Flickner, Eric J Kanury, Sree Ramya Lee, Soohee 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 Wired domain MN 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) TCP doesn’t know exact cause of packet loss, assumes congestion loss. Ideal TCP retransmits without congestion control mechanism MAC layer can explicitly identify what the reason is for packet loss If handoff or non-congestion loss is detected, the TCP layer is immediately notified by the MAC layer through ELN Window size isn’t reduced implying an error and not congestion occurred. 17

4.4.8 WTCP - Reliable Transmission Control Protocol for Wide Area Wireless Networks (WTCP) Rate-based transmission at the source Inter-packet separation as the congestion metric Mechanisms for detecting the reason for packet loss Bandwidth estimation Wireless TCP unique characteristic is separate mechanisms for congestion control and reliability.

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 strategy ACK n acknwoledges correct, in-sequence up to n packets If one packet is missed, the entire sequence beginning at the gap has to be retransmitted, wastes bandwidth Selective retransmit reduces overhead of retransmission on errors Improves performance by allowing sender to retransmit only missing packets based upon selective ACKs provided by receiver. 19

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 Handoff occurs when MN moves from cell to cell Link layer ensures reliable delivery and guarantees zero loss during handoff. TCP unaware and nothing done at transport layer. Disadvantage If handoff latency > TCP timeout, then TCP will become aware of handoff.

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 TCP TCP-Unaware Link Layer Mobile TCP ITCP ELN WTCP TCP SACK TTCP Changes in: AP CN MN Yes No Retransmitting Node NA Single point Failure Yes (AP) Handoff Latency Low High Security Breach at AP No breach End-to-end semantics Retransmission by inter. nodes Slow start Buffer at AP