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Versions: (draft-filsfils-spring-segment-routing-mpls) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Network Working Group                                  A. Bashandy, Ed.
Internet Draft                                               Individual
Intended status: Standards Track                       C. Filsfils, Ed.
Expires: December 2018                                      S. Previdi,
                                                    Cisco Systems, Inc.
                                                            B. Decraene
                                                           S. Litkowski
                                                                 Orange
                                                              R. Shakir
                                                                 Google
                                                          June 11, 2018


                   Segment Routing with MPLS data plane
                 draft-ietf-spring-segment-routing-mpls-14


Abstract

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through a controlled set of instructions, called
   segments, by prepending the packet with an SR header.  In the MPLS
   dataplane, the SR header is instantiated through a label stack. This
   document specifies the forwarding behavior to allow instantiating SR
   over the MPLS dataplane.

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 http://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."

   This Internet-Draft will expire on December 11, 2018.







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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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...................................................3
      1.1. Requirements Language.....................................3
   2. MPLS Instantiation of Segment Routing..........................3
      2.1. Multiple Forwarding Behaviors for the Same Prefix.........4
      2.2. SID Representation in the MPLS Forwarding Plane...........4
      2.3. Segment Routing Global Block and Local Block..............5
      2.4. Mapping a SID Index to an MPLS label......................6
      2.5. Incoming Label Collision..................................7
         2.5.1. Tie-breaking Rules...................................8
         2.5.2. Redistribution between Routing Protocol Instances...11
            2.5.2.1. Illustration...................................11
            2.5.2.2. Illustration 2.................................12
      2.6. Outgoing Label Collision.................................12
      2.7. PUSH, CONTINUE, and NEXT.................................12
         2.7.1. PUSH................................................13
         2.7.2. CONTINUE............................................13
         2.7.3. NEXT................................................13
      2.8. MPLS Label Downloaded to FIB for Global and Local SIDs...13
      2.9. Active Segment...........................................13
      2.10. Forwarding behavior for Global SIDs.....................14
         2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs....14
         2.10.2. Forwarding for NEXT Operation for Global SIDs......15
      2.11. Forwarding Behavior for Local SIDs......................16
         2.11.1. Forwarding for PUSH Operation on Local SIDs........16
         2.11.2. Forwarding for CONTINUE Operation for Local SIDs...16
         2.11.3. Outgoing label for NEXT Operation for Local SIDs...16
   3. IGP Segments Examples.........................................17
      3.1. Example 1................................................18
      3.2. Example 2................................................19


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      3.3. Example 3................................................20
      3.4. Example 4................................................20
      3.5. Example 5................................................20
   4. IANA Considerations...........................................21
   5. Manageability Considerations..................................21
   6. Security Considerations.......................................21
   7. Contributors..................................................21
   8. Acknowledgements..............................................22
   9. References....................................................22
      9.1. Normative References.....................................22
      9.2. Informative References...................................23

1. Introduction

   The Segment Routing architecture [I-D.ietf-spring-segment-routing]
   can be directly applied to the MPLS architecture with no change in
   the MPLS forwarding plane.  This document specifies the forwarding
   plane behavior to allow Segment Routing to operate on top of the MPLS
   data plane. This document does not address the control plane
   behavior. Control plane behavior is specified in other documents such
   as [I-D.ietf-isis-segment-routing-extensions], [I-D.ietf-ospf-
   segment-routing-extensions], and [I-D.ietf-ospf-ospfv3-segment-
   routing-extensions].

   The Segment Routing problem statement is described in [RFC7855].

   Co-existence of SR over MPLS forwarding plane with LDP [RFC5036] is
   specified in [I-D.ietf-spring-segment-routing-ldp-interop].

   Policy routing and traffic engineering using segment routing can be
   found in [I.D. filsfils-spring-segment-routing-policy]

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2. MPLS Instantiation of Segment Routing

   MPLS instantiation of Segment Routing fits in the MPLS architecture
   as defined in [RFC3031] both from a control plane and forwarding
   plane perspective:




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   o  From a control plane perspective, [RFC3031] does not mandate a
      single signaling protocol.  Segment Routing makes use of various
      control plane protocols such as link state IGPs [I-D.ietf-isis-
      segment-routing-extensions], [I-D.ietf-ospf-segment-routing-
      extensions] and [I-D.ietf-ospf-ospfv3-segment-routing-extensions].
      The flooding mechanisms of link state IGPs fits very well with
      label stacking on ingress. Future control layer protocol and/or
      policy/configuration can be used to specify the label stack.

   o  From a forwarding plane perspective, Segment Routing does not
      require any change to the forwarding plane because Segment IDs
      (SIDs) are instantiated as MPLS labels and the Segment routing
      header instantiated as a stack of MPLS labels.

   We call "MPLS Control Plane Client (MCC)" any control plane entity
   installing forwarding entries in the MPLS data plane.  IGPs with SR
   extensions [I-D.ietf-isis-segment-routing-extensions], [I-D.ietf-
   ospf-segment-routing-extensions], [I-D.ietf-ospf-ospfv3-segment-
   routing-extensions] and LDP [RFC5036] are examples of MCCs. Local
   configuration and policies applied on a router are also examples of
   MCCs.

