One document matched: draft-ietf-ccamp-inter-domain-recovery-analysis-02.txt
Differences from draft-ietf-ccamp-inter-domain-recovery-analysis-01.txt
Network Working Group Tomonori Takeda, Ed.
Internet Draft NTT
Intended Status: Informational Yuichi Ikejiri
Expires: March 2008 NTT Communications
Adrian Farrel
Old Dog Consulting
Jean-Philippe Vasseur
Cisco Systems, Inc.
September 2007
Analysis of Inter-domain Label Switched Path (LSP) Recovery
draft-ietf-ccamp-inter-domain-recovery-analysis-02.txt
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Abstract
This document analyzes various schemes to realize Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) Label Switched Path
(LSP) recovery in multi-domain networks based on the existing
framework for multi-domain LSPs.
The main focus for this document is on establishing end-to-end
diverse Traffic Engineering (TE) LSPs in multi-domain networks. It
presents various diverse LSP setup schemes based on existing
functional elements.
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Table of Contents
1. Introduction...................................................2
1.1 Terminology...................................................3
1.2 Domain........................................................4
1.3 Document Scope................................................4
1.4 Note on Other Recovery Techniques.............................5
1.5 Signaling Options.............................................6
2. Diversity in Multi-domain Networks.............................6
2.1 Multi-domain Network Topology.................................6
2.2 Note on Domain Diversity......................................8
3. Factors to Consider............................................8
3.1 Scalability versus Optimality.................................8
3.2 Key Concepts..................................................9
4. Diverse LSP Setup Schemes without Confidentiality.............11
4.1 Management Configuration.....................................11
4.2 Head-end Path Computation (with multi-domain visibility).....11
4.3 Per-domain Path Computation..................................11
4.3.1 Sequential Path Computation................................12
4.3.2 Simultaneous Path Computation..............................12
4.4 Inter-domain Collaborative Path Computation..................13
4.4.1 Sequential Path Computation................................14
4.4.2 Simultaneous Path Computation..............................14
5. Diverse LSP Setup Schemes with Confidentiality................15
5.1 Management Configuration.....................................16
5.2 Head-end Path Computation (with multi-domain visibility).....16
5.3 Per-Domain Path Computation..................................16
5.3.1 Sequential Path Computation................................16
5.3.2 Simultaneous Path Computation..............................17
5.4 Inter-domain Collaborative Path Computation..................17
5.4.1 Sequential Path Computation................................17
5.4.2 Simultaneous Path Computation..............................18
6. Network Topology Specific Considerations......................18
7. Addressing Considerations.....................................19
8. Note on SRLG Diversity........................................19
9. Security Considerations.......................................19
10. References...................................................20
10.1 Normative References........................................20
10.2 Informative References......................................20
11. Acknowledgments..............................................22
12. Authors' Addresses...........................................22
Intellectual Property Consideration..............................23
Full Copyright Statement.........................................23
1. Introduction
This document analyzes various schemes to realize Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) Label Switched Path
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(LSP) recovery in multi-domain networks based on the existing
framework for multi-domain LSPs.
The main focus for this document is on establishing end-to-end
diverse Traffic Engineering (TE) LSPs in multi-domain networks. It
presents various diverse LSP setup schemes based on existing
functional elements.
1.1 Terminology
The reader is assumed to be familiar with the terminology in
[RFC4726] that provides a framework for inter-domain Label Switched
Path (LSP) setup, and [RFC4427] that provides terminology for LSP
recovery.
The following are several key terminologies used within this
document.
- Domain: See [RFC4726]. A domain is considered to be any
collection of network elements within a common sphere of address
management or path computational responsibility. Note that nested
domains continue to be out of scope. Section 1.2 provides some more
details.
- Working LSP: See [RFC4427]. The working LSP transports normal user
traffic. Note that the term LSP and TE LSP will be used
interchangeably.
- Recovery LSP: See [RFC4427]. The recovery LSP transports normal
user traffic when the working LSP fails. The recovery LSP may
transport user traffic even when the working LSP is transporting
normal user traffic (e.g., 1+1 protection). In such a scenario,
the recovery LSP is sometimes referred to as a protecting LSP.
- Diversity: See [RFC4726]. Diversity means the relationship
of multiple LSPs, where those LSPs do not share some specific type
of resource (e.g., link, node, or shared risk link group (SRLG)).
Diverse LSPs may be used for various purposes, such as load-
balancing and recovery. In this document, the recovery aspect of
diversity, specifically the end-to-end diversity of two traffic
engineering (TE) LSPs, is the focus. Those two diverse LSPs are
referred to as the working LSP and recovery LSP hereafter.
Sometimes, diversity is referred to as disjointness.
- Confidentiality: See [RFC4216]. The term confidentiality applies
to the protection of information about the topology and resources
present within one domain from visibility by people or
applications outside that domain.
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1.2 Domain
As defined in [RFC4726], a domain is considered to be any collection
of network elements within a common sphere of address management or
path computational responsibility. Examples of such domains include
IGP areas and Autonomous Systems. A network accessed over the
Generalized Multiprotocol Label Switching (GMPLS) User-to-Network
Interface (UNI) [RFC4208] and a Layer One Virtual Private Network
(L1VPN) [RFC4847] are special cases of multi-domain networks.
Example objectives of domain usage include administrative separation,
scalability, and forming protection domains.
As described in [RFC4726], there could be TE parameters (such as
color and priority) whose meanings are specific to each domain. In
such scenarios, mapping functions could be performed based on policy
agreements between domain administrators.
1.3 Document Scope
This document analyzes various schemes to realize Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) LSP recovery in multi-
domain networks based on the existing framework for multi-domain LSP
setup [RFC4726]. Note that this document does not intend to prevent
development of additional techniques where appropriate (i.e.,
additional to ones described in this document, which are based on the
existing framework described in [RFC4726]). In other words, this
document is informational and intends to show how the existing
framework can be applied with specific procedures described in this
document and the documents referred to by this document.
There are various recovery techniques for LSPs. For TE LSPs, major
techniques are end-to-end recovery [RFC4872], local protection such
as Fast Reroute (FRR) [RFC4090] (in packet switching environments),
and segment recovery [RFC4873] (in GMPLS).
In this document the main focus is the analysis of diverse TE LSP
(hereafter LSP) setup schemes (path computation and signaling
aspects), which can advantageously be used in the context of end-to-
end recovery. This document presents various diverse LSP setup
schemes by combining various functional elements. In particular,
Section 5.5 of [RFC4726] describes some analysis of diverse LSPs in
multi-domain networks, and this document provides more detailed
analysis based on that content.
[RFC4105] and [RFC4216] describe requirements for diverse LSPs. There
could be various types of diversity, and this document focuses on
node/link/SRLG diversity.
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Based on the service grade, both the working LSP and the recovery LSP
may be established at the time of service establishment (e.g.,
service requiring high resiliency). Alternatively, the recovery LSP
may be added later due to a change in the grade of the service. This
document covers both scenarios. Also, there may or may not be
confidentiality requirements among domains. This document covers both
scenarios. Section 3.2 describes more details on confidentiality.
Specific assumptions made in this problem space are described in
Section 2.
Note that the recovery LSP may be used for 1+1 protection, 1:1
protection, or shared mesh restoration. However, ways to compute
diverse paths, and the signaling of these TE LSPs are common across
all uses, and these topics are the main scope of this document.
Also, note that diverse LSPs may be used for various purposes, in
addition to recovery. An example is for load-balancing, so as to
limit the traffic disruption to a portion of the traffic flow in case
of a single network element failure. This document does not preclude
use of diverse LSP setup schemes for those purposes.
The following are beyond the scope of this document.
- Analysis of recovery techniques other than link/node/SRLG diverse
LSPs (see Section 1.4).
- Details of maintenance of diverse LSPs, such as re-optimization and
OAM.
- Comparative evaluation of various diverse LSP setup schemes
mentioned in this document.
1.4 Note on Other Recovery Techniques
Fast Reroute and segment recovery in multi-domain networks are
described in Section 5.4 of [RFC4726], and a more detailed analysis
is provided in Section 5 of [inter-domain-rsvp]. This document does
not cover any additional analysis for Fast ReRoute and segment
recovery in multi-domain networks.
Also, the recovery type of an LSP or service may change at a domain
boundary. That is, the recovery type could remain the same within one
domain, but might be different in the next domain. This may be due to
the capabilities of each domain, administrative policies, or to
topology limitations. An example is where protection at the domain
boundary is provided by link protection on the inter-domain link(s),
but where protection within each domain is achieved through segment
recovery. This mixture of protection techniques is beyond the scope
of this document.
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Domain diversity (that is, the selection of paths that have only the
ingress and egress domains in common) may be considered as one form
of diversity in multi-domain networks, but this is beyond the scope
of this document (see Section 2.2).
1.5 Signaling Options
There are three signaling options for establishing inter-domain TE
LSPs: nesting, contiguous LSPs, and stitching [RFC4726]. The
description in this document of diverse LSP setup is agnostic in
relation to the signaling option used, unless otherwise specified.
Note that signaling option-specific considerations for Fast Reroute
and segment recovery are described in Section 5 of [inter-domain-
rsvp].
2. Diversity in Multi-domain Networks
As described in Section 1.3, analysis of diverse LSP setup schemes in
multi-domain networks is the main focus of this document. This
section describes some assumptions in this problem space made in this
document.
2.1 Multi-domain Network Topology
Figures 1 and 2 show example multi-domain network topologies. In
Figure 1, domains are connected in a linear topology. There are
multiple paths between nodes A and L, but all of them cross domain#1-
domain#2-domain#3 in that order.
+--Domain#1--+ +--Domain#2--+ +--Domain#3--+
| | | | | |
| B------+---+---D-----E--+---+------J |
| / | | \ / | | \ |
| / | | \ / | | \ |
| A | | H | | L |
| \ | | / \ | | / |
| \ | | / \ | | / |
| C------+---+---F-----G--+---+------K |
| | | | | |
+------------+ +------------+ +------------+
Figure 1: Linear Connectivity
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+-----Domain#2-----+
| |
| E--------------F |
| | | |
| | | |
+-+--------------+-+
| |
| |
+--Domain#1-+--+ +-------+------+
| | | | | |
| | | | | |
| A----B--+---+--C----D |
| | | | | |
| | | | | |
+------+-------+ +--+-Domain#4--+
| |
+-+--------------+-+
| | | |
| | | |
| G--------------H |
| |
+-----Domain#3-----+
Figure 2: Mesh Connectivity
In Figure 2, domains are connected in a mesh topology. There are
multiple paths between nodes A and D, and these paths do not
necessarily follow the same set of domains.
Indeed, if inter-domain connectivity forms a mesh, domain level
routing is required (even for the unprotected case). This is tightly
coupled with the next hop domain resolution/discovery mechanisms.
In this document, we assume that domain level routing is given via
configuration, policy, or some external mechanism, and that this is
not part of the path computation process described later in this
document.
In addition, domain level routing may perform "domain re-entry",
where a path enters a domain after the path exits that domain (e.g.,
domain#X-domain#Y-domain#X). This requires specific considerations
when confidentiality is required (described in Section 3.2), and is
beyond the scope of this document.
Furthermore, the working LSP and the recovery LSP may or may not be
routed along the same set of domains in the same order. In this
document, we assume that the working LSP and recovery LSP follow the
same set of domains in the same order (via configuration, policy or
some external mechanism). That is, we assume that the domain mesh
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topology is reduced to a linear domain topology for each pair of
working and recovery LSPs.
In summary,
- There is no assumption about the multi-domain network topology. For
example, there could be more than two domain boundary nodes or
inter-domain links (links connecting a pair of domain boundary
nodes belonging to different domains).
- However, there is an assumption that under such a network topology,
the set of domains that the working LSP and the recovery LSP follow
must be the same and pre-configured.
- Domain re-entry is not considered.
2.2 Note on Domain Diversity
As described in Section 1.4, domain diversity means the selection of
paths that have only the ingress and egress domains in common. This
may provide enhanced resilience against failures, and is a way to
ensure path diversity for most of the path of the LSP.
In Section 2.1 we assumed that the working LSP and the recovery LSP
follow the same set of domains in the same order. Under this
assumption, domain diversity cannot be achieved. However, by relaxing
this assumption, domain diversity could be achieved, e.g., by either
of the following schemes.
- Consider domain diversity as a special case of SRLG diversity
(i.e., assign an SRLG ID to each domain).
- Configure domain level routing to provide domain diverse paths
(e.g., by means of AS_PATH in BGP).
Details are beyond the scope of this document.
3. Factors to Consider
3.1 Scalability versus Optimality
As described in [RFC4726], scalability and optimality are two
conflicting objectives. Note that the meaning of optimality differs
depending on each network operation. Some examples of optimality in
the context of diverse LSPs are:
- Minimizing the sum of their cost while maintaining diversity.
- Restricting the difference of their cost (so as to minimize delay
difference after switch-over) while maintaining diversity.
By restricting TE information distribution to only within each domain
(and not across domain boundaries) as required by RFC4105 and
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RFC4216, it may not be possible to compute an optimal path. As such,
it may not be possible to compute diverse paths, even if they exist.
However, if we assume domain level routing is given (as discussed in
Section 2), it is possible to compute diverse paths using specific
computation schemes, if such paths exist. This is discussed in
Section 4.
3.2 Key Concepts
Three key concepts to classify various diverse LSP computation and
setup schemes are presented below.
o With or without confidentiality
- Without confidentiality
Under this network configuration, it is possible to specify (by
means of the Explicit Route Object - ERO - comprising a list of
strict hops) or obtain record of (by means of the Record Route
Object - RRO) a route across other domains.
Examples of this configuration are multi-area networks, and some
forms of multi-AS networks (especially within the same provider).
- With confidentiality
Under this network configuration, it is not possible to specify
or obtain a full and strict route (by means of ERO/RRO) across
other domains, although paths may be specified/obtained in the
form of an ERO/RRO containing loose hops. Therefore, it is not
possible to specify or obtain a full route at the head-end of a
multi-domain LSP.
Examples of this configuration are some forms of multi-AS
networks (especially inter-provider networks), GMPLS-UNI
networks, and L1VPNs.
o Per domain path computation or inter-domain collaborative path
computation
- Per domain path computation
In this scheme, a path is computed domain by domain as LSP
signaling progresses through the network. This scheme requires
ERO expansion in each domain. The case for unprotected LSPs under
this scheme is covered in [inter-domain-pd].
- Inter-domain collaborative path computation
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In this scheme, path computation is typically done before
signaling. This scheme typically uses communication between
cooperating path computation elements (PCEs) [RFC4655]. An
example of such a scheme is Backward Recursive Path Computation
(BRPC) [brpc]. The use of BRPC for unprotected LSPs under this
scheme is covered in [brpc].
Note that these are path computation techniques. It is also
possible to obtain a path via management configuration, or head-end
path computation (with multi-domain visibility). This is discussed
in Sections 4 and 5.
Note also that it is possible to combine multiple path computation
techniques (including using a different technique for the working
and recovery LSPs), but details are beyond the scope of this
document.
o Sequential path computation or simultaneous path computation
- Sequential path computation
The path of the working LSP is computed (without considering the
recovery LSP), and then the path of the recovery LSP is computed.
Typically, this scheme is applicable when the recovery LSP is
added later as change of the service grade. But this scheme can
also be applicable when both of the working and recovery LSPs are
established from the start. In this scheme, diverse LSPs may not
be correctly computed (without some form of re-optimization or
recomputation technique) in "trap" topologies. Furthermore, such
sequential path computation approaches may prevent the selection
of an "optimal" solution with regards to a specific objective
function.
- Simultaneous path computation
The path of the working LSP and the path of the recovery LSP are
computed simultaneously. Typically, this scheme is applicable
when both the working LSP and the recovery LSP are established
together. In this scheme, it is possible to avoid trap
topologies. Furthermore it may allow for achieving more optimal
results.
Note that LSP setup with or without confidentiality is given as a
per-domain configuration, while the choices of per-domain path
computation or inter-domain collaborative path computation, and
sequential path computation or simultaneous path computation may be a
matter of choice for each individual pair of working/recovery LSPs.
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The analysis of various diverse LSP setup schemes is described in
Sections 4 and 5, based on above criteria.
Some other considerations, such as network topology-specific
considerations, addressing considerations, and SRLG diversity are
described in Sections 6, 7, and 8.
4. Diverse LSP Setup Schemes without Confidentiality
In the following, various schemes for diverse LSP setup are presented
based on different path computation techniques assuming that there is
no requirement for confidentiality between domains. Section 5 makes a
similar examination of schemes where inter-domain confidentiality is
required.
4.1 Management Configuration
Section 3.1 of [RFC4726] describes this path computation technique.
The full explicit paths for the working and recovery LSPs are
specified by a management application at the head-end, and no further
computation or signaling considerations are needed.
4.2 Head-end Path Computation (with multi-domain visibility)
Section 3.2.1 of [RFC4726] describes this path computation technique.
The full explicit paths for the working and recovery LSPs are
computed at the head-end either by the head-end itself or by a PCE.
In either case the computing entity has full TE visibility across
multiple domains and no further computation or signaling
considerations are needed.
4.3 Per-domain Path Computation
Sections 3.2.2, 3.2.3 and 3.3 of [RFC4726] describe this path
computation technique. More detailed procedures are described in
[inter-domain-pd].
In this scheme, the explicit paths of the working and recovery LSPs
are specified as the complete strict path in the source domain
followed by one of the following:
- The complete list of boundary LSRs (or domain identifiers, e.g., AS
numbers) along the path.
- The boundary LSR for the source domain and the LSP destination.
Thus, ERO expansion is needed at domain boundaries. Path computation
is performed by, or by a PCE on behalf of, each domain boundary LSR.
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For establishing diverse LSPs using per-domain computation, there are
two specific schemes, which are described in Sections 4.3.1 and 4.3.2
respectively.
4.3.1 Sequential Path Computation
The RSVP-TE EXCLUDE_ROUTE Object (XRO) [RFC4874] can be used. Details
are as follows. It should be noted that the PRIMARY_PATH_ROUTE Object
defined in [RFC4872] for end-to-end protection is not intended as a
path exclusion mechanism.
