Network Working Group                                    M. JethanandaniInternet-Draft                                            Kloud ServicesUpdates: 5880 (if approved)                                   S. AgarwalIntended status: Standards Track                      Cisco Systems, IncExpires: February 6, 2021                                      A. Mishra                                                            O3b Networks                                                               A. Saxena                                                       Ciena Corporation                                                                A. Dekok                                                     Network RADIUS SARL                                                          August 5, 2020                      Secure BFD Sequence Numbers               draft-ietf-bfd-secure-sequence-numbers-06Abstract   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   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at https://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on February 6, 2021.Copyright Notice   Copyright (c) 2020 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   (https://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 respectJethanandani, et al.    Expires February 6, 2021                [Page 1]Internet-Draft        Securing next sequence number          August 2020   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  . . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . . . . . . . . . .   61.  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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   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 inJethanandani, et al.    Expires February 6, 2021                [Page 2]Internet-Draft        Securing next sequence number          August 2020   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   number   [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   numberJethanandani, et al.    Expires February 6, 2021                [Page 3]Internet-Draft        Securing next sequence number          August 2020   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   number.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.Jethanandani, et al.    Expires February 6, 2021                [Page 4]Internet-Draft        Securing next sequence number          August 20205.  IANA Considerations   This document makes no request of IANA.   Note to RFC Editor: this section may be removed on publication as an   RFC.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.  Acknowledgements8.  References8.1.  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,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,              <https://www.rfc-editor.org/info/rfc5880>.Jethanandani, et al.    Expires February 6, 2021                [Page 5]Internet-Draft        Securing next sequence number          August 20208.2.  Informative References   [I-D.ietf-bfd-optimizing-authentication]              Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,              "Optimizing BFD Authentication", draft-ietf-bfd-              optimizing-authentication-11 (work in progress), July              2020.Authors' Addresses   Mahesh Jethanandani   Kloud Services   Email: mjethanandani@gmail.com   Sonal Agarwal   Cisco Systems, Inc   170 W. Tasman Drive   San Jose, CA  95070   USA   Email: agarwaso@cisco.com   URI:   www.cisco.com   Ashesh Mishra   O3b Networks   Email: mishra.ashesh@gmail.com   Ankur Saxena   Ciena Corporation   3939 North First Street   San Jose, CA  95134   USA   Email: ankurpsaxena@gmail.com   Alan DeKok   Network RADIUS SARL   100 Centrepointe Drive #200   Ottowa, ON  K2G 6B1   Canada   Email: aland@freeradius.orgJethanandani, et al.    Expires February 6, 2021                [Page 6]