On 01:43 PM 6/10/2005, Suresh Balabrahman wrote:
>All,
>
>I would very greatful to someone who tell me what is the Dup Ack & unreassembled packets in Ethereal output.
Duplicate ACK means that the receiver did not receive one more packet(s). The ACK mechanism tells the sender I have received "up to this far of the file" So if the receiver notices a gap in the transmission, he will repeatedly acknowledge with the same ack number. In lay terms, this is the receiver saying "hey, I didn't receive packet #XYZ!"
The sender can decide to wait it out by assuming "perhaps packet XYZ was just delayed momentarily in flight to the receiver."
But in modern TCP stacks, there is a concept of fast retransmission. If it receives three of the same ACK packets, it will retransmit that particular chunk without waiting for the various timers to go off. The assumption here is that if the receiver sent three "I didn't get XYZ" cry for help, the sender can safely assume that the packet XYZ was destroyed in flight.
This is a very high level description of what may be happening. You can look at the various "ACK Generation" RFCs.
hsb
