Segment Routing Conflict Resolution
draft-ietf-spring-conflict-resolution-03

Networking Working Group                                     L. Ginsberg
Internet-Draft                                                 P. Psenak
Intended status: Standards Track                              S. Previdi
Expires: October 29, 2017                                  Cisco Systems
                                                                M. Pilka
                                                          April 27, 2017

                  Segment Routing Conflict Resolution
              draft-ietf-spring-conflict-resolution-03.txt

Abstract

   In support of Segment Routing (SR) routing protocols advertise a
   variety of identifiers used to define the segments which direct
   forwarding of packets.  In cases where the information advertised by
   a given protocol instance is either internally inconsistent or
   conflicts with advertisements from another protocol instance a means
   of achieving consistent forwarding behavior in the network is
   required.  This document defines the policies used to resolve these
   occurrences.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 October 29, 2017.

Ginsberg, et al.        Expires October 29, 2017                [Page 1]
Internet-Draft           sr-conflict-resolution               April 2017

Copyright Notice

   Copyright (c) 2017 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR Global Block Inconsistency . . . . . . . . . . . . . . . .   3
   3.  SR-MPLS Segment Identifier Conflicts  . . . . . . . . . . . .   5
     3.1.  SID Preference  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Conflict Types  . . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Prefix Conflict . . . . . . . . . . . . . . . . . . .   6
       3.2.2.  SID Conflict  . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Processing conflicting entries  . . . . . . . . . . . . .   9
       3.3.1.  Policy: Ignore conflicting entries  . . . . . . . . .   9
       3.3.2.  Policy: Preference Algorithm/Quarantine . . . . . . .  10
       3.3.3.  Policy: Preference algorithm/ignore overlap only  . .  10
       3.3.4.  Preference Algorithm  . . . . . . . . . . . . . . . .  10
       3.3.5.  Example Behavior - Single Topology/Algorithm  . . . .  11
       3.3.6.  Example Behavior - Multiple Topologies  . . . . . . .  12
       3.3.7.  Evaluation of Policy Alternatives . . . . . . . . . .  13
       3.3.8.  Guaranteeing Database Consistency . . . . . . . . . .  14
   4.  Scope of SR-MPLS SID Conflicts  . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Consideration  . . . . . . . . . . . . . . . . . . . . .  15
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informational References  . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Segment Routing (SR) as defined in [SR-ARCH] utilizes forwarding
   instructions called "segments" to direct packets through the network.
   Depending on the forwarding plane architecture in use, routing
   protocols advertise various identifiers which define the permissible

Ginsberg, et al.        Expires October 29, 2017                [Page 2]
Internet-Draft           sr-conflict-resolution               April 2017

   values which can be used as segments, which values are assigned to
   specific prefixes, etc.  Where segments have global scope it is
   necessary to have non-conflicting assignments - but given that the
   advertisements may originate from multiple nodes the possibility
   exists that advertisements may be received which are either
   internally inconsistent or conflicting with advertisements originated
   by other nodes.  In such cases it is necessary to have consistent
   resolution of conflicts network-wide in order to avoid forwarding
   loops.

   The problem to be addressed is protocol independent i.e., segment
   related advertisements may be originated by multiple nodes using
   different protocols and yet the conflict resolution MUST be the same
   on all nodes regardless of the protocol used to transport the
   advertisements.

   The remainder of this document defines conflict resolution policies
   which meet these requirements.  All protocols which support SR MUST
   adhere to the policies defined in this document.

2.  SR Global Block Inconsistency

   In support of an MPLS dataplane routing protocols advertise an SR
   Global Block (SRGB) which defines a set of label ranges reserved for
   use by the advertising node in support of SR.  The details of how
   protocols advertise this information can be found in the protocol
   specific drafts e.g., [SR-OSPF], [SR-OSPFv3], and [SR-IS-IS].
   However the protocol independent semantics are illustrated by the
   following example:

Ginsberg, et al.        Expires October 29, 2017                [Page 3]
Internet-Draft           sr-conflict-resolution               April 2017

   The originating router advertises the following ranges:

         Range 1: (100, 199)
         Range 2: (1000, 1099)
         Range 3: (500, 599)

    The receiving routers concatenate the ranges and build the Segment
    Routing Global Block (SRGB) as follows:

      SRGB = (100, 199)
             (1000, 1099)
             (500, 599)

    The indeces span multiple ranges:

         index=0 means label 100
         ...
         index 99 means label 199
         index 100 means label 1000
         index 199 means label 1099
         ...
         index 200 means label 500
         ...

   Note that the ranges are an ordered set - what labels are mapped to a
   given index depends on the placement of a given label range in the
   set of ranges advertised.

   For the set of ranges to be usable the ranges MUST be disjoint.
   Sender behavior is defined in various SR protocol drafts such as [SR-
   IS-IS] which specify that senders MUST NOT advertise overlapping
   ranges.

   Receivers of SRGB ranges MUST validate the SRGB ranges advertised by
   other nodes.  If the advertised ranges do not conform to the
   restrictions defined in the respective protocol specification
   receivers MUST ignore all advertised SRGB ranges from that node.
   Operationally the node is treated as though it did not advertise any
   SRGB ranges.  [SR-MPLS] defines the procedures for mapping global
   SIDs to outgoing labels.

   Note that utilization of local SIDs (e.g. adjacency SIDs) advertised
   by a node is not affected by the state of the advertised SRGB.

Ginsberg, et al.        Expires October 29, 2017                [Page 4]
Internet-Draft           sr-conflict-resolution               April 2017

3.  SR-MPLS Segment Identifier Conflicts

   In support of an MPLS dataplane Segment identifiers (SIDs) are
   advertised and associated with a given prefix.  SIDs may be
   advertised in the prefix reachability advertisements originated by a
   routing protocol (PFX) . SIDs may also be advertised by a Segment
   Routing Mapping Server (SRMS).

   Information in a SID advertisement is used to construct a mapping
   entry.  A generalized mapping entry can be represented using the
   following definitions:

       Prf - Preference Value (See Section 3.1)
       Pi - Initial prefix
       Pe - End prefix
       L  - Prefix length
       Lx - Maximum prefix length (32 for IPv4, 128 for IPv6)
       Si - Initial SID value
       Se - End SID value
       R  - Range value (See Note 1)
       T  - Topology
       A  - Algorithm

       A Mapping Entry is then the tuple: (Prf, Src, Pi/L, Si, R, T, A)
       Pe = (Pi + ((R-1) << (Lx-L))
       Se = Si + (R-1)

       NOTE 1: The SID advertised in a prefix reachability advertisement
               always has an implicit range of 1.

   Conflicts in SID advertisements may occur as a result of
   misconfiguration.  Conflicts may occur either in the set of
   advertisements originated by a single node or between advertisements
   originated by different nodes.  Conflicts which occur within the set
   of advertisements (P-SID and SRMS) originated by a single node SHOULD
   be prevented by configuration validation on the originating node.

   When conflicts occur, it is not possible for routers to know which of
   the conflicting advertisements is "correct".  In order to avoid
   forwarding loops and/or blackholes, there is a need for all nodes to
   resolve the conflicts in a consistent manner.  This in turn requires
   that all routers have identical sets of advertisements and that they
   all use the same selection algorithm.  This document defines
   procedures to achieve these goals.

Ginsberg, et al.        Expires October 29, 2017                [Page 5]
Internet-Draft           sr-conflict-resolution               April 2017

3.1.  SID Preference

   If a node acts as an SRMS, it MAY advertise a preference to be
   associated with all SRMS SID advertisements sent by that node.  The
   means of advertising the preference is defined in the protocol
   specific drafts e.g., [SR-OSPF], [SR-OSPFv3], and [SR-IS-IS].  The
   preference value is an unsigned 8 bit integer with the following
   properties:

      0 - Reserved value indicating advertisements from that node
          MUST NOT be used.
      1 - 255 Preference value

   Advertisement of a preference value is optional.  Nodes which do not
   advertise a preference value are assigned a preference value of 128.

