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RFC 4907 - Architectural Implications of Link Indications


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Network Working Group                                      B. Aboba, Ed.
Request for Comments: 4907                   Internet Architecture Board
Category: Informational                                              IAB
                                                               June 2007

             Architectural Implications of Link Indications

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

   A link indication represents information provided by the link layer
   to higher layers regarding the state of the link.  This document
   describes the role of link indications within the Internet
   architecture.  While the judicious use of link indications can
   provide performance benefits, inappropriate use can degrade both
   robustness and performance.  This document summarizes current
   proposals, describes the architectural issues, and provides examples
   of appropriate and inappropriate uses of link indications.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements ...............................................3
      1.2. Terminology ................................................3
      1.3. Overview ...................................................5
      1.4. Layered Indication Model ...................................7
   2. Architectural Considerations ...................................14
      2.1. Model Validation ..........................................15
      2.2. Clear Definitions .........................................16
      2.3. Robustness ................................................17
      2.4. Congestion Control ........................................20
      2.5. Effectiveness .............................................21
      2.6. Interoperability ..........................................22
      2.7. Race Conditions ...........................................22
      2.8. Layer Compression .........................................25
      2.9. Transport of Link Indications .............................26
   3. Future Work ....................................................27
   4. Security Considerations ........................................28
      4.1. Spoofing ..................................................28
      4.2. Indication Validation .....................................29
      4.3. Denial of Service .........................................30
   5. References .....................................................31
      5.1. Normative References ......................................31
      5.2. Informative References ....................................31
   6. Acknowledgments ................................................40
   Appendix A. Literature Review .....................................41
     A.1. Link Layer .................................................41
     A.2. Internet Layer .............................................53
     A.3. Transport Layer ............................................55
     A.4. Application Layer ..........................................60
   Appendix B. IAB Members ...........................................60

1.  Introduction

   A link indication represents information provided by the link layer
   to higher layers regarding the state of the link.  While the
   judicious use of link indications can provide performance benefits,
   inappropriate use can degrade both robustness and performance.

   This document summarizes the current understanding of the role of
   link indications within the Internet architecture, and provides
   advice to document authors about the appropriate use of link
   indications within the Internet, transport, and application layers.

   Section 1 describes the history of link indication usage within the
   Internet architecture and provides a model for the utilization of
   link indications.  Section 2 describes the architectural
   considerations and provides advice to document authors.  Section 3
   describes recommendations and future work.  Appendix A summarizes the
   literature on link indications, focusing largely on wireless Local
   Area Networks (WLANs).

1.1.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2.  Terminology

   Access Point (AP)
        A station that provides access to the fixed network (e.g., an
        802.11 Distribution System), via the wireless medium (WM) for
        associated stations.

   Asymmetric
        A link with transmission characteristics that are different
        depending upon the relative position or design characteristics
        of the transmitter and the receiver is said to be asymmetric.
        For instance, the range of one transmitter may be much higher
        than the range of another transmitter on the same medium.

   Beacon
        A control message broadcast by a station (typically an Access
        Point), informing stations in the neighborhood of its continuing
        presence, possibly along with additional status or configuration
        information.

   Binding Update (BU)
        A message indicating a mobile node's current mobility binding,
        and in particular its Care-of Address.

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

   Link
        A communication facility or medium over which nodes can
        communicate at the link layer, i.e., the layer immediately below
        the Internet Protocol (IP).

   Link Down
        An event provided by the link layer that signifies a state
        change associated with the interface no longer being capable of
        communicating data frames; transient periods of high frame loss
        are not sufficient.

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

   Link Layer
        Conceptual layer of control or processing logic that is
        responsible for maintaining control of the link.  The link layer
        functions provide an interface between the higher-layer logic
        and the link.  The link layer is the layer immediately below the
        Internet Protocol (IP).

   Link Up
        An event provided by the link layer that signifies a state
        change associated with the interface becoming capable of
        communicating data frames.

   Maximum Segment Size (MSS)
        The maximum payload size available to the transport layer.

   Maximum Transmission Unit (MTU)
        The size in octets of the largest IP packet, including the IP
        header and payload, that can be transmitted on a link or path.

   Mobile Node
        A node that can change its point of attachment from one link to
        another, while still being reachable via its home address.

   Operable Address
        A static or dynamically assigned address that has not been
        relinquished and has not expired.

   Point of Attachment
        The endpoint on the link to which the host is currently
        connected.

   Routable Address
        Any IP address for which routers will forward packets.  This
        includes private addresses as specified in "Address Allocation
        for Private Internets" [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).

   Strong End System Model
        The Strong End System model emphasizes the host/router
        distinction, tending to model a multi-homed host as a set of
        logical hosts within the same physical host.  In the Strong End
        System model, addresses refer to an interface, rather than to
        the host to which they attach.  As a result, packets sent on an
        outgoing interface have a source address configured on that
        interface, and incoming packets whose destination address does
        not correspond to the physical interface through which it is
        received are silently discarded.

   Weak End System Model
        In the Weak End System model, addresses refer to a host.  As a
        result, packets sent on an outgoing interface need not
        necessarily have a source address configured on that interface,
        and incoming packets whose destination address does not
        correspond to the physical interface through which it is
        received are accepted.

1.3.  Overview

   The use of link indications within the Internet architecture has a
   long history.  In response to an attempt to send to a host that was
   off-line, the ARPANET link layer protocol provided a "Destination
   Dead" indication, described in "Fault Isolation and Recovery"
   [RFC816].  The ARPANET packet radio experiment [PRNET] incorporated
   frame loss in the calculation of routing metrics, a precursor to more
   recent link-aware routing metrics such as Expected Transmission Count
   (ETX), described in "A High-Throughput Path Metric for Multi-Hop
   Wireless Routing" [ETX].

   "Routing Information Protocol" [RFC1058] defined RIP, which is
   descended from the Xerox Network Systems (XNS) Routing Information
   Protocol.  "The OSPF Specification" [RFC1131] defined Open Shortest
   Path First, which uses Link State Advertisements (LSAs) in order to
   flood information relating to link status within an OSPF area.
   [RFC2328] defines version 2 of OSPF.  While these and other routing
   protocols can utilize "Link Up" and "Link Down" indications provided
   by those links that support them, they also can detect link loss
   based on loss of routing packets.  As noted in "Requirements for IP
   Version 4 Routers" [RFC1812]:

   It is crucial that routers have workable mechanisms for determining
   that their network connections are functioning properly.  Failure to
   detect link loss, or failure to take the proper actions when a
   problem is detected, can lead to black holes.

   Attempts have also been made to define link indications other than
   "Link Up" and "Link Down".  "Dynamically Switched Link Control
   Protocol" [RFC1307] defines an experimental protocol for control of
   links, incorporating "Down", "Coming Up", "Up", "Going Down", "Bring
   Down", and "Bring Up" states.

   "A Generalized Model for Link Layer Triggers" [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".  IEEE 802.21
   [IEEE-802.21] defines a Media Independent Handover Event Service
   (MIH-ES) that provides event reporting relating to link
   characteristics, link status, and link quality.  Events defined
   include "Link Down", "Link Up", "Link Going Down", "Link Signal
   Strength", and "Link Signal/Noise Ratio".

   Under ideal conditions, links in the "up" state experience low frame
   loss in both directions and are immediately ready to send and receive
   data frames; links in the "down" state are unsuitable for sending and
   receiving data frames in either direction.

   Unfortunately, links frequently exhibit non-ideal behavior.  Wired
   links may fail in half-duplex mode, or exhibit partial impairment
   resulting in intermediate loss rates.  Wireless links may exhibit
   asymmetry, intermittent frame loss, or rapid changes in throughput
   due to interference or signal fading.  In both wired and wireless
   links, the link state may rapidly flap between the "up" and "down"
   states.  This real-world behavior presents challenges to the
   integration of link indications with the Internet, transport, and
   application layers.

1.4.  Layered Indication Model

   A layered indication model is shown in Figure 1 that includes both
   internally generated link indications (such as link state and rate)
   and indications arising from external interactions such as path
   change detection.  In this model, it is assumed that the link layer
   provides indications to higher layers primarily in the form of
   abstract indications that are link-technology agnostic.

                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Application   |                                               |
   Layer         |                                               |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                               ^     ^   ^
                                               !     !   !
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!-+-+-!-+-!-+-+-+-+
                 |                             !     !   !       |
                 |                             !     ^   ^       |
                 |     Connection Management   !     ! Teardown  |
   Transport     |                             !     !           |
   Layer         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!-+-+-!-+-+-+-+-+-+
                 |                             !     !           |
                 |                             !     !           |
                 |                             ^     !           |
                 |  Transport Parameter Estimation   !           |
                 |(MSS, RTT, RTO, cwnd, bw, ssthresh)!           |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-+-+
                   ^   ^           ^       ^   ^     !
                   !   !           !       !   !     !
                 +-!-+-!-+-+-+-+-+-!-+-+-+-!-+-!-+-+-!-+-+-+-+-+-+
                 | !   ! Incoming  !MIP    !   !     !           |
                 | !   ! Interface !BU     !   !     !           |
                 | !   ! Change    !Receipt!   !     !           |
                 | !   ^           ^       ^   !     ^           |
   Internet      | !   ! Mobility  !       !   !     !           |
   Layer         +-!-+-!-+-+-+-+-+-!-+-+-+-!-+-!-+-+-!-+-+-+-+-+-+
                 | !   ! Outgoing  ! Path  !   !     !           |
                 | !   ! Interface ! Change!   !     !           |
                 | ^   ^ Change    ^       ^   !     ^           |
                 | !                       !   !     !           |
                 | !     Routing           !   !     !           |
                 +-!-+-+-+-+-+-+-+-+-+-+-+-!-+-!-+-+-!-+-+-+-+-+-+
                 | !                       !   v     ! IP        |
                 | !                       !  Path   ! Address   |
                 | !   IP Configuration    ^  Info   ^ Config/   |
                 | !                       !  Cache    Changes   |
                 +-!-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+
                   !                       !
                   !                       !
                 +-!-+-+-+-+-+-+-+-+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+
                 | !                       !                     |
   Link          | ^                       ^                     |
   Layer         | Rate, FER,            Link                    |
                 | Delay                 Up/Down                 |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 1.  Layered Indication Model

1.4.1.  Internet Layer

   One of the functions of the Internet layer is to shield higher layers
   from the specifics of link behavior.  As a result, the Internet layer
   validates and filters link indications and selects outgoing and
   incoming interfaces based on routing metrics.

   The Internet layer composes its routing table based on information
   available from local interfaces as well as potentially by taking into
   account information provided by routers.  This enables the state of
   the local routing table to reflect link conditions on both local and
   remote links.  For example, prefixes to be added or removed from the
   routing table may be determined from Dynamic Host Configuration
   Protocol (DHCP) [RFC2131][RFC3315], Router Advertisements
   [RFC1256][RFC2461], redirect messages, or route updates incorporating
   information on the state of links multiple hops away.

   As described in "Packetization Layer Path MTU Discovery" [RFC4821],
   the Internet layer may maintain a path information cache, enabling
   sharing of Path MTU information between concurrent or subsequent
   connections.  The shared cache is accessed and updated by
   packetization protocols implementing packetization layer Path MTU
   Discovery.

   The Internet layer also utilizes link indications in order to
   optimize aspects of Internet Protocol (IP) configuration and
   mobility.  After receipt of a "Link Up" indication, hosts validate
   potential IP configurations by Detecting Network Attachment (DNA)
   [RFC4436].  Once the IP configuration is confirmed, it may be
   determined that an address change has occurred.  However, "Link Up"
   indications may not necessarily result in a change to Internet layer
   configuration.

   In "Detecting Network Attachment in IPv4" [RFC4436], after receipt of
   a "Link Up" indication, potential IP configurations are validated
   using a bidirectional reachability test.  In "Detecting Network
   Attachment in IPv6 Networks (DNAv6)" [DNAv6], IP configuration is
   validated using reachability detection and Router
   Solicitation/Advertisement.

   The routing sub-layer may utilize link indications in order to enable
   more rapid response to changes in link state and effective
   throughput.  Link rate is often used in computing routing metrics.
   However, in wired networks the transmission rate may be negotiated in
   order to enhance energy efficiency [EfficientEthernet].  In wireless
   networks, the negotiated rate and Frame Error Rate (FER) may change

   with link conditions so that effective throughput may vary on a
   packet-by-packet basis.  In such situations, routing metrics may also
   exhibit rapid variation.

   Routing metrics incorporating link indications such as Link Up/Down
   and effective throughput enable routers to take link conditions into
   account for the purposes of route selection.  If a link experiences
   decreased rate or high frame loss, the route metric will increase for
   the prefixes that it serves, encouraging use of alternate paths if
   available.  When the link condition improves, the route metric will
   decrease, encouraging use of the link.

   Within Weak End System implementations, changes in routing metrics
   and link state may 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.  However,
   link indications such as changes in transmission rate or frame loss
   do not necessarily result in a change of outgoing interface.

   The Internet layer may also become aware of path changes by other
   mechanisms, such as receipt of updates from a routing protocol,
   receipt of a Router Advertisement, dead gateway detection [RFC816] or
   network unreachability detection [RFC2461], ICMP redirects, or a
   change in the IPv4 TTL (Time to Live)/IPv6 Hop Limit of received
   packets.  A change in the outgoing interface may in turn influence
   the mobility sub-layer, 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
   Update [RFC3775]).

1.4.2.  Transport Layer

   The transport layer processes received link indications differently
   for the purposes of transport parameter estimation and connection
   management.

   For the purposes of parameter estimation, the transport layer is
   primarily interested in path properties that impact performance, and
   where link indications may be determined to be relevant to path
   properties they may be utilized directly.  Link indications such as
   "Link Up"/"Link Down" or changes in rate, delay, and frame loss may
   prove relevant.  This will not always be the case, however; where the
   bandwidth of the bottleneck on the end-to-end path is already much
   lower than the transmission rate, an increase in transmission rate
   may not materially affect path properties.  As described in Appendix
   A.3, the algorithms for utilizing link layer indications to improve
   transport parameter estimates are still under development.

   Strict layering considerations do not apply in transport path
   parameter estimation in order to enable the transport layer to make
   use of all available information.  For example, the transport layer
   may determine that a link indication came from a link forming part of
   a path of one or more connections.  In this case, it may utilize the
   receipt of a "Link Down" indication followed by a subsequent "Link
   Up" indication to infer the possibility of non-congestive packet loss
   during the period between the indications, even if the IP
   configuration does not change as a result, so that no Internet layer
   indication would be sent.

   The transport layer may also find Internet layer indications useful
   for path parameter estimation.  For example, path change indications
   can be used as a signal to reset path parameter estimates.  Where
   there is no default route, loss of segments sent to a destination
   lacking a prefix in the local routing table may be assumed to be due
   to causes other than congestion, regardless of the reason for the
   removal (either because local link conditions caused it to be removed
   or because the route was withdrawn by a remote router).

   For the purposes of connection management, layering considerations
   are important.  The transport layer may tear down a connection based
   on Internet layer indications (such as a endpoint address changes),
   but does not take link indications into account.  Just as a "Link Up"
   event may not result in a configuration change, and a configuration
   change may not result in connection teardown, the transport layer
   does not tear down connections on receipt of a "Link Down"
   indication, regardless of the cause.  Where the "Link Down"
   indication results from frame loss rather than an explicit exchange,
   the indication may be transient, to be soon followed by a "Link Up"
   indication.

   Even where the "Link Down" indication results from an explicit
   exchange such as receipt of a Point-to-Point Protocol (PPP) Link
   Control Protocol (LCP)-Terminate or an IEEE 802.11 Disassociate or
   Deauthenticate frame, an alternative point of attachment may be
   available, allowing connectivity to be quickly restored.  As a
   result, robustness is best achieved by allowing connections to remain
   up until an endpoint address changes, or the connection is torn down
   due to lack of response to repeated retransmission attempts.

   For the purposes of connection management, the transport layer is
   cautious with the use of Internet layer indications.  Changes in the
   routing table are not relevant for the purposes of connection
   management, since it is desirable for connections to remain up during
   transitory routing flaps.  However, the transport layer may tear down
   transport connections due to invalidation of a connection endpoint IP
   address.  Where the connection has been established based on a Mobile

   IP home address, a change in the Care-of Address need not result in
   connection teardown, since the configuration change is masked by the
   mobility functionality within the Internet layer, and is therefore
   transparent to the transport layer.

   "Requirements for Internet Hosts -- Communication Layers" [RFC1122],
   Section 2.4, requires Destination Unreachable, Source Quench, Echo
   Reply, Timestamp Reply, and Time Exceeded ICMP messages to be passed
   up to the transport layer.  [RFC1122], Section 4.2.3.9, requires
   Transmission Control Protocol (TCP) to react to an Internet Control
   Message Protocol (ICMP) Source Quench by slowing transmission.

   [RFC1122], Section 4.2.3.9, distinguishes between ICMP messages
   indicating soft error conditions, which must not cause TCP to abort a
   connection, and hard error conditions, which should cause an abort.
   ICMP messages indicating soft error conditions include Destination
   Unreachable codes 0 (Net), 1 (Host), and 5 (Source Route Failed),
   which may result from routing transients; Time Exceeded; and
   Parameter Problem.  ICMP messages indicating hard error conditions
   include Destination Unreachable codes 2 (Protocol Unreachable), 3
   (Port Unreachable), and 4 (Fragmentation Needed and Don't Fragment
   Was Set).  Since hosts implementing classical ICMP-based Path MTU
   Discovery [RFC1191] use Destination Unreachable code 4, they do not
   treat this as a hard error condition.  Hosts implementing "Path MTU
   Discovery for IP version 6" [RFC1981] utilize ICMPv6 Packet Too Big
   messages.  As noted in "TCP Problems with Path MTU Discovery"
   [RFC2923], classical Path MTU Discovery is vulnerable to failure if
   ICMP messages are not delivered or processed.  In order to address
   this problem, "Packetization Layer Path MTU Discovery" [RFC4821] does
   depend on the delivery of ICMP messages.

   "Fault Isolation and Recovery" [RFC816], Section 6, states:

   It is not obvious, when error messages such as ICMP Destination
   Unreachable arrive, whether TCP should abandon the connection.  The
   reason that error messages are difficult to interpret is that, as
   discussed above, after a failure of a gateway or network, there is a
   transient period during which the gateways may have incorrect
   information, so that irrelevant or incorrect error messages may
   sometimes return.  An isolated ICMP Destination Unreachable may
   arrive at a host, for example, if a packet is sent during the period
   when the gateways are trying to find a new route.  To abandon a TCP
   connection based on such a message arriving would be to ignore the
   valuable feature of the Internet that for many internal failures it
   reconstructs its function without any disruption of the end points.

   "Requirements for IP Version 4 Routers" [RFC1812], Section 4.3.3.3,
   states that "Research seems to suggest that Source Quench consumes
   network bandwidth but is an ineffective (and unfair) antidote to
   congestion", indicating that routers should not originate them.  In
   general, since the transport layer is able to determine an
   appropriate (and conservative) response to congestion based on packet
   loss or explicit congestion notification, ICMP Source Quench
   indications are not needed, and the sending of additional Source
   Quench packets during periods of congestion may be detrimental.

   "ICMP attacks against TCP" [Gont] argues that accepting ICMP messages
   based on a correct four-tuple without additional security checks is
   ill-advised.  For example, an attacker forging an ICMP hard error
   message can cause one or more transport connections to abort.  The
   authors discuss a number of precautions, including mechanisms for
   validating ICMP messages and ignoring or delaying response to hard
   error messages under various conditions.  They also recommend that
   hosts ignore ICMP Source Quench messages.

   The transport layer may also provide information to the link layer.
   For example, the transport layer may wish to control the maximum
   number of times that a link layer frame may be retransmitted, so that
   the link layer does not continue to retransmit after a transport
   layer timeout.  In IEEE 802.11, this can be achieved by adjusting the
   Management Information Base (MIB) [IEEE-802.11] variables
   dot11ShortRetryLimit (default: 7) and dot11LongRetryLimit (default:
   4), which control the maximum number of retries for frames shorter
   and longer in length than dot11RTSThreshold, respectively.  However,
   since these variables control link behavior as a whole they cannot be
   used to separately adjust behavior on a per-transport connection
   basis.  In situations where the link layer retransmission timeout is
   of the same order as the path round-trip timeout, link layer control
   may not be possible at all.

1.4.3.  Application Layer

   The transport layer provides indications to the application layer by
   propagating Internet layer indications (such as IP address
   configuration and changes), as well as providing its own indications,
   such as connection teardown.

   Since applications can typically obtain the information they need
   more reliably from the Internet and transport layers, they will
   typically not need to utilize link indications.  A "Link Up"
   indication implies that the link is capable of communicating IP
   packets, but does not indicate that it has been configured;
   applications should use an Internet layer "IP Address Configured"
   event instead.  "Link Down" indications are typically not useful to

   applications, since they can be rapidly followed by a "Link Up"
   indication; applications should respond to transport layer teardown
   indications instead.  Similarly, changes in the transmission rate may
   not be relevant to applications if the bottleneck bandwidth on the
   path does not change; the transport layer is best equipped to
   determine this.  As a result, Figure 1 does not show link indications
   being provided directly to applications.

2.  Architectural Considerations

   The complexity of real-world link behavior poses a challenge to the
   integration of link indications within the Internet architecture.
   While the literature provides persuasive evidence of the utility of
   link indications, difficulties can arise in making effective use of
   them.  To avoid these issues, the following architectural principles
   are suggested and discussed in more detail in the sections that
   follow:

   (1)  Proposals should avoid use of simplified link models in
        circumstances where they do not apply (Section 2.1).

   (2)  Link indications should be clearly defined, so that it is
        understood when they are generated on different link layers
        (Section 2.2).

   (3)  Proposals must demonstrate robustness against spurious link
        indications (Section 2.3).

   (4)  Upper layers should utilize a timely recovery step so as to
        limit the potential damage from link indications determined to
        be invalid after they have been acted on (Section 2.3.2).

   (5)  Proposals must demonstrate that effective congestion control is
        maintained (Section 2.4).

   (6)  Proposals must demonstrate the effectiveness of proposed
        optimizations (Section 2.5).

   (7)  Link indications should not be required by upper layers, in
        order to maintain link independence (Section 2.6).

   (8)  Proposals should avoid race conditions, which can occur where
        link indications are utilized directly by multiple layers of the
        stack (Section 2.7).

   (9)  Proposals should avoid inconsistencies between link and routing
        layer metrics (Section 2.7.3).

   (10) Overhead reduction schemes must avoid compromising
        interoperability and introducing link layer dependencies into
        the Internet and transport layers (Section 2.8).

   (11) Proposals for transport of link indications beyond the local
        host need to carefully consider the layering, security, and
        transport implications (Section 2.9).

2.1.  Model Validation

   Proposals should avoid the use of link models in circumstances where
   they do not apply.

   In "The mistaken axioms of wireless-network research" [Kotz], the
   authors conclude that mistaken assumptions relating to link behavior
   may lead to the design of network protocols that may not work in
   practice.  For example, the authors note that the three-dimensional
   nature of wireless propagation can result in large signal strength
   changes over short distances.  This can result in rapid changes in
   link indications such as rate, frame loss, and signal strength.

   In "Modeling Wireless Links for Transport Protocols" [GurtovFloyd],
   the authors provide examples of modeling mistakes and examples of how
   to improve modeling of link characteristics.  To accompany the paper,
   the authors provide simulation scenarios in ns-2.

   In order to avoid the pitfalls described in [Kotz] [GurtovFloyd],
   documents that describe capabilities that are dependent on link
   indications should explicitly articulate the assumptions of the link
   model and describe the circumstances in which they apply.

   Generic "trigger" models may include implicit assumptions that may
   prove invalid in outdoor or mesh wireless LAN deployments.  For
   example, two-state Markov models assume that the link is either in a
   state experiencing low frame loss ("up") or in a state where few
   frames are successfully delivered ("down").  In these models,
   symmetry is also typically assumed, so that the link is either "up"
   in both directions or "down" in both directions.  In situations where
   intermediate loss rates are experienced, these assumptions may be
   invalid.

   As noted in "Hybrid Rate Control for IEEE 802.11" [Haratcherev],
   signal strength data is noisy and sometimes inconsistent, so that it
   needs to be filtered in order to avoid erratic results.  Given this,
   link indications based on raw signal strength data may be unreliable.
   In order to avoid problems, it is best to combine signal strength
   data with other techniques.  For example, in developing a "Going
   Down" indication for use with [IEEE-802.21] it would be advisable to

   validate filtered signal strength measurements with other indications
   of link loss such as lack of Beacon reception.

2.2.  Clear Definitions

   Link indications should be clearly defined, so that it is understood
   when they are generated on different link layers.  For example,
   considerable work has been required in order to come up with the
   definitions of "Link Up" and "Link Down", and to define when these
   indications are sent on various link layers.

   Link indication definitions should heed the following advice:

   (1)  Do not assume symmetric link performance or frame loss that is
        either low ("up") or high ("down").

        In wired networks, links in the "up" state typically experience
        low frame loss in both directions and are ready to send and
        receive data frames; links in the "down" state are unsuitable
        for sending and receiving data frames in either direction.
        Therefore, a link providing a "Link Up" indication will
        typically experience low frame loss in both directions, and high
        frame loss in any direction can only be experienced after a link
        provides a "Link Down" indication.  However, these assumptions
        may not hold true for wireless LAN networks.  Asymmetry is
        typically less of a problem for cellular networks where
        propagation occurs over longer distances, multi-path effects may
        be less severe, and the base station can transmit at much higher
        power than mobile stations while utilizing a more sensitive
        antenna.

