One document matched: draft-iab-link-indications-01.txt
Differences from draft-iab-link-indications-00.txt
Network Working Group B. Aboba, Ed.
INTERNET-DRAFT Internet Architecture Board
Category: Informational IAB
<draft-iab-link-indications-01.txt>
10 January 2005
Architectural Implications of Link Indications
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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This Internet-Draft will expire on July 22, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document provides an overview of the role of link indications
within the Internet Architecture. While the judicious use of link
indications can provide performance benefits, experience has also
shown that that inapropriate 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 layer indications.
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Table of Contents
1. Introduction.............................................. 3
1.1 Requirements ....................................... 3
1.2 Terminology ........................................ 3
1.3 Link Indications ................................... 5
1.4 Proposals .......................................... 8
1.5 Layering ........................................... 10
2. Architectural considerations ............................. 14
2.1 Model Validation ................................... 14
2.2 Robustness ......................................... 17
2.3 Effectiveness ...................................... 20
2.4 Interoperability Issues ............................ 21
2.5 Race Conditions .................................... 22
2.6 Layer Compression .................................. 24
2.7 Transport of Link Indications ...................... 25
3. Future Work .............................................. 27
4. Security Considerations .................................. 28
5. References ............................................... 28
5.1 Informative References ............................. 28
Appendix A - IAB Members ..................................... 32
Intellectual Property Statement .............................. 33
Disclaimer of Validity ....................................... 33
Copyright Statement .......................................... 33
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1. Introduction
As a performance optimization, proposals have been made for utilizing
link indications (also known as "triggers" or "hints") to influence
the behavior of the Internet, Transport or Application layers.
This document provides an overview of the role of link indications
within the Internet Architecture. While the judicious use of link
indications can provide performance benefits, experience has also
shown that that inapropriate use can degrade both robustness and
performance.
This document summarizes the current understanding of the role of
link indications, and provides advice to document authors considering
the role of link indications within their own work.
In Section 1 of this document we present a brief overview of research
on link behavior as well as proposals for utilization of link
indications. Section 2 provides advice to document authors. Section
3 describes recommendations and future work.
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 distribution services, via the
wireless medium (WM) for associated stations.
Association
The service used to establish an access point/station (AP/STA)
mapping and enable STA access to the Distribution System.
Basic Service Set (BSS)
A set of stations controlled by a single coordination function, where
the coordination function may be centralized (e.g., in a single AP)
or distributed (e.g., for an ad-hoc network). The BSS can be thought
of as the coverage area of a single AP.
Care of Address (CoA)
A unicast routable address associated with a mobile node while
visiting a foreign link; the subnet prefix of this IP address is a
foreign subnet prefix. Among the multiple care-of addresses that a
mobile node may have at any given time (e.g., with different subnet
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prefixes), the one registered with the mobile node's home agent for a
given home address is called its "primary" care-of address.
Correspondent Node
A peer node with which a mobile node is communicating. The
correspondent node may be either mobile or stationary.
Distribution System (DS)
A system used to interconnect a set of basic service sets (BSSs) and
integrated local area networks (LANs) to create an extended service
set (ESS).
Dynamic Host Configuration Protocol (DHCP) client
A DHCP client is an Internet host using DHCP to obtain configuration
parameters such as a network address.
DHCP server
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
Extended Service Set (ESS)
A set of one or more interconnected basic service sets (BSSs) and
integrated local area networks (LANs) that appears as a single BSS to
the logical link control layer at any station associated with one of
those BSSs. The ESS can be thought of as the coverage area provided
by a collection of APs all interconnected by the Distribution System.
It may consist of one or more IP subnets.
Home Address (HoA)
A unicast routable address assigned to a mobile node, used as the
permanent address of the mobile node. This address is within the
mobile node's home link. Standard IP routing mechanisms will deliver
packets destined for a mobile node's home address to its home link.
Mobile nodes can have multiple home addresses, for instance when
there are multiple home prefixes on the home link.
Inter-Access Point Protocol (IAPP)
A protocol used between access points that assures that the station
may only be connected to a single AP within the ESS at a time, and
also provides for transfer of context to the new AP.
Link
A communication facility or medium over which nodes can communicate
at the link layer, such as an Ethernet (simple or bridged). The link
layer is the layer immediately below IP.
Link indication
Information provided by the link layer to higher layers relating to
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the state of the link.
Mobile Node
A node that can change its point of attachment from one link to
another, while still being reachable via its home address.
Point of Attachment
A location within the network where a host may be connected. This
attachment point can be characterized by its address prefix and next
hop routing information.
Most Likely Point of Attachment (MLPA)
The point of attachment heuristically determined by the host to be
most likely, based on hints from the network.
Routable address
In this specification, the term "routable address" refers to any
address other than an IPv4 Link-Local address [RFC3927]. This
includes private addresses as specified in [RFC1918].
Station (STA)
Any device that contains an IEEE 802.11 conformant medium access
control (MAC) and physical layer (PHY) interface to the wireless
medium (WM).
Valid address
The term "valid address" refers to either a static address, or a
dynamically assigned address which has not been relinquished, and has
not expired.
Weak End-System Model
In the Weak End-System Model, packets sent out an interface need not
necessarily have a source address configured on that interface.
1.3. Link Indications
A link indication represents information provided by the link layer
to higher layers relating to the state of the link. While link
indications vary considerably between media, abstraction models have
been proposed. For example, [GenTrig] defines "generic triggers",
including "Link Up", "Link Down", "Link Going Down", "Link Going Up",
"Link Quality Crosses Threshold", "Trigger Rollback", and "Better
Signal Quality AP Available". Other link indications include the
current link rate (which may vary with time and location), link
identifiers (e.g. SSID, BSSID in 802.11), and statistics relating to
link performance (such as the delay or loss rate).
"Link Up" and "Link Down" indications were first developed for wired
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networks, and assume an idealized link behavior model. This model
assumes that links in the "up" state experience low frame loss in
both directions and are ready to send and receive data frames.
Similarly it is assumed that links in the "down" state are unsuitable
for sending and receiving data frames in either direction.
Link indications based on signal quality, such as "Link Doing Down",
"Link Going Up", and "Link Quality Crosses Threshold" are primarily
intended for use in handoff optimization. These indications assume
an idealized model of radio propagation, where signal strength varies
smoothly and frame loss is well predicted by signal strength and
distance.
A number of link performance studies shed light on the applicability
of these assumptions. For the purposes of illustration, we will
focus on literature relating to IEEE 802.11.
In "Measurement and Analysis of the Error Characteristics of an In-
Building Wireless Network" [Eckhardt], the authors characterize the
performance of an AT&T Wavelan 2 Mbps in-building WLAN operating in
Infrastructure mode on the Carnegie-Mellon Campus. In this study,
very low frame loss was experienced. As a result, links could either
be assumed to operate very well or not at all.
In "Performance of Multihop Wireless Networks: Shortest Path is Not
Enough" [Shortest] the authors studied the performance of both an
indoor and outdoor mesh network. By measuring inter-node throughput,
the best path between nodes was computed. The throughput of the best
path was compared with the throughput of the shortest path computed
based on a hop-count metric. In almost all cases, the shortest path
route offered considerably lower throughput than the best path.
In examining link behavior, the authors found that rather than
exhibiting a bi-modal distribution between "up" (low loss rate) and
"down" (high loss rates), many links exhibited intermediate loss
rates. Asymmetry was also common, with 30 percent of links
demonstrating substantial differences between in the loss rates in
each direction. As a result, on wireless networks the measured
throughput can differ substantially from the negotiated rate due to
retransmissions, and successful delivery of routing packets is not
necessarily an indication that the link is useful for delivery of
data.
"Link-level Measurements from an 802.11b Mesh Network" [Aguayo]
analyzes the causes of frame loss in a 38-node urban multi-hop 802.11
ad-hoc network. In most cases, links that are very bad in one
direction tend to be bad in both directions, and links that are very
good in one direction tend to be good in both directions. However,
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30 percent of links exhibited loss rates differing substantially in
each direction.
Signal to noise ratio 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
loss conditions observed in practice. There is a fundamental
difference between indoor infrastructure networks in which site
surveys and careful measurement can assist in promoting ideal
behavior and ad-hoc/mesh networks in which node mobility and external
factors such as weather may not be easily controlled.
In "The mistaken axioms of wireless-network research" [Kotz], the
authors conclude that mistaken assumptions relating to link
performance may lead to the design of network protocols that may not
work in practice. For example, [Kotz] notes that the three-
dimensional nature of wireless propagation can result in large signal
strength changes over short distances, generating short-lived "Link
Down" and "Link Up" indications that are not be predicted by a two
dimensional radio propagation model.
The literature also describes variations in link indication behavior
between implementations.
"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 phases
(connection to an alternative point of attachment). 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, 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
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reduced by initiating the search phase as soon as collisions can be
excluded as the cause of non-acknowledged transmissions; the authors
recommend three consecutive transmission failures as the cutoff.
Where the STA is not sending 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
STAs implementations communicating with two APs. The study found
that there is large variation in handoff latency among STA and AP
implementations and that implementations utilize different message
sequences. For example, one STA sends a Reassociation Request prior
to authentication, which results in receipt of a Deauthenticate
message. The study divided handoff latency into discovery,
authentication and reassociation exchanges, concluding that the
discovery phase was the dominant component of handoff delay. Latency
in the detection phase was not investigated.
"Roaming Interval Measurements" [Alimian] presents data on stationary
STAs after the AP signal has been shut off. This study highlighted
implementation differences in rate adaptation as well as detection,
scanning and handoff. As in [Velayos], performance varied widely
between implementations, from half an order 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 AP is gradually attenuated, which is more
representative of field experience than the shutoff techniques used
in [Velayos]. Stations were configured to initiate handover when
signal strength dipped below a threshold, rather than purely based on
frame loss, so that they could begin handover while still connected
to the current AP. It was noted that stations continue to receive
data frames during the search phase. Station-initiated
Disassociation and pre-authentication were not observed in this
study.
1.4. Proposals
Within the Internet layer, proposals have been made for utilizing
link indications to optimize IP configuration, to improve the
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usefulness of routing metrics, and to optimize aspects of Mobile IP
handoff.
In "Detection of Network Attachment (DNA) in IPv4" [DNAv4], link
indications are utilized to optimize Internet layer configuration.
This enables a host that has moved to a new point of attachment but
remained within the same subnet to rapidly confirm a currently valid
configuration, rather than utilizing the DHCP protocol [RFC2131].
"A High-Throughput Path Metric for Multi-Hop Wireless Routing" [ETX]
describes how routing metrics can be improved by taking link layer
frame loss rates into account, enabling the selection of routes
maximizing available throughput. While the proposed routing metric
utilizes the Expected Transmission Count (ETX), it does not take the
negotiated rate into account, although this was noted as a subject
for further study.
In "L2 Triggers Optimized Mobile IPv6 Vertical Handover: The
802.11/GPRS Example" [Park] the authors propose that the mobile node
send a router solicitation on receipt of a "Link Up" indication in
order provide lower handoff latency than would be possible using
generic movement detection [RFC3775]. The authors also suggest
immediate invalidation of the Care-Of-Address (CoA) on receipt of a
"Link Down" indication.
Within the Transport layer, proposals have focused on countering the
effects of handoff-induced packet loss. This includes proposals for
improving transport parameter estimation, as well as triggering
immediate retransmission on availability of an interface or
intervening link.
"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.
In "TCP Extensions for Immediate Retransmissions" [Eggert], it is
proposed that in addition to regularly scheduled retransmissions that
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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 "The BU-trigger method for improving TCP performance over Mobile
IPv6" [Kim], the authors note that handoff-related packet loss is
interpreted as congestion by the Transport layer. In the case where
the correspondent node is sending to the mobile node, it is proposed
that receipt of a Binding Update by the correspondent node be used as
a signal to the Transport layer to adjust cwnd and ssthresh values,
which may have been reduced due to handoff-induced packet loss. The
authors recommend that cwnd and ssthresh be recovered to pre-timeout
values, regardless of whether the link parameters have changed. The
paper does not discuss the behavior of a mobile node sending a
Binding Update, in the case where the mobile node is sending to the
correspondent node.
At the Application layer, the usage of "Link Down" indications has
been proposed to augment presence systems. In such systems, client
devices periodically refresh their presence state using application
layer protocols such as SIMPLE [RFC3428] or XMPP [RFC3921]. If the
client should become disconnected, their unavailability will not be
detected until the presence status times out, which can take many
minutes. However, if a link goes down, and a disconnect indication
can be sent to the presence server (presumably by the access point,
which remains connected), the status of the user's communication
application can be updated nearly instantaneously.
1.5. Layering
A layered indication model is shown in Figure 1 which includes both
internally generated link indications and indications arising from
external interactions (such as receipt of Mobile IP Binding Updates,
and detection of path changes via routing protocols and TTL changes).
In this model, link indications include frame loss (before
retransmissions), the current link rate, the link state (up/down),
and link identifiers. The indications are inter-dependent, since
rate adjustment and detection algorithms are typically influenced by
frame loss, and in turn a "Link Down" indication may be influenced by
the detection and search process. Link Identifiers are typically
obtained in the process of bringing the link up.
The Internet layer is the primary consumer of link indications, since
one of its functions is to shield applications from the specifics of
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link behavior. The Internet layer may utilize link indications to
optimize aspects of IP configuration, routing and mobility. As noted
in [DNAv4], "Link Up" indications and link identifiers may be useful
in validating the IP configuration. Once the IP configuration is
confirmed, it may be determined that an address change has occurred.
As described in [ETX], the frame loss rate as well as the current
link rate may be utilized in the calculation of routing metrics.
Within "Weak End-System Model" implementations, changes in routing
metrics may in turn result in a change in the outgoing interface for
one or more transport connections. Routes may also be added or
withdrawn, resulting in loss or gain of peer connectivity. The
Internet layer may also become aware of path changes by other
mechanisms, such as by running a routing protocol, receipt of a
Router Advertisement or a change in the IP TTL 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).
However, "Link Up" indications need not result in a change to
Internet layer configuration, and changes in link rate or frame loss
need not result in a change of outgoing interface. By filtering
"Link Up" indications, and selecting outgoing and incoming interfaces
based on the link rate and frame loss, the Internet layer enables
upper layers to avoid writing their own code to filter and validate
link indications.
The Transport layer processes Internet layer and link indications
differently for the purposes of transport parameter estimation and
connection management. For the purposes of parameter estimation, the
Transport layer may be interested in a wide range of Internet and
link layer indications. The Transport layer may wish to use path
change indications from the Internet layer in order to rest parameter
estimates. It may also be useful for the Transport layer to consume
link layer indications such as link rate, frame loss rate and "Link
Up"/"Link Down" in order to improve transport parameter estimates.
However at this point, the algorithms for improving transport
parameter estimates using link layer indications are not well
understood. For example, in transport parameter estimation, layering
considerations may not exist to the same extent as in connection
management. For example, the Internet layer may receive a "Link
Down" indication followed by a subsequent "Link Up" indication. This
information may useful for transport parameter estimation even if IP
configuration does not change, since it may indicate that packet loss
is not caused by congestion.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Application | |
Layer | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | |
| + | | |
| | ^ ^ |
Transport | Transport Parameter + | Teardown |
Layer | Estimation | | |
| (MTU, RTT, RTO, cwnd, + Conxn.| Management|
| ssthresh, Reset) | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^ ^ ^ ^
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | Incoming | MIP | | |
| | | Interface | BU | | |
| | | Change |Receipt| | |
| | ^ ^ ^ ^ |
| | | | | | |
| | | | | | |
| | | Mobility | | | |
Internet | | | | | | |
Layer +-+- -+- - - - - -+- -+- -+- - - - -+- - - - - -+
| | | Outgoing | | | | IP |
| | | Interface | + | | Address |
| | ^ Change ^ | ^ ^ Config/ |
| | Path + | Changes |
| | Change | | |
| | Routing + | IP Configuration |
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ^ ^ ^
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
Link | V ^ ^ ^ |
Layer + Frame -> Rate -> Link Link +
| Loss Adjustment Up/Down Identifiers |
| Rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Layered Indication Model
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For the purposes of connection management, the Transport layer
typically only utilizes Internet layer indications such as changes in
the incoming/outgoing interface and IP configuration changes. For
example, the Transport layer may tear down transport connections due
to invalidation of a connection endpoint IP address.