2.1. Multiple Forwarding Behaviors for the Same Prefix

   The SR architecture does not prohibit having more than one SID for
   the same prefix. In fact, by allowing multiple SIDs for the same
   prefix, it is possible to have different forwarding behaviors (such
   as different paths, different ECMP/UCMP behaviors,...,etc) for the
   same destination.

   Instantiating Segment routing over the MPLS forwarding plane fits
   seamlessly with this principle. An operator may assign multiple MPLS
   labels or indices to the same prefix and assign different forwarding
   behaviors to each label/SID. The MCC in the network downloads
   different MPLS labels/SIDs to the FIB for different forwarding
   behaviors. The MCC at the entry of an SR domain or at any point in
   the domain can choose to apply a particular forwarding behavior to a
   particular packet by applying the PUSH action to that packet using
   the corresponding SID.

2.2. SID Representation in the MPLS Forwarding Plane

   When instantiating SR over the MPLS forwarding plane, a SID is
   represented by an MPLS label or an index [I-D.ietf-spring-segment-
   routing].




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   A global segment MUST be a label, or an index which may be mapped to
   an MPLS label within the Segment Routing Global Block (SRGB) of the
   node installing the global segment in its FIB/receiving the labeled
   packet. Section 2.4 specifies the procedure to map a global segment
   represented by an index to an MPLS label within the SRGB.

   The MCC MUST ensure that any label value corresponding to any SID it
   installs in the forwarding plane follows the following rules:

   o  The label value MUST be unique within the router on which the MCC
      is running. i.e. the label MUST only be used to represent the SID
      and MUST NOT be used to represent more than one SID or for any
      other forwarding purpose on the router.

   o  The label value MUST NOT come from the range of special purpose
      labels [RFC7274].

2.3. Segment Routing Global Block and Local Block

   The concepts of Segment Routing Global Block (SRGB) and global SID
   are explained in [I-D.ietf-spring-segment-routing]. In general, the
   SRGB need not be a contiguous range of labels.

   For the rest of this document, the SRGB is specified by the list of
   MPLS Label ranges [Ll(1),Lh(1)], [Ll(2),Lh(2)],..., [Ll(k),Lh(k)]
   where  Ll(i) =< Lh(i).

   The following rules apply to the list of MPLS ranges representing the
   SRGB

   o  The list of ranges comprising the SRGB MUST NOT overlap.

   o  Every range in the list of ranges specifying the SRGB MUST NOT
      cover or overlap with a reserved label value or range [RFC7274],
      respectively.

   o  If the SRGB of a node does not conform to the structure specified
      in this section or to the previous two rules, then this SRGB MUST
      be completely ignored by all routers in the routing domain and the
      node MUST be treated as if it does not have an SRGB.

   o  The list of label ranges MUST only be used to instantiate global
      SIDs into the MPLS forwarding plane

   Local segments MAY be allocated from the Segment Routing Local Block
   (SRLB) [I-D.ietf-spring-segment-routing] or from any unused label as
   long as it does not use a special purpose label. The SRLB consists of


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   the range of local labels reserved by the node for certain local
   segments.  In a controller-driven network, some controllers or
   applications MAY use the control plane to discover the available set
   of local SIDs on a particular router [I.D. filsfils-spring-segment-
   routing-policy]. Just like SRGB, the SRLB need not be a single
   contiguous range of labels, except the SRGB MUST only be used to
   instantiate global SIDs into the MPLS forwarding plane. Hence it is
   specified the same way and follows the same rules SRGB is specified
   above in this sub-section.

2.4. Mapping a SID Index to an MPLS label

   This sub-section specifies how the MPLS label value is calculated
   given the index of a SID. The value of the index is determined by an
   MCC such as IS-IS [I-D.ietf-isis-segment-routing-extensions] or OSPF
   [I-D.ietf-ospf-segment-routing-extensions]. This section only
   specifies how to map the index to an MPLS label. The calculated MPLS
   label is downloaded to the FIB, sent out with a forwarded packet, or
   both.

   Consider a SID represented by the index "I". Consider an SRGB as
   specified in Section 2.3. The total size of the SRGB, represented by
   the variable "Size", is calculated according to the formula:

   size = Lh(1)- Ll(1) + 1 + Lh(2)- Ll(2) + 1 + ... + Lh(k)- Ll(k) + 1

   The following rules MUST be applied by the MCC when calculating the
   MPLS label value corresponding the SID index value "I".

   o  0 =< I < size. If the index "I" does not satisfy the previous
      inequality, then the label cannot be calculated.

   o  The label value corresponding to the SID index "I" is calculated
      as follows

       o j = 1 , temp = 0

       o While temp + Lh(j)- Ll(j) < I

            . temp = temp + Lh(j)- Ll(j) + 1

            . j = j+1

       o label = I - temp + Ll(j)





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2.5. Incoming Label Collision

   MPLS Architecture [RFC3031] defines Forwarding Equivalence Class
   (FEC) term as the set of packets with similar and / or identical
   characteristics which are forwarded the same way and are bound to the
   same MPLS incoming (local) label. In Segment-Routing MPLS, local
   label serves as the SID for given FEC.

