Internet DRAFT - draft-ietf-teas-interconnected-te-info-exchange
draft-ietf-teas-interconnected-te-info-exchange
Network Working Group A. Farrel (Ed.)
Internet-Draft J. Drake
Intended status: Best Current Practice Juniper Networks
Expires: November 21, 2016
N. Bitar
Nokia
G. Swallow
Cisco Systems, Inc.
D. Ceccarelli
Ericsson
X. Zhang
Huawei
May 21, 2016
Problem Statement and Architecture for Information Exchange
Between Interconnected Traffic Engineered Networks
draft-ietf-teas-interconnected-te-info-exchange-07.txt
Abstract
In Traffic Engineered (TE) systems, it is sometimes desirable to
establish an end-to-end TE path with a set of constraints (such as
bandwidth) across one or more network from a source to a destination.
TE information is the data relating to nodes and TE links that is
used in the process of selecting a TE path. TE information is
usually only available within a network. We call such a zone of
visibility of TE information a domain. An example of a domain may be
an IGP area or an Autonomous System.
In order to determine the potential to establish a TE path through a
series of connected networks, it is necessary to have available a
certain amount of TE information about each network. This need not
be the full set of TE information available within each network, but
does need to express the potential of providing TE connectivity. This
subset of TE information is called TE reachability information.
This document sets out the problem statement for the exchange of TE
information between interconnected TE networks in support of end-to-
end TE path establishment and describes the best current practice
architecture to meet this problem statement. For reasons that are
explained in the document, this work is limited to simple TE
constraints and information that determine TE reachability.
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction .................................................. 5
1.1. Terminology ................................................ 6
1.1.1. TE Paths and TE Connections ............................... 6
1.1.2. TE Metrics and TE Attributes .............................. 6
1.1.3. TE Reachability ........................................... 7
1.1.4. Domain .................................................... 7
1.1.5. Server Network ............................................ 7
1.1.6. Client Network ............................................ 7
1.1.7. Aggregation ............................................... 7
1.1.8. Abstraction ............................................... 8
1.1.9. Abstract Link ............................................. 8
1.1.10. Abstract Node or Virtual Node ............................ 8
1.1.11. Abstraction Layer Network ................................ 9
2. Overview of Use Cases ......................................... 9
2.1. Peer Networks ............................................... 9
2.2. Client-Server Networks ...................................... 11
2.3. Dual-Homing ................................................. 13
2.4. Requesting Connectivity ..................................... 14
2.4.1. Discovering Server Network Information .................... 16
3. Problem Statement ............................................. 16
3.1. Policy and Filters .......................................... 17
3.2. Confidentiality ............................................. 17
3.3. Information Overload ........................................ 18
3.4. Issues of Information Churn ................................. 18
3.5. Issues of Aggregation ....................................... 19
4. Architecture .................................................. 20
4.1. TE Reachability ............................................. 20
4.2. Abstraction not Aggregation ................................. 21
4.2.1. Abstract Links ............................................ 22
4.2.2. The Abstraction Layer Network ............................. 22
4.2.3. Abstraction in Client-Server Networks...................... 25
4.2.4. Abstraction in Peer Networks .............................. 30
4.3. Considerations for Dynamic Abstraction ...................... 32
4.4. Requirements for Advertising Links and Nodes ................ 33
4.5. Addressing Considerations ................................... 34
5. Building on Existing Protocols ................................ 34
5.1. BGP-LS ...................................................... 35
5.2. IGPs ........................................................ 35
5.3. RSVP-TE ..................................................... 35
5.4. Notes on a Solution ......................................... 35
6. Application of the Architecture to Optical Domains and Networks 37
7. Application of the Architecture to the User-to-Network Interface
41
8. Application of the Architecture to L3VPN Multi-AS Environments 43
9. Scoping Future Work ........................................... 44
9.1. Not Solving the Internet .................................... 44
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9.2. Working With "Related" Domains .............................. 44
9.3. Not Finding Optimal Paths in All Situations ................. 44
9.4. Sanity and Scaling .......................................... 44
10. Manageability Considerations ................................. 45
10.1. Managing the Abstraction Layer Network ..................... 45
10.2. Managing Interactions of Client and Abstraction Layer Networks
46
10.3. Managing Interactions of Abstraction Layer and Server Networks
46
11. IANA Considerations .......................................... 47
12. Security Considerations ...................................... 47
13. Acknowledgements ............................................. 48
14. References ................................................... 49
14.1. Informative References ..................................... 49
Authors' Addresses ............................................... 52
Contributors ..................................................... 53
A. Existing Work ................................................. 55
A.1. Per-Domain Path Computation ................................. 55
A.2. Crankback ................................................... 55
A.3. Path Computation Element .................................... 56
A.4. GMPLS UNI and Overlay Networks .............................. 58
A.5. Layer One VPN ............................................... 58
A.6. Policy and Link Advertisement ............................... 59
B. Additional Features ........................................... 60
B.1. Macro Shared Risk Link Groups ............................... 60
B.2. Mutual Exclusivity .......................................... 61
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1. Introduction
Traffic Engineered (TE) systems such as MPLS-TE [RFC2702] and GMPLS
[RFC3945] offer a way to establish paths through a network in a
controlled way that reserves network resources on specified links.
TE paths are computed by examining the Traffic Engineering Database
(TED) and selecting a sequence of links and nodes that are capable of
meeting the requirements of the path to be established. The TED is
constructed from information distributed by the IGP running in the
network, for example OSPF-TE [RFC3630] or ISIS-TE [RFC5305].
It is sometimes desirable to establish an end-to-end TE path that
crosses more than one network or administrative domain as described
in [RFC4105] and [RFC4216]. In these cases, the availability of TE
information is usually limited to within each network. Such networks
are often referred to as Domains [RFC4726] and we adopt that
definition in this document: viz.
For the purposes of this document, 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.
In order to determine the potential to establish a TE path through a
series of connected domains and to choose the appropriate domain
connection points through which to route a path, it is necessary to
have available a certain amount of TE information about each domain.
This need not be the full set of TE information available within each
domain, but does need to express the potential of providing TE
connectivity. This subset of TE information is called TE
reachability information. The TE reachability information can be
exchanged between domains based on the information gathered from the
local routing protocol, filtered by configured policy, or statically
configured.
This document sets out the problem statement for the exchange of TE
information between interconnected TE networks in support of end-to-
end TE path establishment and describes the best current practice
architecture to meet this problem statement. The scope of this
document is limited to the simple TE constraints and information
(such as TE metrics, hop count, bandwidth, delay, shared risk)
necessary to determine TE reachability: discussion of multiple
additional constraints that might qualify the reachability can
significantly complicate aggregation of information and the stability
of the mechanism used to present potential connectivity as is
explained in the body of this document.
An Appendix to this document summarizes relevant existing work that
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is used to route TE paths across multiple domains.
1.1. Terminology
This section introduces some key terms that need to be understood to
arrive at a common understanding of the problem space. Some of the
terms are defined in more detail in the sections that follow (in
which case forward pointers are provided) and some terms are taken
from definitions that already exist in other RFCs (in which case
references are given, but no apology is made for repeating or
summarizing the definitions here).
1.1.1. TE Paths and TE Connections
A TE connection is a Label Switched Path (LSP) through an MPLS-TE or
GMPLS network that directs traffic along a particular path (the TE
path) in order to provide a specific service such as bandwidth
guarantee, separation of traffic, or resilience between a well-known
pair of end points.
1.1.2. TE Metrics and TE Attributes
TE metrics and TE attributes are terms applied to parameters of links
(and possibly nodes) in a network that is traversed by TE
connections. The TE metrics and TE attributes are used by path
computation algorithms to select the TE paths that the TE connections
traverse. A TE metric is a quantifiable value (including measured
characteristics) describing some property of a link or node that can
be used as part of TE routing or planning, while a TE attribute is a
wider term (i.e., including the concept of a TE metric) that refers
to any property or characteristic of a link or node that can be used
as part of TE routing or planning. Thus, the delay introduced by
transmission of a packet on a link is an example of a TE metric while
the geographic location of a router is an example of a more general
attribute.
Provisioning a TE connection through a network may result in dynamic
changes to the TE metrics and TE attributes of the links and nodes in
the network.
These terms are also sometimes used to describe the end-to-end
characteristics of a TE connection and can be derived according to a
formula from the TE metrics and TE attributes of the links and nodes
that the TE connection traverses. Thus, for example, the end-to-end
delay for a TE connection is usually considered to be the sum of the
delay on each link that the connection traverses.
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1.1.3. TE Reachability
In an IP network, reachability is the ability to deliver a packet to
a specific address or prefix. That is, the existence of an IP path
to that address or prefix. TE reachability is the ability to reach a
specific address along a TE path. More specifically, it is the
ability to establish a TE connection in an MPLS-TE or GMPLS sense.
Thus we talk about TE reachability as the potential of providing TE
connectivity.
TE reachability may be unqualified (there is a TE path, but no
information about available resources or other constraints is
supplied) which is helpful especially in determining a path to a
destination that lies in an unknown domain, or may be qualified by TE
attributes and TE metrics such as hop count, available bandwidth,
delay, shared risk, etc.
1.1.4. Domain
As defined in [RFC4726], a domain is any collection of network
elements within a common sphere of address management or path
computational responsibility. Examples of such domains include
Interior Gateway Protocol (IGP) areas and Autonomous Systems (ASes).
1.1.5. Server Network
A Server Network is a network that provides connectivity for another
network (the Client Network) in a client-server relationship. A
Server Network is sometimes referred to as an underlay network.
1.1.6. Client Network
A Client Network is a network that uses the connectivity provided by
a Server Network. A Client Network is sometimes referred to as an
overlay network.
1.1.7. Aggregation
The concept of aggregation is discussed in Section 3.5. In
aggregation, multiple network resources from a domain are represented
outside the domain as a single entity. Thus multiple links and nodes
forming a TE connection may be represented as a single link, or a
collection of nodes and links (perhaps the whole domain) may be
represented as a single node with its attachment links.
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1.1.8. Abstraction
Section 4.2 introduces the concept of abstraction and distinguishes
it from aggregation. Abstraction may be viewed as "policy-based
aggregation" where the policies are applied to overcome the issues
with aggregation as identified in Section 3 of this document.
Abstraction is the process of applying policy to the available TE
information within a domain, to produce selective information that
represents the potential ability to connect across the domain. Thus,
abstraction does not necessarily offer all possible connectivity
options, but presents a general view of potential connectivity
according to the policies that determine how the domain's
administrator wants to allow the domain resources to be used.
1.1.9. Abstract Link
An abstract link is the representation of the characteristics of a
path between two nodes in a domain produced by abstraction. The
abstract link is advertised outside that domain as a TE link for use
in signaling in other domains. Thus, an abstract link represents
the potential to connect between a pair of nodes.
More details of abstract links are provided in Section 4.2.1.
1.1.10. Abstract Node or Virtual Node
An abstract node was defined in [RFC3209] as a group of nodes whose
internal topology is opaque to an ingress node of the LSP. More
generally, an abstract node is the representation as a single node in
a TE topology of some or all of the resources of one or more nodes
and the links that connect them. An abstract node may be advertised
outside the domain as a TE node for use in path computation and
signaling in other domains.
The term virtual node has typically been applied to the aggregation
of a domain (that is, a collection of nodes and links that operate
as a single administrative entity for TE purposes) into a single
entity that is treated as a node for the purposes of end-to-end
traffic engineering. Virtual nodes are often considered a way to
present islands of single vendor equipment in an optical network.
Sections 3.5 and 4.2.2.1 provide more information about the uses
and issues of abstract nodes and virtual nodes.
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1.1.11. Abstraction Layer Network
The abstraction layer network is introduced in Section 4.2.2. It may
be seen as a brokerage layer network between one or more server
network and one or more client network. The abstraction layer
network is the collection of abstract links that provide potential
connectivity across the server networks and on which path computation
can be performed to determine edge-to-edge paths that provide
connectivity as links in the client network.
In the simplest case, the abstraction layer network is just a set of
edge-to-edge connections (i.e., abstract links), but to make the use
of server network resources more flexible, the abstract links might
not all extend from edge to edge, but might offer connectivity
between server network nodes to form a more complex network.
