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
(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
(3) Proposals must demonstrate robustness against spurious link
indications (Section 2.3).
(4) Upper layers should utilize a timely recovery step so as to
limit the potential damage from link indications determined to
be invalid after they have been acted on (Section 2.3.2).
(5) Proposals must demonstrate that effective congestion control is
maintained (Section 2.4).
(6) Proposals must demonstrate the effectiveness of proposed
optimizations (Section 2.5).
(7) Link indications should not be required by upper layers, in
order to maintain link independence (Section 2.6).
(8) Proposals should avoid race conditions, which can occur where
link indications are utilized directly by multiple layers of the
stack (Section 2.7).
(9) Proposals should avoid inconsistencies between link and routing
layer metrics (Section 2.7.3).
(10) Overhead reduction schemes must avoid compromising
interoperability and introducing link layer dependencies into
the Internet and transport layers (Section 2.8).
(11) Proposals for transport of link indications beyond the local
host need to carefully consider the layering, security, and
transport implications (Section 2.9).
2.1. Model Validation
Proposals should avoid the use of link models in circumstances where
they do not apply.
In "The mistaken axioms of wireless-network research" [Kotz], the
authors conclude that mistaken assumptions relating to link behavior
may lead to the design of network protocols that may not work in
practice. For example, the authors note that the three-dimensional
nature of wireless propagation can result in large signal strength
changes over short distances. This can result in rapid changes in
link indications such as rate, frame loss, and signal strength.
In "Modeling Wireless Links for Transport Protocols" [GurtovFloyd],
the authors provide examples of modeling mistakes and examples of how
to improve modeling of link characteristics. To accompany the paper,
the authors provide simulation scenarios in ns-2.
In order to avoid the pitfalls described in [Kotz] [GurtovFloyd],
documents that describe capabilities that are dependent on link
indications should explicitly articulate the assumptions of the link
model and describe the circumstances in which they apply.
Generic "trigger" models may include implicit assumptions that may
prove invalid in outdoor or mesh wireless LAN deployments. For
example, two-state Markov models assume that the link is either in a
state experiencing low frame loss ("up") or in a state where few
frames are successfully delivered ("down"). In these models,
symmetry is also typically assumed, so that the link is either "up"
in both directions or "down" in both directions. In situations where
intermediate loss rates are experienced, these assumptions may be
As noted in "Hybrid Rate Control for IEEE 802.11" [Haratcherev],
signal strength data is noisy and sometimes inconsistent, so that it
needs to be filtered in order to avoid erratic results. Given this,
link indications based on raw signal strength data may be unreliable.
In order to avoid problems, it is best to combine signal strength
data with other techniques. For example, in developing a "Going
Down" indication for use with [IEEE-802.21] it would be advisable to
validate filtered signal strength measurements with other indications
of link loss such as lack of Beacon reception.
2.2. Clear Definitions
Link indications should be clearly defined, so that it is understood
when they are generated on different link layers. For example,
considerable work has been required in order to come up with the
definitions of "Link Up" and "Link Down", and to define when these
indications are sent on various link layers.
Link indication definitions should heed the following advice:
(1) Do not assume symmetric link performance or frame loss that is
either low ("up") or high ("down").
In wired networks, links in the "up" state typically experience
low frame loss in both directions and are ready to send and
receive data frames; links in the "down" state are unsuitable
for sending and receiving data frames in either direction.
Therefore, a link providing a "Link Up" indication will
typically experience low frame loss in both directions, and high
frame loss in any direction can only be experienced after a link
provides a "Link Down" indication. However, these assumptions
may not hold true for wireless LAN networks. Asymmetry is
typically less of a problem for cellular networks where
propagation occurs over longer distances, multi-path effects may
be less severe, and the base station can transmit at much higher
power than mobile stations while utilizing a more sensitive
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
(2) Consider the sensitivity of link indications to transient link
conditions. Due to common effects such as multi-path
interference, signal strength and signal to noise ratio (SNR)
may vary rapidly over a short distance, causing erratic behavior
of link indications based on unfiltered measurements. As noted
in [Haratcherev], signal strength may prove most useful when
utilized in combination with other measurements, such as frame
(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.
Link indication proposals must demonstrate robustness against
misleading indications. Elements to consider include:
Recovery from invalid indications
Damping and hysteresis
2.3.1. Implementation Variation
Variations in link layer implementations may have a substantial
impact on the behavior of link indications. These variations need to
be taken into account in evaluating the performance of proposals.
For example, radio propagation and implementation differences can
impact the reliability of link indications.
In "Link-level Measurements from an 802.11b Mesh Network" [Aguayo],
the authors analyze the cause of frame loss in a 38-node urban
multi-hop IEEE 802.11 ad-hoc network. In most cases, links that are
very bad in one direction tend to be bad in both directions, and
links that are very good in one direction tend to be good in both
directions. However, 30 percent of links exhibited loss rates
differing substantially in each direction.
As described in [Aguayo], wireless LAN links often exhibit loss rates
intermediate between "up" (low loss) and "down" (high loss) states,
as well as substantial asymmetry. As a result, receipt of a "Link
Up" indication may not necessarily indicate bidirectional
reachability, since it could have been generated after exchange of
small frames at low rates, which might not imply bidirectional
connectivity for large frames exchanged at higher rates.
Where multi-path interference or hidden nodes are encountered, signal
strength may vary widely over a short distance. Several techniques
may be used to reduce potential disruptions. Multiple transmitting
and receiving antennas may be used to reduce multi-path effects;
transmission rate adaptation can be used to find a more satisfactory
transmission rate; transmit power adjustment can be used to improve
signal quality and reduce interference; Request-to-Send/Clear-to-Send
(RTS/CTS) signaling can be used to reduce hidden node problems.
These techniques may not be completely effective, so that high frame
loss may be encountered, causing the link to cycle between "up" and
To improve robustness against spurious link indications, it is
recommended that upper layers treat the indication as a "hint"
(advisory in nature), rather than a "trigger" dictating a particular
action. Upper layers may then attempt to validate the hint.
In [RFC4436], "Link Up" indications are rate limited, and IP
configuration is confirmed using bidirectional reachability tests
carried out coincident with a request for configuration via DHCP. As
a result, bidirectional reachability is confirmed prior to activation
of an IP configuration. However, where a link exhibits an
intermediate loss rate, demonstration of bidirectional reachability
may not necessarily indicate that the link is suitable for carrying
IP data packets.
Another example of validation occurs in IPv4 Link-Local address
configuration [RFC3927]. Prior to configuration of an IPv4 Link-
Local address, it is necessary to run a claim-and-defend protocol.
Since a host needs to be present to defend its address against
another claimant, and address conflicts are relatively likely, a host
returning from sleep mode or receiving a "Link Up" indication could
encounter an address conflict were it to utilize a formerly
configured IPv4 Link-Local address without rerunning claim and
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
In Detecting Network Attachment in IPv4 (DNAv4) [RFC4436],
reachability tests are carried out coincident with a request for
configuration via DHCP. Therefore, if the bidirectional reachability
test times out, the host can still obtain an IP configuration via
DHCP, and if that fails, the host can still continue to use an
existing valid address if it has one.
Where a proposal involves recovery at the transport layer, the
recovered transport parameters (such as the Maximum Segment Size
(MSS), RoundTrip Time (RTT), Retransmission TimeOut (RTO), Bandwidth
(bw), congestion window (cwnd), etc.) should be demonstrated to
remain valid. Congestion window validation is discussed in "TCP
Congestion Window Validation" [RFC2861].
Where timely recovery is not supported, unexpected consequences may
result. As described in [RFC3927], early IPv4 Link-Local
implementations would wait five minutes before attempting to obtain a
routable address after assigning an IPv4 Link-Local address. In one
implementation, it was observed that where mobile hosts changed their
point of attachment more frequently than every five minutes, they
would never obtain a routable address. The problem was caused by an
invalid link indication (signaling of "Link Up" prior to completion
of link layer authentication), resulting in an initial failure to
obtain a routable address using DHCP. As a result, [RFC3927]
recommends against modification of the maximum retransmission timeout
(64 seconds) provided in [RFC2131].
