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Versions: 00 01

Internet Engineering Task Force                               Yimin Shen
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                              Ravi Singh
Expires: August 6, 2020                           Individual Contributor
                                                             Yuji Kamite
                                                      NTT Communications
                                                        February 3, 2020


               BGP Flexible Color-Based Tunnel Selection
           draft-shen-idr-flexible-color-tunnel-selection-01

Abstract

   This document discusses color-based tunnel selection for BGP payload
   prefixes.  It defines a set of extended mapping modes, and describes
   how to use them to construct tunnel selection schemes for flexible
   tunnel selection.  Tunnel selection schemes can be implemented as
   policies on routers performing tunnel selection, or signaled by next
   hop routers or a central controller by using the BGP extensions
   specified in this document.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 6, 2020.

Copyright Notice

   Copyright (c) 2020 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



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   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
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   4
   3.  Extended Mapping Modes  . . . . . . . . . . . . . . . . . . .   4
   4.  Tunnel Selection Scheme and Operation . . . . . . . . . . . .   6
   5.  Provisioning of Tunnel Selection Schemes  . . . . . . . . . .   7
   6.  BGP Tunnel Encapsulation Attribute Extensions . . . . . . . .   8
     6.1.  Wildcard Tunnel Type  . . . . . . . . . . . . . . . . . .   8
     6.2.  Color Tunnel Selection Scheme Sub-TLV . . . . . . . . . .   8
       6.2.1.  Extended Mapping Mode Sub-sub-TLV . . . . . . . . . .   9
       6.2.2.  Encoding  . . . . . . . . . . . . . . . . . . . . . .  10
       6.2.3.  Decoding  . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  Association between Color Tunnel Selection Scheme Sub-TLV
           and Tunnel Type . . . . . . . . . . . . . . . . . . . . .  11
   7.  Relationship with Color-Only Bits of Color Extended Community  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In an overlay network using BGP for payload prefix distribution,
   transporting the packets of a payload prefix from a BGP router to the
   next BGP router relies on the selection of a transport tunnel.  This
   selection may be based on various attributes of the prefix, such as
   BGP next hop, color, and information in the Tunnel Encapsulation
   Attribute [BGP-TUNNEL-ENCAP], etc.

   In one tunnel selection model, color is used as a primary criterion
   along with BGP next hop or tunnel endpoint address (contained in a
   Tunnel Endpoint sub-TLV of the Tunnel Encapsulation Attribute).  This
   model is referred to as "color-based" tunnel selection in this
   document.  The model is gaining many use cases today due to its
   general applicability.  In particular, color as a generic notion may
   be used to represent a broad range of network attributes, such as
   virtual topology, network slice, path computation algorithm, TE
   constraint, administrative profile, etc.  For some of the attributes,



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   there may not be a convenient mechanism to associate them with
   payload prefixes or tunnels, or to distribute them in a network,
   especially across domains or to a central controller.  When these
   attributes need to be considered in tunnel selection, mapping them to
   colors and performing color-based tunnel selection will provide a
   generic solution.

   The procedures in this document is relevant to color-based tunnel
   selection.  In general, payload prefixes may be associated with
   colors through configuration or a Color Extended Community [RFC5512].
   Transport tunnels may also be associated with colors through
   configuration (e.g.  RSVP and LDP tunnels), a Color Extended
   Community (e.g.  BGP LU), or a color embedded in BGP NLRI (e.g.  BGP
   SR-TE policy [BGP-SR-POLICY]), etc.  These payload prefixes and
   tunnels are called "colored payload prefixes" and "colored tunnels",
   respectively.

   Normally, a payload prefix of color X is intended to be mapped to a
   tunnel of the same color X.  This is considered as the default
   mapping mode of color-based tunnel selection.  In some cases, when a
   tunnel of color X cannot be found or established, or when a
   previously mapped tunnel of color X fails, a network operator may
   want to map the payload prefix by attempting other modes, e.g.
   selecting a tunnel of another color Y, a tunnel without a color, a
   tunnel of color X but with an IPv4-mapped IPv6 endpoint address, and
   so on.  Note that the colors may represent network slices, virtual
   topologies, path computation algorithms, etc.  Hence, these modes
   will provide the flexibility and enable the operator to take the full
   transport capability of the network.  In this document, these modes
   are called "extended mapping modes", and the procedure of
   automatically executing them in a user-defined order is called
   "fallback".

   This document defines a set of extended mapping modes to complement
   the default mapping mode.  It introduces the notion of "tunnel
   selection scheme".  A tunnel selection scheme is an ordered list of
   extended mapping modes to be automatically executed in tunnel
   selection.  When a tunnel is not selected by using the first mode in
   the list, fallback is performed by attempting the second mode, the
   third mode, and so on, until a tunnel is selected or the list is
   exhausted.

   Color-based tunnel selection for uncolored payload prefixes is also
   considered in this document as a special case.  By using a tunnel
   selection scheme, a colored or uncolored tunnel may be selected for
   an uncolored payload prefix in a flexible manner.





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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] and
   [RFC8174].

3.  Extended Mapping Modes

   This document defines a set of extended mapping modes for flexible
   color-based tunnel selection.  Each mode specifies how a payload
   prefix's endpoint IP address (derived from BGP next hop or a Tunnel
   Endpoint sub-TLV in the Tunnel Encapsulation Attribute
   [BGP-TUNNEL-ENCAP]) and color are used to select a tunnel.  The
   document assumes that each payload prefix SHOULD have a single color
   for tunnel selection purpose or no color, and each tunnel SHOULD have
   a single color or no color.

   In the definitions of the extended mapping modes below, N represents
   a payload prefix's endpoint IP address, and C represents its color.
   An uncolored payload prefix does not have a color.  An extended
   mapping mode may have multiple steps for sub-level fallback within
   it.  The steps are executed sequentially.  The mode is completed as
   soon as a tunnel is successfully selected in one of the steps, and
   the rest steps are skipped.

   (1) IP-color, optionally with a fallback color list of {C1, ...,Cn}

      - If the payload prefix has a color C, select a tunnel with
      endpoint address N and color C.

      - Select a tunnel with endpoint address N and color C1.

      - ...

      - Select a tunnel with endpoint address N and color Cn.

   (2) Color-only, optionally with a fallback color list of {C1, ...,
   Cn}

      - If the payload prefix has a color C, select a tunnel with color
      C, regardless of the tunnel's endpoint address.

      - Select a tunnel with color C1, regardless of tunnel's endpoint
      address.

      - ...




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      - Select a tunnel with color Cn, regardless of tunnel's endpoint
      address.

   (3) IP-any-color

      - Select a tunnel with endpoint address N and any color.

   (4) IP-only

      - Select a tunnel with endpoint address N and without a color.

   (5) Converted-IPv6

   This mode is applicable when N is an IPv4 address.  Assume N' is the
   IPv6 address mapped from N.

      - Select a tunnel with endpoint address N' and without a color.

   (6) Converted-IPv6-color, optionally a fallback color list of {C1,
   ..., Cn}

   This mode is applicable when N is an IPv4 address.  Assume N' is the
   IPv6 address mapped from N.

      - If the payload prefix has a color C, select a tunnel with
      endpoint address N' and color C.

      - Select a tunnel with endpoint address N' and color C1.

      - ...

      - Select a tunnel with endpoint address N' and color Cn.

   (7) Converted-IPv6-any-color

   This mode is applicable when N is an IPv4 address.  Assume N' is the
   IPv6 address mapped from N.

      - Select a tunnel with endpoint address N' and any color.

   (8) Color-profile

      - If the payload prefix has a color C, use C as key to look up a
      profile to construct tunnel selection criteria and select a
      tunnel.






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   As shown above, the IP-color, Color-only, and Converted-IPv6-color
   modes may have a fallback color list for sub-level fallback across
   the colors.

   This list is not exhaustive.  More modes MAY be defined in the
   future.

4.  Tunnel Selection Scheme and Operation

   A tunnel selection scheme is defined as an ordered list of extended
   mapping modes (Section 3) to be automatically executed in tunnel
   selection.  The first mode in the list is called a "primary" mode,
   and all the subsequent modes are called "fallback" modes.  A scheme
   MUST have a primary mode, but MAY or MAY not have any fallback mode.

   When a scheme is executed for a payload prefix, the modes in the list
   are executed sequentially, and within each mode, the steps of sub-
   level fallback are executed sequentially.  When a tunnel is selected
   in a particular step in a particular mode, the scheme is completed,
   and all subsequent steps of the mode and all the subsequent modes in
   the list are skipped.  If no tunnel is selected when the list is
   exhausted, the payload prefix will remain in unresolved state for
   transport.