2. Overview of Use Cases
2.1. Peer Networks
The peer network use case can be most simply illustrated by the
example in Figure 1. A TE path is required between the source (Src)
and destination (Dst), that are located in different domains. There
are two points of interconnection between the domains, and selecting
the wrong point of interconnection can lead to a sub-optimal path, or
even fail to make a path available. Note that peer networks are
assumed to have the same technology type: that is, the same
"switching capability" to use the term from GMPLS [RFC3945].
-------------- --------------
| Domain A | x1 | Domain Z |
| ----- +----+ ----- |
| | Src | +----+ | Dst | |
| ----- | x2 | ----- |
-------------- --------------
Figure 1 : Peer Networks
For example, when Domain A attempts to select a path, it may
determine that adequate bandwidth is available from Src through both
interconnection points x1 and x2. It may pick the path through x1
for local policy reasons: perhaps the TE metric is smaller. However,
if there is no connectivity in Domain Z from x1 to Dst, the path
cannot be established. Techniques such as crankback may be used to
alleviate this situation, but do not lead to rapid setup or
guaranteed optimality. Furthermore RSVP signalling creates state in
the network that is immediately removed by the crankback procedure.
Frequent events of such a kind impact scalability in a non-
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deterministic manner. More details of crankback can be found in
Section A.2.
There are countless more complicated examples of the problem of peer
networks. Figure 2 shows the case where there is a simple mesh of
domains. Clearly, to find a TE path from Src to Dst, Domain A must
not select a path leaving through interconnect x1 since Domain B has
no connectivity to Domain Z. Furthermore, in deciding whether to
select interconnection x2 (through Domain C) or interconnection x3
though Domain D, Domain A must be sensitive to the TE connectivity
available through each of Domains C and D, as well the TE
connectivity from each of interconnections x4 and x5 to Dst within
Domain Z. The problem may be further complicated when the source
domain does not know in which domain the destination node is located,
since the choice of a domain path clearly depends on the knowledge of
the destination domain: this issue is obviously mitigated in IP
networks by inter-domain routing [RFC4271].
Of course, many network interconnection scenarios are going to be a
combination of the situations expressed in these two examples. There
may be a mesh of domains, and the domains may have multiple points of
interconnection.
--------------
| Domain B |
| |
| |
/--------------
/
/x1
--------------/ --------------
| Domain A | | Domain Z |
| | -------------- | |
| ----- | x2| Domain C | x4| ----- |
| | Src | +---+ +---+ | Dst | |
| ----- | | | | ----- |
| | -------------- | |
--------------\ /--------------
\x3 /
\ /
\ /x5
\--------------/
| Domain D |
| |
| |
--------------
Figure 2 : Peer Networks in a Mesh
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2.2. Client-Server Networks
Two major classes of use case relate to the client-server
relationship between networks. These use cases have sometimes been
referred to as overlay networks. In both cases, the client and
server network may have the same switching capability, or may be
built from nodes and links that have different technology types in
the client and server networks.
The first group of use cases, shown in Figure 3, occurs when domains
belonging to one network are connected by a domain belonging to
another network. In this scenario, once connectivity is formed
across the lower layer network, the domains of the upper layer
network can be merged into a single domain by running IGP adjacencies
and by treating the server network layer connectivity as links in the
higher layer network. The TE relationship between the domains
(higher and lower layers) in this case is reduced to determining what
server network connectivity to establish, how to trigger it, how to
route it in the server network, and what resources and capacity to
assign within the server network layer. As the demands in the higher
layer (client) network vary, the connectivity in the server network
may need to be modified. Section 2.4 explains in a little more
detail how connectivity may be requested.
---------------- ----------------
| Client Network | | Client Network |
| Domain A | | Domain B |
| | | |
| ----- | | ----- |
| | Src | | | | Dst | |
| ----- | | ----- |
| | | |
----------------\ /----------------
\x1 x2/
\ /
\ /
\----------------/
| Server Network |
| Domain |
| |
----------------
Figure 3 : Client-Server Networks
The second class of use case of client-server networking is for
Virtual Private Networks (VPNs). In this case, as opposed to the
former one, it is assumed that the client network has a different
address space than that of the server network where non-overlapping
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IP addresses between the client and the server networks cannot be
guaranteed. A simple example is shown in Figure 4. The VPN sites
comprise a set of domains that are interconnected over a core domain,
the provider network, that is the server network in our model.
Note that in the use cases shown in Figures 3 and 4 the client
network domains may (and, in fact, probably do) operate as a single
connected network.
-------------- --------------
| Domain A | | Domain Z |
| (VPN site) | | (VPN site) |
| | | |
| ----- | | ----- |
| | Src | | | | Dst | |
| ----- | | ----- |
| | | |
--------------\ /--------------
\x1 x2/
\ /
\ /
\---------------/
| Core Domain |
| |
| |
/---------------\
/ \
/ \
/x3 x4\
--------------/ \--------------
| Domain B | | Domain C |
| (VPN site) | | (VPN site) |
| | | |
| | | |
-------------- --------------
Figure 4 : A Virtual Private Network
Both use cases in this section become "more interesting" when
combined with the use case in Section 2.1. That is, when the
connectivity between higher layer domains or VPN sites is provided
by a sequence or mesh of lower layer domains. Figure 5 shows how
this might look in the case of a VPN.
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------------ ------------
| Domain A | | Domain Z |
| (VPN site) | | (VPN site) |
| ----- | | ----- |
| | Src | | | | Dst | |
| ----- | | ----- |
| | | |
------------\ /------------
\x1 x2/
\ /
\ /
\---------- ----------/
| Domain X |x5 | Domain Y |
| (core) +---+ (core) |
| | | |
| +---+ |
| |x6 | |
/---------- ----------\
/ \
/ \
/x3 x4\
------------/ \------------
| Domain B | | Domain C |
| (VPN site) | | (VPN site) |
| | | |
------------ ------------
Figure 5 : A VPN Supported Over Multiple Server Domains
2.3. Dual-Homing
A further complication may be added to the client-server relationship
described in Section 2.2 by considering what happens when a client
network domain is attached to more than one domain in the server
network, or has two points of attachment to a server network domain.
Figure 6 shows an example of this for a VPN.
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------------
| Domain B |
| (VPN site) |
------------ | ----- |
| Domain A | | | Src | |
| (VPN site) | | ----- |
| | | |
------------\ -+--------+-
\x1 | |
\ x2| |x3
\ | | ------------
\--------+- -+-------- | Domain C |
| Domain X | x8 | Domain Y | x4 | (VPN site) |
| (core) +----+ (core) +----+ ----- |
| | | | | | Dst | |
| +----+ +----+ ----- |
| | x9 | | x5 | |
/---------- ----------\ ------------
/ \
/ \
/x6 x7\
------------/ \------------
| Domain D | | Domain E |
| (VPN site) | | (VPN site) |
| | | |
------------ ------------
Figure 6 : Dual-Homing in a Virtual Private Network
2.4. Requesting Connectivity
The relationship between domains can be entirely under the control of
management processes, dynamically triggered by the client network, or
some hybrid of these cases. In the management case, the server
network may be requested to establish a set of LSPs to provide client
network connectivity. In the dynamic case, the client network may
make a request to the server network exerting a range of controls
over the paths selected in the server network. This range extends
from no control (i.e., a simple request for connectivity), through a
set of constraints (such as latency, path protection, etc.), up to
and including full control of the path and resources used in the
server network (i.e., the use of explicit paths with label
subobjects).
There are various models by which a server network can be requested
to set up the connections that support a service provided to the
client network. These requests may come from management systems,
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directly from the client network control plane, or through an
intermediary broker such as the Virtual Network Topology Manager
(VNTM) [RFC5623].
The trigger that causes the request to the server network is also
flexible. It could be that the client network discovers a pressing
need for server network resources (such as the desire to provision an
end-to-end connection in the client network or severe congestion on
a specific path), or it might be that a planning application has
considered how best to optimize traffic in the client network or
how to handle a predicted traffic demand.
In all cases, the relationship between client and server networks is
subject to policy so that server network resources are under the
administrative control of the operator or the server network and are
only used to support a client network in ways that the server network
operator approves.
As just noted, connectivity requests issued to a server network may
include varying degrees of constraint upon the choice of path that
the server network can implement.
o Basic Provisioning is a simple request for connectivity. The only
constraints are the end points of the connection and the capacity
(bandwidth) that the connection will support for the client
network. In the case of some server networks, even the bandwidth
component of a basic provisioning request is superfluous because
the server network has no facility to vary bandwidth, but can offer
connectivity only at a default capacity.
o Basic Provisioning with Optimization is a service request that
indicates one or more metrics that the server network must optimize
in its selection of a path. Metrics may be hop count, path length,
summed TE metric, jitter, delay, or any number of technology-
specific constraints.
o Basic Provisioning with Optimization and Constraints enhances the
optimization process to apply absolute constraints to functions of
the path metrics. For example, a connection may be requested that
optimizes for the shortest path, but in any case requests that the
end-to-end delay be less than a certain value. Equally,
optimization my be expressed in terms of the impact on the network.
For example, a service may be requested in order to leave maximal
flexibility to satisfy future service requests.
o Fate Diversity requests ask for the server network to provide a
path that does not use any network resources (usually links and
nodes) that share fate (i.e., can fail as the result of a single
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event) as the resources used by another connection. This allows
the client network to construct protection services over the server
network, for example by establishing links that are known to be
fate diverse. The connections that have diverse paths need not
share end points.
o Provisioning with Fate Sharing is the exact opposite of Fate
Diversity. In this case two or more connections are requested to
to follow same path in the server network. This may be requested,
for example, to create a bundled or aggregated link in the client
network where each component of the client layer composite link is
required to have the same server network properties (metrics,
delay, etc.) and the same failure characteristics.
o Concurrent Provisioning enables the inter-related connections
requests described in the previous two bullets to be enacted
through a single, compound service request.
o Service Resilience requests the server network to provide
connectivity for which the server network takes responsibility to
recover from faults. The resilience may be achieved through the
use of link-level protection, segment protection, end-to-end
protection, or recovery mechanisms.
2.4.1. Discovering Server Network Information
Although the topology and resource availability information of a
server network may be hidden from the client network, the service
request interface may support features that report details about the
services and potential services that the server network supports.
o Reporting of path details, service parameters, and issues such as
path diversity of LSPs that support deployed services allows the
client network to understand to what extent its requests were
satisfied. This is particularly important when the requests were
made as "best effort".
o A server network may support requests of the form "if I was to ask
you for this service, would you be able to provide it?" That is,
a service request that does everything except actually provision
the service.
3. Problem Statement
The problem statement presented in this section is as much about the
issues that may arise in any solution (and so have to be avoided)
and the features that are desirable within a solution, as it is about
the actual problem to be solved.
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The problem can be stated very simply and with reference to the use
cases presented in the previous section.
A mechanism is required that allows TE-path computation in one
domain to make informed choices about the TE-capabilities and exit
points from the domain when signaling an end-to-end TE path that
will extend across multiple domains.
Thus, the problem is one of information collection and presentation,
not about signaling. Indeed, the existing signaling mechanisms for
TE LSP establishment are likely to prove adequate [RFC4726] with the
possibility of minor extensions. Similarly, TE information may
currently be distributed in a domain by TE extensions to one of the
two IGPs as described in OSPF-TE [RFC3630] and ISIS-TE [RFC5305],
and TE information may be exported from a domain (for example,
northbound) using link state extensions to BGP [RFC7752].
An interesting annex to the problem is how the path is made available
for use. For example, in the case of a client-server network, the
path established in the server network needs to be made available as
a TE link to provide connectivity in the client network.
3.1. Policy and Filters
A solution must be amenable to the application of policy and filters.
That is, the operator of a domain that is sharing information with
another domain must be able to apply controls to what information is
shared. Furthermore, the operator of a domain that has information
shared with it must be able to apply policies and filters to the
received information.
Additionally, the path computation within a domain must be able to
weight the information received from other domains according to local
policy such that the resultant computed path meets the local
operator's needs and policies rather than those of the operators of
other domains.
3.2. Confidentiality
A feature of the policy described in Section 3.1 is that an operator
of a domain may desire to keep confidential the details about its
internal network topology and loading. This information could be
construed as commercially sensitive.
