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Versions: (draft-xu-mpls-sr-over-ip) 00 01

Network Working Group                                              X. Xu
Internet-Draft                                              Alibaba Inc.
Intended status: Standards Track                               S. Bryant
Expires: April 21, 2019                                           Huawei
                                                               A. Farrel
                                                      Old Dog Consulting
                                                               S. Hassan
                                                                   Cisco
                                                           W. Henderickx
                                                                   Nokia
                                                                   Z. Li
                                                                  Huawei
                                                        October 18, 2018


                            SR-MPLS over IP
                     draft-ietf-mpls-sr-over-ip-01

Abstract

   MPLS Segment Routing (SR-MPLS) is an MPLS data plane-based source
   routing paradigm in which the sender of a packet is allowed to
   partially or completely specify the route the packet takes through
   the network by imposing stacked MPLS labels on the packet.  SR-MPLS
   could be leveraged to realize a source routing mechanism across MPLS,
   IPv4, and IPv6 data planes by using an MPLS label stack as a source
   routing instruction set while preserving backward compatibility with
   SR-MPLS.

   This document describes how SR-MPLS capable routers and IP-only
   routers can seamlessly co-exist and interoperate through the use of
   SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-in-
   UDP as defined in RFC 7510.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on April 21, 2019.

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.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Procedures of SR-MPLS over IP . . . . . . . . . . . . . . . .   5
     3.1.  Forwarding Entry Construction . . . . . . . . . . . . . .   5
     3.2.  Packet Forwarding Procedures  . . . . . . . . . . . . . .   7
       3.2.1.  Packet Forwarding with Penultimate Hop Popping  . . .   8
       3.2.2.  Packet Forwarding without Penultimate Hop Popping . .   9
       3.2.3.  Additional Forwarding Procedures  . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   MPLS Segment Routing (SR-MPLS) [I-D.ietf-spring-segment-routing-mpls]
   is an MPLS data plane-based source routing paradigm in which the
   sender of a packet is allowed to partially or completely specify the
   route the packet takes through the network by imposing stacked MPLS
   labels on the packet.  SR-MPLS could be leveraged to realize a source
   routing mechanism across MPLS, IPv4, and IPv6 data planes by using an
   MPLS label stack as a source routing instruction set while preserving
   backward compatibility with SR-MPLS.  More specifically, the source
   routing instruction set information contained in a source routed



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   packet could be uniformly encoded as an MPLS label stack no matter
   whether the underlay is IPv4, IPv6, or MPLS.

   This document describes how SR-MPLS capable routers and IP-only
   routers can seamlessly co-exist and interoperate through the use of
   SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-in-
   UDP [RFC7510].

   Section 2 describes various use cases for the tunneling SR-MPLS over
   IP.  Section 3 describes a typical application scenario and how the
   packet forwarding happens.

1.1.  Terminology

   This memo makes use of the terms defined in [RFC3031] and
   [I-D.ietf-spring-segment-routing-mpls].

   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.

2.  Use Cases

   Tunneling SR-MPLS using IPv4 and/or IPv6 tunnels is useful at least
   in the following use cases:

   o  Incremental deployment of the SR-MPLS technology may be
      facilitated by tunneling SR-MPLS packets across parts of a network
      that are not SR-MPLS enabled using an IP tunneling mechanism such
      as MPLS-in-UDP [RFC7510].  The tunnel destination address is the
      address of the next SR-MPLS capable node along the path (i.e., the
      egress of the active node segment).  This is shown in Figure 1.

















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                      ________________________
       _______       (                        )       _______
      (       )     (        IP Network        )     (       )
     ( SR-MPLS )   (                            )   ( SR-MPLS )
    (  Network  ) (                              ) (  Network  )
   (         --------                          --------         )
   (        | Border |    SR-in-UDP Tunnel    | Border |        )
   (        | Router |========================| Router |        )
   (        |   R1   |                        |   R2   |        )
   (         --------                          --------         )
    (           ) (                              ) (           )
     (         )   (                            )   (         )
      (_______)     (                          )     (_______)
                     (________________________)



         Figure 1: SR-MPLS in UDP to Tunnel Between SR-MPLS Sites

   o  If encoding of entropy is desired, IP tunneling mechanisms that
      allow encoding of entropy, such as MPLS-in-UDP encapsulation
      [RFC7510] where the source port of the UDP header is used as an
      entropy field, may be used to maximize the utilization of ECMP
      and/or LAG, especially when it is difficult to make use of entropy
      label mechanism.  Refer to [I-D.ietf-mpls-spring-entropy-label])
      for more discussion about using entropy label in SR-MPLS.

   o  Tunneling MPLS into IP provides a technology that enables SR in an
      IPv4 and/or IPv6 network where the routers do not support SRv6
      capabilities [I-D.ietf-6man-segment-routing-header] and where MPLS
      forwarding is not an option.  This is shown in Figure Figure 2.




















