Network Working Group                                        S. Krishnan
Internet-Draft                                                  Ericsson
Updates: 4861 (if approved)                                     G. Daley
Intended status: Standards Track                        NetStar Networks
Expires: January 14, 2010                                  July 13, 2009

       Simple procedures for Detecting Network Attachment in IPv6

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   Detecting Network Attachment allows hosts to assess if its existing
   addressing or routing configuration is valid for a newly connected

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   This document provides simple procedures for detecting network
   attachment in IPv6 hosts, and procedures for routers to support such

Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Goals  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Link Identification model  . . . . . . . . . . . . . . . .  4
     2.4.  DNA Roles  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.5.  Working Assumptions  . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Host Operations  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Host data structures . . . . . . . . . . . . . . . . . . .  6
     4.2.  Steps involved in detecting link change  . . . . . . . . .  6
     4.3.  Link-Layer Indication  . . . . . . . . . . . . . . . . . .  7
     4.4.  Sending Neighbor Discovery probes  . . . . . . . . . . . .  7
     4.5.  Contents of the Neighbor Discovery messages  . . . . . . .  8
       4.5.1.  Neighbor Solicitation messages . . . . . . . . . . . .  8
       4.5.2.  Router Solicitation messages . . . . . . . . . . . . .  8
     4.6.  Sending DHCPv6 probes  . . . . . . . . . . . . . . . . . .  9
     4.7.  Response Gathering . . . . . . . . . . . . . . . . . . . .  9
     4.8.  Further Host Operations  . . . . . . . . . . . . . . . . . 10
     4.9.  Recommended retransmission behavior  . . . . . . . . . . . 10
   5.  Pseudocode for Simple DNA  . . . . . . . . . . . . . . . . . . 12
   6.  Constants  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Relationship to DNAv4  . . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     11.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   Hosts require procedures to simply and reliably identify if they have
   moved to a different IP network to the one which they have been
   recently connected.  In order to detect change, router and neighbour
   discovery messages are used to collect reachability and configuration
   information.  This information is used to detect whether the existing
   router and address prefixes are likely to be present.

   This document incorporates feedback from host and router operating
   systems implementors, which seeks to make implementation and adoption
   of IPv6 change detection procedures simple for general use.

2.1.  Goals

   The goal of this document is to specify a simple procedure for
   detecting network attachment (Simple DNA) that has the following

   o  Routers do not have to be modified to support this scheme.

   o  Handle only the simplest and most likely use cases.

   o  Work at least as quickly as standard neighbor discovery.

   o  False positives are not acceptable.  A host should not conclude
      that there is no link change when there is one.

   o  False negatives are acceptable.  A host can conclude that there is
      a link change when there is none.

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2.2.  Applicability

   The Simple DNA protocol provides substantial benefits in some
   scenarios and does not provide any benefit at all in certain other
   scenarios.  This is intentional as Simple DNA was designed for
   simplicity rather than completeness.  In particular, the Simple DNA
   protocol provides maximum benefits when a host moves between a small
   set of known links.  When a host moves to a completely new link that
   is previously unknown, the performance of the Simple DNA protocol
   will be identical to that using standard neighbor discovery
   procedures [RFC4861].

2.3.  Link Identification model

   Earlier methods of detecting network attachment, e.g. the procedure
   defined in [I-D.ietf-dna-protocol], relied on detecting whether the
   host was still connected to the same link.  If the host was attached
   to the same link, all information related to the link such as the
   routers, prefixes and configuration parameters was considered to be
   valid.  The Simple DNA protocol follows an alternate approach where
   it relies on probing each previously known router to determine
   whether to use information learnt from THAT router.  This allows
   simple DNA to probe routers learnt from multiple earlier attachments
   to optimize movement between a known set of links.

2.4.  DNA Roles

   Detecting Network Attachment is performed by hosts by sending IPv6
   neighbour discovery and router discovery messages to routers after
   connecting to a network.

