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For this RFC, original HTML is available from the RFC-Editor: RFC8663

PROPOSED STANDARD

Internet Engineering Task Force (IETF)                             X. Xu
Request for Comments: 8663                                  Alibaba, Inc
Category: Standards Track                                      S. Bryant
ISSN: 2070-1721                                   Futurewei Technologies
                                                               A. Farrel
                                                      Old Dog Consulting
                                                               S. Hassan
                                                                   Cisco
                                                           W. Henderickx
                                                                   Nokia
                                                                   Z. Li
                                                                  Huawei
                                                           December 2019


                      MPLS Segment Routing over IP

Abstract

   MPLS Segment Routing (SR-MPLS) is a method of source routing a packet
   through an MPLS data plane by imposing a stack of MPLS labels on the
   packet to specify the path together with any packet-specific
   instructions to be executed on it.  SR-MPLS can 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 making no changes to SR-MPLS specifications and
   interworking with SR-MPLS implementations.

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

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/rfc8663.

Copyright Notice

   Copyright (c) 2019 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
     1.1.  Terminology
   2.  Use Cases
   3.  Procedures of SR-MPLS-over-IP
     3.1.  Forwarding Entry Construction
       3.1.1.  FIB Construction Example
     3.2.  Packet-Forwarding Procedures
       3.2.1.  Packet Forwarding with Penultimate Hop Popping
       3.2.2.  Packet Forwarding without Penultimate Hop Popping
       3.2.3.  Additional Forwarding Procedures
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   MPLS Segment Routing (SR-MPLS) [RFC8660] is a method of source
   routing a packet through an MPLS data plane.  This is achieved by the
   sender imposing a stack of MPLS labels that partially or completely
   specify the path that the packet is to take and any instructions to
   be executed on the packet as it passes through the network.  SR-MPLS
   uses an MPLS label stack to encode a sequence of source-routing
   instructions.  This can be used to realize a source-routing mechanism
   that can operate across MPLS, IPv4, and IPv6 data planes.  This
   approach makes no changes to SR-MPLS specifications and allows
   interworking with SR-MPLS implementations.  More specifically, the
   source-routing instructions in a source-routed packet could be
   uniformly encoded as an MPLS label stack regardless of whether the
   underlay is IPv4, IPv6 (including Segment Routing for IPv6 (SRv6)
   [RFC8354]), or MPLS.

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

   Section 2 describes various use cases for 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 [RFC8660].

   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 (including SRv6) tunnels is
   useful at least in the use cases listed below.  In all cases, this
   can be enabled using an IP tunneling mechanism such as MPLS-over-UDP
   as described in [RFC7510].  The tunnel selected MUST have its remote
   endpoint (destination) address equal to the address of the next node
   capable of SR-MPLS identified as being on the SR path (i.e., the
   egress of the active segment).  The local endpoint (source) address
   is set to an address of the encapsulating node.  [RFC7510] gives
   further advice on how to set the source address if the UDP zero-
   checksum mode is used with MPLS-over-UDP.  Using UDP as the
   encapsulation may be particularly beneficial because it is agnostic
   of the underlying transport.

   *  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 as shown in Figure 1.  This demonstrates how
      islands of SR-MPLS may be connected across a legacy network.  It
      may be particularly useful for joining sites (such as data
      centers).

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

          Figure 1: SR-MPLS-over-UDP to Tunnel between SR-MPLS Sites

   *  If the encoding of entropy [RFC6790] is desired, IP-tunneling
      mechanisms that allow the encoding of entropy, such as MPLS-over-
      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 Equal-Cost Multipath (ECMP) and/or Link Aggregation
      Groups (LAGs), especially when it is difficult to make use of the
      entropy-label mechanism.  This is to be contrasted with [RFC4023]
      where MPLS-over-IP does not provide for an entropy mechanism.
      Refer to [RFC8662]) for more discussion about using entropy labels
      in SR-MPLS.

