MAC Authentication for the Babel Routing Protocol
RFC 8967

Document Type RFC - Proposed Standard (January 2021; No errata)
Obsoletes RFC 7298
Authors Clara Do  , Weronika Kolodziejak  , Juliusz Chroboczek 
Last updated 2021-01-11
Replaces draft-do-babel-hmac
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Document shepherd Donald Eastlake
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IESG IESG state RFC 8967 (Proposed Standard)
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Send notices to Donald Eastlake <d3e3e3@gmail.com>
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Internet Engineering Task Force (IETF)                             C. Dô
Request for Comments: 8967                                W. Kolodziejak
Obsoletes: 7298                                            J. Chroboczek
Category: Standards Track              IRIF, University of Paris-Diderot
ISSN: 2070-1721                                             January 2021

           MAC Authentication for the Babel Routing Protocol

Abstract

   This document describes a cryptographic authentication mechanism for
   the Babel routing protocol that has provisions for replay avoidance.
   This document obsoletes RFC 7298.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8967.

Copyright Notice

   Copyright (c) 2021 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 respect
   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
     1.1.  Applicability
     1.2.  Assumptions and Security Properties
     1.3.  Specification of Requirements
   2.  Conceptual Overview of the Protocol
   3.  Data Structures
     3.1.  The Interface Table
     3.2.  The Neighbour Table
   4.  Protocol Operation
     4.1.  MAC Computation
     4.2.  Packet Transmission
     4.3.  Packet Reception
     4.4.  Expiring Per-Neighbour State
   5.  Incremental Deployment and Key Rotation
   6.  Packet Format
     6.1.  MAC TLV
     6.2.  PC TLV
     6.3.  Challenge Request TLV
     6.4.  Challenge Reply TLV
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informational References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   By default, the Babel routing protocol [RFC8966] trusts the
   information contained in every UDP datagram that it receives on the
   Babel port.  An attacker can redirect traffic to itself or to a
   different node in the network, causing a variety of potential issues.
   In particular, an attacker might:

   *  spoof a Babel packet and redirect traffic by announcing a route
      with a smaller metric, a larger sequence number, or a longer
      prefix;

   *  spoof a malformed packet, which could cause an insufficiently
      robust implementation to crash or interfere with the rest of the
      network;

   *  replay a previously captured Babel packet, which could cause
      traffic to be redirected or otherwise interfere with the network.

   Protecting a Babel network is challenging due to the fact that the
   Babel protocol uses both unicast and multicast communication.  One
   possible approach, used notably by the Babel over Datagram Transport
   Layer Security (DTLS) protocol [RFC8968], is to use unicast
   communication for all semantically significant communication, and
   then use a standard unicast security protocol to protect the Babel
   traffic.  In this document, we take the opposite approach: we define
   a cryptographic extension to the Babel protocol that is able to
   protect both unicast and multicast traffic and thus requires very few
   changes to the core protocol.  This document obsoletes [RFC7298].

1.1.  Applicability

   The protocol defined in this document assumes that all interfaces on
   a given link are equally trusted and share a small set of symmetric
   keys (usually just one, and two during key rotation).  The protocol
   is inapplicable in situations where asymmetric keying is required,
   where the trust relationship is partial, or where large numbers of
   trusted keys are provisioned on a single link at the same time.

   This protocol supports incremental deployment (where an insecure
   Babel network is made secure with no service interruption), and it
   supports graceful key rotation (where the set of keys is changed with
   no service interruption).

   This protocol does not require synchronised clocks, it does not
   require persistently monotonic clocks, and it does not require
   persistent storage except for what might be required for storing
   cryptographic keys.

1.2.  Assumptions and Security Properties

   The correctness of the protocol relies on the following assumptions:

   *  that the Message Authentication Code (MAC) being used is
      invulnerable to forgery, i.e., that an attacker is unable to
      generate a packet with a correct MAC without access to the secret
      key;

   *  that a node never generates the same index or nonce twice over the
      lifetime of a key.

   The first assumption is a property of the MAC being used.  The second
   assumption can be met either by using a robust random number
   generator [RFC4086] and sufficiently large indices and nonces, by
   using a reliable hardware clock, or by rekeying often enough that
   collisions are unlikely.

   If the assumptions above are met, the protocol described in this
   document has the following properties:

   *  it is invulnerable to spoofing: any Babel packet accepted as
      authentic is the exact copy of a packet originally sent by an
      authorised node;

   *  locally to a single node, it is invulnerable to replay: if a node
      has previously accepted a given packet, then it will never again
      accept a copy of this packet or an earlier packet from the same
      sender;

   *  among different nodes, it is only vulnerable to immediate replay:
      if a node A has accepted an authentic packet from C, then a node B
      will only accept a copy of that packet if B has accepted an older
      packet from C, and B has received no later packet from C.

   While this protocol makes efforts to mitigate the effects of a denial
   of service attack, it does not fully protect against such attacks.

1.3.  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.

2.  Conceptual Overview of the Protocol

   When a node B sends out a Babel packet through an interface that is
   configured for MAC cryptographic protection, it computes one or more
   MACs (one per key) that it appends to the packet.  When a node A
   receives a packet over an interface that requires MAC cryptographic
   protection, it independently computes a set of MACs and compares them
   to the MACs appended to the packet; if there is no match, the packet
   is discarded.

   In order to protect against replay, B maintains a per-interface
   32-bit integer known as the "packet counter" (PC).  Whenever B sends
   a packet through the interface, it embeds the current value of the PC
   within the region of the packet that is protected by the MACs and
   increases the PC by at least one.  When A receives the packet, it
   compares the value of the PC with the one contained in the previous
   packet received from B, and unless it is strictly greater, the packet
   is discarded.

   By itself, the PC mechanism is not sufficient to protect against
   replay.  Consider a peer A that has no information about a peer B
   (e.g., because it has recently rebooted).  Suppose that A receives a
   packet ostensibly from B carrying a given PC; since A has no
   information about B, it has no way to determine whether the packet is
   freshly generated or a replay of a previously sent packet.

   In this situation, peer A discards the packet and challenges B to
   prove that it knows the MAC key.  It sends a "Challenge Request", a
   TLV containing a unique nonce, a value that has never been used
   before and will never be used again.  Peer B replies to the Challenge
   Request with a "Challenge Reply", a TLV containing a copy of the
   nonce chosen by A, in a packet protected by MAC and containing the
   new value of B's PC.  Since the nonce has never been used before, B's
   reply proves B's knowledge of the MAC key and the freshness of the
   PC.

   By itself, this mechanism is safe against replay if B never resets
   its PC.  In practice, however, this is difficult to ensure, as
   persistent storage is prone to failure, and hardware clocks, even
   when available, are occasionally reset.  Suppose that B resets its PC
   to an earlier value and sends a packet with a previously used PC n.
   Peer A challenges B, B successfully responds to the challenge, and A
   accepts the PC equal to n + 1.  At this point, an attacker C may send
   a replayed packet with PC equal to n + 2, which will be accepted by
   A.

   Another mechanism is needed to protect against this attack.  In this
   protocol, every PC is tagged with an "index", an arbitrary string of
   octets.  Whenever B resets its PC, or whenever B doesn't know whether
   its PC has been reset, it picks an index that it has never used
   before (either by drawing it randomly or by using a reliable hardware
   clock) and starts sending PCs with that index.  Whenever A detects
   that B has changed its index, it challenges B again.

   With this additional mechanism, this protocol is invulnerable to
   replay attacks (see Section 1.2).

3.  Data Structures

   Every Babel node maintains a set of conceptual data structures
   described in Section 3.2 of [RFC8966].  This protocol extends these
   data structures as follows.

3.1.  The Interface Table

   Every Babel node maintains an interface table, as described in
   Section 3.2.3 of [RFC8966].  Implementations of this protocol MUST
   allow each interface to be provisioned with a set of one or more MAC
   keys and the associated MAC algorithms (see Section 4.1 for suggested
   algorithms and Section 7 for suggested methods for key generation).
   In order to allow incremental deployment of this protocol (see
   Section 5), implementations SHOULD allow an interface to be
   configured in a mode in which it participates in the MAC
   authentication protocol but accepts packets that are not
   authenticated.

   This protocol extends each table entry associated with an interface
   on which MAC authentication has been configured with two new pieces
   of data:

   *  a set of one or more MAC keys, each associated with a given MAC
      algorithm;

   *  a pair (Index, PC), where Index is an arbitrary string of 0 to 32
      octets, and PC is a 32-bit (4-octet) integer.

   We say that an index is fresh when it has never been used before with
   any of the keys currently configured on the interface.  The Index
   field is initialised to a fresh index, for example, by drawing a
   random string of sufficient length (see Section 7 for suggested
   sizes), and the PC is initialised to an arbitrary value (typically
   0).

3.2.  The Neighbour Table

   Every Babel node maintains a neighbour table, as described in
   Section 3.2.4 of [RFC8966].  This protocol extends each entry in this
   table with two new pieces of data:

   *  a pair (Index, PC), where Index is a string of 0 to 32 octets, and
      PC is a 32-bit (4-octet) integer;

   *  a Nonce, which is an arbitrary string of 0 to 192 octets, and an
      associated challenge expiry timer.

   The Index and PC are initially undefined, and they are managed as
   described in Section 4.3.  The Nonce and challenge expiry timer are
   initially undefined, and they are used as described in
   Section 4.3.1.1.

4.  Protocol Operation

4.1.  MAC Computation

   A Babel node computes the MAC of a Babel packet as follows.

   First, the node builds a pseudo-header that will participate in MAC
   computation but will not be sent.  If the packet is carried over
   IPv6, the pseudo-header has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          Src address                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Src port            |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                                                               +
   |                         Dest address                          |
   +                                                               +
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |           Dest port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the packet is carried over IPv4, the pseudo-header has the
   following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Src address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Src port            |        Dest address           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |           Dest port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Src address   The source IP address of the packet.

   Src port      The source UDP port number of the packet.

   Dest address  The destination IP address of the packet.

   Src port      The destination UDP port number of the packet.

   The node takes the concatenation of the pseudo-header and the Babel
   packet including the packet header but excluding the packet trailer
   (from octet 0 inclusive up to (Body Length + 4) exclusive) and
   computes a MAC with one of the implemented algorithms.  Every
   implementation MUST implement HMAC-SHA256 as defined in [RFC6234] and
   Section 2 of [RFC2104], SHOULD implement keyed BLAKE2s [RFC7693] with
   128-bit (16-octet) digests, and MAY implement other MAC algorithms.

4.2.  Packet Transmission

   A Babel node might delay actually sending TLVs by a small amount, in
   order to aggregate multiple TLVs in a single packet up to the
   interface MTU (Section 4 of [RFC8966]).  For an interface on which
   MAC protection is configured, the TLV aggregation logic MUST take
   into account the overhead due to PC TLVs (one in each packet) and MAC
   TLVs (one per configured key).

   Before sending a packet, the following actions are performed:

   *  a PC TLV containing the PC and Index associated with the outgoing
      interface MUST be appended to the packet body;

      -  the PC MUST be incremented by a strictly positive amount
         (typically just 1);

      -  if the PC overflows, a fresh index MUST be generated (as
         defined in Section 3.1);

      a node MUST NOT include multiple PC TLVs in a single packet;

   *  for each key configured on the interface, a MAC is computed as
      specified in Section 4.1 and stored in a MAC TLV that MUST be
      appended to the packet trailer (see Section 4.2 of [RFC8966]).

4.3.  Packet Reception

   When a packet is received on an interface that is configured for MAC
   protection, the following steps are performed before the packet is
   passed to normal processing:

   *  First, the receiver checks whether the trailer of the received
      packet carries at least one MAC TLV; if not, the packet MUST be
      immediately dropped and processing stops.  Then, for each key
      configured on the receiving interface, the receiver computes the
      MAC of the packet.  It then compares every generated MAC against
      every MAC included in the packet; if there is at least one match,
      the packet passes the MAC test; if there is none, the packet MUST
      be silently dropped and processing stops at this point.  In order
      to avoid memory exhaustion attacks, an entry in the neighbour
      table MUST NOT be created before the MAC test has passed
      successfully.  The MAC of the packet MUST NOT be computed for each
      MAC TLV contained in the packet, but only once for each configured
      key.

   *  If an entry for the sender does not exist in the neighbour table,
      it MAY be created at this point (or, alternatively, its creation
      can be delayed until a challenge needs to be sent, see below).

   *  The packet body is then parsed a first time.  During this
      "preparse" phase, the packet body is traversed and all TLVs are
      ignored except PC, Challenge Request, and Challenge Reply TLVs.
      When a PC TLV is encountered, the enclosed PC and Index are saved
      for later processing.  If multiple PCs are found (which should not
      happen, see Section 4.2), only the first one is processed, the
      remaining ones MUST be silently ignored.  If a Challenge Request
      is encountered, a Challenge Reply MUST be scheduled, as described
      in Section 4.3.1.2.  If a Challenge Reply is encountered, it is
      tested for validity as described in Section 4.3.1.3, and a note is
      made of the result of the test.

   *  The preparse phase above yields two pieces of data: the PC and
      Index from the first PC TLV, and a bit indicating whether the
      packet contains a successful Challenge Reply.  If the packet does
      not contain a PC TLV, the packet MUST be dropped, and processing
      stops at this point.  If the packet contains a successful
      Challenge Reply, then the PC and Index contained in the PC TLV
      MUST be stored in the neighbour table entry corresponding to the
      sender (which already exists in this case), and the packet is
      accepted.

   *  Otherwise, if there is no entry in the neighbour table
      corresponding to the sender, or if such an entry exists but
      contains no Index, or if the Index it contains is different from
      the Index contained in the PC TLV, then a challenge MUST be sent
      as described in Section 4.3.1.1, the packet MUST be dropped, and
      processing stops at this stage.

   *  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 the PC contained in the neighbour
      table; if the received PC is smaller or equal than the PC
      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 PC contained in the neighbour table entry
      is set to the received PC, and the packet is accepted.

   In the algorithm described above, Challenge Requests are processed
   and challenges are sent before the (Index, PC) pair is verified
   against the neighbour table.  This simplifies the implementation
   somewhat (the node may simply schedule outgoing requests as it walks
   the packet during the preparse phase) but relies on the rate limiting
   described in Section 4.3.1.1 to avoid sending too many challenges in
   response to replayed packets.  As an optimisation, a node MAY ignore
   all Challenge Requests contained in a packet except the last one, and
   it MAY ignore a Challenge Request in the case where it is contained
   in a packet with an Index that matches the one in the neighbour table
   and a PC that is smaller or equal to the one contained in the
   neighbour table.  Since it is still possible to replay a packet with
   an obsolete Index, the rate limiting described in Section 4.3.1.1 is
   required even if this optimisation is implemented.

   The same is true of Challenge Replies.  However, since validating a
   Challenge Reply has minimal additional cost (it is just a bitwise
   comparison of two strings of octets), a similar optimisation for
   Challenge Replies is not worthwhile.

   After the packet has been accepted, it is processed as normal, except
   that any PC, Challenge Request, and Challenge Reply TLVs that it
   contains are silently ignored.

4.3.1.  Challenge Requests and Replies

   During the preparse stage, the receiver might encounter a mismatched
   Index, to which it will react by scheduling a Challenge Request.  It
   might encounter a Challenge Request TLV, to which it will reply with
   a Challenge Reply TLV.  Finally, it might encounter a Challenge Reply
   TLV, which it will attempt to match with a previously sent Challenge
   Request TLV in order to update the neighbour table entry
   corresponding to the sender of the packet.

4.3.1.1.  Sending Challenges

   When it encounters a mismatched Index during the preparse phase, a
   node picks a nonce that it has never used with any of the keys
   currently configured on the relevant interface, for example, by
   drawing a sufficiently large random string of bytes or by consulting
   a strictly monotonic hardware clock.  It MUST then store the nonce in
   the entry of the neighbour table associated to the neighbour (the
   entry might need to be created at this stage), initialise the
   neighbour's challenge expiry timer to 30 seconds, and send a
   Challenge Request TLV to the unicast address corresponding to the
   neighbour.

   A node MAY aggregate a Challenge Request with other TLVs; in other
   words, if it has already buffered TLVs to be sent to the unicast
   address of the neighbour, it MAY send the buffered TLVs in the same
   packet as the Challenge Request.  However, it MUST arrange for the
   Challenge Request to be sent in a timely manner, as any packets
   received from that neighbour will be silently ignored until the
   challenge completes.

   A node MUST impose a rate limitation to the challenges it sends; the
   limit SHOULD default to one Challenge Request every 300 ms and MAY be
   configurable.  This rate limiting serves two purposes.  First, since
   a challenge may be sent in response to a packet replayed by an
   attacker, it limits the number of challenges that an attacker can
   cause a node to send.  Second, it limits the number of challenges
   sent when there are multiple packets in flight from a single
   neighbour.

4.3.1.2.  Replying to Challenges

   When it encounters a Challenge Request during the preparse phase, a
   node constructs a Challenge Reply TLV by copying the Nonce from the
   Challenge Request into the Challenge Reply.  It MUST then send the
   Challenge Reply to the unicast address from which the Challenge
   Request was sent.  A challenge sent to a multicast address MUST be
   silently ignored.

   A node MAY aggregate a Challenge Reply with other TLVs; in other
   words, if it has already buffered TLVs to be sent to the unicast
   address of the sender of the Challenge Request, it MAY send the
   buffered TLVs in the same packet as the Challenge Reply.  However, it
   MUST arrange for the Challenge Reply to be sent in a timely manner
   (within a few seconds) and SHOULD NOT send any other packets over the
   same interface before sending the Challenge Reply, as those would be
   dropped by the challenger.

   Since a Challenge Reply might be caused by a replayed Challenge
   Request, a node MUST impose a rate limitation to the Challenge
   Replies it sends; the limit SHOULD default to one Challenge Reply for
   each peer every 300 ms and MAY be configurable.

4.3.1.3.  Receiving Challenge Replies

   When it encounters a Challenge Reply during the preparse phase, a
   node consults the neighbour table entry corresponding to the
   neighbour that sent the Challenge Reply.  If no challenge is in
   progress, i.e., if there is no Nonce stored in the neighbour table
   entry or the challenge timer has expired, the Challenge Reply MUST be
   silently ignored, and the challenge has failed.

   Otherwise, the node compares the Nonce contained in the Challenge
   Reply with the Nonce contained in the neighbour table entry.  If the
   two are equal (they have the same length and content), then the
   challenge has succeeded and the nonce stored in the neighbour table
   for this neighbour SHOULD be discarded; otherwise, the challenge has
   failed (and the nonce is not discarded).

4.4.  Expiring Per-Neighbour State

   The per-neighbour (Index, PC) pair is maintained in the neighbour
   table, and is normally discarded when the neighbour table entry
   expires.  Implementations MUST ensure that an (Index, PC) pair is
   discarded within a finite time since the last time a packet has been
   accepted.  In particular, unsuccessful challenges MUST NOT prevent an
   (Index, PC) pair from being discarded for unbounded periods of time.

   A possible implementation strategy for implementations that use a
   Hello history (Appendix A of [RFC8966]) is to discard the (Index, PC)
   pair whenever the Hello history becomes empty.  Another
   implementation strategy is to use a timer that is reset whenever a
   packet is accepted and to discard the (Index, PC) pair whenever the
   timer expires.  If the latter strategy is used, the timer SHOULD
   default to a value of 5 minutes and MAY be configurable.

5.  Incremental Deployment and Key Rotation

   In order to perform incremental deployment, the nodes in the network
   are first configured in a mode where packets are sent with
   authentication but not checked on reception.  Once all the nodes in
   the network are configured to send authenticated packets, nodes are
   reconfigured to reject unauthenticated packets.

   In order to perform key rotation, the new key is added to all the
   nodes.  Once this is done, both the old and the new key are sent in
   all packets, and packets are accepted if they are properly signed by
   either of the keys.  At that point, the old key is removed.

   In order to support the procedures described above, implementations
   of this protocol SHOULD support an interface configuration in which
   packets are sent authenticated but received packets are accepted
   without verification, and they SHOULD allow changing the set of keys
   associated with an interface without a restart.

6.  Packet Format

6.1.  MAC TLV

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 16   |    Length     |     MAC...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type      Set to 16 to indicate a MAC TLV.

   Length    The length of the body, in octets, exclusive of the Type
             and Length fields.  The length depends on the MAC algorithm
             being used.

   MAC       The body contains the MAC of the packet, computed as
             described in Section 4.1.

   This TLV is allowed in the packet trailer (see Section 4.2 of
   [RFC8966]) and MUST be ignored if it is found in the packet body.

6.2.  PC TLV

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 17   |    Length     |             PC                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |            Index...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type      Set to 17 to indicate a PC TLV.

   Length    The length of the body, in octets, exclusive of the Type
             and Length fields.

   PC        The Packet Counter (PC), a 32-bit (4-octet) unsigned
             integer that is increased with every packet sent over this
             interface.  A fresh index (as defined in Section 3.1) MUST
             be generated whenever the PC overflows.

   Index     The sender's Index, an opaque string of 0 to 32 octets.

   Indices are limited to a size of 32 octets: a node MUST NOT send a
   TLV with an index of size strictly larger than 32 octets, and a node
   MAY ignore a PC TLV with an index of length strictly larger than 32
   octets.  Indices of length 0 are valid: if a node has reliable stable
   storage and the packet counter never overflows, then only one index
   is necessary, and the value of length 0 is the canonical choice.

