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OSPF Working Group                                           J. Tantsura
Internet-Draft                                              Apstra, Inc.
Intended status: Standards Track                             U. Chunduri
Expires: April 14, 2019                              Huawei Technologies
                                                               S. Aldrin
                                                             Google, Inc
                                                               P. Psenak
                                                           Cisco Systems
                                                        October 11, 2018


              Signaling MSD (Maximum SID Depth) using OSPF
                 draft-ietf-ospf-segment-routing-msd-22

Abstract

   This document defines a way for an Open Shortest Path First (OSPF)
   Router to advertise multiple types of supported Maximum SID(Segment
   Identifier) Depths (MSDs) at node and/or link granularity.  Such
   advertisements allow entities (e.g., centralized controllers) to
   determine whether a particular SID stack can be supported in a given
   network.  This document defines only one type of MSD, but defines an
   encoding that can support other MSD types.  Here the term OSPF means
   both OSPFv2 and OSPFv3.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on April 14, 2019.

Copyright Notice

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





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Node MSD Advertisement  . . . . . . . . . . . . . . . . . . .   4
   3.  Link MSD sub-TLV  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Procedures for Defining and Using Node and Link MSD
       Advertisements  . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   When Segment Routing (SR) paths are computed by a centralized
   controller, it is critical that the controller learns the Maximum SID
   (Segment Identifier) Depth (MSD) that can be imposed at each node/
   link on a given SR path to ensure that the Segment Identifier (SID)
   stack depth of a computed path doesn't exceed the number of SIDs the
   node is capable of imposing.

   [I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path
   Computation Element communication Protocol (PCEP).  However, if PCEP
   is not supported/configured on the head-end of an SR tunnel or a
   Binding-SID anchor node and controller does not participate in IGP
   routing, it has no way to learn the MSD of nodes and links.  BGP-LS
   (Distribution of Link-State and TE Information using Border Gateway
   Protocol) [RFC7752] defines a way to expose topology and associated
   attributes and capabilities of the nodes in that topology to a
   centralized controller.  MSD signaling by BGP-LS has been defined in
   [I-D.ietf-idr-bgp-ls-segment-routing-msd].  Typically, BGP-LS is
   configured on a small number of nodes that do not necessarily act as



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   head-ends.  In order for BGP-LS to signal MSD for all the nodes and
   links in the network MSD is relevant, MSD capabilities SHOULD be
   advertised by every OSPF router in the network.

   Other types of MSD are known to be useful.  For example,
   [I-D.ietf-ospf-mpls-elc] defines Readable Label Depth Capability
   (RLDC) that is used by a head-end to insert an Entropy Label (EL) at
   a depth that could be read by transit nodes.

   This document defines an extension to OSPF used to advertise one or
   more types of MSD at node and/or link granularity.  In the future it
   is expected, that new MSD-types will be defined to signal additional
   capabilities e.g., entropy labels, SIDs that can be imposed through
   recirculation, or SIDs associated with another dataplane e.g., IPv6.

   MSD advertisements MAY be useful even if Segment Routing itself is
   not enabled.  For example, in a non-SR MPLS network, MSD defines the
   maximum label depth.

1.1.  Terminology

   This memo makes use of the terms defined in [RFC7770]

   BGP-LS: Distribution of Link-State and TE Information using Border
   Gateway Protocol

   OSPF: Open Shortest Path First

   MSD: Maximum SID Depth - the number of SIDs supported by a node or a
   link on a node

   SID: Segment Identifier as defined in [RFC8402]

   Label Imposition: Imposition is the act of modifying and/or adding
   labels to the outgoing label stack associated with a packet.  This
   includes:

   o  replacing the label at the top of the label stack with a new label

   o  pushing one or more new labels onto the label stack

   The number of labels imposed is then the sum of the number of labels
   which are replaced and the number of labels which are pushed.  See
   [RFC3031] for further details.

   PCC: Path Computation Client

   PCE: Path Computation Element



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   PCEP: Path Computation Element Protocol

   SR: Segment Routing

   SID: Segment Identifier

   LSA: Link state advertisement

   RI: OSPF Router Information LSA

1.2.  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.  Node MSD Advertisement

   The node MSD TLV within the body of the OSPF RI Opaque LSA [RFC7770]
   is defined to carry the provisioned SID depth of the router
   originating the RI LSA.  Node MSD is the smallest MSD supported by
   the node on the set of interfaces configured for use by the
   advertising IGP instance.  MSD values may be learned via a hardware
   API or may be provisioned.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Type                       |  Length                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                          Figure 1: Node MSD TLV

   Type: TBD1

   Length: variable (multiple of 2 octets) and represents the total
   length of value field in octets.

