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To: CIPE <cipe-l,AT,inka,DOT,de>
Subject: Re: Checklist (sub-thread of: "Slow file sharing performance: lostpidgeons")
From: James Knott <james.knott,AT,rogers,DOT,com>
Date: Tue, 09 Sep 2003 09:19:35 -0400
In-reply-to: <1063112421.30742.26.camel@les-home.futuresource.com>
References: <002f01c376aa$bb23a860$d620a8c0@pcw_hans.hnsasd.priv> <3F5DC241.90200@rogers.com> <1063112421.30742.26.camel@les-home.futuresource.com>

Les Mikesell wrote:
On Tue, 2003-09-09 at 07:06, James Knott wrote:


I am aware the UDP is best effort and that the services using it are supposed to provide flow control if necessary. Given that FTP, which uses TCP works fine and Samba and NFS, which use UDP have problems, what observations would you make?



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. TCP has a window size mechanism that will
limit the unacked packets sent on a single connection. CIPE
doesn't change this by using UDP - it just wraps the packets
already being sent with another header during delivery.  Things that
use UDP natively for reliable transfers (NFS, etc.) normally
have their own windowing and error correction mechanism that
can differ in size and backoff strategy from TCP's.

I suspect this may be the cause of the problem.

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.

C) If you want to diagnose where the problem is you will have to use tcpdump or some other protocol analyzer to see where the packets are being discarded. If you watch a tcp stream like samba going into the cipe interface you will see the tcp sequence numbers repeat as tcp's error recovery fixes things. On the other end you will only see one instance come out. Then you have to figure out which hardware interface dropped the cipe packet between them.

I posted some tcpdump files to the list yesterday. I had sent them to Allan earlier.



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.



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