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Network Working Group                                 Luca Martini (Ed.)
Internet Draft                                        Cisco Systems Inc.
Expiration Date: December 2013
Intended status: Standards Track                     Matthew Bocci (Ed.)
                                                      Florin Balus (Ed.)
                                                          Alcatel-Lucent

                                                           June 19, 2013


             Dynamic Placement of Multi Segment Pseudowires


                  draft-ietf-pwe3-dynamic-ms-pw-17.txt

Status of this Memo

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

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   This Internet-Draft will expire on December 19, 2013

Abstract

   There is a requirement for service providers to be able to extend the
   reach of pseudowires (PW) 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.




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Table of Contents

    1        Major Co-authors  .....................................   3
    2        Acknowledgements  .....................................   3
    3        Introduction  .........................................   3
    3.1      Scope  ................................................   3
    3.2      Specification of Requirements  ........................   3
    3.3      Terminology  ..........................................   4
    3.4      Architecture Overview  ................................   4
    4        Applicability  ........................................   6
    4.1      Changes to Existing PW Signaling  .....................   6
    5        PW Layer 2 Addressing  ................................   6
    5.1      Attachment Circuit Addressing  ........................   6
    5.2      S-PE Addressing  ......................................   7
    6        Dynamic Placement of MS-PWs  ..........................   7
    6.1      Pseudowire Routing Procedures  ........................   8
    6.1.1    AII PW Routing Table Lookup Aggregation Rules  ........   8
    6.1.2    PW Static Route  ......................................   9
    6.1.3    Dynamic Advertisement with BGP  .......................   9
    6.2      LDP Signaling  ........................................  10
    6.2.1    MS-PW Bandwidth Signaling  ............................  10
    6.2.2    Equal Cost Multi Path (ECMP) in PW Routing  ...........  12
    6.2.3    Active/Passive T-PE Election Procedure  ...............  12
    6.2.4    Detailed Signaling Procedures  ........................  13
    7        Failure Handling Procedures  ..........................  14
    7.1      PSN Failures  .........................................  14
    7.2      S-PE Specfic Failures  ................................  14
    7.3      PW Reachability Changes  ..............................  15
    8        Operations and Maintenance (OAM)  .....................  16
    9        Security Considerations  ..............................  16
   10        IANA Considerations  ..................................  16
   10.1      LDP TLV TYPE NAME SPACE  ..............................  17
   10.2      LDP Status Codes  .....................................  17
   10.3      BGP SAFI  .............................................  17
   11        Normative References  .................................  17
   12        Informative References  ...............................  18
   13        Author's Addresses  ...................................  18












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1. Major Co-authors

   The editors gratefully acknowledge the following additional co-
   authors:  Mustapha Aissaoui, Nabil Bitar, Mike Loomis, David McDysan,
   Chris Metz, Andy Malis, Jason Rusmeisel, Himanshu Shah, Jeff
   Sugimoto.


2. Acknowledgements

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


3. Introduction

3.1. Scope

   [RFC5254] describes the service provider requirements for extending
   the reach of pseudowires across multiple 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 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 proposed
   procedures follow the guidelines defined in [RFC5036] and enable the
   reuse of existing TLVs,
    and procedures defined for SS-PWs in [RFC4447].


3.2. 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.







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3.3. Terminology

   [RFC5659] provides terminology for multi-segment pseudowires.

   This document defines the following additional terms:

     - Source Terminating PE (ST-PE). A Terminating PE (T-PE), which
       assumes the active signaling role and initiates the signaling for
       multi-segment PW.
     - Target Terminating PE (TT-PE). A Terminating PE (T-PE) that
       assumes the passive signaling role. It waits and responds to the
       multi-segment PW signaling message in the reverse direction.
     - Forward Direction: ST-PE to TT-PE.
     - Reverse Direction: TT-PE to ST-PE
     - Forwarding Direction: Direction of control plane, signaling flow
     - Pseudowire Routing (PW routing). The dynamic placement of the
       segments that compose an MS-PW, as well as the automatic
       selection of S-PEs.



3.4. Architecture Overview

   The following figure describes the reference models which are derived
   from [RFC5659] to support PW emulated services across multi-segment
   PWs.

























