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Network Working Group                                        D. Ovsienko
Internet-Draft                                                    Yandex
Updates: 6126 (if approved)                              August 20, 2012
Intended status: Experimental
Expires: February 21, 2013


                Babel HMAC Cryptographic Authentication
              draft-ovsienko-babel-hmac-authentication-00

Abstract

   This document describes a cryptographic authentication mechanism for
   Babel routing protocol, updating, but not superceding 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 21, 2013.

Copyright Notice

   Copyright (c) 2012 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Cryptographic Aspects  . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Neutral Use of Hash Algorithms . . . . . . . . . . . . . .  4
     2.2.  Padding Constant Specifics . . . . . . . . . . . . . . . .  5
     2.3.  Cryptographic Sequence Number Specifics  . . . . . . . . .  6
     2.4.  Definition of HMAC . . . . . . . . . . . . . . . . . . . .  6
   3.  Updates to Protocol Data Structures  . . . . . . . . . . . . .  8
     3.1.  RxAuthRequired . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  LocalTS  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  LocalPC  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  MaxDigestsIn . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  MaxDigestsOut  . . . . . . . . . . . . . . . . . . . . . .  9
     3.6.  ANM Table  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.7.  ANM Timeout  . . . . . . . . . . . . . . . . . . . . . . . 11
     3.8.  Configured Security Associations . . . . . . . . . . . . . 11
     3.9.  Effective Security Associations  . . . . . . . . . . . . . 13
   4.  Updates to Protocol Encoding . . . . . . . . . . . . . . . . . 14
     4.1.  Justification  . . . . . . . . . . . . . . . . . . . . . . 14
     4.2.  TS/PC TLV  . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  HMAC TLV . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Updates to Protocol Operation  . . . . . . . . . . . . . . . . 17
     5.1.  Per-interface TS/PC Number Updates . . . . . . . . . . . . 17
     5.2.  Deriving ESAs from CSAs  . . . . . . . . . . . . . . . . . 19
     5.3.  Updates to Packet Sending  . . . . . . . . . . . . . . . . 21
     5.4.  Updates to Packet Receiving  . . . . . . . . . . . . . . . 23
     5.5.  Authentication-specific Statistics Maintenance . . . . . . 25
   6.  Implementation Notes . . . . . . . . . . . . . . . . . . . . . 26
     6.1.  IPv6 Source Address Selection for Sending  . . . . . . . . 26
     6.2.  Output Buffer Management . . . . . . . . . . . . . . . . . 26
     6.3.  Optimizations of ESAs Deriving . . . . . . . . . . . . . . 27
     6.4.  Internal Representation of CSAs  . . . . . . . . . . . . . 28
   7.  Network Management Aspects . . . . . . . . . . . . . . . . . . 28
     7.1.  Backward Compatibility . . . . . . . . . . . . . . . . . . 28
     7.2.  Multi-Domain Authentication  . . . . . . . . . . . . . . . 29
     7.3.  Migration  . . . . . . . . . . . . . . . . . . . . . . . . 30
     7.4.  Handling of Authentication Keys Exhaustion . . . . . . . . 31
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     11.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Appendix A.  Figures . . . . . . . . . . . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41




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1.  Introduction

   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 operation of a basic Babel routing protocol instance
   ("instance of the original protocol").  This document ("this
   specification") defines data structures, encoding, and operation of
   an extension to 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 such a transparency to an
   operator, 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 key parameters cannot be changed.

   The currently established (see [RIP2-AUTH], [OSPF2-AUTH],
   [OSPF3-AUTH], 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 number, 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 another, 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.

   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



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   from the declared characteristics of an implementation and, whereas
   the implementation allows changing relevant parameters, the effect of
   a reconfiguration.

   The HMAC construct can be combined with any cryptographic hash
   algorithm, although the primary focus of [RIP2-AUTH], [OSPF2-AUTH],
   and [OSPF3-AUTH] is either SHA-1 hash algorithm or SHA-2 family of
   hash algorithms, or both.  This specification does not mandate or
   suggest a use of any particular hash algorithms.  This mechanism can
   be deployed using any appropriate hash algorithms, as long as Babel
   speakers participating in the authenticated exchange are implemented
   and configured consistently.

