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IETF RFC 5088
OSPF Protocol Extensions for Path Computation Element (PCE) Discovery
Last modified on Saturday, January 5th, 2008
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Network Working Group JL. Le Roux, Ed.
Request for Comments: 5088 France Telecom
Category: Standards Track JP. Vasseur, Ed.
Cisco System Inc.
Y. Ikejiri
NTT Communications
R. Zhang
BT
January 2008
OSPF Protocol Extensions for Path Computation Element (PCE) Discovery
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
There are various circumstances where it is highly desirable for a
Path Computation Client (PCC) to be able to dynamically and
automatically discover a set of Path Computation Elements (PCEs),
along with information that can be used by the PCC for PCE selection.
When the PCE is a Label Switching Router (LSR) participating in the
Interior Gateway Protocol (IGP), or even a server participating
passively in the IGP, a simple and efficient way to announce PCEs
consists of using IGP flooding. For that purpose, this document
defines extensions to the Open Shortest Path First (OSPF) routing
protocol for the advertisement of PCE Discovery information within an
OSPF area or within the entire OSPF routing domain.
Le Roux, et al. Standards Track PAGE 1 
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................4
3. Overview ........................................................5
3.1. PCE Discovery Information ..................................5
3.2. Flooding Scope .............................................5
4. The OSPF PCED TLV ...............................................6
4.1. PCE-ADDRESS Sub-TLV ........................................7
4.2. PATH-SCOPE Sub-TLV .........................................8
4.3. PCE-DOMAIN Sub-TLV ........................................10
4.4. NEIG-PCE-DOMAIN Sub-TLV ...................................11
4.5. PCE-CAP-FLAGS Sub-TLV .....................................12
5. Elements of Procedure ..........................................13
6. Backward Compatibility .........................................14
7. IANA Considerations ............................................14
7.1. OSPF TLV ..................................................14
7.2. PCE Capability Flags Registry .............................14
8. Security Considerations ........................................15
9. Manageability Considerations ...................................16
9.1. Control of Policy and Functions ...........................16
9.2. Information and Data Model ................................16
9.3. Liveness Detection and Monitoring .........................16
9.4. Verify Correct Operations .................................16
9.5. Requirements on Other Protocols and Functional
Components ................................................16
9.6. Impact on Network Operations ..............................17
10. Acknowledgments ...............................................17
11. References ....................................................17
11.1. Normative References .....................................17
11.2. Informative References ...................................18
1. Introduction
[RFC 4655] describes the motivations and architecture for a Path
Computation Element (PCE)-based path computation model for
Multi-Protocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
Traffic Engineered Label Switched Paths (TE LSPs). The model allows
for the separation of the PCE from a Path Computation Client (PCC)
(also referred to as a non co-located PCE) and allows for cooperation
between PCEs (where one PCE acts as a PCC to make requests of the
other PCE). This relies on a communication protocol between a PCC
and PCE, and also between PCEs. The requirements for such a
communication protocol can be found in [RFC 4657], and the
communication protocol is defined in [PCEP].
Le Roux, et al. Standards Track PAGE 2
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
The PCE architecture requires that a PCC be aware of the location of
one or more PCEs in its domain, and, potentially, of PCEs in other
domains, e.g., in the case of inter-domain TE LSP computation.
A network may contain a large number of PCEs, each with potentially
distinct capabilities. In such a context, it is highly desirable to
have a mechanism for automatic and dynamic PCE discovery that allows
PCCs to automatically discover a set of PCEs, along with additional
information about each PCE that may be used by a PCC to perform PCE
selection. Additionally, it is valuable for a PCC to dynamically
detect new PCEs, failed PCEs, or any modification to the PCE
information. Detailed requirements for such a PCE discovery
mechanism are provided in [RFC 4674].
Note that the PCE selection algorithm applied by a PCC is out of the
scope of this document.
