CCAMP Working Group                                          Xian Zhang
Internet Draft                                            Haomian Zheng
Category: Standards track                                        Huawei
                                                 Oscar Gonzales de Dios
                                                           Victor Lopez
                                                         Telefonica I+D

Expires: August 14, 2014                              February 14, 2014

    Extensions to Path Computation Element Protocol (PCEP) to Support
                Resource Sharing-based Path Computation


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six
   months   and may be updated, replaced, or obsoleted by other
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   as reference   material or to cite them other than as "work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 14, 2014.

Copyright Notice

   Copyright (c) 2014 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
   ( in effect on the date of
   publication of this document. Please review these documents

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   carefully, as they describe your rights and restrictions with
   respect 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.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].


   Resource sharing in a network means two or more Label Switched Paths
   (LSPs) use common piece(s) of resource along their paths. This can
   help save network resource and useful in scenarios such as LSP
   recovery or two LSPs do not need to be active at the same time. A
   Path Computation Element (PCE) is a centralized entity, responsible
   for path calculation. Given this feature and its access to the
   network resource information and possibly active LSPs information,
   it can be used to support resource-sharing-based path computation
   with better efficiency.

   This document extends the Path Computation Element Protocol (PCEP)
   in order to support resource sharing-based path computation.

Table of Contents

   1. Introduction and Motivation.................................. 3
   2. Motivation .................................................. 4
      2.1. Use Case 1 ............................................. 4
      2.2. Use Case 2 ............................................. 5
   3. Extensions to PCEP .......................................... 7
      3.1. Resource Sharing Object................................. 7
      3.2. Processing Rules........................................ 9
      3.3. Carrying RSO in a PCEP Message .........................10
   4. Security Considerations..................................... 11
   5. IANA Considerations ........................................ 12
      5.1. New Object Type........................................ 12
   6. References ................................................. 13
      6.1. Normative References................................... 13
      6.2. Informative References................................. 13
   7. Authors' Addresses ......................................... 13

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1. Introduction and Motivation

   A Path Computation Element (PCE) provides an alternative way for
   providing path computation function, and it is especially useful in
   the scenarios where complex constraints and/or a demanding amount of
   computation resource are required [RFC4655]. The development of PCE
   standardization has evolved from stateless to stateful. A stateful
   PCE has access to the LSP database information of the network(s) it
   serves as a computation engine [Stateful-PCE]. Unless specified
   otherwise, this document assumes a PCE mentioned is a stateful PCE
   (either passive or active).

   Resource sharing denotes that two or more Label Switched Paths (LSPs)
   share common piece(s) of resource, (such as a common time slot of a
   link in an Optical Transport Network (OTN)). This is usually useful
   in the scenario where only one LSP is active and the benefit herein
   is to save network resources. A simple example of this is
   dynamically calculating a LSP for an existing LSP undergoing a link
   failure. Note that the resource sharing can be worked out using a
   statelss PCE, but the mechanism may be complex and is out the scope
   of this draft.

   This document considers the following requirement: resource sharing
   with one or multiple existing LSPs. In a single domain, this is a
   common requirement in the recovery cases especially in order to
   increase traffic resilience against failure while reducing the amount
   of network resource used for recovery purpose [RFC4428].

   The current protocol supporting the communication between a PCE and
   a Path Computation Client (PCC), i.e. PCE Protocol (PCEP), allows
   for re-optimization of an existing LSP [RFC5440]. This is achieved
   by setting R bit in the Request Parameter (RP) object, together with
   some additional information if applicable, in the Path Computation
   Request (PCReq) message sent from a PCC to the PCE. To support this
   type of resource sharing, a PCC needs to ask a PCE to compute a new
   path with the constraints of sharing resource with one or multiple
   existing LSPs. Current PCEP specifications do not provide such

   As mentioned in [stateful-PCE], the standardization of stateful PCEs
   also facilitates PCEP to meet this requirement since a LSP can be
   identified using a unique number. This simplifies configuration of
   PCCs by making it simpler to for a PCC to request resource sharing
   without having to determine all of the resources to be shared.

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   The resource sharing can also be required across layers. This is
   similar to the previous requirement. However, it is more complex and
   therefore deserves a more detailed explanation here.

   In a multi-layer network, Label Switched Paths (LSPs) in a lower
   layer are used to carry higher-layer LSPs across the lower-layer
   network [RFC5623]. Therefore, the resource sharing constraints in
   the higher layer might actually relate to the resource sharing in
   the lower layer. Thus, it is useful to consider how this can be
   achieved and whether additional extensions are needed using the
   models defined in [RFC5623].

