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rfc:rfc7267

Internet Engineering Task Force (IETF) L. Martini, Ed. Request for Comments: 7267 Cisco Systems, Inc. Updates: 6073 M. Bocci, Ed. Category: Standards Track F. Balus, Ed. ISSN: 2070-1721 Alcatel-Lucent

                                                             June 2014
           Dynamic Placement of Multi-Segment Pseudowires

Abstract

 RFC 5254 describes the service provider requirements for extending
 the reach of pseudowires (PWs) across multiple Packet Switched
 Network domains.  A multi-segment PW is defined as a set of two or
 more contiguous PW segments that behave and function as a single
 point-to-point PW.  This document describes extensions to the PW
 control protocol to dynamically place the segments of the multi-
 segment pseudowire among a set of Provider Edge (PE) routers.  This
 document also updates RFC 6073 by updating the value of the Length
 field of the PW Switching Point PE Sub-TLV Type 0x06 to 14.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7267.

Martini, et al. Standards Track [Page 1] RFC 7267 Dynamic Placement of Multi-Segment PWs June 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
 (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
 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Martini, et al. Standards Track [Page 2] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

Table of Contents

 1. Introduction ....................................................4
    1.1. Scope ......................................................4
    1.2. Specification of Requirements ..............................4
    1.3. Terminology ................................................4
    1.4. Architecture Overview ......................................5
 2. Applicability ...................................................6
    2.1. Changes to Existing PW Signaling ...........................6
 3. PW Layer 2 Addressing ...........................................6
    3.1. Attachment Circuit Addressing ..............................7
    3.2. S-PE Addressing ............................................8
 4. Dynamic Placement of MS-PWs .....................................8
    4.1. Pseudowire Routing Procedures ..............................8
         4.1.1. AII PW Routing Table Lookup Aggregation Rules .......9
         4.1.2. PW Static Route .....................................9
         4.1.3. Dynamic Advertisement with BGP .....................10
    4.2. LDP Signaling .............................................11
         4.2.1. Multiple Alternative Paths in PW Routing ...........13
         4.2.2. Active/Passive T-PE Election Procedure .............14
         4.2.3. Detailed Signaling Procedures ......................15
 5. Procedures for Failure Handling ................................16
    5.1. PSN Failures ..............................................16
    5.2. S-PE Failures .............................................17
    5.3. PW Reachability Changes ...................................17
 6. Operations, Administration, and Maintenance (OAM) ..............18
 7. Security Considerations ........................................18
 8. IANA Considerations ............................................19
    8.1. Correction ................................................19
    8.2. LDP TLV Type Name Space ...................................19
    8.3. LDP Status Codes ..........................................20
    8.4. BGP SAFI ..................................................20
 9. References .....................................................20
    9.1. Normative References ......................................20
    9.2. Informative References ....................................21
 10. Contributors ..................................................22
 11. Acknowledgements ..............................................23

Martini, et al. Standards Track [Page 3] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

1. Introduction

1.1. Scope

 [RFC5254] describes the service provider requirements for extending
 the reach of pseudowires across multiple Packet Switched Network
 (PSN) domains.  This is achieved using a multi-segment pseudowire
 (MS-PW).  An MS-PW is defined as a set of two or more contiguous
 pseudowire (PW) segments that behave and function as a single point-
 to-point PW.  This architecture is described in [RFC5659].
 The procedures for establishing PWs that extend across a single PSN
 domain are described in [RFC4447], while procedures for setting up
 PWs across multiple PSN domains or control plane domains are
 described in [RFC6073].
 The purpose of this document is to specify extensions to the
 pseudowire control protocol [RFC4447], and [RFC6073] procedures, to
 enable multi-segment PWs to be dynamically placed.  The procedures
 follow the guidelines defined in [RFC5036] and enable the reuse of
 existing TLVs, and procedures defined for Single-Segment Pseudowires
 (SS-PWs) in [RFC4447].  Dynamic placement of point-to-multipoint
 (P2MP) PWs is for further study and outside the scope of this
 document.

1.2. Specification of Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

1.3. Terminology

 [RFC5659] provides terminology for multi-segment pseudowires.
 This document defines the following additional terms:
  1. Source Terminating Provider Edge (ST-PE): A Terminating Provider

Edge (T-PE) that assumes the active signaling role and initiates

   the signaling for multi-segment PWs.
  1. Target Terminating Provider Edge (TT-PE): A Terminating Provider

Edge (T-PE) that assumes the passive signaling role. It waits and

   responds to the multi-segment PW signaling message in the reverse
   direction.
  1. Forward Direction: ST-PE to TT-PE.

Martini, et al. Standards Track [Page 4] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

  1. Reverse Direction: TT-PE to ST-PE.
  1. Pseudowire Routing (PW routing): The dynamic placement of the

segments that compose an MS-PW, as well as the automatic selection

   of Switching PEs (S-PEs).

