Network Working Group                                          W. Cheng
Internet Draft                                                  L. Gong
Intended status: Standards Track                           China Mobile
Expires: March 6, 2024                                           C. Lin
                                                                M. Chen
                                                   New H3C Technologies
                                                      September 6, 2023


                         IGP Shortcut Enhancement
                draft-cheng-lsr-igp-shortcut-enhancement-02


Abstract

   IGP shortcut mechanism allows calculating routes to forward traffic
   over Traffic Engineering tunnels. This document describes the
   enhancement of IGP shortcut which can steer routes onto TE-tunnels
   based on colors.

Status of this Memo

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   This Internet-Draft will expire on March 6, 2024.

Copyright Notice

   Copyright (c) 2023 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
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   warranty as described in the Simplified BSD License.

Table of Contents


   1. Introduction...................................................2
      1.1. Requirements Language.....................................3
   2. Extensions for IGP.............................................3
      2.1. IS-IS Color Sub-TLV.......................................3
      2.2. OSPF Color Sub-TLV........................................4
   3. SPF Computation................................................5
   4. Color Values of TE-Tunnels.....................................5
   5. Backward Compatibility.........................................6
   6. Security Considerations........................................6
   7. IANA Considerations............................................6
   8. References.....................................................6
      8.1. Normative References......................................6
      8.2. Informational References..................................7
   Authors' Addresses................................................8

1. Introduction

   [RFC3906] describes how IGP calculate routes to forward traffic over
   Traffic Engineering tunnels. Such mechanism is also referred to as
   IGP shortcut.

   The granularity of IGP shortcut is based on nodes. If the first-hop
   of a node is determined to be a TE-tunnel during the SPF
   computation, all routes to IP prefixes advertised by that node will
   be over that TE-tunnel. For example, in the following topology, X1
   and X2 are IP prefixes advertised by rtrC, and Y1 and Y2 are IP
   prefixes advertised by rtrD. Using IGP shortcut, all routes to X1,
   X2, Y1 and Y2 will be steered onto T1 since T1 has the lowest cost.







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           === T1(10) ===>
           === T2(15) ===>
         rtrA -- rtrB -- rtrC -- rtrD
              10      10  |   10  |
                         X1,X2   Y1,Y2

   However, in some scenarios, there may be requirements to steer the
   routes to different prefixes of the same node onto different TE-
   tunnels. For example, the traffic flows to X1 and Y1 need to be
   forwarded over low-cost tunnel T1, but the traffic flows to X2 and
   Y2 need to be forwarded over low-delay tunnel T2.

   In the BGP-based service, "color" is often used to indicate the
   intent of forwarding [RFC9012] [RFC9252]. The Color Extended
   Community can be attached to BGP routes, and the associated flows
   will be steered into tunnels with the same color.

   This document describes the enhancement of IGP shortcut which can
   steer routes onto TE-tunnels based on colors. It also defines the
   extensions for IGP to advertise colors for prefixes.

   In [RFC3906], the term "TE-tunnel" mainly refers to Label Switched
   Path, such as MPLS RSVP-TE tunnel. With the development of Segment
   Routing (SR) technology, SR Policy [RFC9256] becomes a useful tool
   for Traffic Engineering. In the context of this document, SR
   Policies are also included as TE-tunnels.

1.1. 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. Extensions for IGP

2.1. IS-IS Color Sub-TLV

   The IS-IS Color Sub-TLV is defined in this document to advertise
   colors for prefixes in IS-IS. The Sub-TLV has the following format:







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    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Color                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: 1 octet. The length value is variable.

   o Flags: 2 octets. No flags are defined in this document. Undefined
      flags MUST be set to 0 by the sender, and any unknown flags MUST
      be ignored by the receiver.

   o Color: 4 octets. Contains color value associated with the prefix.

   The IS-IS Color Sub-TLV is applicable to TLVs 27, 135, 235, 236, and
   237.

