Presentation on theme: "Flickner, Eric J Kanury, Sree Ramya Lee, Soohee"— Presentation transcript:
1 Flickner, Eric J Kanury, Sree Ramya Lee, Soohee TCP IN WIRELESS DOMAINFlickner, Eric J Kanury, Sree RamyaLee, Soohee
2 Contents Introduction Traditional TCP TCP Over Wireless Link layer solutions - Snoop TCP - TCP-unaware link layerSplit Approach based solutions - ITCP - M-TCPEnd to end solutions - ELN - WTCP - TCP SACK - TTCP
3 IntroductionIssues with Wireless Domain - High error rates and low bandwidthTraditional TCP guarantees in-order & reliable delivery in wired networkTCP needs to be modified for wireless domain
4 Traditional TCPProvides Connection-oriented - two applications must establish TCP connection before they can exchange dataFull duplex protocolIncludes Flow-control mechanism - allow receiver to limit how much data the sender can transmitImplements Congestion-control mechanismDivides data stream into smaller segments - Segment sequence number is used to provide in-order packet delivery and data loss detection
6 Traditional TCPCongestion Control Mechanism - Initial Window size: Max segment size - Window get doubled for each successful transmission -TCP interpret Timeout as congestion -> initialize Slow Start Threshold as half of current window and reset window as one Max segment size
7 TCP Over WirelessTCP Congestion mechanism causes problem in wireless domain - Wireless has high packet loss and variable latency, which cause Slow Start and retransmission of lost packetsSeveral alternatives are suggested. - Let Link Layer correct all errors FEC (Forward Error Correction): redundancy is encoded into the message Redundancy is introduced only if error possibility is found Retransmission at link layer
8 Snoop TCPBuffer the data as close to Mobile Node as possible to minimize retransmission time.BS buffers the packets and removes them when it sees acknowledgement.BS retransmits if it gets duplicate acknowledgement or no acknowledgement.Several alternatives are suggested. - Let Link Layer correct all errors FEC (Forward Error Correction): redundancy is encoded into the message Redundancy is introduced only if error possibility is found Retransmission at link layer
9 TCP- Unaware Link Layer Main aim is to simulate the behavior of the snoop TCP without requiring the link layer at the BS to be TCP aware.At the BS ,link layer retransmission is used to perform local error recovery.In this, retransmissions are triggered by link level ACKs.
10 TCP receiver (MN) reduces interference between TCP and link level retransmission by delaying third and subsequent dupacks for interval d.TCP receiver responds to the first two packets by sending dupacks immediately.Dupacks for further consecutive packets are delayed for duration d
11 Advantages:Link Layer need not be TCP aware.Works well for small round trip times (RTTs) over the wireless link.Disadvantage:Optimal value of DUPACK delay is dependent on the wireless link.
12 Indirect TCPSplitting of TCP connection into two distinct connections, one between the MN and BS and other between BS and CN.Wireless linkWired domainMNAPCN
14 Customized transport protocol between AP and MN. Advantage:Loss of packets in the wireless domain which cause a retransmission in the wired domain is avoided.Disadvantage:Handoff may take a longer time.
15 Mobile TCPConnection between MN and BS is lost for small intervals of time.This leads to time out by sender or data buffered by AP may be too large or results in slow start.MTCP handles this situation using a supervisory host.MTCP maintains end-end TCP semantics even though TCP connection is split at the supervisory host.
16 Advantages:Avoids RetransmissionAvoids closing of contention windowAvoids slow start at the sender
17 4.4.7 Explicit Loss Notification (ELN) TCP doesn’t know exact cause of packet loss, assumes congestion loss.Ideal TCP retransmits without congestion control mechanismMAC layer can explicitly identify what the reason is for packet lossIf handoff or non-congestion loss is detected, the TCP layer is immediately notified by the MAC layer through ELNWindow size isn’t reduced implying an error and not congestion occurred.17
18 4.4.8 WTCP - Reliable Transmission Control Protocol for Wide Area Wireless Networks (WTCP) Rate-based transmission at the sourceInter-packet separation as the congestion metricMechanisms for detecting the reason for packet lossBandwidth estimationWireless TCP unique characteristic is separate mechanisms for congestion control and reliability.
19 4.4.9 TCP Selective ACK (TCP SACK) Selective retransmission strategy is more complex and requires more buffer space at end-points. (RFC 2018)Traditional TCP uses cumulative ACKs and go-back-N strategyACK n acknwoledges correct, in-sequence up to n packetsIf one packet is missed, the entire sequence beginning at the gap has to be retransmitted, wastes bandwidthSelective retransmit reduces overhead of retransmission on errorsImproves performance by allowing sender to retransmit only missing packets based upon selective ACKs provided by receiver.19
20 4.4.10 Transaction-Oriented TCP (T-TCP) TCP connection setup and tear-down is huge overhead for a small amount of data, uses 3-way handshake3 packets for a single transaction, 2 transactions per connection cycle3 packets for setup + 1 for data + 3 packets for release = 7 packets minimumTo improve performance use T-TCP for small amount of data.Integrate connection setup, tear-down, and data transfer combined into single transactionUsually only 2 or 3 packets are neededAdvantageLarger efficiency (low overhead)DisadvantageChanged TCPMobility not transparent, can’t affect existing framework
21 Impact of MobilityHandoff occurs when MN moves from cell to cellLink layer ensures reliable delivery and guarantees zero loss during handoff.TCP unaware and nothing done at transport layer.DisadvantageIf handoff latency > TCP timeout, then TCP will become aware of handoff.
22 Fast retransmit/fast recovery Handoff usually leads to packet loss during transitTCP reacts with slow-start during handoff even when no congestionSolution: Artificially force fast retransmit mode after handoff. Send duplicate ACK after handoff, instead of entering slow start.AdvantagesSimple changes result in significant higher performanceRequires minimal changes to existing TCP structureDisadvantagesScheme doesn’t consider fact of losses over links during handoff
23 Using MulticastUse multicast to improve performance by transmitting to select group of base stations (likely to visit)These base stations are directed to join multicast group.Multicast group has unique address assigned to the node.Packets destined for mobile node have to be readdressed for multicast group.Only one base station is connected with mobile node and is responsible for TX packets to it.Other BS in multicast group act as buffer so loss of packets can be minimized.DisadvantageLarger buffers, can be minimized by only buffering when handoff is likely to occur.
24 FeatureSnoop TCPTCP-Unaware Link LayerMobile TCPITCPELNWTCPTCP SACKTTCPChanges in:APCNMNYesNoRetransmitting NodeNASingle point FailureYes (AP)Handoff LatencyLowHighSecurityBreach at APNo breachEnd-to-end semanticsRetransmission by inter. nodesSlow startBuffer at AP