[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))

David Borman david.borman at windriver.com
Fri Jan 19 11:03:33 PST 2007

No, it's more than just Large Send Offload or Large Receive Offload.   
That's done on a per-packet basis, without needing to keep much, if  
any state.  In the scenario I'm citing the splitter is also changing  
the window and the MSS option.  The remote host offers a (relatively)  
small window, the splitter offers a much bigger (512K) window to the  
host on the 64K MTU network (in addition to rewriting the MSS  
option).  With the small delay*bandwith to the remote host, the  
splitter has no trouble keeping the pipe full using standard ethernet  
packets.  But if those packets went all the way to the 64K host  
across the large delay*bandwidth 64KMTU network, there'd be a lot of  
idle time waiting for window updates, and you get much lower  
throughput from end-to-end.
			-David Borman

On Jan 19, 2007, at 11:17 AM, Joe Touch wrote:

> rick jones wrote:
>>> In this scenario, the 1500 byte host may be only offering a  
>>> window of,
>>> say 16K.  The splitter offers a window to the 64K host of something
>>> like 512K.  This allows the 64K MTU host to send multiple 64K sized
>>> packets, which the splitter then sends out as ethernet size  
>>> packets to
>>> the remote host.  In other words, for a 16K vs. 512K scenario, for
>>> each window of data transferred between the 64K host and the  
>>> splitter,
>>> there are 32 windows of data transferred out to the remote hosts.
>>> Conversely, as 1500 byte packets arrive from the remote host,  
>>> they are
>>> acked and accumulated into larger packets that are then transferred
>>> over the 64K MTU network in larger packets.
>> Apart from calling it a splitter, superficially at least that  
>> resembles
>> what some 10G NICs can do today, albeit with some explicit
>> knowledge/assistance by the stack.  Large send has the stack(host)
>> giving the NIC(splitter) a large "segment" which the NIC(splitter)
>> resegments for the link.  Those flow across the ethernet to the other
>> NIC(splitter) which if it has Large Receive Offload enabled will
>> "upsegment" the ethernet-sized traffic and give larger segments to  
>> the
>> receiving stack(host).
> Right - this looks like a cooperative outboard processor, which  
> makes a
> lot of sense in some environments when both the outboard processor and
> host are managed/controlled by the same entity, but still makes very
> little sense (to me) when that's not the case.
> Joe
> -- 
> ----------------------------------------
> Joe Touch
> Sr. Network Engineer, USAF TSAT Space Segment

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