Internet DRAFT - draft-xu-mpls-ipo-gmpls-arch


Network Working Group                                     
Internet Draft			
Expiration Date: April 2001	

					Yangguang Xu 		Daniel O. Awduche
					Eve Varma 			Yong Xue
					Ramesh Nagarjan 		UUNet/WorldCom
                                               		Curtis Brownmiller
					Dimitri Papadimitriou 	WorldCom
					Rudy Hoebeke
					Alcatel 			John Eaves
					Osama Aboul-Magd
					Michael Mayer 		Hirokazu Ishimatsu
					Nortel 			Japan Telecom

					Raj Jain

									Nov. 2000

                  Generalized MPLS Control Plane Architecture 
                   for Automatic Switched Transport Network


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   Many solutions have been proposed to enable automatically switched 
   transport networks (ASTN). This document describes a control plane 
   architecture that can be applied to different packet and circuit 
   switching technologies (including fiber, waveband, wavelength, PDH, and 
   SONET/SDH). The control plane technology is based on IP/MPLS control 
   plane protocols. As such, this document is based on the concepts 
   introduced in [MPLambdas] and [GMPLS-SIG] from an architectural 
   perspective. It also describes how this control plane architecture 
   could facilitate control plane integration of networks across technical, 
   administrative, and business domains. This memo includes generic 
   procedures, key concepts, and technical considerations for the 
   generalized MPLS control plane architecture.  It is intended to 
   accentuate understanding of the application domains, create architectural 
   alignment, and serve as guide for protocol engineering.


Table of Contents

    1		Specification  ......................................  3 
    2		Acronyms   ..........................................  4
    3		Introduction to Generalized MPLS Control Plan  ......  4 
    4		Architecture Overview  ..............................  6
    4.1	System Components  ..................................  6
    4.2	Network Overview  ...................................  8
    4.3	Control Plane Transport Network  ....................  9
    5		Detailed Architectural Considerations  ..............  11
    5.1	NE Level Resource Discovery  ........................  11
    5.1.1	NE Level Resource Table  ............................  11
    5.1.2	Resource Discovery Stages  ..........................  12
    5.2	State Information Dissemination  ....................  13
    5.2.1	Logical Link/Bundle  ................................  13
    5.2.2	Link State Information   ............................  14
    5.2.3	Link State Dissemination Mechanisms  ................  15
    5.2.4	Flooding Control  ...................................  15
    5.3	Path Selection  .....................................  16
    5.3.1	Routing Constraints in ASTN  ........................  17

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    5.3.2	Path Selection for Traffic Engineering  .............  20
    5.3.3	Policy Consideration in Path Selection  .............  20
    5.3.4	Result of Path Selection  ...........................  20
    5.4	Path Control  .......................................  21
    5.4.1	MPLS Control Plane Concepts for ASTN  ...............  21	Upstream and Downstream  ............................  21	Generalized Label in ASTN  ..........................  22	Label Space   .......................................  23 Label Binding  ......................................  23	Label Distribution  .................................  24	Circuit LSP Tunnel  .................................  25	Circuit LSP Concatenation   .........................  25
    5.4.2	LSP Information Maintenance  ........................  25
    5.4.3	LSP Restoration  ....................................  25 LSP Restoration Stages  .............................  25 Link Level Restoration  .............................  26 Path Level Restoration  .............................  26
    5.4.4  	Circuit LSP Verification   ..........................  27
    5.4.5 	Control Plane Failure Handling ......................  27
    6		Inter-domain Control Plane Integration  .............  28
    6.1	Technology Domains and Hierarchy  ...................  28
    6.2	Service Interface between Network Domains  ..........  29
    6.3	Control Plane Design for Integration  ...............  30
    6.3.1	NE Level Resource Discovery  ........................  30
    6.3.2	State Information Dissemination  ....................  31	Potential Forwarding Adjacency  .....................  31
    6.3.3	Path Selection  .....................................  32
    6.3.4	Path Control  .......................................  33	Service Invocation  .................................  33	LSPs of Different Technology Domains  ...............  33	LSPs over Multiple ASes  ............................  33	LSP Restoration  ....................................  34
    7		Security Considerations  ............................  34
    8		Intellectual Property  ..............................  34
    9      	Acknowledgment  .....................................  34
    10     	Authors' Addresses  .................................  34
    11    	References  .........................................  36

1. Specification

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

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2. Acronyms
   ASTN:	Automatic Switched Transport Network
   NE:	Network Element
   ENE: 	Edge Network Element
   SNE:	Source Network Element
   DNE:	Destination Network Element
   SLA:	Service Level Agreement
   BER:	Bit Error Ratio
   LSP:	Label Switched Path
   PDM:	Packet Division Multiplexing
   TDM:	Time Division Multiplexing
   SDM:	Space Division Multiplexing
   LO:	Low Order
   HO:	High Order

3. Introduction to Generalized MPLS Control Plane
   A network can be functionally divided into a data or transport plane, a 
   control plane, and a management plane. The control plane consists of 
   network control components and network element (NE) control components. 
   The network control plane components perform network level coordination 
   functions including: state information management (acquisition, 
   representation, dissemination), decision making (e.g., path selection), 
   and action invocation (e.g., signaling).  The network element control 
   components control the local operation of specific network elements. 
   Traditional circuit switched transport networks have virtually no 
   automatic network control, which resulted in very slow service
   provisioning. Automatically switched transport networks, such as optical 
   transport networks, are required and feasible in the emerging data 
   centric networks which are growing very rapidly. In particular,  
   automatic switching of available capacity is needed to quickly and 
   efficiently provision bandwidth wherever and whenever needed to address 
   the increasing demands for network bandwidth.

   Even though differences exist in the technology and operation of 
   different transport networks, especially within the data or transport 
   planes, all switched transport networks share very similar 
   characteristics in their control planes. These control plane commonalties 
   are also shared by connection-oriented data networks, such as MPLS, as 
   noted in [Mplamdas].

   It is now widely recognized that it is desirable to establish generalized 
   control architecture that is applicable to different technologies because 
   it facilitates network inter-working. One of the major applications of 
   MPLS is the set-up of explicitly routed LSPs [MPLS-ARCH] for Traffic 
   Engineering (TE) purposes. Given that a label switched path (LSP) is an 
   instance of a network connection, it is evident that the LSP concept can 

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   be extended to support the set-up of any type of network connection via 
   an MPLS-based control plane as noted in a number of recent IETF drafts 
   (see e.g., [Mplambdas]). This applicability of the control protocols to 
   multiple network domains makes MPLS a good candidate for a generalized 
   control plane protocol. For this reason, there is significant interest 
   throughout the industry to define  a generalized MPLS-based traffic 
   engineering control plane for ASTN. 

   However, in order to generalize and apply the MPLS traffic engineering 
   control plane concepts to ASTNs, it is important to recognize the unique 
   features of ASTNs which demand specific adaptations to the MPLS control 
   protocols. Specically, ASTNs have many unique features that should be 
   taken into consideration in the design of  pertinent  control plane 
   technologies and architectures. Examples of these unique features 

   -- Network level features
      -- ASTN serves as the backbone for user traffic. The traffic 
         load and topology changes are not as dynamic as those for many 
         packet swtiched data networks.
      -- ASTN provides services with different types of SLAs (BER, 
         availability, maximum downtime and etc.) than those that typically 
         prevail for data connections.
      -- ASTN has  additional link state parameters than the link state 
         metrics used for data network topologies. 
      -- Circuit paths are setup with many technology dependent constraints. 
         The path selection process for circuit connections is also impacted 
         by technology-specific considerations.       
      -- Data LSPs are   generally considered to be uni-directional while 
         circuit connections can be  uni-directional or bi-directional.

   -- Network element level features
      -- Circuit switched NEs may have hundreds  or even thousands of 
         physical ports. Many of them  terminate on the same neighbor and 
         share the same attributes.       
      -- Ports in circuit switched NE are not typically assigned IP 
      -- Each physical port includes many technology dependent 
   The characteristics of ASTNs also have an impact on the data network 
   control plane. In particular, it changes the data network's basic 
   assumption that the transport network consists of fixed pipes. In the new 
   paradigm, the transport network can be  conceived as a large circuit 
   switch with a dynamically configurable back plane. Network devices that 
   are connected to a ASTN  can potentially establish direct connectivity 
   with each other. Effort should be expended to enable data networks 
   exploit these new capabilities of ASTNs to achieve true holistic end-to-
   end network traffic engineering.