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                    __________________________________
                 __(           IP Network             )__
              __(                                        )__
             (               --        --        --         )
        --------   --   --  |SR|  --  |SR|  --  |SR|  --   --------
       | Ingress| |IR| |IR| |  | |IR| |  | |IR| |  | |IR| | Egress |
   --->| Router |===========|  |======|  |======|  |======| Router |--->
       |   SR   | |  | |  | |  | |  | |  | |  | |  | |  | |   SR   |
        --------   --   --  |  |  --  |  |  --  |  |  --   --------
             (__             --        --        --       __)
                (__                                    __)
                   (__________________________________)

      Key:
        IR : IP-only Router
        SR : SR-MPLS-capable Router
        == : SR-MPLS in UDP Tunnel



              Figure 2: SR-MPLS Enabled Within an IP Network

3.  Procedures of SR-MPLS over IP

   This section describes the construction of forwarding information
   base (FIB) entries and the forwarding behavior that allow the
   deployment of SR-MPLS when some routers in the network are IP only
   (i.e., do not support SR-MPLS).  Note that the examples in
   Section 3.1 and Section 3.2 assume that OSPF or ISIS is enabled: in
   fact, other mechanisms of discovery and advertisement could be used
   including other routing protocols (such as BGP) or a central
   controller.

3.1.  Forwarding Entry Construction

   This sub-section describes the how to construct the forwarding
   information base (FIB) entry on an SR-MPLS-capable router when some
   or all of the next-hops along the shortest path towards a prefix
   Segment Identifier (prefix-SID) are IP-only routers.

   Consider router A that receives a labeled packet with top label L(E)
   that corresponds to the prefix-SID SID(E) of prefix P(E) advertised
   by router E.  Suppose the i-th next-hop router (termed NHi) along the
   shortest path from router A toward SID(E) is not SR-MPLS capable
   while both routers A and E are SR-MPLS capable.  The following
   processing steps apply:





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   o  Router E is SR-MPLS capable so it advertises the SRGB as described
      in [I-D.ietf-ospf-segment-routing-extensions] and
      [I-D.ietf-isis-segment-routing-extensions].

   o  When Router E advertises the prefix-SID SID(E) of prefix P(E) it
      MUST also advertise the encapsulation endpoint and the tunnel type
      of any tunnel used to reach E.  It does this using the mechanisms
      described in [I-D.ietf-isis-encapsulation-cap] or
      [I-D.ietf-ospf-encapsulation-cap].

   o  If A and E are in different IGP areas/levels, then:

      *  The OSPF Tunnel Encapsulation TLV
         [I-D.ietf-ospf-encapsulation-cap] or the ISIS Tunnel
         Encapsulation sub-TLV [I-D.ietf-isis-encapsulation-cap] is
         flooded domain-wide.

      *  The OSPF SID/label range TLV
         [I-D.ietf-ospf-segment-routing-extensions] or the ISIS SR-
         Capabilities Sub-TLV [I-D.ietf-isis-segment-routing-extensions]
         is advertised domain-wide.  This way router A knows the
         characteristics of the router that originated the advertisement
         of SID(E) (i.e., router E).

      *  When router E advertises the prefix P(E):

         +  If router E is running ISIS it uses the extended
            reachability TLV (TLVs 135, 235, 236, 237) and associates
            the IPv4/IPv6 or IPv4/IPv6 source router ID sub-TLV(s)
            [RFC7794].

         +  If router E is running OSPF it uses the OSPFv2 Extended
            Prefix Opaque LSA [RFC7684] and sets the flooding scope to
            AS-wide.

      *  If router E is running ISIS and advertises the ISIS
         capabilities TLV (TLV 242) [RFC7981], it MUST set the "router-
         ID" field to a valid value or include an IPV6 TE router-ID sub-
         TLV (TLV 12), or do both.  The "S" bit (flooding scope) of the
         ISIS capabilities TLV (TLV 242) MUST be set to "1" .

   o  Router A programs the FIB entry for prefix P(E) corresponding to
      the SID(E) as follows:

      *  If the NP flag in OSPF or the P flag in ISIS is clear:

            pop the top label




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      *  If the NP flag in OSPF or the P flag in ISIS is set:

            swap the top label to a value equal to SID(E) plus the lower
            bound of the SRGB of E

      *  Encapsulate the packet according to the encapsulation
         advertised in [I-D.ietf-isis-encapsulation-cap] or
         [I-D.ietf-ospf-encapsulation-cap]

      *  Send the packet towards the next hop NHi.

