Network Working Group                                      B. Aboba, Ed.
INTERNET-DRAFT                               Internet Architecture Board
Category: Informational                                              IAB
14 October 2004

          Architectural Implications of Link Layer Indications

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   RFC 3668.

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Copyright Notice

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

   As a performance optimization, proposals have been made for utilizing
   link layer indications (also known as "triggers" or "hints") to
   influence the behavior of the Internet, Transport or Application
   layers.  This document briefly summarizes current proposals and
   describes architectural issues relating to link layer indications.

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Table of Contents

1.  Introduction..............................................    3
      1.1 Requirements .......................................    3
      1.2 Terminology ........................................    3
      1.3 Link Indications ...................................    5
      1.4 Proposals ..........................................    6
      1.5 Layering Model .....................................    7
      1.6 Link Behavior ......................................   10
      1.7 Implementation Differences .........................   11
2.  Architectural considerations .............................   12
      2.1 Model Validation ...................................   13
      2.2 Robustness .........................................   15
      2.3 Effectiveness ......................................   18
      2.4 Interoperability Issues ............................   19
      2.5 Race Conditions ....................................   19
      2.6 Layer Compression ..................................   23
      2.7 Remoting Implications ..............................   24
      2.8 Security Considerations ............................   25
3.  Future Work ..............................................   26
4.  References ...............................................   27
      4.1 Normative References ...............................   27
      4.2 Informative References .............................   27
Appendix A - IAB Members .....................................   31
Intellectual Property Statement ..............................   31
Disclaimer of Validity .......................................   31
Copyright Statement ..........................................   32

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1.  Introduction

   As evidence mounts that correct utilization of link layer indications
   can provide real benefits, and that incorrect utilization can degrade
   performance, the importance of understanding the role of link
   indications in the Internet architecture has grown in importance.

   This document is an attempt to summarize current understanding of the
   role of link layer indications, as well as to provide advice to
   document authors considering the role of link layer indications
   within their own work.

   In Section 1 of this document we present a brief overview of current
   proposals, as well as recent research on link behavior.  Based on the
   overview, Section 2 provides advice to document authors.  Section 3
   describes future work.

1.1.  Requirements

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

1.2.  Terminology

Access Point (AP)
     A station that provides access to the distribution services, via
     the wireless medium (WM) for associated stations.

     The service used to establish an access point/station (AP/STA)
     mapping and enable STA access to the Distribution System.

Basic Service Set (BSS)
     A set of stations controlled by a single coordination function,
     where the coordination function may be centralized (e.g., in a
     single AP) or distributed (e.g., for an ad-hoc network).  The BSS
     can be thought of as the coverage area of a single AP.

Care of Address (CoA)
     A unicast routable address associated with a mobile node while
     visiting a foreign link; the subnet prefix of this IP address is a
     foreign subnet prefix.  Among the multiple care-of addresses that a
     mobile node may have at any given time (e.g., with different subnet
     prefixes), the one registered with the mobile node's home agent for
     a given home address is called its "primary" care-of address.

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Correspondent Node
     A peer node with which a mobile node is communicating.  The
     correspondent node may be either mobile or stationary.

Distribution System (DS)
     A system used to interconnect a set of basic service sets (BSSs)
     and integrated local area networks (LANs) to create an extended
     service set (ESS).

Dynamic Host Configuration Protocol (DHCP) client
     A DHCP client or "client" is an Internet host using 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.

Extended Service Set (ESS)
     A set of one or more interconnected basic service sets (BSSs) and
     integrated local area networks (LANs) that appears as a single BSS
     to the logical link control layer at any station associated with
     one of those BSSs.  The ESS can be thought of as the coverage area
     provided by a collection of APs all interconnected by the
     Distribution System.  It may consist of one or more IP subnets.

Home Address (HoA)
     A unicast routable address assigned to a mobile node, used as the
     permanent address of the mobile node.  This address is within the
     mobile node's home link.  Standard IP routing mechanisms will
     deliver packets destined for a mobile node's home address to its
     home link.  Mobile nodes can have multiple home addresses, for
     instance when there are multiple home prefixes on the home link.

Inter-Access Point Protocol (IAPP)
     A protocol used between access points that assures that the station
     may only be connected to a single AP within the ESS at a time, and
     also provides for transfer of context to the new AP.

Link A communication facility or medium over which nodes can communicate
     at the link layer, such as an Ethernet (simple or bridged).  The
     link layer is the layer immediately below IP.

Link Indication
     Information provided by the link layer to higher layers relating to
     the state of the link.

Mobile Node
     A node that can change its point of attachment from one link to

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     another, while still being reachable via its home address.

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].

Station (STA)
     Any device that contains an IEEE 802.11 conformant medium access
     control (MAC) and physical layer (PHY) interface to the wireless
     medium (WM).

Valid address
     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.

Weak End-System Model
     In the Weak End-System Model, packets sent out an interface need
     not necessarily have a source address configured on that interface.

1.3.  Link Indications

   A link indication represents information provided by the link layer
   to higher layers relating to the state of the link.

   While link layer indications vary considerably between media,
   abstraction models have been proposed.  For example, [GenTrig]
   defines "generic triggers", including "Link Up", "Link Down", "Link
   Going Down", "Link Going Up", "Link Quality Crosses Threshold",
   "Trigger Rollback", and "Better Signal Quality AP Available".  Other
   link indications include the current link rate (which may vary with
   time and location), link identifiers (e.g. SSID, BSSID in 802.11),
   and statistics relating to link performance (such as the delay or
   loss rate).

   Among the most commonly implemented link indications are the "Link
   Up" and "Link Down" indications, which are based on an idealized link
   behavior model originally developed for wired networks.  This model

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   assumes that links in the "Up" state experience low frame loss in
   both directions and are ready to send and receive IP data packets.
   Similarly, it is assumed that a link that is in the "Down" state is
   unsuitable for sending and receiving IP data packets in either

   Link indications based on signal quality, such as "Link Doing Down",
   "Link Going Up", and "Link Quality Crosses Threshold" are primarily
   intended for use in handoff optimization.  These indications assume
   an idealized model of radio propagation, where signal strength varies
   smoothly and frame loss is well predicted by signal strength and

1.4.  Proposals

   Within the Internet Layer, proposals have been made for utilizing
   link layer indications to optimize IP configuration, to improve the
   usefulness of routing metrics, and to optimize aspects of Mobile IP

   In "Detection of Network Attachment (DNA) in IPv4" [DNAv4], link
   layer indications are utilized to optimize Internet layer
   configuration.  This enables a host that has moved to a new point of
   attachment but remained within the same subnet to rapidly confirm a
   currently valid configuration, rather than utilizing the DHCP
   protocol [RFC2131].

