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DHC Working Group                                          Bernard Aboba
INTERNET-DRAFT                                     Microsoft Corporation
Category: Proposed Standard
<draft-ietf-dhc-dna-ipv4-09.txt>
14 October 2004



             Detection of Network Attachment (DNA) in IPv4

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   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 to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

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

   This Internet-Draft will expire on April 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   The time required to detect movement (or lack of movement) between
   subnets, and to obtain (or continue to use) a valid IPv4 address may
   be significant as a fraction of the total delay in moving between
   points of attachment.  This document synthesizes experience garnered
   over the years in the deployment of hosts supporting ARP, DHCP and
   IPv4 Link-Local addresses.  A procedure is specified for detection of
   network attachment in order to better accommodate mobile hosts.





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Table of Contents

1.  Introduction..............................................    3
      1.1    Requirements ....................................    3
      1.2    Terminology .....................................    3
2.  Framework ................................................    4
      2.1    Most Likely Point of Attachment .................    5
      2.2    Reachability Test ...............................    6
      2.3    IPv4 Address Acquisition ........................    8
      2.4    IPv4 Link-Local Addresses .......................    9
3.  Constants ................................................   10
4.  IANA Considerations ......................................   10
5.  Security Considerations ..................................   10
6.  References ...............................................   11
      6.1    Normative references ............................   11
      6.2    Informative references ..........................   11
Acknowledgments ..............................................   12
Authors' Addresses ...........................................   12
Appendix A - Link Layer Hints ................................   13
      A.1    Introduction ....................................   13
      A.2    Hints ...........................................   14
Intellectual Property Statement ..............................   15
Disclaimer of Validity .......................................   15
Copyright Statement ..........................................   16



























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1.  Introduction

   The time required to detect movement (or lack of movement) between
   subnets, and to obtain (or continue to use) a valid IPv4 address may
   be significant as a fraction of the total delay in moving between
   points of attachment.  As a result, optimizing detection of network
   attachment is important for mobile hosts.

   This document synthesizes experience in the deployment of hosts
   supporting ARP [RFC826], DHCP [RFC2131], and IPv4 Link-Local
   addresses [IPv4LL], specifying a procedure to be performed for IPv4
   detection of network attachment.   The procedure consists of three
   phases: determination of the Most Likely Point of Attachment (MLPA),
   reachability testing, and IPv4 address acquisition.

   Since this procedure is dependent on the ARP protocol, it is not
   suitable for use on media that do not support ARP [RFC826].

1.1.  Requirements

   In this document, several words are used to signify the requirements
   of the specification.  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
   [RFC2119].

1.2.  Terminology

This document uses the following terms:

ar$sha
     ARP packet field: Source Hardware Address [RFC826]. The hardware
     (MAC) address of the originator of an ARP packet.

ar$spa
     ARP packet field: Source Protocol Address [RFC826].  For IP Address
     Resolution this is the IPv4 address of the sender of the ARP
     packet. If the sender address is unknown, this is set to 0.0.0.0.

ar$tha
     ARP packet field: Target Hardware Address [RFC826].  The hardware
     (MAC) address of the target of an ARP packet, or the broadcast
     address if the target hardware address is unknown.

ar$tpa
     ARP packet field: Target Protocol Address [RFC826].  For IPv4
     Address Resolution, the IPv4 address for which one desires to know
     the hardware address.



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DHCP client
     A DHCP client or "client" is an Internet host using the Dynamic
     Host Configuration protocol (DHCP) [RFC2131] to obtain
     configuration parameters such as a network address.

DHCP server
     A DHCP server or "server" is an Internet host that returns
     configuration parameters to DHCP clients.

Point of Attachment
     A location within the network where a host may be connected.  This
     attachment point can be characterized by its address prefix and
     next hop routing information.

Most Likely Point of Attachment (MLPA)
     The point of attachment heuristically determined by the host to be
     most likely, based on hints from the network.

Routable address
     In this specification, the term "routable address" refers to any
     address other than an IPv4 Link-Local address.  This includes
     private addresses as specified in [RFC1918].

Valid address
     In this specification, the term "valid address" refers to either a
     static IPv4 address, or an address assigned via DHCPv4 which has
     not been relinquished, and whose lease has not yet expired.

2.  Framework

   For Detection of Network Attachment (DNA), the following basic
   principles are suggested:

   [a] Utilization of hints.  As described in Appendix A,
       link layers may provide hints as to whether a
       host remains on the same subnet, despite changing
       its point of attachment.  Based on these hints,
       the host determines the "Most Likely Point of
       Attachment" (MLPA), and the corresponding
       configuration associated with it.  Since
       hints may be misleading, it is important for
       the host to make the correct determination even
       where hints are misleading.

   [b] Response to "Link Up" indications.  On connecting
       to a new point of attachment, the host attempts to
       verify the configuration associated with the MLPA.




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   [c] Handling of IPv4 Link-Local addresses.  Experience has
       shown that IPv4 Link-Local addresses are often assigned
       inappropriately.  This document suggests that hosts
       behave conservatively with respect to assignment of
       IPv4 Link-Local addresses, configuring them only in
       situations in which they can do no harm.

2.1.  Most Likely Point of Attachment

   In order to determine the MLPA, it is assumed that the host saves to
   stable storage parameters relating to the networks it connects to:

    [1] Link layer hints associated with each network.
        For details, see Appendix A.

    [2] The IPv4 and MAC address of the default gateway(s) on
        each network.

    [3] The link type, such as whether the link utilizes
        Ethernet, or 802.11 adhoc or infrastructure mode.

   By matching received hints against network parameters previously
   stored,  the host makes an an educated guess of which network it has
   attached to.  In the absence of other information, the MLPA defaults
   to the network to which the host was most recently attached.

2.1.1.  Alternative Mechanisms

   Aside from utilizing link layer indications, a host may also be able
   to utilize Internet layer information in order to determine the MLPA.

   IPv4 ICMP Router Discovery messages [RFC1256] provide information
   relating to prefix(es) available on the link, as well as the routers
   that serve those prefix(es).  A host may use ICMP Router Discovery to
   conclude that an advertised prefix is available; however it cannot
   conclude the converse -- that prefixes not advertised are
   unavailable.

   However, since [RFC1256] is not widely implemented, in general, it is
   NOT RECOMMENDED that hosts utilize ICMP Router Discovery messages as
   an alternative to the reachability test outlined in Section 2.2.
   Instead, ICMP Router Advertisements can be used to obtain information
   on available prefixes and default gateway(s).  This can provide
   additional resilience in the case where default gateway(s) become
   unavailable.

   Similarly hosts that support routing protocols such as RIP and OSPF
   can use these protocols to determine the prefix(es) available on a



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   link and the default gateway(s) that serve those prefixes.  Full
   support is not required to glean this information.  A host that
   passively observes routing protocol traffic may deduce this
   information without supporting a fully conformant routing protocol
   implementation.

2.2.  Reachability Test

   If the host has a valid routable IPv4 address on the MLPA, the host
   SHOULD perform a reachability test, in order to to confirm that it is
   connected to network on which it has a valid routable IPv4 address.
   If the reachability test is not successful, the host proceeds to the
   IPv4 address acquisition phase, described in Section 2.3.

   The host skips the reachability test and proceeds to the IPv4 address
   acquisition phase if any of the following conditions are true:

   [a] The host does not have a valid routable IPv4
       address on the MLPA.  In this case, the reachability
       test cannot confirm that the host has a valid
       routable IPv4 address, so that completing the
       reachability test would serve no purpose.

   [b] The host does not have information on the default
       gateway(s) on the MLPA.  In this case, insufficient
       information is available to carry out the reachability
       test.

   [c] Reliable hints are unavailable.  Since confirming
       failure of the reachability test requires a timeout,
       mistakes are costly.  In the absence of reliable
       hints, a host SHOULD instead send a DHCPREQUEST from
       the INIT-REBOOT state, as described in [RFC2131],
       Section 3.2 and 4.3.2.  Where reliable hints are
       unavailable, this will typically complete more
       quickly than the reachability test.

   [d] If secure detection of network attachment is required.
       The reachability test utilizes ARP which is insecure,
       whereas DHCP can be secured via DHCP authentication,
       described in [RFC3118].  See Section 5 for details.

   In contrast to a DHCP exchange, which may be between a DHCP client
   and an off-link DHCP server, the reachability test is designed to
   verify bi-directional connectivity to the default gateway(s) on the
   MLPA.

   The host MAY probe only the primary default gateway, or it MAY probe



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   primary and secondary default gateways, in series or in parallel.  If
   the reachability test is successful, the host MAY continue to use a
   valid routable IPv4 address without having to re-acquire it.  This
   reduces roaming latency by allowing the host to bypass DHCP as well
   as subsequent Duplicate Address Detection (DAD).  In order to ensure
   configuration validity,  the host SHOULD only configure default
   gateway(s) which pass the reachability test.

2.2.1.  Packet Format

   The reachability test is performed by sending an ARP Request.  The
   ARP Request SHOULD use the host's MAC address as the source, and the
   broadcast MAC address as the destination.  The host sets the target
   protocol address (ar$tpa) to the IPv4 address of the primary default
   gateway, and uses its own MAC address in the sender hardware address
   field (ar$sha).  The host sets the target hardware address field
   (ar$tha) to 0.

   If the host has a valid routable IPv4 address on the most likely
   point of attachment, then it SHOULD set the sender protocol address
   field (ar$spa) to that address.  It is assumed that the host had
   previously done duplicate address detection so that an address
   conflict is unlikely.

   However if the host has a private address as defined in [RFC1918],
   then it SHOULD set the sender protocol address field (ar$spa) to the
   unspecified address (0.0.0.0).  This is to avoid an address conflict
   in the case where the host has changed its point of attachment from
   one private network to another.

      Note: Some routers may refuse to answer an ARP Request sent with
      the sender protocol address field (ar$spa) set to the unspecified
      address (0.0.0.0). In this case the reachability test will fail.

   If a valid ARP Response is received, the MAC address in the sender
   hardware address field (ar$sha) and the IPv4 address in the sender
   protocol address field (ar$spa) are matched against the list of
   networks and associated default gateway parameters.  If a match is
   found,  then if the host has a valid routable IPv4 address on the
   matched network, the host continues to use that IPv4 address, subject
   to the lease re-acquisition and expiration behavior described in
   [RFC2131], Section 4.4.5.

   Checking for a match on both the IPv4 address and MAC address of the
   default gateway enables the host to confirm reachability even where
   it has moved between two private networks.  In this case the IPv4
   address of the default gateway could remain the same, while the MAC
   address would change, so that both addresses need to be checked.



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   Sending an ICMP Echo Request [RFC792] to the default gateway IPv4
   address does not provide the same level of assurance since this
   requires an ARP Request/Response to be sent first, and typically does
   not allow the MAC address to be checked as well.  It therefore SHOULD
   NOT be used as a substitute.

   Where a host moves from one private network to another, an ICMP Echo
   Request can result in an ICMP Echo Response even when the default
   gateway has changed, as long as the IPv4 address remains the same.
   This can occur,  for example, where a host moves from one home
   network using prefix 192.168/16 to another one.  In addition, if the
   ping is sent with TTL > 1, then an ICMP Echo Response can be received
   from an off-link gateway.

   If the initial ARP Request does not elicit a Response, the host waits
   for REACHABILITY_TIMEOUT and proceeds to the IPv4 address acquisition
   phase.  If a valid ARP Response is received, but cannot be matched
   against known networks, the host assumes it has moved subnets and
   moves on to the IPv4 address acquisition phase.

2.3.  IPv4 Address Acquisition

   If the host has a valid routable IPv4 address on the MLPA but the
   reachability test fails, the host SHOULD verify the configuration by
   entering the INIT-REBOOT state, and sending a DHCPREQUEST to the
   broadcast address as specified in [RFC2131] Section 4.4.2.

   If the host does not have a valid routable IPv4 address on the MLPA,
   the host enters the INIT state and sends a DHCPDISCOVER packet to the
   broadcast address, as described in [RFC2131] Section 4.4.1.  If the
   host supports the Rapid Commit Option [RAPID], it is possible that
   the exchange can be shortened from a 4-message exchange to a
   2-message exchange.

   If the host does not receive a response to a DHCPREQUEST or
   DHCPDISCOVER, then it retransmits as specified in [RFC2131] Section
   4.1.

   As discussed in [RFC2131], Section 4.4.4, a host in INIT or REBOOTING
   state that knows the address of a DHCP server may use that address in
   the DHCPDISCOVER or DHCPREQUEST rather than the IPv4 broadcast
   address.  In the INIT-REBOOT state a DHCPREQUEST is sent to the
   broadcast address so that the host will receive a response regardless
   of whether the previously configured IPv4 address is correct for the
   network to which it has connected.

   Sending a DHCPREQUEST to the unicast address in INIT-REBOOT state is
   not appropriate, since if the DHCP client has moved to another



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   subnet,  a DHCP server response cannot be routed back to the client
   since the DHCPREQUEST will bypass the DHCP relay and will contain an
   invalid source address.

2.4.  IPv4 Link-Local Addresses

   To avoid inappropriate assignment of IPv4 Link-Local addresses, it is
   recommended that hosts behave conservatively with respect to
   assignment of IPv4 Link-Local addresses.  As described in [IPv4LL]
   Section 1.7, use of a routable address is preferred to a IPv4 Link-
   Local address whenever it is available.

   Where the host does not have a valid routable IPv4 address on the
   MLPA, the host MAY configure an IPv4 Link-Local address prior to
   entering the INIT state and sending a DHCPDISCOVER packet, as
   described in Section 2.3.  However, should a routable IPv4 address be
   obtained, the IPv4 Link-Local address is deprecated, as noted in
   [IPv4LL].

   Where a host has a valid routable IPv4 address on the MLPA, but the
   DHCP client does not receive a response after employing the
   retransmission algorithm, [RFC2131] Section 3.2 states that the
   client MAY choose to use the previously allocated network address and
   configuration parameters for the remainder of the unexpired lease.
   Where a host can confirm that it remains connected to a point of
   attachment on which it possesses a valid routable IPv4 address, that
   address SHOULD be used, rather than assigning a IPv4 Link-Local
   address.

   Since a IPv4 Link-Local address is often configured because a DHCP
   server failed to respond to an initial query or is inoperative for
   some time, it is desirable to abandon the IPv4 Link-Local address
   assignment as soon as a valid IPv4 address lease can be obtained.

   As described in [IPv4LL] Appendix A, once a Link-Local IPv4 address
   is assigned, existing implementations do not query the DHCPv4 server
   again for 5 minutes.  This behavior violates [RFC2131] Section 4.1.

   Where a IPv4 Link-Local address is assigned, experience has shown
   that five minutes (see [IPv4LL] Appendix A.2) is too long an interval
   to wait until retrying to obtain a routable IPv4 address using DHCP.
   According to [RFC2131] Section 4.1:

          The retransmission delay SHOULD be doubled with
          subsequent retransmissions up to a maximum of 64 seconds.

   As a result, a DHCP client compliant with [RFC2131] will continue to
   retry every 64 seconds, even after allocating a IPv4 Link-Local



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   address.  Should the DHCP client succeed in obtaining a routable
   address, then as noted in [IPv4LL], the IPv4 Link-Local address is
   deprecated.

   Since it is inevitable that hosts will inappropriately configure IPv4
   Link-Local addresses at some point, hosts with routable IPv4
   addresses SHOULD be able to respond to peers with IPv4 Link-Local
   addresses, as per [IPv4LL].  For example, a host configured with a
   routable address may receive a datagram from a link-local source
   address.  In order to respond, the host will use ARP to resolve the
   target hardware address and send the datagram directly, not to a
   router for forwarding.

3.  Constants

   The suggested default value of REACHABILITY_TIMEOUT is 200 ms.  This
   value was chosen so as to accommodate the maximum retransmission
   timer likely to be experienced on an Ethernet network.

4.  IANA Considerations

   This specification does not request the creation of any new parameter
   registries, nor does it require any other IANA assignments.

5.  Security Considerations

   Detection of Network Attachment (DNA) is based on ARP and DHCP.  ARP
   [RFC826] traffic is inherently insecure, so that the reachability
   test described in Section 1.3 can be easily spoofed by an attacker,
   leading a host to falsely conclude that it is attached to a network
   that it is not connected to.  Similarly, where DHCP traffic is not
   secured, an attacker could masquerade as a DHCP server, in order to
   convince the host that it was attached to a particular network.

   As a result, it is inadvisable for a host to adjust its security
   based on which network it believes it is attached to.  For example,
   it would be inappropriate for a host to disable its personal firewall
   based on the belief that it had connected to a home network.

   Where secure detection of network attachment is required, a host
   SHOULD skip the reachability test since it cannot be secured, and
   instead utilize authenticated DHCP [RFC3118], possibly in combination
   with the Rapid Commit Option [RAPID].








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6.  References

6.1.  Normative References

[RFC792]  Postel, J., "Internet Control Message Protocol", RFC 792,
          USC/Information Sciences Institute, September 1981.

[RFC826]  D. Plummer, "An Ethernet Address Resolution Protocol -or-
          Converting Network Addresses to 48-bit Ethernet Address for
          Transmission on Ethernet Hardware", STD 37, RFC 826, November
          1982.

[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
          PARC, September 1991.

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

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
          March 1997.

[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
          RFC 3118, June 2001.

[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
          of IPv4 Link-Local Addresses",  RFC 3927, October 2004.

6.2.  Informative References

[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
          51, RFC 1661, Daydreamer, July 1994.

[RFC1918] Rekhter, Y.,  Moskowitz, B., Karrenberg, D., de Groot, G. and
          E. Lear, "Address Allocation for Private Internets", RFC 1918,
          February 1996.

[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
          "IEEE 802.1X Remote Authentication Dial In User Service
          (RADIUS) Usage Guidelines", RFC 3580, September 2003.

[RFC3748] Aboba, B.,  Blunk, L., Vollbrecht, J., Carlson, J. and H.
          Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
          3748, June 2004.

[RAPID]   Park, S., Kim, P., and B. Volz, "Rapid Commit Option for
          DHCPv4", Internet draft (work in progress), draft-ietf-dhc-
          rapid-commit-opt-05.txt, June 2004.




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[IEEE8021AB]
          IEEE Standards for Local and Metropolitan Area Networks:
          Station and Media Access Control - Connectivity Discovery,
          IEEE Std 802.1AB/D5, September 2003.

[IEEE8021X]
          IEEE Standards for Local and Metropolitan Area Networks: Port
          based Network Access Control, IEEE Std 802.1X-2004, November
          2004.

[IEEE802] IEEE Standards for Local and Metropolitan Area Networks:
          Overview and Architecture, ANSI/IEEE Std 802, 1990.

[IEEE8021Q]
          IEEE Standards for Local and Metropolitan Area Networks: Draft
          Standard for Virtual Bridged Local Area Networks, P802.1Q,
          January 1998.

[IEEE80211]
          Information technology - Telecommunications and information
          exchange between systems - Local and metropolitan area
          networks - Specific Requirements Part 11:  Wireless LAN Medium
          Access Control (MAC) and Physical Layer (PHY) Specifications,
          IEEE Std. 802.11-2003, 2003.

Acknowledgments

   The authors would like to acknowledge Greg Daley of Monash
   University, Erik Guttman and Erik Nordmark of Sun Microsystems, Ted
   Lemon of Nominum and Thomas Narten of IBM for contributions to this
   document.

Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com
   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329









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Appendix A - Link Layer Hints

A.1  Introduction

   In order to assist in IPv4 network attachment detection, information
   associated with each network may be retained by the host.  Based on
   link-layer information, the host may be able to make an educated
   guess as to whether it has moved between subnets, or has remained on
   the same subnet, as well as whether it has connected to an
   infrastructure or adhoc network.

   If the host is likely to have moved between subnets, it may be
   possible to make an educated guess as to which subnet it has moved
   to.  Since an educated guess may be incorrect, prior to concluding
   that the host remains on the same subnet, further tests (such as a
   reachability test or a DHCPREQUEST sent from the INIT-REBOOT state)
   are REQUIRED.

   In practice, it is necessary for hints to be highly reliable before
   they are worth considering, if the penalty paid for an incorrect hint
   is substantial.

   As an example, assume that a DHCPREQUEST requires DHCPREQUEST_TIME to
   determine if a host has remained on the same subnet, while a
   reachability test typically completes in REACH_TIME and times out in
   REACHABILITY_TIMEOUT, after which a DHCPREQUEST is sent.

