Traffic bursts when using ATM PVC

Discussion in 'Cisco' started by unix, Aug 5, 2009.

  1. unix

    unix Guest


    This is our setup in a nutshell:

    Server (100Mb NIC) --> router (100Mb) --> T1 ATM (IMA) PVC --> 128kb
    client circuit --> client PC (100Mb).

    Certain times during the day, the server needs to send a lot of data
    to client PC. Since the server NIC is 100Mb and it traverses a T1,
    followed by a smal 128kb link at client site, whose job is it to make
    sure the traffic adjusts for speed differences? My captures at server
    side show an enormous amounts of TCP retransmits during the traffic
    bursts. I know frame relay has a mechanism to throttle back , but not
    sure how ATM PVCs handle this.


    unix, Aug 5, 2009
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  2. unix

    Mark Huizer Guest

    The wise unix enlightened me with:
    It will try to buffer (fifo) at first, but that will not really help, so
    when the buffer is filled, you will get tail drops, which will result in
    your tcp session slowing down, doing it's slow start thingie, get up to
    a certain speed again, get drops again, slow down, etc.

    So actually it's doing what it should be doing. If you want to do it
    without the drops and retransmits, you probably need something that will
    play with tcp window sizes (like packeteers and the likes do). Or make
    your server limit itself.

    Mark Huizer, Aug 5, 2009
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  3. ATM is quite close. Is it a single session or multiple ones? What you
    need is per-VC queuing correct ATM shaping parameters for this VC. For
    single session not much else besides making sure there is enough buffer
    to handle 128Kbs worth of traffic multiplied by round-trip time.
    For multiple sessions CBWFQ would be bees knees.

    Andrey Tarasov, Aug 5, 2009
  4. unix

    bod43 Guest

    This is true to some extent however modern TCPs do
    a lot better. There is "fast retransmit" and "selective ack".
    Make sure Selective Ack is enabled on
    both ends if possible. Current Window's TCPs have it on
    by default. Easiest thing if you have captures is to
    look at the two SYNs.

    You don't say what "an enourmous number" might be.
    1% would not seem too bad to me.

    As already mentioned TCP was designed to deal
    with this type of network and, especially with all the
    modern bells and whistles, does pretty well.

    Huge buffers are not the answer either. Andrey's advice
    sounds OK. The two TCPs communicate congestion
    information between each other, if you whack in a huge
    buffer, the main effect is to delay that comminucation.

    The easiest place to see what is going on at
    a high level is to check the interface counters
    for output drops.

    Worth noting that UDP does not have such a
    congestion control mechanism and that if you
    had a high proportion of UDP traffic then you might
    see some issues.
    bod43, Aug 5, 2009
  5. unix

    Stephen Guest

    I know frame relay has a mechanism to throttle back , but not
    Not sure - but it may depend on what type of PVC setup you have.

    the only thing that understands the bandwidth change directly is the
    router - but unless you configure the PVC rate specific to that PVC it
    will just shape to line rate (ie T1 at your end).

    Are you shaping the traffic to the ATM 128 K PVC or are you just
    letting it blast away at the ATM line rate?

    If you are lucky the ATM switches downstream are clever enough to
    discard the entire set of cells associated with a single AAL5 format
    packet on congestion or PVC traffic overload - but this isnt always
    supported (or working properly).

    try altering the outbound Q at the router ATM interface - a smaller
    queue may help, but the key is usually WFQ or WRED to provide feedback
    to the TCP congestion control.

    If you have lots of framing errors at the ATM recieving end, the ATM
    network is dropping some cells within packets - the fix for that is
    probably to shape the per PVC traffic to what you are allowed to send
    in the contract.
    Stephen, Aug 5, 2009
  6. unix

    unix Guest

    When I look at capture I/O graph (using Wireshark/Ethereal), I see a
    burst of around 60 seconds where the T is completely maxed out. (We
    have about 40 clients connected at 128Kb). After the one minute peak,
    I see a definite back-off from the server and the line goes to about
    20% utilization but with about 3% of retransmit packets. Is this
    unix, Aug 8, 2009
  7. unix

    Stephen Guest

    This sounds like each PVC is only limited to the line rate

    After the one minute peak,
    the issue isnt so much the error rate as the "good put" - ie how much
    of the available 128k is actually moving useful info.
    Try moving a big file and measure the time, then work backwards to
    the application bit rate - if you get to 80% of the 128 Kbps then you
    are doing reasonably well.

    what is probably happening is that the 2 devices at the ends of the
    connection "see" the errors and reduce the data rate to roughly match
    the available thruput.

    however - 60 sec is a long time to notice what is going on and adapt,
    and the transfer isnt doing "slow start", so is probably a block
    scheme of some sort.

    Is the connection using a protocol that just blasts traffic in big
    bursts such as windows SMB (file sharing)?

    If so there is a bit of tinkering you can do and the machines to tweak
    the transfer but it isnt straightforward and it may have side effects
    on other windows transfers.
    If you want to try you need selective ACK (at both ends).
    You can reduce the max transfer size, but this will affect local
    transfers - but since it is negotiated at connection setup you only
    need to reduce 1 device.

    Try an FTP if you can and see if a TCP based protocol behaves better,
    and to give you a performance baseline. Or use a test tool such as

    Good luck.
    Stephen, Aug 9, 2009
  8. unix

    alexd Guest

    If you've used Wireshark before, you might find this presentation

    where the author describes how to use Wireshark to diagnose issues where
    Wireshark knows how to decode the protocol [in this example SMB].

    <> (AIM:troffasky) ()
    17:30:41 up 100 days, 8:58, 2 users, load average: 0.31, 0.32, 0.30
    "If being trapped in a tropical swamp with Anthony Worral-Thompson and
    Christine Hamilton is reality then I say, pass the mind-altering drugs"
    -- Humphrey Lyttleton
    alexd, Aug 14, 2009
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