2.3.3. Damping and Hysteresis
Damping and hysteresis can be utilized to limit damage from unstable
link indications. This may include damping unstable indications or
placing constraints on the frequency of link indication-induced
actions within a time period.
While [Aguayo] found that frame loss was relatively stable for
stationary stations, obstacles to radio propagation and multi-path
interference can result in rapid changes in signal strength for a
mobile station. As a result, it is possible for mobile stations to
encounter rapid changes in link characteristics, including changes in
transmission rate, throughput, frame loss, and even "Link Up"/"Link
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
2.4. Congestion Control
Link indication proposals must demonstrate that effective congestion
control is maintained [RFC2914]. One or more of the following
techniques may be utilized:
Rate limiting. Packets generated based on receipt of link
indications can be rate limited (e.g., a limit of one packet per
end-to-end path RTO).
Utilization of upper-layer indications. Applications should
depend on upper-layer indications such as IP address
configuration/change notification, rather than utilizing link
indications such as "Link Up".
Keepalives. In order to improve robustness against spurious link
indications, an application keepalive or transport layer
indication (such as connection teardown) can be used instead of
consuming "Link Down" indications.
Conservation of resources. Proposals must demonstrate that they
are not vulnerable to congestive collapse.
As noted in "Robust Rate Adaptation for 802.11 Wireless Networks"
[Robust], decreasing transmission rate in response to frame loss
increases contention, potentially leading to congestive collapse. To
avoid this, the link layer needs to distinguish frame loss due to
congestion from loss due to channel conditions. Only frame loss due
to deterioration in channel conditions can be used as a basis for
decreasing transmission rate.
Consider a proposal where a "Link Up" indication is used by a host to
trigger retransmission of the last previously sent packet, in order
to enable ACK reception prior to expiration of the host's
retransmission timer. On a rapidly moving mobile node where "Link
Up" indications follow in rapid succession, this could result in a
burst of retransmitted packets, violating the principle of
"conservation of packets".
At the application layer, link indications have been utilized by
applications such as Presence [RFC2778] in order to optimize
registration and user interface update operations. For example,
implementations may attempt presence registration on receipt of a
"Link Up" indication, and presence de-registration by a surrogate
receiving a "Link Down" indication. Presence implementations using
"Link Up"/"Link Down" indications this way violate the principle of
"conservation of packets" since link indications can be generated on
a time scale less than the end-to-end path RTO. The problem is
magnified since for each presence update, notifications can be
delivered to many watchers. In addition, use of a "Link Up"
indication in this manner is unwise since the interface may not yet
even have an operable Internet layer configuration. Instead, an "IP
address configured" indication may be utilized.
Proposals must demonstrate the effectiveness of proposed
optimizations. Since optimizations typically increase complexity,
substantial performance improvement is required in order to make a
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.
While link indications can be utilized where available, they should
not be required by upper layers, in order to maintain link layer
independence. For example, if information on supported prefixes is
provided at the link layer, hosts not understanding those hints must
still be able to obtain an IP address.
Where link indications are proposed to optimize Internet layer
configuration, proposals must demonstrate that they do not compromise
robustness by interfering with address assignment or routing protocol
behavior, making address collisions more likely, or compromising
Duplicate Address Detection (DAD) [RFC4429].
To avoid compromising interoperability in the pursuit of performance
optimization, proposals must demonstrate that interoperability
remains possible (potentially with degraded performance) even if one
or more participants do not implement the proposal.
2.7. Race Conditions
Link indication proposals should avoid race conditions, which can
occur where link indications are utilized directly by multiple layers
of the stack.
Link indications are useful for optimization of Internet Protocol
layer addressing and configuration as well as routing. Although "The
BU-trigger method for improving TCP performance over Mobile IPv6"
[Kim] describes situations in which link indications are first
processed by the Internet Protocol layer (e.g., MIPv6) before being
utilized by the transport layer, for the purposes of parameter
estimation, it may be desirable for the transport layer to utilize
link indications directly.
In situations where the Weak End System model is implemented, a
change of outgoing interface may occur at the same time the transport
layer is modifying transport parameters based on other link
indications. As a result, transport behavior may differ depending on
the order in which the link indications are processed.
Where a multi-homed host experiences increasing frame loss or
decreased rate on one of its interfaces, a routing metric taking
these effects into account will increase, potentially causing a
change in the outgoing interface for one or more transport
connections. This may trigger Mobile IP signaling so as to cause a
change in the incoming path as well. As a result, the transport
parameters estimated for the original outgoing and incoming paths
(congestion state, Maximum Segment Size (MSS) derived from the link
maximum transmission unit (MTU) or Path MTU) may no longer be valid
for the new outgoing and incoming paths.
To avoid race conditions, the following measures are recommended:
Path change re-estimation
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.
Another technique to avoid race conditions is to rely on layering to
damp transient link indications and provide greater link layer
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
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
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
However, prior to sending data packets over the link, the appropriate
routing metric may not easily be predicted. As noted in [Shortest],
a link that can successfully transmit the short frames utilized for
control, management, or routing may not necessarily be able to
reliably transport larger data packets.
Therefore, it may be necessary to utilize alternative metrics (such
as signal strength or Access Point load) in order to assist in
attachment/handoff decisions. However, unless the new interface is
the preferred route for one or more destination prefixes, a Weak End
System implementation will not use the new interface for outgoing
traffic. Where "idle timeout" functionality is implemented, the
unused interface will be brought down, only to be brought up again by
the link enablement algorithm.
Within the link layer, metrics such as signal strength and frame loss
may be used to determine the transmission rate, as well as to
determine when to select an alternative point of attachment. In
order to enable stations to roam prior to encountering packet loss,
studies such as "An experimental study of IEEE 802.11b handover
performance and its effect on voice traffic" [Vatn] have suggested
using signal strength as a mechanism to more rapidly detect loss of
connectivity, rather than frame loss, as suggested in "Techniques to
Reduce IEEE 802.11b MAC Layer Handover Time" [Velayos].
[Aguayo] notes that signal strength and distance are not good
predictors of frame loss or throughput, due to the potential effects
of multi-path interference. As a result, a link brought up due to
good signal strength may subsequently exhibit significant frame loss
and a low throughput. Similarly, an Access Point (AP) demonstrating
low utilization may not necessarily be the best choice, since
utilization may be low due to hardware or software problems. "OSPF
Optimized Multipath (OSPF-OMP)" [Villamizar] notes that link-
utilization-based routing metrics have a history of instability.
2.8. Layer Compression
In many situations, the exchanges required for a host to complete a
handoff and reestablish connectivity are considerable, leading to
proposals to combine exchanges occurring within multiple layers in
order to reduce overhead. While overhead reduction is a laudable
goal, proposals need to avoid compromising interoperability and
introducing link layer dependencies into the Internet and transport
Exchanges required for handoff and connectivity reestablishment may
include link layer scanning, authentication, and association
establishment; Internet layer configuration, routing, and mobility
exchanges; transport layer retransmission and recovery; security
association reestablishment; application protocol re-authentication
and re-registration exchanges, etc.
Several proposals involve combining exchanges within the link layer.
For example, in [EAPIKEv2], a link layer Extensible Authentication
Protocol (EAP) [RFC3748] exchange may be used for the purpose of IP
address assignment, potentially bypassing Internet layer
configuration. Within [PEAP], it is proposed that a link layer EAP
exchange be used for the purpose of carrying Mobile IPv6 Binding
Updates. [MIPEAP] proposes that EAP exchanges be used for
configuration of Mobile IPv6. Where link, Internet, or transport
layer mechanisms are combined, hosts need to maintain backward
compatibility to permit operation on networks where compression
schemes are not available.
