Segment Routing Path MTU in BGP
draft-li-idr-sr-policy-path-mtu-01

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.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 2.1. Requirements Language
• 3. SR Policy for Path MTU
• 3.1. SR Path MTU Sub-TLV
• 4. Operations
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

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

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:

   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 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
             Priority
             Policy Name
             Explicit NULL Label Policy (ENLP)
             Segment List
                 Weight
                 Path MTU
                 Segment
                 Segment
                 ...
             ...

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 segment as defined in [I-D.cheng-spring-mpls-path-segment] and [I-D.li-spring-srv6-path-segment], 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

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.

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

8.2. Informative References

Authors' Addresses