3.2.  Packet Forwarding Procedures

   [RFC7510] specifies an IP-based encapsulation for MPLS, i.e., MPLS-
   in-UDP, which is applicable in some circumstances where IP-based
   encapsulation for MPLS is required and further fine-grained load
   balancing of MPLS packets over IP networks over Equal-Cost Multipath
   (ECMP) and/or Link Aggregation Groups (LAGs) is required as well.
   This section provides details about the forwarding procedure when
   when UDP encapsulation is adopted for SR-MPLS over IP.

   Nodes that are SR-MPLS capable can process SR-MPLS packets.  Not all
   of the nodes in an SR-MPLS domain are SR-MPLS capable.  Some nodes
   may be "legacy routers" that cannot handle SR-MPLS packets but can
   forward IP packets.  An SR-MPLS-capable node may advertise its
   capabilities using the IGP as described in Section 3.  There are six
   types of node in an SR-MPLS domain:

   o  Domain ingress nodes that receive packets and encapsulate them for
      transmission across the domain.  Those packets may be any payload
      protocol including native IP packets or packets that are already
      MPLS encapsulated.

   o  Legacy transit nodes that are IP routers but that are not SR-MPLS
      capable (i.e., are not able to perform segment routing).

   o  Transit nodes that are SR-MPLS capable but that are not identified
      by a SID in the SID stack.

   o  Transit nodes that are SR-MPLS capable and need to perform SR-MPLS
      routing because they are identified by a SID in the SID stack.

   o  The penultimate SR-MPLS capable node on the path that processes
      the last SID on the stack on behalf of the domain egress node.

   o  The domain egress node that forwards the payload packet for
      ultimate delivery.




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3.2.1.  Packet Forwarding with Penultimate Hop Popping

   The description in this section assumes that the label associated
   with each prefix-SID is advertised by the owner of the prefix-SID is
   a Penultimate Hop Popping (PHP) label.  That is, the NP flag in OSPF
   or the P flag in ISIS associated with the prefix SID is not set.



     +-----+       +-----+       +-----+        +-----+        +-----+
     |  A  +-------+  B  +-------+  C  +--------+  D  +--------+  H  |
     +-----+       +--+--+       +--+--+        +--+--+        +-----+
                      |             |              |
                      |             |              |
                   +--+--+       +--+--+        +--+--+
                   |  E  +-------+  F  +--------+  G  |
                   +-----+       +-----+        +-----+


          +--------+
          |IP(A->E)|
          +--------+                 +--------+        +--------+
          |  UDP   |                 |IP(E->G)|        |IP(G->H)|
          +--------+                 +--------+        +--------+
          |  L(G)  |                 |  UDP   |        |  UDP   |
          +--------+                 +--------+        +--------+
          |  L(H)  |                 |  L(H)  |        |Exp Null|
          +--------+                 +--------+        +--------+
          | Packet |     --->        | Packet |  --->  | Packet |
          +--------+                 +--------+        +--------+



               Figure 3: Packet Forwarding Example with PHP

   In the example shown in Figure 3, assume that routers A, E, G and H
   are SR-MPLS-capable while the remaining routers (B, C, D and F) are
   only capable of forwarding IP packets.  Routers A, E, G, and H
   advertise their Segment Routing related information via IS-IS or
   OSPF.

   Now assume that router A (the Domain ingress) wants to send a packet
   to router H (the Domain egress) via the explicit path {E->G->H}.
   Router A will impose an MPLS label stack on the packet that
   corresponds to that explicit path.  Since the next hop toward router
   E is only IP-capable (B is a legacy transit node), router A replaces
   the top label (that indicated router E) with a UDP-based tunnel for
   MPLS (i.e., MPLS-over-UDP [RFC7510]) to router E and then sends the



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   packet.  In other words, router A pops the top label and then
   encapsulates the MPLS packet in a UDP tunnel to router E.

   When the IP-encapsulated MPLS packet arrives at router E (which is an
   SR-MPLS-capable transit node), router E strips the IP-based tunnel
   header and then process the decapsulated MPLS packet.  The top label
   indicates that the packet must be forwarded toward router G.  Since
   the next hop toward router G is only IP-capable, router E replaces
   the current top label with an MPLS-over-UDP tunnel toward router G
   and sends it out.  That is, router E pops the top label and then
   encapsulates the MPLS packet in a UDP tunnel to router G.

   When the packet arrives at router G, router G will strip the IP-based
   tunnel header and then process the decapsulated MPLS packet.  The top
   label indicates that the packet must be forwarded toward router H.
   Since the next hop toward router H is only IP-capable (D is a legacy
   transit router), router G would replace the current top label with an
   MPLS-over-UDP tunnel toward router H and send it out.  However, since
   router G reaches the bottom of the label stack (G is the penultimate
   SR-MPLS capable node on the path) this would leave the original
   packet that router A wanted to send to router H encapsulated in UDP
   as if it was MPLS (i.e., with a UDP header and destination port
   indicating MPLS) even though the original packet could have been any
   protocol.  That is, the final SR-MPLS has been popped exposing the
   payload packet.

   To handle this, when a router (here it is router G) pops the final
   SR-MPLS label, it inserts an explicit null label [RFC3032] before
   encapsulating the packet in an MPLS-over-UDP tunnel toward router H
   and sending it out.  That is, router G pops the top label, discovers
   it has reached the bottom of stack, pushes an explicit null label,
   and then encapsulates the MPLS packet in a UDP tunnel to router H.

3.2.2.  Packet Forwarding without Penultimate Hop Popping

   Figure 4 demonstrates the packet walk in the case where the label
   associated with each prefix-SID advertised by the owner of the
   prefix-SID is not a Penultimate Hop Popping (PHP) label (i.e., the
   the NP flag in OSPF or the P flag in ISIS associated with the prefix
   SID is set).  Apart from the PHP function the roles of the routers is
   unchanged from Section 3.2.1.










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     +-----+       +-----+       +-----+        +-----+        +-----+
     |  A  +-------+  B  +-------+  C  +--------+  D  +--------+  H  |
     +-----+       +--+--+       +--+--+        +--+--+        +-----+
                      |             |              |
                      |             |              |
                   +--+--+       +--+--+        +--+--+
                   |  E  +-------+  F  +--------+  G  |
                   +-----+       +-----+        +-----+

          +--------+
          |IP(A->E)|
          +--------+                 +--------+
          |  UDP   |                 |IP(E->G)|
          +--------+                 +--------+        +--------+
          |  L(E)  |                 |  UDP   |        |IP(G->H)|
          +--------+                 +--------+        +--------+
          |  L(G)  |                 |  L(G)  |        |  UDP   |
          +--------+                 +--------+        +--------+
          |  L(H)  |                 |  L(H)  |        |  L(H)  |
          +--------+                 +--------+        +--------+
          | Packet |     --->        | Packet |  --->  | Packet |
          +--------+                 +--------+        +--------+



              Figure 4: Packet Forwarding Example without PHP

   As can be seen from the figure, the SR-MPLS label for each segment is
   left in place until the end of the segment where it is popped and the
   next instruction is processed.

3.2.3.  Additional Forwarding Procedures

   Non-MPLS Interfaces:  Although the description in the previous two
      sections is based on the use of prefix-SIDs, tunneling SR-MPLS
      packets is useful when the top label of a received SR-MPLS packet
      indicates an adjacency-SID and the corresponding adjacent node to
      that adjacency-SID is not capable of MPLS forwarding but can still
      process SR-MPLS packets.  In this scenario the top label would be
      replaced by an IP tunnel toward that adjacent node and then
      forwarded over the corresponding link indicated by the adjacency-
      SID.

   When to use IP-based Tunnel:  The description in the previous two
      sections is based on the assumption that MPLS-over-UDP tunnel is
      used when the nexthop towards the next segment is not MPLS-
      enabled.  However, even in the case where the nexthop towards the
      next segment is MPLS-capable, an MPLS-over-UDP tunnel towards the



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      next segment could still be used instead due to local policies.
      For instance, in the example as described in Figure 4, assume F is
      now an SR-MPLS-capable transit node while all the other
      assumptions keep unchanged, since F is not identified by a SID in
      the stack and an MPLS-over-UDP tunnel is preferred to an MPLS LSP
      according to local policies, router E would replace the current
      top label with an MPLS-over-UDP tunnel toward router G and send it
      out.

   IP Header Fields:  When encapsulating an MPLS packet in UDP, the
      resulting packet is further encapsulated in IP for transmission.
      IPv4 or IPv6 may be used according to the capabilities of the
      network.  The address fields are set as described in Section 2.
      The other IP header fields (such as DSCP code point, or IPv6 Flow
      Label) on each UDP-encapsulated segment can be set according to
      the operator's policy: they may be copied from the header of the
      incoming packet; they may be promoted from the header of the
      payload packet; they may be set according to instructions
      programmed to be associated with the SID; or they may be
      configured dependent on the outgoing interface and payload.