2.2. OSPF Color Sub-TLV

   The OSPF Color Sub-TLV is defined in this document to advertise
   colors for prefixes in OSPFv2 and OSPFv3. The Sub-TLV has the
   following format:

    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             |            Color              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Color (cont.)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   o Type: TBD.

   o Length: 1 octet. The length value is variable.

   o Flags: 2 octets. No flags are defined in this document.
      Undefined flags MUST be set to 0 by the sender, and any unknown
      flags MUST be ignored by the receiver.

   o Color: 4 octets. Contains color value associated with the prefix.




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   The OSPF Color Sub-TLV is applicable to OSPFv2 Extended Prefix TLV
   OSPFv3 Inter-Area-Prefix TLV OSPFv3 Intra-Area-Prefix TLV OSPFv3
   External-Prefix TLV.

3. SPF Computation

   The SPF computation of IGP shortcut is described in Section 2 of
   [RFC3906]. The key idea is to determine the first-hop information of
   a node with consideration of TE-tunnels:

   o Examine the list of tail-end routers directly reachable via a TE-
      tunnel. If there is a TE-tunnel to this node, we use the TE-
      tunnel as the first-hop.

   o If there is no TE-tunnel, and the node is directly connected, we
      use the first-hop information from the adjacency database.

   o If the node is not directly connected, and is not directly
      reachable via a TE-tunnel, we copy the first-hop information from
      the parent node(s) to the new node.

   This document makes the following changes to the first step of the
   above algorithm:

   o If there is a TE-tunnel to this node, we add the TE-tunnel into
      the first-hop information (without deleting the previous ones).

   This document also adds the following steps when calculating next-
   hops for prefixes advertised by a node:

   o If a prefix is colored, we look up the first-hop information of
      the advertiser node for TE-tunnels with the same color.

        - If there are eligible TE-tunnels, we compare the costs of
          paths over those TE-tunnels, and use the next-hop of the TE-
          tunnel with the lowest path cost.

        - If there is no eligible TE-tunnel, we use the native
          adjacency next-hop.

   o If a prefix has no color, we use the next-hop with the lowest
      path cost.

4. Color Values of TE-Tunnels

   The main idea of this document is to steer the flows to colored
   prefixes into tunnels with the same color values.


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   Some kinds of TE-tunnels, such as SR Policy [RFC9256], have inherent
   color values which can be directly used to match the colors of
   prefixes. For the TE-tunnels which have no inherent color, the color
   values may be determined by local configurations, which is out of
   the scope of this document.

5. Backward Compatibility

   If a head-end node does not support the Color Sub-TLV, it will
   calculate routes ignoring the colors. As a result, the behavior
   would be the same as without this specification.

6. Security Considerations

   TBD.

7. IANA Considerations

   This document defines the following new Sub-TLV in IS-IS:

   TBD - IS-IS Color Sub-TLV

   This document defines the following new Sub-TLV in OSPFv2 and
   OSPFv3:

   TBD - OSPF Color Sub-TLV

8. References

8.1. Normative References

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

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
             Protocol (IGP) Routes Over Traffic Engineering Tunnels",
             RFC 3906, DOI 10.17487/RFC3906, October 2004,
             <https://www.rfc-editor.org/info/rfc3906>.

   [RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
             A., and P. Mattes, "Segment Routing Policy Architecture",
             RFC 9256, DOI 10.17487/RFC9256, July 2022,
             <https://www.rfc-editor.org/info/rfc9256>



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8.2. Informational References

   [RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
             "The BGP Tunnel Encapsulation Attribute", RFC 9012, DOI
             10.17487/RFC9012, April 2021, <https://www.rfc-
             editor.org/info/rfc9012>.

   [RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
             B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
             Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI
             10.17487/RFC9252, July 2022, <https://www.rfc-
             editor.org/info/rfc9252>.




































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Authors' Addresses

   Weiqiang Cheng
   China Mobile
   China
   Email: chengweiqiang@chinamobile.com


   Liyan Gong
   China Mobile
   China
   Email: gongliyan@chinamobile.com


   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com


   Mengxiao Chen
   New H3C Technologies
   China
   Email: chen.mengxiao@h3c.com
























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