   "A High-Throughput Path Metric for Multi-Hop Wireless Routing" [ETX]
   describes how routing metrics can be improved by utilizing the
   Expected Transmission Count (ETX) metric, which takes link layer loss
   rates into account, enabling the selection of routes maximizing
   available throughput.  While the ETX metric did not take the
   negotiated rate into account, this was noted as a subject for further

   In "L2 Triggers Optimized Mobile IPv6 Vertical Handover: The
   802.11/GPRS Example" [Park] the authors propose that the mobile node
   send a router solicitation on receipt of a "Link Up" indication in
   order provide lower handoff latency than would be possible using
   generic movement detection [RFC3775].  The authors also suggest
   immediate invalidation of the Care-Of-Address (CoA) on receipt of a
   "Link Down" indication.

   Within the Transport Layer, proposals have focused on countering the
   effects of handoff-induced packet loss.  This includes proposals for
   improving transport parameter estimation, as well as triggering
   immediate retransmission on availability of an interface or
   intervening link.

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   "Framework and Requirements for TRIGTRAN" [TRIGTRAN] discusses
   optimizations to recover earlier from a retransmission timeout
   incurred during a period in which an interface or intervening link
   was down.

   "Link-layer Triggers Protocol" [Yegin] describes transport issues
   arising from lack of host awareness of link conditions on downstream
   Access Points and routers.  A link-layer trigger remoting is proposed
   to address the issue.

   In "TCP Extensions for Immediate Retransmissions" [Eggert], it is
   proposed that in addition to regularly scheduled retransmissions that
   retransmission be attempted by the transport layer on receipt of an
   indication that connectivity to a peer node may have been restored.
   End-to-end connectivity restoration indications include "Link Up",
   confirmation of first-hop router reachability, confirmation of
   Internet layer configuration, and receipt of other traffic from the

   In "The BU-trigger method for improving TCP performance over Mobile
   IPv6" [Kim], the authors note that handoff-related packet loss is
   interpreted as congestion by the transport layer.  In the case where
   the correspondent node is sending to the mobile node, it is proposed
   that receipt of a Binding Update by the correspondent node be used as
   a signal to the transport layer to adjust cwnd and ssthresh values,
   which may have been reduced due to handoff-induced packet loss.  The
   authors recommend that cwnd and ssthresh be recovered to pre-timeout
   values, regardless of whether the link parameters have changed.  The
   paper does not discuss the behavior of a mobile node sending a
   Binding Update, in the case where the mobile node is sending to the
   correspondent node.

   At the application layer, the usage of "Link Down" indications has
   been proposed to augment presence systems.  In such systems, client
   devices periodically refresh their presence state using application
   layer protocols such as SIMPLE [RFC3428] or XMPP [RFC3921].  If the
   client should become disconnected, their unavailability will not be
   detected until the presence status times out, which can take many
   minutes.  However, if a link goes down, and a disconnect indication
   can be sent to the presence server (presumably by the access point,
   which remains connected), the status of the user's communication
   application can be updated nearly instantaneously.

1.5.  Layering Model

   A simplified layered indication model is shown in Figure 1.  This
   model includes both internally generated link indications as well as
   Internet and Transport layer indications arising out of external

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   interactions (such as receipt of Mobile IP Binding Updates, and
   detection of path changes via routing protocols and TTL changes).

   In this model, link indications provided to higher layers include the
   frame loss rate (before retransmissions), the current link rate, the
   link state (up/down), and link identifiers.  These link indications
   are inter-dependent.  For example, the rate adjustment and detection
   algorithms are typically influenced by frame loss, and in turn, the
   determination of a "Link Down" indication may be influenced by the
   detection and search process.  Link Identifiers are typically
   obtained in the process of bringing the link to the "Up" state.

   Link indications may be utilized by the Internet layer in order to
   optimize aspects of IP configuration, routing and mobility.  As noted
   in [DNAv4], "Link Up" indications and link Identifiers may be useful
   in validation of an existing IP configuration.  Once the IP
   configuration is confirmed, it may be determined that an IP address
   change has occurred.

   As described in [ETX], the frame loss rate as well as the current
   link rate may be utilized in the calculation of routing metrics.
   Within "Weak End-System Model" implementations, changes in routing
   metrics may in turn result in a change in the outgoing interface for
   one or more transport connections.  Routes may also be added or
   withdrawn, resulting in loss or gain of peer connectivity.  The
   Internet layer may also become aware of path changes, by other
   mechanisms such as a change in the IP TTL of received packets.

   A change in the outgoing interface may in turn influence the mobility
   sub-layer if present, causing a change in the incoming interface.
   The mobility sub-layer may also become aware of a change in the
   incoming interface of a peer (via receipt of a Mobile IP binding

   Internet layer indications such as IP address and path changes are
   provided to the Transport layer, which also receives Link layer
   indications such as the loss rate, and "Link Up"/"Link Down".

   Based on these Internet layer indications, the transport layer may
   wish to modify transport parameter estimates (such as by reseting
   parameter estimates of connections undergoing a path change), or tear
   down transport connections (due to invalidation of a connection's
   originating IP address).  It is also possible for the transport layer
   to utilize "Link Up"/"Link Down" indications, and Loss Rate
   information to improve transport parameter estimates.  However, the
   required algorithms not well understood.

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Application   |                                               |
Layer         |                                               |
                                        ^         ^   ^
                                        |         |   |
              |                         | |       |   |       |
              |                         | +       |   |       |
              |                         ^ |       ^   ^       |
Transport     |  Transport Parameter    | +       | Teardown  |
Layer         |  Estimation             | |       |           |
              |  (e.g. RTT, RTO, Reset) | + Conxn.| Management|
              |                         | |       |           |
                ^   ^           ^       ^         ^
                |   |           |       |         |
              | |   | Incoming  | MIP   |         |           |
              | |   | Interface | BU    |         |           |
              | |   | Change    |Receipt|         |           |
              | ^   ^           ^       ^         ^           |
              | |   |           |       |         |           |
              | |   |           |       |         |           |
              | |   | Mobility  |       |         |           |
Internet      | |   |           |       |         |           |
Layer         +-+- -+- - - - - -+- -+- -+- - - - -+- - - - - -+
              | |   | Outgoing  |   |   |         | IP        |
              | |   | Interface |   +   |         | Address   |
              | ^   ^ Change    ^   |   ^         ^ Change    |
              | |              Path +   |                     |
              | |            Change |   |                     |
              | |    Routing        +   |  IP Configuration   |
              | |                   |   |                     |
                ^         ^             ^       ^
                |         |             |       |
              | |         |             |       |             |
Link          | ^         ^             ^       ^             |
Layer         + Frame ->  Rate      -> Link    Link           +
              | Loss     Adjustment   Up/Down  Identifiers    |
              | Rate                                          |

                   Figure 1.  Layered Indication Model

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   In addition to Internet layer indications propagated to the
   Application layer (such as IP address changes), the Transport layer
   propagates its own indications, such as connection teardown.  In most
   cases applications can obtain the information they need from Internet
   and Transport layer indications so that they do not need to directly
   consume link indications.

