Hi everybody,

I am quite confused about the results of the Analyzer and Optimizer.

My ISP (T-Online Business) requests a special MTU of 1456. So are the settings in my computer and the router.
This is what the Optimizer finds out in the Largest MTU tab:

Pinging [66.230.207.58] with 40 bytes ->bytes=40 time=218ms TTL=45
Pinging [66.230.207.58] with 750 bytes ->bytes=750 time=240ms TTL=45
Pinging [66.230.207.58] with 1125 bytes ->bytes=1125 time=254ms TTL=45
Pinging [66.230.207.58] with 1312 bytes ->bytes=1312 time=260ms TTL=45
Pinging [66.230.207.58] with 1406 bytes ->bytes=1406 time=273ms TTL=45
Pinging [66.230.207.58] with 1453 bytes -> ..fragmented
Pinging [66.230.207.58] with 1430 bytes -> ..fragmented
Pinging [66.230.207.58] with 1418 bytes ->bytes=1418 time=262ms TTL=45
Pinging [66.230.207.58] with 1424 bytes ->bytes=1424 time=262ms TTL=45
Pinging [66.230.207.58] with 1427 bytes ->bytes=1427 time=276ms TTL=45
Pinging [66.230.207.58] with 1428 bytes ->bytes=1428 time=269ms TTL=45
Pinging [66.230.207.58] with 1429 bytes -> ..fragmented
The largest possible non-fragmented packet is 1428 (1456 - 28 ICMP & IP headers).
You can set your MTU to 1456


But the Analyzer shows an MTU of 1416:

SpeedGuide.net TCP Analyzer Results
Tested on: 04.02.2010 14:57
IP address: 80.129.xxx.xxx
Client OS: Windows XP

TCP options string: 020405600103030101010402
MSS: 1376
MTU: 1416
TCP Window: 124608 (NOT multiple of MSS)
RWIN Scaling: 1 bits (2^1=2)
Unscaled RWIN : 62304
Recommended RWINs: 63296, 126592, 253184, 506368, 1012736
BDP limit (200ms): 4984kbps (623KBytes/s)
BDP limit (500ms): 1994kbps (249KBytes/s)
MTU Discovery: ON
TTL: 45
Timestamps: OFF
SACKs: ON
IP ToS: 00000000 (0)


How come?

Does anybody have an idea?
Regards