Advertising Seamless Bidirectional Forwarding Detection (S-BFD) Discriminators in Layer 2 Tunneling Protocol, Version 3 (L2TPv3)

defines a simplified mechanism to use Bidirectional Forwarding Detection (BFD) , 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 2 Tunneling Protocol, Version 3 (L2TPv3) is one possible means of advertising these discriminators. This document specifies the encoding to be used when S-BFD discriminators are advertised using L2TPv3.

The reader is expected to be very familiar with the terminology and protocol constructs defined in S-BFD (see Section 2 of ) and L2TPv3 (see Section 1.3 of ).

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.

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):

An LCCE MAY include the S-BFD Discriminator Advertisement AVP in a L2TP Control Protocol message (ICRQ, ICRP, OCRQ, OCRP) . 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 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:

The M bit of the L2TP Control Protocol Message (ICRQ, ICRP, OCRQ, OCRP) MUST NOT be set inside the S-BFD Target Discriminator ID AVP advertisement.

IANA maintains a sub-registry "Message Type AVP (Attribute Type 0) Values" in the "Control Message Attribute Value Pairs" as per . 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

Security concerns for L2TP are addressed in . 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 .

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.

Mallik Mudigonda Cisco Systems, Inc. Email: mmudigon@cisco.com