When PCCs are LSRs participating in the IGP (OSPF or IS-IS), and PCEs
are either LSRs or servers also participating in the IGP, an
effective mechanism for PCE discovery within an IGP routing domain
consists of utilizing IGP advertisements.
This document defines extensions to OSPFv2 [RFC 2328] and OSPFv3
[RFC 2740] to allow a PCE in an OSPF routing domain to advertise its
location, along with some information useful to a PCC for PCE
selection, so as to satisfy dynamic PCE discovery requirements set
forth in [RFC 4674].
Generic capability advertisement mechanisms for OSPF are defined in
[RFC 4970]. These allow a router to advertise its capabilities within
an OSPF area or an entire OSPF routing domain. This document
leverages this generic capability advertisement mechanism to fully
satisfy the dynamic PCE discovery requirements.
This document defines a new TLV (named the PCE Discovery TLV (PCED
TLV)) to be carried within the OSPF Router Information LSA
([RFC 4970]).
The PCE information advertised is detailed in Section 3. Protocol
extensions and procedures are defined in Sections 4 and 5.
The OSPF extensions defined in this document allow for PCE discovery
within an OSPF routing domain. Solutions for PCE discovery across
Autonomous System boundaries are beyond the scope of this document,
and are for further study.
Le Roux, et al. Standards Track PAGE 3
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
2. Terminology
ABR: OSPF Area Border Router.
AS: Autonomous System.
IGP: Interior Gateway Protocol. Either of the two routing protocols,
Open Shortest Path First (OSPF) or Intermediate System to
Intermediate System (IS-IS).
Intra-area TE LSP: A TE LSP whose path does not cross an IGP area
boundary.
Intra-AS TE LSP: A TE LSP whose path does not cross an AS boundary.
Inter-area TE LSP: A TE LSP whose path transits two or more IGP
areas. That is, a TE LSP that crosses at least one IGP area
boundary.
Inter-AS TE LSP: A TE LSP whose path transits two or more ASes or
sub-ASes (BGP confederations). That is, a TE LSP that crosses at
least one AS boundary.
LSA: Link State Advertisement.
LSR: Label Switching Router.
PCC: Path Computation Client. Any client application requesting a
path computation to be performed by a Path Computation Element.
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.
PCED: PCE Discovery.
PCE-Domain: In a PCE context, this refers to any collection of
network elements within a common sphere of address management or path
computational responsibility (referred to as a "domain" in
[RFC 4655]). Examples of PCE-Domains include IGP areas and ASes.
This should be distinguished from an OSPF routing domain.
PCEP: Path Computation Element communication Protocol.
TE LSP: Traffic Engineered Label Switched Path.
TLV: Type-Length-Variable data encoding.
Le Roux, et al. Standards Track PAGE 4
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
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].
IS-IS extensions for PCE discovery are defined in [RFC 5089].
3. Overview
3.1. PCE Discovery Information
The PCE discovery information is composed of:
- The PCE location: an IPv4 and/or IPv6 address that is used to
reach the PCE. It is RECOMMENDED to use an address that is always
reachable if there is any connectivity to the PCE;
- The PCE path computation scope (i.e., intra-area, inter-area,
inter-AS, or inter-layer);
- The set of one or more PCE-Domain(s) into which the PCE has
visibility and for which the PCE can compute paths;
- The set of zero, one, or more neighbor PCE-Domain(s) toward which
the PCE can compute paths;
- A set of communication capabilities (e.g., support for request
prioritization) and path computation-specific capabilities (e.g.,
supported constraints).
PCE discovery information is, by nature, fairly static and does not
change with PCE activity. Changes in PCE discovery information may
occur as a result of PCE configuration updates, PCE
deployment/activation, PCE deactivation/suppression, or PCE failure.
Hence, this information is not expected to change frequently.
3.2. Flooding Scope
The flooding scope for PCE information advertised through OSPF can be
limited to one or more OSPF areas the PCE belongs to, or can be
extended across the entire OSPF routing domain.
