Download TCP in Wireless Networks: Issues, Approaches, and

TCP in Wireless Networks: Issues,
Approaches, and Challenges*
Dr. Ka-Cheong Leung
*Joint work with Professor Victor O. K. Li
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview
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Review on TCP
Issues of Running TCP on Wireless Networks
Contributions
Taxonomy of Solutions
Overview of Existing Solutions
Further Discussion
Open Research Issues
 2006 Ka-Cheong Leung and Victor O.K. Li
2
Review on TCP
• TCP
– byte-stream protocol
• cumulative ACK with next expected octet number
– credit-based flow control
• advertised window set by the destination
• number of unacknowledged data bytes the source
can send to the destination
 2006 Ka-Cheong Leung and Victor O.K. Li
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Review on TCP (Cont’d)
• Congestion control operations
– number of packets within the Internet kept below the
level at which network performance drops significantly
– cwnd set to 1 (maximum segment size) when a new
connection is established
– slow start
• cwnd set to 1
• cwnd incremented by one for each ACK received
– retransmission timer
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•
•
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timer timeout => a segment loss => network congestion
ssthresh set to the half of the amount of outstanding data
slow start until ssthresh
congestion avoidance phase
– cwnd increased by 1 for each RTT
 2006 Ka-Cheong Leung and Victor O.K. Li
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Review on TCP (Cont’d)
• Congestion control operations (Cont’d)
– duplicate ACK
• data octet number of an arriving segment is greater
than the expected one
• destination finds a gap in sequence number space
(sequence hole)
• destination sends a duplicate ACK, an ACK with the
same expected data octet number in the cumulative
acknowledgement field
• in-order communication channel
– reception of duplicate ACK => a segment loss
 2006 Ka-Cheong Leung and Victor O.K. Li
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Review on TCP (Cont’d)
• Congestion control operations (Cont’d)
– fast retransmit
• source receives three duplicate ACKs
• inferred loss segment retransmitted immediately
– fast recovery
• fast retransmission suggests the presence of mild network
congestion
• ssthresh set to the half of the amount of outstanding data
• cwnd set to ssthresh + number of duplicate ACKs received
• cwnd reset to ssthresh and congestion avoidance triggered
when an ACK for a new segment arrives
 2006 Ka-Cheong Leung and Victor O.K. Li
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Review on TCP (Cont’d)
• Congestion control operations (Cont’d)
– popular TCP variants
• TCP Tahoe
– slow start, congestion avoidance, and fast retransmit
– for each inferred segment loss
» ssthresh set to half of the amount of outstanding data
» do slow start
• TCP Reno
– TCP Tahoe + fast recovery
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP
• Taxonomy of wireless networks
– infrastructured networks
• planned, permanent network device installations
• cellular networks and most WLANs
– static infrastructured networks
– set up with fixed topology connected to backbone network
– wireless host can connect via a fixed point (based station or access point)
• satellite networks
– quasi-static or dynamic topology
– space segment: comprises of satellites
– ground segment: a number of base stations (gateway stations) through
which all communications via long-haul satellite links take place
• terminal handoff
– mobile host (MH) moves away from the coverage of its base station
– MH hands over its proxy for communication from one base station to
another one
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Taxonomy of wireless networks (Cont’d)
– Ad hoc networks
• without a fixed topology
• direct communication: the receiver is in the transmission
coverage of the sender
• indirect communication
– send messages to a host in its transmission coverage
– receiving host relays the messages on its way to the destination
• merits: flexibility, more robust
• drawbacks:
– more difficult and complex to perform routing
– more difficult to control or coordinate proper operation of an ad
hoc network (for activities like time synchronization, power
management, and packet scheduling)
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Characteristics of wireless networks
– channel contention
• signals are broadcast and interfere with each other
• transmissions may fail for concurrent transmissions
within the interference range of either sender
• medium access protocol needed for coordination
• TDMA-based multi-hop wireless networks
– limit the number of in-flight segments concurrently
– correlated arrivals of data segments and their ACKs lead
to contention for the wireless channel
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Characteristics of wireless networks (Cont’d)
– signal fading
• signals distorted or weakened
– propagated over an open, unprotected, and ever-changing
medium with irregular boundary
– some signal may disperse and travel on different paths due to
reflection, diffraction, and scattering caused by obstacles
– mobility
• infrastructured networks
– protocol required to ensure seamless transition during a handoff
– packets may be lost during a handoff
• ad hoc networks
– transmission route recomputed to cater for topological changes
– effective and efficient routing protocol needed for frequent
topological changes
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Characteristics of wireless networks (Cont’d)
– limited power and energy
• power source may not be able to deliver power as
much as the one installed in a fixed device
• hard to receive a continuous supply of power
• effective and efficient operations with power
management
– minimize the number of transmissions and receptions for
certain communication operations
» minimize the number of retransmissions for an energy
efficient TCP
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Problems for TCP
– random loss
• dropped due to signal fading
• non-congestive segment losses not negligible
– violate the working assumption of the traditional
congestion control measures for TCP
– congestion control mechanisms react inappropriately by:
» keeping the sending rate of a TCP connection small
» retransmitting some data segments spuriously
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Problems for TCP (Cont’d)
– burst loss
• may be initiated by signal fading
– prolonged uncontrollable channel interferences
• infrastructured networks
– a chain of packets lost due to a handoff event
– frequency: size of coverage region and host mobility
• ad hoc networks
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–
–
–
host mobility => topological change or network partition
re-routing process can take some time to complete
some packets may be lost during the process
frequency: transmission range and host mobility
• can lead to serial timer expirations
– multiple consecutive timer expirations and retransmissions of the same
data segment within a single blackout period
– result in a terribly long period of inactivity of the connection (due to
exponential timer backoff) even after the network conditions has restored
to normal
 2006 Ka-Cheong Leung and Victor O.K. Li
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Issues of TCP (Cont’d)
• Problems for TCP (Cont’d)
– packet reordering
• network behaviour where the receiving order of a flow of
packets differs from its sending order
• persistent and substantial packet reordering violates the (near)
in-order channel assumption
• result in substantial degradation in application throughput and
network performance
• causes:
– link-layer retransmission
– infrastructured networks
» packets take different routes due to handoff
– ad hoc networks
» re-routing due to topological changes
 2006 Ka-Cheong Leung and Victor O.K. Li
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Contributions
• Present an overview of recent developments and explore
some open research issues and challenges
– survey end-to-end solutions proposed to date
• require no intermediaries to scoop the state of a connection
• may require supporting functions implemented at the routers for the
sake of efficiency and performance enhancements
• Give the readers a new angle to view the existing state of
the art
– classify the surveyed solutions based on the way they tackle the
problems
– focus on enhancements that have been implemented in the TCP
clients
• Provide the readers a short tutorials of the surveyed
representative solutions
 2006 Ka-Cheong Leung and Victor O.K. Li
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Taxonomy of Solutions
 2006 Ka-Cheong Leung and Victor O.K. Li
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Taxonomy of Solutions (Cont’d)
• Congestion detection approach
– measure the current network conditions
– determine whether network congestion has actually occurred
– choose a proper traffic control strategy to differentiate congestive
issues from the non-congestive ones based on the measured
information
• State suspension approach
– detect the current network state
– decide when communication activity is suspended and when it can
be resumed to avoid non-congestive losses
• Response postponement approach
– delay triggering a traffic control response to alleviate the problems
in wireless networks
• Hybrid approach
– a collection of methods that can be classified by more than 1
approach described above
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions
• Congestion detection approach
– TCP-Peach (Akyildiz, Morabito, and Palzzo, 2001)
• deal with adverse effects found in satellite networks
with long propagation delays & high link error rates
• dummy segments
– low-priority segments with a copy of recently transmitted
data
– probe for the availability of network resources
