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Versions: 00

Network Working Group                                             Y. Zhu
Internet-Draft                                             China Telecom
Intended status: Standards Track                                   Z. Hu
Expires: May 24, 2021                                            S. Peng
                                                     Huawei Technologies
                                                             R. Mwehaire
                                                         MTN Uganda Ltd.
                                                       November 20, 2020


         Signaling Maximum Transmission Unit (MTU) using BGP-LS
                   draft-ietf-idr-bgp-ls-link-mtu-00

Abstract

   BGP Link State (BGP-LS) describes a mechanism by which link-state and
   TE information can be collected from networks and shared with
   external components using the BGP routing protocol.  The centralized
   controller (PCE/SDN) completes the service path calculation based on
   the information transmitted by the BGP-LS and delivers the result to
   the Path Computation Client (PCC) through the PCEP or BGP protocol.

   Segment Routing (SR) leverages the source routing paradigm, which can
   be directly applied to the MPLS architecture with no change on the
   forwarding plane and applied to the IPv6 architecture, with a new
   type of routing header, called SRH.  The SR uses the IGP protocol as
   the control protocol.  Compared to the MPLS tunneling technology, the
   SR does not require additional signaling.  Therefore, the SR does not
   support the negotiation of the Path MTU.  Since multiple labels or
   SRv6 SIDs are pushed in the packets, it is more likely that the
   packet size exceeds the path mtu of SR tunnel.

   This document specifies the extensions to BGP Link State (BGP-LS) to
   carry maximum transmission unit (MTU) messages of link.  The PCE/SDN
   calculates the Path MTU while completing the service path calculation
   based on the information transmitted by the BGP-LS.

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.




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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Deploying scenarios . . . . . . . . . . . . . . . . . . . . .   5
   4.  BGP_LS Extensions for Link MTU  . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC7752]describes the implementation mechanism of BGP-LS by which
   link-state and TE information can be collected from networks and
   shared with external components using the BGP routing protocol
   [RFC4271].  BGP-LS allows the necessary Link-State Database (LSDB)



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   and Traffic Engineering Database (TEDB) information to be collected
   from the IGP within the network, filtered according to configurable
   policy, and distributed to the PCE as necessary.

   The appropriate MTU size guarantees efficient data transmission.  If
   the MTU size is too small and the packet size is large, fragmentation
   may occur too much and packets are discarded by the QoS queue.  If
   the MTU configuration is too large, packet transmission may be slow.
   Path MTU is the maximum length of a packet that can pass through a
   path without fragmentation.  [RFC1191] describes a technique for
   dynamically discovering the maximum transmission unit (MTU) of an
   arbitrary internet path.

   The traditional MPLS tunneling technology has signaling for
   establishing a path.  [RFC3988] defines the mechanism for
   automatically discovering the Path MTU of LSPs.  For a certain FEC,
   the LSR compares the MTU advertised by all downstream devices with
   the MTU of the FEC output interface in the local device, and
   calculates the minimum value for the upstream device.

   [RFC3209] specify the mechanism of MTU signaling in RSVP-TE.  The
   ingress node of the RSVP-TE tunnel sends a Path message to the
   downstream device.  The Adspec object in the Path message carries the
   MTU.  Each node along the tunnel receives a Path message, compares
   the MTU value in the Adspec object with the interface MTU value and
   MPLS MTU configured on the physical output interface of the local
   tunnel, obtains the minimum MTU value, and puts it into the newly
   constructed Path message and continues to send it to the downstream
   equipment.  Thus, the MTU carried in the Path message received by the
   Egress node is the minimum value of the path MTU.  The Egress node
   brings the negotiated Path MTU back to the Ingress node through the
   Resv message.

   Segment Routing (SR) described in [RFC8402] leverages the source
   routing paradigm.  Segment Routing can be directly applied to the
   MPLS architecture with no change on the forwarding plane [RFC8660]
   and applied to the IPv6 architecture with a new type of routing
   header called the SR header (SRH) [RFC8754].
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] defines SR extensions to
   BGP-LS and specifies the TLVs and sub-TLVs for advertising SR
   information.  Based on the SR information reported by the BGP-LS, the
   SDN can calculate the end-to-end explicit SR-TE paths or SR Policies.

   Nevertheless, Segment Routing is a tunneling technology based on the
   IGP protocol as the control protocol, and there is no additional
   signaling for establishing the path. so the Segment Routing tunnel
   cannot currently support the negotiation mechanism of the MTU.
   Multiple labels or SRv6 SIDs are pushed in the packets.  This causes



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   the length of the packets encapsulated in the Segment Routing tunnel
   to increase during packet forwarding.  This is more likely to cause
   packet size exceed the traditional MPLS packet size.

   This document specify the extension to BGP Link State (BGP-LS) to
   carry link maximum transmission unit (MTU) messages.

2.  Terminology

   This draft refers to the terms defined in [RFC8201], [RFC4821] and
   [RFC3988].


   MTU:  Maximum Transmission Unit, the size in bytes of the largest IP
      packet, including the IP header and payload, that can be
      transmitted on a link or path.  Note that this could more properly
      be called the IP MTU, to be consistent with how other standards
      organizations use the acronym MTU.

   Link MTU:  The Maximum Transmission Unit, i.e., maximum IP packet
      size in bytes, that can be conveyed in one piece over a link.  Be
      aware that this definition is different from the definition used
      by other standards organizations.

      For IETF documents, link MTU is uniformly defined as the IP MTU
      over the link.  This includes the IP header, but excludes link
      layer headers and other framing that is not part of IP or the IP
      payload.

      Be aware that other standards organizations generally define link
      MTU to include the link layer headers.

      For the MPLS data plane, this size includes the IP header and data (or
      other payload) and the label stack but does not include any lower-layer
      headers.  A link may be an interface (such as Ethernet or Packet-over-
      SONET), a tunnel (such as GRE or IPsec), or an LSP.

   Path:  The set of links traversed by a packet between a source node
      and a destination node.

   Path MTU, or PMTU:  The minimum link MTU of all the links in a path
      between a source node and a destination node.









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3.  Deploying scenarios

   This document suggests a solution to extension to BGP Link State
   (BGP-LS) to carry maximum transmission unit (MTU) messages.  The MTU
   information of the link is acquired through the process of collecting
   link state and TE information by BGP-LS.  Concretely, a router
   maintains one or more databases for storing link-state information
   about nodes and links in any given area.  The router's BGP process
   can retrieve topology from these IGP, BGP and other sources, and
   distribute it to a consumer, either directly or via a peer BGP
   speaker (typically a dedicated Route Reflector).  [RFC7176] specifies
   a possible way of using the ISIS mechanism and extensions for link
   MTU Sub-TLV.  In the case of inter-AS scenario (e.g., BGP EPE), the
   link MTU of the inter-AS link can be collected via BGP-LS directly.

   As per [RFC7752], the collection of link-state and TE information and
   its distribution to consumers is shown in the following figure.

                              +-----------+
                              | Consumer  |
                              +-----------+
                                    ^
                                    |
                              +-----------+
                              |    BGP    |               +-----------+
                              |  Speaker  |               | Consumer  |
                              +-----------+               +-----------+
                                ^   ^   ^                       ^
                                |   |   |                       |
                +---------------+   |   +-------------------+   |
                |                   |                       |   |
          +-----------+       +-----------+             +-----------+
          |    BGP    |       |    BGP    |             |    BGP    |
          |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
          +-----------+       +-----------+             +-----------+
                ^                   ^                         ^
                |                   |                         |
          IGP, BGP, Others    IGP, BGP, Others          IGP, BGP, Others

              Figure 1: Collection of Link-State and TE Information

   Please note that this signaled MTU may be different from the actual
   MTU, which is usually from configuration mismatches in a control
   plane and a data plane component.







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4.  BGP_LS Extensions for Link MTU

   [RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link
   NLRI or a Prefix NLRI.  The corresponding BGP-LS attribute is a Node
   Attribute, a Link Attribute or a Prefix Attribute.  [RFC7752] defines
   the TLVs that map link-state information to BGP-LS NLRI and the BGP-
   LS attribute.  Therefore, according to this document, a new sub-TLV
   is added to the Link Attribute TLV.  It is an independent attribute
   TLV that can be used for the link NLRI advertised with all the
   Protocol IDs.

   The format of the sub-TLV is as shown below.


         x  TYPE   - TBD
         x  LENGTH - Total length of the value field, it should be 2
         x  VALUE  - 2-byte MTU value of the link

                              No. of Octets
         +-----------------+
         |    MTU value    |       2
         +-----------------+

                         Figure 2. Sub-TLV Format for Link MTU


   Whenever there is a change in MTU value represented by Link Attribute
   TLV, BGP-LS should re-originate the respective TLV with the new MTU
   value.

5.  IANA Considerations

   This document requests assigning a new code-point from the BGP-LS
   Link Descriptor and Attribute TLVs registry as specified in section
   4.

      Value                  Description                  Reference
      ---------------------- ---------------------------- --------------
      TBD                    Link MTU                     This document


6.  Security Considerations

   This document does not introduce security issues beyond those
   discussed in RFC7752.






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7.  Acknowledgements

8.  Contributors

   Gang Yan
   Huawei
   China

   Email:yangang@huawei.com

   Junda Yao
   Huawei
   China

   Email:yaojunda@huawei.com

9.  References

9.1.  Normative References

   [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>.

9.2.  Informative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 2019.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [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>.

   [RFC3988]  Black, B. and K. Kompella, "Maximum Transmission Unit
              Signalling Extensions for the Label Distribution
              Protocol", RFC 3988, DOI 10.17487/RFC3988, January 2005,
              <https://www.rfc-editor.org/info/rfc3988>.





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   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC7176]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", RFC 7176,
              DOI 10.17487/RFC7176, May 2014,
              <https://www.rfc-editor.org/info/rfc7176>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

Authors' Addresses








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   Yongqing Zhu
   China Telecom
   109, West Zhongshan Road, Tianhe District.
   Guangzhou  510000
   China

   Email: zhuyq8@chinatelecom.cn


   Zhibo Hu
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: huzhibo@huawei.com


   Shuping Peng
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: pengshuping@huawei.com


   Robbins Mwehaire
   MTN Uganda Ltd.
   Uganda

   Email: Robbins.Mwehair@mtn.com



















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