Network Working Group                                    M. Jethanandani
Internet-Draft                                            Kloud Services
Updates: 5880 (if approved)                                    A. Mishra
Intended status: Standards Track                            SES Networks
Expires: January 29, 2021                                      A. Saxena
                                                       Ciena Corporation
                                                               M. Bhatia
                                                           July 28, 2020

                     Optimizing BFD Authentication


   This document describes an optimization to BFD Authentication as
   described in Section 6.7 of BFD RFC 5880.  This document updates RFC

Status of This Memo

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   This Internet-Draft will expire on January 29, 2021.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Authentication Mode . . . . . . . . . . . . . . . . . . . . .   4
   3.  NULL Auth Type  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Authenticating every BFD [RFC5880] control packet with a Simple
   Password, or with a MD5 Message-Digest Algorithm [RFC1321] , or
   Secure Hash Algorithm (SHA-1) algorithms is a computationally
   intensive process.  This makes it difficult, if not impossible to
   authenticate every packet - particularly at faster rates.  Also, the
   recent escalating series of attacks on MD5 and SHA-1 described in
   Finding Collisions in the Full SHA-1 [SHA-1-attack1] and New
   Collision Search for SHA-1 [SHA-1-attack2] raise concerns about their
   remaining useful lifetime as outlined in Updated Security
   Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithm
   [RFC6151] and Security Considerations for the SHA-0 and SHA-1
   Message-Digest Algorithm [RFC6194].  If replaced by stronger
   algorithms, the computational overhead, will make the task of
   authenticating every packet even more difficult to achieve.

   This document proposes that only BFD control packets that signal a
   state change, a demand mode change (to D bit) or a poll sequence
   change (P or F bit change) in a BFD control packet be categorized as
   a significant change.  This document also proposes that all BFD
   control packets which signal a significant change MUST be
   authenticated if the session's bfd.AuthType is non-zero.  Other BFD
   control packets MAY be transmitted and received without the A bit

   Most packets that are transmitted and received have no state change
   associated with them.  Limiting authentication to packets that affect
   a BFD session state allows more sessions to be supported with this
   optimized method of authentication.  Moreover, most BFD control

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   packets that signal a significant change are generally transmitted at
   a slower interval of 1s, leaving enough time to compute the hash.

   To detect a Man In the Middle (MITM) attack, it is also proposed that
   a BFD control packet without a significant change be authenticated
   occasionally.  The interval of the BFD control packets without a
   significant change can be configured depending on the detect
   multiplier and the capability of the system.  As an example, this
   could be equal to the detect multiplier number of packets.

   The rest of the document is structured as follows.  Section 2 talks
   about the changes to authentication mode as described in BFD
   [RFC5880].  Section 3 goes into the details of the new Authentication

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown

1.2.  Terminology

   The following terms used in this document have been defined in BFD

   o  Detect Multiplier

   o  Detection Time

   The following terms are introduced in this document.

   | Term         | Meaning                                            |
   | significant  | State change, a demand model change (to D bit) or  |
   | change       | a poll sequence change (P or F bit).               |
   |              |                                                    |
   | configured   | Interval at which BFD control packets are          |
   | interval     | authenticated in the UP state.                     |

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2.  Authentication Mode

   The cryptographic authentication mechanisms specified in BFD
   [RFC5880] describes enabling and disabling of authentication as a one
   time operation.  As a security precaution, it mentions that
   authentication state be allowed to change at most once.  Once
   enabled, every packet must have Authentication Bit set and the
   associated Authentication Type appended.  In addition, it states that
   an implementation SHOULD NOT allow the authentication state to be
   changed based on the receipt of a BFD control packet.

   This document proposes that the authentication mode be modified to be
   enabled on demand.  Instead of authenticating every packet, BFD peers
   are configured for which packets need to be authenticated, and
   authenticate only those packets.  Rest of the packets can be
   transmitted and received without authentication.  For example, the
   two ends can be configured such that BFD control packets that
   indicate a significant change should be authenticated and enable
   authentication on those packets only.  If the two ends have
   previously been configured as such, but at least one side decides not
   to authenticate a significant change packet, then the BFD session
   will fail to come up.

   This proposal outlines which BFD control packets need to be
   authenticated (carry the A-bit), and which packets can be transmitted
   or received without authentication enabled.  A BFD control packet
   that fails authentication is discarded, or a BFD control packet that
   was supposed to be authenticated, but was not, e.g. a significant
   change packet, is discarded.  However, there is no change to the
   state machine for BFD, as the decision of a significant change is
   still decided by how many valid consecutive packets were received,
   authenticated or otherwise.

   The following table summarizes when the A bit should be set.  The
   table should be read with the column indicating the BFD state the
   receiver is currently in, and the row indicating the BFD state the
   receiver might transition to based on the BFD control packet
   received.  The interesection of the two indicates whether the
   received BFD control packet should have the A bit set (Auth), no
   authentication is needed (NULL), most packets are NULL AUTH (Select)
   or the state transition is not applicable.  The BFD state refers to
   the states in BFD state machine described in Section 6.2 of BFD

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          Read   : On state change from <column> to <row>
          Auth   : Authenticate BFD control packet
          NULL   : No Authentication. Use NULL AUTH Type.
          n/a    : Invalid state transition.
          Select : Most packets NULL AUTH. Selective (periodic)
                   packets authenticated.
         |        | DOWN   | INIT   | UP     |
         | DOWN   |  NULL  |  Auth  |  Auth  |
         | INIT   |  Auth  |  NULL  |  n/a   |
         | UP     |  Auth  |  Auth  | Select |

                       Optimized Authentication Map

   If P or F bit changes value, the BFD control packet MUST be
   authenticated.  If the D bit changes value, the BFD control packet
   MUST be authenticated.

   All packets already carry the sequence number.  The NULL AUTH packets
   MUST contain the Type specified in Section 3.  This enables a
   monotonically increasing sequence number to be carried in each
   packet, and prevents man-in-the-middle from capturing and replaying
   the same packet again.  Since all packets still carry a sequence
   number, the logic for sequence number maintenance remains unchanged
   from BFD [RFC5880].  If at a later time, a different scheme is
   adopted for changing sequence number, e.g.  Secure BFD Sequence
   Numbers [I-D.ietf-bfd-secure-sequence-numbers], this method can use
   the updated scheme without any impact.

   Most packets transmitted on a BFD session are BFD UP packets.
   Authenticating a small subset of these packets, for example, a detect
   multiplier number of packets per configured interval, significantly
   reduces the computational demand for the system while maintaining
   security of the session across the configured interval.  A minimum of
   Detect Multiplier packets MUST be transmitted per configured
   interval.  This ensures that the BFD session should see at least one
   authenticated packet during that interval.

3.  NULL Auth Type

   This section describes a new Authentication Type as:

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        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
       |   Auth Type   |   Auth Len    |  Auth Key ID  |   Reserved    |
       |                        Sequence Number                        |

                              NULL Auth Type


   Auth Type: The Authentication Type, which in this case is TBD (NULL,
   to be assigned by IANA)

   Auth Len: The length of the NULL Auth Type, in bytes i.e. 8 bytes

   Auth Key ID: The authentication key ID in use for this packet.  Must
   be set to zero.

   Reserved: This byte MUST be set to zero on transmit and ignored on

   Sequence Number: The sequence number for this packet.  Implementation
   may use sequence numbers (bfd.XmitAuthSeq) as defined in BFD
   [RFC5880], or secure sequence numbers as defined in Secure BFD
   Sequence Numbers [I-D.ietf-bfd-secure-sequence-numbers].

   The NULL Auth Type must be used for all packets that are not
   authenticated.  This protects against replay-attacks by allowing the
   session to maintain an incrementing sequence number for all packets
   (authenticated and un-authenticated).

   In the future, if a new scheme is adopted for changing the sequence
   number, this method can adopt the new scheme without any impact.

4.  IANA Considerations

   This document requests an update to the registry titled "BFD
   Authentication Types".  IANA is requested to assign a new BFD Auth
   Type for "NULL" (see Section 3).

   Note to RFC Editor: this section may be removed on publication as an

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5.  Security Considerations

   The approach described in this document enhances the ability to
   authenticate a BFD session by taking away the onerous requirement
   that every BFD control packet be authenticated.  By authenticating
   packets that affect the state of the session, the security of the BFD
   session is maintained.  In this mode, packets that are a significant
   change but are not authenticated, are dropped by the system.
   Therefore, a malicious user that tries to inject a non-authenticated
   packet, e.g. with a Down state to take a session down will fail.
   That combined with the proposal of using sequence number defined in
   Secure BFD Sequence Numbers [I-D.ietf-bfd-secure-sequence-numbers]
   further enhances the security of BFD sessions.

6.  References

6.1.  Normative References

              Jethanandani, M., Agarwal, S., Mishra, A., Saxena, A., and
              A. DeKok, "Secure BFD Sequence Numbers", draft-ietf-bfd-
              secure-sequence-numbers-05 (work in progress), February

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

6.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,

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   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,

              Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
              Full SHA-1", 2005.

              Wang, X., Yao, A., and F. Yao, "New Collision Search for
              SHA-1", 2005.

Authors' Addresses

   Mahesh Jethanandani
   Kloud Services


   Ashesh Mishra
   SES Networks


   Ankur Saxena
   Ciena Corporation
   3939 N 1st Street
   San Jose, CA  95134


   Manav Bhatia


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