   In the case where a previously selected tunnel becomes inoperative,
   the scheme SHOULD be run to reselect a tunnel.  In the case where a
   tunnel was previously selected and later another tunnel of higher
   preference (in the tunnel selection scheme or in a fallback color
   list) becomes available, the new tunnel MAY be selected to replace
   the current tunnel.  This procedure is called a reversion.  It may be
   performed manually by a network operator, or triggered automatically
   by the situation.

   The following are some examples of tunnel selection schemes.

   Example 1:

   A payload prefix has a tunnel endpoint IPv4 address 203.0.113.1 and a
   color RED.  It is associated with the following tunnel selection
   scheme:

      (1) IP-color

      (2) Converted-IPv6-color

      (3) IP-only

   The intended tunnel selection procedure is:



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      (1) Find a tunnel with endpoint IPv4 address 203.0.113.1 and color
      RED.

      (2) If the above is unsuccessful, convert the IPv4 address to an
      IPv6 address 2002:cb00:7101::/64.  Find a tunnel with endpoint
      IPv6 address 2002:cb00:7101::/64 and color RED.

      (3) If the above is unsuccessful, find a tunnel with endpoint IPv4
      address 203.0.113.1 and without a color.

   Example 2:

   A prefix has a tunnel endpoint IPv4 address 203.0.113.1 and a color
   RED.  It is associated with the following tunnel selection scheme:

      (1) IP-color, with a fallback color list = {BLUE, GREEN}

      (2) Converted-IPv6-color, with a fallback color list = {WHITE}

      (3) IP-only

   The intended tunnel selection procedure is:

      (1) Find a tunnel with endpoint IPv4 address 203.0.113.1 and color
      RED.  If it is unsuccessful, find a tunnel with endpoint IPv4
      address 203.0.113.1 and color BLUE.  If it is unsuccessful, find a
      tunnel with endpoint IPv4 address 203.0.113.1 and color GREEN.

      (2) If the above is unsuccessful, convert the IPv4 address to an
      IPv6 address 2002:cb00:7101::/64.  Find a tunnel with endpoint
      IPv6 address 2002:cb00:7101::/64 and color RED.  If it is
      unsuccessful, find a tunnel with endpoint IPv6 address
      2002:cb00:7101::/64 and color WHITE.

      (3) If the above is unsuccessful, find a tunnel with endpoint IPv4
      address 203.0.113.1 and without a color.

5.  Provisioning of Tunnel Selection Schemes

   A tunnel selection scheme MAY be provisioned for a payload prefix on
   a router which performs tunnel selection.  In this case, the scheme
   may be implemented as a policy on the router.  The configuration of
   such policy varies by vendors, and hence is out of the scope of this
   document.

   A tunnel selection scheme MAY also be provisioned on a router or a
   central controller which originates the UPDATE message of a payload
   prefix, and then distributed to a router(s) which will perform tunnel



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   selection.  To facilitate this, the document introduces a new "Color
   Tunnel Selection Scheme" sub-TLV (Section 6) to the Tunnel
   Encapsulation Attribute to carry the information.  As color-based
   tunnel selection is typically across all tunnel types, the document
   also introduces a new "Wildcard" tunnel type to the Tunnel
   Encapsulation Attribute.  When the tunnel selection scheme contained
   in a Color Tunnel Selection Scheme sub-TLV is applicable to all
   tunnel types, the top-level Tunnel Encapsulation Attribute TLV SHOULD
   set tunnel type to Wildcard.

   In the case where a payload prefix has one scheme configured as a
   policy on a router, and another scheme received in a Color Tunnel
   Selection Scheme sub-TLV, the router SHOULD treat the policy in
   preference to the received information.

   If a payload prefix does not have a tunnel selection scheme, the
   default mapping mode applicable to colored or non-colored payload
   prefixes SHOULD be used accordingly.

6.  BGP Tunnel Encapsulation Attribute Extensions

   This section specifies a new "Wildcard" tunnel type and a new Color
   Tunnel Selection Scheme sub-TLV for the Tunnel Encapsulation
   Attribute.

6.1.  Wildcard Tunnel Type

   The Wildcard tunnel type has the semantics of "any tunnel types".  It
   allows a Tunnel Encapsulation Attribute TLV to carry information
   which is regardless of tunnel type or applicable to all tunnel types.
   In this document, it is used when a Tunnel Encapsulation Attribute
   TLV contains a Color Tunnel Selection Scheme sub-TLV which is
   applicable to all tunnel types.

6.2.  Color Tunnel Selection Scheme Sub-TLV

   The Color Tunnel Selection Scheme sub-TLV is used to carry the
   information of a tunnel selection scheme.  The sub-TLV is contained
   in a Tunnel Encapsulation Attribute TLV.  If the Tunnel Encapsulation
   Attribute TLV's tunnel type is Wildcard, the tunnel selection scheme
   is regardless of tunnel type.  If the Tunnel Encapsulation Attribute
   TLV's tunnel type is a specific type, the tunnel selection scheme is
   applicable to tunnels of that type.  In any case, a Tunnel
   Encapsulation Attribute TLV MUST not contain more than one Color
   Tunnel Selection Scheme sub-TLV.

   The sub-TLV's Type is TBD (to be allocated by IANA).  The sub-TLV's
   Length is the number of the octets of the sub-TLV's Value field.  The



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   sub-TLV's Value field is composed of one or multiple Extended Mapping
   Mode sub-sub-TLVs.

6.2.1.  Extended Mapping Mode Sub-sub-TLV

   An Extended Mapping Mode sub-sub-TLV contains the information of an
   extended mapping mode.  Its encoding is 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     0x01      |    Length     |             Mode              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Color_1 (optional)                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               ~                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Color_n (optional)                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

   The Extended Mapping Mode sub-sub-TLV's Type is 0x01.

   The Extended Mapping Mode sub-sub-TLV's Length is the total number of
   octets of the sub-sub-TLV's Value field.

   The Extended Mapping Mode sub-sub-TLV's Value field contains a
   2-octet extended mapping mode defined as below, and an optional
   fallback color list.

      1 - IP-color

      2 - Color-only

      3 - IP-any-color

      4 - IP-only

      5 - Converted-IPv6

      6 - Converted-IPv6-color

      7 - Converted-IPv6-any-color

      8 - Color-profile





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   The IP-color, Color-only and Converted-IPv6-color modes MAY have an
   optional fallback color list.  The list contains one or multiple
   4-octet color values, i.e. Color_1, ..., Color_n, in the order from
   the highest preference to the lowest preference.

6.2.2.  Encoding

   Given a tunnel selection scheme, a Color Tunnel Selection Scheme sub-
   TLV is constructed in the following manner:

   o  First, an Extended Mapping Mode sub-sub-TLV containing the primary
      mode is added.  If this mode is IP-Color, Color-Only, or
      Converted-IPv6-Color, and if cross-color fallback is applicable to
      this mode, a fallback color list is added to the sub-sub-TLV.

   o  If there is one or multiple desired fallback modes, an Extended
      Mapping Mode sub-sub-TLV containing the first fallback mode is
      added.  If this mode is IP-Color, Color-Only, or Converted-
      IPv6-Color, and if cross-color fallback is applicable to this
      mode, a fallback color list is added to the sub-sub-TLV.

   o  This process continues, until an Extended Mapping Mode sub-sub-TLV
      containing the last fallback mode is added.  If this mode is IP-
      Color, Color-Only, or Converted-IPv6-Color, and if cross-color
      fallback is applicable to this mode, a fallback color list is
      added to the sub-sub-TLV.

6.2.3.  Decoding

   When decoding a Color Tunnel Selection Scheme sub-TLV, a receiving
   router MUST interpret the preference of the contained Extended
   Mapping Mode sub-sub-TLVs as the order in which they are encoded.  If
   an Extended Mapping Mode sub-sub-TLV contains a mode which is not IP-
   Color, Color-Only, or Converted-IPv6-Color but has a fallback color
   list, the entire Color Tunnel Selection Scheme sub-TLV SHOULD be
   considered as malformatted and ignored.

   A receiving router MUST consider a payload prefix as having a
   modified tunnel selection scheme in any of the following situations,
   and perform tunnel selection accordingly:

   o  The payload prefix did not have a valid Color Tunnel Selection
      Scheme sub-TLV in the previous UPDATE message, and it has one in
      the latest UPDATE message.  Tunnel selection MUST be performed
      based on the latest tunnel selection scheme.

   o  The payload prefix had a valid Color Tunnel Selection Scheme sub-
      TLV in the previous UPDATE message, but it does not have one in



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      the latest UPDATE message.  Tunnel selection MUST revert to the
      default color or non-color mapping mode.

   o  The payload prefix had a valid Color Tunnel Selection Scheme sub-
      TLV in the previous UPDATE message, and it has one with different
      content in the latest UPDATE message.  Tunnel selection MUST be
      performed based on the latest tunnel selection scheme.

