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Interdomain Routing Working Group                                  C. Li
Internet-Draft                                                     Z. Li
Intended status: Standards Track                     Huawei Technologies
Expires: January 3, 2019                                   July 02, 2018


                    Segment Routing Path MTU in BGP
                   draft-li-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.

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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
   3.  SR Policy for Path MTU  . . . . . . . . . . . . . . . . . . .   3
     3.1.  SR Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . .   4
   4.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Segment routing (SR) [I-D.ietf-spring-segment-routing] 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].  For
   distributing SR policies to the headend,
   [I-D.ietf-idr-segment-routing-te-policy] specifies a mechanism by
   using BGP, and new sub-TLVs are defined for SR Policies in BGP UPDATE
   message.

   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




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   the LSP would still function normally.  [RFC3209] specify the
   mechanism of MTU signaling in RSVP.

   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.zhu-idr-bgp-ls-path-mtu], BGP-LS [I-D.hu-lsr-isis-path-mtu] or
   some other means.

2.  Terminology

   This memo makes use of the terms defined in
   [I-D.ietf-spring-segment-routing] and [RFC3209].

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
               Segment List
                   Weight
                   Segment
                   Segment
                   ...

   As introduced in Section 1, each SR path may have a path MTU, an SR
   policy carrying a SR path MTU is expressed as below:

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






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3.1.  SR Path MTU Sub-TLV

   This section defines an SR Path MTU sub-TLV, and it is included in
   the segment list sub-TLV.

   An SR Path MTU sub-TLV is associated with an SR path specified by a
   segment list sub-TLV or path ID as defined in
   [I-D.cheng-spring-mpls-path-segment] and
   [I-D.li-spring-passive-pm-for-srv6-np], 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 (suggested value 11).

   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.  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.



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   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

   TBA

6.  Security Considerations

   TBA

7.  Acknowledgements

   TBA

8.  References

8.1.  Normative References

   [I-D.cheng-spring-mpls-path-segment]
              Cheng, W., Wang, L., Li, H., Chen, M., Zigler, R., and S.
              Zhan, "Path Segment in MPLS Based Sement Routing Network",
              draft-cheng-spring-mpls-path-segment-01 (work in
              progress), March 2018.

   [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-03 (work in
              progress), May 2018.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [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-01 (work in progress), June 2018.



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   [I-D.li-spring-passive-pm-for-srv6-np]
              Li, C. and M. Chen, "Passive Performance Measurement for
              SRv6 Network Programming", draft-li-spring-passive-pm-for-
              srv6-np-00 (work in progress), March 2018.

   [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>.

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.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>.

Authors' Addresses

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

   Email: chengli13@huawei.com


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

   Email: lizhenbin@huawei.com






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