Although it is possible that TE information exchange will take place
only between parties that have significant trust, there are also use
cases (such as the VPN supported over multiple server network domains
described in Section 2.4) where information will be shared between
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domains that have a commercial relationship, but a low level of
trust.
Thus, it must be possible for a domain to limit the information share
to just that which the computing domain needs to know with the
understanding that the less information that is made available the
more likely it is that the result will be a less optimal path and/or
more crankback events.
3.3. Information Overload
One reason that networks are partitioned into separate domains is to
reduce the set of information that any one router has to handle.
This also applies to the volume of information that routing protocols
have to distribute.
Over the years routers have become more sophisticated with greater
processing capabilities and more storage, the control channels on
which routing messages are exchanged have become higher capacity, and
the routing protocols (and their implementations) have become more
robust. Thus, some of the arguments in favor of dividing a network
into domains may have been reduced. Conversely, however, the size of
networks continues to grow dramatically with a consequent increase in
the total amount of routing-related information available.
Additionally, in this case, the problem space spans two or more
networks.
Any solution to the problems voiced in this document must be aware of
the issues of information overload. If the solution was to simply
share all TE information between all domains in the network, the
effect from the point of view of the information load would be to
create one single flat network domain. Thus the solution must
deliver enough information to make the computation practical (i.e.,
to solve the problem), but not so much as to overload the receiving
domain. Furthermore, the solution cannot simply rely on the policies
and filters described in Section 3.1 because such filters might not
always be enabled.
3.4. Issues of Information Churn
As LSPs are set up and torn down, the available TE resources on links
in the network change. In order to reliably compute a TE path
through a network, the computation point must have an up-to-date view
of the available TE resources. However, collecting this information
may result in considerable load on the distribution protocol and
churn in the stored information. In order to deal with this problem
even in a single domain, updates are sent at periodic intervals or
whenever there is a significant change in resources, whichever
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happens first.
Consider, for example, that a TE LSP may traverse ten links in a
network. When the LSP is set up or torn down, the resources
available on each link will change resulting in a new advertisement
of the link's capabilities and capacity. If the arrival rate of new
LSPs is relatively fast, and the hold times relatively short, the
network may be in a constant state of flux. Note that the
problem here is not limited to churn within a single domain, since
the information shared between domains will also be changing.
Furthermore, the information that one domain needs to share with
another may change as the result of LSPs that are contained within or
cross the first domain but which are of no direct relevance to the
domain receiving the TE information.
In packet networks, where the capacity of an LSP is often a small
fraction of the resources available on any link, this issue is
partially addressed by the advertising routers. They can apply a
threshold so that they do not bother to update the advertisement of
available resources on a link if the change is less than a configured
percentage of the total (or alternatively, the remaining) resources.
The updated information in that case will be disseminated based on an
update interval rather than a resource change event.
In non-packet networks, where link resources are physical switching
resources (such as timeslots or wavelengths) the capacity of an LSP
may more frequently be a significant percentage of the available link
resources. Furthermore, in some switching environments, it is
necessary to achieve end-to-end resource continuity (such as using
the same wavelength on the whole length of an LSP), so it is far more
desirable to keep the TE information held at the computation points
up-to-date. Fortunately, non-packet networks tend to be quite a bit
smaller than packet networks, the arrival rates of non-packet LSPs
are much lower, and the hold times considerably longer. Thus the
information churn may be sustainable.
3.5. Issues of Aggregation
One possible solution to the issues raised in other sub-sections of
this section is to aggregate the TE information shared between
domains. Two aggregation mechanisms are often considered:
- Virtual node model. In this view, the domain is aggregated as if
it was a single node (or router / switch). Its links to other
domains are presented as real TE links, but the model assumes that
any LSP entering the virtual node through a link can be routed to
leave the virtual node through any other link (although recent work
on "limited cross-connect switches" may help with this problem
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[RFC7579]).
- Virtual link model. In this model, the domain is reduced to a set
of edge-to-edge TE links. Thus, when computing a path for an LSP
that crosses the domain, a computation point can see which domain
entry points can be connected to which other and with what TE
attributes.
It is of the nature of aggregation that information is removed from
the system. This can cause inaccuracies and failed path computation.
For example, in the virtual node model there might not actually be a
TE path available between a pair of domain entry points, but the
model lacks the sophistication to represent this "limited cross-
connect capability" within the virtual node. On the other hand, in
the virtual link model it may prove very hard to aggregate multiple
link characteristics: for example, there may be one path available
with high bandwidth, and another with low delay, but this does not
mean that the connectivity should be assumed or advertised as having
both high bandwidth and low delay.
The trick to this multidimensional problem, therefore, is to
aggregate in a way that retains as much useful information as
possible while removing the data that is not needed. An important
part of this trick is a clear understanding of what information is
actually needed.
It should also be noted in the context of Section 3.4 that changes in
the information within a domain may have a bearing on what aggregated
data is shared with another domain. Thus, while the data shared is
reduced, the aggregation algorithm (operating on the routers
responsible for sharing information) may be heavily exercised.
4. Architecture
4.1. TE Reachability
As described in Section 1.1, TE reachability is the ability to reach
a specific address along a TE path. The knowledge of TE reachability
enables an end-to-end TE path to be computed.
In a single network, TE reachability is derived from the Traffic
Engineering Database (TED) that is the collection of all TE
information about all TE links in the network. The TED is usually
built from the data exchanged by the IGP, although it can be
supplemented by configuration and inventory details especially in
transport networks.
In multi-network scenarios, TE reachability information can be
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described as "You can get from node X to node Y with the following
TE attributes." For transit cases, nodes X and Y will be edge nodes
of the transit network, but it is also important to consider the
information about the TE connectivity between an edge node and a
specific destination node. TE reachability may be qualified by TE
attributes such as TE metrics, hop count, available bandwidth, delay,
shared risk, etc.
TE reachability information can be exchanged between networks so that
nodes in one network can determine whether they can establish TE
paths across or into another network. Such exchanges are subject to
a range of policies imposed by the advertiser (for security and
administrative control) and by the receiver (for scalability and
stability).
4.2. Abstraction not Aggregation
Aggregation is the process of synthesizing from available
information. Thus, the virtual node and virtual link models
described in Section 3.5 rely on processing the information available
within a network to produce the aggregate representations of links
and nodes that are presented to the consumer. As described in
Section 3, dynamic aggregation is subject to a number of pitfalls.
In order to distinguish the architecture described in this document
from the previous work on aggregation, we use the term "abstraction"
in this document. The process of abstraction is one of applying
policy to the available TE information within a domain, to produce
selective information that represents the potential ability to
connect across the domain.
Abstraction does not offer all possible connectivity options (refer
to Section 3.5), but does present a general view of potential
connectivity. Abstraction may have a dynamic element, but is not
intended to keep pace with the changes in TE attribute availability
within the network.
Thus, when relying on an abstraction to compute an end-to-end path,
the process might not deliver a usable path. That is, there is no
actual guarantee that the abstractions are current or feasible.
While abstraction uses available TE information, it is subject to
policy and management choices. Thus, not all potential connectivity
will be advertised to each client network. The filters may depend on
commercial relationships, the risk of disclosing confidential
information, and concerns about what use is made of the connectivity
that is offered.
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4.2.1. Abstract Links
An abstract link is a measure of the potential to connect a pair of
points with certain TE parameters. That is, it is a path and its
characteristics in the server network. An abstract link represents
the possibility of setting up an LSP, and LSPs may be set up over the
abstract link.
When looking at a network such as that in Figure 7, the link from CN1
to CN4 may be an abstract link. It is easy to advertise it as a link
by abstracting the TE information in the server network subject to
policy.
The path (i.e., the abstract link) represents the possibility of
establishing an LSP from client network edge to client network edge
across the server network. There is not necessarily a one-to-one
relationship between abstract link and LSP because more than one LSP
could be set up over the path.
Since the client network nodes do not have visibility into the server
network, they must rely on abstraction information delivered to them
by the server network. That is, the server network will report on
the potential for connectivity.
4.2.2. The Abstraction Layer Network
Figure 7 introduces the abstraction layer network. This construct
separates the client network resources (nodes C1, C2, C3, and C4, and
the corresponding links), and the server network resources (nodes
CN1, CN2, CN3, and CN4 and the corresponding links). Additionally,
the architecture introduces an intermediary network layer called the
abstraction layer. The abstraction layer contains the client network
edge nodes (C2 and C3), the server network edge nodes (CN1 and CN4),
the client-server links (C2-CN1 and CN4-C3) and the abstract link
CN1-CN4.
The client network is able to operate as normal. Connectivity across
the network can either be found or not found based on links that
appear in the client network TED. If connectivity cannot be found,
end-to-end LSPs cannot be set up. This failure may be reported, but
no dynamic action is taken by the client network.
The server network also operates as normal. LSPs across the server
network between client network edges are set up in response to
management commands or in response to signaling requests.
The abstraction layer consists of the physical links between the
two networks, and also the abstract links. The abstract links are
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created by the server network according to local policy and represent
the potential connectivity that could be created across the server
network and which the server network is willing to make available for
use by the client network. Thus, in this example, the diameter of
the abstraction layer network is only three hops, but an instance of
an IGP could easily be run so that all nodes participating in the
abstraction layer (and in particular the client network edge nodes)
can see the TE connectivity in the layer.
-- -- -- --
|C1|--|C2| |C3|--|C4| Client Network
-- | | | | --
| | | | . . . . . . . . . . .
| | | |
| | | |
| | --- --- | | Abstraction
| |---|CN1|================|CN4|---| | Layer Network
-- | | | | --
| | | | . . . . . . . . . . . . . .
| | | |
| | | |
| | --- --- | | Server Network
| |--|CN2|--|CN3|--| |
--- --- --- ---
Key
--- Direct connection between two nodes
=== Abstract link
Figure 7 : Architecture for Abstraction Layer Network
When the client network needs additional connectivity it can make a
request to the abstraction layer network. For example, the operator
of the client network may want to create a link from C2 to C3. The
abstraction layer can see the potential path C2-CN1-CN4-C3 and can
set up an LSP C2-CN1-CN4-C3 across the server network and make the
LSP available as a link in the client network.
Sections 4.2.3 and 4.2.4 show how this model is used to satisfy the
requirements for connectivity in client-server networks and in peer
networks.
4.2.2.1. Nodes in the Abstraction Layer Network
Figure 7 shows a very simplified network diagram and the reader would
be forgiven for thinking that only client network edge nodes and
server network edge nodes may appear in the abstraction layer
network. But this is not the case: other nodes from the server
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network may be present. This allows the abstraction layer network
to be more complex than a full mesh with access spokes.
Thus, as shown in Figure 8, a transit node in the server network
(here the node is CN3) can be exposed as a node in the abstraction
layer network with abstract links connecting it to other nodes in
the abstraction layer network. Of course, in the network shown in
Figure 8, there is little if any value in exposing CN3, but if it
had other abstract links to other nodes in the abstraction layer
network and/or direct connections to client network nodes, then the
resulting network would be richer.
-- -- -- -- Client
|C1|--|C2| |C3|--|C4| Network
-- | | | | --
| | | | . . . . . . . . .
| | | |
| | | |
| | --- --- --- | | Abstraction
| |--|CN1|========|CN3|========|CN5|--| | Layer Network
-- | | | | | | --
| | | | | | . . . . . . . . . . . .
| | | | | |
| | | | | | Server
| | --- | | --- | | Network
| |--|CN2|-| |-|CN4|--| |
--- --- --- --- ---
Figure 8 : Abstraction Layer Network with Additional Node
It should be noted that the nodes included in the abstraction layer
network in this way are not "abstract nodes" in the sense of a
virtual node described in Section 3.5. While it is the case that
the policy point responsible for advertising server network resources
into the abstraction layer network could choose to advertise abstract
nodes in place of real physical nodes, it is believed that doing so
would introduce significant complexity in terms of:
- Coordination between all of the external interfaces of the abstract
node
- Management of changes in the server network that lead to limited
capabilities to reach (cross-connect) across the Abstract Node. It
may be noted that recent work on limited cross-connect capabilities
such as exist in asymmetrical switches could be used to represent
the limitations in an abstract node [RFC7579], [RFC7580].
