Relaxed Packet Counter Verification for Babel MAC Authentication
draft-ietf-babel-mac-relaxed-02

Network Working Group                                      J. Chroboczek
Internet-Draft                            IRIF, University of Paris-Cité
Updates: 8967 (if approved)                         T. Høiland-Jørgensen
Intended status: Standards Track                                 Red Hat
Expires: 2 March 2023                                     29 August 2022

    Relaxed Packet Counter Verification for Babel MAC Authentication
                    draft-ietf-babel-mac-relaxed-02

Abstract

   This document relaxes packet verification rules defined in the Babel
   MAC Authentication protocol in order to make it more robust in the
   presence of packet reordering.

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   This Internet-Draft will expire on 2 March 2023.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   3
   3.  Relaxing PC validation  . . . . . . . . . . . . . . . . . . .   3
     3.1.  Multiple highest PC values  . . . . . . . . . . . . . . .   3
       3.1.1.  Generalisations . . . . . . . . . . . . . . . . . . .   4
     3.2.  Window-based validation . . . . . . . . . . . . . . . . .   5
     3.3.  Combining the two techniques  . . . . . . . . . . . . . .   6
   4.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Normative references  . . . . . . . . . . . . . . . . . . . .   6
   7.  Informative references  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The design of the Babel MAC authentication mechanism [RFC8967]
   assumes that packet reordering is an exceptional occurrence, and the
   protocol drops any packets that arrive out-of-order.  This assumption
   is generally correct on wired links, but turns out to be incorrect on
   some kinds of wireless links.

   In particular, IEEE 802.11 (Wi-Fi) defines a number of power-saving
   modes that allow stations (mobile nodes) to switch their radio off
   for extended periods of time, ranging in the hundreds of
   milliseconds.  The access point (network switch) buffers all
   multicast packets, and only sends them out after the power-saving
   interval ends.  The result is that multicast packets are delayed by
   up to a few hundred milliseconds with respect to unicast packets,
   which, under some traffic patterns, causes the PC verification
   procedure in RFC 8967 to systematically fail for multicast packets.

   This document defines two ways to relax the PC validation: using two
   separate receiver-side states, one for unicast and one for multicast
   packets (Section 3.1), which allows arbitrary reordering between
   unicast and multicast packets, and using a window of previously
   received PC values (Section 3.2), which allows a bounded amount of
   reordering between arbitrary packets.  Usage of the former is
   RECOMMENDED, while usage of the latter is OPTIONAL.  The two MAY be
   used simultaneously (Section 3.3).  This document updates RFC 8967.

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2.  Specification of Requirements

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

3.  Relaxing PC validation

   The Babel MAC authentication mechanism prevents replay by decorating
   every sent packet with a strictly increasing value, the Packet
   Counter (PC).  Notwithstanding the name, the PC does not actually
   count packets: a sender is permitted to increment the PC by more than
   one between two packets.

   A receiver maintains the highest PC received from each neighbour.
   When a new packet is received, the receiver compares the PC contained
   in the packet with the highest received PC; if the new value is
   smaller or equal, the packet is discarded; otherwise, the packet is
   accepted, and the highest PC value for that neighbour is updated.

   Note that there does not exist a one-to-one correspondence between
   sender states and receiver states: multiple receiver states track a
   single sender state.  The receiver states corresponding to single
   sender state are not necessarily identical, since only a subset of
   receiver states are updated when a packet is sent to a unicast
   address or when a multicast packet is received by a subset of the
   receivers.

3.1.  Multiple highest PC values

   Instead of a single highest PC value maintained for each neighbour,
   an implementation of the procedure described in this section uses two
   values, the highest unicast PC and the highest multicast PC.  More
   precisely, the (Index, PC) pair contained in the Neighbour
   Table (Section 3.2 of [RFC8967]) is replaced by:

   *  a triple (Index, PCm, PCu), where Index is an arbitrary string of
      0 to 32 octets, and PCm and PCu are 32-bit (4-octet) integers.

   When a challenge reply is successful, both highest PC values are
   updated to the value contained in the PC TLV from the packet
   containing the successful challenge.  More precisely, the last
   sentence of the fourth bullet point of Section 4.3 of [RFC8967] is
   replaced by:

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   *  If the packet contains a successful Challenge Reply, then the
      Index contained in the PC TLV MUST be stored in the Index field of
      the Neighbour Table entry corresponding to the sender, and the PC
      contained in the TLV MUST be stored in both the PCm and PCu fields
      of the Neighbour Table entry.

   When a packet that does not contain a successful challenge reply is
   received, the PC value that it contains is compared to either the PCm
   or the PCu field of the corresponding neighbour entry, depending on
   whether the packet was sent to a unicast or a multicast address.  If
   the comparison is successful, then the same value (PCm or PCu) is
   updated.  More precisely, the last bullet point of Section 4.3 of
   [RFC8967] is replaced by:

   *  At this stage, the packet contains no successful challenge reply
      and the Index contained in the PC TLV is equal to the Index in the
      Neighbour Table entry corresponding to the sender.  The receiver
      compares the received PC with either PCm field (if the packet was
      sent to a multicast address) or the PCu field (otherwise) in the
      Neighbour Table; if the received PC is smaller or equal than the
      value contained in the Neighbour Table, the packet MUST be dropped
      and processing stops (no challenge is sent in this case, since the
      mismatch might be caused by harmless packet reordering on the
      link).  Otherwise, the PCm (if the packet was sent to a multicast
      address) or the PCu (otherwise) field contained in the Neighbour
      Table entry is set to the received PC, and the packet is accepted.

