





Network Working Group                                     T. Nadeau, Ed.
Request for Comments: 5085                             C. Pignataro, Ed.
Category: Standards Track                            Cisco Systems, Inc.
                                                           December 2007


      Pseudowire Virtual Circuit Connectivity Verification (VCCV):
                   A Control Channel for Pseudowires

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes Virtual Circuit Connectivity Verification
   (VCCV), which provides a control channel that is associated with a
   pseudowire (PW), as well as the corresponding operations and
   management functions (such as connectivity verification) to be used
   over that control channel.  VCCV applies to all supported access
   circuit and transport types currently defined for PWs.


























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Specification of Requirements  . . . . . . . . . . . . . .  5
   2.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of VCCV . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  CC Types and CV Types  . . . . . . . . . . . . . . . . . . . .  8
   5.  VCCV Control Channel for MPLS PWs  . . . . . . . . . . . . . . 10
     5.1.  VCCV Control Channel Types for MPLS  . . . . . . . . . . . 10
       5.1.1.  In-Band VCCV (Type 1)  . . . . . . . . . . . . . . . . 11
       5.1.2.  Out-of-Band VCCV (Type 2)  . . . . . . . . . . . . . . 12
       5.1.3.  TTL Expiry VCCV (Type 3) . . . . . . . . . . . . . . . 12
     5.2.  VCCV Connectivity Verification Types for MPLS  . . . . . . 13
       5.2.1.  ICMP Ping  . . . . . . . . . . . . . . . . . . . . . . 13
       5.2.2.  MPLS LSP Ping  . . . . . . . . . . . . . . . . . . . . 13
     5.3.  VCCV Capability Advertisement for MPLS PWs . . . . . . . . 13
       5.3.1.  VCCV Capability Advertisement LDP Sub-TLV  . . . . . . 14
   6.  VCCV Control Channel for L2TPv3/IP PWs . . . . . . . . . . . . 15
     6.1.  VCCV Control Channel Type for L2TPv3 . . . . . . . . . . . 16
     6.2.  VCCV Connectivity Verification Type for L2TPv3 . . . . . . 17
       6.2.1.  L2TPv3 VCCV using ICMP Ping  . . . . . . . . . . . . . 17
     6.3.  L2TPv3 VCCV Capability Advertisement for L2TPv3  . . . . . 17
       6.3.1.  L2TPv3 VCCV Capability AVP . . . . . . . . . . . . . . 17
   7.  Capability Advertisement Selection . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  VCCV Interface Parameters Sub-TLV  . . . . . . . . . . . . 19
       8.1.1.  MPLS VCCV Control Channel (CC) Types . . . . . . . . . 19
       8.1.2.  MPLS VCCV Connectivity Verification (CV) Types . . . . 20
     8.2.  PW Associated Channel Type . . . . . . . . . . . . . . . . 21
     8.3.  L2TPv3 Assignments . . . . . . . . . . . . . . . . . . . . 21
       8.3.1.  Control Message Attribute Value Pairs (AVPs) . . . . . 21
       8.3.2.  Default L2-Specific Sublayer Bits  . . . . . . . . . . 21
       8.3.3.  ATM-Specific Sublayer Bits . . . . . . . . . . . . . . 21
       8.3.4.  VCCV Capability AVP Values . . . . . . . . . . . . . . 22
   9.  Congestion Considerations  . . . . . . . . . . . . . . . . . . 23
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     12.2. Informative References . . . . . . . . . . . . . . . . . . 26











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1.  Introduction

   There is a need for fault detection and diagnostic mechanisms that
   can be used for end-to-end fault detection and diagnostics for a
   Pseudowire, as a means of determining the PW's true operational
   state.  Operators have indicated in [RFC4377] and [RFC3916] that such
   a tool is required for PW operation and maintenance.  This document
   defines a protocol called Virtual Circuit Connectivity Verification
   (VCCV) that satisfies these requirements.  VCCV is, in its simplest
   description, a control channel between a pseudowire's ingress and
   egress points over which connectivity verification messages can be
   sent.

   The Pseudowire Edge-to-Edge Emulation (PWE3) Working Group defines a
   mechanism that emulates the essential attributes of a
   telecommunications service (such as a T1 leased line or Frame Relay)
   over a variety of Packet Switched Network (PSN) types [RFC3985].
   PWE3 is intended to provide only the minimum necessary functionality
   to emulate the service with the required degree of faithfulness for
   the given service definition.  The required functions of PWs include
   encapsulating service-specific bit streams, cells, or PDUs arriving
   at an ingress port and carrying them across an IP path or MPLS
   tunnel.  In some cases, it is necessary to perform other operations,
   such as managing their timing and order, to emulate the behavior and
   characteristics of the service to the required degree of
   faithfulness.

   From the perspective of Customer Edge (CE) devices, the PW is
   characterized as an unshared link or circuit of the chosen service.
   In some cases, there may be deficiencies in the PW emulation that
   impact the traffic carried over a PW and therefore limit the
   applicability of this technology.  These limitations must be fully
   described in the appropriate service-specific documentation.

   For each service type, there will be one default mode of operation
   that all PEs offering that service type must support.  However,
   optional modes have been defined to improve the faithfulness of the
   emulated service, as well as to offer a means by which older
   implementations may support these services.

   Figure 1 depicts the architecture of a pseudowire as defined in
   [RFC3985].  It further depicts where the VCCV control channel resides
   within this architecture, which will be discussed in detail shortly.








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            |<-------------- Emulated Service ---------------->|
            |          |<---------- VCCV ---------->|          |
            |          |<------- Pseudowire ------->|          |
            |          |                            |          |
            |          |    |<-- PSN Tunnel -->|    |          |
            |          V    V                  V    V          |
            V    AC    +----+                  +----+     AC   V
      +-----+    |     | PE1|==================| PE2|     |    +-----+
      |     |----------|............PW1.............|----------|     |
      | CE1 |    |     |    |                  |    |     |    | CE2 |
      |     |----------|............PW2.............|----------|     |
      +-----+  ^ |     |    |==================|    |     | ^  +-----+
            ^  |       +----+                  +----+     | |  ^
            |  |   Provider Edge 1         Provider Edge 2  |  |
            |  |                                            |  |
      Customer |                                            | Customer
      Edge 1   |                                            | Edge 2
               |                                            |
               |                                            |
         Native service                               Native service

               Figure 1: PWE3 VCCV Operation Reference Model

   From Figure 1, Customer Edge (CE) routers CE1 and CE2 are attached to
   the emulated service via Attachment Circuits (ACs), and to each of
   the Provider Edge (PE) routers (PE1 and PE2, respectively).  An AC
   can be a Frame Relay Data Link Connection Identifier (DLCI), an ATM
   Virtual Path Identifier / Virtual Channel Identifier (VPI/VCI), an
   Ethernet port, etc.  The PE devices provide pseudowire emulation,
   enabling the CEs to communicate over the PSN.  A pseudowire exists
   between these PEs traversing the provider network.  VCCV provides
   several means of creating a control channel over the PW, between the
   PE routers that attach the PW.