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4.2.3. Abstraction in Client-Server Networks
Figure 9 shows the basic architectural concepts for a client-server
network. The client network nodes are C1, C2, CE1, CE2, C3, and C4.
The server (core) network nodes are CN1, CN2, CN3, and CN4. The
interfaces CE1-CN1 and CE2-CN2 are the interfaces between the client
and server networks.
The technologies (switching capabilities) of the client and server
networks may be the same or different. If they are different, the
client network traffic must be tunneled over a server network LSP.
If they are the same, the client network LSP may be routed over the
server network links, tunneled over a server network LSP, or
constructed from the concatenation (stitching) of client network and
server network LSP segments.
: :
Client Network : Server Network : Client Network
: :
-- -- --- --- -- --
|C1|--|C2|--|CE1|................................|CE2|--|C3|--|C4|
-- -- | | --- --- | | -- --
| |===|CN1|================|CN4|===| |
| |---| | | |---| |
--- | | --- --- | | ---
| |--|CN2|--|CN3|--| |
--- --- --- ---
Key
--- Direct connection between two nodes
... CE-to-CE LSP tunnel
=== Potential path across the server network (abstract link)
Figure 9 : Architecture for Client-Server Network
The objective is to be able to support an end-to-end connection,
C1-to-C4, in the client network. This connection may support TE or
normal IP forwarding. To achieve this, CE1 is to be connected to CE2
by a link in the client network. This enables the client network to
view itself as connected and to select an end-to-end path.
As shown in the figure, three abstraction layer links are formed:
CE1-CN1, CN1-CN2, and CN2-CE2. A three-hop LSP is then established
from CE1 to CE2 that can be presented as a link in the client
network.
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The practicalities of how the CE1-CE2 LSP is carried across the
server network LSP may depend on the switching and signaling options
available in the server network. The LSP may be tunneled down the
server network LSP using the mechanisms of a hierarchical LSP
[RFC4206], or the LSP segments CE1-CN1 and CN2-CE2 may be stitched to
the server network LSP as described in [RFC5150].
Section 4.2.2 has already introduced the concept of the abstraction
layer network through an example of a simple layered network. But it
may be helpful to expand on the example using a slightly more complex
network.
Figure 10 shows a multi-layer network comprising client network nodes
(labeled as Cn for n= 0 to 9) and server network nodes (labeled as Sn
for n = 1 to 9).
-- --
|C3|---|C4|
/-- --\
-- -- -- -- --/ \--
|C1|---|C2|---|S1|---|S2|----|S3| |C5|
-- /-- --\ --\ --\ /--
/ \-- \-- \-- --/ --
/ |S4| |S5|----|S6|---|C6|---|C7|
/ /-- --\ /-- /-- --
--/ -- --/ -- \--/ --/
|C8|---|C9|---|S7|---|S8|----|S9|---|C0|
-- -- -- -- -- --
Figure 10 : An example Multi-Layer Network
If the network in Figure 10 is operated as separate client and server
networks then the client network topology will appear as shown in
-- --
|C3|---|C4|
-- --\
-- -- \--
|C1|---|C2| |C5|
-- /-- /--
/ --/ --
/ |C6|---|C7|
/ /-- --
--/ -- --/
|C8|---|C9| |C0|
-- -- --
Figure 11 : Client Network Topology Showing Partitioned Network
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Figure 11. As can be clearly seen, the network is partitioned and
there is no way to set up an LSP from a node on the left hand side
(say C1) to a node on the right hand side (say C7).
For reference, Figure 12 shows the corresponding server network
topology.
-- -- --
|S1|---|S2|----|S3|
--\ --\ --\
\-- \-- \--
|S4| |S5|----|S6|
/-- --\ /--
--/ -- \--/
|S7|---|S8|----|S9|
-- -- --
Figure 12 : Server Network Topology
Operating on the TED for the server network, a management entity or a
software component may apply policy and consider what abstract links
it might offer for use by the client network. To do this it
obviously needs to be aware of the connections between the layers
(there is no point in offering an abstract link S2-S8 since this
could not be of any use in this example).
In our example, after consideration of which LSPs could be set up in
the server network, four abstract links are offered: S1-S3, S3-S6,
S1-S9, and S7-S9. These abstract links are shown as double lines on
the resulting topology of the abstraction layer network in Figure 13.
As can be seen, two of the links must share part of a path (S1-S9
must share with either S1-S3 or with S7-S9). This could be achieved
--
|C3|
/--
-- -- --/
|C2|---|S1|==========|S3|
-- --\\ --\\
\\ \\
\\ \\-- --
\\ |S6|---|C6|
\\ -- --
-- -- \\-- --
|C9|---|S7|=====|S9|---|C0|
-- -- -- --
Figure 13 : Abstraction Layer Network with Abstract Links
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using distinct resources (for example, separate lambdas) where the
paths are common, but it could also be done using resource sharing.
That would mean that when both paths S1-S3 and S7-S9 carry client-
edge to client-edge LSPs the resources on the path S1-S9 are used and
might be depleted to the point that the path is resource constrained
and cannot be used.
The separate IGP instance running in the abstraction layer network
means that this topology is visible at the edge nodes (C2, C3, C6,
C9, and C0) as well as at a PCE if one is present.
Now the client network is able to make requests to the abstraction
layer network to provide connectivity. In our example, it requests
that C2 is connected to C3 and that C2 is connected to C0. This
results in several actions:
1. The management component for the abstraction layer network asks
its PCE to compute the paths necessary to make the connections.
This yields C2-S1-S3-C3 and C2-S1-S9-C0.
2. The management component for the abstraction layer network
instructs C2 to start the signaling process for the new LSPs in
the abstraction layer.
3. C2 signals the LSPs for setup using the explicit routes
C2-S1-S3-C3 and C2-S1-S9-C0.
4. When the signaling messages reach S1 (in our example, both LSPs
traverse S1) the server network may support them by a number of
means including establishing server network LSPs as tunnels
depending on the mismatch of technologies between the client and
server networks. For example, S1-S2-S3 and S1-S2-S5-S9 might be
traversed via an LSP tunnel, using LSPs stitched together, or
simply by routing the client network LSP through the server
network. If server network LSPs are needed to they can be
signaled at this point.
5. Once any server network LSPs that are needed have been
established, S1 can continue to signal the client-edge to client-
edge LSP across the abstraction layer either using the server
network LSPs as tunnels or as stitching segments, or simply
routing through the server network.
6. Finally, once the client-edge to client-edge LSPs have been set
up, the client network can be informed and can start to advertise
the new TE links C2-C3 and C2-C0. The resulting client network
topology is shown in Figure 14.
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-- --
|C3|-|C4|
/-- --\
/ \--
-- --/ |C5|
|C1|---|C2| /--
-- /--\ --/ --
/ \ |C6|---|C7|
/ \ /-- --
/ \--/
--/ -- |C0|
|C8|---|C9| --
-- --
Figure 14 : Connected Client Network with Additional Links
7. Now the client network can compute an end-to-end path from C1 to
C7.
4.2.3.1 A Server with Multiple Clients
A single server network may support multiple client networks. This
is not an uncommon state of affairs for example when the server
network provides connectivity for multiple customers.
In this case, the abstraction provided by the server network may vary
considerably according to the policies and commercial relationships
with each customer. This variance would lead to a separate
abstraction layer network maintained to support each client network.
On the other hand, it may be that multiple clients networks are
subject to the same policies and the abstraction can be identical.
In this case, a single abstraction layer network can support more
than one client.
The choices here are made as an operational issue by the server
network.
4.2.3.2 A Client with Multiple Servers
A single client network may be supported by multiple server networks.
The server networks may provide connectivity between different parts
of the client network or may provide parallel (redundant)
connectivity for the client network.
In this case the abstraction layer network should contain the
abstract links from all server networks so that it can make suitable
computations and create the correct TE links in the client network.
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That is, the relationship between client network and abstraction
layer network should be one-to-one.
4.2.4. Abstraction in Peer Networks
Figure 15 shows the basic architectural concepts for connecting
across peer networks. Nodes from four networks are shown: A1 and A2
come from one network; B1, B2, and B3 from another network; etc. The
interfaces between the networks (sometimes known as External Network-
to-Network Interfaces - ENNIs) are A2-B1, B3-C1, and C3-D1.
The objective is to be able to support an end-to-end connection A1-
to-D2. This connection is for TE connectivity.
As shown in the figure, abstract links that span the transit networks
are used to achieve the required connectivity. These links form the
key building blocks of the end-to-end connectivity. An end-to-end
LSP uses these links as part of its path. If the stitching
capabilities of the networks are homogeneous then the end-to-end LSP
may simply traverse the path defined by the abstract links across the
various peer networks or may utilize stitching of LSP segments that
each traverse a network along the path of an abstract link. If the
network switching technologies support or necessitate the use of LSP
hierarchies, the end-to-end LSP may be tunneled across each network
using hierarchical LSPs that each each traverse a network along the
path of an abstract link.
: : :
Network A : Network B : Network C : Network D
: : :
-- -- -- -- -- -- -- -- -- --
|A1|--|A2|---|B1|--|B2|--|B3|---|C1|--|C2|--|C3|---|D1|--|D2|
-- -- | | -- | | | | -- | | -- --
| |========| | | |========| |
-- -- -- --
Key
--- Direct connection between two nodes
=== Abstract link across transit network
Figure 15 : Architecture for Peering
Peer networks exist in many situations in the Internet. Packet
networks may peer as IGP areas (levels) or as ASes. Transport
networks (such as optical networks) may peer to provide
concatenations of optical paths through single vendor environments
(see Section 6). Figure 16 shows a simple example of three peer
networks (A, B, and C) each comprising a few nodes.
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Network A : Network B : Network C
: :
-- -- -- : -- -- -- : -- --
|A1|---|A2|----|A3|---|B1|---|B2|---|B3|---|C1|---|C2|
-- --\ /-- : -- /--\ -- : -- --
\--/ : / \ :
|A4| : / \ :
--\ : / \ :
-- \-- : --/ \-- : -- --
|A5|---|A6|---|B4|----------|B6|---|C3|---|C4|
-- -- : -- -- : -- --
: :
: :
Figure 16 : A Network Comprising Three Peer Networks
As discussed in Section 2, peered networks do not share visibility of
their topologies or TE capabilities for scaling and confidentiality
reasons. That means, in our example, that computing a path from A1
to C4 can be impossible without the aid of cooperating PCEs or some
form of crankback.
But it is possible to produce abstract links for reachability across
transit peer networks and to create an abstraction layer network.
That network can be enhanced with specific reachability information
if a destination network is partitioned as is the case with Network C
in Figure 16.
Suppose Network B decides to offer three abstract links B1-B3, B4-B3,
and B4-B6. The abstraction layer network could then be constructed
to look like the network in Figure 17.
-- -- -- --
|A3|---|B1|====|B3|----|C1|
-- -- //-- --
//
//
//
-- --// -- --
|A6|---|B4|=====|B6|---|C3|
-- -- -- --
Figure 17 : Abstraction Layer Network for the Peer Network Example
Using a process similar to that described in Section 4.2.3, Network A
can request connectivity to Network C and abstract links can be
advertised that connect the edges of the two networks and that can be
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used to carry LSPs that traverse both networks. Furthermore, if
Network C is partitioned, reachability information can be exchanged
to allow Network A to select the correct abstract link as shown in
Figure 18.
Network A : Network C
:
-- -- -- : -- --
|A1|---|A2|----|A3|=========|C1|.....|C2|
-- --\ /-- : -- --
\--/ :
|A4| :
--\ :
-- \-- : -- --
|A5|---|A6|=========|C3|.....|C4|
-- -- : -- --
Figure 18 : Tunnel Connections to Network C with TE Reachability
Peer networking cases can be made far more complex by dual homing
between network peering nodes (for example, A3 might connect to B1
and B4 in Figure 17) and by the networks themselves being arranged in
a mesh (for example, A6 might connect to B4 and C1 in Figure 17).
These additional complexities can be handled gracefully by the
abstraction layer network model.
Further examples of abstraction in peer networks can be found in
Sections 6 and 8.
