four-fiber optical path-switched rings (O-BPSR/4) have also been
defined and can provide more advanced capabilities. These rings also
require a wavelength numbering plan, and it is best to choose one that
mirrors the counterpart four-fiber OMS-SPRING scheme (and therefore,
conceptually parallel to four-fiber SONET ring time-slot assignments).
Specifically, due to increased fiber resources, there is no need for
intra-fiber wavelength partitioning, and therefore, two counter-
rotating fibers (i.e., all wavelengths) can be reserved for working
and protection traffic, respectively. Differing directions of a bi-
directional connection are therefore routed on different working fibers
between the same O-ADM nodes. O-BPSR/4 protection schemes are largely
variations of 1:1 protection switching scheme, as illustrated for a
single direction in Figure 7. Furthermore, strong conceptual parallels
exist with O-BLSR/4 line-switching concepts with regards to protection
routing. In the most straightforward case, ubiquitous far-side path
switching can be implemented, with both paths (of a bi-directional
circuit) being switched over on to their corresponding protection fiber
routes on the opposite side of the ring (as per OCh-SPRING O-BPSR/2).
Far-side path switching can protect against failure of all standby
resources on the working side (i.e., complete multi-fiber ring
conduit cut).
Node A Node B
+----------+ +----------+
*********************************************** |
| # @ |<---------------------| * |
| # @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ooooo* |
| # |<---------------------| @o * |
+---#------+ +--@o----*-+
^ # ^ | ** Working ^ @o ^ *
| # | | ## Far-side | @o | *
| # | | @@ Near-side path | @o | X <--Fiber
| # | | 00 Near-side sub-path | @o | * cut
| # | v | @o | *
+---#------+ +--@o----*-+
| # |--------------------->| @oooooo |
| # |<---------------------| @@@@@@@*****> A-D
| ######################################## |
| |<---------------------| |
+----------+ +----------+
Node C Node D
Figure 7: Path protection schemes (O-BPSR/4), one-direction shown
Ghani et. al. [Page 14]
Furthermore, four-fiber ring near-side protection switching concepts
(Section 3.2.2) can also be applied on a path-level. In fact, more
variations are possible, namely edge-to-edge and intermediate near-side
path switching. The first, O-BPSR/4 edge-to-edge near-side path
switching, routes both the working and protection lightpaths from the
working fiber on to the protection fiber in the same direction, see
Figure 7. Meanwhile, to reduce service disruptions and signaling
overheads further, intermediate near-side path switching only performs
partial working path re-routing. This is shown for lightpath A-D in
Figure 7, where the failed segment B-D is switched to the associated
wavelength on the protection set of the second fiber. This largely
limits protection signaling to the two end-point O-ADM nodes, but
cannot overcome node failures. Due to the bi-directionality
requirement, both channel directions are switched regardless of if one
or both failed. For both forms of (O-BPSR/4) near-side switching,
all-optical nodes will have to ensure that wavelength continuity
considerations are met. Note that O-BPSR/2/4/ concepts can also be
applied at the wavelength band level and this can be studied further.
Depending upon the O-ADM node designs, protection resource sharing
can also be achieved for four-fiber rings (and hence multi-level
service definitions). For example, some have proposed a
straightforward fiber protection implementation using 2x1 fiber
switches before any mux/de-mux stages (Figure 1). This implementation
precludes complimentary wavelength-level processing capabilities (such
as pass-through, add, drop), and hence will hinder wavelength sharing
on protection fibers (more restrictive). Clearly, in order to share
wavelengths on the protection spans and improve resource utilization
(i.e., for OMS-SPRING O-BLSR/4), per-wavelength processing is required
for both working and protection fiber channels. This essentially
means that a fiber cut can also be handled by multiple channel-level
re-routing actions. However, since all wavelengths on a failed span
are re-routed along a common route, "batch" control command
implementations can be used. Nevertheless, sharing protection
resources will require larger add/drop or switching fabrics
(Figure 1). Clearly, "full-blown" four-fiber rings can support many
more users of any given service category, as compared to two-
fiber rings.
In general, operators may also want to provision multiple (ring)
protection schemes off of the same fiber infrastructure. In this
regard, a generic limitation of fiber protection is that it treats all
wavelengths (channels) in a fiber equally, and therefore alone it
cannot achieve (channel-specific) service differentiation. However,
span protection can co-exist with channel protection if a priority
mechanism is used to arbitrate between the two recovery mechanisms
(see also Section 4.3). For two-fiber UPSR schemes (O-UPSR/2), span
protection is not applicable for 1+1 channel protection. However,
(signaled) bi-directional OMS/OCh-SPRING schemes (i.e., those using
the O-BLSR/2 or O-BLSR/4 wavelength plans) can support both
mechanisms, with idle protection spans carrying lower-priority traffic.
In such cases, co-existence between channel (O-BPSR) and span (O-BLSR)
protection mechanisms can be achieved in the protection signaling
specification via an appropriate "priority" mechanism. Typically,
Ghani et. al. [Page 15]
span protection should be done first since it represents "lower-level"
(or more coarse) recovery. This can be achieved by inhibiting all
channel failure message responses and only responding to fiber/span
failure messages. Further details and intricacies are outside the
current scope and require careful future considerations.
3.3 Signaling Channel Architectures
Optical rings allow for significant latitude in signaling channel
architectures, and two overall categories are possible, namely in-band
and out-band signaling. In-band signaling requires the use of overhead
framing bytes (as reserved in a SONET/SDH or digital wrapper frame
header) that are reserved on data channels. Such mechanisms are best
suited for O-E based optical node designs, where edge client signals
are mapped into synchronized electronic frames (which already contain
the required signaling bytes). Alternatively, out-band signaling can
be used to more clearly decouple the data and control planes. Out-band
signaling can be done using a dedicated control wavelength, commonly
termed as the optical supervisory channel (OSC), or even via a
physically separate, out-of-band network (such as an Ethernet LAN).
Note that some have termed the OSC approach as in-band also, since
the control wavelength (typically 1510 nm) "physically" resides in
the fiber itself. However, as far as data-control channel interaction
goes, there is no interaction and hence this approach is termed as
out-band. Recently efforts on defining a broad range of OSC standards
are also emerging, see [SZERENYI].
In general, an out-band OSC-based approach is more attractive to some
since it allows for genuine service-transparent optical ring paradigms,
also stated in [SOULLIERE]. Specifically, this approach utilizes the
same fiber plant, precluding limitations with a completely external
out-of-band signaling network, yet still permitting true client
wavelength (payload) transparency. However, out-band signaling systems
need to ensure adequate bandwidth levels for increasingly large data
wavelengths counts (in the hundreds). As a result, further
considerations are needed for the out-band OSC channel approach (see
T1X1 generic proposal [SZERENYI]). For example, some have proposed
using sub-rate (synchronous) TDM circuit streams to partition and
guarantee OSC bandwidth to all data wavelengths. Others have proposed
(asynchronous) packet signaling on the OSC channels. In either case,
whenever fast recovery guarantees are required, some form of bandwidth
scheduling, be it TDM or packet scheduling (possibly with priority drop
mechanisms), will likely be required on the OSC channels. This
introduces added, but necessary, complexity concerns. Additionally,
signaling channel robustness is also of concern and here, backup control
channel provisions can been considered (see [LANG], Section 3.4).
