TCP IN WIRELESS DOMAIN
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Transcript of TCP IN WIRELESS DOMAIN
TCP IN WIRELESS TCP IN WIRELESS DOMAINDOMAIN
Flickner, Eric J Flickner, Eric J Kanury, Sree RamyaKanury, Sree Ramya
Lee, SooheeLee, Soohee
ContentsContents
IntroductionIntroductionTraditional TCPTraditional TCP
TCP Over WirelessTCP Over WirelessLink layer solutionsLink layer solutions- - Snoop TCPSnoop TCP- TCP-unaware link layer- TCP-unaware link layer
Split Approach based solutionsSplit Approach based solutions- - ITCPITCP- M-TCP- M-TCP
End to end solutionsEnd to end solutions- - ELNELN- WTCP- WTCP- TCP SACK- TCP SACK- TTCP- TTCP
IntroductionIntroduction
Issues with Wireless DomainIssues with Wireless Domain
- High error rates and low bandwidth- High error rates and low bandwidth
Traditional TCP guarantees in-order & reliable delivery in wired Traditional TCP guarantees in-order & reliable delivery in wired networknetwork
TCP needs to be modified for wireless domainTCP needs to be modified for wireless domain
Traditional TCPTraditional TCP
Provides Connection-orientedProvides Connection-oriented- two applications must establish TCP connection before - two applications must establish TCP connection before they can exchange datathey can exchange data
Full duplex protocolFull duplex protocol
Includes Flow-control mechanismIncludes Flow-control mechanism- allow receiver to limit how much data the sender can - allow receiver to limit how much data the sender can transmittransmit
Implements Congestion-control mechanismImplements Congestion-control mechanism
Divides data stream into smaller segmentsDivides data stream into smaller segments- Segment sequence number is used to provide in-order - Segment sequence number is used to provide in-order packet delivery and data loss detectionpacket delivery and data loss detection
Traditional TCPTraditional TCP
Traditional TCPTraditional TCP
Congestion 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
TCP Over WirelessTCP Over Wireless
TCP Congestion mechanism causes problem in wireless TCP Congestion mechanism causes problem in wireless domaindomain- Wireless has high packet loss and variable latency, - Wireless has high packet loss and variable latency, which cause Slow Start and retransmission of lost which cause Slow Start and retransmission of lost packetspackets
Several alternatives are suggested.Several alternatives are suggested.
- Let Link Layer correct all errors.- Let Link Layer correct all errors. - FEC (Forward Error Correction): redundancy is - FEC (Forward Error Correction): redundancy is encoded into encoded into the message. the message. - Redundancy is introduced only if error possibility is - Redundancy is introduced only if error possibility is found.found. - Retransmission at link layer - Retransmission at link layer
Snoop TCPSnoop TCP
Buffer the data as close to Mobile Node as possible to minimize Buffer the data as close to Mobile Node as possible to minimize retransmission time.retransmission time.BS buffers the packets and removes them when it sees BS buffers the packets and removes them when it sees acknowledgement.acknowledgement.BS retransmits if it gets duplicate acknowledgement or no BS retransmits if it gets duplicate acknowledgement or no acknowledgement.acknowledgement.
Several alternatives are suggested.Several alternatives are suggested.
- Let Link Layer correct all errors.- Let Link Layer correct all errors. - FEC (Forward Error Correction): redundancy is encoded into - FEC (Forward Error Correction): redundancy is encoded into the message. the message. - Redundancy is introduced only if error possibility is found. - Redundancy is introduced only if error possibility is found. - Retransmission at link layer - Retransmission at link layer
TCP- Unaware Link LayerTCP- Unaware Link Layer
Main aim is to simulate the behavior of Main aim is to simulate the behavior of the snoop TCP without requiring the link layer the snoop TCP without requiring the link layer at the BS to be TCP aware.at the BS to be TCP aware.
At the BS ,link layer retransmission is At the BS ,link layer retransmission is used to perform local error recovery.used to perform local error recovery.
In this, retransmissions are triggered by In this, retransmissions are triggered by link level ACKs. link level ACKs.
TCP receiver (MN) reduces interference between TCP TCP receiver (MN) reduces interference between TCP and link level retransmission by delaying third and and link level retransmission by delaying third and subsequent dupacks for interval subsequent dupacks for interval dd..
TCP receiver responds to the first two packets by TCP receiver responds to the first two packets by
sending dupacks immediatelysending dupacks immediately..
Dupacks for further consecutive packets are delayed for Dupacks for further consecutive packets are delayed for duration duration dd
Advantages:Advantages:
Link Layer need not be TCP aware.Link Layer need not be TCP aware.
Works well for small round trip times (RTTs) over the wireless link.Works well for small round trip times (RTTs) over the wireless link.
Disadvantage:Disadvantage:
Optimal value of DUPACK delay is dependent on the wireless link. Optimal value of DUPACK delay is dependent on the wireless link.
Indirect TCPIndirect TCP
Splitting of TCP connection into two distinct connections, one Splitting of TCP connection into two distinct connections, one between the MN and BS and other between BS and CN.between the MN and BS and other between BS and CN.
MN AP CN Wired domain
Wireless link
Customized transport protocol between AP and MN.Customized transport protocol between AP and MN.
Advantage:Advantage:
Loss of packets in the wireless domain which cause a Loss of packets in the wireless domain which cause a retransmission in the wired domain is avoided.retransmission in the wired domain is avoided.
Disadvantage:Disadvantage:
Handoff may take a longer time.Handoff may take a longer time.
Mobile TCPMobile TCP
Connection between MN and BS is lost for small intervals of time.Connection 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 This leads to time out by sender or data buffered by AP may be too large or results in slow start.large or results in slow start.
