Wireless networks - Lecture 33: TCP over wireless networks. The main topics covered in this chapter include: motivation; TCP variants; issues in heterogeneous wireless networks; TCP schemes for wireless; hard-state transport layer approaches; soft-state cross layer signalling approaches;...
Wireless Networks Lecture 33 TCP Over Wireless Networks Dr Ghalib A Shah Outlines Motivation TCP Variants ► Slow start ► Fast Retransmit/Recovery (TCP Reno) Issues in Heterogeneous Wireless Networks TCP Schemes for Wireless ► ► ► ► Pure Link-level Approaches Soft-state Transport Layer Caching Approaches Soft-state Cross Layer Signalling Approaches Hard-state Transport Layer Approaches Last Lecture MAC Layer ► ► ► ► ► Scalability Single Channel Multi-Channel Some Ideas Research Issues Network Layer ► Routing ► Wish List (Scalability, fast route discovery/repair, mobility, flexibility, QoS, Multicast) ► Route Optimization Criteria ► Routing fairness ► Routing – Cross-layer design QoS Support at each layer WMN Standards Motivation Characteristics of wireless networks ► ► ► ► Lack of infrastructure in ad hoc networks Mobility Shared channel Limited bandwidth Transport protocols typically designed for ► Fixed end-systems ► Fixed, wired networks ► Characteristics of TCP • • • • Window-based: not possible to maintain fine-grained timers on a per-flow basis Slow –start Loss-based congestion indication Dependence on ACKs TCP congestion control Packet loss in fixed networks typically due to overload and is detected as ► 1) Retransmission timeout (RTO) at source ► 2) Arrival of three duplicate ACKs at source ► 3) Receipt of ICMP source quench message Routers discard packets as soon as the buffers are full TCP recognizes congestion only indirectly via missing acknowledgements Retransmissions unwise, they would only contribute to the congestion and make it even worse Slow-start algorithm as reaction which slowly converges to optimal bandwidth TCP Slow Start Sender calculates a congestion window for a receiver Start with a congestion window size equal to one segment Exponential increase of the congestion window up to the congestion threshold, then linear increase Missing acknowledgement causes the reduction of the congestion threshold to one half of the current congestion window Congestion window starts again with one segment TCP Fast Retransmit/Recovery (TCP Reno) TCP sends an acknowledgement only after receiving a packet If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver However, the receiver got all packets up to the gap and is actually receiving packets Therefore, packet loss is not due to congestion, continue with current congestion window (do not use slow-start) When packet loss occurs, congestion window size is reduced ► Due to timeout: cwnd =1 and enter slow start ► Due to duplicate ACKs: cwnd =cwnd/2 +3×segment_size Congestion window size is increased when data is Issues in Heterogeneous Wireless Networks Bit Error Rate (BER): ► 10 or worse are possible upon change in wireless environment Bandwidth ► Very less as compared to wired networks ► TCP underestimated bandwidth in wireless networks Round Trip Time (RTT): ► The wireless media exhibits longer latencies due to long distances or NLOS path ► Large variation in RTT in wireless networks Mobility: ► Addition of mobile devices introduces huge amount of indeterminate delay in rather a stationary network Power consumption 10 Soft-state Cross Layer Signaling Approaches: ► These approaches make the transport layer sender aware of the wireless link and separate the congestion losses from the error losses ► But involve changes at some or all of the intermediate nodes and at the transport layer of the sender’s protocol Hard-state Transport Layer Approaches: ► These solutions encompass all forms of splitting and the end-to-end semantics may be sacrificed ► The advantage of these approaches is that the wireless link is completely shielded from damage loss 14 Pure Linkle ve l Appro ac he s Reliable link-level protocols are implemented on the wireless link which perform local retransmissions to improve the reliability of communication independent of the higher-level protocols ► These protocols employ techniques such as forward error correction (FEC) for error control ► and automatic repeat request (ARQ) for retransmission of lost packets The timeout value for local (link level) retransmissions is of major concern ► Interaction between the link-level retransmission timeouts and the transport-level timeouts for TCP can lead to degraded performance if care is not taken while selecting the timeout values 15 S o fts tate Trans po rt Laye r Cac hing Appro ac he s Snoop 16 Snooping TCP I it involves modification of the network layer (IP) software at the base station (BS) by adding a module called snoop Transparent extension of TCP within the BS/FA buffering of packets sent to the mobile host lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission) the foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs correspondent local retransmission changes of TCP only within the foreign agent (+min MH change) foreign host agent „wired“ Internet mobile host snooping of ACKs buffering of data end-to-end TCP connection 17 Snooping TCP II Data transfer to the mobile host ► ► Data transfer from the mobile host ► ► FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH MH can now retransmit data with only a very short delay Advantages: ► ► ► FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out fast retransmission possible, transparent for the fixed network Maintain