3.4 Fault Detection and Isolation
The ability to quickly detect, and preferably localize, fault events
is crucial to achieving fast service recovery. So far, the above
discussions have focused more upon switchover actions, and assume that
fault detection (possibly localization) is already done. Now a key
differentiating aspect of optical networks, unlike SONET networks, is
that a much wider range of fault detection and localization mechanisms
Ghani et. al. [Page 16]
can be utilized. In order to allow for full flexibility, it is
therefore preferable that optical (ring) network-layer recovery
mechanisms be independent of the exact fault detection and isolation
schemes. A review of the various monitoring solutions is now presented.
Many first-generation and even current-generation WDM systems simply
re-use existing SONET schemes to detect and isolate channel faults
inside the core optical (ring) network. These solutions include re-
using B1 byte monitoring, and loss of framing (LOF)/loss of signal
(LOS) alarm information. Such solutions have been commonly referred to
as opto-electronic (O-E) and/or frame-monitoring schemes [GHANI2],
[CUEPPENS], since they require that all monitored data wavelengths be
"opaque" or "translucent". The digital wrappers approach, which
represents a counterpart to SONET framing, also essentially embodies
a similar O-E based solution, e.g., forward/reverse defect indicator
(FDI/RDI) bytes, etc. Since most operators are quite familiar with
SONET overhead monitoring, O-E type schemes have one definite
advantage, namely, well-defined standards. This permits faster vendor
interoperability (albeit not considering proprietary usages of various
unused overhead bytes). However, opaque monitoring represents some
serious limitations. First of all, per-channel electronic overheads
usually pose increased systems costs and power requirements. More
importantly, such designs are largely unscalable to very large, ultra-
dense WDM systems, and generally inhibit evolutions to truly
transparent networks [BHANDARI]. Furthermore, O-E monitoring requires
mappings for all client payload types. Now although well-known (SONET)
encapsulations exist for IP, ATM, and frame relay protocols, further
extensions may be necessary, e.g., for new gigabit Ethernet standards,
ESCON, cable video signals, etc. Note however, that O-E monitoring
may be suitable for monitoring out-band control channels, since these
are electrically terminated at each node.
To get around the limitations of opaque monitoring, various vendors
have proposed optical monitoring schemes using non-intrusive signal
tapping setups. These include schemes such as fiber/wavelength power
levels, optical signal-to-noise ratio (O-SNR) measurements, Q-factors,
etc (see [GHANI2],[CUEPPENS]). For example, power monitoring can
detect fiber cuts in under 10 ms, and this is capable of meeting the
most stringent of recovery requirements. Power monitoring is also
termed as loss of signal (LOS) fault, and can trigger various
protection actions, such as 1+1 receiver switchovers or MPLS FIS
[GHANI2] alarm messaging (see Section 4.2.1). However, although
optical monitoring is of high interest to vendors and service
providers alike, the current lack of standards (and to an extent,
advanced features) are hindering widescale adoption. Most current
all-optical solutions simply perform line power-level monitoring,
and are therefore best-suited for O-BLSR support. Although per-
wavelength power-level monitoring can also be done, this poses
significant equipment costs. Nevertheless, per-wavelength monitoring
inside the network core will be necessary to support transparent
O-BPSR schemes, in the absence of any O-E (SONET) frame monitoring.
Although optical monitoring resources (such as spectrum scanners)
can be shared between multiple fibers/wavelengths to control costs,
the resulting fault detection times will much longer (beyond the
Ghani et. al. [Page 17]
"50 ms" SONET reference). Regardless, since optical component
technologies are continually undergoing rapid improvements and
miniaturization, it remains to be seen if these concerns, indeed,
may be mitigated in the foreseeable future.
A more timely and cost-effective alternative may be to perform "edge"
channel (OCh) fault isolation, as suggested in [BHANDARI].
Specifically, no channel-level monitoring is performed in the network
core, thereby precluding excessive (expensive) O-E conversions or
OCh-level optical monitoring. Instead, fault isolation is only done
at the channel termination points. This can be achieved using a
variety of techniques, implemented in the receiver line cards (either
optical power monitoring or electronic frame monitoring after O-E
conversion). All that is required is that the channel protection path
be "dis-joint" (Section 4.1.2) from the working paths. This approach
is very attractive in all-optical (ring) networks, where operators
need service transparency and "SONET-like" recovery times. Also,
this solution is well-suited for "edge-to-edge" channel protection
schemes, such as those detailed for O-UPSR/2 or O-BPSR (far-side,
edge-to-edge near-side) setups.
4. MPL(ambda)S Interworking
Recent developments have extended the MPLS protocol framework from the
packet/flow switching domain to the optical lightpath switching domain.
Termed multi-protocol lambda switching, MPL(ambda)S [AWDUCHE],[GHANI1],
[RAJAGOPALAN], this work draws analogies between labels and wavelengths
and intends to re-use/extend signaling and resource discovery protocols
for the optical domain. Optical nodes (such as cross-connects or
add-drop multiplexers) are assigned IP addresses and run modified MPLS
routing and signaling protocols, i.e., lambda switch routers. More
importantly, recent proposals for a generalized MPLS (GMPLS) [XU]
framework are furthering this trend, extending basic MPLS concepts to
provision other entities, e.g., TDM circuits via SONET ADM's, fiber
routes via fiber cross-connects, etc. In parallel, there has been a
lot of focus on defining LSP recovery schemes for MPLS networks,
albeit, mostly at the packet flow level [DOVOLSKY],[OWENS1],[KINI2].
Conceivably, these schemes can also be extended to apply to "optical
LSP" (i.e., lightpath) protection under the MPL(abmda)S framework.
In fact, this application has already been proposed in [GHANI2].
However, owing to the IP-centric origins of the MPLS framework, the
above work is generally tailored for mesh routing networks, even
though its generic nature does not preclude specialized, topology-
specific applications or extensions.
Given all the variations of optical rings (Section 3), it is very
advantageous to develop a comprehensive provisioning framework and align
it with the larger MPL(ambda)S architecture. In particular, two
signaling mechanisms are required for optical rings. First of all,
signaling is required for lightpath provisioning operations, such as
setup/takedown. This mechanism (protocol) must specify both the working
and protection switched O-ADM paths along with all the intricacies of
ring protection switching requirements. Ring resource management will
also be a critical part of the provisioning stage. Secondly, a
signaling mechanism is required to implement fast optical APS actions
Ghani et. al. [Page 18]
for O-ADM (light)path protection schemes, as detailed previously. An
initial look at these two crucial topics is now presented and is
intended to serve as basis for further, more defining work.
4.1 Channel Provisioning
For optical ring channel setup/takedown, the overall provisioning
capabilities developed under the ubiquitous MPL(ambda)S framework are
quite applicable. Namely extensions to MPLS signaling protocols
are already been proposed to handle the specifics of optical lightpath
routing [KOMPELLA3],[YU]. From an operator's point of view, ring
networks will likely interface to (or even migrate into) mesh networks
in the near future (e.g., metro rings to regional/long-haul mesh).
Given the likely adoption of MPL(ambda)S type protocols for optical
mesh provisioning, it is prudent to choose likewise for ring networks,
thereby enabling an even closer interworking. However, provisioning
ring protection paths is not as simple as routing two paths (optical
LSP's) between the source and destination O-ADM nodes. In particular,
optical rings will require added functionalities from both the
MPL(ambda)S information dissemination mechanisms (i.e., interior
gateway protocols, IGP) and connection setup (signaling) protocols.
These two topics are now considered more closely.
