Re: Question about LRO/GRO and TCP acknowledgements

[Ben Hutchings <bhutchings@solarflare.com>]

On Sat, 2011-06-11 at 21:59 +0200, Joris van Rantwijk wrote:
> Hi,
> 
> I'm trying to understand how Linux produces TCP acknowledgements
> for segments received via LRO/GRO.
> 
> As far as I can see, the network driver uses GRO to collect several
> received packets into one big super skb, which is then handled
> during just one call to tcp_v4_rcv(). This will eventually result
> in the sending of at most one ACK packet for the entire GRO packet.
> 
> Conventional wisdom (RFC 5681) says that a receiver should send at
> least one ACK for every two data segments received. The sending TCP
> needs these ACKs to update its congestion window (e.g. slow start).
> 
> It seems to me that the current implementation in Linux may send
> just one ACK for a large number of received segments. This would
> be a deviation from the standard. As a result the congestion
> window of the sender would grow much slower than intended.

This was a problem in older versions of Linux (and still is on other
network stacks that aren't aware of LRO).

> Maybe I misunderstand something in the network code (likely).
> Could someone please explain me how this ACK issue is handled?

LRO implementations (and GRO) are expected to put the actual segment
size in skb_shared_info(skb)->gso_size on the aggregated skb.  TCP will
then use that rather than the aggregated payload size when deciding
whether to defer an ACK.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.