Network Working Group Nabil Bitar (Editor)
Internet Draft Verizon
Raymond Zhang (Editor)
BT Infonet
Intended Status: Informational Kenji Kumaki (Editor)
KDDI Corporation
Expires: April 2007 October 2006
Inter-AS Requirements for the Path Computation Element
Communication Protocol (PCECP)
draft-ietf-pce-interas-pcecp-reqs-01.txt
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Bitar et al. Inter-AS Requirements for PCECP [Page 1]
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Abstract
Multiprotocol Label Switching Traffic Engineered (MPLS-TE)
LabelSwitched Paths (LSPs) may be established wholly within an
Autonomous System (AS) or may cross AS boundaries.
The Path Computation Element (PCE) is a component that is capable of
computing paths for MPLS-TE LSPs. The PCE Communication
Protocol(PCECP) is defined to allow communication between Path
Computation Clients (PCCs) and PCEs, and between PCEs. The PCECP is
used to request paths and to supply computed paths in responses.
Generic requirements for the PCECP are set out in "Path Computation
Element(PCE) Communication Protocol Generic Requirements", RFC 4657.
This document extends those requirements to cover the use of PCECP
in support of inter-AS MPLS-TE.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Table of Contents
1. Introduction.....................................................3
2. Definitions......................................................3
3. Reference Model..................................................4
4. Detailed PCECP Requirements for Inter-AS Computation.............5
4.1. PCE Communication Protocol Requirements........................5
4.1.1. Requirements for path computation requests...................5
4.1.2. Requirements for path computation responses..................6
4.2. Scalability and Performance Requirements.......................7
4.3. Management, Aliveness Detection, and Recovery Requirements.....8
4.4. Confidentiality................................................8
4.5. Policy Controls Affecting inter-AS PCECP.......................9
4.5.1. Inter-AS PCE Peering Policy Controls.........................9
4.5.2. Inter-AS PCE Reinterpretation Policies......................10
5. Security Considerations.........................................10
6. IANA Considerations.............................................11
7. Acknowledgments.................................................11
8. Authors' Addresses..............................................11
9. Normative References............................................11
10. Informative References.........................................12
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1. Introduction
[RFC4216] defines the scenarios motivating the deployment of inter-
AS Multiprotocol Label Switching Traffic Engineering (MPLS-TE).
[RFC4216] specifies the requirements for inter-AS MPLS-TE when the
ASs are under the administration of one Service Provider (SP) or the
administration of different SPs.
Three signaling options are defined for setting up an inter-AS TE
LSP:
1) contiguous TE LSP as documented in [INTERD-TESIG];
2) Stitched inter-AS TE LSP discussed in [LSP-STITCHING];
3) nested TE LSP as in [RFC4206].
[INTERD-TE-PDPC] defines mechanisms for the computation of inter-
domain TE LSPs using network elements along the signaling paths to
compute per-domain path segments. The mechanisms in [INTERD-TE-PDPC]
do not guarantee an optimum path across multiple ASs where an
optimum path for an LSP is one that has the smallest cost, according
to a normalized TE metric (based upon a TE-metric or IGP metric
adopted in each transit AS) among all possible paths that satisfy
the LSP TE-constraints.
The Path Computation Element (PCE) [RFC4655] is a component that is
capable of computing paths for MPLS-TE LSPs. The requirements for a
PCE have come from Service Provider (SP) demands to compute optimum
paths across multiple domains, and to be able to separate the path
computation elements from the forwarding elements.
The PCE Communication Protocol (PCECP) is defined to allow
communication between Path Computation Clients (PCCs) and PCEs, and
between PCEs. The PCECP is used to request paths and to supply
computed paths in responses. Generic requirements for the PCECP are
discussed in [RFC4657]. This document provides a set of PCECP
requirements that are specific to MPLS-TE inter-AS path computation.