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   As more applications enabled by ASTN are discovered, they will affect 
   existing network characteristics and generate new requirements. This 
   demands a control plane architecture  that is flexible and easily 

4. Architecture Overview
   A well-designed control plane architecture should provide service 
   providers with enhanced network control capabilities and relieve 
   operators from unnecessary manual operations.  At the same time, it 
   should facilitate network interoperation and integration between 
   different networks with different transport technologies such as circuit 
   switched networks and data networks. 

   -- It should be applicable to all circuit switched networks; e.g., 
      fiber (Optical Transport Section - OTS), waveband (Optical 
      Multiplex Section - OMS), wavelength (Optical Channel - OCh), 
      SONET/SDH, and PDH. In order to achieve this goal, it is essential 
      to isolate technology dependent aspects from technology 
      independent aspects and to address them separately.

   -- It should be sufficiently flexible to accommodate  different 
      network scenarios (service provider business models). This goal 
      may be achieved by partitioning the control plane into a number of 
      distinct components. This, for example, allows vendors and service 
      providers to decide the logical placement of these components, and 
      also allows the service provider to decide the security and policy 
       aspects of these components. 
   Within the context of ASTNs, the circuit switched network control plane 
   deals primarily with connection services. In a larger sense, this 
   functionality is a subset of the  network management functions; the 
   complement being  aspects of fault management, configuration management, 
   accounting management, performance management, security management, and 
   policy management functions. MPLS control plane components could be 
   applied to ASTN to achieve automatic connection control. It should also 
   be noted that for many circuit connection services, the MPLS based 
   control plane is just one alternative, as capabilities will also be  
   available to provide the same functionality in a less automated fashion. 

   4.1 System Components

   Corresponding to the MPLS traffic engineering control plane model 
   defined in [MPLS-TE], the generalized MPLS control plane can be 
   divided into several modules, namely, element level resource 
   discovery, state information dissemination, path selection and path 
   control modules. These functional components work together to 

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   complement each other. Their structure and the relationships between them  
   establish  the overall architecture. This approach is intended to avoid 
   focus upon certain functional components of the architecture, to the 
   inadvertent exclusion of others, which could result in unnecessary 
   dependencies and  poor network level solutions. The basic modules are 
   described below.

   -- Network element level resource discovery
      Resource discovery is defined as the transaction that establishes, 
      verifies, updates and maintains the NE adjacencies and their port-
      pairs association for their transport (data) plane.  During resource 
      discovery, control plane network should have built up its own 
      adjacencies and is in operation mode already. 

      The resource discovery module generates a complete NE level resource 
      map, which including attributes, neighbor identifiers, and pertinent 
      real-time operational status. The control procedure of this component 
      should be very generic. At the same time, some of its sub-components  
      may have to be technology and perhaps even product specific. 

      Beyond physical resource discovery, this component may also 
      include service/policy negotiation between adjacent NEs. 

   -- State information dissemination
      State information dissemination encompasses  the manner in which 
      NE level resource information is disseminated throughout the 
      network to form network level resource database. State information 
      distribution involves a number of steps. First, the NE level physical 
      resource map is acquired and summarized into logical links 
      (section 5.2.1)  based on link attributes. They are then distributed 
      throughout the network by either piggybacking onto the control plane 
      transport network (section 4.3) IGP, or through some other means such 
      as application layer protocols.  

      Key issues that relate to state information dissemination include the 
      specific types of state information that needs to be distributed and 
      the mechanisms for representing and disseminating this information. 

   -- Path selection
      Transport network routing procedures typically utilize explicit 
      routing, where path (connection) selection can be done either by 
      an operator, through the use of simulation and planning tools, by 
      a NMS or by the network itself. Hop by hop (link by link) routing 
      is also possible and may have  its own benefits under some 

      In a switched transport network, a path (connection) is normally 
      requested with certain constraints, which mainly come from 
      the customers' requirements,  operators' traffic engineering 

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      policies, and network characteristics. Path selection for a connection 
      request should  utilize constrained-based routing algorithms that 
      compute paths in the presence of multiple constraints and objectives:

      -- Conform to constraints such as physical diversity, etc.
      -- Achieve traffic engineering objectives in the transport network
      -- Adhere to operator policies on routing such as preferred routes
	-- Conform to network specific constraints
   -- Path (Connection) control
      Path control deals with path operations such as path creation, 
      deletion and modification (auto-rerouting and protection 
      switching). This is  an area that many MPLS control plane concepts 
      are applicable.

   Details of these functional components are elaborated in Section 5.

   4.2 Network Overview

   A typical connection operation within a single autonomous system (AS) 
   involves the following types of roles played by various entities: 
   service agent, edge NE (ENE), and intermediate NE (INE). 

   -- The service agent represents the system that originates an end-to-
      end circuit connection request. This role could be played by a client 
      NE, NMS, or network scheduling tool. It may or may not be within the 
      scope of ASTN control plane.
      Depending on which network application scenario is supported, and 
      which services are provided by the service provider, the request from 
      the service agent to the source NE may  contain the whole explicit 
      path (connection), or a subset of the explicit route, or only indicate 
      path (connection) end points. The service agent may also include  
      constraints, objective criteria, and policies as part of the request. 

   -- ENE has two functional roles: source NE (SNE) role and destination NE 
      (DNE) role. SNE initiates the connection set-up operation through the 
      associated transport network upon receipt of an authenticated 
      request from the service agent. DNE is the termination point of 
      the connection set-up operation  over the transport network.  
      SNE and ENE are irrelevant to transport traffic flow direction. That 
      is, they do not imply the direction of traffic flow. Furthermore, a 
      particular node may perform the role of SNE for one connection set-up 
      operation, and perform the role of DNE for another connection set-up 

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      A SNE typically supports all the functional components mentioned 
      above.  In addition to having the capability to play the role of INE 
      (see below) and DNE, a SNE  may also support the capability to perform 
      path selection computations. Furthermore,  a SNE should maintain an 
      inventory of path information for all circuit LSPs that were 
      originated from itself. 

   -- Intermediate NE (INE) exists along the path between an SNE and a DNE. 
      The INE accepts signaling requests from adjacent NEs and executes them 
      by rearranging its switch matrix and performing other local operations 
      to facilitate establishment of the connection. In executing connection 
      setup requests,  an INE selects specific physical resource according 
      to its local knowledge and according to pre-defined policies. 

      INE maintains LSP ID as a reference to support other types of 
      requests. It doesn't need to maintain full path (connection) level 
      information. However, it does need to maintain local physical 
      resource information for all LSPs that pass through it. Such local 
      information  need not  exist on other Nes, except in the form of 
      resource availability and usage information which the INE may 
      disseminate to other nodes.
   The roles of ENE and INE are associated with each LSP through the 
   switched transport network. However, each NE in the network could 
   play any or all of these roles.  Specifically, the role a NE plays 
   depends upon context, especially  its role in the establishment of each  
   LSP traversing it. 

   An LSP traversing multiple autonomous systems ASes may be divided into 
   multiple AS-sections, such that each AS-section corresponds  to one AS. 
   Each section may be considered to have its own AS ENE and AS INE. 
   Corresponding interfaces and responsibilities within the context of 
   multiple ASes require further study.

   4.3 Control Plane Transport Network and Requirements

   The underlying control plane infrastructure consists of a set of 
   distributed controllers that communicate and coordinate with each other 
   to facilitate connection set-up. A data communication network is needed 
   to enable control messages to be exchanged between controllers via the  
   distributed control plane. There is no separate transport network for the 
   control plane in traditional IP networks, where control messages are  
   exchanged through the same transport medium as the user data traffic. For 
   circuit based networks, the ability to have an independent network for 
   control message transportation is a fundamental requirement because a 
   typical transport NE  may not have the capability to terminate data 
   traffic through the bearer channels.

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   A control network may be defined as the transport infrastructure  
   that is used to convey control traffic. The control network can be either 
   in-band or out-of-band relative to the user traffic. Thus, the MPLS-based 
   control plane must not make assumptions about its physical transport 
   medium for control traffic. An implication of this is that the control 
   network of a circuit switched network may have a different physical 
   topology than  its transport plane network.

