Static PW status: Pseudowire Status for Static Pseudowires

Internet Engineering Task Force (IETF)                        L. Martini
Request for Comments: 6478                                    G. Swallow
Updates: 5885                                                   G. Heron
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                                 M. Bocci
                                                          Alcatel-Lucent
                                                                May 2012


                Pseudowire Status for Static Pseudowires

Abstract

   This document specifies a mechanism to signal Pseudowire (PW) status
   messages using a PW associated channel (ACh).  Such a mechanism is
   suitable for use where no PW dynamic control plane exits, known as
   static PWs, or where a Terminating Provider Edge (T-PE) needs to send
   a PW status message directly to a far-end T-PE.  The mechanism allows
   PW Operations, Administration, and Maintenance (OAM) message mapping
   and PW redundancy to operate on static PWs.  This document also
   updates RFC 5885 in the case when Bi-directional Forwarding Detection
   (BFD) is used to convey PW status-signaling information.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6478.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................3
   3. Terminology .....................................................3
   4. Applicability ...................................................3
   5. Pseudowire Status Operation .....................................4
      5.1. PW OAM Message .............................................4
      5.2. Sending a PW Status Message ................................5
      5.3. PW OAM Status Message Transmit and Receive .................6
           5.3.1. Acknowledgment of PW Status .........................7
      5.4. MPLS Label Stack ...........................................7
           5.4.1. Label Stack for a Message Destined to the Next PE ...8
           5.4.2. Label Stack for a Message Destined to the Egress PE .8
      5.5. S-PE Bypass Mode ...........................................8
           5.5.1. S-PE Bypass Mode LDP Flag Bit .......................9
   6. S-PE Operation .................................................10
      6.1. Static PW to Another Static PW ............................10
      6.2. Dynamic PW to Static PW or Vice Versa .....................10
   7. Security Considerations ........................................11
   8. IANA Considerations ............................................11
   9. References .....................................................11
      9.1. Normative References ......................................11
      9.2. Informative References ....................................12

1.  Introduction

   The default control plane for Pseudowire (PW) technology, as defined
   in [RFC4447], is based on the Label Distribution Protocol (LDP).
   However, that document also describes a static provisioning mode
   without a control plane.  When a static PW is used, there is no
   method to transmit the status of the PW or attachment circuit (AC)
   between the two Provider Edge (PE) devices at each end of the PW.
   This document defines a method to transport the PW status codes
   defined in Section 5.4.2 of [RFC4447] and elsewhere [REDUNDANCY] in-
   band with the PW data using a generic associated channel [RFC5586].

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 [RFC2119].

3.  Terminology

   ACh:   Associated Channel

   ACH:   Associated Channel Header

   FEC:   Forwarding Equivalence Class

   LDP:   Label Distribution Protocol

   LSP:   Label Switching Path

   MS-PW: Multi-Segment Pseudowire

   PE:    Provider Edge

   PW:    Pseudowire

   SS-PW: Single-Segment Pseudowire

   S-PE:  Switching Provider Edge Node of MS-PW

   T-PE:  Terminating Provider Edge Node of MS-PW

4.  Applicability

   As described in [RFC4447] and [RFC6310], a PE that establishes an
   MPLS PW using means other than LDP, e.g., by static configuration,
   MUST support some alternative method of status reporting.  The
   procedures described in this document are for use when PWs are
   statically configured and an LDP control plane is not available.

   As defined in [RFC4447], a PE that establishes a PW using LDP MUST
   use the PW status TLV mechanism for AC and PW status and defect
   notification on that PW.  In order to avoid duplicate notifications
   and potentially conflicting notifications, such PEs MUST NOT use the
   mechanisms described in this document for those PWs, except that the
   S-PE bypass mode described in Section 5.5 MAY be used when both T-PEs
   at each end of the PW use LDP to establish the PW.

   In order to protect against duplicate notifications and potentially
   conflicting notifications, when the Pseudowire Status protocol for
   Static Pseudowires described in this document is used, the BFD VCCV
   (Virtual Circuit Connectivity Verification) status-signaling
   mechanisms described in [RFC5885] (CV Types 0x08 and 0x20) MUST NOT
   be used.  BFD VCCV for fault detection (CV types 0x04 and 0x10) MAY
   still be used.

5.  Pseudowire Status Operation

5.1.  PW OAM Message

   The PW status TLV as defined in Section 5.4.2 of [RFC4447] is
   transported in a PW OAM message using the PW ACH.

     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    | 0x0027 PW OAM Message         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Refresh Timer         |  TLV Length   |A|   Flags     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                              TLVs                             ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1: ACh PW OAM Message Packet Header

   The first 32 bits are the standard ACH header construct as defined in
   [RFC5586].

   The first nibble (0001b) indicates the ACH instead of PW data.  The
   version and the reserved values are both set to 0 as specified in
   [RFC4385].

   The refresh timer is an unsigned integer and specifies refresh time
   in seconds with a range from 1 to 65535.  The value 0 means that the
   refresh timer is set to infinity, and the PW OAM message will never
   be refreshed, and will never timeout.

   The TLV length field indicates the length of all TLVs only.  This
   document defines only the transport of the PW status TLV, as defined
   in Section 5.4.2, [RFC4447], in the TLV field.  In the future,
   additional TLVs may be defined to be used in this field with code
   points allocated from the IANA registry called "LDP TLV Type Name
   Space".

   The A flag bit is used to indicate an acknowledgment of the PW status
   TLV included.  The rest of the flag bits are reserved and they MUST
   be set to 0 on transmit, and ignored upon receipt.  When the A bit is
   set, the refresh timer value is a requested timer value.

   The PW OAM Message code point value is 0x0027.

5.2.  Sending a PW Status Message

   The PW Status messages are sent in-band using the PW OAM message
   containing the PW Status TLV for a particular PW, as defined in
   [RFC4447].  The PW Status TLV format is almost as defined in
   [RFC4447] and is repeated here for the reader's convenience:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Res|     PW Status (0x096A)    |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Status Code                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: PW Status TLV Format

   Unlike the case in [RFC4447], here, the first 2 bits are reserved,
   and MUST be set to zero on transmit and ignored on receipt.

   The PW Status TLV is prepended with a PW OAM message header and sent
   on the ACh of the PW to which the status update applies.

   To clear a particular status indication, the PE needs to send a new
   PW OAM message containing a PW Status TLV with the corresponding bit
   cleared as defined in [RFC4447].

   The procedures described in [RFC6073] that apply to an S-PE and PW
   using an LDP control plane also apply when sending PW status using
   the PW OAM channel.  The OPTIONAL procedures using the SP-PE TLV
   described in [RFC6073] can also be applied when sending PW status
   using the PW OAM channel.

   The detailed message transmit and message receive procedures are
   specified in the next section.  PW OAM status messages MUST NOT be
   used as a connectivity verification method.

5.3.  PW OAM Status Message Transmit and Receive

   Unlike the PW status procedures defined in [RFC4447], with this
   method there is no TCP/IP session or session management.  Therefore,
   unlike the TCP/IP case, where each message is sent only once, the PW
   OAM message containing the PW status TLV needs to be transmitted
   repeatedly to ensure reliable message delivery.  If a malformed TLV
   or an unknown TLV is received in a PW OAM status message, the TLV
   MUST be ignored, and the PE SHOULD report the event to the operator.

   A PW OAM message containing a PW status TLV with a new status bit set
   or reset will be transmitted immediately by the PE.  Unless the
   message is acknowledged within a second, the PW OAM message will then
   be repeated twice more at an initial interval of one second.
   Subsequently, the PW OAM message will be transmitted with an interval
   specified by the refresh timer value in the packet.  Note that this
   value MAY be updated in the new PW OAM message packet, in which case
   the new refresh timer value becomes the new packet transmit interval.

   The suggested default value for the refresh timer is 600 seconds.
   This default is adequate for typical deployments, and PEs are
   designed to take into account processing these messages at the
   required rate.

