U.S. patent application number 13/894140 was filed with the patent office on 2013-09-26 for mitigating low-rate denial-of-service attacks in packet-switched networks.
This patent application is currently assigned to AT&T Intellectual Property I, L.P.. The applicant listed for this patent is AT&T Intellectual Property I, L.P.. Invention is credited to Chia-Wei Chang, Seungjoon Lee, Bill Lin, Jia Wang.
Application Number | 20130254886 13/894140 |
Document ID | / |
Family ID | 44012331 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130254886 |
Kind Code |
A1 |
Wang; Jia ; et al. |
September 26, 2013 |
Mitigating Low-Rate Denial-Of-Service Attacks in Packet-Switched
Networks
Abstract
A method includes determining, at a network routing device, an
average packet drop rate for a plurality of aggregations of packet
flows. The method also determines a threshold packet drop rate
based on the average packet drop rate, a current packet drop rate
for a select aggregation of the plurality of aggregations, and
whether at least one packet flow of the select aggregation is
potentially subject to a denial-of-service attack based on a
comparison of the current packet drop rate to the threshold packet
drop rate.
Inventors: |
Wang; Jia; (Randolph,
NJ) ; Chang; Chia-Wei; (San Diego, CA) ; Lee;
Seungjoon; (Basking Ridge, NJ) ; Lin; Bill;
(La Jolla, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AT&T Intellectual Property I, L.P. |
Atlanta |
GA |
US |
|
|
Assignee: |
AT&T Intellectual Property I,
L.P.
Atlanta
GA
|
Family ID: |
44012331 |
Appl. No.: |
13/894140 |
Filed: |
May 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12621212 |
Nov 18, 2009 |
8443444 |
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13894140 |
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Current U.S.
Class: |
726/23 |
Current CPC
Class: |
G06F 21/55 20130101;
H04L 63/1458 20130101 |
Class at
Publication: |
726/23 |
International
Class: |
G06F 21/55 20060101
G06F021/55 |
Claims
1. A system for mitigating low-rate denial-of-service attacks in
packet-switched networks, the system comprising: a memory that
stores instructions; a processor that executes the instructions to
perform operations, the operations comprising: determining an
average packet drop rate for a plurality of aggregations of packet
flows; determining a threshold packet drop rate based on the
average packet drop rate determined for the plurality of
aggregations of the packet flows; determining a current packet drop
rate for a select aggregation of the plurality of aggregations of
the packet flows; and determining whether a packet flow of the
select aggregation is potentially subject to a denial-of-service
attack based on a comparison of the current packet drop rate to the
threshold packet drop rate.
2. The system of claim 1, wherein the operations further comprise
selecting packets for transmission that have a higher priority
status over packets having a lower priority status.
3. The system of claim 1, wherein the operations further comprise
assigning a higher priority status to packets of the packet flow of
the select aggregation for transmission based on determining that
the packet flow of the select aggregation is subject to the
denial-of-service attack.
4. The system of claim 1, wherein the operations further comprise
assigning a lower priority status to packets of the packet flow of
the select aggregation for transmission based on determining that
the packet flow of the select aggregation is not subject to the
denial-of-service attack.
5. The system of claim 1, wherein the operations further comprise
determining the average packet drop rate for the plurality of
aggregations of packet flows based on a time sliding window
composed of a contiguous set of time intervals.
6. The system of claim 1, wherein the operations further comprise
determining that the packet flow of the select aggregation is
subject to the denial-of-service attack if the comparison indicates
that the current packet drop rate is greater than the threshold
packet drop rate.
7. The system of claim 1, wherein the operations further comprise
determining that the packet flow of the select aggregation is not
subject to the denial-of-service attack if the comparison indicates
that the current packet drop rate is less than the threshold packet
drop rate.
8. The system of claim 1, wherein the operations further comprise
adjusting the threshold packet drop rate based a type of
application associated with the select aggregation.
9. The system of claim 1, wherein the operations further comprise
selecting packets of the packet flow for transmission based on an
active queue management policy.
10. The system of claim 1, wherein the operations further comprise
selecting packets of the packet flow for transmission based on a
weighted random early detection policy.
11. The system of claim 1, wherein the operations further comprise
determining the average packet drop rate for the plurality of
aggregations of the packet flows by utilizing an arrival counter
and a drop counter.
12. A system for mitigating low-rate denial-of-service attacks in
packet-switched networks, the system comprising: a memory that
stores instructions; a processor that executes the instruction to
perform operations, the operations comprising: determining a first
average packet drop rate for a plurality of aggregations of packet
flows for a first interval; determining a first threshold packet
drop rate based on the first average packet drop rate; assigning a
higher priority status to packets of packet flows of a select
aggregation of the plurality of aggregations at a first time in
response to determining that a current packet drop rate of the
select aggregation at the first time is greater than the first
threshold packet drop rate; and selecting, for transmission,
packets of the packet flows having the higher priority status over
packets of the packet flows having a lower priority status.
13. The system of claim 12, wherein the operations further comprise
determining a second average packet drop rate for the plurality of
aggregations of packet flows for a second interval that is after
the first interval.
14. The system of claim 13, wherein the operations further comprise
determining a second threshold packet drop rate based on the second
average packet drop rate.
15. The system of claim 14, wherein the operations further comprise
assigning a lower priority status to packets of the packet flows of
the select aggregation at a second time in response to determining
a current packet drop rate of the select aggregation at the second
time is greater than the second threshold packet drop rate.
16. The system of claim 15, wherein the second time is subsequent
to the first time.
17. The system of claim 12, wherein the operations further comprise
adjusting the first threshold packet drop rate based a type of
application associated with the select aggregation.
18. The system of claim 12, wherein the operations further comprise
selecting packets of the packet flow for transmission based on an
active queue management policy.
19. The system of claim 12, wherein the operations further comprise
determining first the average packet drop rate for the plurality of
aggregations of packet flows based on a time sliding window
composed of a contiguous set of time intervals.
20. A method for mitigating low-rate denial-of-service attacks in
packet-switched networks, the method comprising: determining, by
utilizing instructions from memory that are executed by a
processor, a first average packet drop rate for a plurality of
aggregations of packet flows for a first interval; determining a
first threshold packet drop rate based on the first average packet
drop rate; assigning a lower priority status to packets of packet
flows of a select aggregation of the plurality of aggregations at a
first time in response to determining that a current packet drop
rate of the select aggregation at the first time is less than the
first threshold packet drop rate; and selecting, for transmission,
packets of the packet flows having a higher priority status over
packets of the packet flows having the lower priority status.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation of and claims
priority to U.S. patent application Ser. No. 12/621,212, filed on
Nov. 18, 2009, now U.S. Pat. No. 8,443,444, which is hereby
incorporated by reference in its entirety.
FIELD OF THE DISCLOSURE
[0002] The present disclosure generally relates to packet-switched
networking, and relates more particularly to protecting against
denial-of-service attacks in packet switched networks.
BACKGROUND
[0003] Packet-switched networks rely on the efficient transmission
of packets across network links. Malicious entities often attempt
to disrupt this efficient data flow using denial-of-service (DoS)
attacks whereby a network device is flooded with a large volume of
network traffic. The resources and bandwidth of the network device
are then consumed in handling this flood of network traffic. As a
result, the network device is forced to begin dropping packets
associated with legitimate packet flows and thus reduce throughput
and quality of legitimate network services provided by the network
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] It will be appreciated that for simplicity and clarity of
illustration, elements illustrated in the Figures have not
necessarily been drawn to scale. For example, the dimensions of
some of the elements are exaggerated relative to other elements.
