Internet-Draft Signaling MTU using BGP-LS November 2021
Zhu, et al. Expires 1 June 2022 [Page]
Network Working Group
Intended Status:
Standards Track
Y. Zhu
China Telecom
Z. Hu
Huawei Technologies
S. Peng
Huawei Technologies
R. Mwehaire
MTN Uganda Ltd.

Signaling Maximum Transmission Unit (MTU) using BGP-LS


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.

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

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 1 June 2022.

Table of Contents

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) 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 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

      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.

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.

[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.

7. Acknowledgements

8. Contributors

Gang Yan

Junda Yao

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.

9.2. Informative References

Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H., and M. Chen, "Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing", Work in Progress, Internet-Draft, draft-ietf-idr-bgp-ls-segment-routing-ext-18, , <>.
Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, , <>.
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, , <>.
Black, B. and K. Kompella, "Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol", RFC 3988, DOI 10.17487/RFC3988, , <>.
Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, , <>.
Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, , <>.
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, , <>.
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, , <>.
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, , <>.
Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, , <>.
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, , <>.
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, , <>.

Authors' Addresses

Yongqing Zhu
China Telecom
109, West Zhongshan Road, Tianhe District.
Zhibo Hu
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Shuping Peng
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Robbins Mwehaire
MTN Uganda Ltd.