1.4. Architecture Overview

 The following figure shows the reference model, derived from
 [RFC5659], to support PW emulated services using multi-segment PWs.
    Native  |<---------Multi-Segment Pseudowire-------->|  Native
    Service |          PSN                PSN           |  Service
     (AC)   |     |<--Tunnel-->|     |<--Tunnel-->|     |   (AC)
       |    V     V     1      V     V     2      V     V     |
       |    +-----+            +-----+            +-----+     |
 +---+ |    |T-PE1|============|S-PE1|============|T-PE2|     | +---+
 |   |------|...... PW.Seg't 1....X....PW.Seg't 3.......|-------|   |
 |CE1| |    |     |            |     |            |     |     | |CE2|
 |   |------|...... PW.Seg't 2....X....PW.Seg't 4.......|-------|   |
 +---+ |    |     |============|     |============|     |     | +---+
     ^      +-----+            +-----+            +-----+       ^
     |   Provider Edge 1          ^          Provider Edge 2    |
     |                            |                             |
     |                            |                             |
     |                    PW switching point                    |
     |                                                          |
     |<-------------------- Emulated Service ------------------>|
                    Figure 1: MS-PW Reference Model
 The PEs that provide services to CE1 and CE2 are Terminating PE1
 (T-PE1) and Terminating PE2 (T-PE2), respectively.  A PSN tunnel
 extends from T-PE1 to Switching PE1 (S-PE1), and a second PSN tunnel
 extends from S-PE1 to T-PE2 .  PWs are used to connect the attachment
 circuits (ACs) attached to PE1 to the corresponding ACs attached to
 T-PE2.
 A PW segment on PSN Tunnel 1 is connected to a PW segment on PSN
 Tunnel 2 at S-PE1 to complete the multi-segment PW (MS-PW) between
 T-PE1 and T-PE2.  S-PE1 is therefore the PW switching point and is
 referred to as the switching provider edge (S-PE).  PW Segment 1 and
 PW Segment 3 are segments of the same MS-PW, while PW Segment 2 and
 PW Segment 4 are segments of another MS-PW.  PW segments of the same
 MS-PW (e.g., PW Segment 1 and PW Segment 3) MUST be of the same PW
 type, and PSN tunnels can be of the same or a different technology.
 An S-PE switches an MS-PW from one segment to another based on the PW
 identifiers (PWid, or Attachment Individual Identifier (AII)).  How

Martini, et al. Standards Track [Page 5] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 the PW protocol data units (PDUs) are switched at the S-PE depends on
 the PSN tunnel technology: in the case of a Multiprotocol Label
 Switching (MPLS) PSN to another MPLS PSN, PW switching involves a
 standard MPLS label swap operation.
 Note that although Figure 1 only shows a single S-PE, a PW may
 transit more than one S-PE along its path.  Although [RFC5659]
 describes MS-PWs that span more than one PSN, this document does not
 specify how the Label Distribution Protocol (LDP) is used for PW
 control [RFC4447] in an inter-AS (Autonomous System) environment.

2. Applicability

 This document describes the case where the PSNs carrying the MS-PW
 are only MPLS PSNs using the Generalized Pseudowire Identifier (PWid)
 Forwarding Equivalence Class (FEC) element (also known as FEC 129).
 Interactions with an IP PSN using the Layer 2 Tunneling Protocol
 version 3 (L2TPv3) as described in Section 8 of [RFC6073] are left
 for further study.

2.1. Changes to Existing PW Signaling

 The procedures described in this document make use of existing LDP
 TLVs and related PW signaling procedures described in [RFC4447] and
 [RFC6073].  The following optional TLV is also defined:
  1. A Bandwidth TLV to address QoS Signaling requirements (see

Section 4.2).

 This document also updates the value of the Length field of the PW
 Switching Point PE Sub-TLV Type 0x06 to 14.

3. PW Layer 2 Addressing

 Single-segment pseudowires on an MPLS PSN can use attachment circuit
 identifiers for a PW using FEC 129.  In the case of a dynamically
 placed MS-PW, there is a requirement for the attachment circuit
 identifiers to be globally unique, for the purposes of reachability
 and manageability of the PW.  Referencing Figure 1 above, individual
 globally unique addresses MUST be allocated to all the ACs and S-PEs
 of an MS-PW.

Martini, et al. Standards Track [Page 6] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

3.1. Attachment Circuit Addressing

 The attachment circuit addressing is derived from AII Type 2
 [RFC5003], as shown here:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AII Type=02  |    Length     |        Global ID              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Global ID (continued)   |        Prefix                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Prefix (continued)      |        AC ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      AC ID                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 2: AII Type 2 TLV Structure
 The fields are defined in Section 3.2 of [RFC5003].
 Addressing schemes based on AII Type 2 permit varying levels of AII
 summarization, thus reducing the scaling burden on PW routing.  PW
 addressing based on AII Type 2 is suitable for point-to-point
 provisioning models where auto-discovery of the address at the TT-PE
 is not required.  That is, it is known a priori by provisioning.
 Implementations of the following procedure MUST interpret the AII
 type to determine the meaning of the address format of the AII,
 irrespective of the number of segments in the MS-PW.  All segments of
 the PW MUST be signaled with the same AII type.
 A unique combination of Global ID, Prefix, and AC ID parts of the
 AII Type 2 are assigned to each AC.  In general, the same Global ID
 and Prefix are be assigned for all ACs belonging to the same T-PE.
 This is not a strict requirement, however.  A particular T-PE might
 have more than one Prefix assigned to it, and likewise a fully
 qualified AII with the same Global ID/Prefix but different AC IDs
 might belong to different T-PEs.
 For the purpose of MS-PWs, the AII MUST be globally unique across all
 PSNs that are spanned by the MS-PW.
 The AII for a local attachment circuit of a given T-PE of an MS-PW
 and the AII of the corresponding attachment circuit on a far-end T-PE
 (with respect to the LDP signaling) are known as the Source
 Attachment Individual Identifier (SAII) and Target Attachment
 Individual Identifier (TAII) as per [RFC6074].

Martini, et al. Standards Track [Page 7] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

3.2. S-PE Addressing

 Each S-PE MUST be assigned an address that uniquely identifies it
 from a pseudowire perspective, in order to populate the PW Switching
 Point PE (SP-PE) TLV specified in [RFC6073].  For this purpose, at
 least one Attachment Identifier (AI) address of the format similar to
 AII Type 2 [RFC5003] composed of the Global ID, and Prefix part,
 only, MUST be assigned to each S-PE.
 If an S-PE is capable of dynamic MS-PW signaling but is not assigned
 with an S-PE address, then on receiving a dynamic MS-PW Label Mapping
 message the S-PE MUST return a Label Release with the "Resources
 Unavailable" (0x38) status code.

4. Dynamic Placement of MS-PWs

 [RFC6073] describes a procedure for concatenating multiple
 pseudowires together.  This procedure requires each S-PE to be
 manually configured with the information required for each segment of
 the MS-PW.  The procedures in the following sections describe a
 method to extend [RFC6073] by allowing the automatic selection of
 predefined S-PEs and dynamically establishing an MS-PW between two
 T-PEs.

4.1. Pseudowire Routing Procedures

 The AII Type 2 described above contains a Global ID, Prefix, and
 AC ID.  The TAII is used by S-PEs to determine the next SS-PW
 destination for LDP signaling.
 Once an S-PE receives an MS-PW Label Mapping message containing a
 TAII with an AII that is not locally present, the S-PE performs a
 lookup in a PW AII routing table.  If this lookup results in an IP
 address for the next-hop PE with reachability information for the AII
 in question, then the S-PE will initiate the necessary LDP messaging
 procedure to set up the next PW segment.  If the PW AII routing table
 lookup does not result in an IP address for a next-hop PE, the
 destination AII has become unreachable, and the PW setup MUST fail.
 In this case, the next PW segment is considered unprovisioned, and a
 Label Release MUST be returned to the T-PE with a status message of
 "AII Unreachable".
 If the TAII of an MS-PW Label Mapping message received by a PE
 contains the Prefix matching the locally provisioned prefix on that
 PE but an AC ID that is not provisioned, then the LDP liberal label
 retention procedures apply, and the Label Mapping message is
 retained.

Martini, et al. Standards Track [Page 8] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 To allow for dynamic end-to-end signaling of MS-PWs, information MUST
 be present in S-PEs to support the determination of the next PW
 signaling hop.  Such information can be provisioned (equivalent to a
 static route) on each S-PE, or disseminated via regular routing
 protocols (e.g., BGP).

4.1.1. AII PW Routing Table Lookup Aggregation Rules

 All PEs capable of dynamic MS-PW path selection MUST build a PW AII
 routing table to be used for PW next-hop selection.
 The PW addressing scheme (AII Type 2 as defined in [RFC5003])
 consists of a Global ID, a 32-bit Prefix, and a 32-bit Attachment
 Circuit ID.
 An aggregation scheme similar to that used for classless IPv4
 addresses can be employed.  A length mask (8 bits) is specified as a
 number ranging from 0 to 96 that indicates which Most Significant
 Bits (MSBs) are relevant in the address field when performing the PW
 address-matching algorithm.
                   0        31 32    63 64    95 (bits)
                  +-----------+--------+--------+
                  | Global ID | Prefix | AC ID  |
                  +-----------+--------+--------+
                    Figure 3: PW Addressing Scheme
 During the signaling phase, the content of the (fully qualified)
 TAII Type 2 field from the FEC 129 TLV is compared against routes
 from the PW routing table.  Similar to the IPv4 case, the route with
 the longest match is selected, determining the next signaling hop and
 implicitly the next PW segment to be signaled.