4.1.1 Signaling Extensions
At the core of ring channel provisioning is the concept of a service
definition, as commonly extended through an "optical user network
interface" (O-UNI). Recently, many such definitions have been tabled
for optical networks, proposing some new signaled interfaces [ARVIND],
[ABOULMAGD],[MCADAMS],[XUE] along with augmentations to MPLS LSP setup
messaging [YU],[KOMPELLA3]. In short, service definitions supply the
signaled information "attributes" for subsequent channel setups.
Channel setup, in turn, implies the more general category of routing
and wavelength assignment (RWA) and policy control [GHANI1]. Setup
information usually includes many details, such as the channel framing
type (e.g., SONET, SDH, PDH, digital wrappers, IEEE Ethernet, none,
etc), rate (2.5 Gb/s OC-48, 10 Gb/s OC-192, 1.0 Gb/s 10 Gb/s Ethernet,
etc), protection type (shared, non-shared), and priority (non-pre-
emptable, pre-emptable), etc, see [ABOULMAGD]. Provisions have
also been suggested for indicating lightpath diversity levels (e.g.,
node, link, etc), see [XUE]. By and large, these generic attributes
also apply to ring networks, although further qualifications are
required to handle the various intricacies. All "attributes" of the
service definition parameters are carried in the RSVP-TE PATH or CR-LDP
LABEL_REQ messages, via appropriately defined TLV objects, see examples
in [KOMPELLA3],[YU] and related references therein for details.
First of all, given the many different optical ring architectures, exact
details must be signaled at setup. Here, a ring-type specification
(e.g., O-UPSR/2, O-BLSR/2/4, O-BPSR/2/4, etc) in the service definition
is required to explicitly indicate the type of provisioning/protection
algorithms to be used. Furthermore, the protection-type can be
Ghani et. al. [Page 19]
augmenting to indicate the required level of protection. Examples
include 1+1 dedicated, 1:1 dedicated, 1:1 shared, non-protected, near/
long-side routing, etc (clearly, not all ring-type/protection-type
combinations are possible). This information is critical for the
channel provisioning phase. For example, in O-UPSR/2 rings, the 1+1
protection-type will convey the requirement for routing two "disjoint"
paths from source to destination along with necessary permanent
bridging/receiving facilities at the source/destination ends.
Alternatively, in O-BLSR/4 rings with a 1:1 shared long-side protection-
type connection, the RWA schemes will only search the long-side of the
ring for protection channel routes, including any assigned protection
wavelengths. Generic discussion of routing diversity (dis-jointness)
is also presented given in [DOVOLSKY],[OWENS2],[XUE].
Since optical rings can utilize a variety of fault detection/isolation
schemes (Section 3.4), this further impacts the O-UNI signaling
requirements. Specifically, fault detection/isolation mechanisms only
serve to notify O-ADM devices about fault events, and this information
is then used to generate alarm messaging (e.g., via an appropriate
"liveness" mechanism [OWENS1]) and initiate "network-level" protection
switchover actions. As such, network level recovery actions are carried
out independent of how exactly the fault condition was detected. In
hybrid O-ADM networks, which can conceivably provide both O-E and
optical monitoring, this information will affect the resource
allocation (routing) algorithms. For example, for O-BLSR/4 intermediate
near-side switching, per-channel fault isolation may imply SONET
overhead monitoring at intermediate nodes and thereby preclude switching
across optically-transparent entities. Some may argue that most service
definitions provide a (related) attribute for framing (e.g., [ARVIND],
[ABOULMAGD],[MCADAMS],[XUE]), and hence the fault detection (even
isolation) mechanisms can be implied thereof. In other words, SONET
framing can imply O-E based fault detection, non-SONET framing can
imply optical fault detection, etc. However, various confusions and/
or ambiguities can result from such a simplistic inference, and
therefore for completeness, fault detection and isolation details
should be specified separately at lightpath request/setup time, e.g.,
new signaled attributes. Overall, the exact details of these signaling
extensions are beyond the scope of this document and for further study.
Note that some of these requirements have also been stated previously
(e.g., 1:1 protection in [OWENS2],[XUE]), and as such are not specific
to optical rings.
Channel setup signaling (within optical rings) for both working and
protection paths is another concern. Specifically, at setup time,
"joint" RWA algorithms are necessary for resolving the routes and
associated wavelengths for both the working and protection (sub)paths.
The actual route computation function can be done in a variety of ways
(distributed or via a centralized route/policy control server).
These associated RWA/policy control procedures are very tightly
associated with ring types, wavelength plans, and wavelength conversion
capabilities (none, full, partial). Overall, many (ring) RWA algorithms
have been proposed in the research literature, and these can be applied
herein, see [GHANI1],[MARCENAC],[ZANG]. Ring RWA algorithms can
optimize various broad constraints such as hop counts, propagation
delays, protection/priority levels, residual resources, revenues,
Ghani et. al. [Page 20]
etc. Furthermore note also that policy control can be implemented
via the generic common open policy service protocol (COPS RFC 2748),
and a sample application for optical networks is presented in [GHANI3].
Regardless, once the (working, protection) sub-connection routes are
resolved, various MPLS LSP signaling capabilities can be exploited for
actual setup. One is explicit route (ER) signaling, which can
explicitly signal the required path and resources. Specifically, ER
inserts the complete route specification in appropriate route
specification objects (i.e., explicit route fields in RSVP-TE PATH or
CR-LDP LABEL_REQ messages). Additionally, MPLS bi-directional LSP
setup proposals [GUO1] can ensure that both uni-directional channels of
a bi-directional connection traverse the same set of nodes. In
conjunction with shared risk link group (SRLG) "disjointness"
information (Section 4.1.2), this signaling feature is directly
applicable to O-BLSR/2/4 and O-BPSR/2/4 setups (i.e., where bi-
directional channels must traverse the same set of O-ADM nodes).
Although the overall MPLS LSP setup mechanisms are a good start,
further augmentations are needed for ring networks. In particular,
the protected entities (e.g., channel paths, sub-paths, spans) and
switching "end-points" must be explicitly indicated, as per the ring
and protection types. For example, the source/destination O-ADM nodes
are the switching end-points for edge-to-edge long/near-side channel
protection (as per O-UPSR and O-BPSR designs), whereas the selected
intermediate nodes are the end-points for near-side intermediate
channel switching (as per some O-BPSR/4 designs). Alternatively, for
span protection, the end-points are the two nodes adjacent to the
failure. Overall, MPL(ambda)S lightpath (LSP) setup messaging (RSVP-TE
PATH, CR-LDP LABEL_REQ) must be augmented to carry such pertinent
information. Note that existing proposals for MPLS mesh LSP protection
already have provisions for indicating intermediate protection points
[OWENS2], and their extension to optical networks is both timely and
germane (e.g., ER protection information objects). The broader
application of mesh-based MPLS (packet) LSP protection concepts to
optical networks, originally proposed in [GHANI1-2], is treated more
completely in Section 4.2.1 for the case of optical rings.
Nevertheless, other critical issues remain. Specifically, MPLS-based
control signaling mechanisms still lack some of the vital "externally-
initiated" [GR1230] features which SONET operators are well-accustomed
to. Namely, the SONET K1/K2 byte protocol enables multiple operating
"modes" via a well-defined message priority structure. For example,
messages are defined (in decreasing order of priority) for lockout,
forced switching, fault events (signal fail, signal degrade), and
manual switching, see [GR1230]. Such procedures are vital to
operations-related tasks and are used during various phases (i.e.,
maintenance, diagnostics, and upgrades). Controlling the "operating
mode" is instrumental in avoiding excessive service disruptions to
live customer traffic. Undoubtedly, similar functions must eventually
be provided by MPL(ambda)S-based optical signaling protocols, in both
ring and mesh networks, if optical channel services are to be deployed
in carrier-class networks. This area has not received much attention
to date and significant further attention is required. For example,
Ghani et. al. [Page 21]
a simple first step would be a new (CR-LDP, RSVP-TE) message type to
enact a forced switching action on to the protection path.