   In the next sections, use cases are provided to show what
   information needs to be exchanged to fulfill these requirements.
   This memo then provides extensions to PCEP to enable this function.

2. Motivation

2.1. Use Case 1

   Figure 1 shows a single domain network with a stateful PCE. Assume a
   working LSP (N1-N2-N3) exists in the network. When there is failure
   on the link N2-N3, it is desired to set up a restoration path for
   this working LSP. Suppose N1 serves as the PCC and sends a request
   to the stateful PCE for such an LSP. Before sending the request, N1
   may need to check what policy is configured locally on N1. For
   example, it might value resource sharing more than effectiveness.
   Effectiveness here denotes whether the traffic can be diverted back
   to the working LSP immediately once the failure on the working LSP
   is repaired. In this case, it would prefer to share as much resource
   with the working LSP as possible and specify this in the PCReq

   On the other hand, if N1 considers effectiveness more important, it
   would prefer to share as few resources as possible. Note this is
   different from path diversity, since diversity is a much stricter
   requirement and it would cause path computation failure if the
   diverse recovery path cannot be found. A simple illustration is
   provided below:

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                 |              |
                 | Stateful PCE |
                 |              |

            +------+          +------+          +------+
            |  N1  +----------+  N2  +-----X---+  N3  |
            +--+---+          +---+--+          +---+--+
               |                  |                 |
               |                  +---------+       |
               |                            |       |
               |     +------+          +------+     |
               +-----+  N5  +----------+  N4  +-----+
                     +------+          +------+

               Figure 1: A Single Domain Example

   Available recovery paths computed by the stateful PCE:

   LSP1: N1-N2-N4-N3
   LSP2: N1-N5-N4-N3

   If resource sharing is preferred, the stateful PCE will reply with
   LSP1 information. Instead, if effectiveness is valued higher, it
   will reply with LSP2 information.

   Another piece of information that needs to be conveyed to the PCE is
   the information about the working path LSP. Note this simple use
   case assumes end-to-end recovery. But in order to be applicable to
   use cases such as shared mesh protection purpose, where the head-end
   and tail-end nodes may be different, this information is necessary
   in the message exchange between PCCs and PCEs, so that the stateful
   PCE knows which LSP the path computation request wants to share the
   resource with.

2.2. Use Case 2

   Figure 2 shows a two-layer network example, with each layer managed
   by a PCE (referred as PCE Hi for higher layer and PCE Lo for lower
   layer later). As Discussed in Section 3 of [RFC5623], there are
   three models for inter-layer path computation. They are single PCE
   computation, multiple PCE with inter-PCE communication and multiple
   PCE without inter-PCE communication, respectively. For the single

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   PCE computation, the process would be similar to that of the use
   case in Section 2.1. Thus, this model is not discussed further.

                             .................................| LSR |
                           .:                                 | H5  |
                         .:                                   /-----
                       .:                                    /   |
       -----    -----.:                       -----    -----/    |
      | LSR |--| LSR |.......................| LSR |--| LSR |   /
      | H1  |  | H2  |                       | H3  |  | H4  |  /
       -----    -----\                       /-----    -----  /
                      \                     /                /
                       \                   /                /
                        \                 /                /
                         \               /                /
                          \-----   -----/                /
                          | LSR |-| LSR |               /
                          | L1  | | L2  |              /
                           -----   -----\             /
                             |           \           /
                             |            \         /
                             |             \       /
                           -----            \-----/
                          | LSR |-----------| LSR |
                          | L3  |           | L4  |
                           -----             -----
               Figure 2: A Two-layer Network Example

   In this example, assume a LSP (LSP1: H2-H3) has been established
   already. A new request comes at H2 to establish a new LSP (LSP2:
   from H2 to H5), given the constraint it can share resource with LSP1.
   This requirement is possible if only one of the LSPs needs to be
   active and resource sharing is the target.

   If multiple PCE with inter-PCE communication model is employed, the
   path computation request sent by H2 to PCE Hi will be passed to PCE
   Lo since there is no resource readily available in the upper layer.
   So it leaves to the PCE Lo to compute a path in the lower layer in
   order to support the upper layer request. In this case, PCE Lo is
   required to compute a path between H2 and H5 under the constraint
   that it can share the resource with that of the LSP1. Assume here
   LSP1 goes from H2, via L1-L2 to H3. So when PCE Lo computes the path
   for LSP2, it can view the resource used by LSP1 available. For
   example, PCE Lo may choose H2-L1-L2-L4-H5 as the computation result.

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   The issue to solve during this procedure is that PCE Hi can only use
   LSP1 information (such as its five-tuple LSP information) as the
   information, how PCE Lo can resolve this information to the actual
   resource usage in its own layer, i.e. lower layer. This could be
   solved by edge LSR L1 reporting this higher-lower layer LSP
   correlation to the Lo PCE as part of the LSP information during the
   LSP state synchronization process. If needed, it can be later
   updated when there is a change in this information. Alternatively,
   the PCE Lo can get this information from other sources, such as
   network management system, where this information should be stored.

   If multiple PCE without inter-PCE communication model is employed,
   the path computation request in the lower layer will be initiated
   the border LSR node, i.e., L1. The process would be similar to that
   of the previous scenario. A point worth noting is that the border
   LSR node may be able to resolve the higher LSP information itself,
   such as mapping it to the corresponding LSP in the lower layer, thus
   PCE Lo do not need to perform this function. Otherwise, the method
   mentioned above can still be used.

3. Extensions to PCEP

   This section provides PCEP extensions to allow a PCC to specify
   resource sharing when sending a PCReq message. It also details the
   processing rule and error codes needed.

3.1. Resource Sharing Object

   The PCEP Resource Sharing Object (RSO) is optional. It MAY be
   carried within a PCRep message so as to indicate the desired
   resource sharing requirements to be applied by the stateful PCE
   during path computation.

   The RSO object format is compliant with the PCEP object format
   defined in [RFC5440].

   The RSO Object-Class is TBA.

   The RSO Object-type is 1.

   The format of the RSO object body is:

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    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
   | RSO codes                 |R|D|       Reserved                |
   |                                                               |
   ~                        Optional TLVs                          ~
   |                                                               |
                        Figure 1: RSO Object Format

   RSO codes (16 bits): the objective of the resource sharing.
   Currently, the following objectives are defined:

   D (1 bit): sharing as little as possible.

   R (1 bit): sharing as much as possible

   If D and R are both set to 0, it denotes the requesting node only
   requires resource sharing without further constraint (i.e., the
   extent of resource sharing). The combination of D=1 and R=1 is not

   Reserved (2 bytes): This field MUST be set to zero on transmission
   and MUST be ignored on receipt.

   Optional TLVs may be needed to indicate the LSP with which the
   resource is shared. The LSP Info TLV is defined as follows, for IPv4
   and IPv6 addresses respectively

    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=[TBD]          |           Length=20           |
   |                 IPv4 tunnel end point address                 |
   |          0                    |     Tunnel ID                 |
   |                       Extended Tunnel ID                      |
   |                   IPv4 tunnel sender address                  |
   |          0                    |            LSP ID             |

                        Figure 2: IPv4 LSP Info TLV

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    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=[TBD]          |           Length=44           |
   |                 IPv6 tunnel end point address                 |
   |                 IPv6 tunnel end point address (cont.)         |
   |                 IPv6 tunnel end point address (cont.)         |
   |                 IPv6 tunnel end point address (cont.)         |
   |          0                    |     Tunnel ID                 |
   |                       Extended Tunnel ID                      |
   |                   IPv6 tunnel sender address                  |
   |                 IPv6 tunnel end point address (cont.)         |
   |                 IPv6 tunnel end point address (cont.)         |
   |                 IPv6 tunnel end point address (cont.)         |
   |          0                    |            LSP ID             |

                        Figure 3: IPv6 LSP Info TLV

3.2. Processing Rules

   To request a path allowing sharing resource with one or multiple
   existing LSPs, a PCC includes a RSO object in the PCReq message.

   On receipt of a PCReq message with a RSO object, a stateful PCE MUST
   proceed as follows:

     - If the RSO object is unknown/unsupported, the PCE will follow
     procedures defined in [RFC5440].  That is, the PCE sends a PCErr
     message with error type 3 or 4 (Unknown / Not supported object)
     and error value 1 or 2 (unknown / unsupported object class /
     object type), and the related path computation request is

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     - If TLV(s) present in the RSO object are unknown/unsupported and
     the P bit is set, the PCE MUST send a PCErr message with error
     type 3 or 4 (Unknown / Not supported object) and error value 4
     (Unrecognized/Unsupported parameter), and the related path
     computation request MUST be discarded as defined in [RFC5440].

     - If the resource sharing information is extracted correctly, the
     PCE MUST apply the requested resource sharing requirement.

