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
- 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.
- It implements local transmission at Base Station.
- Uses link-level acknowledgment to trigger retransmission of the lost packets. Retransmission takes place whenever there is a timeout or ACK from MN.
- 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.
- 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:
- If d is large enough, the wireless loss is recovered.
- If d is small enough, it will interfere with TCP
- 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.