   If a hint that the host has remained on the same subnet is wrong x
   fraction of the time, then it is only worth considering if:

   DHCPREQUEST_TIME = (1 - x) * REACH_TIME +

                      x * (REACHABILITY_TIMEOUT + DHCPREQUEST_TIME)

   x =               DHCPREQUEST_TIME - REACH_TIME
         ----------------------------------------------------
         REACHABILITY_TIMEOUT + DHCPREQUEST_TIME - REACH_TIME

   If we assume that DHCPREQUEST_TIME = 100 ms, REACH_TIME = 10 ms, and
   REACHABILITY_TIMEOUT = 1000ms, then:

   x = (100 - 10)/(1000 + 100 - 10) = 8.2 percent

   Therefore the hint need only be wrong 8.2 percent of the time before
   it is not worth considering.






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A.2  Hints

   For networks running IPv4 over PPP [RFC1661], IPv4 parameters
   negotiated in IPCP provide direct information on whether a previously
   obtained address remains valid on the link.

   On IEEE 802 [IEEE802] wired networks, hints include link-layer
   discovery traffic as well as information exchanged as part of IEEE
   802.1X authentication [IEEE8021X].

   Link-layer discovery traffic includes Link Layer Discovery Protocol
   (LLDP) [IEEE8021AB] traffic as well as network identification
   information passed in the EAP-Request/Identity or within an EAP
   method exchange, as defined in EAP [RFC3748].

   For example, LLDP advertisements can provide information on VLANs
   supported by the device.  When used with IEEE 802.1X authentication
   [IEEE8021X], the EAP-Request/Identity exchange may contain the name
   of the authenticator, also providing information on the potential
   network.  Similarly, during the EAP method exchange the authenticator
   may supply information that may be helpful in identifying the network
   to which the device is attached.   However, as noted in [RFC3580], it
   is possible for the VLANID defined in [IEEE8021Q] to be assigned
   dynamically, so that static advertisements may not prove definitive.

   In IEEE 802.11 [IEEE80211] stations provide information in Beacon
   and/or Probe Response messages, such as the SSID, BSSID, and
   capabilities, as well as information on whether the station is
   operating in Infrastructure or Ad hoc mode.  As described in
   [RFC3580], it is possible to assign a Station to a VLAN dynamically,
   based on the results of IEEE 802.1X [IEEE8021X] authentication.  This
   implies that a single SSID may offer access to multiple VLANs, and in
   practice most large WLAN deployments offer access to multiple
   subnets.

   Thus, associating to the same SSID is a necessary, but not
   necessarily a sufficient condition, for remaining within the same
   subnet: while a Station associating to the same SSID may not
   necessarily remain within the same subnet, a Station associating to a
   different SSID is likely to have changed subnets.

   In IEEE 802.11, the SSID is a non-unique identifier, and SSIDs such
   as "default", "linksys" and "tsunami" are often configured by
   manufacturers by default.  As a result,  matching an advertised SSID
   against those of previously encountered networks may be misleading.
   Where an SSID known to be configured by default is encountered, it is
   recommended that the BSSID be stored and subsequently compared
   against the advertised BSSID to determine whether a match exists.



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   In order to provide additional guidance on the subnets to which a
   given AP offers access, additional subnet-related Information
   Elements (IEs) have been proposed for addition to the IEEE 802.11
   Beacon and Probe Response messages.  As noted earlier, VLANs may be
   determined dynamically so that these information elements may not be
   reliable.

   In IEEE 802.11, the presence of an IBSS can be used as a hint that a
   point of attachment supports adhoc networking, and therefore that
   assignment of a IPv4 Link-Local address is likely.  When running IPv4
   over PPP, if an IP address is not statically configured or  assigned
   via IPCP, this can also be taken as a hint that assignment of an IPv4
   Link-Local address is likely.  In addition, certain media such as USB
   or IEEE 1394 may be considered inherently more likely to support
   adhoc operation, so that connection to these networks may by itself
   be considered a hint.

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Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



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INTERNET-DRAFT                    DNAv4                  14 October 2004


Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Open issues

   Open issues relating to this specification are tracked on the
   following web site:

   http://www.drizzle.com/~aboba/DNA/dnaissues.html







































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