[Gllug] OT: MTU sizes, w/ Broadband

[Richard Jones]

On Wed, May 14, 2003 at 12:49:13PM +0100, Simon Wilcox wrote:
> ATM uses fixed length packets, called cells, which allows highly efficient
> aggregation of data on backbone networks using well understood, and very
> fast, time division multiplexing techniques. It also includes quality of
> service metrics and other features useful for audio/video and other time
> senstive packet systems.
> 
> ATM cells are 53 bytes long, with 48 bytes of user data. The ethernet
> packet is chunked up and reassembled at the other end. Under length cells
> are padded to the exact length. So you can gain *margin* increases in
> speed by making the MTU of your ethernet such that you exactly fill the
> last cell thus saving the padding.

The 48 byte payload size *really* sucks for carrying the most common
type of packet on the Internet - the TCP ACK. This packet is commonly
40 bytes in size, plus some overhead:

40 bytes    IP packet
 8 bytes    SNAP
 8 bytes    AAL5

So it's 8 bytes too large to go in one cell. Instead it takes two
cells. That's 106 bytes to transmit the most common 40 byte packet.

Rich.

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