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 1 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 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 3 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 until ssthresh congestion avoidance phase – cwnd increased by 1 for each RTT 2006 Ka-Cheong Leung and Victor O.K. Li 4 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 5 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 6 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 7 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 8 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 9 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 10 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 11 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 12 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 13 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 14 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 15 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 16 Taxonomy of Solutions 2006 Ka-Cheong Leung and Victor O.K. Li 17 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 18 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 19 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 20 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 21 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 22 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 23 2006 Ka-Cheong Leung and Victor O.K. Li 24 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 25 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 26 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 27 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 28 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 29 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 30 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 31 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 32 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 33 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 34 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 35 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 36 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 37 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 38 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 39 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 40 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 41 2006 Ka-Cheong Leung and Victor O.K. Li 42 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 43 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 44 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 45 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 46 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 47 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 48 2006 Ka-Cheong Leung and Victor O.K. Li 49 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