   This mechanism explicitly allows for multiple HMAC results per an
   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.

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

   These limits establish a basic competition between the configured
   keys and (in 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.

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 RFC 2119 [RFC2119].


2.  Cryptographic Aspects

2.1.  Neutral Use of Hash Algorithms

   The only hash algorithm characteristics meaningful within the scope
   of processing defined herein are digest length and internal block
   size, there is no pre- or post-processing specific to a particular
   hash algorithm.  The following generic requirements affect only the



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   set of options available for an implementation.

   A set of hash algorithms available in an implementation MUST be
   clearly stated, MUST include at least one option and SHOULD include
   multiple options.  Implementers SHOULD consider strong, well-known
   hash algorithms as 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.

   For example, the following hash algorithms meet these requirements at
   the time of this writing:

   o  GOST (256-bit hash)

   o  RIPEMD-160

   o  SHA-224

   o  SHA-256

   o  SHA-384

   o  SHA-512

   o  Tiger (192-bit hash)

   o  Whirlpool (512-bit hash)

   The final choice of particular hash algorithm(s) is left up to the
   implementer.  Whether known weak authentication keys exist for a hash
   algorithm used in an implementation of this mechanism, the
   implementation MUST deny a use of such keys.

2.2.  Padding Constant Specifics

   [RIP2-AUTH] established the reference method of HMAC construct
   application housing the computed authentication data inside the
   message being authenticated.  This involves pre-allocating necessary
   amount of message data space and pre-filling it with some data a
   receiver can reproduce exactly, typically an arbitrary number known
   as a padding constant.  The padding constant used in [RIP2-AUTH] is
   0x878FE1F3 four-octet value.

   Subsequent works (including [OSPF2-AUTH] and [OSPF3-AUTH]) inherited
   both the basic approach and the padding constant.  In particular,
   [OSPF3-AUTH] uses a source IPv6 address to set the first 16 octets of
   the padded area and the padding constant to set any subsequent
   octets.  This mechanism makes the same use for the source IPv6



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   address, but the padding constant size and value are different.

   Since any fixed arbitrary value of a padding constant does not affect
   cryptographic characteristics of a hash algorithm and the HMAC
   construct, and since single-octet padding is more straightforward to
   implement, the padding constant used by this mechanism is 0x00
   single-octet value.  This is respectively addressed in sending
   (Section 5.3 item 5) and receiving (Section 5.4 item 6) procedures.

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 sequence number of the
   original Babel specification (Section 2.5 of [BABEL]) 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               |
   +-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      //

   High-order 32 bits are called "timestamp" (TS) and 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 8).  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].





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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 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], [OSPF2-AUTH], and [OSPF3-AUTH]
   in exactly two regards:

   o  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 having length greater than
      L but not greater than B.

   o  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 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 appended to the end of K,



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       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.


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 changing RxAuthRequired
   value in runtime or by means of Babel speaker restart.  An
   implementation MUST allow the operator discovering the effective
   value of RxAuthRequired in runtime or from the system documentation.

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 operator.  Its initialization
   is explained in Section 5.1.







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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 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 changing the value of MaxDigestsIn in runtime or by means of
   Babel speaker restart.  An implementation MUST allow the operator
   discovering the effective value of MaxDigestsIn in 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.

   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 changing the value of MaxDigestsOut in 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 discovering the effective
   value of MaxDigestsOut in runtime or from the system documentation.

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 term "ANM table" means the table defined herein.
   Indexing of the ANM table is done in exactly the same way as indexing



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   of the neighbour table, but purpose, field set and associated
   procedures are different.

   Conceptual purpose of the ANM table is to provide a 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

      IPv6 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 8).  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
   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 retrieving the current contents of ANM table in runtime
   through common management interfaces such as CLI and SNMP.  An
   implementation SHOULD provide a mean to remove some or all ANM table
   entries in runtime or by means of Babel speaker restart.