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       Native   |<-------------Pseudowire------------>|  Native
       Service  |                                     |  Service
        (AC)    |     |<-PSN1-->|     |<-PSN2-->|     |   (AC)
          |     V     V         V     V         V     V     |
          |     +-----+         +-----+         +-----+
   +----+ |     |T-PE1|=========|S-PE1|=========|T-PE2|     |    +----+
   |    |-------|.....PW.Seg't1........PW Seg't3......|----------|    |
   | CE1| |     |     |         |     |         |     |     |    |CE2 |
   |    |-------|.....PW.Seg't2.......|PW Seg't4......|----------|    |
   +----+ |     |     |=========|     |=========|     |     |    +----+
        ^       +-----+         +-----+         +-----+          ^
        |   Provider Edge 1        ^        Provider Edge 2      |
        |                          |                             |
        |                          |                             |
        |                  PW switching point                    |
        |                                                        |
        |<---------------- Emulated Service -------------------->|


                 Figure 1: PW switching Reference Model


   Figure 1 shows the architecture for a simple multi-segment case. T-
   PE1 and T-PE2 provide an emulated service to CE1 and CE2. These PEs
   reside in different PSNs. A PSN tunnel extends from T-PE1 to S-PE1
   across PSN1, and a second PSN tunnel extends from S-PE1 to T-PE2
   across PSN2. PWs are used to connect the attachment circuits (ACs)
   attached to T-PE1 to the corresponding AC attached to T-PE2. A PW on
   the tunnel across PSN1 is connected to a PW in the tunnel across PSN2
   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 (e.g., PSN1 and PSN2) can be the same or different
   technology. An S-PE switches an MS-PW from one segment to another
   based on the PW identifiers. ( PWid , or AII ) How the PW PDUs are
   switched at the S-PE depends on the PSN tunnel technology: in case of
   an MPLS PSN to another MPLS PSN PW switching the operation is a
   standard MPLS label switch operation.

   Note that although Figure 1 only shows a single S-PE, a PW may
   transit more one S-PE along its path. For instance, in the multi-
   provider case, there can be an S-PE at the border of one provider
   domain and another S-PE at the border of the other provider domain.




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4. Applicability

   In this document we describe the case where the PSNs carrying the
   SS-PW are only MPLS PSNs using the generalized FEC 129. Interactions
   with an IP PSN using L2TPv3 as described in [RFC6073] section 7.4 are
   left for further study.



4.1. 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 TLVs are also defined:
     - A Bandwidth TLV to address QoS Signaling requirements (see "MS-PW
       Next Hop Bandwidth Signaling" section for details).


5. 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 identifiers for the
   attachment circuits 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 composing an MS-PW.


5.1. Attachment Circuit Addressing

   The attachment circuit addressing is derived from [RFC5003] AII type
   2 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 (contd.)      |        Prefix                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Prefix (contd.)         |        AC ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      AC ID                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   AII type 2 based addressing schemes permit varying levels of AII



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   summarization, thus reducing the scaling burden on PW routing. AII
   Type 2 based PW addressing is suitable for point-to-point
   provisioning models where it is not required to auto-discover address
   at Target T-PE (knows the address in 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 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
   interconnected PW domains.


5.2. S-PE Addressing

   Each S-PE MUST be uniquely addressable in terms of pseudowires in
   order to populate the switching point PE TLV specified in [RFC6073].
   For this purpose, at least one 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
   "LDP_RESOURCES_UNAVAILABLE" ( 0x38)" status code.


6. 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 pre-
   defined S-PEs, and dynamically establishing a MS-PW between two T-
   PEs.






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6.1. 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 a 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 a 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 un-provisioned, and a
   label release MUST be returned to the T-PE with a status message of
   "AII Unreachable".

   If the TAI of a MS-PW label mapping message received by a PE contains
   the prefix matching a 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.

   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).


6.1.1. 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 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. An (8 bits) length mask is specified as a
   number ranging from 0 to 96 that indicates which Most Significant
   Bits (MSB) 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  |
   +-----------+--------+--------+



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   During the signaling phase, the content of the (fully qualified) TAII
   type 2 field from the FEC129 TLV is compared against routes from the
   PW Routing table. Similar with 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.


6.1.2. 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 = S-PE1


6.1.3. 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-PE, and S-PEs, MAY choose to use Border
   Gateway Protocol (BGP) [RFC4760] to propagate PW address information
   throughout the PSN.

   Contrary to other l2vpn signaling methods that use BGP [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 to use in
   signaling. 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 needs to be advertised as part
   of well known BGP procedures - see [RFC4760].

   As 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 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
   (pending IANA allocation)). A route target MAY also be advertised
   along with the NLRI.




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   The Next Hop field of MP_REACH_NLRI attribute SHALL be interpreted as
   an IPv4 address, whenever the length of the NextHop address is 4
   octets, and as a 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].

   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  |
    +------+----------------+-----------+--------+--------+


   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.


6.2. 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.


6.2.1. MS-PW Bandwidth Signaling

   In order to enable the QoS objectives for a PW to be met on a
   segment, a PSN tunnel MUST be selected that can support at least the
   required class of service and that has sufficient bandwidth
   available.




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   PW QoS objectives can thus be met 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.

   Where dynamic path selection is used and therefore the next-hop is
   not explicitly configured by the operator at the S-PE, a mechanism is
   required to signal the bandwidth for the PW from the T-PE to the S-
   PEs. 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                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The complete definitions of the content of the SENDER_TSPEC objects
   are found in [TSPEC] section 3.1. The forward SENDER_TSPEC refers to
   the data path in the direction of 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 which 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
   previous hop. When resources are allocated from the PSN for a
   specific PW, then the PSN SHOULD account for the PW usage of the
   resources.

   In the case where PSN resources towards the previous hop are not
   available the following procedure MUST be followed:



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        -i. The PSN MAY allocate more QoS resources, e.g. Bandwidth, to
            the PSN tunnel.
       -ii. The S-PE MAY attempt to setup another PSN tunnel to
            accommodate the new PW QoS requirements.
      -iii. If the S-PE cannot get enough resources to setup 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 for the opposite
   direction if it has already been successfully setup.

   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 that 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 ST-PE MUST send a
   label release message, with a status code of "Duplicate AII" to the
   PE that originate 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
   same LDP peer, regardless of what the PW TAII routing lookup result
   is.


6.2.2. Equal Cost Multi Path (ECMP) in PW Routing

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


6.2.3. Active/Passive T-PE Election Procedure

   When a MS-PW is signaled, each T-PE might independently initiate
   signaling the MS-PW. This could result in a different path being used
   be each direction of the PW. To avoid this situation one of the T-PE
   MUST start the PW signaling (active role), while the other T-PE waits
   to receive the LDP label mapping message before sending the LDP label



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   mapping message for the reverse direction of the PW (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.

   The determination of which T-PE assume the active role SHOULD be done
   as follows: the SAII and TAII are compared as unsigned integers, if
   the SAII is bigger then the T-PE assumes the active role.



6.2.4. 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 then the finagling should be forwarded on to
   another S-PE or T-PE. The S-PE will check if the FEC is already
   installed for the forward direction:
     - If it is already installed, and the received mapping was received
       from the same LDP peer where the forward LDP label mapping was
       sent, then this label mapping represents signaling in the reverse
       direction for this MS-PW segment.
     - If it is already installed, and the received mapping was received
       from a different LDP peer where the forward LDP label mapping was
       sent, then the received label mapping MUST be released with
       status code as "PW_LOOP_DETECTED". If the already installed PW
       segment has not signaled explicit intent for active role then
       installed PW segment MUST be released with status code
       "PW_LOOP_DETECTED".
     - If the FEC is not already installed, then this represents
       signaling in the forward direction.

   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 PW AII routing table in the S-PE then 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 has sent
            the label mapping message then a label Release MUST be
            returned with the status code "PW_LOOP_DETECTED". If the
            SAII in the received label mapping is local to the S-PE then
            a label released MUST be returned with status code
            "PW_LOOP_DETECTED".
       -ii. Check that 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 setup a PSN tunnel.






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      -iii. Check that 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 setup a PSN tunnel.
       -iv. If the S-PE cannot get enough PSN resources to setup the
            segment to the next or previous 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 received FEC for the reverse direction.
       -ii. Determine the next signaling hop by referencing the LDP
            sessions used to setup the PW in the Forward direction.
      -iii. Allocate a new PW label for the reverse direction.
       -iv. Install the FEC for the reverse direction.
        -v. Send the label mapping message with a new label and the FEC
            to the next hop S-PE/ST-PE.


7. Failure Handling Procedures

7.1. PSN Failures

   Failures of the PSN tunnel MUST be handled by PSN mechanisms. If the
   PSN is unable to re-establish the PSN tunnel, then the S-PE SHOULD
   follow the procedures defined in Section 8 of [RFC6073].


7.2. S-PE Specfic 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
   "LOOP_DETECTED", "PW_LOOP_DETECTED", "RESOURCE_UNAVAILBALE",
   "BAD_STRICT_HOP", "AII_UNREACHABLE" etc. All these failure codes
   indicate a generic class of PW failures at an S-PE or T-PE.

