What mtu should I use for fusion to minimize atm padding overhead.
According to my knowledge, fusion does not use ppp, so the 8 bytes of ppp/pppoe overhead would not apply, but it still uses aal5 llc/snap
From the rfc1483 documentation, I see that there are
8 bytes trailer
8 bytes header
2 bytes padding
and then the mac frame which consists of
12 byte mac addresses
2 bytes ethertype
4 bytes checksum for mac frame
atm cell payload = 48 bytes
31 cells * 48 = 1488
1488 - 36 = 1452
Would 1452 be the right size or am I doing something incorrectly?
NormanSI gave her time to steal my mind awayPremium,MVMReviews:
San Jose, CA
Nearly as I can tell, use the highest value which allows ping without fragmentation; or something like that.
There is a FAQ at this site about adjusting MTU:
»Tweaking FAQ »Max MTU: How do I find mine?
With Sonic.net Fusion, I can ping at 1472 without the fragmentation warning:
C:\Utils\dig>ping -f -l 1472 www.dslreports.com
Pinging www.dslreports.com [18.104.22.168] with 1472 bytes of data:
Reply from 22.214.171.124: bytes=1472 time=109ms TTL=50
Reply from 126.96.36.199: bytes=1472 time=110ms TTL=50
Reply from 188.8.131.52: bytes=1472 time=147ms TTL=50
Reply from 184.108.40.206: bytes=1472 time=116ms TTL=50
Ping statistics for 220.127.116.11:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 109ms, Maximum = 147ms, Average = 120ms
Per the FAQ, I can just add 28 to 1472 and set my MTU: 1500 in this case.
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum
|reply to robk2 |
I believe what the OP was asking is the MTU value so that the ATM packets will be optimum. Each ATM cell has a payload of 48 bytes. If you use a value such as 1500, there will be some padding bytes used to make the ATM cell is exactly 48 bytes, resulting in an additional cell with a payload of 1-47 bytes. The highest value without fragmentation is not optimal when using ATM.
I believe that the OP's value is correct as the 14 byte mac header and the 4 byte trailer is not included when you specify the MTU. Subtracting the sum of the overhead from a multiple of 48 will give you the optimal value.