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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 provide not 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
   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
   involving physical moves of network interfaces hardware between
   routers.  Even performed without restarting Babel 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 using a lower
   ANM timeout value to let the entries expire on their own rather than
   having to manually remove them from 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 changing the value of ANM timeout in
   runtime or by means of Babel speaker restart.  An implementation MUST
   allow the operator discovering the effective value of ANM timeout in
   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 ordered
   lists, one list per each interface.  Each interface's list of CSAs is



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   an integral part of the Babel speaker configuration.  The default
   state of an interface's list 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

      An ordered list of items representing authentication keys, each
      item being a structure consisting of the following fields:

      *  LocalKeyID

         An unsigned integer.

      *  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
         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.



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      It is possible for the KeyChain list to be empty, although this is
      not the intended way of CSAs use.

   Since there is no limit imposed on number of CSAs per an interface,
   but number of HMAC computations per a 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.  Ordering of items within a list of CSAs and within a
   KeyChain list is important to make association selection process
   deterministic and transparent.  Once this ordering is deterministic
   at 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 items within a given interface's list of CSAs and within the
   KeyChain list of a given CSA.  Whenever this requirement cannot be
   met, the implementation MUST provide a mean to discover the actual
   item order used.  Whichever order is used by an implementation, it
   MUST be preserved across Babel speaker restarts.

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 establish a runtime
   interface between those procedures and the deriving procedure defined
   in Section 5.2.  All ESAs are managed in ordered, temporary lists,
   which are not intended for any persistent storage.  Item ordering
   within a temporary list of ESAs MUST be preserved as long as the list
   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.




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4.  Updates to Protocol Encoding

4.1.  Justification

   Choice of encoding is very important in the long term.  Protocol
   encoding defines possible options of authentication mechanism design
   and encoding, which in turn define options of future developments of
   the protocol.

   Considering existing implementations of Babel protocol instance
   itself and related modules of packet analysers, 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]:

   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 sequence, 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 sequence of TLVs 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.  Only TLV length matters within
       the sequence, TLV body contents is to be interpreted elsewhere.
       This makes an iteration over the sequence possible without a
       knowledge of body structure of each TLV (with the only
       distinction between a Pad1 TLV and any other TLVs).  The original
       specification allocates TLV types 0 through 10 and defines TLV
       body structure for each.  An instance of the original protocol
       silently ignores any unknown TLV types.

   c.  Within each TLV of the sequence 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.




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

   With these additional two types the Babel TLV types namespace appears
                                as follows:

            +-------+-------------------------+---------------+
            | Value | Code                    | 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 |




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            | 12    | HMAC                    | this document |
            +-------+-------------------------+---------------+

4.2.  TS/PC TLV

   The purpose of a TS/PC TLV is to store a single TS/PC number.  There
   is normally exactly one TS/PC TLV in an authenticated Babel packet.
   Any occurences of this TLV except the first are ignored.

    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 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 ordering of PacketCounter and Timestamp in TLV structure is
   opposite to the ordering of TS and PC in "TS/PC" term and the 48-bit
   equivalent.

   Considering the "expected length" and the "extra data" in the
   definition of Section 4.2 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.



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4.3.  HMAC TLV

   The purpose of a 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 normally 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 a HMAC TLV.

   Length          The length 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, that MUST be at
                   least 16 octets long.

   Considering the "expected length" and the "extra data" in the
   definition of Section 4.2 of [BABEL], the expected length of a HMAC
   TLV body is not defined.  The receiving procedure processes every
   octet of the Digest field, deriving the field boundary from the
   Length field value (Section 5.4 item 6).  The sending procedure
   produces HMAC TLVs with Length field precisely sizing the Digest
   field to match digest length of the hash algorithm used (Section 5.3
   items 5 and 8).

   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

   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



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   is only one property mandatory for the advertised TS/PC number: its
   48-bit equivalent MUST be strictly increasing within the scope of a
   given interface of a Babel speaker as long as the speaker 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 advertised TS/PC numbers would be reused after each
       Babel speaker restart, making neighbouring speakers reject
       authenticated packets until 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
       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



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       across 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.  Whether there is a TS value
                       stored in NVRAM for the current interface, set
                       LocalTS to that 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 speaker's deployed lifetime, relying on NVRAM for storing
       multiple TS numbers, one per each interface.

   As long as the TS/PC number retains its mandatory property stated
   above, an implementer is free to decide, which TS/PC updates
   implementation methods are available to an operator and whether the
   method can be configured per-interface and/or in runtime.  To enable
   the optimal (see Section 3.7) management of ANM timeout in a network
   segment, an implementation MUST allow the operator discovering exact
   matter of the TS/PC update method effective for any interface, either
   in runtime or from the system documentation.

   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 of 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 list of CSAs directly, both (Section 5.4 item 4 and
   Section 5.3 item 4 respectively) derive a list of ESAs from the list
   of CSAs and use the derived list (see Figure 1).  There are two main
   goals achieved through this indirection:



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   o  Filtering of expired and duplicate security associations.  This is
      done earliest possible to keep subsequent procedures focused on
      their respective tasks.

   o  Maintenance of particular sort order in the derived list of ESAs.
      The sort order deterministically depends on the sort order of
      interface's list of CSAs and sort order of KeyChain items of each
      CSA.  Particular correlation maintained by this procedure
      implements a concept of fair (independent of number of keys used
      by each) competition between CSAs.

   The deriving procedure uses the following input arguments:

   o  input list of CSAs

   o  direction (sending or receiving)

   o  current time (CT)

   Processing of input arguments begins with an empty ordered output
   list of ESAs and consists of the following steps:

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

   2.  Remove all expired keys from the copy, that is, any keys such
       that:

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

       *  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.  Remove all duplicate keys from the copy.  A duplicate key (Kd)
       within a list of CSAs is a key, for that another key (Ka) exists
       within the same list of CSAs such that every statement below is
       true:

       *  HashAlgo of the CSA containing Kd is equal to HashAlgo of the
          CSA containing Ka.

       *  LocalKeyID modulo 2^16 of Kd is equal to LocalKeyID modulo
          2^16 of Ka




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       *  AuthKeyOctets of Kd is equal to AuthKeyOctets of Ka

   4.  Use the copy to populate the output list of ESAs as follows:

       1.  Whether the KeyChain list 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 list.

       2.  Whether the KeyChain list 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.  Whether the KeyChain list of the first CSA contains at least
           two keys, use its second key the same way.

       4.  Whether the KeyChain list 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 one time each.

   The resulting list will contain zero or more unique ESAs, ordered in
   a way deterministically correlated with sort order of CSAs within the
   original input list of CSAs and sort orders of keys within each
   KeyChain list.  This ordering maximizes the probability of having
   equal amount of keys per original CSA in any N first items of the
   resulting list.  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.



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   1.  If the current outgoing interface's list 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.  Append a TS/PC TLV to the packet's sequence of TLVs 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 list of ESAs using procedure defined in Section 5.2 with
       the current interface's list of CSAs as the input list of CSAs,
       current time as CT and "sending" as the direction.  Note that
       both the input list of CSAs and the derived list of ESAs are
       sorted.  Proceed to the next step even if the derived list is
       empty.

   5.  Iterate over the derived list using its sort order.  For each ESA
       append a HMAC TLV to the end of the packet's sequence of TLVs
       with fields set as follows:

       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 to the digest length of HashAlgo of the
                current ESA.  Set the first 16 octets to the source IPv6
                address of the current packet (see Section 6.1) and any
                subsequent octets to 0x00 (see Figure 3).

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




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       Note that the current step may involve byte order conversion.

   6.  Update "Body length" field of the current packet header to
       include the total length of TS/PC and HMAC TLVs added to the
       current packet 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 list again, using the same very order
       and amount of items.  For each ESA (and respectively for each
       HMAC TLV recently added to the current packet) compute a 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 Figure 4) of the
       current packet (not the copy).

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

   Note that even if the derived list of ESAs is empty, the packet is
   sent anyway with only a TS/PC TLV appended to its sequence of TLVs.
   Although such a packet is not authenticated, presence of a sole TS/PC
   TLV indicates authentication keys exhaustion to operators of
   neighbouring Babel speakers.  See also Section 7.4.

5.4.  Updates to Packet Receiving

   Perform the following authentication-specific processing after an
   incoming Babel packet is received from local IPv6 stack, but before
   it is processed 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 list of CSAs is empty,
        accept the packet.

   2.   If the current packet does not contain a 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



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        source address of the current packet.  If such an entry exists,
        compare its LastTS and LastPC field values with Timestamp and
        PacketCounter values respectively of the first 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

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

   5.   Make a temporary copy of the current packet.

   6.   For every HMAC TLV present in the temporary copy (not the
        original packet) pad all octets of its Digest field using the
        source IPv6 address of the current packet to set the first 16
        octets and 0x00 to set any subsequent octets (see Figure 3).

   7.   Iterate over all 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 list 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 order of their appearance on the
        full list of ESAs.  Note that nesting the iterations the
        opposite way (over ESAs, then over HMAC TLVs) is wrong.

        For each of these ESAs compute a 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 number of HMAC
        computations done for the current packet 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.   If none of the HMAC results computed during the previous step
        matched, refuse the input packet.



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   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 first TS/PC TLV of the current packet.  That is, either
        add a new ANM table entry or update the existing one, according
        to the result of the entry lookup above.  Reset the entry's
        aging timer to the current value of ANM timeout.

        Note that the current step may involve byte order conversion.

   10.  Accept the input packet.

   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 each
   interface:

   o  Sending of an unauthenticated Babel packet through an interface
      having an empty list of CSAs.

   o  Sending of an unauthenticated Babel packet with a TS/PC TLV but
      without any HMAC TLVs due to an empty list of ESAs.

   o  Sending of an authenticated Babel packet containing both TS/PC and
      HMAC TLVs.

   o  Accepting of a Babel packet received through an interface having
      an empty list of CSAs.

   o  Refusing of a received Babel packet due to an empty list of ESAs.

   o  Refusing of a received Babel packet missing any TS/PC TLVs.

   o  Refusing of a received Babel packet due to the first TS/PC TLV
      failing the ANM table check.

   o  Refusing of a received Babel packet missing any HMAC TLVs.





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   o  Refusing of a received Babel packet due to none of the processed
      HMAC TLVs passing the ESA check.

   o  Accepting of a received Babel packet having both TS/PC and HMAC
      TLVs.

   o  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 of
   RxAuthRequired parameter set to FALSE.  Event counters readings
   SHOULD be available in runtime through common management interfaces
   such as CLI and SNMP.


6.  Implementation Notes

6.1.  IPv6 Source Address Selection for Sending

   Section 3.1 of [BABEL] defines, that Babel datagrams are exchanged
   using IPv6 link-local address as source address.  This implies having
   at least one such address assigned to an interface participating in
   the exchange.  When the interface has more than one link-local
   addresses assigned, selection of one particular link-local address as
   packet source address is left up to the local IPv6 stack, since this
   choice is not meaningful in the scope of the original protocol.
   However, the sending procedure defined in Section 5.3 requires exact
   knowledge of packet source address for proper padding of HMAC TLVs.

   As long as a Babel interface has more than one IPv6 link-local
   addresses assigned, the Babel speaker SHOULD internally choose one
   particular link-local address for Babel packet sending purposes and
   make this choice to both the sending procedure and local IPv6 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 IPv6 address management 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 or an urgent TLV is
   produced.  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



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   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.  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 a HMAC TLV.

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

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

   The maximum safe value of MaxDigestsOut parameter depends on
   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 interface MTU is larger or digest length is smaller,
   higher values of MaxDigestsOut can be used safely.

