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Versions: 00                                                            
LSR Working Group                                               Louis Chan
INTERNET-DRAFT                                                  Krzysztof Szarkowicz
Intended status: Standard Track                                 Juniper Networks
Expires: Sep 6, 2022                                            Mar 6, 2022


              IGP extensions for Advertising Offset for Flex-Algorithm
                      draft-chan-lsr-igp-adv-offset-00.txt


Abstract

   This document describes the IGP extensions to provide predictable Adjacency-SIDs
   per Flex-Algorithm [FLEXALGO] in segment routing.

   We propose some methods to allow the advertisement of additional TLV in IGP so that
   the Flex-Algorithm specific Adjacency-SIDs could be automatically derived.

   With the proposed method, the size of advertisement on per node per link basis is
   greatly reduced. Each participating router would derive the required labels
   automatically.

   Extensions for offset to derive Flex-Algorithm Prefix-SID is also included in the
   document.

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
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   inappropriate to use Internet-Drafts as reference material or to cite them other
   than as "work in progress."

   This Internet-Draft will expire on Sep 6, 2022.

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


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   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. Conventions used in this document..............................2
   3. ISIS extension.................................................2
      3.1. Algorithm Offset for Adj-SID..............................3
      3.2. Algorithm Offset for LAN based Adj-SID....................4
      3.3. Algorithm Offset for Prefix SID...........................5
   4. Compatibility..................................................6
   5. OSPF extension.................................................6
   6. Security Consideration.........................................6
   7. References.....................................................7
      7.1. Normative References......................................7
      7.2. Informative References....................................7
   8. Acknowledgments................................................7

1. Introduction


   The draft proposes methods for routers to announce Flex-Algorithm specific
   Adjacency-SID with minimal advertisement in IGP. When the other routers need a SR
   policy (aka SR-TE) or TI-LFA path, the Flex-Algorithm specific Adjacency-SID would
   be taken into the path label construction.

   Hence, the top most label, either node or link related, in a SR policy stack would
   be used to identify a certain Flex-Algo with full identification.



2. Conventions used in this document

   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].

   In this document, these words will appear with that interpretation only when in ALL
   CAPS. Lower case uses of these words are not to be interpreted as carrying
   significance described in RFC 2119.





3. ISIS extension

   With reference to RFC8667, the information could be advertised in SR-Capabilities
   sub-TLV in Section 3.
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3.1. Algorithm Offset for Adj-SID



    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    |   Flags       |    Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   SID/Label   |      Base offset/Index offset (variable)      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~           Flags/ Algo/ Base offset/ Index offset             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

   Type:                TDB

   Length:              variable; multiple advertisements

   Flags:               1 octet field of the following flags:

                         0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        | |B|V|L|       |
                        +-+-+-+-+-+-+-+-+

                  where:

                  B-Flag:  Backup Flag.  If set, the Adj-SID is eligible
                           for protection (e.g., using IP Fast Reroute
                           (IPFRR) or MPLS Fast Reroute (MPLS-FRR)) as
                           described in [RFC8402].


                  V-Flag:  Value Flag.  If set, then it carries
                           a label value. If not set, it is an index value.

                  L-Flag:  Local Flag.  Always set to 1


   Algorithm:              1 octet
                           Flex-Algo value is between 128 and 255.

   Base Offset:            3 octets
                           Local label base for Adjacency-SIDs for given Flex-
                           Algorithm. The derived Adjacency-SID is the sum of the base
                           offset and the Algo 0 label

   Index Offset:           4 octets
                           If Adjacency-SID is advertised as index, this provides an

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                           offset index value. The new label is sum of (SRGB base +
                           index + index offset)

   The format of Base Offset and Index Offset is the same as RFC8667. The choice of
   advertising Base Offset or Index Offset must match the advertisement of original
   Adjacency-SID method from the same router.

3.2. Algorithm Offset for LAN based Adj-SID


       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    |      Flags    |   Algorithm   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Neighbor System-ID (ID length octets)        |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        SID/Label Base Offset/Index Offset (variable)          ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~    Flags/ Algo/NB Sys-ID/ SID/Label/Index Offset (variable)   ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

   Type:                TDB

   Length:              variable; multiple advertisements

   Flags:               1 octet field of the following flags:

                         0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        | |B|V|L|       |
                        +-+-+-+-+-+-+-+-+

                  where:

                  B-Flag:  Backup Flag.  If set, the Adj-SID is eligible
                           for protection (e.g., using IP Fast Reroute
                           (IPFRR) or MPLS Fast Reroute (MPLS-FRR)) as
                           described in [RFC8402].

