Advertising TE protocols in IS-IS
draft-hegde-isis-advertising-te-protocols-03

Versions: 00 01 02 03                                                   
IS-IS WG                                                        S. Hegde
Internet-Draft                                                 C. Bowers
Intended status: Standards Track                        Juniper Networks
Expires: March 19, 2018                                        P. Mattes
                                                              M. Nanduri
                                                            S. Giacalone
                                                               Microsoft
                                                             I. Mohammad
                                                         Arista Networks
                                                      September 15, 2017


                   Advertising TE protocols in IS-IS
              draft-hegde-isis-advertising-te-protocols-03

Abstract

   This document defines a mechanism to indicate which traffic
   engineering protocols are enabled on a link in IS-IS.  It does so by
   introducing a new traffic-engineering protocol sub-TLV for TLV-22.
   This document also describes mechanisms to address backward
   compatibility issues for implementations that have not yet been
   upgraded to software that understands this new sub-TLV.

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 https://datatracker.ietf.org/drafts/current/.

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   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 March 19, 2018.





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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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Explicit and unambiguous indication of TE protocol  . . .   4
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Traffic-engineering protocol sub-TLV  . . . . . . . . . .   5
     3.2.  Segment Routing flag considerations . . . . . . . . . . .   6
   4.  Backward compatibility  . . . . . . . . . . . . . . . . . . .   7
     4.1.  Scenario with upgraded RSVP-TE transit  router but RSVP-
           TE ingress router not upgraded  . . . . . . . . . . . . .   7
     4.2.  Scenario with upgraded RSVP-TE ingress  router but RSVP-
           TE transit router not upgraded  . . . . . . . . . . . . .   8
     4.3.  Need for a long term solution . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   IS-IS extensions for traffic engineering are specified in [RFC5305].
   [RFC5305] defines several link attributes such as administrative
   group, maximum link bandwidth, and shared risk link groups (SRLGs)
   which can be used by traffic engineering applications.  Additional
   link attributes for traffic engineering have subsequently been
   defined in other documents as well.  Most recently [RFC7810] defined
   link attributes for delay, loss, and measured bandwidth utilization.





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   The primary consumers of these traffic engineering link attributes
   have been RSVP-based applications that use the advertised link
   attributes to compute paths which will subsequently be signalled
   using RSVP-TE.  However, these traffic engineering link attributes
   have also been used by other applications, such as IP/LDP fast-
   reroute using loop-free alternates as described in [RFC7916].  In the
   future, it is likely that traffic engineering applications based on
   Segment Routing [I-D.ietf-spring-segment-routing] will also use these
   link attributes.

   Existing IS-IS standards do not provide a mechanism to explicitly
   indicate whether or not RSVP has been enabled on a link.  Instead,
   different RSVP-TE implementations have used the presence of certain
   traffic engineering sub-TLVs in IS-IS to infer that RSVP signalling
   is enabled on a given link.  A study was conducted with various
   vendor implementations to determine which traffic engineering sub-
   TLVs cause an implementation to infer that RSVP signalling is enabled
   on a link.  The results are shown in Figure 1.

          +--------+--------------------------------------------+
          | TLV/   | Sub-TLV name                |Implementation|
          |sub-TLV |                             +--------------+
          |        |                             | X  | Y  | Z  |
          +--------+--------------------------------------------+
          |22      |Extended IS Reachability TLV | N  | N  | N  |
          |22/3    |Administrative group (color) | N  | Y  | Y  |
          |22/4    |Link Local/Remote ID         | N  | N  | N  |
          |22/6    |IPV4 Interface Address       | N  | N  | N  |
          |22/8    |IPV4 Neighbor Address        | N  | N  | N  |
          |22/9    |Max Link Bandwidth           | N  | Y  | Y  |
          |22/10   |Max Reservable Link Bandwidth| N  | Y  | Y  |
          |22/11   |Unreserved Bandwidth         | Y  | Y  | Y  |
          |22/14   |Extended Admin Group         | N  | Y  | N  |
          |22/18   |TE Default Metric            | N  | N  | N  |
          |22/20   |Link Protection Type         | N  | Y  | Y  |
          |22/21   |Interface Switching          | N  | Y  | Y  |
          |        | Capability                  |    |    |    |
          |22/22   |TE Bandwidth Constraints     | N  | Y  | Y  |
          |22/33-39|TE Metric Extensions(RFC7180)| N  | N  | N  |
          |138     |SRLG TLV                     | N  | Y  | Y  |
          +--------+--------------------------------------------+

