Link-Layer Events: Link-Layer Event Notifications for Detecting Network Attachments

Network Working Group                                   S. Krishnan, Ed.
Request for Comments: 4957                             Ericsson Research
Category: Informational                                     N. Montavont
                                                       GET ENST Bretagne
                                                              E. Njedjou
                                                          France Telecom
                                                           S. Veerepalli
                                                                Qualcomm
                                                           A. Yegin, Ed.
                                                                 Samsung
                                                             August 2007


    Link-Layer Event Notifications for Detecting Network Attachments

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 IETF Trust (2007).

Abstract

   Certain network access technologies are capable of providing various
   types of link-layer status information to IP.  Link-layer event
   notifications can help IP expeditiously detect configuration changes.
   This document provides a non-exhaustive catalogue of information
   available from well-known access technologies.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Link-Layer Event Notifications . . . . . . . . . . . . . . . .  5
     3.1.  GPRS/3GPP  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  cdma2000/3GPP2 . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  IEEE 802.11/WiFi . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  IEEE 802.3 CSMA/CD . . . . . . . . . . . . . . . . . . . .  9
       3.4.1.  Link Integrity Tests in 802.3 Networks . . . . . . . . 10
       3.4.2.  IEEE 802.1D Bridging and Its Effects on Link-layer
               Event Notifications  . . . . . . . . . . . . . . . . . 11
       3.4.3.  802.1AB Link-Layer Discovery Protocol  . . . . . . . . 12
       3.4.4.  Other Heuristics . . . . . . . . . . . . . . . . . . . 13
       3.4.5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   5.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   It is not an uncommon occurrence for a node to change its point of
   attachment to the network.  This can happen due to mobile usage
   (e.g., a mobile phone moving among base stations) or nomadic usage
   (e.g., road-warrior case).

   A node changing its point of attachment to the network may end up
   changing its IP subnet and therefore require reconfiguration of IP-
   layer parameters, such as IP address, default gateway information,
   and DNS server address.  Detecting the subnet change can usually use
   network-layer indications (such as a change in the advertised
   prefixes for IPv6).  But such indications may not be always available
   (e.g., Detecting Network Attachment in IPv6 (DNAv6)) to the node upon
   changing its point of attachment.

   Link-layer event notifications can help IP expeditiously detect
   configuration changes.  This document provides a non-exhaustive
   catalog of information available from some access technologies, and
   discusses the interpretation of this information at the IP layer.
   This document is not intended to specify or change the behavior of
   these access technologies in any manner.

   Additional information can be conveyed along with the event, such as
   the identifier of the network attachment point (e.g., IEEE 802.11
   Basic Service Set Identification (BSSID) and Service Set Identifier
   (SSID)), or network-layer configuration parameters obtained via the
   link-layer attachment process if available.  It is envisaged that
   such event notifications can in certain circumstances be used to
   expedite the inter-subnet movement detection and reconfiguration
   process.  For example, the notification indicating that the node has
   established a new link-layer connection may be used for immediately
   probing the network for a possible configuration change.  In the
   absence of such a notification from the link layer, IP has to wait
   for indications that are not immediately available, such as receipt
   of the next scheduled router advertisement, unreachability of the
   default gateway, etc.

   It should be noted that a link-layer event notification does not
   always translate into a subnet change.  Even if the node has torn
   down a link-layer connection with one attachment point and
   established a new connection with another, it may still be attached
   to the same IP subnet.  For example, several IEEE 802.11 access
   points can be attached to the same IP subnet.  Moving among these
   access points does not warrant any IP-layer configuration change.

   In order to enable an enhanced scheme for detecting change of subnet,
   we need to define link-layer event notifications that can be
   realistically expected from various access technologies.  The
   objective of this document is to provide a catalogue of link-layer
   events and notifications in various architectures.  While this
   document mentions the utility of this information for detecting
   change of subnet (or, detecting network attachment - DNA), the
   detailed usage is left to other documents, namely, DNA solution
   specifications.

   The document limits itself to the minimum set of information that is
   necessary for solving the DNA problem [RFC4135].  A broader set of
   information (e.g., signal strength, packet loss, etc.) and events
   (e.g. link down) may be used for other problem spaces, such as
   anticipation-based Mobile IP fast handovers [RFC4881], [RFC4068],
   etc.

   These event notifications are considered with hosts in mind, although
   they may also be available on the network side (e.g., on the access
   points and routers).  An API or protocol-based standard interface may
   be defined between the link layer and IP for conveying this
   information.  That activity is beyond the scope of this document.

2.  Terminology

   Link: is a communication facility or medium over which network nodes
   can communicate.  Each link is associated with a minimum of two
   endpoints.  An "attachment point" is the link endpoint on the link to
   which the node is currently connected, such as an access point, a
   base station, or a wired switch.

   Link up: is an event provided by the link layer that signifies a
   state change associated with the interface becoming capable of
   communicating data packets.  This event is associated with a link-
   layer connection between the node and an attachment point.

   BSSID: Basic Service Set Identification

   DNA: Detecting Network Attachment

   GPRS: General Packet Radio Service

   PDP: Packet Data Protocol

   SSID: Service Set Identifier

3.  Link-Layer Event Notifications

   Link-layer event notifications are considered to be one of the inputs
   to the DNA process.  A DNA process is likely to take other inputs
   (e.g., presence of advertised prefixes, reachability of default
   gateways) before determining whether IP-layer configuration must be
   updated.  It is expected that the DNA process can take advantage of
   link-layer notifications when they are made available to IP.  While
   by itself a link-layer notification may not constitute all the input
   DNA needs, it can at least be useful for prompting the DNA process to
   collect further information (i.e., other inputs to the process).  For
   example, the node may send a router solicitation as soon as it learns
   that a new link-layer connection is established.

   The link-layer event that is considered most useful to DNA process is
   the link up event.  The associated notifications can be provided to
   the IP-layer after the event concludes successfully.  The link up
   events and notifications are associated with a network interface on
   the node.  The IP module may receive simultaneous independent
   notifications from each one of the network interfaces on the node.

   The actual event is managed by the link layer of the node through
   execution of link-layer protocols and mechanisms.  Once the event
   successfully completes within the link layer, its notification is
   delivered to the IP-layer.  By the time the notification is
   delivered, the link layer of the node must be ready to accept IP
   packets from the IP and the physical layers.  Each time an interface
   changes its point of attachment, a link up event should be generated.

   There is a non-deterministic usage of the link up notification to
   accommodate implementations that desire to indicate the link is up,
   but the data transmission may be blocked in the network (see IEEE
   802.3 discussion).  A link up notification may be generated with an
   appropriate attribute, conveying its non-deterministic nature, to
   convey the event.  Alternatively, the link-layer implementation may
   choose to delay the link up notification until the risk conditions
   cease to exist.

   If a non-deterministic link up was generated, another link up must
   follow as soon as the link layer is capable of generating a
   deterministic notification.  The event attributes may indicate
   whether the packets transmitted since the previous notification were
   presumed to be blocked or allowed by the network, if the link layer
   could determine the exact conditions.

   The deterministic link up event following a non-deterministic link up
   event can be treated differently by consumers of the link up event.
   For example, the second link up event need not trigger a confirmation
   process, if the first one already did.

   A node may have to change its IP-layer configuration even when the
   link-layer connection stays the same.  An example scenario is the
   IPv6 subnet renumbering [RFC2461].  Therefore, there exist cases
   where IP-layer configuration may have to change even without the IP
   layer receiving a link up notification.  Therefore, a link-layer
   notification is not a mandatory indication of a subnet change.

   A link up notification may optionally deliver information relating to
   the attachment point.  Such auxiliary information may include the
   identity of the attachment point (e.g., base station identifier), or
   the IP-layer configuration parameters associated with the attached
   subnet (e.g., subnet prefix, default gateway address, etc.).  While
   merely knowing that a new link-layer connection is established may
   prompt the DNA process to immediately seek other clues for detecting
   a network configuration change, auxiliary information may constitute
   further clues (and even the final answers sometimes).  In cases where
   there is a one-to-one mapping between the attachment point
   identifiers and the IP-layer configurations, learning the former can
   reveal the latter.  Furthermore, IP-layer configuration parameters
   obtained during the link-layer connection may be exactly what the DNA
   process is trying to discover.

   The link-layer process leading to a link up event depend on the link
   technology.  While a link-layer notification must always indicate
   that the link up event occurred, the availability and types of
   auxiliary information on the attachment point depends on the link-
   layer technology as well.  The following subsections examine four
   link-layer technologies and describe when a link-layer notification
   is generated and what information is included in it.

3.1.  GPRS/3GPP

   GSM Packet Radio System (GPRS) provides packet-switched data
   transmission over a cellular network [GPRS][GPRS-LINK].