                  V-Flag:  Value Flag.  If set, then it carries
                           a label value. If not set, it is an index value.

                  L-Flag:  Local Flag.  Always set to 1

   Algorithm:              1 octet. Same as Section 5.1

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   Neighbor System-ID:     IS-IS System-ID of length "ID Length" as
                           defined in [ISO10589].


   Base Offset:            3 octets. Same as Section 5.1

   Index Offset:           4 octets. Same as Section 5.1


3.3. Algorithm Offset for Prefix SID

   Prefix-SID for Algorithm could also be generated by adding an index offset value or
   a base offset for label.

   This advertisement is an sub-TLV in RFC8667 Section 2.

   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    |   Flags       |    Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        SID/Label Base offset/ Index offset                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~      Flags/ Algorithm/ Base Offset/ Index offset             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   Type:                TDB

   Length:              variable

   Flags:               1 octet field of the following flags:

                         0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        |R|N|   |V|L|   |
                        +-+-+-+-+-+-+-+-+

                  where:

                  R-Flag:  Re-advertisement Flag.  If set, then the prefix
                           to which this Prefix-SID is attached has been
                           propagated by the router from either another
                           level (i.e., from Level-1 to Level-2 or the
                           opposite) or redistribution (e.g., from another
                           protocol).

                  N-Flag:  Node-SID Flag.  If set, then the Prefix-SID
                           refers to the router identified by the prefix.
                           Typically, the N-Flag is set on Prefix-SIDs that
                           are attached to a router loopback address.  The

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                           N-Flag is set when the Prefix-SID is a Node-SID
                           as described in [RFC8402].


                  V-Flag:  Value Flag.  If set, then it carries
                           a label value. If not set, it is an index value.

                  L-Flag:  Local Flag.  Always set to 0

   Algorithm:              1 octet
                           Flex-Algo value is between 128 and 255


   Base offset:            3 octets
                           Label base for Prefix-SIDs for given Algorithm
                           The derived Prefix-SID is the sum of the base
                           Offset and the Algo 0 label

   Index Offset:           4 octets
                           Index offset counting from Algo 0
                           The new label for Prefix is sum of (SRGB base +
                           index + index offset)

   The format of Base offset and Index Offset is the same as RFC8667. The choice of
   advertising Base Offset or Index Offset MUST match the advertisement of original
   Prefix-SID method from the same router.

4. Compatibility

   For nodes that do not support Algorithm Offset for Adj-SID extension, the label
   stack could use normal Adj-SID, providing that the node still supports Flex-Algo
   Prefix SID.

   For TI-LFA, there would be two modes of operation - loose or strict. For strict
   mode, all Adj-SID involved in TI-LFA candidate path must be derived with offset
   method.

   For loose mode, Adj-SID in the TI-LFA candidate path could be a mix of Adj-SID's
   with and without offset. This allows backward compatibility with routers which only
   support Flex-Algo.



5. OSPF extension

   TBD





6. Security Consideration

   TBD

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7. References

7.1. Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels",
                 BCP 14, RFC 2119, March 1997.

7.2. Informative References

   [RFC8667]     Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
                 Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
                 Extensions for Segment Routing", RFC 8667,
                 DOI 10.17487/RFC8667, December 2019,
                 <https://www.rfc-editor.org/info/rfc8667>.


   [TI-LFA]      Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
                 Decraene, B., and D. Voyer, "Topology Independent Fast
                 Reroute using Segment Routing",
                 <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-segment-
                 routing-ti-lfa>

   [FLEXAGLO]    S. Hegde, P. Psenak and etc, IGP Flexible Algorithm
                 https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo


   [ISO10589]    International Organization for Standardization,
                 "Information technology -- Telecommunications and
                 information exchange between systems -- Intermediate
                 system to Intermediate system intra-domain routeing
                 information exchange protocol for use in conjunction with
                 the protocol for providing the connectionless-mode network
                 service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
                 November 2002.



8. Acknowledgments


        The following people have contributed to this document:
               Shraddha Hegde, Juniper Networks



     Author Address

     Louis Chan
        Juniper Networks
        2604, Cityplaza One, 1111 King's Road
        Taikoo Shing
        Hong Kong

        Phone: +852-25876659
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        Email: louisc@juniper.net


     Krzysztof Grzegorz Szarkowicz
        Juniper Networks
        Parkring 10
        A-1010 Wien
        Austria

        Phone: +49 89 203012127
        Email: kszarkowicz@juniper.net











































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