Selective repeat ARQ

Question

LLIn selective -repeat when how receiver will know that a frame is lost and send NAK ?Is it when he recieves an out of order packet .?

Answer 1

The sender takes the responsibility of detecting packet loss.Reciever does not have worry about packet loss. No out-of-order packets do not indicate packet loss.

"The SR receiver will acknowledge a correctly received packet whether or not it is in order. Out-of-order packets are buffered until any missing packets (that is,packets with lower sequence numbers) are received, at which point a batch of packets can be delivered in order to the upper layer. "

When the timer associated with the packet expires the sender simply resends the packet.So no need of NAKs.

Comments

What will be the acknowledgement number for out of order received packet means suppose frame 1 is received receiver sends acknowledgement for 1 with number  2 if 2 is  lost while transmitting then  frame 3 is received what will be the acknowledgement number for frame 3?

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In SR protocol, an Ack number defines the sequence number of the error-free packet received.So if packet 1 is received receiver sends ack for 1 with number 1.