   The GPRS architecture consists of a Radio Access Network and a packet
   domain Core Network.

   -  The GPRS Radio Access Network is composed of Mobile Terminals
      (MTs), a Base Station Subsystem and Serving GPRS Support Nodes
      (SGSNs).

   -  An IP Core Network that acts as the transport backbone of user
      datagrams between SGSNs and Gateway GPRS Support Nodes (GGSNs).
      The GGSN ensures the GPRS IP core network connectivity with
      external networks, such as the Internet or Local Area Networks.
      The GGSN acts as the default IP gateway for the MT.

   A GPRS MT that wants to establish IP connectivity establishes first a
   connection to the GPRS network and one or more PDP Context
   associations between the MT and the GGSN.  It is only after the PDP
   Context has been established and after address autoconfiguration and
   tunneling mechanism have taken place that the MT's IP packets can be
   forwarded to and from its remote IP peers.  The aim of PDP Context
   establishment is also to provide IP-level configuration on top of the
   GPRS link-layer attachment.

   Successful establishment of a PDP Context on a GPRS link signifies
   the availability of IP service to the MT.  Therefore, this link-layer
   event generates a link up event notification sent to the IP layer.

   An MT may establish a secondary PDP Context while reusing the IP
   configuration acquired from a previously established and active PDP
   Context.  Such a secondary PDP Context does not provide additional
   information to the IP layer and only allows another quality-of-
   service (QoS) profile to be used.  The activation of such a secondary
   PDP context does not usually generate a link up event since it does
   not require new IP parameters.  However, other additional PDP Context
   activations are to be treated as indicated earlier.

   With IPv4, the auxiliary information carried along with this
   notification is the IPv4 address of the MT that is obtained as part
   of the PDP Context.  With IPv6, the PDP Context activation response
   does not come along with a usable IPv6 address.  Effectively, the
   IPv6 address received from the GGSN in the PDP address field of the
   message does not contain a valid prefix.  The MN actually only uses
   the interface identifier extracted from that field to form a link-
   local address that it uses afterwards to obtain a valid prefix (e.g.,
   by stateless [RFC2462][GPRS-CN] or stateful [RFC3315] [GPRS-GSSA]
   address configuration).  Therefore, no IPv6-related auxiliary
   information is provided to the IP layer.

3.2.  cdma2000/3GPP2

   cdma2000-based 3GPP2 packet data services provide mobile users wide
   area high-speed access to packet switched networks [CDMA2K].  Some of
   the major components of the 3GPP2 packet network architecture consist
   of:

   -  Mobile Station (MS), which allows mobile access to packet-switched
      networks over a wireless connection.

   -  Radio Access Network, which consists of the Base Station
      Transceivers, Base Station Controllers, and the Packet Control
      Function.

   -  Network Access Server known as the Packet Data Switching Node
      (PDSN).  The PDSN also serves as default IP gateway for the IP MS.

   3GPP2 networks use the Point-to-Point Protocol (PPP [RFC1661]) as the
   link-layer protocol between the MS and the PDSN.  Before any IP
   packets may be sent or received, PPP must reach the Network-Layer
   Protocol phase, and the IP Control Protocol (IPCP [RFC1332], IPV6CP
   [RFC2472]) must reach the Opened state.  When these states are
   reached in PPP, a link up event notification is delivered to the IP
   layer.

   When the PPP is used for 3GPP2 Simple (i.e., non-Mobile) IPv4
   Service, IPCP enables configuration of an IPv4 address on the MS.
   This IPv4 address is provided as the auxiliary information along with
   the link up notification.  IPV6CP used for Simple IPv6 service does
   not provide an IPv6 address, but the interface identifiers for local
   and remote endpoints of the PPP link.  Since there is no standards-
   mandated correlation between the interface identifier and other IP-
   layer configuration parameters, this information is deemed not useful
   for DNA (nevertheless, it may be provided as auxiliary information
   for other uses).

3.3.  IEEE 802.11/WiFi

   IEEE 802.11-based WiFi networks are the wireless extension of the
   Local Area Networks.  Currently available standards are IEEE 802.11b
   [IEEE-802.11b], IEEE 802.11g [IEEE-802.11g], and IEEE 802.11a
   [IEEE-802.11a].  The specifications define both the MAC layer and the
   physical layer.  The MAC layer is the same for all these
   technologies.

