Internet DRAFT - draft-awduche-ipo-gmpls-signaling-applicability


IPO Working Group                                         Daniel Awduche
Internet Draft                                            Adrian Farrel
Expiration Date: January 2003                             Movaz Networks

                 GMPLS Signaling Applicability Statement


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026, except that the right to
   produce derivative works is not granted.

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2. Abstract

   This memo describes the applicability of GMPLS signaling to IP over
   Optical (IPO) networks. The underlying premise behind GMPLS
   signaling for IPO networks is discussed and pertinent limitations
   are highlighted. Throughout this discussion, GMPLS-RSVP (RSVP-TE
   with GMPLS extensions) will be used to exemplify the class of
   protocols under consideration. This memo also describes the subsets
   of GMPLS signaling message exchanges that are needed to support
   connection management between optical switches, and between IP/MPLS
   routers and optical switches in IP over Optical networks.

3. Summary for Sub-IP Area

3.1 Summary

   This document describes the applicability of GMPLS signaling to IP
   over Optical Networks.

3.2 Where does it fit in the Picture of the Sub-IP Work

   This work fits in the IPO Working Group.

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3.3 Why is it Targeted at this WG

   This is one of chartered deliverables of the IPO working group.

3.4 Justification

   The WG should consider this document because it describes how the
   existing GMPLS signaling protocols can be used in IP over Optical
   networks. This information is especially useful to the IETF
   community because it delineates the subset of GMPLS signaling
   functionality that are useful in IPO networks.

4. Introduction

   This memo describes the applicability of GMPLS signaling protocols
   to connection management in IP over Optical (IPO) networks. The
   basic function of the GMPLS signaling protocols is to setup,
   maintain, and teardown connection services in different types of
   transport networks. This memo relates only to the applicability of
   the GMPLS signaling capabilities in IPO networks. The applicability
   of GMPLS routing protocols to IPO networks are covered in a
   separate document.

   The term IP over Optical networks generally refers to optical
   transport networks that posses one or more of the following two
   main characteristics: (1) They are used to preponderantly transport
   IP traffic and (2) they employ IP (e.g. GMPLS) protocols in their
   control plane.  An optical network utilizing GMPLS in its control
   plane may also transport other types of digital clients other than
   IP traffic.

   This memo is concerned specifically with optical networks that
   utilize GMPLS signaling protocols in their control plane,
   irrespective of the type of digital clients and payloads carried in
   their transport plane. The definition of IPO networks, for the
   purpose of this applicability statement, encompasses the OCh layer
   of the ITU-T Optical Transport Networks (OTN) described in [8],
   especially when such networks utilize GMPLS protocols in their
   control planes.

   The factors that motivate the application of IP-based control plane
   protocols (such as GMPLS) to optical networks were articulated in
   reference [5].

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5. Overview of GMPLS Signaling

   GMPLS signaling protocols are general purpose control plane
   protocols supporting signaling transactions in different types of
   switched transport networks with different switching
   technologies. The GMPLS signaling protocols are derived from the
   corresponding IETF MPLS signaling protocols, (e.g., RSVP-TE [4])
   through a process of extension and generalization. The GMPLS
   signaling protocols employ the same protocol machinery as the
   underlying protocols from which they were derived, but include
   additional protocol features to support signaling capabilities in
   a multitude of switched transport networks.

   The main aspects of GMPLS signaling are documented in a number of
   IETF memos [1,2,3,6,7]. There are two broad classes of GMPLS
   signaling protocols that are being standardized by the IETF: (1)
   those based on RSVP-TE and (2) those based on CR-LDP. Both classes
   of protocols perform similar functions and either of the two can be
   used to fulfill the necessary signaling transactions in a
   particular context. However, recent surveys conducted within the
   IETF suggest that GMPLS signaling protocols derived from RSVP-TE
   are the most predominantly implemented in the industry (see
   e.g. reference [10]).

   It is significant to point out that the GMPLS architecture does not
   mandate either of the two classes of GMPLS signaling for a
   particular application. Therefore this applicability statement does
   not prevent an implementation from utilizing either of the two
   categories of protocols.

6. Applicability of GMPLS Signaling to IPO Networks

   GMPLS is an appropriate signaling mechanism for establishing end-to
   end connections in optical networks utilizing embedded software
   control planes.  GMPLS signaling provides the ability to perform
   signaling transactions for connection setup and connection release
   operations in optical networks. GMPLS does not impose restrictions
   on the implementation of the data communication network (DCN) that
   is used for conveying signaling messages. This means that GMPLS can
   be employed in optical networks where the control channel is
   implemented (1) in-band, (2) out-of-band in-fiber, (3) or out of
   band and out-of-fiber.

   GMPLS signaling messages provide the flexibility to control the
   path traversed by a particular connection within an optical network
   and to manage the allocation of resources used by the connection
   along the path. Some of the major benefits derived from GMPLS
   signaling in optical networks include the following, which are
   consistent with the requirements stipulated in [12]:

   - Rapid Circuit provisioning
   - Flexibility
   - Enhanced survivability
   - Interoperability

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6.1 Provisioning Capabilities

   One of the key benefits of GMPLS signaling in IPO networks is the
   automation and expediting of the provisioning of optical
   connections within the optical domain. With GMPLS signaling, it is
   possible to specify the route traversed by an optical connection
   (for example, using the Explicit Route Object in GMPLS-RSVP). It is
   also possible to specify the bandwidth for the optical connection.

   In an optical network, the labels exchanged by the GMPLS signaling
   protocol equate directly to an abstract representation of the
   optical resources to be used by the connection.  Depending on the
   characteristics of the optical domain, the labels may represent
   individual wavelengths, wavebands or whole fibers.  While whole
   fibers may be identified using numbered or unnumbered interfaces,
   the identification of ports, wavelengths and wavebands is a local
   matter for resolution between adjacent nodes in the network. In the
   case of wavelength routed optical networks, where the wavelength is
   the basic unit of resource allocation, it might be convenient to
   use a standardized convention for spectrum allocation such as the
   ITU grid.

   The conventional MPLS signaling protocols (e.g., the basic RSVP-TE
   protocol) are based on downstream-on-demand resource allocation and
   label binding. This means that in conventional MPLS signaling,
   label allocation proceeds from downstream nodes to upstream
   nodes. GMPLS signaling improves this scheme by supporting the
   concept of "suggested label" where an upstream node can suggest a
   label to a downstream. This concept is particularly useful in
   optical networks for the following reasons: (1) it can help to
   decrease provisioning latency by enabling anticipatory policies
   that allow optical network elements to reconfigure their switching
   elements during the forward pass of the signaling messages; and (2)
   in wavelength routed optical networks, it can permit wavelength
   assignment by upstream nodes.

   GMPLS signaling also suppports the concept of "label set" which is
   another improvement over conventional MPLS signaling. The
   significance of this concept in wavelength routed optical networks
   is that it allows upstream nodes to restrict the set of labels,
   hence wavelengths, that can be allocated by downstream nodes.

   In contexts in which network edge elements are capable of
   processing signals from many different devices, it may be necessary
   for them to indicate the bandwidth and encoding of the traffic
   associated with a particular connection, and perhaps the required
   switching behavior of the optical devices.  GMPLS signaling
   includes extensions offering this capability.

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   The principles of traffic engineering (that is performance
   optimization of networks) are very important in optical networks.
   In optical networks, traffic engineering is applied almost
   exclusively through routing control. GMPLS signaling allows to
   explicitly specify the path traversed by an optical
   connection. The path itself may be computed online or offline using
   an appropriate path computation mechanism which is not part of
   GMPLS signaling. This facility allows to constrain routes to fully
   or partially constrain the routes traversed by an optical connection.
   This facility can also be used to resolve resource contention
   and blocking problems, thereby optimizing optical resource

   Constraints associated with optical resources may be taken into
   consideration by a path computation engine when selecting the
   explicit route to be traversed by an optical connection. In this
   case, the path computation engine will calculate a path subject to
   pertinent constraints -- together with attendant resources to
   allocate on each hop, and specify these attributes in the explicit
   route object that is embedded in the signaling messages.  This
   capability may be enhanced on a hop-by-hop basis by having each
   network element provide resource availability information and
   suggestions for label allocation to its neighbor.

   Additionally, it may be important to constrain the choice of
   optical resources (wavelengths, wavebands, or fibers) along a path.
   This capability offers a number of advantages. For example, it can
   be used to allow more effective assignment of switching resources
   in optical networks consisting of a combination of electrical
   switching elements (requiring OEO conversion) and photonic
   switching elements (PXCs without OEO conversion). It can also
   substantially reduce wavelength contention and resolve wavelength
   conflicts in transparent sub-networks containing PXCs, where
   wavelength conversion cannot be performed at some or all
   nodes. Additionally, this capability can be used to reduce
   connection setup latency and signaling instability in networks
   containing some classes of PXCs, in which the underlying switching
   technology has a non-negligible finite reconfiguration delay and
   may require a finite stabilization time.  The "suggested label" and
   "label set" features are some of the capabilities of GMPLS
   signaling that allow to constrain the choice of optical resources.

6.2 Advanced Provisioning

6.2.1 Bidirectional Light Paths

   Optical connections are often required to be bidirectional.  This
   can be achieved by sequentially signaling two independent
   unidirectional optical connections, in opposite directions, one
   from each terminating endpoint. However, this approach is not very
   efficient in terms of signaling overhead and connection setup
   latency. This approach also makes it difficult to achieve route
   symmetry, so that both optical channel trails in opposite
   directions take the same physical path through the network.

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   GMPLS addresses the aforementioned issue by supporting the
   establishment of bidirectional optical connections using just one
   round-trip signaling message transaction. The approach adopted by
   GMPLS signaling has the added advantage that both directions of the
   optical connection can follow the same path, achieving the desired
   goal of route symmetry in optical networks. When establishing
   bi-directional optical connections, resources can be independently
   allocated to the two directions, and conceptually other types of
   constraints can be independently imposed as well. However, in an
   operational context, it is likely the two directions of a
   bi-directional optical connection will be assigned similar
   resources and satisfy similar constraints.

   The capability to establish bi-directional optical connections does
   not, however, preclude establishing uni-directional optical
   connections in contexts where uni-directionality is required in IPO
   networks. Clearly, this degree flexibility is very advantageous as
   the requirements imposed on optical networks will evolve and change
   over time.

6.2.2 Signaling Setup Time

   Many optical devices have high stabilization times.  GMPLS allows a
   node to pipeline signaling and device programming such that it can
   suggest to its adjacent nodes which optical resources should be
   used and proceed to program and reconfigure those resources before
   the signaling exchange with its neighbor has completed.  In the
   worst case the node must provision new resources when the signaling
   completes, especially if the suggested resources to be allocated
   are declined by the downstream node.

6.2.3 Alarm-Free Setup and Teardown (Alarm Suppression)

   Many optical networks report alarms if a service is provisioned but
   no signal is present (such as loss of signal and loss of light
   alarms).  For example, if an optical connection is provisioned, but
   a laser somewhere along the path is not turned on in a timely
   fashion, an alarm may be raised. This situation can also occur
   during connection reconfiguration, when the path traversed by an
   existing connection is altered.

   For a variety of reasons, it is often desirable to suppress alarms
   during optical connection setup, reconfiguration, and teardown.
   During these times it is reasonable to expect that lasers may be
   turned on or off sequentially, and it is not necessary to raise
   certain types of alarms, especially alarms suggesting failure
   within the network.

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   GMPLS offers a mechanism to indicate that a connection setup is in
   progress.  When such an indication is present, an optical device
   may suppress certain types of alarms during the signaling
   transactions.  Once the optical connection has been established,
   GMPLS can be used to signal that alarms should be enabled.

   Similarly, GMPLS can be used to indicate that connection teardown
   is in process so that optical devises can suppress pertinent
   alarms during the teardown process.

6.3 Survivability Features

   Survivability is a critical consideration in IPO networks. Within
   the optical domain, there is generally a requirement for
   flexibility in the manner in which resilience properties of optical
   connections are specified and implemented. There is also a general
   requirement to minimize service interruption following network
   outages for services that are intended to be survivable. The
   spectrum of protection/restoration capabilities that can be
   associated with a particular connection can be quite large
   including different protection levels (unprotected, 1+1, 1:1, 1:n,
   m:n), and different types of restoration (local restoration,
   end-to-end restoration, etc). In 1:1, 1:n, and m:n protection
   scenarios, it may also be desirable to route "extra traffic"
   (preemptable low priority traffic) over the protect resources to
   increase the efficiency of asset utilization.

   GMPLS signaling provides a range of capabilities to support
   different types of protection/restoration mechanisms and
   consequently to enable a variety of survivability options in
   optical networks.

6.3.1 Connection Specific Survivability

   A key requirement for the development of signaling protocols for
   the optical domain in IPO networks is the need for features to
   support intelligent fault management, as noted earlier.

   There are four major steps involved in fault management:  (1) fault
   detection, (2) fault localization, (3) fault notification, and (4)
   fault recovery.

   Fault detection and fault localization in optical networks are not
   covered by GMPLS signaling.  These aspects are the responsibility
   of the hardware, the transport plane, and link management protocols
   (such as LMP [11]).

   One of the major attributes of optical networks is that nodes
   downstream of faults are the ones most likely to detect the fault
   (such as loss of signal and loss of light problems). Therefore,
   there is a requirements to notify nodes upstream of the fault so
   they can initiate recovery actions. In the case of 1:1 and general
   m:n protection schemes, it is also important to coordinate the
   actions of the originating node and destination node for a
   connection so that they can both switch to the pre-establish
   protect path.

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   GMPLS signaling protocols include fault notification mechanisms for
   reporting errors across the network, from the nodes that detect the
   faults to the endpoints (originating and destination nodes) of the
   connection.  Furthermore, these fault notification mechanisms are
   defined to traverse the route of the connection, so that each node
   along the path will be informed of the anomaly and can take
   appropriate remedial action to rectify the problem or initiate
   recovery actions to restore affected traffic. The fault
   notification messages can also be targeted to specific nodes using
   an appropriate encapsulation.

   However, where the location of the repair points are predetermined
   and known, this method of propagating fault notifications can be
   slow.  It is to be noted that GMPLS-RSVP specifically includes a
   method that allows repair points to advertise their existence
   during connection setup, or later when the connection is
   modified. This capability allows fault notifications to be sent
   directly to these repair points, expediting the process of
   notification and consequently reducing the time to recover from

   Fault repair (hence service restoration) may be accomplished by
   signaling new optical connections after a fault notification
   message is received (in the case of restoration), or by switching
   traffic to pre-established protection paths (in the case of
   protection).  Some solutions may require coordination with other
   nodes in the network during (or before and/or after) the recovery
   of affected traffic to alternate paths (e.g., some hardware
   solutions for 1+1 and 1:1 protection, bidirectional optical
   connections, removal of extra traffic, etc).  GMPLS-RSVP (but not
   GMPLS-CR-LDP) allows such nodes to advertise their existence during
   optical connection setup or later during modification operations
   associated with connection reconfiguration, so that they can also
   be recipients of fault notification messages which are sent to
   them directly.

6.3.2 Decoupling of Control and Transport Planes

   In IPO networks, it is an important operational requirement on the
   part of carriers to decouple the Control plane from the Transport
   plane within the optical domain, so that faults occurring within
   the control plane will not affect existing services within the
   optical transport plane.

   In many optical networks there is no facility for in-band control
   signaling since that would require both GMPLS signal and bearer
   traffic termination at each node.  Some implementations of optical
   networks may offer in-fiber but out-of-band solutions where a
   particular wavelength is dedicated to function as a control
   channel.  Other alternatives include an out-of-fiber control
   channel that exists in parallel with the fiber carrying bearer
   traffic, and a control channel that is routed separately and
   independently through an external IP network.

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   There are two important implications of the scenarios mentioned
   above. One is the concept that failures in the control does not
   necessarily imply of the transport plane. Hence, unlike
   conventional IP/MPLS networks, failure of the transport plane
   cannot be inferred from failure of the control plane. The second
   important implication is that switching and reconfiguration
   activities in the transport plane should not be initiated by
   control plane failures.

   GMPLS addresses this important operational issue by decoupling the
   path of the GMPLS signaling message (i.e., the signaling data
   communications network) from that of the bearer traffic transported
   by the transport plane (unlike in conventional IP/MPLS networks
   where the two are closely coupled). This decoupling of the control
   and bearer channels implies that any of the in-band or out-of-band
   control channel solutions may be used for GMPLS based optical
   connection management.  In particular, the use of IP encapsulation
   by GMPLS signaling allows control packets to be routed through
   distinct networks that may include other GMPLS-capable nodes that
   must not examine these control packets in order to propagate them

   This decoupling allows the transport plane to be immune to control
   plane failures, whether these are hardware or software related, so
   that existing optical connections are not interrupted by failures
   occurring within the control plane.

   With the decoupling of the control plane and transport plane,
   failure detection associated with transport bearer services is no
   longer the responsibility of the control plane. Rather, fault
   detection and fault localization are the responsibility of the
   hardware and the link management protocols (such as LMP [11]) in
   the network. When such faults are detected using appropriate
   mechanisms within the transport plane or using LMP, they can be
   propagated to the control plane so that the control plane can
   initiate failure notification actions to pertinent nodes within the

   Because failures can occur within the control independent of the
   transport plane, it is now the responsibility of the control plane
   to recover its state (to match that of the transport plane) when
   control plane connectivity is recovered after a failure.
   GMPLS-RSVP includes a graceful restart procedure that allows
   network nodes to re-learn their state from neighbors on recovery.  A
   similar process is being developed for CR-LDP and is likely to
   become available for GMPLS-CR-LDP.

