BGP-LS Extensions for Advertising Path MTU
draft-zhu-idr-bgp-ls-path-mtu-03

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 specify the extension 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.

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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This Internet-Draft will expire on January 14, 2021.

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Table of Contents

• 1. Introduction
• 2. Deploying scenarios
• 3. BGP_LS Extensions for Path MTU
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. Contributors
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

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. PathMTU 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 of LDP tunnels. 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) [I-D.ietf-6man-segment-routing-header]. [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 than traditional MPLS packet size.

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

2. 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 LSDBs and distribute it to a consumer, either directly or via a peer BGP speaker (typically a dedicated Route Reflector). [RFC7176] specifies the ISIS mechanism and extensions for link MTU Sub-TLV.

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                 IGP                       IGP

           Figure 1: Collection of Link-State and TE Information

3. BGP_LS Extensions for Path 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.

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 MTU

4. IANA Considerations

This document requests assigning a new code-points from the BGP-LS Link Descriptor and Attribute TLVs registry as specified in sections 3.

5. Security Considerations

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

6. Acknowledgements

7. Contributors

Gang Yan
Huawei 
China

Email:yangang@huawei.com

Junda Yao
Huawei
China

Email:yaojunda@huawei.com

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses