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Re: ping packet loss when size gt 1500

Adam Hardy <adam.ant@cyberspaceroad.com> wrote:
> I tried lowering the MTU to 1400 but it made no difference.

So ping -s 1372 failed? I thought you said it worked up to -s 1472
(packets of 1500 bytes)?

When you drop your MTU to 1400, that means that your local data packets
are fragmented as necessary to stay within the maximum packet size. If
you generate data with a "do not fragment" ("DF") label attached to it,
you have to ensure that each packet is no larger than the available
size. (All fairly obvious so far, I trust.)

If something between you and the destination is eating the "Hey you're
MTU's too big for this link" messages then there's no way of handling
overly large packets - as far as the recipient is concerned they have
been discarded en route.

Remember that although your packets might be the right size, there is
(AFAICR) nothing there that limits the size of the packets being returned
to you. And if the "too big" ICMP messages relating to data from the
far end back to you are being discarded then the sender can't know it
needs to reduce the packet size in order to reach you.

Does this help any?

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