without joking, the correct answer is yes...
In fact I run some tests for a fixed bandwidth (2 Mbps) and some over BoD
(max of 384 K) with a link delay of 600ms.
Due to TCP congestion avoidance behavior, in both case, the bigger MTU gives
the best results.
Of course, by increasing the MTU, you increase the RTT in the case of low
rate (time needed to transmit the packet) or BoD (ressources required to
transmit the packet). Thus you have to adapt your socket buffer size (and
thus your max window) to the BDP with the "real RTT", and not only the link
delay. In the case of Bod tests, my RTT was around 3-4s, which may be
comparable with large MTU on a low rate link. At the beginning of the
connection, the RTT is small, so you don't have problems with the RTO. When
you reach a steady state, the RTT increase, but the RTO is adjusted.
A good way to test the RTT during a connection without sophisticated tools
is simply to run on the same link a simple ping and a TCP bulk transfer.
With the ping you can get the real RTT, and then adjust your buffer size.
Hope it's helpfull !
Cheers
julien
> -----Original Message-----
> From: Dave [SMTP:[email protected]]
> Sent: Wednesday, February 16, 2000 1:40 AM
> To: Godard, Julien [HAL02:HM90:EXCH]; 'Johnson, Gregory LCDR';
> 'Testasecca, Mariano'
> Subject: Re: Setting the appropriate MTU
>
>
> RE: ..."x Mbps. . ."
> Is that like Bandwidth-On-Demand?
> ;)
>
> Cheers,
> Dave sends
>
> ----- Original Message -----
> From: Julien Godard <mailto:[email protected]>
> To: 'Johnson, Gregory LCDR' <mailto:[email protected]> ;
> 'Testasecca, Mariano' <mailto:[email protected]>
> Cc: '[email protected]' <mailto:'[email protected]'>
> Sent: Tuesday, February 15, 2000 10:53 AM
> Subject: RE: Setting the appropriate MTU
>
>
> Oups ! Yes, correct !
> I completelly forgot to look at the bandwidth. I am too use to run
> test with x Mbps...
>
> -----Original Message-----
> From: Johnson, Gregory LCDR [ SMTP:[email protected]
> <mailto:SMTP:[email protected]>]
> Sent: Tuesday, February 15, 2000 3:39 PM
> To: Godard, Julien [HAL02:HM90:EXCH]; 'Testasecca, Mariano'
> Cc: '[email protected]' <mailto:'[email protected]'>
> Subject: RE: Setting the appropriate MTU
>
> Bigger is not better for slow networks. For a link speed of
> 4kbps you will
> need to use much smaller MTU sizes since the transmission time for a
> packet
> gets too long otherwise. It will also depend upon what the latency
> is in the
> link because the total transmission time for a packet is a function
> of the
> size and delay. For a typical GEO link, I would expect that you have
>
> anywhere up to 600ms depending upon what kind of FEC and
> interleaving is
> done on the link. I have found for Inmarsat-M/Mini-M type links (2.4
> kbps)
> that an MTU size of 500 works well. You also need to adjust the
> initial RTO
> value larger (the default is 3 secs, try 5 secs). If you don't do
> this, you
> will suffer a lot of timeouts and retransmissions. I'm not that
> familiar
> with the protocol Citrix uses, but with only 4kbps of bandwidth, you
> also
> don't want to run multiple simultaneous connections across your link
> as one
> connection will probably saturate the link.
>
>
> -Greg
>
> LCDR Gregory W. Johnson
> Ass't Prof. Electrical Engineering
>
> USCG Academy
> New London CT
>
> 860-444-8683
>
> -----Original Message-----
> From: Julien Godard [ <mailto:[email protected]>]
> Sent: Tuesday, February 15, 2000 4:59 AM
> To: 'Testasecca, Mariano'
> Cc: '[email protected]'
> Subject: RE: Setting the appropiate MTU
>
>
>
> for performance, the bigger the better !
> But it may depend also on your network because you may want to avoid
>
> fragmentation.
> You can test your MTU with some free tools such as easyMTU
> <http://members.tripod.com/~EasyMTU/> <
> <http://members.tripod.com/~EasyMTU/>>
> For example if you use ethernet link, the MTU is 1500, leading to
> 1448 bytes
> of data in each segment (20 IP + 20 TCP + 12 TCP timestamp option +
> 1448
> data). If your router support this size without fragmentation, it's
> fine !
>
> -----Original Message-----
> From: Testasecca, Mariano [SMTP:[email protected]]
>
> Sent: Monday, February 14, 2000 7:49 PM
> To: '[email protected]'
> Subject: Setting the appropiate MTU
>
>
> What MTU value would be appropiate for Windows 95/98
> terminals
> working as a
> Windows NT 4.0 running Citrix MetaFrame under
> a satellite link of a VSAT that provides 4kbps of bandwidth?
>
> Thanks to all.
>
> Mariano Testasecca
> SIEMENS S.A.
> Divisi�n Electromedicina
> Servicios de Salud - SHS
> Bolivar 177 1� Piso
> *+54-11-4340-8400 int. 2796
> * <mailto:[email protected]>
> < <mailto:[email protected]>>
>
>
>
>
>
>
>
This archive was generated by hypermail 2b29 : Wed Feb 16 2000 - 06:13:05 EST