   When an unsolicited label release message with such a failure status
   code is received at T-PE then the T-PE MUST re-attempt to establish
   the PW immediately. However the T-PE MUST throttle its PW setup
   message retry attempts with an exponential backoff in situations



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   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 the following procedures:

        -i. If label release is received from an S-PE or T-PE in the
            forward signaling direction then S-PE MUST tear down both
            segments of the PW. The status code received in label
            release SHOULD be propagated while sending label release for
            the next-segment.
       -ii. If the label release is received from an S-PE or T-PE in the
            reverse Signaling direction do as follows:

            If the PW is set-up at S-PE with an Explicit Intent of Role
            then label release MUST be sent to the next PW segment with
            same status code. The forward signaling path SHOULD NOT be
            tear down in such case.

            If the PW is set-up at S-PE without an Explicit Intent of
            Role then tear down both segments of the PW as described in
            i.


7.3. PW Reachability Changes

   In general an established MS-PW will not be affected by next-hop
   changes in L2 PW reachability information.

   If there is a change in next-hop of the L2 PW reachability
   information in the forward direction, the T-PE MAY elect to tear down
   the MS-PW by sending a label withdraw message to downstream S-PE or
   T-PE. The teardown MUST be also accompanied by a unsolicited label
   release message, and will be followed by and attempt to re-establish
   of the MS-PW by T-PE.

   If there is a change in the L2 PW reachability information in the
   forward direction at S-PE, the S-PE MAY elect to tear down the MS-PW
   in both directions. A label withdrawal is sent on each direction
   followed by a unsolicited label release. The unsolicited label
   releases MUST be accompanied by the Status code "AII_UNREACHABLE".
   This procedure is OPTIONAL.

   A change in L2 reachability information in the reverse direction has
   no effect on an MS-PW.





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8. Operations and Maintenance (OAM)

   The OAM procedures defined in [RFC6073] may be used also for MS-PWs.
   A PW switching point TLV is used [RFC6073] to record the switching
   points that the PW traverses.

   In the case of a MS-PW where the PW Endpoints are identified though
   using a globally unique, FEC 129-based AII addresses, there is no
   PWID defined on a per segment basis. Each individual PW segment is
   identified by the address of adjacent S-PE(s) in conjunction with the
   SAI and TAI. In this case, the following type MUST be used in place
   of type 0x01 in the PW switching point TLV:

   Type      Length    Description
   0x06        14      L2 PW address of PW Switching Point


   The above field MUST be included together with type 0x02 in the TLV
   once per individual PW Switching Point following the same rules and
   procedures as described in [RFC6073]. A more detailed description of
   this field is also in setion 7.4.1 of [RFC6073]


9. Security Considerations

   This document specifies only extensions to the protocols already
   defined in [RFC4447], and [RFC6073]. Each such protocol may have its
   own set of security issues, but those issues are not affected by the
   extensions specified herein. Note that the protocols for dynamically
   distributing PW Layer 2 reachability information may have their own
   security issues, however those protocols specifications are outside
   the scope of this document.



10. IANA Considerations

   IANA needs to correct a minor error in the registry "Pseudowire
   Switching Point PE sub-TLV Type". The entry 0x06 "L2 PW address of
   the PW Switching Point" should have Length 14.











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10.1. LDP TLV TYPE NAME SPACE

   This document uses several new LDP TLV types, IANA already maintains
   a registry of name "TLV TYPE NAME SPACE" defined by RFC5036. The
   following values are suggested for assignment:

      TLV type  Description
       0x096E   Bandwidth TLV


10.2. LDP Status Codes

   This document uses several new LDP status codes, IANA already
   maintains a registry of name "STATUS CODE NAME SPACE" defined by
   RFC5036. The following values have been pre-allocated:

   Range/Value     E     Description                       Reference
   ------------- -----   ----------------------            ---------
    0x00000037     0     Bandwidth resources unavailable   RFCxxxx
    0x00000038     0     Resources Unavailable             RFCxxxx
    0x00000039     0     AII Unreachable                   RFCxxxx


10.3. BGP SAFI

   IANA needs to allocate a new BGP SAFI for "Network Layer Reachability
   Information used for Dynamic Placement of Multi-Segment Pseudiwires"
   from the IANA "Subsequence Address Family Identifiers (SAFI)"
   registry. The following value has been pre-allocated:

   Value    Description                                     Reference
   -----    -----------                                     ---------
   6        Network Layer Reachability Information used [RFCxxxx]
            for Dynamic Placement of Multi-Segment
            Pseudowires


11. Normative References

   [RFC6073] Martini et.al. "Segmented Pseudowire", RFC6073,
        January 2011

   [TSPEC] Wroclawski, J. "The Use of RSVP with IETF Integrated
        Services", RFC 2210, September 1997

   [RFC5036] Andersson, Minei, Thomas. "LDP Specification"
        RFC5036, October 2007




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   [RFC4447] "Pseudowire Setup and Maintenance Using the Label
        Distribution Protocol (LDP)", Martini L.,et al, RFC 4447,
        June 2005.

   [RFC5003] "Attachment Individual Identifier (AII) Types for
        Aggregation", Metz, et al, RFC5003, September 2007


12. Informative References

   [RFC5254] Martini et al, "Requirements for Multi-Segment Pseudowire
        Emulation Edge-to-Edge (PWE3)",
        RFC5254, Bitar, Martini, Bocci, October 2008

   [RFC5659] Bocci at al, "An Architecture for Multi-Segment Pseudo Wire
        Emulation Edge-to-Edge", RFC5659,October  2009.

   [RFC4760] Bates, T., Rekhter, Y., Chandra, R. and D. Katz,
        "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.

   [RFC6074] E. Rosen, W. Luo, B. Davie, V. Radoaca,
        "Provisioning, Autodiscovery, and Signaling in L2VPNs",
        rfc6074, January 2011


13. Author's Addresses


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   e-mail: lmartini@cisco.com


   Matthew Bocci
   Alcatel-Lucent,
   Voyager Place
   Shoppenhangers Road
   Maidenhead
   Berks, UK
   e-mail: matthew.bocci@alcatel-lucent.com









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   Florin Balus
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: florin.balus@alcatel-lucent.com


   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   e-mail: nabil.bitar@verizon.com


   Himanshu Shah
   Ciena Corp
   35 Nagog Park,
   Acton, MA 01720
   e-mail: hshah@ciena.com


   Mustapha Aissaoui
   Alcatel-Lucent
   600 March Road
   Kanata
   ON, Canada
   e-mail: mustapha.aissaoui@alcatel-lucent.com


   Jason Rusmisel
   Alcatel-Lucent
   600 March Road
   Kanata
   ON, Canada
   e-mail: Jason.rusmisel@alcatel-lucent.com


   Yetik Serbest
   SBC Labs
   9505 Arboretum Blvd.
   Austin, TX 78759
   e-mail: Yetik_serbest@labs.sbc.com








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   Andrew G. Malis
   Verizon
   117 West St.
   Waltham, MA 02451
   e-mail: andrew.g.malis@verizon.com


   Chris Metz
   Cisco Systems, Inc.
   3700 Cisco Way
   San Jose, Ca. 95134
   e-mail: chmetz@cisco.com


   David McDysan
   Verizon
   22001 Loudoun County Pkwy
   Ashburn, VA, USA 20147
   e-mail: dave.mcdysan@verizon.com


   Jeff Sugimoto
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: jeffery.sugimoto@alcatel-lucent.com


   Mike Duckett
   Bellsouth
   Lindbergh Center D481
   575 Morosgo Dr
   Atlanta, GA  30324
   e-mail: mduckett@bellsouth.net


   Mike Loomis
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: mike.loomis@alcatel-lucent.com









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   Paul Doolan
   Mangrove Systems
   IO Fairfield Blvd
   Wallingford, CT, USA 06492
   e-mail: pdoolan@mangrovesystems.com


   Ping Pan
   Hammerhead Systems
   640 Clyde Court
   Mountain View, CA, USA 94043
   e-mail: ppan@hammerheadsystems.com


   Prayson Pate
   Overture Networks, Inc.
   507 Airport Blvd, Suite 111
   Morrisville, NC, USA 27560
   e-mail: prayson.pate@overturenetworks.com


   Vasile Radoaca
   Alcatel-Lucent
   Optics Divison, Westford, MA, USA
   email: vasile.radoaca@alcatel-lucent.com


   Yuichiro Wada
   NTT Communications
   3-20-2 Nishi-Shinjuku, Shinjuke-ku
   Tokyo 163-1421, Japan
   e-mail: yuichiro.wada@ntt.com


   Yeongil Seo
   Korea Telecom Corp.
   463-1 Jeonmin-dong, Yusung-gu
   Daejeon, Korea
   e-mail: syi1@kt.co.kr











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