   Entropy and ECMP:  When encapsulating an MPLS packet with an IP
      tunnel header that is capable of encoding entropy (such as
      [RFC7510]), the corresponding entropy field (the source port in
      case UDP tunnel) MAY be filled with an entropy value that is
      generated by the encapsulator to uniquely identify a flow.
      However, what constitutes a flow is locally determined by the
      encapsulator.  For instance, if the MPLS label stack contains at
      least one entropy label and the encapsulator is capable of reading
      that entropy label, the entropy label value could be directly
      copied to the source port of the UDP header.  Otherwise, the
      encapsulator may have to perform a hash on the whole label stack
      or the five-tuple of the SR-MPLS payload if the payload is
      determined as an IP packet.  To avoid re-performing the hash or
      hunting for the entropy label each time the packet is encapsulated
      in a UDP tunnel it MAY be desirable that the entropy value
      contained in the incoming packet (i.e., the UDP source port value)
      is retained when stripping the UDP header and is re-used as the
      entropy value of the outgoing packet.

4.  IANA Considerations

   This document makes no requests for IANA action.








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5.  Security Considerations

   The security consideration of [RFC8354] and [RFC7510] apply.  DTLS
   [RFC6347] SHOULD be used where security is needed on an MPLS-SR-over-
   UDP segment.

   It is difficult for an attacker to pass a raw MPLS encoded packet
   into a network and operators have considerable experience at
   excluding such packets at the network boundaries.

   It is easy for an ingress node to detect any attempt to smuggle an IP
   packet into the network since it would see that the UDP destination
   port was set to MPLS.  SR packets not having a destination address
   terminating in the network would be transparently carried and would
   pose no security risk to the network under consideration.

   Where control plane techniques are used (as described in
   Authors' Addresses it is important that these protocols are
   adequately secured for the environment in which they are run.

6.  Contributors

   Ahmed Bashandy
   Individual
   Email: abashandy.ietf@gmail.com

   Clarence Filsfils
   Cisco
   Email: cfilsfil@cisco.com

   John Drake
   Juniper
   Email: jdrake@juniper.net

   Shaowen Ma
   Juniper
   Email: mashao@juniper.net

   Mach Chen
   Huawei
   Email: mach.chen@huawei.com

   Hamid Assarpour
   Broadcom
   Email:hamid.assarpour@broadcom.com

   Robert Raszuk
   Bloomberg LP



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   Email: robert@raszuk.net

   Uma Chunduri
   Huawei
   Email: uma.chunduri@gmail.com

   Luis M. Contreras
   Telefonica I+D
   Email: luismiguel.contrerasmurillo@telefonica.com

   Luay Jalil
   Verizon
   Email: luay.jalil@verizon.com

   Gunter Van De Velde
   Nokia
   Email: gunter.van_de_velde@nokia.com

   Tal Mizrahi
   Marvell
   Email: talmi@marvell.com

   Jeff Tantsura
   Individual
   Email: jefftant@gmail.com



7.  Acknowledgements

   Thanks to Joel Halpern, Bruno Decraene, Loa Andersson, Ron Bonica,
   Eric Rosen, Jim Guichard, and Gunter Van De Velde for their
   insightful comments on this draft.

8.  References

8.1.  Normative References

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-14
              (work in progress), June 2018.

   [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>.



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   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,
              <https://www.rfc-editor.org/info/rfc7510>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

   [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>.

8.2.  Informative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
              d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-14 (work in
              progress), June 2018.






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   [I-D.ietf-mpls-spring-entropy-label]
              Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy label for SPRING
              tunnels", draft-ietf-mpls-spring-entropy-label-12 (work in
              progress), July 2018.

   [RFC8354]  Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R.,
              Ed., and M. Townsley, "Use Cases for IPv6 Source Packet
              Routing in Networking (SPRING)", RFC 8354,
              DOI 10.17487/RFC8354, March 2018,
              <https://www.rfc-editor.org/info/rfc8354>.

Authors' Addresses

   Xiaohu Xu
   Alibaba Inc.

   Email: xiaohu.xxh@alibaba-inc.com


   Stewart Bryant
   Huawei

   Email: stewart.bryant@gmail.com


   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk


   Syed Hassan
   Cisco

   Email: shassan@cisco.com


   Wim Henderickx
   Nokia

   Email: wim.henderickx@nokia.com


   Zhenbin Li
   Huawei

   Email: lizhenbin@huawei.com



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