TCP in Wireless Networks: Issues, Approaches, and Challenges

1
TCP in Wireless Networks Issues, Approaches,
and Challenges
Dr. Ka-Cheong Leung Joint work with Professor
Victor O. K. Li
2
Overview

• Review on TCP
• Issues of Running TCP on Wireless Networks
• Contributions
• Taxonomy of Solutions
• Overview of Existing Solutions
• Further Discussion
• Open Research Issues

3
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

4
Review on TCP (Contd)

• 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
• timer timeout gt a segment loss gt 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

5
Review on TCP (Contd)

• Congestion control operations (Contd)
• 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 gt a segment loss

6
Review on TCP (Contd)

• Congestion control operations (Contd)
• 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

7
Review on TCP (Contd)

• Congestion control operations (Contd)
• 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

8
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

9
Issues of TCP (Contd)

• Taxonomy of wireless networks (Contd)
• 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)

10
Issues of TCP (Contd)

• 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

11
Issues of TCP (Contd)

• Characteristics of wireless networks (Contd)
• 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

12
Issues of TCP (Contd)

• Characteristics of wireless networks (Contd)
• 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

13
Issues of TCP (Contd)

• 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

14
Issues of TCP (Contd)

• Problems for TCP (Contd)
• 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
• host mobility gt 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

15
Issues of TCP (Contd)

• Problems for TCP (Contd)
• 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

16
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

17
Taxonomy of Solutions
18
Taxonomy of Solutions (Contd)

• 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

19
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

20
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• TCP-Peach (Contd)
• sudden start
• substitute slow start
• aim to open up the congestion window faster
• transmit one dummy data segment for every
  until (awnd 1) dummy segments have been sent
• t estimated RTT
• increment cwnd by 1 segment upon the receipt of
  an ACK for a dummy segment

21
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• TCP-Peach (Contd)
• 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

22
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• TCP-Peach (Contd)
• 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 gt
  congestion at routers
• TCP-Peach no more than 1 NIL segment sent per
  ACK
• all routers configured to implement
  priority-based scheduling

23
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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 non-congestive loss
• SP-TCP
• avoid triggering more than 1 probing cycle in
  small time interval

24
(No Transcript)
25
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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

26
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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 lt threshold gt 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

27
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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 gt 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 gt 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

28
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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 lt 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

29
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• 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

30
Overview of Existing Solutions (Contd)

• Congestion detection approach (Contd)
• TCP-Casablanca (Contd)
• 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

31
Overview of Existing Solutions (Contd)

• 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 destinations response with a positive
  advertised window size
• exit from the persist mode
• resume its transmission as normal

32
Overview of Existing Solutions (Contd)

• State suspension approach (Contd)
• Freeze-TCP (Contd)
• 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

33
Overview of Existing Solutions (Contd)

• State suspension approach (Contd)
• 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 gt network
  congestion gt take regular congestion control
  measures
• otherwise freeze connection state until both
  layers become stable

34
Overview of Existing Solutions (Contd)

• State suspension approach (Contd)
• 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

35
Overview of Existing Solutions (Contd)

• State suspension approach (Contd)
• 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
  re-connection

36
Overview of Existing Solutions (Contd)

• State suspension approach (Contd)
• 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

37
Overview of Existing Solutions (Contd)

• 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

38
Overview of Existing Solutions (Contd)

• Response postponement approach (Contd)
• 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 ß?
• ? 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

39
Overview of Existing Solutions (Contd)

• Response postponement approach (Contd)
• 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 link-layer 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

40
Overview of Existing Solutions (Contd)

• 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 senders 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

41
Overview of Existing Solutions (Contd)

• Hybrid approach (Contd)
• ATCP (Contd)
• 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

42
(No Transcript)
43
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

44
Further Discussion (Contd)

• Congestion detection approach (Contd)
• 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

45
Further Discussion (Contd)

• 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

46
Further Discussion (Contd)

• State suspension approach (Contd)
• 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, short-lived link errors (from signal
  fading)
• TCP-DOOR no mechanisms to deal with
  non-congestive losses

47
Further Discussion (Contd)

• 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

48
Further Discussion (Contd)

• 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

49
(No Transcript)
50
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

51
Open Research Issues (Contd)

• 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