One of the smallest packets commonly seen on networks is a TCP ACK. This has a 20 byte IP header and a 20 byte TCP header, adding up to 40 bytes. Because this is smaller than ethernet's minimum payload size of 46 bytes, it is automatically padded prior to transmission to bring it up to size. It is then wrapped with a 14 byte header and 4 byte CRC, to give the minimum ethernet frame size of 64 bytes.
When transmitted, each packet must also be preceded by a 7-byte preamble and 1-byte start-of-frame delimiter, and must be followed by an inter-frame gap of at least 12 bytes. This makes the smallest transmission in ethernet effectively 84 bytes.
We now have sufficient information to calculate the maximum packet rate on a 10G link:
10Gbps / (84 bytes * 8 bits) = 14.88Mpps
But the data rate that will be reported by most tools is significantly less, due to not counting the overheads:
14.88Mpps * 64 bytes * 8 bits = 7.62Gbps
Finally, we can work out the IP transmission efficiency (as opposed to ethernet efficiency) - it's pretty poor with such small packets:
14.88Mpps * 40 bytes * 8 bits / 10Gbps = 47.6%
No comments:
Post a Comment