However, even where an Internet layer configuration change has
occurred, the configuration change may not be relevant for the
purposes of connection management. For example, where the connection
has been established based on the 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.
Since a "Link Up" indication may or may not result in a change in
Internet layer configuration, the Transport layer cannot draw
conclusions about the implications of "Link Up" for connection
management until the Internet layer has determined whether a
configuration change has occurred.
Similarly, 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 PPP LCP-Terminate or an 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.
In addition to Internet layer indications propagated to the
Application layer (such as IP address configuration and changes), the
Transport layer provides its own indications to the Application
layer, such as connection teardown. The Transport layer may also
provide indications to the link layer. For example, to prevent
excessive retransmissions within the link layer, 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 802.11, this can
be achieved by adjusting the MIB variables dot11ShortRetryLimit
(default: 7) and dot11LongRetryLimit (default: 4), which control the
maximum number of retries for frames shorter and longer in length
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than dot11RTSThreshold, respectively.
In most cases applications can obtain the information they need from
Internet and Transport layer indications so that they do not need to
directly consume link indications. For example, a "Link Up"
indication typically only implies that the link was suitable for
sending and receiving link layer control frames, not that it has been
configured for and is capable of reliably sending and receiving IP
data packets. As a result, applications will typically consume an
Internet layer "IP Address Configured" event instead of a "Link Up"
indication. Similarly, it is typically not useful for applications
to consume "Link Down" indications, since these indications can be
transient in nature. Instead, applications should consume Transport
layer teardown indications.
2. Architectural considerations
While the literature on the usage of Link indications provides
persuasive evidence of their utility, experience shows that a number
of difficulties can arise in making effective use of them. These
issues include:
a. Model validation
b. Robustness
c. Effectiveness
d. Interoperability
e. Race conditions
f. Layer compression
g. Transport of link indications
The sections that follow discuss each of these issues in turn.
2.1. Model Validation
Authors need to be careful to avoid use of simplified link models in
circumstances where they do not apply. In order to avoid the
pitfalls described in [Kotz], documents dependent on link indications
should explicitly articulate the assumptions of the link model and
describe the circumstances in which it applies.
For example, generic "trigger" models often include implicit
assumptions. The use of "Link Up" and "Link Down" indications
implies that a link is either in a state experiencing low frame loss
("up") or in a state where few frames are successfully delivered
("down"). Symmetry may also be assumed, so that the link is either
"up" in both directions or "down" in both directions.
Link indications based on signal quality, such as "Link Doing Down",
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"Link Going Up", and "Link Quality Crosses Threshold" typically
assume the absence of multi-path interference, so that signal to
noise ratio varies smoothly in space, and frame loss is well
predicted by signal strength and distance.
In wireless networks, particularly in outdoor or mesh deployments,
the assumptions underlying generic trigger models may prove invalid.
Where links may exist in intermediate states between "up" and "down"
or asymmetry is encountered, generic "triggers" such as "Link Going
Down", "Link Going Up", and "Link Quality Crosses Threshold" may be
difficult to reliably define and may be unreliable predictors of
future link performance.
Once the network model is defined, considerable effort may be
required to define the link indications model for a given link layer.
For example, the definition of "Link Up" or "Link Down" varies
considerably between link layers. Within PPP [RFC1661], either peer
may send an LCP-Terminate frame in order to terminate the PPP link
layer, and a link may only be assumed to be usable for sending
network protocol packets once NCP negotiation has completed for that
protocol.
Unlike PPP, IEEE 802 does not include facilities for network layer
configuration, and the definition of "Link Up" and "Link Down" varies
between and even within Link types. For example, in IEEE 802.11, the
definition of "Link Up" and "Link Down" depends on whether the
station is mobile or stationary, whether infrastructure or ad-hoc
mode is in use, and whether security and Inter-Access Point Protocol
(IAPP) is implemented.
Where a mobile 802.11 STA encounters a series of consecutive non-
acknowledged frames, the most likely cause is that the station has
moved out of range of the AP. As a result, [Velayos] recommends that
the station begin the search phase after collisions can be ruled out,
after three consecutive non-acknowledged frames. Only when no
alternative 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
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"Link Down" indication may be returned later.
In 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 [IEEE80211F], 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 [IEEE80211i] forwarding of frames from the station to the
distribution system is only feasible after the completion of the
4-way handshake and group-key handshake, so that entering State 3 is
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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 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.
2.2. Robustness
In some situations, improper use of Link indications can result in
operational malfunctions. Given the potential problems, proposals
for consideration of link indications must demonstrate robustness
against misleading indications. Elements to consider include:
a. Indication validation
b. Recovery from invalid indications
c. Damping and hysteresis
2.2.1. Indication Validation
Radio propagation and implementation differences can impact the
reliability of Link indications.
As described in [Aguayo], wireless links often exhibit loss rates
intermediate between "up" (low loss) and "down" (high loss) states,
as well as substantial asymmetry. In these circumstances, a "Link
Up" indication may not imply bi-directional reachability. Also, a
reachability demonstration based on small packets may not mean that
the link is suitable for carrying larger data packets. As a result,
"Link Up" and "Link Down" indications may not reliably determine
whether a link is suitable for carrying IP data packets.
Where multi-path interference or hidden nodes are encountered, frame
loss may vary widely over a short distance. While techniques such as
use of multiple antennas may be used to reduce multi-path effects and
RTS/CTS signaling can be used to address hidden node problems, these
techniques may not be completely effective. As a result, a mobile
host may find itself experiencing widely varying link conditions,
causing the link to rapidly cycle between "up" and "down" states,
with "Going down" or "Going up" indications providing little
predictive value.