   If the received RSO has D bit set, the PCE will find a path that
   shares as much resources as possible with the specified LSP(s).
   Otherwise, if S bit is set, the PCE will find a path that shares as
   little resources as possible with the specified LSP(s). The RSO
   codes may be locally configured on the requesting nodes via external
   entities, such as a network management system or the entity that
   impose the resource sharing requirement.

3.3. Carrying RSO in a PCEP Message

   The RSO is applied to an individual path computation request and the
   format of the PCReq message is updated as follows:

   <PCReq Message> ::= <Common Header>




        <svec-list> ::= <SVEC>




       <request-list> ::= <request> [<request-list>]

       <request> ::= <RP>


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   and where:

      <metric-list> ::= <METRIC>[<metric-list>]

4. Security Considerations

   Security of PCEP is discussed in [RFC5440] and [RFC6952]. The
   extensions in this document do not change the fundamentals of
   security for PCEP.

   However, the introduction of the RSO provides a vector that may be
   used to probe for information from a network. For example, a PCC
   that wants to discover the path of an LSP with which it is not
   involved, can issue a PCReq with an RSO and may be able to get back
   quite a lot of information about the path of the LSP through issuing
   multiple such requests for different endpoints and analyzing the
   received results. To protect against this, a PCE should be
   configured with access and authorization controls such that only
   authorized PCCs (for example, those within the network) can make
   computation requests, only specifically authorized PCCs can make
   requests using the RSO, and resource sharing requests relating to
   specific LSPs are further limited to a select few PCCs. How such
   access controls and authorization is managed is outside the scope of
   this document, but it will at the least include Access Control Lists.

   Furthermore, a PCC must be aware that setting up an LSP that shares
   resources with another LSP may be a way of attacking the other LSP,
   for example by depriving it of the resources it needs to operate
   correctly. Thus it is important that, both in PCEP and the
   associated signaling protocols, only authorized resource sharing is

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5. IANA Considerations

5.1. New Object Type

   IANA manages the PCEP Objects code point registry (see [RFC5440]).
   This is maintained as the "PCEP Objects" sub-registry of the "Path
   Computation Element Protocol (PCEP) Numbers" registry.

   This document defines a new PCEP object, the RSO object, to be
   carried in PCReq messages.  IANA is requested to make the following
   allocation in the "PCEP Objects" sub-registry:

   Object    Name     Object    Name                  Reference
   Class              Type

    TBA      RSO              Resource Sharing     [this document]

   5.2 New RSO TLVs

   IANA is request to create and maintain a new sub-registry named "RSO
   TLVs" and include the following TLVs:

   Value        Description              Reference

     1         IPv4 LSP Info TLV          [this document]

     2         IPv6 LSP Info TLV          [this document]

   5.3 RSO codes

   IANA is requested to create and maintain a new sub-registry named
   "RSO codes". The following codes are defined in this document:

   Bit      Code Name      Meaning                    Reference

    0           D          sharing as much as possible

                                                   [this document]

    1           R          sharing as little as possible

                                                   [this document]

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6. References

6.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
             requirements levels", RFC 2119, March 1997.

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

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

   [Stateful-PCE] Crabbe, E., Medved, J., Minei, I., and R. Varga,
             "PCEP Extensions for Stateful PCE", draft-ietf-pce-
             stateful-pce-07 (work in progress), October 2013.

6.2. Informative References

   [RFC4428] Papadimitriou, D., Mannie., E., "Analysis of Generalized
             Multi-Protocol Label Switching (GMPLS)-based Recovery
             Mechanisms (including Protection and Restoration)",
             RFC4428, March 2006.

   [RFC5623] Oki., E., Takeda, T., Le Roux, JL., Farrel, A., "Framework
             for PCE-Based Inter-Layer MPLS and GMPLS Traffic
             Engineering", RFC5623, September 2009.

   [RFC6952] Jethanandani, M., Patel, K., Zheng, L., "Analysis of BGP,
             LDP, PCEP, and MSDP Issues According to the Keying and
             Authentication for Routing Protocols (KARP) Design Guide",
             RFC6952, May 2013.

7. Authors' Addresses

   Xian Zhang
   Huawei Technologies


   Haomian Zheng

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   Huawei Technologies


   Oscar Gonzalez de Dios
   Telefonica I+D
   Don Ramon de la Cruz 82-84
   Madrid    28045


   Victor Lopez
   Telefonica I+D
   Don Ramon de la Cruz 82-84
   Madrid    28045


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