[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00 01 02 03 04 draft-ietf-ipv6-optimistic-dad

IPv6 Working Group                                 Nick 'Sharkey' Moore
INTERNET-DRAFT                                   Monash University CTIE
                                                        14 October 2002



                 Optimistic Duplicate Address Detection
                <draft-moore-ipv6-optimistic-dad-00.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   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 cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/lid-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This document is an individual submission to the IETF. Comments
   should be directed to the authors.

   Definitions of requirements keywords are in accordance with the IETF
   Best Current Practice - RFC2119 [RFC2119]

Abstract

   Optimistic DAD is an interoperable modification of the existing IPv6
   Neighbour Discovery (RFC2461) and Stateless Address Autoconfiguration
   (RFC2462) process.  The intention is to minimize address
   configuration delays in the successful case without greatly
   increasing disruption in the less likely failure case.








Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 1]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


Table of Contents

   Status of this Memo..........................................  1
   Abstract.....................................................  1
   Table of Contents............................................  2
   1. Introduction..............................................  2
           1.1 Definitions......................................  3
   2. Modifications to RFC-compliant behaviour..................  3
           2.1 Modifications to RFC 2461 Neighbour Discovery....  4
           2.2 Modifications to RFC 2462 SAA....................  4
           2.3 Address Generation...............................  5
   3. Protocol Operation........................................  5
           3.1 Simple case......................................  6
           3.2 Collision case...................................  6
           3.3 Interoperation cases.............................  7
           3.4 Pathological cases...............................  7
   4. Security Considerations...................................  7
   References...................................................  8
   Expired References...........................................  8
   Acknowledgments..............................................  9
   Author's Address.............................................  9


1. Introduction


   Optimistic DAD is an interoperable modification of the existing IPv6
   Neighbour Discovery [RFC2461] and Stateless Address Autoconfiguration
   [RFC2462] process.  The intention is to minimize address
   configuration delays in the successful case without greatly
   increasing disruption in the less likely failure case.

   Optimistic DAD is a useful optimization because DAD is far more
   likely to succeed than fail, by a factor of at least 10,000,000,000
   to one[SOTO].  This makes it worth a little disruption in the failure
   case to provide faster handovers in the successful case, as long as
   the disruption is recoverable.

   It is not the intention of this draft to improve the security,
   reliability or robustness of DAD beyond that of existing standards,
   merely to provide a method to make it faster.

   There is some precedent for this work in previous drafts[KOODLI], and
   in discussions in the mobile-ip WG mailing list and at IETF-54.  This
   version of Optimistic DAD differs somewhat from previous versions in
   that it uses no additional flags or message types beyond those
   already defined, therefore allowing interoperation between Optimistic
   and 'normal' nodes.



Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 2]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


1.1 Definitions


   Tentative - an address for which a node has not completed DAD is
        regarded as Tentative -- a single Neighbour Advertisement
        defending this address will cause the node to deconfigure the
        address and cease using it.

   Optimistic - An Optimistic node assumes that DAD will succeed, and
        allows higher-layer communications on an address even while that
        address is still Tentative.

   Normal - A Normal node is one which is compliant with RFCs 2461 and
        2462.

   Link - A communication facility or medium over which nodes can
        communicate at the link layer.

   Neighbours - Nodes on the same link, which may therefore be competing
        for the same addresses.


2. Modifications to RFC-compliant behaviour


   Modifications are required only to Optimistic nodes -- Optimistic
   nodes will interoperate with Normal nodes without significant
   advantage or incompatibility.

   In order to do this, it is important that an Optimistic node does
   not, while Tentative, send any messages which will override its
   neighbours' Neighbour Cache (NC) entries for the address it is trying
   to configure: doing so would disrupt the rightful owner of the
   address in the case of a collision.

   This is achieved by:

   * clearing the 'Override' bit in Neighbour Advertisements for
        Tentative addresses, which prevents neighbours from overriding
        their existing NC entries. The 'Override' bit is already defined
        [RFC2461] and used for Proxy Neighbour Advertisement.

   * Never attaching a Source Link-Layer Address Option to NSs or RSs
        sent from a Tentative address. This will cause some extra
        signalling if an Optimistic node attempts to establish a
        connection with a neighbour while Tentative, but it prevents the
        overriding of neighbours' NC entries in the collision case.




Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 3]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


2.1 Modifications to RFC 2461 Neighbour Discovery


   * (modifies 7.2.2)  When a Optimistic node sends a Neighbour
        Solicitation or Router Solicitation while Tentative, it MUST NOT
        include the Source Link Layer Address Option.

