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  2006 Ka-Cheong Leung and 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  2006 Ka-Cheong Leung and Victor O.K. Li

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  2006 Ka-Cheong Leung and Victor O.K. Li

4. 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 • timer timeout => a segment loss => network congestion • ssthresh set to the half of the amount of outstanding data • slow start untilssthresh • congestion avoidance phase • cwnd increased by 1 for each RTT  2006 Ka-Cheong Leung and Victor O.K. Li

5. 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

6. 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

7. 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

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  2006 Ka-Cheong Leung and Victor O.K. Li

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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 • 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

15. 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

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  2006 Ka-Cheong Leung and Victor O.K. Li

17. Taxonomy of Solutions  2006 Ka-Cheong Leung and Victor O.K. Li

18. 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

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  2006 Ka-Cheong Leung and Victor O.K. Li

20. 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 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

21. 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

22. 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

23. 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 non-congestive 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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25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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 re-connection  2006 Ka-Cheong Leung and Victor O.K. Li

36. 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

37. 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

38. 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

39. 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 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  2006 Ka-Cheong Leung and Victor O.K. Li

40. 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

41. 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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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  2006 Ka-Cheong Leung and Victor O.K. Li

44. 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

45. 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

46. 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, short-lived link errors (from signal fading) • TCP-DOOR: no mechanisms to deal with non-congestive losses  2006 Ka-Cheong Leung and Victor O.K. Li

47. 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

48. 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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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  2006 Ka-Cheong Leung and Victor O.K. Li