4.1.2. PW Static Route

 For the purpose of determining the next signaling hop for a segment
 of the pseudowire, the PEs MAY be provisioned with fixed-route
 entries in the PW next-hop routing table.  The static PW entries will
 follow all the addressing rules and aggregation rules described in
 the previous sections.  The most common use of PW static provisioned
 routes is this example of the "default" route entry as follows:
 Global ID = 0 Prefix = 0 AC ID = 0, Prefix Length = 0
 Next Signaling Hop = {IP Address of next-hop S-PE or T-PE}

Martini, et al. Standards Track [Page 9] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

4.1.3. Dynamic Advertisement with BGP

 Any suitable routing protocol capable of carrying external routing
 information MAY be used to propagate MS-PW path information among
 S-PEs and T-PEs.  However, T-PEs and S-PEs MAY choose to use the
 Border Gateway Protocol (BGP) [RFC4271] with the Multiprotocol
 Extensions as defined in [RFC4760] to propagate PW address
 information throughout the PSN.  PW address information is only
 propagated by PEs that are capable of PW switching.  Therefore, the
 multiprotocol BGP neighbor topology MUST coincide with the topology
 of T-PEs and S-PEs.
 Contrary to Layer 2 VPN signaling methods that use BGP for
 auto-discovery [RFC6074], in the case of the dynamically placed
 MS-PW, the source T-PE knows a priori (by provisioning) the AC ID on
 the terminating T-PE that signaling should use.  Hence, there is no
 need to advertise a "fully qualified" 96-bit address on a per-PW
 attachment circuit basis.  Only the T-PE Global ID, Prefix, and
 prefix length need to be advertised as part of well-known BGP
 procedures; see [RFC4760].
 Since PW Endpoints are provisioned in the T-PEs, the ST-PE will use
 this information to obtain the first S-PE hop (i.e., first BGP next
 hop) to where the first PW segment will be established.  Any
 subsequent S-PEs will use the same information (i.e., the next BGP
 next hop(s)) to obtain the next signaling hop(s) on the path to the
 TT-PE.
 The PW dynamic path Network Layer Reachability Information (NLRI) is
 advertised in BGP UPDATE messages using the MP_REACH_NLRI and
 MP_UNREACH_NLRI attributes [RFC4760].  The {AFI, SAFI} value pair
 used to identify this NLRI is (AFI=25, SAFI=6).  A route target MAY
 also be advertised along with the NLRI.
 The Next Hop field of the MP_REACH_NLRI attribute SHALL be
 interpreted as an IPv4 address whenever the length of the NextHop
 address is 4 octets, and as an IPv6 address whenever the length of
 the NextHop address is 16 octets.
 The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
 comprising an 8-octet Route Distinguisher, the Global ID, the Prefix,
 and the AC ID, and encoded as defined in Section 4 of [RFC4760].

Martini, et al. Standards Track [Page 10] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 This NLRI is structured as follows:
      Bit
      0     7 8             71 72      103 104  135 136   167
      +------+----------------+-----------+--------+--------+
      |Length|  Route Dist    | Global ID | Prefix | AC ID  |
      +------+----------------+-----------+--------+--------+
                    Figure 4: NLRI Field Structure
 The Length field is the prefix length of the Route Distinguisher +
 Global ID + Prefix + AC ID in bits.
 Except for the default PW route, which is encoded as a 0-length
 Prefix, the minimum value of the Length field is 96 bits.  Lengths of
 128 bits to 159 bits are invalid, as the AC ID field cannot be
 aggregated.  The maximum value of the Length field is 160 bits.  BGP
 advertisements received with invalid Prefix lengths MUST be rejected
 as having a bad packet format.

4.2. LDP Signaling

 The LDP signaling procedures are described in [RFC4447] and expanded
 in [RFC6073].  No new LDP signaling components are required for
 setting up a dynamically placed MS-PW.  However, some optional
 signaling extensions are described below.
 One of the requirements that MUST be met in order to achieve the QoS
 objectives for a PW on a segment is that a PSN tunnel MUST be
 selected that can support at least the required class of service and
 that has sufficient bandwidth available.
 Such PSN tunnel selection can be achieved where the next hop for a PW
 segment is explicitly configured at each PE, whether the PE is a T-PE
 or an S-PE in the case of a segmented PW, without dynamic path
 selection (as per [RFC6073]).  In these cases, it is possible to
 explicitly configure the bandwidth required for a PW so that the T-PE
 or S-PE can reserve that bandwidth on the PSN tunnel.

Martini, et al. Standards Track [Page 11] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 Where dynamic path selection is used and the next hop is therefore
 not explicitly configured by the operator at the S-PE, a mechanism to
 signal the bandwidth for the PW from the T-PE to the S-PEs is
 required.  This is accomplished by including an optional PW Bandwidth
 TLV.  The PW Bandwidth TLV is specified 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1|0|     PW BW TLV  (0x096E)   |          TLV  Length          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Forward SENDER_TSPEC                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Reverse SENDER_TSPEC                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 5: PW Bandwidth TLV Structure
 The PW Bandwidth TLV fields are as follows:
  1. TLV Length: The length of the value fields in octets. Value = 64.
  1. Forward SENDER_TSPEC = the SENDER_TSPEC for the forward direction

of the PW, as defined in Section 3.1 of [RFC2210].