2. Definitions
This document adopts the definitions and acronyms defined in Section
3 of [RFC4216] and Section 2 of [RFC4655]. In addition, we use the
following terminology:
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PCECP: PCE Communication Protocol
Inter-AS (G)MPLS-TE path: An MPLS-TE or Generalized MPLS (GMPLS)
path that traverses two or more ASs.
Intra-AS (G)MPLS-TE path: An MPLS-TE or GMPLS path that is confined
to a single AS. It may traverse one or more IGP areas.
Intra-AS PCE: A PCE responsible for computing MPLS-TE or GMPLS paths
remaining within a single AS.
Inter-AS PCE: A PCE responsible for computing inter-AS MPLS-TE or
GMPLS paths or path segments, possibly by cooperating with intra-AS
PCEs.
3. Reference Model
Figure 1 depicts the reference model for PCEs in an inter-AS
application. We refer to two types of PCE functions in this
document: inter-AS PCEs and intra-AS PCEs. Inter-AS PCEs perform the
procedures needed for inter-AS MPLS-TE or GMPLS path computation
while intra-AS PCEs perform the functions needed for intra-AS MPLS-
TE or GMPLS path computation.
Following is a scenario that depicts the interaction among PCCs,
inter-AS PCEs and intra-area PCEs based on Figure 1. R1 in AS1
wants to setup a TE-LSP or a GMPLS path with certain constraints
to R7 in AS3. R1 determines, using mechanisms out of the scope of
this document, that R7 is an AS-external route and that it needs to
contact Inter-AS PCE1 to compute the path. R1, as a PCC, sends a
PCECP path request to PCE1. PCE1 determines that R7 is reachable via
AS2 and that PCE2 is the PCE to ask for path computation across AS2.
PCE1 sends a PCECP path request to PCE2. Inter-AS PCE2 in turn sends
a PCECP path request to Intra-AS PCE R4 to compute a path within AS2
(In certain cases, the same router such R3 can assume both inter-AS
and intra-AS path computation functions). R4 returns a PCECP path
response to PCE2 with ASBR3 as the entry point to AS2 from AS1 and
ASBR7 as the exit point to AS3. PCE2 then sends a PCECP path request
to PCE3 to compute the path segment across AS3, starting at ASBR7 and
terminating at R7. PCE3 returns a PCECP path response to PCE2 with
the path segment ASBR7-R7. PCE2 then return path ASBR3-ASBR7-R7 to
PCE1 which in turn returns path ASBR1-ASBR3-ASBR7-R7 to PCC R1.
As described in the above scenario, in general, a PCC may contact an
inter-AS PCE to request an inter-AS path, and that PCE may supply
the path itself, or may solicit the services of other PCEs which
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may, themselves be inter-AS PCEs, or may be intra-AS PCEs with the
responsibility for computing path segments within just one AS.
This document describes the PCE Communication Protocol requirements
for inter-AS path computation. That is, for PCCs to communicate path
requests for inter-AS paths to a PCE, and for the PCE to respond. It
also includes the requirements for PCEs to communicate inter-AS path
requests and responses.
Inter-AS Inter-AS Inter AS
PCC <->PCE1<--------->PCE2<--------------->PCE3
:: :: :: ::
R1---ASBR1====ASBR3---R3---ASBR5====ASBR7---R5---R7
| | | | | |
| | | | | |
R2---ASBR2====ASBR4---R4---ASBR6====ASBR8---R6---R8
::
Intra-AS
PCE
<==AS1=> <====AS2======> <=====AS3===>
Figure 1 Inter and Intra-AS PCE Reference Model
4. Detailed PCECP Requirements for Inter-AS Computation
This section discusses detailed PCECP requirements for inter-AS
MPLS-TE and GMPLS. Depending on the deployment environment, some or
all of the requirements described here may be utilized.
Specifically, some requirements are more applicable to inter-
provider inter-AS MPLS-TE and GMPLS operations than to intra-
provider operations.
4.1. PCE Communication Protocol Requirements
Requirements specific to inter-AS PCECP path computation requests
and responses are discussed in Sections 4.1.1 and 4.1.2,
respectively.
