Network Hierarchy and Multilayer Survivability
RFC 3386

Network Working Group                                        W. Lai, Ed.
Request for Comments: 3386                                          AT&T
Category: Informational                                  D. McDysan, Ed.
                                                                WorldCom
                                                           November 2002

            Network Hierarchy and Multilayer Survivability

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document presents a proposal of the near-term and practical
   requirements for network survivability and hierarchy in current
   service provider environments.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

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Table of Contents

   1. Introduction..............................................2
   2. Terminology and Concepts..................................5
   2.1 Hierarchy................................................6
   2.1.1 Vertical Hierarchy.....................................5
   2.1.2 Horizontal Hierarchy...................................6
   2.2 Survivability Terminology................................6
   2.2.1 Survivability..........................................7
   2.2.2 Generic Operations.....................................7
   2.2.3 Survivability Techniques...............................8
   2.2.4 Survivability Performance..............................9
   2.3 Survivability Mechanisms: Comparison....................10
   3. Survivability............................................11
   3.1 Scope...................................................11
   3.2 Required initial set of survivability mechanisms........12
   3.2.1 1:1 Path Protection with Pre-Established Capacity.....12
   3.2.2 1:1 Path Protection with Pre-Planned Capacity.........13
   3.2.3 Local Restoration.....................................13
   3.2.4 Path Restoration......................................14
   3.3 Applications Supported..................................14
   3.4 Timing Bounds for Survivability Mechanisms..............15
   3.5 Coordination Among Layers...............................16
   3.6 Evolution Toward IP Over Optical........................17
   4. Hierarchy Requirements...................................17
   4.1 Historical Context......................................17
   4.2 Applications for Horizontal Hierarchy...................18
   4.3 Horizontal Hierarchy Requirements.......................19
   5. Survivability and Hierarchy..............................19
   6. Security Considerations..................................20
   7. References...............................................21
   8. Acknowledgments..........................................22
   9. Contributing Authors.....................................22
   Appendix A: Questions used to help develop requirements.....23
   Editors' Addresses..........................................26
   Full Copyright Statement....................................27

1. Introduction

   This document is the result of the Network Hierarchy and
   Survivability Techniques Design Team established within the Traffic
   Engineering Working Group.  This team collected and documented
   current and near term requirements for survivability and hierarchy in
   service provider environments.  For clarity, an expanded set of
   definitions is included.  The team determined that there appears to
   be a need to define a small set of interoperable survivability
   approaches in packet and non-packet networks.  Suggested approaches
   include path-based as well as one that repairs connections in

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   proximity to the network fault.  They operate primarily at a single
   network layer.  For hierarchy, there did not appear to be a driving
   near-term need for work on "vertical hierarchy," defined as
   communication between network layers such as Time Division
   Multiplexed (TDM)/optical and Multi-Protocol Label Switching (MPLS).
   In particular, instead of direct exchange of signaling and routing
   between vertical layers, some looser form of coordination and
   communication, such as the specification of hold-off timers, is a
   nearer term need.  For "horizontal hierarchy" in data networks, there
   are several pressing needs.  The requirement is to be able to set up
   many Label Switched Paths (LSPs) in a service provider network with
   hierarchical Interior Gateway Protocol (IGP).  This is necessary to
   support layer 2 and layer 3 Virtual Private Network (VPN) services
   that require edge-to-edge signaling across a core network.

   This document presents a proposal of the near-term and practical
   requirements for network survivability and hierarchy in current
   service provider environments.  With feedback from the working group
   solicited, the objective is to help focus the work that is being
   addressed in the TEWG (Traffic Engineering Working Group), CCAMP
   (Common Control and Measurement Plane Working Group), and other
   working groups.  A main goal of this work is to provide some
   expedience for required functionality in multi-vendor service
   provider networks.  The initial focus is primarily on intra-domain
   operations.  However, to maintain consistency in the provision of
   end-to-end service in a multi-provider environment, rules governing
   the operations of survivability mechanisms at domain boundaries must
   also be specified.  While such issues are raised and discussed, where
   appropriate, they will not be treated in depth in the initial release
   of this document.

   The document first develops a set of definitions to be used later in
   this document and potentially in other documents as well.  It then
   addresses the requirements and issues associated with service
   restoration, hierarchy, and finally a short discussion of
   survivability in hierarchical context.

   Here is a summary of the findings:

   A. Survivability Requirements

   o  need to define a small set of interoperable survivability
      approaches in packet and non-packet networks
   o  suggested survivability mechanisms include
      -  1:1 path protection with pre-established backup capacity (non-
         shared)
      -  1:1 path protection with pre-planned backup capacity (shared)

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      -  local restoration with repairs in proximity to the network
         fault
      -  path restoration through source-based rerouting
   o  timing bounds for service restoration to support voice call cutoff
      (140 msec to 2 sec), protocol timer requirements in premium data
      services, and mission critical applications
   o  use of restoration priority for service differentiation

   B. Hierarchy Requirements

   B.1. Horizontally Oriented Hierarchy (Intra-Domain)

   o  ability to set up many LSPs in a service provider network with
      hierarchical IGP, for the support of layer 2 and layer 3 VPN
      services
   o  requirements for multi-area traffic engineering need to be
      developed to provide guidance for any necessary protocol
      extensions

   B.2. Vertically Oriented Hierarchy

   The following functionality for survivability is common on most
   routing equipment today.

   o  near-term need is some loose form of coordination and
      communication based on the use of nested hold-off timers, instead
      of direct exchange of signaling and routing between vertical
      layers
   o  means for an upper layer to immediately begin recovery actions in
      the event that a lower layer is not configured to perform recovery

   C. Survivability Requirements in Horizontal Hierarchy

   o  protection of end-to-end connection is based on a concatenated set
      of connections, each protected within their area
   o  mechanisms for connection routing may include (1) a network
      element that participates on both sides of a boundary (e.g., OSPF
      ABR) - note that this is a common point of failure; (2) a route
      server
   o  need for inter-area signaling of survivability information (1) to
      enable a "least common denominator" survivability mechanism at the
      boundary; (2) to convey the success or failure of the service
      restoration action; e.g., if a part of a "connection" is down on
      one side of a boundary, there is no need for the other side to
      recover from failures

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2. Terminology and Concepts

2.1 Hierarchy

   Hierarchy is a technique used to build scalable complex systems.  It
   is based on an abstraction, at each level, of what is most
   significant from the details and internal structures of the levels
   further away. This approach makes use of a general property of all
   hierarchical systems composed of related subsystems that interactions
   between subsystems decrease as the level of communication between
   subsystems decreases.

