IETF Internet Draft PCE Working Group Jerry Ash (AT&T)
Proposed Status: Informational Editor
Expires: March 2006 J.L. Le Roux (France Telecom)
Editor
September 2005
draft-ietf-pce-comm-protocol-gen-reqs-02.txt
PCE Communication Protocol Generic Requirements
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Abstract
The PCE model is described in the "PCE Architecture" document and
facilitates path computation requests from Path Computation Clients
(PCCs) to Path Computation Elements (PCEs). This document specifies
generic requirements for a communication protocol between PCCs and
PCEs, and also between PCEs where cooperation between PCEs is
desirable. Subsequent documents will specify application-specific
requirements for the PCE communication protocol.
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Table of Contents
1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Overview of PCE Communication Protocol (PCECP) . . . . . . . . . 4
6. PCE Communication Protocol Generic Requirements . . . . . . . . . 5
6.1 Basic Protocol Requirements . . . . . . . . . . . . . . . . . 7
6.1.1 Commonality of PCC-PCE and PCE-PCE Communication . . . 7
6.1.2 Client-Server Communication . . . . . . . . . . . . . . 7
6.1.3 Transport . . . . . . . . . . . . . . . . . . . . . . . 7
6.1.4 Path Computation Requests . . . . . . . . . . . . . . . 8
6.1.5 Path Computation Responses . . . . . . . . . . . . . . 9
6.1.6 Cancellation of Pending Requests . . . . . . . . . . . 9
6.1.7 Multiple Requests and Responses . . . . . . . . . . . . 9
6.1.8 Reliable Message Exchange . . . . . . . . . . . . . . . 10
6.1.9 Secure Message Exchange . . . . . . . . . . . . . . . . 11
6.1.10 Request Prioritization . . . . . . . . . . . . . . . . 11
6.1.11 Unsolicited Notifications . . . . . . . . . . . . . . 11
6.1.12 Asynchronous Communication . . . . . . . . . . . . . . 11
6.1.13 Communication Overhead Minimization . . . . . . . . . 12
6.1.14 Extensibility . . . . . . . . . . . . . . . . . . . . 12
6.1.15 Scalability . . . . . . . . . . . . . . . . . . . . . 13
6.1.16 Constraints . . . . . . . . . . . . . . . . . . . . . 13
6.2 Deployment Support Requirements . . . . . . . . . . . . . . . 14
6.2.1 Support for Different Service Provider Environments . . 14
6.2.2 Policy Support . . . . . . . . . . . . . . . . . . . . 14
6.3 Detection & Recovery Requirements . . . . . . . . . . . . . . 15
6.3.1 Aliveness Detection . . . . . . . . . . . . . . . . . . 15
6.3.2 PCC/PCE Failure Response . . . . . . . . . . . . . . . 15
6.3.3 Protocol Recovery . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Manageability Considerations . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Normative References . . . . . . . . . . . . . . . . . . . . . . 17
12. Informational References . . . . . . . . . . . . . . . . . . . . 17
13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property Statement . . . . . . . . . . . . . . . . . . 19
Disclaimer of Validity . . . . . . . . . . . . . . . . . . . . . . . 19
Copyright Statement . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Contributors
This document is the result of the PCE Working Group PCE
Communication Protocol (PCECP) requirements design team joint effort.
The following are the design team member authors that contributed to
the present document:
Jerry Ash (AT&T)
Alia Atlas (Google, Inc.)
Arthi Ayyangar (Juniper)
Nabil Bitar (Verizon)
Igor Bryskin (Independent Consultant)
Dean Cheng (Cisco)
Durga Gangisetti (MCI)
Kenji Kumaki (KDDI)
Jean-Louis Le Roux (France Telecom)
Eiji Oki (NTT)
Raymond Zhang (BT Infonet)
2. 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 [RFC2119].
3. Introduction
A Path Computation Element (PCE) [PCE-ARCH] supports requests for
path computation issued by a Path Computation Client (PCC), which may
be 'composite' (co-located) or 'external' (remote) from a PCE. When
the PCC is external from the PCE, a request/response communication
protocol is required to carry the path computation request and return
the response. In order for the PCC and PCE to communicate, the PCC
must know the location of the PCE: PCE discovery is described in
[PCE-DISC-REQ]. The PCE operates on a network graph in order to
compute paths based on the path computation request issued by the
PCC. The path computation request will normally include the source
and destination of the paths to be computed, and a set of constraints
to be applied during the computation. The PCE response includes the
computed paths or the reason for a failed computation.
This document lists a set of generic requirements for the PCE
Communication Protocol (PCECP). Application-specific requirements
are beyond the scope of this document, and will be addressed in
separate documents.
4. Terminology
Domain: any collection of network elements within a common sphere of
address management or path computational responsibility. Examples of
domains include IGP areas, Autonomous Systems (ASs), multiple ASs
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within a service provider network, or multiple ASs across multiple
service provider networks.
GMPLS: Generalized Multi-Protocol Label Switching
LSP: MPLS Label Switched Path.
MPLS: Multi-Protocol Label Switching
PCC: Path Computation Client: any client application requesting a
Path computation to be performed by the PCE.
PCE: Path Computation Element: an entity (component, application or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints (see
further description in [PCE-ARCH]).
TED: Traffic Engineering Database, which contains the topology and
resource information of the network or network segment used by a PCE.
TE LSP: Traffic Engineering MPLS Label Switched Path.
See [PCE-ARCH] for further definitions of terms.
5. Overview of PCE Communication Protocol (PCECP)
In the PCE model, path computation requests are issued by a PCC
to a PCE that may be composite (co-located) or external (remote).
If the PCC and PCE are not composite, a request/response
communication protocol is required to carry the request and return
the response. If the PCC and PCE are composite, a communication
protocol is not required, but implementations may choose to utilize
a protocol for exchanges between the components.
In order that a PCC and PCE can communicate, the PCC must know the
location of the PCE. This can be configured or discovered. The PCE
discovery mechanism is out of scope of this document, but
requirements are documented in [PCE-DISC-REQ].
The PCE operates on a network graph built from the TED in order to
compute paths. The mechanism by which the TED is populated is out of
scope for the PCECP.
A path computation request issued by the PCC includes a specification
of the path(s) needed. The information supplied includes, at a
minimum, the source and destination for the paths, but may also
include a set of further requirements (known as constraints) as
described in Section 6.
The response from the PCE may be positive in which case it will
include the paths that have been computed. If the computation fails
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or cannot be performed, a negative response is required with an
indication of the type of failure.
A request/response protocol is also required for a PCE to communicate
path computation requests to another PCE and for that PCE to return
the path computation response. As described in [PCE-ARCH], there is
no reason to assume that two different protocols are needed, and this
document assumes that a single protocol will satisfy all requirements
for PCC-PCE and PCE-PCE communication.
[PCE-ARCH] describes four models of PCE: composite, external,
multiple PCE path computation, and multiple PCE path computation with
inter-PCE communication. In all cases except the composite PCE model,
a PCECP is required. The requirements defined in this document are
applicable to all models described in the [PCE-ARCH].
6. PCE Communication Protocol Generic Requirements
The following is a summary of the requirements in Section 6:
Requirement Necessity Ref.