   * (modifies 7.2.6)  The Optimistic node SHOULD send an unsolicited
        Neighbour Advertisement to All Nodes when it first configures an
        address. The Override flag on this advertisement MUST be set to
        0.

   * (modifies 7.2.6)  The Optimistic node SHOULD send another
        unsolicited NA to All Nodes when it completes DAD. The Override
        flag on this advertisement SHOULD be set to 1.


2.2 Modifications to RFC 2462 Stateless Address Autoconfiguration


   * (modifies 5.5)  If an initial suffix is not supplied, a new suffix
        SHOULD be generated as per "Address Generation" below.  It MAY
        be derived from the link-layer address as per [RFC2373].

   * (modifies 5.4)  As soon as the initial Neighbour Solicitation (and
        optional unsolicited Neighbour Advertisement) is sent, the
        address is configured on the interface and available for use
        immediately.

   * (modifies 5.4.3) A node MUST reply to a Neighbour Solicitation for
        its address from the unspecified address with a Neighbour
        Advertisement to the All Nodes address.  If the solicitation is
        for an address which is still Tentative, the reply MUST have the
        Override flag set to 0.

   * (modifies 5.4.3) A node MUST reply to a Neighbour Solicitation for
        its address from a unicast address, even while Tentative, but
        the reply MUST have the Override flag set to 0.

   * (modifies 5.4.5) A Tentative address that is determined to be a
        duplicate MUST be deconfigured immediately.  If the address is a
        link-local address formed from a fixed interface identifier, the
        interface SHOULD be disabled.  Otherwise, if the address was
        automatically configured, DAD SHOULD be restarted with a new
        address generated as per "Address Generation" below.






Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 4]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


2.3 Address Generation


   In order for Optimistic DAD to be a useful optimization, the
   probability of a collision must be very small, and the probability of
   multiple collisions even smaller.

   Some interfaces (for example, Ethernet [RFC2464]) offer methods to
   create an address based on a globally unique Interface Identifier,
   however it is conceivable that due to manufacturer or user error that
   the generated address may not in fact be unique.

   * If the interface offers a method to create a supposedly globally
        unique IPv6 address, this address MAY be used for the initial
        attempt.

   * Otherwise, or when creating a new address in the case of a
        collision, a suffix MUST be chosen based on a strongly random
        algorithm (see [RFC1750] for more information on random number
        generation).

   * The algorithm used MAY be one of those documented in [RFC3041].

   * A randomly generated address SHOULD have the Universal/Local bit
        and the Individual/Group bit set to 0 to indicate a locally
        scoped Unicast address (see [RFC2373]).



3. Protocol Operation


   The following cases all consider an Optimistic Node (ON) receiving a
   Router Advertisement containing a new prefix and deciding to
   autoconfigure a new address on that prefix.

   The ON will immediately send out a Neighbour Solicitation to
   determine if its new address is already in use, and a Neighbour
   Advertisement (with Override set to 0) for the address. This NA
   allows communication with neighbours to begin immediately.











Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 5]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


3.1 Simple case


   In the non-collision case, the address being configured by the new
   node is unused and not present in the Neighbour Caches of any of its
   neighbours.

   Therefore, there will be no response to its NS, and the NA with O=0
   will be sufficient to create Neighbour Cache entries in interested
   neighbours.  Since the Optimistic Node already has the link-layer
   address of the router, and the router now has the link-layer address
   of the Optimistic Node, communications can begin immediately.

   After the appropriate DAD delay, the address is marked as non-
   Tentative, and another NA is sent, this time with O=1. This will
   ensure that all Neighbour Caches are up-to-date.


3.2 Collision cases


   In the simplest collision case, the address being configured by the
   new node is already in use by another node, and present in the
   Neighbour Caches (NCs) of neighbours which are communicating with
   this node.  Since the Optimistic advertisement has O=0, it will not
   override existing NC entries, and thus existing traffic will go
   undisturbed. Nodes with no interest in communicating with the new
   address "SHOULD" silently discard the NA [RFC2461 7.2.5], and so will
   likely be undisturbed too.

   If a neighbour is just preparing to begin communication with the
   address, eg: it has a NC entry for the address in state 'INCOMPLETE',
   the optimistic advertisement may cause an incorrect NC entry to be
   created in state 'STALE' and queued packets to be sent to an
   incorrect destination.

   In general, the defending NA will have Override set to 1, and so this
   will correct the incorrect entry almost immediately.  However, if the
   defending NA has Override set to 0 (for example when the address is
   in use by proxy) the defending advertisement will not override this
   incorrect NC entry. In any case, the NC entry will remain in state
   'STALE', and thus the disruption will be recoverable by the standard
   Neighbour Unreachability Detection mechanism.








Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 6]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


3.3 Interoperation cases


   Once the Optimistic Node has completed DAD, it acts exactly like a
   Normal node, and so interoperation cases only arise while an
   Optimistic Node is Tentative.

   If an Optimistic Node attempts to configure an address currently
   Tentatively assigned to a Normal Node, the Normal Node will see the
   Neighbour Solicitation and deconfigure the address.  In contrast, if
   a node attempts to configure an address currently Tentatively
   assigned to an Optimistic Node, the Optimistic Node will not
   deconfigure the address, and instead defend with a Neighbour
   Advertisement, causing the newcomer to reconfigure.  This gives the
   Optimistic Node a slight advantage over Normal nodes, however this is
   justified since the Optimistic node may have already established
   connections while Tentative.



3.4 Pathological cases


   Optimistic DAD suffers from similar problems to Normal DAD, for
   example duplicates are not guaranteed to be detected if packets are
   lost, and if two nodes configure simultaneously, they may each miss
   the other's NS.

   These problems exist, and are not gracefully recoverable, in Normal
   DAD. The probability of such a collision is reduced in Optimistic DAD
   due to the pair of messages (NS, NA) sent.  The probability can be
   further reduced by increasing the RFC2462 DupAddrDetectTransmits
   variable to greater than 1.



4. Security Considerations


   There are existing security concerns with Neighbour Discovery and
   Stateless Address Autoconfiguration, and this draft does not purport
   to fix them.  However, this draft does not significantly increase
   security concerns either.








Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 7]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


References


   [RFC1750] D. Eastlake, S. Crocker, J. Schiller. "Randomness
        Recommendation for Security." Request for Comments 1750, t
        Engineering Task Force, December 1994.

   [RFC2119] S. Bradner.  "Key words for use in RFCs to Indicate
        Requirement Levels." Request for Comments (Best Current
        Practice) 2119 (BCP 14), Internet Engineering Task Force, March
        1997.

   [RFC2373] R. Hinden, S. Deering. "IP Version 6 Addressing
        Architecture." Request for Comments (Proposed Standard) 2373,
        Internet Engineering Task Force, July 1998.

   [RFC2461]  T. Narten, E.Nordmark, W. Simpson. "Neighbor Discovery for
        IP Version 6 (IPv6)." Request for Comments (Draft Standard)
        2461, Internet Engineering Task Force, December 1998.

   [RFC2462] S. Thomson, T. Narten. "IPv6 Stateless Address
        Autoconfiguration."  Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.

   [RFC2464] M. Crawford. "Transmission of IPv6 Packets over Ethernet
        Networks." Request for Comments (Proposed Standard) 2464,
        Internet Engineering Task Force, December 1998.

   [RFC3041] T. Narten, R. Draves. "Privacy Extensions for Stateless
        Address Autoconfiguration in IPv6." Request for Comments
        (Proposed Standard) 3041, Internet Engineering Task Force,
        January 2001.


Expired References


   [DUPONT] F. Dupont. RFC 3041 Considered Harmful.  (draft-dupont-
        ipv6-rfc3041harmful-00.txt).  February 2002 ... Expired August
        2002.

   [KOODLI] R. Koodli, C. Perkins. Fast Handovers in Mobile IPv6.
        (draft-koodli-mobileip-fastv6-00).  October 2000 ... Expired
        April 2001.

   [SOTO] M. Bagnulo, I. Soto, A. Garcia-Martinez, A. Azcorra.  Random
        generation of interface identifiers.  (draft-soto-mobileip-
        random-iids-00).  January 2002 ... Expired July 2002.



Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 8]


INTERNET-DRAFT               Optimistic DAD              14 October 2002


Acknowledgments

   Thanks to Greg Daley and Richard Nelson at CTIE for their feedback,
   and to all the mobile-ip list members who contributed to the debate.

   This work has been supported by the Australian Telecommunications
   Cooperative Research Centre (AT-CRC) <http://www.atcrc.com/>


Author's Address:

   <nick.moore@monash.edu>   <http://www.ctie.monash.edu.au/>

   Nick 'Sharkey' Moore
   Centre for Telecommunications and Information Engineering
   Department of Electrical and Computer Systems Engineering
   Monash University
   Clayton 3800
   Victoria
   Australia































Nick 'Sharkey' Moore     Expires: 14 April 2003                 [Page 9]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/