   It is desirable that routers adopt procedures which allow for fast
   unicast Router Advertisement (RA) messages.  Routers that follow the
   standard neighbor discovery procedure described in [RFC4861] will
   delay the router advertisement by a random period between 0 and
   MAX_RA_DELAY_TIME (defined to be 500ms) as described in Section 6.2.6
   of [RFC4861].  This delay can be significant and may result in
   service disruption.  Please note that support for fast unicast RAs is
   not necessary since the simple dna procedure can continue to work
   using the NS/NA exchange, which will complete earlier than the RA

   The host detects that the link-layer may have changed, and then
   simultaneously probes the network with Router Solicitations (RSs) and
   Neighbour Solicitations (NSs).  The host uses advertisements to
   determine if the routers it currently has configured are still

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   Additionally, on links with no statelessly configured addresses, the
   host may make use of DHCPv6 procedures to identify an operable

2.5.  Working Assumptions

   There are a series of assumptions about the network environment which
   underpin these procedures.

   o  The combination of the link layer address and the link local IPv6
      address of a router is unique across links.

   o  Hosts receive indications when a link-layer comes up.  Without
      this, they would not know when to commence the DNA procedure.

   If these assumptions do not hold, host change detection systems will
   not function optimally.  In that case, they may occasionally detect
   change spuriously, or experience some delay in detecting network
   attachment.  The delays so experienced will be no longer than those
   caused by following the standard neighbor discovery procedure
   described in [RFC4861].

3.  Terminology

   |         Term        |                  Definition                 |
   |  Valid IPv6 address | An IPv6 address configured on the node that |
   |                     |   has a valid lifetime greater than zero.   |
   |                     |                                             |
   |    Operable IPv6    | An IPv6 address configured on the node that |
   |       address       |   can be used safely on the current link.   |

                      Table 1: Simple DNA Terminology

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4.  Host Operations

   When a host has an existing configuration for IP address prefixes and
   next hop routing, it may be disconnected from its link-layer, and
   then subsequently reconnect the link-layer on the same interface.

   When the link-layer becomes available again, it is important to
   determine whether the existing addressing and routing configuration
   are still valid.

   In order to determine this, the host performs the detecting network
   attachment procedure.

4.1.  Host data structures

   In order to correctly perform the procedure described in this
   document the host needs to maintain a data structure called the
   Simple DNA address table (SDAT).  This data structure is maintained
   by the host on a per interface basis.  Each entry in the SDAT table
   consists of at least the following parameters.

   o  IPv6 address and its related parameters like valid lifetime.

   o  Prefix from which the address was formed.

   o  Link-local IPv6 address of the router that advertised the prefix.

   o  Link layer (MAC) address of the router that advertised the prefix.

   o  DHCP Unique IDentifier (DUID) in case DHCPv6 was used to acquire
      the address [RFC3315].

4.2.  Steps involved in detecting link change

   The steps involved in basic detection of network attachment are:

   o  Link-Layer Indication

   o  Sending of neighbour discovery or DHCPv6 probes

   o  Response gathering and assessment

   These steps are described below.

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4.3.  Link-Layer Indication

   In order to start Detection of network attachment procedures, a host
   typically requires a link-layer indication that the medium has become
   available [RFC4957].

   After the indication is received, the host considers all currently
   configured (non-tentative) IP addresses to be deprecated until the
   change detection process completes.  It SHOULD also set all Neighbor
   Cache entries for the routers on its Default Router List to STALE.
   This is done to speed up the acquisition of a new default router when
   link change has occurred.

4.4.  Sending Neighbor Discovery probes

   When a host receives a link-layer "up" indication, it SHOULD
   immediately send both a Router Solicitation and if it retains at
   least one valid IPv6 address, one or more unicast Neighbor
   Solicitations.  The Router Solicitation is sent to the All-routers
   multicast address using a link-local address as the source address
   [RFC4861].  Even if the host is in possession of more than one valid
   IPv6 address, it MUST send only one router solicitation using a valid
   link-local address as the source address.

   For the purpose of sending neighbor solicitations to previous
   routers, the host first identifies the set of operable IPv6 addresses
   (candidate set) that it wishes to use.  If the addresses obtained
   from a previous router are no longer valid, the host does not include
   these addresses in the candidate set for NS based probing.