Assume that the working LSP is established as described in [inter-
domain-pd]. Also, assume that the route of the working LSP (full
route) is available at the head-end through the RRO.
o Path computation aspect
When performing path computation (ERO expansion) for the recovery
LSP as it crosses each domain boundary a path is selected that
avoids the nodes/links/SRLGs used by the working path so that a
diverse path is obtained. When path computation is performed by a
PCE on behalf of each domain boundary LSR, the resources to avoid
can be communicated to a PCE using the XRO extension [PCEP-XRO] to
the PCE Protocol (PCEP) [PCEP].
o Signaling aspect
In order that the computation noted above can be performed, each
computation point must be aware of the path of the working LSP.
This information can be supplied in the XRO included in the Path
message for recovery LSP. The XRO lists nodes, links and SRLGs that
must be avoided by the LSP being signaled, and its contents are
copied from the RRO of the working LSP.
This scheme cannot guarantee to establish diverse LSPs (even if they
could exist) because the first LSP is established without
consideration of the need for a diverse recovery LSP. Crankback
[RFC4920] may be used in combination with this scheme in order to
improve the possibility of successful diverse LSP setup. Furthermore,
re-optimization of the working LSP and the recovery LSP may be used
to achieve fully diverse paths.
Note that even if a solution is found, the degree of optimality of
the solution (i.e., of the set of diverse TE LSPs) might not be
maximal.
4.3.2 Simultaneous Path Computation
o Path computation aspect
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When signaling the working LSP, the path of not only the working
LSP, but also the recovery LSP is computed. For example, in
Figure 1, when node D receives a Path message for the working LSP
between nodes A and L, node D expands the ERO to reach domain#3. At
the same time, node D computes a path for the recovery LSP across
the same domain from node F to reach domain#3. The entry boundary
node for the recovery LSP (node F) needs to be known by the entry
boundary node for the working LSP (node D). In this example the
path for the working LSP may be computed by node D as D-E-domain#3,
and the path for recovery LSP as F-G-domain#3.
o Signaling aspect
Two signaling features are needed in order to make this scheme
work.
- A mechanism is needed to signal, during working LSP setup, the
entry boundary node to be used by the recovery LSP. This
mechanism may grow in complexity as the length of the chain of
domains grows, and as the interconnectivity of the domains
becomes more complex. But it may be perfectly feasible in simpler
topologies.
- There must be a mechanism to force the recovery LSP to follow the
route computed above. One way to realize this is to have a
specific object (with the same format as the ERO) to collect the
route of the recovery LSP in the Path message for the working LSP
and to return is to the head-end in the Resv message. When
signaling the recovery LSP, the content of the ERO is copied from
this object.
Protocol mechanisms to achieve these signaling features are not
currently available. The definition of protocol extensions is
beyond the scope of this document.
This scheme also cannot guarantee to establish diverse LSPs (even if
they could exist) if there are more than two domain boundary nodes
out of each domain. Crankback [RFC4920] may also be used in
combination with this scheme to improve the chances of success.
Note that even if a solution is found, the degree of optimality of
the solution (i.e., of the set of diverse TE LSPs) might not be
maximal.
4.4 Inter-domain Collaborative Path Computation
Section 3.4 of [RFC4726] describes this approach, and [brpc] provides
detail of Backward Recursive Path Computation which is a
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collaborative path computation technique. Path computation is
performed for each of the working and recovery LSPs by the use of
inter-PCE communication before each LSP is signaled.
There are two specific schemes for establishing diverse LSPs using
this scheme, which are described in Sections 4.4.1 and 4.4.2.
4.4.1 Sequential Path Computation
Route exclusion using the XRO [PCEP-XRO] can be requested in PCEP
[PCEP], and this can be used to compute the path of a recovery LSP
after the path of the working LSP has been determined. Details are as
follows.
The working LSP is computed using a collaborative PCE approach such
as that described in [brpc], and the LSP may be immediately
established. Assume that the path of the working LSP (full route) is
available from the computation request or from the RRO.
o Path computation aspect
When requesting path computation for the recovery LSP, an XRO is
included in the PCEP path computation request message (see
[PCEP-XRO]). The content of the XRO is copied from the RRO of the
working LSP. Alternatively, the content of the XRO is copied from
the ERO of the path computation reply for the working LSP. The
latter case is useful when the working LSP is not established at
the time of the path computation request for the recovery LSP. The
computation request specifies that the full route must be returned.
Per-domain PCEs may need to be invoked by the first PCE that is
consulted in order to collaboratively determine the correct path
for the recovery LSP (just as described in [RFC4655] and [RFC4726]
for the computation of a single inter-domain LSP by collaborating
PCEs), and these PCEs exchange the XRO information to ensure that
the computed path is diverse from the working LSP.
o Signaling aspect
The recovery LSP is signaled by including an ERO whose content is
copied from the result returned by the PCE.
This scheme cannot guarantee to establish diverse LSPs (even if they
exist) because the working LSP may block the recovery LSP. In such a
scenario, re-optimization of the working and recovery LSPs may be
used to achieve fully diverse paths.
4.4.2 Simultaneous Path Computation
o Path computation aspect
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The PCEP SVEC Object [PCEP] allows diverse path computation to be
requested. It would be possible to extend BRPC [brpc] to compute
diverse paths through the definition of a specific process. Such
extensions are beyond the scope of this document.
o Signaling aspect
In this scheme the PCE returns the full routes for the working and
recovery LSPs and they are signaled accordingly.
This scheme can guarantee to establish diverse LSPs (if they exist),
assuming domain level routing is given as described in Section 2.
Furthermore, the computed set of TE LSPs can be guaranteed to be
optimal with respect to some objective functions.
5. Diverse LSP Setup Schemes with Confidentiality
In the context of this section, the term confidentiality applies to
the protection of information about the topology and resources
present within one domain from visibility by people or applications
outside that domain. This includes, but is not limited to, recording
of LSP routes, in addition to advertisements of TE information. The
confidentiality does not apply to the protection of user traffic.
Diverse LSP setup schemes with confidentiality are similar to ones
without confidentiality. However, several additional mechanisms are
needed to preserve confidentiality. Examples of such mechanisms are:
- Path key: Provide each per-domain segment of the path in advance to
the domain boundary nodes or to the PCE that computed the path for
a limited period of time (temporary caching) and identify it in the
signaled ERO using a path key. When path computation is done by
PCE, the identify of the PCE containing state for the path may be
required as well (e.g., PCE I-D). The path key is provided by the
PCE to the head-end for inclusion in the ERO [conf-segment].
- LSP segment: Pre-establish each per-domain segments of the path
using hierarchical LSPs [RFC4206] or LSP stitching segments
[LSP-stitch] and signal the end-to-end LSP to use these per-domain
LSPs. This scheme might need additional identifiers (such as LSP
IDs) in the Path message so that the domain boundary node can
identify which LSP segment or tunnel to use for the end-to-end LSP.
Furthermore, this scheme may require additional communication to
pre-establish the LSP segments.
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These techniques may be directly applied, or may be applied with
extensions, depending on specific diverse LSP setup schemes described
below.
Note that in the schemes below, the paths of the working and recovery
LSPs are not impacted by the confidentiality requirements.
5.1 Management Configuration
It is not possible to obtain or specify the full explicit route for
either LSP at the head-end due to confidentiality restrictions.
Therefore, this information cannot be provided to signaling for LSP
setup.
Confidentiality does not prevent the full computation of inter-domain
paths and signaling of sufficient information to distinguish the
paths. However the path information for each domain must be provided
in a way that does not have meaning to other domains. Example
mechanisms to preserve confidentiality as described above, include:
- Path key
- LSP segment
5.2 Head-end Path Computation (with multi-domain visibility)
This scheme is not applicable since multi-domain visibility violates
confidentiality.
5.3 Per-Domain Path Computation
5.3.1 Sequential Path Computation
Assume the working LSP is established as described in [inter-domain-
pd].
It is not possible to obtain the route of the working LSP from the
RRO at the head-end due to confidentiality restrictions. In order to
provide the path of the working LSP through each domain to the
computation point responsible for computing the path of the
protection LSP through each domain additional mechanisms are needed.
Examples of such mechanisms are:
- Information identifying the working LSP is included in the Path
message for the recovery LSP, and the domain boundary node consults
the entity which computed the path of the working LSP (i.e., domain
boundary node or PCE), so that the diverse path can be computed.
When the entity which computed the path of the working LSP is the
PCE, PCE needs to be temporarily stateful. An example of such
information is the Association Object [RFC4872].
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5.3.2 Simultaneous Path Computation
In this scheme the intention is to compute the path of the recovery
LSP at the same time as the working LSP. In order to force the
recovery LSP to follow the computed path as well as to preserve
confidentiality, additional mechanisms are needed to communicate the
computed recovery path from the path of the working LSP (where it was
computed) to the recovery LSP. Examples of such mechanisms, that must
continue to preserve confidentiality, are as follows.
- LSP segment: As described before. The LSP segment for the recovery
LSP is established domain-by-domain as the working LSP setup
progresses. How to initiate such LSP segments for the recovery LSP
is beyond the scope of this document.
- Alternatively, information identifying the working LSP is included
in the Path message for the recovery LSP, and the domain boundary
node consults the entity which computed the path of the recovery
LSP (i.e., domain boundary node or PCE), so as to obtain the path
of the recovery LSP. This requires that the entity which computed
the path of the recovery LSP is temporarily stateful. An example of
such information is the Association Object [RFC4872]. Detailed
protocol specifications are beyond the scope of this document.