Note that some PCEs may belong to multiple areas, in which case the
flooding scope may comprise these areas. This could be the case for
an ABR, for instance, advertising its PCE information within the
backbone area and/or a subset of its attached IGP area(s).
Le Roux, et al. Standards Track PAGE 5
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
4. The OSPF PCED TLV
The OSPF PCE Discovery TLV (PCED TLV) contains a non-ordered set of
sub-TLVs.
The format of the OSPF PCED TLV and its sub-TLVs is identical to the
TLV format used by the Traffic Engineering Extensions to OSPF
[RFC 3630]. That is, the TLV is composed of 2 octets for the type, 2
octets specifying the TLV length, and a value field. The Length
field defines the length of the value portion in octets.
The TLV is padded to 4-octet alignment; padding is not included in
the Length field (so a 3-octet value would have a length of 3, but
the total size of the TLV would be 8 octets). Nested TLVs are also
4-octet aligned. Unrecognized types are ignored.
The OSPF PCED TLV has the following format:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// sub-TLVs //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 6
Length: Variable
Value: This comprises one or more sub-TLVs
Five sub-TLVs are defined:
Sub-TLV type Length Name
1 variable PCE-ADDRESS sub-TLV
2 4 PATH-SCOPE sub-TLV
3 4 PCE-DOMAIN sub-TLV
4 4 NEIG-PCE-DOMAIN sub-TLV
5 variable PCE-CAP-FLAGS sub-TLV
The PCE-ADDRESS and PATH-SCOPE sub-TLVs MUST always be present within
the PCED TLV.
The PCE-DOMAIN and NEIG-PCE-DOMAIN sub-TLVs are optional. They MAY
be present in the PCED TLV to facilitate selection of inter-domain
PCEs.
Le Roux, et al. Standards Track PAGE 6
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
The PCE-CAP-FLAGS sub-TLV is optional and MAY be present in the PCED
TLV to facilitate the PCE selection process.
Malformed PCED TLVs or sub-TLVs not explicitly described in this
document MUST cause the LSA to be treated as malformed according to
the normal procedures of OSPF.
Any unrecognized sub-TLV MUST be silently ignored.
The PCED TLV is carried within an OSPF Router Information LSA defined
in [RFC 4970].
No additional sub-TLVs will be added to the PCED TLV in the future.
If a future application requires the advertisement of additional PCE
information in OSPF, this will not be carried in the Router
Information LSA.
The following sub-sections describe the sub-TLVs that may be carried
within the PCED TLV.
4.1. PCE-ADDRESS Sub-TLV
The PCE-ADDRESS sub-TLV specifies an IP address that can be used to
reach the PCE. It is RECOMMENDED to make use of an address that is
always reachable, provided that the PCE is alive and reachable.
The PCE-ADDRESS sub-TLV is mandatory; it MUST be present within the
PCED TLV. It MAY appear twice, when the PCE has both an IPv4 and
IPv6 address. It MUST NOT appear more than once for the same address
type. If it appears more than once for the same address type, only
the first occurrence is processed and any others MUST be ignored.
The format of the PCE-ADDRESS sub-TLV is as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| address-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// PCE IP Address //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCE-ADDRESS sub-TLV format
Le Roux, et al. Standards Track PAGE 7
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
Type: 1
Length: 8 (IPv4) or 20 (IPv6)
Address-type:
1 IPv4
2 IPv6
Reserved: SHOULD be set to zero on transmission and MUST be ignored
on receipt.
PCE IP Address: The IP address to be used to reach the PCE.
4.2. PATH-SCOPE Sub-TLV
The PATH-SCOPE sub-TLV indicates the PCE path computation scope,
which refers to the PCE's ability to compute or take part in the
computation of paths for intra-area, inter-area, inter-AS, or inter-
layer TE LSPs.
The PATH-SCOPE sub-TLV is mandatory; it MUST be present within the
PCED TLV. There MUST be exactly one instance of the PATH-SCOPE
sub-TLV within each PCED TLV. If it appears more than once, only the
first occurrence is processed and any others MUST be ignored.
The PATH-SCOPE sub-TLV contains a set of bit-flags indicating the
supported path scopes, and four fields indicating PCE preferences.
The PATH-SCOPE sub-TLV has the following format:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|2|3|4|5| Reserved |PrefL|PrefR|PrefS|PrefY| Res |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 2
Length: 4
Value: This comprises a 2-octet flags field where each bit
represents a supported path scope, as well as four
preference fields used to specify PCE preferences.
Le Roux, et al. Standards Track PAGE 8
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
The following bits are defined:
Bit Path Scope
0 L bit: Can compute intra-area paths.
1 R bit: Can act as PCE for inter-area TE LSP
computation.
2 Rd bit: Can act as a default PCE for inter-area TE LSP
computation.
3 S bit: Can act as PCE for inter-AS TE LSP computation.
4 Sd bit: Can act as a default PCE for inter-AS TE LSP
computation.
5 Y bit: Can act as PCE for inter-layer TE LSP
computation.
PrefL field: PCE's preference for intra-area TE LSP computation.
PrefR field: PCE's preference for inter-area TE LSP computation.
PrefS field: PCE's preference for inter-AS TE LSP computation.
PrefY field: PCE's preference for inter-layer TE LSP computation.
Res: Reserved for future use.
The L, R, S, and Y bits are set when the PCE can act as a PCE for
intra-area, inter-area, inter-AS, or inter-layer TE LSP computation,
respectively. These bits are non-exclusive.
When set, the Rd bit indicates that the PCE can act as a default PCE
for inter-area TE LSP computation (that is, the PCE can compute a
path toward any neighbor area). Similarly, when set, the Sd bit
indicates that the PCE can act as a default PCE for inter-AS TE LSP
computation (the PCE can compute a path toward any neighbor AS).
When the Rd and Sd bit are set, the PCED TLV MUST NOT contain a
NEIG-PCE-DOMAIN sub-TLV (see Section 4.4).
When the R bit is clear, the Rd bit SHOULD be clear on transmission
and MUST be ignored on receipt. When the S bit is clear, the Sd bit
SHOULD be clear on transmission and MUST be ignored on receipt.
The PrefL, PrefR, PrefS, and PrefY fields are each three bits long
and allow the PCE to specify a preference for each computation scope,
where 7 reflects the highest preference. Such preferences can be
used for weighted load balancing of path computation requests. An
operator may decide to configure a preference for each computation
scope at each PCE so as to balance the path computation load among
Le Roux, et al. Standards Track PAGE 9
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
them. The algorithms used by a PCC to load balance its path
computation requests according to such PCE preferences is out of the
scope of this document and is a matter for local or network-wide
policy. The same or different preferences may be used for each
scope. For instance, an operator that wants a PCE capable of both
inter-area and inter-AS computation to be preferred for use for
inter-AS computations may configure PrefS higher than PrefR.
When the L, R, S, or Y bits are cleared, the PrefL, PrefR, PrefS, and
PrefY fields SHOULD respectively be set to 0 on transmission and MUST
be ignored on receipt.
Both reserved fields SHOULD be set to zero on transmission and MUST
be ignored on receipt.
4.3. PCE-DOMAIN Sub-TLV
The PCE-DOMAIN sub-TLV specifies a PCE-Domain (area or AS) where the
PCE has topology visibility and through which the PCE can compute
paths.
The PCE-DOMAIN sub-TLV SHOULD be present when PCE-Domains for which
the PCE can operate cannot be inferred by other IGP information: for
instance, when the PCE is inter-domain capable (i.e., when the R bit
or S bit is set) and the flooding scope is the entire routing domain
(see Section 5 for a discussion of how the flooding scope is set and
interpreted).
A PCED TLV may include multiple PCE-DOMAIN sub-TLVs when the PCE has
visibility into multiple PCE-Domains.