   For each of the addresses in the candidate set, the host looks up the
   SDAT to find out the link-local and MAC addresses of the router that
   advertised the prefix used to form the address.  It then sends an
   unicast Neighbor Solicitations to each router's link-local address it
   obtained from the lookup on the SDAT.  The host SHOULD NOT send
   unicast Neighbor Solicitations to a test node corresponding to an
   IPv6 address that is no longer valid.

   Please note that the Neighbour Solicitations SHOULD be sent in
   parallel with the Router Solicitations.  Since sending NSs is just an
   optimization, doing the NSs and RSs in parallel ensures that the
   procedure does not run slower than it would if it only used an RS.

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4.5.  Contents of the Neighbor Discovery messages

4.5.1.  Neighbor Solicitation messages

   This section describes the contents of the neighbor solicitation
   probe messages sent during the probing procedure.

   Source Address:           A link-local address assigned to the
                             probing host.

   Destination Address:      The link-local address of the router being
                             probed as learnt from the SDAT.

   Hop Limit:                255

   ND Options:

   Target Address:           The link-local address of the router being
                             probed as learnt from the SDAT.

   The probing node SHOULD include a Source link-layer address option in
   the probe messages if the address was obtained using DHCPv6 and the
   lease has not expired.  Otherwise the probing node SHOULD NOT include
   the Source link-layer address option in the probe messages.

4.5.2.  Router Solicitation messages

   This section describes the contents of the router solicitation probe
   message sent during the probing procedure.

   Source Address:           A link-local address assigned to the
                             probing host.

   Destination Address:      The all-routers multicast address.

   Hop Limit:                255

   The probing node SHOULD include a Source link-layer address option in
   the probe messages if the address was obtained using DHCPv6 and the
   lease has not expired.  Otherwise the probing node SHOULD NOT include
   the Source link-layer address option in the probe messages.

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4.6.  Sending DHCPv6 probes

   Where the host has acquired addresses from DHCPv6 or the host does
   not have a global prefix, it MAY prefer to use DHCPv6 messages either
   before or simultanously with Neighbour Discovery probing.

   In that case, when the host receives a link-layer indication, it
   sends a DHCPv6 SOLICIT to All_DHCP_Relay_Agents_and_Servers.  This
   message contains an Identity Association for either a Temporary
   Address (IA_TA) or Non-Temporary Address (IA_NA) [RFC3315].  Where an
   existing valid address is being tested for operability, this address
   should be placed in the Identity Association's IAADDR element, and
   the DUID associated with that address should be copied to the DHCP
   SOLICIT from the SDAT.

   In order to quickly acquire a new address in the case that link
   change has occurred, this SOLICIT message MAY contain the Rapid-
   Commit option.

   Where the Rapid-Commit option has not been used, a present DHCP
   server will respond with an ADVERTISE message.  The IP address
   contained in the Identity Association (IA_TA or IA_NA) will contain
   an IP Address which is operable for the link.

   Where Rapid-Commit option has been sent, a DHCPv6 server will respond
   with REPLY.  In addition to being operable, this address is allocated
   to the host for the lease duration indicated in the Identity

4.7.  Response Gathering

   When a responding Neighbour Advertisement is received from a test
   node, the host MUST verify that both the IPv6 and link layer (MAC)
   addresses of the test node match the expected values before utilizing
   the configuration associated with the detected network (prefixes, MTU

   On reception of a Router Advertisement or advertising DHCPv6 message
   (a REPLY or ADVERTISE) which contains prefixes that intersect with
   those previously advertised by a known router, the host utilizes the
   configuration associated with the detected network.

   When the host receives an advertisement containing only prefixes
   which are disjoint from known advertised prefixes, the host MUST
   determine whether the solicited advertisement corresponds to any of
   the routers probed via NS.  If it does, then the host SHOULD conclude
   that the IPv6 addresses corresponding to that router are no longer
   valid.  Since any NS probes to that router will no longer provide

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   useful information, further probing of that router SHOULD be aborted.

