[ On Thursday, November 18, 1999 at 22:03:21 (-0500), Randy Bush wrote: ]
Subject: Re: should TCPs do MTU black hole detection?
o when it breaks, the noc gets the call, debugs it, and it gets fixed ^^^^^^^^^^^^^ Optimist!
but at least the user knows where the problem lies, as opposed to
I certainly agree with you here Randy! However as a reasonably expert user there are times when I want the ability to have my equipment try to work around even ultra-stupid configurations elsewhere on the net, especially when those of us experiencing the problem are far more rare than those who do not. Some time ago when I first began to personally experience this problem with path MTU discovery on my home network I discovered through analysis of the upstream packet traces that it was almost always possible to have the router sending the needs-frag reply to realize when its attempts to do so were futile and thus enforce fragmentation anyway. While this may make some protocols un-usable anyway it could at least allow me to limp along and to allow me to use that same network to communicate with the offending people at the other end using protocols affected by this problem such as SMTP, FTP, HTTP, etc. Unfortunately I have not yet had time (or in this case the need -- I've since increased my link's MTU to 1500 :-), to implement this algorithm in my own upstream router (which thankfully I do have the root password for! ;-). BTW, I think my algorithm provides a much more efficient work-around to the problem than black-hole discovery. Unfortunately it also makes it harder to convince the offending admins to fix their stupid filter definitions (or alternately at least turn off PMTUd in their servers). What I'd really like to do is find some way to enhance my algorithm in such a way that it could send a tiny tactical nuke down the wire after my connection to/through the offending network safely closes. I.e. I want to cause grief to any ICMP filter that has caused my router to have to work around its stupidity! ;-) -- Greg A. Woods +1 416 218-0098 VE3TCP <gwoods@acm.org> <robohack!woods> Planix, Inc. <woods@planix.com>; Secrets of the Weird <woods@weird.com>