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Versions: (draft-pouyllau-pce-enhanced-errors) 00 01 02 03 04 05 06

PCE Working Group                                            H. Pouyllau
Internet-Draft                                            Alcatel-Lucent
Intended status: Standards Track                            R. Theillaud
Expires: January 5, 2013                                 Marben Products
                                                               J. Meuric
                                                   France Telecom Orange
                                                            July 4, 2012


 Extensions to the Path Computation Element Communication Protocol for
                   Enhanced Errors and Notifications
                 draft-ietf-pce-enhanced-errors-01.txt

Abstract

   This document defines new error and notification TLVs for the PCE
   Communication Protocol (PCEP) [RFC5440].  It identifies the possible
   PCEP behaviors in case of error or notification.  Thus, this draft
   defines types of errors and notifications and how they are disclosed
   to other PCEs in order to support predefined PCEP behaviors.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 5, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.      Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.      Conventions used in this document  . . . . . . . . . . . .  4
   3.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  5
   3.1.    Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.1.1.  Error use-case . . . . . . . . . . . . . . . . . . . . . .  5
   3.1.2.  Notification use-case  . . . . . . . . . . . . . . . . . .  6
   3.2.    PCEP Description . . . . . . . . . . . . . . . . . . . . .  6
   3.3.    PCEP Behaviors . . . . . . . . . . . . . . . . . . . . . .  6
   3.3.1.  PCEP Behaviors in Case of Error  . . . . . . . . . . . . .  7
   3.3.2.  PCEP Behaviors in Case of Notification . . . . . . . . . .  7
   3.3.3.  PCE peer identification  . . . . . . . . . . . . . . . . .  8
   4.      Error and Notification Handling in PCEP  . . . . . . . . .  9
   4.1.    Propagation TLV  . . . . . . . . . . . . . . . . . . . . .  9
   4.2.    Error-criticality TLV  . . . . . . . . . . . . . . . . . .  9
   4.3.    Notification type TLV  . . . . . . . . . . . . . . . . . .  9
   4.4.    Behaviors and TLV combinations . . . . . . . . . . . . . . 10
   5.      Propagation Restrictions . . . . . . . . . . . . . . . . . 12
   5.1.    Time-To-Live object  . . . . . . . . . . . . . . . . . . . 12
   5.2.    DIFFUSION-LIST Object (DLO)  . . . . . . . . . . . . . . . 12
   5.3.    Extension rules applied to existing errors and
           notifications  . . . . . . . . . . . . . . . . . . . . . . 14
   6.      Error and Notification Scenarios . . . . . . . . . . . . . 18
   6.1.    Local Error with a low level of criticality  . . . . . . . 18
   6.2.    Propagated Error with a low level of criticality . . . . . 18
   6.3.    Local Error with a medium level of criticality . . . . . . 19
   6.4.    Propagated Error with a medium level of criticality  . . . 20
   6.5.    Local request-specific notification  . . . . . . . . . . . 20
   6.6.    Local non request-specific notification  . . . . . . . . . 21
   6.7.    Propagated request-specific notification . . . . . . . . . 21
   6.8.    Propagated non request-specific notification . . . . . . . 22
   7.      Security Considerations  . . . . . . . . . . . . . . . . . 23
   8.      IANA Considerations  . . . . . . . . . . . . . . . . . . . 24
   8.1.    PCEP TLV Type Indicators . . . . . . . . . . . . . . . . . 24
   8.2.    New DLO object . . . . . . . . . . . . . . . . . . . . . . 24
   9.      References . . . . . . . . . . . . . . . . . . . . . . . . 25
   9.1.    Normative References . . . . . . . . . . . . . . . . . . . 25
   9.2.    Informational References . . . . . . . . . . . . . . . . . 25
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 27





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1.  Terminology

   PCE terminology is defined in [RFC4655].

   PCEP Peer: An element involved in a PCEP session (i.e. a PCC or a
   PCE).

   Source PCC: the PCC which, for a given path computation query,
   initiates the 1st PCEP request, which may trigger a chain of
   successive requests.

   Target PCE: the PCE that can compute a path to the destination
   without having to query any other PCE.






































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2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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3.  Introduction

   The PCE Communication Protocol [RFC5440] is designed to be flexible
   and extensible in order to allow future evolutions or specific
   constraint support such as proposed in
   [I-D.ietf-pce-vendor-constraints].  Crossing different PCE
   implementations (e.g. from different providers or due to different
   releases), a PCEP request may encounter unknown errors or
   notification messages.  In such a case, the PCEP RFC [RFC5440]
   specifies to send a specific error code to the PCEP peer.

   In the context of path computation crossing different routing domains
   or autonomous systems, the number of different PCE system
   specificities is potentially high, thus possibly leading to divergent
   and unstable situations.  Such phenomenon can also occur in
   homogeneous cases since PCE systems have their own policies that can
   introduce differences in requests treatment even for requests having
   the same destination.  In order to generalize PCEP behaviors in the
   case of heterogeneous PCE systems, new objects have to be defined.
   Dealing with heterogeneity is a major challenge considering PCE
   applicability, particularly in multi-domain contexts.  Thus,
   extending such error codes and PCEP behaviors accordingly would
   improve interoperability among different PCEP implementations and
   would solve some of these unstable issues.  However, some of them
   would still remain (e.g. the divergences in request treatment
   introduced by different policies).

