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Network Working Group                                        D. Ovsienko
Internet-Draft                                                    Yandex
Updates: 6126 (if approved)                               April 18, 2014
Intended status: Experimental
Expires: October 20, 2014


                Babel HMAC Cryptographic Authentication
              draft-ovsienko-babel-hmac-authentication-09

Abstract

   This document describes a cryptographic authentication mechanism for
   Babel routing protocol, updating, but not superseding RFC 6126.  The
   mechanism allocates two new TLV types for the authentication data,
   uses HMAC and is both optional and backward compatible.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 October 20, 2014.

Copyright Notice

   Copyright (c) 2014 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
   (http://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.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
   2.  Cryptographic Aspects  . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Mandatory-to-Implement and Optional Hash Algorithms  . . .  6
     2.2.  Definition of Padding  . . . . . . . . . . . . . . . . . .  7
     2.3.  Cryptographic Sequence Number Specifics  . . . . . . . . .  9
     2.4.  Definition of HMAC . . . . . . . . . . . . . . . . . . . .  9
   3.  Updates to Protocol Data Structures  . . . . . . . . . . . . . 11
     3.1.  RxAuthRequired . . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  LocalTS  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  LocalPC  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  MaxDigestsIn . . . . . . . . . . . . . . . . . . . . . . . 12
     3.5.  MaxDigestsOut  . . . . . . . . . . . . . . . . . . . . . . 12
     3.6.  ANM Table  . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.7.  ANM Timeout  . . . . . . . . . . . . . . . . . . . . . . . 14
     3.8.  Configured Security Associations . . . . . . . . . . . . . 15
     3.9.  Effective Security Associations  . . . . . . . . . . . . . 16
   4.  Updates to Protocol Encoding . . . . . . . . . . . . . . . . . 17
     4.1.  Justification  . . . . . . . . . . . . . . . . . . . . . . 17
     4.2.  TS/PC TLV  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.3.  HMAC TLV . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Updates to Protocol Operation  . . . . . . . . . . . . . . . . 21
     5.1.  Per-Interface TS/PC Number Updates . . . . . . . . . . . . 21
     5.2.  Deriving ESAs from CSAs  . . . . . . . . . . . . . . . . . 23
     5.3.  Updates to Packet Sending  . . . . . . . . . . . . . . . . 25
     5.4.  Updates to Packet Receiving  . . . . . . . . . . . . . . . 27
     5.5.  Authentication-Specific Statistics Maintenance . . . . . . 29
   6.  Implementation Notes . . . . . . . . . . . . . . . . . . . . . 30
     6.1.  Source Address Selection for Sending . . . . . . . . . . . 30
     6.2.  Output Buffer Management . . . . . . . . . . . . . . . . . 31
     6.3.  Optimizations of ESAs Deriving . . . . . . . . . . . . . . 32
     6.4.  Security Associations Duplication  . . . . . . . . . . . . 32
   7.  Network Management Aspects . . . . . . . . . . . . . . . . . . 34
     7.1.  Backward Compatibility . . . . . . . . . . . . . . . . . . 34
     7.2.  Multi-Domain Authentication  . . . . . . . . . . . . . . . 34
     7.3.  Migration to and from Authenticated Exchange . . . . . . . 35
     7.4.  Handling of Authentication Keys Exhaustion . . . . . . . . 36
   8.  Implementation Status  . . . . . . . . . . . . . . . . . . . . 37
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 39
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 43
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     12.2. Informative References . . . . . . . . . . . . . . . . . . 45
   Appendix A.  Figures and Tables  . . . . . . . . . . . . . . . . . 48
   Appendix B.  Test Vectors  . . . . . . . . . . . . . . . . . . . . 52



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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 55


















































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1.  Introduction

   [RFC Editor: before publication please remove the sentence below.]
   Comments are solicited and should be addressed to the author.

   Authentication of routing protocol exchanges is a common mean of
   securing computer networks.  Use of protocol authentication
   mechanisms helps in ascertaining that only the intended routers
   participate in routing information exchange, and that the exchanged
   routing information is not modified by a third party.

   [BABEL] ("the original specification") defines data structures,
   encoding, and the operation of a basic Babel routing protocol
   instance ("instance of the original protocol").  This document ("this
   specification") defines data structures, encoding, and the operation
   of an extension to the Babel protocol, an authentication mechanism
   ("this mechanism").  Both the instance of the original protocol and
   this mechanism are mostly self-contained and interact only at
   coupling points defined in this specification.

   A major design goal of this mechanism is transparency to operators
   that is not affected by implementation and configuration specifics.
   A complying implementation makes all meaningful details of
   authentication-specific processing clear to the operator, even when
   some of the operational parameters cannot be changed.

   The currently established (see [RIP2-AUTH], [OSPF2-AUTH],
   [OSPF3-AUTH], [ISIS-AUTH-A], and [RFC6039]) approach to
   authentication mechanism design for datagram-based routing protocols
   such as Babel relies on two principal data items embedded into
   protocol packets, typically as two integral parts of a single data
   structure:

   o  A fixed-length unsigned integer, typically called a cryptographic
      sequence number, used in replay attack protection.

   o  A variable-length sequence of octets, a result of the HMAC
      construct (see [RFC2104]) computed on meaningful data items of the
      packet (including the cryptographic sequence number) on one hand
      and a secret key on the other, used in proving that both the
      sender and the receiver share the same secret key and that the
      meaningful data was not changed in transmission.

   Depending on the design specifics either all protocol packets are
   authenticated or only those protecting the integrity of protocol
   exchange.  This mechanism authenticates all protocol packets.

   Although the HMAC construct is just one of many possible approaches



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   to cryptographic authentication of packets, this mechanism makes use
   of relevant prior experience by using HMAC too and its solution space
   correlates with the solution spaces of the mechanisms above.  At the
   same time, it allows for a future extension that treats HMAC as a
   particular case of a more generic mechanism.  Practical experience
   with the mechanism defined herein should be useful in designing such
   future extension.

   This specification defines the use of the cryptographic sequence
   number in details sufficient to make replay attack protection
   strength predictable.  That is, an operator can tell the strength
   from the declared characteristics of an implementation and, whereas
   the implementation allows to change relevant parameters, the effect
   of a reconfiguration.

   This mechanism explicitly allows for multiple HMAC results per
   authenticated packet.  Since meaningful data items of a given packet
   remain the same, each such HMAC result stands for a different secret
   key and/or a different hash algorithm.  This enables a simultaneous,
   independent authentication within multiple domains.  This
   specification is not novel in this regard, e.g., L2TPv3 allows for 1
   or 2 ([RFC3931] Section 5.4.1) and MANET protocols allow for several
   ([RFC7183] Section 6.1) results per authenticated packet.

   An important concern addressed by this mechanism is limiting the
   amount of HMAC computations done per authenticated packet,
   independently for sending and receiving.  Without these limits the
   number of computations per packet could be as high as the number of
   configured authentication keys (in the sending case) or as the number
   of keys multiplied by the number of supplied HMAC results (in the
   receiving case).

   These limits establish a basic competition between the configured
   keys and (in the receiving case) an additional competition between
   the supplied HMAC results.  This specification defines related data
   structures and procedures in a way to make such competition
   transparent and predictable for an operator.

   Wherever this specification mentions the operator reading or changing
   a particular data structure, variable, parameter, or event counter
   "at runtime", it is up to the implementor how this is to be done.
   For example, the implementation can employ an interactive CLI, or a
   management protocol such as SNMP, or an inter-process communication
   mean such as a local socket, or a combination of these.







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1.1.  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 BCP 14 [RFC2119].


2.  Cryptographic Aspects

2.1.  Mandatory-to-Implement and Optional Hash Algorithms

   [RFC2104] defines HMAC as a construct that can use any cryptographic
   hash algorithm with a known digest length and internal block size.
   This specification preserves this property of HMAC by defining data
   processing that itself does not depend on any particular hash
   algorithm either.  However, since this mechanism is a protocol
   extension case, there are relevant design considerations to take into
   account.

   Section 4.5 of [RFC6709] suggests selecting one hash algorithm as
   mandatory-to-implement for the purpose of global interoperability
   (Section 3.2 ibid.) and selecting another of distinct lineage as
   recommended for implementation for the purpose of cryptographic
   agility.  This specification makes the latter property guaranteed,
   rather than probable, through an elevation of the requirement level.
   There are two hash algorithms mandatory-to-implement, unambiguously
   defined and generally available in multiple implementations each.

   An implementation of this mechanism MUST include support for two hash
   algorithms:

   o  RIPEMD-160 (160-bit digest)

   o  SHA-1 (160-bit digest)

   Besides that, an implementation of this mechanism MAY include support
   for additional hash algorithms, provided each such algorithm is
   publicly and openly specified and its digest length is 128 bits or
   more (to meet the constraint implied in Section 2.2).  Implementors
   SHOULD consider strong, well-known hash algorithms as additional
   implementation options and MUST NOT consider hash algorithms for that
   by the time of implementation meaningful attacks exist or that are
   commonly viewed as deprecated.

   In the latter case it is important to take into account
   considerations both common (such as those made in [RFC4270]) and
   specific to the HMAC application of the hash algorithm.  E.g.,
   [RFC6151] considers MD5 collisions and concludes that new protocol



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   designs should not use HMAC-MD5, while [RFC6194] includes a
   comparable analysis of SHA-1 that finds HMAC-SHA-1 secure for the
   same purpose.

   For example, the following hash algorithms meet these requirements at
   the time of this writing (in alphabetical order):

   o  GOST R 34.11-94 (256-bit digest)

   o  SHA-224 (224-bit digest, SHA-2 family)

   o  SHA-256 (256-bit digest, SHA-2 family)

   o  SHA-384 (384-bit digest, SHA-2 family)

   o  SHA-512 (512-bit digest, SHA-2 family)

   o  Tiger (192-bit digest)

   o  Whirlpool (512-bit digest, 2nd rev., 2003)

   The set of hash algorithms available in an implementation MUST be
   clearly stated.  When known weak authentication keys exist for a hash
   algorithm used in the HMAC construct, an implementation MUST deny a
   use of such keys.

2.2.  Definition of Padding

   Many practical applications of HMAC for authentication of datagram-
   based network protocols (including routing protocols) involve the
   padding procedure, a design-specific conditioning of the message that
   both the sender and the receiver perform before the HMAC computation.
   Specific padding procedure of this mechanism addresses the following
   needs:

   o  Data Initialization

      A design that places the HMAC result(s) computed for a message
      inside the same message after the computation has to allocate in
      the message some data unit(s) purposed for the result(s) (in this
      mechanism it is the HMAC TLV(s), see Section 4.3).  The padding
      procedure sets respective octets of the data unit(s), in the
      simplest case to a fixed value known as the padding constant.

      Particular value of the constant is specific to each design.  For
      instance, in [RIP2-AUTH] as well as works derived from it
      ([ISIS-AUTH-B], [OSPF2-AUTH], and [OSPF3-AUTH]) the value is
      0x878FE1F3.  In many other designs (for instance, [RFC3315],



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      [RFC3931], [RFC4030], [RFC4302], [RFC5176], and [ISIS-AUTH-A]) the
      value is 0x00.

      However, the HMAC construct is defined on the base of a
      cryptographic hash algorithm, that is, an algorithm meeting
      particular set of requirements made for any input message.  Thus
      any padding constant values, whether single- or multiple-octet, as
      well as any other message conditioning methods, don't affect
      cryptographic characteristics of the hash algorithm and the HMAC
      construct respectively.

   o  Source Address Protection

      In the specific case of datagram-based routing protocols the
      protocol packet (that is, the message being authenticated) often
      does not include network layer addresses, although the source and
      (to a lesser extent) the destination address of the datagram may
      be meaningful in the scope of the protocol instance.

      In Babel the source address may be used as a prefix hext hop (see
      Section 3.5.3 of [BABEL]).  A well-known (see Section 2.3 of
      [OSPF3-AUTH]) solution to the source address protection problem is
      to set the first respective octets of the data unit(s) above to
      the source address (yet setting the rest of the octets to the
      padding constant).  This procedure adapts this solution to the
      specifics of Babel, which allows for exchange of protocol packets
      using both IPv4 and IPv6 datagrams (see Section 4 of [BABEL]).
      Even though in the case of IPv6 exchange a Babel speaker currently
      uses only link-local source addresses (Section 3.1 ibid.), this
      procedure protects all octets of an arbitrary given source address
      for the reasons of future extensibility.  The procedure implies
      that future Babel extensions will never use an IPv4-mapped IPv6
      address as a packet source address.

      This procedure does not protect the destination address, which is
      currently considered meaningless (ibid.) in the same scope.  A
      future extension that looks to add such protection would likely
      use a new TLV or sub-TLV to include the destination address into
      the protocol packet (see Section 4.1).

   Description of the padding procedure:

   1.  Set the first 16 octets of the Digest field of the given HMAC TLV
       to:

       *  the given source address, if it is an IPv6 address, or





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       *  the IPv4-mapped IPv6 address (per Section 2.5.5.2 of
          [RFC4291]) holding the given source address, if it is an IPv4
          address.

   2.  Set the remaining (TLV Length - 18) octets of the Digest field of
       the given HMAC TLV to 0x00.

   For an example of a Babel packet with padded HMAC TLVs see Table 3.

2.3.  Cryptographic Sequence Number Specifics

   Operation of this mechanism may involve multiple local and multiple
   remote cryptographic sequence numbers, each essentially being a
   48-bit unsigned integer.  This specification uses a term "TS/PC
   number" to avoid confusion with the route's (Section 2.5 of [BABEL])
   or node's (Section 3.2.1 ibid.) sequence numbers of the original
   Babel specification and to stress the fact that there are two
   distinguished parts of this 48-bit number, each handled in its
   specific way (see Section 5.1):

    0                   1     2 3                   4
    0 1 2 3 4 5 6 7 8 9 0 //  9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         TS         //         |              PC               |
   +-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      //

   The high-order 32 bits are called "timestamp" (TS) and the low-order
   16 bits are called "packet counter" (PC).

   This mechanism stores, updates, compares, and encodes each TS/PC
   number as two independent unsigned integers, TS and PC respectively.
   Such comparison of TS/PC numbers performed in item 3 of Section 5.4
   is algebraically equivalent to comparison of respective 48-bit
   unsigned integers.  Any byte order conversion, when required, is
   performed on TS and PC parts independently.

2.4.  Definition of HMAC

   The algorithm description below uses the following nomenclature,
   which is consistent with [FIPS-198]:

   Text   Is the data on which the HMAC is calculated (note item (b) of
          Section 9).  In this specification it is the contents of a
          Babel packet ranging from the beginning of the Magic field of
          the Babel packet header to the end of the last octet of the
          Packet Body field, as defined in Section 4.2 of [BABEL] (see
          Figure 2).



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   H      Is the specific hash algorithm (see Section 2.1).

   K      Is a sequence of octets of an arbitrary, known length.

   Ko     Is the cryptographic key used with the hash algorithm.

   B      Is the block size of H, measured in octets rather than bits.
          Note that B is the internal block size, not the digest length.

   L      Is the digest length of H, measured in octets rather than
          bits.

   XOR    Is the bitwise exclusive-or operation.

   Opad   Is the hexadecimal value 0x5C repeated B times.

   Ipad   Is the hexadecimal value 0x36 repeated B times.

   The algorithm below is the original, unmodified HMAC construct as
   defined in both [RFC2104] and [FIPS-198], hence it is different from
   the algorithms defined in [RIP2-AUTH], [ISIS-AUTH-B], [OSPF2-AUTH],
   and [OSPF3-AUTH] in exactly two regards:

   o  The algorithm below sets the size of Ko to B, not to L (L is not
      greater than B).  This resolves both ambiguity in XOR expressions
      and incompatibility in handling of keys that have length greater
      than L but not greater than B.

   o  The algorithm below does not change value of Text before or after
      the computation.  Both padding of a Babel packet before the
      computation and placing of the result inside the packet are
      performed elsewhere.

   The intent of this is to enable the most straightforward use of
   cryptographic libraries by implementations of this specification.  At
   the time of this writing implementations of the original HMAC
   construct coupled with hash algorithms of choice are generally
   available.

   Description of the algorithm:

   1.  Preparation of the Key

       In this application, Ko is always B octets long.  If K is B
       octets long, then Ko is set to K. If K is more than B octets
       long, then Ko is set to H(K) with the necessary amount of zeroes
       appended to the end of H(K), such that Ko is B octets long.  If K
       is less than B octets long, then Ko is set to K with zeroes



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       appended to the end of K, such that Ko is B octets long.

   2.  First-Hash

       A First-Hash, also known as the inner hash, is computed as
       follows:

                    First-Hash = H(Ko XOR Ipad || Text)

   3.  Second-Hash

       A second hash, also known as the outer hash, is computed as
       follows:

                 Second-Hash = H(Ko XOR Opad || First-Hash)

   4.  Result

       The resulting Second-Hash becomes the authentication data that is
       returned as the result of HMAC calculation.

   Note that in the case of Babel the Text parameter will never exceed a
   few thousands of octets in length.  In this specific case the
   optimization discussed in Section 6 of [FIPS-198] applies, namely,
   for a given K that is more than B octets long the following
   associated intermediate results may be precomputed only once: Ko,
   (Ko XOR Ipad), and (Ko XOR Opad).


3.  Updates to Protocol Data Structures

3.1.  RxAuthRequired

   RxAuthRequired is a boolean parameter, its default value MUST be
   TRUE.  An implementation SHOULD make RxAuthRequired a per-interface
   parameter, but MAY make it specific to the whole protocol instance.
   The conceptual purpose of RxAuthRequired is to enable a smooth
   migration from an unauthenticated to an authenticated Babel packet
   exchange and back (see Section 7.3).  Current value of RxAuthRequired
   directly affects the receiving procedure defined in Section 5.4.  An
   implementation SHOULD allow the operator to change RxAuthRequired
   value at runtime or by means of Babel speaker restart.  An
   implementation MUST allow the operator to discover the effective
   value of RxAuthRequired at runtime or from the system documentation.







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3.2.  LocalTS

   LocalTS is a 32-bit unsigned integer variable, it is the TS part of a
   per-interface TS/PC number.  LocalTS is a strictly per-interface
   variable not intended to be changed by the operator.  Its
   initialization is explained in Section 5.1.

3.3.  LocalPC

   LocalPC is a 16-bit unsigned integer variable, it is the PC part of a
   per-interface TS/PC number.  LocalPC is a strictly per-interface
   variable not intended to be changed by the operator.  Its
   initialization is explained in Section 5.1.

3.4.  MaxDigestsIn

   MaxDigestsIn is an unsigned integer parameter conceptually purposed
   for limiting the amount of CPU time spent processing a received
   authenticated packet.  The receiving procedure performs the most CPU-
   intensive operation, the HMAC computation, only at most MaxDigestsIn
   (Section 5.4 item 7) times for a given packet.

   MaxDigestsIn value MUST be at least 2.  An implementation SHOULD make
   MaxDigestsIn a per-interface parameter, but MAY make it specific to
   the whole protocol instance.  An implementation SHOULD allow the
   operator to change the value of MaxDigestsIn at runtime or by means
   of Babel speaker restart.  An implementation MUST allow the operator
   to discover the effective value of MaxDigestsIn at runtime or from
   the system documentation.

3.5.  MaxDigestsOut

   MaxDigestsOut is an unsigned integer parameter conceptually purposed
   for limiting the amount of a sent authenticated packet's space spent
   on authentication data.  The sending procedure adds at most
   MaxDigestsOut (Section 5.3 item 5) HMAC results to a given packet,
   concurring with the output buffer management explained in
   Section 6.2.

   The MaxDigestsOut value MUST be at least 2.  An implementation SHOULD
   make MaxDigestsOut a per-interface parameter, but MAY make it
   specific to the whole protocol instance.  An implementation SHOULD
   allow the operator to change the value of MaxDigestsOut at runtime or
   by means of Babel speaker restart, in a safe range.  The maximum safe
   value of MaxDigestsOut is implementation-specific (see Section 6.2).
   An implementation MUST allow the operator to discover the effective
   value of MaxDigestsOut at runtime or from the system documentation.




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3.6.  ANM Table

   The ANM (Authentic Neighbours Memory) table resembles the neighbour
   table defined in Section 3.2.3 of [BABEL].  Note that the term
   "neighbour table" means the neighbour table of the original Babel
   specification, and the term "ANM table" means the table defined
   herein.  Indexing of the ANM table is done in exactly the same way as
   indexing of the neighbour table, but purpose, field set and
   associated procedures are different.

   The conceptual purpose of the ANM table is to provide longer term
   replay attack protection than it would be possible using the
   neighbour table.  Expiry of an inactive entry in the neighbour table
   depends on the last received Hello Interval of the neighbour and
   typically stands for tens to hundreds of seconds (see Appendix A and
   Appendix B of [BABEL]).  Expiry of an inactive entry in the ANM table
   depends only on the local speaker's configuration.  The ANM table
   retains (for at least the amount of seconds set by ANM timeout
   parameter defined in Section 3.7) a copy of TS/PC number advertised
   in authentic packets by each remote Babel speaker.

   The ANM table is indexed by pairs of the form (Interface, Source).
   Every table entry consists of the following fields:

   o  Interface

      An implementation-specific reference to the local node's interface
      that the authentic packet was received through.

   o  Source

      The source address of the Babel speaker that the authentic packet
      was received from.

   o  LastTS

      A 32-bit unsigned integer, the TS part of a remote TS/PC number.

   o  LastPC

      A 16-bit unsigned integer, the PC part of a remote TS/PC number.

   Each ANM table entry has an associated aging timer, which is reset by
   the receiving procedure (Section 5.4 item 9).  If the timer expires,
   the entry is deleted from the ANM table.

   An implementation SHOULD use a persistent memory (NVRAM) to retain
   the contents of ANM table across restarts of the Babel speaker, but



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   only as long as both the Interface field reference and expiry of the
   aging timer remain correct.  An implementation MUST make it clear, if
   and how persistent memory is used for ANM table.  An implementation
   SHOULD allow the operator to retrieve the current contents of ANM
   table at runtime.  An implementation SHOULD allow the operator to
   remove some or all of ANM table entries at runtime or by means of
   Babel speaker restart.

3.7.  ANM Timeout

   ANM timeout is an unsigned integer parameter.  An implementation
   SHOULD make ANM timeout a per-interface parameter, but MAY make it
   specific to the whole protocol instance.  ANM timeout is conceptually
   purposed for limiting the maximum age (in seconds) of entries in the
   ANM table standing for inactive Babel speakers.  The maximum age is
   immediately related to replay attack protection strength.  The
   strongest protection is achieved with the maximum possible value of
   ANM timeout set, but it may not provide the best overall result for
   specific network segments and implementations of this mechanism.

   In the first turn, implementations unable to maintain local TS/PC
   number strictly increasing across Babel speaker restarts will reuse
   the advertised TS/PC numbers after each restart (see Section 5.1).
   The neighbouring speakers will treat the new packets as replayed and
   discard them until the aging timer of respective ANM table entry
   expires or the new TS/PC number exceeds the one stored in the entry.

   Another possible, but less probable, case could be an environment
   using IPv6 for Babel datagrams exchange and involving physical moves
   of network interfaces hardware between Babel speakers.  Even
   performed without restarting the speakers, these would cause random
   drops of the TS/PC number advertised for a given (Interface, Source)
   index, as viewed by neighbouring speakers, since IPv6 link-local
   addresses are typically derived from interface hardware addresses.

   Assuming that in such cases the operators would prefer to use a lower
   ANM timeout value to let the entries expire on their own rather than
   having to manually remove them from the ANM table each time, an
   implementation SHOULD set the default value of ANM timeout to a value
   between 30 and 300 seconds.

   At the same time, network segments may exist with every Babel speaker
   having its advertised TS/PC number strictly increasing over the
   deployed lifetime.  Assuming that in such cases the operators would
   prefer using a much higher ANM timeout value, an implementation
   SHOULD allow the operator to change the value of ANM timeout at
   runtime or by means of Babel speaker restart.  An implementation MUST
   allow the operator to discover the effective value of ANM timeout at



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   runtime or from the system documentation.