------------------------------------------------------------------
Commonality of PCC-PCE and PCE-PCE communication MUST 6.1.1
Client-server communication MUST 6.1.2
Support PCC/PCE request message to request path
computation MUST 6.1.2
Support PCE response message with computed path MUST 6.1.2
Support unsolicited communication PCE-PCC SHOULD 6.1.2
Maintain PCC-PCE session NON-RQMT 6.1.2
Use of existing transport protocol MAY 6.1.3
Transport protocol satisfy reliability & security
requirements MAY 6.1.3
Transport protocol limits size of message MUST NOT 6.1.3
Support path computation requests MUST 6.1.4
include source & destination
support path constraints (e.g., bandwidth, hops,
affinities) to include/exclude MUST 6.1.4
Support path reoptimization & inclusion of a
previously computed path MUST 6.1.4
Allow to select/prefer from advertised list of
standard objective functions/options MUST 6.1.4
Allow to customize objective function/options MUST 6.1.4
Support path computation responses MUST 6.1.5
Negative response support reasons for failure,
constraints to relax to achieve positive result SHOULD 6.1.5
Cancellation of pending requests MUST 6.1.6
Multiple requests and responses MUST 6.1.7
Limit by configuration number of requests within
a message MUST 6.1.7
Support multiple computed paths in response MUST 6.1.7
Support "continuation correlation" where related
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requests or computed paths cannot fit within one
message MUST 6.1.7
Maximum message size & maximum number of requests
per message exchanged through PCE messages to PCC,
or indicated in request message MAY 6.1.7
Reliable message exchange (achieved by PCECP
itself or transport protocol MUST 6.1.8
Allow detection & recovery of lost messages to
occur quickly & not impede operation of PCECP MUST 6.1.8
Handle overload situations without significant
decrease in performance, e.g., through throttling
of requests MUST 6.1.8
Provide acknowledged message delivery with
retransmission, in order message delivery or
facility to restore order, message corruption
detection, flow control & back-pressure to
throttle requests, rapid partner failure
detection, informed rapidly of failure of PCE-PCC
connection MUST 6.1.8
Functionality added to PCECP if transport protocol
provides it SHOULD NOT 6.1.8
Secure message exchange (provided by PCECP or
transport protocol MUST 6.1.9
Support mechanisms to prevent spoofing (e.g.,
authentication), snooping (e.g., encryption),
DOS attacks MUST 6.1.9
Request prioritization MUST 6.1.10
Unsolicited notifications SHOULD 6.1.11
Allow asynchronous communication MUST 6.1.12
PCC has to wait for response before making
another request MUST NOT 6.1.12
Allow order of responses differ from order of
requests MUST 6.1.12
Communication overhead minimization SHOULD 6.1.13
Give particular attention to message size SHOULD 6.1.13
Extensibility without requiring modifications to
the protocol MUST 6.1.14
Easily extensible to support intra-area,
inter-area, inter-AS intra provider, inter-AS
inter-provider, multi-layer path & virtual network
topology path computation MUST 6.1.14
Easily extensible to support future applications
not in scope (e.g., P2MP path computations) SHOULD 6.1.14
Scalability at least linearly with increase in
number of PCCs, PCEs, PCCs communicating with a
single PCE, PCEs communicated to by a single PCC,
PCEs communicated to by another PCE, domains, path
requests, handling bursts of requests MUST 6.1.15
Support path computation constraints MUST 6.1.16
Support different service provider environments
(e.g., MPLS-TE and GMPLS networks, centralized &
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distributed PCE path computation, single &
multiple PCE path computation) MUST 6.2.1
Policy support for policies to accept/reject
requests, PCC to determine reason for rejection,
notification of policy violation MUST 6.2.2
Aliveness detection of PCCs/PCEs, partner failure
detection MUST 6.3.1
PCC/PCE failure response procedures defined for
PCE/PCC failures, PCC able to clear pending
request must 6.3.2
PCC select another PCE upon detection of PCE
failure MUST 6.3.2
PCE able to clear pending requests from a PCC
(e.g. when it detects PCC failure or request
buffer full) must 6.3.2
Protocol recovery support resynchronization of
information & requests between sender & receiver MUST 6.3.3
Minimize repeat data transfer, allow PCE to
respond to computation requests issued before
failure without requests being re-issued SHOULD 6.3.3
Stateful PCE able to resynchronize/recover
states (e.g., LSP status, paths) after restart SHOULD 6.3.3
6.1 Basic Protocol Requirements
6.1.1 Commonality of PCC-PCE and PCE-PCE Communication
A single protocol MUST be defined for PCC-PCE and PCE-PCE
communication. A PCE requesting a path from another PCE can be
considered as a PCC.
6.1.2 Client-Server Communication
PCC-PCE and PCE-PCE communication is by nature client-server based.
The PCECP MUST allow for a PCC or a PCE to send a request message to
a PCE to request path computation, and for a PCE to reply with a
response message to the requesting PCC or PCE, once the path has been
computed.
In addition to this request-response mode, there may be cases where
there is unsolicited communication from the PCE to PCC (see
Requirement 6.1.6).
6.1.3 Transport
The PCECP may utilize an existing transport protocol or operate
directly over IP.
If a transport protocol is used, it may be used to satisfy some
requirements stated in other sections of this document (for example,
reliability and security).
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If a transport protocol is used, it MUST NOT limit the size of the
message used by the PCECP.
Where requirements expressed in this document match the function of
existing transport protocols, consideration MUST be given to the use
of those protocols.
6.1.4 Path Computation Requests
The request message MUST include, at least, a source and a
destination. However, there is no assumption that the receiving PCE
has the complete source/destination domain topology, particularly in
the multiple PCE path computation model [PCE-ARCH]. In the latter
case, the PCE may have incomplete topological information for
multiple domains.
The message MUST support the inclusion of a set of one or more path
constraints, including the requested bandwidth or resources (hops,
affinities, etc.) to include/exclude (e.g., a PCC requests the PCE to
exclude points of failure in the computation of the new path if an
LSP setup fails). The actual inclusion of constraints is a choice
for the PCC issuing the request. A list of core constraints that
MUST be supported by the PCECP is supplied in Section 6.1.16.
Specification of constraints must be future-proofed as described in
Section 6.1.14.
The path computation request message MUST support TE LSP path
reoptimization and the inclusion of a previously computed path. This
will help ensure optimal routing of a reoptimized path, since it will
allow the PCE to avoid double bandwidth accounting and help reduce
blocking issues.
The requester MUST be allowed to select or prefer from an advertised
list or minimal subset of standard objective functions and functional
options. An objective function is used by the PCE to compute a path
metric in order to select the best candidate paths (e.g., minimum hop
path), and corresponds to the optimization criteria used for the
computation of one path, or the synchronized computation of a set of
paths. In case of unsynchronized path computation, this can be, for
example, the path cost or the residual bandwidth on the most loaded
path link. In case of synchronized path computation, this can be,
for example, the global bandwidth consumption or the residual
bandwidth on the most loaded network link.
The requester SHOULD also be able to select a vendor-specific or
experimental objective function or functional option. Furthermore,
the requester MUST be allowed to customize the function/options in
use. That is, individual objective functions will often have
parameters to be set in the request from PCC to PCE. Specification
of objective functions and objective parameters is required in the
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protocol extensibility specified in Section 6.1.14.
Note that a PCC MAY send a request that is based on the set of TE
parameters carried by the MPLS/GMPLS LSP setup signaling protocol,
and as long as those parameters are satisfied, the PCC MAY not care
about which objective function is used. Also, the PCE MAY execute
additional objective functions not explicitly requested by the PCC.
This might include policy based routing path computation for load
balancing instructed by the management plane. The PCC MUST NOT be
allowed to request or cause a computation to fail because it does not
wish the PCE to apply a specific objective function. Allowing such
behavior would constitute a security risk.
6.1.5 Path Computation Responses
The response message MUST allow returning various elements including,
at least, the computed path(s).
The protocol MUST be capable of returning any explicit path that
would be acceptable for use for MPLS and GMPLS LSPs once converted to
an Explicit Route Object for use in RSVP-TE signaling. In addition,
anything that can be expressed in an Explicit Route Object MUST be
capable of being returned in the computed path. Note that the
resultant path(s) may be made up of a set of strict or loose hops, or
any combination of strict and loose hops. Moreover, a hop may have
the form of a non-simple abstract node. See RFC 3209 for the
definition of strict hop, loose hop, and abstract node.
