Network Working Group J.L. Le Roux (Editor)
Internet Draft France Telecom
Category: Standard Track
Expires: October 2007 J.P. Vasseur (Editor)
Cisco System Inc.
Yuichi Ikejiri
NTT Communications
Raymond Zhang
BT Infonet
April 2007
OSPF protocol extensions for Path Computation Element (PCE) Discovery
draft-ietf-pce-disco-proto-ospf-03.txt
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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 (PCE),
along with some information that can be used for PCE selection. When
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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 discover 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.
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. Terminology.................................................3
2. Introduction................................................4
3. Overview....................................................5
3.1. PCE Information.............................................5
3.2. PCE Discovery Information...................................5
3.2.1. PCE Congestion Information..................................6
3.3. Flooding scope..............................................6
4. OSPF extensions.............................................6
4.1. The OSPF PCED TLV...........................................6
4.1.1. PCE-ADDRESS sub-TLV.........................................8
4.1.2. PATH-SCOPE sub-TLV..........................................8
4.1.3. PCE-DOMAIN sub-TLV.........................................10
4.1.4. NEIG-PCE-DOMAIN sub-TLV....................................11
4.1.5. PCE-CAP-FLAGS sub-TLV......................................12
4.1.6. The CONGESTION sub-TLV.....................................14
5. Elements of Procedure......................................14
5.1. CONGESTION sub-TLV specific procedures.....................15
6. Backward compatibility.....................................16
7. IANA Considerations........................................16
7.1. OSPF TLV...................................................16
7.2. PCED sub-TLVs registry.....................................16
7.3. PCE Capability Flags registry..............................17
8. Security Considerations....................................18
9. Manageability Considerations...............................18
9.1. Control of Policy and Functions............................18
9.2. Information and Data Model.................................18
9.3. Liveness Detection and Monitoring..........................18
9.4. Verify Correct Operations..................................18
9.5. Requirements on Other Protocols and Functional
Components...............................................19
9.6. Impact on network operations...............................19
10. Acknowledgments............................................19
11. References.................................................19
11.1. Normative references.......................................19
11.2. Informative references.....................................20
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12. Editor's Addresses:........................................20
13. Contributors' Addresses:...................................21
14. Intellectual Property Statement............................21
1. Terminology
Terminology used in this document
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 (ISIS).
Intra-area TE LSP: A TE LSP whose path does not cross IGP area
boundaries.
Intra-AS TE LSP: A TE LSP whose path does not cross AS boundaries.
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.
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 "domain" in
[RFC4655]). Examples of PCE-Domains include IGP areas and
Autonomous Systems. This should be distinguished from an OSPF
routing domain.
PCEP: Path Computation Element Protocol.
TE LSP: Traffic Engineered Label Switched Path.
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2. Introduction
[RFC4655] describes the motivations and architecture for a 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. This relies on a
communication protocol between PCC and PCE, and between PCEs. The
requirements for such a communication protocol can be found in
[RFC4657] and the communication protocol is defined in [PCEP].
The PCE architecture requires that a PCC be aware of the location of
one or more PCEs in its domain, and also potentially of some PCEs in
other domains, e.g. in case of inter-domain TE LSP computation.
A network may contain a large number of PCEs with potentially
distinct capabilities. In such a context it is highly desirable to
have a mechanism for automatic and dynamic PCE discovery, which
allows PCCs to automatically discover a set of PCEs, along with
additional information about each PCE that may be required for the
PCC to perform PCE selection. Additionally, it is valuable for a PCC
to dynamically detect new PCEs or any modification of the PCE
information. Detailed requirements for such a PCE discovery mechanism
are provided in [RFC4674].
Moreover, it may also be useful to discover when a PCE experiences
processing congestion and when it exits such a state, in order for
the PCCs to take some appropriate actions (e.g. to redirect their
requests to another PCE). 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 OSPF extensions 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 [RFC4674]. This document also
defines extensions allowing a PCE in an OSPF routing domain to
advertise its processing congestion state.
Generic capability advertisement mechanisms for OSPF are defined in
[OSPF-CAP]. 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 aforementioned dynamic PCE discovery requirements.
This document defines a new TLV (named the PCE Discovery (PCED) TLV)
to be carried within the OSPF Router Information LSA ([OSPF-CAP]).
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The PCE information advertised is detailed in section 3. Protocol
extensions and procedures are defined in section 4 and 5.
This document does not define any new OSPF elements of procedure. The
procedures defined in [OSPF-CAP] MUST be used.
The OSPF extensions defined in this document allow for PCE discovery
within an OSPF Routing domain. Solutions for PCE discovery across AS
boundaries are beyond the scope of this document, and for further
study.
In this document, we call TLV any TLV that is carried within an OSPF
LSA. Any TLV that is itself carried within another TLV is referred to
as either a TLV or a sub-TLV.
3. Overview
3.1. PCE Information
The PCE information advertised via OSPF falls into two categories:
PCE Discovery information and PCE Congestion information.
3.2. PCE Discovery Information
The PCE Discovery information is comprised 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;
- The PCE path computation scope (i.e. inter-area, inter-AS, inter-
layer);
- The set of one or more PCE-Domain(s) into which the PCE has
visibility and can compute paths;
- The set of one or more neighbor PCE-Domain(s) towards which a PCE
can compute paths;
- A set of communication capabilities (e.g. support for request
prioritization) and path computation specific capabilities
(e.g. supported constraints).
Optional elements to describe more complex capabilities may also be
advertised.
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
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deployment/activation, PCE deactivation/suppression, or PCE failure.
Hence, this information is not expected to change frequently.
3.2.1. PCE Congestion Information
The PCE Congestion information is optional information and can be
used to report a PCE's processing congestion state along with an
estimated congestion duration. This is dynamic information, which may
change with PCE activity.
Procedures for a PCE to move from a processing congestion state to a
non congestion state are beyond the scope of this document, but the
rate at which a PCE Status change is advertised MUST NOT impact by
any means the IGP scalability. Particular attention MUST be given to
procedures to avoid state oscillations.
3.3. 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).
4. OSPF Extensions
4.1. The OSPF PCED TLV
The OSPF PCE Discovery TLV (PCED TLV) is made of a set of non-ordered
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
[RFC3630]. 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 four-octet alignment; padding is not included in
the Length field (so a three octet value would have a length of
three, but the total size of the TLV would be eight octets). Nested
TLVs are also four-octet aligned. Unrecognized types are ignored.
All Type values between 32768 and 65535 are reserved for vendor-
specific extensions. All other undefined Type codes are reserved for
future assignment by IANA.
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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 To be defined by IANA (suggested value=5)
Length Variable
Value This comprises one or more sub-TLVs
Sub-TLVs types are under IANA control.
Currently five sub-TLVs are defined (type values to be assigned by
IANA):
Sub-TLV type Length Name
1 variable PCE-ADDRESS sub-TLV
2 4 PATH-SCOPE sub-TLV
3 variable PCE-DOMAIN sub-TLV
4 variable NEIG-PCE-DOMAIN sub-TLV
5 variable PCE-CAP-FLAGS sub-TLV
6 4 CONGESTION 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.
The PCE-CAP-FLAGS sub-TLV is optional and MAY be present in the PCED
TLV to facilitate the PCE selection process.
The CONGESTION sub-TLV is optional and MAY be present in the PCED
TLV, to indicate a PCE's processing congestion state.
Any non recognized sub-TLV MUST be silently ignored.
Additional sub-TLVs could be added in the future to advertise
additional information.
The PCED TLV is carried within an OSPF Router Information LSA
defined in [OSPF-CAP].
The following sub-sections describe the sub-TLVs which may be carried
within the PCED sub-TLV.
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4.1.1. PCE-ADDRESS sub-TLV
The PCE-ADDRESS sub-TLV specifies the IP address(es) 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.
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.
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| address-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// PCE IP Address //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCE-ADDRESS sub-TLV format
Type To be assigned by IANA (suggested value =1)
Length 8 (IPv4) or 20 (IPv6)
Address-type:
1 IPv4
2 IPv6
PCE IP Address: The IP address to be used to reach the PCE.
4.1.2. 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 intra-area, inter-area, inter-AS, or inter-layer_TE
LSP(s).
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.
The PATH-SCOPE sub-TLV contains a set of bit flags indicating the
supported path scopes and four fields indicating PCE preferences.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|2|3|4|5| Reserved |PrefL|PrefR|PrefS|PrefY| Res |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type To be defined by IANA (suggested value =2)
Length 4
Value This comprises a 2 byte flag field where each bit
represents a supported path scope, as well as four
preference fields used to specify PCE preferences.
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 compute or take part into the computation
of paths across layers.
Pref-L field: PCE's preference for intra-area TE LSPs computation.
Pref-R field: PCE's preference for inter-area TE LSPs computation.
Pref-S field: PCE's preference for inter-AS TE LSPs computation.
Pref-Y field: PCE's preference for inter-layer TE LSPs computation.
Res: Reserved for future usage.
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 LSPs 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 LSPs computation (that is the PCE can compute a
path towards 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 towards any neighbor AS).
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When the Rd and Sd bit are set the PCED TLV MUST NOT contain any
NEIG-PCE-DOMAIN sub-TLV (see 4.1.4).
When the R/S bit is cleared, the Rd/Sd bit SHOULD be cleared and MUST
be ignored.
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 preference can be used
for weighted load balancing of requests. An operator may decide to
configure a preference for each computation scope to each PCE so as
to balance the path computation load among them. The algorithms used
by a PCC to load balance its path computation requests according to
such PCE preference is out of the scope of this document and is a
matter for local or network wide policy. The same or distinct
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
used preferably for inter-AS computation may configure a PrefS higher
than the PrefR.
When the L bit, R bit, S bit or Y bit are cleared, the PrefL, PrefR,
PrefS, PrefY fields SHOULD respectively be set to 0 and MUST be
ignored.
Both reserved fields SHOULD be set to zero on transmission and MUST
be ignored on receipt.
4.1.3. PCE-DOMAIN sub-TLV
The PCE-DOMAIN sub-TLV specifies a PCE-Domain (areas and/or ASes)
where the PCE has topology visibility and through which the PCE can
compute paths.
The PCE-DOMAIN sub-TLV MAY be present when PCE-Domains 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 in multiple PCE-Domains.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Domain ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCE-DOMAIN sub-TLV format
Type To be assigned by IANA (suggested value =3)
Length Variable
3 domain-type values are defined:
1 IPv4 Area Address
2 IPv6 Area Address
3 AS Number
Domain ID: With the address type 1/2 this indicates the IPv4/v6
address of an area where the PCE has visibility. With address-
type 3 this indicates an AS number where the PCE has
visibility. When coded in two bytes (which is the current
defined format as the time of writing this document), the AS
Number field MUST have its left two bytes set to 0.
.
4.1.4. NEIG-PCE-DOMAIN sub-TLV
The NEIG-PCE-DOMAIN sub-TLV specifies a neighbour PCE-domain (area,
AS) toward which a PCE can compute paths. It means that the PCE can
take part in the computation of inter-domain TE LSPs whose path
transits this neighbour PCE-domain.
A PCED sub-TLV MAY include several NEIG-PCE-DOMAIN sub-TLVs when the
PCE can compute paths towards several neighbour PCE-domains.
The NEIG-PCE-DOMAIN sub-TLV has the same format as the PCE-DOMAIN
sub-TLV:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Domain ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NEIG-PCE-DOMAIN sub-TLV format
Type To be assigned by IANA (suggested value =3)
Length Variable
3 domain-type values are defined:
1 IPv4 Area Address
2 IPv6 Area Address
3 AS Number
Domain ID: With the address type 1/2 this indicates the
IPv4/v6 address of a neighbour area towards which the PCE can
compute paths. With address-type 3 this indicates the AS number
of a neighbour AS towards which the PCE can compute paths. When
coded in two bytes (which is the current defined format as the
time of writing this document), the AS Number field MUST have
its left two bytes set to 0.
The NEIG-PCE-DOMAIN sub-TLV MUST be present if the R bit is set and
the Rd bit is cleared, and/or, if the S bit is set and the Sd bit is
cleared.
4.1.5. 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.
The value field of the PCE-CAP-FLAGS sub-TLV is made up of an array
of units of 32 flags numbered from the most significant as bit zero,
where each bit represents one PCE capability.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// PCE Capability Flags //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type To be assigned by IANA (suggested value =5)
Length Multiple of 4 bytes
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.
IANA is requested to manage the space of the PCE Capability Flags
The following bits are to be 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 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
9-31 Reserved for future assignments by IANA.
These capabilities are defined in [RFC4657].
Reserved bits SHOULD be set to zero on transmission and MUST be
ignored on receipt.
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4.1.6. The CONGESTION sub-TLV
The CONGESTION sub-TLV is used to indicate that a PCE is experiencing
a processing congestion state and may optionally include the expected
PCE congestion duration.
The CONGESTION sub-TLV is optional, it MAY be carried within the PCED
TLV. It MUST NOT be present more than once.
The format of the CONGESTION 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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Reserved | Congestion Duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type To be assigned by IANA (suggested value =6)
Length 4
Value
-C bit: When set this indicates that the PCE is experiencing
congestion and cannot accept any new request. When
cleared this indicates that the PCE is not
experiencing congestion and can accept new requests.
-Congestion Duration: 2-bytes, the estimated PCE congestion
duration in seconds.
When C is set and the Congestion Duration field is equal to 0, this
means that the Congestion Duration is unknown.
When C is cleared the Congestion Duration SHOULD be set to 0 and MUST
be ignored.
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 [OSPF-CAP]. As such,
elements of procedure are inherited from those defined in [OSPF-CAP].
In OSPFv2 the flooding scope is controlled by the opaque LSA type (as
defined in [RFC2370]) and in OSPFv3 by the S1/S2 bits (as defined in
[RFC2740]). If the flooding scope is local to an area 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 cleared. If the flooding scope is the entire domain then the PCED
TLV MUST be carried within an OSPFv2 type 11 Router Information LSA
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or OSPFv3 Router Information LSA with the S1 bit cleared 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.
An OSPF router MUST originate a new Router Information LSA whenever
there is a change in a PCED TLV associated with a PCE it advertises.
When a PCE is deactivated, the OSPF router advertising this PCE MUST
originate a new Router Information LSA that does no longer include
the corresponding PCED TLV.
The PCE address, i.e. the address indicated within the PCE ADDRESS
TLV, MUST be reachable via some prefixes advertised by OSPF; this
allows speeding up the detection of a PCE failure. Note that when the
PCE address is no longer reachable, this means that the PCE node has
failed or has been torn down, or that there is no longer IP
connectivity to the PCE node.
The PCED TLV is OPTIONAL. When an OSPF LSA does not contain any PCED
TLV, this means that the PCE information of that node is unknown.
A change in PCED information MUST NOT trigger any SPF computation at
a receiving router.
The way PCEs determine the information they advertise is out of the
scope of this document. Some information may be configured on the PCE
(e.g., address, preferences, scope) and other information may be
automatically determined by the PCE (e.g., areas of visibility).
5.1. CONGESTION sub-TLV specific procedures
When a PCE enters into a processing congestion state, the conditions
of which are implementation dependent, a Router Information LSA with
a CONGESTION sub-TLV with the C bit set, and optionally a non-null
expected congestion duration MAY be generated.
When a PCE exits from the processing congestion state, the conditions
of which are implementation dependent, two cases are considered:
- If the congestion duration in the previously originated
CONGESITON sub-TLV was null, a CONGESTION sub-TLV with the C bit
cleared SHOULD be generated;
- If the congestion duration in the previously originated
CONGESTION sub-TLV was non null, a CONGESTION sub-TLV with the C bit
cleared MAY be generated. Note that in some particular cases it may
be desired to originate a CONGESTION sub-TLV with the C bit cleared
if the congestion duration was over estimated.
The congestion duration allows a reduction in the amount of OSPF
flooding, as only uncongested-to-congested state transitions need to
be advertised.
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An OSPF implementation SHOULD support an appropriate dampening
algorithm so as to dampen OSPF flooding of PCE Congestion information
in order to not impact the OSPF scalability. It is RECOMMENDED to
introduce some hysteresis for congestion state transition, so as to
avoid state oscillations that may impact OSPF performance. For
instance two thresholds MAY be configured: A resource congestion
upper-threshold and a resource congestion lower-threshold. An LSR
enters the congested state when the CPU load reaches the upper
threshold and leaves the congested state when the CPU load goes under
the lower threshold.
Upon receipt of an updated CONGESTION sub-TLV a PCC SHOULD take
appropriate actions. In particular, the PCC SHOULD stop sending
requests to a congested PCE, and SHOULD gradually start sending
again requests to a PCE that is no longer congested.
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 [OSPF-CAP].
7. IANA Considerations
7.1. OSPF TLV
Once a registry for the Router Information LSA defined in
[OSPF-CAP] will have been assigned, IANA will assign a new
OSPF TLV code-point for the PCED TLV carried within the Router
Information LSA.
Value Sub-TLV References
----- -------- ----------
5 PCED TLV (this document)
7.2. PCED sub-TLVs registry
The PCED TLV referenced above is constructed from sub-TLVs. Each sub-
TLV includes a 16-bit type identifier.
The IANA is requested to create a new registry and manage sub-TLV
type identifiers as follows:
- sub-TLV Type
- sub-TLV Name
- Reference
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This document defines five TLVs as follows (suggested values):
Value TLV name References
----- -------- ----------
1 PCE-ADDRESS This document
2 PATH-SCOPE This document
3 PCE-DOMAIN This document
4 NEIG-PCE-DOMAIN This document
5 PCE-CAP-FLAGS This document
6 CONGESTION This document
New sub-TLV type values may be allocated only by an IETF Consensus
action.
7.3. 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.
The IANA is requested to create a new registry and to 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.
The same registry is defined for IS-IS based PCE discovery [PCED-
ISIS]. A single registry must be defined for both protocols.
New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities:
- Bit number
- Defining RFC
- Capability Description
Several bits are defined in this document. Here are the suggested
values:
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
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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
[RFC2154], and their TLVs, can be used to secure the PCE Discovery
information as well.
OSPF provides no mechanism for protecting the privacy of LSAs, and in
particular the privacy of the PCE discovery information.
9. Manageability Considerations
Manageability considerations for PCE Discovery are addressed in
section 4.10 of [RFC4674].
9.1. Control of Policy and Functions
Requirements on the configuration of PCE discovery parameters on PCCs
and PCEs are discussed in section 4.10.1 of [RFC4674].
Particularly, a PCE implementation SHOULD allow configuring the
following parameters on the PCE:
-The PCE IPv4/IPv6 address(es) (see section 4.1.1)
-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 4.1.2)
-The PCE domains (see section 4.1.3)
-The neighbour PCE domains (see section 4.1.4)
-The PCE capabilities (see section 4.1.5)
9.2. Information and Data Model
A MIB module for PCE Discovery is defined in [PCED-MIB].
9.3. Liveness Detection and Monitoring
PCE Discovery Protocol liveness detection relies upon OSPF liveness
detection. OSPF already includes a liveness detection mechanism
(Hello protocol), and PCE discovery does not require additional
capabilities.
Procedures defined in section 5.1 allow a PCC detecting when a PCE
has been deactivated, or is no longer reachable.
9.4. Verify Correct Operations
The correlation of information advertised against information
received can be achieved by comparing the PCED information in the PCC
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and in the PCE, which is stored in the PCED MIB [PCED-MIB]. The
number of dropped, corrupt, and rejected information elements are
stored in the PCED MIB.
9.5. Requirements on Other Protocols and Functional Components
The OSPF extensions defined in this documents do not imply any
requirement on other protocols.
9.6. Impact on network operations
Frequent changes in PCE information, and particularly in PCE
congestion information, 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 5.1, a PCE implementation SHOULD support an
appropriate dampening algorithm so as to dampen OSPF flooding in
order to not impact the OSPF scalability.
Also, as discussed in section 4.10.4 of [RFC4674], 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 discovery messages
rate-limiting.
- 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.
11. References
11.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[RFC2370] Coltun, R., The OSPF Opaque LSA Option, RFC 2370, July
1998.
[RFC3630] Katz, D., Yeung, D., Kompella, K., "Traffic Engineering
Extensions to OSPF Version 2", RFC 3630, September 2003.
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[OSPF-CAP] Lindem, A., Shen, N., Aggarwal, R., Shaffer, S., Vasseur,
J.P., "Extensions to OSPF for advertising Optional Router
Capabilities", draft-ietf-ospf-cap, work in progress.
[RFC4655] Farrel, A., Vasseur, J.P., Ash, J., "Path Computation
Element (PCE)-based Architecture", RFC4655, August 2006.
[RFC4674] Le Roux, J.L., et al. "Requirements for PCE discovery",
RFC4674, October 2006.
[RFC4203] Kompella, Rekhter, " OSPF Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC4203, October
2005.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
11.2. Informative references
[RFC4657] Ash, J., Le Roux, J.L., " PCE Communication Protocol
Generic Requirements", RFC4657, September 2006.
[PCEP] Vasseur, Le Roux, et al., "Path Computation Element (PCE)
communication Protocol (PCEP) - Version 1", draft-ietf-pce-pcep, work
in progress.
[PCED-MIB] Stephan, E., "Definitions of Managed Objects for Path
Computation Element Discovery", draft-ietf-pce-disc-mib-00, work in
progress.
[PCED-ISIS] Le Roux, Vasseur, et al. "IS-IS protocol extensions for
Path Computation Element (PCE) Discovery", draft-ietf-pce-disco-
proto-isis, work in progress.
12. Editor's 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
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13. Contributors' Addresses:
Yuichi Ikejiri
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019
JAPAN
Email: y.ikejiri@ntt.com
Raymond Zhang
BT Infonet
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@bt.infonet.com
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Le Roux, Vasseur et al. OSPF extensions for PCE Discovery [Page 21]
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Copyright Statement
Copyright (C) The IETF Trust (2007). This document is subject to the
rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.
Le Roux, Vasseur et al. OSPF extensions for PCE Discovery [Page 22]