   *  Tunneling MPLS over IP provides a technology that enables Segment
      Routing (SR) in an IPv4 and/or IPv6 network where the routers do
      not support SRv6 capabilities [IPv6-SRH] and where MPLS forwarding
      is not an option.  This is shown in Figure 2.

                      __________________________________
                   __(           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-over-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 Sections
   3.1.1 and 3.2 assume that OSPF or IS-IS 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 subsection describes 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.  Section 3.1.1 provides a concrete
   example of how the process applies when using OSPF or IS-IS.

   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:

   *  Router E is SR-MPLS capable, so it advertises a Segment Routing
      Global Block (SRGB).  The SRGB is defined in [RFC8402].  There are
      a number of ways that the advertisement can be achieved including
      IGPs, BGP, and configuration/management protocols.  For example,
      see [DC-GATEWAY].

   *  When Router E advertises the Prefix-SID SID(E) of prefix P(E), it
      MUST also advertise the egress endpoint address and the
      encapsulation type of any tunnel used to reach E.  This
      information is flooded domain wide.

   *  If A and E are in different routing domains, then the information
      MUST be flooded into both domains.  How this is achieved depends
      on the advertisement mechanism being used.  The objective is that
      router A knows the characteristics of router E that originated the
      advertisement of SID(E).

   *  Router A programs the FIB entry for prefix P(E) corresponding to
      the SID(E) according to whether a pop or swap action is advertised
      for the prefix.  The resulting action may be:

      -  pop the top label

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

   Once constructed, the FIB can be used by a router to tell it how to
   process packets.  It encapsulates the packets according to the
   appropriate encapsulation advertised for the segment and then sends
   the packets towards the next hop NHi.

3.1.1.  FIB Construction Example

   This section is non-normative and provides a worked example of how a
   FIB might be constructed using OSPF and IS-IS extensions.  It is
   based on the process described in Section 3.1.

   *  Router E is SR-MPLS capable, so it advertises a Segment Routing
      Global Block (SRGB) using [RFC8665] or [RFC8667].

   *  When Router E advertises the Prefix-SID SID(E) of prefix P(E), it
      also advertises the encapsulation endpoint address and the tunnel
      type of any tunnel used to reach E using [ISIS-ENCAP] or
      [OSPF-ENCAP].

   *  If A and E are in different domains, then the information is
      flooded into both domains and any intervening domains.

      -  The OSPF Tunnel Encapsulations TLV [OSPF-ENCAP] or the IS-IS
         Tunnel Encapsulation Type sub-TLV [ISIS-ENCAP] is flooded
         domain wide.

      -  The OSPF SID/Label Range TLV [RFC8665] or the IS-IS SR-
         Capabilities sub-TLV [RFC8667] is advertised domain wide so
         that router A knows the characteristics of router E.

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

         o  If router E is running IS-IS, 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].

         o  If router E is running OSPF, it uses the OSPFv2 Extended
            Prefix Opaque Link-State Advertisement (LSA) [RFC7684] and
            sets the flooding scope to Autonomous System (AS) wide.

      -  If router E is running IS-IS and advertises the IS-IS Router
         CAPABILITY TLV (TLV 242) [RFC7981], it sets the "Router ID"
         field to a valid value or includes an IPv6 TE Router ID sub-TLV
         (TLV 12), or it does both.  The "S" bit (flooding scope) of the
         IS-IS Router CAPABILITY TLV (TLV 242) is set to "1".

   *  Router A programs the FIB entry for prefix P(E) corresponding to
      the SID(E) according to whether a pop or swap action is advertised
      for the prefix as follows:

      -  If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in
         IS-IS is clear:

            pop the top label

      -  If the No-PHP (NP) Flag in OSPF or the Persistent (P) Flag in
         IS-IS is set:

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

   When forwarding the packet according to the constructed FIB entry,
   the router encapsulates the packet according to the encapsulation as
   advertised using the mechanisms described in [ISIS-ENCAP] or
   [OSPF-ENCAP].  It then sends the packets towards the next hop NHi.

