A Ping (Determination of MTU) question

[JonathanB]

I want to know my optimum MTU so I connected to the internet, went to MS DOS, and typed >ping -f -l {my trial MTU}

http://www.{my

ISP} and clicked enter. From a trial number of 549 and above, I got a message "Packet needs to be fragmented but DF set",so I have to try a lower value. The last number possible which is 548 gave me a message:
Request timed out
Hardware error
Hardware error
Hardware error
Packets sent 4 Received 0 Lost 4 (100% loss)
Approximate round trip time Min 0 Max 0 Ave 0
Anybody knows why is this? My OS is Win98SE and a 56K Diamond Supra dial up modem.

 

[tbad1986]

From what I hear, 98 will not accept an MTU lower than 576. I'm guessing you didn't raise your MTU to 1500 (and restart) then ping your ISPs site.

ping -f -l 1472

http://www.yourisp.com

is the max you can do. You take the MTU (576, 1000, 1500 normally) then subtract 28. 1472=1500, 972=1000, 548=576. So it would actually be
MTU - 28 = PingMTU
ping -f -l PingMTU

http://www.yourisp.com

 

[JonathanB]

Yes. After more researching, I now know that MTU-28 is the Ping MTU. When I started doing it at 1472, I got the messages "Packet needs to be fragmented but DF set", so I lowered my settings till 548, but got Request timed out Hardware error on 548. But now I got "Request timed out Hardware error" in all the three numbers 1472, 972, and 548. Does com port speed affect this? My dial adapter in in COM3 so I went to Win.ini and tried COM3 speed to 115200 and the next higher (I installed a software raising the com port speed). And also I'm wondering why my 56K modem is connecting at 33.6k (did I remember the speed right?)Is there adjustments to this or it is due to local ISP conditions.

 

[JonathanB]

Well? I also got my answer. Its the firewall that blocked my echo request. Just for those who want to know.

 

[tbad1986]

Com port speed only effects the speed at which the data goes from the computer to the modem. Once it hits the modem it must slow down to the dial-up speed. Raising it above 115200 is overkill for 56k.

"28.8Kbps to 33.6Kbps (ITU-T V.34 protocol)
Indicates the line is so close to perfect that it would be difficult
for the Telco to actually measure any change that would improve
speed. This means SNR is better than about 32 dB, the bandwidth of
the channels being used is at maximum, and the cable must not be
very long. 33.6Kbps simply means the line is about as good as it can
get. " -

http://www.dslreports.com/faq/2849

 

[JonathanB]

Well, that's add'l info. Thanks. I ping my ISP at 1472 without defragmenting message so I set my IP Packet Size to large and followed this

http://www.optimizing.net/

. I got a boost. I am now connecting at 44.+kbps (previously 33.6K) as indicated in the dual PC icon in the systray. And NetMedic is always green in my systray. I don't know if I still can improve this. BUT IT'S TOO FAR BETTER.