4.3. Considerations for Dynamic Abstraction
It is possible to consider a highly dynamic system where the server
network adaptively suggests new abstract links into the abstraction
layer, and where the abstraction layer proactively deploys new
client-edge to client-edge LSPs to provide new links in the client
network. Such fluidity is, however, to be treated with caution
especially in the case of client-server networks of differing
technologies where hierarchical server network LSPs are used because
of the longer turn-up times of connections in some server networks,
because the server networks are likely to be sparsely connected and
expensive physical resources will only be deployed where there is
believed to be a need for them. More significantly, the complex
commercial, policy, and administrative relationships that may exist
between client and server network operators mean that stability is
more likely to be the desired operational practice.
Thus, proposals for fully automated multi-layer networks based on
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this architecture may be regarded as forward-looking topics for
research both in terms of network stability and with regard to
ecomonic impact.
However, some elements of automation should not be discarded. A
server network may automatically apply policy to determine the best
set of abstract links to offer and the most suitable way for the
server network to support them. And a client network may dynamically
observe congestion, lack of connectivity, or predicted changes in
traffic demand, and may use this information to request additional
links from the abstraction layer. And, once policies have been
configured, the whole system should be able to operate autonomous of
operator control (which is not to say that the operator will not have
the option of exerting control at every step in the process).
4.4. Requirements for Advertising Links and Nodes
The abstraction layer network is "just another network layer". The
links and nodes in the network need to be advertised along with their
associated TE information (metrics, bandwidth, etc.) so that the
topology is disseminated and so that routing decisions can be made.
This requires a routing protocol running between the nodes in the
abstraction layer network. Note that this routing information
exchange could be piggy-backed on an existing routing protocol
instance (subject to different switching capabilities applying to the
links in the different networks, or to adequate address space
separation), or use a new instance (or even a new protocol).
Clearly, the information exchanged is only that which has been
created as part of the abstraction function according to policy.
It should be noted that in many cases the abstract represents the
potential for connectivity across the server network but that no such
connectivity exists. In this case we may ponder how the routing
protocol in the abstraction layer will advertise topology information
for and over a link that has no underlying connectivity. In other
words, there must be a communication channel between the abstract
layer nodes so that the routing protocol messages can flow. The
answer is that control plane connectivity already exists in the
server network and on the client-server edge links, and this can be
used to carry the routing protocol messages for the abstraction layer
network. The same consideration applies to the advertisement, in the
client network of the potential connectivity that the abstraction
layer network can provide although it may be more normal to establish
that connectivity before advertising a link in the client network.
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4.5. Addressing Considerations
The network layers in this architecture should be able to operate
with separate address spaces and these may overlap without any
technical issues. That is, one address may mean one thing in the
client network, yet the same address may have a different meaning in
the abstraction layer network or the server network. In other words
there is complete address separation between networks.
However, this will require some care both because human operators may
well become confused, and because mapping between address spaces is
needed at the interfaces between the network layers. That mapping
requires configuration so that, for example, when the server network
announces an abstract link from A to B, the abstraction layer network
must recognize that A and B are server network addresses and must map
them to abstraction layer addresses (say P and Q) before including
the link in its own topology. And similarly, when the abstraction
layer network informs the client network that a new link is available
from S to T, it must map those addresses from its own address space
to that of the client network.
This form of address mapping will become particularly important in
cases where one abstraction layer network is constructed from
connectivity in multiple server networks, or where one abstraction
layer network provides connectivity for multiple client networks.
5. Building on Existing Protocols
This section is non-normative and is not intended to prejudge a
solutions framework or any applicability work. It does, however,
very briefly serve to note the existence of protocols that could be
examined for applicability to serve in realizing the model described
in this document.
The general principle of protocol re-use is preferred over the
invention of new protocols or additional protocol extensions, and it
would be advantageous to make use of an existing protocol that is
commonly implemented on network nodes and is currently deployed, or
to use existing computational elements such as Path Computation
Elements (PCEs). This has many benefits in network stability, time
to deployment, and operator training.
It is recognized, however, that existing protocols are unlikely to be
immediately suitable to this problem space without some protocol
extensions. Extending protocols must be done with care and with
consideration for the stability of existing deployments. In extreme
cases, a new protocol can be preferable to a messy hack of an
existing protocol.
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5.1. BGP-LS
BGP-LS is a set of extensions to BGP described in [RFC7752]. It's
purpose is to announce topology information from one network to a
"north-bound" consumer. Application of BGP-LS to date has focused on
a mechanism to build a TED for a PCE. However, BGP's mechanisms
would also serve well to advertise abstract links from a server
network into the abstraction layer network, or to advertise potential
connectivity from the abstraction layer network to the client
network.
5.2. IGPs
Both OSPF and IS-IS have been extended through a number of RFCs to
advertise TE information. Additionally, both protocols are capable
of running in a multi-instance mode either as ships that pass in the
night (i.e., completely separate instances using different address
spaces) or as dual instances on the same address space. This means
that either IGP could probably be used as the routing protocol in the
abstraction layer network.
5.3. RSVP-TE
RSVP-TE signaling can be used to set up all traffic engineered LSPs
demanded by this model without the need for any protocol extensions.
If necessary, LSP hierarchy [RFC4206] or LSP stitching [RFC5150] can
be used to carry LSPs over the server network, again without needing
any protocol extensions.
Furthermore, the procedures in [RFC6107] allow the dynamic signaling
of the purpose of any LSP that is established. This means that
when an LSP tunnel is set up, the two ends can coordinate into which
routing protocol instance it should be advertised, and can also agree
on the addressing to be said to identify the link that will be
created.
5.4. Notes on a Solution
This section is not intended to be prescriptive or dictate the
protocol solutions that may be used to satisfy the architecture
described in this document, but it does show how the existing
protocols listed in the previous sections can be combined to provide
a solution with only minor modifications.
A server network can be operated using GMPLS routing and signaling
protocols. Using information gathered from the routing protocol, a
TED can be constructed containing resource availability information
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and Shared Risk Link Group (SRLG) details. A policy-based process
can then determine which nodes and abstract links it wishes to
advertise to form the abstract layer network.
The server network can now use BGP-LS to advertise a topology of
links and nodes to form the abstraction layer network. This
information would most likely be advertised from a single point of
control that made all of the abstraction decisions, but the function
could be distributed to multiple server network edge nodes. The
information can be advertised by BGP-LS to multiple points within the
abstraction layer (such as all client network edge nodes) or to a
single controller.
Multiple server networks may advertise information that is used to
construct an abstraction layer network, and one server network may
advertise different information in different instances of BGP-LS to
form different abstraction layer networks. Furthermore, in the case
of one controller constructing multiple abstraction layer networks,
BGP-LS uses the route target mechanism defined in [RFC4364] to
distinguish the different applications (effectively abstraction layer
network VPNs) of the exported information.
Extensions may be made to BGP-LS to allow advertisement of Macro
Shared Risk Link Groups (MSRLGs) per Appendix B, mutually exclusive
links, and to indicate whether the abstract link has been pre-
established or not. Such extensions are valid options, but do not
form a core component of this architecture.
The abstraction layer network may operate under central control or
use a distributed control plane. Since the links and nodes may be a
mix of physical and abstract links, and since the nodes may have
diverse cross-connect capabilities, it is most likely that a GMPLS
routing protocol will be beneficial for collecting and correlating
the routing information and for distributing updates. No special
additional features are needed beyond adding those extra parameters
just described for BGP-LS, but it should be noted that the control
plane of the abstraction layer network must run in an out of band
control network because the data-bearing links might not yet have
been established via connections in the server network.
The abstraction layer network is also able to determine potential
connectivity from client network edge to client network edge. It
will determine which client network links to create according to
policy and subject to requests from the client network, and will
take four steps:
- First it will compute a path for across the abstraction layer
network.
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- Then, if the support of the abstract links requires the use of
server network LSPs for tunneling or stitching, and if those LSPs
are not already established, it will ask the server layer to set
them up.
- Then, it will signal the client-edge to client-edge LSP.
- Finally, the abstraction layer network will inform the client
network of the existence of the new client network link.
This last step can be achieved either by coordination of the end
points of the LSPs that span the abstraction layer (these points are
client network edge nodes) using mechanisms such as those described
in [RFC6107], or using BGP-LS from a central controller.
Once the client network edge nodes are aware of a new link, they will
automatically advertise it using their routing protocol and it will
become available for use by traffic in the client network.
Sections 6, 7, and 8 discuss the applicability of this architecture
to different network types and problem spaces, while Section 9 gives
some advice about scoping future work. Section 9 on manageability
considerations is particularly relevant in the context of this
section because it contains a discussion of the policies and
mechanisms for indicating connectivity and link availability between
network layers in this architecture.
6. Application of the Architecture to Optical Domains and Networks
Many optical networks are arranged as a set of small domains. Each
domain is a cluster of nodes, usually from the same equipment vendor
and with the same properties. The domain may be constructed as a
mesh or a ring, or maybe as an interconnected set of rings.
The network operator seeks to provide end-to-end connectivity across
a network constructed from multiple domains, and so (of course) the
domains are interconnected. In a network under management control
such as through an Operations Support System (OSS), each domain is
under the operational control of a Network Management System (NMS).
In this way, an end-to-end path may be commissioned by the OSS
instructing each NMS, and the NMSes setting up the path fragments
across the domains.
However, in a system that uses a control plane, there is a need for
integration between the domains.
Consider a simple domain, D1, as shown in Figure 19. In this case,
the nodes A through F are arranged in a topological ring. Suppose
that there is a control plane in use in this domain, and that OSPF is
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used as the TE routing protocol.
-----------------
| D1 |
| B---C |
| / \ |
| / \ |
| A D |
| \ / |
| \ / |
| F---E |
| |
-----------------
Figure 19 : A Simple Optical Domain
Now consider that the operator's network is built from a mesh of such
domains, D1 through D7, as shown in Figure 20. It is possible that
these domains share a single, common instance of OSPF in which case
there is nothing further to say because that OSPF instance will
distribute sufficient information to build a single TED spanning the
whole network, and an end-to-end path can be computed. A more likely
scenario is that each domain is running its own OSPF instance. In
this case, each is able to handle the peculiarities (or rather,
advanced functions) of each vendor's equipment capabilities.
------ ------ ------ ------
| | | | | | | |
| D1 |---| D2 |---| D3 |---| D4 |
| | | | | | | |
------\ ------\ ------\ ------
\ | \ | \ |
\------ \------ \------
| | | | | |
| D5 |---| D6 |---| D7 |
| | | | | |
------ ------ ------
Figure 20 : A Simple Optical Domain
The question now is how to combine the multiple sets of information
distributed by the different OSPF instances. Three possible models
suggest themselves based on pre-existing routing practices.
o In the first model (the Area-Based model) each domain is treated as
a separate OSPF area. The end-to-end path will be specified to
traverse multiple areas, and each area will be left to determine
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the path across the nodes in the area. The feasibility of an end-
to-end path (and, thus, the selection of the sequence of areas and
their interconnections) can be derived using hierarchical PCE.
This approach, however, fits poorly with established use of the
OSPF area: in this form of optical network, the interconnection
points between domains are likely to be links; and the mesh of
domains is far more interconnected and unstructured than we are
used to seeing in the normal area-based routing paradigm.
Furthermore, while hierarchical PCE may be able to solve this type
of network, the effort involved may be considerable for more than a
small collection of domains.
o Another approach (the AS-Based model) treats each domain as a
separate Autonomous System (AS). The end-to-end path will be
specified to traverse multiple ASes, and each AS will be left to
determine the path across the AS.
This model sits more comfortably with the established routing
paradigm, but causes a massive escalation of ASes in the global
Internet. It would, in practice, require that the operator used
private AS numbers [RFC6996] of which there are plenty.
Then, as suggested in the Area-Based model, hierarchical PCE
could be used to determine the feasibility of an end-to-end path
and to derive the sequence of domains and the points of
interconnection to use. But, just as in that other model, the
scalability of this model using a hierarchical PCE must be
questioned given the sheer number of ASes and their
interconnectivity.
