Network Working Group                                    M. Jethanandani
Internet-Draft                                                S. Agarwal
Intended status: Standards Track                      Cisco Systems, Inc
Expires: August 23, 2019                                       A. Mishra
                                                            O3b Networks
                                                               A. Saxena
                                                       Ciena Corporation
                                                                A. Dekok
                                                     Network RADIUS SARL
                                                       February 19, 2019

                      Secure BFD Sequence Numbers


   This document describes a security enhancements for the BFD packet's
   sequence number.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 23, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Theory of operations  . . . . . . . . . . . . . . . . . . . .   2
   3.  Impact of using a hash  . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   BFD [RFC5880] section 6.7 describes the use of monotonically
   incrementing 32-bit sequence numbers for use in authentication of BFD
   packets.  While this method protects against simple replay attacks,
   the monotonically incrementing sequence numbers are predictable and
   vulnerable to more complex attack vectors.  This document proposes
   the use of non-monotonically-incrementing sequence numbers in BFD
   authentication TLVs to enhance the security of BFD sessions.
   Specifically, the document presents a method to generate pseudo-
   random sequence numbers on the frame by algorithmically hashing
   monotonically increasing sequence numbers.  Further security may be
   introduced by resetting un-encrypted sequence to a random value when
   the 32-bit sequence number rolls-over.

2.  Theory of operations

   Instead of monotonically increasing the sequence number or even
   occasionally monotonically increasing the sequence number, the next
   sequence number is generated by computing a hash on what would have
   been the next sequence number using a shared key.  That computed hash
   is then inserted into the sequence number field of the packet.  In
   case of BFD Authentication [I-D.ietf-bfd-optimizing-authentication],
   the sequence number used in computing an authenticated packet would
   be this new computed hash.  Even though the BFD Authentication

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   [I-D.ietf-bfd-optimizing-authentication] sequence number is
   independent of this enhancement, it would benefit by using the
   computed hash.

   A normal BFD packet with authentication will undergo the following
   steps, where:

   [O]: original RFC 5880 packet with monotonically increasing sequence

   [S]: psuedo random sequence number

   [A]: Authentication

                   Sender                    Receiver

                   [O] [S] [A] ------------- [A] [S] [O]

   In order to encode a sequence number, the sender would identify a
   hash algorithm (symmetric) that would create a 32 bit hash.  The
   hashing key is provisioned securely on the sender and receiver of the
   BFD session.  The mechanism of provisioning such a key is outside the
   scope of this draft.  Instead of using the sequence number, the
   sender encodes the sequence number with the hashing key to produce a

   Upon receiving the BFD Control packet, the receiver compares the
   received sequence number against the expected sequence number.  The
   mechanism used for comparing is an implementation detail
   (implementations may pre-calculate the expected hashed sequence
   number, or decrypt the received sequence number before comparing
   against expected value).  To tolerate dropped frames, the receiver
   MUST compare the received sequence number against the current
   expected seuqence number (previous received sequence number + 1) and
   N subsequent expected sequence numbers (where N is greater than or
   equal to the detect multiplier).  Note: The first sequence number can
   be obtained using the same logic as the My Discriminator value.

   k: hashing key

   s: sequence number

   O: original RFC 5880 packet with monotonically increasing sequence

   R: remainder of packet

   H1: hash of s

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   H2: hash of entire packet

   A: H2 + insertion in packet

   hash(s, k) = H1

   hash((H1 + R), k) = H2

   hash'((Packet - H2), k) == H2 ? Good packet : bad packet

   hash'(H1, k) == s ? Good sequence number : bad sequence number

                    Sender                Receiver

                    [O] [H1] [A] -------- [A] [H1] [O]

3.  Impact of using a hash

   Under this proposal, every packet's sequence number is encoded within
   a hash.  Therefore there is some impact on the system and its
   performance while encoding/decoding the hash.  As security measures
   go, this enhancement greatly increases the security of the packet
   with or without authentication of the entire packet.

4.  IANA Considerations

   This document makes no request of IANA.

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

5.  Security Considerations

   While the proposed mechanism improves overall security of BFD
   mechanism, the security consderations are listed below:

   Because of the fast rate of BFD sesions and it is difficult to change
   the keys (used for hashing the sequence number) during the operation
   of a BFD session without affecting the stabiluty of the BFD session.
   It is, therefore, recommended to admistratively disable the BFD
   session before changing the keys.  If the keys are not changed, an
   attacker can use a replay attack.

   Using this method allows the BFD end-points to detect a malicious
   packet (the decrypted sequence number will not be in sequence) the
   behavior of the session when such a packet is detected is based on
   the implementation.  A flood of such malicious packets may cause a
   session to report BFD session to be operationally down.

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   The hashing algorithm and key size will determine the difficulty for
   an attacker to decipher the key from the transmitted BFD frames.
   Sequential nature of the payload (sequence numbers) simplifies the
   decoding of the key.  It is, therefore, recommended to use longer
   keys or more secure hashing algorithms.

6.  Acknowledgements

7.  References

7.1.  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,

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

7.2.  Informative References

              Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,
              "Optimizing BFD Authentication", draft-ietf-bfd-
              optimizing-authentication-07 (work in progress), November

Authors' Addresses

   Mahesh Jethanandani
   Cisco Systems, Inc
   170 West Tasman Drive
   San Jose, CA  95070

   Email: mjethanandani@gmail.com

   Sonal Agarwal
   Cisco Systems, Inc
   170 W. Tasman Drive
   San Jose, CA  95070

   Email: agarwaso@cisco.com
   URI:   www.cisco.com

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   Ashesh Mishra
   O3b Networks

   Email: mishra.ashesh@gmail.com

   Ankur Saxena
   Ciena Corporation
   3939 North First Street
   San Jose, CA  95134

   Email: ankurpsaxena@gmail.com

   Alan DeKok
   Network RADIUS SARL
   100 Centrepointe Drive #200
   Ottowa, ON  K2G 6B1

   Email: aland@freeradius.org

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