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OT - Downloading on Dialup

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  • #31
    Originally posted by saltmine
    I donno....I've got DSL through telephone lines that I suspect Thomas Edison put up himself. Once the net provider got their act together, it's been alright.

    I used to have dial-up, years ago...but I had Windows 3.1 and a 300 Baud modem too. So it didn't matter.

    If Osama Obama gets his mitts in it, we'll be using semaphore flags
    I too have a provider who worked for Edison...as the janitor.

    Obama might have to settle for flags since Bush got the flight jacket. ;<)

    TMT

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    • #32
      Originally posted by whitis
      Frequently on dialup this kind of problem can be caused by badly configured routers that interfere with MTU path discovery.

      Each network interface/link has a maximum amount it can transfer in one packet (MTU). The computer can set a lower value of MTU than imposed by the hardware for a particular interface and for particular connections. Also, the other side of the link may impose an MTU.

      For dialup lines, MTU is usually set smaller. Around 576 bytes typically. The reason being there are no passing lanes on the link - only one packet can be on the link at a time and big packets can tie up the link for a while and prevent other packets, which might be more urgent, from getting through. So your FTP session could make your telnet session almost unusable due to delays, for example. In particular, the router at the other end of your dialup connection probably sets an MTU of 576.

      At the beginning of a TCP network connection, each system tells the other what their Maximum Segment Size is, a number which should be less than or equal to the MTU on that network interface. Each side will try to respect the other sides MTU, but somewhere in between a smaller limit may be imposed. A process called MTU path discovery is used to find the largest MTU supported along the link so all packets will be sent smaller than that. Basically, you send a big packet with some bits set to say the packet can't be fragmented. If it runs into a limit along the way, that node on the network is supposed to send an ICMP error message back indicating the maximum size (which doesn't always happen) and misconfigured routers can block ICMP messages. Some incompetent network administrators block all ICMP messages indiscriminantly because a few of them have been abused for network attacks. Even the ones used for attacks exist for a reason and blocking them outright causes trouble.

      Windoze in particular tends to advertise an MTU (1480) on dialup connections which is bigger than the router on the other end will permit and makes it hard to set this. Real operating systems allow you to set the MTU with the ifconfig program while configuring a link (and gui frontends can do likewise), so setting the MTU is just a few extra keystrokes on the command to configure a network interface. On windoze, this is hidden in the registry.

      TCP/IP has an ability to break packets up into fragments and reassemble them. But this is not supported on all links and the server you are talking to may set the Don't Fragment bit prohibiting this. Since it is more efficient to send the right size packet in the first place, the server will use Path MTU discovery with Don't Fragment set on each packet.

      A feature of TCP/IP called congestion avoidance tries to limit the amount of data sent on each TCP connection so that packets don't get dropped on the network. Part of this is slow start. Each tcp connection starts by sending small packets and if it doesn't get an ICMP source quench message it tries a little bigger on the next and so on until the packets are up to the full mtu size.

      So, initially, the data gets through because it is in small packets. Then as the packets get bigger, they get clobbered. So you get some data through and then you stop.

      If you reconfigure the MTU/RSS on your dialup connection to be less than or equal to the MTU imposed on the other end of your dialup link, the problems should go away, if this is the cause. Try 576.
      http://www.internetweekly.org/llarrow/mtumss.html
      Thank you for an excellent writeup.

      TMT

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