BFD Encapsulated in Large Packets
draft-ietf-bfd-large-packets-00

Abstract

The Bidirectional Forwarding Detection (BFD) protocol is commonly used to verify connectivity between two systems. BFD packets are typically very small. It is desirable in some circumstances to know that not only is the path between two systems reachable, but also that it is capable of carrying a payload of a particular size. This document discusses thoughts on how to implement such a mechanism using BFD in Asynchronous mode.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119] only when they appear in all upper case. They may also appear in lower or mixed case as English words, without normative meaning.

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 https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on August 30, 2019.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. BFD Encapsulated in Large Packets
• 3. Implementation and Deployment Considerations
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Related Features
• Authors' Addresses

1. Introduction

The Bidirectional Forwarding Detection (BFD) [RFC5880] protocol is commonly used to verify connectivity between two systems. However, some applications may require that the Path MTU [RFC1191] between those two systems meets a certain minimum criteria. When the Path MTU decreases below the minimum threshold, those applications may wish to consider the path unusable.

BFD may be encapsulated in a number of transport protocols. An example of this is single-hop BFD [RFC5881]. In that case, the link MTU configuration is typically enough to guarantee communication between the two systems for that size MTU. BFD Echo mode (Section 6.4 of [RFC5880]) is sufficient to permit verification of the Path MTU of such directly connected systems. Previous proposals ([I-D.haas-xiao-bfd-echo-path-mtu]) have been made for testing Path MTU for such directly connected systems. However, in the case of multi-hop BFD [RFC5883], this guarantee does not hold.

The encapsulation of BFD in multi-hop sessions is a simple UDP packet. The BFD elements of procedure (Section 6.8.6 of [RFC5880]) covers validating the BFD payload. However, the specification is silent on the length of the encapsulation that is carrying the BFD PDU. While it is most common that the transport protocol payload (i.e. UDP) length is the exact size of the BFD PDU, this is not required by the elements of procedure. This leads to the possibility that the transport protocol length may be larger than the contained BFD PDU.

2. BFD Encapsulated in Large Packets

Support for BFD between two systems is typically configured, even if the actual session may be dynamically created by a client protocol. A new BFD variable is defined in this document:

bfd.PaddedPduSize

The BFD transport protocol payload size is increased to this value. The contents of this additional payload MUST be zero. The minimum size of this variable MUST NOT be smaller than permitted by the element of BFD procedure; 24 or 26 - see Section 6.8.6 of [RFC5880].

The Don't Fragment bit (Section 2.3 of [RFC0791]) of the IP payload, when using IPv4 encapsulation, MUST be set.

3. Implementation and Deployment Considerations

While this document proposes no change to the BFD protocol, implementations may not permit arbitrarily padded transport PDUs to carry BFD packets. While Section 6 of [RFC5880] warns against excessive pedantry, implementations may not work with this mechanism without additional support. Additional changes to the base BFD protocol may be required to permit negotiation of this functionality and the padding value.

It is also worthy of note that even if an implementation can function with larger transport PDUs, that additional packet size may have impact on BFD scaling. Such systems may support a lower transmission interval (bfd.DesiredMinTxInterval) when operating in large packet mode. This interval may depend on the size of the transport PDU.

Given the impact on scaling larger PDU sizes may have on BFD implementations, operators should consider applying it only in situations where there is appropriate concern for path MTU. An example of this is commercial WAN services.

Since the consideration is path MTU, BFD sessions using this feature only need to use a bfd.PaddedPduSize appropriate to exercise the path MTU for the desired application. This may be significantly smaller than the system's link MTU; e.g. desired path MTU is 1500 bytes while the interface MTU that BFD with large packets is running on is 9000 bytes.

This mechanism also can be applied to other forms of BFD, including S-BFD [RFC7880].

4. Security Considerations

This document does not change the underlying security considerations of the BFD protocol or its encapsulations.

5. IANA Considerations

This document introduces no additional considerations to IANA.

6. References

6.1. Normative References

6.2. Informative References

Appendix A. Related Features

IS-IS supports a Padding feature for its hellos. This provides the ability to detect inconsistent link MTUs.

Authors' Addresses