   The purpose of this draft is to identify and specify new optional
   TLVs and objects in order to generalize PCEP behaviors.

3.1.  Examples

   The two following scenarios underline the need for a normalization of
   the PCEP behaviors according to existing error or notification types.

3.1.1.  Error use-case

   PCE(i-1) has sent a request to PCE(i) which has also sent a request
   to PCE(i+1).  PCE(i-1) and PCE(i+1) have the same error semantic but
   not PCE(i).  If PCE(i+1) throws an error type and value unknown by
   PCE(i).  PCE(i) could then adopt any other behaviors and sends back
   to PCE(i-1) an error of type 2 (Capability not supported), 3 (Unknown
   Object) or 4 (Not supported Object) for instance.  As a consequence,
   the path request would be cancelled but the error has no meaning for
   PCE(i-1) whereas if PCE(i) had simply forwareded the error sent by
   PCE(i+1), it would have been understood by PCE(i-1).





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3.1.2.  Notification use-case

   PCE(i-1) has sent a request to PCE(i) which has also sent a request
   to PCE(i+1) but PCE(i+1) is overloaded.  Without extensions, PCE(i+1)
   should send a notification of type 2 and a value flag giving its
   estimated congestion duration.  PCE(i) can choose to stop the path
   computation and send a NO_PATH reply to PCE(i-1).  Hence, PCE(i-1)
   ignores the congestion duration on PCE(i+1) and could seek it for
   further requests.

3.2.  PCEP Description

   One of the purposes of the PCE architecture is to compute paths
   across networks, but an added value is to compute such paths in
   inter-area/layer/domain environments.  The PCE Communication Protocol
   [RFC5440] is based on the Transport Communication Protocol (TCP).
   Thus, to compute a path within the PCE architecture, several TCP/PCEP
   sessions have to be set up, in a peer-to-peer manner, along a set of
   identified PCEs.

   When the PCEP session is up for 2 PCEP peers, the PCC of the first
   PCE System (the source PCC) sends a PCReq message.  If the PCC does
   not receive any reply before the dead timer is out, then it goes back
   to the idle state.  A PCC can expect two kinds of replies: a PCRep
   message containing one or more valid paths (EROs) or a negative PCRep
   message containing a NO-PATH object.

   Beside PCReq and PCRep messages, notification and error messages,
   named respectively PCNtf and PCErr, can be sent.  There are two types
   of notification messages: type 1 is for cancelling pending requests
   and type 2 for signaling a congestion of the PCE.  Several error
   values are described in [RFC5440].  The error types concerning the
   session phase begin at 2, error type 1 values are dedicated to the
   initialization phase.

   As the PCE Communication Protocol is built to work in a peer-to-peer
   manner (i.e. supported by a TCP Connection), it supposes that the
   ''deadtimer'' of the source PCC is long enough to support the end-to-
   end distributed path computation process.

3.3.  PCEP Behaviors

   The exchange of messages in the PCE Communication Protocol is
   described in details when PCEP is in states OpenWait and KeepWait in
   [RFC5440].  When the session is up, message exchange is defined in
   [RFC5440] but detailed behavior is mostly let free to any specific
   implementation.  [RFC5441] describes the Backward Recursive Path
   Computation (BRPC) procedure, and, because it considers an inter-



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   domain path computation, gives a bigger picture of the possible
   behaviors when the session is up.

3.3.1.  PCEP Behaviors in Case of Error

   [RFC5440] specifies that "a PCEP Error message is sent in several
   situations: when a protocol error condition is met or the request is
   not compliant with the PCEP specification".  On this basis, and
   according to the other RFCs, the identified PCEP behaviors are the
   followings:

      "Propagation": the received message requires to be propagated
      forwardly or backwardly (depending on which PCEP peer has sent the
      message) to a set of PCE peers;

      Criticality level: in the different RFCs, error-types affects the
      state of the PCEP request or session in different manner; hence,
      different level of criticality can be observed:



         Low-level of criticality: the received message does not affect
         the PCEP connection and further answer can still be expected;

         Medium-level of criticality: the received message does not
         affect the PCEP connection but the request(s) is(are)
         cancelled;

         High-level of criticality: the received message indicates that
         the PCE peer will close the session with its peer (and so
         pending requests associated by the error, if any, are
         cancelled)

   The high-level of criticality has been extracted from [RFC5440] which
   associates such a behavior to error-type of 1 (errors raised during
   the PCEP session establishment).  Hence, such errors are quite
   specific for the moment.  For the sake of completeness, they have
   been included in this document.

3.3.2.  PCEP Behaviors in Case of Notification

   Notification messages can be employed in two different manners:
   during the treatment of a PCEP request, or independently from it to
   advertise information (in [RFC5440] the request id list within a
   PCNtf message is optional).  Hence, three different behaviors can be
   identified:





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   o  "Local": the notification is or is not request-specific but does
      not imply any forward or backward propagation of the message;

   o  "Request-specific Propagation": the received message requires to
      be propagated forwardly or backwardly (depending on which peer has
      sent the message) to the PCEP peers;

   o  "Non request-specific Propagation": the received message must be
      propagated to any known peers (e.g. if PCE discovery is activated)
      or to a list of identified peers.

3.3.3.  PCE peer identification

   The propagation of errors and notifications affects the state of the
   PCE peers along the chain.  In some cases, for instance a
   notification that a PCE is overloaded, the identification of the PCE
   peer - or that the sender PCE is not the direct neighbor - might be
   an important information for the PCE peers receiving the message.

































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4.  Error and Notification Handling in PCEP

   This section describes extensions to support error and notification
   messages with respect to the PCEP behavior description defined in
   Section 3.3.1.  This document does not intend to modify errorsand
   notification types previously defined in existing documents (e.g.
   [RFC5440], [RFC5441], etc.).

4.1.  Propagation TLV

   To support the propagation behavior mentioned in Section 3.3.1, we
   extend the PCEP-ERROR and NOTIFICATION objects by creating a new
   optional TLV to indicate whether the message has to be propagateed or
   not.  The allocation from the "PCEP TLV Type Indicators" sub-registry
   will be assigned by IANA and the request is documented in Section 8.

   The description is "Propagation", the length value is 2 bytes.  The
   value field is set to default value 0 meaning that the message MUST
   NOT be propagated.  If the value field is set to 1, the message MUST
   be propagated.  Section 5 specifies the destination and to limit the
   number of messages.

4.2.  Error-criticality TLV

   To support the shutdown behavior mentioned in Section 3.3.1, we
   extend the PCEP-ERROR object by creating a new optional TLV to
   indicate whether the error is recoverable or not.  The allocation
   from the "PCEP TLV Type Indicators" sub-registry will be assigned by
   IANA and the request is documented in Section 8.

   The description is "Error-criticality", the length value is 2 bytes
   and the value field is 1 byte.  The value field is set to default
   value 0 meaning that the error has a low-level of criticality (so
   further messages can be expected for this request).  If the value
   field is set to 1, the error has a medium-level of criticality and
   requests whose the identifiers appear MUST be cancelled (so no
   further messages can be expected for these requests).  If the value
   field is set to 2, the error has a high-level of criticality, the
   connection is closed by the sender PCE peer.

4.3.  Notification type TLV

   To support the request-specific behavior mentioned in Section 3.3.1,
   we extend the NOTIFICATION object by creating a new optional TLV to
   indicate whether the notification is request-specific or not.  The
   allocation from the "PCEP TLV Type Indicators" sub-registry will be
   assigned by IANA and the request is documented in Section 8.




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   The description is "Notification Type", the length value is 2 bytes
   and the value field is 1 byte.  The value field is set to default
   value 0 meaning that the notification is not request-specific.  If
   the value field is set to 1, the notification is request-specific.

   TODO: If according to RFCXXX, the notification contains a request-id,
   then the TLV value field MUST be set to 1.

4.4.  Behaviors and TLV combinations

   The propagation behavior MIGHT be combined with all criticality
   levels, thus leading to 6 different behaviors.  In the case of a
   criticality level of 2, the session is closed by the PCE peer which
   sends the message.  Hence, the criticality level is purely
   informative for the PCE peer which receives the message.  If it is
   combined with a propagation behavior, then the PCE propagating the
   message MUST indicate the same level of criticality if it closes the
   session.  Otherwise, it MUST use a criticality level of 1 if it does
   not close the session.

   The TLVs defined in the sections above allow to cover all the
   possible behaviors listed in Section 3.3.1.  Hence, for an error
   message, the behaviors are covered as ensued, with TLVs included in a
   PCEP-ERROR object:

   o  "Local Error with a low level of criticality" : TLV "Propagation"
      with value 0 and TLV "Error-criticality" with value 0

   o  "Local Error with a medium level of criticality": TLV
      "Propagation" with value 0 and TLV "Error-criticality" with value
      1

   o  "Local Error with a high level of criticality": TLV "Propagation"
      with value 0 and TLV "Error-criticality" with value 2

   o  "Propagated Error with a low level of criticality": TLV
      "Propagation" with value 1 and TLV "Error-criticality" with value
      0

   o  "Propagated Error with a medium level of criticality": TLV
      "Propagation" with value 1 and TLV "Error-criticality" with value
      1

   o  "Propagated Error with a high level of criticality": TLV
      "Propagation" with value 1 and TLV "Error-criticality" with value
      2

   For a notification message, the behaviors are covered as ensued, with



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   TLVs included in a NOTIFICATION object:

   o  "Local request-specific": TLV "Propagation" with value 0 and TLV
      "Notification Type" with value 1

   o  "Local non request-specific": TLV "Propagation" with value 0 and
      TLV "Notification Type" with value 0

   o  "Request-specific Propagation": TLV "Propagation" with value 1 and
      TLV "Notification Type" with value 1

   o  "Non request-specific Propagation": TLV "Propagation" with value 1
      and TLV "Notification Type" with value 0






































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5.  Propagation Restrictions

   In order to limit the propagation of errors and notifications, the
   following mechanisms SHOULD be used:

      A Time-To-Live object: to limit the number of PCEP peers that will
      recursively receive the message;

      A DIFFUSION-LIST object (DLO) which specifies the PCEP peer
      addresses or domains of PCEP peers the message must be propagate
      to;

      History mechanisms: if a PCEP peer keeps track of the messages it
      has relayed, it could avoid propagating an error or notification
      it already received.

   Such mechanisms SHOULD be used jointly or independently depending the
   error or notification behaviors they are associated to.  Note that, a
   non request-specific propagated notification (TLV "Propagation" at
   value 1 and TLV "Notification Type" at value 0) MUST include a DLO
   and SHOULD include a TTL.  The conditions of use for the TTL and
   DIFFUSION-LIST object are described in sections below.

5.1.  Time-To-Live object

   The TTL value is set to any integer value to indicate the number of
   PCEP peers that will recursively receive the message.  This TTL
   SHOULD be used with propagated errors or notifications (TLV
   "Propagation" at value 1 in PCEP-ERROR or NOTIFICATION objects).
   Each PCEP peer MUST decrement the TTL value before propagating the
   message.  When the TTL value is at 0, the message is no more
   propagated.

   If the message has to be propagated, is request-specific (TLV
   "Propagation" at value 1 in PCEP-ERROR or NOTIFICATION objects, and
   TLV "Notification Type" at value 1 in a NOTIFICATION object), and
   there is no TTL or DIFFUSION-LIST object included, the message MUST
   reach the source PCC (or alternatively the target PCE).

5.2.  DIFFUSION-LIST Object (DLO)

   The DIFFUSION-LIST Object can be carried within a PCErr and a PCNtf
   message and can either be used in a message sent by a PCC to a PCE or
   by a PCE to a PCC.  The DLO MAY be used with propagated errors (TLV
   "Propagation"at value 1 in PCEP-ERROR object) and request-specific
   propagated notifications (TLV "Propagation" at value 1 and TLV
   "Notification Type" at value 1), and it MUST be used with non
   request-specific propagated notifications (TLV "Propagation" at value



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   1 and TLV "Notification Type" at value 0).

   DIFFUSION-LIST Object-Class is 25.

   DIFFUSION-LIST Object-Type is 1.

   The format of the DIFFUSION-LIST body object 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Reserved      |  Flags                        | TT            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
   //                        (Sub-objects)                         //
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Reserved (8 bits): This field MUST be set to zero on transmission and
   MUST be ignored on receipt.

   Flags (16 bits): No flags are currently defined.  Unassigned flags
   MUST be set to zero on transmission and MUST be ignored on receipt.

   TT (8 bits): The Target-type restricts the diffusion to certain
   peers.  The following values are currently defined:

      0: Any PCEP peer indicated in the list must be reached.

      1: Only PCEs must be reached (and not PCC).

      2: All PCEP peers with which a session is still opened must be
      reached

   The DLO is made of sub-objects similar to the IRO defined in
   [RFC5440].  The following sub-object types are supported.


     Type Sub-object

     1 IPv4 address
     2 IPv6 address
     4 Unnumbered Interface ID
     5 OSPF area ID
     32 Autonomous System number
     33 Explicit eXclusion Route Sub-object (EXRS)



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   If the error or notification codes target specific PCEP peers, a
   DIFFUSION-LIST object avoids partially flooding all PCEP peers.  Any
   PCEP peer receiving a PCErr or PCNTf message containing a PCEP-ERROR
   object, respectively a NOTIFICATION object, including a TLV
   "Propagation" at value 1, and where a DLO appears MUST remove from
   the DLO the addresses of the PCEP peers to whom it will propagate the
   message, before sending them the message.  This is performed adding
   the PCEP peer addresses to the Explicit eXclusion Route Sub-object of
   the DLO.  If a DIFFUSION-LIST object is empty, the PCEP peer MUST NOT
   propagate the message to any peer.

   Note that, a Diffusion List Object could contain strict or loose
   addresses to refer to a network domain (e.g. an Autonomous System
   number, an OSPF area, an IP address).  Hence, the PCEP peers targeted
   by the message would be the PCEP peers covering the corresponding
   domain.  If an address is loose, each time a PCEP peer forwards a
   message to another PCEP peer of this address, it MUST add it to the
   Explicit eXclusion Route Sub-object (EXRS) of the DLO for any
   forwarded message.  Hence, a PCE SHOULD avoid forwarding several
   times the same message to the same set of peers.  Finally, when an
   address is loose, the forwarding SHOULD be restrained indicating what
   type of PCEP peers are targeted (i.e.  PCE and/or PCC).  Hence, a
   Target-Type is specified.