   We define Segment Routing (SR) FEC as one of the following [I-D.ietf-
   spring-segment-routing]:

   o  (Prefix, Routing Instance, Topology, Algorithm), where a topology
      is identified by a set of links with metrics. For the purpose of
      incoming label collision resolution, the same numerical value
      SHOULD be used on all routers to identify the same set of links
      with metrics. For MCCs where the "Topology" and/or "Algorithm"
      fields are not defined, the numerical value of zero MUST be used
      for these two fields. For the purpose of incoming label collision
      resolution, a routing instance is identified by a single incoming
      label downloader to FIB. Two MCCs running on the same router are
      considered different routing instances if the only way the two
      instances can know about the other's incoming labels is through
      redistribution. The numerical value used to identify a routing
      instance MAY be derived from other configuration or MAY be
      explicitly configured. If it is derived from other configuration,
      then the same numerical value SHOULD be derived from the same
      configuration as long as the configuration survives router reload.
      If the derived numerical value varies for the same configuration,
      then an implementation SHOULD make numerical value used to
      identify a routing instance configurable.

   o  (next-hop, outgoing interface), where the outgoing interface is
      physical or virtual.

   o  (Endpoint, Color) representing an SR policy [I.D. filsfils-spring-
      segment-routing-policy]

   This section covers handling the scenario where, because of an
   error/misconfiguration, more than one SR FEC as defined in this
   section, map to the same incoming MPLS label.

   An incoming label collision occurs if the SIDs of the set of FECs
   {FEC1, FEC2,..., FECk} maps to the same incoming SR MPLS label "L1".

   The objective of the following steps is to deterministically install
   in the MPLS Incoming Label MAP, also known as label FIB, a single FEC



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   with the incoming label "L1". Remaining FECs may be installed in the
   IP FIB without incoming label.

   The procedure in this section relies completely on the local FEC and
   label database within a given router.

   The collision resolution procedure is as follows

   1. Given the SIDs of the set of FECs, {FEC1, FEC2,..., FECk} map to
      the same MPLS label "L1".

   2. Within an MCC, apply tie-breaking rules to select one FEC only and
      assign the label to it. The losing FECs are handled as if no
      labels are attached to them. The losing FECs with a non-zero
      algorithm are not installed in FIB.

       a. If the same set of FECs are attached to the same label "L1",
          then the tie-breaking rules MUST always select the same FEC
          irrespective of the order in which the FECs and the label "L1"
          are received. In other words, the tie-breaking rule MUST be
          deterministic. For example, a first-come-first-serve tie-
          breaking is not allowed.

   3. If there is still collision between the FECs belonging to
      different MCCs, then re-apply the tie-breaking rules to the
      remaining FECs to select one FEC only and assign the label to that
      FEC

   4. Install into the IP FIB the selected FEC and its incoming label in
      the label FIB.

   5. The remaining FECs with a zero algorithm are installed in the FIB
      natively, such as pure IP entries in case of Prefix FEC, without
      any incoming labels corresponding to their SIDs. The remaining
      FECs with a non-zero algorithm are not installed in the FIB.

2.5.1. Tie-breaking Rules

   The default tie-breaking rules SHOULD be as follows:

   1. if FECi has the lowest FEC administrative distance among the
      competing FECs as defined in this section below, filter away all
      the competing FECs with higher administrative distance.

   2. if more than one competing FEC remains after step 1, select the
      smallest numerical FEC value



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   These rules deterministically select the FEC to install in the MPLS
   forwarding plane for the given incoming label.

   This document defines the default tie breaking rules that SHOULD be
   implemented. An implementation MAY choose to implement additional
   tie-breaking rules. All routers in a routing domain SHOULD use the
   same tie-breaking rules to maximize forwarding consistency.

   Each FEC is assigned an administrative distance. The FEC
   administrative distance is encoded as an 8-bit value. The lower the
   value, the better the administrative distance.

   The default FEC administrative distance order starting from the
   lowest value SHOULD be

   o  Explicit SID assignment to a FEC that maps to a label outside the
      SRGB irrespective of the owner MCC. An explicit SID assignment is
      a static assignment of a label to a FEC such that the assignment
      survives router reboot.

       o An example of explicit SID allocation is static assignment of
         a specific label to an adj-SID.

       o An implementation of explicit SID assignment MUST guarantee
         collision freeness on the same router

   o  Dynamic SID assignment:

       o For all FEC types except for SR policy, use the default
         administrative distance depending on the implementation

       o Binding SID [I-D.ietf-spring-segment-routing] assigned to SR
         Policy

   A user SHOULD ensure that the same administrative distance preference
   is used on all routers to maximize forwarding consistency.

