> > I'm stuck. FTP downloads over my PPP link are fine, they pretty much
> > saturate the connection when the remote host can send that
> > fast. Uploads are a different matter: regardless of the remote host
> > I'm uploading to, the xfer rate withers away to sod-all and a huge
> > gob of data remains resolutely stuck in the send-Q.
> I've seen this before, long ago. I fixed the problem by specifying an
> mtu of 1500. What happened was that my ISP specified <mru 1524> in
> the initial negotiation, indicating that I could set my mtu to that;
> pppd did so, but the 1524 size packets were being dropped by a router
> somewhere (at least that's what I surmised, and had some minimal
> evidence to back that up).
OK, I looked into this and it seems some Ascend NASs will advertise a max
mru of 1524 when they really mean 1500, and will throw the packets on the
floor silently, without fragmenting them (or sending ICMP_FRAGNEEDED if DF
is set). I /was/ talking to such a box, but fixing the mtu at 1500 locally
doesn't solve my original problem.
> Your mtu appears to be at 1500 already; try setting it to 576 (I think
> this is the magic "always works" number) and see if that fixes the
> problem.
I tried this too and it seems to help, but I think it's masking the problem
rather than curing it. I'll go grab some more info (tcpdumps and the
RetransSegs count from /proc/net/snmp) and see..
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