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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                          K. Patel
Request for Comments: 8395                                        Arrcus
Updates: 4761                                                 S. Boutros
Category: Standards Track                                         VMware
ISSN: 2070-1721                                                 J. Liste
                                                                   Cisco
                                                                  B. Wen
                                                                 Comcast
                                                              J. Rabadan
                                                                   Nokia
                                                               June 2018


               Extensions to BGP-Signaled Pseudowires to
                  Support Flow-Aware Transport Labels

Abstract

   This document defines protocol extensions required to synchronize
   flow label states among Provider Edges (PEs) when using the BGP-based
   signaling procedures.  These protocol extensions are equally
   applicable to point-to-point Layer 2 Virtual Private Networks
   (L2VPNs).  This document updates RFC 4761 by defining new flags in
   the Control Flags field of the Layer2 Info Extended Community.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8395.













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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Modifications to the Layer2 Info Extended Community .............4
   3. Signaling the Presence of the Flow Label ........................5
   4. IANA Considerations .............................................6
   5. Security Considerations .........................................6
   6. References ......................................................7
      6.1. Normative References .......................................7
      6.2. Informative References .....................................7
   Acknowledgements ...................................................8
   Contributors .......................................................8
   Authors' Addresses .................................................9

1.  Introduction

   The mechanism described in [RFC6391] uses an additional label (Flow
   Label) in the MPLS label stack to allow Label Switching Routers
   (LSRs) to balance flows within Pseudowires (PWs) at a finer
   granularity than the individual PWs across the Equal Cost Multiple
   Paths (ECMPs) that exists within the Packet Switched Network (PSN).

   Furthermore, [RFC6391] defines the LDP protocol extensions required
   to synchronize the flow label states between the ingress and egress
   PEs when using the signaling procedures defined in the [RFC8077].

   A PW [RFC3985] is transported over one single network path, even if
   ECMPs exist between the ingress and egress PW provider edge (PE)
   equipment.  This is required to preserve the characteristics of the
   emulated service.





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   This document introduces an optional mode of operation allowing a PW
   to be transported over ECMPs, for example when the use of ECMPs is
   known to be beneficial to the operation of the PW.  This
   specification uses the principles defined in [RFC6391] and augments
   the BGP-signaling procedures of [RFC4761] and [RFC6624].  The use of
   a single path to preserve the packet delivery order remains the
   default mode of operation of a PW and is described in [RFC4385] and
   [RFC4928].

   High-bandwidth Ethernet-based services are a prime example that use
   of the optional mode benefits from the ability to load-balance flows
   in a PW over multiple PSN paths.  In general, load-balancing is
   applicable when the PW attachment circuit bandwidth and PSN core link
   bandwidth are of the same order of magnitude.

   To achieve the load-balancing goal, [RFC6391] introduces the notion
   of an additional Label Stack Entry (LSE) (flow label) located at the
   bottom of the stack (right after PW LSE).  LSRs commonly generate a
   hash of the label stack in order to discriminate and distribute flows
   over available ECMPs.  The presence of the flow label (closely
   associated to a flow determined by the ingress PE) will normally
   provide the greatest entropy.

   Furthermore, following the procedures for inter-AS scenarios
   described in Section 3.4 of [RFC4761], the flow label should never be
   handled by the ASBRs; only the terminating PEs on each AS will be
   responsible for popping or pushing this label.  This is equally
   applicable to Method B as described in Section 3.4.2 of [RFC4761],
   where ASBRs are responsible for swapping the PW label as traffic
   traverses from ASBR to PE and ASBR to ASBR.  Therefore, the flow
   label will remain untouched across AS boundaries.

1.1.  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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.












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2.  Modifications to the Layer2 Info Extended Community

   The Layer2 Info Extended Community is used to signal control
   information about the PWs to be set up.  The Extended Community
   format is described in [RFC4761].  The format of this Extended
   Community is described as:

            +------------------------------------+
            | Extended Community type (2 octets) |
            +------------------------------------+
            |  Encaps Type (1 octet)             |
            +------------------------------------+
            |  Control Flags (1 octet)           |
            +------------------------------------+
            |  Layer-2 MTU (2 octets)            |
            +------------------------------------+
            |  Reserved (2 octets)               |
            +------------------------------------+

            Figure 1: Layer2 Info Extended Community

   Control Flags:

   This field contains bit flags relating to the control information
   about PWs.  This field is augmented with a definition of two new
   flags fields.

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |Z|Z|Z|Z|T|R|C|S|      (Z = MUST Be Zero)
            +-+-+-+-+-+-+-+-+

            Figure 2: Control Flags Bit Vector

   With reference to the Control Flags Bit Vector, the following bits in
   the Control Flags are defined.  The remaining bits, designated "Z",
   MUST be set to zero when sending and MUST be ignored when receiving
   this Extended Community.

      T   When the bit value is 1, the PE announces the ability to send
          a PW packet that includes a flow label.  When the bit value is
          0, the PE is indicating that it will not send a PW packet
          containing a flow label.

      R   When the bit value is 1, the PE is able to receive a PW packet
          with a flow label present.  When the bit value is 0, the PE is
          unable to receive a PW packet with the flow label present.




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      C   Defined in [RFC4761].

      S   Defined in [RFC4761].

