Tcp In Wirless Domain
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TCP IN WIRELESS DOMAIN Flickner, Eric J Kanury, Sree Ramya Lee, Soohee
Contents Introduction Traditional TCP TCP Over Wireless Link layer solutions - Snoop TCP
- TCP-unaware link layer
Split Approach based solutions - ITCP - M-TCP
End to end solutions - ELN - WTCP - TCP SACK - TTCP
Introduction Issues with Wireless Domain - High error rates and low bandwidth Traditional TCP guarantees in-order & reliable delivery in wired network TCP needs to be modified for wireless domain
Traditional TCP Provides Connection-oriented - two applications must establish TCP connection before they can exchange data Full duplex protocol Includes Flow-control mechanism - allow receiver to limit how much data the sender can transmit Implements Congestion-control mechanism Divides data stream into smaller segments - Segment sequence number is used to provide in-order packet delivery and data loss detection
Traditional TCP
Traditional 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 Wireless TCP Congestion mechanism causes problem in wireless domain - Wireless has high packet loss and variable latency, which cause Slow Start and retransmission of lost packets 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
Snoop TCP Buffer 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
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.
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
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.
Indirect TCP Splitting of TCP connection into two distinct connections, one between the MN and BS and other between BS and CN.
Wireless link MN
Wired domain AP
CN
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.
Mobile TCP 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 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.
Advantages: Avoids Retransmission Avoids closing of contention window Avoids slow start at the sender
4.4.7 Explicit Loss Notification (ELN)
4.4.8 WTCP - Reliable Transmission Control Protocol for Wide Area Wireless Networks (WTCP)
Wireless TCP unique characteristic is separate mechanisms for congestion control and reliability.
4.4.9 TCP Selective ACK (TCP SACK)
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 handshake 3 packets for a single transaction, 2 transactions per connection cycle 3 packets for setup + 1 for data + 3 packets for release = 7 packets minimum To improve performance use T-TCP for small amount of data. Integrate connection setup, tear-down, and data transfer combined into single transaction Usually only 2 or 3 packets are needed Advantage
Larger efficiency (low overhead)
Disadvantage
Changed TCP Mobility not transparent, can’t affect existing framework
4.4.11 Impact of Mobility
Fast retransmit/fast recovery Handoff usually leads to packet loss during transit TCP reacts with slow-start during handoff even when no congestion Solution: Artificially force fast retransmit mode after handoff. Send duplicate ACK after handoff, instead of entering slow start. Advantages •Simple changes result in significant higher performance •Requires minimal changes to existing TCP structure Disadvantages •Scheme doesn’t consider fact of losses over links during handoff
Using Multicast Use 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. Disadvantage •Larger buffers, can be minimized by only buffering when handoff is likely to occur.
Feature
Snoop TCPMobile ITCP TCP Unaware TCP Link Layer
Changes in: AP Yes CN No MN Yes
ELN
WTCP
TCP TTCP SACK
Yes No No
Yes No Yes
Yes No Yes
No Yes Yes
No Yes Yes
No Yes No
No Yes No
RetransmittinAP g Node
AP
NA
AP
NA
NA
NA
NA
Single point No Failure
No
No
Yes (AP) No
No
No
No
Handoff Latency
Low
Low
Low
Low
High
High
High
Security
Breach No breach at AP
NA
Breach at No AP breach
No breach
No No breach breach
High
End-to-end Yes semantics
Yes
Yes
No
Yes
Yes
Yes
Yes
Retransmissi Yes
Yes
No
Yes
No
No
No
No
Contents Introduction Traditional TCP TCP Over Wireless Link layer solutions - Snoop TCP
- TCP-unaware link layer
Split Approach based solutions - ITCP - M-TCP
End to end solutions - ELN - WTCP - TCP SACK - TTCP
Introduction Issues with Wireless Domain - High error rates and low bandwidth Traditional TCP guarantees in-order & reliable delivery in wired network TCP needs to be modified for wireless domain
Traditional TCP Provides Connection-oriented - two applications must establish TCP connection before they can exchange data Full duplex protocol Includes Flow-control mechanism - allow receiver to limit how much data the sender can transmit Implements Congestion-control mechanism Divides data stream into smaller segments - Segment sequence number is used to provide in-order packet delivery and data loss detection
Traditional TCP
Traditional 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 Wireless TCP Congestion mechanism causes problem in wireless domain - Wireless has high packet loss and variable latency, which cause Slow Start and retransmission of lost packets 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
Snoop TCP Buffer 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
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.
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
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.
Indirect TCP Splitting of TCP connection into two distinct connections, one between the MN and BS and other between BS and CN.
Wireless link MN
Wired domain AP
CN
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.
Mobile TCP 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 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.
Advantages: Avoids Retransmission Avoids closing of contention window Avoids slow start at the sender
4.4.7 Explicit Loss Notification (ELN)
4.4.8 WTCP - Reliable Transmission Control Protocol for Wide Area Wireless Networks (WTCP)
Wireless TCP unique characteristic is separate mechanisms for congestion control and reliability.
4.4.9 TCP Selective ACK (TCP SACK)
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 handshake 3 packets for a single transaction, 2 transactions per connection cycle 3 packets for setup + 1 for data + 3 packets for release = 7 packets minimum To improve performance use T-TCP for small amount of data. Integrate connection setup, tear-down, and data transfer combined into single transaction Usually only 2 or 3 packets are needed Advantage
Larger efficiency (low overhead)
Disadvantage
Changed TCP Mobility not transparent, can’t affect existing framework
4.4.11 Impact of Mobility
Fast retransmit/fast recovery Handoff usually leads to packet loss during transit TCP reacts with slow-start during handoff even when no congestion Solution: Artificially force fast retransmit mode after handoff. Send duplicate ACK after handoff, instead of entering slow start. Advantages •Simple changes result in significant higher performance •Requires minimal changes to existing TCP structure Disadvantages •Scheme doesn’t consider fact of losses over links during handoff
Using Multicast Use 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. Disadvantage •Larger buffers, can be minimized by only buffering when handoff is likely to occur.
Feature
Snoop TCPMobile ITCP TCP Unaware TCP Link Layer
Changes in: AP Yes CN No MN Yes
ELN
WTCP
TCP TTCP SACK
Yes No No
Yes No Yes
Yes No Yes
No Yes Yes
No Yes Yes
No Yes No
No Yes No
RetransmittinAP g Node
AP
NA
AP
NA
NA
NA
NA
Single point No Failure
No
No
Yes (AP) No
No
No
No
Handoff Latency
Low
Low
Low
Low
High
High
High
Security
Breach No breach at AP
NA
Breach at No AP breach
No breach
No No breach breach
High
End-to-end Yes semantics
Yes
Yes
No
Yes
Yes
Yes
Yes
Retransmissi Yes
Yes
No
Yes
No
No
No
No