Where the reliability of a link layer indication is suspect, it is
best for upper layers to treat the indication as a "hint" (advisory
in nature), rather than a "trigger" forcing a given action. In order
to provide increased robustness, heuristics can be developed to
assist upper layers in determining whether the "hint" is valid or
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should be discarded.
To provide robustness in the face of potentially misleading link
indications, in [DNAv4] "Link Up" indications are assumed to be
inherently unreliable, so that bi-directional reachability needs to
be demonstrated in the process of validating an existing IP
configuration. However, where a link exhibits an intermediate loss
rate, the success of the [DNAv4] reachability test does not guarantee
that the link is suitable for carrying IP data packets.
Another example of link indication validation occurs 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.
2.2.2. Recovery From Invalid Indications
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.
Recovery is supported within [DNAv4] in the case where link
indications may lead a host to erroneously conclude that the link
prefix remains unchanged when the host has in fact changed subnets.
In this case, the bi-directional reachability test times out, and the
host will eventually realize its mistake and obtain an IP address by
normal means.
Where a proposal involves recovery at the transport layer, the
recovered transport parameters (such as the MTU, RTT, RTO, congestion
window, etc.) must be demonstrated to remain valid. Congestion
window validation is discussed in [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 (signalling of
"Link Up" prior to completion of link layer authentication),
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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].
2.2.3. Damping and Hysteresis
Damping and hysteresis can be utilized to ensure that stability is
maintained in the face of jittery link indications. These limits
typically place constraints on the number of times a given action can
be performed within a time period or introduce damping mechanisms to
prevent instability.
While [Aguayo] found that frame loss was relatively stable for
stationary stations, obstacles to radio propagation and 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 performance, including changes in the
negotiated rate, 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 may be desirable to ignore link indications
entirely. Since it is possible for a host to transition from an ad-
hoc network to a network with centralized address management, a host
receiving a "Link Up" indication cannot necessarily conclude that it
is appropriate to configure a IPv4 Link-Local address prior to
determining whether a DHCP server is available [RFC3927].
As noted in Section 1.5, the Transport layer does not utilize "Link
Up" and "Link Down" indications for the purposes of connection
management. In most cases applications can obtain the information
they need from Internet and Transport layer indications so that they
do not need to directly consume link indications.
Where link indications are used to optimize transport performance,
authors must demonstrate that effective congestion control is
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maintained [RFC2914] in the face of rapidly changing link
indications.
Consider a proposal where a "Link Up" indication is used by a router
to trigger retransmission of the last previously sent packet, in
order to enable ACK reception prior to expiration of the host's
retransmission timer. Where "Link Up" indications follow in rapid
succession, this could 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" when link
indications are generated on a time scale of RTO or less. The
problem is magnified since for each presence update, notifications
can be delivered to many watchers. In addition, use of a "Link Up"
indication in this manner is unwise since the interface may not yet
have a valid Internet layer configuration.
The issue can be addressed by one or more of the following
techniques:
[a] Rate limiting. A limit of one packet per RTO can be imposed on
packets generated from receipt of link indications.
[b] Utilization of upper layer indications. Instead of consuming a
"Link Up" indication, applications can consume alternative upper
layer indications such as an IP address configuration/change
notifications.
[c] Keepalives. Instead of consuming a "Link Down" indication, an
application can utilize an application keepalive or consume
Transport layer indications such as connection teardown.
2.3. Effectiveness
While link indications may show promise, it may be difficult to prove
that processing of a given indication provides benefits in a wide
variety of circumstances. Where link indications are utilized for
the purpose of optimization, proposals need to carefully analyze the
effectiveness of the optimizations in the face of unreliable link
indications. Since optimizations typically bring with them increased
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complexity, an optimization that does not bring about a performance
improvement is not useful.
As with any optimization, the usefulness of link indications lies in
demonstrated effectiveness of the optimization under consideration.
This in turn may depend heavily on the penalty to be paid for false
positives and false negatives.
As noted in [DNAv4], it is simultaneously possible for a link
indication to be highly reliable and provide no net benefit,
depending on the probability of a false indication and the penalty
paid for the false indication.
In the case of [DNAv4], the benefits of successful optimization are
modest, but the penalty for falsely concluding that the subnet
remains unchanged is a lengthy timeout. The result is that link
indications may not be worth considering if they are incorrect even
just a small fraction of the time.
For example, it can be argued that a change in the Service Set
Identifier (SSID) in [IEEE80211] is not a sufficiently reliable
indication of a prefix change. Within IEEE 802.11, the Service Set
Identifier (SSID) functions as a non-unique identifier of the
administrative domain of a Wireless LAN. Since the SSID is non-
unique, many different operators may share the same SSID, and Access
Points typically ship with a default value for the SSID (e.g.
"default"). Since the SSID relates to the administrative domain and
not the network topology, multiple SSIDs may provide access to the
same prefix, and a single SSID may provide access to multiple
prefixes at one or multiple locations.
Given this, it is unreliable to use the SSID alone for the purpose of
movement detection. A host moving from one point of attachment to
another, both with the same SSID, may have remained within the same
subnet, or may have changed subnets. Similarly, a host discovering
that the SSID has changed may have changed subnets, or it may not
have. Moreover, where private address space is in use, it is
possible for the SSID, the prefix (e.g. 192.168/16) and even the
default gateway IP address to remain unchanged, yet for the host to
have moved to a different point of attachment. Were the host to make
decisions relating to configuration of the IP layer (such as address
assignment) based solely on the SSID, address conflicts are likely.