   Network hierarchy is an abstraction of part of a network's topology,
   routing and signaling mechanisms.  Abstraction may be used as a
   mechanism to build large networks or as a technique for enforcing
   administrative, topological, or geographic boundaries.  For example,
   network hierarchy might be used to separate the metropolitan and
   long-haul regions of a network, or to separate the regional and
   backbone sections of a network, or to interconnect service provider
   networks (with BGP which reduces a network to an Autonomous System).

   In this document, network hierarchy is considered from two
   perspectives:

   (1) Vertically oriented: between two network technology layers.
   (2) Horizontally oriented: between two areas or administrative
       subdivisions within the same network technology layer.

2.1.1 Vertical Hierarchy

   Vertical hierarchy is the abstraction, or reduction in information,
   which would be of benefit when communicating information across
   network technology layers, as in propagating information between
   optical and router networks.

   In the vertical hierarchy, the total network functions are
   partitioned into a series of functional or technological layers with
   clear logical, and maybe even physical, separation between adjacent
   layers. Survivability mechanisms either currently exist or are being
   developed at multiple layers in networks [3].  The optical layer is
   now becoming capable of providing dynamic ring and mesh restoration
   functionality, in addition to traditional 1+1 or 1:1 protection.  The
   Synchronous Digital Hierarchy (SDH)/Synchronous Optical NETwork
   (SONET) layer provides survivability capability with automatic
   protection switching (APS), as well as self-healing ring and mesh
   restoration architectures.  Similar functionality has been defined in
   the Asynchronous Transfer Mode (ATM) Layer, with work ongoing to also
   provide such functionality using MPLS [4].  At the IP layer,

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   rerouting is used to restore service continuity following link and
   node outages.  Rerouting at the IP layer, however, occurs after a
   period of routing convergence, which may require a few seconds to
   several minutes to complete [5].

2.1.2 Horizontal Hierarchy

   Horizontal hierarchy is the abstraction that allows a network at one
   technology layer, for instance a packet network, to scale.  Examples
   of horizontal hierarchy include BGP confederations, separate
   Autonomous Systems, and multi-area OSPF.

   In the horizontal hierarchy, a large network is partitioned into
   multiple smaller, non-overlapping sub-networks.  The partitioning
   criteria can be based on topology, network function, administrative
   policy, or service domain demarcation.  Two networks at the *same*
   hierarchical level, e.g., two Autonomous Systems in BGP, may share a
   peer relation with each other through some loose form of coupling.
   On the other hand, for routing in large networks using multi-area
   OSPF, abstraction through the aggregation of routing information is
   achieved through a hierarchical partitioning of the network.

2.2 Survivability Terminology

   In alphabetical order, the following terms are defined in this
   section:

   backup entity, same as protection entity (section 2.2.2)
   extra traffic (section 2.2.2)
   non-revertive mode (section 2.2.2)
   normalization (section 2.2.2)
   preemptable traffic, same as extra traffic (section 2.2.2)
   preemption priority (section 2.2.4)
   protection (section 2.2.3)
   protection entity (section 2.2.2)
   protection switching (section 2.2.3)
   protection switch time (section 2.2.4)
   recovery (section 2.2.2)
   recovery by rerouting, same as restoration (section 2.2.3)
   recovery entity, same as protection entity (section 2.2.2)
   restoration (section 2.2.3)
   restoration priority (section 2.2.4)
   restoration time (section 2.2.4)
   revertive mode (section 2.2.2)
   shared risk group (SRG) (section 2.2.2)
   survivability (section 2.2.1)
   working entity (section 2.2.2)

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2.2.1 Survivability

   Survivability is the capability of a network to maintain service
   continuity in the presence of faults within the network [6].
   Survivability mechanisms such as protection and restoration are
   implemented either on a per-link basis, on a per-path basis, or
   throughout an entire network to alleviate service disruption at
   affordable costs.  The degree of survivability is determined by the
   network's capability to survive single failures, multiple failures,
   and equipment failures.

2.2.2 Generic Operations

   This document does not discuss the sequence of events of how network
   failures are monitored, detected, and mitigated.  For more detail of
   this aspect, see [4].  Also, the repair process following a failure
   is out of the scope here.

   A working entity is the entity that is used to carry traffic in
   normal operation mode.  Depending upon the context, an entity can be
   a channel or a transmission link in the physical layer, an Label
   Switched Path (LSP) in MPLS, or a logical bundle of one or more LSPs.

   A protection entity, also called backup entity or recovery entity, is
   the entity that is used to carry protected traffic in recovery
   operation mode, i.e., when the working entity is in error or has
   failed.

   Extra traffic, also referred to as preemptable traffic, is the
   traffic carried over the protection entity while the working entity
   is active.  Extra traffic is not protected, i.e., when the protection
   entity is required to protect the traffic that is being carried over
   the working entity, the extra traffic is preempted.

   A shared risk group (SRG) is a set of network elements that are
   collectively impacted by a specific fault or fault type.  For
   example, a shared risk link group (SRLG) is the union of all the
   links on those fibers that are routed in the same physical conduit in
   a fiber-span network.  This concept includes, besides shared conduit,
   other types of compromise such as shared fiber cable, shared right of
   way, shared optical ring, shared office without power sharing, etc.
   The span of an SRG, such as the length of the sharing for compromised
   outside plant, needs to be considered on a per fault basis.  The
   concept of SRG can be extended to represent a "risk domain" and its
   associated capabilities and summarization for traffic engineering
   purposes.  See [7] for further discussion.