A positive response from the PCE will include the paths that have
been computed. When a path satisfying the constraints cannot be
found, or if the computation fails or cannot be performed, a
negative response MUST be sent. This response MAY include further
details of the reason(s) for the failure, and potentially advice
about which constraints might be relaxed to be more likely to achieve
a positive result.
6.1.6 Cancellation of Pending Requests
A PCC or PCE MUST be able to cancel a pending request, using an
appropriate notification between PCECP peers. A PCC that has sent a
request to a PCE and no longer needs a response, for instance,
because it received a satisfactory answer from another PCE, MUST be
able to notify the PCE that it must clear the request (i.e. stop the
computation, if already started, and clear the context). Similarly,
a PCE that received a request from a PCC that it cannot serve, for
example, due to congestion, MUST be able to notify the PCC, that the
request will not be served.
6.1.7 Multiple Requests and Responses
It MUST be possible to send multiple path computation requests,
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correlated or not, within the same request message. There are
various motivations for doing so (optimality, path diversity, etc.).
It MUST be possible to limit by configuration of both PCCs and PCEs
the number of requests that can be carried within a single message.
Similarly, it MUST be possible to return multiple computed paths
within the same response message, corresponding either to the same
request (e.g. load balancing) or to distinct requests, correlated or
not, of the same request message or distinct request messages.
It MUST be possible to provide "continuation correlation" where all
related requests or computed paths cannot fit within one message.
Maximum acceptable message sizes and the maximum number of requests
per message supported by a PCE MAY form part of PCE capabilities
advertisement [PCE-DISC-REQ], or MAY be exchanged through information
messages from the PCE as part of the protocol described here.
Maximum acceptable message sizes and the maximum number of computed
paths per message supported by a PCC MAY be indicated in the request
message.
An implementation MAY choose to limit message size to avoid a big
message from unduly delaying a small message.
6.1.8 Reliable Message Exchange
The PCECP MUST include reliability. This may form part of the
protocol itself or may be achieved by the selection of a suitable
transport protocol (see Section 6.1.3).
In particular, it MUST allow for the detection and recovery of lost
messages to occur quickly and not impede the operation of the PCECP.
In some cases (e.g. after link failure), a large number of PCCs may
simultaneously send requests to a PCE, leading to a potential
saturation of the PCEs. The PCECP or the transport protocol it uses
MUST properly handle such overload situations, such as through
throttling of requests. For example, a PCE MUST be able to limit the
rate of incoming request messages to a manageable rate by notifying
PCCs and/or peering PCEs.
The PCECP or the transport protocol it uses MUST provide:
- Acknowledged message delivery with retransmission.
- In order message delivery or the facility (such as message
numbering) to restore the order of received messages.
- Message corruption detection.
- Flow control and back-pressure, as specified above with the
throttling of requests.
- Rapid partner failure detection.
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- Rapid PCE/PCC or PCC-PCE connection failure detection after
failure happens.
If it is necessary to add functions to PCECP to overcome shortcomings
in the chosen transport mechanisms, these functions SHOULD be based
on and re-use where possible techniques developed in other protocols
to overcome the same shortcomings. Functionality SHOULD NOT be added
to the PCECP where the chosen transport protocol already provides it.
6.1.9 Secure Message Exchange
The PCC-PCE and PCE-PCE communication protocol MUST include
provisions to improve the security of the exchanges between the
entities. In particular, it MUST support mechanisms to prevent
spoofing (e.g., authentication), snooping (e.g., encryption) and DOS
attacks (e.g., rate limiting, no promiscuous listening).
This function may be provided by the transport protocol or directly
by the PCECP.
See Section 7 for further discussion of security considerations.
6.1.10 Request Prioritization
The PCECP MUST allow a PCC to specify the priority of a computation
request. This priority MAY be used by a PCE to service high priority
requests before lower priority requests considering all requests
received and queued by a single PCE from all PCCs.
Implementation of priority-based activity within a PCE is subject to
implementation and local policy. This application processing is out
of scope of the PCECP.
6.1.11 Unsolicited Notifications
The normal operational mode is for the PCC to make path computation
requests to the PCE, and for the PCE to respond.
The PCECP SHOULD support unsolicited notifications from PCE to PCC,
PCE to PCE, or PCC to PCE. This requirement facilitates the
unsolicited communication of information and alerts between PCCs and
PCEs and between PCEs.
6.1.12 Asynchronous Communication
The PCC-PCE protocol MUST allow for asynchronous communication. A
PCC MUST NOT have to wait for a response before it can make another
request.
It MUST also be possible to have the order of responses differ from
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the order of the corresponding requests. This may occur, for
instance, when path request messages have different priorities (see
Requirement 6.1.10).
6.1.13 Communication Overhead Minimization
The request and response messages SHOULD be designed so that the
communication overhead is minimized. In particular, the overhead per
message should be minimized, and the number of bytes exchanged to
arrive at a computation answer should be minimized. Note that
compression techniques are not required. Other considerations in
overhead minimization include the following:
- the amount of background messages used by the protocol or its
transport protocol to keep alive any session or association
between the PCE and PCC
- the processing cost at the PCE (or PCC) associated with
request/response messages (as distinct from processing the
computation requests themselves).
6.1.14 Extensibility
The PCECP MUST provide a way for the introduction of new path
computation constraints, diversity types, objective functions,
optimization methods and parameters, etc., without requiring
modifications in the protocol.
The PCECP MUST be easily extensible to support various PCE based
applications that have been currently identified including:
- intra-area path computation
- inter-area path computation
- inter-AS intra provider and inter-AS inter-provider path
computation
The PCECP MUST also allow extensions as more PCE applications will be
introduced in the future. For example, the protocol may be extended
to support PCE-based multi-layer path computation and virtual network
topology computation/reconfiguration.
The PCECP SHOULD also be easily extensible to support future
applications not currently in the scope of the PCE working group,
such as, for instance, P2MP path computations, multi-hop pseudowire
path computation, etc.
Note that application specific requirements are out of the scope of
this document and will be addressed in separate requirements
documents.
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6.1.15 Scalability
The PCECP MUST scale well, at least as good as linearly, with an
increase of any of the following parameters (note, minimum order of
magnitude estimates of what the PCECP should support are given in
parenthesis):
- number of PCCs (1000/domain)
- number of PCEs (100/domain)
- number of PCCs communicating with a single PCE (1000)
- number of PCEs communicated to by a single PCC (100)
- number of PCEs communicated to by another PCE (100)
- number of domains (20)
- number of path request messages (average of 10/second/PCE)
- handling bursts of requests (burst of 100/second/PCE within a 10-
second interval).
Note that path requests can be bundled in path request messages, for
example, 10 path request messages/second may correspond to 100 path
requests/second.
Bursts of requests may arise, for example, after a network outage
when multiple recomputations are requested. It is RECOMMENDED that
the protocol handle the congestion in a graceful way so that it does
not unduly impact the rest of the network, and so that it does not
gate the ability of the PCE to perform computation.
6.1.16 Constraints
This section provides a list of generic constraints that MUST be
supported by the PCECP. Other constraints may be added to service
specific applications as identified by separate application-specific
requirements documents.
Note that the absence of a constraint in this list does not mean that
that constraint must not be supported. Note also that the provisions
of Section 6.1.14 mean that new constraints can be added to this list
without impacting the protocol.
Here is the list of generic constraints that MUST be supported:
o MPLS-TE and GMPLS generic constraints:
- Bandwidth
- Affinities inclusion/exclusion
- Link, Node, SRLG inclusion/exclusion
- Maximum end-to-end delay metrics
- Hop Count
- Maximum end-to-end TE metric (cost)
- Multiple disjoint path computation to allow path protection
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o MPLS-TE specific constraints
- Class-type
- Local protection
- Node protection
- Bandwidth protection
o GMPLS specific constraints
- Switching type, encoding type
- Link protection type
Regarding affinities inclusion/exclusion, note the three categories
used in [RSVP-TE]: exclude-any, include-any, include-all. Regarding
link, node, SRLG inclusion/exclusion, note the mandatory and desired
exclusion approach in [EXCLUDE-ROUTE].
6.2 Deployment Support Requirements
6.2.1 Support for Different Service Provider Environments
The PCECP MUST operate in various different service provider network
environments that utilize an IP-based control plane, including
- MPLS-TE and GMPLS networks
- packet and non-packet networks
- centralized and distributed PCE path computation
- single and multiple PCE path computation
Definitions of centralized, distributed, single, and multiple PCE
path computation can be found in [PCE-ARCH].
6.2.2 Policy Support
The PCECP MUST allow for policies to accept/reject requests, and
include the ability for a PCE to reject requests with sufficient
detail to allow the PCC to determine the reason for rejection or
failure. For example, filtering could be required for intra-AS PCE
path computation such that all requests are rejected that come from
another AS. However, specific policy details are left to
application-specific PCECP requirements. Furthermore, the PCECP MUST
allow for the notification of a policy violation. Actual policies,
configuration of policies, and applicability of policies are out of
scope.
Note that work on supported policy models and the corresponding
requirements/implications is being undertaken as a separate work item
in the PCE working group.
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6.3 Detection & Recovery Requirements
6.3.1 Aliveness Detection
The PCECP MUST allow a PCC to check the liveliness of PCEs it is
using for path computation, and a PCE to check the liveliness of
PCCs it is serving. This includes detection of PCE liveness before a
PCE is used for computation. i.e. during PCE selection. A PCC should
be aware of PCE liveness at all times. The PCECP MUST provide
partner failure detection.
The aliveness detection mechanism MUST ensure reciprocal knowledge of
PCE and PCC liveness.
Note that the PCE or PCC software component can be lost without
losing the connection or the transport end-point, when a transport
protocol is used.
6.3.2 PCC/PCE Failure Response
Appropriate PCC and PCE procedures MUST be defined to deal with PCE
and PCC failures. A PCC must be able to clear any pending request to
a PCE so that it is no longer waiting for a response. Clearing a
pending request does not imply any message exchange; this differs
from pending request cancellation (Section 6.1.6), which requires
message exchange. It is RECOMMENDED that a PCC select another PCE
upon detection of PCE failure or unreachability of a PCE but note
that PCE selection procedure are out of the scope of this document.
Similarly, a PCE must be able to clear pending requests from a PCC,
for instance, when it detects the failure of the requesting PCC or
when its buffer of requests is full. Clearing a pending request does
not imply any message exchange.
6.3.3 Protocol Recovery
Information distributed in asynchronous/unsolicited messages MAY
persist at the recipient in the event of the failure of the sender or
of the communication channel. Upon recovery, the Communication
Protocol MUST support resynchronization of information and requests
between the sender and the receiver, and this SHOULD be arranged so
as to minimize repeat data transfer.
The response to a computation request issued before the PCC is
restarted will not be helpful and could be a waste of effort. Thus
it is better to allow the request to be re-issued in shorthand (e.g.
by request number) if the PCC remembers that it had previously issued
it and is still interested in the response.
The PCECP SHOULD allow a PCE to respond to computation requests
issued before the failure without the requests being re-issued.
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7. Security Considerations
The impact of the use of a PCECP MUST be considered in the light of
the impact that it has on the security of the existing routing and
signaling protocols and techniques in use within the network.
Intra-domain security is impacted since there is a new interface,
protocol and element in the network. Any host in the network could
impersonate a PCC, and receive detailed information on network paths.
Any host could also impersonate a PCE, both gathering information
about the network before passing the request on to a real PCE, and
spoofing responses. Some protection here depends on the PCE
discovery process (if it uses the IGP it relies on IGP security). An
increase in inter-domain information flows may increase the
vulnerability to security attacks, and the facilitation of
inter-domain path may increase the impact of these security attacks.
