Subject: [psg.com #321] preferred lifetime and the 'two-hour' rule

From: [email-masked]
Date: Mon, 09 Feb 2004 05:22:28 +0000

Proposed Resolution (revised): update section 5.5.3 e)
as attached below.

Reason: based on the recent discussion on the wg list.

Key changes are:
  - make it clear that the preferred lifetime is always resest to
    the advertised value
  - note the difference between the two lifetimes and explain the
    reason

Revised text:

    e) If the advertised prefix matches the prefix of an autoconfigured
      address (i.e., one obtained via stateless or stateful address
      autoconfiguration) in the list of addresses associated with the
      interface, the preferred lifetime of the address is reset to the
      Preferred Lifetime in the received advertisement. The specific
      action to perform for the valid lifetime of the address depends on
      the Valid Lifetime in the received advertisement and the remaining
      time to the valid lifetime expiration of the previously
      autoconfigured address. We call the remaining time
      "RemainingLifetime" in the following discussion:

      1.  If the received Valid Lifetime is greater than 2 hours or
          greater than RemainingLifetime, set the valid lifetime of the
          corresponding address to the advertised Valid Lifetime.

      2.  If RemainingLifetime is less than or equal to 2 hours, ignore
          the Prefix Information option with regards to the valid
          lifetime, unless the Router Advertisement from which this
          option was obtained has been authenticated (e.g., via IPSec
          [1]). If the Router Advertisement was authenticated, the valid
          lifetime of the corresponding address should be set to the
          Valid Lifetime in the received option.

      3.  Otherwise, reset the valid lifetime of the corresponding
          address to two hours.

      The above rules address a specific denial of service attack in
      which a bogus advertisement could contain prefixes with very small
      Valid Lifetimes. Without the above rules, a single unauthenticated
      advertisement containing bogus Prefix Information options with
      short Valid Lifetimes could cause all of a node's addresses to
      expire prematurely. The above rules ensure that legitimate
      advertisements (which are sent periodically) will "cancel" the
      short Valid Lifetimes before they actually take effect.

      Note that the preferred lifetime of the corresponding address is
      always reset to the Preferred Lifetime in the received Prefix
      Information option, regardless of whether the valid lifetime is
      also reset or ignored. The difference comes from the fact that the
      possible attack for the preferred lifetime is relatively minor.
      Additionally, it is even undesirable to ignore the preferred
      lifetime when a valid administrator wants to deprecate a
      particular address by sending a short lifetime (and the valid
      lifetime is ignored by accident).

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From: [email-masked]
Date: Mon, 09 Feb 2004 05:22:29 +0000

Key changes are:

Revised text: