BIER MTU Discovery
draft-venaas-bier-mtud-00

Abstract

This document defines an IGP based mechanism for discovering the MTU of a BIER sub-domain. This document defines extensions to OSPF and IS-IS, but other protocols could potentially be extended. MTU discovery is usually done for a given path, while this document defines it for a sub-domain. This allows the computed MTU to be independent of the set of receivers. Also, the MTU is independent of rerouting events within the sub-domain.

Status of This Memo

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Table of Contents

• 1. Introduction
• 2. Conventions used in this document
• 3. IS-IS BIER MTU Sub-sub-TLV
• 4. OSPF BIER MTU Sub-TLV
• 5. IANA considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

1. Introduction

This document defines an IGP based mechanism for discovering the MTU of a BIER sub-domain. The discovered MTU indicates the largest possible BIER payload, such as an IP packet, that can be sent across any link in a BIER sub-domain. This is different from [I-D.ietf-bier-path-mtu-discovery] which performs Path MTU Discovery (PMTUD) for a set of receivers. PMTUD is based on probing, and when there are routing changes, e.g., a link going down, the actual MTU for a path may become less than was previously discovered, and there will be some delay until the next probe is performed. Also, the set of receivers for a flow may change at any time, which may cause the MTU to change. This document instead discovers a BIER sub-domain MTU, which is independent of paths and receivers within the sub-domain.

For convenience we will refer to an interface on a router as a BIER interface if the router has a BIER neighbor on the interface. That is, there is a directly connected router on that interface that is announcing a BIER prefix. We say that it is a BIER interface in a given sub-domain if the router itself announces a prefix tagged with the sub-domain, and there is BIER neighbor on the interface also announcing a prefix tagged with the sub-domain.

In order to allow MTU discovery in a BIER sub-domain, the procedure is as follows. Every BIER router, for each sub-domain with at least one local BIER interface in the sub-domain, per the above definition of a BIER interface, determines the largest payload that can be sent BIER encapsulated out of any of its BIER interfaces in the sub-domain. That is, for each local BIER interface in the sub-domain, it needs to determine the size of the largest BIER encapsulated payload that can be sent out of that interface. We define the local sub-domain MTU of a router to be the minimum of the per BIER interface maximum payload size.

A BIER router announces a BIER prefix in either IS-IS or OSPF as specified in [I-D.ietf-bier-isis-extensions] and [I-D.ietf-bier-ospf-bier-extensions]. They both define a BIER Sub-TLV to be included with the prefix. There is one BIER Sub-TLV included for each sub-domain. This document defines how a router includes its local sub-domain MTU in each of the BIER Sub-TLVs it advertizes.

A router can discover the MTU of a BIER sub-domain by identifying all the prefixes that have a BIER Sub-TLV for the sub-domain. It then computes the minimum of the advertised MTU values for that sub-domain. This includes its local sub-domain MTU. This allows all the routers in the sub-domain to discover the same sub-domain wide MTU.

Note that a router should announce a new local MTU for a sub-domain immediately if the value becomes smaller than what it currently announces. This would happen if the MTU of an interface is configured to a smaller value, or the first BIER neighbor for a sub-domain is detected on an interface, and the MTU of the interface is less than all the other local BIER interfaces in the sub-domain. However, if BIER neighbors go away, or if an interface goes down, so that the local MTU becomes larger, a router SHOULD NOT immediately announce the larger value. A router MAY after some delay announce the new larger MTU. The intention is that dynamic events such as a quick link flap should not cause the announced MTU to be increased.

2. Conventions used in this document

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. IS-IS BIER MTU Sub-sub-TLV

    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    |             MTU               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	
	  

A router uses the BIER MTU Sub-sub-TLV to announce the minimum BIER MTU of all its BIER enabled interfaces. The Sub-sub-TLV MUST be ignored if it is included multiple times.

Type:

TBD

Length:

2

MTU:

MTU in octets

4. OSPF BIER MTU Sub-TLV

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             MTU               |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	
	  

A router uses the BIER MTU Sub-TLV to annonce the minimum BIER MTU of all its BIER enabled interfaces. It is a Sub-TLV of the BIER Sub-TLV, and SHOULD be included exactly once within each of the advertised BIER Sub-TLVs. The Sub-TLV MUST be ignored if it is included multiple times.

Type:

TBD2

Length:

4

MTU:

MTU in octets

5. IANA considerations

An allocation from the "sub-sub-TLVs for BIER Info sub-TLV" registry as defined in [I-D.ietf-bier-isis-extensions] is requested for the IS-IS BIER MTU Sub-sub-TLV. Please replace the string TBD in this document with the appropriate value.

An allocation from the "OSPF Extended Prefix sub-TLV" registry as defined in [RFC7684] is requested for the OSPF BIER MTU Sub-TLV. Please replace the string TBD2 in this document with the appropriate value.

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses