[hpsdr] Packet Length and Fragmentation

[Thierbach, Ed]

Thanks, Jeremy and everyone else, for the explanations.  I think I understand MTU/MRU better now.  I'm looking forward to learning even more as HPSDR dives into the IP pool.

73,
-Ed- AB8OJ

On Sep 21, 2010, at 4:32 PM, Jeremy McDermond wrote:

> On Sep 21, 2010, at 5:05 AM, Thierbach, Ed wrote:
> 
>> Hi, Jeremy -- what happens in the case where 2 systems on the same network segment are set to different MTUs?  Will they simply not communicate?
> 
> It depends on what the Maximum Receive Unit (MRU) is set to on the system.  In MacOS, for example, I don't think it's a settable parameter.  It's usually set to something very large by default.
> 
>> For example, if I have 2 Linux machines, and on one I issue 'ifconfig eth0 mtu 750' and on the other 'ifconfig eth0 mtu 1500', will the receiving PC just generate RX errors and discard the packets?
> 
> Probably not, because when the 1500 MTU sends a packet, the receiving machine will have a > 750 byte MRU, and will happily accept the packet.
> 
> This only works up to a point, though.  If you were to set the MTU to 5000 bytes, it probably won't work at all.  Normal Ethernet MTU is somewhere around 1500 bytes.  To get any larger, your hardware needs to support "Jumbo Frames" to work.  So, if you have one issued "ifconfig eth0 mtu 1500" and the other "ifconfig eth0 mtu 9000", the first would probably not receive the packets if the hardware didn't support jumbo frames, and have them turned on.
> 
>> 
>> Thanks much,
>> -Ed- AB8OJ
>> 
>> On Sep 20, 2010, at 7:34 PM, Jeremy McDermond wrote:
>> 
>>> ***** High Performance Software Defined Radio Discussion List *****
>>> 
>>> Sorry I haven't been around the TS sessions for the last couple of weeks, but I've been at various ham conferences over the last couple of weeks, and will be at DCC next weekend.
>>> 
>>> I was listening to the Teamspeak session from last Friday and noticing Dave WA7YWQ's comments, and have some contributions.
>>> 
>>> If you're getting fragmentation on the same network segment, you have some issues going on.  Yes, any router in the chain is allowed to change the packet length.  In IP parlance, we call it "fragmentation," and it's something that we try to avoid like the plague in network designs because it tends to destroy routers if there's a lot of fragmentation.  This is because the router has to handle the fragmentation and that isn't usually on the fast forwarding path.
>>> 
>>> The only reason why you fragment packets is because the MTU of two interfaces on a router are different.  For example, if you have ethernet on one end and a OC3 on the other, where the MTU of Ethernet is 1500 bytes, and the MTU of the OC3 is 4470 bytes.  If the router receives a packet on the OC3 that is larger than the 1500 byte Ethernet MTU, the router will have to fragment the packet so that it can be transmitted on the Ethernet.  This rarely happens, because both endpoints are usually on 1500 byte MTU ethernet, and IP doesn't "coalesce" packets.  Once they're fragmented, they stay fragmented.
>>> 
>>> So, the upshot of this explanation is that if you're on the same wire, there is no possibility for fragmentation, and you should receive packets exactly as they're transmitted.  The only time when you'll get modified packets is when you go through a router.  Switches are a L2 device, and will never fragment.
>>> 
>>> --
>>> Jeremy McDermond (NH6Z)
>>> Xenotropic Systems
>>> mcdermj at xenotropic.com
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> HPSDR Discussion List
>>> To post msg: hpsdr at openhpsdr.org
>>> Subscription help: http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
>>> HPSDR web page: http://openhpsdr.org
>>> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/
>> 
>> 
> 
> --
> Jeremy McDermond (NH6Z)
> Xenotropic Systems
> mcdermj at xenotropic.com
> 
> 
> 


 1285158236.0