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   Normalization is the sequence of events and actions taken by a
   network that returns the network to the preferred state upon
   completing repair of a failure.  This could include the switching or
   rerouting of affected traffic to the original repaired working
   entities or new routes.  Revertive mode refers to the case where
   traffic is automatically returned to a repaired working entity (also
   called switch back).

   Recovery is the sequence of events and actions taken by a network
   after the detection of a failure to maintain the required performance
   level for existing services (e.g., according to service level
   agreements) and to allow normalization of the network.  The actions
   include notification of the failure followed by two parallel
   processes: (1) a repair process with fault isolation and repair of
   the failed components, and (2) a reconfiguration process using
   survivability mechanisms to maintain service continuity.  In
   protection, reconfiguration involves switching the affected traffic
   from a working entity to a protection entity.  In restoration,
   reconfiguration involves path selection and rerouting for the
   affected traffic.

   Revertive mode is a procedure in which revertive action, i.e., switch
   back from the protection entity to the working entity, is taken once
   the failed working entity has been repaired.  In non-revertive mode,
   such action is not taken.  To minimize service interruption, switch-
   back in revertive mode should be performed at a time when there is
   the least impact on the traffic concerned, or by using the make-
   before-break concept.

   Non-revertive mode is the case where there is no preferred path or it
   may be desirable to minimize further disruption of the service
   brought on by a revertive switching operation.  A switch-back to the
   original working path is not desired or not possible since the
   original path may no longer exist after the occurrence of a fault on
   that path.

2.2.3 Survivability Techniques

   Protection, also called protection switching, is a survivability
   technique based on predetermined failure recovery: as the working
   entity is established, a protection entity is also established.
   Protection techniques can be implemented by several architectures:
   1+1, 1:1, 1:n, and m:n. In the context of SDH/SONET, they are
   referred to as Automatic Protection Switching (APS).

   In the 1+1 protection architecture, a protection entity is dedicated
   to each working entity.  The dual-feed mechanism is used whereby the
   working entity is permanently bridged onto the protection entity at

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   the source of the protected domain.  In normal operation mode,
   identical traffic is transmitted simultaneously on both the working
   and protection entities.  At the other end (sink) of the protected
   domain, both feeds are monitored for alarms and maintenance signals.
   A selection between the working and protection entity is made based
   on some predetermined criteria, such as the transmission performance
   requirements or defect indication.

   In the 1:1 protection architecture, a protection entity is also
   dedicated to each working entity.  The protected traffic is normally
   transmitted by the working entity.  When the working entity fails,
   the protected traffic is switched to the protection entity.  The two
   ends of the protected domain must signal detection of the fault and
   initiate the switchover.

   In the 1:n protection architecture, a dedicated protection entity is
   shared by n working entities.  In this case, not all of the affected
   traffic may be protected.

   The m:n architecture is a generalization of the 1:n architecture.
   Typically m <= n, where m dedicated protection entities are shared by
   n working entities.

   Restoration, also referred to as recovery by rerouting [4], is a
   survivability technique that establishes new paths or path segments
   on demand, for restoring affected traffic after the occurrence of a
   fault.  The resources in these alternate paths are the currently
   unassigned (unreserved) resources in the same layer.  Preemption of
   extra traffic may also be used if spare resources are not available
   to carry the higher-priority protected traffic.  As initiated by
   detection of a fault on the working path, the selection of a recovery
   path may be based on preplanned configurations, network routing
   policies, or current network status such as network topology and
   fault information. Signaling is used for establishing the new paths
   to bypass the fault.  Thus, restoration involves a path selection
   process followed by rerouting of the affected traffic from the
   working entity to the recovery entity.

2.2.4 Survivability Performance

   Protection switch time is the time interval from the occurrence of a
   network fault until the completion of the protection-switching
   operations.  It includes the detection time necessary to initiate the
   protection switch, any hold-off time to allow for the interworking of
   protection schemes, and the switch completion time.

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   Restoration time is the time interval from the occurrence of a
   network fault to the instant when the affected traffic is either
   completely restored, or until spare resources are exhausted, and/or
   no more extra traffic exists that can be preempted to make room.

   Restoration priority is a method of giving preference to protect
   higher-priority traffic ahead of lower-priority traffic.  Its use is
   to help determine the order of restoring traffic after a failure has
   occurred.  The purpose is to differentiate service restoration time
   as well as to control access to available spare capacity for
   different classes of traffic.

   Preemption priority is a method of determining which traffic can be
   disconnected in the event that not all traffic with a higher
   restoration priority is restored after the occurrence of a failure.

2.3 Survivability Mechanisms: Comparison

   In a survivable network design, spare capacity and diversity must be
   built into the network from the beginning to support some degree of
   self-healing whenever failures occur.  A common strategy is to
   associate each working entity with a protection entity having either
   dedicated resources or shared resources that are pre-reserved or
   reserved-on-demand.  According to the methods of setting up a
   protection entity, different approaches to providing survivability
   can be classified.  Generally, protection techniques are based on
   having a dedicated protection entity set up prior to failure.  Such
   is not the case in restoration techniques, which mainly rely on the
   use of spare capacity in the network.  Hence, in terms of trade-offs,
   protection techniques usually offer fast recovery from failure with
   enhanced availability, while restoration techniques usually achieve
   better resource utilization.

   A 1+1 protection architecture is rather expensive since resource
   duplication is required for the working and protection entities.  It
   is generally used for specific services that need a very high
   availability.

