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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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RFC 3386 Hierarchy & Multilayer Survivability November 2002
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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RFC 3386 Hierarchy & Multilayer Survivability November 2002
[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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