Network Working Group                                           JH. Choi
Request for Comments: 4135                                   Samsung AIT
Category: Informational                                         G. Daley
                                                  CTIE Monash University
                                                             August 2005


             Goals of Detecting Network Attachment in IPv6

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   When a host establishes a new link-layer connection, it may or may
   not have a valid IP configuration for Internet connectivity.  The
   host may check for link change (i.e., determine whether a link change
   has occurred), and then, based on the result, it can automatically
   decide whether its IP configuration is still valid.  During link
   identity detection, the host may also collect necessary information
   to initiate a new IP configuration if the IP subnet has changed.  In
   this memo, this procedure is called Detecting Network Attachment
   (DNA).  DNA schemes should be precise, sufficiently fast, secure, and
   of limited signaling.

Table of Contents

   1. Introduction ....................................................2
   2. Problems in Detecting Network Attachment ........................3
      2.1. Wireless Link Properties ...................................3
      2.2. Link Identity Detection with a Single RA ...................3
      2.3. Delays .....................................................4
   3. Goals for Detecting Network Attachment ..........................5
      3.1. Goals List .................................................6
   4. Security Considerations .........................................6
   5. Acknowledgements ................................................7
   6. References ......................................................8
      6.1. Normative References .......................................8
      6.2. Informative References .....................................8

1.  Introduction

   When a host has established a new link-layer connection, it can send
   and receive some IPv6 packets on the link, including those used for
   configuration.  On the other hand, the host has Internet connectivity
   only when it is able to exchange packets with off-link destinations.

   When a link-layer connection is established or re-established, the
   host may not know whether its existing IP configuration is still
   valid for Internet connectivity.  A subnet change might have occurred
   when the host changed its point of attachment.

   In practice, the host doesn't know which of its addresses are valid
   on the newly attached link.  It also doesn't know whether its
   existing default router is on this link or whether its neighbor cache
   entries are valid.  Correct configuration of each of these components
   is necessary in order to send packets on and off the link.

   To examine the status of the existing configuration, a host may check
   whether a 'link change' has occurred.  In this document, the term
   'link' is as defined in RFC 2461 [1].  The notion 'link' is not
   identical with the notion 'subnet', as defined in RFC 3753 [2].  For
   example, there may be more than one subnet on a link, and a host
   connected to a link may be part of one or more of the subnets on the
   link.

   Today, a link change necessitates an IP configuration change.
   Whenever a host detects that it has remained at the same link, it can
   usually assume its IP configuration is still valid.  Otherwise, the
   existing one is no longer valid, and a new configuration must be
   acquired.  Therefore, to examine the validity of an IP configuration,
   all that is required is that the host checks for link change.

   In the process of checking for link change, a host may collect some
   of the necessary information for a new IP configuration, such as on-
   link prefixes.  So, when an IP subnet change has occurred, the host
   can immediately initiate the process of getting a new IP
   configuration.  This may reduce handoff delay and minimize signaling.

   Rapid attachment detection is required for a device that changes
   subnet while having on-going sessions.  This may be the case if a
   host is connected intermittently, is a mobile node, or has urgent
   data to transmit upon attachment to a link.

   Detecting Network Attachment (DNA) is the process by which a host
   collects the appropriate information and detects the identity of its
   currently attached link to ascertain the validity of its IP
   configuration.

   DNA schemes are typically run per interface.  When a host has
   multiple interfaces, the host separately checks for link changes on
   each interface.

   It is important to note that DNA process does not include the actual
   IP configuration procedure.  For example, with respect to DHCP, the
   DNA process may determine that the host needs to get some
   configuration information from a DHCP server.  However, the process
   of actually retrieving the information from a DHCP server falls
   beyond the scope of DNA.

   This document considers the DNA procedure only from the IPv6 point of
   view, unless explicitly mentioned otherwise.  Thus, the term "IP" is
   to be understood to denote IPv6, by default.  For the IPv4 case,
   refer to [7].

2.  Problems in Detecting Network Attachment

   A number of issues make DNA complicated.  First, wireless
   connectivity is not as clear-cut as wired connectivity.  Second, it's
   difficult for a single Router Advertisement (RA) message to indicate
   a link change.  Third, the current Router Discovery specification
   specifies that routers wait a random delay of 0-.5 seconds prior to
   responding with a solicited RA.  This delay can be significant and
   may result in service disruption.

2.1.  Wireless Link Properties

   Unlike in wired environments, what constitutes a wireless link is
   variable both in time and space.  Wireless links do not have clear
   boundaries.  This may be illustrated by the fact that a host may be
   within the coverage area of multiple (802.11) access points at the
   same time.  Moreover, connectivity on a wireless link can be very
   volatile, which may make link identity detection hard.  For example,
   it takes time for a host to check for link change.  If the host
   ping-pongs between two links and doesn't stay long enough at a given
   link, it can't complete the DNA procedure.

2.2.  Link Identity Detection with a Single RA

   Usually, a host gets the information necessary for IP configuration
   from RA messages.  Based on the current definition [1], it's
   difficult for a host to check for link change upon receipt of a
   single RA.

   To detect link identity, a host may compare the information in an RA,
   such as router address or prefixes, with the locally stored
   information.

   The host may use received router addresses to check for link change.
   The router address in the source address field of an RA is of link-
   local scope, however, so its uniqueness is not guaranteed outside a
   link.  If it happens that two different router interfaces on
   different links have the same link-local address, the host can't
   detect that it has moved from one link to another by checking the
   router address in RA messages.

   The set of all global prefixes assigned to a link can represent link
   identity.  The host may compare the prefixes in an incoming RA with
   the currently stored ones.  An unsolicited RA message, however, can
   omit some prefixes for convenience [1], and it's not easy for a host
   to attain and retain all the prefixes on a link with certainty.
   Therefore, neither the absence of a previously known prefix nor the
   presence of a previously unknown prefix in the RA guarantees that a
   link change has occurred.

2.3.  Delays

   The following issues cause DNA delay that may result in communication
   disruption.

   1) Delay for receiving a hint

   A hint is an indication that a link change might have occurred.  This
   hint itself doesn't confirm a link change, but initiates appropriate
   DNA procedures to detect the identity of the currently attached link.

   Hints come in various forms and differ in how they indicate a
   possible link change.  They can be link-layer event notifications
   [6], the lack of RA from the default router, or the receipt of a new
   RA.  The time taken to receive a hint also varies.

   As soon as a new link-layer connection has been made, the link layer
   may send a link-up notification to the IP layer.  A host may
   interpret the new link-layer connection as a hint for a possible link
   change.  With link-layer support, a host can receive such a hint
   almost instantly.

   Mobile IPv6 [4] defines the use of RA Interval Timer expiry for a
   hint.  A host keeps monitoring for periodic RAs and interprets the
   lack of them as a hint.  It may implement its own policy to determine
   the number of missing RAs needed to interpret that as a hint.  Thus,
   the delay depends on the Router Advertisement interval.

   Without schemes such as those above, a host must receive a new RA
   from a new router to detect a possible link change.  The detection
   time then also depends on the Router Advertisement frequency.

   Periodic RA beaconing transmits packets within an interval varying
   randomly between MinRtrAdvInterval to MaxRtrAdvInterval seconds.
   Because a network attachment is unrelated to the advertisement time
   on the new link, hosts are expected to arrive, on average, halfway
   through the interval.  This is approximately 1.75 seconds with
   Neighbor Discovery [1] advertisement rates.

   2) Random delay execution for RS/RA exchange

   Router Solicitation and Router Advertisement messages are used for
   Router Discovery.  According to [1], it is sometimes necessary for a
   host to wait a random amount of time before it may send an RS, and
   for a router to wait before it may reply with an RA.

   According to RFC 2461 [1], the following apply:

   -  Before a host sends an initial solicitation, it SHOULD delay the
      transmission for a random amount of time between 0 and
      MAX_RTR_SOLICITATION_DELAY (1 second).

   -  Furthermore, any RA sent in response to a Router Solicitation MUST
      be delayed by a random time between 0 and MAX_RA_DELAY_TIME (0.5
      seconds).

3.  Goals for Detecting Network Attachment

   The DNA working group has been chartered to define an improved scheme
   for detecting IPv6 network attachment.  In this section, we define
   the goals that any such solution should aim to fulfill.

   DNA solutions should correctly determine whether a link change has
   occurred.  Additionally, they should be sufficiently fast so that
   there would be no or at most minimal service disruption.  They should
   neither flood the link with related signaling nor introduce new
   security holes.

   When defining new solutions, it is necessary to investigate the usage
   of available tools, Neighbor Solicitation/Neighbor Advertisement
   messages, RS/RA messages, link-layer event notifications [6], and
   other features.  This will allow precise description of procedures
   for efficient DNA Schemes.