   All SIDs advertised in prefix reachability advertisements implicitly
   have a preference value of 192.

3.2.  Conflict Types

   Two types of conflicts may occur - Prefix Conflicts and SID
   Conflicts.  Examples are provided in this section to illustrate these
   conflict types.

3.2.1.  Prefix Conflict

   When different SIDs are assigned to the same prefix we have a "prefix
   conflict".  Prefix conflicts are specific to mapping entries sharing
   the same topology and algorithm.

   Example PC1

   (192, 192.0.2.120/32, 200, 1, 0, 0)
   (192, 192.0.2.120/32, 30, 1, 0, 0)

   The prefix 192.0.2.120/32 has been assigned two different SIDs:
     200 by the first advertisement
     30 by the second advertisement

   Example PC2

   (192, 2001:DB8::1/128, 400, 1, 2, 0)
   (192, 2001:DB8::1/128, 50, 1, 2, 0)

   The prefix 2001:DB8::1/128 has been assigned two different SIDs:
    400 by the first advertisement
    50 by the second advertisement

Ginsberg, et al.        Expires October 29, 2017                [Page 6]
Internet-Draft           sr-conflict-resolution               April 2017

   Prefix conflicts may also occur as a result of overlapping prefix
   ranges.

   Example PC3

   (128, 192.0.2.1/32, 200, 200, 0, 0)
   (128, 192.0.2.121/32, 30, 10, 0, 0)

   Prefixes 192.0.2.121/32 - 192.0.2.130/32 are assigned two
   different SIDs:
    320 through 329 by the first advertisement
    30 through 39 by the second advertisement

   Example PC4
   (128, 2001:DB8::1/128, 400, 200, 2, 0)
   (128, 2001:DB8::121/128, 50, 10, 2, 0)

   Prefixes 2001:DB8::121/128 - 2001:DB8::130/128 are assigned
   two different SIDs:
     420 through 429 by the first advertisement
     50 through 59 by the second advertisement

   Examples PC3 and PC4 illustrate a complication - only part of the
   range advertised in the first advertisement is in conflict.  It is
   logically possible to isolate the conflicting portion and try to use
   the non-conflicting portion(s).

   A variant of the overlapping prefix range is a case where we have
   overlapping prefix ranges but no actual SID conflict.

   Example PC5

   (128, 192.0.2.1/32, 200, 200, 0, 0)
   (128, 192.0.2.121/32, 320, 10, 0, 0)

   (128, 2001:DB8::1/128, 400, 200, 2, 0)
   (128, 2001:DB8::121/128, 520, 10, 2, 0)

   Although there is prefix overlap between the two IPv4 entries (and
   the two IPv6 entries) the same SID is assigned to all of the shared
   prefixes by the two entries.

Ginsberg, et al.        Expires October 29, 2017                [Page 7]
Internet-Draft           sr-conflict-resolution               April 2017

   Given two mapping entries:

   (Prf, P1/L1, S1, R1, T1, A1) and
   (Prf, P2/L2, S2, R2, T2, A2)

   where P1 <= P2

   a prefix conflict exists if all of the following are true:

   1)(T1 == T2) && (A1 == A2)
   2)P1 <= P2
   3)The prefixes are in the same address family.
   2)L1 == L2
   3)(P1e >= P2) && ((S1 + (P2 - P1)) != S2)

3.2.2.  SID Conflict

   When the same SID has been assigned to multiple prefixes we have a
   "SID conflict".  SID conflicts are independent of address-family,
   independent of prefix len, independent of topology, and independent
   of algorithm.  A SID conflict occurs when a mapping entry which has
   previously been checked to have no prefix conflict assigns one or
   more SIDs that are assigned by another entry which also has no prefix
   conflicts.

   Example SC1

   (192, 192.0.2.1/32, 200, 1, 0, 0)
   (192, 192.0.2.222/32, 200, 1, 0, 0)
   SID 200 has been assigned to 192.0.2.1/32 by the
   first advertisement.
   The second advertisement assigns SID 200 to 192.0.2.222/32.

   Example SC2

   (192, 2001:DB8::1/128, 400, 1, 2, 0)
   (192, 2001:DB8::222/128, 400, 1, 2, 0)
   SID 400 has been assigned to 2001:DB8::1/128 by the
   first advertisement.
   The second advertisement assigns SID 400 to 2001:DB8::222/128

   SID conflicts may also occur as a result of overlapping SID ranges.

Ginsberg, et al.        Expires October 29, 2017                [Page 8]
Internet-Draft           sr-conflict-resolution               April 2017

   Example SC3

   (128, 192.0.2.1/32, 200, 200, 0, 0)
   (128, 198.51.100.1/32, 300, 10, 0, 0)

   SIDs 300 - 309 have been assigned to two different prefixes.
   The first advertisement assigns these SIDs
   to 192.0.2.101/32 - 192.0.2.110/32.
   The second advertisement assigns these SIDs to
   198.51.100.1/32 - 198.51.100.10/32.

   Example SC4
   (128, 2001:DB8::1/128, 400, 200, 2, 0)
   (128, 2001:DB8:1::1/128, 500, 10, 2, 0)

   SIDs 500 - 509 have been assigned to two different prefixes.
   The first advertisement assigns these SIDs to
   2001:DB8::101/128 - 2001:DB8::10A/128.
   The second advertisement assigns these SIDs to
   2001:DB8:1::1/128 - 2001:DB8:1::A/128.

   Examples SC3 and SC4 illustrate a complication - only part of the
   range advertised in the first advertisement is in conflict.

3.3.  Processing conflicting entries

   Two general approaches can be used to process conflicting entries.

   1.  Conflicting entries can be ignored

   2.  A standard preference algorithm can be used to choose which of
       the conflicting entries will be used

   The following sections discuss these two approaches in more detail.

   Note: This document does not discuss any implementation details i.e.
   what type of data structure is used to store the entries (trie, radix
   tree, etc.) nor what type of keys may be used to perform lookups in
   the database.

3.3.1.  Policy: Ignore conflicting entries

   In cases where entries are in conflict none of the conflicting
   entries are used i.e., the network operates as if the conflicting
   advertisements were not present.

Ginsberg, et al.        Expires October 29, 2017                [Page 9]
Internet-Draft           sr-conflict-resolution               April 2017

   Implementations are required to identify the conflicting entries and
   ensure that they are not used.

3.3.2.  Policy: Preference Algorithm/Quarantine

   For entries which are in conflict properties of the conflicting
   advertisements are used to determine which of the conflicting entries
   are used in forwarding and which are "quarantined" and not used.  The
   entire quarantined entry is not used.

   This approach requires that conflicting entries first be identified
   and then evaluated based on a preference rule.  Based on which entry
   is preferred this in turn may impact what other entries are
   considered in conflict i.e. if A conflicts with B and B conflicts
   with C - it is possible that A does NOT conflict with C.  Hence if as
   a result of the evaluation of the conflict between A and B, entry B
   is not used the conflict between B and C will not be detected.

3.3.3.  Policy: Preference algorithm/ignore overlap only

   A variation of the preference algorithm approach is to quarantine
   only the portions of the less preferred entry which actually
   conflicts.  The original entry is split into multiple ranges.  The
   ranges which are in conflict are quarantined.  The ranges which are
   not in conflict are used in forwarding.  This approach adds
   complexity as the relationship between the derived sub-ranges of the
   original mapping entry have to be associated with the original entry
   - and every time some change to the advertisement database occurs the
   derived sub-ranges have to be recalculated.