– successfully delivered dummy segment indicates that:
» unused network resources exist
» transmission rate can be increased accordingly
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Peach (Cont’d)
• sudden start
– substitute slow start
– aim to open up the congestion window faster

– transmit one dummy data segment for every awnd
until
(awnd – 1) dummy segments have been sent
» τ: estimated RTT
– increment cwnd by 1 segment upon the receipt of an ACK
for a dummy segment
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Peach (Cont’d)
• rapid recovery
– replace fast recovery
– halve cwnd in response to an inferred segment loss
– arrival of an ACK for a data segment
» send 2 dummy segments until a total of 2 cwnd
segments have been transmitted
– increment cwnd by 1 segment
» arrival of an ACK for a data segment
» arrival of an ACK for a dummy segment, after
receiving cwnd ACKs for dummy segments
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Peach (Cont’d)
• merit: maintain ACK-clocking when cwnd is smaller than the
number of unacknowledged data segments
• drawbacks:
– implicitly assumed that more than half of the dummy segments
are lost in transit for a congestive loss event
– all dummy segments can be successfully delivered to the
destination for a non-congestive loss event
– wastage of network resources since the delivery of dummy
segments does not result in any gain in connection goodput
» TCP-Peach+: NIL segments with unacknowledged data in
place of dummy segments
– dummy segments are sent at a rate doubled that before a loss
event is conjectured => congestion at routers
» TCP-Peach+: no more than 1 NIL segment sent per ACK
– all routers configured to implement priority-based scheduling
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Probing (Lahanas and Tsaoussidis, 2002)
• sender-side solution
– aim to enhance performance against random loss and burst loss
• use of probing devices
– determine whether network congestion has occurred when a
segment loss is inferred
• A-TCP
– invoke a probing cycle upon receiving 3 duplicate ACKs or a
retransmission timer expiration
– probe segments sent until the ACKs of a pair of probes are
received within the specified time period
– recovery process depends on the status of network congestion
– drawbacks: costly to perform and respond slowly to noncongestive loss
• SP-TCP
– avoid triggering more than 1 probing cycle in small time interval
 2006 Ka-Cheong Leung and Victor O.K. Li
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 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP Westwood (TCPW) (Casetti et al., 2002)
• sender-side solution
• idea: adjust the size of the congestion window upon an inferred
segment loss by monitoring the rate of acknowledging data
• upon each ACK arrival
– use the amount of new data acknowledged by that ACK to update the
estimate for the available bandwidth of the connection
• when taking congestion control
– ssthresh assigned as:
» (estimated available BW) x (minimum RTT) / (segment size)
• merit: decouple congestion control from error control
• drawbacks:
– some unfriendliness to TCP Reno
– overstate the available bandwidth with the presence of ACK
compression
» TCP Westwood+: bandwidth sample computed every RTT instead
of with each ACK arrival to eliminate the high frequency
components contained in the bandwidth samples
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP Veno (Fu and Liew, 2003)
• sender-side refinements on TCP Reno
– deal with random loss
• estimate the backlog accumulated along the communication path of
the connection
– measured backlog < threshold => no congestion
» inferred segment loss as a random loss
• two refinements
– congestive loss inferred
» cwnd increased by 1 segment every 2 RTTs instead of each RTT
– random loss inferred
» fast retransmit: ssthresh set to 0.8 cwnd instead of 0.5 cwnd
• drawbacks:
– performance improvement fades with high random loss rate
– fail to deal with multiple segment losses in the same congestion window
– may not work well in ad hoc networks
» backlog estimation sensitive to RTT oscillation due to route change
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Jersey (Xu, Tian, and Ansari, 2004)
• follow same idea as TCPW to observe the rate of data acknowledged by ACKs
– simpler estimator for the available bandwidth
• adopt slow start, congestion avoidance, and fast recovery from TCP Reno
• use explicit retransmit instead of fast retransmit
– simply perform a segment retransmission
• congestion warning (CW)
– router marks congestion experienced (CE) bit in the IP header of all packets when
the average queue length > threshold
– destination echoes the congestion information by setting the explicit echo (ECE) bit
of all segments until it receives a segment with the congestion window reduced
(CWR) bit set
• ACK arrival
– estimate the available bandwidth and compute the optimal size of the congestion
window when not run for 1 RTT
– no congestion warning => proceed similarly as TCP Reno
– congestion warning
» apply rate control procedure first to set the size of the congestion window
based on the computed available bandwidth
» follow the congestion control measures as without congestion warning
• drawbacks: aware of CW scheme, fail to handle burst loss
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– JTCP (Wu and Chen, 2004)
• use jitter ratio to determine whether an inferred segment loss
congestive or non-congestive
• fast recovery triggered only when:
– inferred congestive loss is detected
– preceding fast recovery carried out at least 1 RTT ago
• immediate recovery
– set ssthresh as D cwnd, where 0.5 < D ≤ 1
• retransmission timer expires
– congestive loss: slow start
– non-congestive loss: fast retransmit and fast recovery
• drawbacks:
– insert and process timestamps
– unable to handle burst loss satisfactorily
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Casablanca (Biaz and Vaidya, 2005)
• apply a simple biased queue management scheme
• discriminate congestion losses from random losses
• idea: de-randomize congestion losses
– distribution of congestive losses differs from random losses
• label 1 “out” segment for every k segments, else “in” segments
• router experiences congestion
– drop “out” packets before dropping “in” packets
• dropping sequence will show correlated losses if the lost
packets are dropped due to network congestion
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Congestion detection approach (Cont’d)
– TCP-Casablanca (Cont’d)
• extended from TCP Newreno
– same as TCP Reno except that fast recovery exits only when all sent data
segments are acknowledged before it is entered
• destination uses a simple loss discriminator function to diagnose
whether a loss is congestive or not
• non-congestive loss
– mark a duplicate ACK with ELN so that the source does not halve the
size of the congestion window
• merit: identify congestive losses with more than 95% accuracy and
non-congestive losses with more than 75% accuracy
• drawbacks:
– require participating routers to have a differential packet dropping policy
– inferior performance with respect to other TCP-friendly flows since
“out” segments are dropped in advance of any other segments
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach
– Freeze-TCP (Goff et al., 2000)
•
•
•
•
•
improve performance with frequent disconnections
receiver-side solution (for a mobile receiver)
continuously monitor the signal strength of wireless antennas
detect any impending handoffs
about 1 RTT before a handoff
– send some zero window advertisements (ZWAs) to force its peer (sender) into the
persist mode
– ZWA piggybacked into an ACK
• receive a ZWA from the receiver
– persist mode: freeze all retransmission timers and the size of the congestion window
– send zero window probes (ZWPs) with inter-probe time being backed off
exponentially
• receive a destination’s response with a positive advertised window size
– exit from the persist mode
– resume its transmission as normal
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach (Cont’d)
– Freeze-TCP (Cont’d)
• drawbacks:
– must be aware of mobility and need some cross-layer
information exchanges
– need to predict when a disconnection is expected to
happen
– fail to predict and detect an upcoming disconnection event
if it happens at a wireless link along the transmission path
– resumed transmission rate may be set inappropriately
– can only avoid performance degradation due to
disconnections
» fail to avoid and identify occasional segment losses
because of signal fading
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach (Cont’d)
– ILC-TCP (Chinta, Helal, and Lee, 2003)
• sender-side solution (for a mobile sender)
– prevent performance degradation due to temporary
disconnections
• idea: control decision based on the state information
– link layer: link state (good or bad)
– network layer: IP-level handoff started or completed
• upon a timer expiration
– both link and network layers stable => network
congestion => take regular congestion control measures
– otherwise: freeze connection state until both layers become
stable
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach (Cont’d)
– TCP-Feedback (Chandran et al., 2001)
• improve the performance for route failures
• route disruption detected
– failure point transmits a route failure notification (RFN) packet to the
source
– each immediate mobile host invalidates the route
» alternative route exists: reroute packets and discard the RFN packet
» otherwise: relay the RFN packet to the source
• source receives the RFN packet
– bring the TCP connection to the snooze state until the route failure
timeout or a route reestablishment notification (RRN) packet received
• new route learnt by an immediate mobile host
– send an RRN packet to the source
• merit: able to handle route disruption at any wireless link
• drawbacks:
– burst injection
