Advertising Seamless Bidirectional Forwarding Detection (S-BFD) Discriminators in Layer Two Tunneling Protocol, Version 3 (L2TPv3)
draft-ietf-l2tpext-sbfd-discriminator-05

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).

Requirements Language

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].

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/.

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This Internet-Draft will expire on October 17, 2016.

Copyright Notice

Copyright (c) 2016 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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. S-BFD Target Discriminator ID AVP
• 2.1. Encoding Format
• 3. IANA Considerations
• 4. Security Considerations
• 5. Acknowledgements
• 6. Contributing Authors
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

[I-D.ietf-bfd-seamless-base] defines a simplified mechanism to use Bidirectional Forwarding Detection (BFD) [RFC5880], referred to as Seamless Bidirectional Forwarding Detection (S-BFD). The S-BFD mechanisms depend on network nodes knowing the BFD discriminators which 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 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 [I-D.ietf-bfd-seamless-base]) and L2TPv3 (see Section 1.3 of [RFC3931]).

2. S-BFD Target Discriminator ID AVP

The "S-BFD Target Discriminator ID" AVP 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 negotiations.

2.1. Encoding Format

The S-BFD Target Discriminator Identifier (ID) Attribute Value Pair (AVP), Attribute Type "TBA by IANA", is an identifier used to advertise the S-BFD Target Discriminator(s) supported by an 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 Discriminator Advertisement AVP in a L2TP Control Protocol 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 [I-D.ietf-bfd-seamless-base] concerns itself with the advertisement of only one discriminator unless the mapping to 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, 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 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 Protocol Message (ICRQ, ICRP, OCRQ, OCRP) [RFC3931] MUST NOT be set inside the S-BFD Target Discriminator ID AVP advertisement.

3. IANA Considerations

IANA maintains a sub-registry "Message Type AVP (Attribute Type 0) Values" in the "Control Message Attribute Value Pairs" as per [RFC3438]. IANA is requested to assign the first free value from this sub-registry as the Message typ AVP for "S-BFD Target Discriminator ID".

A summary of the new AVPs requested for Attribute Type 0 follows:

Control Message Attribute Value Pairs

 
      Attribute
      Type          Description
      -----------   ------------------
      TBA by IANA   S-BFD Target Discriminator ID
   

4. Security Considerations

Security concerns for L2TP are addressed in [RFC3931]. Introduction of the S-BFD Discriminator Advertisement AVP introduces no new security risks for L2TP.

Advertisement of the S-BFD discriminators does make 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 [I-D.ietf-bfd-seamless-base].

5. Acknowledgements

Authors would like to thank Nobo Akiya, Stewart Bryant and Pawel Sowinski for providing core inputs for the document and for performing thorough reviews and providing number of comments. Authors would like to thank Nagendra Kumar for his reviews.

6. Contributing Authors

Mallik Mudigonda
Cisco Systems, Inc.

Email: mmudigon@cisco.com

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses