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Versions: 00 01 draft-ietf-spring-conflict-resolution

Networking Working Group                                     L. Ginsberg
Internet-Draft                                                 P. Psenak
Intended status: Standards Track                              S. Previdi
Expires: October 14, 2016                                  Cisco Systems
                                                                M. Pilka
                                                   Pantheon Technologies
                                                          April 12, 2016


                  Segment Routing Conflict Resolution
            draft-ginsberg-spring-conflict-resolution-01.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 14, 2016.






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Copyright Notice

   Copyright (c) 2016 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.  Segment Identifier Conflicts  . . . . . . . . . . . . . . . .   5
     3.1.  Conflict Types  . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Prefix Conflict . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  SID Conflict  . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Processing conflicting entries  . . . . . . . . . . . . .  10
       3.2.1.  Policy: Ignore conflicting entries  . . . . . . . . .  10
       3.2.2.  Policy: Preference Algorithm/Quarantine . . . . . . .  10
       3.2.3.  Policy: Preference algorithm/ignore overlap only  . .  10
       3.2.4.  Preference Algorithm  . . . . . . . . . . . . . . . .  11
       3.2.5.  Use of topology in preference . . . . . . . . . . . .  11
       3.2.6.  Example Behavior  . . . . . . . . . . . . . . . . . .  12
       3.2.7.  Evaluation of Policy Alternatives . . . . . . . . . .  13
       3.2.8.  Guaranteeing Database Consistency . . . . . . . . . .  13
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   5.  IANA Consideration  . . . . . . . . . . . . . . . . . . . . .  14
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informational References  . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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
   values which can be used as segments, which values are assigned to
   specific prefixes, etc.  Where segments have global scope it is



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   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] and [SR-IS-IS].  However the protocol
   independent semantics are illustrated by the following example:

























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   The originating router advertises the following ranges:

         Range 1: (100, 199)
         Range 2: (1000, 1099)
         Range 3: (500, 5990

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

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

    The indices 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.








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3.  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.  SIDs may also be advertised by a Segment Routing
   Mapping Server (SRMS).

   Mapping entries have an explicit context which includes the topology
   and the SR algorithm.  A generalized mapping entry can be represented
   using the following definitions:

     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
     T - Topology
     A - Algorithm

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

     Note that the SID advertised in a prefix reachability advertisement
     can be more generally represented as a mapping entry with a 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.  When conflicts occur, it is not
   possible for routers to know which of the conflicting advertisements
   is "correct".  If a router chooses to use one of the conflicting
   entries forwarding loops and/or blackholes may result unless it can
   be guaranteed that all other routers in the network make the same
   choice.  Making the same choice requires that all routers have
   identical sets of advertisements and that they all use the same
   selection algorithm.

3.1.  Conflict Types

   Various types of conflicts may occur.





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3.1.1.  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.  Consider the following sets of
   advertisements:

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

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

   Prefix conflicts may also occur as a result of overlapping prefix
   ranges.  Consider the following sets of advertisements:

   (192.0.2.1/32, 200, 200, 0, 0)
   (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

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

   The second set of examples 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) at the cost of increased
   implementation complexity.

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



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

   (2001:DB8::1/128, 400, 200, 2, 0)
   (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.

   Given two mapping entries:

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

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

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


3.1.2.  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.  Consider the following examples:




















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

   (2001:DB8::1/128, 400, 1, 0, 0)
   (2001:DB8::1/128, 400, 1, 0, 1)
   SID 400 has been assigned to 2001:DB8::1/128 for algorithm 0
   by the first advertisement.
   SID 400 has been assigned to 2001:DB8::1/128 for algorithm 1
   by the second advertisement.

   (192.0.2.1/32, 400, 1, 0, 0)
   (2001:DB8::1/128, 400, 1, 0, 0)
   SID 400 has been assigned to 192.0.2.1/32 by the
   first advertisement.
   SID 400 has been assigned to 2001:DB8::1/128 by the
   second advertisement.

   SID conflicts may also occur as a result of overlapping SID ranges.
   Consider the following sets of advertisements:




























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   (192.0.2.1/32, 200, 200, 0, 0)
   (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.

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

   SIDs 500 - 509 have been assigned to two different prefixes.
   The first advertisement assigns these SIDs to
   2001:DB8::65/128 - 2001:DB8::6E/128.
   The second advertisement assigns these SIDs to
   2001:DB8:1::1 - 2001:DB8:1::A/128.

   (192.0.2.1/32, 200, 200, 0, 0)
   (2001:DB8::1/128, 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
   2001:DB8::1/128 - 2001:DB8::A/128.


   The second set of examples 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) at the cost of increased
   implementation complexity.

   Given two mapping entries:

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

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

   1)S1e >= S2
   2)(AF1 != AF2) || (L1 != L2) || (T1 != T2) || (A1 != A2)
     || (P1 + ((S2-S1) << (Lx-L1)) != P2

   NOTE: The last calculation is valid because it is only done
   when the two mapping entries are in the same address family
   and have the same prefix length.





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3.2.  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.

3.2.1.  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.

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

3.2.2.  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.

3.2.3.  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



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   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.

3.2.4.  Preference Algorithm

   The following algorithm is used to select the preferred mapping entry
   when a conflict exists.  Evaluation is made in the order specified.

   1.  Smaller range wins

   2.  IPv6 entry wins over IPv4 entry

   3.  Smaller algorithm wins

   4.  Smaller prefix length wins

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

   6.  Smaller starting SID wins

   Using smaller range as the highest priority tie breaker makes
   advertisements with a range of 1 the most preferred.  This associates
   a high priority to SID advertisements associated with protocol prefix
   advertisements as these always have an implicit range of one.  SR
   mapping server advertisements (SRMS entries) may have any configured
   range - but in cases where they have a range greater than 1 they will
   be less preferred as compared to any SIDs in prefix advertisements.
   This has the nice property that a single misconfiguratoion of an SRMS
   entry with a large range will not be preferred over a large number of
   SIDs advertised in prefix reachability advertisements.

3.2.5.  Use of topology in preference

   The preference rule defined in the previous section does not include
   a comparison of topologies.  When evaluating prefix conflicts this is
   only done when comparing mapping entries associated with the same
   topology - so this omission is not significant.  However, when
   evaluating a SID conflict the topology associated with two mapping
   entries need not be the same.  The question arises as to what should
   be done when all of the attributes specified in the preference rule
   are identical but the topologies are different?

   The scope of topology identifiers is NOT global.  A given routing
   protocol has topology identifiers which are consistent within the
   protocol area/domain, but if multiple routing protocols are in use in
   a network it cannot be guaranteed that the two routing protocols will



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   use the same identifier for a given topology.  This is, in part, due
   to the fact that different routing protocols have different supported
   ranges for topology identifiers.  It is then NOT possible to
   guarantee a consistent identifier for a topology on all routers in a
   network.  Therefore no preference rule can be defined which will
   guarantee the same result on all routers when the topology is the
   only attribute which differs between two mapping entries.  The
   following preference rule is defined to handle these cases:

   When a SID conflict is detected between two mapping entries and the
   only difference between the two entries is the topology, both entries
   MUST be ignored in their entirety.

3.2.6.  Example Behavior

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

   1.  (192.0.2.1/32, 100, 1)

   2.  (192.0.2.101/32, 200, 1)

   3.  (192.0.2.1/32, 400, 300) !Prefix conflict with entries 1 and 2

   4.  (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:

   +-----------------------------------------------------------------+
   | Policy      | Active Entries          |  Excluded Entries       |
   +-----------------------------------------------------------------+
   | Ignore      |                         | (192.0.2.1/32,100,1)    |
   |             |                         | (192.0.2.101/32,200,1)  |
   |             |                         | (192.0.2.1/32,400,300)  |
   |             |                         | (198.51.100.40/32,200,1)|
   +-----------------------------------------------------------------+
   | Quarantine  | (192.0.2.1/32,100,1)    | (192.0.2.1/32,400,300)  |
   |             | (192.0.2.101/32,200,1)  | (198.51.100.40/32,200,1)|
   +-----------------------------------------------------------------+
   | Overlap-    | (192.0.2.1/32,100,1)    | (198.51.100.40/32,200,1)|
   |  Only       | (192.0.2.101/32,200,1)  |*(192.0.2.1/32,400,1)    |
   |             |*(192.0.2.2/32,401,99)   |*(192.0.2.101/32,500,1)  |
   |             |*(192.0.2.102/32,501,199)|                         |
   +-----------------------------------------------------------------+

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



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3.2.7.  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.  The bias is for mapping entries with the
   smallest range (typically - but not exclusively SIDs advertised in
   prefix reachability advertisements) to be forwarded while the
   destinations included in mapping entries with a larger range but NOT
   covered by entries with a smaller range will not be forwarded.

   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 comapred to quarantine.  Mapping entries with a range greater than
   1 which are in conflict with mapping entries having a smaller range
   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 a mapping entry of smaller range - and
   those which are NOT in conflict with an entry with smaller range.
   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 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.

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

   It is the opinion of the authors that "quarantine" is the best
   alternative.

3.2.8.  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.






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   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.  Security Considerations

   TBD

5.  IANA Consideration

   This document has no actions for IANA.

6.  Acknowledgements

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

7.  References

7.1.  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>.

   [SR-IS-IS]
              "IS-IS Extensions for Segment Routing, draft-ietf-isis-
              segment-routing-extensions-06(work in progress)", December
              2015.

   [SR-MPLS]  "Segment Routing with MPLS dataplane, draft-ietf-spring-
              segment-routing-mpls-04(work in progress)", March 2016.






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   [SR-OSPF]  "OSPF Extensions for Segment Routing, draft-ietf-ospf-
              segment-routing-extensions-07(work in progress)", March
              2016.

   [SR-OSPFv3]
              "OSPFv3 Extensions for Segment Routing, draft-ietf-ospf-
              ospfv3-segment-routing-extensions-05(work in progress)",
              March 2016.

7.2.  Informational References

   [SR-ARCH]  "Segment Routing Architecture, draft-ietf-spring-segment-
              routing-07(work in progress)", December 2015.

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   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


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   Email: sprevidi@cisco.com


   Martin Pilka
   Pantheon Technologies

   Email: martin.pilka@pantheon.tech




Ginsberg, et al.        Expires October 14, 2016               [Page 15]


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