6.3.  Association between Color Tunnel Selection Scheme Sub-TLV and
      Tunnel Type

   A Color Tunnel Selection Scheme sub-TLV MAY be contained in a Tunnel
   Encapsulation Attribute TLV of Wildcard tunnel type, indicating that
   the scheme SHOULD be performed regardless of tunnel type.  The sub-
   TLV MAY also be contained in a Tunnel Encapsulation Attribute TLV of
   a specific tunnel type, indicating that the scheme SHOULD consider
   only the tunnels of that type.  In the case where a Tunnel
   Encapsulation Attribute contains a TLV of Wildcard tunnel type and
   another TLV of a specific tunnel type, and both TLVs contain a Color
   Tunnel Selection Scheme sub-TLV, tunnel selection for that specific
   tunnel type SHOULD be based on the corresponding Color Tunnel
   Selection Scheme sub-TLV.

7.  Relationship with Color-Only Bits of Color Extended Community

   [RFC8402] and [BGP-SR-POLICY] define two "Color-Only" bits (i.e.  CO
   bits) in the BGP Color Extended Community for color-based tunnel
   selection in the context of segment routing.  Each of the four
   combinations of the CO bits corresponds to a predefined fallback
   scheme.

   This document complements these documents by supporting more generic
   and flexible fallback schemes which are user definable.  In fact, the
   predefined fallback schemes of the CO bits can be fully supported by
   using the Color Tunnel Selection Scheme sub-TLV.  When a router
   advertises an UPDATE message, it SHOULD NOT use a Color Extended
   Community with CO bits and a Color Tunnel Selection Scheme sub-TLV at
   the same time, in order to avoid collision between them.  If a router
   receives an UPDATE message containing both objects, it SHOULD give
   preference to CO bits, and ignore the other.  If the tunnel selection
   scheme is implemented as a policy on the receiving router, the router
   SHOULD give the preference to the policy.

8.  IANA Considerations

   This document requires the IANA to allocate a value for the Wildcard
   tunnel type as a new BGP Tunnel Encapsulation Attribute Type, and a
   type value for the new Color Tunnel Selection Scheme sub-TLV.



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9.  Security Considerations

   This document introduces procedures for color-based tunnel selection
   to use tunnel selection schemes.  The procedures can cause traffic to
   be diverted from default path(s).  This requires measures to be taken
   at a number of levels to avoid undesirable transport behaviors and
   security risks.  First, network coloring (i.e. color assignment to
   network resources, attributes, payload prefixes, tunnels, etc.)  MUST
   be carefully planned and validated at a global level to avoid errors
   and collisions.  Second, tunnel selection schemes MUST be legitimate
   and always select valid tunnels leading to desired endpoints.  For
   schemes implemented as policies, this SHOULD be ensured by policy
   configuration.  For schemes distributed via Color Tunnel Selection
   Scheme sub-TLV of BGP Tunnel Encapsulation Attribute, receivers
   SHOULD use BGP security procedures to validate each originator's
   identity and detect unauthorized content modification during
   distribution.

10.  Acknowledgements

   This document leverages work done by Junan Chen.  Thanks to Jeff Hass
   and Srihari Sangli for their kind reviews and comments which helped
   to improve the clarity of this document.

11.  References

11.1.  Normative References

   [RFC5512]  Mohapatra, P. and E. Rosen, "The BGP Encapsulation
              Subsequent Address Family Identifier (SAFI) and the BGP
              Tunnel Encapsulation Attribute", RFC 5512,
              DOI 10.17487/RFC5512, April 2009,
              <https://www.rfc-editor.org/info/rfc5512>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [BGP-SR-POLICY]
              Previdi, S., Filsfils, C., Mattes, P., Rosen, E., Jain,
              D., and S. Lin, "Advertising Segment Routing Policies in
              BGP", draft-previdi-idr-segment-routing-te-policy (work in
              progress), 2019.







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   [BGP-TUNNEL-ENCAP]
              Patel, K., Velde, G., and S. Sangli, "The BGP Tunnel
              Encapsulation Attribute", draft-vandevelde-idr-remote-
              next-hop (work in progress), 2019.

11.2.  Informative 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>.

   [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>.

Authors' Addresses

   Yimin Shen
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Phone: +1 9785890722
   Email: yshen@juniper.net


   Ravi Singh
   Individual Contributor

   Email: ravi.singh.ietf@gmail.com


   Yuji Kamite
   NTT Communications

   Email: y.kamite@ntt.com













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