6.3.  Challenge Request TLV

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 18   |    Length     |     Nonce...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type      Set to 18 to indicate a Challenge Request TLV.

   Length    The length of the body, in octets, exclusive of the Type
             and Length fields.

   Nonce     The nonce uniquely identifying the challenge, an opaque
             string of 0 to 192 octets.

   Nonces are limited to a size of 192 octets: a node MUST NOT send a
   Challenge Request TLV with a nonce of size strictly larger than 192
   octets, and a node MAY ignore a nonce that is of size strictly larger
   than 192 octets.  Nonces of length 0 are valid: if a node has
   reliable stable storage, then it may use a sequential counter for
   generating nonces that get encoded in the minimum number of octets
   required; the value 0 is then encoded as the string of length 0.

6.4.  Challenge Reply TLV

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 19   |    Length     |     Nonce...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type      Set to 19 to indicate a Challenge Reply TLV.

   Length    The length of the body, in octets, exclusive of the Type
             and Length fields.

   Nonce     A copy of the nonce contained in the corresponding
             Challenge Request.

7.  Security Considerations

   This document defines a mechanism that provides basic security
   properties for the Babel routing protocol.  The scope of this
   protocol is strictly limited: it only provides authentication (we
   assume that routing information is not confidential), it only
   supports symmetric keying, and it only allows for the use of a small
   number of symmetric keys on every link.  Deployments that need more
   features, e.g., confidentiality or asymmetric keying, should use a
   more feature-rich security mechanism such as the one described in
   [RFC8968].

   This mechanism relies on two assumptions, as described in
   Section 1.2.  First, it assumes that the MAC being used is
   invulnerable to forgery (Section 1.1 of [RFC6039]); at the time of
   writing, HMAC-SHA256, which is mandatory to implement (Section 4.1),
   is believed to be safe against practical attacks.

   Second, it assumes that indices and nonces are generated uniquely
   over the lifetime of a key used for MAC computation (more precisely,
   indices must be unique for a given (key, source) pair, and nonces
   must be unique for a given (key, source, destination) triple).  This
   property can be satisfied either by using a cryptographically secure
   random number generator to generate indices and nonces that contain
   enough entropy (64-bit values are believed to be large enough for all
   practical applications) or by using a reliably monotonic hardware
   clock.  If uniqueness cannot be guaranteed (e.g., because a hardware
   clock has been reset), then rekeying is necessary.

   The expiry mechanism mandated in Section 4.4 is required to prevent
   an attacker from delaying an authentic packet by an unbounded amount
   of time.  If an attacker is able to delay the delivery of a packet
   (e.g., because it is located at a Layer 2 switch), then the packet
   will be accepted as long as the corresponding (Index, PC) pair is
   present at the receiver.  If the attacker is able to cause the
   (Index, PC) pair to persist for arbitrary amounts of time (e.g., by
   repeatedly causing failed challenges), then it is able to delay the
   packet by arbitrary amounts of time, even after the sender has left
   the network, which could allow it to redirect or blackhole traffic to
   destinations previously advertised by the sender.

   This protocol exposes large numbers of packets and their MACs to an
   attacker that is able to capture packets; it is therefore vulnerable
   to brute-force attacks.  Keys must be chosen in a manner that makes
   them difficult to guess.  Ideally, they should have a length of 32
   octets (both for HMAC-SHA256 and BLAKE2s), and be chosen randomly.
   If, for some reason, it is necessary to derive keys from a human-
   readable passphrase, it is recommended to use a key derivation
   function that hampers dictionary attacks, such as PBKDF2 [RFC8018],
   bcrypt [BCRYPT], or scrypt [RFC7914].  In that case, only the derived
   keys should be communicated to the routers; the original passphrase
   itself should be kept on the host used to perform the key generation
   (e.g., an administrator's secure laptop computer).

   While it is probably not possible to be immune against denial of
   service (DoS) attacks in general, this protocol includes a number of
   mechanisms designed to mitigate such attacks.  In particular,
   reception of a packet with no correct MAC creates no local Babel
   state (Section 4.3).  Reception of a replayed packet with correct
   MAC, on the other hand, causes a challenge to be sent; this is
   mitigated somewhat by requiring that challenges be rate limited
   (Section 4.3.1.1).

