Calculating network throughput

Question

suppose i have a 4MBits network and i want to calculate the data throughput, this is considering the max transfer rate minus overhead from ethernet/IP/TCP headers. Reading on the web i found out that the MSS ( maximum segment size) of a TCP segment is 576 - 20 - 20, these last two being TCP and IP headers overhead, resulting in a 93% of data, meaning i will be only using 93% of my 4MBits link to transfer data. Now where's the link ayer overhead? Shouldn't it be added as well? If im not wrong an ethernet header is around 46 bytes so the final sum would be 576 - 20 - 20 - 46 = 490, resulting in an 85% data throughput, but am i doing something wrong?

Answer 1

Just work bottom up. Regular ethernet frames (no jumbo frames, no vlan tagging) are 1542 bytes in total and can have a payload of 1500 bytes. An Ipv4 header without options is 20 bytes and a TCP header without options also 20 bytes. So you end up with 1460 bytes possible payload of a 1542 byte link-layer frame. So your efficiency is 1460/1542=0.9468223086900129, resulting in a maximum throughput of 3.7872892347600517Mbps.

Notice however this will usually be lower. This is the theoretical maximum rate for a continuous stream you can get on a full duplex link, after the TCP session is established and when you're the only user of that link. Also note that as soon as you're sending at a slightly higher rate for some time your link will get congested, you will see drops and your actual TCP throughput might drop significantly because of slow-start.

If the link is wireless (802.11) the calculation becomes a lot more complex because of RTS/CTS mechanisms, but it's about /2 for only one active user and that's without incorporating loss, which is unrealistic.

Answer 2

In general, the protocol can impact network throughput and much more than simply the packet overhead. You mention that you want to measure throughput on an Ethernet/IP/TCP network but the impact of packet overhead of those protocols is NOT the only thing to consider. TCP is a connection-oriented protocol and uses ACK's to signal if a packet has been received or not. user1777914 missed the mark about ACK's but was on to something - they do not take up any more SPACE but they can DELAY the transmission of packets. As latency increases the overall network throughput can decrease based on how often the application or hosting OS expects a response.

W. Richard Stevens has written an AMAZING book on TCP/IP. Here is an except that explains theoretical TCP performance, what impacts it and how it is calculated.

There too is the Nagle algorithm helps with latency but if disabled can slow down throughput.