5.4 Inter-domain Collaborative Path Computation
5.4.1 Sequential Path Computation
Route exclusion using the XRO [PCEP-XRO] in combination with the path
key [conf-segment] can be requested in PCEP [PCEP] and this can be
used to compute the path of a recovery LSP after the path of the
working LSP has been determined. Details are as follows.
The working LSP is computed as described in [brpc] with the help of
path key [conf-segment], and may be immediately established. It is
not possible to obtain the RRO of the working LSP (full route) at the
head-end due to confidentiality restrictions.
o Path computation aspect
This is similar to the case without confidentiality (Section
4.4.1), but in order to preserve confidentiality, additional
mechanisms are needed.
In the PCEP path computation request for the recovery LSP, an XRO
is included. The content of the XRO is copied from the ERO of the
path computation reply for the working LSP, which is given in the
form of strict hops for the local domain, domain boundaries or
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domain identifiers, and path keys. When a PCE receives XRO
containing one or more path keys, it needs to retrieve the original
content and perform path computation for the recovery LSP excluding
certain nodes/links/SRLGs. It is likely that the content (i.e.,
expansion) of the path key cannot be directly retrieved by a PCE in
one domain from a PCE in another domain since that act would
violate the intended confidentiality. Thus, path key expansion and
the computation of a path across a domain must both be performed
within that domain.
o Signaling aspect
The full route for the recovery LSP can not be returned to the
head-end by PCE because it cannot be collected from the other PCEs
owing to confidentiality restrictions. In order to force the
recovery LSP to follow the path computed above, additional
mechanisms are needed. Examples of such mechanisms are as described
before:
- Path key
- LSP segment
5.4.2 Simultaneous Path Computation
As described in Section 4.4.2, diverse path computation can be
requested by PCEP SVEC Object [PCEP], and [brpc] can be extended for
inter-domain diverse path computation. However, it is not possible
for PCE to return the full route of the working LSP and recovery LSP
to the head-end due to confidentiality. In order to force the working
and recovery LSPs to follow the paths computed, additional mechanisms
are needed. Examples of such mechanisms are as described before:
- Path key: Use this for the working and recovery LSPs.
Note that the LSP segment approach in Section 5 may not be applicable
here because a path cannot be determined until BRPC procedures are
completed.
6. Network Topology Specific Considerations
In some specific network topologies, diverse LSP setup schemes
mentioned above could be drastically simplified.
For example, assume there are only three domains connected linearly,
and the first and the last domain contain only a single node. Assume
that we need to establish diverse LSPs from the node in the first
domain to the node in the last domain. In such a scenario, no BRPC
procedures are needed, because there is no need for path computation
in the first and last domains.
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7. Addressing Considerations
All of the above-mentioned schemes are applicable when a single
address space is used across all domains.
However, there could be cases where private addresses are used within
some of the domains. This case is similar to the one with
confidentiality, since the ERO and RRO are meaningless even if they
are available. This document does not exclude other schemes, but
details are beyond the scope of this document.
8. Note on SRLG Diversity
The above-mentioned schemes are applicable when the nodes and links
in different domains belong to different SRLGs.
However, there could be cases where the nodes and links in different
domains belong to the same SRLG. That is, where SRLGs span domain
boundaries. In such cases, in order to establish SRLG diverse LSPs,
several considerations are needed, such as:
- Record of the SRLGs used by the working LSP.
- Indication of a set of SRLGs to exclude in the computation of the
recovery LSP's path.
Furthermore, SRLG IDs may be assigned independently in each domain,
and might not have global meaning. In such a scenario, some mapping
functions are necessary, similar to the mapping of other TE
parameters mentioned in Section 1.2.
9. Security Considerations
This document does not require additional security considerations
mentioned in [RFC4726], which is the basis of this document. That is,
LSP path computation and setup across domain boundaries is
necessarily a security concern and will be subject to operational
policies. In particular, the trust model across domain boundaries for
computation and signaling must be carefully considered since LSP
setup (whether successful or not) can consume domain network data
resources (bandwidth), and signaling or computation requests can
consume network processing resources (CPU and control channel
bandwidth).
RSVP-TE [RFC3209] and PCEP [PCEP] include policy and authentication
capabilities, and these should be seriously considered in all inter-
domain deployments.
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More discussion on security considerations in MPLG/GMPLS networks is
found in a specific document [security-fw].
10. References
10.1 Normative References
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T.,
Srinivasan, V., and G. Swallow, "RSVP-TE:
Extensions to RSVP for LSP Tunnels", RFC 3209,
December 2001.
[RFC4726] Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A
Framework for Inter-Domain MPLS Traffic
Engineering", RFC 4726, November 2006.
[RFC4427] Mannie, E., Ed. and D. Papadimitriou, Ed.,
"Recovery (Protection and Restoration)
Terminology for Generalized Multi-Protocol Label
Switching (GMPLS)", RFC 4427, March 2006.
