Re: Checklist (sub-thread of: "Slow file sharing performance: lostpidgeons")

I'll send the files directly to you, in case the mail list has dropped them.


Les Mikesell wrote:
> On Tue, 2003-09-09 at 08:19, James Knott wrote:
> 
>>>A) The 'flow control' mechanism when going between a fast medium to
>>>a much slower one is to buffer some, then discard packets when the
>>>buffer is filled. 
> 
> 
>>I suspect this may be the cause of the problem.
> 
> 
> Since it doesn't happen if you don't go out the cable modem the
> packets have to be dropped somewhere beyond your server's ethernet.
> 
> 
> 
>>>B) Samba uses TCP for file transfer, so any conclusions
>>>you made about UDP by observing samba are incorrect.
>>
>>I don't know enough about SMB, but that's what I also thought.  However, 
>>there are some UDP packets as well.
> 
> 
> Netbios uses UDP for name lookups but the connection for file transfer
> is TCP.
> 
> 
> 
>>I posted some tcpdump files to the list yesterday.  I had sent them to 
>>Allan earlier.
>>
> 
> 
> I can't find them in the list archives.  Since you really can't control
> beyond your own ethernet you need to figure out why flow control
> isn't working going into the cipe interface. You mentioned setting
> mtu down.  Does the packet size entering the cipe interface reflect
> the size you set?
> 
> 
> 
>>>D) Wild guess here - I haven't checked but it is possible that
>>>samba sets the DF (don't fragment) bit where ftp doesn't (Microsoft
>>>likes to do that for no particular reason...).  That means if
>>>the medium can't accept the whole packet it has to discard it
>>>instead of fragmenting so it will be rebuilt at the other end.
>>
>>Shouldn't I be seeing some ICMP messages about that?  Also, I tried 
>>using some small MTU values and it didn't help.  Also, I looked at some 
>>of the packets captured with Ethereal.  They don't show the DF flag set.
> 
> 
> Are you looking at the cipe interface?  A quick tcpdump of port 139
> on both windows or samba servers shows that it does set DF on every
> packet.  You should see an ICMP message if a single packet exceeds
> max allowed by the medium for the next hop assuming the router involved
> follows the standards and it isn't blocked by a firewall.  However I
> don't think you'll get that if you just overrun a buffer from the window
> of packets transmitted.  If you are passing several independent tcp
> streams there may not be any way to control it but any single connection
> should control its send window based on the remote acks.
> 
> ---
>   Les Mikesell
>     [email protected]
> 
> 



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