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.