Layer compression schemes may also negatively impact robustness. For
example, in order to optimize IP address assignment, it has been
proposed that prefixes be advertised at the link layer, such as
within the 802.11 Beacon and Probe Response frames. However,
[IEEE-802.1X] enables the Virtual LAN Identifier (VLANID) to be
assigned dynamically, so that prefix(es) advertised within the Beacon
and/or Probe Response may not correspond to the prefix(es) configured
by the Internet layer after the host completes link layer
authentication. Were the host to handle IP configuration at the link
layer rather than within the Internet layer, the host might be unable
to communicate due to assignment of the wrong IP address.
2.9. Transport of Link Indications
Proposals for the transport of link indications need to carefully
consider the layering, security, and transport implications.
As noted earlier, the transport layer may take the state of the local
routing table into account in improving the quality of transport
parameter estimates. While absence of positive feedback that the
path is sending data end-to-end must be heeded, where a route that
had previously been absent is recovered, this may be used to trigger
congestion control probing. While this enables transported link
indications that affect the local routing table to improve the
quality of transport parameter estimates, security and
interoperability considerations relating to routing protocols still
Proposals involving transport of link indications need to demonstrate
(a) Superiority to implicit signals. In general, implicit signals
are preferred to explicit transport of link indications since
they do not require participation in the routing mesh, add no
new packets in times of network distress, operate more reliably
in the presence of middle boxes such as NA(P)Ts, are more likely
to be backward compatible, and are less likely to result in
security vulnerabilities. As a result, explicit signaling
proposals must prove that implicit signals are inadequate.
(b) Mitigation of security vulnerabilities. Transported link
indications should not introduce new security vulnerabilities.
Link indications that result in modifications to the local
routing table represent a routing protocol, so that the
vulnerabilities associated with unsecured routing protocols
apply, including spoofing by off-link attackers. While
mechanisms such as "SEcure Neighbor Discovery (SEND)" [RFC3971]
may enable authentication and integrity protection of router-
originated messages, protecting against forgery of transported
link indications, they are not yet widely deployed.
(c) Validation of transported indications. Even if a transported
link indication can be integrity protected and authenticated, if
the indication is sent by a host off the local link, it may not
be clear that the sender is on the actual path in use, or which
transport connection(s) the indication relates to. Proposals
need to describe how the receiving host can validate the
transported link indication.
(d) Mapping of Identifiers. When link indications are transported,
it is generally for the purposes of providing information about
Internet, transport, or application layer operations at a remote
element. However, application layer sessions or transport
connections may not be visible to the remote element due to
factors such as load sharing between links, or use of IPsec,
tunneling protocols, or nested headers. As a result, proposals
need to demonstrate how the link indication can be mapped to the
relevant higher-layer state. For example, on receipt of a link
indication, the transport layer will need to identify the set of
transport sessions (source address, destination address, source
port, destination port, transport) that are affected. If a
presence server is receiving remote indications of "Link
Up"/"Link Down" status for a particular Media Access Control
(MAC) address, the presence server will need to associate that
MAC address with the identity of the user
(pres:email@example.com) to whom that link status change is
3. Future Work
Further work is needed in order to understand how link indications
can be utilized by the Internet, transport, and application layers.
More work is needed to understand the connection between link
indications and routing metrics. For example, the introduction of
block ACKs (supported in [IEEE-802.11e]) complicates the relationship
between effective throughput and frame loss, which may necessitate
the development of revised routing metrics for ad-hoc networks. More
work is also needed to reconcile handoff metrics (e.g., signal
strength and link utilization) with routing metrics based on link
indications (e.g., frame error rate and negotiated rate).
A better understanding of the use of physical and link layer metrics
in rate negotiation is required. For example, recent work
[Robust][CARA] has suggested that frame loss due to contention (which
would be exacerbated by rate reduction) can be distinguished from
loss due to channel conditions (which may be improved via rate
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.