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6.3.3 Reliable Message Delivery

   Reliable message delivery is an important part of dependable and
   expeditious connection setup, reconfiguration, and teardown.  GMPLS
   includes facilities to ensure prompt and reliable delivery of
   signaling messages.

   GMPLS-CR-LDP achieves this by using a reliable transport (TCP).

   GMPLS-RSVP achieves the same function by applying Message Ids to all
   control messages and retrying the messages until they are

6.4  Architectural Considerations

   The IPO framework document [9] specifies different architectural
   alternatives for deployment of IP over Optical networks. The
   architectural alternatives specifically concern the interconnection
   models between control entities on IP/MPLS routers and control
   entities on optical transport network elements, especially when the
   IP and optical domains both utilize GMPLS control plane protocols.

   GMPLS-RSVP can be used as a signaling protocol between a client
   (e.g. IP/MPLS router) and an optical transport network in an
   overlay interconnection model.

   GMPLS-RSVP can also be used as a signaling protocol between optical
   network elements across an interior NNI (I-NNI), and as a signaling
   protocol between subnetworks across an E-NNI (Exterior NNI)

6.5 Deployment Considerations

   Several distinct deployment scenarios for GMPLS signaling in IPO
   networks can be identified. These include:

   - Deployment within an optical subnetwork with no signaling
     interactions with entities outside of the network.

   - Deployment within a subnetwork with signaling interactions with
     entities across the subnetwork where all subnetworks belong to the
     same administrative entity.

   - Deployment across networks where the networks belong to different
     administrative entities.  In this case, special care should be
     given to security considerations.

   In the case of deployments supporting signaling transactions across
   networks or subnetworks, the network elements performing the
   signaling transactions may utilize similar switching technologies
   or different switching technologies.

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6.5.1 Restrictions

   There are certain restrictions that may limit deployment of the
   existing class of GMPLS signaling protocols in some IPO operational
   contexts. In the following paragraph, we highlight some of the
   operational scenarios in which certain restrictions may apply.

   - Signaling protocol transactions across trust domains: When GMPLS
     signaling is required to traverse administrative domain
     boundaries it may be necessary to take security issues into
     considerations. These security implications across trust domains
     are not adequately covered by the existing protocol machinery.

7. Security Considerations

   The security considerations relating to the GMPLS signaling
   protocols are documented in the pertinent IETF protocol specific
   documents. This applicability statement does not introduce new
   security issues. It should be pointed out, however, that special
   precaution must be taken to ensure that unauthorized entities cannot
   successfully initiate or execute GMPLS signaling protocol
   transactions in IPO networks.

8. References

8.1 Normative References

   [1] L. Berger, et al, "Generalized MPLS - Signaling Functional
       Description," Internet Draft, Work in Progress, 2002.

   [2] L. Berger, et al, "Generalized MPLS Signaling - RSVP-TE
       Extensions," Internet Draft, Work in Progress, 2002.

   [3] P. Ashwood-Smith, et al, "Generalized MPLS Signaling - CR-LDP
       Extensions," Internet Draft, Work in Progress, 2002.

   [4] D. Awduche, et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels",
       RFC 3209, December 2001.

   [5] D. Awduche and Y. Rekhter, "Multiprotocol Lambda Switching," IEEE
       Communications Magazine, June 2001.

8.2 Informative References

   [6] E. Mannie, et al, "Generalized Multi-Protocol Label Switching
       (GMPLS) Architecture," Internet Draft, Work in Progress, 2002.

   [7] D. Papadimitriou, "Generalized MPLS Signalling Extensions for
       G.709 Optical Transport Networks Control," Internet Draft, Work
       in Progress, 2002.

   [8] ITU-T Recommendation G.821: "Optical Transport Network

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   [9] B. Rajagopalan, et al, "IP over Optical Networks: a Framework,"
       Internet Draft, Work in Progress, 2002.

   [10] L. Berger and Y. Rekhter, "Generalized MPLS Signaling -
        Implementation Survey," Internet Draft, Work In Progress, 2002.

   [11] J. Lang, et al, "Link Management Protocol (LMP)," Internet
        Draft, Work In Progress, 2002.

   [12] Xue, et al, "Carrier Optical Services Requirements," Internet
        Draft, Work in Progress, 2002.

9. Author Address

   Daniel Awduche
   Movaz Networks
   One Technology Parkway South
   Norcross, GA 30092
   Phone: 703-298-5291

   Adrian Farrel
   Movaz Networks
   7926 Jones Branch Drive, Suite 615
   McLean VA, 22102
   Phone: 703-847-1867

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