Of particular relevance are the implications for confidentiality
inherent in a PCECP for multi-domain networks. It is not necessarily
the case that a multi-domain PCE solution will compromise security,
but solutions MUST examine their impacts in this area.
Applicability statements for particular combinations of signaling,
routing and path computation techniques are expected to contain
detailed security sections.
It should be observed that the use of an external PCE does introduce
additional security issues. Most notable amongst these are:
- interception of PCE requests or responses
- impersonation of PCE or PCC
- denial of service attacks on PCE or PCE communication mechanisms
It is expected that the PCECP will address these issues in detail
using authentication, encryption and DoS protection techniques. See
also Section 6.1.9.
8. Manageability Considerations
Manageability of the PCECP MUST address the following considerations:
- need for a MIB module for control and monitoring
- need for built-in diagnostic tools (e.g., partner failure
detection, OAM, etc.)
- configuration implications for the protocol
It is expected that PCECP operations will be modeled and controlled
through appropriate MIB modules. Statistics gathering will form an
important part of the operation of the PCECP. The operator must be
able to determine PCECP historical interactions and success rate of
requests. Similarly, it is important for an operator to be able to
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determine PCECP load and whether an individual PCC is responsible for
a disproportionate amount of the load. It will also be important to
be able to record and inspect statistics about the PCECP
communications, including issues such as malformed messages,
unauthorized messages and messages discarded owing to congestion.
The new MIB modules should also be used to provide notifications
(traps) when thresholds are crossed or when important events occur.
PCECP techniques must enable a PCC to determine the liveness of a PCE
both before it sends a request and in the period between sending a
request and receiving a response.
It is also important for a PCE to know about the liveness of PCCs to
gain a predictive view of the likely loading of a PCE in the future,
and to allow a PCE to abandon processing of a received request.
It should be possible for an operator to rate limit the requests that
a PCC sends to a PCE, and a PCE should be able to report impending
congestion (according to a configured threshold) both to the operator
and to its PCCs.
9. IANA Considerations
This document makes no requests for IANA action.
10. Acknowledgements
The authors would like to extend their warmest thanks to (in
alphabetical order) Lou Berger, Adrian Farrel, Thomas Morin, Dimitri
Papadimitriou, and JP Vasseur for their review and suggestions.
11. Normative References
[PCE-ARCH] Farrel, A., Vasseur, JP, Ash, J., "Path Computation
Element (PCE) Architecture", work in progress.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3667] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC
3667, February 2004.
[RFC3668] Bradner, S., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3668, February 2004.
12. Informational References
[PCE-DISC-REQ] Le Roux, JL, et. al., "Requirements for Path
Computation Element (PCE) Discovery," work in progress.
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[RFC3209] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for LSP
Tunnels," RFC 3209, December 2001.
13. Authors' Addresses
Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1-(732)-420-4578
Email: gash@att.com
Alia K. Atlas
Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
Email: akatlas@alum.mit.edu
Arthi Ayyangar
Juniper Networks, Inc.
1194 N.Mathilda Ave
Sunnyvale, CA 94089 USA
Email: arthi@juniper.net
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
Email: nabil.bitar@verizon.com
Igor Bryskin
Independent Consultant
Email: i_bryskin@yahoo.com
Dean Cheng
Cisco Systems Inc.
3700 Cisco Way
San Jose CA 95134 USA
Phone: +1 408 527 0677
Email: dcheng@cisco.com
Durga Gangisetti
MCI
Email: durga.gangisetti@mci.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
Phone: +81-3-6678-3103
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Email: ke-kumaki@kddi.com
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex, FRANCE
Email: jeanlouis.leroux@francetelecom.com
Eiji Oki
NTT
Midori-cho 3-9-11
Musashino-shi, Tokyo 180-8585, JAPAN
Email: oki.eiji@lab.ntt.co.jp
Raymond Zhang
BT INFONET Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90245 USA
Email: Raymond_zhang@bt.infonet.com
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