   Two operating modes are available in the IEEE 802.11 series, either
   infrastructure mode or ad-hoc mode.  In infrastructure mode, all
   link-layer frames are transmitted to an access point (AP) that then
   forwards them to the final receiver.  A station (STA) establishes an
   IEEE 802.11 association with an AP in order to send and receive IP
   packets.  In a WiFi network that uses Robust Secure Network (RSN
   [IEEE-802.11i]), successful completion of the 4-way handshake between
   the STA and AP commences the availability of IP service.  The link up
   event notification is generated upon this event.  In non-RSN-based
   networks, successful association or re-association events on the link
   layer causes a link up notification sent to the IP layer.

   As part of the link establishment, the STA learns the BSSID and SSID
   associated with the AP.  The BSSID is a unique identifier of the AP,
   usually set to the MAC address of the wireless interface of the AP.
   The SSID carries the identifier of the Extended Service Set (ESS) --
   the set composed of APs and associated STAs that share a common
   distribution system.  The BSSID and SSID may be provided as auxiliary
   information along with the link up notification.  Unfortunately, this
   information does not provide a deterministic indication of whether
   the IP-layer configuration must be changed upon movement.  There is
   no standards-mandated one-to-one relation between the BSSID/SSID
   pairs and IP subnets.  An AP with a given BSSID can connect a STA to
   any one of multiple IP subnets.  Similarly, an ESS with the given
   SSID may span multiple IP subnets.  And finally, the SSIDs are not
   globally unique.  The same SSID may be used by multiple independent
   ESSs.  Nevertheless, BSSID/SSID information may be used in a
   probabilistic way by the DNA process; hence, it is provided with the
   link up event notification.

   In ad-hoc mode, mobile stations (STA) in range may directly
   communicate with each other, i.e., without any infrastructure or
   intermediate hop.  The set of communicating STAs is called IBSS for
   Independent Basic Service Set.  In an IBSS, only STA services are
   available, i.e., authentication, deauthentication, privacy, and MAC
   Service Data Unit (MSDU) delivery.  STAs do not associate with each
   other, and therefore may exchange data frames in state 2
   (authenticated and not associated) or even in state 1
   (unauthenticated and unassociated) if the Distribution System is not
   used (i.e., "To DS" and "From DS" bits are clear).  If authentication
   is performed, a link up indication can be generated upon
   authentication.  Concerning the link layer identification, both the
   BSSID (which is a random MAC address chosen by a STA of the IBSS) and
   SSID may be used to identify a link, but not to make any assumptions
   on the IP network configuration.

3.4.  IEEE 802.3 CSMA/CD

   IEEE 802.3 CSMA/CD (commonly referred to as Ethernet) is the most
   commonly deployed Local Area Network technology in use today.  As
   deployed today, it is specified by a physical layer/medium access
   control (MAC) layer specification [IEEE-802.3].  In order to provide
   connection of different LANs together into a larger network, 802.3
   LANs are often bridged together [IEEE-802.1D].

   In this section, the terms 802.3 and Ethernet are used
   interchangeably.  This section describes some issues in providing
   link-layer indications on Ethernet networks, and shows how bridging
   affects these indications.

   In Ethernet networks, hosts are connected by wires or by optic fibre
   to a switch (bridge), a bus (e.g., coaxial cable), a repeater (hub),
   or directly to another Ethernet device.  Interfaces are symmetric, in
   that while many different physical layers may be present, medium
   access control is uniform for all devices.

   In order to determine whether the physical medium is ready for frame
   transfer, IEEE 802.3 Ethernet specifies its own link monitoring
   mechanism, which is defined for some, but not all, classes of media.
   Where available, this Link Integrity Test operation is used to
   identify when packets are able to be received on an Ethernet segment.
   It is applicable to both wired and optical physical layers, although
   details vary between technologies (link pulses in twisted pair
   copper, light levels in fibre).

3.4.1.  Link Integrity Tests in 802.3 Networks

   Link Integrity Tests in 802.3 networks typically occur at initial
   physical connection time (for example, at the auto-negotiation stage)
   and periodically afterwards.  They make use of physical-layer
   specific operations to determine if a medium is able to support link-
   layer frames [IEEE-802.3].

   The status of the link as determined by the Link Integrity Test is
   stored in the variable 'link_status'.  Changes to the value of
   link_status (for example due to Link Integrity Test failure) will
   generate link indications if the technology-dependent interface is
   implemented on an Ethernet device [IEEE-802.3].

   The link_status has possible values of FAIL, READY, and OK.  In FAIL
   state, Link Integrity Tests have failed.  In READY state, the link
   segment has passed integrity tests, but auto-negotiation has not
   completed.  In OK state, the medium is able to send and receive
   packets.

   Upon transition to a particular state, the Physical Medium Attachment
   subsystems generates a PMA_LINK.indicate(link_status).  Indications
   of OK state may be used to generate a link up event notification.
   These indications do not definitively ensure that packets will be
   able to be received through the bridge domain, though (see the next
   section).  Such operations are governed by bridging.

3.4.2.  IEEE 802.1D Bridging and Its Effects on Link-layer Event
        Notifications

   Ethernet networks commonly consist of LANs joined together by
   transparent bridges (usually implemented as switches).  Transparent
   bridges require the active topology to be loop free.  This is
   achieved through the Spanning Tree Protocol (STP) or the Rapid
   Spanning Tree Protocol (RSTP).  These protocols exchange Bridge
   Protocol Data Units (BPDUs), as defined in [IEEE-802.1D]; this leads
   to the blocking of ports (i.e., not forwarding), where required.

   By default, the spanning tree protocol does not know whether a
   particular newly connected piece of Ethernet will cause a loop.

   Therefore, it will block all traffic from and to newly connected
   ports with the exception of some unbridged management frames.  The
   STP will determine if the port can be connected to the network in a
   loop-free manner.

   For these technologies, even though the link layer appears available,
   no data packet forwarding will occur until it is determined that the
   port can be connected to the network in a loop-free environment.

   For hosts that are providing indications to upper-layer protocols,
   even if the host itself does not implement bridging or STP, packet
   delivery across the network can be affected by the presence of
   bridges.

   A host connected to a bridge port does not receive any explicit
   indication that the bridge has started forwarding packets.
   Therefore, a host may not know when STP operations have completed, or
   when it is safe to inform upper layers to transmit packets.

   Where it is not known that forwarding operations are available, a
   host should assume that RSTP or STP is being performed.  Hosts may
   listen to STP/RSTP and 802.1AB messages to gain further information
   about the timing of full connectivity on the link, for example, to
   override an existing indication.

   Notably, though, it is not easy for a host to distinguish between
   disabled bridge ports and non-bridge ports with no active
   transmitters on them, as Disabled ports will have no traffic on them,
   and incur 100% sender loss.

   If no bridge configuration messages are received within the
   Bridge_Max_Age interval (default 20s) then it is likely that there is
   no visible bridge whose port is enabled for bridging (S8.4.5 of
   [IEEE-802.1D]), since at least two BPDU hello messages would have
   been lost.  Upon this timeout, a link up notification is generated,
   if one has not been already.

   If a BPDU is received, and the adjacent bridge is running the
   original Spanning Tree Protocol, then a host cannot successfully send
   packets until at least twice the ForwardDelay value in the received
   BPDU has elapsed.  After this time, a link up notification is
   generated.  If the previous link up notification was non-
   deterministic, then this notification includes an attribute
   signifying that the packets sent within the prior interval were lost.

   If the bridge is identified as performing Rapid Spanning Tree
   Protocol (RSTP), it instead waits Bridge_Max_Age after packet
   reception (advertised in the BPDU's Max Age field), before
   forwarding.  For ports which are known to be point-to-point through
   auto-negotiation, this delay is abbreviated to 3 seconds after auto-
   negotiation completes [IEEE-802.1D].

3.4.3.  802.1AB Link-Layer Discovery Protocol

   The recently defined 802.1AB Link-Layer Discovery Protocol (LLDP)
   provides information to devices that are directly adjacent to them on
   the local LAN [IEEE-802.1ab].

   LLDP sends information periodically and at link status change time to
   indicate the configuration parameters of the device.  Devices may
   send or receive these messages, or do both.

   The LLDP message may contain a System Capabilities TLV, which
   describes the MAC- and IP-layer functions that a device is currently
   using.  Where a host receives the System Capabilities TLV indicating
   that no Bridging is occurring on the LLDP transmitter, no delays for
   STP calculation will be applied to packets sent through this
   transmitter.  This would allow the generation of a link up
   notification.

   Additionally, if a host receives a System Capabilities TLV indicating
   that the LLDP transmitter is a bridge, the host's advertisement that
   it is an (end-host) Station-Only may tell the bridge not to run STP
   and may immediately allow forwarding.