2.4. Interoperability
In general, link indications should only be incorporated by upper
layers for performance optimization, but should not be required, in
order to main link independence.
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To avoid compromising interoperability in the pursuit of performance
optimization, proposals must demonstrate that interoperability
remains possible (though potentially with degraded performance) even
if one or more participants do not implement the proposal.
For example, if link layer prefix hints are provided as a substitute
for Internet layer configuration, hosts not understanding those hints
would be unable 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).
2.5. Race Conditions
It is possible for link indications to be utilized directly by
multiple layers of the stack in situations in which strict layering
may not be observed. In these situations, it is possible for race
conditions to occur.
For example, as discussed earlier, link indications have been shown
to be useful in optimizing aspects of Internet Protocol layer
addressing and configuration as well as routing. Although [Kim]
describes situations in which link indications are first processed by
the Internet Protocol layer (e.g. MIPv6) before being consumed by the
Transport layer, for the purposes of parameter estimation, it may be
desirable for the Transport layer to consume link indications
directly.
For example, in situations where the "Weak End-System Model" is
implemented, a change of outgoing interface may occur at the same
time the Transport layer is modifying transport parameters based on
other link 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 on one of
its interfaces, a routing metric taking frame loss into account will
rise, 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 for the original interface (MTU, congestion
state) may no longer be valid for the new outgoing and incoming
paths.
To avoid race conditions, the following measures are recommended:
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a. Path change processing
b. Layering
c. Metric consistency
2.5.1. Path Change Processing
When the Internet layer detects a path change, such as a change in
the outgoing or incoming interface of the host or the incoming
interface of a peer, or perhaps a substantial change in the TTL of
received IP packets, it may be worth considering whether to reset
transport parameters to their initial values and allow them to be re-
estimated. This ensures that estimates based on the former path do
not persist after they have become invalid.
2.5.2. Layering
Another technique to avoid race conditions is to rely on layering to
damp 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. Since
one of the major objectives of the Internet layer is maintaining link
layer independence, upper layers relying on Internet layer
indications rather than consuming link indications directly can avoid
link layer dependencies.
As described in Section 1.5, it is advisable for applications to
utilize indications from the Internet or Transport layers rather than
consuming link indications directly.
2.5.3. Metric Consistency
Once a link is in the "up" state, its effectiveness in transmission
of data packets can be determined. For example, frame loss may be
used to assist in rate adjustment and to determine when to select an
alternative point of attachment. Also, the effective throughput
depends on the negotiated rate and frame loss, and can be used in
calculation of the routing metric, as described in [ETX].
However, prior to sending data packets over the link, other metrics
are required to determine suitability. 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 data packets.
Since the negotiated rate and frame loss typically cannot be
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predicted prior to utilizing the link for data traffic, existing
implementations often utilize metrics such as signal strength and
access point load in handoff decisions. The "Link Going Down",
"Link Going Up", "Link Quality Crosses Threshold" indications were
developed primarily to assist with handoff between interfaces, and
are oriented toward inferred rather than measured suitability.
Research indicates that this approach may have some promise. In
order to enable stations to roam prior to encountering packet loss,
studies such as [Vatn] have suggested using signal strength as a
detection mechanism, rather than frame loss, as suggested in
[Velayos]. [Vertical] proposes use of signal strength and link
utilization in order to optimize vertical handoff and demonstrates
improved TCP throughput.
However, without careful design, potential differences between link
indications used in routing and those used in roaming and/or link
enablement can result in instability, particularly in multi-homed
hosts. For example, receipt of "Link Going Down" or "Link Quality
Crosses Threshold" indications could be used as a signal to enable
another interface. However, unless the new interface is the
preferred route for one or more destination prefixes, a "Weak End-
System" implementation will not use the new interface for outgoing
traffic. Where "idle timeout" functionality is implemented, the
unused interface will be brought down, only to be brought up again by
the link enablement algorithm.
As noted in [Aguayo], signal strength and distance are not good
predictors of frame loss or negotiated rate, due to the potential
effects of multi-path interference. As a result a link brought up
due to good signal strength may subsequently exhibit significant
frame loss, and a low negotiated rate. Similarly, an AP
demonstrating low utilization may not necessarily be the best choice,
since utilization may be low due to hardware or software problems.
As noted in [Villamizar], link utilization-based routing metrics have
a history of instability, so that they are rarely deployed.
2.6. Layer compression
In many situations, the exchanges required for a host to complete a
handoff and reestablish connectivity are considerable. This includes
link layer scanning, authentication and connectivity establishment;
Internet layer configuration, routing and mobility exchanges;
Transport layer retransmission and recovery; security association re-
establishment; application protocol re-authentication and re-
registration exchanges, etc. Given this, it is natural to consider
combining exchanges occurring within multiple layers into a single
exchange.
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Often this combined exchange occurs within the link layer. For
example, in [EAPIKEv2], a link layer EAP exchange may be used for the
purpose of IP address assignment, potentially bypassing Internet
layer configuration. Within [PEAP], it is proposed that a link layer
EAP exchange be used for the purpose of carrying Mobile IPv6 Binding
Updates. [MIPEAP] proposes that EAP exchanges be used for
configuration of Mobile IPv6.
While the goals of layer compression are laudable, care needs to be
taken to avoid compromising interoperability and introducing link
layer dependencies into the Internet and Transport layers. For
example, where link layer and Internet or Transport layer mechanisms
are combined, it is necessary for hosts to maintain the ability to
interoperate without layer compression schemes, in order to permit
operation on networks where they are not available.
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,
[IEEE8021X] enables the 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.
2.7. Transport of Link Indications
Proposals including the transport of link indications beyond the
local host need to carefully consider the layering, security and
transport implications.
In general, implicit signals are preferred to explicit transport of
link indications since they add no new packets in times of network
distress, operate more reliably in the presence of middle boxes such
as NA(P)Ts, and are more likely to be backward compatible.
While facilities such as ICMP "source quench" were originally
provided at the Internet layer, these facilities have fallen into
disuse due to their questionable value for the Transport layer. In
general, the Transport layer is able to determine an appropriate (and
conservative) response to congestion based on packet loss or explicit
congestion notification, so that ICMP "source quench" indications are
not needed, and in fact the sending of additional "source quench"
packets during periods of congestion may be detrimental.
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Where explicit signalling is required, existing facilities should be
used rather than creating new ones. For example, "TCP Extensions for
Immediate Retransmissions" [Eggert] describes how a Transport layer
implementation may utilize existing "end-to-end connectivity
restored" indications.
For example, routing metrics incorporating link layer indications
such as [ETX] enable hosts participating in the routing mesh to gain
knowledge of path changes and remote link conditions. This can be
accomplished securely if routing protocol security is implemented.
When a link experiences frame loss, routing metrics incorporating
frame loss increase, possibly resulting in selection of an alternate
route. If the troubled link represents the only path to a prefix and
the link experiences high frame loss ("down"), the route will be
withdrawn or the metric will become infinite. Similarly, when the
link becomes operational, the route will appear again.
Proposals involving transport of link indications need to demonstrate
the following:
[a] Absence of alternatives. By default, alternatives not requiring
explicit signalling are preferred. Where these solutions are
shown to be inadequate, proposals must prove that existing
explicit signalling mechanisms (such as path change processing and
link-aware routing metrics) are inadequate.
[b] Conservative behavior. Due to experience with ICMP "source
quench", proposals must demonstrate that they do not violate
conservation of packets.
[c] Security. Proposals need to describe how security issues can be
addressed. Where link indications are transported over the
Internet, an attack can be launched without requiring access to
the link.
[d] Identifiers. When link indications are transported, it is
generally for the purposes of saying something about Internet,
Transport or Application layer operations at a remote element.
These layers use different identifiers, and so it is necessary to
match the link indication with relevant higher layer state.
Therefore proposals need to demonstrate how the link indication
can be mapped to the right higher layer state.
For example, if a presence server is receiving remote indications
of "Link Up"/"Link Down" status for a particular MAC address, the
presence server will need to associate that MAC address with the
identity of the user (pres:user@example.com) to whom that link
status change is relevant.
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3. Future Work
While Figure 1 presents an overview of how link indications are
consumed by the Internet, Transport and Application layers, further
work is needed to investigate this in more detail.
Since recent proposals such as [IEEE80211e] incorporate burst ACKs,
the relationship between 802.11 link throughput and frame loss is
growing more complex, which may necessitate the development of
revised routing metrics, taking the more complex transmission
behavior as well as the negotiated rate into account.
At the Link and Internet layers, more work is needed to reconcile pre
and post-connection metrics, such as reconciling metrics utilized in
handoff (e.g. signal strength and link utilization) with link-aware
routing metrics (e.g. frame loss and negotiated rate).
At the Transport layer, more work is needed to understand how to
react to Internet layer indications such as path changes. For
example, in an early draft of [DCCP], a "Reset Congestion State"
option was proposed in Section 4. This option was removed in part
because the use conditions were not fully understood:
An Half-Connection Receiver sends the Reset Congestion State option
to its sender to force the sender to reset its congestion state --
that is, to "slow start", as if the connection were beginning again.
...
The Reset Congestion State option is reserved for the very few cases
when an endpoint knows that the congestion properties of a path have
changed. Currently, this reduces to mobility: a DCCP endpoint on a
mobile host MUST send Reset Congestion State to its peer after the
mobile host changes address or path.
It may also make sense for the Transport layer to adjust transport
parameter estimates in response to "Link Up"/"Link Down" indications
and frame loss. For example, it is unclear that the Transport layer
should adjust transport parameters as though congestion were detected
when loss is occurring in the link layer or a "Link Down" indication
has been received.
Finally, more work is needed to determine how link layers may utilize
information from the Transport layer. For example, it is undesirable
for a link layer to retransmit so aggressively that the link layer
round-trip time approaches that of the end-to-end transport
connection.
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4. Security Considerations
Since link indications are typically insecure, proposals
incorporating them need to consider the potential security
implications of spoofed or modified link indications, as well as
potential denial of service attacks. This is particularly important
in situations where insecure link indications are used as a
substitute for secure mechanisms operating at a higher layer.
For example, within [IEEE80211F], "Link Up" is considered to occur
when an Access Point sends a Reassociation Response. At that point,
the AP sends a frame with the station's source address to a multicast
address, thereby causing switches within the Distribution System to
learn the station's MAC address, enabling forwarding of frames to the
station at the new point of attachment. Unfortunately, this does not
take security into account, since the station is not capable of
sending and receiving IP packets on the link until completion of the
key exchange protocol defined in [IEEE80211i]. As a result, link
indications as implemented in [IEEE80211F] enable an attacker to
disassociate a station located anywhere within the ESS, by sending a
Reassociation Request frame.
Another example of the potential security implications of link
indications occurs within DNAv4, where link indications are used for
optimization of IP configuration, rather than using a secured
configuration mechanism such as authenticated DHCP [RFC3118], thereby
increasing vulnerability to spoofing.
