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Internet Engineering Task Force                                  H. Chen
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                     O. Gonzalez de Dios
Expires: May 2, 2012                                      Telefonica I+D
                                                        October 30, 2011


       A Forward-Search P2P TE LSP Inter-Domain Path Computation
       draft-chen-pce-forward-search-p2p-path-computation-02.txt

Abstract

   This document presents a forward search procedure for computing paths
   for Point-to-Point (P2P) Traffic Engineering (TE) Label Switched
   Paths (LSPs) crossing a number of domains through using multiple Path
   Computation Elements (PCEs).  In addition, extensions to the Path
   Computation Element Communication Protocol (PCEP) for supporting the
   forward search procedure are described.

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
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   This Internet-Draft will expire on May 2, 2012.

Copyright Notice

   Copyright (c) 2011 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Forward Search Path Computation  . . . . . . . . . . . . . . .  5
     5.1.  Overview of Procedure  . . . . . . . . . . . . . . . . . .  5
     5.2.  Description of Procedure . . . . . . . . . . . . . . . . .  6
     5.3.  Comparing to BRPC  . . . . . . . . . . . . . . . . . . . .  8
   6.  Extensions to PCEP . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  RP Object Extension  . . . . . . . . . . . . . . . . . . .  9
     6.2.  PCE Object . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.3.  Node Flags Object  . . . . . . . . . . . . . . . . . . . . 10
     6.4.  Candidate Node List Object . . . . . . . . . . . . . . . . 11
     6.5.  Request Message Extension  . . . . . . . . . . . . . . . . 12
   7.  Security  Considerations . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Request Parameter Bit Flags  . . . . . . . . . . . . . . . 13
   9.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14























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1.  Introduction

   It would be useful to extend MPLS TE capabilities across multiple
   domains (i.e., IGP areas or Autonomous Systems) to support inter-
   domain resources optimization, to provide strict QoS guarantees
   between two edge routers located within distinct domains.

   RFC 4105 "Requirements for Inter-Area MPLS TE" lists the requirements
   for computing a shortest path for a TE LSP acrossing multiple IGP
   areas; and RFC 4216 "MPLS Inter-Autonomous System (AS) TE
   Requirements" describes the requirements for computing a shortest
   path for a TE LSP acrossing multiple ASes.  RFC 4655 "A PCE-Based
   Architecture" discusses centralized and distributed computation
   models for the computation of a path for a TE LSP acrossing multiple
   domains.

   This document presents a forward search procedure to address these
   requirements through using multiple Path Computation Elements (PCEs).
   This procedure guarantees that the path found from the source to the
   destination is shortest.  It does not depend on any sequence of
   domains from the source node to the destination node.  Navigating a
   mesh of domains is simple and efficient.


2.  Terminology

   ABR: Area Border Router.  Routers used to connect two IGP areas
   (areas in OSPF or levels in IS-IS).

   ASBR: Autonomous System Border Router.  Routers used to connect
   together ASes of the same or different service providers via one or
   more inter-AS links.

   Boundary Node (BN): a boundary node is either an ABR in the context
   of inter-area Traffic Engineering or an ASBR in the context of
   inter-AS Traffic Engineering.

   Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) along
   a determined sequence of domains.

   Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) along
   a determined sequence of domains.

   Inter-area TE LSP: A TE LSP that crosses an IGP area boundary.

   Inter-AS TE LSP: A TE LSP that crosses an AS boundary.

   LSP: Label Switched Path.



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   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(i) is a PCE with the scope of domain(i).

   TED: Traffic Engineering Database.

   This document uses terminologies defined in RFC 5440.


3.  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.


4.  Requirements

   This section summarizes the requirements specific for computing a
   path for a P2P Traffic Engineering (TE) LSP acrossing multiple
   domains (areas or ASes).  More requirements for Inter-Area and
   Inter-AS MPLS Traffic Engineering are described in RFC 4105 and RFC
   4216.

   A number of requirements specific for a solution to compute a path
   for a P2P TE LSP acrossing multiple domains is listed as follows:

   1.  The solution SHOULD provide the capability to compute a shortest
       path dynamically, satisfying a set of specified constraints
       across multiple IGP areas.

   2.  The solution MUST provide the ability to reoptimize in a
       minimally disruptive manner (make before break) an inter-area TE
       LSP, should a more optimal path appear in any traversed IGP area.

   3.  The solution SHOULD provide mechanism(s) to compute a shortest
       end-to-end path for a TE LSP acrossing multiple ASes and
       satisfying a set of specified constraints dynamically.

   4.  Once an inter-AS TE LSP has been established, and should there be
       any resource or other changes inside anyone of the ASes, the



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       solution MUST be able to re-optimize the LSP accordingly and non-
       disruptively, either upon expiration of a configurable timer or
       upon being triggered by a network event or a manual request at
       the TE tunnel Head-End.


5.  Forward Search Path Computation

   This section gives an overview of the forward search path computation
   procedure to satisfy the requirements described above and describes
   the procedure in details.

5.1.  Overview of Procedure

   Simply speaking, the idea of the forward search path computation
   procedure for computing a path for an MPLS TE P2P LSP acrossing
   multiple domains from a source node to a destination node includes:

   Start from the source node and the source domain.

   Consider the optimal path segment from the source node to every exit
   boundary node of the source domain as a special link;

   Consider the optimal path segment from an entry boundary node to
   every exit boundary node of a domain as a special link; and the
   optimal path segment is computed as needed.

   The whole topology consisting of many domains can be considered as a
   special topology, which contains those special links, the normal
   links in the destination domain and the inter-domain links.

   Compute an optimal path in this special topology from the source node
   to the destination node using CSPF.

   The forward search path computation procedure for computing a path
   for an MPLS TE P2P LSP starts at the source domain, in which the
   source (or ingress) node of the MPLS TE LSP locates.  When a PCE in
   the source domain receives a PCReq for the path for the MPLS TE LSP,
   it computes the optimal path from the source node to every exit
   boundary node of the domain towards the destination node.

   There are two lists involved in the path computation.  One list is
   called candidate node list, which contains the nodes with brief
   information about the temporary optimal paths from the source node to
   each of these nodes currently found.  The nodes in the candidate list
   are ordered by the cost of the path.  Initially, the candidate node
   list contains only source node with cost 0.




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   The other is called result path list or tree, which contains the
   final optimal paths from the source node to the boundary nodes or the
   nodes in the destination domain.  Initially, the result path list is
   empty.

   When a PCE responsible for a domain (called current domain) receives
   a PCReq for computing the path for the MPLS TE LSP, it removes the
   node with the minimum cost from the candidate node list and put or
   graft the node to the result path list or tree.

   If the destination node is in the current domain, the PCE tries to
   compute the optimal path from the source node to the destination node
   and sends a PCRep with the optimal path to the PCE or PCC from which
   the PCReq is received.

   Otherwise (i.e., if the destination is not in the domain), the PCE
   computes the optimal path from the source node to every exit boundary
   node of the current domain towards the destination node and further
   to the entry boundary nodes of the domain connected to the current
   domain, puts the new node into the candidate list in order by path
   cost, updates the existing node in the candidate node list with the
   new node with lower cost, and then sends a PCReq with the new
   candidate node list to the PCE that is responsible for the domain
   with the first node in the candidate node list.

5.2.  Description of Procedure

   Suppose that we have the following variables:

   A current PCE named as CurrentPCE which is currently computing the
   path.

   A candidate node list named as CandidateNodeList, which contains the
   nodes to each of which the temporary optimal path from the source
   node is currently found.  The information about each node C in
   CandidateNodeList consists of

   the cost of the path from the source node to node C,

   the previous hop node P and the link between P and C,

   the PCE responsible for C, and

   the flags for C. The flags include

   one bit D indicating that node C is a Destination node if it is set;

   one bit S indicating that C is the Source node if it is set;



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   one bit E indicating that C is an Exit boundary node if it is set;

   one bit I indicating that C is an entry boundary node if it is set;
   and

   one bit N indicating that C is a Node in the destination domain if it
   is set.

   The nodes in CandidateNodeList are ordered by path cost.  Initially,
   CandidateNodeList contains only a Source Node, with path cost 0, PCE
   responsible for the source domain, and flags with S bit set.

   A result path list or tree named as ResultPathTree, which contains
   the final optimal paths from the source node to the boundary nodes or
   the nodes in the destination domain.  Initially, ResultPathTree is
   empty.

   The Forward Search Path Computation procedure for computing the path
   for the MPLS TE P2P LSP is described as follows:

   Initially, a PCC sets ResultPathTree to empty and CandidateNodeList
   to contain the source node and sends PCE responsible for the source
   domain a PCReq with the source node, the destination node,
   CandidateNodeList and ResultPathTree.

   When the PCE responsible for a domain (called current domain)
   receives a request for computing the path for the MPLS TE P2MP LSP,
   it checks whether the current PCE is the PCE responsible for the node
   C with the minimum cost in the CandidateNodeList.  If it is, then
   remove C from CandidateNodeList and graft it into ResultPathTree;
   otherwise, a PCReq message is sent to the PCE for node C.

   Suppose that node C has Flags.  The ResultPathTree is built from C in
   the following steps.

   If the D (Destination Node) bit in the Flags is set, then the optimal
   path from the source node to the destination node is found, and a
   PCRep message with the path is sent to the PCE/PCC which sends the
   request to the current PCE.

   If the N (Node in Destination domain) bit in the Flags is set, then
   for every node N connected to node C and not on ResultPathTree, it is
   merged into CandidateNodeList.  The cost to node N is the sum of the
   cost to node C and the cost of the link between C and N. The PCE for
   node N is the current PCE.

   If the Entry/Incoming Boundary Node (I) bit or the Source Node (S)
   bit is set), then path segments from node C to every exit boundary



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   node of the current domain that is not on the result path tree are
   computed through using CSPF and as special links.  For every node N
   connected to node C through a special link (i.e., a path segment), it
   is merged into CandidateNodeList.  The cost to node N is the sum of
   the cost to node C and the cost of the special link (i.e., path
   segment ) between C and N. The PCE for node N is the current PCE.

   If the Exit Boundary Node (E) bit is set and there exist inter-domain
   links connected to it, then for every node N connected to C and not
   on the result path tree, it is merged into the candidate node list.
   The cost to node N is the sum of the cost to node C and the cost of
   the link between C and N. The PCE for node N is the PCE responsible
   for node N.

   If the CurrentPCE is the same as the PCE of the node with the minimum
   cost in CandidateNodeList, then the node is removed from
   CandidateNodeList, grafted to ResultPathTree, and the above steps are
   repeated; otherwise, the CurrentPCE sends the PCE a request with the
   source node, CandidateNodeList and ResultPathTree.

5.3.  Comparing to BRPC

   RFC 5441 describes the Backward Recursive Path Computation (BRPC)
   algorithm or procedure for computing an MPLS TE P2P LSP path from a
   source node to a destination node crossing multiple domains.
   Comparing to BRPC, there are a number of differences between BRPC and
   the Forward-Search P2P TE LSP Inter-Domain Path Computation.  Some of
   the differences are briefed below.

   First, for BRPC to compute a shortest path from a source node to a
   destination node crossing multiple domains, we MUST provide a
   sequence of domains from the source node to the destination node to
   BRPC in advance.  The Forward-Search P2P TE LSP Inter-Domain Path
   Computation does not need any sequence of domains for computing a
   shortest path.

   Secondly, for a given sequence of domains domain(1), domain(2), ... ,
   domain(n), BRPC searches the shortest path from domain(n), to
   domain(n-1), until domain(1) along the reverse order of the given
   sequence of domain.  It will get the shortest path within the given
   domain sesuence.  The Forward-Search P2P TE LSP Inter-Domain Path
   Computation calculates an optimal path in a special topology from the
   source node to the destination node using CSPF.  It will find the
   shortest path within all the domains.

   Moreover, if the sequence of domains from the source node to the
   destination node provided to BRPC does not contain the shortest path
   from the source to the destionation, then the path computed by BRPC



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   is not optimal.  The Forward-Search P2P TE LSP Inter-Domain Path
   Computation guarantees that the path found is optimal.


6.  Extensions to PCEP

   This section describes the extensions to PCEP for Forward Search Path
   Computation.  The extensions include the definition of a new flag in
   the RP object, a result path list and a candidate node list in the
   PCReq message.

6.1.  RP Object Extension

   The following flag is added into the RP Object:

   The F bit is added in the flag bits field of the RP object to tell
   the receiver of the message that the request/reply is for Forward
   Search Path Computation.

       o F (Forward search Path Computation bit - 1 bit):

           0: This indicates that this is not PCReq/PCRep
              for Forward Search Path Computation.

           1: This indicates that this is PCReq or PCRep message
              for Forward Search Path Computation.


   The IANA request is referenced in Section below (Request Parameter
   Bit Flags) of this document.

   This F bit with the N bit defined in RFC6006 can indicate whether the
   request/reply is for Forward Search Path Computation of an MPLS TE
   P2P LSP or an MPLS TE P2MP LSP.

       o F = 1 and N = 0: This indicates that this is a PCReq/PCRep
                          message for Forward Search Path Computation
                          of an MPLS TE P2P LSP.

       o F = 1 and N = 1: This indicates that this is a PCReq/PCRep
                          message for Forward Search Path Computation
                          of an MPLS TE P2MP LSP.









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6.2.  PCE Object

   The figure below illustrates a PCE IPv4 object body (Object-Type=2),
   which comprises a PCE IPv4 address.  The PCE IPv4 address object
   indicates the IPv4 address of a PCE , with which a PCE session may be
   established and to which a request message may be sent.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       PCE  IPv4 address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 1: PCE Object Body for IPv4

   The format of the PCE object body for IPv6 (Object-Type=2) 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                 PCE  IPv6 address (16 bytes)                  |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 2: PCE Object Body for IPv6

6.3.  Node Flags Object

   The Node Flags object is used to indicate the characteristics of the
   node in a candidate node list in a request or reply message for
   Forward Search Inter-domain Path Computation.  The Node Flags object
   comprises a Reserved field, and a number of Flags.

   The format of the Node Flags object body 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |D|S|I|E|N|          Flags      |         Reserved              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 3: Node Flags Object Body



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   where

       o D = 1:  The node is a destination node.
       o S = 1:  The node is a source node.
       o I = 1:  The node is an entry boundary node.
       o E = 1:  The node is an exit boundary node.
       o N = 1:  The node is a node in a destination domain.


6.4.  Candidate Node List Object

   The candidate-node-list-obj object contains the nodes in the
   candidate node list.  A new PCEP object class and type are requested
   for it.  The format of the candidate-node-list-obj object body 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //               (a list of <candidate-node>s)                 //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 4: Candidate Node List Object

   The following is the definition of candidate node list, which may
   contain Node Flags.

         <candidate-node-list>::= <candidate-node>
                                  [<candidate-node-list>]
         <candidate-node>::= <ERO>
                             <candidate-attribute-list>

         <candidate-attribute-list>::= [<attribute-list>]
                                       [<PCE>]
                                       [<Node-Flags>]



   The ERO in a candidate node contain just the path segment of the last
   link of the path, which is from the previous hop node of the tail end
   node of the path to the tail end node.  With this information, we can
   graft the candidate node into the existing result path list or tree.

   Simply speaking, a candidate node has the same or similar format of a
   path defined in RFC 5440, but the ERO in the candidate node just



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   contain the tail end node of the path and its previous hop, and the
   candidate path may contain two new objects PCE and node flags.

6.5.  Request Message Extension

   Below is the message format for a request message with the extension
   of a result path list and a candidate node list:

           <PCReq Message>::= <Common Header>
                              [<svec-list>]
                              <request-list>
           <request-list>::=<request>[<request-list>]
           <request>::= <RP>
                        <END-POINTS>
                        [<OF>]
                        [<LSPA>]
                        [<BANDWIDTH>]
                        [<metric-list>]
                        [<RRO>[<BANDWIDTH>]]
                        [<IRO>]
                        [<LOAD-BALANCING>]
                        [<result-path-list>]
                        [<candidate-node-list-obj>]

        where:
           <result-path-list>::=<path>[<result-path-list>]
           <path>::= <ERO><attribute-list>
           <attribute-list>::=[<LSPA>]
                              [<BANDWIDTH>]
                              [<metric-list>]
                              [<IRO>]

           <candidate-node-list-obj> contains a <candidate-node-list>


   The definition for the result path list that may be added into a
   request message is the same as that for the path list in a reply
   message that is described in RFC5440.


7.  Security  Considerations

   The mechanism described in this document does not raise any new
   security issues for the PCEP protocols.







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8.  IANA Considerations

   This section specifies requests for IANA allocation.

8.1.  Request Parameter Bit Flags

   A new RP Object Flag has been defined in this document.  IANA is
   requested to make the following allocation from the "PCEP RP Object
   Flag Field" Sub-Registry:


         Bit       Description                         Reference

          18       Forward Path Computation (F-bit)    This I-D




9.  Acknowledgement

   The authors would like to thank Julien Meuric, Daniel King, Cyril
   Margaria, Ramon Casellas, Olivier Dugeon and Dhruv Dhody for their
   valuable comments on this draft.


10.  References

10.1.  Normative References

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

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

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

   [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.






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10.2.  Informative References

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

   [RFC5862]  Yasukawa, S. and A. Farrel, "Path Computation Clients
              (PCC) - Path Computation Element (PCE) Requirements for
              Point-to-Multipoint MPLS-TE", RFC 5862, June 2010.


Authors' Addresses

   Huaimo Chen
   Huawei Technologies
   Boston, MA
   USA

   Email: Huaimochen@huawei.com


   Oscar Gonzalez de Dios
   Telefonica I+D
   Emilio Vargas 6, Madrid
   Spain

   Email: ogondio@tid.es

























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