10.2 Informative References
[RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Y.
Rekhter, "Generalized Multiprotocol Label
Switching (GMPLS) User-Network Interface (UNI):
Resource ReserVation Protocol-Traffic Engineering
(RSVP-TE) Support for the Overlay Model",
RFC 4208, October 2005.
[RFC4847] Takeda, T., Ed., "Framework and Requirements for
Layer 1 Virtual Private Networks", RFC 4847,
April 2007.
[RFC4655] Farrel, A., Vasseur, JP., and J. Ash, "Path
Computation Element (PCE) Architecture",
RFC 4655, August 2006.
[RFC4872] Lang, J., Rekhter, Y., and Papadimitriou, D.
(Eds.), "RSVP-TE Extensions in support of End-to-
End Generalized Multi-Protocol Label Switching
(GMPLS)-based Recovery", RFC 4872, May 2007.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels",
RFC 4090, May 2005.
[RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and
A. Farrel, "GMPLS Based Segment Recovery",
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RFC 4873, May 2007.
[RFC4105] Le Roux, J.-L. , Vasseur, J.-P., and J. Boyle,
"Requirements for Inter-Area MPLS Traffic
Engineering", RFC 4105, June 2005.
[RFC4216] Zhang, R., and Vasseur, J.-P., "MPLS Inter-
Autonomous System (AS) Traffic Engineering (TE)
Requirements", RFC 4216, November 2005
[inter-domain-rsvp] Farrel, A., Ed., "Inter domain Multiprotocol
Label Switching (MPLS) and Generalized MPLS
(GMPLS) Traffic Engineering - RSVP-TE
extensions", draft-ietf-ccamp-inter-domain-rsvp-
te, work in progress.
[RFC4874] Lee, C.Y., Farrel, A., and De Cnodder, S.,
"Exclude Routes - Extension to Resource
ReserVation Protocol-Traffic Engineering
(RSVP-TE)", RFC 4874, April 2007.
[inter-domain-pd] Vasseur JP., Ed., and Ayyangar A., Ed., "A Per-
domain path computation method for establishing
Inter-domain Traffic Engineering (TE) Label
Switched Paths (LSPs)", draft-ietf-ccamp-inter-
domain-pd-path-comp, work in progress.
[brpc] Vasseur, JP., Ed., "A Backward Recursive
PCE-based Computation (BRPC) procedure to compute
shortest inter-domain Traffic Engineering Label
Switched Paths", draft-ietf-pce-brpc, work in
progress.
[PCEP-XRO] Oki, E., and A. Farrel, "Extensions to the Path
Computation Element Communication Protocol (PCEP)
for Route Exclusions", draft-ietf-pce-pcep-xro,
work in progress.
[PCEP] Vasseur, JP., Ed., and Le Roux, JL., Ed., "Path
Computation Element (PCE) communication Protocol
(PCEP)", draft-ietf-pce-pcep, work in progress.
[conf-segment] Bradford, R., Ed., "Preserving Topology
Confidentiality in Inter-Domain Path Computation
using a key based mechanism ", draft-ietf-pce-
path-key, work in progress.
[RFC4920] Farrel, A., Ed., "Crankback Signaling Extensions
for MPLS and GMPLS RSVP-TE ", RFC 4920,
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July 2007.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched
Paths (LSP) Hierarchy with Generalized Multi-
Protocol Label Switching (GMPLS) Traffic
Engineering (TE)", RFC 4206, October 2005.
[LSP-stitch] Ayyangar, A., Kompella, K., Vasseur, JP., and
A. Farrel, "Label Switched Path Stitching with
Generalized Multiprotocol Label Switching Traffic
Engineering (GMPLS TE)", draft-ietf-ccamp-lsp-
stitching, work in progress.
[security-fw] Fang, L., " Security Framework for MPLS and GMPLS
Networks", draft-fang-mpls-gmpls-security-
Framework, work in progress.
11. Acknowledgments
Authors would like to thank Eiji Oki, Ichiro Inoue, Kazuhiro
Fujihara, Dimitri Papadimitriou and Meral Shirazipour for valuable
comments.
12. Authors' Addresses
Tomonori Takeda
NTT Network Service Systems Laboratories, NTT Corporation
3-9-11, Midori-Cho
Musashino-Shi, Tokyo 180-8585 Japan
Email : takeda.tomonori@lab.ntt.co.jp
Yuichi Ikejiri
NTT Communications Corporation
Tokyo Opera City Tower 3-20-2 Nishi Shinjuku, Shinjuku-ku
Tokyo 163-1421, Japan
Email: y.ikejiri@ntt.com
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
Jean-Philippe Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: jpv@cisco.com
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T.Takeda, et al. Expires March 2008 [Page 23]
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