Network Working Group                                    M. Jethanandani
Internet-Draft                                            Kloud Services
Updates: 5880 (if approved)                                   S. Agarwal
Intended status: Standards Track                      Cisco Systems, Inc
Expires: February 6, 2021                                      A. Mishra
                                                            O3b Networks
                                                               A. Saxena
                                                       Ciena Corporation
                                                                A. Dekok
                                                     Network RADIUS SARL
                                                          August 5, 2020

                      Secure BFD Sequence Numbers


   This document describes a security enhancement for the sequence
   number used in BFD control packets.  This document updates RFC 5880.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 6, 2021.

Copyright Notice

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Theory of operation . . . . . . . . . . . . . . . . . . . . .   2
   4.  Impact of using a hash  . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 the BFD
   authentication section 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.  Since the monotonically
   increasing sequence number does not appear on the wire, it is
   difficult for a third party to launch a replay attack.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Theory of operation

   Instead of inserting a monotonically, sometimes occasionally,
   increasing sequence number in BFD control packets, a hash is
   inserted.  The hash is computed, using a shared key, on the sequence
   number.  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

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   computing an authenticated packet would be this new computed hash.
   Even though the BFD Authentication
   [I-D.ietf-bfd-optimizing-authentication] sequence number is
   independent of this enhancement, it would benefit by using the
   computed hash.

   As currently defined in BFD [RFC5880], a BFD packet with
   authentication will undergo the following steps, where:

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

   [S]: pseudo random sequence number

   [A]: Authentication

                   Sender                    Receiver

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

   This document proposes that for enhanced security in sequence number
   encoding, 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 document.
   Instead of using the sequence number, the sender encodes the sequence
   number with the hashing key to produce a hash.

   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 sequence 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 used in determining Local
   Discriminator value for the session or by using a random number.

   k: hashing key

   s: sequence number

   O: original RFC 5880 packet with monotonically increasing sequence

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   R: remainder of packet

   H1: hash of s

   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) > previously received s ? Good sequence number : bad
   sequence number

                    Sender                Receiver

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

   The above diagram describes how the sender encodes and receiver
   decodes the sequence number.  The sender starts by taking the
   monotonically increasing sequence number and hashing it.  It replaces
   the sequence number with the hash.  It then calculates the hash for
   the entire packet and appends the hash value to the end of the
   packet, before transmitting it.

   The receiver hashes the entire packet without H2, and compares the
   hash value with the received hash (H2).  If the hash values are
   equal, it is a good packet, else it is a bad packet.  It then
   calculates the hash on the received sequence number to retreive s.
   If it is greater than the previously received monotically increasing
   sequence number, then the receiver knows it's a valid sequence

4.  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.

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5.  IANA Considerations

   This document makes no request of IANA.

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

6.  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 stability of the BFD session.
   It is, therefore, recommended to administratively 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.

   The hashing algorithm and key size will determine the difficulty for
   an attacker to decipher the key from the transmitted BFD frames.  The
   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.

7.  Acknowledgements

8.  References

8.1.  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,

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8.2.  Informative References

              Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,
              "Optimizing BFD Authentication", draft-ietf-bfd-
              optimizing-authentication-11 (work in progress), July

Authors' Addresses

   Mahesh Jethanandani
   Kloud Services


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


   Ashesh Mishra
   O3b Networks


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


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


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