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I am still not sure why the Ethernet data frame size / MTU was set maximum to 1500 bytes. I found out a document from Xerox which is from 1980. Available at (http://www.pennington.net/archives/ethernet/Ethernet_Version_1.pdf).

At point 6.2.3 it says that

The data field contains a sequence of n octets, where 46 < n < 1500 Within this range, full data transparency is provided, in the sense that any arbitrary sequence of octet values may appear in the data field.

I am not fully understand the reason that provided in this documentation. How does full data transparency provided? Is there any calculations behind this?

Anyone can share to me?

migrated from stackoverflow.com Dec 7 '17 at 19:45

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  • There is a considerable amount of documentation around Ethernet data frames. This type of question is, unfortunately, off-topic for StackOverflow. – David Makogon Nov 1 '17 at 1:37
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    The frame size is limited to 1518 bytes for plain frames. 1500 bytes is the layer 3 MTU, ie. maximum IP packet size in an Ethernet frame. – Zac67 Nov 1 '17 at 8:34
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"Data transparency" in this context means that Ethernet doesn't care what kind of data it transports. You put any payload in a frame, send it to the destination and extract the exact same payload.

Initially, the maximum frame size was a trade-off between efficiency and latency. The larger a frame, the lower the overhead, the better the effiency. However, early Ethernet was a shared medium, so any frame in transit occupied the whole segment - the longer the frame, the longer the network was blocked and other senders had to wait. Additionally, early Ethernet NICs required fast local memory for buffering a frame. Ethernet was aiming to reduce prices, so then-expensive RAM had to be kept at a reasonable minimum.

When switches were introduced in the early 1990s, the maximum size had to be kept for compatibility reasons: a sender has no way to tell whether the frames it sends are going over a shared medium (anywhere) or whether they are switched at all times. Hardware buffers have to sized to maximum frame size as well and can't change on the fly.

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