4.1.1. Requirements for path computation requests
The following are inter-AS specific requirements for PCECP requests
for path computation:
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1. [RFC4657] states the requirement for a priority level to be
associated with each path computation request. This document does
not change that requirement, but in addition it MUST be possible for
an inter-AS PCE to apply local policy to vary the priority of path
computation requests received across AS borders. PCECP MAY include a
mechanism to inform the requesting inter-AS PCE of the change in
priority that was applied.
2. A path computation request by an inter-AS PCE or a PCC to another
inter-AS PCE MUST be able to specify the sequence of ASs and/or
ASBRs across the network by providing ASBRs and/or ASs as hops in
the desired path of the LSP to the destination. For instance, an
inter-AS PCE MUST be be able to specify to the inter-AS PCE serving
the neighboring AS a preferred ASBR for exiting to that AS and reach
the destination. That is, where multiple ASBRs exist, the requester
MUST be able to indicate a non-mandatory preference for one of them.
3. PCECP MUST allow a requester to provide a list of Ass and/or
ASBRs to be excluded from the computed path.
4. A PCECP path request from one inter-AS PCE to another MUST
include the previous AS number in the path of the LSP to enable the
correct application of local policy at the second inter-AS PCE.
5. A path computation request from a PCC to an inter-AS PCE or an
inter-AS PCE to another MUST be able to specify the need for
protection against node, link, or SRLG failure using 1:1 detours or
facility backup. It MUST be possible to request protection across
all ASs or across specific ASs.
6. The disjoint path requirements specified in [RFC4657] are
extended such that it MUST be possible to apply a constraint of AS-
diversity in the computation of a set of two or more paths.
7. A PCECP path computation request message MUST be able to identify
the scope of diversified path computation to be end-to-end (i.e.,
between the endpoints of the (G)MPLS-TE tunnel) or to be limited to
a specific AS.
4.1.2. Requirements for path computation responses
The following are inter-AS specific requirements for PCECresponses
for path computation:
1. A PCECP path computation response from one inter-AS PCE to
another MUST be able to include ASBRs and ASs in the computed path to
maintain path segment and topology confidentiality.
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2. A PCECP path computation response from one inter-AS PCE to the
requesting inter-AS PCE MUST be able to carry an identifier for a path
segment it computes to preserve path segment and topology
confidentiality. The objective of the identifier is to be included in
the LSP signaling, out of scope of this document, to be used for path
expansion during LSP signaling.
3. If a constraint for a desired ASBR (see Section 4.1.1,
requirement 3) cannot be satisfied by a PCE, PCECP SHOULD allow the PCE
to notify the requester of that fact in a positive path computation
response.
4. A PCECP path computation request from an inter-AS PCE to a
requesting inter-AS PCE or a PCC MUST be able to carry a cumulative
inter-AS path cost. Path cost normalization across ASs is out of the
scope of this document.
5. A PCECP path computation response from an inter-AS PCE to a PCC
SHOULD be able to carry the intra-AS cost of the path segment within
the PCC AS.
6. A PCECP path computation response MUST be able to identify
diversified paths for the same (G)MPLS-TE LSP. End-to-end (i.e.,
between the two endpoints of the (G)MPLS-TE tunnel) disjoint paths are
paths that do not share nodes, links or SRLGs except for the LSP head-
end and tail-end. In cases where diversified path segments are desired
within one or more ASes, the disjoint path segments may share only the
ASBRs of the first AS and the ASBR of the last AS across these ASes.
4.2. Scalability and Performance Requirements
PCECP design for use in the inter-AS case SHOULD consider the
following criteria:
- PCE message processing load.
- Scalability as a function of the following parameters:
- number of PCCs within the scope of an inter-AS PCE
- number of intra-AS PCEs within the scope of an inter-AS PCE
- number of peering inter-AS PCEs per inter-AS PCE
- Added complexity caused by inter-AS features.
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4.3. Management, Aliveness Detection, and Recovery Requirements
[RFC4657] specifies generic requirements for PCECP management.
This document addresses new requirements that apply to inter-AS
operations.
The PCECP MIB module MUST provide objects to control the behavior
of PCECP in inter-AS applications, including the ASs within the
scope of an inter-AS PCE, neighboring ASs with whose inter-AS
PCE(s) the inter-AS PCE MUST not communicate, neighboring ASs with
whose inter-AS PCEs the inter-AS PCE can communicate,
confidentiality policies, and traffic engineering policies. Each
of these two latter requirements SHOULD be applicable on inter-AS
PCE-pair basis or neighboring AS basis (i.e., apply to all inter-
AS PCEs that belong to a neighboring AS).
The built-in diagnostic tools MUST enable failure detection and
status checking of PCC/PCE-PCE PCECP. Diagnostic tools include
statistics collection on the historical behavior of PCECP as
specified in [RFC4657], but additionally it MUST be possible to
analyze this statistics on a neighboring AS basis (i.e., across
the inter-AS PCEs that belong to a neighboring AS).
The MIB module MUST support trap functions when thresholds are
crossed or when important events occur as stated in [RFC4657].
These thresholds SHOULD be specifiable per neighbor AS as well as
per peer inter-AS PCE and traps should be accordingly generated.
Basic liveliness detection for PCC/PCE-PCE communication is
described in [RFC4657]. The PCECP MIB module SHOULD allow control
of liveliness check behavior by providing a liveliness message
frequency MIB object and this frequency object SHOULD be specified
per inter-AS PCE peer. In addition, there SHOULD be a MIB object
that specifies the dead-interval as a multiplier of the liveliness
message frequency so that if no liveliness message is received
within that time from an inter-AS PCE, the inter-AS PCE is declared
unreachable.
4.4. Confidentiality
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Confidentiality mainly applies to inter-provider (inter-AS) PCE
communication. It is about protecting the information exchanged
between PCEs and about protecting the topology information within
a provider's network. Confidentiality rules may also apply among
ASs under a single provider. Each SP will in most cases designate
some PCEs for inter-AS MPLS-TE or GMPLS path computation within
its own administrative domain and some other PCEs for inter-
provider inter-AS MPLS-TE or GMPLS path computation. Among the
inter-provider-scoped inter-AS PCEs in each SP domain, there may
also be a subset of the PCEs specifically enabled for path
computation across a specific set of ASs of different peer SPs.
PCECP SHOULD allow an SP to hide from other SPs the set of hops
within its own ASs that are traversed by an inter-AS inter-
provider LSP (c.f., Section 5.2.1 of [RFC4216]). In a multi-SP
administrative domain environment, SPs may want to hide their
network topologies for security or commercial reasons. Thus, for
each inter-AS LSP path segment an inter-AS PCE computes, it may
return to the requesting inter-AS PCE an inter-AS TE LSP path
segment from its own ASs without detailing the explicit intra-AS
hops. As stated earlier, PCECP responses SHOULD be able to carry
path-segment identifiers that replace the details of that path
segment. The potential use of that identifier for path expansion,
for instance, during LSP signaling is out of the scope of this
document.
4.5. Policy Controls Affecting inter-AS PCECP
Section 5.2.2 of [RFC4216] discusses the policy control
requirements for inter-AS RSVP-TE signaling at the AS boundaries
for the enforcement of interconnect agreements, attribute/parameter
translation and security hardening.
This section discusses those policy control requirements that are
similar to what are discussed in section 5.2.2 of [RFC4216], for
PCECP. Please note that SPs may still require policy controls
during signaling of LSPs to enforce their bilateral or multi-
lateral agreements at AS boundaries, but signaling is out of scope
for this document.
4.5.1. Inter-AS PCE Peering Policy Controls
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An inter-AS PCE sends path computation requests to its
neighboring inter-AS PCEs, and an inter-AS PCE that receives
such a request enforces policies applicable to the sender of the
request. These policies may include rewriting some of the
parameters, or rejecting requests based on parameter values.