   A 1:1 architecture is inherently slower in recovering from failure
   than a 1+1 architecture since communication between both ends of the
   protection domain is required to perform the switch-over operation.
   An advantage is that the protection entity can optionally be used to
   carry low-priority extra traffic in normal operation, if traffic
   preemption is allowed.  Packet networks can pre-establish a
   protection path for later use with pre-planned but not pre-reserved
   capacity.  That is, if no packets are sent onto a protection path,

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   then no bandwidth is consumed.  This is not the case in transmission
   networks like optical or TDM where path establishment and resource
   reservation cannot be decoupled.

   In the 1:n protection architecture, traffic is normally sent on the
   working entities.  When multiple working entities have failed
   simultaneously, only one of them can be restored by the common
   protection entity.  This contention could be resolved by assigning a
   different preemptive priority to each working entity.  As in the 1:1
   case, the protection entity can optionally be used to carry
   preemptable traffic in normal operation.

   While the m:n architecture can improve system availability with small
   cost increases, it has rarely been implemented or standardized.

   When compared with protection mechanisms, restoration mechanisms are
   generally more frugal as no resources are committed until after the
   fault occurs and the location of the fault is known.  However,
   restoration mechanisms are inherently slower, since more must be done
   following the detection of a fault.  Also, the time it takes for the
   dynamic selection and establishment of alternate paths may vary,
   depending on the amount of traffic and connections to be restored,
   and is influenced by the network topology, technology employed, and
   the type and severity of the fault.  As a result, restoration time
   tends to be more variable than the protection switch time needed with
   pre-selected protection entities.  Hence, in using restoration
   mechanisms, it is essential to use restoration priority to ensure
   that service objectives are met cost-effectively.

   Once the network routing algorithms have converged after a fault, it
   may be preferable in some cases, to reoptimize the network by
   performing a reroute based on the current state of the network and
   network policies.

3. Survivability

3.1 Scope

   Interoperable approaches to network survivability were determined to
   be an immediate requirement in packet networks as well as in
   SDH/SONET framed TDM networks.  Not as pressing at this time were
   techniques that would cover all-optical networks (e.g., where framing
   is unknown), as the control of these networks in a multi-vendor
   environment appeared to have some other hurdles to first deal with.
   Also, not of immediate interest were approaches to coordinate or
   explicitly communicate survivability mechanisms across network layers
   (such as from a TDM or optical network to/from an IP network).
   However, a capability should be provided for a network operator to

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   perform fault notification and to control the operation of
   survivability mechanisms among different layers.  This may require
   the development of corresponding OAM functionality. However, such
   issues and those related to OAM are currently outside the scope of
   this document.  (For proposed MPLS OAM requirements, see [8, 9]).

   The initial scope is to address only "backhoe failures" in the
   inter-office connections of a service provider network.  A link
   connection in the router layer is typically comprised of multiple
   spans in the lower layers.  Therefore, the types of network failures
   that cause a recovery to be performed include link/span failures.
   However, linecard and node failures may not need to be treated any
   differently than their respective link/span failures, as a router
   failure may be represented as a set of simultaneous link failures.

   Depending on the actual network configuration, drop-side interface
   (e.g., between a customer and an access router, or between a router
   and an optical cross-connect) may be considered either inter-domain
   or inter-layer.  Another inter-domain scenario is the use of intra-
   office links for interconnecting a metro network and a core network,
   with both networks being administered by the same service provider.
   Failures at such interfaces may be similarly protected by the
   mechanisms of this section.

   Other more complex failure mechanisms such as systematic control-
   plane failure, configuration error, or breach of security are not
   within the scope of the survivability mechanisms discussed in this
   document.  Network impairment such as congestion that results in
   lower throughput are also not covered.

3.2 Required initial set of survivability mechanisms

3.2.1   1:1 Path Protection with Pre-Established Capacity

   In this protection mode, the head end of a working connection
   establishes a protection connection to the destination.  There should
   be the ability to maintain relative restoration priorities between
   working and protection connections, as well as between different
   classes of protection connections.

   In normal operation, traffic is only sent on the working connection,
   though the ability to signal that traffic will be sent on both
   connections (1+1 Path for signaling purposes) would be valuable in
   non-packet networks.  Some distinction between working and protection
   connections is likely, either through explicit objects, or preferably
   through implicit methods such as general classes or priorities.  Head
   ends need the ability to create connections that are as failure
   disjoint as possible from each other.  This requires SRG information

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   that can be generally assigned to either nodes or links and
   propagated through the control or management plane.  In this
   mechanism, capacity in the protection connection is pre-established,
   however it should be capable of carrying preemptable extra traffic in
   non-packet networks.  When protection capacity is called into service
   during recovery, there should be the ability to promote the
   protection connection to working status (for non-revertive mode
   operation) with some form of make-before-break capability.

3.2.2   1:1 Path Protection with Pre-Planned Capacity

   Similar to the above 1:1 protection with pre-established capacity,
   the protection connection in this case is also pre-signaled.  The
   difference is in the way protection capacity is assigned.  With pre-
   planned capacity, the mechanism supports the ability for the
   protection capacity to be shared, or "double-booked".  Operators need
   the ability to provision different amounts of protection capacity
   according to expected failure modes and service level agreements.
   Thus, an operator may wish to provision sufficient restoration
   capacity to handle a single failure affecting all connections in an
   SRG, or may wish to provision less or more restoration capacity.
   Mechanisms should be provided to allow restoration capacity on each
   link to be shared by SRG-disjoint failures.  In a sense, this is 1:1
   from a path perspective; however, the protection capacity in the
   network (on a link by link basis) is shared in a 1:n fashion, e.g.,
   see the proposals in [10, 11].  If capacity is planned but not
   allocated, some form of signaling could be required before traffic
   may be sent on protection connections, especially in TDM networks.

   The use of this approach improves network resource utilization, but
   may require more careful planning.  So, initial deployment might be
   based on 1:1 path protection with pre-established capacity and the
   local restoration mechanism to be described next.