   The numerical sort across FECs SHOULD be performed as follows:

   o  Each FEC is assigned a FEC type encoded in 8 bits. The following
      are the type code point for each SR FEC defined at the beginning
      of this Section:

       o 120: (Prefix, Routing Instance, Topology, Algorithm)

       o 130: (next-hop, outgoing interface)



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       o 140: (Endpoint, Color) representing an SR policy

   o  The fields of each FEC are encoded as follows

       o Routing Instance ID represented by 16 bits. For routing
         instances that are identified by less than 16 bits, encode the
         Instance ID in the least significant bits while the most
         significant bits are set to zero

       o Address Family represented by 8 bits, where IPv4 encoded as
         100 and IPv6 is encoded as 110

       o All addresses are represented in 128 bits as follows

            . IPv6 address is encoded natively

            . IPv4 address is encoded in the most significant bits and
               the remaining bits are set to zero

       o All prefixes are represented by 128.

            . A prefix is encoded in the most significant bits and the
               remaining bits are set to zero.

            . The prefix length is encoded before the prefix

       o Topology ID is represented by 16 bits. For routing instances
         that identify topologies using less than 16 bits, encode the
         topology ID in the least significant bits while the most
         significant bits are set to zero

       o Algorithm is encoded in a 16 bits field.

       o The Color ID is encoded using 16 bits

   o  Choose the set of FECs of the smallest FEC type code point

   o  Out of these FECs, choose the FECs with the smallest address
      family code point

   o  Encode the remaining set of FECs as follows

       o Prefix, Routing Instance, Topology, Algorithm: (Prefix Length,
         Prefix, SR Algorithm, routing_instance_id, Topology)

       o (next-hop, outgoing interface): (next-hop,
         outgoing_interface_id)


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       o (Endpoint, Color): (Endpoint_address, Color_id)

   o  Select the FEC with the smallest numerical value



2.5.2. Redistribution between Routing Protocol Instances

   The following rule SHOULD be applied when redistributing SIDs with
   prefixes between routing protocol instances:

   o  If the receiving instance's SRGB is the same as the SRGB of origin
      instance, then

       o the index is redistributed with the route

   o  Else

       o the index is not redistributed and if needed it is the duty of
          the receiving instance to allocate a fresh index relative to
          its own SRGB. Note that in that case, the receiving instance
          MUST compute its local label according section 2.4 and
          install it in FIB.

   It is outside the scope of this document to define local node
   behaviors that would allow to map the original index into a new index
   in the receiving instance via the addition of an offset or other
   policy means.

2.5.2.1. Illustration

           A----IS-IS----B---OSPF----C-192.0.2.1/32 (20001)


   Consider the simple topology above.

   o  A and B are in the IS-IS domain with SRGB [16000-17000]

   o  B and C are in OSPF domain with SRGB [20000-21000]

   o  B redistributes 192.0.2.1/32 into IS-IS domain

   o  In that case A learns 192.0.2.1/32 as an IP leaf connected to B as
      usual for IP prefix redistribution





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   o  However, according to the redistribution rule above rule, B
      decides not to advertise any index with 192.0.2.1/32 into IS-IS
      because the SRGB is not the same.

2.5.2.2. Illustration 2

   Consider the example in the illustration described in Section
   2.5.2.1.

   When router B redistributes the prefix 192.0.2.1/32, router B decides
   to allocate and advertise the same index 1 with the prefix
   192.0.2.1/32

   Within the SRGB of the IS-IS domain, index 1 corresponds to the local
   label 16001

   o  Hence according to the redistribution rule above, router B
      programs the incoming label 16001 in its FIB to match traffic
      arriving from the IS-IS domain destined to the prefix
      192.0.2.1/32.


2.6. Outgoing Label Collision

   For the determination of the outgoing label to use, the ingress node
   pushing new segments, and hence a stack of MPLS labels, MUST use, for
   a given FEC, the same label that has been selected by the node
   receiving the packet with that label exposed as top label. So in case
   of incoming label collision on this receiving node, the ingress node
   MUST resolve this collision using this same "Incoming Label Collision
   resolution procedure", using the data of the receiving node.

   In the general case, the ingress node may not have exactly have the
   same data of the receiving node, so the result may be different. This
   is under the responsibility of the network operator. But in typical
   case, e.g. where a centralized node or a distributed link state IGP
   is used, all nodes would have the same database. However to minimize
   the chance of misforwarding, a FEC that loses its incoming label to
   the tie-breaking rules specified in Section 2.5 MUST NOT be
   installed in FIB with an outgoing segment routing label based on the
   SID corresponding to the lost incoming label.

2.7. PUSH, CONTINUE, and NEXT

   PUSH, NEXT, and CONTINUE are operations applied by the forwarding
   plane. The specifications of these operations can be found in [I-



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   D.ietf-spring-segment-routing]. This sub-section specifies how to
   implement each of these operations in the MPLS forwarding plane.

2.7.1. PUSH

   PUSH corresponds to pushing one or more labels on top of an incoming
   packet then sending it out of a particular physical interface or
   virtual interface, such as UDP tunnel [RFC7510] or L2TPv3 tunnel
   [RFC4817], towards a particular next-hop. Sections 2.10 and 2.11
   specify additional details about forwarding behavior.