        Specifications utilizing a "Link Up" indication should not
        assume that receipt of this indication means that the link is
        experiencing symmetric link conditions or low frame loss in
        either direction.  In general, a "Link Up" event should not be
        sent due to transient changes in link conditions, but only due
        to a change in link layer state.  It is best to assume that a
        "Link Up" event may not be sent in a timely way.  Large handoff
        latencies can result in a delay in the generation of a "Link Up"
        event as movement to an alternative point of attachment is
        delayed.

   (2)  Consider the sensitivity of link indications to transient link
        conditions.  Due to common effects such as multi-path
        interference, signal strength and signal to noise ratio (SNR)
        may vary rapidly over a short distance, causing erratic behavior
        of link indications based on unfiltered measurements.  As noted
        in [Haratcherev], signal strength may prove most useful when

        utilized in combination with other measurements, such as frame
        loss.

   (3)  Where possible, design link indications with built-in damping.
        By design, the "Link Up" and "Link Down" events relate to
        changes in the state of the link layer that make it able and
        unable to communicate IP packets.  These changes are generated
        either by the link layer state machine based on link layer
        exchanges (e.g., completion of the IEEE 802.11i four-way
        handshake for "Link Up", or receipt of a PPP LCP-Terminate for
        "Link Down") or by protracted frame loss, so that the link layer
        concludes that the link is no longer usable.  As a result, these
        link indications are typically less sensitive to changes in
        transient link conditions.

   (4)  Do not assume that a "Link Down" event will be sent at all, or
        that, if sent, it will be received in a timely way.  A good link
        layer implementation will both rapidly detect connectivity
        failure (such as by tracking missing Beacons) while sending a
        "Link Down" event only when it concludes the link is unusable,
        not due to transient frame loss.

   However, existing wireless LAN implementations often do not do a good
   job of detecting link failure.  During a lengthy detection phase, a
   "Link Down" event is not sent by the link layer, yet IP packets
   cannot be transmitted or received on the link.  Initiation of a scan
   may be delayed so that the station cannot find another point of
   attachment.  This can result in inappropriate backoff of
   retransmission timers within the transport layer, among other
   problems.  This is not as much of a problem for cellular networks
   that utilize transmit power adjustment.

2.3.  Robustness

   Link indication proposals must demonstrate robustness against
   misleading indications.  Elements to consider include:

      Implementation variation
      Recovery from invalid indications
      Damping and hysteresis

2.3.1.  Implementation Variation

   Variations in link layer implementations may have a substantial
   impact on the behavior of link indications.  These variations need to
   be taken into account in evaluating the performance of proposals.
   For example, radio propagation and implementation differences can
   impact the reliability of link indications.

   In "Link-level Measurements from an 802.11b Mesh Network" [Aguayo],
   the authors analyze the cause of frame loss in a 38-node urban
   multi-hop IEEE 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.

   As described in [Aguayo], wireless LAN links often exhibit loss rates
   intermediate between "up" (low loss) and "down" (high loss) states,
   as well as substantial asymmetry.  As a result, receipt of a "Link
   Up" indication may not necessarily indicate bidirectional
   reachability, since it could have been generated after exchange of
   small frames at low rates, which might not imply bidirectional
   connectivity for large frames exchanged at higher rates.

   Where multi-path interference or hidden nodes are encountered, signal
   strength may vary widely over a short distance.  Several techniques
   may be used to reduce potential disruptions.  Multiple transmitting
   and receiving antennas may be used to reduce multi-path effects;
   transmission rate adaptation can be used to find a more satisfactory
   transmission rate; transmit power adjustment can be used to improve
   signal quality and reduce interference; Request-to-Send/Clear-to-Send
   (RTS/CTS) signaling can be used to reduce hidden node problems.
   These techniques may not be completely effective, so that high frame
   loss may be encountered, causing the link to cycle between "up" and
   "down" states.

   To improve robustness against spurious link indications, it is
   recommended that upper layers treat the indication as a "hint"
   (advisory in nature), rather than a "trigger" dictating a particular
   action.  Upper layers may then attempt to validate the hint.

   In [RFC4436], "Link Up" indications are rate limited, and IP
   configuration is confirmed using bidirectional reachability tests
   carried out coincident with a request for configuration via DHCP.  As
   a result, bidirectional reachability is confirmed prior to activation
   of an IP configuration.  However, where a link exhibits an
   intermediate loss rate, demonstration of bidirectional reachability
   may not necessarily indicate that the link is suitable for carrying
   IP data packets.

   Another example of validation occurs in IPv4 Link-Local address
   configuration [RFC3927].  Prior to configuration of an IPv4 Link-
   Local address, it is necessary to run a claim-and-defend protocol.
   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 IPv4 Link-Local address without rerunning claim and
   defend.

2.3.2.  Recovery from Invalid Indications

   In some situations, improper use of link indications can result in
   operational malfunctions.  It is recommended that upper layers
   utilize a timely recovery step so as to limit the potential damage
   from link indications determined to be invalid after they have been
   acted on.

   In Detecting Network Attachment in IPv4 (DNAv4) [RFC4436],
   reachability tests are carried out coincident with a request for
   configuration via DHCP.  Therefore, if the bidirectional reachability
   test times out, the host can still obtain an IP configuration via
   DHCP, and if that fails, the host can still continue to use an
   existing valid address if it has one.

   Where a proposal involves recovery at the transport layer, the
   recovered transport parameters (such as the Maximum Segment Size
   (MSS), RoundTrip Time (RTT), Retransmission TimeOut (RTO), Bandwidth
   (bw), congestion window (cwnd), etc.) should be demonstrated to
   remain valid.  Congestion window validation is discussed in "TCP
   Congestion Window Validation" [RFC2861].

   Where timely recovery is not supported, unexpected consequences may
   result.  As described in [RFC3927], early IPv4 Link-Local
   implementations would wait five minutes before attempting to obtain a
   routable address after assigning an IPv4 Link-Local address.  In one
   implementation, it was observed that where mobile hosts changed their
   point of attachment more frequently than every five minutes, they
   would never obtain a routable address.  The problem was caused by an
   invalid link indication (signaling of "Link Up" prior to completion
   of link layer authentication), resulting in an initial failure to
   obtain a routable address using DHCP.  As a result, [RFC3927]
   recommends against modification of the maximum retransmission timeout
   (64 seconds) provided in [RFC2131].

2.3.3.  Damping and Hysteresis

   Damping and hysteresis can be utilized to limit damage from unstable
   link indications.  This may include damping unstable indications or
   placing constraints on the frequency of link indication-induced
   actions within a time period.

   While [Aguayo] found that frame loss was relatively stable for
   stationary stations, obstacles to radio propagation and multi-path
   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 characteristics, including changes in
   transmission rate, throughput, frame loss, and even "Link Up"/"Link
   Down" indications.

   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 is 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 an IPv4 Link-Local address prior to
   determining whether a DHCP server is available [RFC3927] or an
   operable configuration is valid [RFC4436].

   As noted in Section 1.4, the transport layer does not utilize "Link
   Up" and "Link Down" indications for the purposes of connection
   management.

2.4.  Congestion Control

   Link indication proposals must demonstrate that effective congestion
   control is maintained [RFC2914].  One or more of the following
   techniques may be utilized:

      Rate limiting.  Packets generated based on receipt of link
      indications can be rate limited (e.g., a limit of one packet per
      end-to-end path RTO).

      Utilization of upper-layer indications.  Applications should
      depend on upper-layer indications such as IP address
      configuration/change notification, rather than utilizing link
      indications such as "Link Up".

      Keepalives.  In order to improve robustness against spurious link
      indications, an application keepalive or transport layer
      indication (such as connection teardown) can be used instead of
      consuming "Link Down" indications.

      Conservation of resources.  Proposals must demonstrate that they
      are not vulnerable to congestive collapse.

   As noted in "Robust Rate Adaptation for 802.11 Wireless Networks"
   [Robust], decreasing transmission rate in response to frame loss
   increases contention, potentially leading to congestive collapse.  To
   avoid this, the link layer needs to distinguish frame loss due to
   congestion from loss due to channel conditions.  Only frame loss due
   to deterioration in channel conditions can be used as a basis for
   decreasing transmission rate.

   Consider a proposal where a "Link Up" indication is used by a host to
   trigger retransmission of the last previously sent packet, in order
   to enable ACK reception prior to expiration of the host's
   retransmission timer.  On a rapidly moving mobile node where "Link
   Up" indications follow in rapid succession, this could result in a
   burst of retransmitted packets, violating the principle of
   "conservation of packets".

   At the application layer, link 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 de-registration 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" since link indications can be generated on
   a time scale less than the end-to-end path RTO.  The problem is
   magnified since for each presence update, notifications can be
   delivered to many watchers.  In addition, use of a "Link Up"
   indication in this manner is unwise since the interface may not yet
   even have an operable Internet layer configuration.  Instead, an "IP
   address configured" indication may be utilized.

2.5.  Effectiveness

   Proposals must demonstrate the effectiveness of proposed
   optimizations.  Since optimizations typically increase complexity,
   substantial performance improvement is required in order to make a
   compelling case.

   In the face of unreliable link indications, effectiveness may depend
   on the penalty for false positives and false negatives.  In the case
   of DNAv4 [RFC4436], the benefits of successful optimization are
   modest, but the penalty for being unable to confirm an operable
   configuration is a lengthy timeout.  As a result, the recommended
   strategy is to test multiple potential configurations in parallel in
   addition to attempting configuration via DHCP.  This virtually
   guarantees that DNAv4 will always result in performance equal to or
   better than use of DHCP alone.

2.6.  Interoperability

   While link indications can be utilized where available, they should
   not be required by upper layers, in order to maintain link layer
   independence.  For example, if information on supported prefixes is
   provided at the link layer, hosts not understanding those hints must
   still be able to obtain an IP address.

   Where link indications are proposed to optimize Internet layer
   configuration, proposals must demonstrate that they do not compromise
   robustness by interfering with address assignment or routing protocol
   behavior, making address collisions more likely, or compromising
   Duplicate Address Detection (DAD) [RFC4429].

   To avoid compromising interoperability in the pursuit of performance
   optimization, proposals must demonstrate that interoperability
   remains possible (potentially with degraded performance) even if one
   or more participants do not implement the proposal.

2.7.  Race Conditions

   Link indication proposals should avoid race conditions, which can
   occur where link indications are utilized directly by multiple layers
   of the stack.

   Link indications are useful for optimization of Internet Protocol
   layer addressing and configuration as well as routing.  Although "The
   BU-trigger method for improving TCP performance over Mobile IPv6"
   [Kim] describes situations in which link indications are first
   processed by the Internet Protocol layer (e.g., MIPv6) before being
   utilized by the transport layer, for the purposes of parameter
   estimation, it may be desirable for the transport layer to utilize
   link indications directly.

   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

   indications.  As a result, transport behavior may differ depending on
   the order in which the link indications are processed.

   Where a multi-homed host experiences increasing frame loss or
   decreased rate on one of its interfaces, a routing metric taking
   these effects into account will increase, potentially 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.  As a result, the transport
   parameters estimated for the original outgoing and incoming paths
   (congestion state, Maximum Segment Size (MSS) derived from the link
   maximum transmission unit (MTU) or Path MTU) may no longer be valid
   for the new outgoing and incoming paths.

   To avoid race conditions, the following measures are recommended:

      Path change re-estimation
      Layering
      Metric consistency

2.7.1.  Path Change Re-estimation

   When the Internet layer detects a path change, such as a major change
   in transmission rate, a change in the outgoing or incoming interface
   of the host or the incoming interface of a peer, or perhaps even a
   substantial change in the IPv4 TTL/IPv6 Hop Limit of received
   packets, it may be worth considering whether to reset transport
   parameters (RTT, RTO, cwnd, bw, MSS) to their initial values so as to
   allow them to be re-estimated.  This ensures that estimates based on
   the former path do not persist after they have become invalid.
   Appendix A.3 summarizes the research on this topic.

2.7.2.  Layering

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

   In general, it is advisable for applications to utilize indications
   from the Internet or transport layers rather than consuming link
   indications directly.

2.7.3.  Metric Consistency

   Proposals should avoid inconsistencies between link and routing layer
   metrics.  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.

   Once a link is in the "up" state, its effectiveness in transmission
   of data packets can be used to determine an appropriate routing
   metric.  In situations where the transmission time represents a large
   portion of the total transit time, minimizing total transmission time
   is equivalent to maximizing effective throughput.  "A High-Throughput
   Path Metric for Multi-Hop Wireless Routing" [ETX] describes a
   proposed routing metric based on the Expected Transmission Count
   (ETX).  The authors demonstrate that ETX, based on link layer frame
   loss rates (prior to retransmission), enables the selection of routes
   maximizing effective throughput.  Where the transmission rate is
   constant, the expected transmission time is proportional to ETX, so
   that minimizing ETX also minimizes expected transmission time.

   However, where the transmission rate may vary, ETX may not represent
   a good estimate of the estimated transmission time.  In "Routing in
   multi-radio, multi-hop wireless mesh networks" [ETX-Rate], the
   authors define a new metric called Expected Transmission Time (ETT).
   This is described as a "bandwidth adjusted ETX" since ETT = ETX * S/B
   where S is the size of the probe packet and B is the bandwidth of the
   link as measured by a packet pair [Morgan].  However, ETT assumes
   that the loss fraction of small probe frames sent at 1 Mbps data rate
   is indicative of the loss fraction of larger data frames at higher
   rates, which tends to underestimate the ETT at higher rates, where
   frame loss typically increases.  In "A Radio Aware Routing Protocol
   for Wireless Mesh Networks" [ETX-Radio], the authors refine the ETT
   metric further by estimating the loss fraction as a function of
   transmission rate.

   However, prior to sending data packets over the link, the appropriate
   routing metric may not easily be predicted.  As noted in [Shortest],
   a link that can successfully transmit the short frames utilized for
   control, management, or routing may not necessarily be able to
   reliably transport larger data packets.

   Therefore, it may be necessary to utilize alternative metrics (such
   as signal strength or Access Point load) in order to assist in
   attachment/handoff decisions.  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.

   Within the link layer, metrics such as signal strength and frame loss
   may be used to determine the transmission rate, as well as to
   determine when to select an alternative point of attachment.  In
   order to enable stations to roam prior to encountering packet loss,
   studies such as "An experimental study of IEEE 802.11b handover
   performance and its effect on voice traffic" [Vatn] have suggested
   using signal strength as a mechanism to more rapidly detect loss of
   connectivity, rather than frame loss, as suggested in "Techniques to
   Reduce IEEE 802.11b MAC Layer Handover Time" [Velayos].

   [Aguayo] notes that signal strength and distance are not good
   predictors of frame loss or throughput, 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 throughput.  Similarly, an Access Point (AP) demonstrating
   low utilization may not necessarily be the best choice, since
   utilization may be low due to hardware or software problems.  "OSPF
   Optimized Multipath (OSPF-OMP)" [Villamizar] notes that link-
   utilization-based routing metrics have a history of instability.

2.8.  Layer Compression

   In many situations, the exchanges required for a host to complete a
   handoff and reestablish connectivity are considerable, leading to
   proposals to combine exchanges occurring within multiple layers in
   order to reduce overhead.  While overhead reduction is a laudable
   goal, proposals need to avoid compromising interoperability and
   introducing link layer dependencies into the Internet and transport
   layers.

   Exchanges required for handoff and connectivity reestablishment may
   include link layer scanning, authentication, and association
   establishment; Internet layer configuration, routing, and mobility
   exchanges; transport layer retransmission and recovery; security
   association reestablishment; application protocol re-authentication
   and re-registration exchanges, etc.

   Several proposals involve combining exchanges within the link layer.
   For example, in [EAPIKEv2], a link layer Extensible Authentication
   Protocol (EAP) [RFC3748] 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.  Where link, Internet, or transport

   layer mechanisms are combined, hosts need to maintain backward
   compatibility to permit operation on networks where compression
   schemes are not available.

   Layer compression schemes may also negatively impact robustness.  For
   example, in order to optimize IP address assignment, it has been
   proposed that prefixes be advertised at the link layer, such as
   within the 802.11 Beacon and Probe Response frames.  However,
   [IEEE-802.1X] enables the Virtual LAN Identifier (VLANID) to be
   assigned dynamically, so that prefix(es) advertised within the Beacon
   and/or Probe Response may not correspond to the prefix(es) configured
   by the Internet layer after the host completes link layer
   authentication.  Were the host to handle IP configuration at the link
   layer rather than within the Internet layer, the host might be unable
   to communicate due to assignment of the wrong IP address.

2.9.  Transport of Link Indications

   Proposals for the transport of link indications need to carefully
   consider the layering, security, and transport implications.

   As noted earlier, the transport layer may take the state of the local
   routing table into account in improving the quality of transport
   parameter estimates.  While absence of positive feedback that the
   path is sending data end-to-end must be heeded, where a route that
   had previously been absent is recovered, this may be used to trigger
   congestion control probing.  While this enables transported link
   indications that affect the local routing table to improve the
   quality of transport parameter estimates, security and
   interoperability considerations relating to routing protocols still
   apply.

   Proposals involving transport of link indications need to demonstrate
   the following:

   (a)  Superiority to implicit signals.  In general, implicit signals
        are preferred to explicit transport of link indications since
        they do not require participation in the routing mesh, add no
        new packets in times of network distress, operate more reliably
        in the presence of middle boxes such as NA(P)Ts, are more likely
        to be backward compatible, and are less likely to result in
        security vulnerabilities.  As a result, explicit signaling
        proposals must prove that implicit signals are inadequate.

   (b)  Mitigation of security vulnerabilities.  Transported link
        indications should not introduce new security vulnerabilities.
        Link indications that result in modifications to the local
        routing table represent a routing protocol, so that the

        vulnerabilities associated with unsecured routing protocols
        apply, including spoofing by off-link attackers.  While
        mechanisms such as "SEcure Neighbor Discovery (SEND)" [RFC3971]
        may enable authentication and integrity protection of router-
        originated messages, protecting against forgery of transported
        link indications, they are not yet widely deployed.

   (c)  Validation of transported indications.  Even if a transported
        link indication can be integrity protected and authenticated, if
        the indication is sent by a host off the local link, it may not
        be clear that the sender is on the actual path in use, or which
        transport connection(s) the indication relates to.  Proposals
        need to describe how the receiving host can validate the
        transported link indication.

   (d)  Mapping of Identifiers.  When link indications are transported,
        it is generally for the purposes of providing information about
        Internet, transport, or application layer operations at a remote
        element.  However, application layer sessions or transport
        connections may not be visible to the remote element due to
        factors such as load sharing between links, or use of IPsec,
        tunneling protocols, or nested headers.  As a result, proposals
        need to demonstrate how the link indication can be mapped to the
        relevant higher-layer state.  For example, on receipt of a link
        indication, the transport layer will need to identify the set of
        transport sessions (source address, destination address, source
        port, destination port, transport) that are affected.  If a
        presence server is receiving remote indications of "Link
        Up"/"Link Down" status for a particular Media Access Control
        (MAC) address, the presence server will need to associate that
        MAC address with the identity of the user
        (pres:user@example.com) to whom that link status change is
        relevant.

3.  Future Work

   Further work is needed in order to understand how link indications
   can be utilized by the Internet, transport, and application layers.

   More work is needed to understand the connection between link
   indications and routing metrics.  For example, the introduction of
   block ACKs (supported in [IEEE-802.11e]) complicates the relationship
   between effective throughput and frame loss, which may necessitate
   the development of revised routing metrics for ad-hoc networks.  More
   work is also needed to reconcile handoff metrics (e.g., signal
   strength and link utilization) with routing metrics based on link
   indications (e.g., frame error rate and negotiated rate).

   A better understanding of the use of physical and link layer metrics
   in rate negotiation is required.  For example, recent work
   [Robust][CARA] has suggested that frame loss due to contention (which
   would be exacerbated by rate reduction) can be distinguished from
   loss due to channel conditions (which may be improved via rate
   reduction).

   At the transport layer, more work is needed to determine the
   appropriate reaction to Internet layer indications such as routing
   table and path changes.  More work is also needed in utilization of
   link layer indications in transport parameter estimation, including
   rate changes, "Link Up"/"Link Down" indications, link layer
   retransmissions, and frame loss of various types (due to contention
   or channel conditions).

   More work is also 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
   connection.  Instead, it may make sense to do downward rate
   adjustment so as to decrease frame loss and improve latency.  Also,
   in some cases, the transport layer may not require heroic efforts to
   avoid frame loss; timely delivery may be preferred instead.

4.  Security Considerations

   Proposals for the utilization of link indications may introduce new
   security vulnerabilities.  These include:

      Spoofing
      Indication validation
      Denial of service

4.1.  Spoofing

   Where link layer control frames are unprotected, they may be spoofed
   by an attacker.  For example, PPP does not protect LCP frames such as
   LCP-Terminate, and [IEEE-802.11] does not protect management frames
   such as Associate/Reassociate, Disassociate, or Deauthenticate.

   Spoofing of link layer control traffic may enable attackers to
   exploit weaknesses in link indication proposals.  For example,
   proposals that do not implement congestion avoidance can enable
   attackers to mount denial-of-service attacks.

   However, even where the link layer incorporates security, attacks may
   still be possible if the security model is not consistent.  For
   example, wireless LANs implementing [IEEE-802.11i] do not enable

   stations to send or receive IP packets on the link until completion
   of an authenticated key exchange protocol known as the "4-way
   handshake".  As a result, a link implementing [IEEE-802.11i] cannot
   be considered usable at the Internet layer ("Link Up") until
   completion of the authenticated key exchange.

   However, while [IEEE-802.11i] requires sending of authenticated
   frames in order to obtain a "Link Up" indication, it does not support
   management frame authentication.  This weakness can be exploited by
   attackers to enable denial-of-service attacks on stations attached to
   distant Access Points (APs).

   In [IEEE-802.11F], "Link Up" is considered to occur when an AP sends
   a Reassociation Response.  At that point, the AP sends a spoofed
   frame with the station's source address to a multicast address,
   thereby causing switches within the Distribution System (DS) to learn
   the station's MAC address.  While this enables forwarding of frames
   to the station at the new point of attachment, it also permits an
   attacker to disassociate a station located anywhere within the ESS,
   by sending an unauthenticated Reassociation Request frame.

4.2.  Indication Validation

   "Fault Isolation and Recovery" [RFC816], Section 3, describes how
   hosts interact with routers for the purpose of fault recovery:

   Since the gateways always attempt to have a consistent and correct
   model of the internetwork topology, the host strategy for fault
   recovery is very simple.  Whenever the host feels that something is
   wrong, it asks the gateway for advice, and, assuming the advice is
   forthcoming, it believes the advice completely.  The advice will be
   wrong only during the transient period of negotiation, which
   immediately follows an outage, but will otherwise be reliably
   correct.

   In fact, it is never necessary for a host to explicitly ask a gateway
   for advice, because the gateway will provide it as appropriate.  When
   a host sends a datagram to some distant net, the host should be
   prepared to receive back either of two advisory messages which the
   gateway may send.  The ICMP "redirect" message indicates that the
   gateway to which the host sent the datagram is no longer the best
   gateway to reach the net in question.  The gateway will have
   forwarded the datagram, but the host should revise its routing table
   to have a different immediate address for this net.  The ICMP
   "destination unreachable" message indicates that as a result of an
   outage, it is currently impossible to reach the addressed net or host

   in any manner.  On receipt of this message, a host can either abandon
   the connection immediately without any further retransmission, or
   resend slowly to see if the fault is corrected in reasonable time.

   Given today's security environment, it is inadvisable for hosts to
   act on indications provided by routers without careful consideration.
   As noted in "ICMP attacks against TCP" [Gont], existing ICMP error
   messages may be exploited by attackers in order to abort connections
   in progress, prevent setup of new connections, or reduce throughput
   of ongoing connections.  Similar attacks may also be launched against
   the Internet layer via forging of ICMP redirects.

   Proposals for transported link indications need to demonstrate that
   they will not add a new set of similar vulnerabilities.  Since
   transported link indications are typically unauthenticated, hosts
   receiving them may not be able to determine whether they are
   authentic, or even plausible.

   Where link indication proposals may respond to unauthenticated link
   layer frames, they should utilize upper-layer security mechanisms,
   where possible.  For example, even though a host might utilize an
   unauthenticated link layer control frame to conclude that a link has
   become operational, it can use SEND [RFC3971] or authenticated DHCP
   [RFC3118] in order to obtain secure Internet layer configuration.

4.3.  Denial of Service

   Link indication proposals need to be particularly careful to avoid
   enabling denial-of-service attacks that can be mounted at a distance.
   While wireless links are naturally vulnerable to interference, such
   attacks can only be perpetrated by an attacker capable of
   establishing radio contact with the target network.  However, attacks
   that can be mounted from a distance, either by an attacker on another
   point of attachment within the same network or by an off-link
   attacker, expand the level of vulnerability.

   The transport of link indications can increase risk by enabling
   vulnerabilities exploitable only by attackers on the local link to be
   executed across the Internet.  Similarly, by integrating link
   indications with upper layers, proposals may enable a spoofed link
   layer frame to consume more resources on the host than might
   otherwise be the case.  As a result, while it is important for upper
   layers to validate link indications, they should not expend excessive
   resources in doing so.

   Congestion control is not only a transport issue, it is also a
   security issue.  In order to not provide leverage to an attacker, a
   single forged link layer frame should not elicit a magnified response

   from one or more hosts, by generating either multiple responses or a
   single larger response.  For example, proposals should not enable
   multiple hosts to respond to a frame with a multicast destination
   address.

5.  References

5.1.  Normative References

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

5.2.  Informative References

   [RFC816]       Clark, D., "Fault Isolation and Recovery", RFC 816,
                  July 1982.

   [RFC1058]      Hedrick, C., "Routing Information Protocol", RFC 1058,
                  June 1988.

   [RFC1122]      Braden, R., "Requirements for Internet Hosts --
                  Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1131]      Moy, J., "The OSPF Specification", RFC 1131, October
                  1989.

   [RFC1191]      Mogul, J. and S. Deering, "Path MTU discovery", RFC
                  1191, November 1990.

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

   [RFC1305]      Mills, D., "Network Time Protocol (Version 3)
                  Specification, Implementation and Analysis", RFC 1305,
                  March 1992.

   [RFC1307]      Young, J. and A. Nicholson, "Dynamically Switched Link
                  Control Protocol", RFC 1307, March 1992.

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

   [RFC1812]      Baker, F., "Requirements for IP Version 4 Routers",
                  RFC 1812, June 1995.

   [RFC1918]      Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
                  D., and E. Lear, "Address Allocation for Private
                  Internets", BCP 5, RFC 1918, February 1996.

   [RFC1981]      McCann, J., Deering, S. and J. Mogul, "Path MTU
                  Discovery for IP version 6", RFC 1981, June 1996.

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

   [RFC2328]      Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                  1998.

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

   [RFC2778]      Day, M., Rosenberg, J., and H. Sugano, "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.

   [RFC2923]      Lahey, K., "TCP Problems with Path MTU Discovery", RFC
                  2923, September 2000.

   [RFC2960]      Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                  Schwarzbauer, H. Taylor, T., Rytina, I., Kalla, M.,
                  Zhang, L., and V. Paxson, "Stream Control Transmission
                  Protocol" RFC 2960, October 2000.

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

   [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.

   [RFC3366]      Fairhurst, G. and L. Wood, "Advice to link designers
                  on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC
                  3366, August 2002.

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

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

   [RFC3921]      Saint-Andre, P., "Extensible Messaging and Presence
                  protocol (XMPP):  Instant Messaging and Presence", RFC
                  3921, October 2004.

   [RFC3927]      Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
                  Configuration of Link-Local IPv4 Addresses", RFC 3927,
                  May 2005.

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

   [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                  Congestion Control Protocol (DCCP)", RFC 4340, March
                  2006.

   [RFC4423]      Moskowitz, R. and P. Nikander, "Host Identity Protocol
                  (HIP) Architecture", RFC 4423, May 2006.

   [RFC4429]      Moore, N., "Optimistic Duplicate Address Detection
                  (DAD) for IPv6", RFC 4429, April 2006.

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

   [RFC4821]      Mathis, M. and J. Heffner, "Packetization Layer Path
                  MTU Discovery", RFC 4821, March 2007.

   [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.

   [Bakshi]       Bakshi, B., Krishna, P., Vadiya, N., and D.Pradhan,
                  "Improving Performance of TCP over Wireless Networks",
                  Proceedings of the 1997 International Conference on
                  Distributed Computer Systems, Baltimore, May 1997.

   [BFD]          Katz, D. and D. Ward, "Bidirectional Forwarding
                  Detection", Work in Progress, March 2007.

   [Biaz]         Biaz, S. and N. Vaidya, "Discriminating Congestion
                  Losses from Wireless Losses Using Interarrival Times
                  at the Receiver", Proceedings of the IEEE Symposium on
                  Application-Specific Systems and Software Engineering
                  and Technology, Richardson, TX, Mar 1999.

   [CARA]         Kim, J., Kim, S., and S. Choi, "CARA: Collision-Aware
                  Rate Adaptation for IEEE 802.11 WLANs", Korean
                  Institute of Communication Sciences (KICS) Journal,
                  Feb. 2006

   [Chandran]     Chandran, K., Raghunathan, S., Venkatesan, S., and R.
                  Prakash, "A Feedback-Based Scheme for Improving TCP
                  Performance in Ad-Hoc Wireless Networks", Proceedings
                  of the 18th International Conference on Distributed
                  Computing Systems (ICDCS), Amsterdam, May 1998.

   [DNAv6]        Narayanan, S., "Detecting Network Attachment in IPv6
                  (DNAv6)", Work in Progress, March 2007.

   [E2ELinkup]    Dawkins, S. and C. Williams, "End-to-end, Implicit
                  'Link-Up' Notification", Work in Progress, October
                  2003.

   [EAPIKEv2]     Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba,
                  Y., and F. Bersani, "EAP IKEv2 Method", Work in
                  Progress, March 2007.

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

   [Eddy]         Eddy, W. and Y. Swami, "Adapting End Host Congestion
                  Control for Mobility", Technical Report CR-2005-
                  213838, NASA Glenn Research Center, July 2005.

   [EfficientEthernet]
                  Gunaratne, C. and K. Christensen, "Ethernet Adaptive
                  Link Rate: System Design and Performance Evaluation",
                  Proceedings of the IEEE Conference on Local Computer
                  Networks, pp. 28-35, November 2006.

   [Eggert]       Eggert, L., Schuetz, S., and S. Schmid, "TCP
                  Extensions for Immediate Retransmissions", Work in
                  Progress, June 2005.

   [Eggert2]      Eggert, L. and W. Eddy, "Towards More Expressive
                  Transport-Layer Interfaces", MobiArch '06, San
                  Francisco, CA.

   [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.

   [ETX-Rate]     Padhye, J., Draves, R. and B. Zill, "Routing in
                  multi-radio, multi-hop wireless mesh networks",
                  Proceedings of ACM MobiCom Conference, September 2003.

   [ETX-Radio]    Kulkarni, G., Nandan, A., Gerla, M., and M.
                  Srivastava, "A Radio Aware Routing Protocol for
                  Wireless Mesh Networks", UCLA Computer Science
                  Department, Los Angeles, CA.

   [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:
                  <http://www.ieee802.org/handoff/march04_meeting_docs/
                  Generalized_triggers-02.pdf>.

   [Goel]         Goel, S. and D. Sanghi, "Improving TCP Performance
                  over Wireless Links", Proceedings of TENCON'98, pages
                  332-335.  IEEE, December 1998.

   [Gont]         Gont, F., "ICMP attacks against TCP", Work in
                  Progress, October 2006.

   [Gurtov]       Gurtov, A. and J. Korhonen, "Effect of Vertical
                  Handovers on Performance of TCP-Friendly Rate
                  Control", to appear in ACM MCCR, 2004.

   [GurtovFloyd]  Gurtov, A. and S. Floyd, "Modeling Wireless Links for
                  Transport Protocols", Computer Communications Review
                  (CCR) 34, 2 (2003).

   [Haratcherev]  Haratcherev, I., Lagendijk, R., Langendoen, K., and H.
                  Sips, "Hybrid Rate Control for IEEE 802.11", MobiWac
                  '04, October 1, 2004, Philadelphia, Pennsylvania, USA.

   [Haratcherev2] Haratcherev, I., "Application-oriented Link Adaptation
                  for IEEE 802.11", Ph.D. Thesis, Technical University
                  of Delft, Netherlands, ISBN-10:90-9020513-6, ISBN-
                  13:978-90-9020513-7, March 2006.

   [HMP]          Lee, S., Cho, J., and A. Campbell, "Hotspot Mitigation
                  Protocol (HMP)", Work in Progress, October 2003.

   [Holland]      Holland, G. and N. Vaidya, "Analysis of TCP
                  Performance over Mobile Ad Hoc Networks", Proceedings
                  of the Fifth International Conference on Mobile
                  Computing and Networking, pages 219-230.  ACM/IEEE,
                  Seattle, August 1999.

   [Iannaccone]   Iannaccone, G., Chuah, C., Mortier, R., Bhattacharyya,
                  S., and C. Diot, "Analysis of link failures in an IP
                  backbone", Proc. of ACM Sigcomm Internet Measurement
                  Workshop, November, 2002.

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

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

   [IEEE-802.11e] Institute of Electrical and Electronics Engineers,
                  "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
                  - Amendment 8: Medium Access Control (MAC) Quality of
                  Service Enhancements", IEEE 802.11e, November 2005.

   [IEEE-802.11F] 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 (now
                  deprecated).

   [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 LAN Medium
                  Access Control (MAC) and Physical Layer (PHY)
                  Specifications: Specification for Enhanced Security",
                  IEEE 802.11i, July 2004.

   [IEEE-802.11k] Institute of Electrical and Electronics Engineers,
                  "Draft Amendment to Telecommunications and Information
                  Exchange Between Systems - LAN/MAN Specific
                  Requirements - Part 11:  Wireless LAN Medium Access
                  Control (MAC) and Physical Layer (PHY) Specifications
                  - Amendment 7: Radio Resource Management", IEEE
                  802.11k/D7.0, January 2007.

   [IEEE-802.21]  Institute of Electrical and Electronics Engineers,
                  "Draft Standard for Telecommunications and Information
                  Exchange Between Systems - LAN/MAN Specific
                  Requirements - Part 21:  Media Independent Handover",
                  IEEE 802.21D0, June 2005.

   [Kamerman]     Kamerman, A. and L. Monteban, "WaveLAN II: A High-
                  Performance Wireless LAN for the Unlicensed Band",
                  Bell Labs Technical Journal, Summer 1997.

   [Kim]          Kim, K., Park, Y., Suh, K., and Y. Park, "The BU-
                  trigger method for improving TCP performance over
                  Mobile IPv6", 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.

   [Krishnan]     Krishnan, R., Sterbenz, J., Eddy, W., Partridge, C.,
                  and M. Allman, "Explicit Transport Error Notification
                  (ETEN) for Error-Prone Wireless and Satellite
                  Networks", Computer Networks, 46 (3), October 2004.

   [Lacage]       Lacage, M., Manshaei, M., and T. Turletti, "IEEE
                  802.11 Rate Adaptation: A Practical Approach", MSWiM
                  '04, October 4-6, 2004, Venezia, Italy.

   [Lee]          Park, S., Lee, M., and J. Korhonen, "Link
                  Characteristics Information for Mobile IP", Work in
                  Progress, January 2007.

   [Ludwig]       Ludwig, R. and B. Rathonyi, "Link-layer Enhancements
                  for TCP/IP over GSM", Proceedings of IEEE Infocom '99,
                  March 1999.

   [MIPEAP]       Giaretta, C., Guardini, I., Demaria, E., Bournelle,
                  J., and M. Laurent-Maknavicius, "MIPv6 Authorization
                  and Configuration based on EAP", Work in Progress,
                  October 2006.

   [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.

   [Morgan]       Morgan, S. and S. Keshav, "Packet-Pair Rate Control -
                  Buffer Requirements and Overload Performance",
                  Technical Memorandum, AT&T Bell Laboratories, October
                  1994.

   [Mun]          Mun, Y. and J. Park, "Layer 2 Handoff for Mobile-IPv4
                  with 802.11", Work in Progress, March 2004.

   [ONOE]         Onoe Rate Control,
                  <http://madwifi.org/browser/trunk/ath_rate/onoe>.

   [Park]         Park, S., Njedjou, E., and N. Montavont, "L2 Triggers
                  Optimized Mobile IPv6 Vertical Handover: The
                  802.11/GPRS Example", Work in Progress, July 2004.

   [Pavon]        Pavon, J. and S. Choi, "Link adaptation strategy for
                  IEEE802.11 WLAN via received signal strength
                  measurement", IEEE International Conference on
                  Communications, 2003 (ICC '03), volume 2, pages 1108-
                  1113, Anchorage, Alaska, USA, May 2003.

   [PEAP]         Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn,
                  G., and S. Josefsson, "Protected EAP Protocol (PEAP)
                  Version 2", Work in Progress, October 2004.

   [PRNET]        Jubin, J. and J. Tornow, "The DARPA packet radio
                  network protocols", Proceedings of the IEEE, 75(1),
                  January 1987.

   [Qiao]         Qiao D., Choi, S., Jain, A., and Kang G. Shin, "MiSer:
                  An Optimal Low-Energy Transmission Strategy for IEEE
                  802.11 a/h", in Proc. ACM MobiCom'03, San Diego, CA,
                  September 2003.

   [RBAR]         Holland, G., Vaidya, N., and P. Bahl, "A Rate-Adaptive
                  MAC Protocol for Multi-Hop Wireless Networks",
                  Proceedings ACM MOBICOM, July 2001.

   [Ramani]       Ramani, I. and S. Savage, "SyncScan: Practical Fast
                  Handoff for 802.11 Infrastructure Networks",
                  Proceedings of the IEEE InfoCon 2005, March 2005.

   [Robust]       Wong, S., Yang, H ., Lu, S., and V. Bharghavan,
                  "Robust Rate Adaptation for 802.11 Wireless Networks",
                  ACM MobiCom'06, Los Angeles, CA, September 2006.

   [SampleRate]   Bicket, J., "Bit-rate Selection in Wireless networks",
                  MIT Master's Thesis, 2005.

   [Scott]        Scott, J., Mapp, G., "Link Layer Based TCP
                  Optimisation for Disconnecting Networks", ACM SIGCOMM
                  Computer Communication Review, 33(5), October 2003.

   [Schuetz]      Schutz, S., Eggert, L., Schmid, S., and M. Brunner,
                  "Protocol Enhancements for Intermittently Connected
                  Hosts", ACM SIGCOMM Computer Communications Review,
                  Volume 35, Number 2, July 2005.

   [Shortest]     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.

   [TRIGTRAN]     Dawkins, S., Williams, C., and A. Yegin, "Framework
                  and Requirements for TRIGTRAN", 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.

   [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.

   [Vertical]     Zhang, Q., Guo, C., Guo, Z., and W. Zhu, "Efficient
                  Mobility Management for Vertical Handoff between WWAN
                  and WLAN", IEEE Communications Magazine, November
                  2003.

   [Villamizar]   Villamizar, C., "OSPF Optimized Multipath (OSPF-OMP)",
                  Work in Progress, February 1999.

   [Xylomenos]    Xylomenos, G., "Multi Service Link Layers: An Approach
                  to Enhancing Internet Performance over Wireless
                  Links", Ph.D. thesis, University of California at San
                  Diego, 1999.

   [Yegin]        Yegin, A., "Link-layer Triggers Protocol", Work in
                  Progress, June 2002.

6.  Acknowledgments

   The authors would like to acknowledge James Kempf, Phil Roberts,
   Gorry Fairhurst, John Wroclawski, Aaron Falk, Sally Floyd, Pekka
   Savola, Pekka Nikander, Dave Thaler, Yogesh Swami, Wesley Eddy, and
   Janne Peisa for contributions to this document.

Appendix A.  Literature Review

   This appendix summarizes the literature with respect to link
   indications on wireless local area networks.

A.1.  Link Layer

   The characteristics of wireless links have been found to vary
   considerably depending on the environment.

   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 rate), many links exhibited intermediate loss
   rates.  Asymmetry was also common, with 30 percent of links
   demonstrating substantial differences 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
   data.

   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 be
   assumed to operate either very well or not at all.

   In "Link-level Measurements from an 802.11b Mesh Network" [Aguayo],
   the authors analyze 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 (SNR) and distance showed little value in
   predicting 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 interference.

   The findings of [Eckhardt] and [Aguayo] demonstrate the diversity of
   link conditions observed in practice.  While for indoor
   infrastructure networks site surveys and careful measurement can
   assist in promoting ideal behavior, in ad-hoc/mesh networks node
   mobility and external factors such as weather may not be easily
   controlled.

   Considerable diversity in behavior is also observed due to
   implementation effects.  "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 disconnection from the existing point of
   attachment), search (discovery of alternative attachment points), and
   execution (connection to an alternative point of attachment) phases.
   These measurements indicated that the duration of the detection phase
   (the largest component of handoff delay) is determined by the number
   of non-acknowledged frames triggering the search phase and delays due
   to precursors such as RTS/CTS and rate adaptation.

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

   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.
   This approach is both quicker and more immune to multi-path
   interference than monitoring of the SNR.  Where the STA is not
   sending or receiving frames, it is recommended that Beacon reception
   be tracked in order to detect disconnection, 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 and data frame transmission.

   "An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process"
   [Mishra] investigates handoff latencies obtained with three mobile
   STA implementations communicating with two APs.  The study found that
   there is a 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.  Latency
   in the detection phase was not investigated.

   "SyncScan: Practical Fast Handoff for 802.11 Infrastructure Networks"
   [Ramani] weighs the pros and cons of active versus passive scanning.
   The authors point out the advantages of timed Beacon reception, which
   had previously been incorporated into [IEEE-802.11k].  Timed Beacon
   reception allows the station to continually keep up to date on the
   signal to noise ratio of neighboring APs, allowing handoff to occur
   earlier.  Since the station does not need to wait for initial and
   subsequent responses to a broadcast Probe Response (MinChannelTime
   and MaxChannelTime, respectively), performance is comparable to what
   is achievable with 802.11k Neighbor Reports and unicast Probe
   Requests.

   The authors measured the channel switching delay, the time it takes
   to switch to a new frequency and begin receiving frames.
   Measurements ranged from 5 ms to 19 ms per channel; where timed
   Beacon reception or interleaved active scanning is used, switching
   time contributes significantly to overall handoff latency.  The
   authors propose deployment of APs with Beacons synchronized via
   Network Time Protocol (NTP) [RFC1305], enabling a driver implementing
   SyncScan to work with legacy APs without requiring implementation of
   new protocols.  The authors measured the distribution of inter-
   arrival times for stations implementing SyncScan, with excellent
   results.

   "Roaming Interval Measurements" [Alimian] presents data on the
   behavior of 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
   of magnitude variation in rate adaptation to an order of magnitude
   difference in 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 the 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 continued to receive
   data frames during the search phase.  Station-initiated

   Disassociation and pre-authentication were not observed in this
   study.

A.1.1.  Link Indications

   Within a link layer, the definition of "Link Up" and "Link Down" may
   vary according to the deployment scenario.  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 Network Control
   Protocol (NCP) negotiation has completed for that protocol.

   Unlike PPP, IEEE 802 does not include facilities for network layer
   configuration, and the definition of "Link Up" and "Link Down" varies
   by implementation.  Empirical evidence suggests that the definition
   of "Link Up" and "Link Down" may depend 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 STA encounters a series of consecutive non-acknowledged
   frames while having missed one or more Beacons, 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; since this approach does not take rate
   adaptation into account, it may be somewhat aggressive.  Only when no
   alternative workable rate or point of attachment is found is a "Link
   Down" indication returned.

   In a stationary point-to-point installation, the most likely cause of
   an outage is that the link has become impaired, and alternative
   points of attachment may not be available.  As a result,
   implementations configured to operate in this mode tend to be more
   persistent.  For example, within 802.11 the short interframe space
   (SIFS) interval may be increased and MIB variables relating to
   timeouts (such as dot11AuthenticationResponseTimeout,
   dot11AssociationResponseTimeout, dot11ShortRetryLimit, and
   dot11LongRetryLimit) may be set to larger values.  In addition, a
   "Link Down" indication may be returned later.

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

   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
   "Deauthenticate" 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/Reassociation
   Response.

   As described within [IEEE-802.11F], 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 distribution
   system until it sends its first frame at the new location.  Thus, the
   purpose of spoofing is to equalize uplink and downlink handover
   times.  This enables an attacker to deny service to authenticated and
   associated stations by spoofing a Reassociation Request using the
   victim's MAC address, from anywhere within the ESS.  Without
   spoofing, such an attack would only be able to disassociate stations
   on the AP to which the Reassociation Request was sent.

   The signaling 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"
   indications.

   In [IEEE-802.11i], 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.  This has resulted in several observed
   problems.  For example, where a "Link Up" indication is triggered on
   the station by receipt of an Association/Reassociation Response, DHCP
   [RFC2131] or Router Solicitation/Router Advertisement (RS/RA) may be
   triggered prior to when the link is usable by the Internet layer,
   resulting in configuration delays or failures.  Similarly, transport
   layer connections will encounter packet loss, resulting in back-off
   of retransmission timers.

A.1.2.  Smart Link Layer Proposals

   In order to improve link layer performance, several studies have
   investigated "smart link layer" proposals.

   "Advice to link designers on link Automatic Repeat reQuest (ARQ)"
   [RFC3366] provides advice to the designers of digital communication
   equipment and link-layer protocols employing link-layer Automatic
   Repeat reQuest (ARQ) techniques for IP.  It discusses the use of ARQ,
   timers, persistency in retransmission, and the challenges that arise
   from sharing links between multiple flows and from different
   transport requirements.

   In "Link-layer Enhancements for TCP/IP over GSM" [Ludwig], the
   authors describe how the Global System for Mobile Communications
   (GSM)-reliable and unreliable link layer modes can be simultaneously
   utilized without higher layer control.  Where a reliable link layer
   protocol is required (where reliable transports such TCP and Stream
   Control Transmission Protocol (SCTP) [RFC2960] are used), the Radio
   Link Protocol (RLP) can be engaged; with delay-sensitive applications
   such as those based on UDP, the transparent mode (no RLP) can be
   used.  The authors also describe how PPP negotiation can be optimized
   over high-latency GSM links using "Quickstart-PPP".

   In "Link Layer Based TCP Optimisation for Disconnecting Networks"
   [Scott], the authors describe performance problems that occur with
   reliable transport protocols facing periodic network disconnections,
   such as those due to signal fading or handoff.  The authors define a
   disconnection as a period of connectivity loss that exceeds a
   retransmission timeout, but is shorter than the connection lifetime.
   One issue is that link-unaware senders continue to back off during
   periods of disconnection.  The authors suggest that a link-aware
   reliable transport implementation halt retransmission after receiving
   a "Link Down" indication.  Another issue is that on reconnection the
   lengthened retransmission times cause delays in utilizing the link.

   To improve performance, a "smart link layer" is proposed, which
   stores the first packet that was not successfully transmitted on a
   connection, then retransmits it upon receipt of a "Link Up"
   indication.  Since a disconnection can result in hosts experiencing
   different network conditions upon reconnection, the authors do not
   advocate bypassing slow start or attempting to raise the congestion
   window.  Where IPsec is used and connections cannot be differentiated
   because transport headers are not visible, the first untransmitted
   packet for a given sender and destination IP address can be
   retransmitted.  In addition to looking at retransmission of a single
   packet per connection, the authors also examined other schemes such

   as retransmission of multiple packets and simulated duplicate
   reception of single or multiple packets (known as rereception).

   In general, retransmission schemes were superior to rereception
   schemes, since rereception cannot stimulate fast retransmit after a
   timeout.  Retransmission of multiple packets did not appreciably
   improve performance over retransmission of a single packet.  Since
   the focus of the research was on disconnection rather than just lossy
   channels, a two-state Markov model was used, with the "up" state
   representing no loss, and the "down" state representing 100 percent
   loss.

   In "Multi Service Link Layers: An Approach to Enhancing Internet
   Performance over Wireless Links" [Xylomenos], the authors use ns-2 to
   simulate the performance of various link layer recovery schemes (raw
   link without retransmission, go back N, XOR-based FEC, selective
   repeat, Karn's RLP, out-of-sequence RLP, and Berkeley Snoop) in
   stand-alone file transfer, Web browsing, and continuous media
   distribution.  While selective repeat and Karn's RLP provide the
   highest throughput for file transfer and Web browsing scenarios,
   continuous media distribution requires a combination of low delay and
   low loss and the out-of-sequence RLP performed best in this scenario.
   Since the results indicate that no single link layer recovery scheme
   is optimal for all applications, the authors propose that the link
   layer implement multiple recovery schemes.  Simulations of the
   multi-service architecture showed that the combination of a low-error
   rate recovery scheme for TCP (such as Karn's RLP) and a low-delay
   scheme for UDP traffic (such as out-of-sequence RLP) provides for
   good performance in all scenarios.  The authors then describe how a
   multi-service link layer can be integrated with Differentiated
   Services.

   In "WaveLAN-II: A High-Performance Wireless LAN for the Unlicensed
   Band" [Kamerman], the authors propose an open-loop rate adaptation
   algorithm known as Automatic Rate Fallback (ARF).  In ARF, the sender
   adjusts the rate upwards after a fixed number of successful
   transmissions, and adjusts the rate downwards after one or two
   consecutive failures.  If after an upwards rate adjustment the
   transmission fails, the rate is immediately readjusted downwards.

   In "A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks"
   [RBAR], the authors propose a closed-loop rate adaptation approach
   that requires incompatible changes to the IEEE 802.11 MAC.  In order
   to enable the sender to better determine the transmission rate, the
   receiver determines the packet length and signal to noise ratio (SNR)
   of a received RTS frame and calculates the corresponding rate based
   on a theoretical channel model, rather than channel usage statistics.
   The recommended rate is sent back in the CTS frame.  This allows the

   rate (and potentially the transmit power) to be optimized on each
   transmission, albeit at the cost of requiring RTS/CTS for every frame
   transmission.

   In "MiSer: An Optimal Low-Energy Transmission Strategy for IEEE
   802.11 a/h" [Qiao], the authors propose a scheme for optimizing
   transmit power.  The proposal mandates the use of RTS/CTS in order to
   deal with hidden nodes, requiring that CTS and ACK frames be sent at
   full power.  The authors utilize a theoretical channel model rather
   than one based on channel usage statistics.

   In "IEEE 802.11 Rate Adaptation: A Practical Approach" [Lacage], the
   authors distinguish between low-latency implementations, which enable
   per-packet rate decisions, and high-latency implementations, which do
   not.  The former implementations typically include dedicated CPUs in
   their design, enabling them to meet real-time requirements.  The
   latter implementations are typically based on highly integrated
   designs in which the upper MAC is implemented on the host.  As a
   result, due to operating system latencies the information required to
   make per-packet rate decisions may not be available in time.

   The authors propose an Adaptive ARF (AARF) algorithm for use with
   low-latency implementations.  This enables rapid downward rate
   negotiation on failure to receive an ACK, while increasing the number
   of successful transmissions required for upward rate negotiation.
   The AARF algorithm is therefore highly stable in situations where
   channel properties are changing slowly, but slow to adapt upwards
   when channel conditions improve.  In order to test the algorithm, the
   authors utilized ns-2 simulations as well as implementing a version
   of AARF adapted to a high-latency implementation, the AR 5212
   chipset.  The Multiband Atheros Driver for WiFi (MadWiFi) driver
   enables a fixed schedule of rates and retries to be provided when a
   frame is queued for transmission.  The adapted algorithm, known as
   the Adaptive Multi Rate Retry (AMRR), requests only one transmission
   at each of three rates, the last of which is the minimum available
   rate.  This enables adaptation to short-term fluctuations in the
   channel with minimal latency.  The AMRR algorithm provides
   performance considerably better than the existing MadWifi driver.

   In "Link Adaptation Strategy for IEEE 802.11 WLAN via Received Signal
   Strength Measurement" [Pavon], the authors propose an algorithm by
   which a STA adjusts the transmission rate based on a comparison of
   the received signal strength (RSS) from the AP with dynamically
   estimated threshold values for each transmission rate.  Upon
   reception of a frame, the STA updates the average RSS, and on
   transmission the STA selects a rate and adjusts the RSS threshold
   values based on whether or not the transmission is successful.  In
   order to validate the algorithm, the authors utilized an OPNET

   simulation without interference, and an ideal curve of bit error rate
   (BER) vs. signal to noise ratio (SNR) was assumed.  Not surprisingly,
   the simulation results closely matched the maximum throughput
   achievable for a given signal to noise ratio, based on the ideal BER
   vs. SNR curve.

   In "Hybrid Rate Control for IEEE 802.11" [Haratcherev], the authors
   describe a hybrid technique utilizing Signal Strength Indication
   (SSI) data to constrain the potential rates selected by statistics-
   based automatic rate control.  Statistics-based rate control
   techniques include:

   Maximum Throughput

   This technique, which was chosen as the statistics-based technique in
   the hybrid scheme, sends a fraction of data at adjacent rates in
   order to estimate which rate provides the maximum throughput.  Since
   accurate estimation of throughput requires a minimum number of frames
   to be sent at each rate, and only a fraction of frames are utilized
   for this purpose, this technique adapts more slowly at lower rates;
   with 802.11b rates, the adaptation time scale is typically on the
   order of a second.  Depending on how many rates are tested, this
   technique can enable adaptation beyond adjacent rates.  However,
   where maximum rate and low frame loss are already being encountered,
   this technique results in lower throughput.

   Frame Error Rate (FER) Control

   This technique estimates the FER, attempting to keep it between a
   lower limit (if FER moves below, increase rate) and upper limit (if
   FER moves above, decrease rate).  Since this technique can utilize
   all the transmitted data, it can respond faster than maximum
   throughput techniques.  However, there is a tradeoff of reaction time
   versus FER estimation accuracy; at lower rates either reaction times
   slow or FER estimation accuracy will suffer.  Since this technique
   only measures the FER at the current rate, it can only enable
   adaptation to adjacent rates.

   Retry-based

   This technique modifies FER control techniques by enabling rapid
   downward rate adaptation after a number (5-10) of unsuccessful
   retransmissions.  Since fewer packets are required, the sensitivity
   of reaction time to rate is reduced.  However, upward rate adaptation
   proceeds more slowly since it is based on a collection of FER data.
   This technique is limited to adaptation to adjacent rates, and it has
   the disadvantage of potentially worsening frame loss due to
   contention.

   While statistics-based techniques are robust against short-lived link
   quality changes, they do not respond quickly to long-lived changes.
   By constraining the rate selected by statistics-based techniques
   based on ACK SSI versus rate data (not theoretical curves), more
   rapid link adaptation was enabled.  In order to ensure rapid
   adaptation during rapidly varying conditions, the rate constraints
   are tightened when the SSI values are changing rapidly, encouraging
   rate transitions.  The authors validated their algorithms by
   implementing a driver for the Atheros AR5000 chipset, and then
   testing its response to insertion and removal from a microwave oven
   acting as a Faraday cage.  The hybrid algorithm dropped many fewer
   packets than the maximum throughput technique by itself.

   In order to estimate the SSI of data at the receiver, the ACK SSI was
   used.  This approach does not require the receiver to provide the
   sender with the received power, so that it can be implemented without
   changing the IEEE 802.11 MAC.  Calibration of the rate versus ACK SSI
   curves does not require a symmetric channel, but it does require that
   channel properties in both directions vary in a proportional way and
   that the ACK transmit power remains constant.  The authors checked
   the proportionality assumption and found that the SSI of received
   data correlated highly (74%) with the SSI of received ACKs.  Low pass
   filtering and monotonicity constraints were applied to remove noise
   in the rate versus SSI curves.  The resulting hybrid rate adaptation
   algorithm demonstrated the ability to respond to rapid deterioration
   (and improvement) in channel properties, since it is not restricted
   to moving to adjacent rates.

   In "CARA: Collision-Aware Rate Adaptation for IEEE 802.11 WLANs"
   [CARA], the authors propose Collision-Aware Rate Adaptation (CARA).
   This involves utilization of Clear Channel Assessment (CCA) along
   with adaptation of the Request-to-Send/Clear-to-Send (RTS/CTS)
   mechanism to differentiate losses caused by frame collisions from
   losses caused by channel conditions.  Rather than decreasing rate as
   the result of frame loss due to collisions, which leads to increased
   contention, CARA selectively enables RTS/CTS (e.g., after a frame
   loss), reducing the likelihood of frame loss due to hidden stations.
   CARA can also utilize CCA to determine whether a collision has
   occurred after a transmission; however, since CCA may not detect a
   significant fraction of all collisions (particularly when
   transmitting at low rate), its use is optional.  As compared with
   ARF, in simulations the authors show large improvements in aggregate
   throughput due to addition of adaptive RTS/CTS, and additional modest
   improvements with the additional help of CCA.

   In "Robust Rate Adaptation for 802.11 Wireless Networks" [Robust],
   the authors implemented the ARF, AARF, and SampleRate [SampleRate]
   algorithms on a programmable Access Point platform, and

   experimentally examined the performance of these algorithms as well
   as the ONOE [ONOE] algorithm implemented in MadWiFi.  Based on their
   experiments, the authors critically examine the assumptions
   underlying existing rate negotiation algorithms:

   Decrease transmission rate upon severe frame loss
        Where severe frame loss is due to channel conditions, rate
        reduction can improve throughput.  However, where frame loss is
        due to contention (such as from hidden stations), reducing
        transmission rate increases congestion, lowering throughput and
        potentially leading to congestive collapse.  Instead, the
        authors propose adaptive enabling of RTS/CTS so as to reduce
        contention due to hidden stations.  Once RTS/CTS is enabled,
        remaining losses are more likely to be due to channel
        conditions, providing more reliable guidance on increasing or
        decreasing transmission rate.

   Use probe frames to assess possible new rates
        Probe frames reliably estimate frame loss at a given rate unless
        the sample size is sufficient and the probe frames are of
        comparable length to data frames.  The authors argue that rate
        adaptation schemes such as SampleRate are too sensitive to loss
        of probe packets.  In order to satisfy sample size constraints,
        a significant number of probe frames are required.  This can
        increase frame loss if the probed rate is too high, and can
        lower throughput if the probed rate is too low.  Instead, the
        authors propose assessment of the channel condition by tracking
        the frame loss ratio within a window of 5 to 40 frames.

   Use consecutive transmission successes/losses to increase/decrease
        rate
        The authors argue that consecutive successes or losses are not a
        reliable basis for rate increases or decreases; greater sample
        size is needed.

   Use PHY metrics like SNR to infer new transmission rate
        The authors argue that received signal to noise ratio (SNR)
        routinely varies 5 dB per packet and that variations of 10-14 dB
        are common.  As a result, rate decisions based on SNR or signal
        strength can cause transmission rate to vary rapidly.  The
        authors question the value of such rapid variation, since
        studies such as [Aguayo] show little correlation between SNR and
        frame loss probability.  As a result, the authors argue that
        neither received signal strength indication (RSSI) nor
        background energy level can be used to distinguish losses due to
        contention from those due to channel conditions.  While multi-
        path interference can simultaneously result in high signal
        strength and frame loss, the relationship between low signal

        strength and high frame loss is stronger.  Therefore,
        transmission rate decreases due to low received signal strength
        probably do reflect sudden worsening in channel conditions,
        although sudden increases may not necessarily indicate that
        channel conditions have improved.

   Long-term smoothened operation produces best average performance
        The authors present evidence that frame losses more than 150 ms
        apart are uncorrelated.  Therefore, collection of statistical
        data over intervals of 1 second or greater reduces
        responsiveness, but does not improve the quality of transmission
        rate decisions.  Rather, the authors argue that a sampling
        period of 100 ms provides the best average performance.  Such
        small sampling periods also argue against use of probes, since
        probe packets can only represent a fraction of all data frames
        and probes collected more than 150 ms apart may not provide
        reliable information on channel conditions.

   Based on these flaws, the authors propose the Robust Rate Adaptation
   Algorithm (RRAA).  RRAA utilizes only the frame loss ratio at the
   current transmission rate to determine whether to increase or
   decrease the transmission rate; PHY layer information or probe
   packets are not used.  Each transmission rate is associated with an
   estimation window, a maximum tolerable loss threshold (MTL) and an
   opportunistic rate increase threshold (ORI).  If the loss ratio is
   larger than the MTL, the transmission rate is decreased, and if it is
   smaller than the ORI, transmission rate is increased; otherwise
   transmission rate remains the same.  The thresholds are selected in
   order to maximize throughput.  Although RRAA only allows movement
   between adjacent transmission rates, the algorithm does not require
   collection of an entire estimation window prior to increasing or
   decreasing transmission rates; if additional data collection would
   not change the decision, the change is made immediately.

   The authors validate the RRAA algorithm using experiments and field
   trials; the results indicate that RRAA without adaptive RTS/CTS
   outperforms the ARF, AARF, and Sample Rate algorithms.  This occurs
   because RRAA is not as sensitive to transient frame loss and does not
   use probing, enabling it to more frequently utilize higher
   transmission rates.  Where there are no hidden stations, turning on
   adaptive RTS/CTS reduces performance by at most a few percent.
   However, where there is substantial contention from hidden stations,
   adaptive RTS/CTS provides large performance gains, due to reduction
   in frame loss that enables selection of a higher transmission rate.

   In "Efficient Mobility Management for Vertical Handoff between WWAN
   and WLAN" [Vertical], the authors propose use of signal strength and
   link utilization in order to optimize vertical handoff.  WLAN to WWAN

   handoff is driven by SSI decay.  When IEEE 802.11 SSI falls below a
   threshold (S1), Fast Fourier Transform (FFT)-based decay detection is
   undertaken to determine if the signal is likely to continue to decay.
   If so, then handoff to the WWAN is initiated when the signal falls
   below the minimum acceptable level (S2).  WWAN to WLAN handoff is
   driven by both PHY and MAC characteristics of the IEEE 802.11 target
   network.  At the PHY layer, characteristics such as SSI are examined
   to determine if the signal strength is greater than a minimum value
   (S3).  At the MAC layer, the IEEE 802.11 Network Allocation Vector
   (NAV) occupation is examined in order to estimate the maximum
   available bandwidth and mean access delay.  Note that depending on
   the value of S3, it is possible for the negotiated rate to be less
   than the available bandwidth.  In order to prevent premature handoff
   between WLAN and WWAN, S1 and S2 are separated by 6 dB; in order to
   prevent oscillation between WLAN and WWAN media, S3 needs to be
   greater than S1 by an appropriate margin.

A.2.  Internet Layer

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

   In "Analysis of link failures in an IP backbone" [Iannaccone], the
   authors investigate link failures in Sprint's IP backbone.  They
   identify the causes of convergence delay, including delays in
   detection of whether an interface is down or up.  While it is fastest
   for a router to utilize link indications if available, there are
   situations in which it is necessary to depend on loss of routing
   packets to determine the state of the link.  Once the link state has
   been determined, a delay may occur within the routing protocol in
   order to dampen link flaps.  Finally, another delay may be introduced
   in propagating the link state change, in order to rate limit link
   state advertisements, and guard against instability.

   "Bidirectional Forwarding Detection" [BFD] notes that link layers may
   provide only limited failure indications, and that relatively slow
   "Hello" mechanisms are used in routing protocols to detect failures
   when no link layer indications are available.  This results in
   failure detection times of the order of a second, which is too long
   for some applications.  The authors describe a mechanism that can be
   used for liveness detection over any media, enabling rapid detection
   of failures in the path between adjacent forwarding engines.  A path
   is declared operational when bidirectional reachability has been
   confirmed.

   In "Detecting Network Attachment (DNA) in IPv4" [RFC4436], a host
   that has moved to a new point of attachment utilizes a bidirectional
   reachability test in parallel with DHCP [RFC2131] to rapidly
   reconfirm an operable configuration.

