MTU Settings - Help

Hi

I am trying to find the optimal MTU settings - I am with BT broadband and have a netgear dg834n router. But using the "ping -f -l" command, I find that no matter how low I set my MTU, after a while I still get the message that "packets need to be fragmented but DF is set." What is wrong?

P.S I am on interleaved mode - could this be the reason packets are alwyas fragmented?

P.P.S I am using wireless

Edited by deleted (Mon 20-Jul-09 02:48:49)

Hi
I also use a Netgear DG834n / BT option 1
Try using the setting I use see below


Tweak Tester Result
We have collected information from your trial download and upload.
Service: isdndslcablewirelesssatellitefiber
Download Speed kbit/s:
(the download speed advertised for your line)
Operating System: win95win98win98SEwinMEwinNTwin2kwinXPVistaMacLinuxFBSDSolaris
Connection: normalwinXPpppoerouterpppoeraspppoepppoA



1. Your Tweakable SettingsReceive Window (RWIN): 191260
Window Scaling: 2
Path MTU Discovery: ON
RFC1323 Window Scaling: ON
RFC1323 Time Stamping: OFF
Selective Acks: ON
MSS requested: 1460
TTL: unknown
TTL remaining: 48
TOS flags: none set
2. Test DownloadActual data bytes sent: 1024001
Actual data packets: 703
Max packet sent (MTU): 1500
Max packet recd (MTU): 1500
Retransmitted packets: 0
sacks you sent: 0
pushed data pkts: 85
data transmit time: 3.447 secs
our max idletime: 639.6 ms
transfer rate: 241345 bytes/sec
transfer rate: 1930 kbits/sec
transfer efficiency: 100%

3. ICMP (ping) check Target unpingable

If your IP could be pinged then we hit it with various streams to identify any loss, or exessive jitter from our server.

4. Notes and Recommendations RWIN is in range
Looking good

Observations:
Good data stream (no/few rexmits)

Observations:
Become pingable
if you need basic packet loss tests done
Check tweak FAQ

Best of luck Mike A

P/S
I am allso on interleaved / wireless mode
Connection speed 3616
3mb profile
Downstream 2820.45 Kbps
MTU is set to 1500 on Netgear & 1500 on PC
Use Dr TCP to change settings on PC & then reboot PC
Mike A

Probably a better idea to have the router wired to the PC, then set the MTU to 1500 on the PC and the router before trying the cmd prompt with lower settings.

have a look here >>> http://www.kitz.co.uk/adsl/tweak2.htm

Thanks to all your replies - but what ever settings I chose I get "packets need to be fragmented but DF set."

this any use

error

or this

error

Edited by strayaway (Mon 20-Jul-09 10:55:44)

Thanks for those links - but unfortunately they don't answer my question, which is: why do packet sizes lead to messages that "packets need to be fragmented but Df is set"? When I chose a sufficiently low value like 1432, I initially get a good result but then when I reboot and repeat the commnand, I get the "packets need to be fragmented" message again.

P.S So based on the formula "ping -f -l [packet size] + 28" - I have worked out my optimal mtu is 1432. But when I set that, and go back and run the command again, I get "packets need to be fragmented but df is set message" for the figure of 1404, when I previously did not. Is it something to do with the fact that the lower the mtu you set, the lower the maximum size of a packet that can be sent unfragmented?

Edited by deleted (Mon 20-Jul-09 11:22:35)

In reply to a post by has:

Is it something to do with the fact that the lower the mtu you set, the lower the maximum size of a packet that can be sent unfragmented?

Exactly.
You are trying to find out the smallest value you need to set to transmit data efficiently without fragmentation.
Smaller units travel faster, but if they need to fragment you will transmit more.

That's a relief - but I am not quite clear about the explanation. (I am not very technically minded ). Could you explain that a little more? Are you saying that fragmentation is the means by which packets will be transmitted by design?

MTU is the size of the bucket that will carry the data. The smaller the bucket, the more buckets you need. Each device your data passes through has its own buckets.
If the buckets for all the devices are the same size, then the data flow will be optimised.
If some device has smaller buckets, then it will need more buckets.
If some device has larger buckets, there will be wasted space.

So when I discover the maximum mtu which allows packet to be transmitted in an unfragmented form, why does that value not continue to lead to packets being transmitted in an unfragmented form - why in other words, when I run "ping -f -l", do I get the message that the packets need to be fragmented when previously, before I set the new, lower mtu, that value resulted in no fragmentation?

Maybe something is automatically tuning things - Vista for example.

I am using xp - but I have got a clearer picture of what is going on. It seems that the actual mtu is 28bytes lower than the one I set (In other words, the mtu I set is 1432 but the maximu size packet I can send without fragmentation is 1404). Something to do with overheads?