2.7.2. CONTINUE

   In the MPLS forwarding plane, the CONTINUE operation corresponds to
   swapping the incoming label with an outgoing label. The value of the
   outgoing label is calculated as specified in Sections 2.10 and 2.11.

2.7.3. NEXT

   In the MPLS forwarding plane, NEXT corresponds to popping the topmost
   label. The action before and/or after the popping depends on the
   instruction associated with the active SID on the received packet
   prior to the popping. For example suppose the active SID in the
   received packet was an Adj-SID [I-D.ietf-spring-segment-routing],
   then on receiving the packet, the node applies NEXT operation, which
   corresponds to popping the top most label, and then sends the packet
   out of the physical or virtual interface (e.g. UDP tunnel [RFC7510]
   or L2TPv3 tunnel [RFC4817]) towards the next-hop corresponding to the
   adj-SID.

2.8. MPLS Label Downloaded to FIB for Global and Local SIDs

   The label corresponding to the global SID "Si" represented by the
   global index "I" downloaded to FIB is used to match packets whose
   active segment (and hence topmost label) is "Si". The value of this
   label is calculated as specified in Section 2.4.

   For Local SIDs, the MCC is responsible for downloading the correct
   label value to FIB. For example, an IGP with SR extensions I-D.ietf-
   isis-segment-routing-extensions, I-D.ietf-ospf-segment-routing-
   extensions] allocates and downloads the MPLS label corresponding to
   an Adj-SID [I-D.ietf-spring-segment-routing].

2.9. Active Segment

   When instantiated in the MPLS domain, the active segment on a packet
   corresponds to the topmost label on the packet that is calculated


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   according to the procedure specified in Sections 2.10 and 2.11. When
   arriving at a node, the topmost label corresponding to the active SID
   matches the MPLS label downloaded to FIB as specified in Section 2.4.

2.10. Forwarding behavior for Global SIDs

   This section specifies forwarding behavior, including the calculation
   of outgoing labels, that corresponds to a global SID when applying
   PUSH, CONTINUE, and NEXT operations in the MPLS forwarding plane.

   This document covers the calculation of the outgoing label for the
   top label only. The case where the outgoing label is not the top
   label and is part of a stack of labels that instantiates a routing
   policy or a traffic engineering tunnel is covered in other documents
   such as [I.D.filsfils-spring-segment-routing-policy].

2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs

   Suppose an MCC on a router "R0" determines that PUSH or CONTINUE
   operation is to be applied to an incoming packet whose active SID is
   the global SID "Si" represented by the global index "I" and owned by
   the router Ri before sending the packet towards a neighbor "N"
   directly connected to "R0" through a physical or virtual interface
   such as UDP tunnel [RFC7510] or L2TPv3 tunnel [RFC4817].

   The method by which the MCC on router "R0" determines that PUSH or
   CONTINUE operation must be applied using the SID "Si" is beyond the
   scope of this document. An example of a method to determine the SID
   "Si" for PUSH operation is the case where IS-IS [I-D.ietf-isis-
   segment-routing-extensions] receives the prefix-SID "Si" sub-TLV
   advertised with prefix "P/m" in TLV 135 and the destination address
   of the incoming IPv4 packet is covered by the prefix "P/m".

   For CONTINUE operation, an example of a method to determine the SID
   "Si" is the case where IS-IS [I-D.ietf-isis-segment-routing-
   extensions] receives the prefix-SID "Si" sub-TLV advertised with
   prefix "P" in TLV 135 and the top label of the incoming packet
   matches the MPLS label in FIB corresponding to the SID "Si" on the
   router "R0".

   The forwarding behavior for PUSH and CONTINUE corresponding to the
   SID "Si"

   o  If the neighbor "N" does not support SR or "I" does not satisfy
      the inequality specified in Section 2.4 for the SRGB of the
      neighbor "N"



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       o If it is possible to send the packet towards the neighbor "N"
          using standard MPLS forwarding behavior as specified in
          [RFC3031] and [RFC3032], then forward the packet. The method
          by which a router decides whether it is possible to send the
          packet to "N" or not is beyond the scope of this document. For
          example, the router "R0" can use the downstream label
          determined by another MCC, such as LDP [RFC5036], to send the
          packet.

       o Else if there are other useable next-hops, then use other next-
          hops to forward the incoming packet. The method by which the
          router "R0" decides on the possibility of using other next-
          hops is beyond the scope of this document. For example, the
          MCC on "R0" may chose the send an IPv4 packet without pushing
          any label to another next-hop.

       o Otherwise drop the packet.

   o  Else

       o Calculate the outgoing label as specified in Section 2.4 using
          the SRGB of the neighbor "N"

       o If the operation is PUSH

            . Push the calculated label according the MPLS label
               pushing rules specified in [RFC3032]

       o Else

            . swap the incoming label with the calculated label
               according to the label swapping rules in [RFC3032]

       o Send the packet towards the neighbor "N"



2.10.2. Forwarding for NEXT Operation for Global SIDs

   As specified in Section 2.7.3 NEXT operation corresponds to popping
   the top most label. The forwarding behavior is as follows

   o  Pop the topmost label

   o  Apply the instruction associated with the incoming label that has
      been popped



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   The action on the packet after popping the topmost label depends on
   the instruction associated with the incoming label as well as the
   contents of the packet right underneath the top label that got
   popped. Examples of NEXT operation are described in Section 3.