5.3.  Extension rules applied to existing errors and notifications

   Many existing normative references states on error definitions (see
   for instance [RFC5440], [RFC5441],[RFC5455], [RFC5521], [RFC5557],
   [RFC5886], [RFC6006]).  According to the definitions provided in this
   document, the follwoing rules are applicable:

      Error-type 1, described in [RFC5440], relates to PCEP session
      establishement failures.  All errors of this type are local (not
      to be propagated).  Hence, if a TLV "Propagation" is added to the
      error message it MUST be set to value 0.  Error-values 1,2,6,7
      have a high level of criticality.  Hence, if the TLV "Error-
      criticality" is included within a PCErr message of type 1 and
      value 1,2,6 or 7, it MUST have a value of 2.

      Error-type 2,3,4, "Capability not supported", "Unknown object" and
      "Not supported object" respectively, described in [RFC5440]:
      errors of this type MIGHT be propagated using the TLV
      "Propagation".  Their level of criticality is defined as leading
      to cancel the path computation request (cf. [RFC5440]).  Hence, if
      the TLV "Error-criticality" is included, it MUST have a value of
      1.  The error-value 4 of error-type 4 ("Unsupported parameter")
      associated to the BRPC procedure [RFC5441] SHOULD contain the TLV
      "Propagation" with a DIFFUSION-LIST object requesting a



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      propagation to the PCC at the origin of the request.

      Error-type 5 refers to "Policy violation", error values for this
      type have been defined in [RFC5440], [RFC5541], [RFC5557],
      [RFC5886] and [RFC6006].  In [RFC5440], it is specified that the
      path computation request MUST be cancelled when an error of type 5
      occurs.  Hence, if the TLV "Error-criticality" is included, it
      MUST have a value of 1.  As such errors might be conveyed to
      several PCEs, the TLV "Propagation" MIGHT be used.

      Error-type 6 described as "Mandatory object missing" in [RFC5440],
      leads to the cancellation of the path computation request.  Hence,
      if the TLV "Error-criticality" is included, it MUST have a value
      of 1.  The TLV "Propagation" MIGHT be used with such errors.  The
      error-value defined in Monitoring object missing [RFC5886] is no
      exception to the rule.

      Error-type 7 is described as "synchronized path computation
      request missing".  In [RFC5440], it is specified that the reffered
      synchronized path computation request MUST be cancelled when an
      error of type 5 occurs.  Hence, if the TLV "Error-criticality" is
      included, it MUST have a value of 1.  The TLV "Propagation" MIGHT
      be used with such errors.

      Error-type 8 is raised when a PCE receives a PCRep with an unknown
      request reference.  If the TLV "Propagation" is used with error-
      type 8, it SHOULD be set at a value of 0.  The TLV "Error-
      criticality" is not particularly relevant for error-type 8.
      Hence, if it used, it MUST have the value of 0.

      Error-type 9 is raised when a PCE attempts to establish a second
      PCEP session.  The existing session must be preserved.  Hence, if
      the TLV "Error-criticality" is included, it MUST have a value of
      0.  By definition, such an error message SHOULD NOT be propagated.
      Thus, if the TLV "Propagation" is used with error-type 9, it
      SHOULD be set at a value of 0.

      Error-type 10 which refers to the reception of an invalid object
      is described in [RFC5440] no indication is provide on the
      cancellation of the path computation request.  Hence, if the TLV
      "Error-criticality" is included, it MUST have a value of 0.  The
      TLV "Propagation" MIGHT be used with such errors with any value
      depending on the expected behavior.

      Error-type 11 relates to "Unrecognized EXRS subobject" and is
      described in [RFC5521].  No path computation request cancellation
      is required by [RFC5521].  Hence, if the TLV "Error-criticality"
      is included, it MUST have a value of 0.  The TLV "Propagation"



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      MIGHT be used with such errors with any value depending on the
      expected behavior.

      Error-type 12 refers to "Diffserv-aware TE error" and is described
      in [RFC5455].  Such errors are raised when the CLASSTYPE object of
      a PCReq is recognized but not supported by a PCE.  [RFC5455] does
      not state about the path computation request when such errors are
      met.  Hence, both "Propagation" and "Error-criticality" TLVs could
      be used within such error-types' messages and set to any specified
      values.

      Error-type 13 on "BRPC procedure completion failure" is described
      in [RFC5441].  [RFC5441] states that in such cases, the PCErr
      message MUST be relayed to the PCC.  Hence, such messages SHOULD
      contain a TLV "Propagation" and a DIFFUSION-LIST object with a
      Target-Type of 0 and corresponding adresses or with a Target-Type
      of 2.  It is not specified in [RFC5441] whether the path
      computation request should be canceled or not.  If the procedure
      is not supported, it does not necessarily imply to cancel the path
      computation request if another procedure is able to read and write
      VSPT objects.  Thus, the TLV "Error-criticality" MIGHT be used
      with any value depending on the expected behavior.

      Error-type 15 refers to "Global Concurrent Optimization Error"
      defined in [RFC5557].  [RFC5557] states that the corresponding
      global concurrent path optimization MUST be cancelled at the PCC.
      Hence, if the TLV "Error-criticality" is included, it MUST have a
      value of 1.  The TLV "Propagation" MIGHT be used with such errors.

      Error-type 16 relates to "P2MP Capability Error" defined in
      [RFC6006].  Such errors lead to the cancellation of the path
      computation request.  Hence, if the TLV "Error-criticality" is
      included, it MUST have a value of 1.  The TLV "Propagation" MIGHT
      be used with such errors.

      Error-type 17, titled "P2MP END-POINTS Error" is defined
      [RFC6006].  Such errors are thrown when a PCE tries to add or
      prune nodes to or from a P2MP Tree.  [RFC6006] does not specify if
      such errors lead to cancel the path computation request.  Hence,
      TLVs "Error-criticality" and "Propagation" MIGHT be used with this
      type of errors with any value depending on the expected behavior.

      Error-type 18 on "P2MP Fragmentation Error" is described [RFC6006]
      which does not specify whether the path computation request should
      be cancelled.  But, as messages are fragmented, it is natural to
      think that the PCE should wait at least a bit for further
      messages.  The TLV "Error-criticality" MIGHT be included in such
      error messages and is particularly adapted to differ the semantic



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      of the same error-type message: if it is included with a value of
      0 then the PCE will still wait for further fragmented messages,
      when this waiting time ends, the TLV can be included with a value
      of 1 in order to finally cancel the request.  The TLV
      "Propagation" MIGHT also be used with such errors.

   Among the existing normative references, only the [RFC5440] defines
   some notification-types and values.  The recommendations with respect
   to the TLVs definitions provided in this document are the followings:

      Notitification-type=1, Notification-value=1 or 2: a PCC,
      respectively a PCE, cancels a set of pending requests, such a
      notification SHOULD be propagated to the list of PCEs which were
      implied in the path computation requests.  Hence, the NOTIFICATION
      object SHOULD contains the TLV "Propagation" with value 1 and the
      TLV "Notification Type" with value 1, together with a DIFFUSION-
      LIST object containing the list of PCEs.

      Notitification-type=2, Notification-value=1 or 2 : indicates to
      the PCC that the PCE is, respectively is no longer, in an
      overloaded state, such a notification can be propagated or stay
      local.  It is therefore RECOMMENDED to specify this behavior using
      the TLV "Propagation" and associated restriction mechanims.




























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6.  Error and Notification Scenarios

   This section provides some examples depicting how the error and
   notification types described above can be used in a PCEP session.
   The origin of the errors or notifications is only illustrative and
   has no normative purpose.  Sometimes the PCE features behind may be
   implementation-specific (e.g. detection of flooding).  This section
   does not provide scenarios for errors with a high-level of critcity
   since such errors are very specific and until now have been
   normalized only during the session establishment (error-type of 1).

6.1.  Local Error with a low level of criticality

   In this example, a PCC attempts to establish a second PCEP session
   with the same PCE for another request.  Consequently the PCE sent an
   error of error-type 9.  This error stay local and does not affect the
   former session.  The second session is ignored.


                     +-+-+                  +-+-+
                     |PCC|                  |PCE|
                     +-+-+                  +-+-+
   1) Path computation |                      |
    event              |                      |
   2) PCE Selection    |                      |
   3) Path computation |---- PCReq message--->|
    request X sent to  |                      |4) Path computation
    the selected PCE   |                      | request queued
                       |                      |
   5) Path computation |                      |
    event              |                      |
   6) PCE Selection    |                      |
   7) Path computation |---- PCReq message--->|
   request X' sent to  |                      | 8) Session already
   the selected PCE    |                      | opened
                       |                      |
                       |<--- PCErr message----| Error-type=9
                       |                      |


6.2.  Propagated Error with a low level of criticality

   In this example, a PCC sends a path computation requests with no P
   flag set whereas (e.g.  END-POINT object with P-flag cleared).  This
   is detected by another PCE in the sequence.  The path computation
   request can thus be treated but the P-Flag will be ignored.  Hence,
   this error is not critical but the source PCC should be informed of
   this fact.  So, a PCErr message with error-type 10 ("Reception of an



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   invalid object").  The PCEP-ERROR object of the message contains a
   TLV "Propagation" at value 1 and a TLV "Error-criticality" at value
   0.  It is hence propagated backwardly to the source PCC.


    +-+-+              +-+-+-+-+                +-+-+
    |PCC|              |PCE|PCC|                |PCE|
    +-+-+              +-+-+-+-+                +-+-+
     |---- PCReq message-->|                      |
     |                     |                      |
     |                     |---- PCReq message--->|
     |                     |                      |
     |                     |                      |1) Parameter is
     |                     |                      | not supported
     |                     |                      |
     |                     |<--- PCErr message----| Error-type=10
     |<--- PCErr message---|                      |
     |                     |                      |


6.3.  Local Error with a medium level of criticality

   In this example, the PCC sends a DiffServ-aware path computation
   request.  The PCE receiving the request does not support the
   indicated class-type and thus sends back a PCErr message with error-
   type=12, error-value=1, a TLV "Propagation" at value 0 and a TLV
   "Error-criticality" at value 1.  Consequently, the request(s) is
   (are) cancelled.


