wdiff draft-ietf-pce-disco-proto-isis-02.txt draft-ietf-pce-disco-proto-isis-03.txt

Network Working Group J.L. Le Roux (Editor)
Internet Draft France Telecom
Category: Standard Track
Expires: August October 2007 J.P. Vasseur (Editor)
Cisco System Inc.

Yuichi Ikejiri
NTT Communications

Raymond Zhang
BT Infonet

February

April 2007

IS-IS protocol extensions for Path Computation Element (PCE) Discovery

draft-ietf-pce-disco-proto-isis-02.txt

draft-ietf-pce-disco-proto-isis-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 of information that can be used 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 discover PCEs
consists of using IGP flooding. For that purpose this document
defines extensions to the Intermediate System to Intermediate System
(IS-IS) routing protocol for the advertisement of PCE Discovery
information within an IS-IS area or within the entire IS-IS 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
3.1.1. PCE Discovery Information...................................5
3.1.2. PCE Status Information......................................6 Congestion Information..................................6
3.2. Flooding scope..............................................6
4. IS-IS extensions............................................6 extensions............................................7
4.1. The IS-IS PCED TLV..........................................6 sub-TLV......................................7
4.1.1. PCE-ADDRESS sub-TLV.........................................7 sub-TLV.........................................8
4.1.2. The PATH-SCOPE sub-TLV......................................8
4.1.3. PCE-DOMAINS sub-TLV........................................10
4.1.3.1. Area ID DOMAIN sub-TLV...................................10
4.1.3.2. AS Number DOMAIN sub-TLV.................................11 PCE-DOMAIN sub-TLV.........................................10
4.1.4. PCE-NEIG-DOMAINS sub-TLV...................................11 NEIG-PCE-DOMAIN sub-TLV....................................11
4.1.5. PCE-CAP-FLAGS sub-TLV......................................11
4.1.6. The CONGESTION sub-TLV.....................................12
5. Elements of Procedure......................................13
5.1.1. CONGESTION sub-TLV specific procedures.....................14
6. Backward compatibility.....................................15
7. IANA considerations........................................15
7.1. IS-IS sub-TLV..............................................15
7.2. PCED sub-TLVs registry.....................................15
7.3. PCE Capability Flags registry..............................16
8. Security Considerations....................................16
9. Manageability Considerations...............................17
9.1. Control of Policy and Functions............................17
9.2. Information and Data Model.................................17
9.3. Liveness Detection and Monitoring..........................17
9.4. Verify Correct Operations..................................17
9.5. Requirements on Other Protocols and Functional
Components...............................................17
9.6. Impact on network operations...............................18 operations...............................17
10. Acknowledgments............................................18
11. References.................................................18
11.1. Normative references.......................................18
11.2. Informative references.....................................19
12. Editors' Addresses:........................................19
13. Contributors' Adresses:....................................19
14. Intellectual Property Statement............................20

1. Terminology

Terminology used in this document

ABR: IGP Area Border Router (L1L2 router).

AS: Autonomous System.

Domain: any collection of network elements within a common sphere
of address management or path computational responsibility.
Examples of domains include IGP areas and Autonomous Systems.

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 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.

IS-IS LSP: Link State PDU

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 IS-IS
routing domain as defined by [ISO].

PCEP: Path Computation Element communication Protocol.

TE LSP: Traffic Engineered Label Switched Path.

2. Introduction

[RFC4655] 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 PCC Path Computation Client (PCC) (also
referred to as a non co-
located 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. 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, 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 IS-IS extensions to allow a PCE in an IS-IS
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 IS-IS routing domain to
advertise its processing congestion state.

Generic capability advertisement mechanisms for IS-IS are defined in
[IS-IS-CAP]. These allow a router to advertise its capabilities
within an IS-IS area or an entire IS-IS routing domain. This document
leverages this generic capability advertisement mechanism to fully
satisfy the aforementioned dynamic PCE discovery requirements.

This document defines a new sub-TLV (named the PCE Discovery (PCED) to be
carried within the IS-IS Router Capability TLV ([IS-IS-CAP]).

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 IS-IS elements of procedure.
The procedures defined in [IS-IS-CAP] should MUST be used.

The IS-IS extensions defined in this document allow for PCE discovery
within an IS-IS Routing domain. Solutions for PCE discovery across AS
boundaries are beyond the scope of this document, and for further
study.

This document defines a set of sub-TLVs that are nested within each
other. When the degree of nesting TLVs is 2 (a TLV is carried within
another TLV) the TLV carried within a TLV is called a sub-TLV.
Strictly speaking, when the degree of nesting is 3, a subsub-TLV is
carried within a sub-TLV that is itself carried within a TLV. For the
sake of terminology simplicity, we refer to sub-TLV, a TLV carried
within a TLV regardless of the degree of nesting.