3.2.3   Local Restoration

   Due to the time impact of signal propagation, dynamic recovery of an
   entire path may not meet the service requirements of some networks.
   The solution to this is to restore connectivity of the link or span
   in immediate proximity to the fault, e.g., see the proposals in [12,
   13].  At a minimum, this approach should be able to protect against
   connectivity-type SRGs, though protecting against node-based SRGs
   might be worthwhile.  Also, this approach is applicable to support
   restoration on the inter-domain and inter-layer interconnection
   scenarios using intra-office links as described in the Scope Section.

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   Head end systems must have some control as to whether their
   connections are candidates for or excluded from local restoration.
   For example, best-effort and preemptable traffic may be excluded from
   local restoration; they only get restored if there is bandwidth
   available.  This type of control may require the definition of an
   object in signaling.

   Since local restoration may be suboptimal, a means for head end
   systems to later perform path-level re-grooming must be supported for
   this approach.

3.2.4   Path Restoration

   In this approach, connections that are impacted by a fault are
   rerouted by the originating network element upon notification of
   connection failure.  Such a source-based approach is efficient for
   network resources, but typically takes longer to accomplish
   restoration.  It does not involve any new mechanisms.  It merely is a
   mention of another common approach to protecting against faults in a
   network.

3.3 Applications Supported

   With service continuity under failure as a goal, a network is
   "survivable" if, in the face of a network failure, connectivity is
   interrupted for a "brief" period and then recovered before the
   network failure ends.  The length of this interrupted period is
   dependent upon the application supported.  Here are some typical
   applications and considerations that drive the requirements for an
   acceptable protection switch time or restoration time:

   - Best-effort data: recovery of network connectivity by rerouting at
     the IP layer would be sufficient
   - Premium data service: need to meet TCP timeout or application
     protocol timer requirements
   - Voice: call cutoff is in the range of 140 msec to 2 sec (the time
     that a person waits after interruption of the speech path before
     hanging up or the time that a telephone switch will disconnect a
     call)
   - Other real-time service (e.g., streaming, fax) where an
     interruption would cause the session to terminate
   - Mission-critical applications that cannot tolerate even brief
     interruptions, for example, real-time financial transactions

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3.4 Timing Bounds for Survivability Mechanisms

   The approach to picking the types of survivability mechanisms
   recommended was to consider a spectrum of mechanisms that can be used
   to protect traffic with varying characteristics of survivability and
   speed of protection/restoration, and then attempt to select a few
   general points that provide some coverage across that spectrum.  The
   focus of this work is to provide requirements to which a small set of
   detailed proposals may be developed, allowing the operator some
   (limited) flexibility in approaches to meeting their design goals in
   engineering multi-vendor networks.  Requirements of different
   applications as listed in the previous sub-section were discussed
   generally, however none on the team would likely attest to the
   scientific merit of the ability of the timing bounds below to meet
   any specific application's needs.  A few assumptions include:

   1. Approaches in which protection switch without propagation of
      information are likely to be faster than those that do require
      some form of fault notification to some or all elements in a
      network.

   2. Approaches that require some form of signaling after a fault will
      also likely suffer some timing impact.

   Proposed timing bounds for different survivability mechanisms are as
   follows (all bounds are exclusive of signal propagation):

   1:1 path protection with pre-established capacity:  100-500 ms
   1:1 path protection with pre-planned capacity:      100-750 ms
   Local restoration:                                  50 ms
   Path restoration:                                   1-5 seconds

   To ensure that the service requirements for different applications
   can be met within the above timing bounds, restoration priority must
   be implemented to determine the order in which connections are
   restored (to minimize service restoration time as well as to gain
   access to available spare capacity on the best paths).  For example,
   mission critical applications may require high restoration priority.
   At the fiber layer, instead of specific applications, it may be
   possible that priority be given to certain classifications of
   customers with their traffic types enclosed within the customer
   aggregate.  Preemption priority should only be used in the event that
   not all connections can be restored, in which case connections with
   lower preemption priority should be released. Depending on a service
   provider's strategy in provisioning network resources for backup,
   preemption may or may not be needed in the network.

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3.5 Coordination Among Layers

   A common design goal for networks with multiple technological layers
   is to provide the desired level of service in the most cost-effective
   manner.  Multilayer survivability may allow the optimization of spare
   resources through the improvement of resource utilization by sharing
   spare capacity across different layers, though further investigations
   are needed.  Coordination during recovery among different network
   layers (e.g., IP, SDH/SONET, optical layer) might necessitate
   development of vertical hierarchy.  The benefits of providing
   survivability mechanisms at multiple layers, and the optimization of
   the overall approach, must be weighed with the associated cost and
   service impacts.

   A default coordination mechanism for inter-layer interaction could be
   the use of nested timers and current SDH/SONET fault monitoring, as
   has been done traditionally for backward compatibility.  Thus, when
   lower-layer recovery happens in a longer time period than higher-
   layer recovery, a hold-off timer is utilized to avoid contention
   between the different single-layer survivability schemes.  In other
   words, multilayer interaction is addressed by having successively
   higher multiplexing levels operate at a protection/restoration time
   scale greater than the next lowest layer.  This can impact the
   overall time to recover service.  For example, if SDH/SONET
   protection switching is used, MPLS recovery timers must wait until
   SDH/SONET has had time to switch.  Setting such timers involves a
   tradeoff between rapid recovery and creation of a race condition
   where multiple layers are responding to the same fault, potentially
   allocating resources in an inefficient manner.

   In other configurations where the lower layer does not have a
   restoration capability or is not expected to protect, say an
   unprotected SDH/SONET linear circuit, then there must be a mechanism
   for the lower layer to trigger the higher layer to take recovery
   actions immediately.  This difference in network configuration means
   that implementations must allow for adjustment of hold-off timer
   values and/or a means for a lower layer to immediately indicate to a
   higher layer that a fault has occurred so that the higher layer can
   take restoration or protection actions.