Embodiments incorporating teachings of the present disclosure are
shown and described with respect to the drawings presented herein,
in which:
[0005] FIG. 1 is a diagram illustrating network system implementing
a shrew attack protection (SAP) mechanism in accordance with at
least one embodiment of the present disclosure; and
[0006] FIG. 2 is a diagram illustrating a SAP module for
implementing the SAP mechanism at a network routing device of the
network system of FIG. 1 in accordance with at least one embodiment
of the present disclosure.
[0007] The use of the same reference symbols in different drawings
indicates similar or identical items.
DETAILED DESCRIPTION OF THE DRAWINGS
[0008] The numerous innovative teachings of the present application
will be described with particular reference to the presently
preferred example embodiments. However, it should be understood
that this class of embodiments provides only a few examples of the
many advantageous uses of the innovative teachings herein. In
general, statements made in the specification of the present
application do not necessarily delimit any of the various claimed
inventions. Moreover, some statements may apply to some inventive
features but not to others.
[0009] FIGS. 1 and 2 illustrate example techniques for mitigating
low-rate denial-of-service attacks (also commonly referred to as
"shrew" attacks) in packet-switched networks. A shrew attack
exploits the retransmission time-out (RTO) mechanism of the
Transport Communication Protocol (TCP) whereby bursts of connection
requests from an attacker are synchronized to the RTO value
employed by the network routing device that is the target of the
attack. As such, when a node in the network retransmits a packet
after expiration of a retransmission timer, the packet will reach
the network routing device as it is already inundated with the
synchronized burst, thereby leading to repeated packet drops in
packet flows associated with the port being targeted by the shrew
attack. These repeated packet drops can degrade the performance of
TCP-based protocols and services provided by the network routing
device, including Internet routing services such as Border Gateway
Protocol (BGP) services. Accordingly, in one embodiment, the
network routing device can employ a shrew attack protection (SAP)
mechanism whereby the average packet drop rate for packet flows is
dynamically updated and used to determine if any particular subset,
or aggregation, of the packet flows serviced by the network routing
device may include a packet flow that is potentially subject to a
shrew attack. To identify whether a packet flow of a selected
aggregation is a victim to a shrew attack, the network routing
device determines the current, or instant, packet drop rate for the
selected aggregation and compares this current packet drop rate
with the average packet drop rate across all of the aggregations.
In the event that the current packet drop rate for the selected
aggregation is greater than the average packet drop rate across all
of the aggregations, the outgoing packets of the packet flows of
the selected aggregation are marked or otherwise identified as
having a higher priority status. Otherwise, if the current packet
drop rate for the selected aggregation is equal to or less than the
average packet drop rate across all of the aggregations, the
outgoing packets of the packet flows of the selected aggregation
are marked or otherwise identified as having a lower priority
status (a normal priority status, for example).
[0010] The network routing device mitigates any potential shrew
attack by preferentially selecting those packets identified as
having the higher priority status for transmission by the network
routing device over those packets identified as having the lower or
normal priority status. By giving packets of packet flows
potentially victim to a shrew attack a higher priority for output,
the network routing device can more fully ensure that the packets
of the potential victim packet flows are successfully transmitted
and thus diminish the packet drop rate for the potential packet
flows and neutralize the shrew attack. This SAP mechanism can be
employed in conjunction with other conventional
congestion-management schemes that employ preferential packet
dropping, such as the Active Queue Management (AQM) policy or the
Weighted Random Early Detection (WRED) policy.
[0011] FIG. 1 illustrates a portion of a network system 100
employing the shrew attack protection (SAP) mechanism in accordance
with at least one embodiment of the present disclosure. The network
system 100 includes a network routing device 102 connected to one
or more packet-switched network segments 104. The network routing
device 102 can include, for example, a router, a switch, a bridge,
and the like. The network segment 104 can include a network segment
compliant with any of a variety of protocols potentially subject to
a shrew attack. For purposes of illustration, the network segment
104 and the SAP mechanism are described below in the example
context of a TCP-based network, such as an Internet
Protocol/Transmission Control Protocol (IP/TCP) network (the
Internet or a local area network (LAN), for example).
