Network Congestion Symptoms

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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