4.1.2 Resource and State Dissemination
In addition to the above setup information requirements, provisioning
algorithms need to know the existing static topological details and
available dynamic resource levels (as detailed in [BERNSTEIN]) in order
to compute ring routes. Consider the first requirement. Examples of
basic static topological information are the number of fibers, O-ADM
nodes, and their connectivity. For fiber elements, information is
required to indicate the link type (transparent, service-aware), the
number and location of supported wavelength channels (e.g., ITU-T grid
spacing, offsets, guard bands), related analog metrics (loss,
dispersion figures), etc. Meanwhile, for O-ADM node elements, many
(static) details are pertinent. Examples include the ring
configuration type (O-UPSR, O-BLSR, O-BPSR, or multiple), number of
fiber ports (e.g., incoming, outgoing, add, drops), fiber port
protection type (1+1 protected or unprotected), type of ports supported
(e.g., transparent, opaque), performance monitoring capabilities
(e.g., optical, electrical, per-channel, per-span), signal regeneration
(e.g., 2R, 3R), wavelength conversion capabilities (e.g., none,
partial/selected, full), protection switching capabilities (e.g., per-
channel, per-fiber, per-conduit), etc. Since ring schemes are
intricately associated with the directionality and protection
association (working, protection) of fibers or wavelength groups inside
fibers, this information must also be incorporated.
In traditional data networks, interior gateway protocols (IGP) are used
to disseminate static topology and dynamic resource information.
Recent additions for supporting opaque link state attribute (LSA)
definitions (RFC 2370) will help further facilitate extensions to
"non-data" routing applications. More recently, many proposals have
tabled extensions thereof for optical networks, and in fact, many of
the above-discussed requirements (for static topology and dynamic
resource information) have already been proposed with in the context
mesh-routing MPL(ambda)S networks [KOMPELLA1-3]. For example, IGP
provisions have been considered to indicate wavelength conversion
capabilities and dynamic link-level resource (wavelength) utilizations/
levels. Such active resource updates are vital for dynamic ring
RWA algorithms. Delineations between different link-level resource
classes have also been proposed (i.e., active, free, reserved, pre-
emptable wavelength sets), see [KINI1-2]. The actual control/
specification of wavelength plans can be done statically (pre-configured)
by a network management system. As an application here, such resource
class delineations can be leveraged to control intra-fiber wavelength
plans (e.g., per O-BLSR/2, O-BPSR/2 schemes). In addition, a generic
SRLG definition [RAJAGOPALAN] has also been proposed to explicitly
identify entities belonging to the same "risk group" (e.g., fiber
conduit even geographic region). Optical ring "joint" RWA algorithms
can use such information to ensure route diversity between working and
protection channels. For example, (ring) protection paths require
shared-resource (i.e., risk) separation from working entities, i.e.,
"disjoint". In this context, an entity can be a full edge-to-edge
Ghani et. al. [Page 22]
lightpath (as per O-BPSR/2/4 near/far-side and O-UPSR/2), a portion of
a lightpath (i.e., sub-path as per O-BPSR/4 intermediate near-side), or
a complete fiber span (as per O-BLSR/2/4). Moreover, SRLG definitions
can be used to effect inter-fiber delineation between working and
protection fibers (for the case of O-UPSR/2 and O-BLSR/4 rings), i.e.,
working and protection SRLG identifiers. In general, as these advanced
IGP extensions mature, their application to ring channel provisioning
will be highly practical. Additions to or applications of these
augmented LSA's are for future specification.
4.2 Protection Signaling
It is safe to assume that operators will demand SONET SHR (50 ms
ceiling) recovery timescales for protected O-ADM ring services, and
meeting this stringent requirement is perhaps the foremost concern when
trying to leverage MPL(ambda)S signaling for optical ring control
schemes [GHANI2]. Now for almost all optical ring types (excluding
1+1 O-UPSR/2 designs), millisecond recovery requires fast "APS-like"
signaling capabilities, akin to the SONET K1/K2-byte APS protocol.
Generally speaking, all such schemes can be subsumed under a more
encompassing model, namely that of two (or more) switching end-point
nodes and intermediate, physically disjoint protection resource(s).
(This excludes loop-back switching techniques, which are largely deemed
unfavorable for optical networks, Section 3.2.2). For example, for
channel protection, the end-point nodes are the either the source and
destination nodes (O-BPSR/2, edge-to-edge near-side and far-side
O-BPSR/4), or the appropriate intermediate nodes (intermediate near-side
O-BPSR/4). Likewise, for span protection, the end-point nodes are simply
the adjoining O-ADM nodes. By developing appropriate switchover
signaling extensions to implement this generic model, conceivably all
relevant ring protection schemes can be covered.
For the special case of optical ring networks, two possible options
exist for implementing such fast protection switching. One is to
develop enhancements to the existing MPLS LSP protection
(survivability) signaling proposals and tailor them for "optical
lightpath LSP" protection, termed herein as the direct interworking
approach. The other would be to develop an altogether new, dedicated
protection-switching protocol, namely optical APS (O-APS) protocol,
for inclusion in the MPL(ambda)S framework. This new protocol would
only perform protection switchover signaling for fault events but not
any setup provisioning (relegated to existing MPL(ambda)S signaling
mechanisms, as detailed previously in Section 4.1). These two
cases are shown in Figure 8, and further details are now discussed.
+--------------------------------+ +--------------------------------+
| IGP re-routing (e.g., OSPF) | | IGP re-routing (e.g., OSPF) |
+--------------------------------+ +--------------------------------+
| RSVP-TE/CR-LDP (LSP level) | | RSVP-TE/CR-LDP (LSP level) |
+--------------------------------+ +--------------------------------+
| RSVP-TE/CR-LDP (circuit level) | | O-APS protocol (circuit level) |
+--------------------------------+ +--------------------------------+
(a) (b)
Ghani et. al. [Page 23]
Figure 8: Service recovery: (a) MPLS signaling, (b) O-APS protocol
4.2.1 Direct Interworking
It is instructive to first briefly review MPLS LSP protection concepts.
These capabilities and related protection signaling proposals are
beginning to mature within the extended RSVP-TE and CR-LDP protocols
framework [BHANDARI],[HUANG],[KINI1-2],[OWENS1-3]. The basic idea with
MPLS LSP protection is to provision back LSP (sub)-paths, and in case
of fault discovery, perform a signaled switchover. Generic protection
switch LSR (PSL) and protection merge LSR (PML) nodes are defined and
these entities define the edges of the protected LSP segments.
