Network Working Group J. Haas
Internet-Draft Juniper Networks, Inc.
Intended status: Standards Track A. Fu
Expires: May 4, 2020 Bloomberg L.P.
November 1, 2019
BFD Encapsulated in Large Packets
draft-ietf-bfd-large-packets-02
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 http://datatracker.ietf.org/drafts/current/.
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 May 4, 2020.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. BFD Encapsulated in Large Packets . . . . . . . . . . . . . . 3
3. Implementation and Deployment Considerations . . . . . . . . 3
3.1. Implementations that do not support Large BFD Packets . . 3
3.2. Selecting MTU size to be detected . . . . . . . . . . . . 4
3.3. Detecting MTU mismatches . . . . . . . . . . . . . . . . 4
3.4. Equal Cost Multiple Paths (ECMP) or other Load Balancing
Considerations . . . . . . . . . . . . . . . . . . . . . 5
3.5. S-BFD . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Related Features . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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
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(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
3.1. Implementations that do not support Large BFD Packets
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.
[RFC5880], section 6.8.6, discusses the procedures for receiving BFD
Control packets. When an implementation is incapable of processing
Large BFD Packets, it could manifest in one of two possible ways:
o A receiving BFD implementation is incapable of accepting Large BFD
Packets. This is identical to the packet being discarded.
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o A receiving BFD implementation is capable of accepting Large BFD
Packets, but the Control packet is improperly rejected during
validation procedures. This is identical to the packet being
discarded.
In each of these cases, the BFD state machine would behave as if it
were not receiving Control packets and the implementation would
follow normal BFD procedures with regards to not having received
Control packets.
3.2. Selecting MTU size to be detected
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.
In the case multiple BFD clients desire to test the same BFD
endpoints using different bfd.PaddedPduSize parameters,
implementations should select the largest bfd.PaddedPduSize parameter
from the configured sessions. This is similar to how implementations
of BFD select the most aggressive timing parameters for multiple
sessions to the same endpoint.
3.3. Detecting MTU mismatches
The accepted MTU for an interface is impacted by packet encapsulation
considerations at a given layer; e.g. layer 2, layer 3, tunnel, etc.
A common misconfiguration of interface parameters is inconsistent
MTU. In the presence of inconsistent MTU, it is possible for
applications to have unidirectional connectivity.
When it is necessary for an application using BFD with Large Packets
to test the bi-directional Path MTU, it is necessary to configure the
bfd.PaddedPduSize parameter on both sides of an interface. E.g., if
the desire is to verify a 1500 byte MTU in both directions on an
Ethernet or point to point link, each side of the BFD session must
have bfd.PaddedPduSize set to 1500. In the absence of such
consistent configuration, BFD with Large Packets may correctly
determine unidirectional connectivity at the tested MTU, but bi-
directional MTU may not be properly validated.
It should be noted that some interfaces may intentionally have
different MTUs. Setting the bfd.PaddedPduSize appropriately for each
side of the interface supports such scenarios.
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3.4. Equal Cost Multiple Paths (ECMP) or other Load Balancing
Considerations
Various mechanisms are utilized to increase throughput between two
endpoints at various network layers. Such features include Link
Aggregate Groups (LAGs) or ECMP forwarding. Such mechanisms balance
traffic across multiple physical links while hiding the details of
that balacing from the higher networking layers. The details of that
balancing are highly implementation specific.
In the presence of such load balancing mechanisms, it is possible to
have member links that are not properly forwarding traffic. In such
circumstances, this will result in dropped traffic when traffic is
chosen to be load balanced across those member links.
Such load balancing mechanisms may not permit all link members to be
properly tested by BFD. This is because the BFD Control packets may
be forwarded only along links that are up. BFD on LAG, [RFC7130],
was developed to help cover one such scenario. However, for testing
forwarding over multiple hops, there is no such specified general
purpose BFD mechanism for exercising all links in an ECMP. This may
result in a BFD session being in the Up state while some traffic may
be dropped or otherwise negatively impacted along some component
links.
Some BFD implementations utilize their internal understanding of the
component links and their resultant forwarding to exercise BFD in
such a way to better test the ECMP members and to tie the BFD session
state to the health of that ECMP. Due to the implementation specific
load balancing, it is not possible to standardize such additional
mechanisms for BFD.
Mis-configuration of some member MTUs may lead to Load Balancing that
may have an inconsistent Path MTU depending on how the traffic is
balanced. While the intent of BFD with Large Packets is to verify
path MTU, it is subject to the same considerations above.
3.5. S-BFD
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.
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5. IANA Considerations
This document introduces no additional considerations to IANA.
6. Acknowledgments
The authors would like to thank Les Ginsberg, Mahesh Jethandani,
Robert Raszuk, and Ketan Talaulikar, for their valuable feedback on
this proposal.
7. References
7.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981, <https://www.rfc-
editor.org/info/rfc791>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010, <https://www.rfc-
editor.org/info/rfc5881>.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <https://www.rfc-editor.org/info/rfc5883>.
[RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
Forwarding Detection (BFD) on Link Aggregation Group (LAG)
Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
2014, <https://www.rfc-editor.org/info/rfc7130>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<https://www.rfc-editor.org/info/rfc7880>.
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7.2. Informative References
[I-D.haas-xiao-bfd-echo-path-mtu]
Haas, J. and M. Xiao, "Application of the BFD Echo
function for Path MTU Verification or Detection", draft-
haas-xiao-bfd-echo-path-mtu-01 (work in progress), July
2011.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990, <https://www.rfc-
editor.org/info/rfc1191>.
[RFC3719] Parker, J., Ed., "Recommendations for Interoperable
Networks using Intermediate System to Intermediate System
(IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004,
<https://www.rfc-editor.org/info/rfc3719>.
Appendix A. Related Features
IS-IS [RFC3719] supports a Padding feature for its hellos. This
provides the ability to detect inconsistent link MTUs.
Authors' Addresses
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
US
Email: jhaas@juniper.net
Albert Fu
Bloomberg L.P.
Email: afu14@bloomberg.net
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