[0012] The network routing device 102 includes a line interface 106
having a network input 108 and a network output 110 coupled to one
or more network segments 104, an input buffer 112 to buffer packets
received from a network segment 104 via the network input 108, and
an output buffer 114 to buffer packets for transmission to a
network segment 104 from the network output 110. For ease of
illustration, the network input 108 and the network output 110 are
depicted as connected to the same network segment 104. However, it
will be appreciated that the network routing device 102 may be
connected to multiple network segments 104 and thus have multiple
network inputs and network outputs. The line interface 106 further
includes an output control module 116 to control the processing of
packets buffered in the output buffer 114 for output via the
network output 110 and a routing mechanism 118 to implement various
routing processes for packets within the network routing device
102. These routing processes can include, for example, routing
packets to the appropriate output, filtering packets, encapsulating
packets, encrypting/decrypting packets, and the like. The line
interface 106 also includes a SAP module 120 to implement the SAP
mechanism described herein for outgoing packets. The illustrated
components of the network routing device 102 can be implemented in
hardware, in one or more processors to execute software
representative of the functionality described herein, or a
combination thereof. To illustrate, the network routing device 102
can include, for example, a processor 122 and a memory 124, whereby
the memory 124 includes a computer program 126 comprising a set of
executable instructions that are accessed from the memory 124 and
executed by the processor 122 to perform one or more of the
functions described herein.
[0013] In operation, incoming packets (such as packets 131, 132,
and 133) are received at the network input 108 of the network
routing device 102, buffered in the input buffer 112, and then
routed to the appropriate output of the network routing device 102
by the routing mechanism 118. In the process, the SAP module 120
monitors the incoming packets and the outgoing packets for the
various packet flows processed by the network routing device 102 so
as to dynamically determine the average packet drop rate for the
packet flows in total, as well as to determine the current packet
drop rates related to individual packet flows or aggregations
thereof. Although the packet drop rate can be maintained on a
per-flow basis, the resources necessary to maintain such statistics
typically are difficult for a typically network routing device to
maintain. Accordingly, rather than monitor the packet drop rate for
each packet flow, the SAP module 120 groups the packet flows into a
plurality of aggregations based on one or more aggregation criteria
and determines the packet drop rate on a per-aggregation basis. Any
of a variety of aggregation criteria may be used, although
different aggregation criteria may have different performance
trade-offs between accuracy and memory/computation requirements. In
one embodiment, the SAP module 120 employs an application-level
granularity to the aggregation of packet flows by using the
destination port found in the destination port field of the TCP/IP
header of the packets as the aggregation criterion. Accordingly,
all packet flows directed to the same destination port are combined
by the SAP module 120 into the same aggregation of packet flows.
Other examples of aggregation criteria include the destination IP
address, the source IP address, a hash of various fields of the
packets, and the like. The aggregation criterion also can include a
combination of other aggregation criteria, such as a combination of
the destination IP address and the source IP address. As the SAP
mechanism is not limited to aggregation of multiple packet flows
but instead can be performed on an individual packet flow basis,
reference to an aggregation of packet flows also includes a single
packet flow for implementations of the SAP mechanism on a per-flow
basis (that is, the aggregation criterion in such instances can be
an identifier unique to a single packet flow).