   When a PW OAM message containing a status TLV is received, a timer is
   started according to the refresh rate specified in the packet.  If
   another non-zero PW status message is not received within 3.5 times
   the specified timer value, the status condition will timeout in 3.5
   times the last refresh timer value received, and the default status
   of zero is assumed on the PW.  It is also a good practice to
   introduce some jitter in the delay between refresh transmissions, as
   long as the maximum jitter delay is within the prescribed maximum
   refresh time of 3.5 times the specified timer value for 3 consecutive
   refresh packets.

   To clear a particular status fault, the PE need only send an updated
   message with the corresponding bit cleared.  If the PW status code is
   zero, the PW OAM message will be sent like any other PW OAM status
   message using the procedures described above; however, transmission
   will cease after 3 PW status messages have been sent at one second
   intervals and before the refresh timer expires.  A PW status message
   of zero MAY be acknowledged using the procedures described in Section
   5.3.1.  If it is acknowledged, then a timer value of zero MUST be
   used.  This SHOULD cause the PE sending the PW status notification
   message with a PW status code equal to zero to stop sending and to
   continue normal operation.

5.3.1.  Acknowledgment of PW Status

   A PE receiving a PW OAM message containing a PW status message MAY
   acknowledge the PW status message by simply building a reply packet
   with the same format and status code as the received PW OAM message,
   but with the A bit set, and transmitting it on the PW ACh back to the
   source of the PW OAM message.  The receiving PE MAY use the refresh
   timer field in the acknowledgement packet to request a new refresh
   interval from the originator of the PW OAM message.  The timer value
   set in the reply packet SHOULD then be used by the originator of the
   PW OAM message as the new transmit interval.  If the requested
   refresh timer value is to be used, then, when the the current timer
   expires, the PW OAM message transmission interval is set to the new
   value and the new value is sent in the PW OAM message.  If the
   transmitting PE does not want to use the new timer value (for local
   policy reasons, or because it simply cannot support it), it MUST
   refresh the PW OAM message with the timer value it desires.  The
   receiving PE will then set its timeout timer according to the new
   refresh timer value that is in the packet received, regardless of
   what timer value it requested.  The receiving PE MUST NOT request a
   new refresh timer value more than once per refresh interval.

   The suggested default value for the refresh timer value in the
   acknowledgment packet is 600 seconds.

   If the sender PE receives an acknowledgment message that does not
   match the current active PW status message being sent, it simply
   ignores the acknowledgment packet.

   If a PE that has received a non-zero status code for a PW detects by
   any means that the far end PE has become unreachable, it will follow
   the standard defect entry procedures of [RFC6310], Section 6.2.

5.4.  MPLS Label Stack

   With one exception, all PW OAM status messages are sent to the
   adjacent PE across the PSN tunnel.  In many cases, the transmitting
   PE has no way to determine whether the adjacent PE is an S-PE or a
   T-PE.  This is a necessary behavior to preserve backward
   compatibility with PEs that do not understand MS-PWs.  In the
   procedures described in this document, there are two possible
   destinations for the PW OAM status messages: the adjacent PE or the
   T-PE.  Sending a PW status message directly to the T-PE is an
   enhanced method that is only applicable using PW OAM status messages
   sent in the PW ACH.

5.4.1.  Label Stack for a Message Destined to the Next PE

   A PE that needs to forward a PW OAM status message to the adjacent PE
   across the PSN tunnel MUST set the PW label TTL field to 1.
   Furthermore, if the control word is not in use on the particular PW,
   the PE MUST place the GAL reserved label [RFC5586] below the PW label
   with the TTL field set to 1.

5.4.2.  Label Stack for a Message Destined to the Egress PE

   This is also known as "S-PE bypass mode"; see below.  A T-PE that
   requires sending a PW OAM status message directly to the
   corresponding T-PE at the other end of the PW MUST set the TTL of the
   PW label to a value that is sufficient to reach the corresponding
   T-PE.  This value will be greater than one, but will be set according
   to the local policy on the transmitting T-PE.  Furthermore, if the
   control word is not in use on the particular PW, the PE MUST also
   place the GAL reserved label [RFC5586] below the PW label with the
   TTL field set to 1.

5.5.  S-PE Bypass Mode

   S-PE bypass mode enables a T-PE that uses LDP as the PW setup and
   control protocol to bypass all S-PEs that might be present along the
   MS-PW and to send a message directly to the remote T-PE.  This is
   used for very fast message transmission in-band with the PW PDUs.
   This mode is OPTIONAL and MUST be supported by both T-PEs to be
   enabled.  This mode MUST NOT be used if the first PW segment
   connected to each T-PE is not using LDP.

   Note that this method MUST NOT be used to send messages that are
   permitted to originate at an S-PE.  Otherwise, race conditions could
   occur between messages sent via the control plane by S-PEs and
   messages sent via the data plane by T-PEs.

   Status codes, except for those listed below, MUST NOT be sent using
   the S-PE bypass procedure and MUST be ignored on reception.

      0x00000002 - Local Attachment Circuit (ingress) Receive Fault

      0x00000004 - Local Attachment Circuit (egress) Transmit Fault

      0x00000020 - PW forwarding standby
      0x00000040 - Request switchover to this PW

   Note that since "clear all failures" may be sent by an S-PE, it MUST
   NOT be sent using the S-PE bypass mode.

   When S-PE bypass mode is enabled, all PW Status TLVs received using
   this method have priority over PW Status TLVs sent via control
   protocols such as LDP [RFC4447].  However, the same PW Status TLVs
   MUST also be sent in LDP to keep the S-PEs state updated.

5.5.1.  S-PE Bypass Mode LDP Flag Bit

   When a PW Segment along an MS-PW is using the LDP control protocol
   and wishes to request the use of the S-PE bypass status message mode,
   it sets the B bit in the generic protocol flags interface parameters
   sub-TLV as shown in Figure 3.  This flag can only be set by a T-PE
   using LDP as the PW configuration and management protocol.  If the
   S-PE bypass mode LDP flag bit in the generic protocol flags interface
   parameter does not match in the FEC advertisement for directions of a
   specific PW, that PW MUST NOT be enabled.

   The interface parameter is defined 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type=0x18  |   Length=4    |R R R R R R R R R R R R R R R B|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: PW Generic Protocol Flags Sub-TLV

   TLV Type

      Type 0x18 - PW Generic Protocol Flags.

   Length

      TLV length is always 4 octets.

   Flags

      Bit B, in position 31 above, is set to request the S-PE bypass
      mode.  R bits are to be allocated by IANA as described in the IANA
      section.  If they are not allocated, they are to be considered as
      reserved for future use and MUST be zero on transmission and
      ignored on reception of this TLV.

      If the T-PE receives an LDP label mapping message containing a
      generic protocol flags interface parameter TLV with the bit B set,
      then the T-PE receiving the label mapping message MAY send S-PE
      bypass status messages in the PW ACh.  If bit B of said TLV is not
      set, or the TLV is not present, then the T-PE receiving the label
      mapping message MUST NOT send S-PE bypass status messages in the
      PW ACh.

6.  S-PE Operation

   The S-PE will operate according to the procedures defined in
   [RFC6073].  The following additional procedures apply to the case
   where a static PW segment is switched to a dynamic PW segment that
   uses LDP, and the case where a static PW segment is switched to
   another static PW segment.

6.1.  Static PW to Another Static PW

   The procedures that are described in [RFC6073] Section 10 also apply
   to the case of a static PW switched to another static PW.  The LDP
   header is simply replaced by the PW OAM header; otherwise, the packet
   format will be identical.  The information that is necessary to form
   an SP-PE TLV MUST be configured in the S-PE, or no SP-PE TLV will be
   sent.  [RFC6073] defines the IANA "Pseudowire Switching Point PE TLV
   Type" registry.  In order to support the static PW configuration and
   addressing scheme, the following new code point has been assigned:

      Type  Length   Description
      ----  ------   -----------
      0x07      24   Static PW/MPLS-TP PW segment ID of last
                     PW segment traversed

   The format of this TLV is that of the "Static Pseudowire Sub-TLV"
   defined in [RFC6426].