3.8.  Configured Security Associations

   A Configured Security Association (CSA) is a data structure
   conceptually purposed for associating authentication keys and hash
   algorithms with Babel interfaces.  All CSAs are managed in finite
   sequences, one sequence per interface ("interface's sequence of CSAs"
   hereafter).  Each interface's sequence of CSAs, as an integral part
   of the Babel speaker configuration, MAY be intended for a persistent
   storage as long as this conforms with the implementation's key
   management policy.  The default state of an interface's sequence of
   CSAs is empty, which has a special meaning of no authentication
   configured for the interface.  The sending (Section 5.3 item 1) and
   the receiving (Section 5.4 item 1) procedures address this convention
   accordingly.

   A single CSA structure consists of the following fields:

   o  HashAlgo

      An implementation-specific reference to one of the hash algorithms
      supported by this implementation (see Section 2.1).

   o  KeyChain

      A finite sequence of elements ("KeyChain sequence" hereafter)
      representing authentication keys, each element being a structure
      consisting of the following fields:

      *  LocalKeyID

         An unsigned integer of an implementation-specific bit length.

      *  AuthKeyOctets

         A sequence of octets of an arbitrary, known length to be used
         as the authentication key.

      *  KeyStartAccept

         The time that this Babel speaker will begin considering this
         authentication key for accepting packets with authentication
         data.

      *  KeyStartGenerate

         The time that this Babel speaker will begin considering this



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         authentication key for generating packet authentication data.

      *  KeyStopGenerate

         The time that this Babel speaker will stop considering this
         authentication key for generating packet authentication data.

      *  KeyStopAccept

         The time that this Babel speaker will stop considering this
         authentication key for accepting packets with authentication
         data.

   Since there is no limit imposed on the number of CSAs per interface,
   but the number of HMAC computations per sent/received packet is
   limited (through MaxDigestsOut and MaxDigestsIn respectively), only a
   fraction of the associated keys and hash algorithms may appear used
   in the process.  The ordering of elements within a sequence of CSAs
   and within a KeyChain sequence is important to make the association
   selection process deterministic and transparent.  Once this ordering
   is deterministic at the Babel interface level, the intermediate data
   derived by the procedure defined in Section 5.2 will be
   deterministically ordered as well.

   An implementation SHOULD allow an operator to set any arbitrary order
   of elements within a given interface's sequence of CSAs and within
   the KeyChain sequence of a given CSA.  Regardless if this requirement
   is or isn't met, the implementation MUST provide a mean to discover
   the actual element order used.  Whichever order is used by an
   implementation, it MUST be preserved across Babel speaker restarts.

   Note that none of the CSA structure fields is constrained to contain
   unique values.  Section 6.4 explains this in more detail.  It is
   possible for the KeyChain sequence to be empty, although this is not
   the intended manner of CSAs use.

   The KeyChain sequence has a direct prototype, which is the "key
   chain" syntax item of some existing router configuration languages.
   Whereas an implementation already implements this syntax item, it is
   suggested to reuse it, that is, to implement a CSA syntax item
   referring to a key chain item instead of reimplementing the latter in
   full.

3.9.  Effective Security Associations

   An Effective Security Association (ESA) is a data structure
   immediately used in sending (Section 5.3) and receiving (Section 5.4)
   procedures.  Its conceptual purpose is to determine a runtime



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   interface between those procedures and the deriving procedure defined
   in Section 5.2.  All ESAs are temporary data units managed as
   elements of finite sequences that are not intended for a persistent
   storage.  Element ordering within each such finite sequence
   ("sequence of ESAs" hereafter) MUST be preserved as long as the
   sequence exists.

   A single ESA structure consists of the following fields:

   o  HashAlgo

      An implementation-specific reference to one of the hash algorithms
      supported by this implementation (see Section 2.1).

   o  KeyID

      A 16-bit unsigned integer.

   o  AuthKeyOctets

      A sequence of octets of an arbitrary, known length to be used as
      the authentication key.

   Note that among the protocol data structures introduced by this
   mechanism ESA is the only one not directly interfaced with the system
   operator (see Figure 1), it is not immediately present in the
   protocol encoding either.  However, ESA is not just a possible
   implementation technique, but an integral part of this specification:
   the deriving (Section 5.2), the sending (Section 5.3), and the
   receiving (Section 5.4) procedures are defined in terms of the ESA
   structure and its semantics provided herein.  ESA is as meaningful
   for a correct implementation as the other protocol data structures.


4.  Updates to Protocol Encoding

4.1.  Justification

   Choice of encoding is very important in the long term.  The protocol
   encoding limits various authentication mechanism designs and
   encodings, which in turn limit future developments of the protocol.

   Considering existing implementations of Babel protocol instance
   itself and related modules of packet analysers, the current encoding
   of Babel allows for compact and robust decoders.  At the same time,
   this encoding allows for future extensions of Babel by three (not
   excluding each other) principal means defined by Section 4.2 and
   Section 4.3 of [BABEL] and further discussed in



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   [I-D.chroboczek-babel-extension-mechanism]:

   a.  A Babel packet consists of a four-octet header followed by a
       packet body, that is, a sequence of TLVs (see Figure 2).  Besides
       the header and the body, an actual Babel datagram may have an
       arbitrary amount of trailing data between the end of the packet
       body and the end of the datagram.  An instance of the original
       protocol silently ignores such trailing data.

   b.  The packet body uses a binary format allowing for 256 TLV types
       and imposing no requirements on TLV ordering or number of TLVs of
       a given type in a packet.  [BABEL] allocates TLV types 0 through
       10 (see Table 1), defines TLV body structure for each and
       establishes the requirement for a Babel protocol instance to
       ignore any unknown TLV types silently.  This makes it possible to
       examine a packet body (to validate the framing and/or to pick
       particular TLVs for further processing) considering only the type
       (to distinguish between a Pad1 TLV and any other TLV) and the
       length of each TLV, regardless if and how many additional TLV
       types are eventually deployed.

   c.  Within each TLV of the packet body there may be some "extra data"
       after the "expected length" of the TLV body.  An instance of the
       original protocol silently ignores any such extra data.  Note
       that any TLV types without the expected length defined (such as
       PadN TLV) cannot be extended with the extra data.

   Considering each principal extension mean for the specific purpose of
   adding authentication data items to each protocol packet, the
   following arguments can be made:

   o  Use of the TLV extra data of some existing TLV type would not be a
      solution, since no particular TLV type is guaranteed to be present
      in a Babel packet.

   o  Use of the TLV extra data could also conflict with future
      developments of the protocol encoding.

   o  Since the packet trailing data is currently unstructured, using it
      would involve defining an encoding structure and associated
      procedures, adding to the complexity of both specification and
      implementation and increasing the exposure to protocol attacks
      such as fuzzing.

   o  A naive use of the packet trailing data would make it unavailable
      to any future extension of Babel.  Since this mechanism is
      possibly not the last extension and since some other extensions
      may allow no other embedding means except the packet trailing



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      data, the defined encoding structure would have to enable
      multiplexing of data items belonging to different extensions.
      Such a definition is out of the scope of this work.

   o  Deprecating an extension (or only its protocol encoding) that uses
      purely purpose-allocated TLVs is as simple as deprecating the
      TLVs.

   o  Use of purpose-allocated TLVs is transparent for both the original
      protocol and any its future extensions, regardless of the
      embedding mean(s) used by the latter.

   Considering all of the above, this mechanism neither uses the packet
   trailing data nor uses the TLV extra data, but uses two new TLV
   types: type 11 for a TS/PC number and type 12 for an HMAC result (see
   Table 1).

4.2.  TS/PC TLV

   The purpose of a TS/PC TLV is to store a single TS/PC number.  There
   is exactly one TS/PC TLV in an authenticated Babel packet.

    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 = 11   |     Length    |         PacketCounter         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type            Set to 11 to indicate a TS/PC TLV.

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

   PacketCounter   A 16-bit unsigned integer in network byte order, the
                   PC part of a TS/PC number stored in this TLV.

   Timestamp       A 32-bit unsigned integer in network byte order, the
                   TS part of a TS/PC number stored in this TLV.

   Note that the ordering of PacketCounter and Timestamp in the TLV
   structure is opposite to the ordering of TS and PC in "TS/PC" term
   and the 48-bit equivalent (see Section 2.3).

   Considering the "expected length" and the "extra data" in the



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   definition of Section 4.3 of [BABEL], the expected length of a TS/PC
   TLV body is unambiguously defined as 6 octets.  The receiving
   procedure correctly processes any TS/PC TLV with body length not less
   than the expected, ignoring any extra data (Section 5.4 items 3 and
   9).  The sending procedure produces a TS/PC TLV with body length
   equal to the expected and Length field set respectively (Section 5.3
   item 3).

   Future Babel extensions (such as sub-TLVs) MAY modify the sending
   procedure to include the extra data after the fixed-size TS/PC TLV
   body defined herein, making necessary adjustments to Length TLV
   field, "Body length" packet header field and output buffer management
   explained in Section 6.2.

4.3.  HMAC TLV

   The purpose of an HMAC TLV is to store a single HMAC result.  To
   assist a receiver in reproducing the HMAC computation, LocalKeyID
   modulo 2^16 of the authentication key is also provided in the TLV.
   There is at least one HMAC TLV in an authenticated Babel packet.

    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 = 12   |    Length     |             KeyID             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Digest...
   +-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type            Set to 12 to indicate an HMAC TLV.

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

   KeyID           A 16-bit unsigned integer in network byte order.

   Digest          A variable-length sequence of octets, which is at
                   least 16 octets long (see Section 2.2).

   Considering the "expected length" and the "extra data" in the
   definition of Section 4.3 of [BABEL], the expected length of an HMAC
   TLV body is not defined.  The receiving and the padding procedures
   process every octet of the Digest field, deriving the field boundary
   from the Length field value (Section 5.4 item 7 and Section 2.2
   respectively).  The sending procedure produces HMAC TLVs with Length
   field precisely sizing the Digest field to match digest length of the



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   hash algorithm used (Section 5.3 items 5 and 8).

   The HMAC TLV structure defined herein is final, future Babel
   extensions MUST NOT extend it with any extra data.


5.  Updates to Protocol Operation

5.1.  Per-Interface TS/PC Number Updates

   The LocalTS and LocalPC interface-specific variables constitute the
   TS/PC number of a Babel interface.  This number is advertised in the
   TS/PC TLV of authenticated Babel packets sent from that interface.
   There is only one property mandatory for the advertised TS/PC number:
   its 48-bit equivalent (see Section 2.3) MUST be strictly increasing
   within the scope of a given interface of a Babel speaker as long as
   the protocol instance is continuously operating.  This property
   combined with ANM tables of neighbouring Babel speakers provides them
   with the most basic replay attack protection.

   Initialization and increment are two principal updates performed on
   an interface TS/PC number.  The initialization is performed when a
   new interface becomes a part of a Babel protocol instance.  The
   increment is performed by the sending procedure (Section 5.3 item 2)
   before advertising the TS/PC number in a TS/PC TLV.

   Depending on particular implementation method of these two updates
   the advertised TS/PC number may possess additional properties
   improving the replay attack protection strength.  This includes, but
   is not limited to the methods below.

   a.  The most straightforward implementation would use LocalTS as a
       plain wrap counter, defining the updates as follows:

       initialization  Set LocalPC to 0, set LocalTS to 0.

       increment       Increment LocalPC by 1.  If LocalPC wraps (0xFFFF
                       + 1 = 0x0000), increment LocalTS by 1.

       In this case the advertised TS/PC numbers would be reused after
       each Babel protocol instance restart, making neighbouring
       speakers reject authenticated packets until the respective ANM
       table entries expire or the new TS/PC number exceeds the old (see
       Section 3.6 and Section 3.7).

   b.  A more advanced implementation could make a use of any 32-bit
       unsigned integer timestamp (number of time units since an
       arbitrary epoch) such as the UNIX timestamp, whereas the



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       timestamp itself spans a reasonable time range and is guaranteed
       against a decrease (such as one resulting from network time use).
       The updates would be defined as follows:

       initialization  Set LocalPC to 0, set LocalTS to 0.

       increment       If the current timestamp is greater than LocalTS,
                       set LocalTS to the current timestamp and LocalPC
                       to 0, then consider the update complete.
                       Otherwise increment LocalPC by 1 and, if LocalPC
                       wraps, increment LocalTS by 1.

