wdiff rfc7399v1.txt rfc7399.txt

Internet Engineering Task Force (IETF) A. Farrel
Request for Comments: 7399 Juniper Networks
Category: Informational D. King
ISSN: 2070-1721 Old Dog Consulting
November
October 2014

Unanswered Questions in the Path Computation Element Architecture

Abstract

The Path Computation Element (PCE) architecture is set out in RFC
4655. The architecture is extended for multi-layer networking with
the introduction of the Virtual Network Topology Manager (VNTM) in
RFC 5623 and generalized to Hierarchical PCE (H-PCE) in RFC 6805.

These three architectural views of PCE deliberately leave some key
questions unanswered, especially with respect to the interactions
between architectural components. This document draws out those
questions and discusses them in an architectural context with
reference to other architectural components, existing protocols, and
recent IETF efforts.

This document does not update the architecture documents and does not
define how protocols or components must be used. It does, however,
suggest how the architectural components might be combined to provide
advanced PCE function.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

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/rfc7399.

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 ....................................................3
1.1. Terminology ................................................4
2. What Is Topology Information? ...................................4
3. How Is Topology Information Gathered? ...........................5
4. How Do I Find My PCE? ...........................................6
5. How Do I Select a PCE? ..........................................7 between PCEs? ...................................7
6. How Do Redundant PCEs Synchronize TEDs? .........................8
7. Where Is the Destination? .......................................9
8. Who Runs or Owns a Parent PCE? .................................10
9. How Do I Find My Parent PCE? ...................................11
10. How Do I Find My Child PCEs? ..................................11
11. How Is the Parent PCE Domain Topology Built? ..................12
12. Does H-PCE Solve the Internet? ................................12
13. What are Sticky Resources? ....................................13
14. What Is a Stateful PCE for? ...................................14
15. How Is the LSP-DB Built? ......................................14
16. How Do Redundant Stateful PCEs Synchronize State? .............15
17. What Is an Active PCE? What Is a Passive PCE? .................16
18. What is LSP Delegation? .......................................17
19. Is an Active PCE with LSP Delegation Just a Fancy NMS? ........18
20. Comparison of Stateless and Stateful PCE ......................18
21. How Does a PCE Work with a Virtual Network Topology? ..........19
22. How Does PCE Communicate with VNTM ............................21
23. How Does Service Scheduling and Calendering Work? .............21
24. Where Does Policy Fit In? .....................................22
25. Does PCE Play a Role in SDN? ..................................23
26. Security Considerations .......................................23
27. References ....................................................25
27.1. Normative References .....................................25
27.2. Informative References ...................................25
Acknowledgements ..................................................29
Authors' Addresses ................................................29

1. Introduction

Over the years since the architecture for the Path Computation
Element (PCE) was documented in [RFC4655], many new people have
become involved in the work of the PCE working group and wish to use
or understand the PCE architecture. These people often missed out on
early discussions within the working group and are unfamiliar with
questions that were raised during the development of the
documentation.

Furthermore, the base architecture has been extended to handle other
situations and requirements: the architecture was extended for multi-
layer networking with the introduction of the Virtual Network
Topology Manager (VNTM) [RFC5623] and was generalized to include
Hierarchical PCE (H-PCE) [RFC6805].

1.1. Terminology

Readers are assumed to be thoroughly familiar with terminology
defined in [RFC4655], [RFC4726], [RFC5440], [RFC5623], and [RFC6805].
More information about terms related to stateful PCE can be found in
[STATEFUL-PCE].

Throughout this document, the term "area" is used to refer equally to
an OSPF area and an IS-IS level. It is assumed that the reader is
able to map the small differences between these two use cases.

2. What Is Topology Information?

[RFC4655] specifies that a PCE performs path computations based on a
view of the available network resources and network topology. This
information is collected into a Traffic Engineering Database (TED).

However, [RFC4655] does not provide a detailed description of what
information is present in the TED. It simply says that the TED
"contains the topology and resource information of the domain." The
precise information that needs to be held in a TED depends on the
type of network and nature of the computation that has to be
performed. As a basic minimum, the TED must contain the nodes and
links that form the domain, and it must identify the connectivity in
the domain.

For most traffic-engineering needs (for example, MPLS Traffic
Engineering - MPLS-TE), the TED would additionally contain a basic
metric for each link and knowledge of the available (unallocated)
resources on each link.

More advanced use cases might require that the TED contain additional
data that represents qualitative information such as:

- link delay
- link jitter
- node throughput capabilities
- optical impairments
- switching capabilities
- limited node cross-connect capabilities

Additionally, an important information element for computing paths,
especially for protected services, is the Shared Risk Group (SRG).
This is an indication of resources in the TED that have a common risk
of failure. That is, they have a shared risk of failure from a
single event.

In short, the TED needs to contain as much information as is needed
to satisfy the path computation requests subject to the objective
functions (OFs). This, in itself, may not be a trivial issue in some
network technologies. For example, in some optical networks, the
path computation for a new Label Switched Path (LSP) may need to
consider the impact that turning up a new laser would have on the
optical signals already being carried by fibers. It may be possible
to abstract this information as parameters of the optical links and
nodes in the TED, but it may be easier to capture this information
through a database of existing LSPs (see Sections 14 and 15).

3. How Is Topology Information Gathered?

Clearly, the information in the TED discussed in Section 2 needs to
be gathered and maintained somehow. [RFC4655] simply says "The TED
may be fed by Interior Gateway Protocol (IGP) extensions or
potentially by other means." In this context, "fed" means built and
maintained.

Thus, one way that the PCE may construct its TED is by participating
in the IGP running in the network. In an MPLS-TE network, this would
depend on OSPF TE [RFC3630] and IS-IS TE [RFC5305]. In a GMPLS
network, it would utilize the GMPLS extensions to OSPF and IS-IS,
[RFC4203] and [RFC5307].

However, participating in an IGP, even as a passive receiver of IGP
information, can place a significant load on the PCE. The IGP can be
quite "chatty" when there are frequent updates to the use of the
network, meaning that the PCE must dedicate significant processing to
parsing protocol messages and updating the TED. Furthermore, to be
truly useful, a PCE implementation would need to support OSPF and IS-
IS.