   Note that [RFC7510] specifies the use of port number 6635 to indicate
   that the payload of a UDP packet is MPLS, and port number 6636 for
   MPLS-over-UDP utilizing DTLS.  However, [ISIS-ENCAP] and [OSPF-ENCAP]
   provide dynamic protocol mechanisms to configure the use of any
   Dynamic Port for a tunnel that uses UDP encapsulation.  Nothing in
   this document prevents the use of an IGP or any other mechanism to
   negotiate the use of a Dynamic Port when UDP encapsulation is used
   for SR-MPLS, but if no such mechanism is used, then the port numbers
   specified in [RFC7510] are used.

3.2.  Packet-Forwarding Procedures

   [RFC7510] specifies an IP-based encapsulation for MPLS, i.e., MPLS-
   over-UDP.  This approach is applicable where IP-based encapsulation
   for MPLS is required and further fine-grained load balancing of MPLS
   packets over IP networks over ECMP and/or LAGs is also required.
   This section provides details about the forwarding procedure when UDP
   encapsulation is adopted for SR-MPLS-over-IP.  Other encapsulation
   and tunneling mechanisms can be applied using similar techniques, but
   for clarity, this section uses UDP encapsulation as the exemplar.

   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.  A node capable of SR-MPLS MAY advertise its
   capabilities using the IGP as described in Section 3.  There are six
   types of nodes in an SR-MPLS domain:

   *  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.

   *  Legacy transit nodes that are IP routers but that are not SR-MPLS
      capable (i.e., are not able to perform Segment Routing).

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

   *  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.

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

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

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 as
   a Penultimate Hop-Popping (PHP) label.  That is, if one of the IGP
   flooding mechanisms is used, the NP-Flag in OSPF or the P-Flag in IS-
   IS 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 capable of SR-MPLS 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, such as 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
   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 a
   transit node capable of SR-MPLS), router E strips the IP-based tunnel
   header and then processes 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
   node capable of SR-MPLS 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 (e.g., the
   NP-Flag in OSPF or the P-Flag in IS-IS associated with the Prefix-SID
   is set).  Apart from the PHP function, the roles of the routers are
   unchanged from Section 3.2.1.

     +-----+       +-----+       +-----+        +-----+        +-----+
     |  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 Tunnels:  The description in the previous two
      sections is based on the assumption that an MPLS-over-UDP tunnel
      is used when the next hop towards the next segment is not MPLS
      enabled.  However, even in the case where the next hop towards the
      next segment is MPLS capable, an MPLS-over-UDP tunnel towards the
      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 a transit node capable of SR-MPLS while all the other
      assumptions remain 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 replaces the current top
      label with an MPLS-over-UDP tunnel toward router G and sends it
      out.  (Note that if an MPLS LSP was preferred, the packet would be
      forwarded as native SR-MPLS.)

   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 the ECN field [RFC6040], the
      Differentiated Services Code Point (DSCP) [RFC2983], or IPv6 Flow
      Label) on each UDP-encapsulated segment SHOULD be configurable
      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.
      The TTL field setting in the encapsulating packet header is
      handled as described in [RFC7510], which refers to [RFC4023].

   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
      the case of a 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 recalculating 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 reused as the
      entropy value of the outgoing packet.

   Congestion Considerations:  Section 5 of [RFC7510] provides a
      detailed analysis of the implications of congestion in MPLS-over-
      UDP systems and builds on Section 3.1.3 of [RFC8085], which
      describes the congestion implications of UDP tunnels.  All of
      those considerations apply to SR-MPLS-over-UDP tunnels as
      described in this document.  In particular, it should be noted
      that the traffic carried in SR-MPLS flows is likely to be IP
      traffic.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   The security consideration of [RFC8354] (which redirects the reader
   to [RFC5095]) and [RFC7510] apply.  DTLS [RFC6347] SHOULD be used
   where security is needed on an SR-MPLS-over-UDP segment including
   when the IP segment crosses the public Internet or some other
   untrusted environment.  [RFC8402] provides security considerations
   for Segment Routing, and Section 8.1 of [RFC8402] is particularly
   applicable to SR-MPLS.