   Furthermore, faults at higher layers should not trigger restoration
   or protection actions at lower layers [3, 4].

   It was felt that the current approach to coordination of
   survivability approaches currently did not have significant
   operational shortfalls.  These approaches include protecting traffic
   solely at one layer (e.g., at the IP layer over linear WDM, or at the
   SDH/SONET layer).  Where survivability mechanisms might be deployed

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   at several layers, such as when a routed network rides a SDH/SONET
   protected network, it was felt that current coordination approaches
   were sufficient in many cases.  One exception is the hold-off of MPLS
   recovery until the completion of SDH/SONET protection switching as
   described above.  This limits the recovery time of fast MPLS
   restoration.  Also, by design, the operations and mechanisms within a
   given layer tend to be invisible to other layers.

3.6 Evolution Toward IP Over Optical

   As more pressing requirements for survivability and horizontal
   hierarchy for edge-to-edge signaling are met with technical
   proposals, it is believed that the benefits of merging (in some
   manner) the control planes of multiple layers will be outlined.  When
   these benefits are self-evident, it would then seem to be the right
   time to review whether vertical hierarchy mechanisms are needed, and
   what the requirements might be.  For example, a future requirement
   might be to provide a better match between the recovery requirements
   of IP networks with the recovery capability of optical transport.
   One such proposal is described in [14].

4. Hierarchy Requirements

   Efforts in the area of network hierarchy should focus on mechanisms
   that would allow more scalable edge-to-edge signaling, or signaling
   across networks with existing network hierarchy (such as multi-area
   OSPF).  This appears to be a more urgent need than mechanisms that
   might be needed to interconnect networks at different layers.

4.1 Historical Context

   One reason for horizontal hierarchy is functionality (e.g., metro
   versus backbone).  Geographic "islands" or partitions reduce the need
   for interoperability and make administration and operations less
   complex.  Using a simpler, more interoperable, survivability scheme
   at metro/backbone boundaries is natural for many provider network
   architectures.  In transmission networks, creating geographic islands
   of different vendor equipment has been done for a long time because
   multi-vendor interoperability has been difficult to achieve.
   Traditionally, providers have to coordinate the equipment on either
   end of a "connection," and making this interoperable reduces
   complexity.  A provider should be able to concatenate survivability
   mechanisms in order to provide a "protected link" to the next higher
   level.  Think of SDH/SONET rings connecting to TDM DXCs with 1+1
   line-layer protection between the ADM and the DXC port.  The TDM
   connection, e.g., a DS3, is protected but usually all equipment on
   each SDH/SONET ring is from a single vendor.  The DXC cross
   connections are controlled by the provider and the ports are

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   physically protected resulting in a highly available design.  Thus,
   concatenation of survivability approaches can be used to cascade
   across a horizontal hierarchy.  While not perfect, it is workable in
   the near to mid-term until multi-vendor interoperability is achieved.

   While the problems associated with multi-vendor interoperability may
   necessitate horizontal hierarchy as a practical matter in the near to
   mid-term (at least this has been the case in TDM networks), there
   should not be a technical reason for it in the standards developed by
   the IETF for core networks, or even most access networks.
   Establishing interoperability of survivability mechanisms between
   multi-vendor equipment in core IP networks is urgently required to
   enable adoption of IP as a viable core transport technology and to
   facilitate the traffic engineering of future multi-service IP
   networks [3].

   Some of the largest service provider networks currently run a single
   area/level IGP.  Some service providers, as well as many large
   enterprise networks, run multi-area Open Shortest Path First (OSPF)
   to gain increases in scalability.  Often, this was from an original
   design, so it is difficult to say if the network truly required the
   hierarchy to reach its current size.

   Some proposals on improved mechanisms to address network hierarchy
   have been suggested [15, 16, 17, 18, 19].  This document aims to
   provide the concrete requirements so that these and other proposals
   can first aim to meet some limited objectives.

4.2 Applications for Horizontal Hierarchy

   A primary driver for intra-domain horizontal hierarchy is signaling
   capabilities in the context of edge-to-edge VPNs, potentially across
   traffic-engineered data networks.  There are a number of different
   approaches to layer 2 and layer 3 VPNs and they are currently being
   addressed by different emerging protocols in the provider-provisioned
   VPNs (e.g., virtual routers) and Pseudo Wire Edge-to-Edge Emulation
   (PWE3) efforts based on either MPLS and/or IP tunnels.  These may or
   may not need explicit signaling from edge to edge, but it is a common
   perception that in order to meet SLAs, some form of edge-to-edge
   signaling may be required.

   With a large number of edges (N), scalability is concerned with
   avoiding the O(N^2) properties of edge-to-edge signaling.  However,
   the main issue here is not with the scalability of large amounts of
   signaling, such as in O(N^2) meshes with a "connection" between every
   edge-pair.  This is because, even if establishing and maintaining
   connections is feasible in a large network, there might be an impact
   on core survivability mechanisms which would cause

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   protection/restoration times to grow with N^2, which would be
   undesirable.  While some value of N may be inevitable, approaches to
   reduce N (e.g. to pull in from the edge to aggregation points) might
   be of value.

   Thus, most service providers feel that O(N^2) meshes are not
   necessary for VPNs, and that the number of tunnels to support VPNs
   would be within the scalability bounds of current protocols and
   implementations.  That may be the case, as there is currently a lack
   of ability to signal MPLS tunnels from edge to edge across IGP
   hierarchy, such as OSPF areas.  This may require the development of
   signaling standards that support dynamic establishment and
   potentially the restoration of LSPs across a 2-level IGP hierarchy.

   For routing scalability, especially in data applications, a major
   concern is the amount of processing/state that is required in the
   variety of network elements.  If some nodes might not be able to
   communicate and process the state of every other node, it might be
   preferable to limit the information.  There is one school of thought
   that says that the amount of information contained by a horizontal
   barrier should be significant, and that impacts this might have on
   optimality in route selection and ability to provide global
   survivability are accepted tradeoffs.