   Another implication is that the health of the control plane and 
   transport plane should be separately maintained. In the event of 
   control plane failure (for example, communications channel or control 
   entity failure), new circuit LSP (connection) operations shall not be 
   accepted, but existing connections shall not be dropped. Control 
   network failure  should still allow dissemination of  failure events 
   to a management system for operations, maintenance, and management 
   purposes.  This implies a need for separate notifications and status 
   codes for the control plane and transport plane. In general, because of 
   the importance of the control plane to the overall operation of the 
   network, the control plane transport infrastructure should be highly 
   survivable. Therefore, additional procedures need to be defined for 
   control plane failure recovery.

   Inter-working of the control networks is the first step towards 
   control plane inter-operation.  To maintain a certain level of  
   interoperability, it is desirable to have an  IP based control network. 
   Control networks from different administrative domains or different types 
   of transport networks can be interconnected in various ways to facilitate 
   control interaction  between different domains, or types of networks.

   The control network is an integral but independent part of the overall 
   control plane. It is critical to the scalability, reliability, and 
   performance of the overall control plane. To maintain the integrity of 
   control message delivery, the control network has several important 
   requirements, some of which are listed below:
   -- Control plane message transport must be secure. This requirement 
      stems from the fact that the information exchanged over the 
      control plane is service-provider specific and security is of 
      utmost importance.  Thus, service providers may not want to expose 
      internal network information outside their network boundaries even 
      to their own customers. In some cases, control traffic may need to 
      have its own dedicated physical medium or be carried on its own 
      virtual private network.     
   -- Control message transport reliability has to be guaranteed in 
      almost all situations, even during what might be considered 
      catastrophic failure scenarios of the service transport networks.  
      Since the control plane also supports connection restoration 
      functions, failures in the service transport network that also 

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      affect the control plane may result in the inability to restore 
      traffic or a degradation in the service restoration time. 

   -- The transport plane connection service performance largely depends 
      on its message transport. Time sensitive operations, such as 
      protection switching, may need certain QoS guarantees. 
      Furthermore, message transport should be guaranteed or recovered 
      quickly in  event of control network failure.
   -- The control network needs to be extensible and scalable in order to 
      make the control plane scalable as the requirements imposed upon it

5. Detailed Architectural Considerations

   This section covers what happens within an AS of a single technical 
   domain (section 6.1). Network interactions are covered in section 6.

   5.1 Network Element Level Resource Discovery

   5.1.1 Element Level Resource Table
   For circuit switched NEs, physical resources are usually dictated by 
   interface ports and their characteristics. Each NE maintains its own 
   resource table indexed by local port ID. The content of the NE level 
   resource table is as below. Physical and logical attributes may span 
   multiple columns. Figure 1 shows a NE level resource table depicting some 
   of the attributes that can be associated with interface ports.
| Local Port |  Neighbor   |  Neighbor  | Physical | Logical |  Oper.  |
|     ID     |  NE addr.   |  Port ID   |   attrs  |  attrs  |  State  |   
                   Figure 1: NE Level Resource Table
   Physical attributes are technology and vendor specific. Examples are:
   -- Signal format (SONET, Ethernet, etc.)
   -- Transmission bit rate
   -- Optics Type (SR, LR, etc.)
   -- Multiplexing structure
   -- Wavelength
   -- Directionality (Transmit, Receive, Bi-directional)

   Several physical attributes can be abstracted into one logical 
   attribute. Meanwhile, logical attributes may not be assigned to a 
   particular physical port directly, instead, they may describe 
   characteristics of a physical port pool. Logical attribute examples 
   -- VPN ID

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   -- SRLG (Shared risk link group)[OPT-BUND]
   -- Protection type 

   Operational state indicates the real-time state of a physical port. 
   It includes:
   -- Unequipped
   -- In-service
   -- Standby
   -- Failed
   -- Mis-configured
   -- Unknown

   5.1.2 Resource Discovery Stages

   Resource discovery can be achieved through either manual provisioning 
   or automated procedures. The procedures are generic while the 
   specific mechanisms and control information can be technology dependent. 
   Typically, there are several steps involved in resource discovery:

   -- Self resource awareness/discovery
      The results of self resource awareness/discovery is to populate the 
      local ID, physical attributes, logical constraints parameters in the 
      element resource table. 

   -- Neighbor discovery and port association
      This step discovers the adjacencies in the transport plane and 
      their port association and  populates the neighbor NE address and port 
      ID fields in the resource table.

      Because the control plane network may be different from the 
      transport plane network in ASTN, NEs that are not adjacent in the 
      control plane network may be adjacent in the transport network. In 
      order to unambiguously identify the transport plane neighbors and 
      their port associations, it is essential to have in-band events (along 
      the transport plane) coordinated  with other control messages (along 
      the control plane). 

      An example of in-band events for NE capable of electrical signal 
      processing in the transport plane is a byte stream containing the 
      local NE address and port ID. Both [LMP] and [ND-SONET/SDH] adopt this 
      approach.  Bi-directional links may use in-band signaling between 
      both ends. Uni-directional links may employ messages through the 
      control network to coordinate with in-band signals to achieve 
      bidirectional control coordination. [AND] 

      For a pure OXC without O-E-O capability, an analog signal (power 
      on/off) could principally be used as the in-band event. Control 
      messages over the control network  can then be used to co-ordinate  

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      and associate additional significance with the in-band event to 
      identify neighbor adjacency and port associations. Details of this  
      type of scenario are for further study.  
   -- Resource verification and monitoring
      After neighbor resource discovery, neighbors should detect their 
      operation state and verify their configurations such as physical 
      attributes in order to  ensure  compatibility. Such verification can 
      be done through control message over the control plane network without 
      using in-band signals. In case of any mismatch, the port should be 
      marked as mis-configured, and notification should be issued to 

      Resource monitoring is a  continual process. Neighbor discovery and 
      port association procedures are repeated periodically. Resource 
      monitoring procedures update resource state and report changes to 
      relevant control entities. 
   -- Service negotiation
      Service negotiation essentially  covers all aspects related to service 
      related to rules/policy negotiation between neighbors. 

      A typical example is the negotiation of rules for label binding and 
      assignment. In this case, neighbors build up a label binding and 
      look-up table according to the physical connectivity map. They may 
      exchange information on technology specific details such as 
      multiplexing schemes. They may also 

      negotiate the semantics for complex labels, and decide upon label 
      assignment order. 

   5.2 State Information Dissemination
   In link state routing, ASTN links can be represented either as point-to-
   point or non-broadcast multiple access links.  

   5.2.1 Logical Link/Bundle

   Typically, core circuit switches contain thousands of physical ports. 
   Therefore, scalability requires minimizing global information and keeping 
   information and decision making locally as much as possible. For this 
   reason, link aggregation plays a key role in ASTN topology  
   representation. Parallel link connections can be summarized into an 
   aggregate logical link, which hides the link connection details not 
   necessary for network level control functions, such as path selection.  
   The summarized information is then distributed to the rest of the 

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   A logical link or bundle is defined as a collection of link connections 
   that share some common logical or physical attributes that are 
   significant for path selection purposes. Link connections within a bundle 
   are equivalent for routing purposes. However, the notion of equivalence 
   is not static. It is configurable according to the service providers' 
   requirements. The granularity of bundling abstraction can range from 
   physical tributary to node and even to AS. 

   The granularity of bundling  can also be used to balance between network 
   operational performance optimality and control plane scalability. The 
   more details that are depicted in the topology database, the more 
   accurate the path selection could be. However, too much information could 
   result in an implementation that is not scalable. The trade off needs to 
   be carefully considered and engineered. 

   Bundling details are technology specific. Below are some examples of 
   bundling for different technologies:

      4 OC48s with the same multiplexing structure and going to the same 

   -- Wavelength
      100 protected wavelengths with wavelength convertibility and going 
      to the same neighbor 

   5.2.2 Link State Information 

   Link state information includes both static information and dynamic 
   information. This  information  may be  technology dependent.