   Where the conclusions obtained from the Neighbor Solicitation/
   Advertisement from a given router and the RS/RA exchange with the
   same router differ, the results obtained from the RS/RA will be
   considered definitive.

   When the host receives a Router Advertisement in reply to the Router
   Solicitation it sent, the host SHOULD look for a Neighbor Cache entry
   for the sending router and SHOULD mark that router's Neighbor Cache
   Entry as REACHABLE if one was found.  The host SHOULD add a new
   Neighbor Cache Entry in the REACHABLE state for the sending router if
   one does not currently exist.

4.8.  Further Host Operations

   Operations subsequent to detecting network attachment depend upon
   whether change was detected.

   After confirming the reachability of the associated router using an
   NS/NA pair, the host performs the following steps.

   o  The host SHOULD rejoin any solicited nodes' multicast groups for
      addresses it continues to use.

   o  The host SHOULD select a default router as described in [RFC4861].

   If the host has determined that there has been no link change, it
   SHOULD NOT perform duplicate address detection on the addresses that
   have been confirmed to be operable.

   If the NS based probe with a router did not complete or if the RS
   based probe on the same router completed with different prefixes than
   the ones in the SDAT the host MUST unconfigure all the existing
   addresses received from the given router, and MUST begin address
   configuration techniques, as indicated in the received Router
   Advertisement [RFC4861][RFC4862].

4.9.  Recommended retransmission behavior

   In situations where Neighbor Solicitation probes and Router
   Solicitation probes are used on the same link, it is possible that
   the NS probe will complete successfully, and then the RS probe will
   complete later with a different result.  If this happens, the
   implementation SHOULD abandon the results obtained from the NS probe
   of the router that responded to the RS and the implementation SHOULD
   behave as if the NS probe did not successfully complete.  If the
   confirmed address was assigned manually, the implementation SHOULD

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   NOT unconfigure the manually assigned address and SHOULD log an error
   about the mismatching prefix.

   Where the NS probe does not complete successfully, it usually implies
   that the host is not attached to the network whose configuration is
   being tested.  In such circumstances, there is typically little value
   in aggressively retransmitting unicast neighbor solicitations that do
   not elicit a response.

   Where unicast Neighbor Solicitations and Router Solicitations are
   sent in parallel, one strategy is to forsake retransmission of
   Neighbor Solicitations and to allow retransmission only of Router
   Solicitations or DHCPv6.  In order to reduce competition between
   unicast Neighbor Solicitations and Router Solicitations and DHCPv6
   retransmissions, a DNAv6 implementation that retransmits may utilize
   the retransmission strategy described in the DHCPv6 specification
   [RFC3315], scheduling DNAv6 retransmissions between Router
   Solicitation or DHCPv6 retransmissions.

   If a response is received to any unicast Neighbor Solicitation,
   Router Solicitation or DHCPv6 message, pending retransmissions MUST
   be canceled [RFC3315][RFC3736].  A Simple DNA implementation SHOULD
   NOT retransmit a Neighbor Solicitation more than twice.  To provide
   damping in the case of spurious Link Up indications, the host SHOULD
   NOT perform the Simple DNA procedure more than once a second.

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5.  Pseudocode for Simple DNA

       /* Link up indication received on INTERFACE */
       /* Start Simple DNA process */

       /* Mark All Addresses as deprecated */
       foreach Configured_Address in Configured_Address_List
         if (Get_Address_State(Configured_Address)!=AS_TENTATIVE)

       /* Mark all routers' NC entries as STALE to speed up */
       /* acquisition of new router if link change has occurred */
       foreach Router_Address in DEFAULT_ROUTER_LIST

       /* Thread A : Send Router Solicitation */

       /* Thread B : Send Neighbor Solicitation(s) */

       foreach Router_Address in Previously_Known_Router_List
         if (Get_Any_Valid_Address_from_SDAT(Router_Address))

       /* Thread C : Response collection */

       /* Received Router Advertisement processing */
       /* Only for RAs received as response to DNA RSs */