Specifically, a desired LSP segment, termed working (or active) path
[OWENS1], is setup for protection by having the PML/PSL nodes source
and sink two distinct (sub)-paths, working and protection, as shown in
Figure 9. As a generalization, PSL/PML node pairs can protect multiple
LSP segments, termed protected MPLS traffic group (PMTG) [OWENS1],
reducing signaling overheads for improved scalability. Downstream
nodes detecting a fault event propagate a failure indication signal
(FIS) in the upstream direction, containing a list of protected LSP's
on the failed PMTG entity. Various timer mechanisms are used to
control the inter-FIS packet timing, duration of FIS transmissions,
and hold-off time for initial FIS indication, see [OWENS1] for
discussions on timer settings. Upon receiving the FIS message, the
PML node performs a switchover from the working to protection sub-paths
for all affected LSP's specified in the PMTG. Additionally, a
failure recovery signal (FRS) is also propagated after the fault has
been repaired (along the same route as the FIS message). Similar
timer mechanisms as with the FIS message also exist for the FRS
message, and neither message type requires reliable transport, e.g., no
TCP connection. Note that both the FIS and FRS message types are
"protection-related" additions to the MPLS signaling framework (CR-LDP,
RSVP). Owing to the generic nature of this specification, the PML and
PSL nodes need not be the "end-point" source and destination nodes,
respectively, and hence technically speaking, judicious placement
thereof allows this framework to incorporate path, sub-path, and hop
protection schemes. Although this overall framework seems most
applicable to 1:1 or 1:N protection schemes (downstream nodes signal
fault switchover requests to upstream nodes), a 1+1 protection type
is also mentioned in [OWENS3]. Finally, proposals for sharing
protection resources between multiple protection paths (and lower-
priority traffic) are also beginning to emerge [BHANDARI],
[GHANI1],[KINI2].
********************************
* +---+ *
* +-------------| C |------------+ *
* / +---+ \ *
* / \ *
*********** / \ ************>
+---+ +---+ ** Working +---+ +---+
| A |------| B | PSL (A-B-C-D-E) PML | D |-----| E |
+---+ +---+ ## Protection (PMTG) +---+ +---+
# \ (A-B-F-G-D-E) / #
Ghani et. al. [Page 24]
# \ / #
# \ +---+ +---+ / #
# +------| F |--------| G |------+ #
# +---+ +---+ #
################################
Figure 9: MPLS LSP protection concept (PSL/PML LSR nodes)
A paramount concern for network operators is fast recovery times. The
MPLS LSP protection proposals are increasingly aware of this need,
especially in comparison with the relatively longer timescales of IP
re-routing schemes. Along these lines, the MPLS LSP protection
framework includes the concept of a reverse notification tree (RNT)
[OWENS1-2] entity that traverses from the PSL to the PML node. This
provides an "express" signaling path for protection and recovery
messages, significantly more efficient that "flooding-type" recovery
schemes. The RNT is basically an "inverse" label-lookup (cross-connect)
table that is constructed at the time of working/protection LSP setup
and allows for resolving the incoming links on which to forward the
backwards-propagating FIS message. By using the RNT, hop-by-hop routing
of FIS messages can be avoided, helping to expedite switchover times.
In the latest specification, hop-by-hop routing (layer 3), packet LSP
(MPLS), or SONET K1/K2 bytes (layer 2) mechanisms can be used to
implement the RNT (see [OWENS1]). Also note that the RNT concept
extends to multicast LSP's and is implemented for both working and
protected paths. The latter allows it to be used to indicate failures
on the protection path (requiring subsequent manual operator
intervention, however). Overall, LSP protection setup is implemented
via extensions to the ER field of CR-LDP and RSVP-TE setup messages,
e.g., LABEL_REQ, PATH [HUANG],[OWENS3]. Specifically, attributes are
added for identifying the PSL/PML pair, protection type (1:1, 1+1) RNT
implementation, timer values, etc. Note that there have also been
related proposals for augmenting IGP protocols to support LSP
protection (e.g., delineate active/back bandwidths), see [KINI1].
These can be extended to the optical case to specify active/backup
wavelength sets, etc.
Now consider the application of the above MPLS "packet LSP" protection
framework within the context of MPL(ambda)S optical (ring) networks.
For the case of channel (OCh) protection, the optical (O-ADM, OXC) LSR
devices can now serve as PML and PSL nodes and "disjoint" protection
lightpaths (or hops) can be specified between the two nodes, as per
[GHANI2]. The PMTG entity at this level is the lightpath channel. For
example, for edge-to-edge channel protection (e.g., O-BPSR/2, O-BPSR/4),
the PSL/PML nodes can be the (sub)connection end-points themselves.
Alternatively, for intermediate near-side channel protection (O-BLSR/4
case only), the PSL/PML pair can be the appropriate intermediate O-ADM
nodes. Given the appropriate information (via requirements specified
in Section 4.1.2), RWA algorithms can appropriately setup the ring
working/protection routes and switching points by using the ER
signaling function.
The case of line protection, as proposed in O-BLSR/2/4 schemes, is
somewhat different, since spans are more static (physical) entities and
not dynamically created ones, as are lightpaths. However, using the
protection group concept, all wavelengths on a given fiber span can be
grouped into a common "span" PMTG and the diverse PMTG "span" route
Ghani et. al. [Page 25]
established. This route can either be a single span (O-BLSR/4) or a
series of spans (O-BLSR/2 with loop-back), with the two adjacent nodes
serving as the PML/PSL pair. Note that depending upon the span-
switching implementation, wavelength switching may not be required.
More clearly, O-BLSR/4 schemes using simple 2x1 switches for fiber
protection do not permit wavelength re-use on protection fibers. In
this case, a FIS message (pertaining to a fiber cut) will simply
trigger a 2x1 span switch. However, simpler O-BLSR/2 schemes and
more elaborate O-BLSR/4 schemes (e.g., without 2x1 span switches)
can carry lower-priority traffic on protection wavelengths. In
these cases, all individual channels of a PMTG have to be switched.
Although the above high-level interworking seems amenable, there are
some concerns regarding recovery timing, particularly with regards to
RNT setups and fault signaling. Consider the RNT issue first. During
MPLS LSP setup, LSR nodes must keep track of the upstream node,
incoming link and interface, and list of LSP(s) (unicast case) in order
to construct the RNT. The procedure assumes bi-directional links
between intermediate LSR nodes, since FIS messages are subsequently
transmitted on the "reverse-table" incoming link interface. However,
in optical rings (even meshes), especially transparent rings (meshes),
there is likely a much higher degree of orthogonality between control
and data flows. For example, if control signaling is done on OSC
channels and not embedded in data wavelengths, even though RNT setups
can extract the above-detailed state at channel setup time, the actual
FIS (and FRS) messages are not sent on the "reverse-lookup" incoming
interface links. Additionally, the current MPLS RNT setup performs
near-side protection signaling, since fault messaging traverses the
same set of nodes but in the opposite direction. For long-side
protection signaling (as required per some O-BLSR/O-BPSR designs,
Section 3.2), however, protection signaling is required on the RNT
of the protection path. This is slightly different from the existing
possibility of MPLS protection-path RNT signaling [OWENS1], since it
implies failure of the working and not protection side. All of these
intricacies will require further setup signaling considerations.
Now consider the MPLS fault signaling message types, namely FIS and
FRS, and their use for optical channel protection. Initially, the
various fault detection (isolation) schemes, Section 3.4, are expected
to trigger FIS message transmissions within a few milliseconds of an
occurring fault (note that associated FIS hold-off timers must set
appropriately). Once the FIS messages are generated, the remaining
recovery latency is largely controlled by MPLS-layer signaling protocols
and ensuing optical switchover times. The latter issue depends upon
the actual switching technology used in the O-ADM protection stage,
Figure 1, and realistically, millisecond timeframes can be expected
via solutions such as MEMS or (O-E based) EXC designs. Meanwhile,
this stresses the need for expedient FIS processing in order to match
stringent benchmarks set by SONET APS. Here, the RNT architecture
is of particular importance (as detailed above). It is expected
that high-priority MPLS packet LSP's (routed on the OSC) will be
required to expedite fault message transmissions along the reverse
Ghani et. al. [Page 26]
path. Specifically, improvements can be achieved using a variety
of solutions. One is to use priority queuing for (reverse) FIS
messages, and dedicate a fixed minimum amount of bandwidth via
scheduler mechanisms. A further extension would be to perform
FIS message processing (e.g., RNT label lookups and fast switchover)
via dedicated hardware, such as FPGA devices. Clearly, both of
these schemes entail added system complexity, and demonstrable
evidence is required to determine if SONET recovery times can be
matched. Overall, the MPLS LSP protection proposals pose some
complexity concerns and may prove rather slow for achieving the
desired millisecond-level recovery timescales.
