RFC 7926
Problem Statement and Architecture for Information Exchange between Interconnected Traffic-Engineered Networks
Pages: 67
Best Current Practice: 206
Best Current Practice: 206
Part 1 of 3 – Pages 1 to 21
Internet Engineering Task Force (IETF) A. Farrel, Ed. Request for Comments: 7926 J. Drake BCP: 206 Juniper Networks Category: Best Current Practice N. Bitar ISSN: 2070-1721 Nokia G. Swallow Cisco Systems, Inc. D. Ceccarelli Ericsson X. Zhang Huawei July 2016 Problem Statement and Architecture for Information Exchange between Interconnected Traffic-Engineered NetworksAbstract
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 networks 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 this document, this work is limited to simple TE constraints and information that determine TE reachability.
Status of This Memo This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7926. 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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 ...............................................15 2.4. Requesting Connectivity ...................................15 2.4.1. Discovering Server Network Information .............17 3. Problem Statement ..............................................18 3.1. Policy and Filters ........................................18 3.2. Confidentiality ...........................................19 3.3. Information Overload ......................................19 3.4. Issues of Information Churn ...............................20 3.5. Issues of Aggregation .....................................21 4. Architecture ...................................................22 4.1. TE Reachability ...........................................22 4.2. Abstraction, Not Aggregation ..............................22 4.2.1. Abstract Links .....................................23 4.2.2. The Abstraction Layer Network ......................23 4.2.3. Abstraction in Client-Server Networks ..............26 4.2.4. Abstraction in Peer Networks .......................32 4.3. Considerations for Dynamic Abstraction ....................34 4.4. Requirements for Advertising Links and Nodes ..............35 4.5. Addressing Considerations .................................36 5. Building on Existing Protocols .................................36 5.1. BGP-LS ....................................................37 5.2. IGPs ......................................................37 5.3. RSVP-TE ...................................................37 5.4. Notes on a Solution .......................................37 6. Application of the Architecture to Optical Domains and Networks .......................................................39
7. Application of the Architecture to the User-Network Interface ..44 8. Application of the Architecture to L3VPN Multi-AS Environments .46 9. Scoping Future Work ............................................47 9.1. Limiting Scope to Only Part of the Internet ...............47 9.2. Working with "Related" Domains ............................47 9.3. Not Finding Optimal Paths in All Situations ...............48 9.4. Sanity and Scaling ........................................48 10. Manageability Considerations ..................................48 10.1. Managing the Abstraction Layer Network ...................49 10.2. Managing Interactions of Abstraction Layer and Client Networks ..........................................49 10.3. Managing Interactions of Abstraction Layer and Server Networks ..........................................50 11. Security Considerations .......................................51 12. Informative References ........................................52 Appendix A. Existing Work .........................................58 A.1. Per-Domain Path Computation ...............................58 A.2. Crankback .................................................59 A.3. Path Computation Element ..................................59 A.4. GMPLS UNI and Overlay Networks ............................61 A.5. Layer 1 VPN ...............................................62 A.6. Policy and Link Advertisement .............................62 Appendix B. Additional Features ...................................63 B.1. Macro Shared Risk Link Groups .............................63 B.2. Mutual Exclusivity ........................................64 Acknowledgements ..................................................65 Contributors ......................................................66 Authors' Addresses ................................................67
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 Interior Gateway Protocol (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 (ASes). 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.
Appendix A summarizes relevant existing work that 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.
1.1.3. TE Reachability
In an IP network, reachability is the ability to deliver a packet to a specific address or prefix, i.e., 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); this is helpful especially in determining a path to a destination that lies in an unknown domain or that may be qualified by TE attributes and TE metrics such as hop count, available bandwidth, delay, and shared risk.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 IGP areas and 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.
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 it 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 regarding 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.
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 networks and one or more client networks. 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), which 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 suboptimal 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 such techniques do not lead to rapid setup or guaranteed optimality. Furthermore, RSVP signaling creates state in the network that is immediately removed by the crankback
procedure. Frequent events of this kind will impact scalability in a non-deterministic manner. More details regarding crankback can be found in Appendix 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 through Domain D, Domain A must be sensitive to the TE connectivity available through each of Domains C and D, as well as 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 Mesh2.2. 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 of these classes of use case, the client and server networks may have the same switching capability, or they 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 relating to 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 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 (i.e., 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. ------------ ------------ | 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. ------------ | 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 Network2.4. 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 asked 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 (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 asked to set up the connections that support a service provided to the client network. These requests may come from management systems, 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 and 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 may 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 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 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 follow the 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 interrelated connection
requests described in the previous two bullets to be enacted
through a single, compound service request.
o "Service resilience" requests that the server network 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 were 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.
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 2.2) where information will be shared between domains that have a commercial relationship but a low level of trust. Thus, it must be possible for a domain to limit the shared information to only 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 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 that 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 are considerably longer. Thus, the information churn may be sustainable.
3.5. Issues of Aggregation
One possible solution to the issues raised in other subsections 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 [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 others, and with what TE attributes. Part of the nature of aggregation is 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.
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