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 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.




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       The procedure would obviously return the same result for the same
       input arguments (list 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 list 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 list 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 list.  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 list of CSAs or
       after reaching one of the critical points mentioned above.

   b.  Considering, that the sending procedure iterates over at most
       MaxDigestsOut items of the ordered list of derived ESAs
       (Section 5.3 item 5), there is little sense in the case of
       "sending" direction in appending ESA items to the end of the
       output list once the list already contains MaxDigestsOut number
       of items.  Note that a similar optimization is impossible in the
       case of "receiving" direction, since number of ESAs actually used
       in examining a particular packet cannot be determined in advance.

6.4.  Internal Representation of CSAs

   Note that the KeyChain list of the CSA structure is a direct
   equivalent of 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.


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 not



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   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.

   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 avoided 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 list 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.

   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)



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   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 consequences of a
   possible misconfiguration to the scope of one CSA.  See also item (e)
   of Section 8.

7.3.  Migration

   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 for such 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:

   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).




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   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
       RxAuthRequired parameter to TRUE.

   Likewise, temporarily setting RxAuthRequired to FALSE can be used to
   migrate smoothly from 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).

   o  Stopping routing information exchange over the interface.  This
      behaviour is likely to impact regular traffic routing and is
      commonly viewed as "not advisable" (ibid.).

   o  Use of the "most recently expired" key over its intended lifetime
      range.  This behaviour is commonly recommended for implementation
      (ibid.), although it may become a problem due to an offline
      cryptographic attack (see item (e) of Section 8) 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 implementer (in a way
   resembling the "fail secure" configuration knob described in Section
   5.1 of [RIP2-AUTH]).



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   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.  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 amount 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.

   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:

   a.  Peer Entity Authentication

       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 the 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.






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   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
       IPv6 source address of the datagram (ibid.  Section 3.5.3).  This
       mechanism authenticates both parts using a 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 hash algorithm 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.  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).  Implementer'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.

   d.  Denial of Service

       Proper deploy 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 for 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 is seen in embedded systems and hardware routers.  The
       MaxDigestsIn parameter, which is purposed to limit the maximum



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       amount of CPU time spent on a single received Babel packet,
       addresses this concern to some extent.

   The following in-scope concerns are not addressed:

   e.  Offline Cryptographic Attacks

       This mechanism is an obvious 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
       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 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 sufficient amount 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) 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 3), but a 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
       value 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.






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

   g.  Confidentiality Violations

       The original Babel protocol does not encrypt any of the
       information contained in its packets.  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.

   h.  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.

   i.  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.








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9.  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 to the
   TS/PC and HMAC TLV types by Juliusz Chroboczek, designer of the
   original Babel protocol.  Therefore, this document has no IANA
   actions.


10.  Acknowledgements

   Thanks to Ran Atkinson and Matthew Fanto for their comprehensive 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 for his works on mesh networking in
   general and Babel routing protocol in particular, and also for
   feedback on early revisions of this document.  This work would not be
   possible without prior works on Babel.

   Thank 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.


11.  References

11.1.  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.



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   [FIPS-198]
              US National Institute of Standards & Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
              198 , March 2002.

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

11.2.  Informative References

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

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

   [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.