   It is difficult for an attacker to pass a raw MPLS-encoded packet
   into a network, and operators have considerable experience in
   excluding such packets at the network boundaries, for example, by
   excluding all packets that are revealed to be carrying an MPLS packet
   as the payload of IP tunnels.  Further discussion of MPLS security is
   found in [RFC5920].

   It is easy for a network 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, and such filtering SHOULD be
   applied.  If, however, the mechanisms described in [RFC8665] or
   [RFC8667] are applied, a wider variety of UDP port numbers might be
   in use making port filtering harder.

   SR packets not having a destination address terminating in the
   network would be transparently carried and would pose no different
   security risk to the network under consideration than any other
   traffic.

   Where control-plane techniques are used (as described in Section 3),
   it is important that these protocols are adequately secured for the
   environment in which they are run as discussed in [RFC6862] and
   [RFC5920].

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>.

   [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>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              DOI 10.17487/RFC5095, December 2007,
              <https://www.rfc-editor.org/info/rfc5095>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

   [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>.

   [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>.

   [RFC8660]  Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with the
              MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December
              2019, <https://www.rfc-editor.org/info/rfc8660>.

6.2.  Informative References

   [DC-GATEWAY]
              Farrel, A., Drake, J., Rosen, E., Patel, K., and L. Jalil,
              "Gateway Auto-Discovery and Route Advertisement for
              Segment Routing Enabled Domain Interconnection", Work in
              Progress, Internet-Draft, draft-ietf-bess-datacenter-
              gateway-04, 21 August 2019, <https://tools.ietf.org/html/
              draft-ietf-bess-datacenter-gateway-04>.

   [IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", Work in Progress, Internet-Draft, draft-ietf-6man-
              segment-routing-header-26, 22 October 2019,
              <https://tools.ietf.org/html/draft-ietf-6man-segment-
              routing-header-26>.

   [ISIS-ENCAP]
              Xu, X., Decraene, B., Raszuk, R., Chunduri, U., Contreras,
              L., and L. Jalil, "Advertising Tunnelling Capability in
              IS-IS", Work in Progress, Internet-Draft, draft-ietf-isis-
              encapsulation-cap-01, 24 April 2017,
              <https://tools.ietf.org/html/draft-ietf-isis-
              encapsulation-cap-01>.

   [OSPF-ENCAP]
              Xu, X., Decraene, B., Raszuk, R., Contreras, L., and L.
              Jalil, "The Tunnel Encapsulations OSPF Router
              Information", Work in Progress, Internet-Draft, draft-
              ietf-ospf-encapsulation-cap-09, 10 October 2017,
              <https://tools.ietf.org/html/draft-ietf-ospf-
              encapsulation-cap-09>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <https://www.rfc-editor.org/info/rfc5920>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC6862]  Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
              Authentication for Routing Protocols (KARP) Overview,
              Threats, and Requirements", RFC 6862,
              DOI 10.17487/RFC6862, March 2013,
              <https://www.rfc-editor.org/info/rfc6862>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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>.

   [RFC8662]  Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy Label for Source
              Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
              DOI 10.17487/RFC8662, December 2019,
              <https://www.rfc-editor.org/info/rfc8662>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

Acknowledgements

   Thanks to Joel Halpern, Bruno Decraene, Loa Andersson, Ron Bonica,
   Eric Rosen, Jim Guichard, Gunter Van De Velde, Andy Malis, Robert
   Sparks, and Al Morton for their insightful comments on this document.

   Additional thanks to Mirja Kuehlewind, Alvaro Retana, Spencer
   Dawkins, Benjamin Kaduk, Martin Vigoureux, Suresh Krishnan, and Eric
   Vyncke for careful reviews and resulting comments.

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
   Mellanox Technologies
   Email: mashaowen@gmail.com

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

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

   Robert Raszuk
   Bloomberg LP
   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
   Apstra, Inc.
   Email: jefftant.ietf@gmail.com

Authors' Addresses

   Xiaohu Xu
   Alibaba, Inc

   Email: xiaohu.xxh@alibaba-inc.com


   Stewart Bryant
   Futurewei Technologies

   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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