   Value: consists of one or more pairs of a 1 octet MSD-type and 1
   octet MSD-Value.





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   MSD-Type: one of the values defined in the IGP MSD Types registry
   defined in [I-D.ietf-isis-segment-routing-msd].

   MSD-Value: a number in the range of 0-255.  For all MSD-Types, 0
   represents lack of the ability to impose MSD stack of any depth; any
   other value represents that of the node.  This value MUST represent
   the lowest value supported by any link configured for use by the
   advertising OSPF instance.

   This TLV is applicable to OSPFv2 and to OSPFv3 and is optional.  The
   scope of the advertisement is specific to the deployment.

   When multiple Node MSD TLVs are received from a given router, the
   receiver MUST use the first occurrence of the TLV in the Router
   Information LSA.  If the Node MSD TLV appears in multiple Router
   Information LSAs that have different flooding scopes, the Node MSD
   TLV in the Router Information LSA with the area-scoped flooding scope
   MUST be used.  If the Node MSD TLV appears in multiple Router
   Information LSAs that have the same flooding scope, the Node MSD TLV
   in the Router Information (RI) LSA with the numerically smallest
   Instance ID MUST be used and other instances of the Node MSD TLV MUST
   be ignored.  The RI LSA can be advertised at any of the defined
   opaque flooding scopes (link, area, or Autonomous System (AS)).  For
   the purpose of Node MSD TLV advertisement, area-scoped flooding is
   RECOMMENDED.

3.  Link MSD sub-TLV

   The link sub-TLV is defined to carry the MSD of the interface
   associated with the link.  MSD values may be learned via a hardware
   API or may be provisioned.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Type                       |  Length                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




                        Figure 2: Link MSD Sub-TLV

   Type:



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   For OSPFv2, the Link level MSD-Value is advertised as an optional
   Sub-TLV of the OSPFv2 Extended Link TLV as defined in [RFC7684], and
   has a type of TBD2.

   For OSPFv3, the Link level MSD-Value is advertised as an optional
   Sub-TLV of the E-Router-LSA TLV as defined in [RFC8362], and has a
   type of TBD3.

   Length: variable and same as defined in Section 2.

   Value: consists of one or more pairs of a 1 octet MSD-type and 1
   octet MSD-Value.

   MSD-Type: one of the values defined in the MSD Types registry defined
   in [I-D.ietf-isis-segment-routing-msd].

   MSD-Value field contains Link MSD of the router originating the
   corresponding LSA as specified for OSPFv2 and OSPFv3.  Link MSD is a
   number in the range of 0-255.  For all MSD-Types, 0 represents lack
   of the ability to impose MSD stack of any depth; any other value
   represents that of the particular link when used as an outgoing
   interface.

   If this sub-TLV is advertised multiple times in the same OSPFv2
   Extended Link Opaque LSA/E-Router-LSA, only the first instance of the
   TLV MUST be used by receiving OSPF routers.  This situation SHOULD be
   logged as an error.

   If this sub-TLV is advertised multiple times for the same link in
   different OSPF Extended Link Opaque LSAs/E-Router-LSAs originated by
   the same OSPF router, the OSPFv2 Extended Link TLV in the OSPFv2
   Extended Link Opaque LSA with the smallest Opaque ID or in the OSPFv3
   E-Router-LSA with the smallest Link State ID MUST be used by
   receiving OSPF routers.  This situation MAY be logged as a warning.

4.  Procedures for Defining and Using Node and Link MSD Advertisements

   When Link MSD is present for a given MSD-type, the value of the Link
   MSD MUST take precedence over the Node MSD.  When a Link MSD-type is
   not signaled but the Node MSD-type is, then the Node MSD-type value
   MUST be considered as the MSD value for that link.

   In order to increase flooding efficiency, it is RECOMMENDED that
   routers with homogenous link MSD values advertise just the Node MSD
   value.

   The meaning of the absence of both Node and Link MSD advertisements
   for a given MSD-type is specific to the MSD-type.  Generally it can



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   only be inferred that the advertising node does not support
   advertisement of that MSD-type.  However, in some cases the lack of
   advertisement might imply that the functionality associated with the
   MSD-type is not supported.  The correct interpretation MUST be
   specified when an MSD-type is defined.

5.  IANA Considerations

   This document requests IANA to allocate TLV type (TBD1) from the OSPF
   Router Information (RI) TLVs Registry as defined by [RFC7770].  IANA
   has allocated the value 12 through the early assignment process.


      Value     Description                      Reference
      -----     ---------------                  -------------
      12        Node MSD                         This document


                           Figure 3: RI Node MSD

   Also, this document requests IANA to allocate a sub-TLV type (TBD2)
   from the OSPFv2 Extended Link TLV Sub-TLVs registry.  IANA has
   allocated the value 6 through the early assignment process.


      Value     Description                      Reference
      -----     ---------------                  -------------
      6         OSPFv2 Link MSD                   This document


                         Figure 4: OSPFv2 Link MSD

   Finally, this document requests IANA to allocate a sub-TLV type
   (TBD3) from the OSPFv3 Extended-LSA Sub-TLV registry.


      Value     Description                      Reference
      -----     ---------------                  -------------
      TBD3      OSPFv3 Link MSD                  This document


                         Figure 5: OSPFv3 Link MSD

6.  Security Considerations

   Security concerns for OSPF are addressed in [RFC7474], [RFC4552] and
   [RFC7166].  Further security analysis for OSPF protocol is done in




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   [RFC6863].  Security considerations, as specified by [RFC7770],
   [RFC7684] and [RFC8362] are applicable to this document.

   Implementations MUST assure that malformed TLV and Sub-TLV defined in
   this document are detected and do not provide a vulnerability for
   attackers to crash the OSPF router or routing process.  Reception of
   malformed TLV or Sub-TLV SHOULD be counted and/or logged for further
   analysis.  Logging of malformed TLVs and Sub-TLVs SHOULD be rate-
   limited to prevent a Denial of Service (DoS) attack (distributed or
   otherwise) from overloading the OSPF control plane.

   Advertisement of an incorrect MSD value may have negative
   consequences.  If the value is smaller than supported, path
   computation may fail to compute a viable path.  If the value is
   larger than supported, an attempt to instantiate a path that can't be
   supported by the head-end (the node performing the SID imposition)
   may occur.

   The presence of this information also may inform an attacker of how
   to induce any of the aforementioned conditions.

   There's no Denial of Service risk specific to this extension, and it
   is not vulnerable to replay attacks.

7.  Contributors

   The following people contributed to this document:

   Les Ginsberg

   Email: ginsberg@cisco.com

8.  Acknowledgments

   The authors would like to thank Acee Lindem, Ketan Talaulikar, Tal
   Mizrahi, Stephane Litkowski and Bruno Decraene for their reviews and
   valuable comments.

9.  References

9.1.  Normative References

   [I-D.ietf-isis-segment-routing-msd]
              Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling MSD (Maximum SID Depth) using IS-IS", draft-
              ietf-isis-segment-routing-msd-19 (work in progress),
              October 2018.




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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

9.2.  Informative References

   [I-D.ietf-idr-bgp-ls-segment-routing-msd]
              Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling MSD (Maximum SID Depth) using Border Gateway
              Protocol Link-State", draft-ietf-idr-bgp-ls-segment-
              routing-msd-02 (work in progress), August 2018.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-07 (work in progress), September 2018.




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   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-12 (work in progress), June
              2018.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC6863]  Hartman, S. and D. Zhang, "Analysis of OSPF Security
              According to the Keying and Authentication for Routing
              Protocols (KARP) Design Guide", RFC 6863,
              DOI 10.17487/RFC6863, March 2013,
              <https://www.rfc-editor.org/info/rfc6863>.

   [RFC7166]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166,
              DOI 10.17487/RFC7166, March 2014,
              <https://www.rfc-editor.org/info/rfc7166>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   Uma Chunduri
   Huawei Technologies

   Email: uma.chunduri@huawei.com







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   Sam Aldrin
   Google, Inc

   Email: aldrin.ietf@gmail.com


   Peter Psenak
   Cisco Systems

   Email: ppsenak@cisco.com









































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