4.2.2 O-APS Protocol
As an alternative to generalizing MPLS LSP protection capabilities, a
specialized, fast optical APS (O-APS) protocol is possible for optical
rings. This entity can be considered as a "lower-layer" (i.e., layer
two) type of protocol, as illustrated in Figure 8, somewhat akin to the
LMP [LANG],[FREDETTE]. For some, there are various compelling reasons
to develop such an alternative. First of all, given the relatively
stringent recovery requirements, many may argue that modifying or
specializing MPLS signaling protocols (e.g., added failure-recovery
messages, prioritized processing/implementations) may become too
complicated and lengthy a process. Instead, a lightweight O-APS
protocol can be designed, and this would be functionally equivalent
to an "optical" version of the ubiquitous SONET K1/K2 byte protocol.
Nevertheless, unlike the SONET K1/K2 byte APS protocol [GR1230], for
IP integration purposes, this protocol must be packet-based. In other
words, the implementation should not be restricted to a fixed number
of signaling bytes in a synchronous (e.g., SONET) framing standard.
Such a setup would be totally counter to packet-oriented control done
in IP networks. Instead, a "fast" packet-based protection protocol
must be developed. How these packets are actually carried in the
control channel, however, can be left open to vendor implementation,
but bandwidth guarantees are necessary in order to meet recovery timing
requirements (similar to discussions for FIS message transport,
Section 4.2.1). It is here that some vendors may choose to embed these
O-APS control packets into SONET overhead framing bytes on the
(out-band) OSC, but note that this is distinctly different from the
case standardizing explicit signaling bytes, i.e., "non-packetized"
O-APS implementation.
Some of the key components of an O-APS protocol are briefly highlighted
here, although a more detailed specification is clearly beyond the
scope of this discussion and intended for further study. Among other
things, message fields must identify the switching nodes, lightpath
channels/spans, fault type (channel, span, node), and requested
protection actions (channel or span switching, near-side, far-side),
etc. Additional parameters must also be specified for alarm messaging,
such as durations, spacings, even priorities (e.g., span, channel).
A complete state machine definition and related rules are also required,
and examples include triggering recovery actions, starting/stopping
alarm messaging, alarm squelching for multiple types of alarms (e.g.,
channel versus span, etc). Another issue is inter-node keepalive
Ghani et. al. [Page 27]
messaging. Such "hello" message formats are common in IGP protocols
and are directly embedded into the SONET APS protocol, i.e., non-alarm
K1/K2 byte fields serve as constant "hello" updates. O-APS peer nodes
must also have this capability, and one alternative it to add explicit
hello messaging for non-failure time periods. Note that the LMP
protocol also has some provisions for "liveness" message updates, but
this protocol is currently more geared towards mesh network support,
i.e., OXC-to-OXC or router-to-OXC connectivity maintenance, see [LANG],
[FREDETTE]. Hence a fast, dedicated liveness/hello mechanism is
desirable for optical rings. Finally, since the O-APS protocol will
be "new" protocol, it presents a good opportunity to properly define
crucial "operator-initiated" functionalities, Section 4.1.1. For
example, explicit message types (or fields, as appropriate) and
appropriate priorities can be assigned for features such as resource
lockout, forced and/or manual protection switching, etc. In fact,
this option is one clear advantage of defining an altogether new
protection O-APS switching protocol. However, significant further
work is required to specify a truly generalized O-APS framework to
implement the previously-defined transparent optical ring
architectures, Section 3.
>From a broader perspective, a dedicated O-APS protocol can also be
deployed in a "standalone" manner, an added benefit. This is important
for many vendors need to provide O-ADM ring solutions, but at the same
time, want to gradually transition into a full-blown "IP-based"
MPL(ambda)S-based control framework. In such cases, a standardized
O-APS protocol will allow such vendors to provide NMS (network
management system) based provisioning solutions. Here, the NMS
controller(s) will explicitly setup/takedown ring channel lightpaths
and "fill in" the required information for the O-APS protocols to
operate from. Nevertheless, further migration to dynamic MPL(ambda)S
(GMPLS) provisioning paradigms brings up some notable architectural
issues with regards to a new "layer-two" O-APS protocol. In
particular, proper interfaces have to be identified (or developed) to
enact the necessary information exchange between the setup and
protection signaling entities. This is because the O-APS protocol
will likely only implement switchover messaging and not any O-ADM
channel provisioning capabilities (performed instead by extended
MPLS signaling protocols, Section 4.1). This setup requires that
CR-LDP (or RSVP-TE) signaling "instantiate" the appropriate
operational "state" for the O-APS protocol. This includes identifying
the ring connection maps/tables, protection switching nodes, protection
types (channel, wavelength), protection signaling routes, etc. Most
of this information will reside in ubiquitous MPLS constructs, such
as the incoming label map (ILM) and next hop label forwarding entries
(NHLFE). Moreover, in many ways, these requirements overlap with
constructs already proposed for LSP protection, such as the RNT, and
possibly these can be utilized also. Further, more defining work is
required here.
4.2.3 Multi-Layer Escalation Strategies
Assuming that fast optical (ring) lightpath protection schemes will
emerge, inter-layer protection "collisions" will be of concern. Since
Ghani et. al. [Page 28]
multiple protocols provide recovery mechanisms, duplicated
functionalities (e.g., optical lightpath protection, SONET APS, MPLS
LSP protection switching, IP flow re-routing) can lead to reduced
resource utilization and data routing instabilities [DEMEESTER]. For
example, optical lightpath recovery times can overlap with (client)
SONET or MPLS LSP protection timescales. Clearly, a mechanism is
required to coordinate recovery actions between the various layers
(packet, circuit, wavelength, fiber). This issue is commonly termed
as escalation strategy design and has been treated in the broader
research literature [GHANI1]. Specifically, two types of escalation
strategies have been proposed, namely bottom-up and top-down approaches,
see [DEMEESTER] for full details. The former scheme assumes that
"lower-level" recovery schemes (e.g., optical ring protection) are
more efficient and expedient, and therefore inhibits higher-layer
protection switching (such as IP re-routing or MPLS/ATM LSP protection
switching). Alternatively, the top-down approach attempts service
recovery at the higher layers first before invoking "lower layer"
(e.g., optical) recovery. The reasoning here is that higher-layer
protection can be more service selective, and therefore efficient.
Clearly, these are both advanced mechanisms and require complex
signaling and hold-off timer mechanisms [GHANI2] to coordinate the
different layer recovery procedures.
As far as the proposed optical ring protection framework is concerned,
there are two cases of escalation strategies that can be derived, namely
MPLS (GMPLS) control-specific, and non-MPLS (non-GMPLS) control-specific.
