North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Broken PMTU (was: Who does source address validation? (wasRe:what's that smell?))
On Wed, 9 Oct 2002, Stephen J. Wilcox wrote: > On a related issue (pMTU) I recently discovered that using a link with MTU < > 1500 breaks a massive chunk of the net - specifically mail and webservers who > block all inbound icmp.. the servers assume 1500, send out the packets with DF > set, they hit the link generating an icmp frag, icmp is filtered and data > stops. Culprits included several major ISP/Telcos ... I'd love to tell the > customer the link is fine its the rest of the Internet at fault but in the end I > just forced the DF bit clear as a temp workaround before finally swapping out to > MTU 1500! I'm not going to say what I think of these people in order to avoid another semi-flame fest, but limit my comments to: You can also get around this by making the first hop the one with the lowest MTU. This is no fun for ethernet-connected stuff, but for dial-up this is easy. Then this box will announce a smaller TCP MSS when the connection is established and there aren't any problems.
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