   Figure 2 depicts how the VCCV control channel is associated with the
   pseudowire protocol stack.















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       +-------------+                                +-------------+
       |  Layer2     |                                |  Layer2     |
       |  Emulated   |       < Emulated Service >     |  Emulated   |
       |  Services   |                                |  Services   |
       +-------------+                                +-------------+
       |             |            VCCV/PW             |             |
       |Demultiplexer|       < Control Channel >      |Demultiplexer|
       +-------------+                                +-------------+
       |    PSN      |          < PSN Tunnel >        |    PSN      |
       +-------------+                                +-------------+
       |  Physical   |                                |  Physical   |
       +-----+-------+                                +-----+-------+
             |                                              |
             |             ____     ___       ____          |
             |           _/    \___/   \    _/    \__       |
             |          /               \__/         \_     |
             |         /                               \    |
             +--------|      MPLS or IP Network         |---+
                       \                               /
                        \   ___      ___     __      _/
                         \_/   \____/   \___/  \____/

     Figure 2: PWE3 Protocol Stack Reference Model including the VCCV
                              Control Channel

   VCCV messages are encapsulated using the PWE3 encapsulation as
   described in Sections 5 and 6, so that they are handled and processed
   in the same manner (or in some cases, a similar manner) as the PW
   PDUs for which they provide a control channel.  These VCCV messages
   are exchanged only after the capability (expressed as two VCCV type
   spaces, namely the VCCV Control Channel and Connectivity Verification
   Types) and desire to exchange such traffic has been advertised
   between the PEs (see Sections 5.3 and 6.3), and VCCV types chosen.

1.1.  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 [RFC2119].

2.  Abbreviations

   AC      Attachment Circuit [RFC3985].

   AVP     Attribute Value Pair [RFC3931].

   CC      Control Channel (used as CC Type).




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   CE      Customer Edge.

   CV      Connectivity Verification (used as CV Type).

   CW      Control Word [RFC3985].

   L2SS    L2-Specific Sublayer [RFC3931].

   LCCE    L2TP Control Connection Endpoint [RFC3931].

   OAM     Operation and Maintenance.

   PE      Provider Edge.

   PSN     Packet Switched Network [RFC3985].

   PW      Pseudowire [RFC3985].

   PW-ACH  PW Associated Channel Header [RFC4385].

   VCCV    Virtual Circuit Connectivity Verification.

3.  Overview of VCCV

   The goal of VCCV is to verify and further diagnose the pseudowire
   forwarding path.  To this end, VCCV is comprised of different
   components:

   o  a means of signaling VCCV capabilities to a peer PE,

   o  an encapsulation for the VCCV control channel messages that allows
      the receiving PE to intercept, interpret, and process them locally
      as OAM messages, and

   o  specifications for the operation of the various VCCV operational
      modes transmitted within the VCCV messages.

   When a pseudowire is first signaled using the Label Distribution
   Protocol (LDP) [RFC4447] or the Layer Two Tunneling Protocol version
   3 (L2TPv3) [RFC3931], a message is sent from the initiating PE to the
   receiving PE requesting that a pseudowire be set up.  This message
   has been extended to include VCCV capability information (see
   Section 4).  The VCCV capability information indicates to the
   receiving PE which combinations of Control Channel (CC) and
   Connectivity Verification (CV) Types it is capable of receiving.  If
   the receiving PE agrees to establish the PW, it will return its
   capabilities in the subsequent signaling message to indicate which CC




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   and CV Types it is capable of processing.  Precedence rules for which
   CC and CV Type to choose in cases where more than one is specified in
   this message are defined in Section 7 of this document.

   Once the PW is signaled, data for the PW will flow between the PEs
   terminating the PW.  At this time, the PEs can begin transmitting
   VCCV messages based on the CC and CV Type combinations just
   discussed.  To this end, VCCV defines an encapsulation for these
   messages that identifies them as belonging to the control channel for
   the PW.  This encapsulation is designed to both allow the control
   channel to be processed functionally in the same manner as the data
   traffic for the PW in order to faithfully test the data plane for the
   PE, and allow the PE to intercept and process these VCCV messages
   instead of forwarding them out of the AC towards the CE as if they
   were data traffic.  In this way, the most basic function of the VCCV
   control channel is to verify connectivity of the pseudowire and the
   data plane used to transport the data path for the pseudowire.  It
   should be noted that because of the number of combinations of
   optional and mandatory data-plane encapsulations for PW data traffic,
   VCCV defines a number of Control Channel (CC) and Connectivity
   Verification (CV) types in order to support as many of these as
   possible.  While designed to support most of the existing
   combinations (both mandatory and optional), VCCV does define a
   default CC and CV Type combination for each PW Demultiplexer type, as
   will be described in detail later in this document.

   VCCV can be used both as a fault detection and/or a diagnostic tool
   for pseudowires.  For example, an operator can periodically invoke
   VCCV on a timed, on-going basis for proactive connectivity
   verification on an active pseudowire, or on an ad hoc or as-needed
   basis as a means of manual connectivity verification.  When invoking
   VCCV, the operator triggers a combination of one of its various CC
   Types and one of its various CV Types.  The CV Types include LSP Ping
   [RFC4379] for MPLS PWs, and ICMP Ping [RFC0792] [RFC4443] for both
   MPLS and L2TPv3 PWs.  We define a matrix of acceptable CC and CV Type
   combinations further in this specification.

   The control channel maintained by VCCV can additionally carry fault
   detection status between the endpoints of the pseudowire.
   Furthermore, this information can then be translated into the native
   OAM status codes used by the native access technologies, such as ATM,
   Frame-Relay or Ethernet.  The specific details of such status
   interworking is out of the scope of this document, and is only noted
   here to illustrate the utility of VCCV for such purposes.  Complete
   details can be found in [MSG-MAP] and [RFC4447].






