Internet Engineering Task Force (IETF) V. Govindan
Request for Comments: 7886 C. Pignataro
Category: Standards Track Cisco
ISSN: 2070-1721 June 2016
Advertising Seamless Bidirectional Forwarding Detection (S-BFD)
Discriminators in the Layer Two Tunneling Protocol Version 3 (L2TPv3)
Abstract
This document defines a new Attribute-Value Pair (AVP) that allows
L2TP Control Connection Endpoints (LCCEs) to advertise one or more
Seamless Bidirectional Forwarding Detection (S-BFD) Discriminator
values using the Layer Two Tunneling Protocol version 3 (L2TPv3).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7886.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. S-BFD Target Discriminator ID AVP . . . . . . . . . . . . . . 2
2.1. Encoding Format . . . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Normative References . . . . . . . . . . . . . . . . . . 4
5.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
[RFC7880] defines a simplified mechanism to use Bidirectional
Forwarding Detection (BFD) [RFC5880], referred to as Seamless
Bidirectional Forwarding Detection (S-BFD). The S-BFD mechanism
depends on network nodes knowing the BFD Discriminators that each
node in the network has reserved for this purpose. S-BFD requires
the usage of unique discriminators within an administrative domain.
The use of the Layer Two Tunneling Protocol version 3 (L2TPv3)
[RFC3931] is one possible means of advertising these discriminators.
This document specifies the encoding to be used when S-BFD
Discriminators are advertised using L2TPv3.
1.1. Terminology
The reader is expected to be very familiar with the terminology and
protocol constructs defined in S-BFD (see Section 2 of [RFC7880]) and
L2TPv3 (see Section 1.3 of [RFC3931]).
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 [RFC2119].
2. S-BFD Target Discriminator ID AVP
The S-BFD Target Discriminator Identifier (ID) Attribute Value Pair
(AVP) advertisement is exchanged using the ICRQ (Incoming-Call-Request), ICRP
(Incoming-Call-Reply), OCRQ (Outgoing-Call-Request), and OCRP
(Outgoing-Call-Reply) control messages during session negotiation.
2.1. Encoding Format
The S-BFD Target Discriminator ID AVP, Attribute Type 102, is an
identifier used to advertise the S-BFD target discriminator(s)
supported by an L2TP Control Connection Endpoint (LCCE) for the S-BFD
reflector operation. This AVP indicates that the advertiser
implements an S-BFD reflector supporting the specified target
discriminator(s) and is ready for S-BFD reflector operation. The
receiving LCCE MAY use this AVP if it wants to monitor connectivity
to the advertising LCCE using S-BFD.
The Attribute Value field for this AVP has the following format:
S-BFD Target Discriminator ID (ICRQ, ICRP, OCRQ, OCRP):
No. of octets
+-----------------------------+
| Discriminator Value(s) | 4/Discriminator
: :
+-----------------------------+
An LCCE MAY include the S-BFD Target Discriminator ID AVP
advertisement in an L2TP control message (ICRQ, ICRP, OCRQ, OCRP)
[RFC3931]. If the other LCCE does not wish to monitor connectivity
using S-BFD, it MAY safely discard this AVP without affecting the
rest of session negotiation. While [RFC7880] concerns itself with
the advertisement of only one discriminator unless the mapping of
discriminators to entities is specified, the AVP encoding allows the
specification of an arbitrary number of S-BFD Discriminators (at
least one) for extensibility.
When an LCCE uses the S-BFD Target Discriminator ID AVP
advertisement, multiple S-BFD Discriminators MAY be included, and at
least one S-BFD Discriminator MUST be included. When one S-BFD
Discriminator is advertised, such an S-BFD Discriminator is
associated with the L2TPv3 session. When multiple S-BFD
Discriminators are advertised, how a given discriminator is mapped to
a specific use case is out of scope for this document.
The S-BFD Target Discriminator ID AVP allows for advertising at least
one S-BFD Discriminator value:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discriminator 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discriminator 2 (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discriminator n (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The M bit of the L2TP control message (ICRQ, ICRP, OCRQ, OCRP)
[RFC3931] MUST NOT be set inside the S-BFD Target Discriminator
ID
AVP advertisement. AVP.
3. IANA Considerations
IANA maintains the "Control Message Attribute Value Pairs"
sub-registry as per [RFC3438]. IANA has assigned the following value
to the S-BFD Target Discriminator ID:
Control Message Attribute Value Pairs
Attribute
Type Description
----------- ------------------
102 S-BFD Target Discriminator ID
4. Security Considerations
Security concerns for L2TP are addressed in [RFC3931]. The
introduction of the S-BFD Target Discriminator ID AVP advertisement
introduces no new security risks for L2TP.
Advertising the S-BFD Discriminators makes it possible for attackers
to initiate S-BFD sessions using the advertised information. The
vulnerabilities this poses and how to mitigate them are discussed in
the Security Considerations section of [RFC7880].
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA) Considerations
Update", BCP 68, RFC 3438, DOI 10.17487/RFC3438, December
2002, <http://www.rfc-editor.org/info/rfc3438>.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC3931, March 2005,
<http://www.rfc-editor.org/info/rfc3931>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, June 2016,
<http://www.rfc-editor.org/info/rfc7880>.
5.2. Informative References
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.
Acknowledgements
The authors would like to thank Nobo Akiya, Stewart Bryant, and Pawel
Sowinski for providing core inputs for the document, performing
thorough reviews, and providing a number of comments. The authors
would also like to thank Nagendra Kumar for his reviews.
Contributors
Mallik Mudigonda
Cisco Systems, Inc.
Email: mmudigon@cisco.com
Authors' Addresses
Vengada Prasad Govindan
Cisco Systems, Inc.
Email: venggovi@cisco.com
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com