3.  Signaling the Presence of the Flow Label

   As part of the PW signaling procedures described in [RFC4761], a
   Layer2 Info Extended Community is advertised in the Virtual Private
   LAN Service (VPLS) BGP Network Layer Reachability Information (NLRI).

   A PE that wishes to send a flow label in a PW packet MUST include in
   its VPLS BGP NLRI a Layer2 Info Extended Community using Control
   Flags field with T = 1.

   A PE that is willing to receive a flow label in a PW packet MUST
   include in its VPLS BGP NLRI a Layer2 Info Extended Community using
   Control Flags field with R = 1.

   A PE that receives a VPLS BGP NLRI containing a Layer2 Info Extended
   Community with R = 0 MUST NOT include a flow label in the PW packet.

   Therefore, a PE sending a Control Flags field with T = 1 and
   receiving a Control Flags field with R = 1 MUST include a flow label
   in the PW packet.  With any other combination, a PE MUST NOT include
   a flow label in the PW packet.

   A PE MAY support the configuration of the flow label (T and R bits)
   on a per-service basis (e.g., a VPLS VPN Forwarding Instance (VFI)).
   Furthermore, it is also possible that on a given service, PEs may not
   share the same flow label settings.  The presence of a flow label is
   therefore determined on a per-peer basis and according to the local
   and remote T and R bit values.  For example, a PE part of a VPLS and
   with a local T = 1 must only transmit traffic with a flow label to
   those peers that signaled R = 1.  If the same PE has local R = 1, it
   must only expect to receive traffic with a flow label from peers with
   T = 1.  Any other traffic must not have a flow label.  A PE expecting
   to receive traffic from a remote peer with a flow label MAY drop
   traffic that has no flow label.  A PE expecting to receive traffic
   from a remote peer with no flow label MAY drop traffic that has a
   flow label.

   Modification of flow label settings may impact traffic over a PW, as
   these could trigger changes in the PEs data-plane programming (i.e.,
   imposition/disposition of the flow label).  This is an
   implementation-specific behavior and is outside the scope of this
   document.





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   The signaling procedures in [RFC4761] state that the unspecified bits
   in the Control Flags field (bits 0-5) MUST be set to zero when
   sending and MUST be ignored when receiving.  The signaling procedure
   described here is therefore backwards compatible with existing
   implementations.  A PE not supporting the extensions described in
   this document will always advertise a value of zero in the R bit;
   therefore, a flow label will never be included in a packet sent to it
   by one of its peers.  Similarly, it will always advertise a value of
   zero in the T bit; therefore, a peer will know that a flow label will
   never be included in a packet sent by it.

   Note that what is signaled is the desire to include the flow LSE in
   the label stack.  The value of the flow label is a local matter for
   the ingress PE, and the label value itself is not signaled.

4.  IANA Considerations

   Although [RFC4761] defined a Control Flags Bit Vector as part of the
   Layer2 Info Extended Community, it did not ask for the creation of a
   registry.

   Per this document, IANA has created the "Layer2 Info Extended
   Community Control Flags Bit Vector" registry
   <https://www.iana.org/assignments/bgp-extended-communities>.

   Based on [RFC4761] and this document, the initial contents of this
   registry are as follows:

   Value   Name                               Reference
   -----   --------------------------------   --------------
   T       Request to send a flow label       This document
   R       Ability to receive a flow label    This document
   C       Presence of a Control Word         RFC 4761
   S       Sequenced delivery of frames       RFC 4761

   As per [RFC4761] and this document, the remaining bits are
   unassigned, and MUST be set to zero when sending and MUST be ignored
   when receiving the Layer2 Info Extended Community.

5.  Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in [RFC4271] and [RFC4761].








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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed.,
              "A Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4761]  Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network",
              RFC 6391, DOI 10.17487/RFC6391, November 2011,
              <https://www.rfc-editor.org/info/rfc6391>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [RFC3985]  Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <https://www.rfc-editor.org/info/rfc3985>.

   [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, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC8077]  Martini, L., Ed., and G. Heron, Ed., "Pseudowire Setup and
              Maintenance Using the Label Distribution Protocol (LDP)",
              STD 84, RFC 8077, DOI 10.17487/RFC8077, February 2017,
              <https://www.rfc-editor.org/info/rfc8077>.





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   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, DOI 10.17487/RFC4928, June 2007,
              <https://www.rfc-editor.org/info/rfc4928>.

   [RFC6624]  Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2
              Virtual Private Networks Using BGP for Auto-Discovery and
              Signaling", RFC 6624, DOI 10.17487/RFC6624, May 2012,
              <https://www.rfc-editor.org/info/rfc6624>.

Acknowledgements

   The authors would like to thank Bertrand Duvivier and John Drake for
   their review and comments.

Contributors

   In addition to the authors listed above, the following individuals
   also contributed to this document:

      Eric Lent

      John Brzozowski

      Steven Cotter


























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Authors' Addresses

   Keyur Patel
   Arrcus

   Email: keyur@arrcus.com


   Sami Boutros
   VMware

   Email: boutros.sami@gmail.com


   Jose Liste
   Cisco

   Email: jliste@cisco.com


   Bin Wen
   Comcast

   Email: bin_wen@cable.comcast.com


   Jorge Rabadan
   Nokia

   Email: jorge.rabadan@nokia.com





















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