strange MTU behavior!

nigthlord · 02-23-07, 06:46 AM

Hi!
Something is stragne to me; maybe someone can explain it to me!
Here are the results from the analyzer:
________________________________________________
« SpeedGuide.net TCP Analyzer Results »
Tested on: 02.23.2007 06:24
IP address: 213.143.xxx.xx

TCP options string: 020405ac01010402
MSS: 1452
MTU: 1492
TCP Window: 33396 (multiple of MSS)
RWIN Scaling: 0
Unscaled RWIN : 33396
Reccomended RWINs: 63888, 127776, 255552, 511104
BDP limit (200ms): 1336kbps (167KBytes/s)
BDP limit (500ms): 534kbps (67KBytes/s)
MTU Discovery: ON
TTL: 46
Timestamps: OFF
SACKs: ON
IP ToS: 00000000 (0)
_____________________________________________________
Ok! Thing I dont understand is when I try to ping with ping -f -l 1492 speedguiede.net I get the hint that the packet has to be fragmented but df-flag is set....
I have to lower the the value till 1464 until I get packages back!
But according to the analyzer it should be 1492.

Why do this happen ???
Thanks in advance!

best regards
roland

**trogers** · 02-23-07, 06:57 AM

When you use the DOS command for ping -f -l, you have to chose a packet size which is 28 bytes less than your MTU.

Thus if your MTU is 1500, you can ping with a packet size not greater than 1472. If greater, you get fragmentation.

For MTU 1492, the largest packet size is 1464.