Re: When are payload lengths greater than the negotiated MSS?

[Guy Harris <guy () alum mit edu>]

On Nov 26, 2009, at 10:01 AM, Ashwin Rao wrote:

> Clearly data payloads with length greater than the MSS negotiated are
> being exchanged. I would like to know under which conditions are
> payloads greater than the MSS exchanged?

When TCP segmentation offloading (TSO) is being done?

Linux's loopback interface might claim to support TSO (if somebody  
does, say, a 4K write to a TCP socket connected to another socket on  
the same machine, there's no point in breaking that up into MSS-sized  
chunks, as the MTU is just a convention for the loopback interface -  
there's no hardware or link-layer-standard reason for any particular  
MTU), and it might implement it by accepting larger-than-MSS chunks of  
data, *not* splitting it up before "sending" it (as, again, the MTU is  
just a convention), so that when the packet is received it looks like  
a big packet.

libpcap, on the Linux loopback device, hands the incoming copy of the  
packet, not the outgoing copy of the packet, to the application, so  
what you're seeing are the incoming packets.

> Further, is there a way to
> limit the payloads to the specified MSS value?

If TSO is the cause, you could see whether you can disable TSO on the  
loopback interface, somehow (and possibly get a reduction in  
throughput as a result).
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