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