4.3 Horizontal Hierarchy Requirements

   Mechanisms are required to allow for edge-to-edge signaling of
   connections through a network.  One network scenario includes medium
   to large networks that currently have hierarchical interior routing
   such as multi-area OSPF or multi-level Intermediate System to
   Intermediate System (IS-IS).  The primary context of this is edge-
   to-edge signaling, which is thought to be required to assure the SLAs
   for the layer 2 and layer 3 VPNs that are being carried across the
   network.  Another possible context would be edge-to-edge signaling in
   TDM SDH/SONET networks with IP control, where metro and core networks
   again might be in a hierarchical interior routing domain.

   To support edge-to-edge signaling in the above network scenarios
   within the framework of existing horizontal hierarchies, current
   traffic engineering (TE) methods [20, 6] may need to be extended.
   Requirements for multi-area TE need to be developed to provide
   guidance for any necessary protocol extensions.

5. Survivability and Hierarchy

   When horizontal hierarchy exists in a network technology layer, a
   question arises as to how survivability can be provided along a
   connection that crosses hierarchical boundaries.

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   In designing protocols to meet the requirements of hierarchy, an
   approach to consider is that boundaries are either clean, or are of
   minimal value.  However, the concept of network elements that
   participate on both sides of a boundary might be a consideration
   (e.g., OSPF ABRs).  That would allow for devices on either side to
   take an intra-area approach within their region of knowledge, and for
   the ABR to do this in both areas, and splice the two protected
   connections together at a common point (granted it is a common point
   of failure now).  If the limitations of this approach start to appear
   in operational settings, then perhaps it would be time to start
   thinking about route-servers and signaling propagated directives.
   However, one initial approach might be to signal through a common
   border router, and to consider the service as protected as it
   consists of a concatenated set of connections which are each
   protected within their area.  Another approach might be to have a
   least common denominator mechanism at the boundary, e.g., 1+1 port
   protection.  There should also be some standardized means for a
   survivability scheme on one side of such a boundary to communicate
   with the scheme on the other side regarding the success or failure of
   the recovery action.  For example, if a part of a "connection" is
   down on one side of such a boundary, there is no need for the other
   side to recover from failures.

   In summary, at this time, approaches as described above that allow
   concatenation of survivability schemes across hierarchical boundaries
   seem sufficient.

6. Security Considerations

   The set of SRGs that are defined for a network under a common
   administrative control and the corresponding assignment of these SRGs
   to nodes and links within the administrative control is sensitive
   information and needs to be protected.  An SRG is an acknowledgement
   that nodes and links that belong to an SRG are susceptible to a
   common threat.  An adversary with access to information contained in
   an SRG could use that information to design an attack, determine the
   scope of damage caused by the attack and, therefore, be used to
   maximize the effect of an attack.

   The label used to refer to a particular SRG must allow for an
   encoding such that sensitive information such as physical location,
   function, purpose, customer, fault type, etc. is not readily
   discernable by unauthorized users.

   SRG information that is propagated through the control and management
   plane should allow for an encryption mechanism.  An example of an
   approach would be to use IPSEC [21] on all packets carrying SRG
   information.

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7. References

   [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  K. Owens, V. Sharma, and M. Oommen, "Network Survivability
        Considerations for Traffic Engineered IP Networks", Work in
        Progress.

   [4]  V. Sharma, B. Crane, S. Makam, K. Owens, C. Huang, F.
        Hellstrand, J. Weil, L. Andersson, B. Jamoussi, B. Cain, S.
        Civanlar, and A. Chiu, "Framework for MPLS-based Recovery", Work
        in Progress.

   [5]  M. Thorup, "Fortifying OSPF/ISIS Against Link Failure",
        http://www.research.att.com/~mthorup/PAPERS/lf_ospf.ps

   [6]  Awduche, D., Chiu, A., Elwalid, A., Widjaja, I. and X. Xiao,
        "Overview and Principles of Internet Traffic Engineering", RFC
        3272, May 2002.

   [7]  S. Dharanikota, R. Jain, D. Papadimitriou, R. Hartani, G.
        Bernstein, V. Sharma, C. Brownmiller, Y. Xue, and J. Strand,
        "Inter-domain routing with Shared Risk Groups", Work in
        Progress.

   [8]  N. Harrison, P. Willis, S. Davari, E. Cuevas, B. Mack-Crane, E.
        Franze, H. Ohta, T. So, S. Goldfless, and F. Chen, "Requirements
        for OAM in MPLS Networks," Work in Progress.

   [9]  D. Allan and M. Azad, "A Framework for MPLS User Plane OAM,"
        Work in Progress.

   [10] S. Kini, M. Kodialam, T.V. Lakshman, S. Sengupta, and C.
        Villamizar, "Shared Backup Label Switched Path Restoration,"
        Work in Progress.

   [11] G. Li, C. Kalmanek, J. Yates, G. Bernstein, F. Liaw, and V.
        Sharma, "RSVP-TE Extensions For Shared-Mesh Restoration in
        Transport Networks", Work in Progress.

   [12] P. Pan (Editor), D.H. Gan, G. Swallow, J. Vasseur, D. Cooper, A.
        Atlas, and M. Jork, "Fast Reroute Extensions to RSVP-TE for LSP
        Tunnels", Work in Progress.

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   [13] A. Atlas, C. Villamizar, and C. Litvanyi, "MPLS RSVP-TE
        Interoperability for Local Protection/Fast Reroute", Work in
        Progress.

   [14] A. Chiu and J. Strand, "Joint IP/Optical Layer Restoration after
        a Router Failure", Proc. OFC'2001, Anaheim, CA, March 2001.

   [15] K. Kompella and Y. Rekhter, "Multi-area MPLS Traffic
        Engineering", Work in Progress.

   [16] G. Ash, et. al., "Requirements for Multi-Area TE", Work in
        Progress.