[0014] The SAP module 120 uses the average packet drop rate across
all of the aggregations (P.sub.avg.sub.--.sub.tot) to set a
threshold packet drop rate (P.sub.thresh), which is then compared
with the current packet drop rates for each of the aggregations to
identify any aggregations having packet flows that are potentially
subject to a shrew attack. In one embodiment, the SAP module 120
identifies an aggregation as having a packet flow potentially
victim to a shrew attack when the current packet drop rate of the
aggregation is greater than the current threshold packet drop rate
for all of the aggregations. Conversely, the SAP module 120
identifies an aggregation as not having any packet flows
potentially victim to a shrew attack when the current packet drop
rate of the aggregation is less than the current threshold packet
drop rate. The SAP module 120 then assigns a priority status to
packets based on whether their associated aggregation has been
identified as a potential victim of a shrew attack. For an
aggregation identified as a potential victim, the SAP module 120
marks the packets of the packet flows of the aggregation as having
a higher priority status. For an aggregation identified as not
potentially subject to a shrew attack, the SAP module 120 marks the
packets of the packet flows of the aggregation as having a lower
priority status.
[0015] The output control module 116 uses the priority status of
the packets buffered in the output buffer 114 to control the output
of the packets (packets 141-143, for example) to the network
segment 104. In one embodiment, the output control module 116
preferentially selects for output those packets having the higher
priority status over those packets having the lower priority
status. As a result, the packets associated with the aggregations
identified by the SAP module 120 as being potentially subject to a
shrew attack are less likely to be dropped by the congestion
control mechanism implemented by the output control module 116, and
thereby mitigating the shrew attack. The output control module 116
can employ any of a variety of congestion control mechanisms that
implement a preferential drop policy, such as the AQM policy or
WRED policy identified above.
[0016] FIG. 2 illustrates an example implementation of the SAP
module 120 of the network routing device 102 of FIG. 1 in
accordance with at least one embodiment of the present disclosure.
In the depicted example, the SAP module 120 includes a packet drop
rate calculator 202, a fair prioritization controller 204, and a
prioritization module 206. As noted above, the output control
module 116 processes packets buffered in the output buffer 114 for
output from the network routing device 102 to a network segment
104. In processing the packets for output, congestion on the
network segment 104 (either normal congestion or congestion
resulting from a shrew attack) may require that the output control
module 116 drop packets (that is, cease attempting the transmission
of a packet) such that only a subset of the packets buffered by the
output buffer 114 are successfully transmitted by the network
routing device 102 to the network segment 104. Thus, the
preferential selection of packets marked as having a higher
priority status over packets marked as having a lower priority
status results in the higher priority status packets being less
likely to be dropped in the event of network congestion potentially
caused by a shrew attack.
[0017] The packet drop rate calculator 202 receives signaling from
the line interface 106 regarding the incoming packets and the
dropped packets, and from this information the packet drop rate
calculator 202 determines the average packet drop rate across all
of the packet flows and the current packet drop rate for each
aggregation of packet flows. In one embodiment, these packet drop
rates are calculated by the packet drop rate calculator 202 in
terms of cumulative bytes dropped up to time t (identified as d(t))
to cumulative bytes received up to time t (identified as a(t)).
Accordingly, the packet drop rate calculator 202 implements two
counters for each aggregation: an arrival counter (Count(in)) to
accumulate the number of bytes received for the corresponding
aggregation; and a drop counter (Count(out)) to accumulate the
number of bytes dropped for the corresponding aggregation. In one
embodiment the packet drop rate calculator 202 implements these
counters in a memory 208. To illustrate, FIG. 2 depicts four
counter pairs 211-214 implemented in a SRAM 208, each counter pair
corresponding to a different destination port (which serves as the
aggregation criterion in this example).
[0018] To provide packet drop rate statistics that are less
susceptible to small fluctuations in packet drop rates, the packet
drop rate calculator 202, in one embodiment, calculates the average
packet drop rate for each aggregation over a time sliding window
(TSW). The time sliding window is composed of a contiguous set of N
time intervals, wherein each time interval is a fixed duration of
t.sub.s seconds. The duration of t.sub.s is selected so as to be
sufficiently short so as to permit identification of the instant
high packet drop rates, whereas N should be large enough to
consider the previous instant packet drop rates. At the beginning
of each time interval, the packet drop rate calculator 202
initializes the arrival counter and the drop counter for each
aggregation with the cumulative bytes for the counters from the
previous time interval (that is, a(t)=a(t-1) and d(t)=d(t-1)).