Consider the former case, in which the "higher layers" (e.g., TDM
circuit, packet LSP) are also controlled (provisioned and protected) by
the MPLS (GMPLS) framework. Assuming a generalized MPLS LSP restoration
framework [XU] at all layers, escalation strategy timing is facilitated
by this common control framework. The appropriate LSP protection timer
mechanisms can specify hold-off times, alarm message (FIS) spacings, and
alarm message durations. Clearly, judicious choices of these parameters
at different LSP levels (packet, circuit, wavelength lightpath, fiberpath)
can be used to design advanced "inter-layer" escalation strategies. For
example, at the wavelength LSP level, small hold-off times and FIS
spacings can be used to enact fast (sub-50 ms) recovery. Additionally,
the duration of lightpath-level FIS messaging can be restricted to a
timescale window, beyond which lightpath FIS notification is terminated.
This duration (plus an acceptable guard-time) can be the hold-off time
for "higher-layer" packet LSP FIS message generation. Note that this
example details a "bottom-up" recovery case, and a complimentary
"top-down" case can also be detailed.
Meanwhile, for non-MPLS (non-GMPLS) control specific case, escalation
strategy design can be more complicated since generalized timing control
and signaling mechanisms may not exist at all protocol layers. In
particular, this situation will arise if MPLS (GMPLS) protection is not
used at the various networking "levels", e.g., O-APS at optical
lightpath level, SONET APS at the circuit tributary level, MPLS LSP
protection at flow level, etc. In general, this makes it more difficult
to control inter-layer protection recovery timings. In such cases,
simpler "all-or-nothing" interworkings are more feasible. For example,
for the case of traditional SONET over WDM rings, either optical ring
Ghani et. al. [Page 29]
or SONET APS recovery can be disabled. Nevertheless, for higher-layer
IP packet traffic, "bottom-up" escalation strategies can usually be
implemented safely by simply ensuring small enough FIS message windows,
i.e., versus IGP re-routing timescales. In general, escalation
strategy design (both MPLS, non-MPLS based) needs significant
further attention.
4.3 Advanced Evolutions
Albeit detailed, the above discussions have only focused on basic
optical ring definitions and provisioning issues. Clearly, much more
advanced concerns relating to optical rings can be tabled, but their
detailed treatment is beyond the scope of this document. Nevertheless,
a brief synopsis is presented in order to stimulate further work.
In most current SONET networks, multi-ring architectures are very
common. Specifically, smaller rings are used to aggregate traffic from
local domains onto larger rings spanning increased distances (metro,
regional), and standards exist for interconnection between multiple
SONET rings [GR1230]. Likewise, as optical rings emerge (possibly on
the same fiber infrastructures), there will be a strong requirement for
similar O-ADM ring interworkings, namely to route lightpaths spanning
multiple rings. Inter-connection can be achieved by simpler, static
"back-to-back" O-ADM co-location or via more advanced, dynamic OXC
switching devices [ARIJIS]. Conceivably, different optical ring types
(e.g., O-BLSR, O-BLSR, O-UPSR) can be used for an end-to-end path,
along with their "localized" respective protection mechanisms (i.e.,
protection domains). This will also provide for more ranges in the
service definitions. Moreover, individual rings may likely belong to
different domains, e.g., routing autonomous systems (AS), and therefore
inter-ring routing will have to be done under the broader framework of
inter-domain provisioning [GHANI1],[GUO2],[PAPADIMITRIOU],[RAJAGOPALAN].
Most likely, inter-ring protection provisioning will come under
enhancements to emerging (optical) network interface (O-NNI)
definitions [PAPADIMITRIOU]. Hence, signaling requirements for channel
provisioning (i.e., setup) between multiple rings and protection
switching between rings need consideration.
As optical mesh optical networks evolve, mesh channel protection
schemes can also leverage off of ring protection schemes. Specifically,
mesh span protection or mesh end-to-end channel protection (via diverse
routing) can re-use (extend) the (working, protection) channel setup and
protection signaling mechanisms developed for optical rings. In fact,
this analogy can be taken one step further in the form of "virtual
rings." Here, different ring types (O-UPSR, O-BLSR, O-BPSR) can be
overlaid on top of generic mesh networks, thereby exploiting the
advantages of fast ring-based protection switching. Such overlay
applications are a very powerful feature, especially for operators
familiar with operating ring networks, and will allow them to provision
virtual private optical rings (VPOR). Although various extensions are
required, early work along these lines is emerging [GUO2], e.g., "ring
ID" and "ring type" sub-TLV's for opaque LSA.
Ghani et. al. [Page 30]
So far, only "full-granularity" lightpaths have been considered, i.e.,
ring channels utilizing full wavelength capacity, such as OC-48, OC-192.
However, new generations of advanced integrated edge devices (IED's)
are beginning to appear, integrating packet, circuit (i.e., sub-rate
lambda), and wavelength switching capabilities into a single box.
In a timely manner, the broader MPLS framework is also being extended
to provision related sub-rate tributary channels (e.g., OC-12, OC-3,
even DS3), see [XU]. Therefore, for true generality, future ring
provisioning mechanisms must also apply to sub-rate tributary
channels. Sub-rate rings can improve wavelength utilization and
provide more granular connectivity between smaller users. As a result,
advanced "multi-ring" architectures can be envisioned, with "embedded"
sub-rate ring channels (LSP's) being carried in larger granularity
wavelength ring channels. Here, a given sub-rate channel can traverse
multiple wavelength channels (depending upon location of the IED
nodes). Again, escalation strategies will be required here to
arbitrate between multiple different ring protection mechanisms, akin
to O-BPSR/O-BLSR interworking, e.g., Section 3.2.1. However, for this
particular case, the escalation strategies can be built into a single
protocol, such as the O-APS protocol, simplifying the architecture.
Similarly, there has recently also been significant interest
and developments in resilient packet ring (RPR) architectures. Since
these rings operate on a "higher" level and require visibility into
the data stream, they are markedly different from the optical rings
proposed herein. However, RPR's can be embedded onto optical
wavelength ring channels, and given the increased detection/recovery
speeds being proposed (in comparison to IP re-routing or LSP
recovery), there can be a much higher likelihood of protection
"collisions" between optical ring and RPR recovery mechanisms. Hence
protection escalation strategies will be needed for arbitration
between the respective ring protocols (packet, circuit). Additionally,
traffic/resource engineering synergies are also possible, and all
of these topics need further investigation.
5. Security Considerations
Security considerations are for future study, in particular with
regards to signaling extensions and a possibly new O-APS protocol.
The overall optical ring provisioning framework, however, poses the
same security requirements as those present in existing MPL(ambda)S
or GMPLS provisioning architectures.
6. References
[ABOULMAGD] O. Aboul-Magd, "Signaling Requirements of the Optical
UNI," Internet Draft, draft-bala-mpls-optical-uni-signaling-01.txt,
November 2000.
[ANSI] "Synchronous Optical Network (SONET): Basic Description
Including Multiplex Structure, Rates, and Formats," ANSI
T1.105-1995, 1995.
[ARIJS] P. Arijs, et al, "Design of Ring and Mesh Based WDM Transport
Networks," Optical Networks, July 2000.
Ghani et. al. [Page 31]
[ARVIND] K. Arvind, et al, "Optical Domain Service Interconnect (ODSI)
Signaling Control Specification," Version 1.4.5, ODSI Coalition,
November 2000.
[AWDUCHE] D. Awduche, Y. Rekhter, J. Drake, R. Coltun, "Multi-Protocol
Lambda Switching: Combining MPLS Traffic Engineering Control with
Optical Crossconnects," Internet Draft, draft-awduche-mpls-te-
optical-00.trt, October 1999.
[BERNSTEIN] G. Bernstein, V. Sharma, "Some Comments on GMPLS and Optical
Technologies," Internet Draft, draft-bernstein-gmpls-optical-00.txt,
November 2000.
