Using GAL as a VCCV Channel Indicator
draft-ietf-pals-vccv-for-gal-01

PWE3                                                           T. Nadeau
Internet-Draft                                               lucidvision
Intended status: Standards Track                              L. Martini
Expires: August 3, 2015                                        S. Bryant
                                                           Cisco Systems
                                                        January 30, 2015

                 Using GAL as a VCCV Channel Indicator
                    draft-ietf-pals-vccv-for-gal-01

Abstract

   This document specifies a new Virtual Circuit Connectivity
   Verification (VCCV) (RFC5085) control channel type for use with
   pseudowires (PW) carried over an MPLS network.  This new channel type
   uses the Generic Associated Channel Label (GAL) (RFC5586) to
   distinguish VCCV packets from packets carrying user data.

Status of This Memo

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  GAL VCCV Control Channel Type . . . . . . . . . . . . . . . .   3
   4.  FAT PWs . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Multi-Segment Pseudowires . . . . . . . . . . . . . . . . . .   4
   6.  VCCV Capability Advertisement . . . . . . . . . . . . . . . .   5
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  MPLS VCCV Control Channel (CC) Type 4 . . . . . . . . . .   6
     9.2.  LDP Status Code . . . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document specifies a new Virtual Circuit Connectivity
   Verification (VCCV) [RFC5085] control channel (CC) type for use with
   pseudowires (PW) carried over an MPLS network that do not use the PW
   Control Word (CW) [RFC4385].  This new VCCV CC type uses the Generic
   Associated Channel Label (GAL) [RFC5586] to distinguish VCCV packets
   from packets carrying user data.  This new VCCV CC type introduces
   compatibility with the method of MPLS Label Switched Path (LSP)
   Operations, Administration, and Maintenance (OAM) identification,
   particularly in MPLS-TP networks [RFC5921].

   VCCV currently specifies three CC types.  VCCV CC Type 1 uses the PW
   Control Word (CW) to distinguish VCCV packets from packets carrying
   user data.  VCCV CC Types 2 and 3 require IP encapsulation for OAM
   packets they carry.  This was not an issue when [RFC5085] was
   designed, but is in conflict with the design goals of MPLS-TP
   [RFC5921] which does not otherwise require the availability of IP.
   VCCV CC Type 2 is not applicable to multi-segment PWs (MS-PWs)
   [RFC6073].  A MS-PW operating without the CW therefore has to use
   VCCV CC Type 3 which identifies VCCV packets on the basis of TTL
   expiry.  Whilst less of an issue with a single segment PW (SS-PW), on
   an MS-PW this need to be accurately set to cause TTL expiry at the
   egress Terminating Provider Edge (T-PE) [RFC6073].  In the event of a
   error in the setting of the PW LSE TTL this can result in VCCV
   packets leaking into the attachment circuit which may disrupt the

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   operation of the PW, or the user service, and is a security risk.
   The new VCCV CC type defined in this specification addresses these
   problems for PWs that do not use the CW.

   Note that VCCV CC Type 3 remains mandatory for multi-segment PW (MS-
   PW) OAM and it is currently combined with VCCV CC Type 1 for MS-PWs
   that use the CW [RFC6073].  This specification enables the
   combination of VCCV CC Type 3 with VCCV Type 4 for PWs that do not
   support the CWSection 5.

   For reasons of network efficiency and due to hardware constraints it
   is not possible to address these issue by mandating that all PWs use
   the PW CW, hence the introduction of this new VCCV CC type.  PWs
   without the CW are widely deployed, and hence mandating that all PWs
   use the CW is not a viable way to address this issue.

2.  Requirements Language

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

3.  GAL VCCV Control Channel Type

   When the PW CW is not used, the GAL VCCV Control Channel (CC) type
   defined in this section MAY be used.  This is referred to as VCCV CC
   Type4 throughout the rest of this of this document.  VCCV Type 4 uses
   the encapsulation shown in Figure 1.

   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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            PW LSE                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           GAL LSE                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 1|Version|   Reserved    |        Channel Type           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                        VCCV Message Body                      ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

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   The VCCV message body is preceded by a Generic Associated Channel
   Header as defined in [RFC5586], in which the Channel Type identifies
   the type and format of the OAM message carried in the VCCV message
   body.

   The GAL LSE MUST contain the GAL reserved label as defined in
   [RFC5586].

   The PW LSE is constructed according to the existing procedures that
   apply to the type of pseudowire that is in use.

   Note that the inclusion of a GAL following the PW LSE over a label
   switched path subject to Equal-Cost Multi-path (ECMP) load balancing
   can cause the OAM packet to take a different path through the network
   from the corresponding PW data packets.  If that is not acceptable,
   then an alternative VCCV type MUST be used.

4.  FAT PWs

   [RFC6391] specifies that when the flow-aware transport (FAT) of
   pseudowires over an MPLS packet switched network has been signalled
   or configured, the Flow LSE MUST be present.  It further specifies
   that "the flow label MUST NOT be an MPLS reserved label (values in
   the range 0..15) [RFC3032]", and that "If a flow LSE is present, it
   MUST be checked to determine whether it carries a reserved label.  If
   it is a reserved label, the packet is processed according to the
   rules associated with that reserved label; otherwise, the LSE is
   discarded."

   This document specifies that if the flow-aware transport of
   pseudowires over an MPLS packet switched network has been signalled
   or configured then the presence of VCCV message is indicated by the
   use of a GAL in place of the flow LSE.

   This is consistent with [RFC6391], and the packet structure is
   identical to that shown in Figure 1.

   Note that the use of a GAL in place of the flow label over a label
   switched path subject to ECMP can cause the OAM packet to take a
   different path through the network from the corresponding PW data
   packets.  If that is not acceptable, then an alternative VCCV type
   MUST be used.

5.  Multi-Segment Pseudowires

   When using VCCV CC Type 4 for MS-PWs, a PE transmitting the VCCV
   packet to a Switching PE (S-PE) MUST set the TTL to the appropriate
   value to expire at that S-PE.  An S-PE that supports this

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   specification MUST inspect packets PW packet that are received as a
   result of TTL expiry, determine whether a GAL follows the PW LSE.  If
   a GAL is present the S-PE then processes the VCCV packet.

   An S-PE that does not support this specification would be expected to
   reject as malformed a VCCV CC Type 4 packet that was received.  This
   is because the S-PE would expect the PW LSE to be bottom of stack
   (the non FAT case) and for the LSE at bottom of stack not to be a
   reserved label (both the FAT and the non-FAT cases).  An S-PE that
   did not make this reserved label check would then find that the first
   nibble following the label stack was 0x1 and not the expected start
   of an IP packet.  It would hence be expected to also reject the
   packet.  This update to the behaviour of S-PEs is therefore backwards
   compatible.

6.  VCCV Capability Advertisement

   The VCCV capability advertisement MUST match the c-bit setting that
   is advertised in the PW FEC element [RFC4447].  If the c-bit is set,
   indicating the use of the PW CW, then VCCV CC Type 4 MUST NOT be
   advertised.  If the c-bit is not set, indicating that the PW CW is
   not in use, then an equipment supporting this specification MUST
   advertise VCCV CC Type 4.  Advertisement of VCCV CC Types 1 and 4 are
   therefore mutually exclusive.

   A PE supporting VCCV CC Type 4 MAY advertise other VCCV CC types as
   defined in [RFC5085] .

   If the remote PE supports VCCV CC Type 4, and the PW CW is not in
   use, then the following capability advertisement precedence rules
   supersede those defined in Section 7 of [RFC5085] :

   1.  Type 4: GAL VCCV Control Channel.

   2.  Type 2: MPLS Router Alert Label.

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

   If the remote PE finds that VCCV CC Types 1 and 4 are both
   advertised, or that c-bit is set and VCCV CC Type 4 is advertised,
   then it should report the error to the operator through the
   management interface in use, and send a Label Release Message with a
   status code "VCCV Type Error".

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7.  Manageability Considerations

   Whilst the introduction of this additional VCCV CC type increases the
   number of VCCV CC types that the operator needs to manage, it
   addresses the issues with VCCV CC Types 2 and 3 described in .
   (Section 1).

   In the event of a misconfiguration of this VCCV CC type, the PW is
   taken out of service and the operator advised as described in
   Section 6.

   Attention is drawn to the possible absence of fate sharing between PW
   data packets and VCCV CC Type 4 packets described in Section 3 and
   Section 4.

8.  Security Considerations

   This document does not by itself raise any new security
   considerations beyond those described in [RFC5085].  It addresses the
   possibility of packet leaking that can occur with VCCV CC Type 3.

9.  IANA Considerations

9.1.  MPLS VCCV Control Channel (CC) Type 4

   IANA is requested to assign a new bit from the MPLS VCCV Control
   Channel (CC) Types registry in the PWE3-parameters name space in
   order to identify VCCV type 4.  It is recommended that Bit 3 be
   assigned to this purpose which would have a value of 0x08.

   MPLS VCCV Control Channel (CC) Types

         Bit (Value)    Description   Reference
         ============   ===========   ==================
         Bit X (0x0Y)   Type 4        This Specification

9.2.  LDP Status Code

   IANA is requested to assign a new Status Code from the Label
   Distribution Protocol (LDP) Parameters name space:

   Status Code Name Space

         Range/Value  E  Description      Reference
         ===========  =  ===============  =========
         0x000000xx   0  VCCV Type Error  This Specification

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10.  Acknowledgments

   The authors wish to thank Alexander (Sasha) Vainshtein for his review
   comments and for his proposal to make the GAL and Flow labels
   mutually exclusive.  This proposal let to a significant
   simplification of this design.

11.  References

11.1.  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., 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. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

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

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

   [RFC6391]  Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
              J., and S. Amante, "Flow-Aware Transport of Pseudowires
              over an MPLS Packet Switched Network", RFC 6391, November
              2011.

11.2.  Informative References

   [RFC5921]  Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks", RFC
              5921, July 2010.

Authors' Addresses

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   Thomas D. Nadeau
   lucidvision

   Email: tnadeau@lucidvision.com

   Luca Martini
   Cisco Systems

   Email: lmartini@cisco.com

   Stewart Bryant
   Cisco Systems

   Email: stbryant@cisco.com

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