Effect of MTU setting

Posted on 2005-05-02
Last Modified: 2013-12-14
In response to a question about how best to cable for a dsl modem it was suggested that I find the best MTU setting by testing with ping -f -l 1470 and finding out how low the setting has to go before it's not fragmented. I have to step all the way down to 1370. Given the default is something like 1492, what's that say about the quality of my connection?
Question by:juststeve
    LVL 13

    Expert Comment

    Hi Steve,
    It probably doesn't say much about the quality of the connection, just that you have some devices in between that add overhead to the packets. I have a similar setting; mine comes from the VPN tunnels I need... Have you tried other web sites? Maybe web sites based at your internet provider (to check if it's on the way to the ISP or from the ISP to his partners)?

    Author Comment

    I've tried a few different both directly to my isp and elsewhere with no change. I've also tested to my VPN server both with the connection live and with it change. I'm stepping thru a Linksys router and Norton's Personal Firewall. Without the router there I can go higher but it's gotta be there so...

    The big picture issue here is understanding why my 1.5mp service level is only getting me ~.6-.7. At some points in the past (have only had DSL for a month or so) I've seen it exceed 1.6mp for extended download but lately nothing approaching that performance. I'm fairly close to concluding I've done what i can configuration-wise and am preparing to string a new cable direct to the modem's jack...thankfully not too hard to do in my home.

    But then...any other advise is welcome.
    LVL 13

    Assisted Solution

    Hmm, DSL is usually sold as "best-effort", meaning the provider will try to get that speed going. (Speed is also a function of the line to the nearest phone-station - whatever they call those things. If your line is too long or the quality of the line is not too hot, then this will have a great influence on your DSL speed. You should be able to get the speed with that MTU anyhow. Have you talked to your provider? Do you have other people in the same neigherborhood with the same provider - how are their lines? The main problem (at least where I live) is that the physical lines are owned by one company, the DSL is provided by a different one - meaning that the problem is usually "not mine" for each company, pushing you back and forth without helping solve the issue... Lets see what the others have to say -- Good luck!
    LVL 41

    Assisted Solution

    Go here and run the tweak test, should help
    LVL 7

    Accepted Solution

    Not much.  Says a lot about the devices in the path the packet takes, but that's not == quality.   Packet loss is quality.  MTU reflects the devices in the (routing) path between two hosts.

    Remember, the tcp/ip protocol handles this just fine - it fragments packets as it needs to.  Say, for whatever reason, there's a link which only supports 1000 bytes - your 1500 byte packet gets chopped into a 1K and 500 byte fragment, gets sent on and reassembled at the host.

    HOWEVER, fragmentation takes time and buffers and - worst of all - significantly increases the chance that your packet needs to get resent on poor quality lines.  (say really bad, like losing 1 in 10 - tcp/ip handles this, but if each packet gets split in two, your EFFECTIVE loss is now 1 in 5...)

    So, the recommendation to find and set your MTU is designed to eliminate fragmentation and thus improve performance.

    Great, right?  Maybe / Maybe not.

    The tcp/ip protocol itself includes MTU discovery - so as part of the start up of a connection, if it finds packets are being fragmented it does 'discovery' to find the best value and uses that for the connection.

    So by doing MTU discovery, you are substituting this automatic per connection process with a permanent one.  That's good if the small MTU link is always part of your path.  But if it's not, you are incuring extra packets (and thus maybe more ACK time) on connections that don't require it.

    So before you go setting your MTU based on the path to some far distant site, figure out what's close in the path to your ISP (use traceroute/tracert) and check THAT.  You'll get better results.


    LVL 40

    Assisted Solution

    Yes.  I usually recommend using MTU path discovery on the ISP's DNS servers, or the ISP Gateway...  In other words, keep it local...

    I would find it hard to believe that your router would be causing latency, but I know the Norton Security sure does...  Personally, I would pull that off your system and if you need a software firewall, just use XPs built-in firewall.  Not as secure, but not near the overhead of Norton.  And using the router to hide your private LAN addresses will provide you with attack prevention also.

    LVL 40

    Expert Comment



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