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4.  CC Types and CV Types

   The VCCV Control Channel (CC) Type defines several possible types of
   control channel that VCCV can support.  These control channels can in
   turn carry several types of protocols defined by the Connectivity
   Verification (CV) Type.  VCCV potentially supports multiple CV Types
   concurrently, but it only supports the use of a single CC Type.  The
   specific type or types of VCCV packets that can be accepted and sent
   by a router are indicated during capability advertisement as
   described in Sections 5.3 and 6.3.  The various VCCV CV Types
   supported are used only when they apply to the context of the PW
   demultiplexer in use.  For example, the LSP Ping CV Type should only
   be used when MPLS Labels are utilized as PW Demultiplexer.

   Once a set of VCCV capabilities is received and advertised, a CC Type
   and CV Type(s) that match both the received and transmitted
   capabilities can be selected.  That is, a PE router needs to only
   allow Types that are both received and advertised to be selected,
   performing a logical AND between the received and transmitted bitflag
   fields.  The specific CC Type and CV Type(s) are then chosen within
   the constraints and rules specified in Section 7.  Once a specific CC
   Type has been chosen (i.e., it matches both the transmitted and
   received VCCV CC capability), transmitted and replied to, this CC
   Type MUST be the only one used until such time as the pseudowire is
   re-signaled.  In addition, based on these rules and the procedures
   defined in Section 5.2 of [RFC4447], the pseudowire MUST be re-
   signaled if a different set of capabilities types is desired.  The
   relevant portion of Section 5.2 of [RFC4447] is:

         Interface Parameter Sub-TLV

         Note that as the "interface parameter sub-TLV" is part of the
         FEC, the rules of LDP make it impossible to change the
         interface parameters once the pseudowire has been set up.

   The CC and CV Type indicator fields are defined as 8-bit bitmasks
   used to indicate the specific CC or CV Type or Types (i.e., none,
   one, or more) of control channel packets that may be sent on the VCCV
   control channel.  These values represent the numerical value
   corresponding to the actual bit being set in the bitfield.  The
   definition of each CC and CV Type is dependent on the PW type
   context, either MPLS or L2TPv3, within which it is defined.









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   Control Channel (CC) Types:

      The defined values for CC Types for MPLS PWs are:

         MPLS Control Channel (CC) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - Type 1: PWE3 Control Word with 0001b as
                        first nibble (PW-ACH, see [RFC4385])
         Bit 1 (0x02) - Type 2: MPLS Router Alert Label
         Bit 2 (0x04) - Type 3: MPLS PW Label with TTL == 1
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved
         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved

      The defined values for CC Types for L2TPv3 PWs are:

         L2TPv3 Control Channel (CC) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - L2-Specific Sublayer with V-bit set
         Bit 1 (0x02) - Reserved
         Bit 2 (0x04) - Reserved
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved
         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved

   Connectivity Verification (CV) Types:

      The defined values for CV Types for MPLS PWs are:

         MPLS Connectivity Verification (CV) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - ICMP Ping
         Bit 1 (0x02) - LSP Ping
         Bit 2 (0x04) - Reserved
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved




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         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved

      The defined values for CV Types for L2TPv3 PWs are:

         L2TPv3 Connectivity Verification (CV) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - ICMP Ping
         Bit 1 (0x02) - Reserved
         Bit 2 (0x04) - Reserved
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved
         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved

   If none of the types above are supported, the entire CC and CV Type
   Indicator fields SHOULD be transmitted as 0x00 (i.e., all bits in the
   bitfield set to 0) to indicate this to the peer.

   If no capability is signaled, then the peer MUST assume that the peer
   has no VCCV capability and follow the procedures specified in this
   document for this case.

5.  VCCV Control Channel for MPLS PWs

   When MPLS is used to transport PW packets, VCCV packets are carried
   over the MPLS LSP as defined in this section.  In order to apply IP
   monitoring tools to a PW, an operator may configure VCCV as a control
   channel for the PW between the PE's endpoints [RFC3985].  Packets
   sent across this channel from the source PE towards the destination
   PE either as in-band traffic with the PW's data, or out-of-band.  In
   all cases, the control channel traffic is not forwarded past the PE
   endpoints towards the Customer Edge (CE) devices; instead, VCCV
   messages are intercepted at the PE endpoints for exception
   processing.

5.1.  VCCV Control Channel Types for MPLS

   As already described in Section 4, the capability of which control
   channel types (CC Type) are supported is advertised by a PE.  Once
   the receiving PE has chosen a CC Type mode to use, it MUST continue
   using this mode until such time as the PW is re-signaled.  Thus, if a
   new CC Type is desired, the PW must be torn-down and re-established.





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   Ideally, such a control channel would be completely in-band (i.e.,
   following the same data-plane faith as PW data).  When a control word
   is present on the PW, it is possible to indicate the control channel
   by setting a bit in the control word header (see Section 5.1.1).

   Section 5.1.1 through Section 5.1.3 describe each of the currently
   defined VCCV Control Channel Types (CC Types).

5.1.1.  In-Band VCCV (Type 1)

   CC Type 1 is also referred to as "PWE3 Control Word with 0001b as
   first nibble".  It uses the PW Associated Channel Header (PW-ACH);
   see Section 5 of [RFC4385].

   The PW set-up protocol [RFC4447] determines whether a PW uses a
   control word.  When a control word is used, and that CW uses the
   "Generic PW MPLS Control Word" format (see Section 3 of [RFC4385]), a
   Control Channel for use of VCCV messages can be created by using the
   PW Associated Channel CW format (see Section 5 of [RFC4385]).

   The PW Associated Channel for VCCV control channel traffic is defined
   in [RFC4385] as shown in Figure 3:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1|Version|   Reserved    |         Channel Type          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: PW Associated Channel Header

   The first nibble is set to 0001b to indicate a channel associated
   with a pseudowire (see Section 5 of [RFC4385] and Section 3.6 of
   [RFC4446]).  The Version and the Reserved fields are set to 0, and
   the Channel Type is set to 0x0021 for IPv4 and 0x0057 for IPv6
   payloads.

   For example, Figure 4 shows how the Ethernet [RFC4448] PW-ACH would
   be received containing an LSP Ping payload corresponding to a choice
   of CC Type of 0x01 and a CV Type of 0x02:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1|0 0 0 0|0 0 0 0 0 0 0 0|   0x21 (IPv4) or 0x57 (IPv6)  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: PW Associated Channel Header for VCCV



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   It should be noted that although some PW types are not required to
   carry the control word, this type of VCCV can only be used for those
   PW types that do employ the control word when it is in use.  Further,
   this CC Type can only be used if the PW CW follows the "Generic PW
   MPLS Control Word" format.  This mode of VCCV operation MUST be
   supported when the control word is present.

5.1.2.  Out-of-Band VCCV (Type 2)

   CC Type 2 is also referred to as "MPLS Router Alert Label".

   A VCCV control channel can alternatively be created by using the MPLS
   router alert label [RFC3032] immediately above the PW label.  It
   should be noted that this approach could result in a different Equal
   Cost Multi-Path (ECMP) hashing behavior than pseudowire PDUs, and
   thus result in the VCCV control channel traffic taking a path which
   differs from that of the actual data traffic under test.  Please see
   Section 2 of [RFC4928].

   CC Type 2 can be used whether the PW is set-up with a Control Word
   present or not.

   This is the preferred mode of VCCV operation when the Control Word is
   not present.

   If the Control Word is in use on this PW, it MUST also be included
   before the VCCV message.  This is done to avoid the different ECMP
   hashing behavior.  In this case, the CW uses the PW-ACH format
   described in Section 5.1.1 (see Figures 3 and 4).  If the Control
   Word is not in use on this PW, the VCCV message follows the PW Label
   directly.

5.1.3.  TTL Expiry VCCV (Type 3)

   CC Type 3 is also referred to as "MPLS PW Label with TTL == 1".

   The TTL of the PW label can be set to 1 to force the packet to be
   processed within the destination router's control plane.  This
   approach could also result in a different ECMP hashing behavior and
   VCCV messages taking a different path than the PW data traffic.

   CC Type 3 can be used whether the PW is set-up with a Control Word
   present or not.

   If the Control Word is in use on this PW, it MUST also be included
   before the VCCV message.  This is done to avoid the different ECMP
   hashing behavior.  In this case, the CW uses the PW-ACH format




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   described in Section 5.1.1 (see Figures 3 and 4).  If the Control
   Word is not in use on this PW, the VCCV message follows the PW Label
   directly.

5.2.  VCCV Connectivity Verification Types for MPLS

5.2.1.  ICMP Ping

   When this optional connectivity verification mode is used, an ICMP
   Echo packet using the encoding specified in [RFC0792] (ICMPv4) or
   [RFC4443] (ICMPv6) achieves connectivity verification.
   Implementations MUST use ICMPv4 [RFC0792] if the signaling for VCCV
   used IPv4 addresses, or ICMPv6 [RFC4443] if IPv6 addresses were used.
   If the pseudowire is set up statically, then the encoding MUST use
   that which was used for the pseudowire in the configuration.

5.2.2.  MPLS LSP Ping

   The LSP Ping header MUST be used in accordance with [RFC4379] and
   MUST also contain the target FEC Stack containing the sub-TLV of sub-
   Type 8 for the "L2 VPN endpoint", 9 for "FEC 128 Pseudowire
   (deprecated)", 10 for "FEC 128 Pseudowire", or 11 for the "FEC 129
   Pseudowire".  The sub-TLV value indicates the PW to be verified.

5.3.  VCCV Capability Advertisement for MPLS PWs

   To permit the indication of the type or types of PW control
   channel(s) and connectivity verification mode or modes over a
   particular PW, a VCCV parameter is defined in Section 5.3.1 that is
   used as part of the PW establishment signaling.  When a PE signals a
   PW and desires PW OAM for that PW, it MUST indicate this during PW
   establishment using the messages defined in Section 5.3.1.
   Specifically, the PE MUST include the VCCV interface parameter sub-
   TLV (0x0C) assigned in [RFC4446] in the PW set-up message [RFC4447].

   The decision of the type of VCCV control channel is left completely
   to the receiving control entity, although the set of choices is given
   by the sender in that it indicates the control channels and
   connectivity verification type or types that it can understand.  The
   receiver SHOULD choose a single Control Channel Type from the match
   between the choices sent and received, based on the capability
   advertisement selection specified in Section 7, and it MUST continue
   to use this type for the duration of the life of the control channel.
   Changing Control Channel Types after one has been established to be
   in use could potentially cause problems at the receiving end and
   could also lead to interoperability issues; thus, it is NOT
   RECOMMENDED.




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   When a PE sends a label mapping message for a PW, it uses the VCCV
   parameter to indicate the type of OAM control channels and
   connectivity verification type or types it is willing to receive and
   can send on that PW.  A remote PE MUST NOT send VCCV messages before
   the capability of supporting the control channel(s) (and connectivity
   verification type(s) to be used over them) is signaled.  Then, it can
   do so only on a control channel and using the connectivity
   verification type(s) from the ones indicated.

   If a PE receives VCCV messages prior to advertising capability for
   this message, it MUST discard these messages and not reply to them.
   In this case, the PE SHOULD increment an error counter and optionally
   issue a system and/or SNMP notification to indicate to the system
   administrator that this condition exists.

   When LDP is used as the PW signaling protocol, the requesting PE
   indicates its configured VCCV capability or capabilities to the
   remote PE by including the VCCV parameter with appropriate options in
   the VCCV interface parameter sub-TLV field of the PW ID FEC TLV (FEC
   128) or in the interface parameter sub-TLV of the Generalized PW ID
   FEC TLV (FEC 129).  These options indicate which control channel and
   connectivity verification types it supports.  The requesting PE MAY
   indicate that it supports multiple control channel options, and in
   doing so, it agrees to support any and all indicated types if
   transmitted to it.  However, it MUST do so in accordance with the
   rules stipulated in Section 5.3.1 (VCCV Capability Advertisement Sub-
   TLV.)

   Local policy may direct the PE to support certain OAM capability and
   to indicate it.  The absence of the VCCV parameter indicates that no
   OAM functions are supported by the requesting PE, and thus the
   receiving PE MUST NOT send any VCCV control channel traffic to it.
   The reception of a VCCV parameter with no options set MUST be ignored
   as if one is not transmitted at all.

   The receiving PE similarly indicates its supported control channel
   types in the label mapping message.  These may or may not be the same
   as the ones that were sent to it.  The sender should examine the set
   that is returned to understand which control channels it may
   establish with the remote peer, as specified in Sections 4 and 7.
   Similarly, it MUST NOT send control channel traffic to the remote PE
   for which the remote PE has not indicated it supports.

5.3.1.  VCCV Capability Advertisement LDP Sub-TLV

   [RFC4447] defines an Interface Parameter Sub-TLV field in the LDP PW
   ID FEC (FEC 128) and an Interface Parameters TLV in the LDP
   Generalized PW ID FEC (FEC 129) to signal different capabilities for



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   specific PWs.  An optional sub-TLV parameter is defined to indicate
   the capability of supporting none, one, or more control channel and
   connectivity verification types for VCCV.  This is the VCCV parameter
   field.  If FEC 128 is used, the VCCV parameter field is carried in
   the Interface Parameter sub-TLV field.  If FEC 129 is used, it is
   carried as an Interface Parameter sub-TLV in the Interface Parameters
   TLV.

   The VCCV parameter ID is defined as follows in [RFC4446]:

   Parameter ID   Length     Description
     0x0c           4           VCCV

   The format of the VCCV parameter field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0x0c     |       0x04    |   CC Types    |   CV Types    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Control Channel Type field (CC Type) defines a bitmask used to
   indicate the type of control channel(s) (i.e., none, one, or more)
   that a router is capable of receiving control channel traffic on.  If
   more than one control channel is specified, the router agrees to
   accept control traffic over either control channel; however, see the
   rules specified in Sections 4 and 7 for more details.  If none of the
   types are supported, a CC Type Indicator of 0x00 SHOULD be
   transmitted to indicate this to the peer.  However, if no capability
   is signaled, then the PE MUST assume that its peer is incapable of
   receiving any of the VCCV CC Types and MUST NOT send any OAM control
   channel traffic to it.  Note that the CC and CV Types definitions are
   consistent regardless of the PW's transport or access circuit type.
   The CC and CV Type values are defined in Section 4.

6.  VCCV Control Channel for L2TPv3/IP PWs

   When L2TPv3 is used to set up a PW over an IP PSN, VCCV packets are
   carried over the L2TPv3 session as defined in this section.  L2TPv3
   provides a "Hello" keepalive mechanism for the L2TPv3 control plane
   that operates in-band over IP or UDP (see Section 4.4 of [RFC3931]).
   This built-in Hello facility provides dead peer and path detection
   only for the group of sessions associated with the L2TP Control
   Connection.  VCCV, however, allows individual L2TP sessions to be
   tested.  This provides a more granular mechanism which can be used to
   troubleshoot potential problems within the data plane of L2TP
   endpoints themselves, or to provide additional connection status of
   individual pseudowires.



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   The capability of which Control Channel Type (CC Type) to use is
   advertised by a PE to indicate which of the potentially various
   control channel types are supported.  Once the receiving PE has
   chosen a mode to use, it MUST continue using this mode until such
   time as the PW is re-signaled.  Thus, if a new CC Type is desired,
   the PW must be torn down and re-established.

   An LCCE sends VCCV messages on an L2TPv3-signaled pseudowire for
   fault detection and diagnostic of the L2TPv3 session.  The VCCV
   message travels in-band with the Session and follows the exact same
   path as the user data for the session, because the IP header and
   L2TPv3 Session header are identical.  The egress LCCE of the L2TPv3
   session intercepts and processes the VCCV message, and verifies the
   signaling and forwarding state of the pseudowire on reception of the
   VCCV message.  It is to be noted that the VCCV mechanism for L2TPv3
   is primarily targeted at verifying the pseudowire forwarding and
   signaling state at the egress LCCE.  It also helps when L2TPv3
   Control Connection and Session paths are not identical.

6.1.  VCCV Control Channel Type for L2TPv3

   In order to carry VCCV messages within an L2TPv3 session data packet,
   the PW MUST be established such that an L2-Specific Sublayer (L2SS)
   that defines the V-bit is present.  This document defines the V-bit
   for the Default L2-Specific Sublayer [RFC3931] and the ATM-Specific
   Sublayer [RFC4454] using the Bit 0 position (see Sections 8.3.2 and
   8.3.3).  The L2-Specific Sublayer presence and type (either the
   Default or a PW-Specific L2SS) is signaled via the L2-Specific
   Sublayer AVP, Attribute Type 69, as defined in [RFC3931].  The V-bit
   within the L2-Specific Sublayer is used to identify that a VCCV
   message follows, and when the V-bit is set the L2SS has the format
   shown in Figure 5:

      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 0 0|Version|   Reserved    |         Channel Type          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 5: L2-Specific Sublayer Format when the V-bit (bit 0) is set

   The VCCV messages are distinguished from user data by the V-bit.  The
   V-bit is set to 1, indicating that a VCCV session message follows.
   The next three bits MUST be set to 0 when sending and ignored upon
   receipt.  The remaining fields comprising 28 bits (i.e., Version,
   Reserved, and Channel Type) follow the same definition, format, and
   number registry from Section 5 of [RFC4385].




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   The Version and Reserved fields are set to 0.  For the CV Type
   currently defined of ICMP Ping (0x01), the Channel Type can indicate
   IPv4 (0x0021) or IPv6 (0x0057) (see [RFC4385]) as the VCCV payload
   directly following the L2SS.

6.2.  VCCV Connectivity Verification Type for L2TPv3

   The VCCV message over L2TPv3 directly follows the L2-Specific
   Sublayer with the V-bit set.  It MUST contain an ICMP Echo packet as
   described in Section 6.2.1.

6.2.1.  L2TPv3 VCCV using ICMP Ping

   When this connectivity verification mode is used, an ICMP Echo packet
   using the encoding specified in [RFC0792] for (ICMPv4) or [RFC4443]
   (for ICMPv6) achieves connectivity verification.  Implementations
   MUST use ICMPv4 [RFC0792] if the signaling for the L2TPv3 PW used
   IPv4 addresses, or ICMPv6 [RFC4443] if IPv6 addresses were used.  If
   the pseudowire is set-up statically, then the encoding MUST use that
   which was used for the pseudowire in the configuration.

   The ICMP Ping packet directly follows the L2SS with the V-bit set.
   In the ICMP Echo request, the IP Header fields MUST have the
   following values: the destination IP address is set to the remote
   LCCE's IP address for the tunnel endpoint, the source IP address is
   set to the local LCCE's IP address for the tunnel endpoint, and the
   TTL or Hop Limit is set to 1.

6.3.  L2TPv3 VCCV Capability Advertisement for L2TPv3

   A new optional AVP is defined in Section 6.3.1 to indicate the VCCV
   capabilities during session establishment.  An LCCE MUST signal its
   desire to use connectivity verification for a particular L2TPv3
   session and its VCCV capabilities using the VCCV Capability AVP.

   An LCCE MUST NOT send VCCV packets on an L2TPv3 session unless it has
   received VCCV capability by means of the VCCV Capability AVP from the
   remote end.  If an LCCE receives VCCV packets and it is not VCCV
   capable or it has not sent VCCV capability indication to the remote
   end, it MUST discard these messages.  It should also increment an
   error counter.  In this case the LCCE MAY optionally issue a system
   and/or SNMP notification.

6.3.1.  L2TPv3 VCCV Capability AVP

   The "VCCV Capability AVP", Attribute Type 96, specifies the VCCV
   capabilities as a pair of bitflags for the Control Channel (CC) and
   Connectivity Verification (CV) Types.  This AVP is exchanged during



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   session establishment (in ICRQ (Incoming-Call-Request), ICRP
   (Incoming-Call-Reply), OCRQ (Outgoing-Call-Request), or OCRP
   (Outgoing-Call-Reply) messages).  The value field has the following
   format:

   VCCV Capability AVP (ICRQ, ICRP, OCRQ, OCRP)

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   CC Types    |   CV Types    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   CC Types:

      The Control Channel (CC) Types field defines a bitmask used to
      indicate the type of control channel(s) that may be used to
      receive OAM traffic on for the given Session.  The router agrees
      to accept VCCV traffic at any time over any of the signaled VCCV
      control channel types.  CC Type values are defined in Section 4.
      Although there is only one value defined in this document, the CC
      Types field is included for forward compatibility should further
      CC Types need to be defined in the future.

      A CC Type of 0x01 may only be requested when there is an L2-
      Specific Sublayer that defines the V-bit present.  If a CC Type of
      0x01 is requested without requesting an L2-Specific Sublayer AVP
      with an L2SS type that defines the V-bit, the session MUST be
      disconnected with a Call-Disconnect-Notify (CDN) message.

      If no CC Type is supported, a CC Type Indicator of 0x00 SHOULD be
      sent.

   CV Types:

      The Connectivity Verification (CV) Types field defines a bitmask
      used to indicate the specific type or types (i.e., none, one, or
      more) of control packets that may be sent on the specified VCCV
      control channel.  CV Type values are defined in Section 4.

   If no VCCV Capability AVP is signaled, then the LCCE MUST assume that
   the peer is incapable of receiving VCCV and MUST NOT send any OAM
   control channel traffic to it.








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   All L2TP AVPs have an M (Mandatory) bit, H (Hidden) bit, Length, and
   Vendor ID.  The Vendor ID for the VCCV Capability AVP MUST be 0,
   indicating that this is an IETF-defined AVP.  This AVP MAY be hidden
   (the H bit MAY be 0 or 1).  The M bit for this AVP SHOULD be set to
   0.  The Length (before hiding) of this AVP is 8.

7.  Capability Advertisement Selection

   When a PE receives a VCCV capability advertisement, the advertisement
   may potentially contain more than one CC or CV Type.  Only matching
   capabilities can be selected.  When multiple capabilities match, only
   one CC Type MUST be used.

   In particular, as already specified, once a valid CC Type is used by
   a PE (traffic sent using that encapsulation), the PE MUST NOT send
   any traffic down another CC Type control channel.

   For cases where multiple CC Types are advertised, the following
   precedence rules apply when choosing the single CC Type to use:

   1.  Type 1: PWE3 Control Word with 0001b as first nibble

   2.  Type 2: MPLS Router Alert Label

   3.  Type 3: MPLS PW Label with TTL == 1

   For MPLS PWs, the CV Type of LSP Ping (0x02) is the default, and the
   CV Type of ICMP Ping (0x01) is optional.

8.  IANA Considerations

8.1.  VCCV Interface Parameters Sub-TLV

   The VCCV Interface Parameters Sub-TLV codepoint is defined in
   [RFC4446].  IANA has created and will maintain registries for the CC
   Types and CV Types (bitmasks in the VCCV Parameter ID).  The CC Type
   and CV Type new registries (see Sections 8.1.1 and 8.1.2,
   respectively) have been created in the Pseudo Wires Name Spaces,
   reachable from [IANA.pwe3-parameters].  The allocations must be done
   using the "IETF Consensus" policy defined in [RFC2434].

8.1.1.  MPLS VCCV Control Channel (CC) Types

   IANA has set up a registry of "MPLS VCCV Control Channel Types".
   These are 8 bitfields.  CC Type values 0x01, 0x02, and 0x04 are
   specified in Section 4 of this document.  The remaining bitfield
   values (0x08, 0x10, 0x20, 0x40, and 0x80) are to be assigned by IANA
   using the "IETF Consensus" policy defined in [RFC2434].  A VCCV



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   Control Channel Type description and a reference to an RFC approved
   by the IESG are required for any assignment from this registry.

      MPLS Control Channel (CC) Types:

      Bit (Value)    Description
      ============   ==========================================
      Bit 0 (0x01) - Type 1: PWE3 Control Word with 0001b as
                     first nibble (PW-ACH, see [RFC4385])
      Bit 1 (0x02) - Type 2: MPLS Router Alert Label
      Bit 2 (0x04) - Type 3: MPLS PW Label with TTL == 1
      Bit 3 (0x08) - Reserved
      Bit 4 (0x10) - Reserved
      Bit 5 (0x20) - Reserved
      Bit 6 (0x40) - Reserved
      Bit 7 (0x80) - Reserved

   The most significant (high order) bit is labeled Bit 7, and the least
   significant (low order) bit is labeled Bit 0, see parenthetical
   "Value".

8.1.2.  MPLS VCCV Connectivity Verification (CV) Types

   IANA has set up a registry of "MPLS VCCV Control Verification Types".
   These are 8 bitfields.  CV Type values 0x01 and 0x02 are specified in
   Section 4 of this document.  The remaining bitfield values (0x04,
   0x08, 0x10, 0x20, 0x40, and 0x80) are to be assigned by IANA using
   the "IETF Consensus" policy defined in [RFC2434].  A VCCV Control
   Verification Type description and a reference to an RFC approved by
   the IESG are required for any assignment from this registry.

      MPLS Connectivity Verification (CV) Types:

      Bit (Value)    Description
      ============   ==========================================
      Bit 0 (0x01) - ICMP Ping
      Bit 1 (0x02) - LSP Ping
      Bit 2 (0x04) - Reserved
      Bit 3 (0x08) - Reserved
      Bit 4 (0x10) - Reserved
      Bit 5 (0x20) - Reserved
      Bit 6 (0x40) - Reserved
      Bit 7 (0x80) - Reserved

   The most significant (high order) bit is labeled Bit 7, and the least
   significant (low order) bit is labeled Bit 0, see parenthetical
   "Value".




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8.2.  PW Associated Channel Type

   The PW Associated Channel Types used by VCCV as defined in Sections
   5.1.1 and 6.1 rely on previously allocated numbers from the
   Pseudowire Associated Channel Types Registry [RFC4385] in the Pseudo
   Wires Name Spaces reachable from [IANA.pwe3-parameters].  In
   particular, 0x21 (Internet Protocol version 4) MUST be used whenever
   an IPv4 payload follows the Pseudowire Associated Channel Header, or
   0x57 MUST be used when an IPv6 payload follows the Pseudowire
   Associated Channel Header.

8.3.  L2TPv3 Assignments

   Section 8.3.1 through Section 8.3.3 are registrations of new L2TP
   values for registries already managed by IANA.  Section 8.3.4 is a
   new registry that has been added to the existing L2TP name spaces,
   and will be maintained by IANA accordingly.  The Layer Two Tunneling
   Protocol "L2TP" Name Spaces are reachable from
   [IANA.l2tp-parameters].

8.3.1.  Control Message Attribute Value Pairs (AVPs)

   An additional AVP Attribute is specified in Section 6.3.1.  It was
   defined by IANA as described in Section 2.2 of [RFC3438].

      Attribute
      Type        Description
      ---------   ----------------------------------
      96          VCCV Capability AVP

8.3.2.  Default L2-Specific Sublayer Bits

   The Default L2-Specific Sublayer contains 8 bits in the low-order
   portion of the header.  This document defines one reserved bit in the
   Default L2-Specific Sublayer in Section 6.1, which was assigned by
   IANA following IETF Consensus [RFC2434].

      Default L2-Specific Sublayer bits - per [RFC3931]
      ---------------------------------
      Bit 0 - V (VCCV) bit

8.3.3.  ATM-Specific Sublayer Bits

   The ATM-Specific Sublayer contains 8 bits in the low-order portion of
   the header.  This document defines one reserved bit in the ATM-
   Specific Sublayer in Section 6.1, which was assigned by IANA
   following IETF Consensus [RFC2434].




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      ATM-Specific Sublayer bits - per [RFC4454]
      --------------------------
      Bit 0 - V (VCCV) bit

8.3.4.  VCCV Capability AVP Values

   This is a new registry that IANA maintains in the L2TP Name Spaces.

   IANA created and maintains a registry for the CC Types and CV Types
   bitmasks in the VCCV Capability AVP, defined in Section 6.3.1.  The
   allocations must be done using the "IETF Consensus" policy defined in
   [RFC2434].  A VCCV CC or CV Type description and a reference to an
   RFC approved by the IESG are required for any assignment from this
   registry.

   IANA has reserved the following bits in this registry:

      VCCV Capability AVP (Attribute Type 96) Values
      ---------------------------------------------------

      L2TPv3 Control Channel (CC) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - L2-Specific Sublayer with V-bit set
         Bit 1 (0x02) - Reserved
         Bit 2 (0x04) - Reserved
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved
         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved

      L2TPv3 Connectivity Verification (CV) Types:

         Bit (Value)    Description
         ============   ==========================================
         Bit 0 (0x01) - ICMP Ping
         Bit 1 (0x02) - Reserved
         Bit 2 (0x04) - Reserved
         Bit 3 (0x08) - Reserved
         Bit 4 (0x10) - Reserved
         Bit 5 (0x20) - Reserved
         Bit 6 (0x40) - Reserved
         Bit 7 (0x80) - Reserved






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   The most significant (high order) bit is labeled Bit 7, and the least
   significant (low order) bit is labeled Bit 0, see parenthetical
   "Value".

9.  Congestion Considerations

   The bandwidth resources used by VCCV are recommended to be minimal
   compared to those of the associated PW.  The bandwidth required for
   the VCCV channel is taken outside any allocation for PW data traffic,
   and can be configurable.  When doing resource reservation or network
   planning, the bandwidth requirements for both PW data and VCCV
   traffic need to be taken into account.

   VCCV applications (i.e., Connectivity Verification (CV) Types) MUST
   consider congestion and bandwidth usage implications and provide
   details on bandwidth or packet frequency management.  VCCV
   applications can have built-in bandwidth management in their
   protocols.  Other VCCV applications can have their bandwidth
   configuration-limited, and rate-limiting them can be harmful as it
   could translate to incorrectly declaring connectivity failures.  For
   all other VCCV applications, outgoing VCCV messages SHOULD be rate-
   limited to prevent aggressive connectivity verification consuming
   excessive bandwidth, causing congestion, becoming denial-of-service
   attacks, or generating an excessive packet rate at the CE-bound PE.

   If these conditions cannot be followed, an adaptive loss-based scheme
   SHOULD be applied to congestion-control outgoing VCCV traffic, so
   that it competes fairly with TCP within an order of magnitude.  One
   method of determining an acceptable bandwidth for VCCV is described
   in [RFC3448] (TFRC); other methods exist.  For example, bandwidth or
   packet frequency management can include any of the following: a
   negotiation of transmission interval/rate, a throttled transmission
   rate on "congestion detected" situations, a slow-start after shutdown
   due to congestion and until basic connectivity is verified, and other
   mechanisms.

   The ICMP and MPLS LSP PING applications SHOULD be rate-limited to
   below 5% of the bit-rate of the associated PW.  For this purpose, the
   considered bit-rate of a pseudowire is dependent on the PW type.  For
   pseudowires that carry constant bit-rate traffic (e.g., TDM PWs) the
   full bit-rate of the PW is used.  For pseudowires that carry variable
   bit-rate traffic (e.g., Ethernet PWs), the mean or sustained bit-rate
   of the PW is used.








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   As described in Section 10, incoming VCCV messages can be rate-
   limited as a protection against denial-of-service attacks.  This
   throttling or policing of incoming VCCV messages should not be more
   stringent than the bandwidth allocated to the VCCV channel to prevent
   false indications of connectivity failure.

10.  Security Considerations

   Routers that implement VCCV create a Control Channel (CC) associated
   with a pseudowire.  This control channel can be signaled (e.g., using
   LDP or L2TPv3 depending on the PWE3) or statically configured.  Over
   this control channel, VCCV Connectivity Verification (CV) messages
   are sent.  Therefore, three different areas are of concern from a
   security standpoint.

   The first area of concern relates to control plane parameter and
   status message attacks, that is, attacks that concern the signaling
   of VCCV capabilities.  MPLS PW Control Plane security is discussed in
   Section 8.2 of [RFC4447].  L2TPv3 PW Control Plane security is
   discussed in Section 8.1 of [RFC3931].  The addition of the
   connectivity verification negotiation extensions does not change the
   security aspects of Section 8.2 of [RFC4447], or Section 8.1 of
   [RFC3931].  Implementation of IP source address filters may also aid
   in deterring these types of attacks.

   A second area of concern centers on data-plane attacks, that is,
   attacks on the associated channel itself.  Routers that implement the
   VCCV mechanisms are subject to additional data-plane denial-of-
   service attacks as follows:

      An intruder could intercept or inject VCCV packets effectively
      providing false positives or false negatives.

      An intruder could deliberately flood a peer router with VCCV
      messages to deny services to others.

      A misconfigured or misbehaving device could inadvertently flood a
      peer router with VCCV messages which could result in denial of
      services.  In particular, if a router has either implicitly or
      explicitly indicated that it cannot support one or all of the
      types of VCCV, but is sent those messages in sufficient quantity,
      it could result in a denial of service.

   To protect against these potential (deliberate or unintentional)
   attacks, multiple mitigation techniques can be employed:

      VCCV message throttling mechanisms can be used, especially in
      distributed implementations which have a centralized control-plane



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      processor with various line cards attached by some control-plane
      data path.  In these architectures, VCCV messages may be processed
      on the central processor after being forwarded there by the
      receiving line card.  In this case, the path between the line card
      and the control processor may become saturated if appropriate VCCV
      traffic throttling is not employed, which could lead to a complete
      denial of service to users of the particular line card.  Such
      filtering is also useful for preventing the processing of unwanted
      VCCV messages, such as those which are sent on unwanted (and
      perhaps unadvertised) control channel types or VCCV types.

      Section 8.1 of [RFC4447] discusses methods to protect the data
      plane of MPLS PWs from data-plane attacks.  However the
      implementation of the connectivity verification protocol expands
      the range of possible data-plane attacks.  For this reason
      implementations MUST provide a method to secure the data plane.
      This can be in the form of encryption of the data by running IPsec
      on MPLS packets encapsulated according to [RFC4023], or by
      providing the ability to architect the MPLS network in such a way
      that no external MPLS packets can be injected (private MPLS
      network).

      For L2TPv3, data packet spoofing considerations are outlined in
      Section 8.2 of [RFC3931].  While the L2TPv3 Session ID provides
      traffic separation, the optional Cookie field provides additional
      protection to thwart spoofing attacks.  To maximize protection
      against a variety of data-plane attacks, a 64-bit Cookie can be
      used.  L2TPv3 can also be run over IPsec as detailed in Section
      4.1.3 of [RFC3931].

   A third and last area of concern relates to the processing of the
   actual contents of VCCV messages, i.e., LSP Ping and ICMP messages.
   Therefore, the corresponding security considerations for these
   protocols (LSP Ping [RFC4379], ICMPv4 Ping [RFC0792], and ICMPv6 Ping
   [RFC4443]) apply as well.

11.  Acknowledgements

   The authors would like to thank Hari Rakotoranto, Michel Khouderchah,
   Bertrand Duvivier, Vanson Lim, Chris Metz, W. Mark Townsley, Eric
   Rosen, Dan Tappan, Danny McPherson, Luca Martini, Don O'Connor, Neil
   Harrison, Danny Prairie, Mustapha Aissaoui, and Vasile Radoaca for
   their valuable comments and suggestions.








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12.  References

12.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

12.2.  Informative References

   [IANA.l2tp-parameters]
              Internet Assigned Numbers Authority, "Layer Two Tunneling
              Protocol "L2TP"", April 2007,
              <http://www.iana.org/assignments/l2tp-parameters>.

   [IANA.pwe3-parameters]
              Internet Assigned Numbers Authority, "Pseudo Wires Name
              Spaces", June 2007,
              <http://www.iana.org/assignments/pwe3-parameters>.




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   [MSG-MAP]  Nadeau, T., "Pseudo Wire (PW) OAM Message Mapping",
              Work in Progress, March 2007.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC3438]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
              Internet Assigned Numbers Authority (IANA) Considerations
              Update", BCP 68, RFC 3438, December 2002.

   [RFC3448]  Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 3448, January 2003.

   [RFC3916]  Xiao, X., McPherson, D., and P. Pate, "Requirements for
              Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
              September 2004.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
              MPLS in IP or Generic Routing Encapsulation (GRE)",
              RFC 4023, March 2005.

   [RFC4377]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
              Matsushima, "Operations and Management (OAM) Requirements
              for Multi-Protocol Label Switched (MPLS) Networks",
              RFC 4377, February 2006.

   [RFC4448]  Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, April 2006.

   [RFC4454]  Singh, S., Townsley, M., and C. Pignataro, "Asynchronous
              Transfer Mode (ATM) over Layer 2 Tunneling Protocol
              Version 3 (L2TPv3)", RFC 4454, May 2006.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, June 2007.









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Appendix A.  Contributors' Addresses

   George Swallow
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA 01719
   USA

   EMail: swallow@cisco.com


   Monique Morrow
   Cisco Systems, Inc.
   Glatt-com
   CH-8301 Glattzentrum
   Switzerland

   EMail: mmorrow@cisco.com


   Yuichi Ikejiri
   NTT Communication Corporation
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo 100-8019
   Shinjuku-ku
   JAPAN

   EMail: y.ikejiri@ntt.com


   Kenji Kumaki
   KDDI Corporation
   KDDI Bldg. 2-3-2
   Nishishinjuku
   Tokyo 163-8003
   JAPAN

   EMail: ke-kumaki@kddi.com


   Peter B. Busschbach
   Alcatel-Lucent
   67 Whippany Road
   Whippany, NJ, 07981
   USA

   EMail: busschbach@alcatel-lucent.com




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   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   USA

   EMail: rahul@juniper.net


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   USA

   EMail: lmartini@cisco.com

Authors' Addresses

   Thomas D. Nadeau (editor)
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA  01719
   USA

   EMail: tnadeau@lucidvision.com


   Carlos Pignataro (editor)
   Cisco Systems, Inc.
   7200 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC  27709
   USA

   EMail: cpignata@cisco.com















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Full Copyright Statement

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