During the time interval t, new byte arrivals or byte drops
increment a(t) or d(t), respectively, by the appropriate amount.
Thus, the packet drop rate calculator 202 can calculate the average
packet drop rate P.sub.avg[x] for each aggregation x over a sliding
window of the last N time intervals as represented by the following
equation:
P avg [ x ] = .DELTA. d ( t ) .DELTA. a ( t ) = d ( t ) - d ( t - N
) a ( t ) - a ( t - N ) ##EQU00001##
To calculate this average, the packet drop rate calculator 202 can
maintain N pairs of counters for each aggregation. Using the time
sliding window, the packet drop rate calculator 202 then can
recursively free and reuse counters using a circular modulo counter
allocation. Thus, the total number of counters needed per
aggregation is 2*N in this instance. The average packet drop rate
across all of the aggregations at time t
(P.sub.avg.sub.--.sub.tot(t)) therefore can be calculated by the
packet drop rate calculator 202 as:
P avg_tot ( t ) = i = 1 N Pavg [ i ] N = i = 1 N .DELTA. d ( t ) i
= 1 N .DELTA. a ( t ) ##EQU00002##
The average packet drop rate across all of the aggregations can be
updated, for example, every t.sub.s seconds. The current or instant
packet drop rate P.sub.i[x] for an aggregation x at time t can be
calculated by the packet drop rate calculator 202 as the ratio of
the cumulative arrived bytes for the aggregation to the cumulative
dropped bytes for the aggregation at the time t, or:
P i ( t ) [ x ] = d ( t ) [ x ] a ( t ) [ x ] ##EQU00003##
[0019] The fair prioritization controller 204 determines the
threshold packet drop rate (P.sub.thresh(t)) used to identify
aggregations having a packet flow that is potentially victim to a
shrew attack. In one embodiment, the threshold packet drop rate is
determined based on the average packet drop rate across all
aggregations, and further based on a minimum packet drop rate
P.sub.min such that the threshold packet drop rate is set to the
greater of the average packet drop rate or the minimum packet drop
rate P. The minimum packet drop rate P.sub.min is predetermined or
otherwise set by a user or provider of the network routing device
102 and specifies the minimum packet drop rate at which the SAP
mechanism does not intervene. Accordingly, the minimum packet drop
rate P.sub.min preferably is set to a value sufficient to encompass
small fluctuations in packet drop rates typically encountered in
the packet flows in the absence of a shrew attack, but low enough
to trigger the SAP mechanism quickly in the event of a shrew
attack. An example value of 0.1% for P.sub.min has been identified
as acceptable under certain conditions in simulations performed for
the SAP mechanism.
[0020] The prioritization module 206 uses the dynamically-updated
threshold packet drop rate P.sub.thresh to identify those
aggregations having packet flows potentially subject to a shrew
attack and to prioritize the packets of the identified aggregations
accordingly so as to more fully ensure their successful
transmission by the network routing device 102. In one embodiment,
the prioritization module 206 makes this identification on the
basis of a comparison of the instant packet drop rate P.sub.i(t)[x]
of a selected aggregation x with the threshold packet drop rate
P.sub.thresh(t). In the event that the instant packet drop rate for
the selected aggregation is greater than the threshold packet drop
rate (that is, when P.sub.i(t)[x]>P.sub.thresh(t)), the
prioritization module 206 identifies the selected aggregation as
potentially subject to a shrew attack. Conversely, in the event
that the instant packet drop rate is not greater than the threshold
packet drop rate (that is, when P.sub.i(t)[x]<=P.sub.thresh(t)),
the prioritization module 206 identifies the selected aggregation
as not subject to a shrew attack.
[0021] Rather than using a single threshold packet drop rate, in
one embodiment the applicable threshold packet drop rate can be
selected from multiple threshold packet drop rates. To illustrate,
each application can have a separate threshold packet drop rate
based on an average packet drop rate for just those aggregations
associated with the particular application. To illustrate, one
threshold packet drop rate could be calculated for packet flows
associated with real-time applications and another threshold packet
drop rate could be calculated for packet flows associated with file
transfer applications. Alternately, the same average packet drop
rate can be calculated, but different minimum threshold packet drop
rates P.sub.min can be used for different aggregations.
[0022] Upon receipt of an outgoing packet, the prioritization
module 206 identifies the aggregation to which the packet is
associated and marks the packet as having either the higher
priority status or the lower priority status based on whether the
identified aggregation was determined to be potentially subject to
a shrew attack. Upon marking the packet as having either the higher
priority status or the lower priority status, the packet is placed
in the output buffer 114. The output control module 116 then can
employ a preferential packet dropping scheme, such as the AQM
policy or the WRED policy, to preferentially select for
transmission those packets from the output buffer 114 that have the
higher priority status over those packets having the lower priority
status (or, put another way, to preferentially drop those packets
from the output buffer 114 that have the lower priority status over
those packets having the higher priority status).
[0023] By selectively prioritizing outgoing packets of aggregations
of packet flows having at least one packet flow that may be victim
to a shrew attack, the SAP mechanism described above can mitigate
the effects of the shrew attack by more fully ensuring that the
packets of the victim packet flow are effectively transmitted. As
such, the SAP mechanism can reduce the risk of TCP sessions closing
during a shrew attack, as well as help maintain a normal throughput
for packet flows during a shrew attack.
[0024] Although the present specification describes components and
functions that may be implemented in particular embodiments with
reference to particular standards and protocols, the invention is
not limited to such standards and protocols. Such standards are
periodically superseded by faster or more efficient equivalents
having essentially the same functions. Accordingly, replacement
standards and protocols having the same or similar functions as
those disclosed herein are considered equivalents thereof.
[0025] The illustrations of the embodiments described herein are
intended to provide a general understanding of the structure of the
various embodiments. The illustrations are not intended to serve as
a complete description of all of the elements and features of
apparatus and systems that utilize the structures or methods
described herein. Many other embodiments may be apparent to those
of skill in the art upon reviewing the disclosure. Other
embodiments may be utilized and derived from the disclosure, such
that structural and logical substitutions and changes may be made
without departing from the scope of the disclosure. Additionally,
the illustrations are merely representational and may not be drawn
to scale. Certain proportions within the illustrations may be
exaggerated, while other proportions may be minimized. Accordingly,
the disclosure and the figures are to be regarded as illustrative
rather than restrictive.
[0026] The Abstract of the Disclosure is provided to comply with 37
C.F.R. .sctn.1.72(b) and is submitted with the understanding that
it will not be used to interpret or limit the scope or meaning of
the claims. In addition, in the foregoing Detailed Description of
the Drawings, various features may be grouped together or described
in a single embodiment for the purpose of streamlining the
disclosure. This disclosure is not to be interpreted as reflecting
an intention that the claimed embodiments require more features
than are expressly recited in each claim. Rather, as the following
claims reflect, inventive subject matter may be directed to less
than all of the features of any of the disclosed embodiments. Thus,
the following claims are incorporated into the Detailed Description
of the Drawings, with each claim standing on its own as defining
separately claimed subject matter.
[0027] The above disclosed subject matter is to be considered
illustrative, and not restrictive, and the appended claims are
intended to cover all such modifications, enhancements, and other
embodiments which fall within the true spirit and scope of the
present disclosed subject matter. Thus, to the maximum extent
allowed by law, the scope of the present disclosed subject matter
is to be determined by the broadest permissible interpretation of
the following claims and their equivalents, and shall not be
restricted or limited by the foregoing detailed description.
* * * * *