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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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