   Proprietary extensions may also indicate that data forwarding is
   already available on such a port.  Discussion of such optimizations
   is out of scope for this document.

   Because the protocol is new and not widely deployed, it is unclear
   how this protocol will eventually affect DNA in IPv4 or IPv6
   networks.

3.4.4.  Other Heuristics

   In 802.3 networks, Network Interface Cards (NICs) are often capable
   of returning a speed and duplex indication to the host.  Changes in
   these characteristics may indicate a connection to a new layer 2
   network.

3.4.5.  Summary

   Link-layer indications in Ethernet-like networks are complicated by
   additional unadvertised delays due to spanning tree calculations.
   This may cause re-indication or retraction of indications previously
   sent to upper layer protocols.

4.  Security Considerations

   Attackers may spoof various indications at the link layer, or
   manipulate the physical medium directly in an effort to confuse the
   host about the state of the link layer.  For instance, attackers may
   spoof error messages or disturb the wireless medium to cause the host
   to move its connection elsewhere or even to disconnect.  Attackers
   may also spoof information to make the host believe it has a
   connection when, in reality, it does not.  In addition, wireless
   networks such as 802.11 are susceptible to an attack called the "Evil
   Twin" attack where an attacker sets up an Access Point with the same
   SSID as a legitimate one and gets the use to connect to the fake
   access point instead of the real one.  These attacks may cause use of
   non-preferred networks or even denial of service.

   This specification does not provide any protection of its own for the
   indications from the lower layers.  But the vulnerabilities can be
   mitigated through the use of techniques in other parts of the
   protocol stack.  In particular, it is recommended that
   authentication, replay, and integrity protection of link-layer
   management messages are enabled when available.  For example, the
   IEEE 802.1ae standard [IEEE-802.1ae] defines such mechanisms for IEEE
   802-compliant MAC layers.  Additionally, the protocol stack may also
   use some network-layer mechanisms to achieve partial protection.  For
   instance, SEND [RFC3971] could be used to confirm secure reachability
   with a router.  However, network layer mechanisms are unable to deal
   with all problems, such as insecure lower-layer notifications that
   lead to the link not functioning properly.

5.  Contributors

   In addition to the people listed in the author list, text for the
   specific link-layer technologies covered by this document was
   contributed by Thomas Noel (IEEE 802.11b) and Greg Daley (IEEE
   802.3).  The authors would like to thank them for their efforts in
   bringing this document to fruition.

6.  Acknowledgements

   The authors would like to acknowledge Bernard Aboba, Sanjeev Athalye,
   JinHyeock Choi, John Loughney, Pekka Nikander, Brett Pentland, Tom
   Petch, Dan Romascanu, Pekka Savola, Steve Bellovin, Thomas Narten,
   Matt Mathis, Alfred Hoenes, and Muhammad Mukarram bin Tariq for their
   useful comments and suggestions.

7.  References

7.1.  Normative References

   [CDMA2K]        "cdma2000 Wireless IP Network Standard",  ,
                   December 2000.

   [GPRS]          "Digital cellular telecommunications system (Phase
                   2+); General Packet Radio Service (GPRS) Service
                   description; Stage 2", 3GPP TS 03.60 version 7.9.0
                   Release 98.

   [GPRS-LINK]     "Digital cellular telecommunications system (Phase
                   2+); Radio subsystem link control", 3GPP GSM 03.05
                   version 7.0.0 Release 98.

   [IEEE-802.11a]  Institute of Electrical and Electronics Engineers,
                   "IEEE Std 802.11a-1999, supplement to IEEE Std
                   802.11-1999, Part 11: Wireless MAN Medium Access
                   Control (MAC) and Physical Layer (PHY)
                   specifications: High-speed Physical Layer in the 5
                   GHZ band", IEEE Standard 802.11a, September 1999.

   [IEEE-802.11b]  Institute of Electrical and Electronics Engineers,
                   "IEEE Std 802 Part 11, Information technology -
                   Telecomunications 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 Standard 802.11b, August 1999.

   [IEEE-802.11g]  Institute of Electrical and Electronics Engineers,
                   "IEEE Std 802.11g-2003, Amendment to IEEE Std 802.11,
                   1999 edition, Part 11: Wireless MAN Medium Access
                   Control (MAC) and Physical Layer (PHY)
                   specifications.  Amendment 4: Further Higher Data
                   Rate Extension in the 2.4 GHz Band", IEEE Standard
                   802.11g, June 2003.

   [IEEE-802.11i]  Institute of Electrical and Electronics Engineers,
                   "Supplement to STANDARD FOR Telecommunications and
                   Information Exchange between Systems - LAN/MAN
                   Specific Requirements - Part 11: Wireless Medium
                   Access Control (MAC) and physical layer (PHY)
                   specifications: Specification for Enhanced Security",
                   IEEE 802.11i, December 2004.

   [IEEE-802.1D]   Institute of Electrical and Electronics Engineers,
                   "IEEE standard for local and metropolitan area
                   networks - common  specifications - Media access
                   control (MAC) Bridges", ISO/IEC IEEE Std 802.1D,
                   2004.

   [IEEE-802.1ab]  Institute of Electrical and Electronics Engineers,
                   "Draft Standard for Local and Metropolitan Networks:
                   Station and Media Access Control Connectivity
                   Discovery (Draft 13)", IEEE draft Std 802.1AB, 2004.

   [IEEE-802.1ae]  Institute of Electrical and Electronics Engineers,
                   "IEEE Std 802.1AE, Local and Metropolitan Area
                   Networks - Media Access Control (MAC) Security",
                   IEEE Standard 802.1ae, June 2006.

   [IEEE-802.3]    Institute of Electrical and Electronics Engineers,
                   "IEEE standard for local and metropolitan area
                   networks -  Specific Requirements, Part 3: Carrier
                   Sense Multiple Access with Collision Detection
                   (CSMA/CD) Access Method and Physical Layer
                   Specifications", ISO/IEC IEEE Std 802.3, 2002.

   [RFC1332]       McGregor, G., "The PPP Internet Protocol Control
                   Protocol (IPCP)", RFC 1332, May 1992.

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

   [RFC2462]       Thomson, S. and T. Narten, "IPv6 Stateless Address
                   Autoconfiguration", RFC 2462, December 1998.

   [RFC2472]       Haskin, D. and E. Allen, "IP Version 6 over PPP",
                   RFC 2472, December 1998.

   [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.

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

   [RFC4135]       Choi, JH. and G. Daley, "Goals of Detecting Network
                   Attachment in IPv6", RFC 4135, August 2005.

7.2.  Informative References

   [GPRS-CN]       "Technical Specification Group Core Network;
                   Internetworking between the Public Land Mobile
                   Network (PLMN) supporting packet based services and
                   Packet Data Networks (PDN) (Release 6)", 3GPP TS
                   29.061 version 6.1.0 2004-06.

   [GPRS-GSSA]     "Technical Specification Group Services and System
                   Aspect; General Packet Radio Service (GPRS) Service
                   description; Stage 2 (Release 6)", 3GPP TS 23.060
                   version 6.5.0 2004-06.

   [RFC2461]       Narten, T., Nordmark, E., and W. Simpson, "Neighbor
                   Discovery for IP Version 6 (IPv6)", RFC 2461,
                   December 1998.

   [RFC4068]       Koodli, R., "Fast Handovers for Mobile IPv6",
                   RFC 4068, July 2005.

   [RFC4881]       El Malki, K., "Low-Latency Handoffs in Mobile IPv4",
                   RFC 4881, June 2007.

Authors' Addresses

   Suresh Krishnan (editor)
   Ericsson Research
   8400 Decarie Blvd.
   Town of Mount Royal, QC
   Canada

   EMail: suresh.krishnan@ericsson.com


   Nicolas Montavont
   GET ENST Bretagne
   2, rue de la chataigneraie
   Cesson-Sevigne  35576
   France

   Phone: (33) 2 99 12 70 23
   EMail: nicolas.montavont@enst-bretagne.fr


   Eric Njedjou
   France Telecom
   4, Rue du Clos Courtel BP 91226
   Cesson Sevigne  35512
   France

   Phone: +33 299124878
   EMail: eric.njedjou@orange-ftgroup.com


   Siva Veerepalli
   Qualcomm
   5775 Morehouse Drive
   San Diego, CA  92131
   USA

   Phone: +1 858 658 4628
   EMail: sivav@qualcomm.com


   Alper E. Yegin (editor)
   Samsung
   Istanbul
   Turkey

   Phone: +90 533 348 2402
   EMail: a.yegin@partner.samsung.com

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