Such policies may be applied for PCEs belonging to different SPs
or to PCEs responsible for ASs within a single SP administrative
domain. Parameters that might be subject to policy include
bandwidth, setup/holding priority, Fast Reroute request,
Differentiated Services Traffic Engineering (DS-TE) Class Type
(CT), and others as specified in section 5.2.2.1 of [RFC4216].
For path computation requests that are not compliant with
locally configured policies, PCECP SHOULD enable a PCE to send
an error message to the requesting PCC or PCE indicating that
the request has been rejected because a specific parameter did
not satisfy the local policy.
4.5.2. Inter-AS PCE Reinterpretation Policies
Each SP may have different definitions in its use of, for
example, DS-TE TE classes. An inter-AS PCE receiving a path
computation request needs to interpret the parameters and
constraints and adapt them to the local environment.
Specifically, a request constructed by a PCC or PCE in one AS
may have parameters and constraints that should be interpreted
differently by the receiving PCE that is in a different AS. A
list of signaling parameters subject to policy reinterpretation
at AS borders can be found in section 5.2.2.2 of [RFC4216], and
the list for patch computation request parameters and
constraints is the same. In addition, the transit SPs along the
inter-AS TE path may be GMPLS transport providers which may
require reinterpretation of MPLS specific PCECP path request
objects to enable path computation over a GMPLS network.
5. Security Considerations
Security concerns arise between any two communicating
elements especially when the elements belong to different
administrative entities. In this case, there are security concerns
that need to be addressed for communication among inter-AS PCEs and
other PCEs in a single SP administrative domain as well among inter-
AS PCEs under different SP administrative domains. [RFC4657]
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specifies requirements on PCECP to protect against spoofing,
snooping and DoS attacks. These requirements become especially
important in the multi-AS case. An inter-AS PCE MUST be able to
authenticate a peering inter-AS PCE as a legitimate peer. Since
inter-AS PCEs can be auto-discovered by an inter-AS PCE and peering
sessions are formed dynamically, mechanisms for securely exchanging
authentication keys across SP boundaries MUST be defined. The
autodiscovery process itself MUST also be authenticated.
6. IANA Considerations
This document makes no requests for IANA action.
7. Acknowledgments
We would like to thank Adrian Farrel, Jean-Philippe Vasseur, and
Jean Louis Le Roux for their useful comments and suggestions.
8. Authors' Addresses
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02451
Email: nabil.n.bitar@verizon.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
Phone: +81-3-6678-3103
Email: ke-kumaki@kddi.com
Raymond Zhang
BT INFONET Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90245 USA
Email: Raymond_zhang@bt.infonet.com
9. Normative References
[RFC4216] Zhang and Vasseur, "MPLS Inter-AS Traffic Engineering
Requirements", RFC 4216, November 2005.
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[RFC4655] Farrel, A., Vasseur, J.P, & Ash, J., "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4657] Ash, J., Le Roux, J.L, et al., "PCE Communication Protocol
Generic Requirements", RFC 4657, September 2006.
10. Informative References
[INTERD-TESIG] Ayyangar, A., and Vasseur, J.P, "Inter domain GMPLS
Traffic Engineering - RSVP-TE extensions", draft-ietf-ccamp-inter-
domain-rsvp-te-03.txt, March 2006 (Work in Progress)
[LSP-STITCHING] Ayyangar A., Vasseur J,P., "Label Switched Path
Stitching with Generalized MPLS Traffic Engineering", draft-ietf-
ccamp-lsp-stitching-03.txt, March 2006, (work in progress).
[RFC4206] Kompella, K., Rekhter, Y., "Label switched Paths(LSP)
Hierarchy with Generalized MPLS TE", RFC4206, October 2005.
[INTERD-TE-PDPC] Vasseur,J.P, Ayyangar, A., and Zhang, R., "A Per-
domain path computation method for establishing Inter-domain Traffic
Engineering (TE) Label Switched Paths (LSPs)", draft-ietf-ccamp-inter
-domain-pd-path-comp-03.txt, August 2006, (Work in Progress).
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