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





















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Appendix A.  Figures

      +-------------------------------------------------------------+
      |              authentication-specific statistics             |
      +-------------------------------------------------------------+
        ^                            |                            ^
        |                            v                            |
        |    +-----------------------------------------------+    |
        |    |                system operator                |    |
        |    +-----------------------------------------------+    |
        |              ^ |          ^ |          ^ |              |
        |              | v          | v          | v              |
      +---+  +----------------+ +---------+ +----------------+  +---+
      |   |  | MaxDigestsIn   | |         | | MaxDigestsOut  |  |   |
      |   |<-| ANM timeout    | |   CSAs  | |                |->|   |
      | R |  | RxAuthRequired | |         | |                |  | T |
      | x |  +----------------+ +---------+ +----------------+  | x |
      |   |                      |       |                      |   |
      | p |                      v       v                      | p |
      | r |  +---------------------+   +---------------------+  | r |
      | o |<-| Rx ESAs (temporary) |   | Tx ESAs (temporary) |->| o |
      | c |  +---------------------+   +---------------------+  | c |
      | e |  +---------------------+   +---------------------+  | e |
      | s |->|         ANM         |   |       LocalTS       |->| s |
      | s |<-|        table        |   |       LocalPC       |<-| s |
      | i |  +---------------------+   +---------------------+  | i |
      | n |  +------------------------------+----------------+  | n |
      | g |  |     instance of              | output buffers |=>| g |
      |   |=>|     the original             +----------------+  |   |
      |   |  |     protocol                 | source address |->|   |
      +---+  +------------------------------+----------------+  +---+
       /\                                            |            ||
       ||                                            v            \/
      +-------------------------------------------------------------+
      |                         IPv6 stack                          |
      +-------------------------------------------------------------+
         /\ ||       /\ ||                       /\ ||       /\ ||
         || \/       || \/                       || \/       || \/
      +---------+ +---------+                 +---------+ +---------+
      | speaker | | speaker |       ...       | speaker | | speaker |
      +---------+ +---------+                 +---------+ +---------+

      Flow of Babel datagrams: ===>         Flow of contol data: --->


                       Figure 1: Interaction Diagram





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   The diagram below depicts structure of two Babel datagrams.  The left
   datagram contains an unauthenticated Babel packet and an optional
   trailing data block.  The right datagram, besides these, contains
   authentication-specific TLVs in the Babel packet body.

      +-------------------+ -------     ------- +-------------------+
      |   Babel packet    |      ^       ^      |   Babel packet    |
      |      header       |      |       |      |      header       |
      +-------------------+ --   |       |   -- +-------------------+
      |     other TLV     | ^    |       |    ^ |     other TLV     |
      +-------------------+ |    |       |    | +-------------------+
      |     other TLV     | |    | P     |    | |     other TLV     |
      +-------------------+ |    |       |    | +-------------------+
      |       (...)       | | B  |       |    | |       (...)       |
      +-------------------+ |    |       |    | +-------------------+
      |     other TLV     | |    |     P |    | |     other TLV     |
      +-------------------+ |    |       |    | +-------------------+
      |     other TLV     | v    v       |  B | |     other TLV     |
      +-------------------+ -------      |    | +-------------------+
      | optional trailing |              |    | |     TS/PC TLV     |
      |    data block     |              |    | +-------------------+
      +-------------------+              |    | |     HMAC TLV      |
                                         |    | +-------------------+
                                         |    | |       (...)       |
                                         |    | +-------------------+
          P: Babel packet                v    v |     HMAC TLV      |
          B: Babel packet body          ------- +-------------------+
                                                | optional trailing |
                                                |    data block     |
                                                +-------------------+

                    Figure 2: Babel Datagram Structure



















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   The diagram below depicts a sample HMAC TLV corresponding to a hash
   algorithm with digest length of 20 octets (such as RIPEMD-160).  Its
   Digest field is fully padded using IPv6 address
   fe80::0a11:96ff:fe1c:10c8 for the first 16 octets and 0x00 for the
   subsequent octets.

    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 = 22  |         KeyID = 12345         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Digest = 0xFE         80              00              00      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       00              00              00              00      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0A              11              96              FF      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       FE              1C              10              C8      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       00              00              00              00      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 3: A Padded HMAC TLV

   The diagram below depicts the same HMAC TLV with all 20 octets of a
   sample HMAC result written to the Digest field.

    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 = 22  |         KeyID = 12345         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Digest = 0x4F         C8              C8              9D      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       57              83              91              9B      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       81              B0              90              47      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       B4              2F              E3              37      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       A7              BE              93              83      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: A HMAC TLV with a HMAC Result







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Internet-Draft   Babel HMAC Cryptographic Authentication     August 2012


Author's Address

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

   Email: infrastation@yandex.ru










































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