By: Lawrence Baldwin
Subject: Optimal MTU configuration for PPPoE ADSL Connections
NOTE: I believe I may have left out a critical piece in the analysis below...I've
reviewing RFCs now and will update this later today...the basic concepts below
are sound, but my conclusions regarding the exact MTU size that is optimal may
change. Be advised that this same issue is at play for Bridged ADSL (RFC 1483)
and PPPoA connections...I just need to do the calculations for them seperatly.
(LB 04-Mar-2001 11:03am)
There is often an incorrect assumption that bigger is always better. The maximum allowable packet size (MTU) for a PPPoE connection is 1492 bytes, however, the optimal MTU is actually 1454 bytes.
The protocol overhead (TCP, IP, PPP, PPPoE, Ethernet, AAL5, and ATM) of using a 1492 byte MTU is 16.80%.
By contrast, the protocol overhead using a 1454 byte MTU is 16.20%.
Although not a staggering difference, using a lower MTU actually reduces ATM overhead by about 0.6% and will thus yield a corresponding increase in user thoroughput: .06% * 1.5Mbps = ~+90Kbps
If you want to understand the details of exactly why overhead is lower, read on:
PPPoE 1492 byte Frame:
|Total Frame Size||1518|
Thus the total frame size when using a 1492 byte MTU, is actually 1518 bytes.
When a packet is converted to an ATM cell stream it is broken down into multiple cells of 53bytes each (48 bytes of user data and bytes of ATM header information).
1518 / 48 bytes= 31 cells plus a 30 byte remainder
Unlike Ethernet, ATM cells are fixed length...they must ALWAYS be 53 bytes long. Thus, the first 31 cells will contain 48 bytes of user data, however, the last cell will only be partially filled with 30 bytes of data. The last cell will also hold the Segmentation and Reassmbly (SAR) Trailer which is 8 bytes long. The SAR Trailer tells your ASDL modem how to reassmble the 32 cells back into the 1518 byte packet. Since the last cell will only have 30 data bytes, plus 8 SAR Trailer bytes, the remaining 10 bytes will simply be marked as cell padding (and is thus, wasted space).
The 10 bytes of cell padding is precisely where the 0.6% overhead comes from
(10 bytes / 1452 bytes) = 0.68%
Note: When using a 1492 byte MTU, TCP and IP headers account for 40 bytes, thus the amount of user data is 1452 bytes (hence, we use 1452 bytes in the above calculations).
By contrast, here's what happens when we reduce our MTU to 1454 bytes:
PPPoE 1454 byte Frame:
|Total Frame Size||1480|
1480 / 48 bytes= 30 cells plus a 40 byte remainder
Thus, the first 30 cells will contain 48 bytes of user data, and the last cell will 40 bytes of data, plus the 8 byte SAR Trailer. Since the frame and associated ATM overhead fit exactly in to 31 cells, a 1454 byte MTU eliminates all ATM cell padding overhead.
The following graphic summarizes this:
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