ATM backbones, an example of MTU tuning

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Sometimes it is preferable from the point of view of efficiency to artificially declare a reduced MTU in software below the true maximum possible length supported. One example of this is the case where IP traffic is carried over an ATM (Asynchronous Transfer Mode) network. Some providers, particularly those with a telephony background, use ATM on their internal backbone network.

Using ATM at optimum efficiency is achieved when packet length is a multiple of 48 bytes. This is because ATM is sent as a stream of fixed-length packets (known as 'cells'), each of which can carry a payload of 48 bytes of user data with 5 bytes of overhead for a total cost of 53 bytes per cell. So the total length of the transmitted data length is 53 * ncells bytes, where ncells = the number of required cells of = INT((payload_length+47)/48). So in the worst case, where the total length = (48*n+1) bytes, one additional cell is needed to transmit the one last byte of payload, the final cell costing an extra 53 transmitted bytes 47 of which are padding. For this reason, artificially declaring a reduced MTU in software maximises protocol efficiency at the ATM layer by making the ATM AAL5 total payload length be a multiple of 48 bytes whenever possible.

For example, 31 completely filled ATM cells carry a payload of 31*48=1488 bytes. Taking this figure of 1488 and subtracting from it any overheads contributed by all relevant higher protocols we can obtain a suggested value for an artificially reduced optimal MTU. In the case where the user would normally send 1500 byte packets, sending between 1489 and 1536 bytes requires an additional fixed cost of a 53 bytes transmitted, in the form of one extra ATM cell.

For the example of IP over DSL connections using PPPoA/VC-MUX, again choosing to fill 31 ATM cells as before, we obtain a desired optimal reduced MTU figure of 1478 = 31*48-10 taking into account an overhead of 10 bytes consisting of a Point-to-Point Protocol overhead of 2 bytes, and an AAL5 overhead of 8 bytes. This gives a total cost of 31*53=1643 bytes transmitted via ATM from a 1478 byte packet passed to PPPoA. In the case of IP sent over ADSL using PPPoA the figure of 1478 would be the total length of the IP packet including IP headers. So in this example keeping to a self-imposed reduced MTU of 1478 as opposed to sending IP packets of total length 1500 saves 53 bytes per packet at the ATM layer at a cost of a 22 byte reduction of the length of IP packets.

A maximum MTU for PPPoE/DSL connections is 1492, per RFC 2516: 6 bytes being PPPoE header, leaving enough room for a 1488 byte payload, or 31 full ATM cells.