Architectural Implications of Link Indications
RFC 4907
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.
IAB Informational [Page 1]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 2]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 3]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 4]
RFC 4907 Link Indications June 2007
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].
IAB Informational [Page 5]
RFC 4907 Link Indications June 2007
"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.
IAB Informational [Page 6]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 7]
RFC 4907 Link Indications June 2007
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
IAB Informational [Page 8]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 9]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 10]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 11]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 12]
RFC 4907 Link Indications June 2007
"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
IAB Informational [Page 13]
RFC 4907 Link Indications June 2007
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).
IAB Informational [Page 14]
RFC 4907 Link Indications June 2007
(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
IAB Informational [Page 15]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 16]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 17]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 18]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 19]
RFC 4907 Link Indications June 2007
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".
IAB Informational [Page 20]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 21]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 22]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 23]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 24]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 25]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 26]
RFC 4907 Link Indications June 2007
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).
IAB Informational [Page 27]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 28]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 29]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 30]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 31]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 32]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 33]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 34]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 35]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 36]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 37]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 38]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 39]
RFC 4907 Link Indications June 2007
[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.
IAB Informational [Page 40]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 41]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 42]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 43]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 44]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 45]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 46]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 47]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 48]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 49]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 50]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 51]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 52]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 53]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 54]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 55]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 56]
RFC 4907 Link Indications June 2007
"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
IAB Informational [Page 57]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 58]
RFC 4907 Link Indications June 2007
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.
IAB Informational [Page 59]
RFC 4907 Link Indications June 2007
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
IAB Informational [Page 60]
RFC 4907 Link Indications June 2007
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/
IAB Informational [Page 61]
RFC 4907 Link Indications June 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
IAB Informational [Page 62]