  1. Reverse SENDER_TSPEC = the SENDER_TSPEC for the reverse direction

of the PW, as defined in Section 3.1 of [RFC2210].

 The complete definitions of the content of the SENDER_TSPEC objects
 are found in Section 3.1 of [RFC2210].  The forward SENDER_TSPEC
 refers to the data path in the direction ST-PE to TT-PE.  The reverse
 SENDER_TSPEC refers to the data path in the direction TT-PE to ST-PE.
 In the forward direction, after a next-hop selection is determined, a
 T/S-PE SHOULD reference the forward SENDER_TSPEC object to determine
 an appropriate PSN tunnel towards the next signaling hop.  If such a
 tunnel exists, the MS-PW signaling procedures are invoked with the
 inclusion of the PW Bandwidth TLV.  When the PE searches for a PSN
 tunnel, any tunnel that points to a next hop equivalent to the next
 hop selected will be included in the search (the LDP address TLV is
 used to determine the next-hop equivalence).
 When an S/T-PE receives a PW Bandwidth TLV, once the PW next hop is
 selected, the S/T-PE MUST request the appropriate resources from the
 PSN.  The resources described in the reverse SENDER_TSPEC are
 allocated from the PSN toward the originator of the message or

Martini, et al. Standards Track [Page 12] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 previous hop.  When resources are allocated from the PSN for a
 specific PW, the allocation SHOULD account for the resource usage of
 the PW.
 In the case where PSN resources towards the previous hop are not
 available, the following procedure MUST be followed:
   i. The PSN MAY allocate more QoS resources, e.g., bandwidth, to the
      PSN tunnel.
  ii. The S-PE MAY attempt to set up another PSN tunnel to accommodate
      the new PW QoS requirements.
 iii. If the S-PE cannot get enough resources to set up the segment in
      the MS-PW, a Label Release MUST be returned to the previous hop
      with a status message of "Bandwidth resources unavailable".
 In the latter case, the T-PE receiving the status message MUST also
 withdraw the corresponding PW Label Mapping message for the opposite
 direction if it has already been successfully set up.
 If an ST-PE receives a Label Mapping message, the following procedure
 MUST be followed:
 If the ST-PE has already sent a Label Mapping message for this PW,
 then the ST-PE MUST check to see if this Label Mapping message
 originated from the same LDP peer to which the corresponding Label
 Mapping message for this particular PW was sent.  If it is the same
 peer, the PW is established.  If it is a different peer, then the
 ST-PE MUST send a Label Release message with a status code of "PW
 Loop Detected" to the PE that originated the LDP Label Mapping
 message.
 If the PE has not yet sent a Label Mapping message for this
 particular PW, then it MUST send the Label Mapping message to this
 LDP peer, regardless of what the PW TAII routing lookup result is.

4.2.1. Multiple Alternative Paths in PW Routing

 A next-hop selection for a specific PW may find a match with a PW
 route that has multiple next hops associated with it.  Multiple next
 hops may be either configured explicitly as static routes or learned
 through BGP routing procedures.  Implementations at an S-PE or T-PE
 MAY use selection algorithms, such as CRC32 on the FEC TLV or flow-
 aware transport of PWs [RFC6391], for load balancing of PWs across
 multiple next hops, so that each PW has a single next hop.  The
 details of such selection algorithms are outside the scope of this
 document.

Martini, et al. Standards Track [Page 13] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

4.2.2. Active/Passive T-PE Election Procedure

 When an MS-PW is signaled, each T-PE might independently initiate
 signaling the MS-PW.  This could result in a different path being
 used by each direction of the PW.  To avoid this situation, one T-PE
 MUST initiate PW signaling (i.e., take an active role), while the
 other T-PE waits to receive the LDP Label Mapping message before
 sending the LDP Label Mapping message for the reverse direction of
 the PW (i.e., take a passive role).  The active T-PE (the ST-PE) and
 the passive T-PE (the TT-PE) MUST be identified before signaling
 begins for a given MS-PW.  Both T-PEs MUST use the same method for
 identifying which is active and which is passive.
 A T-PE SHOULD determine whether it assumes the active role or the
 passive role using procedures similar to those of [RFC5036],
 Section 2.5.2, Bullet 2.  The T-PE compares the Source Attachment
 Individual Identifier (SAII) [RFC6074] with the Target Attachment
 Individual Identifier (TAII) [RFC6074] as unsigned integers, and if
 the SAII > TAII, the T-PE assumes the active role.  Otherwise, it
 assumes the passive role.
 The following procedure for comparing the SAII and TAII as unsigned
 integers SHOULD be used:
  1. If the SAII Global ID > TAII Global ID, then the T-PE is active
  1. else if the SAII Global ID < TAII Global ID, then the T-PE is

passive

  1. else if the SAII Prefix > TAII Prefix, then the T-PE is active
  1. else if the SAII Prefix < TAII Prefix, then the T-PE is

passive

  1. else if the SAII AC ID > TAII AC ID, then the T-PE is

active

  1. else if the SAII AC ID < TAII AC ID, then the T-PE is

passive

  1. else there is a configuration error

Martini, et al. Standards Track [Page 14] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

4.2.3. Detailed Signaling Procedures

 On receiving a Label Mapping message, the S-PE MUST inspect the FEC
 TLV.  If the receiving node has no local AII matching the TAII for
 that Label Mapping message, then the Label Mapping message SHOULD be
 forwarded on to another S-PE or T-PE.  The S-PE will check to see if
 the FEC is already installed for the forward direction:
  1. If the FEC is already installed and the received Label Mapping was

received from the same LDP peer to which the forward LDP Label

   Mapping was sent, then this Label Mapping represents signaling in
   the reverse direction for this MS-PW segment.
  1. If the FEC is already installed and the received Label Mapping was

received from a different LDP peer to which the forward LDP Label

   Mapping was sent, then the received Label Mapping MUST be released
   with a status code of "PW Loop Detected".
  1. If the FEC is not already installed, then this represents signaling

in the forward direction.

 The following procedures are then executed, depending on whether the
 Label Mapping was determined to be for the forward or the reverse
 direction of the MS-PW.
 For the forward direction:
    i. Determine the next-hop S-PE or T-PE according to the procedures
       above.  If next-hop reachability is not found in the S-PE's PW
       AII routing table, then a Label Release MUST be sent with
       status code "AII Unreachable".  If the next-hop S-PE or T-PE is
       found and is the same LDP peer that sent the Label Mapping
       message, then a Label Release MUST be returned with status code
       "PW Loop Detected".  If the SAII in the received Label Mapping
       is local to the S-PE, then a Label Release MUST be returned
       with status code "PW Loop Detected".
   ii. Check to see if a PSN tunnel exists to the next-hop S-PE or
       T-PE.  If no tunnel exists to the next-hop S-PE or T-PE, the
       S-PE MAY attempt to set up a PSN tunnel.
  iii. Check to see if a PSN tunnel exists to the previous hop.  If no
       tunnel exists to the previous-hop S-PE or T-PE, the S-PE MAY
       attempt to set up a PSN tunnel.

Martini, et al. Standards Track [Page 15] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

   iv. If the S-PE cannot get enough PSN resources to set up the
       segment to the next-hop or previous-hop S-PE or T-PE, a Label
       Release MUST be returned to the T-PE with a status message of
       "Resources Unavailable".
    v. If the Label Mapping message contains a Bandwidth TLV, allocate
       the required resources on the PSN tunnels in the forward and
       reverse directions according to the procedures above.
   vi. Allocate a new PW label for the forward direction.
  vii. Install the FEC for the forward direction.
 viii. Send the Label Mapping message with the new forward label and
       the FEC to the next-hop S-PE/T-PE.
 For the reverse direction:
   i. Install the FEC received in the Label Mapping message for the
      reverse direction.
  ii. Determine the next signaling hop by referencing the LDP sessions
      used to set up the PW in the forward direction.
 iii. Allocate a new PW label for the next hop in the reverse
      direction.
  iv. Install the FEC for the next hop in the reverse direction.
   v. Send the Label Mapping message with a new label and the FEC to
      the next-hop S-PE/ST-PE.

5. Procedures for Failure Handling

5.1. PSN Failures

 Failures of the PSN tunnel MUST be handled by PSN mechanisms.  An
 example of such a PSN mechanism is MPLS fast reroute [RFC4090].  If
 the PSN is unable to re-establish the PSN tunnel, then the S-PE
 SHOULD follow the procedures defined in Section 10 of [RFC6073].

Martini, et al. Standards Track [Page 16] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

5.2. S-PE Failures

 For defects in an S-PE, the procedures defined in [RFC6073] SHOULD be
 followed.  A T-PE or S-PE may receive an unsolicited Label Release
 message from another S-PE or T-PE with various failure codes, such as
 "Loop Detected", "PW Loop Detected", "Resources Unavailable", "Bad
 Strict Node Error", or "AII Unreachable".  All these failure codes
 indicate a generic class of PW failures at an S-PE or T-PE.
 If an unsolicited Label Release message with such a failure status
 code is received at a T-PE, then it is RECOMMENDED that the T-PE
 attempt to re-establish the PW immediately.  However, the T-PE MUST
 throttle its PW setup message retry attempts with an exponential
 backoff in situations where PW setup messages are being constantly
 released.  It is also RECOMMENDED that a T-PE detecting such a
 situation take action to notify an operator.
 S-PEs that receive an unsolicited Label Release message with a
 failure status code SHOULD follow this procedure:
 i. If the Label Release is received from an S-PE or T-PE in the
    forward or reverse signaling direction, then the S-PE MUST tear
    down both segments of the PW.  The status code received in the
    Label Release message SHOULD be propagated when sending the Label
    Release for the next segment.