– resumed transmission rate may be set inappropriately
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach (Cont’d)
– ELFN (Holland and Vaidya, 2002)
• similar to TCP-Feedback
• differences with TCP-Feedback
– ELFN relies on the route failure messages for dynamic
source routing (DSR) to notify a source about link and
route failures
– no route maintenance or invalidation at immediate hosts
– no need for any immediate hosts to send or forward a
RRN packet to a source to re-activate a suspended
connection
» source probes the network periodically for reconnection
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• State suspension approach (Cont’d)
– TCP-DOOR (Wang and Zhang, 2002)
• detect route changes through out-of-order events
• out-of-order data/ACK detection
– insert the TCP packet sequence number and ACK
duplication sequence number, or current timestamps, into
each data and ACK segment, respectively
• temporarily disable congestion control
– source keeps its state variable unchanged for a time period
• instant recovery during congestion avoidance
– source recovers immediately to the state before the
congestion response invoked within a time period
• drawbacks:
– transmission rate may be set inappropriately after a route change
– fail to perform well in a congested network environment with substantial
persistent packet reordering
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Response postponement approach
– DelAck (Altman and Jiménez, 2003)
• use of delayed ACK techniques to improve performance in multi-hop
wireless ad hoc networks
• receiver-side solution
– reduce channel contentions among data segments and ACKs of the same
TCP connection
– as a side-effect to reduce performance degradation due to packet
reordering
• idea: delay acknowledging the arrivals of data segments and reduce
the number of ACKs sent to the source
– generate an ACK for every d data segments or the first unacknowledged
data segment has been received for a certain time period (e.g. 0.1 s)
• merit:
– reduce the connection overhead and hence the channel contentions
• drawbacks:
– d is orthogonal to the segment sequence number in general, but DelAck
sets it to increase with the sequence number
– burst injection due to delayed acknowledgement
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Response postponement approach (Cont’d)
– TCP-ADA (Singh and Kankipati, 2004)
• receiver-side solution, similar to DelAck
• postpone acknowledgement for a time period
– defer sending an ACK of a segment for βΔ
» Δ is an EWMA of the inter-segment arrival time
– deferment period restarted every time data segment arrives
– ACK sent when deferment timer expires or maximum
deferment period is reached
• drawbacks:
– send bursts of new segments once every RTT
– significant drop in throughput if ACKs are lost
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Response postponement approach (Cont’d)
– TCP-DCR (Bhandarkar et al., 2005)
• sender-side solution
– meliorate the TCP robustness to non-congestive events
• idea: delay a congestion response for a time interval after the first
duplicate ACK is received
• set the time interval as RTT to deal with packet reordering due to linklayer retransmissions
• merit:
– perform significantly better than TCP with SACK and TCPW in the
presence of channel errors
• drawback:
– chosen period of deferment highly dependent on RTT
» several retransmissions of the same packet can be delayed longer
than one RTT
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Hybrid approach
– ATCP (Liu and Singh, 2001)
• resolve problems with TCP in ad hoc networks
– high bit error rates, frequent route changes, network partitions,
and packet reordering
• idea: introduce a layer (ATCP layer) between TCP and IP at the
sender’s protocol stack
– ATCP layer: monitor the current TCP state and spoof TCP from
triggering its congestion control mechanisms inappropriately
• apply ECN and ICMP to sense the onset of network congestion
and integrity of the transmission path
 2006 Ka-Cheong Leung and Victor O.K. Li
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Overview of Existing Solutions (Cont’d)
• Hybrid approach (Cont’d)
– ATCP (Cont’d)
• four states
– normal: do nothing and transparent
– congested: take congestion behaviour of TCP
– loss: put TCP in the persist mode and send unacknowledged data
segments
– disconnected: place TCP into the persist mode and send probes to the
destination; slow start is invoked when leaving the state
• merit:
– able to handle most of the problems relating to wireless networks
• drawbacks:
– inefficient in using the available bandwidth with frequent route changes
and network partition
– aware of and implemented with ECN
– do not allow a source to send new data segments in the loss state
 2006 Ka-Cheong Leung and Victor O.K. Li
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 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion
• Congestion detection approach