3.1.  Goals List

   G1  DNA schemes should detect the identity of the currently attached
       link to ascertain the validity of the existing IP configuration.
       They should recognize and determine whether a link change has
       occurred and initiate the process of acquiring a new
       configuration if necessary.

   G2  DNA schemes should detect the identity of an attached link with
       minimal latency lest there should be service disruption.

   G3  If a host has not changed a link, DNA schemes should not falsely
       assume a link change, and an IP configuration change should not
       occur.

   G4  DNA schemes should not cause undue signaling on a link.

   G5  DNA schemes should make use of existing signaling mechanisms
       where available.

   G6  DNA schemes should make use of signaling within the link
       (particularly link-local scope messages), because communication
       off-link may not be achievable in the case of a link change.

   G7  DNA schemes should be compatible with security schemes such as
       Secure Neighbor Discovery [3].

   G8  DNA schemes should not introduce new security vulnerabilities.
       The node supporting DNA schemes should not expose itself or other
       nodes on a link to additional man-in-the-middle, identity-
       revealing, or denial-of-service attacks.

   G9  Nodes (such as routers or hosts) that support DNA schemes should
       work appropriately with unmodified nodes that do not.

   G10 Hosts, especially in wireless environments, may perceive routers
       reachable on different links.  DNA schemes should take into
       consideration the case where a host is attached to more than one
       link at the same time.

4.  Security Considerations

   The DNA process is intimately related to the Neighbor Discovery
   protocol [1] and its trust model and threats have much in common with
   those presented in RFC 3756 [5].  Nodes connected over wireless
   interfaces may be particularly susceptible to jamming, monitoring,
   and packet-insertion attacks.

   With unsecured DNA schemes, 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 because it believed that it had connected to a home
   network.

   Even in the case where authoritative information (routing and prefix
   state) are advertised, wireless network attackers may still prevent
   soliciting nodes from receiving packets.  This may cause unnecessary
   IP configuration change in some devices.  Such attacks may be used to
   make a host preferentially select a particular configuration or
   network access.

   Devices receiving confirmations of reachability (for example, from
   upper-layer protocols) should be aware that unless these indications
   are sufficiently authenticated, reachability may falsely be asserted
   by an attacker.  Similarly, even if such reachability tests are known
   to originate from a trusted source, they should be ignored for
   reachability confirmation if the packets are not fresh or have been
   replayed.  This may reduce the effective window for attackers
   replaying otherwise authentic data.

   It may be dangerous to receive link-change notifications from the
   link layer and network layer, if they are received from devices that
   are insufficiently authenticated.  In particular, notifications that
   authentication has completed at the link layer may not imply that a
   security relationship is available at the network layer.  Additional
   authentication may be required at the network layer to justify
   modification of IP configuration.

5.  Acknowledgements

   Erik Nordmark has contributed significantly to work predating this
   document.  Also Ed Remmell's comments on the inconsistency of RA
   information were most illuminating.  The authors wish to express our
   appreciation to Pekka Nikander for valuable feedback.  We gratefully
   acknowledge the generous assistance we received from Shubhranshu
   Singh for clarifying the structure of the arguments.  Thanks to Brett
   Pentland, Nick Moore, Youn-Hee Han, JaeHoon Kim, Alper Yegin, Jim
   Bound, and Jari Arkko for their contributions to this document.

6.  References

6.1.  Normative References

   [1]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [2]  Manner, J. and M. Kojo, "Mobility Related Terminology", RFC
        3753, June 2004.

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

6.2.  Informative References

   [4]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.

   [5]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
        Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

   [6]  Yegin, A., "Link-layer Event Notifications for Detecting Network
        Attachments", work in progress, July 2005.

   [7]  Aboba, B., "Detecting Network Attachment (DNA) in IPv4", work in
        progress, June 2005.

Authors' Addresses

   JinHyeock Choi
   Samsung AIT
   Communication & N/W Lab
   P.O.Box 111 Suwon 440-600
   KOREA

   Phone: +82 31 280 9233
   EMail: jinchoe@samsung.com


   Greg Daley
   CTIE Monash University
   Centre for Telecommunications and Information Engineering
   Monash University
   Clayton 3800 Victoria
   Australia

   Phone: +61 3 9905 4655
   EMail: greg.daley@eng.monash.edu.au

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