Delayed Duplicate Acknowledgement(DDA)

Pre-requisites: TCP Snooping

The delayed duplicate acknowledgment or DDA approach is intended to be a modification of the Snooping TCP approach. As you know Snooping TCP is designed to solve the end-to-end semantics loss in I-TCP, DDA is also no different. It uses link-level transmission to recover from losses. Whenever there is a packet loss detected by the TCP, it reacts by reducing the congestion window even when the loss is due to wireless non-congestion-related losses, which heavily affects the TCP performance. DDA tries to improve this aspect of TCP. 

Working of DDA

1. Unlike TCP Snooping, Base Station(BS) is not TCP aware whereas BS is TCP aware in the case of Snooping which means it depends on link levels and not TCP levels. 
2. It implements local transmission at Base Station.
3. Uses link-level acknowledgment to trigger retransmission of the lost packets. Retransmission takes place whenever there is a timeout or ACK from MN.
4. Mobile Host delays third and subsequent DUPACK by d units to reduce interference with the TCP sender which gives chance for Base Station to recover from packet losses.
5. Now one of the most important and confusing topics in DDA is to find the delayed interval d. We have to find out the value of d with utmost care as: 
   1. If d is large enough, the wireless loss is recovered.
   2. If d is small enough, it will interfere with TCP
6. If the loss is not recovered within interval d, DUPACK is released.

So, when MN detects packet loss for the first time or we can say when MN receives out-of-order packets for the first time, it responds by sending a DUPACK. But for the next out-of-order packets, each DUPACK is delayed by a time interval d. But if the following packets are in order by their sequence number, the MN does not need to send DUPACK. 

Advantages of DDA

• BS need not be TCP aware so TCP headers can be ciphered.   
• Recovery from wireless loss is possible without a response from Foreign Host.
• It can be used with IPsec.
• It can work even when the IP payload is encrypted.

Disadvantages of DDA

• Choosing the right value of d is difficult.
• Performs poorly in the face of real congestion, as it delays third retransmission effectively delaying fast retransmission, thus only works when congestion losses are minimal.