An alternative feed from the network to the PCE's TED is offered by
BGP-LS [LS-DISTRIB]. This approach offers the alternative of
leveraging an in-network BGP speaker (such as an autonomous system
border router Autonomous System
Border Router or a route reflector) Route Reflector) that already has to participate
in the IGP and that is specifically designed to apply filters to IGP
advertisements. In this usage, the BGP speaker filters and
aggregates topology information according to configured policy before
advertising it "north-bound" to the PCE to update the TED. The PCE
implementation has to support just a simplified subset of BGP rather
than two full IGPs.

But BGP might not be convenient in all networks (for example, where
BGP is not run, such as in an optical network or a BGP-free core).
Furthermore, not all relevant information is made available through
standard TE extensions to the IGPs. In these cases, the TED must be
built or supplemented from other sources such as the Network
Management System (NMS), inventory management systems, and directly
configured data.

It has also been proposed that the PCE Communication Protocol (PCEP)
[RFC5440] could be extended to serve as an information collection
protocol to supply information from network devices to a PCE. The
logic is that the network devices may already speak PCEP; so, the
protocol could easily be used to report details about the resources
and state in the network, including the LSP state discussed in
Sections 14 and 15.

Note that a PCE that is responsible for more than one domain must, of
course, collect TE information from each domain to build its TED or
TEDs.

4. How Do I Find My PCE?

A Path Computation Client (PCC) needs to know the identity/location
of a PCE in order to be able to make computation requests. This is
because PCEP is a transaction-based protocol carried over TCP, and
the architectural decision made in Section 6.4 of RFC 4655 required
targeted PCC-PCE communications.

As described in [RFC4655], a PCC could be configured with the
knowledge of the IP address of its PCE. This is a relatively
lightweight option considering all of the other configuration that a
router may require, but it is open to configuration errors, and does
not meet the need for minimal-configuration operation. Furthermore,
configuration communication with multiple PCEs could become onerous,
while handling changes in PCE identities and coping with failure
events would be an issue for a configured system.

[RFC4655] offers the possibility for PCEs to advertise themselves in
the IGP, and this requirement is developed in [RFC4674] and made
possible in OSPF and IS-IS through [RFC5088] and [RFC5089]. In
general, these mechanisms should be sufficient for PCCs in a network
where an IGP is used and where the PCE participates in the IGP.

Note, however, that not all PCEs will participate in the IGP (see
Section 3). In these cases, assuming configuration is not
appropriate as a discovery mechanism, some other server
announcement/discovery function may be needed, such as DNS [RFC4848]
as used for discovery of the Local Location Information Server (LIS)
[RFC5986] and in the Application-Layer Traffic Optimization (ALTO)
discovery function [RFC7286]. [ALTO-SERVER-DISC].

5. How Do I Select a PCE? between PCEs?

When more than one PCE is discovered or configured, a PCC will need
to select which PCE to use. It may make this decision on any
arbitrary algorithm (for example, first-listed, or round robin), but
it may also be the case that different PCEs have different
capabilities and path computation scope; in which case, the PCC will
want to select the PCE most likely to be able to satisfy any one
request. The first requirement, of course, is that the PCE can
compute paths for the relevant domain.

PCE advertisement in OSPF or IS-IS per [RFC5088] and [RFC5089] allows
a PCE to announce its capabilities as required in [RFC4657]. A PCC
can select between PCEs based on the capabilities that they have
announced. However, these capabilities are expressed as flags in the
PCE advertisement so only the core capabilities are presented, and
there is not scope for including detailed information (such as
support for specific objective functions) in the advertisement.

Additional and more complex PCE capabilities, including the
capability to perform point-to-multipoint (P2MP) path computations
[RFC6006], may be announced by the PCE as optional PCEP type-length-
value (TLV) Type Indicators in the Open message described in
[RFC5440]. This mechanism is not limited to just a set of flags, and
detailed capability information may be presented in sub-TLVs.

Note that this exchange of PCE capabilities is in the form of an
announcement, not a negotiation. That is, a PCC that wants specific
function from a PCE must examine the advertised capabilities and
select which PCE to use for a specific request. There is no scope
for a PCC to request a PCE to support features or functions that it
does not offer or announce.

A PCC may also vary which PCE it uses according to congestion
information reported by the PCEs using the Notification Object and
Notification Type [RFC5440]. In a heavily overloaded PCE system,
note that reports from one PCE that it is overloaded may simply
result in all PCCs switching to another PCE, which will, itself,
immediately become overloaded. Thus, PCCs should exercise a certain
amount of discretion and queueing theory before selecting a PCE
purely based on reported load.

Note that a PCC could send all requests to all PCEs that it knows
about. It can then select between the results, perhaps choosing the
first result it receives, but this approach is very likely to
overload all the PCEs in the network considering that one of the
reasons for multiple PCEs is to share the load.

6. How Do Redundant PCEs Synchronize TEDs?

A network may have more than one PCE, as discussed in the previous
sections. These PCEs may provide redundancy for load-sharing,
resilience, or partitioning of computation features.

In order to achieve some consistency between the results of different
PCEs, it is desirable that they operate on the same TE information.

The TED reflects the actual state of the network and is not a
resource reservation or booking scheme. Therefore, a PCE-based
system does not prevent competition for network resources during the
provisioning phase, although a process of "sticky resources" that are
temporarily reduced in the TED after a computation may be applied
purely as a local implementation feature.

One option for ensuring that multiple PCEs use the same TE
information is simply to have the PCEs driven from the same TED.
This could be achieved in implementations by utilizing a shared
database, but it is unlikely to be efficient.

More likely is that each PCE is responsible for building its own TED
independently, using the techniques described in Section 3. If the
PCEs participate in the IGP, it is likely that they will attach at
different points in the network; so, there may be minor and temporary
inconsistencies between their TEDs caused by IGP convergence issues.
If the PCEs gather TE information via BGP-LS [LS-DISTRIB] from
different sources, the same inconsistencies may arise. However, if
the PCEs attach to the same BGP speaker, it may be possible to
achieve consistency between TEDs modulo the BGP-LS process itself.

A final option is to provide an explicit synchronization process
between the TED of a "master" PCE and the TEDs of other PCEs. Such a
process could be achieved using BGP-LS or a database synchronization
protocol (which would allow check-pointing and sequential updates).
This approach is fraught with issues around selection of the master
PCE and handling failures. It is, in fact, a mirrored database
scenario: a problem that is well known and the subject of plenty of
work.

Noting that the provisioning protocols such as RSVP-TE [RFC3209]
already handle contention for resources, that the differences between
TEDs are likely to be relatively small with moderate arrival rates
for new services, and that contention in all but the most busy
networks is relatively unlikely, there may be no value in any attempt
to synchronize TEDs between PCEs.

However, see Section 16 for a discussion of synchronizing other state
between redundant PCEs.

7. Where Is the Destination?

Path computation provides an end-to-end path between a source and a
destination. If the destination lies in the source domain, then its
location will be known to the PCE and there are no issues to be
solved. However, in a multi-domain system a path must be found to a
remote domain that contains the destination, and that can only be
achieved by knowledge of the location of the destination or at least
knowing the next domain in the path toward the domain that contains
the destination.

The simplest solution here is achieved when a PCE has visibility into
multiple domains. Such may be the case in a multi-area network where
the PCE is aware of the contents of all of the IGP areas. This
approach is only likely to be appropriate where the number of nodes
is manageable, and it is unlikely to extend over administrative
boundaries.

The per-domain path computation method for establishing inter-domain
traffic engineering LSPs [RFC5152] simply requires a PCE to compute a
path to the next domain toward the destination. As the LSP setup
(through signaling) progresses domain by domain, the Label Switching
Router (LSR) at the entry point to each domain requests its local PCE
to compute the next segment of the path, that is from that LSR to the
next domain in the sequence toward the destination. Thus, it is not
necessary for any PCE (except the last) to know in which domain the
destination exists. But, in this approach, each PCE must somehow
determine the next domain toward the destination, and it is not
obvious how this is achieved.

[RFC5152] suggests that, in an IP/MPLS network, it is good enough to
leverage the IP reachability information distributed by BGP and
assume that TE reachability can follow the same Autonomous System
(AS) path. This approach might not guarantee the optimal TE path
and, of course, might result in no path being found in degenerate
cases. Furthermore, in many network technologies (such as optical
networks operated by GMPLS) there may be limited or no end-to-end IP
connectivity.

The Backward Recursive PCE-based Computation (BRPC) procedure
[RFC5441] is able to achieve a more optimal end-to-end path than the
per-domain method, but depends on the knowledge of both the domain in
which the destination is located and the sequence of domains toward
the destination. This information is described in [RFC5441] as being
known a priori. Clearly, however, information is not always known a
priori, and it may be hard for the PCE that serves the source PCC to
discover the necessary details. While there are several approaches
to solving the question of establishing the domain sequence (for
example, BRPC trial and error or H-PCE [RFC6805]), none of them
addresses the issue of determining where the destination lies.

One argument that is often made is that an end-to-end connection
expressed as an LSP is a feature of a service agreement between
source and destination. If that is the case, it is argued, it stands
to reason that the location of the destination must be known to the
source node in the same way that the source has determined the IP
address of the destination. Presumably, this would be through a
commercial process or an administrative protocol.

[RFC4974] introduced the concept of Calls and Connections for LSPs.
A Call does not provide the actual connectivity for transmitting user
traffic, but builds a relationship that will allow subsequent
Connections to be made. A Call might be considered an agreement to
support an end-to-end LSP that is made between the endpoint nodes.
Call messages are sent and routed as normal IP messages, so the
sender does not need to know the location of the destination.

Furthermore, Call requests are responded, and the Call Response can
carry information (such as the identity of the domain containing the
destination) for use by Call initiator. Thus, the use of GMPLS Calls
might provide a mechanism to discover destination's location.

8. Who Runs or Owns a Parent PCE?

A parent PCE [RFC6805] is responsible for selecting inter-domain path
by coordinating with child PCEs and maintaining a domain topology
map.

In the case of multi-domains (e.g., IGP areas or multiple ASes)
within a single service provider network, the management
responsibility for the parent PCE would most likely be handled by the
service provider.

In the case of multiple ASes within different service provider
networks, it may be necessary for a third party to manage the parent
PCEs according to commercial and policy agreements from each of the
participating service providers. Note that the H-PCE architecture
does not require disclosure of internals of a child domain to the
parent PCE. Thus, there is ample scope for a parent PCE to be run by
one of the connected service providers or by a third party without
compromising commercial issues. In fact, each service provider could
run its own parent PCE while allowing its child PCEs to be contacted
by outsider parent PCEs according to configured policy and security.

9. How Do I Find My Parent PCE?

[RFC6805] specifies that a child PCE must be configured with the
address of its parent PCE in order for it to interact with its parent
PCE. There is no scope for parent PCEs to advertise their presence;
however, there is potential for directory systems (such as DNS
[RFC4848] as used in the ALTO discovery function [RFC7286]) [ALTO-SERVER-DISC])
to be used as described in Section 4.

According to [RFC6805], note that the child PCE must also be
authorized to peer with the parent PCE. This is discussed from the
viewpoint of the parent PCE in Section 10. The child PCE may need to
participate in a key distribution protocol in order to properly
authenticate its identity to the parent PCE.

10. How Do I Find My Child PCEs?

Within the hierarchical PCE framework [RFC6805], the parent PCE must
only accept path computation requests from authorized child PCEs. If
a parent PCE receives a request from an unauthorized child PCE, the
request should be dropped.

This requires a parent PCE to be configured with the identities and
security credentials of all of its child PCEs, or there must be some
form of shared secret that allows an unknown child PCE to be
authorized by the parent PCE.