2.11. Forwarding Behavior for Local SIDs

   This section specifies the forwarding behavior for local SIDs when SR
   is instantiated over the MPLS forwarding plane.

2.11.1. Forwarding for PUSH Operation on Local SIDs

   Suppose an MCC on a router "R0" determines that PUSH operation is to
   be applied to an incoming packet using the local SID "Si" before
   sending the packet towards a neighbor "N" directly connected to R0
   through a physical or virtual interface such as UDP tunnel [RFC7510]
   or L2TPv3 tunnel [RFC4817].

   An example of such local SID is an Adj-SID allocated and advertised
   by IS-IS [I-D.ietf-isis-segment-routing-extensions]. The method by
   which the MCC on "R0" determines that PUSH operation is to be applied
   to the incoming packet is beyond the scope of this document. An
   example of such method is backup path used to protect against a
   failure using TI-LFA [I.D.bashandy-rtgwg-segment-routing-ti-lfa].

   As mentioned in [I-D.ietf-spring-segment-routing], a local SID is
   specified by an MPLS label. Hence the PUSH operation for a local SID
   is identical to label push operation [RFC3032] using any MPLS label.
   The forwarding action after pushing the MPLS label corresponding to
   the local SID is also determined by the MCC. For example, if the PUSH
   operation was done to forward a packet over a backup path calculated
   using TI-LFA, then the forwarding action may be sending the packet to
   a certain neighbor that will in turn continue to forward the packet
   along the backup path

2.11.2. Forwarding for CONTINUE Operation for Local SIDs

   A local SID on a router "R0" corresponds to a local label such as an
   Adj-SID. In such scenario, the outgoing label towards a next-hop "N"
   is determined by the MCC running on the router "R0"and the forwarding
   behavior for CONTINUE operation is identical to swap operation
   [RFC3032] on an MPLS label.

2.11.3. Outgoing label for NEXT Operation for Local SIDs

   NEXT operation for Local SIDs is identical to NEXT operation for
   global SIDs specified in Section 2.10.2.


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3. IGP Segments Examples

   Consider the network diagram of Figure 1 and the IP address and IGP
   Segment allocation of Figure 2. Assume that the network is running
   IS-IS with SR extensions [I-D.ietf-isis-segment-routing-extensions]
   and all links have the same metric. The following examples can be
   constructed.

                                +--------+
                               /          \
                R0-----R1-----R2----------R3-----R8
                              | \        / |
                              |  +--R4--+  |
                              |            |
                              +-----R5-----+

                   Figure 1: IGP Segments - Illustration






























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       +-----------------------------------------------------------+
       | IP address allocated by the operator:                     |
       |                      192.0.2.1/32 as a loopback of R1     |
       |                      192.0.2.2/32 as a loopback of R2     |
       |                      192.0.2.3/32 as a loopback of R3     |
       |                      192.0.2.4/32 as a loopback of R4     |
       |                      192.0.2.5/32 as a loopback of R5     |
       |                      192.0.2.8/32 as a loopback of R8     |
       |              198.51.100.9/32 as an anycast loopback of R4 |
       |              198.51.100.9/32 as an anycast loopback of R5 |
       |                                                           |
       | SRGB defined by the operator as 1000-5000                 |
       |                                                           |
       | Global IGP SID indices allocated by the operator:         |
       |                      1 allocated to 192.0.2.1/32          |
       |                      2 allocated to 192.0.2.2/32          |
       |                      3 allocated to 192.0.2.3/32          |
       |                      4 allocated to 192.0.2.4/32          |
       |                      8 allocated to 192.0.2.8/32          |
       |                   1009 allocated to 198.51.100.9/32       |
       |                                                           |
       | Local IGP SID allocated dynamically by R2                 |
       |                     for its "north" adjacency to R3: 9001 |
       |                     for its "north" adjacency to R3: 9003 |
       |                     for its "south" adjacency to R3: 9002 |
       |                     for its "south" adjacency to R3: 9003 |
       +-----------------------------------------------------------+

        Figure 2: IGP Address and Segment Allocation - Illustration


3.1. Example 1

   Suppose R1 wants to send an IPv4 packet P1 to R8. In this case, R1
   needs to apply PUSH operation to the IPv4 packet.

   Remember that the SID index "8" is a global IGP segment attached to
   the IP prefix 192.0.2.8/32. Its semantic is global within the IGP
   domain: any router forwards a packet received with active segment 8
   to the next-hop along the ECMP-aware shortest-path to the related
   prefix.

   R2 is the next-hop along the shortest path towards R8. By applying
   the steps in Section 2.8 the local label downloaded to R1's FIB
   corresponding to the global SID index 8 is 1008 because the SRGB of
   R2 is [1000,5000] as shown in Figure 2.



