1. Flickner, Eric J Kanury, Sree Ramya Lee, Soohee
TCP IN WIRELESS DOMAIN
Flickner, Eric J Kanury, Sree Ramya
Lee, Soohee

2. 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

3. 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

4. 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

5. Traditional TCP

6. 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

7. 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

8. 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

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 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

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 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.

16. Advantages:
Avoids Retransmission
Avoids closing of contention window
Avoids 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 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

18. 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.

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 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

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 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

21. 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.

22. 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

23. 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.

24. 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