1.6.  Link Behavior

   In order to understand the applicability of the layered indication
   model it is instructive to review recent research relating to link

   In "Measurement and Analysis of the Error Characteristics of an In-
   Building Wireless Network" [Eckhardt], the authors characterize the
   performance of an AT&T Wavelan 2 Mbps in-building WLAN operating in
   Infrastructure mode on the Carnegie-Mellon Campus.  In this study,
   very low frame loss was experienced.  As a result, links could either
   be assumed to operate very well or not at all.

   In "Performance of Multihop Wireless Networks: Shortest Path is Not
   Enough" [Shortest] the authors studied the performance of both an
   indoor and outdoor mesh network.  By measuring inter-node throughput,
   the best path between nodes was computed.  The throughput of the best
   path was compared with the throughput of the shortest path computed
   based on a hop-count metric.  In almost all cases, the shortest path
   route offered considerably lower throughput than the best path.

   In examining link behavior, the authors found that rather than
   exhibiting a bi-modal distribution between "up" (low loss rate) and
   "down" (high loss rates), many links exhibited intermediate loss
   rates.  Asymmetry was also common, with 30 percent of links
   demonstrating substantial differences between in the loss rates in
   each direction.  As a result, on wireless networks the measured
   throughput can differ substantially from the negotiated rate due to
   retransmissions, and successful delivery of routing packets is not
   necessarily an indication that the link is  useful for delivery of

   "Link-level Measurements from an 802.11b Mesh Network" [Aguayo]
   analyzes the causes of frame loss in a 38-node urban multi-hop 802.11
   ad-hoc network.  In most cases,  links that are very bad in one
   direction tend to be bad in both directions, and links that are very
   good in one direction tend to be good in both directions.  However,
   30 percent of links exhibited loss rates differing substantially in
   each direction.

   Signal to noise ratio and distance showed little value in predicting

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   loss rates, and rather than exhibiting a step-function transition
   between "up" (low loss) or "down" (high loss) states,  inter-node
   loss rates varied widely, demonstrating a nearly uniform distribution
   over the range at the lower rates.  The authors attribute the
   observed effects to multi-path fading, rather than attenuation or

   The findings of [Eckhardt] and [Aguayo] demonstrate the diversity of
   loss conditions observed in practice.  There is a fundamental
   difference between infrastructure networks in which site surveys and
   careful measurement can assist in promoting ideal behavior and ad-
   hoc/mesh networks in which node mobility and external factors such as
   weather may not be easily controlled.

1.7.  Implementation Differences

   The literature also describes the effect of implementation
   differences on link indications.  For the purposes of illustration,
   we will restrict ourself to literature relating to IEEE 802.11

   "An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process"
   [Mishra] investigates handoff latencies obtained with three mobile
   STAs implementations communicating with two APs.  The study found
   that there is large variation in handoff latency among STA and AP
   implementations and that implementations utilize different message
   sequences.  For example, one STA sends a Reassociation Request prior
   to authentication, which results in receipt of a Deauthenticate
   message.  The study divided handoff latency into discovery,
   authentication and reassociation exchanges, concluding that the
   discovery phase was the dominant component of handoff delay.
   Detection was not investigated.

   "Techniques to reduce IEEE 802.11b MAC layer handover time" [Velayos]
   measured handover times for a stationary STA after the AP was turned
   off.  This study divided handover times into detection (determination
   of the need for handover), search (discovery of alternative
   attachment points), and execution phases (authentication and
   association exchanges).  These measurements indicated that the
   detection phase was longest in duration.  The duration of the
   detection phase is determined by the number of non-acknowledged
   frames triggering the search phase and precursors such as RTS/CTS and
   rate adaptation.

   Detection behavior varied widely between implementations.  For
   example, NICs designed for desktops attempted more retransmissions
   prior to triggering search as compared with laptop designs, since
   they assumed that the AP is always in range, regardless of Beacon

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   The study recommends that the duration of the detection phase be
   reduced by initiating the search phase as soon as collisions can be
   excluded as the cause of non-acknowledged transmissions; the authors
   recommend three consecutive transmission failures as the cutoff.
   Where the STA is not sending,  it is recommended that Beacon
   reception be tracked if no data frames are being received, and that
   Beacon spacing be reduced to 60 ms in order to reduce detection
   times.  In order to compensate for more frequent triggering of the
   search phase, the authors recommend algorithms for wait time
   reduction, as well as interleaving of search with data transmission.

   "Roaming Interval Measurements" [Alimian] presents data on stationary
   STAs after the AP signal has been shut off.  This study highlighted
   implementation differences in rate adaptation as well as detection,
   scanning and handoff.  As in [Velayos], performance varied widely
   between implementations, from  half an order variation in rate
   adaptation to an order of magnitude difference in connectivity
   detection times, two orders of magnitude in scanning, and one and a
   half orders of magnitude in handoff times.

   "An experimental study of IEEE 802.11b  handoff performance and its
   effect on voice traffic" [Vatn] describes handover behavior observed
   when the signal from AP is gradually attenuated, which is more
   representative of field experience than the shutoff techniques used
   in [Velayos].  Stations were configured to initiate handover when
   signal strength dipped below a threshold, rather than purely based on
   frame loss, so that they could begin handover while still connected
   to the current AP.  It was noted that stations continue to receive
   data frames during the search phase.  Station-initiated
   Disassociation and pre-authentication were not observed in this

2.  Architectural considerations

   While the literature on the usage of link layer indications provides
   persuasive evidence of their utility,  experience shows that a number
   of difficulties can arise in making effective use of them.  These
   issues include:

        a.  Model validation
        b.  Robustness
        c.  Effectiveness
        d.  Interoperability Issues
        e.  Race conditions
        f.  Layer compression
        g.  Remoting implications

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        h.  Security implications

   The sections that follow discuss each of these issues in turn.

2.1.  Model Validation

   In "The mistaken axioms of wireless-network research" [Kotz], the
   authors conclude that mistaken assumptions relating to link
   performance may lead to  the design of network protocols that may not
   work in practice.  In order to avoid these pitfalls, documents
   dependent on link indications should explicitly articulate the
   assumptions of the link model and describe the circumstances in which
   it applies.

   Authors need to be careful to avoid use of simplified network models
   in circumstances where the model does not apply.  For example,
   generic "trigger" models assume that a link is either in a state
   experiencing low frame loss ("Link Up") or in a state where few if
   any frames are delivered ("Link Down").  Often symmetry is assumed as
   well, so that a link is assumed to be either "Up" in both directions
   or "Down" in both directions.  In wireless networks, particularly in
   the case of ad-hoc or mesh deployments, these assumptions may prove

   Furthermore, where links are in intermediate states between "Up" and
   "Down" and asymmetry is encountered, generic "triggers" such as "Link
   Going Down", "Link Going Up", "Link Quality Crosses Threshold" may
   prove difficult to define and may prove to be unreliable predictors
   of future link performance.

   Once the network model is defined, considerable effort may be
   required to map the model to common link types.  In practice the
   definition of "Link Up" or "Link Down" may vary according to the link
   layer.  For example, within PPP [RFC1661], either peer may send an
   LCP-Terminate frame in order to terminate the PPP link layer, and a
   link may only be assumed to be usable for sending network protocol
   packets once NCP negotiation has completed for that protocol.

   Within IEEE 802.11, the definition of "Link Up" and "Link Down"
   depends on whether the station is mobile or stationary, whether
   infrastructure or ad-hoc mode is in use, and whether security and
   Inter-Access Point Protocol (IAPP) is implemented.

   Where a mobile 802.11 STA encounters a series of consecutive non-
   acknowledged frames, the most likely cause is that the station has
   moved out of range of the AP.  As a result, [Velayos] recommends that
   the station begin the search phase after collisions can be ruled out,
   after three consecutive non-acknowledged frames.  Only when no

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   alternative point of attachment is found is a "Link Down" indication

   In a stationary point-to-point installation, the most likely cause of
   an outage is that the link has become impaired.  As a result,
   implementations tend to be more persistent and a "Link Down"
   indication may be returned later.

   In Infrastructure mode, IEEE 802.11-2003 enables reception of data
   frames only in State 3 ("Authenticated" and "Associated").  As a
   result, a transition to State 3 (e.g. completion of a successful
   Association or Reassociation exchange) enables sending and receiving
   of network protocol packets and a transition from State 3 to State 2
   (reception of a "Disassociate" frame) or State 1 (reception of a "De-
   authenticate" frame) disables sending and receiving of network
   protocol packets.  As a result, IEEE 802.11 stations typically signal
   "Link Up" on receipt of a successful Association or Reassociation

   Within the [IEEE80211f] specification, after sending a Reassociation
   Response, an Access Point will send a frame with the station's source
   address to a multicast destination.  This causes switches within the
   Distribution System (DS) to update their learning tables, readying
   the DS to forward frames to the station at its new point of
   attachment.  Were the AP to not send this "spoofed" frame, the
   station's location would not be updated within the DS until it sent
   its first frame at the new location.  Thus IAPP serves to equalize
   uplink and downlink handover times.

   The signalling of "Link Down" is considerably more complex.  Even
   though a transition to State 2 or State 1 results in the station
   being unable to send or receive IP packets, this does not necessarily
   imply that such a transition should be considered a "Link Down"
   indication.  In an infrastructure network, a station may have a
   choice of multiple access points offering connection to the same
   network.  In such an environment, a station that is unable to reach
   State 3 with one access point may instead choose to attach to another
   access point.  Rather than registering a "Link Down" indication with
   each move, the station may instead register a series of "Link Up"

   In [IEEE80211i] forwarding of frames from the station to the
   distribution system is only feasible after the completion of the
   4-way handshake and group-key handshake, so that entering State 3 is
   no longer sufficient.

   Unfortunately, other elements of the IEEE 802.11 specification such
   as IEEE 802.11f continue to recognize the Reassociation Response as

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   the "Link Up" definition.  By spoofing a multicast frame with the
   station's source address once it sends a Reassociation Response,
   Access Points implementing IEEE 802.11f cause the learning tables
   within switches comprising the DS to be updated.  This enables an
   attacker to deny service to attached stations by sending a
   Reassociation Request from anywhere within the ESS.  Without the
   spoofing recommended in IEEE 802.11f, such an attack would only be
   able to disassociate stations on the AP to which the Reassociation
   Request was sent.

   [IEEE80211i] implementations utilizing the "Link Up" definition from
   [IEEE80211] or [IEEE80211f] have also encountered difficulty in IP
   address assignment, since they may trigger DHCP [RFC2131] or RS/RA
   prior to when the link is usable by the Internet layer.  As a result,
   Internet layer configuration may fail.

   In contrast, in 802.11 ad-hoc mode with no security, reception of
   data frames is enabled in State 1 ("Unauthenticated" and "Un-
   associated").  As a result, reception of data frames is enabled at
   any time, and no explicit "Link Up" indication exists.

2.2.  Robustness

   Implementation experience provides us with several examples of
   situations in which improper consideration of link layer indications
   can result in operational malfunctions.  Given the potential
   problems, proposals for consideration of link layer indications must
   demonstrate robustness against misleading indications.  Elements of
   robustness include:

        a.  Indication validation
        b.  Damping and hysterisis

2.2.1.  Indication Validation

   As noted in Section 1.6 and 1.7, radio propagation and implementation
   differences can impact the reliability of link layer indications.
   [Kotz] notes that the three-dimensional nature of wireless
   propagation can result in large signal strength changes over short
   distances, generating short-lived "Link Down" and "Link Up"
   indications that are not be predicted by a two dimensional radio
   propagation model.

   As described in [Aguayo], wireless links often exhibit loss rates
   intermediate between "up" (low loss) and "down" (high loss) states,
   as well as substantial asymmetry.  In these circumstances, a "Link
   Up" indication may not imply bi-directional reachability.  Also,  a
   reachability demonstration based on small packets may not mean that

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   the link is suitable for carrying larger data packets.  As a result,
   "Link Up" and "Link Down" indications may not reliably determine
   whether a link is suitable for carrying IP traffic.

   Where the reliability of a link layer indication is suspect, it is
   best to treat the indication as a "hint" that is advisory in nature,
   rather than a "trigger" forcing a given action.  In order to provide
   increased robustness, heuristics can be developed to determine
   whether the "hint" is valid or should be discarded.

   In addition, a recovery step may be utilized in order to limit the
   potential damage from link indications determined to be invalid after
   they have been acted on.

   To provide robustness in the face of potentially misleading link
   indications, in [DNAv4] "Link Up" indications are assumed to be
   inherently unreliable, so that bi-directional reachability needs to
   be demonstrated prior to validating an existing IP configuration.
   However, in the case of a link of intermediate loss rate, success
   with the [DNAv4] reachability test does not guarantee that the link
   is suitable for carrying data.

   Another example of link indication validation occurs occurs in IPv4
   Link-Local address configuration.  Prior to configuration of an IPv4
   Link-Local address, it is necessary to run a claim and defend
   protocol [RFC3927].  Since a host needs to be present to defend its
   address against another claimant, and address conflicts are
   relatively likely, a host returning from sleep mode or receiving a
   "Link Up" indication could encounter an address conflict were it to
   utilize a formerly configured Link-Local Link-Local address without
   rerunning claim and defend.

2.2.2.  Damping and Hysterisis

   Damping and hysterisis can be utilized to ensure that stability is
   maintained in the face of jittery link indications.  These limits
   typically place constraints on the number of times a given action can
   be performed within a time period or introduce damping mechanisms to
   prevent instability.