3.3.4.  Preference Algorithm

   The following algorithm is used to select the preferred mapping entry
   when a conflict exists.  Evaluation is made in the order specified.
   Prefix conflicts are evaluated first.  SID conflicts are then
   evaluated on the Active entries remaining after Prefix Conflicts have
   been resolved.

   1.  Higher preference value wins

   2.  Smaller range wins

   3.  IPv6 entry wins over IPv4 entry

   4.  Longer prefix length wins

   5.  Smaller algorithm wins

Ginsberg, et al.        Expires October 29, 2017               [Page 10]
Internet-Draft           sr-conflict-resolution               April 2017

   6.  Smaller starting address (considered as an unsigned integer
       value) wins

   7.  Smaller starting SID wins

   8.  If topology IDs are NOT identical both entries MUST be ignored

   As SIDs associated with prefix reachability advertisements have a
   preference of 192 while by default SRMS preference is 128, the
   default behavior is then to prefer SIDs advertised in prefix
   reachability advertisements over SIDs advertised by SRMSs, but an
   operator can choose to override this behavior by setting SRMS
   preference higher than 192.

   Preferring advertisements with smaller range has the nice property
   that a single misconfiguration of an SRMS entry with a large range
   will not be preferred over a large number of advertisements with
   smaller ranges.

   Since topology identifiers are locally scoped, it is not possible to
   make a consistent choice network wide when all elements of a mapping
   entry are identical except for the topology.  This is why both
   entries MUST be ignored in such cases (Rule #8 above).  Note that
   Rule #8 only applies when considering SID conflicts since Prefix
   conflicts are restricted to a single topology.

3.3.5.  Example Behavior - Single Topology/Algorithm

   The following mapping entries exist:in the database.  For brevity,
   Topology/Algorithm is omitted and assumed to be (0,0) in all entries.

   1.  (192, 192.0.2.1/32, 100, 1)

   2.  (192, 192.0.2.101/32, 200, 1)

   3.  (128, 192.0.2.1/32, 400, 255) !Prefix conflict with entries 1 and
       2

   4.  (128, 198.51.100.40/32, 200,1) !SID conflict with entry 2

   The table below shows what mapping entries will be used in the
   forwarding plane (Active) and which ones will not be used (Excluded)
   under the three candidate policies:

Ginsberg, et al.        Expires October 29, 2017               [Page 11]
Internet-Draft           sr-conflict-resolution               April 2017

  +--------------------------------------------------------------------+
  |Policy     | Active Entries           |  Excluded Entries           |
  +--------------------------------------------------------------------+
  |Ignore    |                           |(192,192.0.2.1/32,100,1)     |
  |          |                           |(192,192.0.2.101/32,200,1)   |
  |          |                           |(128,192.0.2.1/32,400,255)   |
  |          |                           |(128,198.51.100.40/32,200,1) |
  +--------------------------------------------------------------------+
  |Quarantine|(192,192.0.1.1/32,100,1)   |(128,192.0.2.1/32,400,255)   |
  |          |(192,192.0.2.101/32,200,1) |(128,198.51.100.40/32,200,1) |
  +--------------------------------------------------------------------+
  |Overlap-  |(192,192.0.2.1/32,100,1)   |(128,198.51.100.40/32,200,1) |
  | Only     |(192,192.0.2.101/32,200,1) |*(128,192.0.2.1/32,400,1)    |
  |          |*(128,192.0.2.2/32,401,99) |*(128,192.0.2.101/32,500,1)  |
  |          |*(128,192.0.2.102/32,                                    |
  |          |      501,153)             |                             |
  +--------------------------------------------------------------------+

  * Derived from (128,192.0.2.1/32,400,300)

3.3.6.  Example Behavior - Multiple Topologies

   When using a preference rule the order in which conflict resolution
   is applied has an impact on what entries are usable when entries for
   multiple topologies (or algorithms) are present.  The following
   mapping entries exist in the database:

   1.  (192, 192.0.2.1/32, 100, 1, 0, 0) !Topology 0

   2.  (192, 192.0.2.1/32, 200, 1, 0, 0) !Topology 0, Prefix Conflict
       with entry #1

   3.  (192, 198.51.100.40/32, 200,1,1,0) ! Topology 1, SID conflict
       with entry 2

   The table below shows what mapping entries will be used in the
   forwarding plane (Active) and which ones will not be used (Excluded)
   under the Quarantine Policy based on the order in which conflict
   resolution is applied.

Ginsberg, et al.        Expires October 29, 2017               [Page 12]
Internet-Draft           sr-conflict-resolution               April 2017

   +------------------------------------------------------------------+
   |Order   | Active Entries             | Excluded Entries           |
   +------------------------------------------------------------------+
   |Prefix- |(192,192.0.2.1/32,100,1,0,0)|(192,192.0.2.101/32,200,1,0)|
   |Conflict|(192,198.51.100.40/32,200,1,|                            |
   |First   |    1,0)                    |                            |
   +------------------------------------------------------------------+
   |SID-    |(192,192.0.2.1/32,100,1,0,0)|(192,192.0.2.101/32,200,1,0)|
   |Conflict|                            |(192,198.51.100.40/32,200,1,|
   |First   |                            |    1,0)                    |
   +------------------------------------------------------------------+

   This illustrates the advantage of evaluating prefix conflicts within
   a given topology (or algorithm) before evaluating topology (or
   algorithm) independent SID conflicts.  It insures that entries which
   will be excluded based on intratopology preference will not prevent a
   SID assigned in another topology from being considered Active.

3.3.7.  Evaluation of Policy Alternatives

   The previous sections have defined three alternatives for resolving
   conflicts - ignore, quarantine, and ignore overlap-only.

   The ignore policy impacts the greatest amount of traffic as
   forwarding to all destinations which have a conflict is affected.

   Quarantine allows forwarding for some destinations which have a
   conflict to be supported.

   Ignore overlap-only maximizes the destinations which will be
   forwarded as all destinations covered by some mapping entry
   (regardless of range) will be able to use the SID assigned by the
   winning range.  This alternative increases implementation complexity
   as compared to quarantine.  Mapping entries with a range greater than
   1 which are in conflict with other mapping entries have to internally
   be split into 2 or more "derived mapping entries".  The derived
   mapping entries then fall into two categories - those that are in
   conflict with other mapping entries and those which are NOT in
   conflict.  The former are ignored and the latter are used.  Each time
   the underived mapping database is updated the derived entries have to
   be recomputed based on the updated database.  Internal data
   structures have to be maintained which maintain the relationship
   between the advertised mapping entry and the set of derived mapping
   entries.  All nodes in the network have to achieve the same behavior
   regardless of implementation internals.

Ginsberg, et al.        Expires October 29, 2017               [Page 13]
Internet-Draft           sr-conflict-resolution               April 2017

   There is then a tradeoff between a goal of maximizing traffic
   delivery and the risks associated with increased implementation
   complexity.

   Consensus of the working group is that maximizing traffic delivery is
   the most important deployment consideration - therefore ignore-
   overlap-only is specified as the standard policy which MUST be
   implemented by all nodes which support SR-MPLS.

3.3.8.  Guaranteeing Database Consistency

   In order to obtain consistent active entries all nodes in a network
   MUST have the same mapping entry database.  Mapping entries can be
   obtained from a variety of sources.

   o  SIDs can be configured locally for prefixes assigned to interfaces
      on the router itself.  Only SIDs which are advertised to protocol
      peers can be considered as part of the mapping entry database.

   o  SIDs can be received in prefix reachability advertisements from
      protocol peers.  These advertisements may originate from peers
      local to the area or be leaked from other areas and/or
      redistributed from other routing protocols.

   o  SIDs can be received from SRMS advertisements - these
      advertisements can originate from routers local to the area or
      leaked from other areas

   o  In cases where multiple routing protocols are in use mapping
      entries advertised by all routing protocols MUST be included.