The PCE-DOMAIN sub-TLV has the following format:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCE-DOMAIN sub-TLV format
Type: 3
Length: 8
Le Roux, et al. Standards Track PAGE 10
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
Two domain-type values are defined:
1 OSPF Area ID
2 AS Number
Domain ID: With the domain-type set to 1, this indicates the
32-bit Area ID of an area where the PCE has visibility and can
compute paths. With domain-type set to 2, this indicates an AS
number of an AS where the PCE has visibility and can compute
paths. When the AS number is coded in two octets, the AS Number
field MUST have its first two octets set to 0.
4.4. NEIG-PCE-DOMAIN Sub-TLV
The NEIG-PCE-DOMAIN sub-TLV specifies a neighbor PCE-Domain (area or
AS) toward which a PCE can compute paths. It means that the PCE can
take part in the computation of inter-domain TE LSPs with paths that
transit this neighbor PCE-Domain.
A PCED sub-TLV may include several NEIG-PCE-DOMAIN sub-TLVs when the
PCE can compute paths towards several neighbor PCE-Domains.
The NEIG-PCE-DOMAIN sub-TLV has the same format as the PCE-DOMAIN
sub-TLV:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NEIG-PCE-DOMAIN sub-TLV format
Type: 4
Length: 8
Two domain-type values are defined:
1 OSPF Area ID
2 AS Number
Domain ID: With the domain-type set to 1, this indicates the
32-bit Area ID of a neighbor area toward which the PCE can compute
paths. With domain-type set to 2, this indicates the AS number of
Le Roux, et al. Standards Track PAGE 11
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
a neighbor AS toward which the PCE can compute paths. When the AS
number is coded in two octets, the AS Number field MUST have its
first two octets set to 0.
The NEIG-PCE-DOMAIN sub-TLV MUST be present at least once with
domain-type set to 1 if the R bit is set and the Rd bit is cleared,
and MUST be present at least once with domain-type set to 2 if the S
bit is set and the Sd bit is cleared.
4.5. PCE-CAP-FLAGS Sub-TLV
The PCE-CAP-FLAGS sub-TLV is an optional sub-TLV used to indicate PCE
capabilities. It MAY be present within the PCED TLV. It MUST NOT be
present more than once. If it appears more than once, only the first
occurrence is processed and any others MUST be ignored.
The value field of the PCE-CAP-FLAGS sub-TLV is made up of an array
of units of 32-bit flags numbered from the most significant bit as
bit zero, where each bit represents one PCE capability.
The format of the PCE-CAP-FLAGS sub-TLV is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// PCE Capability Flags //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 5
Length: Multiple of 4 octets
Value: This contains an array of units of 32-bit flags
numbered from the most significant as bit zero, where
each bit represents one PCE capability.
Le Roux, et al. Standards Track PAGE 12
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
IANA will manage the space of the PCE Capability Flags.
The following bits have been assigned by IANA:
Bit Capabilities
0 Path computation with GMPLS link constraints
1 Bidirectional path computation
2 Diverse path computation
3 Load-balanced path computation
4 Synchronized path computation
5 Support for multiple objective functions
6 Support for additive path constraints
(max hop count, etc.)
7 Support for request prioritization
8 Support for multiple requests per message
9-31 Reserved for future assignments by IANA.
These capabilities are defined in [RFC 4657].
Reserved bits SHOULD be set to zero on transmission and MUST be
ignored on receipt.
5. Elements of Procedure
The PCED TLV is advertised within OSPFv2 Router Information LSAs
(Opaque type of 4 and Opaque ID of 0) or OSPFv3 Router Information
LSAs (function code of 12), which are defined in [RFC 4970]. As such,
elements of procedure are inherited from those defined in [RFC 4970].