6.2.  Dynamic PW to Static PW or Vice Versa

   The procedures that are described in Section 10 of [RFC6073] also
   apply to this situation.  However, if the PW label of the LDP-
   controlled PW segment is withdrawn by the adjacent PE, the S-PE will
   set the PW status code "0x00000001 - Pseudowire Not Forwarding" to
   the adjacent PW on the static PW segment.

   The S-PE will only withdraw its label for the dynamic, LDP-controlled
   PW segment if the S-PE is not provisioned.

7.  Security Considerations

   The security measures described in [RFC4447], [RFC5085], and
   [RFC6073] are adequate for the proposed mechanism.

8.  IANA Considerations

   IANA has set up the registry of "PW Generic Protocol Flags".  These
   are bit strings of length 16.  Bit 0 is defined in this document.
   Bits 1 through 15 are to be assigned by IANA using the "IETF Review"
   policy defined in [RFC5226].

   Any requests for allocation from this registry require a description
   of up to 65 characters.

   Initial PW Generic Protocol Flags value allocations are as follows:

   Bit Mask     Description
   ====================================================================
   0x0001    -  S-PE bypass mode                              [RFC6478]

   This document uses a new Associated Channel Type.  IANA already
   maintains the "Pseudowire Associated Channel Types" registry.  The
   value 0x0027 has been assigned with the description "PW OAM Message".

   This document uses a new Pseudowire Switching Point PE TLV Type.
   IANA already maintains the "Pseudowire Switching Point PE sub-TLV
   Type" registry.  A value of 0x07 has been assigned with the
   description "Static PW/MPLS-TP PW segment ID of last PW segment
   traversed".

   This document uses a new interface parameter type.  IANA already
   maintains the "Pseudowire Interface Parameters Sub-TLV type
   Registry".  A value of 0x18 has been assigned with the description
   "PW Generic Protocol Flags".

9.  References

9.1.  Normative References

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

   [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.

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

   [RFC5085]    Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
                Virtual Circuit Connectivity Verification (VCCV): A
                Control Channel for Pseudowires", RFC 5085, December
                2007.

   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.

   [RFC6073]    Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
                Aissaoui, "Segmented Pseudowire", RFC 6073, January
                2011.

   [RFC6310]    Aissaoui, M., Busschbach, P., Martini, L., Morrow, M.,
                Nadeau, T., and Y(J). Stein, "Pseudowire (PW)
                Operations, Administration, and Maintenance (OAM)
                Message Mapping", RFC 6310, July 2011.

   [RFC6426]    Gray, E., Bahadur, N., Boutros, S., and R. Aggarwal,
                "MPLS On-Demand Connectivity Verification and Route
                Tracing", RFC 6426, November 2011.

9.2.  Informative References

   [REDUNDANCY] Muley, P., Ed., and M. Aissaoui, Ed., "Pseudowire
                Preferential Forwarding Status Bit", Work in Progress,
                September 2011.

   [RFC5885]    Nadeau, T., Ed., and C. Pignataro, Ed., "Bidirectional
                Forwarding Detection (BFD) for the Pseudowire Virtual
                Circuit Connectivity Verification (VCCV)", RFC 5885,
                June 2010.

   [RFC5586]    Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
                "MPLS Generic Associated Channel", RFC 5586, June 2009.

Authors' Addresses

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


   George Swallow
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, Massachusetts  01719
   United States
   EMail: swallow@cisco.com


   Giles Heron
   Cisco Systems
   9-11 New Square
   Bedfont Lakes
   Feltham
   Middlesex
   TW14 8HA
   United Kingdom
   EMail: giheron@cisco.com


   Matthew Bocci
   Alcatel-Lucent
   Voyager Place
   Shoppenhangers Road
   Maidenhead
   Berks
   SL6 2PJ
   United Kingdom
   EMail: matthew.bocci@alcatel-lucent.com