4.  Scope of SR-MPLS SID Conflicts

   The previous section defines the types of SID conflicts and
   procedures to resolve such conflicts when using an MPLS dataplane.
   The mapping entry database used MUST be populated with entries for
   destinations for which the associated SID will be used to derive the
   labels installed in the forwarding plane of routers in the network.
   This consists of entries associated with intra-domain routes.

   There are cases where destinations which are external to the domain
   are advertised by protocol speakers running within that network - and
   it is possible that those advertisements have SIDs associated with
   those destinations.  However, if reachability to a destination is
   topologically outside the forwarding domain of the protocol instance
   then the SIDs for such destinations will never be installed in the
   forwarding plane of any router within the domain - so such
   advertisements cannot create a SID conflict within the domain.  Such

Ginsberg, et al.        Expires October 29, 2017               [Page 14]
Internet-Draft           sr-conflict-resolution               April 2017

   entries therefore MUST NOT be installed in the database used for
   intra-domain conflict resolution.

   Consider the case of two sites "A and B" associated with a given
   [RFC4364] VPN.  Connectivity between the sites is via a provider
   backbone.  SIDs associated with destinations in Site A will never be
   installed in the forwarding plane of routers in Site B.  Reachability
   between the sites (assuming SR is being used across the backbone)
   only requires using a SID associated with a gateway PE.  So a
   destination in Site A MAY use the same SID as a destination in Site B
   without introducing any conflict in the forwarding plane of routers
   in Site A.

   Such cases are handled by insuring that the mapping entries in the
   database used by the procedures defined in the previous section only
   include entries associated with advertisements within the site.

5.  Security Considerations

   The ability to introduce SID conflicts into a deployment may
   compromise traffic forwarding.  Protocol specific security mechanisms
   SHOULD be used to insure that all SID advertisements originate from
   trusted sources.

6.  IANA Consideration

   This document has no actions for IANA.

7.  Acknowledgements

   The authors would like to thank Jeff Tantsura, Wim Henderickx, and
   Bruno Decraene for their careful review and content suggestions.

8.  References

8.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <http://www.rfc-editor.org/info/rfc4364>.

Ginsberg, et al.        Expires October 29, 2017               [Page 15]
Internet-Draft           sr-conflict-resolution               April 2017

   [SR-IS-IS]
              "IS-IS Extensions for Segment Routing, draft-ietf-isis-
              segment-routing-extensions-11(work in progress)", March
              2017.

   [SR-MPLS]  "Segment Routing with MPLS dataplane, draft-ietf-spring-
              segment-routing-mpls-08(work in progress)", March 2017.

   [SR-OSPF]  "OSPF Extensions for Segment Routing, draft-ietf-ospf-
              segment-routing-extensions-12(work in progress)", March
              2017.

   [SR-OSPFv3]
              "OSPFv3 Extensions for Segment Routing, draft-ietf-ospf-
              ospfv3-segment-routing-extensions-09(work in progress)",
              March 2016.

8.2.  Informational References

   [SR-ARCH]  "Segment Routing Architecture, draft-ietf-spring-segment-
              routing-11(work in progress)", February 2017.

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA  95035
   USA

   Email: ginsberg@cisco.com

   Peter Psenak
   Cisco Systems
   Apollo Business Center Mlynske nivy 43
   Bratislava  821 09
   Slovakia

   Email: ppsenak@cisco.com

Ginsberg, et al.        Expires October 29, 2017               [Page 16]
Internet-Draft           sr-conflict-resolution               April 2017

   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   Email: sprevidi@cisco.com

   Martin Pilka

   Email: martin@infobox.sk

Ginsberg, et al.        Expires October 29, 2017               [Page 17]