end-to-end semantics No change to correspondent node No major state transfer during handover Problems ► ► Snooping TCP does not isolate the wireless link well Snooping might be useless depending on encryption schemes 18 S o fts tate Cro s s Laye r S ig naling Appro ac he s Explicit Congestion Notification (ECN) ► is an extension proposed to Random Early Detection (RED) ► marks a packet instead of dropping in when the average queue size is between minth and maxth ► Upon receipt of congestion marked packet, the TCP receiver informs the sender about incipient congestion, ► which in turn will trigger the congestion avoidance algorithm at the sender Explicit Bad State Notification (EBSN) ► proposes a mechanism to update the TCP timer at the source to prevent source form decreasing its congestion window ► EBSN’s are sent to the source after every unsuccessful attempt by the base station to transmit packets over the wireless link ► EBSN would cause the previous timeouts to be cancelled and new timeouts put in place, based on existing estimate of round trip time and variance 19 Explicit Loss Notification (ELN) ► Add ELN option to TCP acks When a packet is dropped on the wireless networks, ► future cumulative acknowledgements corresponding to the lost packet are marked to identify that a noncongestion related loss has occurred 20 Hards tate Trans po rt Laye r Appro ac he s Indirect TCP or I-TCP segments the connection ► no changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol ► optimized TCP protocol for mobile hosts ► splitting of the TCP connection at, e.g., the foreign agent into TCP connections, no real end-to-end connection any longer ► hosts in the fixed part of the net not notice the characteristics of the wireless part mobile host access point (foreign agent) “wireless” TCP wired Internet standard TCP 21 Indirect TCP II Advantages ► no changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work ► transmission errors on the wireless link not propagate into the fixed network ► simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host ► therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known Disadvantages ► loss of end-to-end semantics, an acknowledgement to a sender does not any longer mean that a receiver really got a packet, foreign agents might crash ► higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent 22 Wireless TCP preserve the end-to-end semantics This Protocol tries to distinguish Random losses from Congestion losses by measuring the packet inter arrival time with the packet inter departure time WTCP uses rate-based rather than windowbased transmission control Hence it shapes its data traffic, never allows a burst of packet transmissions, and is fair when competing connections have different round-trip times 23 Mobile TCP Special handling of lengthy and/or frequent disconnections with low BER links M-TCP splits as I-TCP does ► ► unmodified TCP fixed network to supervisory host (SH) optimized TCP SH to MH Supervisory host ► ► no caching, no retransmission monitors all packets, if disconnection detected • • ► old or new SH reopen the window Advantages ► set sender window size to sender automatically goes into persistent mode maintains semantics, supports disconnection, no buffer forwarding Disadvantages ► ► loss on wireless link propagated into fixed network adapted TCP on wireless link 24 Ad Hoc Transport Protocol (ATP) Layer coordination ► Rate based transmissions ► Congestion control uses feedback from the network; Reliability is ensured through receiver feedback and selective ACK Assisted congestion control ► Avoids impact of bursty traffic Decoupling of congestion control and reliability ► Uses feedback from network nodes for congestion detection, avoidance, and control Adapts sending rate based on feedback from intermediate nodes TCP friendliness and fairness ► ► Achieved through feedback from intermediate nodes But fairness yet an issue 25 ATCP Approach ATCP utilizes network layer feedback (from the intermediate nodes) to take appropriate actions Network feedback is: ► ICMP: The Destination Unreachable ICMP message indicates route disruption ► ECN: Indicates network congestion With ECN enabled, time out and dup ACKs are assumed to no longer be due to congestion 26 Transport Layer Challenges New transport layer protocols need to be developed that avoids the shortcomings of TCP while being compatible with it Transport layer protocols for supporting real-time traffic in wireless meshes are desirable Integration of transport layer with other layers; or inferring and reacting with respect to the observations at other layers Impact of mobility on transport layer 27 Summary Motivation TCP Variants ► Slow start ► Fast Retransmit/Recovery (TCP Reno) Issues in Heterogeneous Wireless Networks TCP Schemes for Wireless ► ► ► ► Pure Link-level Approaches Soft-state Transport Layer Caching Approaches Soft-state Cross Layer Signaling Approaches Hard-state Transport Layer Approaches 28 ... Motivation TCP Variants ► Slow start ► Fast Retransmit/Recovery (TCP Reno) Issues in Heterogeneous Wireless Networks TCP Schemes for Wireless ► ► ► ► Pure Link-level Approaches Soft-state Transport... Motivation TCP Variants ► Slow start ► Fast Retransmit/Recovery (TCP Reno) Issues in Heterogeneous Wireless Networks TCP Schemes for Wireless ► ► ► ► Pure Link-level Approaches Soft-state Transport... have different round-trip times 23 Mobile TCP Special handling of lengthy and/or frequent disconnections with low BER links M -TCP splits as I -TCP does ► ► unmodified TCP fixed network to