5.3. PW Reachability Changes

 In general, an established MS-PW will not be affected by next-hop
 changes in AII reachability information.
 If there is a next-hop change in AII reachability information in the
 forward direction, the T-PE MAY elect to tear down the MS-PW by
 sending a Label Withdraw message to the downstream S-PE or T-PE.  The
 teardown MUST also be accompanied by an unsolicited Label Release
 message and will be followed by an attempt by the T-PE to
 re-establish the MS-PW.
 If there is a change in the AII reachability information in the
 forward direction at an S-PE, the S-PE MAY elect to tear down the
 MS-PW in both directions.  A label withdrawal is sent in each
 direction followed by an unsolicited Label Release.  The unsolicited
 Label Release messages MUST be accompanied by the status code "AII
 Unreachable".  This procedure is OPTIONAL.  Note that this procedure
 is likely to be disruptive to the emulated service.  PW Redundancy
 [RFC6718] MAY be used to maintain the connectivity used by the
 emulated service in the case of a failure of the PSN or S-PE.

Martini, et al. Standards Track [Page 17] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 A change in AII reachability information in the reverse direction has
 no effect on an MS-PW.

6. Operations, Administration, and Maintenance (OAM)

 The OAM procedures defined in [RFC6073] may also be used for
 dynamically placed MS-PWs.  A PW Switching Point PE TLV [RFC6073] is
 used to record the switching points that the PW traverses.
 In the case of an MS-PW where the PW Endpoints are identified by
 using globally unique AII addresses based on FEC 129, there is no
 pseudowire identifier (PWid) defined on a per-segment basis.  Each
 individual PW segment is identified by the address of the adjacent
 S-PE(s) in conjunction with the SAII and TAII.
 In this case, the following TLV type (0x06) MUST be used in place of
 type 0x01 in the PW Switching Point PE TLV:
    Type      Length    Description
    ----      ------    -----------------------------------
    0x06        14      L2 PW address of PW Switching Point
 The above sub-TLV MUST be included in the PW Switching Point PE TLV
 once per individual PW switching point, following the same rules and
 procedures as those described in [RFC6073].  A more detailed
 description of this sub-TLV is also given in Section 7.4.1 of
 [RFC6073].  However, the length value MUST be set to 14 ([RFC6073]
 states that the length value is 12, but this does not correctly
 represent the actual length of the TLV).

7. Security Considerations

 This document specifies extensions to the protocols already defined
 in [RFC4447] and [RFC6073].  The extensions defined in this document
 do not affect the security considerations for those protocols, but
 [RFC4447] and [RFC6073] do impose a set of security considerations
 that are applicable to the protocol extensions specified in this
 document.
 It should be noted that the dynamic path selection mechanisms
 specified in this document enable the network to automatically select
 the S-PEs that are used to forward packets on the MS-PW.  Appropriate
 tools, such as the Virtual Circuit Connectivity Verification (VCCV)
 trace mechanisms specified in [RFC6073], can be used by an operator
 of the network to verify the path taken by the MS-PW and therefore be
 satisfied that the path does not represent an additional security
 risk.

Martini, et al. Standards Track [Page 18] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 Note that the PW control protocol may be used to establish and
 maintain an MS-PW across administrative boundaries.  Section 13 of
 [RFC6073] specifies security considerations applicable to LDP used in
 this manner, including considerations for establishing the integrity
 of, and authenticating, LDP control messages.  These considerations
 also apply to the protocol extensions specified in this document.
 Note that the protocols for dynamically distributing AII reachability
 information may have their own security considerations.  However,
 those protocol specifications are outside the scope of this document.

8. IANA Considerations

8.1. Correction

 IANA has corrected a minor error in the "Pseudowire Switching Point
 PE sub-TLV Type" registry.  The entry 0x06 "L2 PW address of the PW
 Switching Point" has been corrected to Length 14 and the reference
 changed to [RFC6073] and this document as follows:
 Type  Length  Description                          Reference
 ----  ------  -----------------------------------  ------------------
 0x06    14    L2 PW Address of PW Switching Point  [RFC6073][RFC7267]

8.2. LDP TLV Type Name Space

 This document defines one new LDP TLV type.  IANA already maintains a
 registry for LDP TLV types, called the "TLV Type Name Space"
 registry, within the "Label Distribution Protocol (LDP) Parameters"
 registry as defined by [RFC5036].  IANA has assigned the following
 value.
   Value    Description     Reference       Notes/Registration Date
   ------   -------------   -------------   -----------------------
   0x096E   Bandwidth TLV   This document

Martini, et al. Standards Track [Page 19] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

8.3. LDP Status Codes

 This document defines three new LDP status codes.  IANA maintains a
 registry of these codes, called the "Status Code Name Space"
 registry, in the "Label Distribution Protocol (LDP) Parameters"
 registry as defined by [RFC5036].  The IANA has assigned the
 following values.
 Range/Value    E     Description                       Reference
 -----------  -----   -------------------------------   -------------
 0x00000037     0     Bandwidth resources unavailable   This document
 0x00000038     0     Resources Unavailable             This document
 0x00000039     0     AII Unreachable                   This document

