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    SubjectRe: tcp stalls with 2.4 (but not 2.2)
    Brian Grossman wrote:

    > I'm seeing stalls sending packets to some clients. I see this problem
    > under 2.4 (2.4.1 and 2.4.1ac17) but not under 2.2.17.

    compiled in ECN support? SYNcookies? try disabling through /proc
    tcp or udp? if udp check /proc/net/ipv4/ip_udpdloose or such

    > My theory is there is an ICMP black hole between my server and some of its
    > clients. Is there a tool to pinpoint that black hole if it exists?

    ping is your friend. -s lets you set size of packet. (to
    check for fragmentation) use tcpdump to capture
    a trace of this or a tcp session.

    email trace to me private if you want.

    > Can anyone suggest another cause or a direction for investigation?
    > Why does this affect 2.4 but not 2.2?
    > The characteristics I've discovered so far:
    > From strace of the server process, each write to the network is
    > preceeded by a select on the output fd. The select waits for a
    > long time, after which the write succeeds.
    > The packets are received by the client a couple minutes after my
    > server sends them.
    > The clients I have tested with are win98 and winNT.
    > The router for both 2.4 and 2.2 servers is running 2.2.18 with
    > ipvs (ipvs-1.0.2-2.2.18).
    > That router does not block any ICMP.
    > The behavior occurs on the 2.4 machine whether the packets are
    > routed directly or are mangled by ipvs.
    > I've tried the same machine with both 2.4 and 2.2, as well as
    > another machine with just 2.2. 2.2 works. 2.4 doesn't.
    > Both of my servers and the router I mentioned have two tulip
    > network cards.
    > The clients I've tested with are behind a modem through earthlink.
    > Another I suspect to have same problem is behind a modem
    > through Juno.
    > I've tried adjusting both /proc/sys/net/ipv4/route/min_adv_mss and
    > /proc/sys/net/ipv4/route/min_pmtu downward. Do these require an
    > ifconfig down/up to take effect?

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