   Receiving a replayed packet with an obsolete index causes an entry to
   be created in the neighbour table, which, at first sight, makes the
   protocol susceptible to resource exhaustion attacks (similarly to the
   familiar "TCP SYN Flooding" attack [RFC4987]).  However, the MAC
   computation includes the sender address (Section 4.1), and thus the
   amount of storage that an attacker can force a node to consume is
   limited by the number of distinct source addresses used with a single
   MAC key (see also Section 4 of [RFC8966], which mandates that the
   source address is a link-local IPv6 address or a local IPv4 address).

   In order to make this kind of resource exhaustion attacks less
   effective, implementations may use a separate table of uncompleted
   challenges that is separate from the neighbour table used by the core
   protocol (the data structures described in Section 3.2 of [RFC8966]
   are conceptual, and any data structure that yields the same result
   may be used).  Implementers might also consider using the fact that
   the nonces included in Challenge Requests and Replies can be fairly
   large (up to 192 octets), which should in principle allow encoding
   the per-challenge state as a secure "cookie" within the nonce itself;
   note, however, that any such scheme will need to prevent cookie
   replay.

8.  IANA Considerations

   IANA has allocated the following values in the Babel TLV Types
   registry:

                 +======+===================+===========+
                 | Type | Name              | Reference |
                 +======+===================+===========+
                 | 16   | MAC               | RFC 8967  |
                 +------+-------------------+-----------+
                 | 17   | PC                | RFC 8967  |
                 +------+-------------------+-----------+
                 | 18   | Challenge Request | RFC 8967  |
                 +------+-------------------+-----------+
                 | 19   | Challenge Reply   | RFC 8967  |
                 +------+-------------------+-----------+

                                 Table 1

9.  References

9.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7693]  Saarinen, M-J., Ed. and J-P. Aumasson, "The BLAKE2
              Cryptographic Hash and Message Authentication Code (MAC)",
              RFC 7693, DOI 10.17487/RFC7693, November 2015,
              <https://www.rfc-editor.org/info/rfc7693>.

   [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/info/rfc8174>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
              <https://www.rfc-editor.org/info/rfc8966>.

9.2.  Informational References

   [BCRYPT]   Niels, P. and D. Mazières, "A Future-Adaptable Password
              Scheme", Proceedings of the FREENIX Track: 1999 USENIX
              Annual Technical Conference, June 1999.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <https://www.rfc-editor.org/info/rfc4987>.

   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, DOI 10.17487/RFC6039, October 2010,
              <https://www.rfc-editor.org/info/rfc6039>.

   [RFC7298]  Ovsienko, D., "Babel Hashed Message Authentication Code
              (HMAC) Cryptographic Authentication", RFC 7298,
              DOI 10.17487/RFC7298, July 2014,
              <https://www.rfc-editor.org/info/rfc7298>.

   [RFC7914]  Percival, C. and S. Josefsson, "The scrypt Password-Based
              Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
              August 2016, <https://www.rfc-editor.org/info/rfc7914>.

   [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              RFC 8018, DOI 10.17487/RFC8018, January 2017,
              <https://www.rfc-editor.org/info/rfc8018>.

   [RFC8968]  Décimo, A., Schinazi, D., and J. Chroboczek, "Babel
              Routing Protocol over Datagram Transport Layer Security",
              RFC 8968, DOI 10.17487/RFC8968, January 2021,
              <https://www.rfc-editor.org/info/rfc8968>.

Acknowledgments

   The protocol described in this document is based on the original HMAC
   protocol defined by Denis Ovsienko [RFC7298].  The use of a pseudo-
   header was suggested by David Schinazi.  The use of an index to avoid
   replay was suggested by Markus Stenberg.  The authors are also
   indebted to Antonin Décimo, Donald Eastlake, Toke Høiland-Jørgensen,
   Florian Horn, Benjamin Kaduk, Dave Taht, and Martin Vigoureux.

Authors' Addresses

   Clara Dô
   IRIF, University of Paris-Diderot
   75205 Paris CEDEX 13
   France

   Email: clarado_perso@yahoo.fr

   Weronika Kolodziejak
   IRIF, University of Paris-Diderot
   75205 Paris CEDEX 13
   France

   Email: weronika.kolodziejak@gmail.com

   Juliusz Chroboczek
   IRIF, University of Paris-Diderot
   Case 7014
   75205 Paris CEDEX 13
   France

   Email: jch@irif.fr