3. Overview

3.1. PCE Information

The PCE information advertised via IS-IS falls into two categories:
PCE Discovery information and PCE Status Congestion information.

3.1.1. 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 inter-domain functions: PCE path computation scope (i.e. inter-area, inter-AS, inter-layer…); inter-
layer);

- The PCE domain(s): set of one or more domain(s) PCE-Domain(s) into which the PCE has
visibility and can compute paths;

- The PCE neighbor domain(s): set of one or more neighbor domain(s) 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
deployment/activation, PCE deactivation/suppression, or PCE failure.
Hence, this information is not expected to change frequently.

3.1.2. PCE Status Congestion Information

The PCE Status Congestion information is optional and can be used to report
a PCE's processing congestion state along with an estimated
congestion duration. This is a 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 should be given
on procedures to avoid state oscillations.

3.2. Flooding scope

The flooding scope for PCE information advertised through IS-IS can
be limited to one or more IS-IS areas a single L1 area, a L1 area and the PCE belongs to, L2 sub-domain, or can be
extended across the entire
IS-IS 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 a
L1L2 router for instance advertising its PCE information within the
L2 area and/or a subset of its attached L1 area(s).

4. IS-IS extensions

4.1. The IS-IS PCED TLV sub-TLV

The IS-IS PCED TLV sub-TLV is made of a set of non ordered sub-TLVs.

The format of the IS-IS PCED TLV sub-TLV and its sub-TLVs is the identical to
the TLV format used by the Traffic Engineering Extensions to IS-IS
[RFC3784]. That is, the TLV is composed of 1 octet for the type, 1
octet specifying the TLV length, and a value field. The Length field
defines the length of the value portion in octets. bytes.

The IS-IS PCED TLV sub-TLV has the following format:

TYPE: To be assigned by IANA (suggested value = 5)
LENGTH: Variable
VALUE: set of sub-TLVs

Sub-TLVs types are under IANA control.

Currently six sub-TLVs are defined (suggested type values to be
assigned by IANA):
Sub-TLV type Length Name
1 variable PCE-ADDRESS sub-TLV
2 3 PATH-SCOPE sub-TLV
3 variable PCE-DOMAINS PCE-DOMAIN sub-TLV
4 variable PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLV
5 variable PCE-CA-FLAGS PCE-CAP-FLAGS sub-TLV
6 1 CONGESTION sub-TLV

The PCE-ADDRESS and PATH-SCOPE sub-TLVs MUST always be present within
the PCED TLV. sub-TLV.

The PCE-DOMAINS PCE-DOMAIN and PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLVs are optional. They may
MAY be present in the PCED TLV sub-TLV to facilitate selection of inter-domain inter-
domain PCEs.

The PCE-CAP-FLAGS sub-TLVs are sub-TLV is optional and MAY be present in the PCED TLV
sub-TLV to facilitate the PCE selection process.

The CONGESTION sub-TLV is optional and MAY be present in the PCED
TLV,
sub-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 PCE information.

The PCED TLV sub-TLV is carried within an IS-IS CAPABILITY TLV defined in
[IS-IS-CAP].

The following sub-sections describe the sub-TLVs which may be carried
within the PCED sub-TLV.

4.1.1. PCE-ADDRESS sub-TLV

The PCE-ADDRESS sub-TLV specifies the IP address that can be
used to reach the PCE. It is RECOMMENDED to make use of an address
that is always reachable, provided the PCE is alive.

The PCE-ADDRESS sub-TLV is mandatory; it MUST be present within the
PCED TLV. sub-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 PCE-ADDRESS sub-TLV has the following format:

TYPE: To be assigned by IANA (Suggested value =1)
LENGTH: 5 for IPv4 address and 17 for IPv6 address
VALUE: This comprises one octet indicating the address-type and 4
or 16 octets bytes encoding the IPv4 or IPv6 address to be used
to reach the PCE PCE.

Address-type:
1 IPv4
2 IPv6

4.1.2. The 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. sub-TLV. There MUST be exactly one instance of the PATH-SCOPE sub-
TLV
sub-TLV within each PCED TLV. sub-TLV.

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:

TYPE: To be assigned by IANA (Suggested value =2)
LENGTH: 3
VALUE: This comprises a one-byte flags field where flag
represents a supported path scope, followed by a 2-bytes
preferences field indicating PCE preferences.