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   Because the packet is IPv4, R1 applies the PUSH operation using the
   label value 1008 as specified in Section 2.10.1. The resulting MPLS
   header will have the "S" bit [RFC3032] set because it is followed
   directly by an IPv4 packet.

   The packet arrives at router R2. Because the top label 1008
   corresponds to the IGP SID "8", which is the prefix-SID attached to
   the prefix 192.0.2.8/32 owned by the R8, then the instruction
   associated with the SID is "forward the packet using all ECMP/UCMP
   interfaces and all ECMP/UCMP next-hop(s) along the shortest path
   towards R8". Because R2 is not the penultimate hop, R2 applies the
   CONTINUE operation to the packet and sends it to R3 using one of the
   two links connected to R3 with top label 1008 as specified in Section
   2.10.1.

   R3 receives the packet with top label 1008. Because the top label
   1008 corresponds to the IGP SID "8", which is the prefix-SID attached
   to the prefix 192.0.2.8/32 owned by the R8, then the instruction
   associated with the SID is "send the packet using all ECMP interfaces
   and all next-hop(s) along the shortest path towards R8". Because R3
   is the penultimate hop, R3 applies NEXT operation then sends the
   packet to R8. The NEXT operation results in popping the outer label
   and sending the packet as a pure IPv4 packet to R8. The

   In conclusion, the path followed by P1 is R1-R2--R3-R8.  The ECMP-
   awareness ensures that the traffic be load-shared between any ECMP
   path, in this case the two north and south links between R2 and R3.

3.2. Example 2

   Suppose the right most router R0 wants to send a packet P2 to R8 over
   the path <R2, (north link between R2 and R3)>. In that case, the
   router R0 needs to use the SID list <2, 9001, 8>. Using the
   calculation techniques specified in Section 2.10 and 2.11 the
   resulting label stack starting from the topmost label is <1002, 9001,
   1008>.

   The MPLS label 1002 is the MPLS instantiation of the global IGP
   segment index 2 attached to the IP prefix 192.0.2.2/32. Its semantic
   is global within the IGP domain: any router forwards a packet
   received with active segment 1002 to the next-hop along the shortest-
   path to the related prefix.

   The MPLS label 9001 is a local IGP segment attached by node R2 to its
   north link to R3.  Its semantic is local to node R2: R2 applies NEXT
   operation, which corresponding to popping the outer label, then



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   switches a packet received with active segment 9001 towards the north
   link to R3.

   In conclusion, the path followed by P2 is R0-R1-R2-north-link-R3-R8.

3.3. Example 3

   R0 may send a packet P3 along the same exact path as P2 using a
   different segment list <2,9003,8> which corresponds to the label
   stack <1002, 9003, 1008>.

   9003 is a local IGP segment attached by node R2 to both its north and
   south links to R3.  Its semantic is local to node R2: R2 applies NEXT
   operation, which corresponds to popping the top label, then switches
   a packet received with active segment 9003 towards either the north
   or south links to R3 (e.g. per-flow loadbalancing decision).

   In conclusion, the path followed by P3 is R0-R1-R2-any-link-R3-R8.

3.4. Example 4

   R0 may send a packet P4 to R8 while avoiding the links between R2 and
   R3 by pushing the SID list <4,8>, which corresponds to the label
   stack <1004, 1008>.

   1004 is a global IGP segment attached to the IP prefix 192.0.2.4/32.
   Its semantic is global within the IGP domain: any router forwards a
   packet received with active segment 1004 to the next-hop along the
   shortest-path to the related prefix.

   In conclusion, the path followed by P4 is R0-R1-R2-R4-R3-R8.

3.5. Example 5

   R0 may send a packet P5 to R8 while avoiding the links between R2 and
   R3 and still benefiting from all the remaining shortest paths (via R4
   and R5) by pushing the SID list <1009, 8> which corresponds to the
   label stack <2009, 1008> using the steps specified in Sections 2.10
   and 2.11.

   1009 is a global anycast-SID [I-D.ietf-spring-segment-routing]
   attached to the anycast IP prefix 198.51.100.9/32.  Its semantic is
   global within the IGP domain: any router forwards a packet received
   with top label 2009 (corresponding to the active segment 1009) to the
   next-hop along the shortest-path to the related prefix.




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   In conclusion, the path followed by P5 is either R0-R1-R2-R4-R3-R8 or
   R0-R1-R2-R5-R3-R8.

4. IANA Considerations

   This document does not make any request to IANA.

5. Manageability Considerations

   This document describes the applicability of Segment Routing over the
   MPLS data plane.  Segment Routing does not introduce any change in
   the MPLS data plane.  Manageability considerations described in [I-
   D.ietf-spring-segment-routing] applies to the MPLS data plane when
   used with Segment Routing. SR OAM use cases for the MPLS data plane
   are defined in [I-D.ietf-spring-oam-usecase].  SR OAM procedures for
   the MPLS data plane are defined in [RFC8287].

