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Interdomain Routing Working Group                                  C. Li
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                  Y. Zhu
Expires: October 30, 2020                                  China Telecom
                                                                A. Sawaf
                                                   Saudi Telecom Company
                                                                   Z. Li
                                                     Huawei Technologies
                                                          April 28, 2020


                    Segment Routing Path MTU in BGP
                  draft-ietf-idr-sr-policy-path-mtu-00

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
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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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 October 30, 2020.

Copyright Notice

   Copyright (c) 2020 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
   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 . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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



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   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] .

   The host may discover the PMTU by Path MTU Discovery (PMTUD)
   [RFC8201] or other mechanisms.  But the ingress still needs to
   examine the packet size for dropping too large packets to avoid
   malicious traffic or error traffic.  Also, the packet size may
   exceeds the PMTU because of the new encapsulation of SR-MPLS or SRv6
   packet at the ingress.

   In order to check whether the Packet size exceeds the PMTU or not,
   the ingress node needs to know the Path MTU associated to the
   forwarding path.  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 Link MTU information can be
   obtained via BGP-LS [I-D.zhu-idr-bgp-ls-path-mtu] or some other
   means.  With the Link MTU, the controller can compute the PMTU and
   convey the information via the BGP SR policy.

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:










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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
                    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:


      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 an Optional sub-TLV.  When it appears, it
   must appear only once at most within a Segment List sub-TLV.  If
   multiple Path MTU sub-TLVs appear within a Segment List sub-TLV, the
   first one will be processed, and the rest will be ignored.  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].  It 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     |               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.

   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.





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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.  Contributors

   Jun Qiu

   Huawei Technologies

   China

   Email: qiujun8@huawei.com

8.  Acknowledgements

   Authors would like to thank Ketan Talaulikar, Aijun Wang, Weiqiang
   Cheng, Huanan Chen, Chongfeng Xie, Stefano Previdi, Taishan Tang,
   Keqiang Guo, Chen Zhang, Susan Hares, Weiguo Hao, Gong Xia, Bing
   Yang, Linda Dunbar, Shunwan Zhuang, Huaimo Chen, Mach Chen, Jingring
   Xie, Zhibo Hu, Jimmy Dong and Jianwei Mao for their proprefessional
   comments and help.

9.  References

9.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
              Rosen, E., Jain, D., and S. Lin, "Advertising Segment
              Routing Policies in BGP", draft-ietf-idr-segment-routing-
              te-policy-08 (work in progress), November 2019.








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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-06 (work in progress),
              December 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>.

9.2.  Informative References

   [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-02 (work in progress),
              February 2020.

   [I-D.li-spring-srv6-path-segment]
              Li, C., Cheng, W., Chen, M., Dhody, D., and R. Gandhi,
              "Path Segment for SRv6 (Segment Routing in IPv6)", draft-
              li-spring-srv6-path-segment-05 (work in progress), March
              2020.

   [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-02 (work in progress), January 2020.

   [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>.



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   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

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


   Ahmed El Sawaf
   Saudi Telecom Company
   Riyadh
   Saudi Arabia

   Email: aelsawaf.c@stc.com.sa


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

   Email: lizhenbin@huawei.com











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