5. References
5.1. Informative References
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, D. and
E. Lear, "Address Allocation for Private Internets", RFC 1918,
February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2778] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
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[RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion Window
Validation", RFC 2861, June 2000.
[RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914, BCP 41,
September 2000.
[RFC3118] Droms, R. and B. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[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, October 2004.
[Alimian] Alimian, A., "Roaming Interval Measurements",
11-04-0378-00-roaming-intervals-measurements.ppt, IEEE 802.11
submission (work in progress), March 2004.
[Aguayo] Aguayo, D., Bicket, J., Biswas, S., Judd, G. and R. Morris,
"Link-level Measurements from an 802.11b Mesh Network",
SIGCOMM '04, September 2004, Portland, Oregon.
[DCCP] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", Internet drafts (work in progress),
draft-ietf-dccp-spec-08.txt, October 2004.
[DNAv4] Aboba, B., "Detection of Network Attachment in IPv4", draft-
ietf-dhc-dna-ipv4-09.txt, Internet draft (work in progress),
October 2004.
[E2ELinkup]
Dawkins, S. and C. Williams, "End-to-end, Implicit 'Link-Up'
Notification", draft-dawkins-trigtran-linkup-01.txt, Internet
draft (work in progress), October 2003.
[EAPIKEv2]
Tschofenig, H., D. Kroeselberg and Y. Ohba, "EAP IKEv2
Method", draft-tschofenig-eap-ikev2-05.txt, Internet draft
(work in progress), October 2004.
IAB Informational [Page 29]
INTERNET-DRAFT Link Indications 10 January 2005
[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.
[Eggert] Eggert, L., Schuetz, S. and S. Schmid, "TCP Extensions for
Immediate Retransmissions", draft-eggert-tcpm-tcp-retransmit-
now-01.txt, Internet draft (work in progress), September 2004.
[ETX] Douglas S. J. De Couto, Daniel Aguayo, John Bicket, and Robert
Morris, "A High-Throughput Path Metric for Multi-Hop Wireless
Routing", Proceedings of the 9th ACM International Conference
on Mobile Computing and Networking (MobiCom '03), San Diego,
California, September 2003.
[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
[IEEE8021X]
Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X, December 2004.
[IEEE80211]
Institute of Electrical and Electronics Engineers, "Wireless
LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications", IEEE Standard 802.11, 2003.
[IEEE80211e]
Institute of Electrical and Electronics Engineers, "Draft
Amendment 7: Medium Access Control (MAC) Quality of Service
(QoS) Enhancements", IEEE 802.11e Draft 10.0, October 2004.
[IEEE80211F]
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.
[IEEE80211i]
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
IAB Informational [Page 30]
INTERNET-DRAFT Link Indications 10 January 2005
(PHY) Specifications: Specification for Enhanced Security",
IEEE 802.11i, November 2004.
[Kim] Kim, K., Park, Y., Suh, K., and Y. Park, "The BU-trigger
method for improving TCP performance over Mobile IPv6", draft-
kim-tsvwg-butrigger-00.txt, Internet draft (work in progress),
August 2004.
[Kotz] Kotz, D., Newport, C. and C. Elliot, "The mistaken axioms of
wireless-network research", Dartmouth College Computer Science
Technical Report TR2003-467, July 2003.
[MIPEAP] Giaretta, C., Guardini, I., Demaria, E., Bournelle, J. and M.
Laurent-Maknavicius, "MIPv6 Authorization and Configuration
based on EAP", draft-giaretta-mip6-authorization-eap-02.txt,
Internet draft (work in progress), October 2004.
[Mishra] Mitra, A., Shin, M., and W. Arbaugh, "An Empirical Analysis of
the IEEE 802.11 MAC Layer Handoff Process", CS-TR-4395,
University of Maryland Department of Computer Science,
September 2002.
[PEAP] Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G. and S.
Josefsson, "Protected EAP Protocol (PEAP) Version 2", draft-
josefsson-pppext-eap-tls-eap-10.txt, Internet draft (work in
progress), October 2004.
[Park] Park, S., Njedjou, E. and N. Montavont, "L2 Triggers Optimized
Mobile IPv6 Vertical Handover: The 802.11/GPRS Example",
draft-daniel-mip6-optimized-vertical-handover-00.txt, July
2004.
[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", draft-dawkins-trigtran-
framework-00.txt, Internet draft (work in progress), August
2003.
[Vatn] Vatn, J., "An experimental study of IEEE 802.11b handover
performance and its effect on voice traffic", TRITA-IMIT-
TSLAB R 03:01, KTH Royal Institute of Technology, Stockholm,
IAB Informational [Page 31]
INTERNET-DRAFT Link Indications 10 January 2005
Sweden, July 2003.
[Yegin] Yegin, A., "Link-layer Triggers Protocol", draft-yegin-
l2-triggers-00.txt, Internet Draft (work in progress), June
2002.
[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)", draft-
ietf-ospf-omp-02.txt, Internet draft (work in progress),
February 1999.
Appendix A. IAB Members at the time of this writing
Bernard Aboba
Rob Austein
Leslie Daigle
Patrik Falstrom
Sally Floyd
Mark Handley
Bob Hinden
Geoff Huston
Jun-Ichiro Itojun Hagino
Eric Rescorla
Pete Resnick
Jonathan Rosenberg
IAB Informational [Page 32]
INTERNET-DRAFT Link Indications 10 January 2005
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IAB Informational [Page 33]
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