       In this case the advertised TS/PC number would remain unique
       across the speaker's deployed lifetime without the need for any
       persistent storage.  However, a suitable timestamp source is not
       available in every implementation case.

   c.  Another advanced implementation could use LocalTS in a way
       similar to the "wrap/boot counter" suggested in Section 4.1.1 of
       [OSPF3-AUTH], defining the updates as follows:

       initialization  Set LocalPC to 0.  If there is a TS value stored
                       in NVRAM for the current interface, set LocalTS
                       to the stored TS value, then increment the stored
                       TS value by 1.  Otherwise set LocalTS to 0 and
                       set the stored TS value to 1.

       increment       Increment LocalPC by 1.  If LocalPC wraps, set
                       LocalTS to the TS value stored in NVRAM for the
                       current interface, then increment the stored TS
                       value by 1.

       In this case the advertised TS/PC number would also remain unique
       across the speaker's deployed lifetime, relying on NVRAM for
       storing multiple TS numbers, one per interface.

   As long as the TS/PC number retains its mandatory property stated
   above, it is up to the implementor, which TS/PC number updates
   methods are available and if the operator can configure the method
   per-interface and/or at runtime.  However, an implementation MUST
   disclose the essence of each update method it includes, in a
   comprehensible form such as natural language description, pseudocode,
   or source code.  An implementation MUST allow the operator to
   discover, which update method is effective for any given interface,
   either at runtime or from the system documentation.  These
   requirements are necessary to enable the optimal (see Section 3.7)
   management of ANM timeout in a network segment.




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   Note that wrapping (0xFFFFFFFF + 1 = 0x00000000) of LastTS is
   unlikely, but possible, causing the advertised TS/PC number to be
   reused.  Resolving this situation requires replacing all
   authentication keys of the involved interface.  In addition to that,
   if the wrap was caused by a timestamp reaching its end of epoch,
   using this mechanism will be impossible for the involved interface
   until some different timestamp or update implementation method is
   used.

5.2.  Deriving ESAs from CSAs

   Neither receiving nor sending procedures work with the contents of
   interface's sequence of CSAs directly, both (Section 5.4 item 4 and
   Section 5.3 item 4 respectively) derive a sequence of ESAs from the
   sequence of CSAs and use the derived sequence (see Figure 1).  There
   are two main goals achieved through this indirection:

   o  Elimination of expired authentication keys and deduplication of
      security associations.  This is done as early as possible to keep
      subsequent procedures focused on their respective tasks.

   o  Maintenance of particular ordering within the derived sequence of
      ESAs.  The ordering deterministically depends on the ordering
      within the interface's sequence of CSAs and the ordering within
      KeyChain sequence of each CSA.  The particular correlation
      maintained by this procedure implements a concept of fair
      (independent of number of keys contained by each) competition
      between CSAs.

   The deriving procedure uses the following input arguments:

   o  input sequence of CSAs

   o  direction (sending or receiving)

   o  current time (CT)

   The processing of input arguments begins with an empty output
   sequence of ESAs and consists of the following steps:

   1.  Make a temporary copy of the input sequence of CSAs.

   2.  Remove all expired authentication keys from each KeyChain
       sequence of the copy, that is, any keys such that:

       *  for receiving: KeyStartAccept is greater than CT or
          KeyStopAccept is less than CT




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       *  for sending: KeyStartGenerate is greater than CT or
          KeyStopGenerate is less than CT

       Note well that there are no special exceptions.  Remove all
       expired keys, even if there are no keys left after that (see
       Section 7.4).

   3.  Use the copy to populate the output sequence of ESAs as follows:

       1.  When the KeyChain sequence of the first CSA contains at least
           one key, use its first key to produce an ESA with fields set
           as follows:

           HashAlgo       Set to HashAlgo of the current CSA.

           KeyID          Set to LocalKeyID modulo 2^16 of the current
                          key of the current CSA.

           AuthKeyOctets  Set to AuthKeyOctets of the current key of the
                          current CSA.

           Append this ESA to the end of the output sequence.

       2.  When the KeyChain sequence of the second CSA contains at
           least one key, use its first key the same way and so forth
           until all first keys of the copy are processed.

       3.  When the KeyChain sequence of the first CSA contains at least
           two keys, use its second key the same way.

       4.  When the KeyChain sequence of the second CSA contains at
           least two keys, use its second key the same way and so forth
           until all second keys of the copy are processed.

       5.  And so forth until all keys of all CSAs of the copy are
           processed, exactly once each.

       In the description above the ordinals ("first", "second", and so
       on) with regard to keys stand for an element position after the
       removal of expired keys, not before.  For example, if a KeyChain
       sequence was { Ka, Kb, Kc, Kd } before the removal and became
       { Ka, Kd } after, then Ka would be the "first" element and Kd
       would be the "second".

   4.  Deduplicate the ESAs in the output sequence, that is, wherever
       two or more ESAs exist that share the same (HashAlgo, KeyID,
       AuthKeyOctets) triplet value, remove all of these ESAs except the
       one closest to the beginning of the sequence.



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   The resulting sequence will contain zero or more unique ESAs, ordered
   in a way deterministically correlated with ordering of CSAs within
   the original input sequence of CSAs and ordering of keys within each
   KeyChain sequence.  This ordering maximizes the probability of having
   equal amount of keys per original CSA in any N first elements of the
   resulting sequence.  Possible optimizations of this deriving
   procedure are outlined in Section 6.3.

5.3.  Updates to Packet Sending

   Perform the following authentication-specific processing after the
   instance of the original protocol considers an outgoing Babel packet
   ready for sending, but before the packet is actually sent (see
   Figure 1).  After that send the packet regardless if the
   authentication-specific processing modified the outgoing packet or
   left it intact.

   1.  If the current outgoing interface's sequence of CSAs is empty,
       finish authentication-specific processing and consider the packet
       ready for sending.

   2.  Increment TS/PC number of the current outgoing interface as
       explained in Section 5.1.

   3.  Add to the packet body (see the note at the end of this section)
       a TS/PC TLV with fields set as follows:

       Type            Set to 11.

       Length          Set to 6.

       PacketCounter   Set to the current value of LocalPC variable of
                       the current outgoing interface.

       Timestamp       Set to the current value of LocalTS variable of
                       the current outgoing interface.

       Note that the current step may involve byte order conversion.

   4.  Derive a sequence of ESAs using procedure defined in Section 5.2
       with the current interface's sequence of CSAs as the input
       sequence of CSAs, the current time as CT and "sending" as the
       direction.  Proceed to the next step even if the derived sequence
       is empty.

   5.  Iterate over the derived sequence using its ordering.  For each
       ESA add to the packet body (see the note at the end of this
       section) an HMAC TLV with fields set as follows:



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       Type     Set to 12.

       Length   Set to 2 plus digest length of HashAlgo of the current
                ESA.

       KeyID    Set to KeyID of the current ESA.

       Digest   Size exactly equal to the digest length of HashAlgo of
                the current ESA.  Pad (see Section 2.2) using the source
                address of the current packet (see Section 6.1).

       As soon as there are MaxDigestsOut HMAC TLVs added to the current
       packet body, immediately proceed to the next step.

       Note that the current step may involve byte order conversion.

   6.  Increment the "Body length" field value of the current packet
       header by the total length of TS/PC and HMAC TLVs appended to the
       current packet body so far.

       Note that the current step may involve byte order conversion.

   7.  Make a temporary copy of the current packet.

   8.  Iterate over the derived sequence again, using the same order and
       number of elements.  For each ESA (and respectively for each HMAC
       TLV recently appended to the current packet body) compute an HMAC
       result (see Section 2.4) using the temporary copy (not the
       original packet) as Text, HashAlgo of the current ESA as H, and
       AuthKeyOctets of the current ESA as K. Write the HMAC result to
       the Digest field of the current HMAC TLV (see Table 4) of the
       current packet (not the copy).

   9.  After this point, allow no more changes to the current packet
       header and body and consider it ready for sending.

   Note that even when the derived sequence of ESAs is empty, the packet
   is sent anyway with only a TS/PC TLV appended to its body.  Although
   such a packet would not be authenticated, the presence of the sole
   TS/PC TLV would indicate authentication key exhaustion to operators
   of neighbouring Babel speakers.  See also Section 7.4.

   Also note that it is possible to place the authentication-specific
   TLVs in the packet's sequence of TLVs in a number of different valid
   ways so long as there is exactly one TS/PC TLV in the sequence and
   the ordering of HMAC TLVs relative to each other, as produced in step
   5 above, is preserved.




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   For example, see Figure 2.  The diagrams represent a Babel packet
   without (D1) and with (D2, D3, D4) authentication-specific TLVs.  The
   optional trailing data block that is present in D1 is preserved in
   D2, D3, and D4.  Indexing (1, 2, ..., n) of the HMAC TLVs means the
   order in which the sending procedure produced them (and respectively
   the HMAC results).  In D2 the added TLVs are appended: the previously
   existing TLVs are followed by the TS/PC TLV, which is followed by the
   HMAC TLVs.  In D3 the added TLVs are prepended: the TS/PC TLV is the
   first and is followed by the HMAC TLVs, which are followed by the
   previously existing TLVs.  In D4 the added TLVs are intermixed with
   the previously existing TLVs and the TS/PC TLV is placed after the
   HMAC TLVs.  All three packets meet the requirements above.

   Implementors SHOULD use appending (D2) for adding the authentication-
   specific TLVs to the sequence, this is expected to result in more
   straightforward implementation and troubleshooting in most use cases.

5.4.  Updates to Packet Receiving

   Perform the following authentication-specific processing after an
   incoming Babel packet is received from the local network stack, but
   before it is acted upon by the Babel protocol instance (see
   Figure 1).  The final action conceptually depends not only upon the
   result of the authentication-specific processing, but also on the
   current value of RxAuthRequired parameter.  Immediately after any
   processing step below accepts or refuses the packet, either deliver
   the packet to the instance of the original protocol (when the packet
   is accepted or RxAuthRequired is FALSE) or discard it (when the
   packet is refused and RxAuthRequired is TRUE).

   1.   If the current incoming interface's sequence of CSAs is empty,
        accept the packet.

   2.   If the current packet does not contain exactly one TS/PC TLV,
        refuse it.

   3.   Perform a lookup in the ANM table for an entry having Interface
        equal to the current incoming interface and Source equal to the
        source address of the current packet.  If such an entry does not
        exist, immediately proceed to the next step.  Otherwise, compare
        the entry's LastTS and LastPC field values with Timestamp and
        PacketCounter values respectively of the TS/PC TLV of the
        packet.  That is, refuse the packet, if at least one of the
        following two conditions is true:

        *  Timestamp is less than LastTS





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        *  Timestamp is equal to LastTS and PacketCounter is not greater
           than LastPC

        Note that the current step may involve byte order conversion.

   4.   Derive a sequence of ESAs using procedure defined in Section 5.2
        with the current interface's sequence of CSAs as the input
        sequence of CSAs, current time as CT and "receiving" as the
        direction.  If the derived sequence is empty, refuse the packet.

   5.   Make a temporary copy of the current packet.

   6.   Pad (see Section 2.2) every HMAC TLV present in the temporary
        copy (not the original packet) using the source address of the
        original packet.

   7.   Iterate over all the HMAC TLVs of the original input packet (not
        the copy) using their order of appearance in the packet.  For
        each HMAC TLV look up all ESAs in the derived sequence such that
        2 plus digest length of HashAlgo of the ESA is equal to Length
        of the TLV and KeyID of the ESA is equal to value of KeyID of
        the TLV.  Iterate over these ESAs in the relative order of their
        appearance on the full sequence of ESAs.  Note that nesting the
        iterations the opposite way (over ESAs, then over HMAC TLVs)
        would be wrong.

        For each of these ESAs compute an HMAC result (see Section 2.4)
        using the temporary copy (not the original packet) as Text,
        HashAlgo of the current ESA as H, and AuthKeyOctets of the
        current ESA as K. If the current HMAC result exactly matches the
        contents of Digest field of the current HMAC TLV, immediately
        proceed to the next step.  Otherwise, if the number of HMAC
        computations done for the current packet so far is equal to
        MaxDigestsIn, immediately proceed to the next step.  Otherwise
        follow the normal order of iterations.

        Note that the current step may involve byte order conversion.

   8.   Refuse the input packet unless there was a matching HMAC result
        in the previous step.

   9.   Modify the ANM table, using the same index as for the entry
        lookup above, to contain an entry with LastTS set to the value
        of Timestamp and LastPC set to the value of PacketCounter fields
        of the TS/PC TLV of the current packet.  That is, either add a
        new ANM table entry or update the existing one, depending on the
        result of the entry lookup above.  Reset the entry's aging timer
        to the current value of ANM timeout.



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        Note that the current step may involve byte order conversion.

   10.  Accept the input packet.

   An implementation SHOULD before the authentication-specific
   processing above perform those basic procedures of the original
   protocol that don't take any protocol actions upon the contents of
   the packet but discard it unless the packet is sufficiently well-
   formed for further processing.  Although exact composition of such
   procedures belongs to the scope of the original protocol, it seems
   reasonable to state that a packet SHOULD be discarded early,
   regardless if any authentication-specific processing is due, unless
   its source address conforms to Section 3.1 of [BABEL] and is not the
   receiving speaker's own address (see item (e) of Section 9).

   Note that RxAuthRequired affects only the final action, but not the
   defined flow of authentication-specific processing.  The purpose of
   this is to preserve authentication-specific processing feedback (such
   as log messages and event counters updates) even with RxAuthRequired
   set to FALSE.  This allows an operator to predict the effect of
   changing RxAuthRequired from FALSE to TRUE during a migration
   scenario (Section 7.3) implementation.

5.5.  Authentication-Specific Statistics Maintenance

   A Babel speaker implementing this mechanism SHOULD maintain a set of
   counters for the following events, per protocol instance and per
   interface:

   a.  Sending of an unauthenticated Babel packet through an interface
       having an empty sequence of CSAs (Section 5.3 item 1).

   b.  Sending of an unauthenticated Babel packet with a TS/PC TLV but
       without any HMAC TLVs due to an empty derived sequence of ESAs
       (Section 5.3 item 4).

   c.  Sending of an authenticated Babel packet containing both TS/PC
       and HMAC TLVs (Section 5.3 item 9).

   d.  Accepting of a Babel packet received through an interface having
       an empty sequence of CSAs (Section 5.4 item 1).

   e.  Refusing of a received Babel packet due to an empty derived
       sequence of ESAs (Section 5.4 item 4).

   f.  Refusing of a received Babel packet that does not contain exactly
       one TS/PC TLV (Section 5.4 item 2).




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   g.  Refusing of a received Babel packet due to the TS/PC TLV failing
       the ANM table check (Section 5.4 item 3).  In the view of future
       extensions this event SHOULD leave out some small amount, per
       current (Interface, Source, LastTS, LastPC) tuple, of the packets
       refused due to Timestamp value being equal to LastTS and
       PacketCounter value being equal to LastPC.

   h.  Refusing of a received Babel packet missing any HMAC TLVs
       (Section 5.4 item 8).

   i.  Refusing of a received Babel packet due to none of the processed
       HMAC TLVs passing the ESA check (Section 5.4 item 8).

   j.  Accepting of a received Babel packet having both TS/PC and HMAC
       TLVs (Section 5.4 item 10).

   k.  Delivery of a refused packet to the instance of the original
       protocol due to RxAuthRequired parameter set to FALSE.

   Note that terms "accepting" and "refusing" are used in the sense of
   the receiving procedure, that is, "accepting" does not mean a packet
   delivered to the instance of the original protocol purely because the
   RxAuthRequired parameter is set to FALSE.  Event counters readings
   SHOULD be available to the operator at runtime.


6.  Implementation Notes

6.1.  Source Address Selection for Sending

   Section 3.1 of [BABEL] allows for exchange of protocol datagrams
   using IPv4 or IPv6 or both.  The source address of the datagram is a
   unicast (link-local in the case of IPv6) address.  Within an address
   family used by a Babel speaker there may be more than one addresses
   eligible for the exchange and assigned to the same network interface.
   The original specification considers this case out of scope and
   leaves it up to the speaker's network stack to select one particular
   address as the datagram source address.  But the sending procedure
   requires (Section 5.3 item 5) exact knowledge of packet source
   address for proper padding of HMAC TLVs.

   As long as a network interface has more than one addresses eligible
   for the exchange within the same address family, the Babel speaker
   SHOULD internally choose one of those addresses for Babel packet
   sending purposes and make this choice to both the sending procedure
   and the network stack (see Figure 1).  Wherever this requirement
   cannot be met, this limitation MUST be clearly stated in the system
   documentation to allow an operator to plan network address management



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   accordingly.

6.2.  Output Buffer Management

   An instance of the original protocol buffers produced TLVs until the
   buffer becomes full or a delay timer has expired.  This is performed
   independently for each Babel interface with each buffer sized
   according to the interface MTU (see Sections 3.1 and 4 of [BABEL]).

   Since TS/PC and HMAC TLVs and any other TLVs, in the first place
   those of the original protocol, share the same packet space (see
   Figure 2) and respectively the same buffer space, a particular
   portion of each interface buffer needs to be reserved for 1 TS/PC TLV
   and up to MaxDigestsOut HMAC TLVs.  The amount (R) of this reserved
   buffer space is calculated as follows:

                    R = St + MaxDigestsOut * Sh =
                      = 8  + MaxDigestsOut * (4 + Lmax)

   St      Is the size of a TS/PC TLV.

   Sh      Is the size of an HMAC TLV.

   Lmax    Is the maximum digest length in octets possible for a
           particular interface.  It SHOULD be calculated based on
           particular interface's sequence of CSAs, but MAY be taken as
           the maximum digest length supported by particular
           implementation.

   An implementation allowing for per-interface value of MaxDigestsOut
   or Lmax has to account for different value of R across different
   interfaces, even having the same MTU.  An implementation allowing for
   runtime change of the value of R (due to MaxDigestsOut or Lmax) has
   to take care of the TLVs already buffered by the time of the change,
   especially when the value of R increases.

   The maximum safe value of MaxDigestsOut parameter depends on the
   interface MTU and maximum digest length used.  In general, at least
   200-300 octets of a Babel packet should be always available to data
   other than TS/PC and HMAC TLVs.  An implementation following the
   requirements of Section 4 of [BABEL] would send packets sized 512
   octets or larger.  If, for example, the maximum digest length is 64
   octets and MaxDigestsOut value is 4, the value of R would be 280,
   leaving less than a half of a 512-octet packet for any other TLVs.
   As long as the interface MTU is larger or digest length is smaller,
   higher values of MaxDigestsOut can be used safely.





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6.3.  Optimizations of ESAs Deriving

   The following optimizations of the ESAs deriving procedure can reduce
   amount of CPU time consumed by authentication-specific processing,
   preserving an implementation's effective behaviour.

   a.  The most straightforward implementation would treat the deriving
       procedure as a per-packet action.  But since the procedure is
       deterministic (its output depends on its input only), it is
       possible to significantly reduce the number of times the
       procedure is performed.

       The procedure would obviously return the same result for the same
       input arguments (sequence of CSAs, direction, CT) values.
       However, it is possible to predict when the result will remain
       the same even for a different input.  That is, when the input
       sequence of CSAs and the direction both remain the same but CT
       changes, the result will remain the same as long as CT's order on
       the time axis (relative to all critical points of the sequence of
       CSAs) remains unchanged.  Here, the critical points are
       KeyStartAccept and KeyStopAccept (for the "receiving" direction)
       and KeyStartGenerate and KeyStopGenerate (for the "sending"
       direction) of all keys of all CSAs of the input sequence.  In
       other words, in this case the result will remain the same as long
       as both none of the active keys expire and none of the inactive
       keys enter into operation.

       An implementation optimized this way would perform the full
       deriving procedure for a given (interface, direction) pair only
       after an operator's change to the interface's sequence of CSAs or
       after reaching one of the critical points mentioned above.

   b.  Considering that the sending procedure iterates over at most
       MaxDigestsOut elements of the derived sequence of ESAs
       (Section 5.3 item 5), there would be little sense in the case of
       "sending" direction in returning more than MaxDigestsOut ESAs in
       the derived sequence.  Note that a similar optimization would be
       relatively difficult in the case of "receiving" direction, since
       the number of ESAs actually used in examining a particular
       received packet (not to be confused with the number of HMAC
       computations) depends on additional factors besides just
       MaxDigestsIn.

6.4.  Security Associations Duplication

   This specification defines three data structures as finite sequences:
   a KeyChain sequence, an interface's sequence of CSAs, and a sequence
   of ESAs.  There are associated semantics to take into account during



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   implementation, in that the same element can appear multiple times at
   different positions of the sequence.  In particular, none of CSA
   structure fields (including HashAlgo, LocalKeyID, and AuthKeyOctets)
   alone or in a combination has to be unique within a given CSA, or
   within a given sequence of CSAs, or within all sequences of CSAs of a
   Babel speaker.

   In the CSA space defined this way, for any two authentication keys
   their one field (in)equality would not imply their another field
   (in)equality.  In other words, it is acceptable to have more than one
   authentication key with the same LocalKeyID or the same AuthKeyOctets
   or both at a time.  It is a conscious design decision that CSA
   semantics allow for duplication of security associations.
   Consequently, ESA semantics allow for duplication of intermediate
   ESAs in the sequence until the explicit deduplication (Section 5.2
   item 4).

   One of the intentions of this is to define the security association
   management in a way that allows the addressing of some specifics of
   Babel as a mesh routing protocol.  For example, a system operator
   configuring a Babel speaker to participate in more than one
   administrative domain could find each domain using its own
   authentication key (AuthKeyOctets) under the same LocalKeyID value,
   e.g., a "well-known" or "default" value like 0 or 1.  Since
   reconfiguring the domains to use distinct LocalKeyID values isn't
   always feasible, the multi-domain Babel speaker using several
   distinct authentication keys under the same LocalKeyID would make a
   valid use case for such duplication.

   Furthermore, if in this situation the operator decided to migrate one
   of the domains to a different LocalKeyID value in a seamless way,
   respective Babel speakers would use the same authentication key
   (AuthKeyOctets) under two different LocalKeyID values for the time of
   the transition (see also item (f) of Section 9).  This would make a
   similar use case.

   Another intention of this design decision is to decouple security
   association management from authentication key management as much as
   possible, so that the latter, be it manual keying or a key management
   protocol, could be designed and implemented independently (as
   respective reasoning made in Section 3.1 of [RIP2-AUTH] still
   applies).  This way the additional key management constraints, if
   any, would remain out of scope of this authentication mechanism.  A
   similar thinking justifies LocalKeyID field having bit length in ESA
   structure definition, but not in that of CSA.






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7.  Network Management Aspects

7.1.  Backward Compatibility

   Support of this mechanism is optional, it does not change the default
   behaviour of a Babel speaker and causes no compatibility issues with
   speakers properly implementing the original Babel specification.
   Given two Babel speakers, one implementing this mechanism and
   configured for authenticated exchange (A) and another not
   implementing it (B), these would not distribute routing information
   uni-directionally or form a routing loop or experience other protocol
   logic issues specific purely to the use of this mechanism.

   The Babel design requires a bi-directional neighbour reachability
   condition between two given speakers for a successful exchange of
   routing information.  Apparently, in the case above neighbour
   reachability would be uni-directional.  Presence of TS/PC and HMAC
   TLVs in Babel packets sent by A would be transparent to B. But lack
   of authentication data in Babel packets send by B would make them
   effectively invisible to the instance of the original protocol of A.
   Uni-directional links are not specific to use of this mechanism, they
   naturally exist on their own and are properly detected and coped with
   by the original protocol (see Section 3.4.2 of [BABEL]).

7.2.  Multi-Domain Authentication

   The receiving procedure treats a packet as authentic as soon as one
   of its HMAC TLVs passes the check against the derived sequence of
   ESAs.  This allows for packet exchange authenticated with multiple
   (hash algorithm, authentication key) pairs simultaneously, in
   combinations as arbitrary as permitted by MaxDigestsIn and
   MaxDigestsOut.