11. How Is the Parent PCE Domain Topology Built?

The parent PCE maintains a domain topology map of the child domains
and their interconnectivity. This map does not include any
visibility into the child domains. Where inter-domain connectivity
is provided by TE links, the capabilities of those links may also be
known to the parent PCE.

The parent PCE maintains a TED for the parent domain in the same way
that any PCE does. The nodes in the parent domain will be
abstractions of the child domains (connected by real or virtual TE
links), but the parent domain may also include real nodes and links.

The mechanism for building the parent TED is likely to rely heavily
on administrative configuration and commercial issues because the
network was probably partitioned into domains specifically to address
these issues. However, note that in some configurations (for
example, collections of small optical domains) a separate instance of
a routing protocol (probably an IGP) may be run within the parent
domain to advertise the domain interconnectivity. Additionally,
since inter-domain TE links can be advertised by the IGPs operating
in the child domains, this information could be exported to the
parent PCE either by the child PCEs or using a north-bound export
mechanism such as BGP-LS [LS-DISTRIB].

12. Does H-PCE Solve the Internet?

The model described in [RFC6805] introduced a hierarchical
relationship between domains. It is applicable to environments with
small groups of domains where visibility from the ingress LSRs is
limited. Applying the hierarchical PCE model to large groups of
domains such as the Internet is not considered feasible or desirable.

This does open up a harder question: how many domains can be handled
by an H-PCE system? In other words: what is a small group of
domains? The answer is not clear and might be "I know it when I see
it." At the moment, a rough guide might be around 20 domains as a
maximum.

An associated question would be: how many hierarchy levels can be
handled by H-PCE? Architecturally, the answer is that there is no
limit, but it is hard to construct practical examples where more than
two or possibly three levels are needed.

13. What are Sticky Resources?

When a PCE computes a path, it has a reasonable idea that an LSP will
be set up and that resources will be allocated within the network.
If the arrival rate of computation requests is faster than the LSP
setup rate combined with the IGP convergence time, it is quite
possible that the PCE will perform its next computation before the
TED has been updated to reflect the setup of the previous LSP. This
can result in LSP setup failures if there is contention for
resources. The likelihood of this problem is particularly high
during recovery from network failures when a large number of LSPs
might need new paths.

A PCE may choose to make a provisional assignment of the resources
that would be needed for an LSP and to reduce the available resources
in its TED so that the problem is mitigated. Such resources are
informally known as "sticky resources".

Note that using sticky resources introduces a number of other
problems that can make managing the TED difficult. For example:

- When the TED is updated as a result of new information from the
IGP, how does the PCE know whether the reduction in available
resources is due to the successful setup of the LSP for which it
is holding sticky resources or due to some other network event
(such as the setup of another LSP)? This problem may be
particularly evident if there are multiple PCEs that do not
synchronize their sticky resources or if not all LSPs utilize PCE
computation.

- When LSP setup fails, how are the sticky resources released?
Since the PCE doesn't know about the failure of the LSP setup, it
needs some other mechanism to release them.

- What happens if a path computation was made only to investigate
the potential for an LSP but not to actually set one up?

- What if the path used by the LSP does not match that provided by
the PCE (for example, because the control plane routes around some
problem)?

Some of these issues can be mitigated by using a Stateful PCE (see
Section 14) or by timers.

14. What Is a Stateful PCE for?

A Stateless PCE can perform path computations that take into account
the existence of other LSPs if the paths of those LSPs are supplied
on the computation request. This function can be particularly useful
when arranging protection paths so that a working and protection LSP
do not share any links or nodes. It can also be used when a group of
LSPs are to be reoptimized at the same time in the process known as
Global Concurrent Optimization (GCO) [RFC5557].

However, this mechanism can be quite a burden on the protocol
messages, especially when large numbers of LSP paths need to be
reported.

A Stateful PCE [STATEFUL-PCE] maintains a database of LSPs (the LSP-
DB) that are active in the network, i.e., have been provisioned such
that they use network resources although they might or might not be
carrying traffic. This database allows a PCC to refer to an LSP
using only its identifier -- all other details can be retrieved by
the PCE from the LSP-DB.

A Stateful PCE can use the LSP-DB for many other functions, such as
balancing the distribution of LSPs in the network. Furthermore, the
PCE can correlate LSPs with network resource availability placing new
LSPs more cleverly.

A Stateful PCE that is also an Active PCE (see Section 17) can
respond to changes in network resource availability and predicted
demands to reroute LSPs that it knows about.

Section 20 offers a brief comparison of the different modes of PCE
with reference to stateful and stateless PCE.

15. How Is the LSP-DB Built?

The LSP-DB contains information about the LSPs that are active in the
network, as mentioned in Section 14. This state information can be
constructed by the PCE from information it receives from a number of
sources including from provisioning tools and from the network, but
no matter how the information is gleaned, a Stateful PCE needs to
synchronize its LSP-DB with the state in the network. Just as
described in Section 13, the PCE cannot rely on knowledge about
previous computations it has made, but it must find out the actual
LSPs in the network.

A simple solution is for all ingress LSRs to report all LSPs to the
PCE as they are set up, modified, or torn down. Since PCEP already
has the facility to fully describe LSP routes and resources in the
protocol messages, this is not a difficult problem, and the LSP State
Report (PCRpt) message has been defined for this purpose
[STATEFUL-PCE].

The situation can be more complex, however, if there are ingress LSRs
that do not support PCEP, support PCEP but not the PCRpt, or that are
unaware of the requirement to report LSPs to the PCE. This might
happen if the LSRs are able to compute paths themselves or if they
receive LSP setup instructions with pre-computed paths from an NMS.

An alternative approach is to note that any LSR on the path of an LSP
can probably see the whole path (through the Record Route object in
RSVP-TE signaling [RFC3209]) and knows the bandwidth reserved for the
LSP. Thus, any LSR could report the LSP to the PCE, noting that it
will not hurt (beyond additional message processing and potential
overload of the PCE or the network) for the LSP to be reported
multiple times because it is clearly identified. In fact, this would
also provide a cross-check mechanism.

Nevertheless, it is possible that some LSPs will traverse only LSRs
that are not aware of the PCE's need to learn LSP state and build an
LSP-DB. In these cases, the stateful PCE must either only have
limited knowledge of the LSPs in the network or must learn about LSPs
through some other mechanism (such as reading the MPLS and GMPLS MIB
modules [RFC3812] [RFC4802]).

Ultimately, there may be no substitute for all LSRs being aware of
Stateful PCEs and able to respond to requests for reports on all LSPs
that they know about. This will allow a Stateful PCE to build its
LSP-DB from scratch (which it may need to do at start of day) and to
verify its LSP-DB against the network (which may be important if the
PCE has suffered some form of outage).

16. How Do Redundant Stateful PCEs Synchronize State?

It is important that two PCEs operating in a network have similar
views of the available resources. That is, they should have the same
or substantially similar TEDs. This is easy to achieve either by
building the TEDs from the network in the same way or by one PCE
synchronizing its TED to the other PCE using a TED export protocol
such as BGP-LS [LS-DISTRIB] or the Network Configuration Protocol
(NETCONF) [RFC6241] (see Section 6).

Synchronizing the LSP-DB can be a more complicated issue. As
described in Section 15, building the LSP-DB can be an involved
process, so it would be best to not have multiple PCEs each trying to
build an LSP-DB from the network. However, it is still important
that where multiple PCEs operate in the network (either as
distributed PCEs or with one acting as a backup for the other), their
LSP-DBs are kept synchronized.

Thus, there is likely to be a need for a protocol mechanism for one
PCE to update its LSP-DB with that of another PCE. This is no
different from any other database-synchronization problem and could
use existing mechanisms or a new protocol. Note, however, that in
the case of distributed PCEs that are also Active PCEs (see Section
17), each PCE will be creating entries in its own LSP-DB; so, the
synchronization of databases must be incremental and bidirectional,
not just simply a database dump.

It may be helpful to clarify the word "redundant" in the context of
this question. One interpretation is that a redundant PCE exists
solely as a backup such that it only performs a function in the
network in the event of a failure of the primary PCE. This seems
like a waste of expensive resources, and it would make more sense for
the redundant PCE to take its share of computation load all the time.
However, that scenario of two (or more) active PCEs creates exactly
the state synchronization issue described above.

Various deployment options have been suggested where one PCE serves a
set of PCCs as the primary computation server, and only addresses
requests from other PCCs in the event of the failure of some other
PCE; however, this mode of operation still raises questions about the
need for synchronized state even in non-failure scenarios if the LSPs
that will be computed by the different PCEs may traverse the same
network resources.

17. What Is an Active PCE? What Is a Passive PCE?

A Passive PCE is one that only responds to path computation requests.
It takes no autonomous actions. A Passive PCE may be stateless or
stateful.

An Active PCE is one that issues provisioning "recommendations" to
the network. These recommendations may be new routes for existing
LSPs or routes for new LSPs (that is, an Active PCE may recommend the
instantiation of new LSPs). An Active PCE may be stateless or
stateful, but in order for it to reroute existing LSPs effectively,
it is likely to hold state for at least those LSPs that it will
reroute.

Many people consider that the PCE, itself, cannot be Active. That
is, they hold that the PCE's function is purely to compute paths. In
that worldview, the "Active PCE" is actually the combination of a
normal, passive PCE and an additional architectural component
responsible for issuing commands or recommendations to the network.

In some configurations, the VNTM discussed in Sections 21 and 22
provides this additional component.

Section 20 offers a brief comparison of the different modes of PCE
with reference to passive and active PCE.

18. What is LSP Delegation?

LSP delegation [STATEFUL-PCE] is the process where a PCC (usually an
ingress LSR) passes responsibility for triggering updates to the
attributes of an LSP (such as bandwidth or path) to the PCE. In this
case, the PCE would need to be both Stateful and Active.

LSP delegation allows an LSP to be set up under the control of the
ingress LSR potentially using the services of a PCE. Once the LSP
has been set up, the LSR (a PCC) tells the PCE about the LSP by
providing details of the path and resources used. It delegates
responsibility for the LSP to the PCE so that the PCE can make
adjustments to the LSP as dictated by changes to the TED and the
policies in force at the PCE. The PCE makes the adjustments by
sending a new path to the LSR with the instruction/recommendation
that the LSP be re-signaled.

There may be some debate over whether the PCE "owns" the LSP after
delegation. That is, if the PCE supplies a new path, is the ingress
LSR required to act or can it take the information "under
advisement"? It may be too soon to answer this question
definitively; however, there is certainly an expectation that the LSR
will act on the advice it receives. A comparison may be drawn with a
visit to the doctor: the doctor has an expectation that the patient
will take the medicine, but the patient has free will.

It is important, however, to distinguish between an LSP established
within the network and subsequently delegated to a PCE and an LSP
that was established as the result of an Active PCE's recommendation
for LSP instantiation.

Section 20 offers a brief comparison of the different modes of PCE
with reference to LSP delegation.

19. Is an Active PCE with LSP Delegation Just a Fancy NMS?

In many ways the answer here is "yes". But the PCE architecture
forms part of a new way of looking at network operation and
management. In this new view, the network operation is more dynamic
and under the control of software applications without direct
intervention from operators. This is not to say that the operator
has no say in how their network runs, but it does mean that the
operator sets policies (see Section 24) and that new components (such
as an Active PCE) are responsible for acting on those policies to
dynamically control the network.

There is a subtle distinction between an NMS and an Active PCE with
LSP delegation. An NMS is in control of the LSPs in the network and
can command that they are set up, modified, or torn down. An Active
PCE can only make suggestions about LSPs that have been delegated to
the PCE by a PCC, or make recommendations for the instantiation of
new LSPs.

For more details, see the discussion of an architecture for
Application-Based Network Operation (ABNO) in [NET-OPS]

20. Comparison of Stateless and Stateful PCE

Table 1 shows a comparison of stateless and stateful PCEs to show how
they how might be instantiated as passive or active PCEs with or
without control of LSPs. The terms used relate to the concepts
introduced in the previous sections. The entries in the table refer
to the notes that follow.

| Stateless | Stateful |
------------------------+-----------+-----------+
Passive | 1 | 2 |
Active delegated LSPs | 3 | 4 |
Active suggest new LSPs | 5 | 6 |
Active instantiate LSPs | 7 | 7 |

Notes:
1. Passive is the normal mode for a stateless PCE.
2. A passive mode stateful PCE may have value for more complex
environments and for computing protected services.
3. Delegation of LSPs to a stateless PCE is relatively pointless,
but could add value at moment of delegation.
4. This is the normal mode for a stateful PCE.
5. There is only marginal potential for a stateless PCE to
recommend new LSPs because without a view of existing LSPs, the
PCE cannot determine when new ones might be needed.
6. This mode has potential for recommending the instantiation of
new LSPs.
7. These modes are out of scope for PCE as currently described.
That is, the PCE can recommend instantiation, but cannot
actually instantiate the LSPs.

Table 1 : Comparing Stateless and Stateful PCE

21. How Does a PCE Work with a Virtual Network Topology?

A Virtual Network Topology (VNT) is described in [RFC4397] as a set
of Hierarchical LSPs that is created (or could be created) in a
particular network layer to provide network flexibility (data links)
in other layers. Thus, the TE topology of a network can be
constructed from TE links that are simply data links, from TE links
that are supported by LSPs in another layer of the network, or from
TE links that could be supported by LSPs ("potential LSPs") that
would be set up on demand in another network layer. This third type
of TE link is known as a Virtual TE Link in [RFC5212].

[RFC5212] also gives a more detailed explanation of a VNT, and it
should be noted that the network topology in a packet network could
be supported by LSPs in a number of different lower-layer networks.
For example, the TE links in the packet network could be achieved by
connections (LSPs) in underlying Synchronous Optical Network or
Synchronous Digital Hierarchy (SONET/SDH) and photonic networks.
Furthermore, because of the hierarchical nature of MPLS, the TE links
in a packet network may be achieved by setting up packet LSPs in the
same packet network.

A PCE obviously works with the TED that contains information about
the TE links in the network. Those links may be already established
or may be virtual TE links. In a simple TED, there is no distinction
between the types of TE link; however, there may be advantages to
selecting TE links that are based on real data links over those based
on dynamic LSPs in lower layers because the data links may be more
stable. Conversely, the TE links based on dynamic LSPs may be able
to be repaired dynamically giving better resilience. Similarly, a
PCE may prefer to select a TE link that is supported by a data link
or existing LSP in preference to using a virtual TE link because the
latter may need to be set up (taking time) and the setup could
potentially fail. Thus, a PCE might want to employ additional
metrics or indicators to help it view the TED and select the right
path for LSPs.

If a PCE uses a virtual TE link, then some action will be needed to
establish the LSP that supports that link. Some models (such as that
in [RFC5212]) trigger the setup of the lower-layer LSPs on-demand
during the signaling of the upper-layer LSP (i.e., when the upper
layer comes to use the virtual TE link, the upper-layer signaling is
paused and the lower-layer LSP is established). Another view,
described in [RFC5623], is that when the PCE computes a path that
will use a virtual TE link, it should trigger the setup of the lower-
layer LSP to properly create the TE link so that the path it returns
will be sure to be viable. This latter mode of operation can be
extended to allow the PCE to spot the need for additional TE links
and to trigger LSPs in lower layers in order to create those links.

Of course, such "interference" in a lower-layer network by a PCE
responsible for a higher-layer network depends heavily on policy. In
order to make a clean architectural separation and to facilitate
proper policy control, [RFC5623] introduces the Virtual Network
Topology Manager (VNTM) as a functional element that manages and
controls the VNT. [RFC5623] notes that the PCE and VNT Manager are
distinct functional elements that may or may not be collocated.
indeed, it should be noted that there will be a PCE for the upper
layer, and a PCE for each lower layer, and a VNTM responsible for
coordinating between the PCEs and for triggering LSP setup in the
lower layers. Therefore, the combination of all of the PCEs and the
VNTM produces functionally similar to an Active, multi-layer PCE.

See [TE-INFO] for additional discussion of the construction of
networks using virtual and potential links.

22. How Does PCE Communicate with VNTM

The VNTM described in Section 21 and [RFC5623] has several interfaces
(see also [NET-OPS]).

- The In order to make decisions on whether to create new TE links, the
VNTM will need needs to learn from the upper-layer PCE about resource
shortages and the need for additional TE links from the upper-layer PCE in order that it links. It can then make
policy-based decisions to determine whether and which
LSPs to set up to create new TE links. This interface is
currently undefined. links
and how to support them through existing or new LSPs.

- The VNTM will need to coordinate with the PCEs in the lower
layers, but this is simply a normal use of PCEP.

- The VNTM will need to issue provisioning requests/commands (via
the Provisioning Manager described in [NET-OPS]) to the lower-
layer networks to cause LSPs to be set up to act as TE links in
the higher layer network. A number of potential protocols exist
for this function as described in [NET-OPS], but it should be
noted that it makes a lot of sense for this interface to be the
same as that used by an Active PCE when providing paths to the
network.

23. How Does Service Scheduling and Calendering Work?

LSP scheduling or calendaring is a process where LSPs are planned
ahead of time, and they are only set up when needed. The challenge
here is to ensure that the resources needed by an LSP and that were
available when the LSP's path was computed are still available when
the LSP needs to be set up. This needs to be achieved using a
mechanism that allows those resources to be used in the meantime.

Previous discussion of this topic has suggested that LSPs should be
pre-signaled so that each LSR along the path could make a "temporal
reservation" of resources. But this approach can become very
complicated requiring each network node to store multi-dimensional
state.

Conversely, a centralized database of resources and LSPs (such as the
database maintained by a Stateful PCE) can be enhanced with a time-
based booking system. If the PCE is also Active, then when the time
comes for the LSP to be set up (or later, when it is to be torn
down), the PCE can issue recommendations to the network.

In a busy network (and why would one bother with a scheduling service
in a network that is not busy?), it should be noted that the
computation algorithm can be quite complex. It may also be necessary
to reposition existing or planned LSPs as new bookings arrive.