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       foreach SDAT_Entry in SDAT_Entry_List
         if (Exists_PIO(RECEIVED_MESSAGE,Get_Prefix(SDAT_Entry)))
           /* Address is operable. Configure on Interface */
           /* Rejoin solicited-node multicast group for address */
           /* If address is configured on interface, remove it */
           /* This could be because of a NA arriving before RA */

       /* Mark router as reachable */
       if (NCEntry is not NULL)

       /* Ignore further NAs from this router */

       /* Received Neighbor Advertisement processing */
       /* Only for NAs received as response to DNA NSs */


       if (Is_Router_on_NA_Ignore_List(L3_Source)) {
         /* Ignore message and wait for next message */


       foreach SDAT_Entry in SDAT_Entry_List
           /* Address is operable. Configure on Interface */

                    Figure 1: Pseudocode for Simple DNA

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6.  Constants


         Definition: An optional period to wait after Neighbour
         Solicitation before adopting a non-SEND RA's link change

         Value: 40 milliseconds

7.  Relationship to DNAv4

   DNAv4 [RFC4436] specifies a set of steps that optimize the (common)
   case of re-attachment to an IPv4 network that one has been connected
   to previously by attempting to re-use a previous (but still valid)
   configuration.  This document shares the same goal as DNAv4 (that of
   minimizing the handover latency in moving between points of
   attachment) but differs in the steps it performs to achieve this
   goal.  Another difference is that this document also supports
   stateless autoconfiguration of addresses in addition to addresses
   configured using DHCPv6.

8.  IANA Considerations

   There are no changes to IANA registries required in this document.

9.  Security Considerations

   A host may receive Router Advertisements from non SEND devices, after
   receiving a link-layer indications.  While it is necessary to assess
   quickly whether a host has moved to another network, it is important
   that the host's current secured SEND [RFC3971] router information is
   not replaced by an attacker which spoofs an RA and purports to change
   the link.

   As such, the host SHOULD send a Neighbour Solicitation to the
   existing SEND router upon link-up indication as described above in
   Section 4.3.  The host SHOULD then ensure that unsecured router
   information does not cause deletion of existing SEND state, within
   MIN_DELAY_BETWEEN_RAS, in order to allow for a present SEND router to

   If the current default router is a SEND-secured router, the host
   SHOULD wait SEND_NA_GRACE_TIME after transmission before adopting a
   new default router.

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   Even if SEND signatures on RAs are used, it may not be immediately
   clear if the router is authorized to make such advertisements.  As
   such, a host SHOULD NOT treat such devices as secure until and unless
   authorization delegation discovery is successful.

10.  Acknowledgments

   This document is the product of a discussion between the authors had
   with Bernard Aboba, Thomas Narten, Erik Nordmark and Dave Thaler at
   IETF 69.  The authors would like to thank them for clearly detailing
   the requirements of the solution and the goals it needed to meet and
   for helping to explore the solution space.  The authors would like to
   thank the authors and editors of the complete DNA specification for
   detailing the overall problem space and solutions.  The authors would
   like to thank Jari Arkko for driving the evolution of a simple and
   probabilistic DNA solution.  The authors would like to thank Bernard
   Aboba, Thomas Narten, Jari Arkko, Sathya Narayan, Julien Laganier,
   Domagoj Premec, Jin Hyeock-Choi, Alfred Hoenes and Frederic Rossi for
   performing reviews on the document and providing valuable comments to
   drive the document forward.

11.  References

11.1.  Normative References

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

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

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

11.2.  Informative References

              Narayanan, S., "Detecting Network Attachment in IPv6

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              Networks (DNAv6)", draft-ietf-dna-protocol (work in
              progress), June 2007.

   [RFC4957]  Krishnan, S., Montavont, N., Njedjou, E., Veerepalli, S.,
              and A. Yegin, "Link-Layer Event Notifications for
              Detecting Network Attachments", RFC 4957, August 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4436]  Aboba, B., Carlson, J., and S. Cheshire, "Detecting
              Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.

Authors' Addresses

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Phone: +1 514 345 7900 x42871

   Greg Daley
   NetStar Networks
   Level 9/636 St Kilda Rd
   Melbourne, Victoria  3004

   Phone: +61 3 8532 4042

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