Here is the structure of the bits flag:

+-+-+-+-+-+-+-+-+
|0|1|2|3|4|5|Res|
+-+-+-+-+-+-+-+-+

Bit Path Scope

0 L bit: Can compute intra-area path
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 LSPs
computation
5 Y bit: Can compute or take part into the computation of
paths across layers
6-7 Reserved for future usage.

Here is the structure of the preferences field

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PrefL|PrefR|PrefS|PrefY| Res |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Res: Reserved for future usage.

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 bits 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 LSP 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).

When the Rd bit is set, the PCE-NEIG-DOMAIN TLV (see 5.1.4) MUST NOT
contain any Area ID DOMAIN sub-TLVs.

Similarly, when the and Sd bit is are set, the PCE-NEIG-DOMAIN TLV PCED sub-TLV MUST NOT contain any AS-DOMAIN sub-TLVs.
NEIG-PCE-DOMAIN sub-TLV (see 4.1.4).

When the R/S bit is cleared, the RD/Sd 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 balance its path computation requests according to such
PCE preference are 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 scopes. 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-DOMAINS PCE-DOMAIN sub-TLV

The PCE-DOMAINS PCE-DOMAIN sub-TLV specifies the set of domains a PCE-Domain (areas and/or ASes)
where the PCE has topology visibility and through which the PCE can
compute paths. It contains a set of one or more sub-TLVs where
each sub-TLV identifies a domain.

The PCE-DOMAINS PCE-DOMAIN sub-TLV MAY be present when PCE domains 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 sub-TLV MAY include multiple PCE-DOMAIN sub-TLVs when the PCE
has visibility in multiple PCE-Domains.

The PCE-DOMAINS PCE-DOMAIN sub-TLV has the following format:

TYPE: To be assigned by IANA (Suggested value =3)
LENGTH: Variable
VALUE: This comprises a set of one octet indicating the domain-type (area ID
or more DOMAIN sub-TLVs where
each DOMAIN sub-TLV identifies AS Number) and a domain variable length IS-IS area ID or a 32 bits AS
number, identifying a PCE-domain where the PCE has
topology visibility and can compute paths. visibility.

Two DOMAIN sub-TLVs domain types are defined

Sub-TLV type Length Name defined:
1 Variable Area ID sub-TLV
2 4 AS number sub-TLV

At least one DOMAIN sub-TLV MUST be present in the PCE-DOMAINS sub-
TLV. Note than when the PCE visibility is an entire AS, the PCE-
DOMAINS sub-TLV MUST include exactly one AS number sub-TLV, and MUST
not contain an area-ID sub-TLV.

4.1.3.1. Number

The Area ID DOMAIN sub-TLV

This sub-TLV carries an IS-IS area ID. It has the following format

TYPE: 1
LENGTH: Variable
VALUE: This comprises a variable length IS-IS area ID. This is the
combination of an Initial Domain Part (IDP) and High Order
part of the Domain Specific part (HO-DSP)

4.1.3.2. AS Number DOMAIN sub-TLV

The AS Number sub-TLV carries an AS number. It has the following
format:

TYPE: 2
LENGTH: 4
VALUE: AS number identifying an AS. area address as defined in [ISO].

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. PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLV

The PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLV specifies the set of a neighbour domains
(areas, ASes) PCE-domain (area,
AS) toward which a PCE can compute paths. It means that the PCE can compute or
take part in the computation of inter-domain TE LSPs whose path
transits one of these domains. It contains a set of
one or more DOMAIN sub-TLVs where each DOMAIN this neighbour PCE-domain.

A PCED sub-TLV identifies a
domain. MAY include several NEIG-PCE-DOMAIN sub-TLVs when the
PCE can compute paths towards several neighbour PCE-domains.

The PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLV has the following format: same format as the PCE-DOMAIN
sub-TLV:

TYPE: To be assigned by IANA (Suggested value =4)
LENGTH: Variable
VALUE: This comprises a set of one octet indicating the domain-type (area ID
or more AS Number) and a variable length IS-IS area or/and ID or a 32 bits AS DOMAIN sub-
TLVs where each sub-TLV identifies
number, identifying a neighbour domain toward PCE-domain towards which a the PCE can compute path.
paths.

Two domain types are defined:
1 Area ID
2 AS Number

The PCE-NEIG-DOMAINS sub-TLV MUST be present if the R bit Area ID is set and the Rd bit area address as defined in [ISO].

When coded in two bytes (which is cleared, and/or, if the S bit is set and current defined format as the Sd bit is
cleared.
time of writing this document), the AS Number field MUST have its
left two bytes set to 0.

The PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN sub-TLV MUST include at least one DOMAIN sub-
TLV. It MUST include at least one Area ID sub-TLV, be present if the R bit of
the PATH-SCOPE TLV is set and
the Rd bit of the PATH-SCOPE TLV is
cleared. Similarly, it MUST include at least one AS number sub-TLV cleared, and/or, if the S bit of the PATH-SCOPE TLV is set and the Sd bit of the PATH-
SCOPE TLV is
cleared.

4.1.5. PCE-CAP-FLAGS sub-TLV

The PCE-CAP-FLAGs sub-TLV is an optional TLV sub-TLV used to indicate
PCEP related capabilities. It MAY be present within the PCED TLV. sub-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 bit flags numbered from the most significant as bit
zero, where each bit represents one PCE capability.

The GENERAL-CAP PCE-CAP-FLAGS sub-TLV has the following format:

TYPE: To be assigned by IANA (Suggested value =4)
LENGTH: Multiple of 4
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 Flags.

The following bits are to be assigned by IANA:

Bit Capabilities

0 Capability to handle Path computation with GMPLS link constraints
1 Capability to compute bidirectional paths Bidirectional path computation
2 Capability to compute PSC Diverse path computation
3 Capability to compute a TDM Load-balanced path computation
4 Capability to compute a LSC path
5 Capability to compute a FSC path

6 Capability to compute link/node/SRLG diverse paths
7 Capability to compute load-balanced paths
8 Capability to compute a set of Synchronized paths in a
synchronized Manner
9 computation
5 Support for multiple objective functions
10 Capability to handle
6 Support for additive path constraints (e.g. max
(max hop count,
max path metric)
11 etc.)
7 Support for Request prioritization.
12 request prioritization
8 Support for multiple requests within the same
request message.

13-31 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.

4.1.6. The CONGESTION sub-TLV

The CONGESTION sub-TLV is used to indicate that a PCE's experiences PCE is experiencing
a processing congestion state and may optionally include expected PCE
congestion duration.
The CONGESTION sub-TLV is optional, it MAY be carried within the PCED
TLV.
sub-TLV. It MUST NOT be present more than once.

The format of the CONGESTION sub-TLV is as follows:

TYPE: To be assigned by IANA (Suggested value =6)
LENGTH: 3
VALUE: This comprises a one-byte one byte of bit flags indicating the
congestion status, followed by a 2-bytes field indicating the
congestion duration.

Here is the TLV structure

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Reserved| Congestion Duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 sub-TLV is advertised within an IS-IS Router Capability TLV
defined in [IS-IS-CAP]. A As such, elements of procedures are inherited
from those defined in [IS-IS-CAP].

The flooding scope is controlled by the S flag in the IS-IS Router
Capability TLV (see [IS-IS-CAP]). When the scope of the PCED TLV sub-TLV
is area local it MUST be carried within an IS-IS CAPABILITY Router Capability
TLV having the S bit cleared. When the scope of the PCED TLV sub-TLV is
the entire IGP IS-IS routing domain, itMUST it MUST be carried within an IS-IS CAPABILITY
Router Capability TLV having the S bit set. When Note that when only the L
bit of the PATH-SCOPE sub-TLV is set, the flooding scope MUST be area
local.

A PCE

Note that a L1L2 node may include both in its L1 and L2 LSPs a PCED
TLV in a Router Capability TLV with the S bit cleared. This allows
restricting the flooding scope to the L1 area and the L2 sub-domain.

An IS-IS router MUST originate a new IS-IS LSP whenever the content
of any of the there is a
change in a PCED TLV changes or whenever required by the regular
IS-IS procedure. associated with a PCE it advertises.

When the a PCE function is deactivated on a node, deactivated, the node IS-IS Router advertising this PCE MUST
originate a new IS-IS LSP with that does no longer any PCED TLV. A PCC MUST be
able to detect that include the
corresponding PCED TLV has been removed from an IS-IS LSP. TLV.

The PCE address, address(s), i.e. the address address(s) indicated within the PCE
ADDRESS sub-TLV, MUST must be distributed as part of IS-IS routing; reachable via some prefix(es)
advertised by IS-IS; 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 sub-TLV is OPTIONAL. When an IS-IS LSP 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 (e.g.,
address, preferences, scope) and other information may be
automatically determined by the PCE (e.g. areas of visibility).

5.1.1.

5.1. CONGESTION sub-TLV specific procedures

When a PCE enters into a processing congestion state, the conditions
of which are implementation dependent, it MAY originate a new IS-IS LSP with a
CONGESTION sub-TLV with the C bit set and optionally a non-null
expected congestion duration. duration MAY be generated.