   [17] A. Iwata, N. Fujita, G.R. Ash, and A. Farrel, "Crankback Routing
        Extensions for MPLS Signaling", Work in Progress.

   [18] C-Y Lee, A. Celer, N. Gammage, S. Ghanti, G. Ash, "Distributed
        Route Exchangers", Work in Progress.

   [19] C-Y Lee and S. Ghanti, "Path Request and Path Reply Message",
        Work in Progress.

   [20] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
        McManus, "Requirements for Traffic Engineering Over MPLS", RFC
        2702, September 1999.

   [21] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

8. Acknowledgments

   A lot of the direction taken in this document, and by the team in its
   initial effort was steered by the insightful questions provided by
   Bala Rajagoplan, Greg Bernstein, Yangguang Xu, and Avri Doria.  The
   set of questions is attached as Appendix A in this document.

   After the release of the first draft, a number of comments were
   received.  Thanks to the inputs from Jerry Ash, Sudheer Dharanikota,
   Chuck Kalmanek, Dan Koller, Lyndon Ong, Steve Plote, and Yong Xue.

9. Contributing Authors

   Jim Boyle (PDNets), Rob Coltun (Movaz), Tim Griffin (AT&T), Ed Kern,
   Tom Reddington (Lucent) and Malin Carlzon.

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Appendix A: Questions used to help develop requirements

   A. Definitions

   1. In determining the specific requirements, the design team should
      precisely define the concepts "survivability", "restoration",
      "protection", "protection switching", "recovery", "re-routing"
      etc. and their relations.  This would enable the requirements doc
      to describe precisely which of these will be addressed. In the
      following, the term "restoration" is used to indicate the broad
      set of policies and mechanisms used to ensure survivability.

   B. Network types and protection modes

   1. What is the scope of the requirements with regard to the types of
      networks covered?  Specifically, are the following in scope:

      Restoration of connections in mesh optical networks (opaque or
      transparent)
      Restoration of connections in hybrid mesh-ring networks
      Restoration of LSPs in MPLS networks (composed of LSRs overlaid on
      a transport network, e.g., optical)
      Any other types of networks?
      Is commonality of approach, or optimization of approach more
      important?

   2. What are the requirements with regard to the protection modes to
      be supported in each network type covered? (Examples of protection
      modes include 1+1, M:N, shared mesh, UPSR, BLSR, newly defined
      modes such as P-cycles, etc.)

   3. What are the requirements on local span (i.e., link by link)
      protection and end-to-end protection, and the interaction between
      them?  E.g.: what should be the granularity of connections for
      each type (single connection, bundle of connections, etc).

   C. Hierarchy

   1. Vertical (between two network layers):
      What are the requirements for the interaction between restoration
      procedures across two network layers, when these features are
      offered in both layers?  (Example, MPLS network realized over pt-
      to-pt optical connections.)  Under such a case,

      (a) Are there any criteria to choose which layer should provide
          protection?

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      (b) If both layers provide survivability features, what are the
          requirements to coordinate these mechanisms?

      (c) How is lack of current functionality of cross-layer
          coordination currently hampering operations?

      (d) Would the benefits be worth additional complexity associated
          with routing isolation (e.g. VPN, areas), security, address
          isolation and policy / authentication processes?

   2. Horizontal (between two areas or administrative subdivisions
      within the same network layer):

      (a) What are the criteria that trigger the creation of protocol or
          administrative boundaries pertaining to restoration? (e.g.,
          scalability?  multi-vendor interoperability?  what are the
          practical issues?)  multi-provider?  Should multi-vendor
          necessitate hierarchical separation?

      When such boundaries are defined:

      (b) What are the requirements on how protection/restoration is
          performed end-to-end across such boundaries?

      (c) If different restoration mechanisms are implemented on two
          sides of a boundary, what are the requirements on their
          interaction?

      What is the primary driver of horizontal hierarchy? (select one)
          - functionality (e.g. metro -v- backbone)
          - routing scalability
          - signaling scalability
          - current network architecture, trying to layer on TE on top
            of an already hierarchical network architecture
          - routing and signalling

      For signalling scalability, is it
          - manageability
          - processing/state of network
          - edge-to-edge N^2 type issue

      For routing scalability, is it
          - processing/state of network
          - are you flat and want to go hierarchical
          - or already hierarchical?
          - data or TDM application?

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   D. Policy

   1. What are the requirements for policy support during
      protection/restoration, e.g., restoration priority, preemption,
      etc.

   E. Signaling Mechanisms

   1. What are the requirements on the signaling transport mechanism
      (e.g., in-band over SDH/SONET overhead bytes, out-of-band over an
      IP network, etc.) used to communicate restoration protocol
      messages between network elements?  What are the bandwidth and
      other requirements on the signaling channels?

   2. What are the requirements on fault detection/localization
      mechanisms (which is the prelude to performing restoration
      procedures) in the case of opaque and transparent optical
      networks? What are the requirements in the case of MPLS
      restoration?

   3. What are the requirements on signaling protocols to be used in
      restoration procedures (e.g., high priority processing, security,
      etc)?

   4. Are there any requirements on the operation of restoration
      protocols?

   F. Quantitative

   1. What are the quantitative requirements (e.g., latency) for
      completing restoration under different protection modes (for both
      local and end-to-end protection)?

   G. Management

   1. What information should be measured/maintained by the control
      plane at each network element pertaining to restoration events?

   2. What are the requirements for the correlation between control
      plane and data plane failures from the restoration point of view?

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Editors' Addresses

   Wai Sum Lai
   AT&T
   200 Laurel Avenue
   Middletown, NJ 07748, USA

   Phone: +1 732-420-3712
   EMail: wlai@att.com

   Dave McDysan
   WorldCom
   22001 Loudoun County Pkwy
   Ashburn, VA 20147, USA

   EMail: dave.mcdysan@wcom.com

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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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