Furthermore, the booking database that contains both the scheduled
LSPs and their impact on the network resources can become quite
large. A very important factor in the size of the active database
(depending on implementation) may be the timeslices that are
available in the calendering process.

24. Where Does Policy Fit In?

Policy is critical to the operation of a network. In a PCE context,
it provides control and management of how a PCE selects network
resources for use by different PCEs.

[RFC5394] introduced the concept of PCE-based policy-enabled path
computation. It is based on the Policy Core Information Model (PCIM)
[RFC3060] as extended by [RFC3460], and provides a framework for
supporting path computation policy.

Policy enters into all aspects of the use of a PCE starting from the
very decision to use a PCE to off-load computation function from the
LSRs.

- Each PCC must select which computations will be delegated to a
PCE.

- Each PCC must select which PCEs it will use.

- Each PCE must determine which PCCs are allowed to use its services
and for what computations.

- The PCE must determine how to collect the information in its TED,
who to trust for that information, and how to refresh/update the
information.

- Each PCE must determine which objective functions and which
algorithms to apply.

- Inter-domain (and particularly H-PCE) computations will need to be
sensitive to commercial and reliability information about domains
and their interactions.

- Stateful PCEs must determine what state to hold, when to refresh
it, and which network elements to trust for the supply of the
state information.

- An Active PCE must have a policy relationship with its LSRs to
determine which LSPs can be modified or triggered, and what LSP
delegation is supported.

- Multi-layer interactions (especially those using virtual or
dynamic TE links) must provide policy control to stop server layer
LSPs (which are fat and expensive by definition) from being set up
on a whim to address micro-flows or speculative computations in
higher layers.

- A PCE may supply, along with a computed path, policy information
that should be signaled during LSP setup for use by the LSRs along
the path.

It may be seen, therefore, that a PCE is substantially a policy
engine that computes paths. It should also be noted that the work of
the PCE can be substantially controlled by configured policy in a way
that will reduce the options available to the PCC, but also
significantly reduce the need for the use of optional parameters in
the PCEP messages.

25. Does PCE Play a Role in SDN?

Software-Defined Networking (SDN) is the latest shiny thing in
networking. It refers to a separation between the control elements
and the forwarding components so that software running in a
centralized system called a controller, can act to program the
devices in the network to behave in specific ways.

A required element in an SDN architecture is a component that plans
how the network resources will be used and how the devices will be
programmed. It is possible to view this component as performing
specific computations to place flows within the network given
knowledge of the availability of network resources, how other
forwarding devices are programmed, and the way that other flows are
routed. This, it may be concluded, is the same function that a PCE
might offer in a network operated using a dynamic control plane.
Thus, a PCE could form part of the infrastructure for an SDN.

A view of how PCE integrates into a wider network control system
including SDN is presented in [NET-OPS].

26. Security Considerations

The use of a PCE-based architecture and subsequent impact on network
security must, itself, be considered in the context of existing
routing and signaling protocols and techniques. The nature of multi-
domain network scenarios and establishment of relationships between
PCCs and PCEs may increase the vulnerability of the network to
security attacks. However, this informational document does not
define any new protocol elements or mechanism. As such, it does not
introduce any new security issues and security is deemed to be a
"previously answered question" even if the answers previously
supplied are not perfect. Previous PCE RFCs have given some
attention to security concerns in the use of PCE (RFC 4655), PCE
discovery (RFC 4674, RFC 5088, and RFC 5089), and PCEP (RFC 4657 and
RFC 5440).

It is worth noting that PCEP operates over TCP. An analysis of the
security issues for routing protocols that use TCP (including PCEP)
is provided in [RFC6952], while [PCE-PCEPS] discusses an experimental
approach to provide secure transport for PCEP.

A number of the questions raised and answered in this document should
be given consideration in the light of security requirements. Some
of these are called out explicitly (Sections 8 and 10), but attention
should also be paid to security in all aspects of the use of PCE.
For example:

- Topology and other information about the network needs to be kept
private and protected from modification or forgery. That means
that access to the TED, LSP-DB, etc., needs to be secured and that
mechanisms used to gather topology and other information (Sections
2, 11, 14, and 15) need to include security.

- PCE discovery (Sections 4, 5, 9, and 10) needs to protect against
impersonation or misconfiguration so that PCCs know that they are
getting correct paths and so that PCEs know that they are only
serving legitimate computation requests.

- Synchronization of information and state between PCEs (Sections 6
and 16) is subject to the same security requirements in that the
information exchanged is sensitive and needs to be protected
against interception and modification.

- PCE computes paths for components that may provision the network.
Those component are responsible for the security of the
provisioning mechanisms, however, if PCE operates as a
provisioning protocol (Sections 17, 18, 19, and 25).

- A PCE may also need to interface with other network components
(Sections 19, 21, 22, and 25). Those communications, if external
to an implementation, also need to be secure.

27. References

27.1. Normative References

[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC
4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.

[RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
Computation Element (PCE) Communication Protocol
(PCEP)", RFC 5440, March 2009,
<http://www.rfc-editor.org/info/rfc5440>.

[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,
<http://www.rfc-editor.org/info/rfc5623>.

[RFC6805] King, D., Ed., and A. Farrel, Ed., "The Application of
the Path Computation Element Architecture to the
Determination of a Sequence of Domains in MPLS and
GMPLS", RFC 6805, November 2012,
<http://www.rfc-editor.org/info/rfc6805>.

27.2. Informative References

[ALTO-SERVER-DISC]
Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M.,
and H. Song, "ALTO Server Discovery", Work in
Progress, draft-ietf-alto-server-discovery-10,
September 2013.

[LS-DISTRIB] Gredler, H., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", Work in Progress,
draft-ietf-idr-ls-distribution-06, September 2014.

[NET-OPS] King, D., and A. Farrel, "A PCE-based Architecture for
Application-based Network Operations", Work in
Progress, draft-farrkingel-pce-abno-architecture-13,
October 2014.

[PCE-PCEPS] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"Secure Transport for PCEP", Work in Progress,
draft-ietf-pce-pceps-02, October 2014.

[RFC3060] Moore, B., Ellesson, E., Strassner, J., and A.
Westerinen, "Policy Core Information Model -- Version
1 Specification", RFC 3060, February 2001,
<http://www.rfc-editor.org/info/rfc3060>.

[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,
<http://www.rfc-editor.org/info/rfc3209>.

[RFC3460] Moore, B., Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460, January 2003
<http://www.rfc-editor.org/info/rfc3460>.

[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003,
<http://www.rfc-editor.org/info/rfc3630>.

[RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Traffic
Engineering (TE) Management Information Base (MIB)",
RFC 3812, June 2004,
<http://www.rfc-editor.org/info/rfc3812>.

[RFC4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 4203, October 2005,
<http://www.rfc-editor.org/info/rfc4203>.

[RFC4397] Bryskin, I. and A. Farrel, "A Lexicography for the
Interpretation of Generalized Multiprotocol Label
Switching (GMPLS) Terminology within the Context of
the ITU-T's Automatically Switched Optical Network
(ASON) Architecture", RFC 4397, February 2006,
<http://www.rfc-editor.org/info/rfc4397>.

[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006,
<http://www.rfc-editor.org/info/rfc4657>.

[RFC4674] Le Roux, J., Ed., "Requirements for Path Computation
Element (PCE) Discovery", RFC 4674, October 2006,
<http://www.rfc-editor.org/info/rfc4674>.

[RFC4726] Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A
Framework for Inter-Domain Multiprotocol Label
Switching Traffic Engineering", RFC 4726, November
2006, <http://www.rfc-editor.org/info/rfc4726>.

[RFC4802] Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
Multiprotocol Label Switching (GMPLS) Traffic
Engineering Management Information Base", RFC 4802,
February 2007,
<http://www.rfc-editor.org/info/rfc4802>.

[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery
Service (DDDS)", RFC 4848, April 2007,
<http://www.rfc-editor.org/info/rfc4848>.

[RFC4974] Papadimitriou, D. and A. Farrel, "Generalized MPLS
(GMPLS) RSVP-TE Signaling Extensions in Support of
Calls", RFC 4974, August 2007,
<http://www.rfc-editor.org/info/rfc4974>.

[RFC5088] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
R. Zhang, "OSPF Protocol Extensions for Path
Computation Element (PCE) Discovery", RFC 5088,
January 2008,
<http://www.rfc-editor.org/info/rfc5088>.

[RFC5089] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
R. Zhang, "IS-IS Protocol Extensions for Path
Computation Element (PCE) Discovery", RFC 5089,
January 2008,
<http://www.rfc-editor.org/info/rfc5089>.

[RFC5152] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
Per-Domain Path Computation Method for Establishing
Inter-Domain Traffic Engineering (TE) Label Switched
Paths (LSPs)", RFC 5152, February 2008,
<http://www.rfc-editor.org/info/rfc5152>.

[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,
<http://www.rfc-editor.org/info/rfc5212>.

[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008,
<http://www.rfc-editor.org/info/rfc5305>.

[RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 5307, October 2008,
<http://www.rfc-editor.org/info/rfc5307>.

[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J.
Ash, "Policy-Enabled Path Computation Framework", RFC
5394, December 2008,
<http://www.rfc-editor.org/info/rfc5394>.

[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
Roux, "A Backward-Recursive PCE-Based Computation
(BRPC) Procedure to Compute Shortest Constrained
Inter-Domain Traffic Engineering Label Switched
Paths", RFC 5441, April 2009,
<http://www.rfc-editor.org/info/rfc5441>.

[RFC5557] Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
Computation Element Communication Protocol (PCEP)
Requirements and Protocol Extensions in Support of
Global Concurrent Optimization", RFC 5557, July 2009,
<http://www.rfc-editor.org/info/rfc5557>.

[RFC5986] Thomson, M. and J. Winterbottom, "Discovering the
Local Location Information Server (LIS)", RFC 5986,
September 2010,
<http://www.rfc-editor.org/info/rfc5986>.

[RFC6006] Zhao, Q., Ed., King, D., Ed., Verhaeghe, F., Takeda,
T., Ali, Z., and J. Meuric, "Extensions to the Path
Computation Element Communication Protocol (PCEP) for
Point-to-Multipoint Traffic Engineering Label Switched
Paths", RFC 6006, September 2010,
<http://www.rfc-editor.org/info/rfc6006>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J.,
Ed., and A. Bierman, Ed., "Network Configuration
Protocol (NETCONF)", RFC 6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.

[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis
of BGP, LDP, PCEP, and MSDP Issues According to the
Keying and Authentication for Routing Protocols (KARP)
Design Guide", RFC 6952, May 2013,
<http://www.rfc-editor.org/info/rfc6952>.

[RFC7286] Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M.,
and H. Song, "Application-Layer Traffic Optimization
(ALTO) Server Discovery", RFC 7286, November 2014,
<http://www.rfc-editor.org/info/rfc7286>.

[STATEFUL-PCE] Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
Extensions for Stateful PCE", Work in Progress,
draft-ietf-pce-stateful-pce-10, October 2014.

[TE-INFO] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D, and X. Zhang, "Problem Statement and
Architecture for Information Exchange Between
Interconnected Traffic Engineered Networks", Work in
Progress, draft-farrel-interconnected-te-info-
exchange-07, September 2014.

Acknowledgements

Thanks for constructive comments go to Fatai Zhang, Oscar Gonzalez de
Dios, Xian Zhang, Cyril Margaria, Denis Ovsienko, Ina Minei, Dhruv
Dhody, and Qin Wu.

This work was supported in part by the FP-7 IDEALIST project under
grant agreement number 317999.

This work received funding from the European Union's Seventh
Framework Programme for research, technological development and
demonstration through the PACE project under grant agreement no.
619712.

Authors' Addresses

Adrian Farrel
Juniper Networks
EMail: adrian@olddog.co.uk

Daniel King
Old Dog Consulting
EMail: daniel@olddog.co.uk