Furthermore, determining the mesh of domains (i.e., the inter-AS
connections) conventionally requires the use of BGP as an inter-
domain routing protocol. However, not only is BGP not normally
available on optical equipment, but this approach indicates that
the TE properties of the inter-domain links would need to be
distributed and updated using BGP: something for which it is not
well suited.
o The third approach (the ASON model) follows the architectural
model set out by the ITU-T [G.8080] and uses the routing protocol
extensions described in [RFC6827]. In this model the concept of
"levels" is introduced to OSPF. Referring back to Figure 20, each
OSPF instance running in a domain would be construed as a "lower
level" OSPF instance and would leak routes into a "higher level"
instance of the protocol that runs across the whole network.
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This approach handles the awkwardness of representing the domains
as areas or ASes by simply considering them as domains running
distinct instances of OSPF. Routing advertisements flow "upward"
from the domains to the high level OSPF instance giving it a full
view of the whole network and allowing end-to-end paths to be
computed. Routing advertisements may also flow "downward" from the
network-wide OSPF instance to any one domain so that it has
visibility of the connectivity of the whole network.
While architecturally satisfying, this model suffers from having to
handle the different characteristics of different equipment
vendors. The advertisements coming from each low level domain
would be meaningless when distributed into the other domains, and
the high level domain would need to be kept up-to-date with the
semantics of each new release of each vendor's equipment.
Additionally, the scaling issues associated with a well-meshed
network of domains each with many entry and exit points and each
with network resources that are continually being updated reduces
to the same problem as noted in the virtual link model.
Furthermore, in the event that the domains are under control of
different administrations, the domains would not want to distribute
the details of their topologies and TE resources.
Practically, this third model turns out to be very close to the
methodology described in this document. As noted in Section 6.1 of
[RFC6827], there are policy rules that can be applied to define
exactly what information is exported from or imported to a low level
OSPF instance. The document even notes that some forms of
aggregation may be appropriate. Thus, we can apply the following
simplifications to the mechanisms defined in RFC 6827:
- Zero information is imported to low level domains.
- Low level domains export only abstracted links as defined in this
document and according to local abstraction policy and with
appropriate removal of vendor-specific information.
- There is no need to formally define routing levels within OSPF.
- Export of abstracted links from the domains to the network-wide
routing instance (the abstraction routing layer) can take place
through any mechanism including BGP-LS or direct interaction
between OSPF implementations.
With these simplifications, it can be seen that the framework defined
in this document can be constructed from the architecture discussed
in RFC 6827, but without needing any of the protocol extensions that
that document defines. Thus, using the terminology and concepts
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already established, the problem may solved as shown in Figure 21.
The abstraction layer network is constructed from the inter-domain
links, the domain border nodes, and the abstracted (cross-domain)
links.
Abstraction Layer
-- -- -- -- -- --
| |===========| |--| |===========| |--| |===========| |
| | | | | | | | | | | |
..| |...........| |..| |...........| |..| |...........| |......
| | | | | | | | | | | |
| | -- -- | | | | -- -- | | | | -- -- | |
| |_| |_| |_| | | |_| |_| |_| | | |_| |_| |_| |
| | | | | | | | | | | | | | | | | | | | | | | |
-- -- -- -- -- -- -- -- -- -- -- --
Domain 1 Domain 2 Domain 3
Key Optical Layer
... Layer separation
--- Physical link
=== Abstract link
Figure 21 : The Optical Network Implemented Through the
Abstraction Layer Network
7. Application of the User-to-Network Interface
The User-to-Network Interface (UNI) is an important architectural
concept in many implementations and deployments of client-server
networks especially those where the client and server network have
different technologies. The UNI can be seen described in [G.8080],
and the GMPLS approach to the UNI is documented in [RFC4208]. Other
GMPLS-related documents describe the application of GMPLS to specific
UNI scenarios: for example, [RFC6005] describes how GMPLS can support
a UNI that provides access to Ethernet services.
Figure 1 of [RFC6005] is reproduced here as Figure 22. It shows the
Ethernet UNI reference model, and that figure can serve as an example
for all similar UNIs. In this case, the UNI is an interface between
client network edge nodes and the server network. It should be noted
that neither the client network nor the server network need be an
Ethernet switching network.
There are three network layers in this model: the client network, the
"Ethernet service network", and the server network. The so-called
Ethernet service network consists of links comprising the UNI links
and the tunnels across the server network, and nodes comprising the
client network edge nodes and various server network nodes. That is,
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the Ethernet service network is equivalent to the abstraction layer
network with the UNI links being the physical links between the
client and server networks, and the client edge nodes taking the
role of UNI Client-side (UNI-C) and the server edge nodes acting as
the UNI Network-side (UNI-N) nodes.
Client Client
Network +----------+ +-----------+ Network
-------------+ | | | | +-------------
+----+ | | +-----+ | | +-----+ | | +----+
------+ | | | | | | | | | | | | +------
------+ EN +-+-----+--+ CN +-+----+--+ CN +--+-----+-+ EN +------
| | | +--+--| +-+-+ | | +--+-----+-+ |
+----+ | | | +--+--+ | | | +--+--+ | | +----+
| | | | | | | | | |
-------------+ | | | | | | | | +-------------
| | | | | | | |
-------------+ | | | | | | | | +-------------
| | | +--+--+ | | | +--+--+ | |
+----+ | | | | | | +--+--+ | | | +----+
------+ +-+--+ | | CN +-+----+--+ CN | | | | +------
------+ EN +-+-----+--+ | | | | +--+-----+-+ EN +------
| | | | +-----+ | | +-----+ | | | |
+----+ | | | | | | +----+
| +----------+ |-----------+ |
-------------+ Server Networks +-------------
Client UNI UNI Client
Network <-----> <-----> Network
Scope of This Document
Legend: EN - Client Network Edge Node
CN - Server Network (Core) Node
Figure 22 : Ethernet UNI Reference Model
An issue that is often raised concerns how a dual-homed client
network edge node (such as that shown at the bottom left-hand corner
of Figure 22) can make determinations about how they connect across
the UNI. This can be particularly important when reachability across
the server network is limited or when two diverse paths are desired
(for example, to provide protection). However, in the model
described in this network, the edge node (the UNI-C) is part of the
abstraction layer network and can see sufficient topology information
to make these decisions. If the approach introduced in this document
is used to model the UNI as described in this section, there is no
need to enhance the signaling protocols at the GMPLS UNI nor to add
routing exchanges at the UNI.
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8. Application of the Architecture to L3VPN Multi-AS Environments
Serving layer-3 VPNs (L3PVNs) across a multi-AS or multi-operator
environment currently provides a significant planning challenge.
Figure 6 shows the general case of the problem that needs to be
solved. This section shows how the abstraction layer network can
address this problem.
In the VPN architecture, the CE nodes are the client network edge
nodes, and the PE nodes are the server network edge nodes. The
abstraction layer network is made up of the CE nodes, the CE-PE
links, the PE nodes, and PE-PE tunnels that are the abstract links.
In the multi-AS or multi-operator case, the abstraction layer network
also includes the PEs (maybe ASBRs) at the edges of the multiple
server networks, and the PE-PE (maybe inter-AS) links. This gives
rise to the architecture shown in Figure 23.
The policy for adding abstract links to the abstraction layer network
will be driven substantially by the needs of the VPN. Thus, when a
new VPN site is added and the existing abstraction layer network
cannot support the required connectivity, a new abstract link will be
created out of the underlying network.
........... .............
VPN Site : : VPN Site
-- -- : : -- --
|C1|-|CE| : : |CE|-|C2|
-- | | : : | | --
| | : : | |
| | : : | |
| | : : | |
| | : -- -- -- -- : | |
| |----|PE|=========|PE|---|PE|=====|PE|----| |
-- : | | | | | | | | : --
........... | | | | | | | | ............
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | - - | | | | - | |
| |-|P|-|P|-| | | |-|P|-| |
-- - - -- -- - --
Figure 23 : The Abstraction Layer Network for a Multi-AS VPN
It is important to note that each VPN instance can have a separate
abstraction layer network. This means that the server network
resources can be partitioned and that traffic can be kept separate.
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This can be achieved even when VPN sites from different VPNs connect
at the same PE. Alternatively, multiple VPNs can share the same
abstraction layer network if that is operationally preferable.
Lastly, just as for the UNI discussed in Section 7, the issue of
dual-homing of VPN sites is a function of the abstraction layer
network and so is just a normal routing problem in that network.
9. Scoping Future Work
The section is provided to help guide the work on this problem and to
ensure that oceans are not knowingly boiled. This guidance is non-
normative for this architecture description.
9.1. Not Solving the Internet
The scope of the use cases and problem statement in this document is
limited to "some small set of interconnected domains." In
particular, it is not the objective of this work to turn the whole
Internet into one large, interconnected TE network.
9.2. Working With "Related" Domains
Subsequent to Section 9.1, the intention of this work is to solve
the TE interconnectivity for only "related" domains. Such domains
may be under common administrative operation (such as IGP areas
within a single AS, or ASes belonging to a single operator), or may
have a direct commercial arrangement for the sharing of TE
information to provide specific services. Thus, in both cases, there
is a strong opportunity for the application of policy.
9.3. Not Finding Optimal Paths in All Situations
As has been well described in this document, abstraction necessarily
involves compromises and removal of information. That means that it
is not possible to guarantee that an end-to-end path over
interconnected TE domains follows the absolute optimal (by any measure
of optimality) path. This is taken as understood, and future work
should not attempt to achieve such paths which can only be found by a
full examination of all network information across all connected
networks.
9.4. Sanity and Scaling
All of the above points play into a final observation. This work is
intended to bite off a small problem for some relatively simple use
cases as described in Section 2. It is not intended that this work
will be immediately (or even soon) extended to cover many large
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interconnected domains. Obviously the solution should as far as
possible be designed to be extensible and scalable, however, it is
also reasonable to make trade-offs in favor of utility and
simplicity.
10. Manageability Considerations
Manageability should not be a significant additional burden. Each
layer in the network model can and should be managed independently.
That is, each client network will run its own management systems and
tools to manage the nodes and links in the client network: each
client network link that uses an abstract link will still be
available for management in the client network as any other link.
Similarly, each server network will run its own management systems
and tools to manage the nodes and links in that network just as
normal.
Three issues remain for consideration:
- How is the abstraction layer network managed?
- How is the interface between the client network and the abstraction
layer network managed?
- How is the interface between the abstraction layer network and the
server network managed?
10.1. Managing the Abstraction Layer Network
Management of the abstraction layer network differs from the client
and server networks because not all of the links that are visible in
the TED are real links. That is, it is not possible to run OAM on
the links that constitute the potential of a link.
Other than that, however, the management should be essentially the
same. Routing and signaling protocols can be run in the abstraction
layer (using out of band channels for links that have not yet been
established), and a centralized TED can be constructed and used to
examine the availability and status of the links and nodes in the
network.
Note that different deployment models will place the "ownership" of
the abstraction layer network differently. In some case the
abstraction layer network will be constructed by the operator of the
server network and run by that operator as a service for one or more
client networks. In other cases, one or more server networks will
present the potential of links to an abstraction layer network run
by the operator of the client network. And it is feasible that a
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business model could be built where a third-party operator manages
the abstraction layer network, constructing it from the connectivity
available in multiple server networks, and facilitating connectivity
for multiple client networks.
10.2. Managing Interactions of Client and Abstraction Layer Networks
The interaction between the client network and the abstraction layer
network is a management task. It might be automated (software
driven) or it might require manual intervention.
This is a two-way interaction:
- The client network can express the need for additional
connectivity. For example, the client network may try and fail to
find a path across the client network and may request additional,
specific connectivity (this is similar to the situation with
Virtual Network Topology Manager (VNTM) [RFC5623]). Alternatively,
a more proactive client network management system may monitor
traffic demands (current and predicted), network usage, and network
"hot spots" and may request changes in connectivity by both
releasing unused links and by requesting new links.
- The abstraction layer network can make links available to the
client network or can withdraw them. These actions can be in
response to requests from the client network, or can be driven by
processes within the abstraction layer (perhaps reorganizing the
use of server network resources). In any case, the presentation of
new links to the client network is heavily subject to policy since
this is both operationally key to the success of this architecture
and the central plank of the commercial model described in this
document. Such policies belong to the operator of the abstraction
layer network and are expected to be fully configurable.
Once the abstraction layer network has decided to make a link
available to the client network it will install it at the link end
points (which are nodes in the client network) such that it appears
and can be advertised as a link in the client network.
