Performance at different speeds and packet loss characteristics

 The transmission control protocol (TCP) is very sensitive to packet loss. Some researchers measured TCP performance at different speeds and packet loss characteristics and the result is known as the Mathis Equation. The short summary is that packet loss of more than .001% of all packets causes significant decreases in throughput. That’s a packet loss rate of one packet out of 100,000 (1 out of 10E5). That translates into a bit error rate (BER) of about 10E-10. (The figures are approximate because of differences in packet sizes).

Before you say that this error threshold is too small, let’s look at it differently. How long do you think a link should run before it experiences a packet loss? Using the 10E-11 figure, a one gigabit per second (1Gbps) link would run about 10 seconds between errors, while a computer science degree jobs link would experience an error every second. You can use this information to determine your network management system packet loss thresholds.

Network management systems (NMS) should be collecting interface performance data from all network interfaces within the organization, including errors and drops/discards. Your selection of an alerting threshold for errors/drops/discards will depend on what error rates you are willing to tolerate for your network and what threshold setting the network management tools will support. I was recently surprised to find an NMS in which packet drop thresholds couldn’t be set smaller than one percent. In these cases, it may be better to use absolute count values as thresholds. Also, note that management systems typically count errors separately from drops/discards.