In OSPFv2, the flooding scope is controlled by the opaque LSA type
(as defined in [RFC 2370]) and in OSPFv3, by the S1/S2 bits (as
defined in [RFC 2740]). If the flooding scope is area local, then the
PCED TLV MUST be carried within an OSPFv2 type 10 router information
LSA or an OSPFV3 Router Information LSA with the S1 bit set and the
S2 bit clear. If the flooding scope is the entire IGP domain, then
the PCED TLV MUST be carried within an OSPFv2 type 11 Router
Information LSA or OSPFv3 Router Information LSA with the S1 bit
clear and the S2 bit set. When only the L bit of the PATH-SCOPE
sub-TLV is set, the flooding scope MUST be area local.
When the PCE function is deactivated, the OSPF speaker advertising
this PCE MUST originate a new Router Information LSA that no longer
includes the corresponding PCED TLV, provided there are other TLVs in
the LSA. If there are no other TLVs in the LSA, it MUST either send
an empty Router Information LSA or purge it by prematurely aging it.
Le Roux, et al. Standards Track PAGE 13
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
The PCE address (i.e., the address indicated within the PCE-ADDRESS
sub-TLV) SHOULD be reachable via some prefixes advertised by OSPF.
The PCED TLV information regarding a specific PCE is only considered
current and useable when the router advertising this information is
itself reachable via OSPF calculated paths in the same area of the
LSA in which the PCED TLV appears.
A change in the state of a PCE (activate, deactivate, parameter
change) MUST result in a corresponding change in the PCED TLV
information advertised by an OSPF router (inserted, removed, updated)
in its LSA. The way PCEs determine the information they advertise,
and how that information is made available to OSPF, is out of the
scope of this document. Some information may be configured (e.g.,
address, preferences, scope) and other information may be
automatically determined by the PCE (e.g., areas of visibility).
A change in information in the PCED TLV MUST NOT trigger any SPF
computation at a receiving router.
6. Backward Compatibility
The PCED TLV defined in this document does not introduce any
interoperability issues.
A router not supporting the PCED TLV will just silently ignore the
TLV as specified in [RFC 4970].
7. IANA Considerations
7.1. OSPF TLV
IANA has defined a registry for TLVs carried in the Router
Information LSA defined in [RFC 4970]. IANA has assigned a new TLV
codepoint for the PCED TLV carried within the Router Information LSA.
Value TLV Name Reference
----- -------- ----------
6 PCED (this document)
7.2. PCE Capability Flags Registry
This document provides new capability bit flags, which are present in
the PCE-CAP-FLAGS TLV referenced in Section 4.1.5.
Le Roux, et al. Standards Track PAGE 14
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
The IANA has created a new top-level OSPF registry, the "PCE
Capability Flags" registry, and will manage the space of PCE
capability bit flags numbering them in the usual IETF notation
starting at zero and continuing at least through 31, with the most
significant bit as bit zero.
New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities:
- Bit number
- Capability Description
- Defining RFC
Several bits are defined in this document. The following values have
been assigned:
Bit Capability Description
0 Path computation with GMPLS link constraints
1 Bidirectional path computation
2 Diverse path computation
3 Load-balanced path computation
4 Synchronized paths computation
5 Support for multiple objective functions
6 Support for additive path constraints
(max hop count, etc.)
7 Support for request prioritization
8 Support for multiple requests per message
8. Security Considerations
This document defines OSPF extensions for PCE discovery within an
administrative domain. Hence the security of the PCE discovery
relies on the security of OSPF.
Mechanisms defined to ensure authenticity and integrity of OSPF LSAs
[RFC 2154], and their TLVs, can be used to secure the PCE Discovery
information as well.
OSPF provides no encryption mechanism for protecting the privacy of
LSAs and, in particular, the privacy of the PCE discovery
information.
Le Roux, et al. Standards Track PAGE 15
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
9. Manageability Considerations
Manageability considerations for PCE Discovery are addressed in
Section 4.10 of [RFC 4674].
9.1. Control of Policy and Functions
Requirements for the configuration of PCE discovery parameters on
PCCs and PCEs are discussed in Section 4.10.1 of [RFC 4674].