                     +-+-+                  +-+-+
                     |PCC|                  |PCE|
                     +-+-+                  +-+-+
   1) Path computation |                      |
    event              |                      |
   2) PCE Selection    |                      |
   3) Path computation |---- PCReq message--->|
    request X sent to  |                      |4) Path computation
    the selected PCE   |                      | request queued
                       |                      |
                       |                      |5) DiffServ class-type
                       |                      | not supported
                       |                      |6) Path computation
                       |                      | request X
                       |                      | cancelled
                       |<--- PCErr message----| Error-type=12
                       |                      |




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6.4.  Propagated Error with a medium level of criticality

   In this example, PCEs are using the BRPC procedure to treat a path
   computation request [RFC5441].  However, one of the PCE does not
   support a parameter of the request.  Hence, a PCErr message with
   error-type 4 and error-value 4 is sent by this PCE and has to be
   forwarded to the source PCC.  The PCEP-ERROR object includes a TLV
   "Propagation" at value 1 and TLV "Error-criticality" at value 1 and
   the message is propagated backwardly to the source PCC.
   Consequently, the request(s) is (are) cancelled.


    +-+-+               +-+-+-+-+                +-+-+
    |PCC|               |PCE|PCC|                |PCE|
    +-+-+               +-+-+-+-+                +-+-+
      |---- PCReq message-->|                      |
      |                     |                      |
      |                     |---- PCReq message--->|
      |                     |                      |
      |                     |                      |1) Unsupported
      |                     |                      | Parameter BRPC
      |                     |                      |2) Path
      |                     |                      | computation
      |                     |                      | request X
      |                     |                      | cancelled
      |                     |<--- PCErr message----| Error-type=4
      |<--- PCErr message---|                      |
      |                     |                      |


6.5.  Local request-specific notification

   In this example, a PCE sends a request-specific notification
   indicating that, a set of pending requests are cancelled (e.g.
   notification-type=1, notification-value=1 as described in [RFC5440]).
   Hence, a PCNtf message is sent to the PCC with a NOTIFICATION object
   including a TLV "Propagation" at value 0 and a TLV "Notification
   Type" at value 1.













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                     +-+-+                  +-+-+
                     |PCC|                  |PCE|
                     +-+-+                  +-+-+
   1) Path computation |                      |
    event              |                      |
   2) PCE Selection    |                      |
   3) Path computation |---- PCReq message--->|
    request X sent to  |                      |4) Path
    the selected PCE   |                      | computation
                       |                      | request queued
                       |                      |
                       |                      |5) Pending requests
                       |                      | cancelled
                       |                      |
                       |<--- PCNtf message----|Notification-Type=1
                       |                      |


6.6.  Local non request-specific notification

   In this example, a PCE sends a non request-specific notification
   indicating that, due to multiple sendings (or for other reason),
   further requests from this PCC will be ignored.  Hence, a PCNtf
   message is sent to the PCC with a NOTIFICATION object including a TLV
   "Propagation" at value 0 and a TLV "Notification Type" at value 0.


                     +-+-+                  +-+-+
                     |PCC|                  |PCE|
                     +-+-+                  +-+-+
                       |                      |
                       |                      |1) Further requests
                       |                      | will be ignored
                       |                      |
                       |<--- PCNtf message----|
                       |                      |


6.7.  Propagated request-specific notification

   In this example, a PCE receives a request but it is temporarily
   congested.  However, it can treat the request after few minutes which
   might cause some time-out in the predecessor PCEs.  Hence, a PCNtf
   message with a NOTIFICATION object containing a TLV "Propagation" at
   value 1 and a TLV "Notification Type" at value 1 is send to the PCC
   and propagated backwardly in the PCE sequence.  Such a notification
   could include an OVERLOAD object as described in [RFC5886].




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   +-+-+               +-+-+-+-+                +-+-+
   |PCC|               |PCE|PCC|                |PCE|
   +-+-+               +-+-+-+-+                +-+-+
     |---- PCReq message-->|                      |
     |                     |                      |
     |                     |---- PCReq message--->|
     |                     |                      |
     |                     |                      |1)PCE is busy but
     |                     |                      | will answer to X
     |                     |                      | in M minutes
     |                     |                      | (time-out update)
     |                     |<--- PCNtf message----| Notification-type=2
     |<--- PCNtf message---|                      |


6.8.  Propagated non request-specific notification

   In this example, a PCE is temporarily congested.  A PCNtf message
   with a NOTIFICATION object containing a TLV "Propagation" at value 1
   and a TLV "Notification Type" at value 0 is send to a PCE and
   propagated to a sequence of PCEs.  Here, PCEk is congested and send a
   PCNtf message to PCEi with the approapriate TLVs, an OVERLOAD object
   as described in [RFC5886], and a DIFFUSION-LIST object indicating
   PCEj as a target of the notification.