When a PCE exists from the processing congestion state, the
conditions of which are implementation dependent, two cases are
considered:
- If the congestion duration in the previously originated
CONGESTION sub-TLV was null, it SHOULD originate a CONGESTION sub-TLV with the C bit
cleared and a null congestion duration; SHOULD be generated;
- If the congestion duration in the previously originated
CONGESTION sub-TLV was non null, it MAY originate a CONGESTION sub-
TLV sub-TLV with the C bit cleared.
cleared MAY be generated. Note that in some particular cases it may
be desired to originate a PCES TLV 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 IS-IS
flooding, as only uncongested-to-congested state transitions need are
advertised.

A PCE

An IS-IS implementation SHOULD support an appropriate dampening
algorithm so as to dampen IS-IS flooding of PCE Congestion information in
order to not impact the IS-IS scalability. It is RECOMMENDED to
introduce some hysteresis for congestion state transition, so as to
avoid state oscillations that may impact IS-IS 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 congested.

6. Backward compatibility

The PCED TLV sub-TLV defined in this document does not introduce any
interoperability issues.

An IS-IS router not supporting the PCED TLV sub-TLV will just silently
ignore the TLV as specified in [IS-IS-CAP].

7. IANA considerations

7.1. IS-IS sub-TLV

Once a registry for the IS-IS Router Capability TLV defined in
[IS-IS-CAP] will have has been assigned, IANA will assign a new TLV code-point
for the PCED TLV sub-TLV carried within the Router Capability TLV.

Value Sub-TLV References
----- -------- ----------
5 PCED TLV sub-TLV (this document)

7.2. PCED sub-TLVs registry

The PCED TLV sub-TLV referenced above is constructed from sub-TLVs. Each sub-
TLV
sub-TLV includes a 8-bit type identifier.

The IANA is requested to create a new registry and manage TLV sub-TLV
type identifiers as follows:

- TLV sub-TLV Type
- TLV sub-TLV Name
- Reference

This document defines five TLVs sub-TLVs as follows (suggested values):

Value TLV name References
----- -------- ----------
1 PCE-ADDRESS This document
2 PATH-SCOPE This document
3 PCE-DOMAINS PCE-DOMAIN This document
4 PCE-NEIG-DOMAINS NEIG-PCE-DOMAIN This document
5 PCE-CAP-FLAGS This document
6 CONGESTION This document

New TLV 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 OSPF based PCE discovery [PCED-OSPF].
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 paths path computation
2 PSC paths
3 TDM paths
4 LSC paths
5 FSC paths
6 Diverse paths
7 path computation
3 Load-balanced paths
8 path computation
4 Synchronized paths computation
9 Multiple
5 Support for multiple objective functions
10 Additive
6 Support for additive path constraints (e.g. max
(max hop count)
11 Request count, etc.)
7 Support for request prioritization
12 Multiple
8 Support for multiple requests per message

8. Security Considerations

This document defines IS-IS extensions for PCE discovery within an
administrative domain. Hence the security of the PCE discovery relies
on the security of IS-IS.

Mechanisms defined to ensure authenticity and integrity of IS-IS LSPs
[RFC3567], and their TLVs, can be used to secure the PCED TLV sub-TLV as
well.

IS-IS provides no mechanism for protecting the privacy of LSAs, LSPs, 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 PCE 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 IS-IS liveness
detection. IS-IS already includes a liveness detection mechanism
(Hello PDUs), and PCE discovery does not require additional
capabilities.

Procedures defined in section 5 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
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 IS-IS extensions defined in this documents does 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 IS-IS and
might destabilize the operation of the network by causing the PCCs to
swap between PCEs.

As discussed in section 5, 5.1, a PCE implementation SHOULD support an
appropriate dampening algorithm so as to dampen IS-IS flooding in
order to not impact the IS-IS 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 and Wong, Adrian Farrel 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.

[ISO] "Intermediate System to Intermediate System Intra-Domain
Routeing Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-mode Network Service
(ISO 8473)", ISO DP 10589, February 1990.

[RFC3784] Li, T., Smit, H., "IS-IS extensions for Traffic
Engineering", RFC 3784, June 2004.

[IS-IS-CAP] Vasseur, J.P. et al., "IS-IS extensions for advertising
router information", draft-ietf-isis-caps, 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.

[RFC4205] Kompella, Rekhter, " IS-IS Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC4205, October
2005.

[RFC3567] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic Authentication", RFC 3567,
July 2003.

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-OSPF] Le Roux, Vasseur, "OSPF protocol extensions for Path
Computation Element (PCE) Discovery", draft-ietf-pce-disco-proto-
ospf, work in progress.

12. Editors' 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

13. Contributors' Adresses:

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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