   -- Static information is usually required for circuit operation. It 
      includes neighbor connectivity, logical link attributes (which 
      could be the same as individual attributes), total bandwidth and 

   -- Some dynamic information are required for circuit LSP operation, 
      while others are mainly used for traffic engineering purposes. 
      Dynamic information includes information that changes constantly,  
      such as bandwidth availability, current bandwidth 
      fragmentation information,  etc.
   The total number of attributes in a ASTN can be huge. The decision as to  
   what should be distributed and what could be negotiated during connection 
   setup requires a compromise  between control plane scalability 
   and connection operation performance. The more the number of attributes 
   that are conveyed in link state information, the more accurate the path 
   selection decisions. However, it may result in reduced  control plane 

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   scalability. In general, scalability and simplicity should be favored 
   above abundance of information and call setup speed in case of 
   irreconcilable conflicts. Stability is another crucial consideration. The 
   more the amount of information conveyed and the more frequent their rate 
   of change, the more the network will tend towards less stable operating 
   The objective should be to distribute just enough link state information 
   to support connection set-up operations. More information could be 
   distributed  provided it does not affect control plane scalability and 

   Furthermore, information reduction mechanisms should be utilized whenever 
   possible. The  basic concept is to only flood what is needed to 
   accomplish specific control goals. For example, static information and 
   dynamic information could be flooded independently. To build up the 
   initial network topology database, all pertinent static and dynamic 
   information needs to  first be disseminated. Subsequently, as long as 
   static information  remain unchanged, only the dynamically changing 
   information needs to be flooded. 

   5.2.3 Link State Dissemination Mechanisms

   Link state information can be piggybacked through IGP or EGP of the 
   control plane network according to network application scenarios. For 
   example, mechanisms introduced by [OSFP-OMPLS]/[ISIS-OMPLS] can be used 
   to disseminate ASTN link state information within a single area. 
   Alternately, optical link state and general ASTN advertisement can be 
   built as a separate application on top of any control network. This 
   solution can tailor existing IGP/EGP's behaviors to ASTN and meanwhile 
   permits flexibility in choice of the underlying control plane transport 

   5.2.4 Flooding Control

   Topology updates should happen only when a significant topology 
   change  occurs or upon expiration of a periodic timer. 

   -- Static information change requires immediate link state 
      information flooding. 
   -- Flooding of dynamic information change is triggered by thresholds 
      as defined below. 
   -- Periodic topology refreshing is needed to ensure that network 
      topology information in all NEs is consistent and correct.
   Some dynamic attributes are important for connection operation. For 

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   example, to SONET/SDH network, bandwidth continuity is critical for 
   connection operation. Because of frequent connection operations, a 
   SONET/SDH connection may become fragmented. An OC48 pipe with total 
   24 free STS-1s left doesn't mean that it can accommodate an OC-12c 
   connection request. So bandwidth continuity change should also be 
   distributed in order to avoid rejection of connection requests.

   Other dynamic attributes are important for traffic engineering in ASTN. 
   For example, the bandwidth consumption state of a logical link may  
   be used to adjust link cost and balance network traffic.

   The frequent change of link states requires frequent flooding in 
   order to keep the topology database updated. However, there is a 
   tradeoff between accuracy of the topology and the scalability of the 
   control plane as note earlier. Operators can control link state flooding 
   by configuring thresholds and frequencies according to a particular 
   network situation. Generally, there are two types of thresholds: absolute 
   threshold and relative threshold. 

   -- Absolute threshold defines change relative to total resource. 
   -- Relative threshold describes changes relative to previous 
      available resource. 
   Relative thresholding is preferable to absolute thresholding. Using 
   bandwidth availability as an example, it is desirable in general to 
   advertise less frequently when a link is lightly loaded and increase 
   the frequency as the link loading increases. This is automatically 
   accomplished by defining a threshold for the relative change in 
   available bandwidth. 

   There could be thresholds for key attributes of a logical link. A 
   significant change of any relative attribute could trigger flooding. 
   Details of how to set thresholds to control flooding are outside 
   the scope of this document.

   5.3 Path Selection

   Path selection could be done either off-line or on-line. The path 
   selection algorithms may also be executed in real-time or non-real time 
   depending upon their computational complexity, implementation, and  
   specific network context. Also, off-line computation is typically a 
   centralized operation whereas on-line computation is  typically 

   -- Off-line computation  may be facilitated by simulation and/or network 
      planning tools. Off-line computation  can help provide guidance to 
      subsequent real-time computations.

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   -- On-line computation  may be done whenever a connection request is 
   Off-line and on-line path selection  may be used together to make 
   network operation more efficient. Operators could  use on-line 
   computation to handle  a subset of path selection decisions and use off-
   line computation for complicated traffic engineering and policy related 
   issues such as demand planning, service scheduling, cost modeling and 
   global optimization. 

   In case of on-line computation, there are two choices: explicit 
   routing or hop-by-hop routing. 

   -- In ASTN, explicit routing is preferred as it provides superior 
      traffic engineering capabilities. Explicit routing may in fact be 
      mandatory if multiple constraints are considered in the path selection 

   -- Hop by hop routing can make more accurate next hop choice because 
      each node can consult its local resource information that may be 
      unavailable to other nodes. However, hop-by-hop routing requires 
      path calculation at each node and needs loop-prevention mechanisms. 
      Hop-by-hop routing is also more difficult when constraints other than 
      additive link metrics are taken into account.

   5.3.1 Routing Constraints in ASTN 

   There are potentially many routing constraints of interest in ASTNs. Some 
   of them are technology specific, such as optical constrains addressed by 
   [OLCP]. In the following, we highlight some of the major constraints of 
   interest, but the list is not meant to be exhaustive. Also, the 
   discussion is in general equally applicable to all types of circuit 
   networks. However, we highlight any differences where applicable.

   One of the major services provided by a transport network is restoration. 
   Restoration introduces several routing constraints. A major one is that of 
   physically diverse routing. Restoration is often provided by either pre-
   computing or finding  alternate routes for connections in real-time. These 
   alternate routes need to be physically diverse from the primary route in at 
   least the failed link  for efficiency purposes  or completely diverse. The 
   simplest form of diversity is node disjointness of the primary and alternate 
   routes. More complete notions of diversity can be addressed by logical 
   attributes such as shared risk link groups (SRLG). These logical attributes 
   are abstracted by operators from several physical attributes such as fiber 
   trench ID and destructive areas. Because methods of automatically identifying 
   physical separation do not exist yet these physical attributes require 
   operators' configuration. 

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   In order to address this constraint, path selection algorithms are 
   needed that can find a diverse pair of routes in the specified fiber 
   graph. Algorithms for node-disjoint diverse routes have been 
   developed [DISJ-PATH]. 

   Another restoration constraint is related to shared capacity mesh 
   restoration architectures [ON-REST]. In these architectures, 
   restoration capacity is shared among multiple demands. In order to 
   guarantee that restoration capacity is available on a chosen route, 
   one needs information on the free capacity on that route, demands 
   whose alternate routes share that route or some links on it and their 
   failure sets. Consider two demands that do not share any link on 
   their primary routes but share parts of an alternate route. In this 
   case, the capacity on the alternate route can be shared between the 
   two demands since the primary routes (and hence demands) can hardly 
   at the same time under single-failure scenarios. Optimal routing in 
   these cases requires considerable network level information and good 
   heuristics that limited information about the network and routing of 
   demands. Some heuristic approaches are proposed in [ON-REST]. 
   However, further study is needed to develop a complete solution.

   Other constraints of interest include node/LSP inclusion/exclusion, 
   propagation delay, wavelength convertibility and connection bandwidth 
   among others. Node exclusion involves specifying a set of nodes that 
   should be avoided for routing. These are typically specified on a 
   connection basis. An interesting application of node exclusion has 
   been proposed by Japan Telecom [DIST_AREA]. Certain office sites of 
   this carrier are in earthquake prone areas and to the extent possible 
   it is desired to route traffic of premium customers so as to avoid 
   these sites. A possible application of node inclusion is to route 
   traffic through a site that has some special equipment such as for 
   connection monitoring. Since transport connections are physical 
   connections involving fixed time slots or wavelengths, there are no 
   queuing delays like in packet networks. However, propagation delay 
   may be a factor for large global networks and especially networks 
   with satellite links. Consider traffic from New York to London on a 
   global network. It may be undesirable to route this traffic the long 
   way around the world (over the Pacific Ocean) and rather choose the 
   short route over the Atlantic Ocean. This can be realized by 
   requiring the propagation delay of the chosen route to be less than a 
   certain value.

   Wavelength convertibility is a problem encountered in 
   wavelength/waveband networks. It refers to the ability to cross-
   connect two different wavelengths. The wavelengths may be completely 
   disparate or just slightly different due to differences in the ITU 
   grid wavelengths supported by a network vendor. Wavelength 
   convertibility is typically accomplished via use of electrical 
   transponders. Owing to the cost of OEO conversions, operators may 

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   choose to selectively deploy transponders. In this case, end to end 
   connections have to employ the same wavelength through the network. 
   This constraint introduces considerable challenges in path selection. 
   In order to determine if a connection can be supported on a given 
   route, path selection needs to determine if the same wavelength, say 
   red, is available on each leg of the route. This requires that the 
   path selection be aware of all the available wavelengths on each leg 
   of the route. This information may be unavailable if link bundling 
   includes all wavelengths on a link into the same bundle. When routing 
   is carried out in a distributed manner at source NE of the connection, 
   this implies that considerable local information about the status of 
   network links needs to be distributed across the network. An alternate 
   less efficient approach might be to guess availability of wavelengths on 
   a computed route and determine feasibility of the route via local probing 
   of link resources along the computed route. However, this approach is 
   likely to result in considerable connection crank-backs and wasted 
   network processing resources. When wavelength convertibility is available 
   at each intermediate NE, however, no information about availability of 
   particular wavelengths on a link is needed. In this case, the total 
   number of available wavelengths is sufficient for routing purposes. 
   The above discussion applies equally to TDM circuit networks by 
   considering time slots rather than particular color wavelengths. Time 
   slot interchange capabilities are analogous to wavelength 

   Bandwidth availability is an important consideration in routing. This 
   is simplified in a wavelength optical network since all requests are 
   for end to end wavelength paths. However, in a TDM transport network 
   such as SONET/SDH, connection requests can be variable bandwidth 
   ranging from VT1.5 to as high as OC-768 with current technologies. 
   Routing needs to ensure that sufficient network capacity is available 
   on the chosen route to support the connection. Further challenges are 
   introduced by different concatenation schemes in SONET/SDH networks. 
   An example would be a contiguous concatenated STS-3c signal, which 
   require three adjacent time slots be allocated. This implies that a 
   time slot map of the link be distributed to all nodes in the network 
   to facilitate routing decisions. Alternately, in the logical link 
   representation, one would need N different logical links to represent 
   all possible STS-N signals. This could be expensive in terms of 
   control network bandwidth. A preferable approach is to advertise 
   just the largest contiguous block of time slots available on a 
   logical link instead of the entire time slot map. This is sufficient 
   to determine supportability of a connection on a link. Note that the 
   detailed information on local resource availability is only used for 
   routing decisions. Actual assignment of local resources to a 
   connection is performed by individual NEs.

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   5.3.2 Path Selection for Traffic Engineering

   5.3.3 Policy Consideration in Path Selection

   Operators' policy aspects of routing should also be considered. While 
   distributed, dynamic path selection has significant advantages such 
   as scalability, it is often advantageous to provide the network 
   operator some degree of control over path selection. Such control can 
   be provided in multiple ways. One possibility is to allow the 
   operator to provide a set of preferred routes between a source 
   destination pair. Automatic path selection may then choose between 
   these preferred routes. Alternately, static link weights could be 
   employed. The automatic path selection may then choose routes, when 
   feasible, that minimizes the static link weight. But the preferred 
   routes should typically be used only as a guide as in the extreme 
   case they reduce to fixed pre-determined routes and one loses the 
   flexibility to correct any mismatch between network capacity and 
   traffic patterns. 

   5.3.4. Result of Path Selection

   The result of explicit path selection is a linked list of explicit 
   routed hops. In order to provide just enough information for each NE 
   to choose the right physical resource for circuit connection 
   operation, it should be in logical link level of details.  

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |                      ER-Hop           (Mandatory)             |
  |                    Logical Link ID    (Optional)              |
                     Figure 2: Path Selection Result

   Explicit hop can be either node ID, AS ID or existing LSP ID as 
   defined in current MPLS signaling protocols. 

   The logical link (bundle) is defined in 5.2.1. It's flexible in 
   usage. For example, a logical link ID can be a VPN ID. If a low order 
   SONET LSP needs to tunnel through a existing high order SONET VPN, it 
   can use this VPN ID to restrict which resource to use. This concept 
   is similar to LSP tunnel, except, instead within a LSP, it's within a 
   pre-established circuit VPN.

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   5.4 Path Control   

   5.4.1 MPLS Control Plane Concepts for ASTN

   In data networks, MPLS covers both the control plane (label binding, 
   label distribution and etc.) and the transport plane (packet 
   forwarding). Label binding associates FECs to labels; label 
   distribution distributes label-binding information. Together with 
   mapping between FEC and next hop, the label switching forwarding 
   table is constructed to forward packets. 

   In circuit switched networks, there is no packet forwarding. Labels 
   are only used to set up LSPs by making cross connects. After LSPs are 
   set up, there is no dynamic label switching at all. 
   So only the MPLS control plane components are truly applicable to circuit 
   switched networks. 

   There are several ways to apply MPLS control plane concepts in 
   circuit switched networks. Given that circuit paths are service-
   driven, one possible understanding is to treat the service request as 
   a FEC. It contains next hop (or destination address for hop by hop 
   routing) and incoming physical interface (e.g., if negotiation is not 
   required at connection creation time) or path constraints ( e.g., if  
   negotiation is required). Because labels are used for traffic 
   forwarding, they are analogous to cross connect ID. Label binding is 
   then conceptually equivalent to choosing cross-connection across the 
   switch matrix. 

   An alternate interpretation can be constructed as follows: In circuit 
   switched networks, setting up a LSP means making cross connects along 
   the desirable path. Labels are used to determine cross connect 
   configurations. Assuming a non-blocking switch fabric, determining a 
   cross-connect means determining external ingress and egress physical 
   interfaces. So labels are used to indicate specific physical interfaces. 
   Label binding  entails choosing the correct ingress or egress interface 
   according to path selection decision and local policies. 

   In both of the above interpretations, making cross connects is equivalent 
   to writing to a label switching forwarding table. After the path is set 
   up, user traffic simply follows the dedicated physical path (cross 
   connect). The concepts below  elaborate on the second interpretation. Upstream and Downstream 

   Upstream and downstream in circuit switched network are based on path 
   create request direction. They are irrelevant to user traffic 
   direction. The NE that receives the request first is treated as an 

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   upstream NE relative to the NE that receives the request later. Generalized Label in ASTN

   Because the label is used to identify the physical resource in circuit 
   switches, first we should understand how physical resource is 
   represented in circuit switches. Typically, a specific resource in 
   circuit switches can be described as a concatenation of port ID and 
   channel/sub-channel ID within the port (assuming non-blocking switch 
    -- Port identifiers are product dependent. Different vendors have 
       different naming and addressing structures for their products. 
       For example, a port ID may be prefixed with bay ID, shelf ID and 
       slot ID. 

       However, neighbors don't need to understand the semantic of each 
       other's port ID. The port association information in the NE resource 
       table can serve as a look-up table. In the example below, when A 
       asks B for a connection, A will find B's port ID through its 
       resource table and use it when communicating with B.

                  	+-----+                  +-----+
            	------|\    |                  |    /|--------
                  	| \   |                  |   / |
                  	|  \  |                  |  /  |
            	    	|   \ | x               y| /   |
      	     	     	|    \|------------------|/    |
           			|     |                  |     |
           			|     |                  |     |
           			|     |                  |     |
           			|  A  |                  |  B  |
           			+-----+	             +-----+
                         Figure 3: Label Illustration 

   -- Channel/Sub-channel ID is technology dependent. Generally, there 
      are different multiplexing structures for different technologies. 
      Channel/Sub-channel ID indicates the channel/sub-channel within a 
      port. For example, within an OC-48 port with STS-1 granularity, 
      each STS-1 may have a channel/sub-channel ID. For two physical 
      ports connected by a physical link, these two ports should share 
      common physical attributes including multiplexing structure, which 
      are technology specific and clearly defined by standards.

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   So the label format can be defined as follows:

    0                   1                   2                   3  
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
    |                  G-label    (port ID section)                 | 
    |                      ...                                      |
    |                  G-label    (channel/sub-channel ID section)  | 
    |                      ...                                      |
                          Figure 4: G-Label Format 

   For G-label channel/sub-channel section, technology specific content 
   can be either encoded in certain fixed format as suggested by [GMPLS-
   SIG] for each technology or mapped through a separate label 
   association procedure at resource discovery time with technology 
   specific rules.
   Furthermore, for complex labels, such as different concatenations in 
   SONET/SDH, extra rules and information may need to be provided. Again 
   these rules are technology specific. They can be either hard-coded as 
   suggested by [GMPLS-SIG] or negotiated between neighbors at resource 
   discovery time. Label Space

   For out-of-band control signaling, only "per-platform label space" is 
   applicable to circuit switches. In this case, both port ID and 
   channel/sub-channel ID sections are required. In case of in-band 
   control signaling, the port section is not necessary, labels may have 
   "per-interface label space". Label Binding

   Label binding include both FEC to label binding and label to next hop 
   binding. Label to next hop binding in ASTN is hard-wired. FEC to 
   label binding in ASTN is selecting a specific physical resource 
   according to path selection result and local rules. It is a service-
   driven and happens at the same time when label distribution happens.

   The procedure for choosing a specific physical resource requires local 
   intelligence. A path create request message only contains logical link 
   level information. When a node receives the request, it typically has 
   many physical resources to choose. In order to smooth path creation

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   operation, there could be local rules for choosing a specific resource. 
   For example, in bi-directional path create request, in order to minimize 
   the chance of neighbors choosing the same interface, adjacent nodes may 
   form a master-slave relationship and decide different orders of selecting 
   interfaces. Label Distribution 

   Label distribution in circuit switched network is equivalent to 
   signaling mechanism for circuit path operations. It's always ordered 
   and on-demand. MPLS signaling mechanisms can be extended as signaling 
   protocol in circuit switched networks.

   Path operations consist of a set of basic transactions between 
   adjacent NEs. Each transaction may involve multiple signaling 
   messages.  The basic transactions are:

   -- Create:  This transaction creates a link of an end-to-end path.
      Although there is only one create transaction for the end-to-end 
      path, the resource allocation is made on a link-by-link basis. 

      MPLS signaling protocols always involve label request and 
      response. They can be mapped into path create request and response 
      directly. In general, most of circuit path constraints should be 
      considered at path selection time rather than negotiated at path 
      creation time. However, some constrains, for example, the label 
      set [GMPLS-SIG], could be considered at the path creation time by 
      using label request and response messages. These constrains are 
      normally expensive to be considered at path selection time because 
      that either control plane scalability may be hurt or path 
      selection algorithms could become too complicated. 

      In the explicit routing case, a create request should contain 
      information that is essential for path creation, e.g., ER hop 
      list, current ER hop pointer. If the incoming label can be 
      decided, it should be given at the request time so that NEs can 
      minimize operation delay by allowing circuit switches to perform 
      cross connect and wait for response from other NEs in parallel. In 
      the example of figure 3, NE A could issue the cross connect 
      request to the local system controller and send label the request 
      downstream to B simultaneously. 

      Create response return LSP ID.

   -- Delete:  This transaction de-allocates a specific link of a LSP 
      identified by its LSP ID. 


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      This request contains the LSP ID to be torn down. After a NE 
      receives this request, it releases local cross connect according 
      to the LSP ID and meanwhile forwards the request downstream. Release 
      and withdraw messages in MPLS signaling can be mapped to delete 
      request and delete response.

   -- Query:  This transaction queries the status of a specific link of 
      an existing LSP identified by a LSP ID. Current MPLS signaling 
      needs to be extended to support this function. Circuit LSP Tunnel

   New LSPs can tunnel through existing LSPs of higher granularity. 
   After a circuit LSP is created, it can be treated as a FA in the topology 
   database. A new circuit LSP can tunnel through it by indicating the 
   LSP ID in its ER hop list. For example, if a low order (LO) SONET LSP 
   needs to tunnel through an existing high order (HO) SONET LSP, the LO 
   LSP create request includes the HO SONET LSP ID in its ER hop list. 
   Its label only needs to indicate LO resource within the existing HO 
   LSP. Circuit LSP Concatenation

   A new LSP can also be concatenated through several existing LSPs of 
   the same type. These LSPs must share the same characteristics. Their 
   LSP IDs are also included in path create request ER hop list.

   5.4.2 LSP Information Maintenance

   LSPs can be uniquely identified by LSP IDs. In circuit switched 
   networks, A LSP ID can be defined as a source NE address + a source 
   NE ingress label. 

   An LSP has two sets of information: ER hop list and local resource 
   usage information. ER hop list should be maintained at the ENE. Both ENEs 
   and INEs should maintain the LSP ID and associated local physical 
   resource usage information.

   5.4.3 Circuit LSP Restoration Restoration Stages

   Circuit LSP restoration is very important for transport network and 
   quite different from packet LSP in terms of both requirements and 

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   mechanisms. Circuit LSP restoration typically includes following four 

   -- Failure Detection
   -- Failure Notification
   -- Traffic Restoration 
   -- Post-Restoration Cleanup 

   Mechanisms of these four stages are technology dependent. Some 
   technologies provide special infrastructure to speed up path 
   restoration. For example, SONET/SDH detects LOS and LOF very quickly 
   and uses in-band overhead bytes for failure notification and co-
   ordinate actions. They also adopt special topology (ring) and 
   algorithms (BLSR, UPSR and etc.) for fast restoration. 

   In order to speed up overall restoration time, each stage has to be 
   designed carefully and utilize existing infrastructure as effectively 
   as possible. For example, failure notification should avoid message-
   based indication if byte/bit stream based indication is available. In 
   case of message based notification, direct notification to end points 
   is more desirable than hop-by-hop processing. For traffic restoration, 
   handshaking and negotiation should be avoided as much as possible. 

   Circuit LSPs can be either link protected or path protected. Link level protection.
   In link level protection, restorations happen around the point of 
   failure, e.g., either the failed NE or link. In SONET/SDH, BLSR/SPRing is 
   a typical link level protection. This type of restoration requires 
   accurate fault localization, i.e., pinpointing the precise location of 
   the fault along the LSP so that rerouting can be performed around it. In 
   optical networks, fault localization typically involves OEO conversion at 
   every network element so that digital monitoring of the optical channel 
   (OCh) overhead can be carried out.  However, installing such digital 
   monitoring equipment to determine performance degradation might be very 
   expensive. Alternatively, controlling the expense by sharing the 
   monitoring equipment over many optical channels leads to an unacceptably 
   long detection time for degraded signal condition (fault) owing to 
   correlation that have to be performed across multiple monitoring nodes. Path level protection 
   Path level protection is a more popular choice for mesh network. In 
   this case path restoration is done from the endpoints. Failure 
   notification need to be propagated to end points where the restoration is 


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  There are several types of path protections.

   -- Dedicated protection
      This type of protection is the fastest and also the least cost 
      effective. It requires a dedicated physically diverse protection 
      path for each working path. Physically diversity could have 
      different meanings depending on the degree of threat to be 
      protected against. It can be node disjoint, link disjoint and span 
      disjoint. SRLG could be assigned to physical link by operators to 
      meet different requirements. For example, an operator can assign 
      physical links passing same disaster zone the same SRLG ID to 
      guarantee services even in case of catastrophic disasters such as 
      earthquakes, and floods.

   -- Shared protection
      Shared protection is fast and more cost effective than dedicated 
      protection. In this case, multiple paths could share the same 
      reserved protection sections (not necessarily paths). However, this 
      requires more information to be distributed in the network and 
      complicated algorithms. A scalable, effective and efficient 
      solution needs to be designed. 

   -- Auto-rerouting
      Auto-rerouting is a non-preplanned protection. No dedicated resource     
      is reserved for working paths. A protection path is created after 
      detecting the failure of a working LSP. The auto-rerouting doesn't 
      need to wait for network topology convergence. A new path can be 
      created by excluding failed nodes in case that they can be 
      identified on-the-fly.  Alternatively, when failures cannot be 
      precisely located on-the-fly, a new path can be created  simply by 
      excluding all nodes along the failed path.

   5.4.4 Circuit LSP Verification
   Serving as transport backbone, circuit LSPs are normally required to 
   be strictly verified before delivering to customers. 

   For ASTN, some verification procedures can be automated. After a 
   circuit LSP is created, OAM&P signals or simulated user traffic can 
   be inserted and tested to verify against path connectivity, constraints 
   and some aspects of SLA.

   5.4.5 Control Plane Failure Handling

   Failures may happen at both the control plane and the transport 
   plane. Many current MPLS mechanisms have the assumption that control 
   plane failure indicates transport plane failure because control 

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   traffic and data traffic are mixed together in the same physical path 
   in IP network. So MPLS signaling mechanisms require existing LSPs to 
   be torn down in case of control plane failure. This is undesirable 
   for circuit switched network, where control plane and transport plane 
   may be separated and circuit LSPs are usually backbones for user 
   traffic. One way to handle control plane failures is to keep current 
   circuit LSPs in operation but abort requests that are being 

   Details for performing graceful control plane failure handling and 
   recovery need to be furthered studied. For example, notification 
   messages should be extended for both types of failures. Also control 
   plane recovery mechanism should be introduced. In case that a NE 
   controller recovered, it may need to consult with its neighbors to 
   synchronize link state information and current LSPs status.

   With control plane taking over many responsibilities from the 
   management plane, control plane failure also has significant impact 
   on management plane. For example, the billing system may rely on NE 
   control entity for resource usage information. Details of these 
   implications need to be further studied.

6. Inter-domain Control Plane Integration
   One of major purposes of a generalized MPLS control plane is to 
   facilitate network inter-operation. 

   Domains are defined to facilitate network partitioning for connection 
   management purposes. Typically, networks can be divided into technology 
   and business domains. Consequently, there are two types of interfaces 
   between domains:

   -- Business related interfaces are between administrative domains. 

   -- Technology related interfaces are between different technology
      domains. The following sections focuses on this topic.

   6.1 Technology Domains and Hierarchy

   Circuit networks can be divided into logical network layers according 
   to networks' switching granularity even though they may be within a 
   single administrative domain. These layers form typical client/server 
   relations, with lower layer network provides transport service 
   to upper layer networks. Figure 6 shows examples of this hierarchy 
   (not exact signal mapping and encapsulation). For example, Optical 
   networks (OCh granularity) provide transport service to SONET/SDH HO 
   networks, which in turn provide transport service to SONET/SDH LO 

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   An IP network can use different circuit switched transport networks as 
   its direct server network. Edge/Access routers use DS-1 or DS-3 
   networks as their server network, while core routers may request STS 
   or optical channel services directly.

          PDM (packet/cell)    
                                        |          |          |                   
          TDM LO (DS-1, VT1.5 etc.)     |          |          |          
          ______________________________V_______   |          |          
                                            |      |          |          
          TDM HO (DS-3, STS-1 etc.)         |      |          |          
          __________________________________V______V____      |          
                                                |             |          
          WDM (OCh/OMS)                         |             |          
          SDM (OTS)                                 |                    
                      Figure 6: Network Hierarchy

   6.2 Service Interfaces Between Network Domains

   There is a clear business related service interface between networks 
   owned by different service providers. This interface is identified as 
   public service interface [CARR-REQ]. This type of interface includes 
   service requests from both the same type and different types of 
   networks. A carrier will not relinquish control of its resources 
   outside of its administrative boundaries.  No outside entity will be 
   allowed to reconfigure the network directly. They can only take 
   requests through the service interface. 

   For different types of networks owned by the same service provider 
   and provisioned within a single AS, there are still logical service 
   interfaces between any two of them. These interfaces are identified 
   as private service interfaces [CARR-REQ]. For example, if a service 
   provider owns both an IP network and a switched optical core transport 
   network, traffic that flows in the IP network shouldn't affect the 
   optical network directly. Optical LSPs are created by different 
   reasons and set up at different time from packet LSPs. For example, 
   it's not practical to set up an OC48 pipe for a 64Kps IP stream. LSPs 

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   in transport network are normally created according to the traffic 
   pattern and aggregated traffic requirements of the IP network. Then 
   they serve as tunnels for IP traffic.

   Functions at the private service interface forms a base set for 
   network interaction. This base set includes four functional 
   components covered in section 5 and additional service triggering 
   function, which is normally the traffic engineering function in the 
   client network that analyzes client network traffic and request 
   service from server ASTN accordingly. At the public service interface, 
   functions include the base set and stricter security and policy 

   6.3 Control Plane Design for Integration  

   Each technology has its own unique features, requirements, and evolution 
   path. It is preferable for networks of different technologies to have 
   independent control entities. While it is, in principle, feasible to 
   integrate them, it is impractical and unnecessary for efficient network 

   However, different control entities don't require different control 
   plane protocols. On the contrary, unified control plane protocols 
   with technology specific extensions could facilitate network inter-
   operation and network application migration.

   It should be noticed that different control entities don't need to be 
   physically separated. In reality, there may be no clear physical 
   boundary between networks. For example, a network can be made of 
   hybrid IP/SONET/DWDM NEs. In this case, service interfaces are within 
   the NEs. 

   The four functional modules discussed in section 5 for a single 
   domain are revisited below in a multiple technology domain environment to  
   examine how they can support interactions of network from different 
   technology domains, what are the extensions, and how security and policy 
   control should be apply. 

   6.3.1 Resource Discovery

   Resource discovery happens between neighbors. A mechanism designed 
   for a technology domain can be applied to any pair of NEs interconnected 
   through interfaces of the same technology. 

   However, because resource discovery means certain information  
   disclosure between two business domains, it's under service providers' 
   security and policy control. In certain network scenario, a service 

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   provider who owns the transport network may not be willing to disclose 
   any internal addressing scheme to its client. So a client NE may not 
   have the neighbor NE address and port ID in its NE level resource 

   6.3.2 State Information Dissemination

   Topology information may be proprietary to service providers. So state 
   information dissemination should be under careful control. 

   There are different levels of information dissemination controls 
   between networks of different technology domains. Routing mechanisms 
   introduced in [IPO-FRAM] could be extended to networks of other 
   technologies. Normally, a single type of information dissemination 
   control can not meet all needs of a service provider. ASTN is 
   configurable to provide service provider controls for different 

   -- Total Dissemination without Control
      Topology dissemination is done through control plane network IGPs 
      or high layer applications. 
   -- Controlled Dissemination
      In this case, partial ASTN topology information is disseminated to 
      designated targets. Policy based EGPs can be extended to apply here. 
      For one type of circuit VPN services, a customer may lease 
      specific resources (internal bandwidth, not only access points) 
      from a service provider and want to manage these resource by 
      itself. The customer may require the service provider to only leak its 
      VPN topology to the customer. 
   -- No Information Dissemination
      This is the most widely used practice. For different types of networks 
      that belonging to different service providers, there is normally no 
      link state information dissemination between them at all. 
      For another type of circuit VPN services, customers lease access 
      points instead of specific internal resource of a backbone ASTN. 
      Then, from the customer's view, the network appears as a non-blocking 
      backplane, independent of any internal network structure. Potential Forwarding Adjacency

   NEs of different technology domains don't need to know each other's 
   network topology. They should be transparent to each other. What a NE 
   should care is the topology of its own technology domain. 

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   Enabled by ASTN, there are three types of routes in a client topology 
   database: (1) fixed routes, (2) Forwarding Adjacencies (FA) and (3) 
   Potential Forwarding Adjacencies (PFA). 
   -- Fixed routes are conventional in current IGP and EGP. 
   -- FA is introduced by [LSP-HIER]. It's the route that were 
      dynamically created and can be torn down if necessary. 
   -- PFAs are routes that can be connected through a low layer network, 
      but are not connected yet. Enabled by ASTN, NEs that directly 
      connected to a ASTN become FFAs.