3.1.1.  Generalisations

   Modern networking hardware tends to maintain more than just two
   queues, and it might be tempting to generalise the approach taken to
   more than just two last PC values.  For example, one might be tempted
   to use distinct last PC values for packets received with different
   values of the Type of Service (ToS) field, or with different IEEE
   802.11e access categories.  However, choosing a highest PC field by
   consulting a value that is not protected by the MAC (Section 4.1 of
   [RFC8967]) would no longer protect against replay.  In practice, this
   means that only the destination address and port number and data
   stored in the packet body may be used for choosing the highest PC
   value, since these are the only fields that are protected by the MAC
   (in addition to the source address and port number, which are already
   used when choosing the Neighbour Table entry and therefore provide no
   additional information).

   The following example shows why it would be unsafe to select the
   highest PC depending on the ToS field.  Suppose that a node B were to
   maintain distinct highest PC values for different values T1 and T2 of
   the ToS field, and that initially all of the highest PC fields at B

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   have value 42.  Suppose now that a node A sends a packet P1 with ToS
   equal to T1 and PC equal to 43; when B receives the packet, it sets
   the highest PC value associated with ToS T1 to 43.  If an attacker
   were now to send an exact copy of P1 but with ToS equal to T2, B
   would consult the highest PC value associated with T2, which is still
   equal to 42, and accept the replayed packet.

3.2.  Window-based validation

   Window-based validation is similar to what is described in
   Section 3.4.3 of [RFC4303].  When using window-based validation, in
   addition to retaining within its neighbour table the highest PC value
   PCh seen from every neighbour, an implementation maintains a fixed-
   size window of booleans corresponding to PC values directly below
   PCh.  More precisely, the (Index, PC) pair contained in the Neighbour
   Table (Section 3.2 of [RFC8967]) is replaced by:

   *  a triple (Index, PCh, Window), where Index is an arbitrary string
      of 0 to 32 octets, PCh is a 32-bit (4-octet) integer, and Window
      is a vector of booleans of size S (the default value S=128 is
      RECOMMENDED).

   The window is a vector of S boolean values numbered from 0 (the "left
   edge" of the window) up to S-1 (the "right edge"); the boolean
   associated with the index i indicates whether a packet with PC value
   (PCh - (S-1) + i) has been seen before.  Shifting the window to the
   left by an integer amount k is defined as moving all values so that
   the value previously at index n is now at index (n - k); k values are
   discarded at the left edge, and k new unset values are inserted at
   the right edge.

   Whenever a packet is received, the receiver computes its _index_ i =
   (PC - PCh + S - 1).  It then proceeds as follows:

   1.  If the index i is negative, the packet is considered too old, and
       MUST be discarded.

   2.  If the index i is non-negative and strictly less than the window
       size S, the window value at the index is checked; if this value
       is already set, the received PC has been seen before and the
       packet MUST be discarded.  Otherwise, the corresponding window
       value is marked as set, and the packet is accepted.

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   3.  If the index i is larger or equal to the window size (i.e., PC is
       strictly larger than PCh), the window MUST be shifted to the left
       by (i - S + 1) values (or, equivalently, by the difference PC -
       PCh) and the highest PC value PCh MUST be set to the received PC.
       The value at the right of the window (the value with index S - 1)
       MUST be set, and the packet is accepted.

   When receiving a successful Challenge Reply, the remembered highest
   PC value PCh MUST be set to the value received in the challenge
   reply, and all of the values in the window MUST be reset except the
   value at index S - 1, which MUST be set.

3.3.  Combining the two techniques

   The two techniques described above serve complementary purposes:
   splitting the state allows multicast packets to be reordered with
   respect to unicast ones by an arbitrary number of PC values, while
   the window-based technique allows arbitrary packets to be reordered
   but only by a bounded number of PC values.  Thus, they can profitably
   be combined.

   An implementation that uses both techniques MUST maintain, for every
   entry of the Neighbour table, two distinct windows, one for multicast
   and one for unicast packets.  When a successful challenge reply is
   received, both windows MUST be reset.  When a packet that does not
   contain a challenge reply is received, then if the packet's
   destination address is a multicast address, the multicast window MUST
   be consulted and possibly updated, as described in Section 3.2;
   otherwise, the unicast window MUST be consulted and possibly updated.

4.  Security considerations

   The procedures described in this document do not change the security
   properties described in Section 1.2 of RFC 8967.  While they do
   slightly increase the amount of per-neighbour state maintained by
   each node, this increase is marginal (between 4 and 36 octets,
   depending on implementation choices), and should not significantly
   impact the ability of nodes to survive denial-of-service attacks.

5.  Acknowledgments

   The authors are indebted to Daniel Gröber, who first identified
   the problem that the procedures in this document aim to solve.

6.  Normative references

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   [RFC8967]  Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
              Authentication for the Babel Routing Protocol", RFC 8967,
              DOI 10.17487/RFC8967, January 2021,
              <https://www.rfc-editor.org/info/rfc8967>.

   [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/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

7.  Informative references

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

Authors' Addresses

   Juliusz Chroboczek
   IRIF, University of Paris-Cité
   Case 7014
   75205 Paris CEDEX 13
   France
   Email: jch@irif.fr

   Toke Høiland-Jørgensen
   Red Hat
   Email: toke@toke.dk

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