rfc3041.txt

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   portion of an address over time and generate new addresses from the
   interface identifier for some address scopes.  Changing the interface
   identifier can make it more difficult to look at the IP addresses in
   independent transactions and identify which ones actually correspond
   to the same node, both in the case where the routing prefix portion
   of an address changes and when it does not.

   Many machines function as both clients and servers.  In such cases,
   the machine would need a DNS name for its use as a server.  Whether
   the address stays fixed or changes has little privacy implication
   since the DNS name remains constant and serves as a constant
   identifier.  When acting as a client (e.g., initiating
   communication), however, such a machine may want to vary the
   addresses it uses.  In such environments, one may need multiple
   addresses: a "public" (i.e., non-secret) server address, registered



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   in the DNS, that is used to accept incoming connection requests from
   other machines, and a "temporary" address used to shield the identity
   of the client when it initiates communication.  These two cases are
   roughly analogous to telephone numbers and caller ID, where a user
   may list their telephone number in the public phone book, but disable
   the display of its number via caller ID when initiating calls.

   To make it difficult to make educated guesses as to whether two
   different interface identifiers belong to the same node, the
   algorithm for generating alternate identifiers must include input
   that has an unpredictable component from the perspective of the
   outside entities that are collecting information.  Picking
   identifiers from a pseudo-random sequence suffices, so long as the
   specific sequence cannot be determined by an outsider examining
   information that is readily available or easily determinable (e.g.,
   by examining packet contents).  This document proposes the generation
   of a pseudo-random sequence of interface identifiers via an MD5 hash.
   Periodically, the next interface identifier in the sequence is
   generated, a new set of temporary addresses is created, and the
   previous temporary addresses are deprecated to discourage their
   further use.  The precise pseudo-random sequence depends on both a
   random component and the globally unique interface identifier (when
   available), to increase the likelihood that different nodes generate
   different sequences.

3.  Protocol Description

   The goal of this section is to define procedures that:

   1) Do not result in any changes to the basic behavior of addresses
      generated via stateless address autoconfiguration [ADDRCONF].

   2) Create additional global-scope addresses based on a random
      interface identifier for use with global scope addresses.  Such
      addresses would be used to initiate outgoing sessions.  These
      "random" or temporary addresses would be used for a short period
      of time (hours to days) and would then be deprecated.  Deprecated
      address can continue to be used for already established
      connections, but are not used to initiate new connections.  New
      temporary addresses are generated periodically to replace
      temporary addresses that expire, with the exact time between
      address generation a matter of local policy.

   3) Produce a sequence of temporary global-scope addresses from a
      sequence of interface identifiers that appear to be random in the
      sense that it is difficult for an outside observer to predict a





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      future address (or identifier) based on a current one and it is
      difficult to determine previous addresses (or identifiers) knowing
      only the present one.

   4) Generate a set of addresses from the same (randomized) interface
      identifier, one address for each prefix for which a global address
      has been generated via stateless address autoconfiguration.  Using
      the same interface identifier to generate a set of temporary
      addresses reduces the number of IP multicast groups a host must
      join.  Nodes join the solicited-node multicast address for each
      unicast address they support, and solicited-node addresses are
      dependent only on the low-order bits of the corresponding address.
      This decision was made to address the concern that a node that
      joins a large number of multicast groups may be required to put
      its interface into promiscuous mode, resulting in possible reduced
      performance.

3.1.  Assumptions

   The following algorithm assumes that each interface maintains an
   associated randomized interface identifier.  When temporary addresses
   are generated, the current value of the associated randomized
   interface identifier is used.  The actual value of the identifier
   changes over time as described below, but the same identifier can be
   used to generate more than one temporary address.

   The algorithm also assumes that for a given temporary address, an
   implementation can determine the corresponding public address from
   which it was generated.  When a temporary address is deprecated, a
   new temporary address is generated.  The specific valid and preferred
   lifetimes for the new address are dependent on the corresponding
   lifetime values in the public address.

   Finally, this document assumes that when a node initiates outgoing
   communication, temporary addresses can be given preference over
   public addresses.  This can mean that all connections initiated by
   the node use temporary addresses by default, or that applications
   individually indicate whether they prefer to use temporary or public
   addresses.  Giving preference to temporary address is consistent with
   on-going work that addresses the topic of source-address selection in
   the more general case [ADDR_SELECT].  An implementation may make it a
   policy that it does not select a public address in the event that no
   temporary address is available (e.g., if generation of a useable
   temporary address fails).







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3.2.  Generation Of Randomized Interface Identifiers.

   We describe two approaches for the maintenance of the randomized
   interface identifier.  The first assumes the presence of stable
   storage that can be used to record state history for use as input
   into the next iteration of the algorithm across system restarts.  A
   second approach addresses the case where stable storage is
   unavailable and there is a need to generate randomized interface
   identifiers without previous state.

3.2.1.  When Stable Storage Is Present

   The following algorithm assumes the presence of a 64-bit "history
   value" that is used as input in generating a randomized interface
   identifier.  The very first time the system boots (i.e., out-of-the-
   box), a random value should be generated using techniques that help
   ensure the initial value is hard to guess [RANDOM].  Whenever a new
   interface identifier is generated, a value generated by the
   computation is saved in the history value for the next iteration of
   the algorithm.

   A randomized interface identifier is created as follows:

   1) Take the history value from the previous iteration of this
      algorithm (or a random value if there is no previous value) and
      append to it the interface identifier generated as described in
      [ADDRARCH].
   2) Compute the MD5 message digest [MD5] over the quantity created in
      the previous step.
   3) Take the left-most 64-bits of the MD5 digest and set bit 6 (the
      left-most bit is numbered 0) to zero.  This creates an interface
      identifier with the universal/local bit indicating local
      significance only.  Save the generated identifier as the
      associated randomized interface identifier.
   4) Take the rightmost 64-bits of the MD5 digest computed in step 2)
      and save them in stable storage as the history value to be used in
      the next iteration of the algorithm.

   MD5 was chosen for convenience, and because its particular properties
   were adequate to produce the desired level of randomization.  IPv6
   nodes are already required to implement MD5 as part of IPsec [IPSEC],
   thus the code will already be present on IPv6 machines.

   In theory, generating successive randomized interface identifiers
   using a history scheme as above has no advantages over generating
   them at random.  In practice, however, generating truly random
   numbers can be tricky.  Use of a history value is intended to avoid
   the particular scenario where two nodes generate the same randomized



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   interface identifier, both detect the situation via DAD, but then
   proceed to generate identical randomized interface identifiers via
   the same (flawed) random number generation algorithm.  The above
   algorithm avoids this problem by having the interface identifier
   (which will often be globally unique) used in the calculation that
   generates subsequent randomized interface identifiers.  Thus, if two
   nodes happen to generate the same randomized interface identifier,
   they should generate different ones on the followup attempt.

3.2.2.  In The Absence of Stable Storage

   In the absence of stable storage, no history value will be available
   across system restarts to generate a pseudo-random sequence of
   interface identifiers.  Consequently, the initial history value used
   above will need to be generated at random.  A number of techniques
   might be appropriate.  Consult [RANDOM] for suggestions on good
   sources for obtaining random numbers.  Note that even though machines
   may not have stable storage for storing a history value, they will in
   many cases have configuration information that differs from one
   machine to another (e.g., user identity, security keys, serial
   numbers, etc.).  One approach to generating a random initial history
   value in such cases is to use the configuration information to
   generate some data bits (which may remain constant for the life of
   the machine, but will vary from one machine to another), append some
   random data and compute the MD5 digest as before.

3.3.  Generating Temporary Addresses

   [ADDRCONF] describes the steps for generating a link-local address
   when an interface becomes enabled as well as the steps for generating
   addresses for other scopes.  This document extends [ADDRCONF] as
   follows.  When processing a Router Advertisement with a Prefix
   Information option carrying a global-scope prefix for the purposes of
   address autoconfiguration (i.e., the A bit is set), perform the
   following steps:

   1) Process the Prefix Information Option as defined in [ADDRCONF],
      either creating a public address or adjusting the lifetimes of
      existing addresses, both public and temporary.  When adjusting the
      lifetimes of an existing temporary address, only lower the
      lifetimes.  Implementations must not increase the lifetimes of an
      existing temporary address when processing a Prefix Information
      Option.
   2) When a new public address is created as described in [ADDRCONF]
      (because the prefix advertised does not match the prefix of any
      address already assigned to the interface, and the Valid Lifetime
      in the option is not zero), also create a new temporary address.




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   3) When creating a temporary address, the lifetime values are derived
      from the corresponding public address as follows:

      -  Its Valid Lifetime is the lower of the Valid Lifetime of the
         public address or TEMP_VALID_LIFETIME.
      -  Its Preferred Lifetime is the lower of the Preferred Lifetime
         of the public address or TEMP_PREFERRED_LIFETIME -
         DESYNC_FACTOR.

      A temporary address is created only if this calculated Preferred
      Lifetime is greater than REGEN_ADVANCE time units.  In particular,
      an implementation must not create a temporary address with a zero
      Preferred Lifetime.
   4) New temporary addresses are created by appending the interface's
      current randomized interface identifier to the prefix that was
      used to generate the corresponding public address.  If by chance
      the new temporary address is the same as an address already
      assigned to the interface, generate a new randomized interface
      identifier and repeat this step.
   5) Perform duplicate address detection (DAD) on the generated
      temporary address.  If DAD indicates the address is already in
      use, generate a new randomized interface identifier as described
      in Section 3.2 above, and repeat the previous steps as appropriate
      up to 5 times.  If after 5 consecutive attempts no non-unique
      address was generated, log a system error and give up attempting
      to generate temporary addresses for that interface.

      Note: because multiple temporary addresses are generated from the
      same associated randomized interface identifier, there is little
      benefit in running DAD on every temporary address.  This document
      recommends that DAD be run on the first address generated from a
      given randomized identifier, but that DAD be skipped on all
      subsequent addresses generated from the same randomized interface
      identifier.

3.4.  Expiration of Temporary Addresses

   When a temporary address becomes deprecated, a new one should be
   generated.  This is done by repeating the actions described in
   Section 3.3, starting at step 3).  Note that, except for the
   transient period when a temporary address is being regenerated, in
   normal operation at most one temporary address corresponding to a
   public address should be in a non-deprecated state at any given time.
   Note that if a temporary address becomes deprecated as result of
   processing a Prefix Information Option with a zero Preferred
   Lifetime, then a new temporary address must not be generated.  The
   Prefix Information Option will also deprecate the corresponding
   public address.



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   To insure that a preferred temporary address is always available, a
   new temporary address should be regenerated slightly before its
   predecessor is deprecated.  This is to allow sufficient time to avoid
   race conditions in the case where generating a new temporary address
   is not instantaneous, such as when duplicate address detection must
   be run.  It is recommended that an implementation start the address
   regeneration process REGEN_ADVANCE time units before a temporary
   address would actually be deprecated.

   As an optional optimization, an implementation may wish to remove a
   deprecated temporary address that is not in use by applications or
   upper-layers.  For TCP connections, such information is available in
   control blocks.  For UDP-based applications, it may be the case that
   only the applications have knowledge about what addresses are
   actually in use.  Consequently, one may need to use heuristics in
   deciding when an address is no longer in use (e.g., the default