8.4. BGP SAFI

 IANA has allocated a new BGP SAFI for "Network Layer Reachability
 Information used for Dynamic Placement of Multi-Segment Pseudowires"
 in the IANA "SAFI Values" registry [RFC4760] within the "Subsequent
 Address Family Identifiers (SAFI) Parameters" registry.  The IANA has
 assigned the following value.
 Value   Description                                   Reference
 -----   -------------------------------------------   -------------
 6       Network Layer Reachability Information        This document
         used for Dynamic Placement of Multi-Segment
         Pseudowires

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.
 [RFC2210]  Wroclawski, J., "The Use of RSVP with IETF Integrated
            Services", RFC 2210, September 1997.
 [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
            G. Heron, "Pseudowire Setup and Maintenance Using the
            Label Distribution Protocol (LDP)", RFC 4447, April 2006.
 [RFC5003]  Metz, C., Martini, L., Balus, F., and J. Sugimoto,
            "Attachment Individual Identifier (AII) Types for
            Aggregation", RFC 5003, September 2007.

Martini, et al. Standards Track [Page 20] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
            "LDP Specification", RFC 5036, October 2007.
 [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
            Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.

9.2. Informative References

 [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
            Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
            May 2005.
 [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
            Border Gateway Protocol 4 (BGP-4)", RFC 4271,
            January 2006.
 [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
            "Multiprotocol Extensions for BGP-4", RFC 4760,
            January 2007.
 [RFC5254]  Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
            "Requirements for Multi-Segment Pseudowire Emulation Edge-
            to-Edge (PWE3)", RFC 5254, October 2008.
 [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
            Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
            October 2009.
 [RFC6074]  Rosen, E., Davie, B., Radoaca, V., and W. Luo,
            "Provisioning, Auto-Discovery, and Signaling in Layer 2
            Virtual Private Networks (L2VPNs)", RFC 6074,
            January 2011.
 [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
            Regan, J., and S. Amante, "Flow-Aware Transport of
            Pseudowires over an MPLS Packet Switched Network",
            RFC 6391, November 2011.
 [RFC6718]  Muley, P., Aissaoui, M., and M. Bocci, "Pseudowire
            Redundancy", RFC 6718, August 2012.

Martini, et al. Standards Track [Page 21] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

10. Contributors

 The editors gratefully acknowledge the following people for their
 contributions to this document:
 Nabil Bitar
 Verizon
 40 Sylvan Road
 Waltham, MA  02145
 US
 EMail: nabil.bitar@verizon.com
 Himanshu Shah
 Ciena Corp.
 35 Nagog Park
 Acton, MA  01720
 US
 EMail: hshah@ciena.com
 Mustapha Aissaoui
 Alcatel-Lucent
 600 March Road
 Kanata
 ON, Canada
 EMail: mustapha.aissaoui@alcatel-lucent.com
 Jason Rusmisel
 Alcatel-Lucent
 600 March Road
 Kanata
 ON, Canada
 EMail: Jason.rusmisel@alcatel-lucent.com
 Andrew G. Malis
 Huawei
 2330 Central Expressway
 Santa Clara, CA  95050
 US
 EMail: agmalis@gmail.com

Martini, et al. Standards Track [Page 22] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

 Chris Metz
 Cisco Systems, Inc.
 3700 Cisco Way
 San Jose, CA  95134
 US
 EMail: chmetz@cisco.com
 David McDysan
 Verizon
 22001 Loudoun County Pkwy.
 Ashburn, VA  20147
 US
 EMail: dave.mcdysan@verizon.com
 Jeff Sugimoto
 Alcatel-Lucent
 701 E. Middlefield Rd.
 Mountain View, CA  94043
 US
 EMail: jeffery.sugimoto@alcatel-lucent.com
 Mike Loomis
 Alcatel-Lucent
 701 E. Middlefield Rd.
 Mountain View, CA  94043
 US
 EMail: mike.loomis@alcatel-lucent.com

11. Acknowledgements

 The editors also gratefully acknowledge the input of the following
 people:  Paul Doolan, Mike Duckett, Pranjal Dutta, Ping Pan, Prayson
 Pate, Vasile Radoaca, Yeongil Seo, Yetik Serbest, and Yuichiro Wada.

Martini, et al. Standards Track [Page 23] RFC 7267 Dynamic Placement of Multi-Segment PWs June 2014

Authors' Addresses

 Luca Martini (editor)
 Cisco Systems, Inc.
 9155 East Nichols Avenue, Suite 400
 Englewood, CO  80112
 US
 EMail: lmartini@cisco.com
 Matthew Bocci (editor)
 Alcatel-Lucent
 Voyager Place
 Shoppenhangers Road
 Maidenhead
 Berks, UK
 EMail: matthew.bocci@alcatel-lucent.com
 Florin Balus (editor)
 Alcatel-Lucent
 701 E. Middlefield Rd.
 Mountain View, CA  94043
 US
 EMail: florin@nuagenetworks.net

Martini, et al. Standards Track [Page 24]

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