Network Working Group Nabil Bitar
(Editor)
Verizon
Internet Draft Raymond Zhang
(Editor)
BT Infonet
Intended Status: Informational Kenji Kumaki
(Editor)
KDDI Corporation
Expires: January 2008 July 2007
Inter-AS Requirements for the Path Computation Element
Communication Protocol (PCECP)
draft-ietf-pce-interas-pcecp-reqs-02.txt
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Bitar, Zhang and Kumaki .Inter-AS Requirements for PCECP [Page 1]
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Abstract
Multiprotocol Label Switching Traffic Engineered (MPLS-TE) Label
Switched 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 PCEP are set out in "Path Computation
Element(PCE) Communication Protocol Generic Requirements", RFC 4657.
This document extends those requirements to cover the use of PCEP 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 Considerations.....................7
4.3. Management Considerations......................................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............................................12
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) and
specifies the requirements for inter-AS MPLS-TE when the ASes 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 ASes 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 areas and/or 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 ASes.
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.
Lets follow a scenario that illustrates the interaction among PCCs,
inter-AS PCEs and intra-AS PCEs shown Figure 1. R1 in AS1 wants to
setup a MPLS-TE or a GMPLS path, call LSP1, with certain constraints
to R7 in AS3. R1 determines using mechanisms out of the scope of
this document that R7 is an inter-AS route and that it needs to
contact its 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
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the path itself, or may solicit the services of other PCEs which
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 LSPs. 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 the following sections.
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 ASes and/or
ASBRs across the network by providing ASBRs and/or ASes 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 ASes 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 ASes or across specific ASes.
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 PCECP responses
for path computation:
1. A PCECP path computation response from one inter-AS PCE to
another MUST be able to include both ASBRs and ASes in the computed
path while preserving 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, whose mechanism is 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 as an error in a path
computation response.
4. A PCECP path computation 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 ASes is out of 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 Considerations
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 Considerations
[RFC4657] specifies generic requirements for PCECP management. This
document addresses new requirements that apply to inter-AS
perations.
The PCECP MIB module MUST provide objects to control the behavior of
PCECP in inter-AS applications. They include the ASes within the
scope of an inter-AS PCE, Inter-AS PCEs in neighboring ASes to which
the requesting PCE will or will not communicate, confidentiality and
policies, etc..
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 PCECP 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-A PCE, the inter-AS PCE is declared unreachable.
4.4. Confidentiality
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
ASes 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
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domain, there may also be a subset of the PCEs specifically enabled
for path computation across a specific set of ASes of different peer
SPs.
PCECP SHOULD allow an SP to hide from other SPs the set of hops
within its own ASes 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 ASes
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 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
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 ASes
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].
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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 path 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
PCC/PCEs especially when they belong to different administrative
entities. Security concerns that need to be addressed are 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] specifies requirements on PCECP to
protect against spoofing, snooping and DoS attacks. These
requirements become especially critical in the multi-AS case.
Additionally, two aspects of operations specific to inter-AS PCEs
require careful security considerations. There are two modes of
determining peering PCEs across the AS boundary manual
configuration and auto-discovery. In the manual mode, mechanisms
for securely exchanging authentication keys across SP boundaries
MUST be defined. For example, PCE registration MAY be served as a
mechanism for securely exchanging authentication keys across SP
boundaries. In the auto-discovery mode, inter-as PCEs can be auto-
discovered only if it is configured to participate in that discovery
scope. An inter-AS PCE is not necessarily able to establish PCEP
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sessions with the discovered PCEs in its scope(s), it MUST be able
to authenticate with the peering inter-AS PCE, therefore mechanisms
for securely exchanging authentication keys across SP boundaries
MUST also be defined in this case. Furthermore, the auto-discovery
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
2160 E. Grand Ave.
El Segundo, CA 90245 USA
Email: Raymond_zhang@bt.com
9. Normative References
[RFC4216] Zhang and Vasseur, "MPLS Inter-AS Traffic Engineering
Requirements", RFC 4216, November 2005.
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[RFC4655] Farrel, Vasseur & Ash, "A Path Computation Element (PCE)-
Based Architecture", RFC 4755, August 2006.
[RFC4657] J. Ash, J.L Le Roux et al., "PCE Communication Protocol
Generic Requirements", RFC 4657, September 2006.
10. Informative References
[INTERD-TESIG] Ayyangar and Vasseur, "Inter domain GMPLS Traffic
Engineering - RSVP-TE extensions", draft-ietf-ccamp-inter-domain-
rsvp-te-06.txt, April 2006 (Work in Progress)
[LSP-STITCHING] Ayyangar A., Vasseur JP., "LSP Stitching with
Generalized MPLS TE", draft-ietf-ccamp-lsp-stitching-06.txt,
September 2005, (work in progress).
[RFC4206] Kompella K., Rekhter Y., "Label switched Paths(LSP)
Hierarchy with Generalized MPLS TE", RFC4206, October 2005.
[INTERD-TE-PDPC] Vasseur, Ayyangar and Zhang, "A Per-domain path
computation method for computing Inter-domain Traffic Engineering
(TE) Label Switched Path (LSP)", draft-ietf-ccamp-inter-domain-pd-
path-comp-05.txt, February 2006, (Work in Progress).
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