In particular, a PCE implementation SHOULD allow the following
parameters to be configured on the PCE:
- The PCE IPv4/IPv6 address(es) (see Section 4.1).
- The PCE Scope, including the inter-domain functions
(inter-area, inter-AS, inter-layer), the preferences,
and whether the PCE can act as default PCE (see Section 4.2).
- The PCE-Domains (see Section 4.3).
- The neighbor PCE-Domains (see Section 4.4).
- The PCE capabilities (see Section 4.5).
9.2. Information and Data Model
A MIB module for PCE Discovery is defined in [PCED-MIB].
9.3. Liveness Detection and Monitoring
This document specifies the use of OSPF as a PCE Discovery Protocol.
The requirements specified in [RFC 4674] include the ability to
determine liveness of the PCE Discovery protocol. Normal operation
of the OSPF protocol meets these requirements.
9.4. Verify Correct Operations
The correlation of information advertised against information
received can be achieved by comparing the information in the PCED TLV
received by the PCC with that stored at the PCE using the PCED MIB
[PCED-MIB]. The number of dropped, corrupt, and rejected information
elements are available through the PCED MIB.
9.5. Requirements on Other Protocols and Functional Components
The OSPF extensions defined in this document do not imply any
requirement on other protocols.
Le Roux, et al. Standards Track PAGE 16
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
9.6. Impact on Network Operations
Frequent changes in PCE information advertised in the PCED TLV, may
have a significant impact on OSPF and might destabilize the operation
of the network by causing the PCCs to swap between PCEs.
As discussed in Section 4.10.4 of [RFC 4674], it MUST be possible to
apply at least the following controls:
- Configurable limit on the rate of announcement of changed
parameters at a PCE.
- Control of the impact on PCCs, such as through rate-limiting
the processing of PCED TLVs.
- Configurable control of triggers that cause a PCC to swap to
another PCE.
10. Acknowledgments
We would like to thank Lucy Wong, Adrian Farrel, Les Ginsberg, Mike
Shand, and Lou Berger for their useful comments and suggestions.
We would also like to thank Dave Ward, Lars Eggert, Sam Hartman, Tim
Polk, and Lisa Dusseault for their comments during the final stages
of publication.
11. References
11.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC 2370] Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July
1998.
[RFC 2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[RFC 3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC 3630,
September 2003.
Le Roux, et al. Standards Track PAGE 17
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
[RFC 4970] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R.,
and S. Shaffer, "Extensions to OSPF for Advertising
Optional Router Capabilities", RFC 4970, July 2007.
11.2. Informative References
[PCED-MIB] Stephan, E., "Definitions of Managed Objects for Path
Computation Element Discovery", Work in Progress, March
2007.
[PCEP] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
Computation Element (PCE) communication Protocol (PCEP)
", Work in Progress, November 2007.
[RFC 4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC 4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC 4674] Le Roux, J., Ed., "Requirements for Path Computation
Element (PCE) Discovery", RFC 4674, October 2006.
[RFC 5089] 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.
Le Roux, et al. Standards Track PAGE 18
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
Authors' Addresses
Jean-Louis Le Roux (Editor)
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
FRANCE
EMail: jeanlouis.leroux@orange-ftgroup.com
Jean-Philippe Vasseur (Editor)
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
EMail: jpv@cisco.com
Yuichi Ikejiri
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019
JAPAN
EMail: y.ikejiri@ntt.com
Raymond Zhang
BT
2160 E. Grand Ave.
El Segundo, CA 90025
USA
EMail: raymond.zhang@bt.com
Le Roux, et al. Standards Track PAGE 19
RFC 5088 OSPF Protocol Extensions for PCE Discovery January 2008
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Le Roux, et al. Standards Track PAGE 20
OSPF Protocol Extensions for Path Computation Element (PCE) Discovery
RFC TOTAL SIZE: 40936 bytes
PUBLICATION DATE: Saturday, January 5th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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