[BHANDARI] R. Bhandari, S. Sankaranarayanan, E. Varma, "High-Level
Requirements for Optical Shared Mesh Restoration," Internet Draft,
draft-bhandari-optical-restoration-00.txt, May 2001.
[CEUPPENS] L. Ceuppens, et al, "Performance Monitoring in Photonic
Networks in Support of MPL(ambda)S," Internet Draft, draft-ceuppens-
mpls-optical-00.txt, March 2000.
[CHEN] J. Chen, T. Shiragaki, "Routing of OCh Shared Protection
Ring," T1X1 Forum, T1X1.5/99-256R1, October 1999.
[CVIJETIC1] M. Cvijetic, T. Shiragaki, "Standardization of OCh Shared
Protection Ring and Its Open Issue List," T1X1 Forum, T1X1.5/99-255R1,
October 1999.
[CVIJETIC2] M. Cvijetic, T. Shiragaki, A. Weissberger, "OCh Shared
Protection Ring," T1X1 Forum, T1X1.5/99-178, July 1999.
[DEMMESTER] P. Demmester, et al, "Resiliency in Multilayer Networks,"
IEEE Communications Magazine, March 2000.
[DOVOLSKY] D. Dovolsky, I. Bryskin, "Calculation of Protection Paths
and Proxy Interfaces in Optical Networks Using OSPF," Internet Draft,
draft-dovolsky-bryskin-ospf-pathprotect-proxy-00.txt, June 2000.
[FREDETTE] A. Fredette, et al, "Link Management Protocol (LMP) for
WDM Transmission Systems," Internet Draft, draft-fredette-lmp-
wdm-00.txt, December 2000.
[GHANI1] N. Ghani, "Lambda-Labeling: A Framework for IP over WDM
Using MPLS," Optical Networks, April 2000.
[GHANI2] N. Ghani, "Survivability Provisioning in Optical MPLS
Networks," 5th European Conference on Networks and Optical
Communications, June 2000.
[GHANI3] N. Ghani, et al, "COPS Usage for ODSI," Version 2, ODSI
Coalition, August 2000.
[GR1230] GR-1230-CORE, SONET Bi-directional Line-Switched Ring
Equipment Generic Criteria, Issue 4, December 1998.
Ghani et. al. [Page 32]
[GR3009] GR-3009-CORE, Optical Cross-Connect Generic Requirements,
Issue 1, January 1999.
[GUO1] D. Guo, et al, "Extensions to RSVP-TE for Bi-directional
Optical Path Setup," Internet Draft, draft-sorrento-rsvp-bi-
osp-00.txt, July 2000.
[GUO2] D. Guo, et al, "Hybrid Mesh-Ring Optical Networks and Their
Routing Information Distribution Using Opaque LSA," Internet Draft,
draft-guo-optical-mesh-ring-00.txt, December 2000.
[HUANG] C. Huang, V. Sharma, S. Makam, K. Owens, "Extensions to RSVP-
TE for MPLS Path Protection", Internet Draft, draft-chang-rsvpte-path-
protection-ext-00.txt, June 2000.
[ITU] "Network Node Interface for the Synchronous Digital Hierarchy
(SDH)," International Telecommunication Union, G.707, March 1996.
[KINI1] S. Kini, M. Kodialam, T. V. Lakshman, "Open Shortest Path
First (OSPF) Protocol Extensions for Label Switched Path Restoration,"
Internet Draft, draft-kini-ospf-lsp-restoration-00.txt, October 2000.
[KINI2] S. Kini, M. Kodialam, T. V. Lakshman, "Shared Backup Label
Switched Path Restoration," Internet Draft, draft-kini-restoration-
shared-backup-00.txt, October 2000.
[KOMPELLA1] K. Kompella, et al, "OSPF Extensions in Support of
MPL(ambda)S," Internet Draft, draft-kompella-ospf-ompls-
extensions-00.txt, July 2000.
[KOMPELLA2] K. Kompella, et al, "IS-IS Extensions in Support of
MPL(ambda)S," Internet Draft, draft-kompella-isis-ompls-
extensions-00.txt, July 2000.
[KOMPELLA3] K. Kompella, et al, "Extensions to IS-IS/OSPF and RSVP
in Support of MPL(ambda)S," Internet Draft, draft-kompella-mpls-
optical-00.txt, March 2000.
[LANG] J. Lang, et al, "Link Management Protocol (LMP)," Internet
Draft, draft-lang-mpls-lmp-01.txt, July 2000.
[MARCENAC] D. Marcenac, "Benefits of Wavelength Conversion in Optical
Ring-Based Networks," Optical Networks, April 2000.
[MCADAMS] L. McAdams, J. Yates, K. Bala, "User to Network Interface
(UNI) Service Definition and Lightpath Attributes," OIF Forum,
OIF2000.061, September 2000.
[OWENS1] K. Owens, et al, "A Path Protection/Restoration Mechanism
for MPLS Networks", Internet Draft, draft-chang-mpls-path-
protection-02.txt, November 2000.
[OWENS2] K. Owens, V. Sharma, M. Oommen, "Network Survivability
Considerations for Traffic Engineered IP Networks", Internet Draft,
draft-owens-te-network-survivability-00.txt, March 2000.
Ghani et. al. [Page 33]
[OWENS3] K. Owens, et al, "Extensions to CR-LDP for MPLS Path
Protection," Internet Draft, draft-owens-crldp-path-protection-
ext-00.txt, December 2001.
[PAPADIMITRIOU] D. Papadimitriou, et al, "Optical Network-to-Network
Interface Framework and Signaling Requirements," Internet Draft,
draft-papadimitriou-onni-frame-01.txt, November 2000.
[RAJAGOPALAN] B. Rajagopalan, et al, "IP Over Optical Networks-
A Framework," Internet draft, draft-many-optical-framework-02.txt,
November 2000.
[SOULLIERE] "Proposed ITU-T Contribution on Transparent OCh SPRings,"
T1X1 Forum, T1X1.5/2001-027, January 2001.
[SZERENYI] L. Szerenyi, "Approach to OSC Standardization," T1X1
Forum, T1X1.5/2001-002, January 2001.
[XU] Y. Xu, et al, "Generalized MPLS Control Plane Architecture for
Automatic Switched Transport Network," Internet Draft, draft-xu-mpls-
ipo-gmpls-arch-00.txt, November 2000.
[XUE] Y. Xue, et al, "Carrier Optical Services Framework and Associated
UNI Requirements," Internet Draft, draft-many-carrier-framework-
uni-00.txt, November 2000.
[YU] J. Yu, et al, "RSVP Extensions in Support for OIF Optical UNI
Signaling," Internet Draft, draft-yu-mpls-rsvp-oif-uni-00.txt,
July 2000.
[ZANG] H. Zang, J. Jue, B. Mukherjee, "A Review of Routing and
Wavelength Assignment Approaches for Wavelength-Routed Optical WDM
Networks," Optical Networks, January 2000.
7. Authors Information
Nasir Ghani
Sorrento Networks Inc.,
Email: nghani@sorrentonet.com
James Fu
Sorrento Networks Inc.,
Email: jfu@sorrentonet.com
Dan Guo
Sorrento Networks Inc.,
Email: dguo@sorrentonet.com
Xinyi Liu
Sorrento Networks Inc.,
Email: zzhang@sorrentonet.com
Zhensheng Zhang
Sorrento Networks Inc.,
Email: xliu@sorrentonet.com
Ghani et. al. [Page 34]
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