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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Path-MTU-discovery
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: [email protected] TTFN, patrick
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