   +-+--+                +-+--+                 +-+--+
   |PCEj|                |PCEi|                 |PCEk|
   +-+--+                +-+--+                 +-+--+
     |                     |                      |
     |                     |                      |1)PCE is busy
     |                     |                      | for M minutes
     |                     |                      | (time-out update)
     |                     |<--- PCNtf message----| Notification-type=2
     |<--- PCNtf message---|                      |
















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7.  Security Considerations

   Within the introduced set of TLVs , the TLV "Propagation" affects
   PCEP security considerations since it forces propagation behaviors.
   Thus, a PCEP implementation SHOULD activate stateful mechanism when
   receiving PCEP-ERROR or NOTIFICATION object including this TLV in
   order to avoid DoS attacks.












































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8.  IANA Considerations

   IANA maintains a registry of PCEP parameters.  This includes a sub-
   registry for PCEP Objects.

   IANA is requested to make an allocation from the sub-registry as
   follows.  The values here are suggested for use by IANA.

8.1.  PCEP TLV Type Indicators

   As described in Section 5 the newly defined TLVs allows a PCE to
   enforce specific error and notification behaviors within PCEP-ERROR
   and NOTIFICATION objects.  IANA is requested to make the following
   allocations from the "PCEP TLV Type Indicators" sub-registry.


    Value     Description                 Reference
      7       Propagation                this document

      8       Error-criticality           this document

      9       Notification type           this document


8.2.  New DLO object


   Object-class Value    Object-Type and Name       Reference
      25               1: Diffusion list object       this document


   Target-Type Value     Meaning                    Reference
      0               Any PCEP peers                  this document

      1               PCEs but excludes
                      PCC-only peers                  this document

      2               PCEs and PCCs                   this document
                      with which a session
                      is still opened

      Subobjects                                    Reference
      1: IPv4 prefix                                  this document
      2: IPv6 prefix                                  this document
      4: Unnumbered Interface ID                      this document
      5: OSPF Area ID                                 this document
      32: Autonomous system number                    this document
      33: Explicit Exclusion Route subobject (EXRS)   this document



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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5440]  Vasseur, JP. and JL. Le Roux, "Path Computation Element
              (PCE) Communication Protocol (PCEP)", RFC 5440,
              March 2009.

   [RFC5441]  Vasseur, JP., Zhang, R., Bitar, N., and JL. Le Roux, "A
              Backward-Recursive PCE-Based Computation (BRPC) Procedure
              to Compute Shortest Constrained Inter-Domain Traffic
              Engineering Label Switched Paths", RFC 5441, April 2009.

   [RFC5455]  Sivabalan, S., Parker, J., Boutros, S., and K. Kumaki,
              "Diffserv-Aware Class-Type Object for the Path Computation
              Element Communication Protocol", RFC 5455, March 2009.

   [RFC5521]  Oki, E., Takeda, T., and A. Farrel, "Extensions to the
              Path Computation Element Communication Protocol (PCEP) for
              Route Exclusions", RFC 5521, April 2009.

   [RFC5541]  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
              Objective Functions in the Path Computation Element
              Communication Protocol (PCEP)", RFC 5541, June 2009.

   [RFC5557]  Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
              Computation Element Communication Protocol (PCEP)
              Requirements and Protocol Extensions in Support of Global
              Concurrent Optimization", RFC 5557, July 2009.

   [RFC5886]  Vasseur, JP., Le Roux, JL., and Y. Ikejiri, "A Set of
              Monitoring Tools for Path Computation Element (PCE)-Based
              Architecture", RFC 5886, June 2010.

   [RFC6006]  Zhao, Q., King, D., Verhaeghe, F., Takeda, T., Ali, Z.,
              and J. Meuric, "Extensions to the Path Computation Element
              Communication Protocol (PCEP) for Point-to-Multipoint
              Traffic Engineering Label Switched Paths", RFC 6006,
              September 2010.

9.2.  Informational References

   [I-D.ietf-pce-vendor-constraints]
              Zhang, F., Farrel, A., and G. Bernstein, "Conveying
              Vendor-Specific Constraints in the Path Computation



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              Element Protocol", draft-ietf-pce-vendor-constraints-05
              (work in progress), December 2011.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, August 2006.














































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Authors' Addresses

   Helia Pouyllau
   Alcatel-Lucent
   Route de Villejust
   NOZAY  91620
   FRANCE

   Phone: + 33 (0)1 30 77 63 11
   Email: helia.pouyllau@alcatel-lucent.com


   Remi Theillaud
   Marben Products
   176 rue Jean Jaures
   Puteaux  92800
   FRANCE

   Phone: + 33 (0)1 79 62 10 22
   Email: remi.theillaud@marben-products.com


   Julien Meuric
   France Telecom Orange
   2, avenue Pierre Marzin
   Lannion  22307
   FRANCE

   Email: julien.meuric@orange-ftgroup.com






















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