In all cases, it is important that the operators of both networks are
able to track the requests and responses, and the operator of the
client network should be able to see which links in that network are
"real" physical links, and which are presented by the abstraction
layer network.
10.3. Managing Interactions of Abstraction Layer and Server Networks
The interactions between the abstraction layer network and the server
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network a similar to those described in Section 10.2, but there is a
difference in that the server network is more likely to offer up
connectivity, and the abstraction layer network is less likely to ask
for it.
That is, the server network will, according to policy that may
include commercial relationships, offer the abstraction layer network
a set of potential connectivity that the abstraction layer network
can treat as links. This server network policy will include:
- how much connectivity to offer
- what level of server network redundancy to include
- how to support the use of the abstraction links,
This process of offering links from the server network may include a
mechanism to indicate which links have been pre-established in the
server network, and can include other properties such as:
- link-level protection ([RFC4202])
- SRLG and MSRLG (see Appendix A)
- mutual exclusivity (see Appendix B).
The abstraction layer network needs a mechanism to tell the server
network which links it is making use of. This mechanism could also
include the ability to request additional connectivity from the
server network, although it seems most likely that the server network
will already have presented as much connectivity as it is physically
capable of subject to the constraints of policy.
Finally, the server network will need to confirm the establishment of
connectivity, withdraw links if they are no longer feasible, and
report failures.
Again, it is important that the operators of both networks are able
to track the requests and responses, and the operator of the server
network should be able to see which links are in use.
11. IANA Considerations
This document makes no requests for IANA action. The RFC Editor may
safely remove this section.
12. Security Considerations
Security of signaling and routing protocols is usually administered
and achieved within the boundaries of a domain. Thus, and for
example, a domain with a GMPLS control plane [RFC3945] would apply
the security mechanisms and considerations that are appropriate to
GMPLS [RFC5920]. Furthermore, domain-based security relies strongly
on ensuring that control plane messages are not allowed to enter the
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domain from outside.
In this context, additional security considerations arising from this
document relate to the exchange of control plane information between
domains. Messages are passed between domains using control plane
protocols operating between peers that have predictable relationships
(for example, UNI-C to UNI-N, between BGP-LS speakers, or between
peer domains). Thus, the security that needs to be given additional
attention for inter-domain TE concentrates on authentication of
peers, assertion that messages have not been tampered with, and to a
lesser extent protecting the content of the messages from inspection
since that might give away sensitive information about the networks.
The protocols described in Appendix A and which are likely to provide
the foundation to solutions to this architecture already include
such protection and further can be run over protected transports
such as IPsec [RFC6701], TLS [RFC5246], and the TCP Authentication
Option (TCP-AO) [RFC5925].
It is worth noting that the control plane of the abstraction layer
network is likely to be out of band. That is, control plane messages
will be exchanged over network links that are not the links to which
they apply. This models the facilities of GMPLS (but not of MPLS-TE)
and the security mechanisms can be applied to the protocols operating
in the out of band network.
13. Acknowledgements
Thanks to Igor Bryskin for useful discussions in the early stages of
this work and to Gert Grammel for discussions on the extent of
aggregation in abstract nodes and links.
Thanks to Deborah Brungard, Dieter Beller, Dhruv Dhody, Vallinayakam
Somasundaram, Hannes Gredler, Stewart Bryant, Brian Carpenter, and
Hilarie Orman for review and input.
Particular thanks to Vishnu Pavan Beeram for detailed discussions and
white-board scribbling that made many of the ideas in this document
come to life.
Text in Section 4.2.3 is freely adapted from the work of Igor
Bryskin, Wes Doonan, Vishnu Pavan Beeram, John Drake, Gert Grammel,
Manuel Paul, Ruediger Kunze, Friedrich Armbruster, Cyril Margaria,
Oscar Gonzalez de Dios, and Daniele Ceccarelli in
[I-D.beeram-ccamp-gmpls-enni] for which the authors of this document
express their thanks.
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14. References
14.1. Informative References
[G.8080] ITU-T, "Architecture for the automatically switched optical
network (ASON)", Recommendation G.8080.
[I-D.beeram-ccamp-gmpls-enni]
Bryskin, I., Beeram, V. P., Drake, J. et al., "Generalized
Multiprotocol Label Switching (GMPLS) External Network
Network Interface (E-NNI): Virtual Link Enhancements for
the Overlay Model", draft-beeram-ccamp-gmpls-enni, work in
progress.
[I-D.ietf-ccamp-rsvp-te-srlg-collect]
Zhang, F. (Ed.) and O. Gonzalez de Dios (Ed.), "RSVP-TE
Extensions for Collecting SRLG Information", draft-ietf-
ccamp-rsvp-te-srlg-collect, work in progress.
[RFC7752] Gredler, H., Medved, J., Previdi, S., Farrel, A., and Ray,
S., "North-Bound Distribution of Link-State and Traffic
Engineering (TE) Information Using BGP", RFC 7752, March
2016.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
McManus, J., "Requirements for Traffic Engineering Over
MPLS", RFC 2702, September 1999.
[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.
[RFC3473] L. Berger, "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RC 3473, January 2003.
[RFC3630] Katz, D., Kompella, and K., Yeung, D., "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC3945] Mannie, E., (Ed.), "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4105] Le Roux, J.-L., Vasseur, J.-P., and Boyle, J.,
"Requirements for Inter-Area MPLS Traffic Engineering",
RFC 4105, June 2005.
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[RFC4202] Kompella, K. and Y. Rekhter, "Routing Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4202, October 2005.
[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.
[RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
"User-Network Interface (UNI): Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Support for the
Overlay Model", RFC 4208, October 2005.
[RFC4216] Zhang, R., and Vasseur, J.-P., "MPLS Inter-Autonomous
System (AS) Traffic Engineering (TE) Requirements",
RFC 4216, November 2005.
[RFC4271] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4726] Farrel, A., Vasseur, J.-P., and Ayyangar, A., "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, November 2006.
[RFC4847] T. Takeda (Ed.), "Framework and Requirements for Layer 1
Virtual Private Networks," RFC 4847, April 2007.
[RFC4874] Lee, CY., Farrel, A., and S. De Cnodder, "Exclude Routes -
Extension to Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE)", RFC 4874, April 2007.
[RFC4920] Farrel, A., Satyanarayana, A., Iwata, A., Fujita, N., and
Ash, G., "Crankback Signaling Extensions for MPLS and GMPLS
RSVP-TE", RFC 4920, July 2007.
[RFC5150] Ayyangar, A., Kompella, K., Vasseur, JP., and A. Farrel,
"Label Switched Path Stitching with Generalized
Multiprotocol Label Switching Traffic Engineering (GMPLS
TE)", RFC 5150, February 2008.
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[RFC5152] Vasseur, JP., Ayyangar, A., and Zhang, R., "A Per-Domain
Path Computation Method for Establishing Inter-Domain
Traffic Engineering (TE) Label Switched Paths (LSPs)",
RFC 5152, February 2008.
[RFC5195] Ould-Brahim, H., Fedyk, D., and Y. Rekhter, "BGP-Based
Auto-Discovery for Layer-1 VPNs", RFC 5195, June 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5251] Fedyk, D., Rekhter, Y., Papadimitriou, D., Rabbat, R., and
L. Berger, "Layer 1 VPN Basic Mode", RFC 5251, July 2008.
[RFC5252] Bryskin, I. and L. Berger, "OSPF-Based Layer 1 VPN Auto-
Discovery", RFC 5252, July 2008.
[RFC5305] Li, T., and Smit, H., "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5440] Vasseur, JP. and Le Roux, JL., "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440, March 2009.
[RFC5441] Vasseur, JP., Zhang, R., Bitar, N, and Le Roux, JL., "A
Backward-Recursive PCE-Based Computation (BRPC) Procedure
to Compute Shortest Constrained Inter-Domain Traffic
Engineering Label Switched Paths", RFC 5441, April 2009.
[RFC5523] L. Berger, "OSPFv3-Based Layer 1 VPN Auto-Discovery", RFC
5523, April 2009.
[RFC5553] Farrel, A., Bradford, R., and JP. Vasseur, "Resource
Reservation Protocol (RSVP) Extensions for Path Key
Support", RFC 5553, May 2009.
[RFC5623] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
"Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic
Engineering", RFC 5623, September 2009.
[RFC5920] L. Fang, Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
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[RFC6005] Nerger, L., and D. Fedyk, "Generalized MPLS (GMPLS) Support
for Metro Ethernet Forum and G.8011 User Network Interface
(UNI)", RFC 6005, October 2010.
[RFC6107] Shiomoto, K., and A. Farrel, "Procedures for Dynamically
Signaled Hierarchical Label Switched Paths", RFC 6107,
February 2011.
[RFC6701] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap", RFC 6701,
February 2011.
[RFC6805] King, D., and A. Farrel, "The Application of the Path
Computation Element Architecture to the Determination of a
Sequence of Domains in MPLS and GMPLS", RFC 6805, November
2012.
[RFC6827] Malis, A., Lindem, A., and D. Papadimitriou, "Automatically
Switched Optical Network (ASON) Routing for OSPFv2
Protocols", RFC 6827, January 2013.
[RFC6996] J. Mitchell, "Autonomous System (AS) Reservation for
Private Use", BCP 6, RFC 6996, July 2013.
[RFC7399] Farrel, A. and D. King, "Unanswered Questions in the Path
Computation Element Architecture", RFC 7399, October 2014.
[RFC7579] Bernstein, G., Lee, Y.,et al., "General Network Element
Constraint Encoding for GMPLS-Controlled Networks", RFC
7579, June 2015.
[RFC7580] Zhang, F., Lee, Y,. Han, J, Bernstein, G., and Xu, Y.,
"OSPF-TE Extensions for General Network Element
Constraints", RFC 7580, June 2015.
Authors' Addresses
Adrian Farrel
Juniper Networks
EMail: adrian@olddog.co.uk
John Drake
Juniper Networks
EMail: jdrake@juniper.net
Nabil Bitar
Nokia
EMail: nbitar40@gmail.com
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George Swallow
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
EMail: swallow@cisco.com
Xian Zhang
Huawei Technologies
Email: zhang.xian@huawei.com
Daniele Ceccarelli
Ericsson
Via A. Negrone 1/A
Genova - Sestri Ponente
Italy
EMail: daniele.ceccarelli@ericsson.com
Contributors
Gert Grammel
Juniper Networks
Email: ggrammel@juniper.net
Vishnu Pavan Beeram
Juniper Networks
Email: vbeeram@juniper.net
Oscar Gonzalez de Dios
Email: ogondio@tid.es
Fatai Zhang
Email: zhangfatai@huawei.com
Zafar Ali
Email: zali@cisco.com
Rajan Rao
Email: rrao@infinera.com
Sergio Belotti
Email: sergio.belotti@alcatel-lucent.com
Diego Caviglia
Email: diego.caviglia@ericsson.com
Jeff Tantsura
Email: jeff.tantsura@ericsson.com
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Khuzema Pithewan
Email: kpithewan@infinera.com
Cyril Margaria
Email: cyril.margaria@googlemail.com
Victor Lopez
Email: vlopez@tid.es
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Appendix A. Existing Work
This appendix briefly summarizes relevant existing work that is used
to route TE paths across multiple domains. It is non-normative.
A.1. Per-Domain Path Computation
The per-domain mechanism of path establishment is described in
[RFC5152] and its applicability is discussed in [RFC4726]. In
summary, this mechanism assumes that each domain entry point is
responsible for computing the path across the domain, but that
details of the path in the next domain are left to the next domain
entry point. The computation may be performed directly by the entry
point or may be delegated to a computation server.
This basic mode of operation can run into many of the issues
described alongside the use cases in Section 2. However, in practice
it can be used effectively with a little operational guidance.
For example, RSVP-TE [RFC3209] includes the concept of a "loose hop"
in the explicit path that is signaled. This allows the original
request for an LSP to list the domains or even domain entry points to
include on the path. Thus, in the example in Figure 1, the source
can be told to use the interconnection x2. Then the source computes
the path from itself to x2, and initiates the signaling. When the
signaling message reaches Domain Z, the entry point to the domain
computes the remaining path to the destination and continues the
signaling.
