At 08:26 AM 7/17/00 +0200, Mikael Abrahamsson wrote:

On the other hand, at work we're doing some tunneling using ciscos. Due to routing etc the ICMP "need-to-frag"-messages get lost and the people behind those tunnels cannot use 90% of the www sites (so they have to resort to proxies). Seems to me that PMTUd works better than most people think.

Wow, why would the ICMPs get lost? Also, cisco has a feature on tunnels now where the routers will frag & de-frag making the tunnel MTU effectively 1500 bytes (or whatever you set).

I do believe that NT and Win2k comes default with a registry setting that makes it send all TCP traffic with the DF flag set (which I can see no reason for unless M$ IP stack cannot do refragmentation properly). This setting is changable as far as I know but I cannot seem to find the information at this time. Anyone?

I have no clue if that is really a setting. (Do not run any MS web servers.) However, end stations do not do fragmentation. They do re-assembly, but the receiving station has no control over whether something gets fragmented in transit to it. If the MTU of a path is less than, say, 1500, the end station just sends out smaller packets, not fragments. Since it is likely that you have visited an MS-power site, and you say you can reach all sites, then the MS IP-stack can probably send out packets < 1500 bytes long. If the setting you describe does exist (and it may very well considering MS' history), it is probably just another screw up from the world's black-hole for bad programmers.

Mikael Abrahamsson email: swmike@swm.pp.se

TTFN, patrick