    Figure 1: Traffic engineering Sub-TLVs that cause implementation X,
        Y, or Z to infer that RSVP signalling is enabled on a link

   The study indicates that the different implementations use the
   presence of different sub-TLVs under TLV 22 (or the presence of TLV
   138) to infer that RSVP signalling is enabled on a link.  It is



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   possible that other implementations may use other sub-TLVs to infer
   that RSVP is enabled on a link.

   This document defines a standard way to indicate whether or not RSVP,
   segment routing, or another future protocol is enabled on a link.  In
   this way, implementations will not have to infer whether or not RSVP
   is enabled based on the presence of different sub-TLVs, but can use
   the explicit indication.  When network operators want to use a non-
   RSVP traffic engineering application (such as IP/LDP FRR or segment
   routing), they will be able to advertise traffic engineering sub-TLVs
   and explicitly indicate what traffic engineering protocols are
   enabled on a link.

2.  Goals

   1.  The solution should allow the TE protocol enabled on a link to be
       communicated unambiguously.

   2.  The solution should decouple the advertisement of which TE
       protocols are enabled on a link from the advertisement of other
       TE attributes.

   3.  The solution should be backward compatible so that nodes that do
       not understand the new advertisement do not cause issues for
       existing RSVP deployments.

   4.  The solution should be extensible for new protocols.

   5.  The solution should try to limit any increases to the quantity
       and size of link state advertisements.

2.1.  Explicit and unambiguous indication of TE protocol

   Communicating unambiguously which TE protocol is enabled on a link is
   important to be able to share this information with other consumers
   through other protocols, aside from just the IGP.  For example, for a
   network running both RSVP-TE and SR, it will be useful to communicate
   which TE protocols are enabled on which links via BGP-LS [RFC7752] to
   a central controller.  Typically, BGP-LS relies on the IGP to
   distribute IGP topology and traffic engineering information so that
   only a few BGP-LS sessions with the central controller are needed.
   In order for a router running a BGP-LS session to a central
   controller to correctly communicate what TE protocols are enabled on
   the links in the IGP domain, that information first needs to be
   communicated unambiguously within the IGP itself.  As Figure 1
   illustrates, that is currently not the case.





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3.  Solution

3.1.  Traffic-engineering protocol sub-TLV

   A new Traffic-engineering protocol sub-TLV is added in the TLV 22
   [RFC5305] or TLV 222 to indicate the protocols enabled on the link.
   The sub-TLV has flags in the value field to indicate the protocol
   enabled on the link.  The length field is variable to allow the flags
   field to grow for future requirements.


    Type  : TBD suggested value 40
    Length: Variable
    Value :
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Flags                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


              Figure 2: Traffic-Engineering Protocol sub-TLV

   Type : TBA (suggested value 40)

   Length: variable (in bytes)

   Value: The value field consists of bits indicating the protocols
   enabled on the link.  This document defines the two protocol values
   below.

               +----------+-------------------------------+
               | Value    | Protocol Name                 |
               +----------+-------------------------------+
               |0x01      | RSVP                          |
               +----------+-------------------------------+
               |0x02      | Segment Routing               |
               +----------+-------------------------------+


                     Figure 3: Flags for the protocols

   The RSVP flag is set to one to indicate that RSVP-TE is enabled on a
   link.  The RSVP flag is set to zero to indicate that RSVP-TE is not
   enabled on a link.






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   The Segment Routing flag is set to one to indicate that Segment
   Routing is enabled on a link.  The Segment Routing flag is set to
   zero to indicate that Segment Routing is not enabled on a link.

   All undefined flags MUST be set to zero on transmit and ignored on
   receipt.

   An implementation that supports the TE protocol sub-TLV and sends TLV
   22 MUST advertise the TE protocol sub-TLV in TLV 22 for that link,
   even when both the RSVP and SR flags are set to zero.  In other
   words, whenever the TE protocol sub-TLV is supported, it MUST be
   sent, even if no TE protocols are enabled on the link.  This allows a
   receiving router to determine whether or not the sending router is
   capable of sending the TE protocol sub-TLV.

   A router supporting the TE protocol sub-TLV which receives an
   advertisement for a link containing TLV 22 with the TE protocol sub-
   TLV present SHOULD respect the values of the flags in the TE protocol
   sub-TLV.  The receiving router SHOULD only consider links with a
   given TE protocol enabled for inclusion in a path using that TE
   protocol.  Conversely, links for which the TE protocol sub-TLV is
   present, but for which the TE protocol flag is not set to one, SHOULD
   NOT be included in any TE CSPF computations on the receiving router
   for the protocol in question.

   The ability for a receiving router to determine whether or not the
   sending router is capable of sending the TE protocol sub-TLV is also
   used for backward compatibility as described in Section 4.

   An implementation that supports the TE protocol sub-TLV SHOULD be
   able to advertise TE sub-TLVs without enabling RSVP-TE signalling on
   the link.

3.2.  Segment Routing flag considerations

   The Segment Routing (SR) architecture assumes that the SR topology is
   congruent with the IGP topology.  The path described by a prefix
   segment is computed using the SPF algorithm applied to the IGP
   topology, which is the same as the SR topology.  Therefore, the
   presence or absence of the Segment Routing flag MUST NOT be
   interpreted as modifying the SR topology, which is always congruent
   with the IGP topology.

   It is however useful for a centralized application (or an ingress
   router) to know whether or not it should expect to be able to forward
   traffic over a given link using labels distributed via SR.  If a link
   is advertised with the TE protocol sub-TLV and the SR flag set to
   zero, then a centralized application can assume that traffic sent



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   with a prefix segment whose path crosses that link is unlikely to be
   forwarded across that link.  With this information, a centralized
   application can decide to use a different path for that traffic by
   using a different label stack.

4.  Backward compatibility

   Routers running older software that do not understand the new
   Traffic-Engineering protocol sub-TLV will continue to interpret the
   presence of some sub-TLVs in TLV 22 or the presence of TLV 138 as
   meaning that RSVP is enabled a link.  A network operator may not want
   to or be able to upgrade all routers in the domain at the same time.
   There are two backward compatibility scenarios to consider depending
   on whether the router that doesn't understand the new TE protocol
   sub-TLV is an RSVP-TE ingress router or an RSVP-TE transit router.

4.1.  Scenario with upgraded RSVP-TE transit router but RSVP-TE ingress
      router not upgraded

   An upgraded RSVP-TE transit router is able to explicitly indicate
   that RSVP is not enabled on a link by advertising the TE protocol
   sub-TLV with the RSVP flag set to zero.  However, an RSVP-TE ingress
   router that has not been upgraded to understand the new TE protocol
   sub-TLV will not understand that RSVP-TE is not enabled on the link,
   and may include the link on a path computed for RSVP-TE.  When the
   network tries to signal an explicit path LSP using RSVP-TE through
   that link, it will fail.  In order to avoid this scenario, an
   operator can use the mechanism described below.

   For this scenario, the basic idea is to use the existing
   administrative group link attribute as a means of preventing existing
   RSVP implementations from using a link.  The network operator defines
   an administrative group to mean that RSVP is not enabled on a link.
   We call this admin group the RSVP-not-enabled admin group.  If the
   operator needs to advertise a TE sub-TLV (maximum link bandwidth, for
   example) on a link, but doesn't want to enable RSVP on that link,
   then the operator also advertises the RSVP-not-enabled admin group on
   that link.  The operator can then use existing mechanisms to exclude
   links advertising the RSVP-not-enabled admin group from the
   constrained shortest path first (CSPF) computation used by RSVP.
   This will prevent RSVP implementations from attempting to signal
   RSVP-TE LSPs across links that do not have RSVP enabled.  Once the
   entire network domain is upgraded to understand the TE protocol sub-
   TLV in this draft, the configuration involving the RSVP-not-enabled
   admin group is no longer needed for this network.