Another alternative suggested in [RFC5152] is to make TE routing
attempt to follow inter-domain IP routing. Thus, in the example
shown in Figure 2, the source would examine the BGP routing
information to determine the correct interconnection point for
forwarding IP packets, and would use that to compute and then signal
a path for Domain A. Each domain in turn would apply the same
approach so that the path is progressively computed and signaled
domain by domain.
Although the per-domain approach has many issues and drawbacks in
terms of achieving optimal (or, indeed, any) paths, it has been the
mainstay of inter-domain LSP set-up to date.
A.2. Crankback
Crankback addresses one of the main issues with per-domain path
computation: what happens when an initial path is selected that
cannot be completed toward the destination? For example, what
happens if, in Figure 2, the source attempts to route the path
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through interconnection x2, but Domain C does not have the right TE
resources or connectivity to route the path further?
Crankback for MPLS-TE and GMPLS networks is described in [RFC4920]
and is based on a concept similar to the Acceptable Label Set
mechanism described for GMPLS signaling in [RFC3473]. When a node
(i.e., a domain entry point) is unable to compute a path further
across the domain, it returns an error message in the signaling
protocol that states where the blockage occurred (link identifier,
node identifier, domain identifier, etc.) and gives some clues about
what caused the blockage (bad choice of label, insufficient bandwidth
available, etc.). This information allows a previous computation
point to select an alternative path, or to aggregate crankback
information and return it upstream to a previous computation point.
Crankback is a very powerful mechanism and can be used to find an
end-to-end path in a multi-domain network if one exists.
On the other hand, crankback can be quite resource-intensive as
signaling messages and path setup attempts may "wander around" in the
network attempting to find the correct path for a long time. Since
RSVP-TE signaling ties up networks resources for partially
established LSPs, since network conditions may be in flux, and most
particularly since LSP setup within well-known time limits is highly
desirable, crankback is not a popular mechanism.
Furthermore, even if crankback can always find an end-to-end path, it
does not guarantee to find the optimal path. (Note that there have
been some academic proposals to use signaling-like techniques to
explore the whole network in order to find optimal paths, but these
tend to place even greater burdens on network processing.)
A.3. Path Computation Element
The Path Computation Element (PCE) is introduced in [RFC4655]. It is
an abstract functional entity that computes paths. Thus, in the
example of per-domain path computation (see A.1) the source node and
each domain entry point is a PCE. On the other hand, the PCE can
also be realized as a separate network element (a server) to which
computation requests can be sent using the Path Computation Element
Communication Protocol (PCEP) [RFC5440].
Each PCE has responsibility for computations within a domain, and has
visibility of the attributes within that domain. This immediately
enables per-domain path computation with the opportunity to off-load
complex, CPU-intensive, or memory-intensive computation functions
from routers in the network. But the use of PCE in this way does not
solve any of the problems articulated in A.1 and A.2.
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Two significant mechanisms for cooperation between PCEs have been
described. These mechanisms are intended to specifically address the
problems of computing optimal end-to-end paths in multi-domain
environments.
- The Backward-Recursive PCE-Based Computation (BRPC) mechanism
[RFC5441] involves cooperation between the set of PCEs along the
inter-domain path. Each one computes the possible paths from
domain entry point (or source node) to domain exit point (or
destination node) and shares the information with its upstream
neighbor PCE which is able to build a tree of possible paths
rooted at the destination. The PCE in the source domain can
select the optimal path.
BRPC is sometimes described as "crankback at computation time". It
is capable of determining the optimal path in a multi-domain
network, but depends on knowing the domain that contains the
destination node. Furthermore, the mechanism can become quite
complicated and involve a lot of data in a mesh of interconnected
domains. Thus, BRPC is most often proposed for a simple mesh of
domains and specifically for a path that will cross a known
sequence of domains, but where there may be a choice of domain
interconnections. In this way, BRPC would only be applied to
Figure 2 if a decision had been made (externally) to traverse
Domain C rather than Domain D (notwithstanding that it could
functionally be used to make that choice itself), but BRPC could be
used very effectively to select between interconnections x1 and x2
in Figure 1.
- Hierarchical PCE (H-PCE) [RFC6805] offers a parent PCE that is
responsible for navigating a path across the domain mesh and for
coordinating intra-domain computations by the child PCEs
responsible for each domain. This approach makes computing an end-
to-end path across a mesh of domains far more tractable. However,
it still leaves unanswered the issue of determining the location of
the destination (i.e., discovering the destination domain) as
described in Section 2.1. Furthermore, it raises the question of
who operates the parent PCE especially in networks where the
domains are under different administrative and commercial control.
It should also be noted that [RFC5623] discusses how PCE is used in a
multi-layer network with coordination between PCEs operating at each
network layer. Further issues and considerations of the use of PCE
can be found in [RFC7399].
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A.4. GMPLS UNI and Overlay Networks
[RFC4208] defines the GMPLS User-to-Network Interface (UNI) to
present a routing boundary between an overlay (client) network and
the server network, i.e. the client-server interface. In the client
network, the nodes connected directly to the server network are known
as edge nodes, while the nodes in the server network are called core
nodes.
In the overlay model defined by [RFC4208] the core nodes act as a
closed system and the edge nodes do not participate in the routing
protocol instance that runs among the core nodes. Thus the UNI
allows access to and limited control of the core nodes by edge nodes
that are unaware of the topology of the core nodes. This respects
the operational and layer boundaries while scaling the network.
[RFC4208] does not define any routing protocol extension for the
interaction between core and edge nodes but allows for the exchange
of reachability information between them. In terms of a VPN, the
client network can be considered as the customer network comprised
of a number of disjoint sites, and the edge nodes match the VPN CE
nodes. Similarly, the provider network in the VPN model is
equivalent to the server network.
[RFC4208] is, therefore, a signaling-only solution that allows edge
nodes to request connectivity cross the server network, and leaves
the server network to select the paths for the LSPs as they traverse
the core nodes (setting up hierarchical LSPs if necessitated by the
technology). This solution is supplemented by a number of signaling
extensions such as [RFC4874], [RFC5553], [I-D.ietf-ccamp-xro-lsp-
subobject], [I-D.ietf-ccamp-rsvp-te-srlg-collect], and [I-D.ietf-
ccamp-te-metric-recording] to give the edge node more control over
path within the server network and by allowing the edge nodes to
supply additional constraints on the path used in the server network.
Nevertheless, in this UNI/overlay model, the edge node has limited
information of precisely what LSPs could be set up across the server
network, and what TE services (such as diverse routes for end-to-end
protection, end-to-end bandwidth, etc.) can be supported.
A.5. Layer One VPN
A Layer One VPN (L1VPN) is a service offered by a layer 1 server
network to provide layer 1 connectivity (TDM, LSC) between two or
more customer networks in an overlay service model [RFC4847].
As in the UNI case, the customer edge has some control over the
establishment and type of the connectivity. In the L1VPN context
three different service models have been defined classified by the
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semantics of information exchanged over the customer interface:
Management Based, Signaling Based (a.k.a. basic), and Signaling and
Routing service model (a.k.a. enhanced).
In the management based model, all edge-to-edge connections are set
up using configuration and management tools. This is not a dynamic
control plane solution and need not concern us here.
In the signaling based service model [RFC5251] the CE-PE interface
allows only for signaling message exchange, and the provider network
does not export any routing information about the server network.
VPN membership is known a priori (presumably through configuration)
or is discovered using a routing protocol [RFC5195], [RFC5252],
[RFC5523], as is the relationship between CE nodes and ports on the
PE. This service model is much in line with GMPLS UNI as defined in
[RFC4208].
In the enhanced model there is an additional limited exchange of
routing information over the CE-PE interface between the provider
network and the customer network. The enhanced model considers four
different types of service models, namely: Overlay Extension, Virtual
Node, Virtual Link and Per-VPN service models. All of these
represent particular cases of the TE information aggregation and
representation.
A.6. Policy and Link Advertisement
Inter-domain networking relies on policy and management input to
coordinate the allocation of resources under different administrative
control. [RFC5623] introduces a functional component called the
Virtual Network Topology Manager (VNTM) for this purpose.
An important companion to this function is determining how
connectivity across the abstraction layer network is made available
as a TE link in the client network. Obviously, if the connectivity
is established using management intervention, the consequent client
network TE link can also be configured manually. However, if
connectivity from client edge to client edge is achieved using
dynamic signalling then there is need for the end points to exchange
the link properties that they should advertise within the client
network, and in the case of support for more than one client network,
it will be necessary to indicate which client network or networks can
use the link. This capability it provided in [RFC6107].
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Appendix B. Additional Features
This Appendix describes additional features that may be desirable and
that can be achieved within this architecture. It is non-normative.
B.1. Macro Shared Risk Link Groups
Network links often share fate with one or more other links. That
is, a scenario that may cause a link to fail could cause one or more
other links to fail. This may occur, for example, if the links are
supported by the same fiber bundle, or if some links are routed down
the same duct or in a common piece of infrastructure such as a
bridge. A common way to identify the links that may share fate is to
label them as belonging to a Shared Risk Link Group (SRLG) [RFC4202].
TE links created from LSPs in lower layers may also share fate, and
it can be hard for a client network to know about this problem
because it does not know the topology of the server network or the
path of the server network LSPs that are used to create the links in
the client network.
For example, looking at the example used in Section 4.2.3 and
considering the two abstract links S1-S3 and S1-S9 there is no way
for the client network to know whether the links C2-C0 and C2-C3
share fate. Clearly, if the client layer uses these links to provide
a link-diverse end-to-end protection scheme, it needs to know that
the links actually share a piece of network infrastructure (the
server network link S1-S2).
Per [RFC4202], an SRLG represents a shared physical network resource
upon which the normal functioning of a link depends. Multiple SRLGs
can be identified and advertised for every TE link in a network.
However, this can produce a scalability problem in a mutli-layer
network that equates to advertising in the client network the server
network route of each TE link.
Macro SRLGs (MSRLGs) address this scaling problem and are a form of
abstraction performed at the same time that the abstract links are
derived. In this way, links that actually share resources in the
server network are advertised as having the same MSRLG, rather than
advertising each SRLG for each resource on each path in the server
network. This saving is possible because the abstract links are
formulated on behalf of the server network by a central management
agency that is aware of all of the link abstractions being offered.
It may be noted that a less optimal alternative path for the abstract
link S1-S9 exists in the server network (S1-S4-S7-S8-S9). It would
be possible for the client network request for connectivity C2-C0 to
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ask that the path be maximally disjoint from the path C2-C3. While
nothing can be done about the shared link C2-S1, the abstraction
layer could request to use the link S1-S9 in a way that is diverse
from use of the link S1-S3, and this request could be honored if the
server network policy allows.
Note that SRLGs and MSRLGs may be very hard to describe in the case
of multiple server networks because the abstraction points will not
know whether the resources in the various server layers share
physical locations.
B.2. Mutual Exclusivity
As noted in the discussion of Figure 13, it is possible that some
abstraction layer links can not be used at the same time. This
arises when the potentiality of the links is indicated by the server
network, but the use the links would actually compete for server
network resources. In Figure 13 this arose when both link S1-S3 and
link S7-S9 were used to carry LSPs: in that case the link S1-S9 could
no longer be used.
Such a situation need not be an issue when client-edge to client-edge
LSPs are set up one by one because the use of one abstraction layer
link and the corresponding use of server network resources will cause
the server network to withdraw the availability of the other
abstraction layer links, and these will become unavailable for
further abstraction layer path computations.
Furthermore, in deployments where abstraction layer links are only
presented as available after server network LSPs have been
established to support them, the problem is unlikely exist.
However, when the server network is constrained, but chooses to
advertise the potential of multiple abstraction layer links even
though they compete for resources, and when multiple client-edge to
client-edge LSPs are computed simultaneously (perhaps to provide
protection services) there may be contention for server network
resources. In the case that protected abstraction layer LSPs are
being established, this situation would be avoided through the use of
SRLGs and/or MSRLGs since the two abstraction layer links that
compete for server network resources must also fate share across
those resources. But in the case where the multiple client-edge to
client-edge LSPs do not care about fate sharing, it may be necessary
to flag the mutually exclusive links in the abstraction layer TED so
that path computation can avoid accidentally attempting to utilize
two of a set of such links at the same time.
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