   While [Aguayo] found that frame loss was relatively stable for
   stationary stations, obstacles to radio propagation and multipath
   interference can result in rapid changes in signal strength for a
   mobile station.  As a result, it is possible for mobile stations to
   encounter rapid changes in link performance, including changes in the
   negotiated rate, frame loss and even "Link Up"/"Link Down"

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   Where link-aware routing metrics are implemented, this can result in
   rapid metric changes, potentially resulting in frequent changes in
   the outgoing interface for "Weak End-System" implementations.  As a
   result, it may be necessary to introduce route flap dampening.

   However, the benefits of damping need to be weighed against the
   additional latency that can be introduced.  For example, in order to
   filter out spurious "Link Down" indications, these indications may be
   delayed until it can be determined that a "Link Up" indication will
   not follow shortly thereafter.  However, in situations where multiple
   Beacons are missed such a delay may not be needed, since there is no
   evidence of a suitable point of attachment in the vicinity.

   In some cases, it may be desirable to ignore link indications
   entirely.  Since it is possible for a host to transition from an ad-
   hoc network to a network with centralized address management, a host
   receiving a "Link Up" indication cannot necessarily conclude that it
   is appropriate to configure a IPv4 Link-Local address at all.

   It can be argued that reliable transport protocol implementations
   should ignore "Link Down" indications, rather than tearing down
   connections, regardless of the cause of the "Link Down" indication.

   Where the "Link Down" indication results from frame loss rather than
   an explicit exchange, the indication may be transient, rapidly
   followed by a "Link Up" indication.  Even where the "Link Down"
   indication results from an explicit exchange such as a PPP LCP-
   Terminate or an 802.11 Disassociation or Deauthenticate, an
   alternative point of attachment may be available, allowing
   connectivity to be quickly restored.  As a result, robustness is best
   served by allowing connections to remain up until the connection
   source address is invalidated by an address change, or the connection
   times out.

   Where link indications are used to optimize transport performance,
   authors must demonstrate that effective congestion control is
   maintained [RFC2914] in the face of rapidly changing link

   In addition, where a proposal involves "recovery" from a handoff
   event, it is important to demonstrate that the recovered parameters
   (such as the adjusted RTT, RTO, congestion window, etc.) remain
   valid, as noted in [RFC2861].

   Consider a proposal where a "Link Up" indication is used by a router
   to signal retransmission a previously sent packet, in order to enable
   ACK reception prior to expiration of the host's retransmission timer.
   Where "Link Up" indications follow in rapid succession,  this could

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   result in a burst of retransmitted packets, violating the law of
   conservation of packets.

   At the Application Layer, link layer indications have been utilized
   by applications such as Presence [RFC2778] in order to optimize
   registration and user interface update operations.  For example,
   implementations may attempt presence registration on receipt of a
   "Link Up" indication, and presence deregistration by a surrogate
   receiving a "Link Down" indication.  Presence implementations using
   "Link Up"/"Link Down" indications this way violate the principle of
   "conservation of packets" when link indications are generated on a
   time scale of RTO or less.  The problem is magnified since for each
   presence update, notifications can be delivered to many watchers.

   The issue can be addressed by one or more of the following

[a]  Rate limiting.  A limit of one packet per RTO can be imposed on
     packets generated from receipt of link indications.

[b]  Utilization of upper layer indications.  Instead of consuming a
     "Link Up" indication, applications can consume alternative upper
     layer indications such as an IP address change notification.

[c]  Keepalives.  Instead of consuming a "Link Down" indication, an
     application can utilize an application keepalive or consume
     transport layer indications such as connection teardown.

2.3.  Effectiveness

   While link layer indications may show promise, it may be difficult to
   prove that processing of a given indication provides benefits in a
   wide variety of circumstances.  Where link layer indications are
   utilized for the purpose of optimization, proposals need to carefully
   analyze the effectiveness of the optimizations in the face of
   unreliable link layer indications.  Since optimizations typically
   bring with them increased complexity, an optimization that does not
   bring about a performance improvement is not useful.

   As with any optimization, the usefulness of link layer indications
   lies in demonstrated effectiveness of the optimization under
   consideration.  This in turn may depend heavily on the penalty to be
   paid for false positives and false negatives.

   As noted in [DNAv4], it is simultaneously possible for a link layer
   indication to be highly reliable, as well as for that indication to
   provide no net benefit, depending on the probability of a false
   indication and the penalty paid for the false indication.

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   In the case of [DNAv4], the benefits of successful optimization are
   modest, but the penalty for falsely concluding that the subnet
   remains unchanged is a lengthy timeout.  The result is that link
   layer indications may not be worth considering if they are incorrect
   even just a small fraction of the time.

   For example, it can be argued that a change in the Service Set
   Identifier (SSID) in [IEEE80211] is not a sufficiently reliable
   indication of subnet change.  Within IEEE 802.11, the Service Set
   Identifier (SSID) functions as a non-unique identifier of the
   administrative domain of a Wireless LAN.  Since the SSID is non-
   unique, many different operators may share the same SSID, and Access
   Points typically ship with a default value for the SSID (e.g.
   "default").  Since the SSID relates to the administrative domain and
   not the network topology, multiple SSIDs may provide access to the
   same prefix, and a single SSID may provide access to multiple
   prefixes at one or multiple locations.

   Given this, it is unreliable to use the SSID alone for the purpose of
   movement detection.  A host moving from one point of attachment to
   another, both with the same SSID, may have remained within the same
   subnet, or may have changed subnets.  Similarly, a  host discovering
   that the SSID has changed may have changed subnets, or it may not
   have.  Moreover, where private address space is in use, it is
   possible for the SSID,  the prefix (e.g. 192.168/16) and even the
   default gateway IP address to remain unchanged, yet for the host to
   have moved to a different point of attachment.  Were the host to make
   decisions relating to configuration of the IP layer (such as address
   assignment) based solely on the SSID, address conflicts are likely.

2.4.  Interoperability Issues

   Since link layer indications are often processed by upper layers for
   the purpose of optimization, proposals must demonstrate that
   interoperability remains possible (though potentially with degraded
   performance) even if one or more participants do not implement the

   Where link layer indications are proposed for use in optimizing
   configuration of the Internet layer, it is necessary to demonstrate
   that the proposal does not interfere with routing protocol behavior,
   make address collisions more likely,  or compromise Duplicate Address
   Detection (DAD).

2.5.  Race Conditions

   It is possible for link layer indications to be utilized directly by
   multiple layers of the stack in situations in which strict layering

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   may not be observed.  In these situations, it is possible for race
   conditions to occur.

   For example, as discussed earlier, link layer indications have been
   shown to be useful in optimizing aspects of Internet Protocol layer
   addressing and configuration as well as routing.  Although [Kim]
   describes situations in which link layer indications are first
   processed by the Internet Protocol layer (e.g. MIPv6) before being
   consumed by the Transport Layer,  in some situations it may be
   desirable for the Transport Layer to consume link layer indications

   For example, in situations where the "Weak End-System Model" is
   implemented, a  change of outgoing interface may occur at the same
   time the Transport Layer is modifying transport parameters based on
   other link layer indications.  As a result, transport behavior may
   differ depending on the order in which the  link indications are

   Whee a multi-homed host experiences high frame loss on one of its
   interfaces,  the ETX metric computed for that interface will rise,
   causing a change in the outgoing interface for one or more transport
   connections.  This may trigger Mobile IP signaling so as to cause a
   change in the incoming path as well.  At the same time, the Transport
   Layer may be estimating transport parameters based on the former
   outgoing interface, and may not properly adjust for the changes in
   outgoing and incoming paths.

   To avoid race conditions, the following measures are recommended:

        a.  Path change processing
        b.  Layering
        c.  Metric consistency

2.5.1.  Path Change Processing

   When the Internet layer detects a path change, such as a change in
   the outgoing or incoming interface of the host or the incoming
   interface of a peer, or perhaps a substantial change in the TTL of
   received IP packets, it may be worth considering whether to reset
   transport parameters to their initial values and allow them to be re-
   estimated.  This ensures that estimates based on the former path do
   not persist after they have become invalid.

2.5.2.  Layering

   Another technique to avoid race conditions is to rely on layering to
   damp potentially misleading indications and provide greater link

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   layer independence.

   The Internet layer is responsible for routing as well as IP
   configuration, and mobility, providing higher layers with an
   abstraction that is independent of link layer technologies.  Since
   one of the major objectives of the Internet layer is maintaining link
   layer independence, upper layers relying on Internet Layer
   indications rather than consuming link layer indications directly can
   avoid link layer dependencies.

   For example, in order to provide robustness, it is necessary to
   demonstrate that a link providing a "Link Up" indication is likely to
   be usable for the transmission of IP data packets prior to using it.
   While applications can in principle incorporate their own versions of
   such a test, efficiency and maintenance considerations argue for
   providing this facility within the Internet layer.

   Many "Link Up" indications do not result in a change of Internet
   Layer configuration, and many changes in link rate or frame loss do
   not result in a change of outgoing interface.  By filtering "Link Up"
   indications, and selecting outgoing and incoming interfaces based on
   the link rate and frame loss, the Internet Layer enables upper layers
   to avoid writing their own code to filter and validate link

   The transport layer consumes Internet layer indication such as
   changes in the incoming/outgoing interface and Internet layer
   configuration changes, as well as potentially utilizing link layer
   indications directly.  For example, the Internet layer may receive a
   "Link Down" indication followed by a subsequent "Link Up" indication.
   This information may be of interest to the Transport layer even if
   the Internet layer configuration does not change, since it may
   provide information about the validity of the transport parameters

   In general, it is advisable for applications to consume indications
   from the Internet or Transport layers rather than consuming link
   indications directly, since this enables applications to leverage
   layered indication processing, resulting in improved robustness and
   scalability.  For example, instead of directly consuming "Link Down"
   indications, applications may wish to rely on Application layer
   keepalives or the Transport layer to determine whether connections
   should be torn down;  instead of consuming "Link Up" indications,
   applications can consume IP address change indications.

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2.5.3.  Metric Consistency

   Once a link is in the "Up" state, its effectiveness in transmission
   of data packets can be determined.  For example, frame loss may be
   used in rate adjustment and detection of when to roam to an
   alternative point of attachment.  While connected,  the effective
   throughput may be determined based on the negotiated rate and frame
   loss, and used in calculation of the routing metric, as described in

   However, prior to sending data packets over the link, other measures
   of suitability are required.  As noted in [Shortest], the ability of
   a link to successfully transmit short frames utilized for control,
   management or routing is not indicative of its usefulness in carrying
   IP data packets.  As a result, it is typically not possible to
   predict the negotiated rate or data frame loss rate in advance of a
   roaming decision.

   As a result,  802.11 stations evaluating the suitability of candidate
   APs often utilize received signal strength and/or AP load as a
   primary selection criteria.  Similarly, in order to enable stations
   to roam prior to encountering packet loss, studies such as [Vatn]
   have suggested using signal strength as a detection mechanism, rather
   than frame loss, as suggested in [Velayos].

   The "Link Going Down",  "Link Going Up", "Link Quality Crosses
   Threshold" indications were developed primarily to assist with
   handoff between interfaces, and are oriented toward inferred rather
   than measured suitability.

   Research indicates that this approach may have some promise.  For
   example, [Vertical] proposes use of signal strength and link
   utilization in order to optimize vertical handoff and demonstrates
   improved TCP throughput.  However, without careful design, potential
   differences between link indications used in routing and those used
   in roaming and/or link enablement can result in instability,
   particularly in multi-homed hosts.

   For example, receipt of "Link Going Down" or "Link Quality Crosses
   Threshold" indications could be used as a signal to enable another
   interface.  However, unless the new interface is the preferred route
   for one or more destination prefixes, a "Weak End-System"
   implementation will not use the new interface for outgoing traffic.
   Where "idle timeout" functionality is implemented, the unused
   interface will be brought down, only to be brought up again by the
   link enablement algorithm.

   As noted in [Aguayo], signal strength and distance are not good

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   predictors of frame loss or negotiated rate, due to the potential
   effects of multi-path interference.  As a result a link brought up
   due to good signal strength may subsequently exhibit significant
   frame loss, and a low negotiated rate.  Similarly, an AP
   demonstrating low utilization may not necessarily be the best choice,
   since utilization may be low due to hardware or software problems.
   As noted in [Villamizar], link utilization-based routing metrics have
   a history of instability, so that they are rarely deployed.

2.6.  Layer compression

   In many situations, the exchanges required for a host to complete a
   handoff and reestablish connectivity are considerable.  This includes
   link layer scanning, authentication and connectivity establishment;
   Internet layer configuration, routing and mobility exchanges;
   transport layer retransmission and recovery; security association re-
   establishment;  application protocol reauthentication and
   reregistration exchanges, etc.  Given this, it is natural to consider
   combining exchanges occurring within multiple layers into a single

   Often this combined exchange occurs within the link layer.  For
   example, in [EAPIKEv2], a link layer EAP exchange may be used for the
   purpose of IP address assignment, potentially bypassing Internet
   layer configuration.  Within [PEAP], it is proposed that a link layer
   EAP exchange be used for the purpose of carrying Mobile IPv6 Binding
   Updates.  [MIPEAP] proposes that EAP exchanges be used for
   configuration of Mobile IPv6.

   While the goals of layer compression are laudable, care needs to be
   taken to avoid compromising interoperability and introducing link
   layer dependencies into the  Internet and Transport layers.  For
   example, where Link layer and Internet or Transport layer mechanisms
   are combined, it is necessary for hosts to maintain the ability to
   interoperate without layer compression schemes, in order to permit
   operation on networks where they are not available.

   As noted earlier, while the layered handling of link indications
   introduces latency,  it also increases robustness.  As a result,
   layer compression schemes need to take care to avoid introducing
   unnecessary brittleness.  For example, in order to optimize IP
   address assignment, it has been proposed that prefixes be advertised
   at the link layer.  While in theory such a proposal addresses the
   non-uniqueness issues found with the use of the SSID for movement
   detection, it suffers from its own set of problems.