MTCP handles this situation using a supervisory host.MTCP handles this situation using a supervisory host.
MTCP maintains end-end TCP semantics even though TCP MTCP maintains end-end TCP semantics even though TCP connection is split at the supervisory host.connection is split at the supervisory host.
Advantages:Advantages:
Avoids RetransmissionAvoids Retransmission
Avoids closing of contention windowAvoids closing of contention window
Avoids slow start at the senderAvoids slow start at the sender
4.4.7 Explicit Loss Notification 4.4.7 Explicit Loss Notification (ELN)(ELN)
4.4.8 WTCP - Reliable 4.4.8 WTCP - Reliable Transmission Control Protocol Transmission Control Protocol
for Wide Area Wireless for Wide Area Wireless Networks (WTCP)Networks (WTCP)
Wireless TCP unique characteristic is separate mechanisms for congestion control and reliability.
4.4.9 TCP Selective ACK (TCP 4.4.9 TCP Selective ACK (TCP SACK)SACK)
4.4.10 Transaction-Oriented 4.4.10 Transaction-Oriented TCP (T-TCP)TCP (T-TCP)
TCP connection setup and tear-down is huge overhead for a small amount TCP connection setup and tear-down is huge overhead for a small amount of data, uses 3-way handshake of data, uses 3-way handshake 3 packets for a single transaction, 2 transactions per connection cycle3 packets for a single transaction, 2 transactions per connection cycle3 packets for setup + 1 for data + 3 packets for release = 7 packets 3 packets for setup + 1 for data + 3 packets for release = 7 packets minimumminimumTo improve performance use T-TCP for small amount of data.To improve performance use T-TCP for small amount of data.Integrate connection setup, tear-down, and data transfer combined into Integrate connection setup, tear-down, and data transfer combined into single transactionsingle transactionUsually only 2 or 3 packets are neededUsually only 2 or 3 packets are neededAdvantageAdvantage
Larger efficiency (low overhead)Larger efficiency (low overhead)DisadvantageDisadvantage
Changed TCPChanged TCP Mobility not transparent, can’t affect existing frameworkMobility not transparent, can’t affect existing framework
4.4.11 Impact of Mobility4.4.11 Impact of Mobility
Fast retransmit/fast recoveryFast retransmit/fast recovery
Handoff usually leads to packet loss during transitHandoff usually leads to packet loss during transit
TCP reacts with slow-start during handoff even when no TCP reacts with slow-start during handoff even when no congestioncongestion
SolutionSolution: Artificially force fast retransmit mode after handoff. : Artificially force fast retransmit mode after handoff. Send duplicate ACK after handoff, instead of entering slow start.Send duplicate ACK after handoff, instead of entering slow start.
AdvantagesAdvantages•Simple changes result in significant higher performanceSimple changes result in significant higher performance•Requires minimal changes to existing TCP structureRequires minimal changes to existing TCP structure
DisadvantagesDisadvantages•Scheme doesn’t consider fact of losses over links during handoffScheme doesn’t consider fact of losses over links during handoff
Using MulticastUsing Multicast
Use multicast to improve performance by transmitting to select Use multicast to improve performance by transmitting to select group of base stations (likely to visit)group of base stations (likely to visit)These base stations are directed to join multicast group.These base stations are directed to join multicast group.Multicast group has unique address assigned to the node.Multicast group has unique address assigned to the node.Packets destined for mobile node have to be readdressed for Packets destined for mobile node have to be readdressed for multicast group.multicast group.Only one base station is connected with mobile node and is Only one base station is connected with mobile node and is responsible for TX packets to it.responsible for TX packets to it.Other BS in multicast group act as buffer so loss of packets can Other BS in multicast group act as buffer so loss of packets can be minimized.be minimized.Disadvantage Disadvantage •Larger buffers, can be minimized by only buffering when Larger buffers, can be minimized by only buffering when handoff is likely to occur.handoff is likely to occur.
FeatureFeature Snoop Snoop TCPTCP
TCP-TCP-Unaware Unaware Link Link LayerLayer
Mobile Mobile TCPTCP
ITCPITCP ELNELN WTCPWTCP TCP TCP SACKSACK
TTCPTTCP
Changes Changes in:in:APAPCNCNMNMN
YesYesNoNoYesYes
YesYesNoNoNoNo
YesYesNoNoYesYes
YesYesNoNoYesYes
NoNoYesYesYesYes
NoNoYesYesYesYes
NoNoYesYesNoNo
NoNoYesYesNoNo
RetransmittRetransmitting Nodeing Node
APAP APAP NANA APAP NANA NANA NANA NANA
Single Single point point FailureFailure
NoNo NoNo NoNo Yes Yes (AP)(AP)
NoNo NoNo NoNo NoNo
Handoff Handoff LatencyLatency
LowLow LowLow LowLow LowLow HighHigh HighHigh HighHigh HighHigh
SecuritySecurity Breach Breach at APat AP
No breachNo breach NANA Breach Breach at APat AP
No No breachbreach
No No breachbreach
No No breacbreachh
No No breacbreachh
End-to-end End-to-end semanticssemantics
YesYes YesYes YesYes NoNo YesYes YesYes YesYes YesYes
RetransmisRetransmission by sion by inter. inter. nodesnodes
YesYes YesYes NoNo YesYes NoNo NoNo NoNo NoNo
Slow startSlow start YesYes YesYes NoNo NANA NoNo NoNo NoNo YesYes
Buffer at Buffer at APAP
YesYes YesYes NoNo YesYes NoNo NoNo NoNo NoNo