Segment Routing Path MTU in BGP
draft-li-idr-sr-policy-path-mtu-02

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Interdomain Routing Working Group                                  C. Li
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                  Y. Zhu
Expires: January 2, 2020                                   China Telecom
                                                                   Z. Li
                                                     Huawei Technologies
                                                            July 1, 2019

                    Segment Routing Path MTU in BGP
                   draft-li-idr-sr-policy-path-mtu-02

Abstract

   Segment Routing is a source routing paradigm that explicitly
   indicates the forwarding path for packets at the ingress node.  An SR
   policy is a set of candidate SR paths consisting of one or more
   segment lists with necessary path attributes.  However, the path
   maximum transmission unit (MTU) information for SR path is not
   available in the SR policy since the SR does not require signaling.
   This document defines extensions to BGP to distribute path MTU
   information within SR policies.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 2, 2020.

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  SR Policy for Path MTU  . . . . . . . . . . . . . . . . . . .   3
     3.1.  SR Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . .   4
   4.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node.  The ingress node steers packets into a specific path according
   to the Segment Routing Policy ( SR Policy) as defined in
   [I-D.ietf-spring-segment-routing-policy].  In order to distribute SR
   policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
   specifies a mechanism by using BGP.

   The maximum transmission unit (MTU) is the largest size packet or
   frame, in bytes, that can be sent in a network.  An MTU that is too
   large might cause retransmissions.  Too small an MTU might cause the
   router to send and handle relatively more header overhead and
   acknowledgments.  When an LSP is created across a set of links with
   different MTU sizes, the ingress router needs to know what the
   smallest MTU is on the LSP path.  If this MTU is larger than the MTU
   of one of the intermediate links, traffic might be dropped, because
   MPLS packets cannot be fragmented.  Also, the ingress router may not
   be aware of this type of traffic loss, because the control plane for
   the LSP would still function normally.  [RFC3209] specify the
   mechanism of MTU signaling in RSVP.  Likewise, SRv6 pakcets will be
   dropped if the packet size is larger than path MTU, since IPv6 packet
   can not be fragmented on transmission [RFC8200] .

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   However, the path maximum transmission unit (MTU) information for SR
   path is not available since the SR does not require signaling.  This
   document defines extensions to BGP to distribute path MTU information
   within SR policies.  The MTU information can be obtained via IGP
   [I-D.hu-lsr-isis-path-mtu], BGP-LS [I-D.zhu-idr-bgp-ls-path-mtu] or
   some other means.

2.  Terminology

   This memo makes use of the terms defined in [RFC8402] and [RFC3209].

2.1.  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.

3.  SR Policy for Path MTU

   As defined in [I-D.ietf-idr-segment-routing-te-policy] , the SR
   policy encoding structure is as follows:

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Segment List
                    Weight
                    Segment
                    Segment
                    ...
                ...

   As introduced in Section 1, each SR path has it's path MTU.  SR
   policy with SR path MTU information is expressed as below:

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      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Segment List
                    Weight
                    Path MTU
                    Segment
                    Segment
                    ...
                ...

3.1.  SR Path MTU Sub-TLV

   An SR Path MTU sub-TLV is associated with an SR path specified by a
   segment list sub-TLV or path segment as defined in
   [I-D.ietf-spring-mpls-path-segment] and
   [I-D.li-spring-srv6-path-segment], and it MUST appear only once
   within a Segment List sub-TLV.  It 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     |               RESERVED        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Path MTU                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 1. Path MTU sub-TLV

   Where:

   Type: to be assigned by IANA.

   Length: the total length of the value field not including Type and
   Length fields.

   Reserved: 16 bits reserved and MUST be set to 0 on transmission and
   MUST be ignored on receipt.

   Path MTU: 4 bytes value of path MTU in octets.  The value can be
   calculated by a central controller or other devices based on the
   information that learned via IGP of BGP-LS or other means.

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   Whenever the path MTU of a physical or logical interface is changed,
   a new SR policy with new path MTU information should be updated
   accordingly by BGP.

4.  Operations

   The document does not bring new operation beyong the description of
   operations defined in [I-D.ietf-idr-segment-routing-te-policy].  The
   existing operations defined in
   [I-D.ietf-idr-segment-routing-te-policy] can apply to this document
   directly.

   Typically but not limit to, the SR policies carrying path MTU
   infomation are configured by a controller.

   After configuration, the SR policies carrying path MTU infomation
   will be advertised by BGP update messages.  The operation of
   advertisement is the same as defined in
   [I-D.ietf-idr-segment-routing-te-policy], as well as the receiption.

   The consumer of the SR policies is not the BGP process.  The
   operation of sending information to consumers is out of scope of this
   document.

5.  IANA Considerations

   This document defines a new Sub-TLV in registries "SR Policy List
   Sub- TLVs" [I-D.ietf-idr-segment-routing-te-policy]:

   Value    Description                                  Reference
   ---------------------------------------------------------------------
    TBA     Path MTU sub-TLV                            This document

6.  Security Considerations

   TBA

7.  Acknowledgements

   TBA

8.  References

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8.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen,
              E., and S. Lin, "Advertising Segment Routing Policies in
              BGP", draft-ietf-idr-segment-routing-te-policy-06 (work in
              progress), May 2019.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), May 2019.

   [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>.

   [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>.

   [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>.

8.2.  Informative References

   [I-D.hu-lsr-isis-path-mtu]
              Hu, Z., Zhu, Y., Li, Z., and L. Dai, "IS-IS Extensions for
              Path MTU", draft-hu-lsr-isis-path-mtu-00 (work in
              progress), June 2018.

   [I-D.ietf-spring-mpls-path-segment]
              Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
              "Path Segment in MPLS Based Segment Routing Network",
              draft-ietf-spring-mpls-path-segment-00 (work in progress),
              March 2019.

   [I-D.li-spring-srv6-path-segment]
              Li, C., Cheng, W., Chen, M., Dhody, D., Li, Z., Dong, J.,
              and R. Gandhi, "Path Segment for SRv6 (Segment Routing in
              IPv6)", draft-li-spring-srv6-path-segment-01 (work in
              progress), June 2019.

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   [I-D.zhu-idr-bgp-ls-path-mtu]
              Zhu, Y., Hu, Z., Yan, G., and J. Yao, "BGP-LS Extensions
              for Advertising Path MTU", draft-zhu-idr-bgp-ls-path-
              mtu-00 (work in progress), June 2018.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

Authors' Addresses

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: chengli13@huawei.com

   YongQing Zhu
   China Telecom
   109, West Zhongshan Road, Tianhe District.
   Guangzhou
   China

   Email: zhuyq.gd@chinatelecom.cn

   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: lizhenbin@huawei.com

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