   For example, consider three Babel speakers with one interface each,
   configured with the following CSAs:

   o  speaker A: (hash algorithm H1; key SK1), (hash algorithm H1; key
      SK2)

   o  speaker B: (hash algorithm H1; key SK1)

   o  speaker C: (hash algorithm H1; key SK2)

   Packets sent by A would contain 2 HMAC TLVs each, packets sent by B
   and C would contain 1 HMAC TLV each.  A and B would authenticate the
   exchange between themselves using H1 and SK1; A and C would use H1
   and SK2; B and C would discard each other's packets.




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   Consider a similar set of speakers configured with different CSAs:

   o  speaker D: (hash algorithm H2; key SK3), (hash algorithm H3; key
      SK4)

   o  speaker E: (hash algorithm H2; key SK3), (hash algorithm H4, keys
      SK5 and SK6)

   o  speaker F: (hash algorithm H3; keys SK4 and SK7), (hash algorithm
      H5, key SK8)

   Packets sent by D would contain 2 HMAC TLVs each, packets sent by E
   and F would contain 3 HMAC TLVs each.  D and E would authenticate the
   exchange between themselves using H2 and SK3; D and F would use H3
   and SK4; E and F would discard each other's packets.  The
   simultaneous use of H4, SK5, and SK6 by E, as well as use of SK7, H5,
   and SK8 by F (for their own purposes) would remain insignificant to
   A.

   An operator implementing a multi-domain authentication should keep in
   mind that values of MaxDigestsIn and MaxDigestsOut may be different
   both within the same Babel speaker and across different speakers.
   Since the minimum value of both parameters is 2 (see Section 3.4 and
   Section 3.5), when more than 2 authentication domains are configured
   simultaneously it is advised to confirm that every involved speaker
   can handle sufficient number of HMAC results for both sending and
   receiving.

   The recommended method of Babel speaker configuration for multi-
   domain authentication is not only using a different authentication
   key for each domain, but also using a separate CSA for each domain,
   even when hash algorithms are the same.  This allows for fair
   competition between CSAs and sometimes limits the consequences of a
   possible misconfiguration to the scope of one CSA.  See also item (f)
   of Section 9.

7.3.  Migration to and from Authenticated Exchange

   It is common in practice to consider a migration to authenticated
   exchange of routing information only after the network has already
   been deployed and put to an active use.  Performing the migration in
   a way without regular traffic interruption is typically demanded, and
   this specification allows a smooth migration using the RxAuthRequired
   interface parameter defined in Section 3.1.  This measure is similar
   to the "transition mode" suggested in Section 5 of [OSPF3-AUTH].

   An operator performing the migration needs to arrange configuration
   changes as follows:



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   1.  Decide on particular hash algorithm(s) and key(s) to be used.

   2.  Identify all speakers and their involved interfaces that need to
       be migrated to authenticated exchange.

   3.  For each of the speakers and the interfaces to be reconfigured
       first set RxAuthRequired parameter to FALSE, then configure
       necessary CSA(s).

   4.  Examine the speakers to confirm that Babel packets are
       successfully authenticated according to the configuration
       (supposedly, through examining ANM table entries and
       authentication-specific statistics, see Figure 1) and address any
       discrepancies before proceeding further.

   5.  For each of the speakers and the reconfigured interfaces set the
       RxAuthRequired parameter to TRUE.

   Likewise, temporarily setting RxAuthRequired to FALSE can be used to
   migrate smoothly from an authenticated packet exchange back to
   unauthenticated one.

7.4.  Handling of Authentication Keys Exhaustion

   This specification employs a common concept of multiple authenticaion
   keys co-existing for a given interface, with two independent lifetime
   ranges associated with each key (one for sending and another for
   receiving).  It is typically recommended to configure the keys using
   finite lifetimes, adding new keys before the old keys expire.
   However, it is obviously possible for all keys to expire for a given
   interface (for sending or receiving or both).  Possible ways of
   addressing this situation raise their own concerns:

   o  Automatic switching to unauthenticated protocol exchange.  This
      behaviour invalidates the initial purposes of authentication and
      is commonly viewed as "unacceptable" ([RIP2-AUTH] Section 5.1,
      [OSPF2-AUTH] Section 3.2, [OSPF3-AUTH] Section 3, [OSPF3-AUTH-BIS]
      Section 3).

   o  Stopping routing information exchange over the interface.  This
      behaviour is likely to impact regular traffic routing and is
      commonly viewed as "not advisable" ([RIP2-AUTH], [OSPF2-AUTH],
      [OSPF3-AUTH]), although [OSPF3-AUTH-BIS] is different in this
      regard.

   o  Use of the "most recently expired" key over its intended lifetime
      range.  This behaviour is recommended for implementation in
      [RIP2-AUTH], [OSPF2-AUTH], [OSPF3-AUTH], but not in



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      [OSPF3-AUTH-BIS].  The use may become a problem due to an offline
      cryptographic attack (see item (f) of Section 9) or a compromise
      of the key.  In addition, telling a recently expired key from a
      key never ever been in a use may be impossible after a router
      restart.

   Design of this mechanism prevents the automatic switching to
   unauthenticated exchange and is consistent with similar
   authentication mechanisms in this regard.  But since the best choice
   between two other options depends on local site policy, this decision
   is left up to the operator rather than the implementor (in a way
   resembling the "fail secure" configuration knob described in Section
   5.1 of [RIP2-AUTH]).

   Although the deriving procedure does not allow for any exceptions in
   expired keys filtering (Section 5.2 item 2), the operator can
   trivially enforce one of the two remaining behaviour options through
   local key management procedures.  In particular, when using the key
   over its intended lifetime is more preferred than regular traffic
   disruption, the operator would explicitly leave the old key expiry
   time open until the new key is added to the router configuration.  In
   the opposite case the operator would always configure the old key
   with a finite lifetime and bear associated risks.


8.  Implementation Status

   [RFC Editor: before publication please remove this section and the
   reference to [RFC6982], along the offered experiment of which this
   section exists to assist document reviewers.]

   At the time of this writing the original Babel protocol is available
   in two free, production-quality implementations, both of which
   support IPv4 and IPv6 routing but exchange Babel packets using IPv6
   only:

   o  The "standalone" babeld, a BSD-licensed software with source code
      publicly available [1].

      That implementation does not support this authentication
      mechanism.

   o  The integrated babeld component of Quagga-RE, a work derived from
      Quagga routing protocol suite, a GPL-lisensed software with source
      code publicly available [2].

      That implementation supports this authentication mechanism as
      defined in revision 09 of this document.  It supports both



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      mandatory-to-implement hash algorithms (RIPEMD-160 and SHA-1) and
      a few additional algorithms (SHA-224, SHA-256, SHA-384, SHA-512
      and Whirlpool).  It does not support more than one link-local IPv6
      address per interface.  It does not distinguish refused replayed
      packets for purpose of logging in the sense of item (g) of
      Section 5.5 and does not check the packet source address before
      the authentication-specific processing as suggested in
      Section 5.4.  It implements authentication-specific parameters,
      data structures and methods as follows (whether a parameter can be
      "changed at runtime", it is done by means of CLI and can also be
      set in a configuration file):

      *  MaxDigestsIn value is fixed to 4.

      *  MaxDigestsOut value is fixed to 4.

      *  RxAuthRequired value is specific to each interface and can be
         changed at runtime.

      *  ANM Table contents is not retained across speaker restarts, can
         be retrieved and reset (all entries at once) by means of CLI.

      *  ANM Timeout value is specific to the whole protocol instance,
         has a default value of 300 seconds and can be changed at
         runtime.

      *  Ordering of elements within each interface's sequence of CSAs
         is arbitrary as set by operator at runtime.  CSAs are
         implemented to refer to existing key chain syntax items.
         Elements of an interface's sequence of CSAs are constrained to
         be unique reference-wise, but not contents-wise, that is, it is
         possible to duplicate security associations using a different
         key chain name to contain the same keys.

      *  Ordering of elements within each KeyChain sequence is fixed to
         the sort order of LocalKeyID.  LocalKeyID is constrained to be
         unique within each KeyChain sequence.

      *  TS/PC number updates method can be configured at runtime for
         the whole protocol instance to one of two methods standing for
         items (a) and (b) of Section 5.1.  The default method is (b).

      *  Most of the authentication-specific statistics counters listed
         in Section 5.5 are implemented (per protocol instance and per
         each interface) and their readings are available by means of
         CLI with an option to log respective events into a file.

      No other implementations of this authentication mechanism are



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      known to exist, thus interoperability can only be assessed on
      paper.  The only existing implementation has been tested to be
      fully compatible with itself regardless of a speaker CPU
      endianness.


9.  Security Considerations

   Use of this mechanism implies requirements common to a use of shared
   authentication keys, including, but not limited to:

   o  holding the keys secret,

   o  including sufficient amounts of random bits into each key,

   o  rekeying on a regular basis, and

   o  never reusing a used key for a different purpose

   That said, proper design and implementation of a key management
   policy is out of scope of this work.  Many publications on this
   subject exist and should be used for this purpose (BCP 107 [RFC4107],
   BCP 132 [RFC4962], and [RFC6039] may be suggested as starting
   points).

   It is possible for a network that exercises authentication keys
   rollover to experience accidental expiration of all the keys for a
   network interface as discussed at greater length in Section 7.4.
   With that and the guidance of Section 5.1 of [RIP2-AUTH] in mind, in
   such an event the Babel speaker MUST send a "last key expired"
   notification to the operator (e.g. via syslog, SNMP, and/or other
   implementation-specific means), most likely in relation to the item
   (b) of Section 5.5.  Also, any actual occurrence of an authentication
   key expiration MUST cause a security event to be logged by the
   implementation.  The log item MUST include at least a note that the
   authentication key has expired, the Babel routing protocol
   instance(s) affected, the network interface(s) affected, the
   LocalKeyID that is affected, and the current date/time.  Operators
   are encouraged to check such logs as an operational security
   practice.

   Considering particular attacks being in-scope or out of scope on one
   hand and measures taken to protect against particular in-scope
   attacks on the other, the original Babel protocol and this
   authentication mechanism are in line with similar datagram-based
   routing protocols and their respective mechanisms.  In particular,
   the primary concerns addressed are:




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   a.  Peer Entity Authentication

       The Babel speaker authentication mechanism defined herein is
       believed to be as strong as is the class itself that it belongs
       to.  This specification is built on fundamental concepts
       implemented for authentication of similar routing protocols: per-
       packet authentication, use of HMAC construct, use of shared keys.
       Although this design approach does not address all possible
       concerns, it is so far known to be sufficient for most practical
       cases.

   b.  Data Integrity

       Meaningful parts of a Babel datagram are the contents of the
       Babel packet (in the definition of Section 4.2 of [BABEL]) and
       the source address of the datagram (Section 3.5.3 ibid.).  This
       mechanism authenticates both parts using the HMAC construct, so
       that making any meaningful change to an authenticated packet
       after it has been emitted by the sender should be as hard as
       attacking the HMAC construct itself or successfully recovering
       the authentication key.

       Note well that any trailing data of the Babel datagram is not
       meaningful in the scope of the original specification and does
       not belong to the Babel packet.  Integrity of the trailing data
       is respectively not protected by this mechanism.  At the same
       time, although any TLV extra data is also not meaningful in the
       same scope, its integrity is protected, since this extra data is
       a part of the Babel packet (see Figure 2).

   c.  Denial of Service

       Proper deployment of this mechanism in a Babel network
       significantly increases the efforts required for an attacker to
       feed arbitrary Babel PDUs into protocol exchange (with an intent
       of attacking a particular Babel speaker or disrupting exchange of
       regular traffic in a routing domain).  It also protects the
       neighbour table from being flooded with forged speaker entries.

       At the same time, this protection comes with a price of CPU time
       being spent on HMAC computations.  This may be a concern for low-
       performance CPUs combined with high-speed interfaces, as
       sometimes seen in embedded systems and hardware routers.  The
       MaxDigestsIn parameter, which is used to limit the maximum amount
       of CPU time spent on a single received Babel packet, addresses
       this concern to some extent.