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4.2.  Scenario with upgraded RSVP-TE ingress router but RSVP-TE transit
      router not upgraded

   The other scenario to consider is when the RSVP-TE ingress router has
   been upgraded to understand the TE protocol sub-TLV, but the RSVP-TE
   transit router has not.  In this case, the transit router has not
   been upgraded, so it is not yet capable of sending the TE protocol
   sub-TLV.  If the transit router has RSVP-TE enabled on a link, we
   would like for the RSVP-TE ingress router to still be able to use the
   link for RSVP-TE paths.  While it is possible to describe a solution
   for this scenario that makes use of administrative groups, we
   describe a simpler solution below.

   The solution for this scenario relies on the following observation.
   If the RSVP-TE ingress router can understand that the transit router
   is not capable of sending the TE protocol sub-TLV, then it can
   continue inferring whether or not RSVP-TE is enabled on the transit
   router links based on the presence of TE sub-TLVs, just as it does
   today.

   To accomplish this, we require an upgraded router to send the TE
   protocol sub-TLV if it sends TLV 22, even when both the RSVP and SR
   flags are set to zero.  In other words, whenever the TE protocol sub-
   TLV is supported, it MUST be sent, even if no TE protocols are
   enabled on the link.  see Section 3.  This allows the receiving
   router to interpret the absence of the TE-protocol sub-TLV together
   with presence of TLV 22 to mean that the sending router has not been
   upgraded.  This allows the upgraded RSVP-TE ingress router to
   distinguish between transit routers that have been upgraded and those
   that haven't.  When the transit router has been upgraded, then the
   RSVP-TE ingress router uses the information in the TE protocol sub-
   TLV.  When the transit router has not been upgraded, then RSVP-TE
   ingress router contines to infer whether or not RSVP-TE is enabled on
   the transit router links based on the presence of TE sub-TLVs, just
   as it does today.  The solution for this scenario requires no
   configuration on the part of network operators.

4.3.  Need for a long term solution

   The use of the adminstrative group link attribute to prevent an RSVP-
   TE ingress router from computing a path using a given link is an
   effective short term workaround to allow networks to incrementally
   upgrade the routers to software that understands the new TE-protocol
   sub-TLV.  One might also consider a long term solution based solely
   on the use of operator-defined adminstrative groups to communicate
   the TE protocol enabled on a link.  However, we do not consider this
   workaround to be an effective long term solution because it relies on
   operator configuration that would have to be maintained in the long



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   term.  As discussed in Section 2, continuing to have to infer which
   TE protocol is enabled on a link also limits our ability to
   communicate this information unambiguously in an interoperable manner
   for use by other applications such as central controllers.

5.  Security Considerations

   This document does not introduce any further security issues other
   than those discussed in [RFC1195] and [RFC5305].

6.  IANA Considerations

   This specification updates one IS-IS registry:


   The extended IS reachability TLV Registry

   i) Traffic-engineering Protocol sub-tlv = Suggested value 40

7.  Acknowledgements

   The authors thank Alia Atlas, Les Ginsberg, and Peter Psenak for
   helpful discussions on the topic of this draft.

8.  References

8.1.  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-09 (work in progress), July 2016.

   [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>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
              Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
              RFC 7810, DOI 10.17487/RFC7810, May 2016,
              <https://www.rfc-editor.org/info/rfc7810>.





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8.2.  Informative References

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [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>.

   [RFC7916]  Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational Management of
              Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
              July 2016, <https://www.rfc-editor.org/info/rfc7916>.

Authors' Addresses

   Shraddha Hegde
   Juniper Networks
   Embassy Business Park
   Bangalore, KA  560093
   India

   Email: shraddha@juniper.net


   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: cbowers@juniper.net


   Paul Mattes
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: pamattes@microsoft.com







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   Mohan Nanduri
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: mnanduri@microsoft.com


   Spencer Giacalone
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: Spencer.Giacalone@microsoft.com


   Imtiyaz Mohammad
   Arista Networks
   Global Tech Park
   Bangalore, KA  560103
   India

   Email: imtiyaz@arista.com


























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