By using a network protocol analyzer such as Wireshark, a Network Administrator can determine if congestion is present on their network. The key is looking for duplicate ACKs.
Consistent duplicate ACKs are an indication that network congestion
may be present. The problem is worth investigating.
To manage the reliability of data between two endpoints, TCP
uses ordered sequence numbers (SYN) and acknowledgements (ACKs).
For example, data is broken up into packets, and each packet
is given a sequence number.
(Data to be sent) = Packet A – SYN 1, Packet B – SYN2, Packet C – SYN3
Each packet is sent in sequential order, and as each packet
arrives to its destination, the destination endpoint sends a unique acknowledgement
(ACK) back to the transmitter as a way to verify the data has been received in its
proper order.
ACK
1 [Packet A – SYN1], ACK 2 ACK 1 [Packet B – SYN2], ACK 3 [Packet A
– SYN3],
If the data arrives out of order, a duplicate ACK is sent to
the transmitting endpoint, because it did not receive the next expected ACK in
the proper sequence. Three duplicate ACKS sent in a row will trigger the
request for a re-transmission of the packet.
Network congestion will equate to performance issues on the
network, and packets arriving out of order are a sign that it is present. There are a couple of areas a Network
Administrator can look, one being Layer – One (physical cabling, EMI
interference), network adapter or a problem with the actual transmitting
endpoint. For instance, the processor or
other component at the endpoint.
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