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   d.  Reflection Attacks

       Given the approach discussed in item (b), the only potential
       reflection attack on this mechanism could be replaying exact
       copies of Babel packets back to the sender from the same source
       address.  The mitigation in this case is straightforward and is
       discussed in Section 5.4.

   The following in-scope concern is only partially addressed:

   e.  Replay Attacks

       This specification establishes a basic replay protection measure
       (see Section 3.6), defines a timeout parameter affecting its
       strength (see Section 3.7), and outlines implementation methods
       also affecting protection strength in several ways (see
       Section 5.1).  The implementor's choice of the timeout value and
       particular implementation methods may be suboptimal due to, for
       example, insufficient hardware resources of the Babel speaker.
       Furthermore, it may be possible that an operator configures the
       timeout and the methods to address particular local specifics and
       this further weakens the protection.  An operator concerned about
       replay attack protection strength should understand these factors
       and their meaning in a given network segment.

       That said, a particular form of replay attack on this mechanism
       remains possible anyway.  Whether there are two or more network
       segments using the same CSA and there is an adversary that
       captures Babel packets on one segment and replays on another (and
       vice versa due to the bi-directional reachability requirement for
       neighbourship), some of the speakers on one such segment will
       detect the "virtual" neighbours from another and may prefer them
       for some destinations.  This applies even more so as Babel
       doesn't require a common pre-configured network prefix between
       neighbours.

       A reliable solution to this particular problem, which Section 4.5
       of [RFC7186] discusses as well, is not currently known.  It is
       recommended that the operators use distinct CSAs for distinct
       network segments.

   The following in-scope concerns are not addressed:

   f.  Offline Cryptographic Attacks

       This mechanism is obviously subject to offline cryptographic
       attacks.  As soon as an attacker has obtained a copy of an
       authenticated Babel packet of interest (which gets easier to do



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       in wireless networks), he has got all the parameters of the
       authentication-specific processing performed by the sender,
       except authentication key(s) and choice of particular hash
       algorithm(s).  Since digest lengths of common hash algorithms are
       well-known and can be matched with those seen in the packet,
       complexity of this attack is essentially that of the
       authentication key attack.

       Viewing the cryptographic strength of particular hash algorithms
       as a concern of its own, the main practical means of resisting
       offline cryptographic attacks on this mechanism are periodic
       rekeying and use of strong keys with a sufficient number of
       random bits.

       It is important to understand that in the case of multiple keys
       being used within single interface (for a multi-domain
       authentication or during a key rollover) the strength of the
       combined configuration would be that of the weakest key, since
       only one successful HMAC test is required for an authentic
       packet.  Operators concerned about offline cryptographic attacks
       should enforce the same strength policy for all keys used for a
       given interface.

       Note that a special pathological case is possible with this
       mechanism.  Whenever two or more authentication keys are
       configured for a given interface such that all keys share the
       same AuthKeyOctets and the same HashAlgo, but LocalKeyID modulo
       2^16 is different for each key, these keys will not be treated as
       duplicate (Section 5.2 item 4), but an HMAC result computed for a
       given packet will be the same for each of these keys.  In the
       case of sending procedure this can produce multiple HMAC TLVs
       with exactly the same value of the Digest field, but different
       values of KeyID field.  In this case the attacker will see that
       the keys are the same, even without the knowledge of the key
       itself.  Reuse of authentication keys is not the intended use
       case of this mechanism and should be strongly avoided.

   g.  Non-repudiation

       This specification relies on a use of shared keys.  There is no
       timestamp infrastructure and no key revocation mechanism defined
       to address a shared key compromise.  Establishing the time that a
       particular authentic Babel packet was generated is thus not
       possible.  Proving that a particular Babel speaker had actually
       sent a given authentic packet is also impossible as soon as the
       shared key is claimed compromised.  Even with the shared key not
       being compromised, reliably identifying the speaker that had
       actually sent a given authentic Babel packet is not possible any



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       better than proving the speaker belongs to the group sharing the
       key (any of the speakers sharing a key can impose any other
       speaker sharing the same key).

   h.  Confidentiality Violations

       The original Babel protocol does not encrypt any of the
       information contained in its packets.  The contents of a Babel
       packet is trivial to decode, revealing network topology details.
       This mechanism does not improve this situation in any way.  Since
       routing protocol messages are not the only kind of information
       subject to confidentiality concerns, a complete solution to this
       problem is likely to include measures based on the channel
       security model, such as IPSec and WPA2 at the time of this
       writing.

   i.  Key Management

       Any authentication key exchange/distribution concerns are left
       out of scope.  However, the internal representation of
       authentication keys (see Section 3.8) allows for diverse key
       management means, manual configuration in the first place.

   j.  Message Deletion

       Any message deletion attacks are left out of scope.  Since a
       datagram deleted by an attacker cannot be distinguished from a
       datagram naturally lost in transmission and since datagram-based
       routing protocols are designed to withstand a certain loss of
       packets, the currently established practice is treating
       authentication purely as a per-packet function without any added
       detection of lost packets.


10.  IANA Considerations

   [RFC Editor: please do not remove this section.]

   At the time of this publication Babel TLV Types namespace did not
   have an IANA registry.  TLV types 11 and 12 were assigned (see
   Table 1) to the TS/PC and HMAC TLV types by Juliusz Chroboczek,
   designer of the original Babel protocol.  Therefore, this document
   has no IANA actions.


11.  Acknowledgements

   Thanks to Randall Atkinson and Matthew Fanto for their comprehensive



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   work on [RIP2-AUTH] that initiated a series of publications on
   routing protocols authentication, including this one.  This
   specification adopts many concepts belonging to the whole series.

   Thanks to Juliusz Chroboczek, Gabriel Kerneis, and Matthieu Boutier.
   This document incorporates many technical and editorial corrections
   based on their feedback.  Thanks to all contributors to Babel,
   because this work would not be possible without the prior works.
   Thanks to Dominic Mulligan for editorial proofreading of this
   document.  Thanks to Riku Hietamaki for suggesting the test vectors
   section.

   Thanks to Joel Halpern, Jim Schaad, Randall Atkinson, and Stephen
   Farrell for providing (in chronological order) valuable feedback on
   draft versions of this document.

   Thanks to Jim Gettys and Dave Taht for developing CeroWrt wireless
   router project and collaborating on many integration issues.  A
   practical need for Babel authentication emerged during a research
   based on CeroWrt that eventually became the very first use case of
   this mechanism.

   Thanks to Kunihiro Ishiguro and Paul Jakma for establishing GNU Zebra
   and Quagga routing software projects respectively.  Thanks to Werner
   Koch, the author of Libgcrypt.  The very first implementation of this
   mechanism was made on base of Quagga and Libgcrypt.

   This document was produced using the xml2rfc ([RFC2629]) authoring
   tool.


12.  References

12.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [FIPS-198]
              US National Institute of Standards & Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS



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              PUB 198-1, July 2008.

   [BABEL]    Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              April 2011.

12.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270, November 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RIP2-AUTH]
              Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

   [RFC5176]  Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
              Aboba, "Dynamic Authorization Extensions to Remote
              Authentication Dial In User Service (RADIUS)", RFC 5176,
              January 2008.

   [ISIS-AUTH-A]
              Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, October 2008.

   [ISIS-AUTH-B]



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              Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

   [OSPF2-AUTH]
              Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009.

   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, October 2010.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, March 2011.

   [OSPF3-AUTH]
              Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 6506,
              February 2012.

   [RFC6709]  Carpenter, B., Aboba, B., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              September 2012.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              July 2013.

   [I-D.chroboczek-babel-extension-mechanism]
              Chroboczek, J., "Extension Mechanism for the Babel Routing
              Protocol", draft-chroboczek-babel-extension-mechanism-00
              (work in progress), June 2013.

   [OSPF3-AUTH-BIS]
              Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166, March 2014.

   [RFC7183]  Herberg, U., Dearlove, C., and T. Clausen, "Integrity
              Protection for the Neighborhood Discovery Protocol (NHDP)
              and Optimized Link State Routing Protocol Version 2
              (OLSRv2)", RFC 7183, April 2014.




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   [RFC7186]  Yi, J., Herberg, U., and T. Clausen, "Security Threats for
              the Neighborhood Discovery Protocol (NHDP)", RFC 7186,
              April 2014.

URIs

   [1]  <https://github.com/jech/babeld>

   [2]  <https://github.com/Quagga-RE/quagga-RE>










































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Appendix A.  Figures and Tables

      +-------------------------------------------------------------+
      |              authentication-specific statistics             |
      +-------------------------------------------------------------+
        ^                            |                            ^
        |                            v                            |
        |    +-----------------------------------------------+    |
        |    |                system operator                |    |
        |    +-----------------------------------------------+    |
        |        ^ |      ^ |       ^ |       ^ |      ^ |        |
        |        | v      | |       | |       | |      | v        |
      +---+  +---------+  | |       | |       | |  +---------+  +---+
      |   |->|   ANM   |  | |       | |       | |  | LocalTS |->|   |
      | R |<-|  table  |  | |       | |       | |  | LocalPC |<-| T |
      | x |  +---------+  | v       | v       | v  +---------+  | x |
      |   |  +----------------+ +---------+ +----------------+  |   |
      | p |  | MaxDigestsIn   | |         | | MaxDigestsOut  |  | p |
      | r |<-| ANM timeout    | |  CSAs   | |                |->| r |
      | o |  | RxAuthRequired | |         | |                |  | o |
      | c |  +----------------+ +---------+ +----------------+  | c |
      | e |  +-------------+     |       |     +-------------+  | e |
      | s |  |   Rx ESAs   |     |       |     |   Tx ESAs   |  | s |
      | s |<-| (temporary) |<----+       +---->| (temporary) |->| s |
      | i |  +-------------+                   +-------------+  | i |
      | n |  +------------------------------+----------------+  | n |
      | g |  |     instance of              | output buffers |=>| g |
      |   |=>|     the original             +----------------+  |   |
      |   |  |     protocol                 | source address |->|   |
      +---+  +------------------------------+----------------+  +---+
       /\                                            |            ||
       ||                                            v            \/
      +-------------------------------------------------------------+
      |                        network stack                        |
      +-------------------------------------------------------------+
         /\ ||       /\ ||                       /\ ||       /\ ||
         || \/       || \/                       || \/       || \/
      +---------+ +---------+                 +---------+ +---------+
      | speaker | | speaker |       ...       | speaker | | speaker |
      +---------+ +---------+                 +---------+ +---------+


      Flow of control data           : --->
      Flow of Babel datagrams/packets: ===>

                       Figure 1: Interaction Diagram





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                  P
   |<---------------------------->|                                 (D1)
   |                B             |
   |  |<------------------------->|
   |  |                           |
   +--+-----+-----+...+-----+-----+--+   P: Babel packet
   |H |some |some |   |some |some |T |   H: Babel packet header
   |  |TLV  |TLV  |   |TLV  |TLV  |  |   B: Babel packet body
   |  |     |     |   |     |     |  |   T: optional trailing data block
   +--+-----+-----+...+-----+-----+--+

                               P
   |<----------------------------------------------------->|        (D2)
   |                             B                         |
   |  |<-------------------------------------------------->|
   |  |                                                    |
   +--+-----+-----+...+-----+-----+------+------+...+------+--+
   |H |some |some |   |some |some |TS/PC |HMAC  |   |HMAC  |T |
   |  |TLV  |TLV  |   |TLV  |TLV  |TLV   |TLV 1 |   |TLV n |  |
   |  |     |     |   |     |     |      |      |   |      |  |
   +--+-----+-----+...+-----+-----+------+------+...+------+--+

                               P
   |<----------------------------------------------------->|        (D3)
   |                             B                         |
   |  |<-------------------------------------------------->|
   |  |                                                    |
   +--+------+------+...+------+-----+-----+...+-----+-----+--+
   |H |TS/PC |HMAC  |   |HMAC  |some |some |   |some |some |T |
   |  |TLV   |TLV 1 |   |TLV n |TLV  |TLV  |   |TLV  |TLV  |  |
   |  |      |      |   |      |     |     |   |     |     |  |
   +--+------+------+...+------+-----+-----+...+-----+-----+--+