6. Security Considerations

   This document does not introduce additional security requirements and
   mechanisms other than the ones described in [I-D.ietf-spring-segment-
   routing].

7. Contributors

   The following contributors have substantially helped the definition
   and editing of the content of this document:






















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   Martin Horneffer
   Deutsche Telekom
   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com

   Jeff Tantsura
   Email: jefftant@gmail.com
   Edward Crabbe
   Email: edward.crabbe@gmail.com

   Igor Milojevic
   Email: milojevicigor@gmail.com

   Saku Ytti
   Email: saku@ytti.fi

8. Acknowledgements

   The authors would like to thank Les Ginsberg and Himanshu Shah for
   their comments on this document.

   This document was prepared using 2-Word-v2.0.template.dot.

9. References

9.1. Normative References

   [I-D.ietf-spring-segment-routing] Filsfils, C., Previdi, S.,
             Decraene, B., Litkowski, S., and R. Shakir, "Segment
             Routing Architecture", draft-ietf-spring-segment-routing-12
             (work in progress), June 2017.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, DOI
             0.17487/RFC2119, March 1997, <http://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, <http://www.rfc-
             editor.org/info/rfc3031>.





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   [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,
             <http://www.rfc-editor.org/info/rfc3032>.

   [RFC7274] K. Kompella, L. Andersson, and A. Farrel, "Allocating and
             Retiring Special-Purpose MPLS Labels", RFC7274 DOI
             10.17487/RFC7274, May 2014  <http://www.rfc-
             editor.org/info/rfc7274>

   [RFC8174] B. Leiba, " Ambiguity of Uppercase vs Lowercase in RFC 2119
             Key Words", RFC7274 DOI 10.17487/RFC8174, May 2017
             <http://www.rfc-editor.org/info/rfc8174>



9.2. Informative References

   [I-D.ietf-isis-segment-routing-extensions] Previdi, S., Filsfils, C.,
             Bashandy, A., Gredler, H., Litkowski, S., Decraene, B., and
             j. jefftant@gmail.com, "IS-IS Extensions for Segment
             Routing", draft-ietf-isis-segment-routing-extensions-13
             (work in progress), June 2017.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions] Psenak, P.,
             Previdi, S., Filsfils, C., Gredler, H., Shakir, R.,
             Henderickx, W., and J. Tantsura, "OSPFv3 Extensions for
             Segment Routing", draft-ietf-ospf-ospfv3-segment-routing-
             extensions-09 (work in progress), March 2017.

   [I-D.ietf-ospf-segment-routing-extensions] Psenak, P., Previdi, S.,
             Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and
             J. Tantsura, "OSPF Extensions for Segment Routing", draft-
             ietf-ospf-segment-routing-extensions-16 (work in progress),
             May 2017.

   [I-D.ietf-spring-segment-routing-ldp-interop] Filsfils, C., Previdi,
             S., Bashandy, A., Decraene, B., and S. Litkowski, "Segment
             Routing interworking with LDP", draft-ietf-spring-segment-
             routing-ldp-interop-08 (work in progress), June 2017.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
             Litkowski, S., Horneffer, M., and R. Shakir, "Source Packet
             Routing in Networking (SPRING) Problem Statement and
             Requirements", RFC 7855, DOI 10.17487/RFC7855, May 2016,
             <http://www.rfc-editor.org/info/rfc7855>.



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   [RFC5036] Andersson, L., Acreo, AB, Minei, I., Thomas, B., " LDP
             Specification", RFC5036, DOI 10.17487/RFC5036, October
             2007, <https://www.rfc-editor.org/info/rfc5036>

   [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>.

   [RFC4817] Townsley, M., Pignataro, C., Wainner, S., Seely, T., Young,
             T., "Encapsulation of MPLS over Layer 2 Tunneling Protocol
             Version 3", RFC4817, DOI 10.17487/RFC4817, March 2007,
             <https://www.rfc-editor.org/info/rfc4817>

   [RFC8287] N. Kumar, C. Pignataro, G. Swallow, N. Akiya, S. Kini, and
             M. Chen " Label Switched Path (LSP) Ping/Traceroute for
             Segment Routing (SR) IGP-Prefix and IGP-Adjacency Segment
             Identifiers (SIDs) with MPLS Data Planes" RFC8287, DOI
             10.17487/RFC8287, December 2017, <https://www.rfc-
             editor.org/info/rfc8287>

   [I.D. filsfils-spring-segment-routing-policy] Filsfils, C.,
             Sivabalan, S., Raza, K., Liste,  J. , Clad, F., Voyer,  D.,
             Lin, S.,  Bogdanov, A.,  Horneffer, M.,  Steinberg, D.,
             Decraene, B. , Litkowski, S., " Segment Routing Policy for
             Traffic Engineering",  draft-filsfils-spring-segment-
             routing-policy-01 (work in progress), July 2017




















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             Authors' Addresses

   Ahmed Bashandy
   Individual

   Email: abashandy.ietf@gmail.com


   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Italy

   Email: stefano@previdi.net


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Rob Shakir
   Google
   US

   Email: robjs@google.com







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