One document matched: draft-farrel-interconnected-te-info-exchange-00.txt
Network Working Group A. Farrel
Internet-Draft J. Drake
Intended status: Standards Track Juniper Networks
Expires: August 18, 2013
N. Bitar
Verizon Networks
G. Swallow
Cisco Systems, Inc.
D. Ceccarelli
Ericsson
February 18, 2013
Problem Statement and Architecture for Information Exchange
Between Interconnected Traffic Engineered Networks
draft-farrel-interconnected-te-info-exchange-00.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. The availability of TE
information is usually limited to within a network (such as an IGP
area) often referred to as a domain.
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 and architecture for the
exchange of TE information between interconnected TE networks in
support of end-to-end TE path establishment. For reasons that are
explained in the document, this work is limited to simple TE
constraints and information that determine TE reachability.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Copyright Notice
Copyright (c) 2013 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. What is TE Reachability? ................................... 6
2. Overview of Use Cases ........................................ 6
2.1. Peer Networks and the E-NNI ................................ 6
2.1.1. Where is the Destination? ................................ 7
2.2. Client-Server (Overlay) Networks ........................... 8
2.3. Dual-Homing ................................................ 10
3. Problem Statement ............................................ 11
3.1. Use of Existing Protocol Mechanisms ........................ 12
3.2. Policy and Filters ......................................... 12
3.3. Confidentiality ............................................ 13
3.4. Information Overload ....................................... 13
3.5. Issues of Information Churn ................................ 14
3.6. Issues of Aggregation ...................................... 15
3.7. Virtual Network Topology ................................... 15
4. Existing Work ................................................ 17
4.1. Per-Domain Path Computation ................................ 17
4.2. Crankback .................................................. 18
4.3. Path Computation Element ................................... 18
4.4. GMPLS UNI and Overlay ...................................... 20
4.5. Layer One VPN .............................................. 20
4.6. VNT Manager and Link Advertisement ......................... 21
4.7. What Else is Needed and Why? ............................... 22
5. Architectural Concepts ....................................... 22
5.1. Basic Components ........................................... 22
5.1.1. Peer Interconnection ..................................... 22
5.1.2. Overlay Interconnection .................................. 23
5.2. TE Reachability ............................................ 24
5.3. Abstraction not Aggregation ................................ 24
5.3.1. Abstract Links ........................................... 25
5.3.2. Abstract Nodes ........................................... 26
5.3.3. Abstraction in Peer Networks ............................. 26
5.3.4. Abstraction in Overlay Networks .......................... 26
5.4. Considerations for Dynamic Abstraction ..................... 26
5.5. Requirements for Advertising Abstracted Links and Nodes .... 26
6. Building on Existing Protocols ............................... 26
6.1. BGP ........................................................ 26
6.1.1. Current Uses of BGP ...................................... 26
6.1.1.1. IP Reachability ........................................ 26
6.1.1.2. VPNs ... ............................................... 26
6.1.1.3. Link State Distribution................................. 26
6.1.2. Potential Extensions to BGP for TE Reachability .......... 26
6.2. IGPs ....................................................... 27
6.3. RSVP-TE .................................................... 27
7. Scoping Future Work .......................................... 27
7.1. Not Solving the Internet ................................... 27
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7.2. Working With "Related" Domains ............................. 27
7.3. Not Breaking Existing Protocols ............................ 27
7.4. Sanity and Scaling ......................................... 27
8. Manageability Considerations ................................. 28
9. IANA Considerations .......................................... 28
10. Security Considerations ..................................... 28
11. Acknowledgements ............................................ 28
12. References .................................................. 28
12.1. Normative References....................................... 28
12.2. Informative References .................................... 28
Authors' Addresses ............................................... 31
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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 and architecture for the
exchange of TE information between interconnected TE domains in
support of end-to-end TE path establishment. The scope of this
document is limited to the simple TE constraints and information
(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.
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1.1. What is 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.
TE reachability may be unqualified (there is a TE path) which is
helpful especially in determining a path to a destination that lies
in an unknown domain, or may be qualified by TE attributes such as TE
metrics, hop count, available bandwidth, delay, shared risk, etc.
2. Overview of Use Cases
2.1. Peer Networks and the E-NNI
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.
For example, when Domain A attempts to select a path, it may
determine that adequate bandwidth is available on 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 (see
Section 4.2) may be used to allieviate this situation, but do not
lead to rapid setup or guaranteed optimality.
-------------- --------------
| Domain A | x1 | Domain Z |
| +----+ |
| ----- | | ----- |
| | Src | | | | Dst | |
| ----- +----+ ----- |
| | x2 | |
-------------- --------------
Figure 1 : Peer Networks
There are countless more complicated examples of the problem of peer
networks. Figure 2 shows the case where there is a simple mesh of
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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.
--------------
| Domain B |
| |
| |
/--------------
/
/
/x1
--------------/ --------------
| Domain A | | Domain Z |
| | -------------- | |
| ----- | x2| Domain C | x4| ----- |
| | Src | +---+ +---+ | Dst | |
| ----- | | | | ----- |
| | -------------- | |
--------------\ /--------------
\x3 /
\ /
\ /x5
\--------------/
| Domain D |
| |
| |
--------------
Figure 2 : Peer Networks in a Mesh
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.
2.1.1. Where is the Destination?
A variation of the problems expressed in Section 2.1 arises when the
source domain (Domain A in both figures) does not know where the
destination is located. That is, when the domain in which the
destination node is located is not known to the source domain.
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This is most easily seen in consideration of Figure 2 where the
decision about which interconnection to select needs to be based on
building a path toward the destination domain. Yet this can only be
achieved if it is known in which domain the destination node lies, or
at least if there is some indication in which direction the
destination lies. This function is obviously provided in IP networks
by inter-domain routing [RFC4271].
2.2. Client-Server (Overlay) Networks
Two specific use cases relate to the client-server (overlay)
relationship between networks.
The first case, shown is Figure 3, occurs when domains belonging to
one network are connected by a domain belonging to another network.
In this scenario, once connections (or tunnels) are formed across the
lower layer network, the domains of the upper layer network can be
merged into a single domain by running IGP adjacencies over the
tunnels, and treating the tunnels as links in the higher layer
network. The TE relationship between the domains (higher and lower
layer) in this case is reduced to determining which tunnels to set
up, how to trigger them, how to route them, and what capacity to
assign them. As the demands in the higher layer network vary, these
tunnels may need to be modified.
-------------- --------------
| Domain A | | Domain Z |
| | | |
| ----- | | ----- |
| | Src | | | | Dst | |
| ----- | | ----- |
| | | |
--------------\ /--------------
\x1 x2/
\ /
\ /
\---------------/
| Server Domain |
| |
| |
---------------
Figure 3 : Client-Server (Overlay) Networks
The second use case relating to client-server networking is for
Virtual Private Networks (VPNs). In this case, as opposed to the
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former one, it is assumed that the client network has a different
address space than that of the server layer 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,
the provider 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
Note that in the use cases shown in Figures 3 and 4 the client layer
domains may (and, in fact, probably do) operate as a single connected
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
domain is attached to more than one server domain, or has two points
of attachment to a server domain. Figure 6 shows an example of this
for a VPN.
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------------
| Domain A |
| (VPN site) |
------------ | ----- |
| Domain B | | | Src | |
| (VPN site) | | ----- |
| | | |
------------\ -+--------+-
\x1 | |
\ x2| |x3
\ | | ------------
\--------+- -+-------- | Domain Z |
| Domain X | x8 | Domain Y | x4 | (VPN site) |
| (core) +----+ (core) +----+ ----- |
| | | | | | Dst | |
| +----+ +----+ ----- |
| | x9 | | x5 | |
/---------- ----------\ ------------
/ \
/ \
/x6 x7\
------------/ \------------
| Domain C | | Domain D |
| (VPN site) | | (VPN site) |
| | | |
------------ ------------
Figure 6 : Dual-Homing in a Virtual Private Network
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 path computation in one domain
to make informed choices about the exit point 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
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TE LSP establishment are likely to prove adequate [RFC4726] with the
possibility of minor extensions.
An interesting annex to the problem is how the path is made available
for use. For example, in the case of an overlay 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. Use of Existing Protocol Mechanisms
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]. TE information may be exported from a domain
(for example, northbound) using link state extensions to BGP
[I-D.ietf-idr-ls-distribution].
It is desirable that a solution to the problem described in this
document does not require the implementation of a new, network-wide
protocol. Instead, it would be advantageous to make use of an
existing protocol that is commonly implemented on routers 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.
3.2. 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.
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3.3. Confidentiality
A feature of the policy described in Section 3.3 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 domains
described in Section 2.4) 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 information share
to just that which the computing domain needs to know with the
understanding that 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.
3.4. 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 3.2 because such filters might not
always be enabled.
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3.5. 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 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.
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3.6. 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.
- 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 3.5 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 in
reduced, the aggregation algorithm (operating on the routers
responsible for sharing information) may be heavily exercised.
3.7. Virtual Network Topology
The terms "virtual topology" and "virtual network topology" have
become overloaded in a relatively short time. We draw on [RFC5212]
and [RFC5623] for inspiration to provide a definition for use in this
document. Our definition is based on the fact that a topology at the
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client network layer is constructed of nodes and links. Typically,
the nodes are routers in the client layer, and the links are data
links. However, a layered network provides connectivity through the
lower layer as LSPs, and these LSPs can provide links in the client
layer. Furthermore, those LSPs may have been established in advance,
or might be LSPs that could be set up if required. This leads to the
definition:
A Virtual Network Topology (VNT) is made up of links in a network
layer. Those links may be realized as direct data links or as
multi-hop connections (LSPs) in a lower network layer. Those
underlying LSPs may be established in advance or created on demand.
The creation and management of a VNT requires interaction with
management and policy. Activity is needed in both the client and
server layer:
- In the server layer, LSPs need to be set up either in advance in
response to management instructions or in answer to dynamic
requests subject to policy considerations.
- In the server layer, evaluation of available TE resources can lead
to the announcement of potential connectivity (i.e., LSPs that
could be set up on demand).
- In the client layer, connectivity (lower layer LSPs or potential
LSPs) needs to be announced in the IGP as a normal TE link. Such
links may or may not be made available to IP routing: but, they are
never made available to IP until fully instantiated.
- In the client layer, requests to establish lower layer LSPs need to
be made either when links supported by potential LSPs are about to
be used (i.e., when a higher layer LSP is signalled to cross the
link, the setup of the lower layer LSP is triggered), or when the
client layer determines it needs more connectivity or capacity.
It is a fundamental of the use of a VNT that there is a policy point
at the point of instantiation of a lower-layer LSP. At the moment
that the setup of a lower-layer LSP is triggered, whether from a
client-layer management tool or from signaling in the client layer,
the server layer must be able to apply policy to determine whether to
actually set up the LSP. Thus, fears that a micro-flow in the client
layer might cause the activation of 100G optical resources in the
server layer can be completely controlled by the policy of the server
layer network's operator (and could even be subject to commercial
terms).
These activities require an architecture and protocol elements as
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well as management components and policy elements.
4. Existing Work
This section briefly summarizes relevant existing work that is used
to route TE paths across multiple domains.
4.1. 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.
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4.2. 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
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 in a multi-domain network if one exists.
On the other hand, crankback can be quite resouce-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 cranback 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.)
4.3. 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 (Section 4.1) 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].
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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 Sections 4.1 and 4.2.
Two significant mechanisms for cooperation between PCEs have been
described. These mechanisms are intended to specifically address the
problems of computing optimal emd-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 PCE. 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 2.1.1. Furthermore, it raises the question of
who operates the parent PCE especially in networks where the
domains are under different administrative and commercial control.
Further issues and considerations of the use of PCE can be found in
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[I-D.farrkingel-pce-questions].
4.4. GMPLS UNI and Overlay
[RFC4208] defines the GMPLS User-to-Network Interface (UNI) to
present a routing boundary between an overlay network and the core
network. In the overlay network, the nodes connected directly to the
core network are known as edge nodes, while the nodes in the core
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.
[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
overlay 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 core network.
[RFC4208] is, therefore, a signaling-only solution that allows edge
nodes to request connectivity cross the core network, and leaves the
core network to select the paths and set up the core LSPs. This
solution is supplemented by a number of signaling extensions such as
[RFC5553], [I-D.ietf-ccamp-xro-lsp-subobject], and
[I-D.ietf-ccamp-te-metric-recording] to give the edge node more
control over the LSP that the core network will set up by exchanging
information about core LSPs that have been established and by
allowing the edge nodes to supply additional constraints on the core
LSPs that are to be set up.
Nevertheless, in this UNI/overlay model, the edge node has limited
information of precisely what LSPs could be set up across the core,
and what TE services (such as diverse routes for end-to-end
protection, end-to-end bandwidth, etc.) can be supported.
4.5. Layer One VPN
A Layer One VPN (L1VPN) is a service offered by a core layer 1
network to provide layer 1 connectivity (TDM, LSC) between two or
more customer networks in an overlay environment [RFC4847].
As in the UNI case, the customer edge has some control over the
establishment and type of the connectivity. In the L1VPN context
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three different service models have been defined classified by the
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 core 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.
4.6. VNT Manager and Link Advertisement
As discussed in Section 3.7, operation of a VNT requires policy and
management input. In order to handle this, [RFC5623] introduces the
concept of the Virtual Network Topology Manager. This is a
functional component that applies policy to requests from client
networks (or agents of the client network, such as a PCE) for the
establishment of LSPs in the server network to provide connectivity
in the client network.
The VNT Manager would, in fact, form part of the provisioning path
for all server network LSPs whether they are set up ahead of client
network demand or triggered by end-to-end client network LSP
signaling.
An important companion to this function is determining how the LSP
set up across the server network is made available as a TE link in
the client network. Obviously, if the LSP is established using
management intervention, the subsequent client network TE link can
also be configured manually. However, if the LSP is signaled
dynamically 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 a server network that supports more than one client,
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it will be necessary to indicate which client or clients can use the
link. This capability it provided in [RFC6107].
Note that a potential server network LSP that is advertised as a TE
link in the client network might to be determined dynamically by
the edge nodes. In this case there will need to be some effort to
ensure that both ends of the link have the same view of the available
TE resources, or else the advertised link will be asymmetrical.
4.7. What Else is Needed and Why?
As can be seen from Sections 4.1 through 4.6, a lot of effort has
focused on overlay networks as described in Figure 3. Far less
consideration has been given to network peering or the combination of
the two use cases.
<TBD>
5. Architectural Concepts
5.1. Basic Components
This section revisits the use cases from Section 2 to present the
basic architectural components that provide connectivity in the
peer and overlay cases. These component models can then be used in
later sections to enable discussion.
5.1.1. Peer Interconnection
Figure 7 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 (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 LSP tunnels that span the transit networks are
used to achieve the required connectivity. These transit LSPs form
the key building blocks of the end-to-end connectivity and may be
advertised to the source network to enable it to determine the right
way to route a TE connection to the destination.
The transit tunnels can be used as hierarchical LSPs [RFC4206] to
carry the end-to-end LSP, or can become stitching segments [RFC5150]
of the end-to-end LSP. Two different abstraction models may be
applied (as described further in Section 5.3): the connection B1-B3
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can be expressed as an abstract link; or the network {C1, C2, C3} can
be represented as an abstract node.
-- -- -- -- -- -- -- -- -- --
|A1|--|A2|--|B1|--|B2|--|B3|--|C1|--|C2|--|C3|--|D1|--|D2|
-- -- | | -- | | | | -- | | -- --
| |========| | | |========| |
-- -- -- --
Key
--- Direct connection between two nodes
=== LSP tunnel across transit network
Figure 7 : Architecture for Peering
5.1.2. Overlay Interconnection
Figure 8 shows the basic architectural concepts for an overlay
network. The client network nodes are C1, C2, CE1, CE2, C3, and C4.
The core network nodes are CN1, CN2, CN3, and CN4. The interfaces
CE1-CN1 and CE2-CN2 are the UNIs between the client and core
networks.
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 layer that is supported by a core network
LSP.
As shown in the figure, two LSPs are used to achieve the required
connectivity. One LSP is set up across the core from CN1 to CN2.
This core LSP then supports a three-hop LSP from CE1 to CE2. The
three-hop LSP is often called the UNI-LSP, and its middle hop is
comprised of the core LSP. It is the UNI-LSP that is presented as a
link in the client network.
The practicalities of how the UNI-LSP is carried across the core LSP
may depend on the switching and signaling options available in the
core network. The UNI-LSP may be tunneled down the core LSP using
the mechanisms of a hierarchical LSP [RFC4206], or the LSP segments
CE1-CN1 and CN2-CE2 may be stitched to the core LSP as described in
[RFC5150].
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-- -- --- --- -- --
|C1|--|C2|--|CE1|.................................|CE2|---|C3|---|C4|
-- -- | | --- --- | | -- --
| |--|CN1|===================|CN4|--| |
--- | | --- --- | | ---
| |---|CN2|---|CN3|---| |
--- --- --- ---
Key
--- Direct connection between two nodes
... CE-to-CE LSP tunnel (UNI-LSP)
=== LSP tunnel across the core
Figure 8 : Architecture for Overlay
5.2. 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
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 reaching a specific destination node from an edge
node.
TE reachability may be unqualified (there is a TE path), or may be
qualified by TE attributes such as TE metrics, hop count, available
bandwidth, delay, shared risk, etc.
TE reachability information is exchanged between networks so that
nodes in one network can determine whether they can establish TE
paths across or into another network.
5.3. Abstraction not Aggregation
Aggregation is the process of synthesizing from available
information. Thus, the virtual node and virtual link models rely on
processing the information available within a network to produce the
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aggregate representations of links and nodes that are presented to
the consumer. As described in Section 3, dynamic aggregation is
subject is subject to a number of pitfalls.
In order to distinguish this work 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 an 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 3.6), 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.
However, when dealing with requested TE parameters that are only a
small percentage of the available resources, abstraction is likely to
prove more than adequate. For example, when setting up an end-to-end
LSP that needs 64 MB bandwidth, an abstraction that offers 100 GB
connectivity is unlikely to result in a setup failure.
While abstraction uses available TE information, it will be subject
to policy and management choice. Thus, not all potential
connectivity will be advertised to each client. 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.
5.3.1. Abstract Links
An abstract link is a measure of the potential to connect a pair of
points with certain TE parameters. An abstract link may be realized
by an existing LSP, or may represent the possibility of setting up an
LSP.
When looking at an overlay network such as that in Figure 8, the link
from CE1 to CE2 may be an abstract link. If the LSP has already been
set up, it is easy to advertise it into the client layer IGP with
known TE attributes. However, if the LSP is not yet established, the
potential for an LSP must be abstracted from the TE information in
the core network. Since the client nodes (CE1 and CE2) do not have
visibility into the core network, they must rely on abstraction
information delivered to them by the core network. That is, the core
network will report on the potential for connectivity from CN1 to
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CN4, and CE1 will build on this to generate the abstraction for the
UNI connectivity.
5.3.2. Abstract Nodes
<TBD>
5.3.3. Abstraction in Peer Networks
<TBD>
5.3.4. Abstraction in Overlay Networks
<TBD>
5.4. Considerations for Dynamic Abstraction
<TBD>
5.5. Requirements for Advertising Abstracted Links and Nodes
<TBD>
6. Building on Existing Protocols
6.1. BGP
<TBD>
6.1.1. Current Uses of BGP
<TBD>
6.1.1.1. IP Reachability
<TBD>
6.1.1.2. VPNs
<TBD>
6.1.1.3. Link State Distribution
<TBD>
6.1.2. Potential Extensions to BGP for TE Reachability
<TBD>
Farrel, et al. [Page 26]
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6.2. IGPs
<TBD>
6.3. RSVP-TE
<TBD>
7. Scoping Future Work
The section is provided to help guide the work on this problem and to
ensure that oceans are not knowingly boiled.
7.1. 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.
7.2. Working With "Related" Domains
Subsequent to Section 7.1, 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.
7.3. Not Breaking Existing Protocols
It is a clear objective of this work to not break existing protocols.
The Internet relies on the stability of a few key routing protocols,
and so it is critical that any new work must not make these protocols
brittle or unstable.
7.4. 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
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.
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8. Manageability Considerations
<TBD>
9. IANA Considerations
This document makes no requests for IANA action.
10. Security Considerations
<TBD>
11. Acknowledgements
Thanks to Vishnu Pavan Beeram for useful discussions.
12. References
12.1. Normative References
12.2. Informative References
[I-D.farrkingel-pce-questions]
Farrel, A., and D. King, "Unanswered Questions in the Path
Computation Element Architecture", draft-farrkingel-pce-
questions, work in progress.
[I-D.ietf-ccamp-xro-lsp-subobject]
Z. Ali, et al., "Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) LSP Route Diversity using Exclude
Routes," draft-ali-ccamp-xro-lsp-subobject, work in
progress.
[I-D.ietf-ccamp-te-metric-recording]
Z. Ali, et al., "Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) extension for recording TE Metric of
a Label Switched Path," draft-ali-ccamp-te-metric-
recording, work in progress.
[I-D.ietf-idr-ls-distribution]
Gredler, H., Medved, J., Previdi, S., Farrel, A., and Ray,
S., "North-Bound Distribution of Link-State and TE
Information using BGP", draft-ietf-idr-ls-distribution,
work in progress.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
McManus, J., "Requirements for Traffic Engineering Over
MPLS", RFC 2702, September 1999.
Farrel, et al. [Page 28]
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[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.
[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.
[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.
[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.
Farrel, et al. [Page 29]
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[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.
[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.
[RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
M., and D. Brungard, "Requirements for GMPLS-Based Multi-
Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
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.
Farrel, et al. [Page 30]
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[RFC6107] Shiomoto, K., and A. Farrel, "Procedures for Dynamically
Signaled Hierarchical Label Switched Paths", RFC 6107,
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.
Authors' Addresses
Adrian Farrel
Juniper Networks
EMail: adrian@olddog.co.uk
John Drake
Juniper Networks
EMail: jdrake@juniper.net
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
EMail: nabil.bitar@verizon.com
George Swallow
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
EMail: swallow@cisco.com
Daniele Ceccarelli
Ericsson
Via A. Negrone 1/A
Genova - Sestri Ponente
Italy
EMail: daniele.ceccarelli@ericsson.com
Farrel, et al. [Page 31]
| PAFTECH AB 2003-2026 | 2026-04-23 09:05:43 |