                                  P
   |<------------------------------------------------------------>| (D4)
   |                                B                             |
   |  |<--------------------------------------------------------->|
   |  |                                                           |
   +--+-----+------+-----+------+...+-----+------+...+------+-----+--+
   |H |some |HMAC  |some |HMAC  |   |some |HMAC  |   |TS/PC |some |T |
   |  |TLV  |TLV 1 |TLV  |TLV 2 |   |TLV  |TLV n |   |TLV   |TLV  |  |
   |  |     |      |     |      |   |     |      |   |      |     |  |
   +--+-----+------+-----+------+...+-----+------+...+------+-----+--+


                    Figure 2: Babel Datagram Structure




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            +-------+-------------------------+---------------+
            | Value | Name                    | Reference     |
            +-------+-------------------------+---------------+
            |     0 | Pad1                    | [BABEL]       |
            |     1 | PadN                    | [BABEL]       |
            |     2 | Acknowledgement Request | [BABEL]       |
            |     3 | Acknowledgement         | [BABEL]       |
            |     4 | Hello                   | [BABEL]       |
            |     5 | IHU                     | [BABEL]       |
            |     6 | Router-Id               | [BABEL]       |
            |     7 | Next Hop                | [BABEL]       |
            |     8 | Update                  | [BABEL]       |
            |     9 | Route Request           | [BABEL]       |
            |    10 | Seqno Request           | [BABEL]       |
            |    11 | TS/PC                   | this document |
            |    12 | HMAC                    | this document |
            +-------+-------------------------+---------------+

                   Table 1: Babel TLV Types 0 through 12

    +--------------+-----------------------------+-------------------+
    | Packet field | Packet octets (hexadecimal) | Meaning (decimal) |
    +--------------+-----------------------------+-------------------+
    | Magic        | 2a                          | 42                |
    | Version      | 02                          | version 2         |
    | Body length  | 00:14                       | 20 octets         |
    | [TLV] Type   | 04                          | 4 (Hello)         |
    | [TLV] Length | 06                          | 6 octets          |
    | Reserved     | 00:00                       | no meaning        |
    | Seqno        | 09:25                       | 2341              |
    | Interval     | 01:90                       | 400 (4.00 s)      |
    | [TLV] Type   | 08                          | 8 (Update)        |
    | [TLV] Length | 0a                          | 10 octets         |
    | AE           | 00                          | 0 (wildcard)      |
    | Flags        | 40                          | default router-id |
    | Plen         | 00                          | 0 bits            |
    | Omitted      | 00                          | 0 bits            |
    | Interval     | ff:ff                       | infinity          |
    | Seqno        | 68:21                       | 26657             |
    | Metric       | ff:ff                       | infinity          |
    +--------------+-----------------------------+-------------------+

            Table 2: A Babel Packet without Authentication TLVs








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   +---------------+-------------------------------+-------------------+
   | Packet field  | Packet octets (hexadecimal)   | Meaning (decimal) |
   +---------------+-------------------------------+-------------------+
   | Magic         | 2a                            | 42                |
   | Version       | 02                            | version 2         |
   | Body length   | 00:4c                         | 76 octets         |
   | [TLV] Type    | 04                            | 4 (Hello)         |
   | [TLV] Length  | 06                            | 6 octets          |
   | Reserved      | 00:00                         | no meaning        |
   | Seqno         | 09:25                         | 2341              |
   | Interval      | 01:90                         | 400 (4.00 s)      |
   | [TLV] Type    | 08                            | 8 (Update)        |
   | [TLV] Length  | 0a                            | 10 octets         |
   | AE            | 00                            | 0 (wildcard)      |
   | Flags         | 40                            | default router-id |
   | Plen          | 00                            | 0 bits            |
   | Omitted       | 00                            | 0 bits            |
   | Interval      | ff:ff                         | infinity          |
   | Seqno         | 68:21                         | 26657             |
   | Metric        | ff:ff                         | infinity          |
   | [TLV] Type    | 0b                            | 11 (TS/PC)        |
   | [TLV] Length  | 06                            | 6 octets          |
   | PacketCounter | 00:01                         | 1                 |
   | Timestamp     | 52:1d:7e:8b                   | 1377664651        |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:c8                         | 200               |
   | Digest        | fe:80:00:00:00:00:00:00:0a:11 | padding           |
   |               | 96:ff:fe:1c:10:c8:00:00:00:00 |                   |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:64                         | 100               |
   | Digest        | fe:80:00:00:00:00:00:00:0a:11 | padding           |
   |               | 96:ff:fe:1c:10:c8:00:00:00:00 |                   |
   +---------------+-------------------------------+-------------------+

   Table 3: A Babel Packet with Each HMAC TLV Padded Using IPv6 Address
                         fe80::0a11:96ff:fe1c:10c8













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   +---------------+-------------------------------+-------------------+
   | Packet field  | Packet octets (hexadecimal)   | Meaning (decimal) |
   +---------------+-------------------------------+-------------------+
   | Magic         | 2a                            | 42                |
   | Version       | 02                            | version 2         |
   | Body length   | 00:4c                         | 76 octets         |
   | [TLV] Type    | 04                            | 4 (Hello)         |
   | [TLV] Length  | 06                            | 6 octets          |
   | Reserved      | 00:00                         | no meaning        |
   | Seqno         | 09:25                         | 2341              |
   | Interval      | 01:90                         | 400 (4.00 s)      |
   | [TLV] Type    | 08                            | 8 (Update)        |
   | [TLV] Length  | 0a                            | 10 octets         |
   | AE            | 00                            | 0 (wildcard)      |
   | Flags         | 40                            | default router-id |
   | Plen          | 00                            | 0 bits            |
   | Omitted       | 00                            | 0 bits            |
   | Interval      | ff:ff                         | infinity          |
   | Seqno         | 68:21                         | 26657             |
   | Metric        | ff:ff                         | infinity          |
   | [TLV] Type    | 0b                            | 11 (TS/PC)        |
   | [TLV] Length  | 06                            | 6 octets          |
   | PacketCounter | 00:01                         | 1                 |
   | Timestamp     | 52:1d:7e:8b                   | 1377664651        |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:c8                         | 200               |
   | Digest        | c6:f1:06:13:30:3c:fa:f3:eb:5d | HMAC result       |
   |               | 60:3a:ed:fd:06:55:83:f7:ee:79 |                   |
   | [TLV] Type    | 0c                            | 12 (HMAC)         |
   | [TLV] Length  | 16                            | 22 octets         |
   | KeyID         | 00:64                         | 100               |
   | Digest        | df:32:16:5e:d8:63:16:e5:a6:4d | HMAC result       |
   |               | c7:73:e0:b5:22:82:ce:fe:e2:3c |                   |
   +---------------+-------------------------------+-------------------+

   Table 4: A Babel Packet with Each HMAC TLV Containing an HMAC Result


Appendix B.  Test Vectors

   The test vectors below may be used to verify the correctness of some
   procedures performed by an implementation of this mechanism, namely:

   o  appending of TS/PC and HMAC TLVs to the Babel packet body,

   o  padding of the HMAC TLV(s),




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   o  computation of the HMAC result(s), and

   o  placement of the result(s) in the TLV(s).

   This verification isn't exhaustive, there are other important
   implementation aspects that would require testing methods of their
   own.

   The test vectors were produced as follows.

   1.  A Babel speaker with a network interface with IPv6 link-local
       address fe80::0a11:96ff:fe1c:10c8 was configured to use two CSAs
       for the interface:

       *  CSA1={HashAlgo=RIPEMD-160, KeyChain={{LocalKeyID=200,
          AuthKeyOctets=Key26}}}

       *  CSA2={HashAlgo=SHA-1, KeyChain={{LocalKeyId=100,
          AuthKeyOctets=Key70}}}

       The authentication keys above are:

       *  Key26 in ASCII:

   ABCDEFGHIJKLMNOPQRSTUVWXYZ

       *  Key26 in hexadecimal:

   41:42:43:44:45:46:47:48:49:4a:4b:4c:4d:4e:4f:50
   51:52:53:54:55:56:57:58:59:5a

       *  Key70 in ASCII:

  This=key=is=exactly=70=octets=long.=ABCDEFGHIJKLMNOPQRSTUVWXYZ01234567

       *  Key70 in hexadecimal:

   54:68:69:73:3d:6b:65:79:3d:69:73:3d:65:78:61:63
   74:6c:79:3d:37:30:3d:6f:63:74:65:74:73:3d:6c:6f
   6e:67:2e:3d:41:42:43:44:45:46:47:48:49:4a:4b:4c
   4d:4e:4f:50:51:52:53:54:55:56:57:58:59:5a:30:31
   32:33:34:35:36:37

       The length of each key was picked to relate (in the terms of
       Section 2.4) with the properties of respective hash algorithm as
       follows:





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       *  the digest length (L) of both RIPEMD-160 and SHA-1 is 20
          octets,

       *  the internal block size (B) of both RIPEMD-160 and SHA-1 is 64
          octets,

       *  the length of Key26 (26) is greater than L but less than B,
          and

       *  the length of Key70 (70) is greater than B (and thus greater
          than L).

       KeyStartAccept, KeyStopAccept, KeyStartGenerate and
       KeyStopGenerate were set to make both authentication keys valid.

   2.  The instance of the original protocol of the speaker produced a
       Babel packet (PktO) to be sent from the interface.  Table 2
       provides a decoding of PktO, contents of which is below:

   2a:02:00:14:04:06:00:00:09:25:01:90:08:0a:00:40
   00:00:ff:ff:68:21:ff:ff

   3.  The authentication mechanism appended one TS/PC TLV and two HMAC
       TLVs to the packet body, updated the "Body length" packet header
       field and padded the Digest field of the HMAC TLVs using the
       link-local IPv6 address of the interface and necessary amount of
       zeroes.  Table 3 provides a decoding of the resulting temporary
       packet (PktT), contents of which is below:

   2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
   00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
   0c:16:00:c8:fe:80:00:00:00:00:00:00:0a:11:96:ff
   fe:1c:10:c8:00:00:00:00:0c:16:00:64:fe:80:00:00
   00:00:00:00:0a:11:96:ff:fe:1c:10:c8:00:00:00:00

   4.  The authentication mechanism produced two HMAC results,
       performing the computations as follows:

       *  For H=RIPEMD-160, K=Key26, and Text=PktT the HMAC result is:

   c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a:ed:fd:06:55
   83:f7:ee:79

       *  For H=SHA-1, K=Key70, and Text=PktT the HMAC result is:

   df:32:16:5e:d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82
   ce:fe:e2:3c




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   5.  The authentication mechanism placed each HMAC result into
       respective HMAC TLV, producing the final authenticated Babel
       packet (PktA), which was eventually sent from the interface.
       Table 4 provides a decoding of PktA, contents of which is below:

   2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
   00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
   0c:16:00:c8:c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a
   ed:fd:06:55:83:f7:ee:79:0c:16:00:64:df:32:16:5e
   d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82:ce:fe:e2:3c

   Interpretation of this process is to be done in the view of Figure 1,
   differently for the sending and the receiving directions.

   For the sending direction, given a Babel speaker configured using the
   IPv6 address and the sequence of CSAs as described above, the
   implementation SHOULD (see notes in Section 5.3) produce exactly the
   temporary packet PktT if the original protocol instance produces
   exactly the packet PktO to be sent from the interface.  If the
   temporary packet exactly matches PktT, the HMAC results computed
   afterwards MUST exactly match respective results above and the final
   authenticated packet MUST exactly match the PktA above.

   For the receiving direction, given a Babel speaker configured using
   the sequence of CSAs as described above (but a different IPv6
   address), the implementation MUST (assuming the TS/PC check didn't
   fail) produce exactly the temporary packet PktT above if its network
   stack receives through the interface exactly the packet PktA above
   from the source IPv6 address above.  The first HMAC result computed
   afterwards MUST match the first result above.  The receiving
   procedure doesn't compute the second HMAC result in this case, but if
   the implementor decides to compute it anyway for the verification
   purpose, it MUST exactly match the second result above.


Author's Address

   Denis Ovsienko
   Yandex
   16, Leo Tolstoy St.
   Moscow,   119021
   Russia

   Email: infrastation@yandex.ru







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