   For example, [IEEE8021X] enables the VLANID to by assigned
   dynamically.  As a result, a prefix advertised at the link layer need

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   not correspond to the prefix assigned to the host once it connects to
   the link.  Were IP configuration to be based on such hints, errors
   are likely.

2.7.  Remoting implications

   Proposals which include support for remoting of link layer
   indications need to carefully consider the layering, security and
   transport implications.

   While facilities such as ICMP "source quench" were originally
   provided at the Internet layer, these facilities have fallen into
   disuse due to their questionable value for the Transport layer.  In
   general, the Transport layer is able to determine an appropriate (and
   conservative) response to congestion based on packet loss or explicit
   congestion notification, so that ICMP "source quench" indications are
   not needed, and in fact the sending of additional "source quench"
   packets during periods of congestion may be detrimental.

   On the other hand, proposals such as [ETX] imply that hosts
   participating in the routing mesh may gain knowledge of remote link
   conditions where link-aware routing metrics are used.  This can be
   accomplished securely if routing protocol security is implemented.

   For example,  when a link experiences frame loss, the ETX metric will
   increase, possibly resulting in selection of an alternate route.  If
   the troubled link represents the only path to a prefix and the link
   experiences high frame loss ("Down"), the route will be withdrawn or
   the metric will become infinite.  Thus, where link-indication aware
   routing metrics are implemented, "link indication remoting" can be
   accomplished via host participation in the routing mesh, and
   transport layer response to path changes.

   Proposals involving remoting of link layer indications need to
   demonstrate the following:

[a]  Absence of alternatives.  By default, alternative solutions not
     requiring explicit remoting of link layer indications are
     preferred, and the burden of proof rests on remoting advocates to
     show that alternatives (including link-indication aware routing
     metrics) are unsuitable.

[b]  Conservative behavior.  Due to past experience with ICMP "source
     quench", the burden of proof rests on advocates of remoting
     proposals to demonstrate that proposals do not violate conservation
     of packets.

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[c]  Security.  Remoting proposals need to describe how security issues
     can be addressed. Where insecure remoted link layer indications are
     transported over the Internet,  an attack can be launched without
     requiring access to the link.

[d]  Identifiers.  When link indications are remoted, it is generally
     for the purposes of saying something about Internet, Transport or
     Application layer operations at a remote element.  These layers use
     different identifiers, and so it is necessary to match the link
     indication with relevant higher layer state.  The burden rests on
     remoting advocates to demonstrate how the link indication can be
     mapped to the right higher layer state.  As an example, if a
     presence server is receiving remote indications about "Link
     Up"/"Link Down" status for a particular MAC address, the presence
     server will need to associate that MAC address with the identity of
     the user ( to whom that link status change is

2.8.  Security Considerations

   Since link layer indications are typically insecure, proposals
   incorporating them need to consider the potential security
   implications of spoofed or modified link layer indications, as well
   as the potential denial of service attacks.  This is particularly
   important in situations where insecure link layer indications are as
   a substitute for secure mechanisms operating at a higher layer.

   For example, within [IEEE80211f], "Link Up" is considered to occur
   when an Access Point sends a Reassociation Response.  At that point,
   the AP sends a frame with the station's source address to a multicast
   address, thereby causing switches within the Distribution System to
   learn the station's MAC address, enabling forwarding of frames to the
   station at the new point of attachment.  Unfortunately, this does not
   take security into account, since the station is not capable of
   sending and receiving IP packets on the link until completion of the
   key exchange protocol defined in [IEEE80211i].  As a result, link
   layer indications as implemented in [IEEE80211f] enable an attacker
   to disassociate a station located anywhere within the ESS, simply by
   sending a Reassociation Request frame.

   Another example of the potential security implications of link layer
   indications occurs within DNAv4, where link layer indications are
   used for optimization of IP configuration, rather than using a
   secured configuration mechanism such as authenticated DHCP [RFC3118],
   thereby increasing vulnerability to spoofing.

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3.  Further work

   While Figure 1 presents an overview of how link indications are
   consumed by the Internet, Transport and Application layers, further
   work is needed to investigate this in more detail.

   Given that recent proposals such as [IEEE80211e] incorporate burst
   ACKs, the relationship between 802.11 link throughput and frame loss
   is growing more complex, which may necessitate the development of
   revised routing metrics, taking the more complex transmission
   behavior as well as the negotiated rate into account.

   At the link and Internet layers, more work is needed to reconcile pre
   and post-connection metrics, such as reconciling metrics utilized in
   handoff (e.g. signal strength and link utilization) with link-aware
   routing metrics (e.g. frame loss and negotiated rate).

   At the Transport layer, more work is needed to understand how to
   react Internet layer indications such as path changes.  For example,
   in an early draft of DCCP [DCCP], a "Reset Congestion State" option
   was proposed in Section 4.  This option was removed in part because
   the conditions under which it was to be used were not fully

     An HC-Receiver sends the Reset Congestion State option to its sender
     to force the sender to reset its congestion state -- that is, to
     "slow start", as if the connection were beginning again. ...
     The Reset Congestion State option is reserved for the very few cases
     when an endpoint knows that the congestion properties of a path have
     changed.  Currently, this reduces to mobility: a DCCP endpoint on a
     mobile host MUST send Reset Congestion State to its peer after the
     mobile host changes address or path.

   It may also make sense for the Transport layer to adjust transport
   parameter estimates in response to "Link Up"/"Link Down" indications
   and frame loss.  For example, it is unclear that the Transport layer
   should adjust transport parameters as though congestion were detected
   when loss is occurring in the link layer or a "Link Down" indication
   has been received.

   Finally, more work is needed to determine how link layers may utilize
   information from the transport layer.  For example, it is undesirable
   for a link layer to retransmit so aggressively that the link layer
   round-trip time approaches that of the end-to-end transport

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4.  References

4.1.  Normative References

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

4.2.  Informative References

[RFC791]  Postel, J., "Internet Protocol", RFC 791, USC/Information
          Sciences Institute, September 1981.

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

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

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

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

[RFC2778] Day, M., Rosenberg, J., Sugano, H., "A Model for Presence and
          Instant Messaging", RFC 2778, February 2000.

[RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion Window
          Validation", RFC 2861, June 2000.

[RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914, BCP 41,
          September 2000.

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

[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
          D. Gurle, "Session Initiation Protocol (SIP) Extension for
          Instant Messaging", RFC 3428, December 2002.

[RFC3748] Aboba, B., , "Extensible Authentication Protocol (EAP)", RFC
          3748, June 2004.

[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
          IPv6", RFC 3775, June 2004.

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[RFC3921] Saint-Andre, P., "Extensible Messaging and Presence protocol
          (XMPP): Instant Messaging and Presence", RFC 3921, October

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

          McCann, P., "Mobile IPv6 Fast Handovers for 802.11 Networks",
          draft-ietf-mipshop-80211fh-01.txt, Internet draft (work in
          progress), July 2004.

[Alimian] Alimian, A., "Roaming Interval Measurements",
          11-04-0378-00-roaming-intervals-measurements.ppt, IEEE 802.11
          submission (work in progress), March 2004.

[Aguayo]  Aguayo, D., Bicket, J., Biswas, S., Judd, G. and R. Morris,
          "Link-level Measurements from an 802.11b Mesh Network",
          SIGCOMM '04, September 2004, Portland, Oregon.

[DCCP]    Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion
          Control Protocol (DCCP)", Internet drafts (work in progress),
          draft-ietf-dccp-spec-07.txt, July 2004.

[DNAv4]   Aboba, B., "Detection of Network Attachment in IPv4", draft-
          ietf-dhc-dna-ipv4-08.txt, Internet draft (work in progress),
          July 2004.

          Tschofenig, H., D. Kroeselberg and Y. Ohba, "EAP IKEv2
          Method", draft-tschofenig-eap-ikev2-03.txt, Internet draft
          (work in progress), February 2004.

          Eckhardt, D. and P. Steenkiste, "Measurement and Analysis of
          the Error Characteristics of an In-Building Wireless Network",
          SIGCOMM '96, August 1996, Stanford, CA.

[Eggert]  Eggert, L., Schuetz, S. and S. Schmid, "TCP Extensions for
          Immediate Retransmissions", draft-eggert-tcpm-tcp-retransmit-
          now-00.txt, Internet draft (work in progress), July 2004.

[ETX]     Douglas S. J. De Couto, Daniel Aguayo, John Bicket, and Robert
          Morris, "A High-Throughput Path Metric for Multi-Hop Wireless
          Routing", Proceedings of the 9th ACM International Conference
          on Mobile Computing and Networking (MobiCom '03), San Diego,
          California, September 2003.

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[GenTrig] Gupta, V. and D. Johnston, "A Generalized Model for Link Layer
          Triggers", submission to IEEE 802.21 (work in progress), March
          2004, available at:


          Institute of Electrical and Electronics Engineers, "Local and
          Metropolitan Area Networks: Port-Based Network Access
          Control", IEEE Standard 802.1X, November 2004.

          Institute of Electrical and Electronics Engineers, "Wireless
          LAN Medium Access Control (MAC) and Physical Layer (PHY)
          Specifications", IEEE Standard 802.11, 2003.

          Institute of Electrical and Electronics Engineers, "Amendment
          7: Medium Access Control (MAC) Quality of Service (QoS)
          Enhancements", IEEE 802.11e, October 2003.

          Institute of Electrical and Electronics Engineers, "IEEE
          Trial-Use Recommended Practice for Multi-Vendor Access Point
          Interoperability via an Inter-Access Point Protocol Across
          Distribution Systems Supporting IEEE 802.11 Operation", IEEE
          802.11f, June 2003.

          Institute of Electrical and Electronics Engineers, "Supplement
          to Standard for Telecommunications and Information Exchange
          Between Systems - LAN/MAN Specific Requirements - Part 11:
          Wireless LAN Medium Access Control (MAC) and Physical Layer
          (PHY) Specifications: Specification for Enhanced Security",
          IEEE 802.11i, November 2004.

[Kim]     Kim, K., Park, Y., Suh, K., and Y. Park, "The BU-trigger
          method for improving TCP performance over Mobile IPv6", draft-
          kim-tsvwg-butrigger-00.txt, Internet draft (work in progress),
          August 2004.

[Kotz]    Kotz, D., Newport, C. and C. Elliot, "The mistaken axioms of
          wireless-network research", Dartmouth College Computer Science
          Technical Report TR2003-467, July 2003.

[MIPEAP]  Giaretta, C., Guardini, I., Demaria, E., Bournelle, J., and M.
          Laurent-Maknavicius, "MIPv6 Authorization and Configuration
          based on EAP", draft-giaretta-mip6-authorization-eap-01.txt,

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INTERNET-DRAFT           Link Layer Indications          14 October 2004

          Internet draft (work in progress), July 2004.

[Mishra]  Mitra, A., Shin, M., and W. Arbaugh, "An Empirical Analysis of
          the IEEE 802.11 MAC Layer Handoff Process", CS-TR-4395,
          University of Maryland Department of Computer Science,
          September 2002.

[PEAP]    Palekar, A., et al, "Protected EAP Protocol (PEAP) Version 2",
          draft-josefsson-pppext-eap-tls-eap-09.txt, Internet draft
          (work in progress), October 2004.

[Park]    Park, S., Njedjou, E. and N. Montavont, "L2 Triggers Optimized
          Mobile IPv6 Vertical Handover: The 802.11/GPRS Example",
          draft-daniel-mip6-optimized-vertical-handover-00.txt, July

[PPPIANA] Schryver, V., "IANA Considerations for the Point to Point
          Protocol (PPP)", draft-schryver-pppext-iana-01.txt, August

          Douglas S. J. De Couto, Daniel Aguayo, Benjamin A. Chambers,
          and Robert Morris, "Performance of Multihop Wireless Networks:
          Shortest Path is Not Enough", Proceedings of the First
          Workshop on Hot Topics in Networking (HotNets-I), Princeton,
          New Jersey, October 2002.

          Dawkins, S., et al., "Framework and Requirements for
          TRIGTRAN", draft-dawkins-trigtran-framework-00.txt, Internet
          draft (work in progress), August 2003.

[Vatn]    Vatn, J., "An experimental study of IEEE 802.11b handover
          performance and its effect on voice traffic", TRITA-IMIT-TSLAB
          R 03:01, KTH Royal Institute of Technology, Stockholm, Sweden,
          July 2003.

[Yegin]   Yegin, A., "Link-layer Triggers Protocol", draft-yegin-
          l2-triggers-00.txt, Internet Draft (work in progress), June

[Velayos] Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE
          802.11b MAC Layer Handover Time", TRITA-IMIT-LCN R 03:02, KTH
          Royal Institute of Technology, Stockholm, Sweden, April 2003.

          Zhang, Q., Guo, C., Guo, Z. and W. Zhu, "Efficient Mobility
          Management for Vertical Handoff between WWAN and WLAN", IEEE

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          Communications Magazine, November 2003.

          Villamizar, C., "OSPF Optimized Multipath (OSPF-OMP)", draft-
          ietf-ospf-omp-02.txt, Internet draft (work in progress),
          February 1999.

Appendix A.  IAB Members at the time of this writing

   Bernard Aboba
   Rob Austein
   Leslie Daigle
   Patrik Falstrom
   Sally Floyd
   Mark Handley
   Bob Hinden
   Geoff Huston
   Jun-Ichiro Itojun Hagino
   Eric Rescorla
   Pete Resnick
   Jonathan Rosenberg

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Copyright Statement

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