PPP link between two PC's - Losing every 5th packet - PPP

This is a discussion on PPP link between two PC's - Losing every 5th packet - PPP ; Hi there, I'm trying to setup a test environment that consists of two linux machines networked using a serial connection using ppp. I bought two high speed serial cards using the 16850 uart to do this. I'm having some problems ...

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Thread: PPP link between two PC's - Losing every 5th packet

  1. PPP link between two PC's - Losing every 5th packet

    Hi there,

    I'm trying to setup a test environment that consists of two linux
    machines networked using a serial connection using ppp. I bought two
    high speed serial cards using the 16850 uart to do this. I'm having
    some problems achieving the throughput I'm expecting.

    With an ftp transfer with the link running at 115.2K I get about
    6kbytes/sec using the 16850 equiped cards. I expected closer to
    13kbytes/sec. I can achieve this rate (13K) when I create a 115.2K
    link using the inbuilt serial ports (16550 uart) in the two machines.

    I did a tcpdump on the ppp connection using the 16850 equiped cards,
    performed an ftp transfer, and looked at the results. I found every
    5th packet sent was not being received. I then halved the mtu size of
    the link and retried and still every 5th packet sent was being lost. I
    had thought that if the loss was related to something like a buffer
    overflow or time related I would have found approx. every 10th packet
    lost with the halved mtu size.

    Does anybody have any ideas as to what may be going on or what I
    should look at ?

    I am using the 2.6.7 kernel. I started off trying to use 2.4.20 but
    had problems with it's serial driver and the 16850 uart serial cards.

    Alan

  2. Re: PPP link between two PC's - Losing every 5th packet

    alsecure2002@yahoo.com.au (Al) writes:
    > I did a tcpdump on the ppp connection using the 16850 equiped cards,
    > performed an ftp transfer, and looked at the results. I found every
    > 5th packet sent was not being received. I then halved the mtu size of
    > the link and retried and still every 5th packet sent was being lost. I
    > had thought that if the loss was related to something like a buffer
    > overflow or time related I would have found approx. every 10th packet
    > lost with the halved mtu size.


    At a guess, that sounds like a flow control problem, a serial driver
    problem, incorrect IRQ assignments, or something more complex like
    interrupt latency. If everything is set correctly with 'setserial',
    and you don't want to hack about with the kernel, then I'd complain to
    the vendor of those cards.

    --
    James Carlson, IP Systems Group
    Sun Microsystems / 1 Network Drive 71.234W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.497N Fax +1 781 442 1677

  3. Re: PPP link between two PC's - Losing every 5th packet

    James Carlson wrote in message news:...
    > alsecure2002@yahoo.com.au (Al) writes:
    > > I did a tcpdump on the ppp connection using the 16850 equiped cards,
    > > performed an ftp transfer, and looked at the results. I found every
    > > 5th packet sent was not being received. I then halved the mtu size of
    > > the link and retried and still every 5th packet sent was being lost. I
    > > had thought that if the loss was related to something like a buffer
    > > overflow or time related I would have found approx. every 10th packet
    > > lost with the halved mtu size.

    >
    > At a guess, that sounds like a flow control problem, a serial driver
    > problem, incorrect IRQ assignments, or something more complex like
    > interrupt latency. If everything is set correctly with 'setserial',
    > and you don't want to hack about with the kernel, then I'd complain to
    > the vendor of those cards.


    James,

    I think it's a flow control problem as well but I suspect it's to do
    with how the linux serial driver is handling the 16850 uart on the
    card. I haven't been able to find many references to using the 16850
    uart under linux.

    I get similar rates of packet losses no matter what speed I run the
    serial link.

    Al

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