   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 to 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.  However, this is problematic where a "Link
   Down" indication can be followed by a "Link Up" indication without a
   resulting change in IP configuration, as described in [RFC4436].

   In "Layer 2 Handoff for Mobile-IPv4 with 802.11" [Mun], the authors
   suggest that MIPv4 Registration messages be carried within
   Information Elements of IEEE 802.11 Association/Reassociation frames,
   in order to minimize handoff delays.  This requires modification to
   the mobile node as well as 802.11 APs.  However, prior to detecting
   network attachment, it is difficult for the mobile node to determine
   whether or not the new point of attachment represents a change of
   network.  For example, even where a station remains within the same
   ESS, it is possible that the network will change.  Where no change of
   network results, sending a MIPv4 Registration message with each
   Association/Reassociation is unnecessary.  Where a change of network
   results, it is typically not possible for the mobile node to
   anticipate its new CoA at Association/Reassociation; for example, a
   DHCP server may assign a CoA not previously given to the mobile node.
   When dynamic VLAN assignment is used, the VLAN assignment is not even
   determined until IEEE 802.1X authentication has completed, which is
   after Association/Reassociation in [IEEE-802.11i].

   In "Link Characteristics Information for Mobile IP" [Lee], link
   characteristics are included in registration/Binding Update messages
   sent by the mobile node to the home agent and correspondent node.
   Where the mobile node is acting as a receiver, this allows the
   correspondent node to adjust its transport parameters window more
   rapidly than might otherwise be possible.  Link characteristics that
   may be communicated include the link type (e.g., 802.11b, CDMA (Code
   Division Multiple Access), GPRS (General Packet Radio Service), etc.)
   and link bandwidth.  While the document suggests that the
   correspondent node should adjust its sending rate based on the
   advertised link bandwidth, this may not be wise in some
   circumstances.  For example, where the mobile node link is not the
   bottleneck, adjusting the sending rate based on the link bandwidth
   could cause congestion.  Also, where the transmission rate changes
   frequently, sending registration messages on each transmission rate

   change could by itself consume significant bandwidth.  Even where the
   advertised link characteristics indicate the need for a smaller
   congestion window, it may be non-trivial to adjust the sending rates
   of individual connections where there are multiple connections open
   between a mobile node and correspondent node.  A more conservative
   approach would be to trigger parameter re-estimation and slow start
   based on the receipt of a registration message or Binding Update.

   In "Hotspot Mitigation Protocol (HMP)" [HMP], it is noted that Mobile
   Ad-hoc NETwork (MANET) routing protocols have a tendency to
   concentrate traffic since they utilize shortest-path metrics and
   allow nodes to respond to route queries with cached routes.  The
   authors propose that nodes participating in an ad-hoc wireless mesh
   monitor local conditions such as MAC delay, buffer consumption, and
   packet loss.  Where congestion is detected, this is communicated to
   neighboring nodes via an IP option.  In response to moderate
   congestion, nodes suppress route requests; where major congestion is
   detected, nodes rate control transport connections flowing through
   them.  The authors argue that for ad-hoc networks, throttling by
   intermediate nodes is more effective than end-to-end congestion
   control mechanisms.

A.3.  Transport Layer

   Within the transport layer, proposals have focused on countering the
   effects of handoff-induced packet loss and non-congestive loss caused
   by lossy wireless links.

   Where a mobile host moves to a new network, the transport parameters
   (including the RTT, RTO, and congestion window) may no longer be
   valid.  Where the path change occurs on the sender (e.g., change in
   outgoing or incoming interface), the sender can reset its congestion
   window and parameter estimates.  However, where it occurs on the
   receiver, the sender may not be aware of the path change.

   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.

   In "Effect of Vertical Handovers on Performance of TCP-Friendly Rate
   Control" [Gurtov], the authors examine the effect of explicit
   handover notifications on TCP-friendly rate control (TFRC).  Where
   explicit handover notification includes information on the loss rate
   and throughput of the new link, this can be used to instantaneously
   change the transmission rate of the sender.  The authors also found
   that resetting the TFRC receiver state after handover enabled
   parameter estimates to adjust more quickly.

   In "Adapting End Host Congestion Control for Mobility" [Eddy], the
   authors note that while MIPv6 with route optimization allows a
   receiver to communicate a subnet change to the sender via a Binding
   Update, this is not available within MIPv4.  To provide a
   communication vehicle that can be universally employed, the authors
   propose a TCP option that allows a connection endpoint to inform a
   peer of a subnet change.  The document does not advocate utilization
   of "Link Up" or "Link Down" events since these events are not
   necessarily indicative of subnet change.  On detection of subnet
   change, it is advocated that the congestion window be reset to
   INIT_WINDOW and that transport parameters be re-estimated.  The
   authors argue that recovery from slow start results in higher
   throughput both when the subnet change results in lower bottleneck
   bandwidth as well as when bottleneck bandwidth increases.

   In "Efficient Mobility Management for Vertical Handoff between WWAN
   and WLAN" [Vertical], the authors propose a "Virtual Connectivity
   Manager", which utilizes local connection translation (LCT) and a
   subscription/notification service supporting simultaneous movement in
   order to enable end-to-end mobility and maintain TCP throughput
   during vertical handovers.

   In an early version of "Datagram Congestion Control Protocol (DCCP)"
   [RFC4340], a "Reset Congestion State" option was proposed in Section
   11.  This option was removed in part because the use conditions were
   not fully understood:

      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.

   "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.  "End-to-end, Implicit 'Link-Up' Notification" [E2ELinkup]
   describes methods by which a TCP implementation that has backed off
   its retransmission timer due to frame loss on a remote link can learn
   that the link has once again become operational.  This enables
   retransmission to be attempted prior to expiration of the backed-off
   retransmission timer.

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

   "TCP Extensions for Immediate Retransmissions" [Eggert] describes how
   a transport layer implementation may utilize existing "end-to-end
   connectivity restored" indications.  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 peer.

   In "Discriminating Congestion Losses from Wireless Losses Using
   Interarrival Times at the Receiver" [Biaz], the authors propose a
   scheme for differentiating congestive losses from wireless
   transmission losses based on inter-arrival times.  Where the loss is
   due to wireless transmission rather than congestion, congestive
   backoff and cwnd adjustment is omitted.  However, the scheme appears
   to assume equal spacing between packets, which is not realistic in an
   environment exhibiting link layer frame loss.  The scheme is shown to
   function well only when the wireless link is the bottleneck, which is
   often the case with cellular networks, but not with IEEE 802.11
   deployment scenarios such as home or hotspot use.

   In "Improving Performance of TCP over Wireless Networks" [Bakshi],
   the authors focus on the performance of TCP over wireless networks
   with burst losses.  The authors simulate performance of TCP Tahoe
   within ns-2, utilizing a two-state Markov model, representing "good"
   and "bad" states.  Where the receiver is connected over a wireless
   link, the authors simulate the effect of an Explicit Bad State
   Notification (EBSN) sent by an Access Point unable to reach the
   receiver.  In response to an EBSN, it is advocated that the existing
   retransmission timer be canceled and replaced by a new dynamically

   estimated timeout, rather than being backed off.  In the simulations,
   EBSN prevents unnecessary timeouts, decreasing RTT variance and
   improving throughput.

   In "A Feedback-Based Scheme for Improving TCP Performance in Ad-Hoc
   Wireless Networks" [Chandran], the authors proposed an explicit Route
   Failure Notification (RFN), allowing the sender to stop its
   retransmission timers when the receiver becomes unreachable.  On
   route reestablishment, a Route Reestablishment Notification (RRN) is
   sent, unfreezing the timer.  Simulations indicate that the scheme
   significantly improves throughput and reduces unnecessary
   retransmissions.

   In "Analysis of TCP Performance over Mobile Ad Hoc Networks"
   [Holland], the authors explore how explicit link failure notification
   (ELFN) can improve the performance of TCP in mobile ad hoc networks.
   ELFN informs the TCP sender about link and route failures so that it
   need not treat the ensuing packet loss as due to congestion.  Using
   an ns-2 simulation of TCP Reno over 802.11 with routing provided by
   the Dynamic Source Routing (DSR) protocol, it is demonstrated that
   TCP performance falls considerably short of expected throughput based
   on the percentage of the time that the network is partitioned.  A
   portion of the problem was attributed to the inability of the routing
   protocol to quickly recognize and purge stale routes, leading to
   excessive link failures; performance improved dramatically when route
   caching was turned off.  Interactions between the route request and
   transport retransmission timers were also noted.  Where the route
   request timer is too large, new routes cannot be supplied in time to
   prevent the transport timer from expiring, and where the route
   request timer is too small, network congestion may result.

   For their implementation of ELFN, the authors piggybacked additional
   information (sender and receiver addresses and ports, the TCP
   sequence number) on an existing "route failure" notice to enable the
   sender to identify the affected connection.  Where a TCP receives an
   ELFN, it disables the retransmission timer and enters "stand-by"
   mode, where packets are sent at periodic intervals to determine if
   the route has been reestablished.  If an acknowledgment is received,
   then the retransmission timers are restored.  Simulations show that
   performance is sensitive to the probe interval, with intervals of 30
   seconds or greater giving worse performance than TCP Reno.  The
   effect of resetting the congestion window and RTO values was also
   investigated.  In the study, resetting the congestion window to one
   did not have much of an effect on throughput, since the
   bandwidth/delay of the network was only a few packets.  However,
   resetting the RTO to a high initial value (6 seconds) did have a
   substantial detrimental effect, particularly at high speed.  In terms
   of the probe packet sent, the simulations showed little difference

   between sending the first packet in the congestion window, or
   retransmitting the packet with the lowest sequence number among those
   signaled as lost via the ELFNs.

   In "Improving TCP Performance over Wireless Links" [Goel], the
   authors propose use of an ICMP-DEFER message, sent by a wireless
   Access Point on failure of a transmission attempt.  After exhaustion
   of retransmission attempts, an ICMP-RETRANSMIT message is sent.  On
   receipt of an ICMP-DEFER message, the expiry of the retransmission
   timer is postponed by the current RTO estimate.  On receipt of an
   ICMP-RETRANSMIT message, the segment is retransmitted.  On
   retransmission, the congestion window is not reduced; when coming out
   of fast recovery, the congestion window is reset to its value prior
   to fast retransmission and fast recovery.  Using a two-state Markov
   model, simulated using ns-2, the authors show that the scheme
   improves throughput.

   In "Explicit Transport Error Notification (ETEN) for Error-Prone
   Wireless and Satellite Networks" [Krishnan], the authors examine the
   use of explicit transport error notification (ETEN) to aid TCP in
   distinguishing congestive losses from those due to corruption.  Both
   per-packet and cumulative ETEN mechanisms were simulated in ns-2,
   using both TCP Reno and TCP SACK over a wide range of bit error rates
   and traffic conditions.  While per-packet ETEN mechanisms provided
   substantial gains in TCP goodput without congestion, where congestion
   was also present, the gains were not significant.  Cumulative ETEN
   mechanisms did not perform as well in the study.  The authors point
   out that ETEN faces significant deployment barriers since it can
   create new security vulnerabilities and requires implementations to
   obtain reliable information from the headers of corrupt packets.

   In "Towards More Expressive Transport-Layer Interfaces" [Eggert2],
   the authors propose extensions to existing network/transport and
   transport/application interfaces to improve the performance of the
   transport layer in the face of changes in path characteristics
   varying more quickly than the round-trip time.

   In "Protocol Enhancements for Intermittently Connected Hosts"
   [Schuetz], the authors note that intermittent connectivity can lead
   to poor performance and connectivity failures.  To address these
   problems, the authors combine the use of the Host Identity Protocol
   (HIP) [RFC4423] with a TCP User Timeout Option and TCP Retransmission
   trigger, demonstrating significant improvement.

A.4.  Application Layer

   In "Application-oriented Link Adaptation for IEEE 802.11"
   [Haratcherev2], rate information generated by a link layer utilizing
   improved rate adaptation algorithms is provided to a video
   application, and used for codec adaptation.  Coupling the link and
   application layers results in major improvements in the Peak Signal
   to Noise Ratio (PSNR).  Since this approach assumes that the link
   represents the path bottleneck bandwidth, it is not universally
   applicable to use over the Internet.

   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 SIP for Instant Messaging and Presence
   Leveraging Extensions (SIMPLE) [RFC3428] or Extensible Messaging and
   Presence Protocol (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.

Appendix B.  IAB Members at the Time of This Writing

   Bernard Aboba
   Loa Andersson
   Brian Carpenter
   Leslie Daigle
   Elwyn Davies
   Kevin Fall
   Olaf Kolkman
   Kurtis Lindqvist
   David Meyer
   David Oran
   Eric Rescorla
   Dave Thaler
   Lixia Zhang

Author's Address

   Bernard Aboba, Ed.
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

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

   IAB

   EMail: iab@iab.org
   URI:   http://www.iab.org/

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