– either use probes or information stored in the data segments and
ACKs to detect the congestion conditions
– TCP-Peach and TCP-Peach+
• send low-priority segments to quickly seize the available bandwidth
– TCP-Probing
• transmit probes to detect whether the network is congested based on
the estimated RTT
– TCPW, TCP Westwood+, TCP Veno, TCP-Jersey, and JTCP
• estimate the network congestion level based on the spatial and
temporal information carried by the ACKs
– TCP-Casablanca
• infer the network congestion status based on the ratio of the marked
segments being dropped by the network
 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion (Cont’d)
• Congestion detection approach (Cont’d)
– merit:
• generally able to distinguish between congestive
loss and non-congestive random loss
• determine the appropriate traffic control measure
strategy to improve the TCP performance
– drawbacks:
• generally fail to gracefully handle multiple segment
losses in the same congestion window as most
approaches extended from TCP Reno
• no specific mechanisms to avoid burst loss due to
temporary disconnections
 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion (Cont’d)
• State suspension approach
– utilize the state information as well as route failure and restoration
notifications
– decide whether the communication activity of a connection is suspended
or resumed
– Freeze-TCP
• use the signal strength information to infer the occurrence of a temporary
disconnection
– ILC-TCP
• freeze the communication whenever either link or network errors are
experienced
– TCP-Feedback and ELFN
• stop the communication activity when a route failure notification is received
• resume communication after a route is established for a suspended connection
– TCP-DOOR
• temporarily disable congestion control or perform instant recovery during
congestion avoidance after detecting an out-of-order event
 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion (Cont’d)
• State suspension approach (Cont’d)
– merit:
• successful at suspending any congestion control measures and
stopping further segment losses when a temporary
disconnection is encountered
• TCP-DOOR: alleviate some performance problems caused by
packet reordering
– drawback:
• fail to deal with occasional random losses due to transit, shortlived link errors (from signal fading)
• TCP-DOOR: no mechanisms to deal with non-congestive
losses
 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion (Cont’d)
• Response postponement approach
– defer taking any traffic control measures to gather more network
information to see if the decision needs to be changed
– DelAck and TCP-ADA
• delay the issuance of an ACK
• reduce the load of the control traffic and thus channel contention
• as a side-effect to deal with packet reordering
– TCP-DCR
• postpone triggering the congestion control response to a newly
received ACK
– merits: with the presence of packet reordering
• able to reduce spurious retransmissions
• maintain a larger congestion window
– drawbacks:
• fail to clock out traffic during the deferment of congestion response
(except for TCP-DCR)
• no mechanisms to deal with non-congestive losses
 2006 Ka-Cheong Leung and Victor O.K. Li
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Further Discussion (Cont’d)
• Hybrid approach
– use ECN information and source quench messages to
detect the occurrence of network congestion
– utilize the destination unreachable messages to detect
temporary disconnections
– merit:
• able to handle most of the problems (random loss, burst loss,
and packet reordering) relating to wireless networks
– drawbacks:
• inefficient in using the available bandwidth with frequent route
changes and network partition
• aware of and implemented with ECN
• do not allow a source to send new data segments in the loss
state
 2006 Ka-Cheong Leung and Victor O.K. Li
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 2006 Ka-Cheong Leung and Victor O.K. Li
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Open Research Issues
• Integrated solutions for all types of wireless
problems
– except for ATCP, none of the surveyed solutions can
deal with all of the aforementioned problems
– ATCP
• loss state: stop transmitting new data segments until a new
ACK arrives
• presence of persistent packet reordering
– block the regular new data segment transmissions
– reduce the connection goodput
– shrink the battery lifetime of a wireless host due to unnecessary
retransmissions
 2006 Ka-Cheong Leung and Victor O.K. Li
50
Open Research Issues (Cont’d)
• Improved discriminator for wireless problems
– problem mistaken
• improper traffic control may exacerbate the problem
(congestive loss as non-congestive)
– need an efficient and effective discriminator to correctly
identify congestive loss, random loss, burst loss, and
packet reordering
• Theoretical basis for an efficient and optimal
scheme to handle the identified non-congestive
losses
 2006 Ka-Cheong Leung and Victor O.K. Li
51