   Link state dissemination (for PFAs and potential routes that are 
   reachable through PFAs) can be done either through existing network 
   connections or the control plane network of the ASTN that provides 
   transport services. In the later case, link state dissemination is 
   under the control of server ASTN client management. The server ASTN 
   should maintain client information such as where a client is attached, 
   which user group a client belongs to, and what's a client's service 
   contract/policies. Serving as a EGP proxy with configurable filters, 
   the server ASTN determines whose information to distribute to which 

   With these three types of route information in the topology database, 
   the service agent (section 4.2) at the client network could decide when 
   and where to set up or tear down FAs for more efficient network   
   resource utilization according to client network data traffic pattern 
   and requirements. It then requests the service from ASTN that 
   provide the transport service accordingly. One of PFA's applications is 
   enabling the virtual terabit router. It's especially useful for hybrid 
   NEs with both IP switch fabric and circuit switch fabric. When 
   interconnected together, they can aggregate traffic and choose 
   appropriate circuit shortcuts by dynamically adjusting their circuit 
   connections through circuit switch fabric in order to avoid unnecessary 
   packet switching and minimize user traffic port consumption for NE 
   6.3.3 Path Selection

   Path selection can be done by the party having the access to network 
   level resource information. As path selection is very technology 
   dependent, it's suggested that it be done within domains (both business 
   domain and technical domain). However, service providers have the 
   flexibility to do whatever they want.


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   6.3.4 Path Control Service Invocation

   In packet networks, LSPs are created either because of topology 
   changes  (control-driven) or data traffic flow (data-driven). In 
   ASTN, circuit LSPs are service-driven (could be view as one type of 
   control-driven). They are created either because of customer requests 
   or client network traffic engineering decisions. 

   In the signaling perspective, even though intra-domain and inter-
   domain signaling may use the same protocol, their message content are 
   quite different.

   In the network view, service invocation happens between a service agent 
   and a SNE. Service requests include create, delete, modify and query. 

   Details are being studied by OIF and ODSI. LSPs of Different Technology Domains

   LSPs in different technology domains are treated as different 
   entities. They are set up at different time and triggered by 
   different reasons. Coarse granularity LSPs can serve as tunnels for 
   fine granularity LSPs. LSPs over Multiple ASes

   In data networks, label stacking is only used for packet forwarding 
   over multiple sub-networks. There is no such equivalence in circuit 
   switched network. Nevertheless, ER hops can be stacked for LSP 
   creation through multiple ASes.

   In case of explicit routing, an SNE may only include AS IDs in its ER 
   hop list. When a path create request enters a transmit AS, its AS SNE 
   needs to compute a ER hop list for the AS and push it up to the 
   original ER hop stack. Before the request leaving the AS, its AS DNE 
   needs to pop up the domain ER hop list. 

   When path operation messages passing across AS boundaries, they 
   usually should include some authentication information. Each AS needs 
   to validate these information in order to guarantee network operation 


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   -- A single network failure may affect LSPs of different technical 
      domains. This topic involves multi-layer restoration. For details, 
      please refer to [MUL-SURV].

   -- For a LSP over multiple ASes (business domains), restoration could 
      be done either end-to-end or within each AS depending on SLAs of the 
      user to service provider and service provider to service provider. 
      Details and implications to signaling protocols need further 

7. Security Considerations

   Security is critical where different business domains interact. When 
   service invocations happen between business domains, encryption and 
   authentication mechanisms are required at the service interfaces. 
   Within a single business domain, strict security needs further study. 
   However, the control plane network should be isolated from user traffic 
   to avoid sensitive information being leaked out. Many details need 
   further study. 

8. Intellectual Property

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed

9. Acknowledgment

   The authors would like to thank Patrice Lamy, Wyley Robinson, George 
   Newsome, John Ellson, Harvey Epstein, Zhi-wei Lin, Siva Sankaranarayanan, 
   Dave Chen, and Girish Pahlya for their helpful suggestions and comments 
   on this work.

10. Authors' Addresses

    Yangguang Xu
    21-2A41, 1600 Osgood Street
    Lucent Technologies, Inc.
    North Andover, MA 01845

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    Eve Varma
    3C 509, 101 Crawfords Corner Rd
    Lucent Technologies, Inc.
    Holmdel, NJ  07733-3030 

    Ramesh Nagarajan
    3M-335, 101 Crawfords Corner Rd.
    Lucent Technologies, Inc.
    Holmdel, NJ 007733

    Rudy Hoebeke 				
    Francis Wellesplein 1, 		
    2018 Antwerp, Belgium 		

    Dimitri Papadimitriou
    F. Wellesplein 3,
    B-2018 Antwerpen, Belgium

    Hirokazu Ishimatsu
    Japan Telecom Co. Ltd.
    2-9-1 Hatchobori, 
    Chuo-ku, Tokyo, 104-0032 Japan

    Osama S. Aboul-Magd 			
    Nortel Networks 				
    P.O. Box 3511, Station "C" 		
    Ottawa, Ontario, Canada 			
    K1Y - 4H7  

    Michael Mayer
    Nortel Networks
    P.O. Box 402    
    Ogdensburg NY 13669    

    Daniel O.
    22001 Loudoun County Parkway
    Ashburn, VA 20147

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    Yong Xue
    Global Network Architecture
    Ashburn, Virginia

    Curtis Brownmiller
    Richardson, TX 75082

    John Eaves
    1C-240, 250 Industrial Way West
    Eatontown, NJ 07724

    Raj Jain
    Nayna Networks, Inc.
    157 Topaz St.
    Milpitas, CA 95035

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11. References

[MPLambdaS] D. Awduche, etc. al., "Multi-Protocol Lambda Switching: Combining 
            MPLS Traffic Engineering Control with Optical Crossconnects", draft-
            awduche-mpls-te-optical-02.txt, Work in Progress, July 2000.

[MPLS-ARCH]	E. Rosen, et. al. "Multiprotocol Label Switching Architecture", 
            draft-ietf-mpls-arch-07.txt, Work in Progress, July 2000.

[MPLS-TE]	D. O. Awduche et al., "Requirements for Traffic Engineering over 
            MPLS", RFC 2702, September 1999.

[GMPLS-SIG]	P. Ashwood-Smith, et al., "Generalized MPLS - Signaling Functional 
            Description", draft-ietf-MPLS-generalized-mpls-signaling-00.txt, 
            Work in Progress, Oct. 2000.

[OPT-BUND]	B. Rajagopalan et al., "Link Bundling in Optical Networks", draft-
            rs-optical-bundling-01.txt, Work in Progress, September, 2000.

[AND]		H. Koay, Y. Xu, "Automatic Neighbor Discovery for Optical Networks", 
            draft-koay-mpls-and-optical-00.txt, Work in Progress, Sept,2000

[LMP]		J. Lang et al., " Link Management Protocol", draft-ietf-mpls-lmp-
            00.txt, Work in Progress, September, 2000.

[ND-OIF]	G. Bernstein et al., "Methods for Neighbor Discovery in SONET & 
            SDH", OIF2000-159, August, 2000.

[OLCP]	A. Chiu et al., "Unique Features and Requirements for The Optical 
            Layer Control Plane", draft-chiu-strand-unique-OLCP-00.txt, Work in          
            Progress, July 2000.

[LSP-HIER]	K. Kompella et al., "LSP Hierarchy with MPLS TE", draft-kompella-
            lsp-hierarchy-00.txt, Work in Progress, June 2000.

[IPO-FRAM]  B. Rajagopalan, et al., "IP over Optical Networks: A Framework", 
            work in progress, July, 2000

[DIST-AREA]	Y. Hayata, et al., "Carrier needs regarding survivability 
            and maintenance of switched optical networks", draft-hayata-ipo-
            carrier-needs-00.txt, Work in Progress, June, 2000.

[OSPF-OMPLS]K. Kompella, et al., "OSPF Extensions in Support of MPL(ambda)S", 
            draft-kompella-ospf-ompls-extensions-00.txt, Work in Progress, June, 

[ISIS-OMPLS]K. Kompella, et al., "IS-IS Extensions in Support of MPL(ambda)S", 
            draft-kompella-isis-ompls-extensions-00.txt, Work in Progress, June, 

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[MUL-SRUV]	J. Meijen, et al., "Multi-layer Survivability", White Paper, 
  , Oct, 

[CARR-REQ]	Y. Xue, M. Lazar, "Carrier Optical Services Framework and 
            Associated Requirement for UNI", OIF2000-155, Work in Progress, Oct, 

[ON-REST] 	B. Doshi et al., "Optical Network Design and Restoration," Bell Labs 
            Technical Journal, Vol. 4, No. 1, Jan-Mar 1999.

[DISJ-PATH] J. W. Suurballe, "Disjoint Paths in a Network," Networks, Vol. 4, 
            1974, pp. 125145.

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